Thermomechanical and diffusive studies in films prepared from copolymers of ethylene-1-octene subject to longitudinal and transversal induced stretching

The effects of induced stretching on the relaxation behaviour and on gas transport properties in coextruded linear low-density polyethylene (LLDPE) films, prepared from copolymers of ethylene-1-octene, are reported. The spectra show that the stretching, regardless of the direction of its application, does not produce significant changes in the gamma and beta relaxation processes. However, in the alpha region, there are some differences in the relaxation response depending on the direction of the induced stretching. The longitudinal stretching gives rise to a displacement of the temperature at which the alpha '' process occurs in LLDPE2 film with respect to LLDPE1 film. On the other hand, the transverse stretching does not produce a displacement of the alpha '' relaxation, but it does produce a decrease of intensity of the alpha' relaxation peak in both films. Transport coefficients have been measured for CO2 and O-2 in stretched polyethylene samples over temperatures ranging from 25 to 85 degrees C. In all cases, a linear plot of the logarithm of the gas transport coefficients as a function of the reciprocal absolute temperature is observed, indicating that activated diffusion is occurring. Similarly, a break in these Arrhenius plots can be noted at nearly the same temperature range as reported in the thermomechanical study. The activation energies associated with the transport mechanism are reported. On the other hand, the results reveal that the films are dependent on thermal history. In order to obtain reproducible results, a suitable control of the thermal conditioning of the films is advisable. (C) 1997 Elsevier Science Ltd.