Study of the morphology and its effects on the mechanical properties of linear low density polyethylene

Abstract

A linear low density polyethylene, with 1-octene as the co-unit, forms a semicrystalline matrix when cooled from the melt. This matrix is composed of a spherulitic superstructure with a lamellar microstructure. The careful study of characterized fractions allowed the quantitative description of the melting profile of the whole copolymer, which corresponds to the crystallite size distribution. This description was achieved by the calculation of the most probable unbranched ethylene sequence, and whereby it was assumed that the lamellar thickness is directly linked to this sequence. This lamellar thickness had to be adjusted by the number average molecular weight to achieve the final distribution. Another method extrapolated the effect of branching on this sequence from a region of low branching frequency to a region of high frequency. This lamellar thickness also had to be adjusted by the number average molecular weight to achieve the final distribution. The crystallite size distribution of the fractions and the whole copolymer was changed by annealing procedures. The rate of change was accelerated by the use of cool-raise and cool-raise-cool procedures. The rate and extent of change is governed by the branch and molecular weight distribution. The total crystallinity was rather invariant for this copolymer. This was explained by segregation of low-molecular weight fractions. At very low strain, the stress relaxation was found to be dependent on the total crystallinity, but independent of the crystallite size distribution. Frequency-temperature studies indicated that the modulus may be predicted, at least up to the melting point, by the extrapolation of low temperature data and by taking into account changes in the degree of crystallinity. However, the imaginary modulus is affected more than the real modulus by the distribution change.