Relations between thermomechanical properties and chain folding of polyethylene terephthalate fibers

Abstract

The molecular mechanism of the nonlinear relationship between the transition temperatures (T g resp.Tinm) and the outer tensile stress is discussed. On the basis of thermomechanical curves (deformation-temperature) taken at different external tensile force on drawn annealed PET fibers and films it has been shown that the transition temperatures depend nonlinearly on the applied tension stress. The maxima observed for bothT g andT m confirm the theoretical results ofCiferri andSmith andFrenkel assuming a crystallization of oriented polymer with chains in folded or helical conformation. A reasonable explanation is proposed for the increase ofT m followed by decrease and again increase with progressively rising of the applied tension stress using the model ofBonart-Hosemann for structure of semicrystalline polymers: at low tension values an orientation of macromolecules in noncrystalline zones takes place followed by defolding of chains from crystallites and finally (at highest tension values) an extreme stretching with additional orientation proceeds. This mechanism is supported by infra-red measurements. The thermomechanical data plotted as external tension divided by the corresp. melting temperature versus deformation confirm the theoretical curve derived byFlory. The experimental curves demonstrate that (1) the crystallization under strain with negative elongation as well as (2) the regeneration of the amorphous phase and its additional stretching are physically realisable situations when the crystallization is accompanied by chain folding or building of helices. It is shown that the thermomechanical method could be used as a simple tool for investigating the chain folding problem. The data reported are an additional proof of the existence of regular folded chains in the crystalline PET too.