Polymer-solvent interactions diffusion in constrained elastomers

Abstract

Prolonged liquid absorption by the constrained elastomer sample results in an equilibrium pressure, P∞, and an equilibrium amount of liquid, V∞, in the sample. P∞ is a measure of the strength of polymer solvent interactions. P∞ as a function of the cohesive energy density of solvent did not give a good correlation for various solvents and natural rubber. No distinct correlation was found in comparing equilibrium volumes from free-swelling data to P∞ values in the corresponding solvent-natural rubber systems.

The linear relationship found between reduced pressure and reduced absorption over a large portion of the data indicates a distinct relationship between stress (pressure) generation and liquid absorption during the time where the polymer can be considered to be a semi-infinite medium. The magnitudes of equilibrium p~essure and equilibrium concentration of solvent for various systems may be significantly different, but the same type of a stress-diffusion mechanism seems to operate over a considerable range of solvent-natural rubber systems.

It has been found very useful to convert the results into terms of reduced variables; i.e., pressure generation as a function of time is normalized by dividing by P∞, and liquid absorption by the sample is normalized by dividing by V∞ For good and intermediate strength solvents (P∞ = 150-266 atms.), much of the observed data for natural rubber systems could be duplicated by the empirical equations: P(t)/P∞ = A+ B log (time) V(t)/V∞ = C + D log (time)

Molar volume of the solvent was found to give a better correlation with P∞ than the cohesive energy density. In the constant volume system large stresses are generated and markedly increase the importance of chain length and pendant side groups on the diffusion of solvents into the elastomer.

An apparatus has been developed to study polymer-liquid interactions and diffusion in elastomers where the polymer is constrained to constant volume. The present study is an attempt to investigate the interactions of various liquids (solvents) with lightly crosslinked elastomers without the conformational entropic changes which accompany unrestrained swelling of a polymer. The measured variables in the constant volume apparatus were liquid absorption and pressure generation as functions of time. Most of the data were taken for natural rubber and various polar and nonpolar solvents at 25°C and a solvent hydraulic head of 100 psia. Some information on cis-polybutadiene and three styrene-butadiene random copolymers is also included.