Waterborne UV-curable comb-shaped (meth)acrylate graft copolymer containing long fluorinated and/or polysiloxane side chains

Abstract

A series of waterborne UV-curable comb-shaped (meth)acrylate graft copolymers containing long fluorinated and/or siloxane side chains were synthesized by conventional radical copolymerization of a novel mono-methacryloyloxy terminated fluorinated macromonomer (PHFA-GMA) and/or polysiloxane macromonomer (SiOHMAC) with (meth)acrylate monomers. The separate effects of PHFA-GMA and SiOHMAC, as well as the synergic effect of these two components, on the properties, especially the surface properties were investigated in detail. A hydrophobic surface could be obtained with extremely low content of PHFA-GMA and/or SiOHMAC due to the strong tendency of the macromonomers to migrate towards the outmost layer, resulting in abundant enrichment of fluorine and silicon atoms on the surface. XPS (X-ray photoelectron spectroscopy) results revealed that for a given weight of the two macromonomers, Si atomic concentration of the copolymer modified by SiOHMAC is higher than F atomic concentration of the copolymer modified by PHFA-GMA. AFM (atomic force microscopy) images showed that surface of the copolymer modified by SiOHMAC is rougher than that modified by PHFA-GMA. Compared to PHFA-GMA, SiOHMAC had higher efficiency and effectiveness in creating hydrophobic surfaces. In addition, the influence of PHFA-GMA and/or SiOHMAC on the physical properties, such as water dispersion particle size, water absorption, pencil hardness, adhesion, mechanical properties and thermal properties were also investigated. The novel comb-shaped copolymers prepared via conventional radical polymerization not only had excellent properties but also have potential applications in large scale industrialization.

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1. Introduction

Waterborne UV curing technology has received much attention, especially in the coatings industry, because it combines the advantages of both UV curing technology and waterborne technology, such as rapid curing process, mild curing conditions, being free of volatile organic compounds (VOC), etc.^1–4 Among waterborne UV-curing materials, polyacrylate is one of the most widely used materials for free-radical type UV-cure coating systems due to their suitability to be shaped in various molecular structures with different properties, good adhesion, excellent film formation ability, as well as their versatility and relatively low costs.^1,5 However, the hydrophilic groups which endow the polymer with the ability to disperse in water have negative effects on the properties, especially the surface property, of the coatings.

Fluorinated polymers and siloxane polymers are two famous families of materials with extremely low surface energy, outstanding water and oil repellency, excellent thermal and chemical properties, photochemical stability.^6–18

Fluorosilicone materials, which integrating the advantages of both fluorinated polymers and silicone-based polymers, create synergistic effects of fluorine and silicon.⁷ The synergistic effects of fluorine and silicon can improve the comprehensive performance of the materials, especially the surface property. Numerous researchers had focused on the preparation and properties of the fluoro-silicone containing poly(meth)acrylate.

Two major methods have been employed to incorporate fluorinated moieties and siloxane segments into poly(meth)acrylate, including the fluorosilicone core–shell latex nanoparticles method^19–25 and the fluorosilicone block copolymer method.^26–29

It has been found that in order to prepare coatings with extremely low surface energy, it is necessary to aggregate as many hydrophobic moieties, such as CF₃ and Si–O, on the surface of the coatings as possible.^22,23 The hydrophobic groups in the polymer films derived from fluorosilicone core–shell latex nanoparticles have been constricted in great extent since, on the one hand, the length of hydrophobic groups in fluorinated monomers are relatively short and easily buried in the bulk, on the other hand, the Si–O segments in silicone monomers usually form crosslinking structures. Thus the migration of the hydrophobic moieties towards the surface is extremely difficult.

Even though block copolymers with long fluorinated or long siloxane segments possess extremely low surface energy since the long hydrophobic segments have stronger thermodynamic driving force to migrate towards the surface of the films than that of the fluorosilicone core–shell latex nanoparticles, the preparation of fluorosilicone block copolymer requires reagents which are not facile for industry. It seemed that fluorosilicone block copolymers are not the proper candidate for the large scale industrialization.

Despite of block copolymers, another route to prepare low surface energy materials is the comb-shaped copolymers.³⁰ Furthermore, comb-shaped copolymers can be prepared via much more versatile and economical technique.^31–39 In this study, we synthesized a novel waterborne UV curable comb-shaped (meth)acrylate graft copolymer with long fluorinated and siloxane side chains via radical polymerization of mono methacryloyloxy terminated fluorinated macromonomer (PHFA-GMA), mono methacryloyloxy terminated siloxane macromonomer (SiOHMAC) and other (meth)acrylate monomers.

To our best knowledge, not much literature concerning the preparation and properties of comb-shaped poly(meth)acrylate copolymers with long fluorinated and/or polysiloxane side chains. Furthermore, no detailed research has been done to investigate the separate effects of fluorinated macromonomers and polysiloxane macromonomers, as well as the synergy of these two components, on the properties, especially the surface property, of waterborne comb-shaped UV-curable polyacrylate. This study might fill in this blank.

The preparation and investigation of the comb-shaped polyacrylate copolymers with long fluorinated and polysiloxane side chains might not only be of great significance to the better understanding of the influence of comb-shaped topological structures on the properties of poly(meth)acrylate, but also open up an effective but economical route to prepare low surface energy materials.

1.1 Materials

Hexafluorobutyl acrylate (HFA) was purchased from Harbin Xeogia Fluorine-Silicon Chemical Co., Ltd, China. Azobisisobutyronitrile (AIBN), 2,6-di-tert-butyl-4-methylphenol (BHT), 1,4-dioxane (DO), petroleum ether and triethylamine (TEA) were purchased from Tianjin Fuchen chemical reagents factory. 3-Mercaptopropionic acid (MPA), methyl methacrylate (MMA), isobutyl acrylate (IBA), acrylic acid (AA), glycidyl methacrylate (GMA), methacryloyl chloride (MAC), benzophenone (BP) were purchased from Aladdin Chemistry Co., Ltd, China. Tetrabutylammonium bromide (TBAB) was obtained from Tianjin Kemiou Chemical Reagent Co., Ltd. Dihydroxypropyl-terminated poly(dimethyl siloxane) (SiOH, M_n ≈ 3500, hydroxyl value ≈ 32.1 mg KOH per g) was purchased from Ark(FoGang) Chemicals Industry Co. Ltd. Dichloromethane (DCM) and sodium carbonate were purchased from Sinopharm Chemical Reagent Co. Ltd. Benzophenone (BP) was obtained from Shanghai Lingfeng chemical reagents Co., Ltd. All the materials were used as received except for AIBN which was purified by recrystallization from absolute ethyl alcohol.

1.2 Synthesis of mono-methacryloyloxy terminated fluorinated macromonomer (PHFA-GMA)

Solution of HFA (40.00 g), AIBN (0.40 g), MPA (3.20 g) and DO (40.00 g) was added dropwisely during 3 h into a three-neck flask, containing DO (40.00 g), immersed in an oil bath at 80 °C under nitrogen atmosphere. After added, the system was kept at 80 °C for 8 h, and then at 90 °C for 16 h. Subsequently the system was heated to 102 °C. GMA (8.57 g) was added dropwisely into the solution during 1 h after TBAB (0.41 g) and BHT (0.21 g) were added. The solution was kept at 102 °C for another 4 h after added. The purification procedure was as follows. The obtained polymer solution was added into 10 times weight of petroleum ether and the precipitate was collected. Subsequently the precipitate was dissolved into DO and then precipitated in petroleum ether again. After the precipitation–dissolution process was done for 5 times, the collected precipitate was dried at 60 °C for 24 h under vacuum condition. The yield of PHFA-GMA was 90.2%.

1.3 Synthesis of mono-methacryloyloxy terminated siloxane macromonomer (SiOHMAC)

In a three-necked flask containing SiOH (35.00 g) under a nitrogen atmosphere, a DCM solution (100.00 g) of MAC (0.523 g) was slowly added dropwisely for 6 h at room temperature by using a dropping funnel. After the end of addition, the solution was heated and refluxed for 5 h. Thereafter, the solution was cooled to room temperature, and the mixture was filtered and the filtrate was collected. Then the filtrate was successively washed with water, saturated sodium carbonate solution, water for three times. The organic layer was dried with anhydrous sodium sulfate, and the solvent was removed under reduced pressure. SiOHMAC was obtained as transparent oil liquid and the yield was 95.4%.

1.4 Synthesis of F series, Si series and Si/F series

Synthesis of F series, Si series and Si/F series can be divided into two parts: copolymerization and GMA grafted. Copolymerization is different while GMA grafted are the same among F series, Si series and Si/F series.

Copolymerization: for the synthesis of F series, Si series and Si/F series, the reaction formulation (see Table 1) of appropriate PHFA-GMA and/or SiOHMAC with half of the total DO (the same weight of the total monomers) was placed into a three-neck flask and immersed in an oil bath at 80 °C. Appropriate solution of MMA, IBA, AA, AIBN and the rest DO was added dropwisely into the above solution under nitrogen atmosphere for 3 h. After added, the polymerization was carried out at 80 °C for another 8 h, and then at 90 °C for 16 h.

Table 1 Reaction formulation of F series, Si series and Si/F series

Samples		PHFA-GMA (g)	SiOHMAC (g)	MMA (g)	IBA (g)	AA (g)	AIBN (g)	GMA (g)
F series	0Si/F	—	—	8.400	8.400	3.200	0.300	3.950
	0.125F	0.025	—	8.3875	8.3875	3.200	0.300	3.950
	0.25F	0.050	—	8.375	8.375	3.200	0.300	3.950
	0.5F	0.100	—	8.350	8.350	3.200	0.300	3.950
	1F	0.200	—	8.300	8.300	3.200	0.300	3.950
	2F	0.400	—	8.200	8.200	3.200	0.300	3.950
	4F	0.800	—	8.000	8.000	3.200	0.300	3.950
	8F	1.600	—	7.600	7.600	3.200	0.300	3.950
Si series	0.125Si	—	0.025	8.3875	8.3875	3.200	0.300	3.950
	0.25Si	—	0.050	8.375	8.375	3.200	0.300	3.950
	0.5Si	—	0.100	8.350	8.350	3.200	0.300	3.950
	1Si	—	0.200	8.300	8.300	3.200	0.300	3.950
	2Si	—	0.400	8.200	8.200	3.200	0.300	3.950
	4Si		0.800	8.000	8.000	3.200	0.300	3.950
	8Si		1.600	7.600	7.600	3.200	0.300	3.950
Si/F series	0.125Si/F	0.0125	0.0125	8.3875	8.3875	3.200	0.300	3.950
	0.25Si/F	0.025	0.025	8.375	8.375	3.200	0.300	3.950
	0.5Si/F	0.050	0.050	8.350	8.350	3.200	0.300	3.950
	1Si/F	0.100	0.100	8.300	8.300	3.200	0.300	3.950
	2Si/F	0.200	0.200	8.200	8.200	3.200	0.300	3.950
	4Si/F	0.400	0.400	8.000	8.000	3.200	0.300	3.950
	8Si/F	0.800	0.800	7.600	7.600	3.200	0.300	3.950

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GMA grafted: GMA was added dropwisely into the polymer solution during 1 h after TBAB (0.20 g) and BHT (0.10 g) were added. The solution was kept at 102 °C for another 4 h after added.

The purification procedure was the same with that subscribed in Section 1.2.

1.5 Preparation of water dispersion

TEA (1.69 g) was added into the polymer solution prepared in Section 1.4 to react with carboxylic group in the polymeric chains to form a salt-like structure at 40 °C in order to improve the hydrophilicity. In addition, an amount of BP (5 wt% the weight of the copolymer) was dissolved in the polymer solution. Deionized water was added dropwisely into the systems with vigorous stirring at 40 °C for 0.5 h. Subsequently, DO was removed by rotary evaporation and water dispersion with 35% solid content was obtained.

1.6 Preparation UV-cured films

The dispersions were cast onto various substrates and dried at 50 °C for 24 h and then heated overnight in an oven at 50 °C under vacuum to remove all water until constant weight. Subsequently, the coated films were irradiated by a high-pressure mercury lamp (500 W) for 120 s with a distance of 20 cm from lamp to the surface of samples in air atmosphere. The film thickness is around 100 μm. Freestanding films were prepared by pouring the dispersion into a Teflon™ mold (10 mm × 75 mm × 1 mm) and dried at room temperature for 7 days. Then the films were heated at 50 °C to reach a constant weight. Finally, the formulations were irradiated by a high-pressure mercury lamp (500 W) for 600 s.

1.7 Characterization

The molecular weight distributions of the polymer samples were measured at 30 8C by gel permeation chromatography (GPC) on a Waters 2410 instrument with THF as the solvent (1.0 ml min⁻¹) and polystyrene as the calibration standards.

The FTIR spectra were obtained using a TENSOR27 (Bruker, Germany) spectrometer over the range 400–4000 cm⁻¹.

¹H NMR was performed on a 400 MHz Bruker NMR spectrometer using CDCl₃ as solvent and tetramethylsilane as an internal reference. Chemical shifts of the ¹H NMR were related to the CDCl₃ signal at 7.24 ppm.

The particle size of the water dispersion was characterized by dynamic light scattering on Malvern (Zetasizer Nano ZS 90).

The contact angle of water was measured on the air-side surface of the coating films with a contact goniometer (Shanghai Zhongchen, China) by the sessile drop method with a micro-syringe at 25 °C. The average contact angle of each sample was measured more than five times at different locations. In this study, the surface free energy was calculated by means of geometric-mean equation which was described by Owens and Wendt.⁴⁰ According to Owens and Wendt, the surface energy of a given solid can be determined using an equation applied to two liquids.

(1 + cos θ)γ_l = 2(γ^d_sγ^d_l)^1/2 + 2(γ^nd_sγ^nd_l)^1/2

where γ_s and γ_l are the surface free energies of the solid and pure liquid, respectively. The superscripts ‘d’ and ‘nd’ represent the dispersive and non-dispersive contributions to the total surface energy, respectively. Water (γ_l = 72.8 mJ m⁻², γ^d_l = 21.8 mJ m⁻², γ^nd_l = 51 mJ m⁻²), diiodomethane (γ_l = 50.8 mJ m⁻², γ^d_l = 48.5 mJ m⁻², γ^nd_l = 2.3 mJ m⁻²).

Surface composition was carried by XPS using a Perkin-Elmer PHI-5400 X-ray photoelectron spectroscope in ultra-high vacuum with Al K radiation. Ar ion beam was employed to etch.

AFM images were obtained using a tapping mode at room temperature on Veeco (Multimode IIIa).

The gel content was performed with the following procedure. The UV-cured films were extracted with acetone for 24 h. After extraction, the UV-cured films was dried in a vacuum oven until constant weight. The equation summarized below: gel content% = W_t/W₀ × 100%, where, W₀ is the weight of the UV-cured film before extraction. W_t is the final weight after extraction.

Water absorption measurements of UV-cured samples were carried out in distilled water at 25 °C according to ASTM D 570. Weighed samples were kept in water for at least 48 h until equilibrium was attained. The water absorption value of UV-cured samples was calculated from the ratio of the weight of absorbed water to that of the dry polymer, Q = (W_s − W_d)/W_d, where W_s and W_d are weights of a swollen and dry sample, respectively.

The adhesion and the pencil hardness of the films were performed in accordance with ASTM 3359 and ASTM 3363, respectively. Sample preparation is the same as the one described in contact angle measurement.

Mechanical properties of the films were determined by standard tensile stress–strain tests. Stress–strain measurements were carried out at room temperature by using a universal testing machine (Reger, extension rate of 5 mm min⁻¹).

Differential scanning calorimetry (DSC) measurements were performed on DSC 204 (NETZSCH Germany) under N₂ atmosphere. Each sample (about 10 mg) was scanned from −30 to 200 °C at a heating rate of 20 °C min⁻¹ and held at 200 °C for 5 min to remove the thermal history. Glass transition temperature (T_g) values were recorded during the second heating scan taken as the midpoint of the heat capacity change.

Thermogravimetric analysis (TGA, TG209F3 NETZSCH Germany) was performed on a TG209F3 to study the thermal stability of fluorinated UV-cured films under N₂ atmosphere. Each sample was about 5 mg and heated at the heating rate of 10 °C min⁻¹ from 30 °C to 800 °C.

2. Results and discussion

2.1 Synthesis and characterization of macromonomers and copolymers

The synthetic routes and structure of PHFA-GMA, SiOHMAC, Si series, F series and Si/F series are shown in Schemes 1–3, respectively. Molecular weight and molecular weight distribution of the macromonomers and copolymers were summarized in Table 2. The molecular weight of PHFA-GMA was adjusted by changing the ratio of MPA to AIBN.

Scheme 1 Synthetic route of PHFA-GMA.

Scheme 2 Synthetic route of SiOHMAC.

Scheme 3 Synthetic route of F series, Si series and Si/F series.

Table 2 Molecular weight and molecular weight distribution of the macromonomers and copolymers

Samples	Mn^a	Mn^b	Mn^c	PDI
a Mn from end group titration.b Mn from GPC.c Mn from NMR.
PHFA-GMA	3470	3590	3684	1.67
SiOHMAC	3618	3495	—	1.60
4Si	—	26 498	—	2.21
4Si/F	—	29 391	—	2.25
4F	—	26 980	—	2.30

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In order to characterize the structure of macromonomer specifically, it is necessary to determine the functionality of the macromonomer. In this article, end group titration,^41,42 GPC and ¹H NMR were employed to determine the functionality of PHFA-GMA. GPC profiles were summarized in Fig. 1. The methacryloyloxy group content of macromonomer can be calculated by the integral ratio of the protons (–CH₂=C(CH₃)–COO–, 6.05 ppm and 5.63 ppm) of the double bond in methacryloyloxy to methane proton of –CHF– (5.06–4.78 ppm). The results show that the number-average molecular weight determined by end group titration were well consistent with the molecular weight measured by GPC and ¹H NMR, indicating that there is one carboxyl acid group per PHFA–COOH chain. Furthermore, mercapto compounds are efficient and effective chain transfer agent due to the relatively weak S–H bond. The functionality of the macromonomer can be over 90% in most cases.^41–46 These results are in fair agreement with the articles reported previously.^41–46

Fig. 1 GPC profiles of the macromonomers and copolymers.

In addition, for polysiloxane macromonomer, the end-group titration result showed that the hydroxyl value after the esterification between MAC and SiOH was 16.5 mg KOH per g, which is very close to half of the original value (32.1 mg KOH per g). Furthermore, during the preparation of SiOHMAC, slowly dropping technique was employed and the usage of MAC was half of the mole amount of hydroxyl. This result suggested that there is one methacryloyloxy group per SiOHMAC.

The chemical structures of PHFA-GMA, SiOHMAC, 4Si, 4F and 4Si/F were confirmed by FTIR and ¹H NMR spectra.

Fig. 2 shows the FTIR spectra of PHFA-GMA (trace a), 4F (trace b), 4Si (trace c), 4Si/F (trace d) and SiOHMAC (trace e), respectively. The wide and dispersive absorption peak at 3500 cm⁻¹ confirmed the presence of OH in the polymer. The absorbance at 2875 and 2930 cm⁻¹ are ascribed to the C–H stretching's. Absorbance at 1729 cm⁻¹ confirms the presence of C=O stretching and peaks at 1635 cm⁻¹ are assigned to the C=C stretching vibration. Absorption at 1247 cm⁻¹ ascribed to the C–O–C stretching vibration could be observed clearly except for SiOHMAC in which the amount of C–O–C is quite low. For PHFA-GMA, the characteristic peaks at 1300 cm⁻¹ and 1190 cm⁻¹ are ascribed to the characteristic absorbance of –CF₃ and –CF₂, respectively. For 4F and 4Si/F, the characteristic absorbance of –CF₃ and –CF₂ might be overlapped. For SiOHMAC, peaks at 1264 cm⁻¹ are assigned to the CH₃ connecting Si. Peaks at 809 cm⁻¹ are ascribed to the rocking CH₃ connecting Si, which can be observed only in SiOHMAC, 4Si and 4Si/F. The FTIR results implied that fluorinated and/or polysiloxane macromonomer had been successfully incorporated into the poly(meth)acrylates.

Fig. 2 FTIR spectra of the macromonomers and copolymers.

Fig. 3 and 4 show the ¹H NMR spectra of PHFA-GMA and SiOHMAC, and 4Si/F in CDCl₃ solvent, respectively. In Fig. 3, the characteristic signals at 6.14 ppm and 5.63 ppm are assigned to the protons of the double bond in methacryloyloxy –CH₂=C(CH₃)COO– (a, b); the signals ranging from 4.78 to 5.18 ppm are ascribed to the methane proton of –CHF– (k); the chemical shift of 4.23–4.58 ppm belong to the methylene protons of d, e, j. The peaks at 2.60–2.80 ppm represent the methylene protons of f, g. The signals of 1.00–2.40 ppm represent the proton of c and h, i.

Fig. 3 ¹H NMR spectra of PHFA-GMA and SiOHMAC.

Fig. 4 ¹H NMR spectra of 4Si/F.

With regard to the ¹H NMR spectra of SiOHMAC, the characteristic signals of the protons of the double bond in methacryloyloxy –CH₂=C(CH₃)COO– (a, b) at 6.14 ppm and 5.63 ppm are clearly detected. The signals at 4.30 ppm are ascribed to the protons of d. The chemical shifts of 3.42–3.80 ppm belong to the protons of j and k. The characteristic resonance signals at 1.95 ppm belong to the protons of c. The signals located from 1.50–1.80 ppm are ascribed to the protons of e and i. The chemical shifts at 0.55 ppm are assigned to f and h. The peaks at 0.07 ppm are the characteristic signals of g.

In Fig. 4, the characteristic resonance signals of the double bond (a, b) in 6.14 ppm and 5.59 ppm were observed. The resonance signals at 4.78–5.18 ppm and 4.22–4.56 ppm are assigned to the protons of –CHF– (k) and –OCH₂–CF₂ (j), respectively. The resonance signals at 3.83–4.40 ppm belong to the protons of methylene e, h, j and methine d, respectively. Peaks at 3.50–3.71 ppm are ascribed to the methyl protons of i. The chemical shift ranging from 0.75–2.45 ppm represents the methylene protons of f, g and methyl protons of c, respectively. The resonance signals of Si–CH₃ protons (l) are located in 0.05 ppm.

2.2 Particle size of the dispersion

Particle size of the dispersion is summarized in Fig. 5. With increasing the amount of PHFA-GMA and/or SiOHMAC, the average particle size of the dispersion increased since more hydrophobic segments in the polymer enhanced the hydrophobicity, thus aggregation of the polymer segments occurred and larger particle was formed. In addition, at a given content of macromonomers, the sequence of particle size is that Si series > Si/F series > F series. Due to the polar acrylate structural unit in PHFA-GMA, the polarity of PHFA-GMA is stronger than SiOHMAC. Thus Si series performed larger particle size than F series.

Fig. 5 Average particle size of the water dispersions.

2.3 Contact angle and surface energy

The water and diiodomethane contact angle values and the surface energy of the samples were summarized in Fig. 6 and 7, respectively. As can be seen, very small amount of the PHFA-GMA and/or SiOHMAC (0.125 wt%) added during polymerization can improve the contact angles significantly. Water contact angle increased from initially 80.5° to 98.6°(0.125Si), 100.0°(0.125Si/F) and 92.5°(0.125F), respectively. Similar trend was observed in diiodomethane contact angle. With increasing the amount of macromonomers, the surface energy decreased, resulting in slightly increasing of contact angle. It seemed that the decrement of the surface energy began to level-off at just 2 wt% macromonomers, indicating that the macromonomers possessed the extremely high efficiency and effectiveness, which are rarely reported, in lowering the surface energy. This was attributed to reasons: on one hand, the thermodynamic incompatibility among the poly(meth)acrylate and the two macromonomers forced the chains containing fluorinated moieties and siloxane groups migrate to the outmost layer of the polymer. On the other hand, the soft nature of PHFA-GMA and SiOHMAC was beneficial for the fluorinated moieties and/or siloxane groups to migrate to the surface.

Fig. 6 Water and diiodomethane contact angle of the UV-cured films.

Fig. 7 Surface energy of the UV-cured films.

In addition, from Fig. 7, for given the same weight of the two macromonomers, the surface energy of Si series was smaller than that of F series in the range we investigated. The contact angles showed the corresponding trend. The results suggested that the ability of SiOHMAC in lowering the surface energy was stronger than that of PHFA-GMA. The most likely reasons are as follows: firstly, the structural units of PHFA-GMA were acrylates which are quite similar to that of the poly(meth)acrylate matrix, resulting in thermodynamic incompatibility between PHFA-GMA and polyacrylate matrix is not as strong as that between non-polar SiOHMAC and poly(meth)acrylate. Thus the fluorinated segments are easily buried in the bulk than that of SiOHMAC. Secondly, the fluorinated alkyls in PHFA-GMA structural units are relatively short (–CF₂CHFCF₃) and there are a lot of polar ester groups in PHFA-GMA, which will weaken the hydrophobicity to some extent. Thirdly, the mobility of fluorinated acrylates is weaker than that of polysiloxane chains, which is harmful for fluorinated groups to migrate to the surface. Thus the ability of SiOHMAC to improve the hydrophobicity is stronger.

Of interest to note is that the combination of PHFA-GMA and SiOHMAC could generate the highest efficiency in lowering the surface energy. The contact angles for 0.5Si/F were slightly larger than both 0.5Si and 0.5F. Correspondingly, the surface energy was lower than that of both 0.5Si and 0.5F. This result indicated that synergic effect of F and Si occurred.

2.4 Surface composition

In order to quantitatively characterize the surface property of the copolymers, X-ray photoelectron spectroscopy (XPS) analysis was employed. XPS was recorded at different etching time (reflecting different analytical depth of the polymer films). Fig. 8 shows the XPS survey spectras of 0.5Si, 0.5Si/F and 0.5F at etching time 0 s (at the surface) and 50 s (∼15 nm depth from the surface). Peaks corresponding to carbon (C 1s), oxygen (O 1s), fluorine (F 1s) and silicon (Si 2p) are seen. Atomic concentration is summarized in Table 3. At 0 s, for these three samples, the atomic concentration of F and Si at the surface is far higher than the feed. This is attributed to the strong tendency to migrate to the surface for fluorinated macromonomer and/or polysiloxane macromonomer segments. However, at 50 s, the atomic concentration for F and Si declined. XPS results revealed that both F and Si performed concentration gradient distribution in the polymer film. The closer to the surface, the higher F and Si concentration is.

Fig. 8 XPS curves of 0.5Si, 0.5Si/F and 0.5F UV-cured films at etching 0 s and 50 s.

Table 3 Surface composition of 0.5Si, 0.5Si/F and 0.5F UV-cured films measured by XPS

Samples	Atomic concentration (%)
	Etched 0 s				Etched 50 s
	C 1s	O 1s	F 1s	Si 2p	C 1s	O 1s	F 1s	Si 2p
0.5Si	66.12	24.62	0	9.26	87.58	10.97	0	1.45
0.5Si/F	65.91	22.89	3.74	7.46	89.76	8.04	1.04	1.16
0.5F	71.22	24.74	4.04	0	90.82	8.13	1.05	0

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Since the molecular weight of PHFA-GMA and SiOHMAC is similar, the relative atomic mass of F (18.99) is smaller than Si (28.08) and the element weight fraction of F in PHFA-GMA is approximately 0.456 while Si in SiOHMAC is approximately 0.378. It's worthy to note that, in the case of the same weight of these two macromonomers, the molar amount of Si is lower than that of F. Conversely, Si atomic concentration of 0.5Si was higher than the F atomic concentration of 0.5F. This result strongly confirms the phenomenon observed in contact angle measurement that the higher efficiency and effectiveness for SiOHMAC to create hydrophobic surface than PHFA-GMA. Furthermore, the total atomic concentration of F and Si for 0.5Si/F (F + Si = 11.20%) is higher than that of 0.5Si (F + Si = 9.26%) and 0.5F (F + Si = 4.04%). These results implied the synergic effect of F and Si that the combination of PHFA-GMA and SiOHMAC is beneficial for both F and Si enriching at the surface.^21,24 The XPS results show a high degree of consistency with the results from contact angle measurement.

2.5 Surface morphology

Hydrophobicity of polymeric films is not only determined by the film surface composition but also the surface roughness.²¹ The AFM images of the films are summarized in Fig. 9 and the corresponding surface rootmean-square roughness (R_q) was also calculated and recorded. Rougher surface is beneficial to improving the hydrophobicity of the films. R_q for 0.5Si and 0.5F are 3.38 nm and 2.53 nm, respectively. This result further reveals that the ability to roughen the surface for Si series is stronger than that of F series. In AFM images, the aggregates forming on the samples are probably attributed to the incorporation of SiOHMAC and/or PHFA-GMA. Due to the thermodynamic incompatibility among the polysiloxane macromonomer and/or fluorinated macromonomer with the matrix, micro phase segregation might occur, thus aggregates and domains might be formed at the surface. As can be seen, the numbers and the size of the aggregates for the sample 0.5Si were larger than that of 0.5F since SiOHMAC showed stronger incompatibility than PHFA-GMA. 0.5Si/F had the most and largest aggregates might be attributed to the synergistic effect between Si and F besides their incompatibility with the matrix. As a result, the sequence of surface roughness was 0.5Si/F > 0.5Si > 0.5F. AFM images showed consistent with the results from contact angle measurement and it provides a good supplement. From XPS and AFM, the stronger hydrophobicity (higher contact angle values and lower surface energy) for Si series is not only attributed to the higher Si atomic concentration, but also to the rougher surface.

Fig. 9 AFM images of 0.5Si, 0.5F and 0.5Si/F.

2.6 Gel content and water absorption

Gel content tests were employed to measure the curing and crosslink degree of the UV-cured films. Gel content results are summarized in Table 4. Gel content of all the UV-cured films exceeded 90%, which indicated outstanding curing and crosslink degree for the films.

Table 4 Gel content, water absorption, pencil hardness and adhesion of the UV-cured films

Samples	Gel content (%)	Water absorption (%)	Pencil hardness	Adhesion
				On PC	On PET	On PMMA
0Si/F	97	3.27	2H	5B	5B	5B
0.25Si	97	3.10	2H	5B	5B	5B
1Si	95	2.55	2H	5B	5B	5B
4Si	96	2.05	2H	5B	4B	4B
8Si	93	1.34	2H	4B	3B	4B
0.25F	95	3.25	2H	5B	5B	5B
1F	96	2.97	2H	5B	5B	5B
4F	95	2.76	2H	5B	5B	5B
8F	95	2.20	2H	5B	5B	5B
0.25Si/F	94	3.14	2H	5B	5B	5B
1Si/F	95	2.64	2H	5B	5B	5B
4Si/F	96	2.10	2H	5B	5B	5B
8Si/F	95	1.69	2H	5B	5B	5B

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Water absorption results were summarized in Table 4. Water absorption of the samples modified by siloxane macromonomer and/or fluorinated macromonomer decreased with increasing the content of the hydrophobic components. It was obvious that the decreasing of water absorption was more significant with increasing the content of siloxane macromonomer than that of the fluorinated macromonomer. This might be attributed to the excellent water repellency of siloxane side chains which endowed the films with outstanding hydrophobicity. The polar ester groups exist in the fluorinated macromonomer resulted in higher water absorption for the samples modified by fluorinated macromonomer. It can be seen that the water absorption of Si/F series was between Si series and F series.

2.7 Pencil hardness and adhesion

Pencil hardness and adhesion test results are summarized in Table 4. Most of the samples show pencil hardness with 2H. This was attributed to three main reasons: firstly, MMA, AA are hard monomers and they hindered the motion of the segments; secondly, the long fluorinated side chains intensified the entanglement among the polymer chains; thirdly, in the micro scale, three dimensional crosslinking network structures among the polymers was formed during the UV process. All these three factors hindered the mobility of the segments, thus the UV-cured films showed excellent pencil hardness in the macro scale.

Most of the samples show excellent adhesion (5B) on the substrates (PC, PET and PMMA), except for 4Si and 8Si. The explanation was as follows: firstly, the UV-cured films contained much polar groups such as ester groups and carboxyl acid groups, which had strong interaction with the substrate. Secondly, the UV-cured films possessed extremely high cohesive strength due to three dimensional crosslinking network structures. For 4Si and 8Si, the weaker adhesion was attributed to the non-polarity of polysiloxane side chains, which showed weak interaction forces with the polar substrates. Overall, the adhesion of the modified UV-cured films was excellent.

2.8 Mechanical properties

Mechanical properties, including tensile strength and modulus were summarized in Fig. 10. As can be seen, all the samples showed the tendency that the tensile strength and modulus decreased with increasing the amount of fluorinated macromonomer and/or polysiloxane macromonomer. Segments in PHFA-GMA and SiOHMAC are soft, as a result, the addition of them suppressed the amount of the hard segments, resulting in lower tensile strength and modulus. Furthermore, as the polysiloxane segments were softer than that of the fluorinated acrylates, the reduction of mechanical properties was greater in Si than in F. In addition, the mechanical properties of Si/F series were between Si series and F series.

Fig. 10 Tensile strength and modulus of the UV-cured films.

2.9 Thermal properties

DSC thermograms of the samples and the T_g values are summarized in Fig. 11. The copolymer films performed totally different physical properties before and after UV-cured that the UV-cured films were hard and smooth while it was soft and thick before UV-cured. The forming of three dimensional crosslinking network structures greatly hindered the mobility of the segments, leading to the T_g that ranging from 45.17–47.42 °C. T_g declined with increasing the amount of the fluorinated and/or polysiloxane macromonomers. This was attributed to the softness of the macromonomer segments incorporated into the polymers. Since the dosage of the fluorinated and/or polysiloxane macromonomer was small, there is no obvious T_g difference among different series.

Fig. 11 DSC thermograms of the UV-cured films.

TGA curves are shown in Fig. 12. All the samples show similar thermal decomposition process. It was obvious that thermal stability of the copolymer was enhanced significantly when PHFA-GMA and/or SiOHMAC was incorporated. From Fig. 12, initial degradation temperature at weight loss of 5 wt% (T_5%) for 0Si/F, 4Si, 4Si/F and 4F are 265.5 °C, 284.2 °C, 282.5 °C and 274.1 °C, respectively. Furthermore, weight of residual at 800 °C for 0Si/F, 4Si, 4Si/F and 4F are 1.54%, 5.29%, 4.37% and 2.45%, respectively. Apparantly, the sequence of thermal stability is Si series > Si/F series > F series. The results from TGA demonstrated that incorporation of SiOHMAC endows the copolymer with better thermal stability than PHFA-GMA. These results are quite consistent with that reported in the ref. 8 and 21.

Fig. 12 TGA thermograms of the UV-cured films.

3. Conclusions

A series of waterborne UV-curable comb-shaped (meth)acrylate graft copolymers containing long fluorinated and/or siloxane side chains were synthesized via radical copolymerization of a novel mono-methacryloyloxy terminated fluorinated macromonomer (PHFA-GMA) and/or polysiloxane macromonomer (SiOHMAC) with (meth)acrylate monomers. Chemical structures of the copolymers were successfully characterized by FTIR, GPC and ¹H NMR. DLS results showed that the sequence of particle size is that Si series > Si/F series > F series. Contact angle measurement, XPS and AFM were employed to investigate the surface properties of the copolymer. Contact angle measurement showed that hydrophobic surface was created by just using extremely low content (just 0.125 wt%) of PHFA-GMA and/or SiOHMAC added during polymerization. Furthermore, XPS results revealed that for given weight of the two macromonomers, Si atomic concentration of 0.5Si is higher than F atomic concentration of 0.5F. AFM images showed that surface of 0.5Si is rougher than 0.5F. In all, compared to PHFA-GMA, SiOHMAC performed higher efficiency and effectiveness in creating hydrophobic surface. Additionally, Si series performed lower water absorption, lower pencil hardness, weaker adhesion and weaker mechanical properties, lower T_g but higher thermal stability than that of F series in the range of macromonomers we investigated. Our study provided better understanding of the separate effects of fluorinated macromonomers and polysiloxane macromonomers, as well as the synergy of these two components, on the properties, especially the surface property, of waterborne UV cured polyacrylate. Moreover, the novel comb-shaped copolymers could be prepared via conventional radical polymerization, which endows it with the prominent potential application in large scale industrialization.

Acknowledgements

This study was supported by the Key Laboratory of Cellulose and Lignocellulosics, Guangzhou Institute of Chemistry, Chinese Academy of Sciences, Provincial Science and technology project of Guangdong Province (No. 2015B090925019) and Guangzhou Municipal Science and Technology Program (No. 201509010019).

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