The thermochemistry of polymer solutions and compositions in the phase separation region

Abstract

The heats of mixing of oligomeric polyoxypropylenediol (POPD) with water and of the following polymers: cellulose nitrate (NC) with polyvinylacetate (PVA), NC with cellulose acetate (CA), CA with PVA and polyvinylchloride (PVC) with polymethyl-methacrylate (PMMA) have been obtained in the region of phase separation. For the polymer compositions the heats of mixing were calculated according to Hess's law, fow which the heats of solution of the polymers and polymer mixtures in a common solvent were determined. The POPD-water system, which has a lower critical temperature of mixing (LCTM) is exothermic over the entire range of compositions and temperatures. At T<LCTM, i.e. in the region of homogeneous solution, the concentration dependence of the enthalpy of mixing is expressed by smooth curves, but in the phase separation region ( T>LCTM) the curves contain a linear section. The curves of the concentration dependence of the enthalpy of mixing of NC with PVA are uninterrupted and the system is exothermic. The enthalpy of mixing-composition curves of the other polymer compositions contain a linear section, indicating phase separation. Analysis of the curves showed that the temperature coefficients of solubility of each component in the two layers have the same sign, which is possible only if there is one sided diffusion of one of the components. From the evidence presented it follows that PVC diffuses into PMMA, CA into NC and CA into PVA. This is accompanied by loosening up of one part of the transitional layer (in which diffusion takes place), with an endothermic effect, and compacting of the other part of the transitional layer, with an exothermic effect. This is in good agreement with the ideas developed by Voyutskii and Kuleznev.