PERMEATION OF GASES THROUGH POLYESTER FILMS CRYSTALLIZED BY HEAT TREATMENT

Abstract

Recently the measurement of permeability of gases through polymer film is utilized as a method to obtain the information about the internal fine structure of polymer solid. In this paper, the permeability coefficients of He, O2 and N2 for several semicrystalline polyester films, crystallized by heat treatment, were determined by an apparatus based on the low vacuum method.From the data of permeability measurements, it was found that the permeability coefficient was decreased with the increase in the polymer crystallinity.To obtain the information about the fine structure of amorphous region of the polymer solid, the permeability coefficients of gases for amorphous region of semicrystalline polymer, Pa, were calculated from the permeability data, assuming a structural model of semicrystalline polymer solid. The value of Pa was determined by Pa=P•k/αa, where k is the geometric factor and αa is the volume fraction of amorphous region. Pa value obtained were identical at each temperature for almost all films determined with small gas molecule such as He, but there were also systematic variations with the crystallinity of polymer films for larger gas molecules such as N2 and O2 above Tg of polymer. The correlation of Pa with crystallinity could be explained by the concept of segmental mobility and the number of possible conformational forms of chain molecule existing in amorphous region of polymer, using a suitable assumption to simplify the complicated problems about the chain conformation.To consider the relation between the permeability coefficient in amorphous region and the dynamic modulus of the polymer at each temperature, which was regarded as a measure of flexibility and mobility of polymer chain segments, log Pa were plotted against log E′ The log Pa vs. log E′ curves were identical for individual gas molecule for each polymer films independent of crystallinity.From these results, it was concluded that the permeability coefficient was strongly related to the internal fine structure of polymer solid, especially to the segmental mobility of polymer chain molecule and to the number of possible conformations of chain molecule existed in the amorphous region of polymer.