Endowing recyclability to anti-adhesion materials via designing physically crosslinked polyurethane

Minhuan Liu a, Danfeng Yu a, Xiubin Xu a, Hui Yang b, Ian Wyman c, Jinben Wang b and Xu Wu *a
aSchool of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou 510006, China
bCAS Key Lab of Colloid, Interface and Chemical Thermodynamics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China
cDepartment of Chemistry, Queen's University, Kingston K7L 3N6, Canada. E-mail: xuwu@gzhu.edu.cn

Received 16th July 2019 , Accepted 20th August 2019

First published on 21st August 2019


The existing principles regarding the design of anti-adhesion materials involving molecular thickness, crystallinity, thermosetting chemical crosslinking, sophisticated structures, and complex compositions significantly restrict the recyclability of these functional materials. Herein, we report a homogeneous thermoplastic film generated from a physically crosslinked linear polyurethane that exhibits anti-adhesion performance against water and various oily liquids, while exhibiting facile recyclability via dissolution in selective solvents or under slight heating, and these treatments could also heal the material if it became damaged. Moreover, the film exhibits a maximal fracture strain exceeding 1570% with a tensile strength of over 5.66 MPa, high optical transparency with a transmittance exceeding 98.5% at a thickness of 0.5 mm, and it can bond strongly to various kinds of substrates. Due to the convenient preparation and excellent recyclability of this novel anti-adhesion polymer-based material, it may have significant potential for numerous applications. In addition, this work may ultimately allow researchers to endow various other anti-adhesion materials with facile recyclability, thus improving resource utilization and abating environmental pollution.


1. Introduction

Due to the shortage of valuable resources and the increase of environmental pollution, recyclability has become a highly desirable feature for many materials.1–3 Numerous examples of anti-adhesion materials exhibiting low affinity to both water and oil have been designed and prepared, and their liquid repellency is of broad interest for various practical applications such as self-cleaning,4–6 anti-fouling,7,8 deicing,9,10 antifogging,11,12 corrosion resistance,13 liquid transport,14,15 and drag reduction.16,17 The existing anti-adhesion materials rely on various features with regard to their surface chemistry and surface roughness, which ultimately limit their recyclability. Consequently, it is very difficult to develop a material that combines anti-adhesion performance with recyclability.

In general, the existing anti-adhesion materials can be classified into two categories according to their compositions, known as homogeneous and hybrid materials.18–20 Homogeneous anti-adhesion materials are based on hydrophobic organic compounds, and they can be further divided into thermoplastic materials with linear molecular structures, such as monolayers21,22 and Teflon (polytetrafluoroethylene),23–25 and thermosetting polymer materials which are obtained via chemical crosslinking.26–28 Smooth surfaces of these homogeneous materials can exhibit anti-adhesion performance against both water and oil with moderate contact angles below 120°.20 Meanwhile, the roughening of these materials via methods such as etching or fibration can increase the contact angles to over 150° and impart otherwise amphiphobic materials with superamphiphobicity.29,30 Meanwhile, the typical examples of hybrid anti-adhesion materials are lotus-inspired superamphiphobic surfaces with hierarchical roughness (on both the micro- and nanoscale)4 and the Nepenthes-inspired slippery lubricant-infused porous surfaces (SLIPSs) along with micro/nanoscale inner structures.5 These hybrid superamphiphobic materials have been developed using various composites such as fluorinated compounds with a diverse range of fillers including SiO2,31 TiO2,32 ZnO,33 multi-walled carbon nanotubes,34 or raspberry-like polymer particles.35 The smooth liquid-like surfaces of SLIPS materials achieved via locking the lubricants within porous substrates can also remain clean after exposure to various liquids, while exhibiting moderate contact angles. The lubricants employed in these materials are typically fluorinated liquids (such as poly(n-hexafluoropropylene oxide)/poly(hexafluoroisopropylene oxide)) or silicon oil (such as poly(dimethylsiloxane)) with low surface energies and insolubility in the test liquids, while the porous substrates include Teflon nanofibers,5 silica nanoparticles,36 and nano-textured alumina gels.37 The difficulties encountered with the separation and reshaping of hybrid composites, especially those possessing micro-/nanoscale roughness and porous features, and the poor solubility in common solvents and high melting points (∼330 °C) of Teflon as well as the homogeneous thermoset composites present significant challenges at the end of their service lifetime and impede their recovery and recyclability.38

In addition, extensive efforts have been devoted toward addressing the inherent drawbacks of the existing anti-adhesion materials such as the limitations regarding their transparency, stretchability, and the bonding of these materials to other substrates. The crystallinity of Teflon23–25 and the micro-/nanoscale structures of hybrid materials tend to scatter light, thus inducing optical blur.39–41 Meanwhile, materials incorporating monolayers, rigid crystalline phases, and micro-/nanoscale structures are prone to damage via mechanical deformation.39–41 The structured and low surface energy components do not readily bond to other substrates, and a combination of paint and adhesives has been employed to enhance the bonding strengths.42

In contrast to traditional strategies involving molecular thickness, crystallinity, thermosetting chemical crosslinking, sophisticated structures, and complex compositions, herein we report a homogeneous thermoplastic film generated from physically crosslinked polyurethane linear chains, exhibiting anti-adhesion toward water and various oils, while also offering facile recyclability. In contrast to the existing anti-adhesion materials, recyclability can be readily achieved in this case via dissolution in selective solvents or slight heating. Moreover, the film exhibits outstanding stretchability and transparency because of its physical crosslinking and amorphous nature. It can also bond strongly to a wide range of substrates as the fluorinated content is sufficiently low that it does not affect the interfacial binding chemistry between the material and the substrates. In addition, the film can exhibit self-healing performance via contact with selective solvents or slight heating.

2. Experimental section

2.1 Materials

Poly(adipate)-(2-methyl-1,3-propanediol)-(1,4-butanediol) ester diol (PMBA, with an average equivalent weight of 1000 g mol−1) was supplied by Jining Huakai Resin Co., Ltd. and distilled under vacuum prior to use. Isophorone diisocyanate (IPDI, >99.0%) was provided by Hengtai Chemical Co., Ltd. and was used as received. 1H,1H,2H,2H-Tridecafluoro-1-octanol (TFO) was purchased from Guangdong Wengjiang Chemical Reagent Co., Ltd. and purified under vacuum prior to use. N,N-Dimethylacetamide (DMAC) was purchased from Damao Chemical Reagent Co., Ltd and was dehydrated for 48 h with 4.0 Å molecular sieves prior to use.

2.2 Synthesis of the polyurethane

The synthesis of polyurethane was performed as follows: first, PMBA (6.66 g, 6.66 mmol), IPDI (3.70 g, 16.6 mmol), DMAC (12.00 g, 138 mmol) and DBTDL (0.0050 g, 0.0080 mmol) were added into a 100 mL flask. This mixture was then heated to 80 °C with stirring, and the reaction proceeded for 2 h at 80 °C. TFO (1.22 g, 3.35 mmol) was subsequently added to the above mixture, and the reaction was allowed to continue for 2 h to obtain the polyurethane prepolymer (PU–NCO). This reaction mixture was then cooled down to 50 °C, and H2O (0.300 g, 16.6 mmol) was added to react with the remaining –NCO groups. Finally, a 50.00 wt% solution was obtained after the reaction had proceeded at 50 °C for 24 h.

2.3 Preparation of the polyurethane film

The polymer solution was poured into a culture dish and kept at 50 °C for 24 h to form the film. The film was then peeled away from the culture dish, and cut into rectangular sections measuring 5.5 cm (length) × 1.0 cm (width) × 0.5 mm (thickness) for further testing.

2.4 Characterization

Fourier-transform infrared (FTIR, Bruker Optics, Spectrum 100) spectra were recorded using a Tensor-27 spectrometer at room temperature to verify the conversion of the isocyanate groups, and KBr was used as the sample matrix. Size-exclusion chromatography (SEC) measurements were performed to analyze the weight average molecular weight (Mw) and the number average molecular weight (Mn), as well as the polydispersity index (PDI). Polystyrene was used as the standard and N,N-dimethylformamide (DMF) was used as the mobile phase at a flow rate of 1.0 mL min−1. Dynamic thermomechanical analysis (DMA) was performed with a dynamic mechanical analyzer (TA instrument, Q800) using stretching mode at a constant frequency of 1 Hz. The storage modulus (G′) and loss modulus (G′′) were measured from −70 to 200 °C at a heating rate of 3 °C min−1, and the samples were used with dimensions of 30 mm (length) × 5 mm (width) × 0.5 mm (height). Differential scanning calorimetry (DSC, PerkinElmer, DSC8000) measurements were performed under a nitrogen atmosphere at heating and cooling rates of 5 °C min−1. Thermogravimetric analysis (PerkinElmer, TGA, TGA4000) was conducted to evaluate the thermal stability of the film by using dry nitrogen as the purge gas. The optical transmittance of the film was measured using a UV-visible spectrometer (PerkinElmer LAMBDA 950). Air was used as a reference. The transmission at 500 nm was used as an index to evaluate the optical transmittance of the sample. An atomic force microscope (Bruker, Dimension) was used to analyze the surface roughness of the film. Meanwhile, the elemental analysis of the cross-section and surfaces of the film was performed via X-ray photoelectron spectroscopy (XPS, Thermo Scientific, ESCALAB 250Xi). The morphology and the energy-dispersive X-ray spectroscopy (EDX) mapping of the film were obtained with a scanning electron microscope (SEM, Hitachi, SU8010) at a 20 kV accelerating voltage. The static contact angles (θS), sliding angles (θslide), advancing contact angles (θA) and receding contact angles (θR) were measured using a contact angle goniometer (SDC200, Shengding, China). The droplet volume employed for the θS measurements was 2 μL while that used for the θslide measurements was 20 μL. Meanwhile, the θA and θR values were measured by injecting the test liquids into or withdrawing them from a droplet of the same liquid resting on the film until it advanced or receded. The reported data were obtained by averaging the data from five measurements. The microphotographs were recorded using an optical microscope (Mshot, MD30). Mechanical tensile-stress tests were carried out using a universal testing machine (SUNS, UT4304) with a load cell of 100 kN, using a sample with a width of 10 mm, thickness of 0.5 mm, and a gauge length of 5 mm. The recovery of the film after the release of stress was calculated using the equation Rr = 1 − ε(t)/εmax, where εmax denotes the strain measured at the beginning of the recovery process and ε(t) represents the time-dependent strain. The adhesion between the coatings and the substrates was evaluated according to the ASTM D3359 standard protocol.

3. Results and discussion

As shown in Fig. 1, the film was molded in a culture dish at 50 °C for 24 h from fluorine-terminated physically crosslinked polyurethane linear chains that were dispersed in N,N-dimethylacetamide. This polyurethane was synthesized via the polycondensation of isophorone diisocyanate (IPDI) with polyols. 1H,1H,2H,2H-Tridecafluoro-1-octanol (TFO) was used to provide a low surface energy, and poly(adipate)-(2-methyl-1,3-propanediol)-(1,4-butanediol) ester diol (PMBA) was introduced as a flexible hydrophobic macrodiol. A non-covalent physical crosslinking phenomenon was achieved via the macrocyclic urea hydrogen bonding that resulted from the reactions between water and IPDI. SEC and FTIR data confirming the structure of the polyurethane are shown in Fig. S1 and S2 in the ESI.
image file: c9ta07675a-f1.tif
Fig. 1 Schematic illustration depicting the preparation of the polyurethane and the film. The structures of IPDI, PMBA, and TFO are also shown.

The homogeneous composition of the film was confirmed by energy-dispersive X-ray spectroscopy (EDX) elemental mapping and X-ray photoelectron spectroscopy (XPS). The scanning electron microscopy (SEM) image and EDX mappings of the cross-section of the film (Fig. 2a) revealed that the elements including C, O, N, and F were uniformly distributed throughout the entire film matrix. In addition, XPS analysis of both sides of the film also indicated that the elemental distribution did not change significantly (Fig. 2b), which was consistent with the EDX mappings of both surfaces of the film (Fig. S3 in the ESI).


image file: c9ta07675a-f2.tif
Fig. 2 The SEM and EDX elemental mapping of the film's cross-section (a). The XPS analysis of both sides of the film (b). Still frames from videos recording the contraction of water, hexadecane, cooking oil, and pump oil on the film (c). Images recorded during static (2 μL in volume) and dynamic (advancing and receding) contact angle measurements for water, hexadecane, cooking oil, and pump oil on the film (d). Images of 20 μL various test droplets sliding off the film (e). Time-lapsed contact angles of various liquids after their application onto the film (f).

The surface of a film containing well-distributed low surface energy components can remain clean after exposure to water, hexadecane, cooking oil and pump oil, respectively. After the film had been immersed in these liquids, stained water (blue) was unable to wet the film and hexadecane rapidly glided off the film without leaving any residue (Fig. 2c and Video S1 in the ESI). Meanwhile, viscous cooking oil (∼80 cP at 20 °C) and pump oil (∼200 cP at 20 °C) required a few seconds to contract and slide off the surface of the film.43 The static and dynamic contact angles of various test liquids on the film are shown in Fig. 2d, and these values closely match those observed on the smooth surfaces of chemically crosslinked polymers described in our previous work.8,26 The sliding processes of these test liquids at their respective sliding angles were recorded, and the images and video are provided in Fig. 2e and Video S2 (ESI).

With regard to the wettability of the film over time, the contact angles of hexadecane, cooking oil, and pump oil were found to remain stable, while that of water decreased (Fig. 2f). The images of these droplets recorded at various times after their application onto the film are shown in Fig. S4 in the ESI. The increasing wettability for water alone can be explained by the rapid evaporation of water and the hydrophilicity of the polar urea bonds, which would be attracted by water and migrate onto the film surface and subsequently lead to surface reconstruction.44 The water absorption of the film after several immersion periods have been investigated, and the results are given in Fig. S5 in the ESI. On the other hand, hexadecane, cooking oil, and pump oil were incompatible with the composition of the polyurethane, and they thus did not induce surface reconstruction during extended contact with the film.

The film exhibited good foldability and flexibility, showing no damage when it was subjected to a minimum bending radius of 1 mm (Fig. 3a). This outstanding flexibility can be attributed to the unique hard-soft nanophase of the polyurethane structure,45 which endows the film with a wide range of applications for flexible materials and devices such as coatings, wearable electronics, textiles, biosensors, or soft robotics.46–51


image file: c9ta07675a-f3.tif
Fig. 3 Photograph taken during the measurement of the film minimum bending radius (a). The application of the film onto various substrates including Teflon, tin, glass, wood and PET, respectively (from left to right, (b)). The photograph taken during the measurement of the minimum bending radius for a coated Teflon plate (c). Cross-cut patterns remaining on the coated tin, glass, wood, and PET after the adhesion test (left to right, (d)). The transmittance curve for the film, including an inset image of the film that had been applied onto QR code-labeled paper (e). The tensile stress curve for the film stretched at 15 mm min−1 for a sample with a width of 10 mm, thickness of 0.5 mm, and gauge length of 5 mm (f). The recovery of the film after it had been stretched by 1500% at 15 mm min−1. The inset photographs show the initial recovery of the film after release (g). Contact angles of the films with various strains (h). Sliding angles of the films with various strains (i). Images of various liquid droplets sliding off the film with different strains (j).

In addition, the bonding of the film onto various substrates such as Teflon, tin, glass, wood, and polyethylene terephthalate (PET) was studied (Fig. 3b). Teflon is a typical anti-adhesion material, while the film can bond firmly to Teflon without showing signs of rupture even at a bending radius of 1 mm (Fig. 3c). During ASTM D3359 standard tests, with tin, glass, wood, and PET substrates, no squares were peeled from the cross-cut patterns by tape (Fig. 3d), demonstrating that the adhesion of the film onto these substrates reached up to the highest ranking of 5B.52 The outstanding bonding to various substrates could be attributed to the polyurethane bonds and the relatively low content of low surface energy components.53 Moreover, Fig. 3e demonstrates that the film with a thickness of 0.5 ± 0.1 mm had a transmittance exceeding 98.5% at 500 nm, and the QR code remained very clear and legible when it was covered by the film. Due to its superb transparency, this film is a good candidate for anti-adhesion surface functionalization of optical devices, such as electronic displays, optical lenses and eye glasses.

The film exhibited high stretchability and good mechanical strength as shown in Fig. 3f and Video S3 (ESI). For a film having a thickness of 0.5 mm, gauge length of 5 mm, and width of 10 mm, the maximum fracture strain reached 1570 ± 100% at a stretching rate of 15 mm min−1 with the tensile strength reaching up to 5.66 MPa. This outstanding stretchability could be attributed to the cleavage and reformation of physical bonds during the deformation process.54 After stretching to 1500% at a rate of 15 mm min−1, the film exhibited rapid self-recovery upon release, as shown in Fig. 3g and Video S4 (ESI). The recovery ratio of the stretched film was up to 83.0% within 10 min of stress release and reached 88.0% and 94.3% within 30 min and 24 h, respectively. The microscale damage and roughness that the film had incurred during the tensile process were observed via optical microscopy in combination with a camera (Fig. S6, ESI). The incorporation of this microscale roughness has been shown to yield a larger contact angle if that of the intrinsic smooth surface exceeds 65°.55,56 This increase in the contact angles and sliding angles was also observed for this film (Fig. 3h and i). Notably, the film retained its anti-adhesion performance against water and hexadecane when the elongation reached up to 900%, and it repelled both cooking oil and pump oil within 600% of elongation. As shown in Fig. 3j and Video S5 (ESI), droplets of water, hexadecane, cooking oil, and pump oil cleanly slid off the stretched films.

The system was designed with the use of physical crosslinking and exhibited selective solubility and a thermoplastic nature. The film can repel water and various oils while remaining soluble in polar organic solvents, such as ethanol, acetone, and N,N-dimethylacetamide. Consequently, this film has potential applications as a recyclable material. As shown in Fig. 4a and Video S6 (ESI), the film with a flower-like shape was cut into millimeter-sized sections and subsequently saturated with ethanol (well resolved within 30 min at room temperature), and the solution can be used to reshape the recycled butterfly-like film via facile pouring into a mold and curing at 50 °C for 24 h. As indicated by the AFM images in Fig. 4b, the surface roughness of the films was less than 2 nm both before and after recycling, and the optical transmission of the recycled film reached 98.3% at 500 nm, which was consistent with the intrinsic film (Fig. 4c).


image file: c9ta07675a-f4.tif
Fig. 4 Procedure for the recycling of the film (a). AFM images of the film surfaces before and after the recycling process (b). Transmittance curves for the film before and after recycling (c). Tensile stress curves of the film before and after recycling (d). Stress–strain curves for the films stretched at 15 mm min−1 for 10 stretching and recovery cycles with elongations of 200% (e) and 1000% (f), respectively. Images of static and dynamic (advancing and receding) contact angles and the sliding angles of various liquids on the recycled film (g).

As shown in the stress–strain curve, the recycled film displayed mechanical performance that closely matched that of the original film (Fig. 4d). Moreover, the stress–strain curves of the pristine and recycled films for each of the 10 stretching and recovery cycles with elongations of 200% and 1000% (Fig. 4e and f, respectively) demonstrated the sustained reversible stretchability of the film, although energy dissipation-induced hysteresis loops were observed as the number of cycles increased.45 In addition, the recycled film showed identical anti-adhesion performance to water, hexadecane, cooking oil, and pump oil in comparison with the pristine film, and the contact angles and sliding angles of these liquids are given in Fig. 4g.

As shown in Fig. 5a, the thermal stability of the film was evaluated via thermogravimetric analysis (TGA) and derivative thermogravimetry (DTG). According to the curves, no significant weight loss was observed up to 200 °C, and the film subsequently underwent a three-stage thermal degradation when the temperature was increased further. The initial weight loss was attributed to the loss of solvent vapor and the degradation of the oligomers. The hard domains in the backbone of the polyurethane began to decompose when the temperature exceeded 280 °C, and the soft segments decomposed in the temperature range between 300 and 470 °C.57 In addition, the viscoelastic behavior of the film was characterized via dynamic thermomechanical analysis (DMA), and the temperature dependence of the storage modulus (G′), loss modulus (G′′), and tan[thin space (1/6-em)]δ(G′′/G′) is shown in Fig. 5b. A large storage modulus in excess of 65 ± 5 MPa at 40 °C was obtained, indicating that the film had sufficient mechanical strength for room temperature applications. To the best of our knowledge, only a few elastomers with higher stretchabilities than this film (but exhibiting much less tensile strength of less than 0.4 MPa) have been reported, such as those based on metal–ligand complexes58 or nanocomposite hydrogels.59 When the temperature was increased to 80 °C, the film became ruptured with a deformation of only 1% during the DMA experiments. The dynamic reversible physical hydrogen bonds would dissociate above 80 °C, resulting in a viscous state and significantly diminished strength.45,60 The glass transition temperature (Tg) observed from tan[thin space (1/6-em)]δ was −37 °C. A broad peak at approximately −40 °C corresponding to the Tg was observed via differential scanning calorimetry (DSC) analysis, which was consistent with the results from the DMA tan[thin space (1/6-em)]δ (Fig. 5c). DSC analysis also demonstrated the amorphous nature of this film because it did not display sharp peaks corresponding to a crystalline phase in the temperature range from −60 to 80 °C.


image file: c9ta07675a-f5.tif
Fig. 5 TGA and DTG curves of the film (a). G′, G′′, and tan[thin space (1/6-em)]δ(G′′/G′) curves of the film obtained via DMA (b). The DSC curve of the film (c). Photographs of the damaged and healed film (d). Optical microscope images of the damaged (e) and healed (f) film.

The thermoplastic nature and the dissociation and reformation of dynamic reversible physical bonds endow the film with self-healing performance.61 To demonstrate this self-healing behavior, the film was scratched and subsequently thermally cured at 120 °C for 2 min. In comparison with the scratched film, the scratches on the film disappeared after treatment (Fig. 5d), indicating that the film presented good self-healing capabilities. The scratched film was observed via optical microscopy before and after the self-healing62 (Fig. 5e and f), and an ink droplet was added near an intersection between two scratches to enhance the contrast. After curing at 120 °C for 2 min, the permeation of ink into the film can be observed during the process of bond dissociation and reformation, and the scratches were found to have completely disappeared after treatment. The re-formation of physical bonds and crosslinking could provide healing from the scratches and strain-induced damage, thus extending the lifetime of this recyclable anti-adhesion material even for applications involving continuous stretching.

4. Conclusions

In summary, a recyclable anti-adhesion material has been developed herein via the synthesis of a fluorine-terminated physically crosslinked linear polyurethane and the molding of this polymer into a film. The as-prepared homogeneous thermoplastic amorphous film exhibited anti-adhesion performance against various liquids, high transparency with a transmittance exceeding 98.5% at a film thickness of 0.5 ± 0.1 mm, maximal fracture strain exceeding 1570% with a tensile strength of over 5.66 MPa, and the ability to bond strongly to various kinds of substrates. The thermoplastic nature endowed the film with desirable recyclability via dissolution in selective solvents or slight heating, and these materials also exhibit solvent- or thermally triggered healing from damage. The facile preparation and recycling conditions of these novel anti-adhesion materials may provide them with significant potential for applications in various fields including the next of generation flexible and stretchable materials and devices. We anticipate that this work will ultimately lead to the introduction of recyclability into a wide range of anti-adhesion materials.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We gratefully thank the Important National Science and Technology Specific Project of China (2017ZX05013003004), the National Natural Science Foundation of China (21878059 and 21603240), and the Science and Technology Project of Guangdong Province (2017A050501040) for sponsoring this research.

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Footnotes

Electronic supplementary information (ESI) available. See DOI: 10.1039/c9ta07675a
M. H. Liu and D. F. Yu contributed equally to this work.

This journal is © The Royal Society of Chemistry 2019