Phase-separation behavior basis

Based on the pioneering work of Molau [64], it is evident that phase separation can occur in blends of two or more copolymers produced from the same monomers when the composition difference between the blend components exceeds some critical value. The mean field theory for random copolymer-copolymers blends has been applied to ES-ES blends differing in styrene content to determine the miscibility behavior of blends [65,66]. On the basis of the solubility parameter difference between PS and PE, it was predicted that the critical comonomer difference in styrene content at which phase separation occurs is about 10 wt% S for ESI with molecular weight around 105. DMS plots for ES73 and ES66 copolymers and their 1 1 blend are presented in Figure 26.8. 

The solution behavior of polymers has been intensively investigated in the past. Dilute solutions, where polymer-polymer interactions may be excluded, have become the basis for the characterization of the primary structure of macromolecules and their dimensions in solution. Besides this "classical" aspect of macromolecular science, interest has focussed on systems, where - due to strong polymer/polymer interactions - association of polymers causes supermolecular structures in homogeneous thermo-dynamically-stable isotropic and anisotropic solutions or in phase-separated multi-component systems. The association of polymers in solutions gives rise to unconventional properties, yielding new aspects for applications and multiple theoretical aspects. 

We had earlier encountered specific examples where deviations from ideal behavior of binary solutions led to the phenomenon of phase separation. We now introduce several generalizations on the basis of qualitative sketches introduced below. 

The principle of distillation is the use of differences in volatiHties of the components to be separated. Distillation processes are usually carried out in countercurrent mode in multistage units. The differences that can be obtained in concentrations of the components in the vapor and liquid phases are determined by the vapor-liquid equihbrium (VLE). Until the 1970s reliable data for vapor-liquid equilibria could only be obtained by measurement, which, for a mixture containing more than two components, required a large number of time-consuming measurements. Advances in chemical thermodynamics have resulted in methods activity coefficient models (g models or equations of state) for the calculation of the phase-equihbrium behavior of multicomponent mixtures on the basis of binary subsystems. In the case that no information about the binary subsystems is available, predictive methods (group contribution methods) are available to allow estimation of the required phase equilibria. 

This chapter will review the relationships among synthetic detail, morphology, and resulting mechanical behavior. While effects on the glass-rubber transition and modulus will be emphasized, aspects of toughness and impact resistance will be touched upon and applications discussed. In order to generalize this critique, polymer I is defined as the first synthesized polymer, and polymer II as the second synthesized polymer. Even when the order of synthesis is immaterial, as in mechanical blends, this notation will prove useful. Since the basis of this chapter lies in the two-phased nature of these materials, it is appropriate to examine first the fundamental reasons underlying phase separation. 

Sherma et al. (1983) reported that their amino acid separations on C g reversed-phase thin layers were similar to those previously aehieved on conventional silica gel and cellulose by Sleckman and Sherma (1982). Sherma et al. (1983) concluded that it may be difficult to predict the relative separation behavior of numerous compounds including amino acids on the basis of chemical notions of reversed - or normal -phase chromatography. 

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