MECHANICAL RELAXATIONS AND DIFFUSIVE CHANGES IN LINEAR LOW-DENSITY POLYETHYLENE (LLDPE) FILMS SUBJECT TO INDUCED STRETCHING

An analysis of the diffusion of oxygen and carbon dioxide through line ar low density polyethylene (LLDPE) subjected to longitudinal and tran sversal induced stretching in the ratio 2:1, is presented in this pape r. The relaxation behavior of two coextruded LLDPE films prepared from copolymers of ethylene-1-octene is reported as well. The spectra, exp ressed in terms of loss tan delta, present a gamma relaxation shifted 5 degrees C to 10 degrees C in the LLDPE1 with respect to the LLDPE2 w hen the stretching was longitudinal. This relaxation is lower in inten sity than the one exhibited for conventional low-density polyethylene of the same crystallinity. Increasing the temperature order, a beta re laxation process appears as an ostensible shoulder of the first of two relaxation processes, called alpha' and alpha '', detected in the alp ha region. The beta relaxation, which is believed to be produced by mo tion taking place in the amorphous and interfacial regions, appears as two overlapping peaks centered at -36 degrees C and -30 degrees C for a longitudinal stretching and at -34 degrees C and -28 degrees C for a transversal stretching at 1 Hz. In relation to this fact, we observe d a slight increase in the T-alpha of the LLDPE2 with respect to the L LDPE1, which is greater in the transversally stretched polymers than i n the longitudinal ones. The values of the activation energy correspon ding to the second peak of the beta relaxation were obtained for the f ilms subjected to stretching in longitudinal and transversal direction s to the processing orientation. The study of the diffusional characte ristics of oxygen and carbon dioxide through the films shows the tempe rature is related to the region for which the alpha processes are give n. An anomalous behavior of the diffusion coefficient with the tempera ture is observed suggesting general movements around the amorphous seg ments and crystalline entities. The increase with the temperature of b oth parameters (diffusion and permeability) can be attributed to a cha nge in the gas solubility. This increase is greater for CO2 than for O -2, which we interpret as a plastificant effect of the CO2. Finally, t he activation energies from diffusion coefficient and permeability are analyzed in terms of Arrhenius. The results show that the temperature dependence of the diffusive parameters may not be a simply activated process as a consequence of the fact that the diffusional characterist ics of the films depend on their morphology which, in turn, is changin g with temperature. Little changes are observed when the films are sub jected to any kind of stretching. In this sense, we think that the ori entation by tensile drawing will decrease the conformational entropy i nvolved in melting processes and, as a consequence, will reduce both t he permeability and the apparent diffusion coefficients. (C) 1996 John Wiley & Sons, Inc.