Effect of Supercritical CO2 Treatment on Mechanical and Gas Transport Characteristics of Polyimides Based on Diethyl Toluene Diamine Isomers

Abstract

It has been shown that supercritical CO₂ (sc-CO₂) treatment leads to a significant increase in free volume and gas permeability and a certain decrease in selectivity for films of amorphous polyimides based on a mixture of diethyl toluene diamine isomers and 6FDA, BPDA and BPADA dianhydrides. At the same time, higher permeability coefficients are attributed to the growth of both diffusion coefficients and solubility. The sc-CO₂ treatment leads to a certain increase in the selectivity of diffusion for PIs based on 6FDA and BPADA dianhydrides, which is interpreted as ordering of the chains packing in the polymer matrix, and the selectivity of diffusion decreases for PI based on BPDA dianhydride. Mechanical tests of PI films demonstrate an increase in the modulus of elasticity after the sc-CO₂ treatment, which indicates the ordering of the chains packing. However, an increase in the free volume due to the sc-CO₂ treatment leads to an increase in the brittleness of the films for PIs based on 6FDA and BPADA dianhydrides. Such drastic changes in mechanical characteristics are not observed for PIs based on BPDA dianhydride. In general, the analysis of mechanical characteristics allows one to supplement the analysis of gas separation data and confirm the connection between ordering of chains packing and gas separation characteristics of polymers. Comparison of data on the treatment of sc-CO₂ and uniform biaxial deformation shows that the swelling of polymer films in sc-CO₂ is equivalent to three-axial uniform stretching complicated by significant defect formation.