Thermodynamic Aspects of Polymer Compatibility

Abstract

Polymer compatibility, to be understood here in the sense of thermodynamic stability, depends in a subtle way on molecular parameters. Some of these are traceable with the aid of model calculations based upon Flory and Huggins’ expression for the free enthalpy of mixing. It appears that, in oligomeric mixtures in particular, the average chain lengths and the distributions of chain lengths in the two constituents markedly affect the location and shape of the miscibility gap. The entropy of mixing tends to shift the gap towards the composition region in which the constituent with the smaller average chain length dominates. The interaction parameter, if depending on concentration, can outweigh this effect. Examples, taken from literature and our own data, indicate that this situation does occur in practice. Hence, the lattice-based free enthalpy expression is an accurate tool for describing actual phase relations; however, it cannot quantitatively predict their temperature, chain length, and concentration dependence. In the last respect Florys equation of state theory appears to be very superior. This was recently shown by McMaster, who analysed Flory’s new free enthalpy function and found that small differences inter al. in the thermal expansion coefficients of two compatible polymers will cause the system to become incompatible upon a rise in temperature. The latter phenomenon is known to occur; examples are given. Compatibility being a subtle phenomenon, the question seems to be justified whether two samples of the same polymer of widely differing chain length might be immiscible. For linear polyethylene and anionic polystyrene the available data point to chain length compatibility. Two methods for the determination of compatibility are discussed, viz. the mutual solvent method and a light-scattering technique. © 1974, Walter de Gruyter. All rights reserved.