PS-PDMS

Much work on the preparation of nonaqueous polymer dispersions has involved the radical polymerization of acrylic monomers in the presence of copolymers having the A block the same as the acrylic polymer in the particle core 2). The preparation of polymer dispersions other than polystyrene in the presence of a PS-PDMS diblock copolymer is of interest because effective anchoring of the copolymer may be influenced by the degree of compatibility between the PS anchor block and the polymer molecules in the particle core. The present paper describes the interpretation of experimental studies performed with the aim of determining the mode of anchoring of PS blocks to polystyrene, poly(methyl methacrylate), and poly(vinyl acetate) (PVA) particles. 

Flocculation studies (6) indicated that the mechanism of steric stabilization operates for the PMMA dispersions. The stability of PMMA dispersions was examined further by redispersion of the particles in cyclohexane at 333 K. Above 307 K, cyclohexane is a good solvent for PS and PDMS, and if the PS-PDMS block copolymer was not firmly anchored, desorption of stabilizer by dissolution should occur at 333 K followed by flocculation of the PMMA dispersion. However, little change in dispersion stability was observed over a period of 60 h. Consequently, we may conclude that the PS blocks are firmly anchored within the hard PMMA matrix. However, the indication from neutron scattering of aggregates of PS(D) blocks in PMMA particles may be explained by the observation that two different polymers are often not very compatible on mixing (10) so that the PS(D) blocks are tending to... 

Polymers - The PS, PDMS, polyhexylisocyanate (PHIC), and polylso-prene (PI) samples had been extensively characterized to determine molecular weights, molecular sizes, and thermodynamic parameters (5, 6, 7 ). The samples were anionlcally polymerized using butyl lithium as the initiator. The pertinent data are shown in Table L Polylsobutylene/PIB polymers were obtained by fractionation of commercial polymers and their molecular weights were measured (8). 

Figure 3. Irradiation of PS/PDMS blends at 1.55 Mrad. Experimental data ( ) "additive line (-)...

Fig. 6.21 Reduction of interfacial tension of a PS/PDMS blend (PS, Af = 4.0kgmol PDMS, M = 4.5kgmoP ) upon addition of a PS-PDMS diblock (M = 13 kgmol-1, 50wt% PS) (Hu et al. 1995). The interfacial tension of the blend without diblock is y0 = 4.85 O.OSdyncm . All measurements were performed at 140 °C.

Chen, Gardella and Kumler106 have studied a series of polydimethylsiloxane-polystyrene block copolymers and examined the surface composition by ATR-FTIR and XPS. For AB-type PS-PDMS diblock copolymers (26) with siloxane block molecular weights of between 38,000 and 99,000, the surface was found to be exclusively polydimethylsiloxane down to a depth of 10 nm by XPS. ATR-FTIR, which samples... 

It is possible to convert the essentially spherical PS particles just described into ellipsoids.18 58-62 First, the PS-PDMS composite is raised to a temperature well above the Tg of PS. It is then deformed, and cooled while in the stretched state. The particles are thereby deformed into ellipsoids, and retain this shape when cooled. Uniaxial deformations of the composite give prolate (needle-shaped) ellipsoids, and biaxial deformations give oblate (disc-shaped) ellipsoids.59,63 Prolate particles can be thought of as a conceptual bridge between the roughly spherical particles used to reinforce elastomers and... 

The macromonomer method was used by Fujimoto et al. for the preparation of (PS)(PDMS)(PtBuMA) stars [53], as described in Scheme 20. The lithium salt of the p-(dimethylhydroxy)silyl-a-phenyl styrene was synthesized and used as initiator for the polymerization of hexamethylcyclotrisiloxane (D3). Living PS chains were reacted with the end double bond of the macromonomer, followed by the anionic polymerization of the f-BuMA. 

Fig. 2 Viscosity ratio X versus PDMS drop size ((D)) for PS/PDMS blends [8]...

They observed that the range of compositions was broader for the PS/PDMS pair having the smaller viscosity ratio. This tendency is in good agreement with the predicted relationship [11-14] ... 

Since they act as surfactants, copolymers are added in only small amounts, typically from a thousandth parts to a few hundredth parts. Theoretically, Leibler [30] showed that only 2% of a diblock copolymer may thermodynamically stabilize an 80%/20% incompatible blend with an optimum morphology (submicronic droplets). However, in practice kinetic control and micelle formation interfere in this best-case scenario. To a some extent, compatibilization increases with copolymer concentration [8,31,32], Beyond a critical concentration (critical micellar concentration cmc) little or no improvement is observed (moreover, for high amounts, the copolymer can act as a plasticizer). Copolymer molecular weight influence is similar to that of the concentration effect. For example, in a PS/PDMS system [8,31,32], when the copolymer molecular weight increases, domain size decreases to a certain extent. Hu et al. [31] correlated their experimental results with theoretical prediction of the Leibler s brush theory [30]. Leibler distinguishes two regimes to characterize the behaviour of the copolymer at the interface... 

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