RHEOOPTICAL RAMAN-STUDY OF CHAIN DEFORMATION IN UNIAXIALLY STRETCHED BULK POLYETHYLENE

A rheo-optical study has been developed to examine the nature of the s tructural changes, other than the well known chain orientation, that c an appear during uniaxial drawing of polyethylene samples. Stretching was performed at room temperature in samples of melt-crystallized high - and low-density polyethylene. Raman spectra were simultaneously coll ected along with stress and strain data in order to gain insight into the molecular basis of the properties resulting from the mechanical pr ocess. The need for real-time studies was to detect possible transient occurrences that cannot be observed with non-rheo-optical studies. Th ese facts originate from the rapid relaxation phenomena of this polyme r. By these means, a band shift and asymmetric band broadening were ob served on the C-C asymmetric stretching Raman mode (1064 cm(-1)), whic h has been attributed to some degree of backbone chain deformation. Th e band shift and, accordingly, the rate of bond deformation did not sh ow a linear stress dependence. There is no substantial band shift duri ng the first moments of the stretching. That initial zone was followed by a stage, during formation of the transition fronts, where the band shift begins to be appreciable. Finally, a third stage was observed w hen the observation zone falls into the neck. In that stage, a linear dependence was found between the band shift and the applied load. Qual itatively, the observed trend can be thought of as the consequence of the initial isotropic nature of the stretched sample. For that kind of sample, the transference of the load on the polymer backbone is not e ffective until the sample shows a high degree of orientation in the st retching direction. The band shifts were reversible as the load was re moved.