Block copolymer phase mixing effects

PVA Particles. Dispersions were prepared in order to examine stabilization for a core polymer having a glass transition temperature below the dispersion polymerization temperature. PVA particles prepared with a block copolymer having M PS) x 10000 showed a tendency to flocculate at ambient temperature during redispersion cycles to remove excess block copolymer, particularly if the dispersion polymerization had not proceeded to 100 conversion of monomer. It is well documented that on mixing solutions of polystyrene and poly(vinyl acetate) homopolymers phase separation tends to occur (10,11), and solubility studies (12) of PS in n-heptane suggest that PS blocks with Mn(PS) 10000 will be close to dissolution when dispersion polymerizations are performed at 3 +3 K. Consequently, we may postulate that for soft polymer particles the block copolymer is rejected from the particle because of an incompatibility effect and is adsorbed at the particle surface. If the block copolymer desorbs from the particle surface, then particle agglomeration will occur unless rapid adsorption of other copolymer molecules occurs from a reservoir of excess block copolymer. 

The effect of adding homopolymer to ordered block copolymer melts was interpreted by considering how homopolymer is distributed within the microstructure (Matsen 19956). Within the A-rich domains of a microslructure, the A blocks tend to segregate to the interfaces, whereas homopolymer A is preferentially located in the domain centre due to tension in the A blocks. This is countered by the entropy of mixing which favours a more uniform distribution of homopolymer. To allow homopolymer near the interface, a phase transition may occur to a microstructure where the interface is less curved towards the A-rich domains (this is the reverse of the process shown in Fig. 6.9). Even without such a phase transition, increasing homopolymer content leads to an increasing... 

Sundararaj and Macosko (1995) and Beck Tan et al. (1996) observed that the addition of a block copolymer to the droplet phase before mixing it with the matrix phase had little effect on the resulting droplet size at low droplet volume fraction. Although a block copolymer should reduce the interfacial tension between the two phases, and thereby lead to smaller droplets, the diffusion time of the block copolymer may be too long for it to saturate the new interfacial area that must form rapidly if a droplet is to fragment. However, block copolymers do seem to suppress coalescence, possibly by immobilizing the interface... 

The hydrophobic polyurethane block copolymer was chosen because phase-separated samples had been shown by an in vitro assay to exhibit a greater platelet adhesion from human blood than samples having a mixed, nonphase-separated structure (7). The hydrophobic styrene-butadiene-styrene (SBS) block copolymer was chosen because of the ease with which the morphology could be controlled (8). The block copolymer having hydrophilic blocks of poly(ethylene oxide) and hydrophobic polystyrene blocks (PS-PEO) was chosen to examine the effect of the more hydrophilic blocks on the protein interaction. 

The polyurethane in the phase-separated state (ANN), however, behaved in a manner similar to certain other hydrophobic surfaces (3), including siliconized glass, in that platelet adhesion was independent of the time of exposure to plasma, that is, no difference between 3 s and 3 min existed. The hydrophobic SBS block copolymer, on the other hand, attached significantly (P < 0.025) less platelets at 3 min of plasma exposure. The effect was relatively more predominant in the mixed state as the percent decrease in platelets attached at 3 min (conversion) was 56 compared with 34 for the phase-separated state (Table II). The conversion, however, was far less than the 99% observed on the hydrophilic glass. 

It has been found in the study of PVME and SBS triblock copolymer that solubility of PVME in PS block copolymer domains is larger than in PS homopolymer. This may indicate that the mixing enthalpy has an effect on the blend miscibility [Xie et al., 1993]. The behavior has been attributed to the effect of PB segments in SBS. The phase equilibria and miscibility in polymer blends containing random or block copolymer was reviewed [Roe and Rigby, 1987]. More recent data are presented in Chapter 4 Interphase and Compatibilization by Addition of a Compatibilizer in this Handbook. 

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