Solution of Gases in Polyethylene Terephthalate

Abstract

The solubilities of helium, nitrogen, oxygen, argon, methane, carbon dioxide, and ethane in glassy amorphous and crystalline polyethylene terephthalate have been studied by time‐lag and/or static sorption methods. Solubilities of all the gases but ethane were also determined in the rubbery crystalline polymer. The only deviations from Henry's law were displayed by ethane at 25°C and carbon dioxide at 25° and 40° C in the glassy polymer. Correlation of solubility constants for oxygen and nitrogen obtained by the static sorption method with values obtained dynamically indicate that the glassy crystalline polymer is an isotropic diffusion medium. Sorption in the glassy amorphous and crystalline polymers generally takes place by two processes operating concurrently: ordinary dissolution plus ``hole‐filling.'' The ``hole‐filling'' process obeys a Langmuir expression and dissolution obeys Henry's law for sorption of carbon dioxide in the amorphous polymer at 25°C. Crystallinity reduces gas solubility in the glassy polymer, but not generally in direct proportion to the decrease in amorphous volume. An exception is helium, which appears to obey the simple law, k=k*α. The glassy state of the partially crystalline polymer extends to about 80–85°C where a transition region is noted on a van't Hoff plot for solubility constants. The other extremity of this region (about 95°C) marks the onset of rubber‐like behavior in the polymer. Heats of sorption in the glassy crystalline polymer are more exothermic than in the rubbery crystalline polymer, characteristic of the transition from sorption by dissolution and void‐filling processes to dissolution alone. The heat of solution of helium is slightly positive in both the glassy and rubbery states of the crystalline polymer, with no evidence of a break at T_g. It appears that the major contribution to helium sorption in either state of the crystalline polymer is ordinary dissolution. In both the glassy and rubbery states, carbon dioxide shows evidence of a polar interaction with the polymer. Solubility constants are correlated with gas force constants ε/k̄ in both the glassy amorphous and rubbery crystalline polymers.