Mixing Polymer Pairs

Most polymer pairs are thermodynamically incompatible, in the sense that their free energy of mixing is positive. This does not mean that there is absolutely no interdiffusion at all at the interface between them adjacent to the interface limited interdiffusion occurs, which can be seen as an increasing of the low surface entropy implied by a smooth surface [30-33]. This nanoscale roughening of an interface can increase the adhesion between the polymers. 

Flory-Huggins Approach. One explanation of blend behavior lies in the thermodynamics of the preceding section, where instead of a polymer-solvent mixture, we now have a polymer-polymer mixture. In these instances, the heat of mixing for polymer pairs (labeled 1 and 2) tends to be endothermic and can be approximated using the solubility parameter. The interaction parameter for a polymer-polymer mixture, Xi2, can be approximated by... 

Without doubt the simple mixing of two or more polymers is the easiest, most direct and the cheapest way of producing new materials. However, the properties of such mixtures are very dependent on the miscibility of the components. Of the few polymer pairs which are thermodynamically miscible, some examples are ... 

Table 1 Scale of homogeneous mixing estimated for representative blends of cellulose/synthetic polymer pairs, quoted from a previous review [42]... 

By rough count, more than 50 miscible systems have been reported in the literature, and from intensive studies of these systems a more optimistic view of the potential for miscibility is beginning to develop. Most of these miscible polymer pairs have chemical structures that are capable of forming strong specific interactions such as donor-acceptor complexes and hydrogen bonds. While they have not all been completely studied, those that have show behavior which can only result from the presence of exothermic or negative heats of mixing. 

It is possible to simulate the spinodal curves of the phase diagram of polymer pairs using the Equation-of-state theory developed by Flory and co-workers. It is only, however, possible to do this using the adjustable non-combinatorial entropy parameter, Qjj. Another problem arises in the choice of a value for the interaction parameter Xjj. This is introduced into the theory as a temperature independent constant whereas we know that in many cases the heat of mixing, and hence is strongly temperature dependent. The problem arises because Xj was never intended to describe the interaction between two polymers which are dominated by a temperature dependent specific interaction. 

Olabisi (79) evaluated the miscibility of polymer pairs by gas chromatr raphy with mixed poly(e-caprolactone)-poly(vinyl chloride) stationa phases. Some interaction parameters for the pure and mixed stationary phases are given in Table 10. 

This chapter provides a broad overview of the subjects of polymer blends and lonomers. Specific topics concerning polymer blends Include the thermodynamics of mixing of polymer-polymer pairs, polymer Interfaces, rheology, and mechanical properties. For lonomers, the chemistry, structure, rheology and solution properties are discussed. 

According to the second law of thermodynamics, polymer pairs will be miscible if the Gibbs free energy of mixing AG is negative. The AG may be described by the following three contributions [9-11] ... 

Among the experimental findings were the inhibiting effect of the polymer, the yield restriction in the course of the reaction, and the decelerating effect of salts dissociating during dissolution. These results can be explained by the displacement of equilibrium toward the formation of mixed ion pairs. 

Although it is generally difficult to find polymer pairs that mix homogeneously, PHOST partially protected with THP or tert-butoxycarbonylmethyl group has been reported to be miscible with a novolac resin or PHOST and to function as a dissolution inhibitor [124,132]. Another acid-labile polymeric dissolution inhibitor is a terpolymer of tert-butyl methacrylate (TBMA), methyl methacrylate (MMA), and methacrylic acid (MAA), which is miscible with a novolac resin [179] while PTBMA is not [159]. This methacrylic terpolymer is only marginally miscible with PHOST but more compatible with C- and... 

Brief reviews covering redistribution reactions in polyester and in polycarbonate binary blends have been prepared by Porter et al. [1989] and Porter and Wang [1992]. Selected references for redistribution processes in PEST/PEST blends are listed in Table 5.7. Early studies of these processes focused on measuring the extent of redistribution under specific processing conditions rather than on producing compatibilized polymer blends with an attractive balance of properties. A number of more recent studies have reported the limits of miscibility for certain melt-mixed polyester pairs in the absence of transesterification — see for example the NMR study of PC/PET blends [Abis et al., 1994]. 

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