Ultraviolet-blocking polymers and composites: recent advances and future perspectives

Abstract

Due to the many negative effects of ultraviolet light (UV) irradiation on human health, material stability, and food deterioration, substantial efforts have been dedicated to the development of UV-blocking materials. Here, we present a comprehensive overview of recent innovations in strategies and mechanisms for creating UV-blocking polymers and composites. Basic models of UV-blocking composites will be mentioned, including those that incorporate organic, inorganic, or polymeric UV filters that exhibit strong UV scattering and/or absorption capability. In addition, strategies to enable UV-blocking properties in pure polymers are discussed in detail, including polymer structure design and surface/in situ chemical modification. We will emphasize the importance of the design of UV absorbing motifs to the ultimate performance in UV-blocking. Eventually, we will elaborate on the significant challenges and new opportunities in this field. This review aims to motivate further innovations in this emerging field of UV-blocking materials.

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Huan Zhang

Dr Huan Zhang received her B.S. degree from Northwest University (China) and her PhD degree from the Institute of Chemistry, Chinese Academy of Sciences, under the supervision of Prof. Jian Xu. She completed her postdoctoral training at Shenzhen University with Prof. Ning Zhao and Prof. Cuihua Li. Currently, she holds a lecturer position at the School of Materials, Henan University, China. Her research focuses on polymeric materials with high performance and versatility, including high strength, high stretchability, high toughness, recyclability, self-healing as well as UV-blocking, and more.

Jie Ju

Prof. Jie Ju received her B.S. degree from Jilin University and her PhD from the Institute of Chemistry, Chinese Academy of Sciences, under the supervision of Prof. Lei Jiang. She finished her postdoctoral training at Brigham and Women's Hospital, Harvard Medical School with Prof. Ali Khademhosseini and Tufts University with Prof. Brian P. Timko. She is currently a full professor at the School of Materials, Henan University, China. Her research interests include materials for the water-energy nexus, including zero energy-input fog harvesting, solar desalination, liquid super-spreading enabled heat dissipation and electric-energy harvesting through manipulating interactions between liquid and surfaces with special wettability.

Xi Yao

Prof. Xi Yao received his PhD from Jilin University in 2014, majoring in physical chemistry. During 2015–2018, he worked with Prof. Zhigang Suo as a postdoctoral research fellow at Harvard University. He is currently working as a full professor in the School of Materials, Key Laboratory for Special Functional Materials at Henan University, Henan, China. His research focuses on superwetting materials for desalination and heat management; bio-inspired hydrogel coatings with superwettability and special mechanical features for engineering and biomedical applications.

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1. Introduction

Ultraviolet light (UV for short) produced by natural (sun) or artificial UV sources (e.g., electric arcs, mercury-vapor lamps, and germicidal cabinets) usually spans the wavelength range of 10–400 nm, featuring higher frequency and shorter wavelengths than visible light (referred to as vis, 400–800 nm). In general, UV is distinguished by three bands: UVA (near-UV, 320–400 nm), UVB (far-UV, 280–320 nm), and UVC (ultra-UV, 200–280 nm) (Fig. 1(a)). The exposure to UV radiation has its pros and cons (Fig. 1(b)). On the positive side, as a form of energy input, UV has been widely used in resin curing¹ and UVC disinfection.² In addition, small amounts of UVB radiation on the skin play a crucial role in vitamin D production to maintain calcium homeostasis in the body.³ However, excessive or prolonged UV exposure induces photochemical reactions, causing skin cancer,^4,5 loss of food nutrients,⁶ aging and degradation of organic materials,⁷ among many adverse effects. Over the years, the ongoing depletion of the ozone layer in our atmosphere has caused an increase in solar UV radiation intensity at the Earth's surface, thereby posing a more significant threat to human health and the ecosystem. Therefore, there is a compelling need to develop UV-blocking materials for diverse application scenarios.

Fig. 1 Ultraviolet light (abbreviated as UV) and UV-blocking polymers and composites. (a) Different categories of UV (including UVA, UVB, and UVC) and their wavelength ranges in the electromagnetic spectrum. (b) Advantages and disadvantages of UV. (c) UV-blocking mechanism and potential applications of UV-blocking polymers and composites.

Among many solid materials, polymers are ubiquitous and utilized in almost every field of application. Polymeric materials are aggregates of molecular chains, with widely tunable optical as well as mechanical properties. The freedom in designing chemical and physical structures makes polymers a great ideal starting point for creating UV-blocking materials and devices. One of the most important applications for UV-blocking polymers is in the electronics industry. The fast growing dependence on electronic devices poses tremendous demands on UV-blocking polymers and composites, whose function is to scatter and/or absorb UV, which is either shone upon or emitted from the electronic devices. In fact, UV-blocking polymeric materials have already been indispensable in practical applications, such as foldable displays,⁸ crop cultivation,^9,10 physical protection,¹¹ photovoltaic cells,^12,13 food preservation,⁶ preservation of historical artifacts,^14–16 and so many more (Fig. 1(c)).

To date, there have been many successful attempts at developing UV-blocking polymers^17–19 and composites.^8,20–22 Usually, composite polymer systems are established by loading various UV filters (organic,⁸ inorganic,²⁰ and polymeric,^21,22 in the form of particles, sheets, or fibers) into polymer matrices (Fig. 2(a)). These UV filters are capable of scattering and/or absorbing UV. Comparatively, pure polymer systems (that are not doped with UV filters of any sort) inherently contain strong UV absorption motifs, accountable for the UV-blocking properties. The UV-absorbing motifs are either incorporated into the backbone or sidechains of polymers¹⁷ or grafted to the surface or bulk^18,19 (Fig. 2(b)).

Fig. 2 Strategies for the creation of UV-blocking polymers and composites. (a) Composite polymer systems. Diverse UV filters, including organic, inorganic, and polymeric (in the form of particles, sheets, or fibers), are loaded into polymer matrices. (b) Pure polymer systems. UV-blocking polymers inherently contain UV absorbing motifs, which can be incorporated into the main chains, side chains, or crosslinks through polymer structure design. In addition, these motifs can also be introduced into existing polymers through surface or in situ chemical modification.

In the literature, there are several reviews on specific topics, such as UV-blocking materials in food packaging,^6,23–25 novel designs based on specific photo-additives such as lignin^26–29 or nanomaterials.^30,31 Despite this, there lacks a comprehensive review on UV-blocking polymers and composites, which in an angle of materials science, covers a broad design and optimization methodology related to both the chemical and physical structures of polymers and/or their composites. Such a review can serve as essential guidance for the vast number of end-users who are committed to accurately assembling specific polymer compositions and matrices or choosing an appropriate UV-blocking filter for particular needs. It would also provide insights into the trade-off between UV-blocking and vis-transparency for optimization in fields related to optical applications. Here, we focus on recent advances and future perspectives in UV-blocking polymers and composites. First, we briefly organize existing methods for evaluating the UV-blocking properties in Section 2. Second, we categorize the synthesis and compare the performances of UV-blocking composites and pure polymer systems in Sections 3 and 4, respectively. Finally, we summarize our perspectives on the promises and challenges of the current UV-blocking polymers and composites.

2. Evaluation methods of UV-blocking properties

UV-vis spectroscopy is an effective tool for evaluating the UV-blocking properties of polymer and composite films. It assesses their absorbance and transmittance responses.^24,32,33 The wavelength range of 200–400 nm is particularly important as it affects the stability of organic materials, food quality, nutritional values, and skin health. In addition, when UV-blocking polymers are applied in food packaging, automotive window tinting, medical equipment, optical components, or visualization of pipeline systems, it is highly desirable for them to maintain vis-transparency. Therefore, the light transmittance of polymer and composite films in the wavelength range of 200–800 nm is measured to assess both the UV-blocking and the vis-transparency. The vis-transparency is usually demonstrated by the light transmittance (T) at a specific wavelength (e.g., T₆₀₀ represents the transmittance at a wavelength of 600 nm) in the wavelength range of 400–800 nm. The UV-blocking properties can be quantified by calculating the average transmittance within a specific wavelength range or at a particular UV wavelength. For example, eqn (1)–(4) are used to calculate the UV-blocking properties in UVA (320–400 nm), UVB (280–320 nm), and UVC (200–280 nm), and at a specific UV wavelength λ′, respectively.³⁴

UV-blocking of λ′ (UB_λ′, %) = 100 − T(λ′) (4)

where T(λ) is the average transmittance of the polymer film at the wavelength λ, dλ is the polymer film's bandwidth, and T(λ′) is the average transmittance of the polymer film at a specific wavelength λ′. In addition, the UV-blocking properties of fabrics are expressed using UV protection factor 21 (UPF) values.³⁵

3. Composite polymer systems

It is widely accepted that incorporating UV filters into polymer matrices is a common and effective approach to enhancing UV-blocking properties. These filters have the ability of scattering and/or absorbing UV. Both the UV filters and the polymer matrices have an impact on the UV-blocking properties. Currently, three types of UV filters, including organic, inorganic, and polymeric (in the form of particles, sheets, or fibers), have been utilized. Thus, we categorize the composite polymer systems into three groups, namely organic UV filter/polymer, inorganic UV filter/polymer, and polymeric UV filter/polymer systems. This section will thoroughly discuss these composite polymer systems. We summarize the optical properties, including UV-blocking and vis-transparency, of these UV-blocking polymer composites discussed in this section in Table 1.

Table 1 Summary of the optical properties, including UV-blocking and vis-transparency, of these UV-blocking polymer composites discussed in Section 3

Polymer composite systems	Polymer matrices	Organic/inorganic/polymeric UV filters	UV-blocking (UB)	Vis-transparency (T)	Ref.
Organic UV filter/polymer	Hemicellulose and polyvinyl alcohol (PVA)	Potassium cinnamate	100% of UB200–280, ∼98% of UB320, ∼85% of UB400 (C2-P)	∼40% of T600 (C2-P)	36
	Sodium alginate	Thymol	100% of UB200–280, <90% of UB280–400 (Thy-10)	∼15% of T600 (Thy-10)	37
	Chitosan	Ellagic acid, caffeic acid	∼100% of UB200–400 (CH/EA_5.0, CCAF-1)	<20% of T660 (CH/EA_5.0, CCAF-1)	38 and 39
	Pectin	Curcumin	(97.8 ± 0.03)% of UB280 (Pec/Cur/SNP^1)	(49.5 ± 0.9)% of T660 (Pec/Cur/SNP^1)	40
	Soy protein isolate	Cardanol derivative	100% of UB200–350 and ∼92% of UB400 (HBPE-GDE-MCGE)	∼40% of T550 (HBPE-GDE-MCGE)	41
	Poly(lactic acid) (PLA)	Benzoxazine	98.3% of UB350 and ∼75% of UB400 (BOZ30)	90% of T550 (BOZ30)	42
	Cellulose	Naringin	100% of UB200–320 and ∼88% of UB320–400 (CN-20)	∼85% of T600 (CN-20)	43
	Acrylic resin	Dimethyl 2-(4-(dimethylamino)benzylidene)malonate) and ethyl 2-cyano-3,3-diphenylacrylate	∼100% of UB200–400	∼100% of T550	8 and 44
Inorganic UV filter (particle)/polymer (blending)	Polydimethylsiloxane	Silicon nanocrystal	∼100% of UB200–400	∼16% (ncSi:H/PDMS) and ∼5% (ncSi-decyl/PDMS) of T600	45
	PVA	(N-doped) carbon quantum dot	90–100% of UB280,320,400 (CQDs-PVA), 70–85% of UB280,400 (N-CD (50 μL mL^−1)/PVA)	∼80% of T600 (CQDs-PVA), ∼78% of T600 (N-CD (50 μL mL^−1)/PVA)	46 and 47
	PLA, 2-hydroxyethyl cellulose (HEC), 2-hydroxyethyl starch (HES), cellulose	ZnO	∼98% of UB200–400 (PLA/ACNC/7%ZnO), 80–95% of UB320,400 (HEC/ZnO), 30–45% of UB320,400 (HES/ZnO), ∼100% of UB200–380 and ∼89% of UB400 (MNF/PDA0.5-ZnO)	∼8% of T600 (PLA/ACNC/7%ZnO), ∼60% of T600 (HEC/ZnO), ∼80% of T600 (HES/ZnO), ∼10% of T600 (MNF/PDA0.5-ZnO)	48–50
Inorganic UV filter (sheet)/polymer	PVA, chitosan	Boron nitride	∼100% of UB280–400 (PVA/70% sucrose-g-BNNSs), 95–100% of UB280,320,400 (CS:hBNNSs = 95 : 5)	∼30% of T600 (PVA/70% sucrose-g-BNNSs), ∼15% of T600 (CS:hBNNSs = 95 : 5)	51 and 52
	Konjac glucomannan	Montmorillonite	∼100% of UB200–250, ∼90% of UB280 and 20–40% of UB320,400 (MMT-KGM (30%))	∼90% of T600 (MMT-KGM (30%))	53
	Cellulose	Graphene oxide	∼100% of UB200–400 (CP40GO0.8)	∼60% of T600 (CP40GO0.8)	54
	PLA, sodium alginate	Mica	∼96% of UB280 and ∼55% of UB400 (PLA/50 wt% mica, sodium alginate/1 wt% mica)	70–75% of T600 (PLA/50 wt% mica, sodium alginate/1 wt% mica)	20 and 55
Inorganic UV filter (particle)/polymer (surface modification)	Cotton and wood fabrics, cellulosic paper	TiO2, ZnO	90–99.95% of UB280–320 and 85–99.8% of UB320–400 (TiO2, ZnO coated cotton and wood fabrics, cellulosic paper)	<5% of T600 (ZnO coated cotton fabric), <15% of T600 (TiO2 coated cellulosic paper)	56–60 and 61
	Knit polyester fabric	Graphene oxide	∼100% of UB330–380 and ∼97.5% of UB390 (graphene oxide coated fabric)	—	62
	Cotton and silk textiles	Metal–organic framework	99.5–100% of UB280–400 (In-MIL-2 or Zn/Zr MOF coated cotton or silk)	—	63 and 64
Polymeric UV filter (particle)/polymer	PVA, poly(butylene adipate-co-terephthalate), PLA-PCL, waterborne epoxy, cellulose	(Modified-)lignin, lignosulfonic acid, lignin-melanin, tannic acid-modified sodium lignosulfonate, quaternized sodium lignosulfonate, esterified lignin	∼95% of UB320 and ∼60% of UB400 (PLA/LMNP4), 100% of UB200–400 (LA-10), ∼100% of UB280–350 and ∼85% of UB400 (LNP-5.0%), 100% of UB200–400 (TA@LS-Ag-5), 100% of UB200–350 and ∼90% of UB400 (PLA/PCL-12L), ∼100% of UB200–320 and ∼76% of UB400 (5-QLS/WEP), 100% of UB200–350 and ∼85% of UB400 (RPC-EP1–10)	∼80% of T600 (PLA/LMNP4), ∼25% of T600 (LA-10), ∼55% of T600 (LNP-5.0%), ∼45% of T600 (TA@LS-Ag-5), ∼50% of T600 (PLA/PCL-12L), ∼78% of T600 (5-QLS/WEP), ∼58% of T600 (RPC-EP1–10)	65–71
	PVA	Cinnamate-functionalized cellulose nanocrystal (Cin-CNC), dopamine-melanin hollow nanoparticle (Dpa-h NP)	100% of UB200–320 and ∼35% of UB400 (PVA/6 wt% Cin-CNCs), ∼100% of UB200–400 (PVA/5 vol% Dpa-h NPs)	∼80% of T600 (PVA/6 wt% Cin-CNCs), ∼58% of T600 (PVA/5 vol% Dpa-h NPs)	72 and 73
Polymeric UV filter (sheet)/polymer	PVA	Modified-cellulose nanofiber	∼92% of UB280–320 and ∼53% of UB320–400 (10 wt% BT-CNF)	∼87% of T550 (10 wt% BT-CNF)	74
	Gelatin	Polyphenol-modified microfibrillated cellulose	∼100% of UB280–320 and 85% of UB320–400 (TA@MFC-G 20)	∼48% of T550 (TA@MFC-G 20)	75
Polymeric UV filter (polymeric dispersed phase)/polymer	Poly(ethylene terephthalate-co-1,4-cylclohexylenedi methylene terephthalate) (PETG)	Citrate-based polyesters (CPs)	100% of UB200–400 (PETG/2% CPs)	∼80% of T600 (PETG/2% CPs)	76
	PVA	Catechol-functionalized chitosan	60–82% of UB280,320,400 (CP20)	∼50% of T660 (CP20)	77
	Soy protein isolate (SPI) and polyethyleneimine (PEI)	Polydopamine (PDA)	∼100% of UB200–380 and ∼98% of UB400 (SPI-PEI-PDA)	∼25% of T500 (SPI-PEI-PDA)	78
	PLA	Polyphosphate (PPD), polybis(2-amino-6-hydroxypurine)phosphazene (PAHP)	100% of UB280–400 (PLA/6% PPD),100% of UB280–320 and ∼70% of UB400 (PLA/5% PAHP)	5.5% of visible light transmittance (PLA/6% PPD)	79 and 80
	Polymethyl methacrylate (PMMA)	Sulfur-rosin (Sx-Ro)	100% of UB200–300, ∼85% of UB320 and ∼20% of UB400 (PMMA/S50-Ro (0.25%))	∼90% of T600 (PMMA/S50-Ro (0.25%))	81

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3.1 Organic UV filter/polymer systems

In general, organic UV filters are known for their specific UV absorption abilities and high blocking efficiency. This is attributed to the presence of chromophore groups, such as unsaturated bonds like aromatic rings (C=O and C=C), auxochrome groups like –NH₂, –OH, and –OR, and conjugated structures like –C=C–C=C– and –C=C–C=O–. Many organic UV filters exhibit good UV absorption abilities. Some examples include salicylic acid,⁸² potassium cinnamate,³⁶ thymol,³⁷ 4-dimethylaminopyridine,³² dibenzoylmethane derivatives,⁸³ camphor derivatives,⁸³N-heterocycle-containing benzotriazoles,⁸⁴ ellagic acid,³⁸ curcumin,⁴⁰ maleic cardanol derivatives,⁴¹ benzoxazine,⁴² naringin,⁴³ benzophenone derivatives,⁸⁵p-aminobenzoic acid derivatives,⁸³ caffeic acid,³⁹ crylenes,⁸³ and more (Fig. 3).

Fig. 3 Chemical structures of various organic UV filters.

Polymers containing organic UV filters have demonstrated excellent UV-blocking properties. For example, potassium cinnamate was incorporated into a hemicellulose/polyvinyl alcohol (PVA) composite system to create a UV-blocking food packaging film.³⁶ The presence of potassium cinnamate resulted in approximately 100% UV-blocking in the 200–300 nm UV region. Similarly, sodium alginate-based films infused with thymol exhibited UV-blocking capability.³⁷ The concentration of thymol played a crucial role in controlling the optical properties, with higher concentrations offering better UV-blocking properties and lower vis-transparency. For example, with a thymol concentration of 10 mg mL⁻¹, the UV-blocking reached close to 100% in the wavelength range of 250–295 nm, while lower concentrations of 0.01–1 mg mL⁻¹ showed less than 100% UV-blocking. The transmittance at 550 nm decreased from approximately 70% (0.01 mg mL⁻¹ thymol) to around 15% (10 mg mL⁻¹ thymol). To evade the trade-off between UV-blocking and vis-transparency, Heredia-Guerrero and coworkers developed naringin/cellulose composites.⁴³ When the naringin content was 20 wt%, the composites exhibited UV-blocking properties of approximately 88% and 100% in the UVA and UVB regions, respectively, while also maintaining high vis-transparency with around 85% transmittance at a wavelength of 550 nm. Moreover, composite systems using ellagic acid/chitosan³⁸ and curcumin/pectin composites⁴⁰ have also demonstrated UV-blocking properties. Notably, the UV filters employed in these composites are natural organic phytochemicals, ensuring food safety and instilling trust. Other natural compounds, such as tannic acid, can also act as UV filters due to the presence of gallic acid moieties and numerous benzene ring structures. Tannic acid has an ability to form physical crosslinking with polymer matrices through hydrogen bonding or π–π stacking, leading to an enhanced supramolecular structure that effectively blocks UV radiation.⁸⁶ Inspired by mussel adhesion chemistry, oxidized tannic acid with abundant phenol-quinone structures has shown exceptional UV-blocking capabilities (approximately 99.99%) in an all-polymer piezoelectric elastomer.⁸⁷

In addition to natural organic UV filters, some synthetic organic UV filters have also been used to create UV-blocking polymer composites. One common and typical synthetic organic UV filter is benzoxazine, which is a six-membered heterocyclic ring compound containing nitrogen and oxygen. Benzoxazine exhibits strong absorption in the wavelength range of 300–400 nm, with a peak absorption wavelength around 350 nm. By incorporating benzoxazine into poly(lactic acid) (PLA), the UV-blocking properties of the composite are significantly enhanced. For example, the addition of benzoxazine results in a UV-blocking efficiency of approximately 98.3% at 350 nm, compared to only 6.7% for the pristine PLA film.⁴² Another synthetic organic UV filter, synthesized by Li and coworkers, is maleic anhydride-cardanol glycidyl ether (MCGE).⁴¹ MCGE absorbs UV radiation due to the presence of unsaturated bonds and phenolic hydroxyl groups. To prevent its migration, MCGE is anchored onto the polymer chains through hydrogen bonds. The resulting MCGE-based composite film exhibits excellent UV-blocking capabilities, blocking 100% of UVB and UVC, as well as over 90% of UVA radiation. In addition, Luo and coworkers have designed low-migration macromolecular benzophenones to address the migration issues associated with small organic UV filters.⁸⁵ These benzophenone-based composites are suitable for food packaging materials, ensuring food safety.

Currently, there is a high demand for advanced optically clear adhesives (OCAs) with UV-blocking properties in the development of organic light-emitting diode panels for foldable displays. However, incorporating UV filters into OCAs poses a challenge as it restricts the use of conventional UV-curing methods. To address this challenge, Kwon and coworkers developed UV-blocking OCAs using a new acrylic resin that could be efficiently cured under visible light without the need for oxygen removal (Fig. 4(a)).⁸ The system combined novel photocatalysts (PCs), such as 2,4,5,6-tetra(9H-carbazol-9-yl)isophthalonitrile, and amine reductants, such as 2-dimethylaminoethyl acetate, which facilitated the curing of acrylic resin in the presence of two UV filters (Fig. 4(b)). The resultant OCAs effectively blocked UV radiation (98.8% UV-blocking at 373 nm) while maintaining transparency in the visible light region (approximately 100% transmittance at 452 nm, Fig. 4(c)). Dynamic folding tests demonstrated that the prepared UV-blocking OCAs were suitable for use in foldable displays. Although the visible light curing method for fabricating UV-blocking OCAs was presented, there was a barrier to commercialization due to the slow curing speed. To achieve rapid fabrication of UV-blocking OCAs using visible light, Kwon and coworkers subsequently chose 2,4,5,6-tetrakis(diphenylamino)isophthalonitrile (4DP-IPN) as the PC to minimize energy and electron transfer to UV filters. They also selected [4-(octyloxy)phenyl](phenyl)iodonium hexafluoroantimonate (HNu 254) and 2-butanoyloxyethyl(trimethyl)azanium butyl(triphenyl)boranuide (Borate V) as ionic co-initiators to facilitate faster electron transfer and more rapid PC regeneration (Fig. 4(e)).⁴⁴ Compared to previous amine-based co-initiators,⁸ the use of these advanced ionic co-initiators resulted in a tenfold increase in the fabrication speed of UV-blocking OCAs. As shown in Fig. 4(d), the prepared UV-blocking OCA exhibited exceptional effectiveness in blocking UV radiation (98.7% UV-blocking at 345 nm) while maintaining remarkable vis-transparency (approximately 100% transmittance at 455 nm). The combination of two UV filters (Fig. 4(b)) in these two studies ensured exceptional UV-blocking properties. UV filter-1 (dimethyl(2-(4-(dimethylamino)benzylidene)malonate)) blocked UV radiation in the wavelength range of 350–400 nm (maximum UV absorption peak at 368 nm), while UV filter-2 (ethyl 2-cyano-3,3-diphenylacrylate) blocked UV radiation below 350 nm (maximum UV absorption peak at 300 nm).

Fig. 4 Optically clear adhesives (OCAs) with UV-blocking properties prepared from visible light curable acrylic resins. (a) Schematic illustration for film curing of UV-blocking OCAs and the composition of pre-polymers. (b) Chemical structures of two UV filters used in this study. (c) UV-vis spectrum and an optical photograph of the prepared UV-blocking OCA. Reproduced from ref. 8 with permission from Wiley. (d) UV-vis spectra of the UV-blocking and non-UV-blocking OCAs and UV light-emitting diode (LED) emission spectra. (e) Schematic illustration of the rapid production of UV-blocking OCAs using 4DP-IPN as the photocatalyst and HNu 254/Borate V as the ionic co-initiator. An optical image showing the prepared UV-blocking OCA. Reproduced from ref. 44 with permission from Springer Nature.

3.2 Inorganic UV filter/polymer systems

Adding inorganic UV filters to polymeric matrices is an effective method to enhance the UV-blocking properties of polymeric materials. Examples of inorganic UV filters include inorganic particles like titanium dioxide (TiO₂),^88,89 nanocrystals,⁴⁵ quantum dots,^46,47,90 zinc oxide (ZnO),^{48–50,91,92} and silica,⁹³ as well as inorganic sheets such as hexagonal boron nitride nanosheets (BNNSs),^51,52 montmorillonite (MMT),⁵³ graphene oxide (GO),⁵⁴ and mica nanosheets.^20,55 Although these filters may have some drawbacks, such as blocking visible light and poor compatibility, they provide broad-band blocking and excellent thermal stability. In addition, there is growing interest in the surface modification of polymeric materials using inorganic UV filters.^56–60

3.2.1 Blending inorganic particles with polymers. UV-blocking coatings with self-healing capabilities not only protect organic polymers from photochemical degradation caused by UV radiation but also prolong their service time compared to traditional coatings. To achieve these properties, Zhang and coworkers developed a UV-blocking coating by connecting a UV filter (β-cyclodextrin (βCD)-modified TiO₂ nanoparticle) with a polymer matrix (poly(hydroxyethyl methacrylate-co-butyl acrylate), abbreviated as P(HEMA-co-BA)), through host–guest interactions.⁸⁸ This modification of the TiO₂ surface with βCD served two key purposes: facilitating the formation of host–guest interactions and the enhancement of TiO₂ dispersity. The host–guest interactions were responsible for the impressive self-healing capability of the coating, exhibiting a healing efficiency of 84%. In addition, the composite showed significantly stronger UV-absorbing properties in the 200–350 nm wavelength range compared to the polymer raw material P(HEMA-co-BA), due to the UV absorbing and scattering capacities of TiO₂. In another study conducted by Ozin and coworkers, they fabricated polydimethylsiloxane (PDMS) composites using hydrogen-terminated silicon nanocrystals (ncSi:H) and decyl-terminated silicon nanocrystals (ncSi-decyl) (Fig. 5(a1)).⁴⁵ These composites, in contrast to pure PDMS (which blocked approximately 49% of UV radiation), exhibited 100% UV-blocking capability in the 280–315 nm wavelength range due to the UV absorption properties of the two types of silicon nanocrystals. Even when extended to 400 nm, the ncSi-decyl/PDMS composite still maintained close to 100% UV-blocking (Fig. 5(a2)). However, the addition of silicon nanocrystals did lead to a decrease in vis-transparency. At 550 nm, the transmittance of ncSi:H/PDMS and ncSi-decyl/PDMS was approximately 15% and 4%, respectively, both lower than that of pure PDMS (approximately 58%).

Fig. 5 Blending an inorganic UV filter (particle or sheet) with a polymer matrix. (a) Optical properties influenced by the modification of inorganic particles. (a1) Schematic illustrations of the hydrogen-terminated silicon nanocrystals/polydimethylsiloxane (PDMS) composite (ncSi:H/PDMS) and decyl-terminated silicon nanocrystals/PDMS composite (ncSi-decyl/PDMS). (a2) Comparison of UV-blocking at 280–315 and 400 nm (UB_280–315 and UB₄₀₀) and vis-transparency at 550 nm (T₅₅₀) of pure PDMS, ncSi:H/PDMS, and ncSi-decyl/PDMS composites. Reproduced from ref. 45 with permission from Wiley. (b) Optical properties influenced by the content of inorganic particles. (b1) UV-blocking at 400 nm (UB₄₀₀) and (b2) transmittance at 550 nm (T₅₅₀) as a function of zinc oxide (ZnO) contents. Reproduced from ref. 48 with permission from Elsevier. (c) Inorganic particle distribution influenced by the polymer matrix. (c1) Schemes of the assembly structures of ZnO nanoparticles with 2-hydroxyethyl cellulose (HEC) and 2-hydroxyethyl starch (HES) before and after drying. (c2) Comparison of UV-blocking at 354 nm (UB₃₅₄) between ZnO/HEC and ZnO/HES composites. Reproduced from ref. 49 with permission from the ACS. (d) Inorganic sheet (nano-mica)/polymer (sodium alginate) composite. (d1) Schematic illustration of the structure of the nano-mica/sodium alginate composite prepared by a water-evaporation-induced self-assembly method. (d2) UV-vis spectra of the pure sodium alginate and nano-mica/sodium alginate composite films. (d3) Photographs showing the storage results of tomatoes packaged without/with a nano-mica/sodium alginate composite film. Reproduced from ref. 55 with permission from Elsevier.

ZnO has been considered a potential candidate for enhancing the UV-blocking properties of polymeric materials due to its inherent scattering and UV-absorbing capacities. The optical properties of the ZnO-loaded polymeric materials are highly influenced by factors such as nanoparticle content, size, and distribution. For example, the pristine PLA/acetylated cellulose nanocrystal (ACNC) film showed high vis-transparency (approximately 75% of T₅₅₀) but had low UV-blocking (around 40% of UB₄₀₀, Fig. 5(b)). However, with the addition of ZnO, the UV-blocking properties of the PLA/ACNC film improved, and the degree of UV-blocking increased with higher concentrations of ZnO (e.g., approximately 75% and 98% of UB₄₀₀ with 1 and 5 wt% ZnO, respectively, Fig. 5(b1)). It is important to note that the vis-transparency decreased significantly, reaching only about 8% of T₅₅₀ with 5 wt% ZnO (Fig. 5(b2)).⁴⁸ The conformational characteristics of the polymer matrices also played a significant role in nanoparticle aggregation and consequently affected the optical properties.⁴⁹ 2-Hydroxyethyl starch (HES) and 2-hydroxyethyl cellulose (HEC) have similar chemical compositions but possess different conformational characteristics. HES adopts a more coil-like conformation, while HEC exhibits a more extended rod-like conformation. The conformational differences led to distinct distribution states of ZnO nanoparticles. The coiled structure of HES induced the formation of aggregates with numerous ZnO nanoparticles (Fig. 5(c1)), resulting in limited contact between UV radiation and the nanoparticle surface and consequently lower UV-blocking for the ZnO/HES composite (approximately 40% of UB₃₅₄, Fig. 5(c2)). In contrast, the extended structure of HEC allowed for the formation of loose fractal clusters, exposing a large surface area of nanoparticles to interact with UV. As a result, the ZnO/HEC composite exhibited a high UV-blocking capability (approximately 95% of UB₃₅₄).

3.2.2 Blending inorganic sheets with polymers. Hexagonal BNNSs show high transparency to visible light and strong absorption to UV, making them attractive fillers for fabricating vis-transparent UV-blocking composites such as BNNSs/PVA and BNNSs/chitosan.^51,52 Efficient preparation and good dispersibility are crucial factors for the application of BNNSs. Cheng and coworkers proposed a simple yet efficient sugar-assisted mechanochemical exfoliation method that achieved a remarkable exfoliation yield of 87.3% while simultaneously functionalizing BNNSs.⁵¹ The resultant functionalization endowed BNNSs with excellent dispersibility, enabling their incorporation into PVA to produce transparent composite films with superior UV-blocking properties, effectively blocking over 95% of UV below 400 nm. In addition, the combination of MMT nanosheets and an ordered layered structure inspired by nacre resulted in a nacre-inspired MMT-konjac glucomannan-glycerin film with high vis-transparency (over 70% of T₅₅₀) and exceptional UV-blocking properties (approximately 100% of UB₂₈₀).⁵³ Moreover, the presence of GO for UV absorption and the chiral nematic structure of cellulose nanocrystals (CNCs) for high UV reflection contributed to exceptional UV-blocking capabilities and high transmission of visible light in GO/CNC/polyethylene glycol composites. These composites exhibited impressive UV-blocking of 98.3% for UVA (UB_UVA) and a transmittance of 60.5% at 600 nm (T₆₀₀).⁵⁴

It is essential to develop biodegradable biomimetic films that combine UV-blocking and excellent mechanical properties for packaging materials. Recently, Yu and coworkers successfully fabricated a nacre-inspired nano-mica/PLA nanocomposite film with a “brick and mortar” structure using a PLA-assisted exfoliation and dispersion method.²⁰ The resulting nanocomposite film presented low UV transmittance (60–98% of UB₂₈₀) while maintaining high visible light transmittance (65–85% of T₅₅₅), indicating its good UV-blocking properties and vis-transparency. This optical selectivity was due to the polarization and interlayer optical interference of mica nanosheets and the layered crystal structure. In addition, attributed to the “brick and mortar” structure, the nacre-inspired nanocomposite film demonstrated higher strength (97 MPa) and modulus (8 GPa) compared to commercial plastic films (approximately 50 MPa strength and 1–4 GPa modulus). Furthermore, a natural seaweed-based nanocomposite material with a similar “brick and mortar” structure was developed by self-assembly of nano-mica in sodium alginate through water evaporation (Fig. 5(d1)).⁵⁵ Compared to pure sodium alginate films, the resultant nano-mica/sodium alginate composite film showed strong UV-blocking properties (approximately 96% of UB₂₈₀, Fig. 5(d2)), which is highly favorable for preserving the freshness of packaged products. As shown in Fig. 5(d3), unprotected tomatoes exhibited signs of rot and mildew on their surfaces, whereas tomatoes stored in a nano-mica/sodium alginate bag remained plump and fresh.

3.2.3 Surface modification of polymeric materials with inorganic UV filters. Textiles, such as cotton, wood, knit polyester, and silk fabrics, occupy many corners of our lives. For outdoor workers, it is crucial to have protective textiles that shield their skin from harmful UV radiation, preventing the risk of diseases. Surface modification using inorganic UV filters, such as TiO₂,^56,57 ZnO,^57–59 graphene or graphene oxide,^60,62 and metal–organic framework (MOF),^63,64 has proven to be an effective method for creating UV-blocking textiles without compromising their macroscopic properties. For example, TiO₂ and ethanolic stearic acid-TiO₂ coatings on cotton fabrics have demonstrated high UV-blocking (over 99% of UB_280–400) due to the high UV absorption and scattering properties of TiO₂.⁵⁶ ZnO nanoparticles, on the other hand, exhibit better UV absorption ability and are more nontoxic and biocompatible with human cells compared to TiO₂.⁵⁷ In addition, the white appearance of ZnO nanoparticles matches the color of most fabrics, making them a preferred choice in the synthesis of UV-blocking functional textiles.^57–59 To illustrate, Chen and coworkers successfully anchored ZnO nanoparticles onto the cotton fabric surface using aminopropyltriethoxysilane (APTES) as a silane crosslinker (Fig. 6(a1)).⁵⁹ The pristine cotton fabric had a smooth morphology (Fig. 6(a2)). After coating with ZnO nanoparticles over the pristine cotton fabric, the nanoparticles distributed uniformly and covered the fabric surface completely without visible agglomeration (Fig. 6(a3)). The resulting ZnO-coated fabrics showed excellent UV-blocking properties, with over 99.5% blocking efficiency for UVA and UVB, surpassing the pristine cotton fabrics (Fig. 6(a4)). GO coatings have also been applied to knit polyester fabrics, utilizing their strong absorption peaks at 232.26 nm and 301.65 nm, corresponding to the π–π* and n–π* transitions of aromatic C=C bonds and C=O bonds, respectively. This characteristic imparts UV-blocking properties to the GO-coated fabrics.⁶² In addition, the combination of MOFs has been explored for the fabrication of UV-blocking textiles.^63,64 For example, Hussain and coworkers developed a cotton fabric functionalized with zinc (Zn)/zirconium (Zr) bimetallic MOF nanoflowers, resulting in significantly enhanced UV-blocking properties (up to UPF 200) compared to the pristine fabric (UPF 10).⁶⁴ This improvement can be attributed to the strong UV absorption of chromophoric amine groups and Zn–Zr clusters within the MOF nanoflowers.

Fig. 6 Surface modification of polymeric materials with inorganic UV filters. (a) Cotton fabric surface modification with ZnO. (a1) Schematic illustration of the synthesis process of ZnO-coated cotton fabrics. Scanning electron microscope (SEM) images of the (a2) pristine and (a3) ZnO-coated cotton fabrics. (a4) Comparison of UV-blocking in the UVA region (UB_320–400) and UV-blocking in the UVB region (UB_280–320) between the pristine and ZnO-coated cotton fabrics. Reproduced from ref. 59 with permission from Wiley. (b) Cellulosic paper surface modification with TiO₂/beeswax. Schematic illustrations of the (b1) NFC-1 (TiO₂/beeswax coating paper composite (un-annealed)) and (b2) NFC-2 (TiO₂/beeswax coating paper composite (annealed)) composites. (b3) UV-vis spectra of the nonwoven fabric, NFC-1 and NFC-2. Reproduced from ref. 61 with permission from the ACS.

In addition to UV light protective textiles, Wang and coworkers have utilized a simple surface treatment method to fabricate environmentally friendly UV-blocking biofiber composites.⁶¹ In their study, TiO₂ was dispersed into a hot beeswax-anhydrous ethanol solution and then sprayed onto the biofiber assemblies (cellulosic paper) to create a TiO₂/beeswax coating paper composite (un-annealed) (NFC-1) through the rapid volatilization of anhydrous ethanol (Fig. 6(b1)). Subsequently, the researchers obtained a TiO₂/beeswax coating paper composite (annealed) (NFC-2) by annealing NFC-1 (Fig. 6(b2)). Both composites demonstrated enhanced UV-blocking properties compared to the pristine biofiber assemblies, thanks to the excellent UV protection provided by TiO₂ nanoparticles (Fig. 6(b3)). However, it is worth noting that the UV-blocking efficiency of NFC-2 was slightly lower than that of NFC-1. This can be attributed to the beeswax coating that enveloped the TiO₂ nanoparticles during the annealing process.

3.3 Polymeric UV filter/polymer system

It is widely accepted that incorporating organic and inorganic UV filters into polymers is a common and effective strategy to enhance their UV-blocking properties. However, there are challenges associated with the use of small organic filters. These filters have a tendency to escape during high-temperature polymer processing, which can result in the degradation or failure of UV-blocking properties and raise concerns about public health.^94–96 In addition, the presence of small organic filters weakens the interactions between polymer chains, leading to a deterioration of the mechanical properties of polymer composites.⁷⁶ Meanwhile, inorganic filters also have drawbacks such as poor compatibility, accelerated photodegradation of polymers, and blocking of visible light.^46,97–99 To address these concerns, various polymeric UV filters have been developed, which can be loaded into polymer matrices in the form of particles, fibers, or dispersed phases.

3.3.1 Blending polymeric particles with polymers. Lignin is the second most abundant organic carbon biopolymer. Commercial lignin is primarily classified into five categories: lignin sulfonate, alkaline lignin, organosolv lignin, enzymatic lignin, and Kraft lignin, based on the separation process.²⁸ Due to its plentiful aromatic structures, phenolic hydroxyl, and ketone structures, lignin has emerged as a feasible UV filter for producing polymer composites.^29,100–104 However, the application of commercial lignin is limited by its low activity, poor solubility and dispersibility. Therefore, lignin is usually transformed into micro/nanoparticles to effectively improve its physical properties. Li and coworkers have proposed a simple and environmentally friendly strategy to prepare lignin micro/nanoparticles from organosolv lignin using recyclable γ-valerolactone.⁶⁵ Various parameters, including lignin concentration, stirring speed, lignin solution dropping speed, water/lignin solution volume ratio, and lignin structure, greatly influence the yield, size, morphology, and properties of the resulting lignin micro/nanoparticles. These lignin micro/nanoparticles have been successfully incorporated into PVA films, resulting in composites that can completely block UVC and UVB radiation. Qiu and coworkers have also developed lignosulfonic acid/PLA nanocomposites derived from biomass, which exhibit excellent UV-blocking properties, blocking 100% of full-band UV radiation in the wavelength range of 200–400 nm.⁶⁶ To further enhance lignin dispersibility, interfacial compatibility, photostability, and reactivity, various modified lignin nanoparticles have been synthesized and loaded into polymer matrices to create UV-blocking polymer composites. For example, Wang and coworkers incorporated a bioinspired melanin-like polydopamine thin layer into lignin nanoparticles, resulting in UV-blocking LMNPs with improved compatibility and durability.⁶⁷ By compounding LMNPs with poly(butylene adipate-co-terephthalate) (PBAT), they prepared LMNPs/PBAT composites with enhanced UV-blocking properties, achieving over 98% UV-blocking in the UVB region and nearly 80% UV-blocking in the UVA region. The polydopamine layer played a crucial role in preventing lignin photodegradation, while the core–shell structure ulteriorly improved lignin photostability. Furthermore, modified lignin nanoparticles, such as tannic acid-modified sodium lignosulfonate nanoparticles,⁶⁸ poly(lactide ε-caprolactone) copolymer-grafted lignin nanoparticles,⁶⁹ quaternized sodium lignosulfonate (QLS),⁷⁰ esterified lignin nanoparticles,⁷¹ and acetylated lignin,¹⁰⁵ have been synthesized to enhance lignin dispersibility, interfacial compatibility, and UV-blocking properties. For example, QLS demonstrated high dispersibility and exceptional interfacial compatibility in waterborne epoxy (WEP) systems. By incorporating QLS into WEP composites, a green UV-blocking coating (QLS/WEP) was fabricated (Fig. 7(a1)).⁷⁰ The UV-vis spectra of the coatings before and after UV radiation showed that the UV-blocking properties of 5-QLS/WEP and 5-sodium lignosulfonate (LS)/WEP did not change significantly (Fig. 7(a2)). Overall, the utilization of lignin and its modified nanoparticles in polymer composites holds great potential for producing UV-blocking materials.

Fig. 7 Blending polymeric particles with a polymer. (a) Quaternized sodium lignosulfonate (QLS)/waterborne epoxy (WEP) composite. (a1) Schematic illustration of the QLS/WEP composite coatings. (a2) UV-vis spectra of WEP, 5-sodium lignosulfonate (LS)/WEP, and 5-QLS/WEP before and after UV radiation. Reproduced from ref. 70 with permission from Wiley. (b) Dopamine-melanin nanoparticle/poly(vinyl alcohol) (PVA) composite. Comparison of UV-blocking in the (b1) UVA region and (b2) UVB region of the pristine PVA, dopamine-melanin solid nanoparticles (Dpa-s NPs)/PVA, and dopamine-melanin hollow nanoparticles (Dpa-h NPs)/PVA composite films. (b3) Schematic illustrations of the UV-blocking mechanism between Dpa-s NPs and Dpa-h NPs. Reproduced from ref. 73 with permission from the ACS.

Cellulose nanocrystals (CNCs) are sustainable rod-like nanoparticles that can disperse in water. When esterified with cinnamoyl chloride, the resulting cinnamate-functionalized CNCs (Cin-CNCs) show strong UV absorption while maintaining high visible light transmittance.⁷² As a result, Cin-CNCs are utilized as UV filters in hydrophobic (polystyrene) and hydrophilic (PVA) polymer-based composites. These composite films display excellent UV-blocking properties, effectively blocking almost 100% of UV radiation below 310 nm. In addition, CNCs play a significant role in the development of UV-blocking PCL-based multifunctional composite films for food packaging.¹⁰⁶ Traditionally, most studies have focused on using solid nanoparticles as UV filters. However, hollow nanostructures have shown enhanced optical properties compared to their solid nanoparticles. Dong and coworkers discovered that when incorporating bioinspired dopamine-melanin hollow nanoparticles (Dpa-h NPs) into PVA, the resulting Dpa-h NPs (>2 vol%)/PVA composite films showed even stronger UV-blocking properties, effectively blocking full-band UV radiation (Fig. 7(b1) and (b2)).⁷³ This performance surpassed that of the original PVA and dopamine-melanin solid nanoparticles (Dpa-s NPs)/PVA composite films. The UV-blocking mechanisms are illustrated in Fig. 7(b3). In general, solid nanoparticles reflect UV radiation on their surfaces, leading to scattering. In contrast, UV radiation penetrates the holes of Dpa-h NPs and is trapped and absorbed within the nanoparticles through multiple reflections and absorptions. Furthermore, polymerized caffeic acid phenethyl ester¹⁰⁷ and shellac nanoparticles¹⁰⁸ are used as UV filters in the construction of UV-blocking composites.

3.3.2 Blending polymeric fibers with polymers. Surface modification of cellulose nanofibers (CNFs) was achieved through esterification reactions in the heterogeneous phase with diisocyanate, epoxidized soybean oil, and benzophenone.⁷⁴ The incorporation of benzophenone imparted strong UV absorption to the surface-modified CNFs, making them ideal for use as UV filters. These modified CNFs were then combined with PVA to create composite films with UV-absorbing capacity. The composite films exhibited high UV-blocking properties, especially in the UVB region, when the CNF loading was below 5 wt%. With increased CNF loading to 10 wt%, the composite films blocked not only approximately 92% of UVB but also 52% of UVA radiation. To address the pollution caused by traditional plastics, an effective strategy is to utilize natural biomass resources for the fabrication of degradable membrane materials. Lu and coworkers developed a degradable biofilm by mechanochemically modifying microfibrillated cellulose with tannic acid (TA@MFC) and incorporating gelatin.⁷⁵ The successful surface modification of cellulose with tannins was demonstrated by the dark layer observed on the surface of TA@MFC prepared through ball milling (Fig. 8(a1)). In comparison to the dense and smooth surface of the pure gelatin film (Fig. 8(a2)), the cross-sectional surface of the TA@MFC-G 20 composite film, which contained 20% TA@MFC and gelatin, appeared rough, with TA@MFC evenly dispersed and tightly bound to gelatin (Fig. 8(a3)). The presence of benzene rings in tannic acid contributed to its strong UV absorption within the 270–400 nm wavelength range. Consequently, the prepared composite film exhibited excellent UV-blocking properties. A comparison of UV-blocking in the UVA and UVB regions between the pure gelatin film and composite films revealed that the pure gelatin film had weak UV-blocking properties (Fig. 8(a4)). In contrast, all composite films showed enhanced UV-blocking properties, with the TA@MFC-G 20 composite film achieving 85% UV-blocking in the UVA region and over 98% UV-blocking in the UVB region.

Fig. 8 Blending polymeric fibers or dispersed phases with a polymer. (a) Tannic acid-modified microfibrillated cellulose (TA@MFC)/gelatin (G) composite (TA@MFC-G). (a1) A transmission electron microscope image showing TA@MFC prepared by ball milling. SEM images of the cross-section of the (a2) pure gelatin film and (a3) composite film TA@MFC-G 20. (a4) Comparison of UV-blocking in the UVA and UVB regions between the pure gelatin film and composite films. Reproduced from ref. 75 with permission from Wiley. (b) Polyphosphazene (PAHP)/polylactic acid (PLA) composite. UV-vis spectra of neat PLA, 5% PAHP/PLA, and 9% PAHP/PLA. Reproduced from ref. 80 with permission from Elsevier. (c) Sulfur-rosin copolymer (S_x-Ro)/poly(methyl methacrylate) (PMMA) composite. (c1) Chemical structures of rosin, PMMA, and S_x-Ro. (c2) UV-vis spectra of the pure PMMA, Ro/PMMA, and S_x-Ro/PMMA composite films. Reproduced from ref. 81 with permission from Wiley.

3.3.3 Blending polymeric dispersed phases with polymers. Chitosan (CS), the second most abundant polysaccharide obtained from the waste of the shellfish industry, has been reported to possess UV-blocking properties by Velazquez and coworkers.¹⁰⁹ In their work, CS was functionalized with a catechol-containing compound and blended with PVA to create catechol-functionalized chitosan (C-CS)/PVA composite films that showed excellent UV-blocking properties due to the presence of unsaturated bonds and catechol groups in the C-CS.⁷⁷ To meet the requirements of versatile application scenarios while embracing the concepts of carbon neutrality and sustainability, UV-blocking polymers with high mechanical properties and additional features, such as self-healing,⁷⁸ biodegradability,^78,110,111 flame retardant,^79,80 and antibacterial properties,³⁴ have garnered significant development. For example, Yu and coworkers prepared a self-healable and biodegradable UV-protective film based on soy protein isolate (SPI) with high mechanical strength.⁷⁸ By introducing high-flowability polyethyleneimine (PEI), the strong hydrogen bonds between SPI molecules were disrupted, improving the mobility of SPI. Subsequently, polydopamine was utilized as a macromolecular crosslinker to create a hydrogen bonding-based reversible network, resulting in satisfactory self-healing properties. Analogously to melanin, the quinone structure of polydopamine endowed the SPI-PEI-PDA films with UV-blocking properties as high as 93.22%. PLA is considered one of the most promising bioplastics. However, it is inherently flammable and susceptible to UV radiation, limiting its applications in fields such as packaging, electronics industry, and engineering safety fields. Therefore, the development of UV-blocking PLA with excellent flame retardancy is highly desirable. Recently, Song and coworkers synthesized a novel bioderived polyphosphate (PPD) as a phosphorus-based flame retardant.⁷⁹ The addition of 6 wt% PPD to PLA increased its LOI value to 27.1%, achieving a desirable UL-94V-0 rating. The aromatic rings in PPD possessed strong UV absorption capabilities, enabling the resultant PPD/PLA composite films with more than 6 wt% PPD to show full-band UV-blocking properties. In addition, they synthesized a bio-based singly substituted polyphosphazene flame retardant called polybis(2-amino-6-hydroxypurine)phosphazene (PAHP).⁸⁰ Just 5 wt% of PAHP enabled the corresponding PLA-based composite to meet the fire-safety requirements of engineering applications (UL-94V-0 rating). Interestingly, PAHP also effectively improved the UV-blocking properties of the PLA. As illustrated in Fig. 8(b), the PAHP/PLA composites showed nearly 100% UV-blocking in the wavelength range of 280–310 nm, surpassing the UV-blocking capabilities of pristine PLA. Furthermore, transparent composites, such as a sodium alginate-κ carrageenan-lignin composite,³⁴ and a strong and biodegradable spent coffee ground-based all-biomass film,¹¹¹ were fabricated, both displaying excellent UV-blocking properties.

Thermoplastic resins, such as poly(ethylene terephthalate-co-1,4-cyclohexylenedimethylene terephthalate) (PETG) and poly(methyl methacrylate) (PMMA), have been widely utilized as substitutes for inorganic glass in optical lenses, windows, and various other applications. However, these polymeric materials often lack effective UV-blocking capabilities. To enhance the UV-blocking properties of PETG, Yuan and coworkers synthesized a new type of citrate-based polyester (CP) through the polycondensation of thiazolopyridine dicarboxylic acid (TPDA) and a series of diols.⁷⁶ TPDA provided the CPs with high UV absorption efficiency, resulting in PETG composites with CPs (2% loading) showing 100% UV-blocking across the entire UV region. Similarly, PMMA has inherent UVC absorption capability, but improving its absorption towards harmful UVB and UVA is crucial. Zhou and coworkers recently prepared a novel sulfur (S)-rosin (Ro) copolymer (S_x-Ro) through the inverse vulcanization of S and natural Ro (Fig. 8(c1)).⁸¹ The S_x-Ro copolymer showed exceptional UV absorption capacity, making it an effective UV filer to improve the UV-blocking properties of PMMA in the UVA and UVB regions. As shown in Fig. 8(c2), when the S_x-Ro copolymer concentration was 0.50%, the S₅₀-Ro/PMMA composite effectively blocked almost all UVC and UVB, as well as a significant portion of UVA. In comparison, the addition of pure Ro only improved the UV-blocking properties of PMMA in the wavelength range of 250–275 nm, with little effect on the UVA and UVB.

4. Pure polymer systems

Even though various UV filters can be engineered to provide UV-blocking properties for polymers, the dispersion of UV filters is still a challenge. In general, the poorly dispersed UV filters lead to a decrease in optical properties (e.g., UV-blocking and vis-transparency) and mechanical properties.^49,112 In addition, both organic and inorganic UV filters are at risk of leakage, leading to poor stability of polymeric materials.^94–96,113 To avoid these disadvantages, constructing pure polymer systems would be conducive to excellent optical properties, mechanical properties, and stability. Until now, an important observation in pure polymer systems based on polymer structure design and polymer surface/in situ chemical modification strategies has been the achievement of high and stable optical properties. These pure polymer systems are to be discussed exhaustively in this section. We summarize the optical properties, including UV-blocking and vis-transparency, of these pure UV-blocking polymers discussed here in Table 2.

Table 2 Summary of the optical properties, including UV-blocking and vis-transparency, of these pure UV-blocking polymers discussed in Section 4

Pure polymer systems	Polymers	UV absorbing motifs	UV-blocking (UB)	Vis-transparency (T)	Ref.
Polymer structure design (main chain regulation)	Poly(ethylene bifuranoate)	Conjugated bifuran structure	∼100% of UB200–380 and ∼90% of UB400	∼75% of T600	114
	Poly(ethylene glycol) (PEG)-functionalized carbazole-based polymer	Conjugated carbazole structure	∼70% of UB300 and ∼30% of UB400 (KP-PEG5000)	∼85% of T600 (KP-PEG5000)	115
	Schiff-base crosslinked modified dialdehyde starch	Benzene ring	∼100% of UB200–400 (DS/DDA2)	∼15% of T600 (DS/DDA2)	116
	Polytelluoxane	Te–O polymer skeleton	100% of UB280–320 and ∼40% of UB400	∼80% of T600	117
Polymer structure design (side chain regulation)	Lignin-based triblock copolymers, chitosan-based cardanol glycidyl ether	Aromatic ring	100% of UB200–300 and ∼10% of UB400 (PMSMA-based triblock copolymers), ∼100% of UB200–320 and ∼90% of UB400 (CS-CGE)	∼95% of T600 (PMSMA-based triblock copolymers), ∼30% of T600 (CS-CGE)	17 and 118
	Polyurethane (PU), fluorinated acrylic polymer	Benzotriazole group	∼98% of UB300 and ∼35% of UB400 (PU-0.5%)	∼90% of T600 (PU-0.5%)	113 and 119
	Ethyl cellulose phenyl propylene ketone ethers	Aromatic ring and –O–C=C moieties	∼100% of UB200-400 (DS0.41)	∼80% of T600	120
Polymer structure design (crosslink regulation)	Itaconic anhydride crosslinked polyglycerol	Double bonds	100% of UB300–400 (HPG/IA-2/1-Zn)	∼45% of T600 (HPG/IA-2/1-Zn)	121
	Waterborne PU	Furan structure	∼100% of UB200–380 and ∼90% of UB400 (WPU-SP60)	∼90% of T600 and ∼60% of T550 (WPU-SP60)	122
	Lignin-based polymers	Aromatic skeleton and phenolic hydroxyl groups	100% of UB200–350 and ∼88% of UB400 (Lignin-BV19.4), 100% of UB400 (LPU98/2), 100% of UB300–400 (lig-PU-PDMS 0.5%)	∼80% of T600 (Lignin-BV19.4), ∼50% of T600 (LPU98/2), ∼60% of T600 (lig-PU-PDMS0.5%)	123–125
Surface chemical modification	Cotton textile, cellulose, PVA	Conjugated –C=C–C=O structure	∼98.5% of UB280–400 (CAA-SA), 100% of UB200–400 (CAA-CA-ODA/HA-PVA)	86% of T550 (CAA-CA-ODA), ∼90% of T550 (HA-PVA)	18, 126 and 127
	Cellulose	Conjugated –C=C–C=O structure and benzene ring	100% UB200–400 (CAA-V (4 h), CL-SCNF4)	∼75% of T550 (CAA-V (4 h)), ∼84% of T600 (CL-SCNF4)	128 and 129
	Polydopamine	Heterocycle structure	∼82% of UB320 and ∼55% of UB400 (PC@PDA-24)	∼58% of T550 (PC@PDA-24)	130
In situ chemical modification	Cellulose acetoacetate	Conjugated –C=C–C=O structure	∼100% of UB200–380 and ∼60% of UB400 (FC-FF film)	∼85% of T600 (FC-FF film)	19
	PVA	Conjugated –C=C–C=O and –C=C–C=C– structures	100% of UB200–400 (PVAA-CA/PVAA-CA-Gen)	∼87% of T550 (PVAA-CA and PVAA-CA-Gen)	131

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4.1 Polymer structure design

The incorporation of UV-absorbing motifs through chemical reactions is currently the widely used strategy for creating UV-blocking polymers. To date, a plethora of polymers with tunable UV-blocking properties have been designed. In these UV-blocking polymers, the UV-absorbing motifs are either chemically attached to the main/side chains or serve as chemical crosslinks.

4.1.1 UV-absorbing motifs with main chains (main chain regulation). Furans, which are derived from C5 and C6 sugars through dehydration, have emerged as key intermediates derived from biomass. Currently, furans are being explored as potential precursors for a wide range of polymeric materials, including thermoplastics and thermosets. For example, Heiskanen and coworkers synthesized a furan-based synthetic biopolymer known as poly(ethylene bifuranoate) (PEBF, Fig. 9(a1)) through the melt polycondensation of a biomass-based bifuran diester and ethylene glycol.¹¹⁴ The bifuran structure in PEBF contains a conjugation system that effectively absorbs UV, resulting in a PEBF film that demonstrates excellent UV-blocking behavior. This film can block all UV in the wavelength of 200–380 nm and most UV beyond 380 nm, while maintaining high vis-transparency with a transmittance of up to 80% in the wavelength range of 420–800 nm. In addition, self-cleaning and UV-protected coatings are widely utilized in various technologies. Recently, Koyuncu and coworkers focused on developing an antifog surface with high UV-blocking properties and vis-transparency.¹¹⁵ To achieve these desired properties, they synthesized a polymer (KP-PEG, Fig. 9(a2)), which combines a poly(ethylene glycol) (PEG) subunit for hydrophilicity and a carbazole-based conjugated backbone for UV absorption. The resulting film surfaces exhibited good wettability and antifogging properties, with water contact angles ranging from 60° to 29°. The carbazole-based conjugated structure, with its electro-donating capability, effectively absorbs UV and contributes to the UV-blocking behavior. Starch-based films have also received considerable attention in the field. Yang and coworkers synthesized a series of bio-based bifunctional benzoxazine compounds with varying carbon chain lengths.¹¹⁶ These compounds were then incorporated into dialdehyde starch (DS) to fabricate DS/DAA films by the Schiff-base reaction (Fig. 9(a3)). When the content of the bio-based bifunctional benzoxazine compound (e.g., benzoxazine with 1,12-dodecanediamine, BOZ-DDA) based on the mass of DS was 20 wt%, the resulting DS/DDA2 film exhibited almost 100% UV-blocking across the entire UV region (200–400 nm). These exceptional UV-blocking properties can be attributed to the presence of benzene rings as the chromophobe groups from BOZ-DDA and the C=N groups from the Schiff-base.

Fig. 9 Polymer structure design for main chain regulation. (a) Chemical structures of various main chains in UV-blocking polymers. (a1) Poly(ethylene bifuranoate) (PEBF). Reproduced from ref. 114 with permission from the ACS. (a2) Poly(ethylene glycol) (PEG)-functionalized carbazole-based polymers (KP-PEG). Reproduced from ref. 115 with permission from Elsevier. (a3) Mian chain fragment of the DS/DDA Schiff-base crosslinked film. Reproduced from ref. 116 with permission from Elsevier. (b) Polytelluoxane. (b1) Schemes for the fabrication of PTeO via a facile interfacial oxidative polymerization of dialkyl tellurides at room temperature. (b2) Preparation of a PTeOC12 sheet by the processing of PTeOC12 powder via hot press molding. UV-vis spectra of PTeO and three traditional optical materials, including quartz, sapphire, and PMMA, in the (b3) UV and (b4) visible regions. Reproduced from ref. 117 with permission from Cell Press.

In general, carbon is the predominant component in polymer main chains. However, Xu and coworkers introduced a novel approach by incorporating tellurium (Te), a group VI element, to create a long linear polytelluoxane (PTeO) through a facile interfacial oxidative polymerization of dialkyl tellurides at room temperature.¹¹⁷ As shown in Fig. 9(b1), the process involves combining an organic phase containing dialkyl tellurides in ethyl acetate with an aqueous phase containing hydrogen peroxide (H₂O₂), leading to oxidative polymerization at the interface between the two phases. This results in the formation of PTeO with a periodic Te–O polymer skeleton. Analogous to many commercial polymers, the PTeOC12 powder is subsequently processed via hot pressing molding to obtain PTeOC12 (Fig. 9(b2)). The noteworthy aspect is the unique optical properties exhibited by PTeO. Fig. 9(b3) and (b4) compare the UV-blocking properties and vis-transparency of PTeO with several other traditional optical materials, such as quartz, sapphire, and PMMA. PTeOC12 demonstrates exceptional UV-blocking capabilities in the UVA and UVB regions compared to other optical materials. In addition, PTeOC12 shows a comparable transparency to quartz in the visible light range. These findings demonstrate PTeOC12 as a promising candidate for a novel functional optical material.

4.1.2 UV-absorbing motifs at side chains. Functional cellulose derivatives prepared through chemical modification offer a wide range of promising applications. However, conventional methods often face environmental challenges due to the use of toxic reagents and the generation of significant waste. Recently, Fan and coworkers proposed a more sustainable method called the hydroxyl-yne click reaction to prepare a novel cellulose derivative named ECPPK, which possesses UV-blocking properties.¹²⁰ Specifically, ECPPK was obtained by reacting the hydroxyl groups from ethyl cellulose with the alkyne groups from 1-phenyl-2-propargyl-1-ketone (PPK). The presence of aromatic rings in the side chains of the PPK moieties (Fig. 10(a1)) allows the ECPPK to effectively block 100% of UVC, most UVB, and over 50% of UVA. In another exciting development, Cao and coworkers drew inspiration from the advantages of chitosan and cardanol glycidyl ether (CGE) and succeeded in creating a novel, fully bio-based coating known as CS-xCGE (Fig. 10(a2)). This coating shows outstanding UV-blocking properties while also being biodegradable.¹¹⁸ The UV-blocking properties primarily stem from the presence of the benzene ring from CGE, which facilitates the absorption of UV. Both chitosan and CGE contribute to the biodegradability of CS-xCGE, making it an environmentally friendly option.

Fig. 10 Polymer structure design for side chain regulation. (a) Chemical structures of various side chains in UV-blocking polymers. (a1) Ethyl cellulose phenyl propylene ketone ether (ECPPK). Reproduced from ref. 120 with permission from the RSC. (a2) Chitosan-derived cardanol glycidyl ether (CS-xCGE). Reproduced from ref. 118 with permission from Elsevier. (a3) Lignin-based triblock copolymers. Reproduced from ref. 17 with permission from Wiley. (a4) Benzotriazole-based acrylic polymer/polyurethane (PU). Reproduced from ref. 113 with permission from the ACS. (b) Lignin-based triblock copolymers. Chemical structures of (b1) various monomers and (b2) Lewis pairs. (b3) Comparison of UV-blocking properties and vis-transparency between syringyl methacrylate (SMA)-based, 4-(methacryloyloxy)-3,5-dimethoxybenzoate (MSMA)-based, and methyl methacrylate (MMA)-based thermoplastic elastomers. Reproduced from ref. 17 with permission from Wiley. (c) Benzotriazole-containing UV-blocking polyurethane (PU). (c1) UV-vis spectra of ordinary glasses, ordinary glasses covered with a PU-0.5% film, and sunglasses. (c2) Schematic illustration of UV-blocking glasses under UV radiation. (c3) A digital image showing a glass covered with a pentagram-shaped PU-0.5% film after UV radiation. (c4) UV-blocking mechanism of the benzotriazole groups. Reproduced from ref. 113 with permission from the ACS.

Lignin, a commonly used UV filer, has been widely employed in the production of UV-blocking polymer composites.^66,70,71,100 Similarly, lignin-based methacrylate monomers, such as syringyl methacrylate (SMA) and 4-(methacryloyloxy)-3,5-dimethoxybenzoate (MSMA), also possess strong UV absorption capacity. Taking advantage of this, Zhang and coworkers utilized these lignin-based methacrylate monomers, along with other monomers (e.g., n-butyl acrylate (nBA) and methyl methacrylate (MMA)), to construct well-defined triblock copolymers through a one-step block copolymerization method (Fig. 10(b1)).¹⁷ During the copolymerization process, they employed Lewis pairs (Fig. 10(b2)), comprising a bulky organoaluminum (such as BHT(ⁱBu)₂Al) and a tethered bis-organophosphorus superbase (such as μ^Oct[P(mMPy)Ph₂]₂), to achieve a highly efficient synthesis. Compared to the MMA-based thermoplastic elastomer (TPE), the TPEs based on SMA and MSMA exhibited superior UV-blocking properties due to the presence of aromatic rings inherited from lignin (Fig. 10(a3) and (b3)). Among them, the MSMA-based TPE demonstrated the highest performance, with UV-blocking of 100% at 245 nm, 100% at 300 nm, and 8% at 400 nm. In addition, the transmittance at 550 nm exceeded 95%, indicating excellent vis-transparency for all TPEs. These results underscore the potential of lignin-based TPEs for applications in optical devices.

In addition, the incorporation of benzotriazole pendants has been proven to be an effective method for the construction of UV-blocking polymers.^113,119 For example, He and coworkers introduced benzotriazole pendants into polyurethane (PU) chains through in situ polymerization to produce non-migrating intrinsic UV-blocking PU films (Fig. 10(a4)) with colorless vis-transparency (more than 88% in the visible region).¹¹³ These PU films not only exhibited UV-blocking properties but also maintained high vis-transparency, making them suitable for use as UV protection membranes. As shown in Fig. 10(c1), when ordinary glasses were covered with a PU-0.5% film, their UV-blocking capabilities in the UVA region were enhanced while retaining high vis-transparency (over 85% of T₅₅₀). In contrast, sunglasses provided UV-blocking abilities, but wearing them may darken the scenery observed (approximately 10% of T₅₅₀). Fig. 10(c2) and (c3) further demonstrate the UV-blocking properties of glasses covered with a PU-0.5% film. The UV-blocking mechanism is shown in Fig. 10(c4), wherein the benzotriazole pendants in the PU films absorb UV and form an intramolecular hydrogen bonding chelation ring through proton transfer. This allows the absorbed UV to be converted into heat, thereby protecting the film from UV damage. Importantly, this transformation is reversible, enabling the PU film to exhibit repeatable UV-blocking properties.

4.1.3 UV-absorbing motifs as crosslinks. Taking inspiration from non-traditional fluorescence polymers and vitrimers, Zhang and coworkers utilized the esterification reaction between hyper-branched polyglycerol (HPG) and itaconic anhydride (IA) to fabricate an adaptive dynamic covalent network (HPG/IA, Fig. 11(a1)) with UV-blocking and fluorescence properties.¹²¹ By intentionally preserving the double bonds from IA, which have strong absorption capabilities, all polymer films showed no noticeable UV transmittance in the wavelength range below 380 nm and even in the entire UV region for HPG/IA-2/1-Zn (Fig. 11b). In contrast to traditional fluorescent polymers with conjugated structural units, the fluorescence properties of HPG/IA were attributed to the unique branching structure of HPG and its crosslinking structure. Sorbitan monooleate (SP) is a typical derivative containing three adjacent hydroxyl groups and a rigid furan ring, making it a suitable raw material when combined with castor oil to fabricate a UV-blocking waterborne PU (WPU) film (Fig. 11(a2)).¹²² The rigid furan ring in SP, along with the high crosslinking structure, contributed to the UV-blocking properties. As shown in Fig. 11(c1), all film samples completely blocked UVC, with the exception of the WPU-SP₀ film, which also provided complete UV-blocking in the UVB region. In the UVA region, the UV-blocking capability gradually increased with the increase in SP content. For example, the UV-blocking at 400 nm improved from 30% for the WPU-SP₀ film to approximately 95% for the WPU-SP₆₀ film. To further confirm the UV-blocking properties, the authors compared the color changes of UV test cards covered with and without quartz glass, as well as WPU-SP₀ and WPU-SP₂₀ coated quartz glasses (Fig. 11(c2)–(c5)). The card covered with WPU-SP₂₀ coated quartz glass exhibited minimal color development compared to the others, demonstrating that SP enabled the WPU to possess a stronger UV-blocking effect.

Fig. 11 Polymer structure design for crosslink regulation. (a) Chemical structures of various crosslinks in UV-blocking polymers. (a1) Itaconic anhydride crosslinked hyperbranched polyglycerol (HPG/IA). Reproduced from ref. 121 with permission from Elsevier. (a2) Sorbitan monooleate (SP)-based waterborne PU (WPU). Reproduced from ref. 122 with permission from Elsevier. (a3) Lignin-based PU. Reproduced from ref. 123 and 124 with permission from Elsevier and ref. 125 with permission from the ACS. (b) UV-vis spectra of a series of itaconic anhydride crosslinked hyperbranched polyglycerol (HPG/IA) films. Reproduced from ref. 121 with permission from Elsevier. (c) UV-blocking properties of sorbitan monooleate (SP)-based waterborne PU (WPU). (c1) UV-vis spectra of a series of SP-based WPU films. (c2–c5) Optical images of UV irradiated test cards covered with and without quartz glass, and WPU-SP₀ and WPU-SP₂₀ coated quartz glasses. Reproduced from ref. 122 with permission from Elsevier.

In addition to its use as an excellent filler,^66,70,71,100 lignin also served as a building block for constructing various UV-blocking polymers (Fig. 11(a3)) due to the presence of numerous active groups, such as hydroxyl, carboxyl, and carbonyl groups, in its structure.^123–125 For example, Wang and coworkers utilized lignin as a raw material with a polyhydroxyl structure to construct UV-blocking PU elastomers. By combining these PU elastomers with fabrics through hot pressing, the resulting fabrics exhibited enhanced UV-blocking properties, with over 85% full-band UV-blocking and an average UPF value of over 50.¹²⁴ Similarly, Lei and coworkers used biomass lignin as a renewable polyol to fabricate a translucent and omniphobic PU coating.¹²⁵ These PU coatings effectively blocked full-band UV, thanks to the aromatic skeleton and conjugated structure of lignin. In addition, the PU coatings demonstrated the ability to repel various contaminating liquids, including paint, ink, and other complex liquids, which easily slid off without leaving any traces. These studies have greatly expanded the high-value utilization of lignin biomass. Furthermore, magnolol, a lignin-derived renewable compound possessing biphenyl groups, phenol hydroxyl groups, and allyl groups, offers multiple possibilities for the synthesis of functional polymers. Recently, Gao and coworkers developed magnolol-based PU materials with excellent UV-absorbing properties, thereby providing UV-blocking capabilities.¹³²

4.2 Polymer surface chemical modification

It has been realized that surface chemical modification is an effective method for enhancing the UV- blocking properties of fabric textiles and polymeric films. Surface chemical modification mainly occurs in the surface-accessible regions of polymers while preserving their physical and optical characteristics. Multicomponent reactions (MCRs) are powerful one-pot reactions that involve at least three starting reactants and offer many advantages, including efficiency, mild conditions, the use of green solvents, atom economy, and high convergence.¹³³ Examples of MCRs include the Hantzsch,^134,135 Biginelli,^136,137 Mannich,¹³⁸ Ugi,¹³⁹ and Passerini reactions.¹⁴⁰ The Hantzsch reaction is particularly useful for generating a 1,4-dihydropyridine (DHP) ring with a conjugated structure by combining amine, aldehyde, and β-diketone components. Importantly, the conjugated structure has strong UV absorption properties. In light of this, Qi and coworkers employed the Hantzsch reaction to perform surface chemical modification, resulting in UV-blocking multifunctional cotton textiles,¹²⁶ cellulose acetoacetate (CAA),¹²⁷ and PVA¹⁸ films (Fig. 12(a1)). For UV-blocking multifunctional cotton textiles, the researchers attached DHP rings and gentamycin sulfate (GS) molecules to the surface of cotton fibers simultaneously using a one-pot method. The resulting textiles exhibited integrated UV-blocking properties (over 98.5% UV-blocking in the UVA and UVB regions and a UPF value of up to 69.2). In addition, they displayed hydrophobic behavior (water contact angle of up to 134°), bright fluorescence, and excellent antibacterial capability.¹²⁶ In the case of CAA films, the researchers bonded DHP rings and hydrophobic octadecylamine molecules to cellulose chains through the Hantzsch reaction. This led to the production of UV-blocking films with bright fluorescence behavior and good hydrophobicity (water angle of up to 112°).¹²⁷ The films achieved a UV-blocking efficiency of 100% while maintaining a transmittance of 86% at 550 nm, comparable to that of the pure CAA film. Previously, PVA-based UV-blocking films were typically fabricated through composite systems incorporating UV filters. However, this approach significantly compromised the inherent vis-transparency of PVA.^{51,65,68,77,79} To address this, the researchers developed a transparent UV-blocking PVA-based film using the Hantzsch reaction (Fig. 12(b1)).¹⁸ Initially, they introduced acetoacetyl groups onto PVA chains through transesterification, resulting in a polyvinyl alcohol acetoacetate (PVAA) film. The subsequent formation of DHP rings on the surface of the PVVA film involved the reaction of acetoacetyl groups, ammonium acetate and aldehyde components (Fig. 12(b2)). By utilizing different aldehyde components, such as hexaldehyde (HA), dodecyl aldehyde (DA), and stearaldehyde (SA), the researchers prepared a series of PVA-based films. Notably, a large-sized DA-PVA film was easily fabricated through surface modification and used for packaging a large box (Fig. 12(b3) and (b4)). These PVA-based films effectively blocked full-band UV and exhibited high vis-transparency, with approximately 90% transmittance at 550 nm (Fig. 12(b5)).

Fig. 12 UV-blocking polymers prepared through surface chemical modification. (a) Various structures on the UV-blocking polymer surface. (a1) 1,4-Dihydropyridine (DHP) ring with a conjugated structure from the Hantzsch reaction. Reproduced from ref. 18, 126 and 127 with permission from the ACS, Springer, and RSC, respectively. (a2) 3,4-Dihydropyrimidin-2(1H)-ones (DHPM) structure from the Biginelli reaction. Reproduced from ref. 128 with permission from Wiley. (a3) Curcumin-modified polymer surface by the Mannich reaction. Reproduced from ref. 129 with permission from Elsevier. (a4) Thiol-heterocycle (TH) derivative-modified polydopamine coatings. Reproduced from ref. 130 with permission from the RSC. (b) UV-blocking PVA prepared by the Hantzsch reaction. (b1) A schematic illustration for the preparation of a UV-blocking PVA film. (b2) Synthesis equations of polyvinyl alcohol acetoacetate (PVAA) and a 1,4-dihydropyridine (DHP) ring through the Hantzsch reaction. (b3) A photograph showing a large-sized dodecyl aldehyde (DA)-PVA film (60 cm length and 60 cm width) and (b4) a glove box wrapped in the large-sized DA-PVA film. (b5) UV-vis spectra of the pure PVAA, hexaldehyde (HA)-PVA, DA-PVA, and stearaldehyde (SA)-PVA films. Reproduced from ref. 18 with permission from the RSC.

Apart from the Hantzsch reaction, the Biginelli reaction is another environmentally friendly and efficient MCR that involves the combination of urea (or thiourea), aldehyde, and acetoacetate to create a 3,4-dihydropyrimidin-2(1H)-ones (DHPM) structure using a one-pot method. Due to the presence of a biconjugate effect (–C=C–C=O and a benzene ring), the DHPM structure exhibits strong UV absorption properties. Therefore, Qi and coworkers utilized the Biginelli reaction to prepare a cellulose-based UV-blocking film (Fig. 12(a2)).¹²⁸ Importantly, the reaction predominantly occurred on the surface of the cellulose film, making it more efficient and causing minimal damage to the surface microstructure and morphology of the cellulose film. The resulting cellulose-based film demonstrated excellent UV-blocking properties, effectively blocking UVA and UVB. The UV-blocking efficiency was highly dependent on the loading of DHPM, which was determined by the reaction time and vanillin concentration. Increasing the reaction time allowed for the formation of more DHPM, thereby enhancing the UV-blocking properties. For example, after a reaction time of 0.5, 1, 3, and 4 hours, the UV-blocking at 400 nm increased to approximately 62.7%, 85.5%, 94.8%, and 100%, respectively. Similar trends were observed with an increase in vanillin concentration. Furthermore, the researchers also developed a durable antimicrobial and UV-blocking cellulose surface by incorporating curcumin and lysine onto the cellulose surface through the Mannich reaction.¹²⁹ The presence of two phenolic hydroxyl groups and accessible 5 and 5′-positions in the curcumin molecule facilitated its cooperation with the Mannich reaction, resulting in the formation of a stable product (Fig. 12(a3)). The resulting curcumin-modified cellulose surface effectively blocked full-band UV while maintaining 82.1% vis-transparency.

The UV-blocking properties are a crucial feature of mussel-inspired polydopamine coatings. However, the current strategy for achieving UV-blocking relies solely on regulating the thickness of the coatings. This approach can introduce additional problems such as color variation and reduced visible light transmittance. Recently, a simple and versatile method was developed to enhance the UV-blocking properties of polydopamine coatings by incorporating thiol-heterocycle (TH) derivatives.¹³⁰ In this method, the coatings were post-modified by grafting TH components onto the surface of the polydopamine coatings through covalent bonds (Fig. 12(a4)). The incorporation of TH components led to an increase in the UV-blocking of the post-modified polydopamine coatings by 20.4% to 29.5% compared to conventional polydopamine coatings. This improvement was attributed to the strong UV absorption of the TH components. Importantly, the post-modification approach did not significantly affect the vis-transparency of polydopamine coatings. In addition, TH-doped polydopamine coatings were also prepared using a pre-modification strategy by copolymerizing dopamine monomers with TH components. Similar to the post-modified polydopamine coatings, the TH-doped polydopamine coatings exhibited enhanced UV-blocking properties, with enhancements ranging from 43.8% to 62% across the wavelength range of 290–400 nm. The improved UV-blocking properties can be attributed not only to the strong UV absorption of the TH components but also to the hindrance of conjugated structure formation by the doped TH components and the formation of new D–D intermediates with larger bandgaps within polydopamine coatings. The work provides valuable insights into the design and fabrication of polydopamine coatings, particularly for enhancing UV-blocking properties in bioinspired polydopamine coatings.

4.3 Polymer in situ chemical modification

The utilization of in situ chemical modification represents a highly promising approach for the fabrication of UV-blocking polymers. Recently, Qi and coworkers successfully demonstrated the in situ chemical modification of a cellulose film¹⁹ and a PVA film¹³¹ through the Hantzsch reaction. As shown in Fig. 13(a), the acetoacetylation of cellulose was performed through transesterification with tert-butylacetoacetate in an ionic liquid. Subsequently, the acetoacetyl groups in the prepared CAA hydrogel film served as reaction sites for the Hantzsch reaction, leading to the creation of DHP rings on cellulose chains in the presence of aldehyde and ammonium acetate. The final product was an UV-blocking cellulose film obtained by drying the cellulose hydrogel film.¹⁹ Remarkably, it was found that different functional groups could be connected to the DHP rings by utilizing various aldehyde group-containing components. For example, the FC-FA film, prepared from formaldehyde, exhibited strong UV-blocking ability, effectively blocking almost 100% of UVC and UVB as well as a significant portion of UVA. At the same time, it maintained a high level of vis-transparency (approximately 90% of T₆₀₀, Fig. 13(b)). It is worth noting that the choice of functional groups connected to the DHP rings significantly influenced the UV-blocking properties. When a benzene or a tetrahydrofuran ring was connected to a DHP ring, the extended aromatic ring structure enhanced the conjugation effect, resulting in improved UV-blocking properties (Fig. 13(c)). In comparison, four other cellulose films demonstrated complete UV-blocking in the UVC and UVB regions and a noticeable improvement in UV-blocking within the wavelength range of 320–380 nm, compared to the FC-FA film. In addition, the DHP could dissipate energy in the form of visible fluorescence emission under UV radiation. Together with UV-blocking capability and vis-transparency, the prepared cellulose film holds promise as an outdoor sun protection film (Fig. 13(d)).

Fig. 13 UV-blocking cellulose acetoacetate (CAA) prepared via in situ chemical modification. (a) A schematic illustration of the preparation of a CAA-based UV-blocking film. (b) and (c) UV-vis spectra of a series of prepared CAA-based films through the utilization of various aldehyde group-containing components. (d) A scheme showing the CAA-based film for UV protection. Reproduced from ref. 19 with permission from the RSC.

Subsequently, they also utilized a PVA film containing acetoacetyl groups as the base material for facilely preparing a multifunctional film.¹³¹ The acetoacetyl groups served as reaction sites for both the Hantzsch reaction to form DHP rings and the fabrication of the dynamic covalent enamine bonds to graft gentamicin sulfate (Gen) onto the PVA chains. As expected, the incorporation of DHP rings endowed the PVA-based film with full-band UV-blocking and fluorescent properties, making it highly versatile for various optical and sensing applications. In addition, the successful grafting of Gen onto the film gave the PVA-based film excellent antibacterial properties, further demonstrating its potential for applications in antimicrobial coverings and medical protection.

Overall, the incorporation of diverse optical motifs enhances UV absorption and enables excellent UV-blocking. Aromatic ring structures, including furan and benzene rings, possess a conjugated π-electron system that efficiently absorbs UV radiation, preventing it from reaching the underlying layers of the material. Unsaturated bonds, such as C=C and C=N, contribute to UV absorption by undergoing electronic transitions upon UV irradiation. Conjugated structures, like bifuran and carbazole structures, exhibit extended π-conjugation, allowing for greater UV absorption. The presence of –C=C–C=O and –C=C–C=C– structures further enhances UV-blocking capabilities due to their extended conjugation and absorption in the UV region. Auxochromes, such as phenolic hydroxyl groups, can also contribute to UV absorption by acting as electron-donating groups and extending the conjugation system. Further research and application of these optical motifs hold significant importance in providing better UV protection and improving overall health and quality of life.

5. Conclusion and outlook

UV-blocking polymers and composites have significant potential for providing effective physical UV protection for the human body, historic preservation, food preservation, and more. Over many years, numerous advances in synthetic UV-blocking polymers and composites have allowed for precise control over the types and distribution of UV filters, polymer matrix types, and UV absorption motifs. Various UV filters, including organic, inorganic, and polymeric UV filters, can be engineered to enhance the UV-blocking properties of polymers. However, achieving a uniform dispersion of UV filters remains a challenge, as the poorly dispersed UV filters can inversely affect not only optical properties (such as UV-blocking and vis-transparency) but also mechanical properties. In addition, both organic and inorganic UV filters are at risk of leakage, compromising the stability of polymeric materials. To overcome these disadvantages, constructing pure polymer systems through polymer structure design and surface/in situ chemical modification strategies would be conducive to excellent optical properties, mechanical properties, and stability.

Despite the significant progress made in this field, there are still several fundamental issues that require further investigation. This review of current studies reveals many materials with exceptional UV-blocking and even full-band UV-blocking properties. However, most of these materials exhibit relatively low vis-transparency, with less than 80% transmittance in the visible region (Tables 1 and 2). These results underline a considerable challenge in the field, namely the inherent trade-off between achieving high/full-band UV-blocking and maintaining high vis-transparency (>90% transmittance in the visible region). In polymer composite systems, adding a high amount of UV filters can enhance the UV-blocking properties but can also severely impact vis-transparency. Consequently, effective approaches should focus on enhancing the UV-blocking efficiency and dispersibility of UV filters, as well as achieving a suitable refractive index matching between UV filters and polymer matrices.^141–143 Moreover, future research should focus on investigating novel strong UV absorption motifs and their integration into pure polymer systems to overcome the inherent trade-off between UV-blocking properties and vis-transparency. In addition to providing strong UV absorption, these motifs can be customized to exhibit lower absorption in the visible light range, thereby ensuring the materials' transparency. By fine-tuning parameters such as the motif type, concentration, and distribution within the polymer matrix, it is feasible to optimize both UV-blocking properties and vis-transparency of the material. In addition, the selection or synthesis of amorphous polymers is highly recommended to improve vis-transparency.^144,145

The objective of developing UV-blocking polymers and composites with self-healing behavior, recyclability, or biodegradability extends beyond providing UV protection. It also aims to achieve carbon neutrality and promote sustainable developments. Recent literature demonstrates that only a limited number of UV-blocking polymers or composites have been successfully recycled or shown self-healing capabilities. For instance, examples include the TiO₂/P(HEMA-co-BA) composite utilizing host–guest interactions,⁸⁸ SPI-PEI-PDA based on hydrogen bonds,⁷⁸ lignin-based imine vitrimers,¹⁴⁶ MXene composited organohydrogels relying on hydrogen bonds,¹⁴⁷ HPG/IA film,¹²¹ lignin-based PU elastomer,¹²⁴ and castor oil-based epoxy resin-itaconic acid vitrimer¹⁴⁸ employing dynamic transesterification. Consequently, there is an urgent need to develop innovative UV-blocking polymers and composites with self-healing behavior and/or recyclability. In the field of self-healing or recyclable UV-blocking polymers and composites, there are several areas that warrant development. These include maintaining optical properties ever after the healing or recycling process, enhancing the healing efficiency of healing, expanding the range of environmental conditions in which these materials can function effectively, increasing the number of healing or recyclable cycles, and exploring cost-effective approaches. In addition, significant efforts have been invested in the development of biodegradable UV-blocking polymers and composites. Examples include all-biobased materials (e.g., oxidized corn starch-based nonionic biopolymer/gelatin¹⁴⁹ and tannic acid/gelatin/transparent wood film¹⁵⁰), PVA-based materials,^151–154 pectin-based plastic film,¹⁵⁵ and lignin-based film.^156,157 However, several emerging trends can be observed in the area. Researchers are focusing on developing biodegradable materials that not only provide UV-blocking but also possess additional functionalities, such as antimicrobial activity,^149,154 self-healing capabilities, mechanical reinforcement, or plasticity.¹⁵⁵ These multifunctional materials aim to address various needs in various applications. Another trend is the development of biodegradable UV-blocking materials that can respond to external stimuli, such as temperature or UV radiation. These smart materials can alter their UV-blocking properties based on environmental conditions, providing more efficient protection. Furthermore, the integration of nanoparticles into biodegradable polymers and composites is gaining traction. This approach offers enhanced UV-blocking capabilities and opens up new possibilities for material design. As the field progresses, there is also an increasing emphasis on evaluating the environmental impact of biodegradable UV-blocking materials throughout their lifecycle. This includes assessing their biodegradability, ecotoxicity, and potential for releasing microplastics or harmful byproducts during degradation. These considerations are crucial for ensuring the sustainability of such materials.

To date, there have been limited literature studies where these strategies have been used in the development of UV-blocking polymers or composites that can satisfy daily life needs (usually low-power UV radiation). To address the requirements of a broader range of applications, future research in this field should prioritize the optical stability of UV-blocking polymers and composites under high-power UV radiation. Therefore, it is highly desirable to explore additional UV-blocking mechanisms. Moreover, more effort should be dedicated to elucidating the relationship between the optical structure and function.

In addition to optimizing the manufacturing process of UV-blocking polymers and composites, it is crucial to prioritize enhancing their practicality. Firstly, several strategies, such as streamlining the production steps, minimizing material waste, and utilizing advanced technologies, should be employed to reduce overall manufacturing costs and enhance accessibility. Secondly, UV-blocking polymers and composites should be designed to withstand prolonged exposure to sunlight and maintain their protective properties over time. This can be achieved by selecting suitable additives, reinforcing agents, or stabilizers that enhance the material's resistance to UV degradation. Furthermore, it is crucial to consider the compatibility and versatility of UV-blocking polymers and composites with different manufacturing processes and applications. This can be accomplished by designing these materials to be easily moldable, injectable, or extrudable, enabling their integration into a wide range of products, such as coatings, films, textiles, and construction materials. Lastly, it is essential to consider the environmental impact of the manufacturing process and the end-of-life disposal of UV-blocking polymers and composites. Developing sustainable strategies, such as using bio-based or recyclable materials, reducing energy consumption, and promoting responsible waste management, can contribute to the overall practicality and market acceptance of these materials. Overall, this review highlights recent developments in UV-blocking polymers or composites and serves as a guide for fabricating novel sophisticated UV-blocking polymers or composites using straightforward and effective structural designs.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors thank the National Natural Science Foundation of China (Grant No. 52203008, 22172045, U23A20122, and 22472047) and the Key Science Foundation Project of Henan Province (Grant No. 232300421146).

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