Sorption and transport of pure gases in random styrene/methyl methacrylate copolymers

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Abstract
Previous work has shown that permeability coefficients for CO2 can increase with pressure for poly(methyl methacrylate) (PMMA); whereas, those for polystyrene decrease slightly as found for many glassy polymers. This response is attributed to a greater propensity for PMMA to be plasticized by CO2. This issue is considered in detail here by examining the behavior of a series of random styrene/methyl methacrylate copolymers in order to learn how the plasticization response varies with MMA content. At low pressure, the sorption and transport of CO2 and other gases in these copolymers depend on copolymer composition in ways expected from simple theories for multicomponent polymers. The change in CO2 permeability coefficient upon pressurization from 1 to 20 atm ranged from –6% for polystyrene to +57% for poly (methyl methacrylate). Furthermore, upon holding at 20 atm of CO2 driving pressure, there was a significant increase in the CO2 permeability coefficient with time for PMMA; whereas this conditioning effect was much smaller for polystyrene. Conditioning and plasticization effects seem to be related to the same molecular causes. The responses change progressively for the copolymers but not directly in proportion to MMA content. The greater effects of CO2 for PMMA are, to a significant extent, but not entirely, due to its much higher level of CO2 sorption compared with that of polystyrene. The results are discussed in terms of relevant theories.

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