Colloid Polym Sci (2013) 291:1513–1518 DOI 10.1007/s00396-012-2862-0

SHORT COMMUNICATION

Preparation of PDMS-ormosil and its application in miniemulsion polymerization Shouji Qiu & Lei Yang & Ya Zhang & Yongshen Xu

Received: 15 July 2012 / Revised: 22 October 2012 / Accepted: 13 November 2012 / Published online: 14 December 2012 # Springer-Verlag Berlin Heidelberg 2012

Abstract With the assistance of ultrasound, octamethylcyclotetrasiloxane (D4) and slight silane coupling agent γ(2,3-epoxypropoxy)propyltrimethoxysilane (KH560) were sufficiently mixed with silica sol, and the in situ ringopening polymerization of D4 on the surface of silica nanoparticles, catalyzed by dodecylbenzene sulfonic acid (DBSA), was enhanced as well. Thus, polydimethylsiloxane (PDMS)-modified silica (PDMS-ormosil) mixture was obtained. A slight addition of silane coupling agent γ-(2,3epoxypropoxy)propyltrimethoxysilane (KH560) could significantly enhance the modified efficiency. The PDMSormosil mixtures were directly dispersed in the mixed monomer methyl methacrylate and butyl methacrylate. Then, miniemulsion polymerizations of acrylate monomers containing PDMS-ormosils were carried out, with free PDMS as hydrophobe and neutralized DBSA as emulsifier, both preexisted in the PDMS-ormosil mixture. Thus, the troubles of separation, purification, and redispersion in the traditional techniques can be omitted. Fourier transform infrared spectroscopy, thermogravimetric analysis, transmission electron microscope, scanning electron microscope, and water contact angle tests were utilized to demonstrate that nanocomposite particles with a core–shell structure were synthesized; when the silica content was 3 wt% of the monomers, the average particle size was 97 nm, and the generated PDMS improved the hydrophobicity of the nanocomposite latex. Keywords Ormosil . Ultrasonic . Polydimethylsiloxane . Miniemulsion polymerization . Nanocomposite particles

S. Qiu (*) : L. Yang : Y. Zhang : Y. Xu School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China e-mail: [email protected]

Introduction Polymer/inorganic nanocomposites are a new advanced material with excellent properties [1–3] and are becoming a foreland of polymer materials science. Researchers have proposed varied approaches to prepare these materials. Hua et al. [4] reviewed the blend, sol–gel process, emulsion polymerization, self-assembly, etc. Among which, Bourgeat-Lami and Lansalot [5] indicated that waterborne organic/inorganic colloids elaborated through emulsion polymerization had developed into a well-established technology. And they summarized two main approaches: the polymerization of organic monomers in the presence of preformed inorganic particles and the reverse approach by which inorganic materials are synthesized in the presence of preformed polymer latexes. The first route combined with miniemulsion polymerization [6] is more facile and conveniently performed, and the modification of inorganic particles is crucial. Generally, multifunctional modifier was used to convert the hydrophilic state to hydrophobic for better compatibility between organic and inorganic parts [7, 8]. Farzi et al. [8] used cryo-transmission electron microscope (TEM) to discover that the ormosil could easily be dispersed in vinyl monomers, and after the subsequent polymerization step, the ormosil would be buried inside the latex particles. In the former researches, consuming separation, purification, and redispersion of the ormosil were needed to prevent the damages to subsequent emulsion polymerization caused by solvents, modifiers, or free polymers. An economic feasible design of systems without the complicated purification processes was first to be proposed in the present research. To the best of our knowledge, no papers exist in the literature on this subject. Polydimethylsiloxane (PDMS) with lower surface energy and strong hydrophobic might be used as the co-stabilizer in miniemulsion polymerization to inhibit the Ostwald ripening

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[9]. It could be obtained via the ring-opening polymerization of octamethylcyclotetrasiloxane (D4) catalyzed by acid with the aid of ultrasound [10]. Meanwhile, the ultrasonic technique was usually used in the modification of organic relying on the multi-effect of the ultrasound [11]. PDMS-modified silica (ormosil) was obtained by ultrasonic method and the effects of ultrasound were also discussed in former papers [12, 13]. They laid the foundation of the economic feasible design of this present research. Additionally, the residual modifiers were inevitable in former papers; thus, elaborate purification cycles were needed as discussed above. However, in this present work, the “by-product” was free PDMS; it can be utilized as a co-stabilizer of the subsequent miniemulsion polymerization. Furthermore, choosing dodecylbenzene sulfonic acid (DBSA) as acid catalyst of the ring-opening polymerization of D4, DBSA can also be used as an emulsifier of the subsequent miniemulsion polymerization; thus, the exclusion of acid catalyst impurity in the former researches can be omitted. Therefore, the consuming separation, purification, and redispersion processes in former researches were avoided via this economic design. In this present work, firstly, PDMS-ormosils (PDMS-modified silica) were prepared by in situ ring-opening polymerization of octamethylcyclotetrasiloxane (D4) enhanced by ultrasound on the surface of silica nanoparticles. Then, the PDMS-ormosils were used in the subsequent miniemulsion polymerization to produce polyacrylates/silica nanocomposite particles. The PDMS-ormosils and the final product were characterized by Fourier transform infrared spectroscopy (FTIR), TG, TEM, scanning electron microscope (SEM), Malvern Nanosizer, and water contact angle test.

Experimental part

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Fig. 1 FTIR spectra of modified SiO2 and pure SiO2

Preparation of PDMS-modified silica (PDMS-ormosils) Ten grams of silica sol was adjusted to pH01 by adding DBSA. While adding the mixture of 6 g D4 or 6 g D4 and KH560(1 wt% of D4), the ormosil was prepared under magnetic stirring by ultrasonicating the system for 30 min, 100 W with a ultrasonic cell crusher of model JY92-II by Scientz Biotechnology Co., Ltd. To reduce any rise in temperature that may occur during the emulsification process, the beaker containing mixture was immersed in a water bath. Then, certain Na2CO3 solution was added to adjust the PDMS-ormosil mixture to adjust pH07. Preparation of polymer/silica nanocomposite particles A 9-g sample of monomer (MMA/BA050:50, based on weight) and the respective amount former mixture were mixed first and then added to 36 g of water. The miniemulsion was prepared under magnetic stirring by ultrasonicating

Materials Potassium persulfate (KPS), DBSA, stearyl alcohol (SA), and sodium bicarbonate (NaHCO 3) (Tianjin Reagent Chemical Ltd, China) were analytical grade and used as received. Silica sol (silica size, 10–20 nm; 30 wt%, Beijing Xingdaxin Chemical Ltd, China), octamethylcyclotetrasiloxane (D4) (Beijing Xingdaxin Chemical Ltd, China), and γ-(2,3-epoxypropoxy)propyltrimethoxysilane (KH560) (Danyang Chenguang Coupling Agent Ltd, China) were used as received. Methyl methacrylates (MMA) and butyl acrylate (BA) (Tianjin Guang-Fu Fine Chemical Institute, China) were distilled under vacuum before use. The monomers were kept refrigerated before used. Deionized water was applied for polymerization and treatment processes.

Fig. 2 TGA curves of ormosils

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the emulsion for 30 min with an ice bath. For polymerization, the temperature was increased to 75 °C, and typically 180 mg of KPS was added. The time between miniemulsification and initiation was minimized to 5 min to reduce the droplet degradation (Ostwald ripening) period. The initial charge and monomer feeds were protected with nitrogen during the polymerization. The reaction is usually completed after 3 h. The theoretical solid content of the latex product is approximately 20 %. Determination of the PDMS-ormosils The components of ormosils were characterized qualitatively by Bio-Rad FTS3000 FTIR using KBr pellets and pure silica as contrast. The ormosil samples were extracted with acetone for 30 h till no free polymers and other residual molecules left. The components of ormosils were determined quantitatively by DSC (NETZSCH, DSC-204, Germany) with a heating rate of 10 °C/min from room temperature to 800 °C under the protection of nitrogen. The samples of ormosils were extracted or unextracted while without KH560 and extracted while adding KH560 to verify the modifying efficiency. And TEM was performed with a JEOL JEM-100CX II electron microscope operating at 100 kV. In a typical experiment, one drop of the colloidal dispersion was put on a carbon film supported by a copper grid and allowed to air dry before observation. The particles were treated with an aqueous solution of phosphotungstic acid for negative staining. Determination of the polymer/silica nanocomposite particles TEM was used to characterize the nanocomposite particles with the same method above. Malvern Nanosizer was used to test the particle size and size distribution of the emulsion before and after polymerization. The samples were diluted with water before tested.

Fig. 4 The effect of different hydrophobe on the rate of miniemulsion polymerization

The fractured surfaces were observed by SEM (Philips, XL30, Holland). The samples were obtained by the tensile tests. Choosing the fracture parts and coating them with gold avoid charging under the electron beam. Mechanical tests were carried out at ambient conditions using a universal material tensile testing machine (Testometric, M350-10KN, Britain). The water contact angle was tested by a contact angle tester (CA-A, Xiehe, Japan). Conversion of monomers The conversions of monomers during miniemulsion polymerization were evaluated by gravimetry. Different stages of polymerization were terminated by benzoquinone then dried at 80 °C under vacuum condition. The conversion was calculated by: c 0(mt − mb) / (me × a) × 100 % (1) where mt, mb, and me are the amount of dried mass (in grams), terminator benzoquinone (in grams), and

Fig. 3 TEM images of nanocomposite particles: a untreated silica dispersed in monomers, b ormosils dispersed in monomers, and c PDMS of (b) fused under TEM condition

1516 Fig. 5 Particle size distributions of monomer droplets and polymer latexes with 3wt%SiO2 based on monomers. a Particle size distribution of monomer droplets before polymerization. b Particle size distribution of polymer latexes after polymerization

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(a)

(b)

emulsion (in grams), respectively, and a is the solid content of the emulsion.

The in situ ring-opening polymerization of D4 would produce hydroxyl-terminated PDMS; under the ultrasonic condition, PDMS might condensate with silanols on the surface of silica nanoparticles forming ormosils. However,

because of the different properties between silica and PDMS, small amount of PDMS could graft onto the silica surface. Slightly adding coupling agent KH560 could enhance the grafting efficiency significantly. And the free PDMS became inevitable. In Fig. 1, in contrast to pure silica, the characteristic peak of C–H stretching vibration at 2,964 cm−1 was observed. It also can be clearly seen that the peaks of –OH stretching vibration at 3,440 cm−1 and bending vibration at 1,636 cm−1 were weakened in ormosils. This meant that PDMS had grafted onto the surface of silica and inhibited the –OH. The thermogravimetric analysis (TGA) curves of Fig. 2

Fig. 6 TEM image of polymer/silica nanocomposite particles

Fig. 7 TEM image of polyacrylates/silica in larger area

Results and discussion Synthesis of ormosils

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Table 1 The mechanical properties of nanocomposites with varied [SiO2] Sample

[SiO2]

Elongation %

Tensile strength/MPa

Matrix 1 2 3

0 1% 3% 5%

354 385 411 376

5.66 7.79 9.06 9.86

quantitatively showed the results of the grafting efficiency. Above the decomposition temperature, PDMS would decompose. After the decomposition, pure silica was left. As shown in Fig. 2, only 5.4 % PDMS was grafted on the silica surface without KH560, while 13 % PDMS was absorbed on the surface of the silica [14], showing a two-stage decomposition of the before extracted curve. And as much as 26.4 % PDMS was grafted onto the silica surface when

Fig. 8 SEM image of the fractured surface of polyacrylates/silica nanocomposites

Fig. 9 The water contact angles of nanocomposites with SA or PDMS as co-stabilizer

adding a slight amount of KH560; coupling agent KH560 could efficiently enhance the grafting efficiency. The qualitative and quantitative analysis demonstrated that ormosils were obtained. The ormosils could be well dispersed in monomers. Figure 3b showed that the ormosil with core–shell structure could be well dispersed in monomers. The dark part was silica surrounded by light part of PDMS. And there was free PDMS dispersed around. In contrast, the untreated silica aggregated strongly when dispersed in monomers shown in Fig. 3a. Under TEM condition, polymer would fuse gradually. The same phenomenon was observed by Luo et al. [15]; (b) became (c), and (c) clearly showed the perfect dispersion of ormosils in monomers. Synthesis of polymer/silica nanocomposite particles To verify the effect of by-product PDMS on the miniemulsion polymerization, the conversions of different stages were tested and the traditional co-stabilizer SA was used for comparison. In a former study [16], there did not appear to be a clear dependency of conversion of monomers on silica content. Thus, despite the absence of silica particles in SA co-stabilized miniemulsion polymerization, it is reasonable to study the difference of the present work between SA co-stabilized miniemulsion polymerization. As shown in Fig. 4, there was no obvious difference between the two curves, and it resembled with the results of the former study of miniemulsion polymerization[16], demonstrating that free PDMS obtained by the ring-opening polymerization of D4 could act like the traditional co-stabilizer SA of the miniemulsion polymerization. Moreover, co-stabilizer works by the means of inhibiting the Ostwald ripening in miniemulsion polymerization, which can lead to the widening particle size distribution and instability of the emulsion. By testing the size distribution, the effect of PDMS can be determined. Figure 5 showed the particle size distributions of monomer droplets and polymer latexes with 3 wt% SiO2 based on monomers. And the polydispersity indices were 0.084 and 0.093, respectively, which can be considered to be monodisperse [16]. Moreover, the average size and size distribution made a little change, which further demonstrated the co-stabilization of PDMS.

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The TEM image of the nanocomposite particles prepared by miniemulsion polymerization was shown in Figs. 6 and 7. It can be clearly seen that nanocomposite particles with (multi) core–shell structure were successfully prepared; the average size was about 97 nm. The composition of silica nanoparticles would significantly enhance the properties of the matrix. The mechanical properties of nanocomposites were shown in Table 1. The tensile strength of nanocomposite increased with the silica content from 0 to 5 %, displaying good strengthening effect. It demonstrated that the silica particles greatly influenced the mechanical properties of the polymer films. Due to the very high surface area of the silica in composite, the applied stress was expected to be easily transferred from matrix to the silica particles, resulting in an enhancement of the tensile strength [17]. And the silica particles could act as stress concentrators of failure points of nanocomposites, leading to a slight increase of the elongation at break. As shown in Fig. 8, clear evidence of moderate plastic deformation of sample 2 was detected, and the dispersed silica nanoparticles were trapped in the composites with strong binding, resulting in good mechanical properties as discussed above. Additionally, using the by-product PDMS could improve the hydrophobicity of the polymer/silica nanocomposites. In Fig. 9, using traditional co-stabilizer SA, the water contact angle of the final polymer matrix was 45.9°, while using the by-product free PDMS generated in the modification process as co-stabilizer, the water contact angle increased to 90°, showing better water resistance.

Conclusion PDMS-modified silica (ormosils) nanoparticles were prepared by the in situ ring-opening polymerization of D4 on the surface of silica. A small amount of KH560 could significantly enhance the grafting efficiency. The byproduct free PDMS was inevitable, but it could be used as co-stabilizer in subsequent miniemulsion polymerization and improve the water resistance of the nanocomposites. And the acid catalyst DBSA could act as emulsifier of the subsequent miniemulsion polymerization, consuming separation, purification, and redispersion needed in traditional approaches were avoided. Polymer/silica nanocomposite particles with (multi)core–shell structure were successfully synthesized via miniemulsion polymerization;

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the average size was 97 nm when silica content was 3 wt% of the monomers. And the nanocomposites possessed better performance than the polymer matrix for potential applications.

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