- Email: [email protected]
Synthesis and characterization of organic fluorine and nano-SiO2 modified polyacrylate emulsifier-free latex
Progress in Organic Coatings 89 (2015) 192–198
Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Synthesis and characterization of organic fluorine and nano-SiO2 modified polyacrylate emulsifier-free latex Jianhua Zhou ∗ , Xin Chen, Hao Duan, Jianzhong Ma ∗ College of Resource and Environment, Shaanxi University of Science and Technology, Xi’an 710021, China
a r t i c l e
i n f o
Article history: Received 27 February 2015 Received in revised form 3 August 2015 Accepted 12 September 2015 Keywords: Nano-SiO2 Organic fluorine Polyacrylate Emulsifier-free emulsion
a b s t r a c t In this research, organic fluorine and nano-SiO2 modified polyacrylate emulsifier-free emulsion was successfully synthesized via emulsifier-free emulsion polymerization with ethyl silicate (TEOS) as precursor for nano-SiO2 and dodecafluoroheptyl methacrylate (DFMA) as fluorinated monomer. The stability of latex prepared in the presence of alkyl vinyl sulfonate was much higher than that of latex prepared in the presence of sodium dodecyl benzene sulfonate. With increasing DFMA content, the latex particle size and hydrophobicity of hybrid film increased. Furthermore, the hybrid film presented highly solvent-resistant and good mechanical properties. In addition, the FT-IR spectrum indicated that the DFMA and nano-SiO2 were successfully introduced into the hybrid polymer. AFM and SEM measurements confirmed that the hybrid film had a rough surface. At last, the formation mechanism of the hybrid film was established. © 2015 Elsevier B.V. All rights reserved.
1. Introduction In recent years, organic–inorganic composite materials have been given much attention and applied to many areas [1–3] such as coatings, adhesives, catalysis and fuels due to their unique chemical and physical properties including excellent optics, electricity property, mechanical property, hydrophobicity, thermal stability and flame retardant [4]. Among the organic components, fluorinecontaining polyacrylate polymers have many excellent properties including high thermal, chemical, aging, solvent and weather resistance, low dielectric constant and surface free energy due to low polarizability and strong electronegativity of fluorine atom. Hence, the fluorine-containing polyacrylate polymers have been widely studied in recent years as useful materials [5–7] in the field of biomaterials and coatings for leather, textile, paper and walls of buildings. Meanwhile, many kinds of nano-particles such as nano-SiO2 , TiO2 , Al2 O3 , ZrO2 and CaCO3 [8–10] are used to fabricate nanocomposites. Among them, nano-SiO2 can improve the mechanical property and thermal stability of organic polymers, and has been extensively used as inorganic phase in inorganic–organic hybrid system [11–13]. Up to now, many approaches have been applied to prepare organic–inorganic composite materials. And an effective way is to synthesize the organic–inorganic composite materials by emulsion
∗ Corresponding authors. E-mail addresses: [email protected] (J. Zhou), [email protected] (J. Ma). http://dx.doi.org/10.1016/j.porgcoat.2015.09.016 0300-9440/© 2015 Elsevier B.V. All rights reserved.
polymerization. Emulsion polymerization has many advantages, including the use of an environmentally friendly solvent, high heat transfer rate, low viscosity, fast polymerization rate and easy processability [14–16]. However, the residual emulsifier in materials will have negative effects on the properties of product and pollute the environmental [17]. Studies have proved that the emulsifier-free emulsion polymerization can effectively eliminate the disadvantages of emulsifier to the properties of the materials because there is no emulsifier migration during film formation [18,19]. Therefore, in the emulsion polymerization, reactive surfactants have been widely used, especially in the preparation of fluorinated polyacrylate polymer/inorganic composite materials, as they can react with fluorinated monomers and acrylate monomers and become a part of fluorinated polyacrylate polymer, endowing the latex films with the excellent water and oil repellence properties [20,21]. In this work, the organic fluorine and nano-SiO2 modified polyacrylate emulsifier-free emulsion was synthesized by emulsifier-free emulsion polymerization with ethyl silicate (TEOS) as precursor for nano-SiO2 and dodecafluoroheptyl methacrylate (DFMA) as fluorinated monomer, as shown in Scheme 1. The influences of the amount of TEOS and DFMA on the properties of the copolymer were discussed. Fourier transform infrared (FT-IR) spectrometry, dynamic light scattering (DLS) analysis, contact angle (CA) analysis, atom force microscopy (AFM) and scanning electron microscopy (SEM) were used to characterize the obtained organic fluorine and nano-SiO2 modified polyacrylate hybrid latex particles and the corresponding latex films. In addition, the formation
J. Zhou et al. / Progress in Organic Coatings 89 (2015) 192–198
193
Scheme 1. Synthetic scheme of organic fluorine and nano-SiO2 modified polyacrylate emulsifier-free latex.
O C9H19
O
O
2.2. Synthesis of nano-SiO2 modified fluorine-containing polyacrylate emulsifier-free emulsion
(CH2CH2O)10SO3NH4
The recipes for nano-SiO2 modified fluorine-containing polyacrylate emulsifier-free emulsion were described in Table 1. For a typical experiment, 6.16 g of BA, 4.62 g of MMA, 0.20 g of AVS, and 7.50 g of deionized water were mixed in the beaker and stirred vigorously to form the pre-emulsion I. 2.64 g of BA, 1.83 g of MMA, 2.00 g of DFMA, 0.50 g of SA, 0.30 g of AVS, and 7.50 g of deionized water were mixed in the beaker and then intensively homogenized to form the pre-emulsion II. A 250-mL three-neck round-bottomed flask equipped with a reflux condenser, a thermometer and a mechanical stirrer was filled with 0.25 g of AVS, 1/3 APS aqueous solution (0.22 g of APS was solved in 15.00 g water), and 1/4 pre-emulsion I and 7.50 mL of deionized water with a stirring rate of 250 rpm at 70 ◦ C and the mixture was kept still for 30 min. Then, both the 1/3 APS aqueous solution and remnant pre-emulsion-were added dropwise into the reacting mixture for 120 min. After that, the reaction was carried out continuously for another 2 h. Then, the polymerization continued with a slow addition of the pre-emulsion II and remnant APS aqueous solution during 120 min. 1.40 g of KH-570 was added to the flask. The reaction mixture was kept at 80–85 ◦ C for 2 h, then cooled down to 40 ◦ C. Then, 0.80 g of TEOS was added to emulsion, and contents of the flask were stirred at 50 ◦ C for 12 h.
Fig. 1. Structure of AVS.
mechanism of organic fluorine and nano-SiO2 modified polyacrylate emulsifier-free emulsion was investigated.
2. Experimental 2.1. Materials Methyl methacrylate (MMA), butyl acrylate (BA), ammonium persulfate (APS), ethyl silicate (TEOS) were purchased from Tianjin Kemiou Chemical Co. Ltd., analytical pure and used as received. ␥-methacryloxypropyltrimethoxysilane (KH-570) was purchased from Nanjing Shuguang Chemical Company, and used as received. Alkyl vinyl sulfonate (AVS) (Fig. 1) was produced by Hanerche Chemical Company, industrial purity and used as received. Dodecafluoroheptyl methacrylate (DFMA) was obtained from Harbin Xuejia Fluorin Silicon Chemical Co., Ltd. Stearyl acrylate (SA) was purchased from Tianjin Tianjiao Chemical Co., Ltd. and used as received.
Table 1 Recipes for organic fluorine and nano-SiO2 modified polyacrylate emulsifier-free emulsion. Sample
P0
P1
P2
P3
P4
P5
F1
F2
F3
F4
MMA/g BA/g APS/g AVS/g KH-570/g SA/g TEOS/g DFMA/g DI water/g
6.45 8.80 0.22 0.75 1.40 0.50 0.00 0.00 37.50
6.45 8.80 0.22 0.75 1.40 0.50 0.00 2.00 37.50
6.45 8.80 0.22 0.75 1.40 0.50 0.40 2.00 37.50
6.45 8.80 0.22 0.75 1.40 0.50 0.80 2.00 37.50
6.45 8.80 0.22 0.75 1.40 0.50 1.20 2.00 37.50
6.45 8.80 0.22 0.75 1.40 0.50 1.60 2.00 37.50
6.45 8.80 0.22 0.75 1.40 0.50 0.80 1.00 37.50
6.45 8.80 0.22 0.75 1.40 0.50 0.80 3.00 37.50
6.45 8.80 0.22 0.75 1.40 0.50 0.80 4.00 37.50
6.45 8.80 0.22 0.75 1.40 0.50 0.80 5.00 37.50
194
J. Zhou et al. / Progress in Organic Coatings 89 (2015) 192–198
2.3. Preparation of the latex film The hybrid emulsion was cast onto a pyrex glass plate and became film at room temperature. Then the film was dried at 80 ◦ C for 3 h to remove water inside the film. At last, the film was annealed at 120 ◦ C for 30 min. 2.4. Characterization The particle size of the synthesized emulsion was measured by Malvern Nano-ZS instrument (Malvern Instruments, UK) at a fixed scattering angle of 90◦ at room temperature. Contact angles (CA) measurements were performed on a contact angle goniometer (OCA 20, Dataphysics Company, Germany) by the sessile drop method with a microsyringe at room temperature. Static contact angles were obtained from the injection volume of 5 L water droplets on the surface of hybrid film. The average of nine readings of contact angle at different locations of one sample was used as the final value of each sample. The film specimen (dimensions = 20 mm × 20 mm) was immersed in toluene for 24 h at room temperature to attain swelling and dissolution equilibrium. The remaining film specimen was removed from the solvent, and the weight of the film (W1 ) was measured. The film was then dried to a constant weight (W2 ) at room temperature. The swelling ratio was calculated from W1 /W2 . The thickness of the films in the midline was measured via YQ981 Leather Thickness Measure Instrument made by Middle Mountain Sword Tool Factory. The measuring points were chosen to be 3, and then the average was obtained. Samples were fixed between the clamps extending at speed 500 mm/min. The burthen number at break was recorded. X = F/S, X is the tensile strength of specimen (N/mm2 ), F is the burthen number at break (N) and S is the transect acreage at break (mm2 ). Elongation at break was determined during the measure of tensile strength. When the specimen was extended, the distance between the two standard lines was recorded. X = (L1 − L2 )/L × 100%, in which, X is the elongation at break (%), L1 the distance between midlines of specimen at break (mm) and L is the distance between midlines of the original sample (mm2 ). The FT-IR spectrum was obtained on a BRUKER Avance VECTOR-22 spectrometer. The film was prepared on the clean glass surface, dried under vacuum condition at room temperature to remove all the water. Next, the film was immersed in ionized water for 30 min, followed by sonication for 15 min to remove unreacted AVS monomers which are soluble in water, and then dried at 120 ◦ C for 90 min to remove unreacted volatile monomers. The film was peeled off from the glass substrate with forceps. Atomic force microscope (AFM) was performed using a SPA-400 AFM (Seiko Instruments Inc. Japan). Images were acquired under ambient conditions in tapping mode using a nanoprobe cantilever. For AFM measurements, the corresponding organic fluorine and nano-SiO2 modified polyacrylate emulsifierfree emulsion (1 wt% solid content) were cast onto the prepared silicon wafers and then dried at room temperature. The morphology of the films was carried out with a field emission scanning electron microscopy (FE-SEM) (Hitachi S-4800, Japan).
Fig. 2. FT-IR spectrum of organic fluorine and nano-SiO2 modified polyacrylate.
The peak at 1241 cm−1 is characteristic absorption peak of C F. The characteristic peak at 1453 cm−1 originates from the C6 H5 groups of the alkyl vinyl sulfonate (AVS). The peak at 1100–1000 cm−1 is the stretching vibration absorption peak of Si O and C O. The peak at 759 cm−1 originates from the C Si stretching vibration of the CH2 Si groups. FT-IR result demonstrates that DFMA and nanoSiO2 have effectively participated in the emulsion polymerization and the organic fluorine and nano-SiO2 modified polyacrylate has been obtained. 3.2. Particle size of latexes Fig. 3 shows the influence of dodecafluoroheptyl methacrylate (DFMA) content on the diameter of organic fluorine and nanoSiO2 modified polyacrylate latex particles. The diameter of the hybrid emulsion particles increases from 104.3 nm to 210.3 nm as the DFMA amount increases from 5 wt% to 25 wt% based on total monomer weight. In emulsion polymerization, the reaction relies on the diffusion of the monomers from monomer droplets through the aqueous phase to micelles. DFMA monomers with low aqueous solubility diffuse into the interior of the micelles, and then react with acrylate monomers to form fluorinated polyacrylate, resulting in the increase of the latex particle size with increasing DFMA amount. However, when DFMA concentration is too high, the excess part of DFMA monomers cannot enter the
3. Results and discussion 3.1. FT-IR analysis The chemical structure of organic fluorine and nano-SiO2 modified polyacrylate (sample P3 in Table 1) was characterized by FT-IR, as shown in Fig. 2. The bands at 2959 cm−1 and 2870 cm−1 can be assigned to characteristic stretching vibration peaks of methyl group ( CH3 ) and methylene group ( CH2 ), respectively. The characteristic stretching vibration peak of C O appears at 1732 cm−1 .
Fig. 3. Effect of dosage of DFMA on latex particle size.
J. Zhou et al. / Progress in Organic Coatings 89 (2015) 192–198
195
Table 2 Appearance, solid content, freeze-thaw stability and storage stability of the latexes. Emulsion properties
SDBS
AVS
Appearance Theoretical solid content (%) Experimental solid content (%) Freeze-thaw stability (cycle number) Storage stability at 50 ◦ C
White 35.43 28.14 0 Precipitation
White and blue light 35.43 32.52 5 Stability
micellar interior, and these DFMA monomers can be homoploymerized. Consequently, heterocoagulation can be formed between fluorine-containing homopolymers and hybrid latex particles by intermolecular forces, leading to the significant increase of the latex particle size. 3.3. Stability of hybrid dispersion Surfactants are essential for emulsification of monomer droplets, fast nucleation of latex particles, and stabilization of latex particles during the course of polymerization and storage. However, as small molecular surfactants are attached to the surface of latex particles by the physical adsorption, the stability of emulsion becomes worse. In the present article, the polymerizable surfactant AVS has been used to synthesize organic fluorine and nano-SiO2 modified polyacrylate emulsifier-free emulsion. During the reaction, AVS can react with other monomer and becomes a part of copolymer, eliminating the disadvantages of small molecular emulsifier. The experimental solid content of hybrid latex prepared in the presence of AVS is 32.52%, which is higher than the experimental solid content (28.14%) of hybrid latex prepared in the presence of sodium dodecyl benzene sulfonate (SDBS), and is close to the theory value (35.43%), as shown in Table 2. The reaction between surfactant and other monomer is essential to improve the stability of emulsion. In contrast to the emulsion stabilized with sodium dodecyl benzene sulfonate (SDBS), the emulsion stabilized with alkyl vinyl sulfonate (AVS) shows remarkable polymerization stability, freeze-thaw stability and storage stability, as shown in Table 2. This results from the fact that the SDBS is attached to the surface of latex particles by the physical adsorption. However, AVS has surface activity and reactive group, high participation efficiency in the process of polymerization and AVS can be bonded with the surface of latex particles by covalent bond. The conversion of alkyl vinyl sulfonate (AVS) was 98.68% by water extraction experiment reported by Zhang et al. [22]. Colloidal stability of hybrid latex after TEOS addition was tested by rapid centrifugation method. The results show that there was no precipitation and emulsification phenomenon in the hybrid latex, indicating that the hybrid latex had a remarkable centrifugal stability. Meanwhile, hybrid latex is still a uniform system after 6 months of storage. This phenomenon can be explained by the fact that silane coupling agent (KH-570) with silyloxymethyl and unsaturated double bond can combine the nano-SiO2 by the silanol groups, on the other hand, the double bond can copolymerize with acrylate monomers. Consequently, fluorine-containing polyacrylate polymers and nano-SiO2 can be combined by chemical bonds, rendering high interfacial compatibility to organic and inorganic phases.
Fig. 4. Effect of dosage of DFMA on contact angles of film.
of the film increases from 65.3◦ to 92.4◦ with the increment of the amount of DFMA from 5 wt% to 25 wt%. The increase in the water contact angle of the film indicates that the surface energy of latex film reduces with the increase of the amount of DFMA. This phenomenon can be attributed to the extremely low surface energy and self-aggregation property of the fluorine atoms, which cause the fluorinated segments to be preferentially oriented to the filmair interface during the film formation so as to decrease the surface energy of film. The result is consistent with those reported in the literatures [25,7]. However, the less increment of contact angle for the latex films with more than 15 wt% DFMA may be ascribed to the factor that the migration of fluorine atom onto the film surface is hindered by the steric effect of the fluorine atoms migrating to the surface of film [26]. 3.5. Solvent resistance and mechanical property Nano-SiO2 modified polyacrylate by the combination of radical heterophase polymerization and hydrolytic condensation to incorporate nano-SiO2 particles into polyacrylate polymer [27] exhibits good solvent resistance and mechanical performance due to the large specific surface area and high chemical reactivity of nanoSiO2 . The solvent resistance of the composite film was characterized by swelling measurements and solvent extraction [28], as shown in Fig. 5. Compared with the film obtained from the organic fluorine modified polyacrylate emulsifier-free emulsion, the film obtained
3.4. Hydrophobicity The water contact angle is commonly used as a criterion for the evaluation of the hydrophobicity of a solid surface [23]. The surface is hydrophobic when  is higher than 90◦ and the surface is hydrophilic when  is lower than 90◦ [24]. The water contact angles of composite films with different content of DFMA are shown in Fig. 4. It can be observed from Fig. 4 that the water contact angle
Fig. 5. Effect of dosage of TEOS on solvent resistance of the hybrid film.
196
J. Zhou et al. / Progress in Organic Coatings 89 (2015) 192–198
Fig. 8. Various roughness parameters for polymer films formed from polyacrylate (PA) emulsifier-free emulsion, organic fluorine modified polyacrylate (PFA) emulsifier-free emulsion and organic fluorine and nano-SiO2 modified polyacrylate (PFSiA) emulsifier-free emulsion. Fig. 6. Effect of dosage of TEOS on mechanical properties of film.
from the organic fluorine and nano-SiO2 modified polyacrylate emulsifier-free emulsion has smaller swelling ratios and higher gel contents. The solvent resistance of the hybrid films increases along with the increase of TEOS content. This can be explained by the fact that the fluorined polyacrylate chain and silica component can form three-dimensional crosslinking network by the cocondensation of silanol groups on the silane coupling agent and nano-silica formed by the hydrolysis and polycondensation of TEOS. The crosslinking densities of the hybrid films increase along with the increase of TEOS dosage, rendering excellent solvent resistance to hybrid film. The mechanical properties of composite films with different content of TEOS are showed in Fig. 6. The tensile strength of hybrid film obtained from organic fluorine and nano-SiO2 modified polyacrylate emulsifier-free emulsion is higher than that of the film obtained from the organic fluorine modified polyacrylate emulsifier-free emulsion. The tensile strength of hybrid film increases from 9.6 MPa to 12.5 MPa with the increment of TEOS amount from 0 wt% to 4 wt%, and decreases with the continuous increase of TEOS content. The higher tensile strength of hybrid film is attributed to the three-dimensional crosslinking network of hybrid polymers containing silica component as a crosslinking point. However, further increase of TEOS content above 4% leads to the decreases of the tensile strength. This phenomenon can be explained by the fact that the increasing amount of nano-SiO2 hinders interparticle polymer diffusion by generating a sheating layer that may cause brittleness of the hybrid film, leading to the decrease of the mechanical properties. 3.6. Microstructure morphology of latex film Atomic force microscopy (AFM) was used in this work to observe surface morphologies of the polymer film and the results are presented in Figs. 7 and 8. The parameters, such as root-mean-square
(RMS) roughness and average roughness (Ra), were obtained from AFM software analysis. As shown in Fig. 7, compared with the surface structure of the film formed from polyacrylate emulsifier-free emulsion (sample P0 in Table 1), the film obtained from the organic fluorine modified polyacrylate emulsifier-free emulsion (sample P1 in Table 1) is covered by more little protuberances or peaks on its surface. In addition, the Ra and RMS of the polyacrylate film (Fig. 8) are lower than the organic fluorine modified polyacrylate film. This phenomenon can be explained by the fact that the organic fluorine segment migrates toward the film surface during the heat treatment, and phase separation between fluorinated and non-fluorinated components occurs for organic fluorine modified polyacrylate film, improving the roughness of the latex film [29]. On the other hand, the controllability of polymerization process may be poor due to the low aqueous solubility of DFMA, and a small amount of DFMA monomers can be homoploymerized. The phase separation occurred during the film-forming process due to the presence of fluorine-containing homopolymers, which maybe also improve the roughness of hybrid film. What is more, it can be observed from Figs. 7 and 8 that the organic fluorine and nanoSiO2 modified polyacrylate film (sample P3 in Table 1) have more protuberances or peaks and higher Ra and RMS than organic fluorine modified polyacrylate film, indicating that the nano-SiO2 can improve the roughness of hybrid film. Fig. 9 shows the surface morphology of the films cast from the polyacrylate soap-free emulsion (sample P0 in Table 1) by scanning electron microscopy (SEM), the organic fluorine modified polyacrylate emulsifier-free emulsion (sample P1 in Table 1), and the organic fluorine and nano-SiO2 modified polyacrylate emulsifierfree emulsion (sample P3 in Table 1). It can be noted that the polyacrylate films (Fig. 9a, d, and g) appear to be quite smooth and continuous, and the organic fluorine modified polyacrylate films (Fig. 9b, e, and h) have some salients on the surface, as a result of the enrichment of fluorinated polymers and the micro-phase
Fig. 7. AFM images of sample films formed from polyacrylate emulsifier-free emulsion (a), organic fluorine modified polyacrylate emulsifier-free emulsion (b) and organic fluorine and nano-SiO2 modified polyacrylate emulsifier-free emulsion (c).
J. Zhou et al. / Progress in Organic Coatings 89 (2015) 192–198
197
Fig. 9. SEM images of sample films formed from polyacrylate emulsifier-free emulsion (a, d, g), organic fluorine modified polyacrylate emulsifier-free emulsion (b, e, h) and organic fluorine and nano-SiO2 modified polyacrylate emulsifier-free emulsion (c, f, i).
separation. Meanwhile, on the organic fluorine and nano-SiO2 modified polyacrylate films (Fig. 9c, f, and i), as there are some nanoparticles attached to the surface, the roughness of hybrid film has been improved. These SEM results are in good agreement with AFM results.
3.7. Formation mechanism of hybrid film Scheme 2, based on the aforementioned experimental results, shows the schematic representation of the formation process of the organic fluorine and nano-SiO2 modified polyacrylate film. During
Scheme 2. Film formation process of organic fluorine and nano-SiO2 modified polyacrylate.
198
J. Zhou et al. / Progress in Organic Coatings 89 (2015) 192–198
water evaporation, the latex particles contact with each other, deform into polyhedra and pack together. At last, the diffusion and permeation occur among the molecular chains, forming the thin homogeneous film with high mechanical property. The cocondensation polymerization between the silanol groups on silane coupling agents and SiO2 colloidal particles forms Si O Si bonds between the fluorinated polyacrylate and the silica components during the polymerization, therefore, the SiO2 components disperse within the fluorinated polyacrylate component. The fluorinated groups in organic fluorine and nano-SiO2 modified polyacrylate have low surface free energy due to the low polarizability of C F bond, leading to strong tendency of the surface. During the latex film formation, the fluorinated groups will preferentially migrate and accumulate on the surface. When the surface is uniformly covered with fluorinated array, a very low energy surface can be achieved, rendering excellent water repellency to the hybrid film. 4. Conclusions Organic fluorine and nano-SiO2 modified polyacrylate emulsifier-free emulsion was successfully synthesized by the emulsifier-free emulsion polymerization. Compared with stability of latex prepared in the presence of sodium dodecyl benzene sulfonate, the stability of latex prepared in the presence of AVS was much higher. The DLS analysis confirmed that the latex particle size increased with the increase of DFMA content. The contact angle of the hybrid latex film increased along with the increase of DFMA content. The solvent extraction and swelling measurements indicated that the obtained hybrid latex film exhibited better solvent resistance than pure organic fluorine modified polyacrylate emulsifier-free emulsion film. At the same time, the hybrid film presented good mechanical properties. In addition, the FT-IR spectrum indicated that the DFMA and nano-SiO2 were successfully introduced into organic fluorine and nano-SiO2 modified polyacrylate. The AFM and SEM images confirmed that the hybrid film had a rough surface with many little protuberances or peaks. Acknowledgments This work was supported by the National Natural Science Fund of China (No. 21206088), the Science and Technology Innovation Engineering Program of Shaanxi Province (No. 2011KTCL01-13), the Academic Backbone Cultivation Program of Shaanxi University of Science and Technology (No. XSGP201205). References [1] C. Ni, G. Ni, L. Zhang, J. Mi, B. Yao, C. Zhu, Syntheses of silsesquioxane (POSS)-based inorganic/organic hybrid and the application in reinforcement for an epoxy resin, J. Colloid Interface Sci. 362 (1) (2011) 94–99. [2] M. Wu, R. Wu, R. Li, H. Qin, J. Dong, Z. Zhang, H. Zou, Polyhedral oligomeric silsesquioxane as a cross-linker for preparation of inorganic–organic hybrid monolithic columns, Anal. Chem. 82 (13) (2010) 5447–5454. [3] C. Sanchez, K.J. Shea, S. Kitagawa, Recent progress in hybrid materials science, Chem. Soc. Rev. 40 (2) (2011) 471–472. [4] B. Li, S. Zhang, Q. Xu, B. Wang, Preparation of composite polyacrylate latex particles with in situ-formed methylsilsesquioxane cores, Polym. Adv. Technol. 20 (12) (2009) 1190–1194.
[5] J. Hu, J. Ma, W. Deng, Properties of acrylic resin/nano-SiO2 leather finishing agent prepared via emulsifier-free emulsion polymerization, Mater. Lett. 62 (17) (2008) 2931–2934. [6] W. Xu, Q. An, L. Hao, D. Zhang, M. Zhang, Synthesis of self-crosslinking fluorinated polyacrylate soap-free latex and its waterproofing application on cotton fabrics, Fibers Polym. 15 (3) (2014) 457–464. [7] J. Wang, X. Zeng, H. Li, Preparation and characterization of soap-free fluorine-containing acrylate latex, J. Coat. Technol. Res. 7 (4) (2010) 469–476. [8] X. Song, Y. Zhao, H. Wang, Q. Du, Fabrication of polymer microspheres using titania as a photocatalyst and pickering stabilizer, Langmuir 25 (8) (2009) 4443–4449. [9] H. Liu, H. Ye, T. Lin, T. Zhou, Synthesis and characterization of PMMA/Al2 O3 composite particles by in situ emulsion polymerization, Particuology 6 (2008) 207–213. [10] L. Jiang, K. Pan, Y. Dan, Synthesis and characterization of well-defined poly(methyl methacrylate)/CaCO3 /SiO2 three-component composite particles via reverse atom transfer radical polymerization, Colloid Polym. Sci. 285 (2006) 65–74. [11] S. Yu, X. Zuo, R. Bao, X. Xu, J. Wang, J. Xu, Effect of SiO2 nanoparticle addition on the characteristics of a new organic–inorganic hybrid membrane, Polymer 50 (2) (2009) 553–559. [12] M. Percy, C. Barthet, J. Lobb, M. Khan, S. Lascelles, M. Vamvakaki, S. Armes, Synthesis and characterization of vinyl polymer–silica colloidal nanocomposites, Langmuir 16 (17) (2000) 6913–6920. [13] M. Comes, M.D. Marcos, R. Martinez-Manez, F. Sancenon, L.A. Villaescusa, A. Graefe, G.J. Mohr., Hybrid functionalised mesoporous silica–polymer composites for enhanced analyte monitoring using optical sensors, J. Mater. Chem. 18 (47) (2008) 5815–5823. [14] X. Cheng, M. Chen, S. Zhou, L. Wu, Preparation of SiO2 /PMMA composite particles via conventional emulsion polymerization, Polym. Sci. A: Polym. Chem. 44 (2006) 3807–3816. [15] K. Schmidtke, G. Lieser, M. Klapper, K. Müllen, Complex inorganic/organic core–shell architectures via an inverse emulsion process, Colloid Polym. Sci. 288 (3) (2010) 333–339. [16] A.R. Mahdavian, M. Ashjari, A.B. Makoo, Preparation of poly (styrene-methyl methacrylate)/SiO2 composite nanoparticles via emulsion polymerization. An investigation into the compatiblization, Eur. Polym. J. 43 (2) (2007) 336–344. [17] G. Riess, C. Labbe, Block copolymers in emulsion and dispersion polymerization, Macromol. Rapid Commun. 25 (2) (2004) 401–435. [18] Y. Ohtsuka, H. Kawaguchi, Y. Sugi, Copolymerization of styrene with acrylamide in an emulsifier-free aqueous medium, J. Appl. Polym. Sci. 26 (5) (1981) 1637–1647. [19] X. Cui, S. Zhong, H. Wang, Synthesis and characterization of emulsifier-free core–shell fluorine-containing polyacrylate latex, Colloids Surf. A 303 (3) (2007) 173–178. [20] J. Hu, J. Ma, W. Deng, Properties of acrylic resin/nano-SiO2 leather finishing agent prepared via emulsifier-free emulsion polymerization, Mater. Lett. 62 (2008) 2931–2934. [21] A. Guyot, Advances in reactive surfactants, Adv. Colloid Interface Sci. 108 (2004) 3–22. [22] Y. Zhang, C. Rocco, F. Karasu, L.G.J. van der Ven, R.A.T.M. van Benthem, X. Allonas, C. Croutxe-Barghorn, A.C.C. Esteves, G. de With, UV-cured self-replenishing hydrophobic polymer films, Polymer 69 (9) (2015) 384–393. [23] J. Ha, I. Park, S. Lee, Hydrophobicity and sliding behavior of liquid droplets on the fluorinated latex films, Macromolecules 38 (3) (2005) 736–744. [24] B. Zhang, B. Liu, X. Deng, S. Cao, X. Hou, H. Chen, A novel approach for the preparation of organic-siloxane oligomers and the creation of hydrophobic surface, Appl. Surf. Sci. 254 (2) (2007) 452–458. [25] W. Zheng, L. He, J. Liang, G. Chang, N. Wang, Preparation and properties of core–shell nanosilica/poly (methyl methacrylate–butyl acrylate–2,2,2-trifluoroethyl methacrylate) latex, J. Appl. Polym. Sci. 120 (2) (2011) 1152–1161. [26] J. Zhou, L. Zhang, J. Ma, Fluorinated polyacrylate emulsifier-free emulsion mediated by poly (acrylic acid)-b-poly (hexafluorobutyl acrylate) trithiocarbonate via ab initio RAFT emulsion polymerization, Chem. Eng. J. 223 (2013) 8–17. [27] X. Cui, S. Zhong, H. Wang, Emulsifier-free core–shell polyacrylate latex nanoparticles containing fluorine and silicon in shell, Polymer 48 (25) (2007) 7241–7248. [28] T. Tamai, M. Watanabe, Acrylic polymer/silica hybrids prepared by emulsifier free emulsion polymerization and the sol–gel process, J. Polym. Sci. Part A: Polym. Chem. 44 (1) (2006) 273–280. [29] W. Xu, Q. An, L. Hao, D. Zhang, M. Zhang, Synthesis and characterization of self-crosslinking fluorinated polyacrylate soap-free lattices with core–shell structure, Appl. Surf. Sci. 268 (2013) 373–380.
Contents lists available at ScienceDirect
Progress in Organic Coatings journal homepage: www.elsevier.com/locate/porgcoat
Synthesis and characterization of organic fluorine and nano-SiO2 modified polyacrylate emulsifier-free latex Jianhua Zhou ∗ , Xin Chen, Hao Duan, Jianzhong Ma ∗ College of Resource and Environment, Shaanxi University of Science and Technology, Xi’an 710021, China
a r t i c l e
i n f o
Article history: Received 27 February 2015 Received in revised form 3 August 2015 Accepted 12 September 2015 Keywords: Nano-SiO2 Organic fluorine Polyacrylate Emulsifier-free emulsion
a b s t r a c t In this research, organic fluorine and nano-SiO2 modified polyacrylate emulsifier-free emulsion was successfully synthesized via emulsifier-free emulsion polymerization with ethyl silicate (TEOS) as precursor for nano-SiO2 and dodecafluoroheptyl methacrylate (DFMA) as fluorinated monomer. The stability of latex prepared in the presence of alkyl vinyl sulfonate was much higher than that of latex prepared in the presence of sodium dodecyl benzene sulfonate. With increasing DFMA content, the latex particle size and hydrophobicity of hybrid film increased. Furthermore, the hybrid film presented highly solvent-resistant and good mechanical properties. In addition, the FT-IR spectrum indicated that the DFMA and nano-SiO2 were successfully introduced into the hybrid polymer. AFM and SEM measurements confirmed that the hybrid film had a rough surface. At last, the formation mechanism of the hybrid film was established. © 2015 Elsevier B.V. All rights reserved.
1. Introduction In recent years, organic–inorganic composite materials have been given much attention and applied to many areas [1–3] such as coatings, adhesives, catalysis and fuels due to their unique chemical and physical properties including excellent optics, electricity property, mechanical property, hydrophobicity, thermal stability and flame retardant [4]. Among the organic components, fluorinecontaining polyacrylate polymers have many excellent properties including high thermal, chemical, aging, solvent and weather resistance, low dielectric constant and surface free energy due to low polarizability and strong electronegativity of fluorine atom. Hence, the fluorine-containing polyacrylate polymers have been widely studied in recent years as useful materials [5–7] in the field of biomaterials and coatings for leather, textile, paper and walls of buildings. Meanwhile, many kinds of nano-particles such as nano-SiO2 , TiO2 , Al2 O3 , ZrO2 and CaCO3 [8–10] are used to fabricate nanocomposites. Among them, nano-SiO2 can improve the mechanical property and thermal stability of organic polymers, and has been extensively used as inorganic phase in inorganic–organic hybrid system [11–13]. Up to now, many approaches have been applied to prepare organic–inorganic composite materials. And an effective way is to synthesize the organic–inorganic composite materials by emulsion
∗ Corresponding authors. E-mail addresses: [email protected] (J. Zhou), [email protected] (J. Ma). http://dx.doi.org/10.1016/j.porgcoat.2015.09.016 0300-9440/© 2015 Elsevier B.V. All rights reserved.
polymerization. Emulsion polymerization has many advantages, including the use of an environmentally friendly solvent, high heat transfer rate, low viscosity, fast polymerization rate and easy processability [14–16]. However, the residual emulsifier in materials will have negative effects on the properties of product and pollute the environmental [17]. Studies have proved that the emulsifier-free emulsion polymerization can effectively eliminate the disadvantages of emulsifier to the properties of the materials because there is no emulsifier migration during film formation [18,19]. Therefore, in the emulsion polymerization, reactive surfactants have been widely used, especially in the preparation of fluorinated polyacrylate polymer/inorganic composite materials, as they can react with fluorinated monomers and acrylate monomers and become a part of fluorinated polyacrylate polymer, endowing the latex films with the excellent water and oil repellence properties [20,21]. In this work, the organic fluorine and nano-SiO2 modified polyacrylate emulsifier-free emulsion was synthesized by emulsifier-free emulsion polymerization with ethyl silicate (TEOS) as precursor for nano-SiO2 and dodecafluoroheptyl methacrylate (DFMA) as fluorinated monomer, as shown in Scheme 1. The influences of the amount of TEOS and DFMA on the properties of the copolymer were discussed. Fourier transform infrared (FT-IR) spectrometry, dynamic light scattering (DLS) analysis, contact angle (CA) analysis, atom force microscopy (AFM) and scanning electron microscopy (SEM) were used to characterize the obtained organic fluorine and nano-SiO2 modified polyacrylate hybrid latex particles and the corresponding latex films. In addition, the formation
J. Zhou et al. / Progress in Organic Coatings 89 (2015) 192–198
193
Scheme 1. Synthetic scheme of organic fluorine and nano-SiO2 modified polyacrylate emulsifier-free latex.
O C9H19
O
O
2.2. Synthesis of nano-SiO2 modified fluorine-containing polyacrylate emulsifier-free emulsion
(CH2CH2O)10SO3NH4
The recipes for nano-SiO2 modified fluorine-containing polyacrylate emulsifier-free emulsion were described in Table 1. For a typical experiment, 6.16 g of BA, 4.62 g of MMA, 0.20 g of AVS, and 7.50 g of deionized water were mixed in the beaker and stirred vigorously to form the pre-emulsion I. 2.64 g of BA, 1.83 g of MMA, 2.00 g of DFMA, 0.50 g of SA, 0.30 g of AVS, and 7.50 g of deionized water were mixed in the beaker and then intensively homogenized to form the pre-emulsion II. A 250-mL three-neck round-bottomed flask equipped with a reflux condenser, a thermometer and a mechanical stirrer was filled with 0.25 g of AVS, 1/3 APS aqueous solution (0.22 g of APS was solved in 15.00 g water), and 1/4 pre-emulsion I and 7.50 mL of deionized water with a stirring rate of 250 rpm at 70 ◦ C and the mixture was kept still for 30 min. Then, both the 1/3 APS aqueous solution and remnant pre-emulsion-were added dropwise into the reacting mixture for 120 min. After that, the reaction was carried out continuously for another 2 h. Then, the polymerization continued with a slow addition of the pre-emulsion II and remnant APS aqueous solution during 120 min. 1.40 g of KH-570 was added to the flask. The reaction mixture was kept at 80–85 ◦ C for 2 h, then cooled down to 40 ◦ C. Then, 0.80 g of TEOS was added to emulsion, and contents of the flask were stirred at 50 ◦ C for 12 h.
Fig. 1. Structure of AVS.
mechanism of organic fluorine and nano-SiO2 modified polyacrylate emulsifier-free emulsion was investigated.
2. Experimental 2.1. Materials Methyl methacrylate (MMA), butyl acrylate (BA), ammonium persulfate (APS), ethyl silicate (TEOS) were purchased from Tianjin Kemiou Chemical Co. Ltd., analytical pure and used as received. ␥-methacryloxypropyltrimethoxysilane (KH-570) was purchased from Nanjing Shuguang Chemical Company, and used as received. Alkyl vinyl sulfonate (AVS) (Fig. 1) was produced by Hanerche Chemical Company, industrial purity and used as received. Dodecafluoroheptyl methacrylate (DFMA) was obtained from Harbin Xuejia Fluorin Silicon Chemical Co., Ltd. Stearyl acrylate (SA) was purchased from Tianjin Tianjiao Chemical Co., Ltd. and used as received.
Table 1 Recipes for organic fluorine and nano-SiO2 modified polyacrylate emulsifier-free emulsion. Sample
P0
P1
P2
P3
P4
P5
F1
F2
F3
F4
MMA/g BA/g APS/g AVS/g KH-570/g SA/g TEOS/g DFMA/g DI water/g
6.45 8.80 0.22 0.75 1.40 0.50 0.00 0.00 37.50
6.45 8.80 0.22 0.75 1.40 0.50 0.00 2.00 37.50
6.45 8.80 0.22 0.75 1.40 0.50 0.40 2.00 37.50
6.45 8.80 0.22 0.75 1.40 0.50 0.80 2.00 37.50
6.45 8.80 0.22 0.75 1.40 0.50 1.20 2.00 37.50
6.45 8.80 0.22 0.75 1.40 0.50 1.60 2.00 37.50
6.45 8.80 0.22 0.75 1.40 0.50 0.80 1.00 37.50
6.45 8.80 0.22 0.75 1.40 0.50 0.80 3.00 37.50
6.45 8.80 0.22 0.75 1.40 0.50 0.80 4.00 37.50
6.45 8.80 0.22 0.75 1.40 0.50 0.80 5.00 37.50
194
J. Zhou et al. / Progress in Organic Coatings 89 (2015) 192–198
2.3. Preparation of the latex film The hybrid emulsion was cast onto a pyrex glass plate and became film at room temperature. Then the film was dried at 80 ◦ C for 3 h to remove water inside the film. At last, the film was annealed at 120 ◦ C for 30 min. 2.4. Characterization The particle size of the synthesized emulsion was measured by Malvern Nano-ZS instrument (Malvern Instruments, UK) at a fixed scattering angle of 90◦ at room temperature. Contact angles (CA) measurements were performed on a contact angle goniometer (OCA 20, Dataphysics Company, Germany) by the sessile drop method with a microsyringe at room temperature. Static contact angles were obtained from the injection volume of 5 L water droplets on the surface of hybrid film. The average of nine readings of contact angle at different locations of one sample was used as the final value of each sample. The film specimen (dimensions = 20 mm × 20 mm) was immersed in toluene for 24 h at room temperature to attain swelling and dissolution equilibrium. The remaining film specimen was removed from the solvent, and the weight of the film (W1 ) was measured. The film was then dried to a constant weight (W2 ) at room temperature. The swelling ratio was calculated from W1 /W2 . The thickness of the films in the midline was measured via YQ981 Leather Thickness Measure Instrument made by Middle Mountain Sword Tool Factory. The measuring points were chosen to be 3, and then the average was obtained. Samples were fixed between the clamps extending at speed 500 mm/min. The burthen number at break was recorded. X = F/S, X is the tensile strength of specimen (N/mm2 ), F is the burthen number at break (N) and S is the transect acreage at break (mm2 ). Elongation at break was determined during the measure of tensile strength. When the specimen was extended, the distance between the two standard lines was recorded. X = (L1 − L2 )/L × 100%, in which, X is the elongation at break (%), L1 the distance between midlines of specimen at break (mm) and L is the distance between midlines of the original sample (mm2 ). The FT-IR spectrum was obtained on a BRUKER Avance VECTOR-22 spectrometer. The film was prepared on the clean glass surface, dried under vacuum condition at room temperature to remove all the water. Next, the film was immersed in ionized water for 30 min, followed by sonication for 15 min to remove unreacted AVS monomers which are soluble in water, and then dried at 120 ◦ C for 90 min to remove unreacted volatile monomers. The film was peeled off from the glass substrate with forceps. Atomic force microscope (AFM) was performed using a SPA-400 AFM (Seiko Instruments Inc. Japan). Images were acquired under ambient conditions in tapping mode using a nanoprobe cantilever. For AFM measurements, the corresponding organic fluorine and nano-SiO2 modified polyacrylate emulsifierfree emulsion (1 wt% solid content) were cast onto the prepared silicon wafers and then dried at room temperature. The morphology of the films was carried out with a field emission scanning electron microscopy (FE-SEM) (Hitachi S-4800, Japan).
Fig. 2. FT-IR spectrum of organic fluorine and nano-SiO2 modified polyacrylate.
The peak at 1241 cm−1 is characteristic absorption peak of C F. The characteristic peak at 1453 cm−1 originates from the C6 H5 groups of the alkyl vinyl sulfonate (AVS). The peak at 1100–1000 cm−1 is the stretching vibration absorption peak of Si O and C O. The peak at 759 cm−1 originates from the C Si stretching vibration of the CH2 Si groups. FT-IR result demonstrates that DFMA and nanoSiO2 have effectively participated in the emulsion polymerization and the organic fluorine and nano-SiO2 modified polyacrylate has been obtained. 3.2. Particle size of latexes Fig. 3 shows the influence of dodecafluoroheptyl methacrylate (DFMA) content on the diameter of organic fluorine and nanoSiO2 modified polyacrylate latex particles. The diameter of the hybrid emulsion particles increases from 104.3 nm to 210.3 nm as the DFMA amount increases from 5 wt% to 25 wt% based on total monomer weight. In emulsion polymerization, the reaction relies on the diffusion of the monomers from monomer droplets through the aqueous phase to micelles. DFMA monomers with low aqueous solubility diffuse into the interior of the micelles, and then react with acrylate monomers to form fluorinated polyacrylate, resulting in the increase of the latex particle size with increasing DFMA amount. However, when DFMA concentration is too high, the excess part of DFMA monomers cannot enter the
3. Results and discussion 3.1. FT-IR analysis The chemical structure of organic fluorine and nano-SiO2 modified polyacrylate (sample P3 in Table 1) was characterized by FT-IR, as shown in Fig. 2. The bands at 2959 cm−1 and 2870 cm−1 can be assigned to characteristic stretching vibration peaks of methyl group ( CH3 ) and methylene group ( CH2 ), respectively. The characteristic stretching vibration peak of C O appears at 1732 cm−1 .
Fig. 3. Effect of dosage of DFMA on latex particle size.
J. Zhou et al. / Progress in Organic Coatings 89 (2015) 192–198
195
Table 2 Appearance, solid content, freeze-thaw stability and storage stability of the latexes. Emulsion properties
SDBS
AVS
Appearance Theoretical solid content (%) Experimental solid content (%) Freeze-thaw stability (cycle number) Storage stability at 50 ◦ C
White 35.43 28.14 0 Precipitation
White and blue light 35.43 32.52 5 Stability
micellar interior, and these DFMA monomers can be homoploymerized. Consequently, heterocoagulation can be formed between fluorine-containing homopolymers and hybrid latex particles by intermolecular forces, leading to the significant increase of the latex particle size. 3.3. Stability of hybrid dispersion Surfactants are essential for emulsification of monomer droplets, fast nucleation of latex particles, and stabilization of latex particles during the course of polymerization and storage. However, as small molecular surfactants are attached to the surface of latex particles by the physical adsorption, the stability of emulsion becomes worse. In the present article, the polymerizable surfactant AVS has been used to synthesize organic fluorine and nano-SiO2 modified polyacrylate emulsifier-free emulsion. During the reaction, AVS can react with other monomer and becomes a part of copolymer, eliminating the disadvantages of small molecular emulsifier. The experimental solid content of hybrid latex prepared in the presence of AVS is 32.52%, which is higher than the experimental solid content (28.14%) of hybrid latex prepared in the presence of sodium dodecyl benzene sulfonate (SDBS), and is close to the theory value (35.43%), as shown in Table 2. The reaction between surfactant and other monomer is essential to improve the stability of emulsion. In contrast to the emulsion stabilized with sodium dodecyl benzene sulfonate (SDBS), the emulsion stabilized with alkyl vinyl sulfonate (AVS) shows remarkable polymerization stability, freeze-thaw stability and storage stability, as shown in Table 2. This results from the fact that the SDBS is attached to the surface of latex particles by the physical adsorption. However, AVS has surface activity and reactive group, high participation efficiency in the process of polymerization and AVS can be bonded with the surface of latex particles by covalent bond. The conversion of alkyl vinyl sulfonate (AVS) was 98.68% by water extraction experiment reported by Zhang et al. [22]. Colloidal stability of hybrid latex after TEOS addition was tested by rapid centrifugation method. The results show that there was no precipitation and emulsification phenomenon in the hybrid latex, indicating that the hybrid latex had a remarkable centrifugal stability. Meanwhile, hybrid latex is still a uniform system after 6 months of storage. This phenomenon can be explained by the fact that silane coupling agent (KH-570) with silyloxymethyl and unsaturated double bond can combine the nano-SiO2 by the silanol groups, on the other hand, the double bond can copolymerize with acrylate monomers. Consequently, fluorine-containing polyacrylate polymers and nano-SiO2 can be combined by chemical bonds, rendering high interfacial compatibility to organic and inorganic phases.
Fig. 4. Effect of dosage of DFMA on contact angles of film.
of the film increases from 65.3◦ to 92.4◦ with the increment of the amount of DFMA from 5 wt% to 25 wt%. The increase in the water contact angle of the film indicates that the surface energy of latex film reduces with the increase of the amount of DFMA. This phenomenon can be attributed to the extremely low surface energy and self-aggregation property of the fluorine atoms, which cause the fluorinated segments to be preferentially oriented to the filmair interface during the film formation so as to decrease the surface energy of film. The result is consistent with those reported in the literatures [25,7]. However, the less increment of contact angle for the latex films with more than 15 wt% DFMA may be ascribed to the factor that the migration of fluorine atom onto the film surface is hindered by the steric effect of the fluorine atoms migrating to the surface of film [26]. 3.5. Solvent resistance and mechanical property Nano-SiO2 modified polyacrylate by the combination of radical heterophase polymerization and hydrolytic condensation to incorporate nano-SiO2 particles into polyacrylate polymer [27] exhibits good solvent resistance and mechanical performance due to the large specific surface area and high chemical reactivity of nanoSiO2 . The solvent resistance of the composite film was characterized by swelling measurements and solvent extraction [28], as shown in Fig. 5. Compared with the film obtained from the organic fluorine modified polyacrylate emulsifier-free emulsion, the film obtained
3.4. Hydrophobicity The water contact angle is commonly used as a criterion for the evaluation of the hydrophobicity of a solid surface [23]. The surface is hydrophobic when  is higher than 90◦ and the surface is hydrophilic when  is lower than 90◦ [24]. The water contact angles of composite films with different content of DFMA are shown in Fig. 4. It can be observed from Fig. 4 that the water contact angle
Fig. 5. Effect of dosage of TEOS on solvent resistance of the hybrid film.
196
J. Zhou et al. / Progress in Organic Coatings 89 (2015) 192–198
Fig. 8. Various roughness parameters for polymer films formed from polyacrylate (PA) emulsifier-free emulsion, organic fluorine modified polyacrylate (PFA) emulsifier-free emulsion and organic fluorine and nano-SiO2 modified polyacrylate (PFSiA) emulsifier-free emulsion. Fig. 6. Effect of dosage of TEOS on mechanical properties of film.
from the organic fluorine and nano-SiO2 modified polyacrylate emulsifier-free emulsion has smaller swelling ratios and higher gel contents. The solvent resistance of the hybrid films increases along with the increase of TEOS content. This can be explained by the fact that the fluorined polyacrylate chain and silica component can form three-dimensional crosslinking network by the cocondensation of silanol groups on the silane coupling agent and nano-silica formed by the hydrolysis and polycondensation of TEOS. The crosslinking densities of the hybrid films increase along with the increase of TEOS dosage, rendering excellent solvent resistance to hybrid film. The mechanical properties of composite films with different content of TEOS are showed in Fig. 6. The tensile strength of hybrid film obtained from organic fluorine and nano-SiO2 modified polyacrylate emulsifier-free emulsion is higher than that of the film obtained from the organic fluorine modified polyacrylate emulsifier-free emulsion. The tensile strength of hybrid film increases from 9.6 MPa to 12.5 MPa with the increment of TEOS amount from 0 wt% to 4 wt%, and decreases with the continuous increase of TEOS content. The higher tensile strength of hybrid film is attributed to the three-dimensional crosslinking network of hybrid polymers containing silica component as a crosslinking point. However, further increase of TEOS content above 4% leads to the decreases of the tensile strength. This phenomenon can be explained by the fact that the increasing amount of nano-SiO2 hinders interparticle polymer diffusion by generating a sheating layer that may cause brittleness of the hybrid film, leading to the decrease of the mechanical properties. 3.6. Microstructure morphology of latex film Atomic force microscopy (AFM) was used in this work to observe surface morphologies of the polymer film and the results are presented in Figs. 7 and 8. The parameters, such as root-mean-square
(RMS) roughness and average roughness (Ra), were obtained from AFM software analysis. As shown in Fig. 7, compared with the surface structure of the film formed from polyacrylate emulsifier-free emulsion (sample P0 in Table 1), the film obtained from the organic fluorine modified polyacrylate emulsifier-free emulsion (sample P1 in Table 1) is covered by more little protuberances or peaks on its surface. In addition, the Ra and RMS of the polyacrylate film (Fig. 8) are lower than the organic fluorine modified polyacrylate film. This phenomenon can be explained by the fact that the organic fluorine segment migrates toward the film surface during the heat treatment, and phase separation between fluorinated and non-fluorinated components occurs for organic fluorine modified polyacrylate film, improving the roughness of the latex film [29]. On the other hand, the controllability of polymerization process may be poor due to the low aqueous solubility of DFMA, and a small amount of DFMA monomers can be homoploymerized. The phase separation occurred during the film-forming process due to the presence of fluorine-containing homopolymers, which maybe also improve the roughness of hybrid film. What is more, it can be observed from Figs. 7 and 8 that the organic fluorine and nanoSiO2 modified polyacrylate film (sample P3 in Table 1) have more protuberances or peaks and higher Ra and RMS than organic fluorine modified polyacrylate film, indicating that the nano-SiO2 can improve the roughness of hybrid film. Fig. 9 shows the surface morphology of the films cast from the polyacrylate soap-free emulsion (sample P0 in Table 1) by scanning electron microscopy (SEM), the organic fluorine modified polyacrylate emulsifier-free emulsion (sample P1 in Table 1), and the organic fluorine and nano-SiO2 modified polyacrylate emulsifierfree emulsion (sample P3 in Table 1). It can be noted that the polyacrylate films (Fig. 9a, d, and g) appear to be quite smooth and continuous, and the organic fluorine modified polyacrylate films (Fig. 9b, e, and h) have some salients on the surface, as a result of the enrichment of fluorinated polymers and the micro-phase
Fig. 7. AFM images of sample films formed from polyacrylate emulsifier-free emulsion (a), organic fluorine modified polyacrylate emulsifier-free emulsion (b) and organic fluorine and nano-SiO2 modified polyacrylate emulsifier-free emulsion (c).
J. Zhou et al. / Progress in Organic Coatings 89 (2015) 192–198
197
Fig. 9. SEM images of sample films formed from polyacrylate emulsifier-free emulsion (a, d, g), organic fluorine modified polyacrylate emulsifier-free emulsion (b, e, h) and organic fluorine and nano-SiO2 modified polyacrylate emulsifier-free emulsion (c, f, i).
separation. Meanwhile, on the organic fluorine and nano-SiO2 modified polyacrylate films (Fig. 9c, f, and i), as there are some nanoparticles attached to the surface, the roughness of hybrid film has been improved. These SEM results are in good agreement with AFM results.
3.7. Formation mechanism of hybrid film Scheme 2, based on the aforementioned experimental results, shows the schematic representation of the formation process of the organic fluorine and nano-SiO2 modified polyacrylate film. During
Scheme 2. Film formation process of organic fluorine and nano-SiO2 modified polyacrylate.
198
J. Zhou et al. / Progress in Organic Coatings 89 (2015) 192–198
water evaporation, the latex particles contact with each other, deform into polyhedra and pack together. At last, the diffusion and permeation occur among the molecular chains, forming the thin homogeneous film with high mechanical property. The cocondensation polymerization between the silanol groups on silane coupling agents and SiO2 colloidal particles forms Si O Si bonds between the fluorinated polyacrylate and the silica components during the polymerization, therefore, the SiO2 components disperse within the fluorinated polyacrylate component. The fluorinated groups in organic fluorine and nano-SiO2 modified polyacrylate have low surface free energy due to the low polarizability of C F bond, leading to strong tendency of the surface. During the latex film formation, the fluorinated groups will preferentially migrate and accumulate on the surface. When the surface is uniformly covered with fluorinated array, a very low energy surface can be achieved, rendering excellent water repellency to the hybrid film. 4. Conclusions Organic fluorine and nano-SiO2 modified polyacrylate emulsifier-free emulsion was successfully synthesized by the emulsifier-free emulsion polymerization. Compared with stability of latex prepared in the presence of sodium dodecyl benzene sulfonate, the stability of latex prepared in the presence of AVS was much higher. The DLS analysis confirmed that the latex particle size increased with the increase of DFMA content. The contact angle of the hybrid latex film increased along with the increase of DFMA content. The solvent extraction and swelling measurements indicated that the obtained hybrid latex film exhibited better solvent resistance than pure organic fluorine modified polyacrylate emulsifier-free emulsion film. At the same time, the hybrid film presented good mechanical properties. In addition, the FT-IR spectrum indicated that the DFMA and nano-SiO2 were successfully introduced into organic fluorine and nano-SiO2 modified polyacrylate. The AFM and SEM images confirmed that the hybrid film had a rough surface with many little protuberances or peaks. Acknowledgments This work was supported by the National Natural Science Fund of China (No. 21206088), the Science and Technology Innovation Engineering Program of Shaanxi Province (No. 2011KTCL01-13), the Academic Backbone Cultivation Program of Shaanxi University of Science and Technology (No. XSGP201205). References [1] C. Ni, G. Ni, L. Zhang, J. Mi, B. Yao, C. Zhu, Syntheses of silsesquioxane (POSS)-based inorganic/organic hybrid and the application in reinforcement for an epoxy resin, J. Colloid Interface Sci. 362 (1) (2011) 94–99. [2] M. Wu, R. Wu, R. Li, H. Qin, J. Dong, Z. Zhang, H. Zou, Polyhedral oligomeric silsesquioxane as a cross-linker for preparation of inorganic–organic hybrid monolithic columns, Anal. Chem. 82 (13) (2010) 5447–5454. [3] C. Sanchez, K.J. Shea, S. Kitagawa, Recent progress in hybrid materials science, Chem. Soc. Rev. 40 (2) (2011) 471–472. [4] B. Li, S. Zhang, Q. Xu, B. Wang, Preparation of composite polyacrylate latex particles with in situ-formed methylsilsesquioxane cores, Polym. Adv. Technol. 20 (12) (2009) 1190–1194.
[5] J. Hu, J. Ma, W. Deng, Properties of acrylic resin/nano-SiO2 leather finishing agent prepared via emulsifier-free emulsion polymerization, Mater. Lett. 62 (17) (2008) 2931–2934. [6] W. Xu, Q. An, L. Hao, D. Zhang, M. Zhang, Synthesis of self-crosslinking fluorinated polyacrylate soap-free latex and its waterproofing application on cotton fabrics, Fibers Polym. 15 (3) (2014) 457–464. [7] J. Wang, X. Zeng, H. Li, Preparation and characterization of soap-free fluorine-containing acrylate latex, J. Coat. Technol. Res. 7 (4) (2010) 469–476. [8] X. Song, Y. Zhao, H. Wang, Q. Du, Fabrication of polymer microspheres using titania as a photocatalyst and pickering stabilizer, Langmuir 25 (8) (2009) 4443–4449. [9] H. Liu, H. Ye, T. Lin, T. Zhou, Synthesis and characterization of PMMA/Al2 O3 composite particles by in situ emulsion polymerization, Particuology 6 (2008) 207–213. [10] L. Jiang, K. Pan, Y. Dan, Synthesis and characterization of well-defined poly(methyl methacrylate)/CaCO3 /SiO2 three-component composite particles via reverse atom transfer radical polymerization, Colloid Polym. Sci. 285 (2006) 65–74. [11] S. Yu, X. Zuo, R. Bao, X. Xu, J. Wang, J. Xu, Effect of SiO2 nanoparticle addition on the characteristics of a new organic–inorganic hybrid membrane, Polymer 50 (2) (2009) 553–559. [12] M. Percy, C. Barthet, J. Lobb, M. Khan, S. Lascelles, M. Vamvakaki, S. Armes, Synthesis and characterization of vinyl polymer–silica colloidal nanocomposites, Langmuir 16 (17) (2000) 6913–6920. [13] M. Comes, M.D. Marcos, R. Martinez-Manez, F. Sancenon, L.A. Villaescusa, A. Graefe, G.J. Mohr., Hybrid functionalised mesoporous silica–polymer composites for enhanced analyte monitoring using optical sensors, J. Mater. Chem. 18 (47) (2008) 5815–5823. [14] X. Cheng, M. Chen, S. Zhou, L. Wu, Preparation of SiO2 /PMMA composite particles via conventional emulsion polymerization, Polym. Sci. A: Polym. Chem. 44 (2006) 3807–3816. [15] K. Schmidtke, G. Lieser, M. Klapper, K. Müllen, Complex inorganic/organic core–shell architectures via an inverse emulsion process, Colloid Polym. Sci. 288 (3) (2010) 333–339. [16] A.R. Mahdavian, M. Ashjari, A.B. Makoo, Preparation of poly (styrene-methyl methacrylate)/SiO2 composite nanoparticles via emulsion polymerization. An investigation into the compatiblization, Eur. Polym. J. 43 (2) (2007) 336–344. [17] G. Riess, C. Labbe, Block copolymers in emulsion and dispersion polymerization, Macromol. Rapid Commun. 25 (2) (2004) 401–435. [18] Y. Ohtsuka, H. Kawaguchi, Y. Sugi, Copolymerization of styrene with acrylamide in an emulsifier-free aqueous medium, J. Appl. Polym. Sci. 26 (5) (1981) 1637–1647. [19] X. Cui, S. Zhong, H. Wang, Synthesis and characterization of emulsifier-free core–shell fluorine-containing polyacrylate latex, Colloids Surf. A 303 (3) (2007) 173–178. [20] J. Hu, J. Ma, W. Deng, Properties of acrylic resin/nano-SiO2 leather finishing agent prepared via emulsifier-free emulsion polymerization, Mater. Lett. 62 (2008) 2931–2934. [21] A. Guyot, Advances in reactive surfactants, Adv. Colloid Interface Sci. 108 (2004) 3–22. [22] Y. Zhang, C. Rocco, F. Karasu, L.G.J. van der Ven, R.A.T.M. van Benthem, X. Allonas, C. Croutxe-Barghorn, A.C.C. Esteves, G. de With, UV-cured self-replenishing hydrophobic polymer films, Polymer 69 (9) (2015) 384–393. [23] J. Ha, I. Park, S. Lee, Hydrophobicity and sliding behavior of liquid droplets on the fluorinated latex films, Macromolecules 38 (3) (2005) 736–744. [24] B. Zhang, B. Liu, X. Deng, S. Cao, X. Hou, H. Chen, A novel approach for the preparation of organic-siloxane oligomers and the creation of hydrophobic surface, Appl. Surf. Sci. 254 (2) (2007) 452–458. [25] W. Zheng, L. He, J. Liang, G. Chang, N. Wang, Preparation and properties of core–shell nanosilica/poly (methyl methacrylate–butyl acrylate–2,2,2-trifluoroethyl methacrylate) latex, J. Appl. Polym. Sci. 120 (2) (2011) 1152–1161. [26] J. Zhou, L. Zhang, J. Ma, Fluorinated polyacrylate emulsifier-free emulsion mediated by poly (acrylic acid)-b-poly (hexafluorobutyl acrylate) trithiocarbonate via ab initio RAFT emulsion polymerization, Chem. Eng. J. 223 (2013) 8–17. [27] X. Cui, S. Zhong, H. Wang, Emulsifier-free core–shell polyacrylate latex nanoparticles containing fluorine and silicon in shell, Polymer 48 (25) (2007) 7241–7248. [28] T. Tamai, M. Watanabe, Acrylic polymer/silica hybrids prepared by emulsifier free emulsion polymerization and the sol–gel process, J. Polym. Sci. Part A: Polym. Chem. 44 (1) (2006) 273–280. [29] W. Xu, Q. An, L. Hao, D. Zhang, M. Zhang, Synthesis and characterization of self-crosslinking fluorinated polyacrylate soap-free lattices with core–shell structure, Appl. Surf. Sci. 268 (2013) 373–380.