Mutual Solubility of Polymers and Properties of Mixtures

Abstract

Abstract If a homogeneous mixture of two polymers is to be obtained, both components must exhibit fluidity. It is, therefore, necessary first of all to analyze the process of mixing of two amorphous linear polymers capable of developing true irreversible deformation. However, though the mixing of two such polymers, from the thermodynamic point of view, constitutes the mutual solution of two liquid phases, it does not, in fact, take place spontaneously, since the exceptionally high viscosity of polymers1 results in a negligible rate of diffusion of the long-chain molecules, even at elevated temperatures. For this reason, special procedures (mixed milling, evaporation of solvent from mixed solutions, etc.) are used to obtain polymer mixtures which exhibit no heterogeneity, even when examined by means of a good optical microscope. It is, however, quite evident that, should two polymers be mutually insoluble, the spontaneous separation of such artificially prepared mixtures into macroscopically distinct phases must proceed at infinitesimal rates as a result of the enormous viscosity of the system. In such cases, the flexibility of the chain, which permits the relatively rapid translation of individual segments of the chain, is an important factor. This property of chain molecules will result in a process by which the mixture of mutually insoluble polymers becomes resolved in extremely small volume elements, i.e., to the formation of two types of regions whose composition will accord with the thermodynamic conditions of phase separations. Such mixtures of two amorphous polymers structurally closely resemble crystalline polymers, which are also microheterogeneous. In both cases, the dimensions of the regions differing in phase character formed by the segregation of segments are considerably smaller than the length of the chain molecules. For this reason, the macroscopic miscibility of two polymers is by no means equivalent to their microscopic miscibility, i.e., to true mutual solubility of the polymers.