Gas sorption and transport in substituted polycarbonates

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Abstract
The effects of molecular structure manipulation of polycarbonates on sorption and transport of various gases were studied using tetramethyl, tetrachloro, and tetrabromo substitutions onto the aromatic rings of bisphenol A polycarbonate. Solubility and permeability measurements were made at 35°C over the pressure range of 1–20 atm for a variety of gases, namely CO2, CH4, O2, N2, and He. A threefold to fourfold increase in permeability was caused by the tetramethyl substitution, whereas the tetrachloro and tetrabromo substitutions reduced the permeability relative to the tetramethyl substitution. Lower activation energies for transport were found for the tetramethyl polycarbonate relative to the unsubstituted polycarbonate. Permeability coefficients were factored into solubility and diffusion coefficients. Sorption levels increased for all substitutions, but among the substituted polymers the levels remain practically the same. Solubility data were analyzed in terms of the dual sorption model. The Henry's law solubility coefficients obtained from this analysis were found to be consistent with a predictive equation developed for rubbery polymers. The usual correlation for predicting the Langmuir sorption capacity of the model overestimates the values for the substituted polycarbonates, and a proposal for the cause of this is offered. Thermal expansion of these polymers was measured using dilatometry, and the results are used in the interpretation of the sorption data. Diffusion phenomena are explained by segmental mobility and free volume considerations. The effects of CO2 exposure history on sorption and transport were also investigated.

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