Correlation between the transport behavior of polyimides and the conformational rigidity of their chains

For a number of polyimides, Kuhn segment lengths have been calculated by the Monte Carlo method under the free-rotation approximation and the energies of cohesion have been estimated by the group-contribution method. It has been shown that, in the case of isomeric polyimides characterized by close energies of interchain interactions, a decrease in the rigidity of chains always leads to reductions in the free volume and gas permeability. For polyimides of allied structures, the simultaneous (and frequently oppositely directed) effects of chain rigidity and the energy of interchain interactions ambiguously influence gas permeability. Both factors should be taken into account during estimation of the influence of chain rigidity on the transport characteristics of polymers. Small-scale effects of packing and mobility of chain fragments exert the decisive effect on the transport characteristics of polyimides possessing a decreased mobility of chains.