Sorptive properties of glasslike polymers

Abstract

AbstractThe swelling and solution of elastic polymers, like the mixing of liquids, results in the formation of equilibrium systems. In the interaction of conventional glasses and liquids, however, quite different phenomena appear and cause nonequilibrium systems to form. The density of molecules in conventional glasses is similar t o that of tightly packed liquids, but the diffusion rates are very low. Such glasses sorb practically nothing at low vapor pressures. Sorption begins only when the vapor pressure and the concentration of sorbed liquid in the surface layer are sufficient to cause the glass to melt. It should be noted that in this case sorption shows a sharp rise and an equilibrium solution forms. The degree of tight packing of polymeric glasses varies considerably, decreasing with increasing molecular weight and rigidity of the chain molecules. This packing can be estimated by comparing the densities of the polymers with the volume of the molecular groups of which these polymers consist, the latter being obtained from x‐ray data, or by comparing the density of the monomer with that of the polymer. Cellulose, which unlike tightly packed glucose sorbs water over the whole range of vapor pressures, is a typical example of a loosely packed polymer. The effect of the density of packing on the sorbing properties of glasses can be more clearly demonstrated by the sorption of ethylbenzene by polystyrenes of different molecular weights. Low molecular weight polystyrenes dissolve in ethylbenzene without any heat production and sorb ethylbenzene from the vapor at high pressures only. This process results in the formation of a solution. High molecular weight polystyrenes, however, dissolve in ethylbenzene, producing heat, and sorb ethylbenzene a t all vapor pressures. The loose packing of polymers is the result of the long relaxation times existent in glasses. When the deformation time of the polymer is less than the volume relaxation time, the polymer becomes porous. Thus both the sorbing properties and the heats of solution of polystyrene become more pronounced during orientation. The degree of tight packing again increases at heats close to the glass point. Thus polymeric glasses hold an intermediate position between liquids and hard porous sorbents, such as silica gel, in regard to their sorbing properties. Unlike hard sorbents, however, they are apt to change their structure not only under external influence but also in the process of sorption.