Nucleation of Emulsion Polymerization in the Presence of Small Silica

Chapter 24

Nucleation of Emulsion Polymerization in the Presence of Small Silica Particles 1

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Philippe Espiard , André Revillon , Alain Guyot, and James E. Mark 1

Laboratoire des Matériaux Organiques, Centre National de la Recherche Scientifique, B.P. 24, 69390 Vernaison, France Polymer Research Center and Department of Chemistry, University of Cincinnati, Cincinnati, OH 45221-0172

Downloaded by CORNELL UNIV on August 6, 2012 | http://pubs.acs.org Publication Date: June 3, 1992 | doi: 10.1021/bk-1992-0492.ch024

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Several silica were used : either non functionalized silica from commercial origin or prepared according to the Stöber method, or functionalized silica prepared upon surface coverage of non functionalized silica with functional coupling agents such as X(CH ) Si(OR) where X is either a methacryloyl group or the precursor amino group for an azo compound. Some organophobic silica can be prepared via a sol-gel processfromco-hydrolysis and co-condensation of tetraethoxysilane (TEOS) and the functional coupling agent with water in inverse microemulsion in the presence of a suitable surfactant. The average diameter of the silica particles is in the range 20-90 nm. When the silica is hydrophilic, encapsulation with polymer upon emulsion copolymerization of highly hydrophobic monomers (styrene, butyl acrylate) is not observed : a regular latex is produced independently from the silica, even if coverage of silica with surfactant has been previously carried out. Partial success has been obtained using limited amounts of methyl methacrylate. Very good results have been obtained for ethyl acrylate polymerization with a semi-continuous feed process, provided a suitable nonionic surfactant is used. The dispersability of functionalized silica is poor, but may be improved after a suitable treatment with ammonia, and a combination of an alcohol and a nonionic surfactant. Then, each latex particle is nucleated by the silica, again in the case of ethyl acrylate. 2

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Dispersion of small mineral particles in polymer matrices has received an increasing amount of interest these last few years. A popular approach is to prepare these particles in situ, and this has been carried out extensively by one of us (J.E.M) using various elastomer matrices (1). An extension of this approach leads to the preparation of new materials called ceramers (2). On the other hand, some years ago a Japanese team gave the first reports of emulsion 0O97-6156/92/0492-O387$06.00/0 © 1992 American Chemical Society In Polymer Latexes; Daniels, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.


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polymerization of methyl methacrylate (MMA) in the presence of various inorganic materials such as calcium carbonate (3,4), graphite (5), calcium sulfite (6), or barium sulfate (7,8). Most of these experiments led to encapsulation of the particles, even if polymerizations have been carried out without emulsifier. A patent from Union Carbide (9) claims a general method for polymer encapsulation of dispersed solids. Basic research involving encapsulation of silica has been reported recently. For instance Hereerth et al (10-14) have covered quartz powder with a diameter of 26 nm, with methyl methacrylate or vinyl acetate. In some cases previous treatments have been carried out m order to anchor organic materials onto the silica (15,16) or onto Ti02 (17,18). Surface active monomer has been used in one case (19) andfinallythere is one report about vinyl modified silica covered with polyvinylpyrrolidone (20). The present paper describes ourfirstwork along this line, in order to try to understand the possibilities and the limitations of this approach, and to determine the necessary conditions to reach the final goal which is to obtain latex particles containing one and, hopefully, only one silica particle in its core. Therefore, we have used a variety of silica, functionalized or not, as well as various monomers which were more or less hydrophilic. Experimental The Ludox AS40 (Dupont) and Degussa Aerosil A 200 V are commercial products. The preparation of silica via a sol-gel process in inverse microemulsion has been previously described (23). Trimethoxypropylmethacrylate silane (MPS), 7-aminopropyl triethoxy silane (APS) and azooiscyanovaleric acid (ACVA) are commercial products from Fluka. Functionalization of Commercial Silica. A first technique to prepare these functionalized silicas is to react a typical coupling agent for composite materials with the silanol groups of a commercial silica, (Degussa A 200 V) according to scheme I. The typical procedure for reaction la or lb (scheme I) is as follows : Dried Degussa A 200 V silica is suspended in dry toluene, i.e. toluene distilled over molecular sieve undef nitrogen atmosphere. After addition of an excess of MPS or APS (8 fimol/m ), the mixture is refluxed for 16 hours under a N 2 atmosphere, the weight ratio of toluene/silica being 30. The support was washed 3 times with toluene, separated by centrifugation and dried in vacuum. The grafting of A C V A onto APS modified silica (scheme Ic) is carried out by reaction of 4.5 g of the modified silica with 1 g of A C V A in 250 ml of dry t6trahydrofuran (THF), to which 0.8 ml of triethylamine and 0.5 g of chloroformiate are added. Temperature is kept at -78°C for the first half hour and then allowed to reach -10 C, where the reaction was run for 17 hours. Grafted silica is separated from T H F by centrifugation. 1

Emulsion Polymerizations with Silica Ludox Silica and Modified Degussa Silica. Emulsion polymerizations are performed at 60°C in a 1.5 L jacketed flask under a nitrogen atmosphere. Prior to polymerization, silica particles are dispersed in water which is tnen flushed with nitrogen in order to remove oxygen. These silica particles are emulsified with different surfactants : sodium dodecyl sulfate (SDS, Prolabo), or dodecyl trimethylammomum bromide (Sigma), or polyoxyethylene lauryl ether (E4C12, Aldrich), or nonylphenyl polyoxyethylene (NP30). All experiments were carried out with deionized water. Different kinds of monomers were involved : styrene

In Polymer Latexes; Daniels, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

24. ESPIARD ET AL.

Nucleation of Emulsion Polymerization

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(Prolabo), butyl acrylate (Norsolor), methyl methacrylate (Aldrich) and ethyl acrylate (Kodak). Each of them was distilled at reduced pressure to remove the inhibitor. The monomer/water weight ratio was kept constant at 10 % whereas the silica content was variable. Silica/monomer weight ratios from 1/1 to 1/64 were investigated. When experiments were carried out using a semi-continuous feed process, an addition rate of 0.34 ml/min was chosen in order to prevent the formation of polymer particles without silica inside them. Potassium persulfate (K2S2O8, Fisher) was employed as initiator at 0.7 g/L. The emulsion was Monomer conversion was determined by taking samples during the entire course of the polymerization and by measuring their solids content. Hydroquinone was aaded to each sample in order to stop the reaction. Verification of silica encapsulation was undertaken by different techniques on the final latex: - by measuring the particle size (transmission electron microscopy, photon correlation spectroscopy, hydrodynamic chromatography) - by centrifugation at 18000 rpm and analyzing the different observed layers (if any) by elemental analysis and Fourier Transform Infra-red spectroscopy. - by comparison of the number of initial silica particles with the number of final latex particles Silica from the Water-in-Oil (W/O) Emulsion. Polymerization with this last kind of silica was undertaken by a three step process : 1 step : w/o polymerization of 0.8 g of acrylic acid (Aldrich) at 60°C in a 100 ml round bottom flask containing 50 ml of the emulsified silica obtained by the sol-gel process from hydrolysis and condensation of TEOS and MPS in a water-in-toluene process (23) ; the average particle diameter being 64 nm. 0.017 g of 2,2 azoisobutyronitrile (AIBN, Prolabo) was used as initiator. The resulting emulsion had some characteristic bluish effects. 2 step : the w/o emulsion was inverted in an o/w emulsion and the remaining toluene evaporated with an evaporator. 3 step : emulsion polymerization of ethyl acrylate took place in this emulsion at 60°C using semi-continuous feed process with 0.85 g/L of K2S2O8 as initiator. st

n d

r a

Results and Discussion Silica Particles Hydrophilic Silica. We have first used non-functionalized commercial silica, Ludox AS40, i.e., silica containing only hydroxyl groups on its surface, which consists of non-porous uniform particles with an average diameter of 22 nm, and a surface area of 140 m / g ; the particles are stabilized by ammonium ions and carry a negative charge, the pH 01 a 40 weight % suspension being 9.2. z

Hydrophobic silica. Then we have been working with a second kind of silica ; functionalized silica, i.e silica carrying functional, alkyl groups on the surface.

In Polymer Latexes; Daniels, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1992.

390

POLYMER LATEXES

SCHEME I. FUNCTIONALIZATION OF SILICA WITH COUPLING AGENTS


a) grafting of a methacryloyl comonomer (MPS) b) grafting of the aimnopropyltriethoxysilane (APS) c) reaction of the azobiscyanovaleric acid (ACVA) with the APS modified silica.

b

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%i-0-Si4CHjiNH?

(C H 0J3'SUCH ipH 2

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O C) ^ i - 0 - S i ( C H ) - N H

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CM

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+ \:(CH ) C-N^-C-(CH ) c' " 2

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OH

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Si-0-Si(CH ) -NHC 2

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(CH ) -C-N-N-C-(CH ) COOH CH CH 2

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Grafting of methacryloylpropyl trimethoxy silane (MPS) gives a grafted reactive monomer (scheme la) while successive reactions of 7aminopropyltriethoxy silane (APS) with surface silanol groups and of azobiscyanovaleric acid (ACVA) with the amino groups of the grafted APS silica gives a radical generator (schemes I b and c). These reactions were already well documented in the literature (20-22). These grafted silicas were characterized from elemental analysis (C for MPS grafted and N for APS and A C V A grafted materials) from which it can be estimated according to the Berendsen formula (24) that 90 % of the silanol groups have been reacted with MPS while the yield was 72 % in the case of APS, the conversion from APS to ACVAieing 61 %. Infrared spectroscopy (Figures 1-3) and ^ S i CPMAS NMR spectra (Figures 4 and 5) have been used for fiirther characterization. The grafting of MPS is evidenced by the infrared bands at 1708.-1719 cm" KC=0), 1637 cm" KC=C) 2737 cm" v(CHj) and 2922-2944 cm" i

Nucleation of Emulsion Polymerization in the Presence of Small Silica

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