EFFECT OF THE STRUCTURAL DISTRIBUTION ON THE MECHANICAL-PROPERTIES OF LINEAR LOW-DENSITY POLYETHYLENES

Stress strain curves and tensile impact strength were measured for fractionated specimens of the molecular weight range of 1 x 10(4)-4 x 10(5) (M(w)BAR/M(n)BAR = 1.1-1.3) for various kinds of linear low-density polyethylene (LLDPE). The fractions of molecular weight less than 2 x 10(4) were extremely brittle and elongation was not possible. Elongation with stress increase was observed for the fractions of molecular weight above 5 x 10(4) for ethylene/butene-1 type LLDPEs (EB) and for the fractions of molecular weight above 3 x 10(4) for ethylene, 4-methyl pentene-1 (EMP) and ethylene/octene-1 (EO) type LLDPEs. Tensile impact strength (sigma(TI)) increased with increasing molecular weight, reached a maximum at the molecular weight range of about 1-2 x 10(5) and then decreased slightly. Molecular weight range above which the impact strength could be observed was 4 x 10(4) for EB, 2 x 10(4) for EMP and EO, and 8 x 10(4) for the fractions of ethylene homopolymer, respectively. Comparing the results among fractions of the same molecular weight, it was found that sigma(TI) increased with narrowing the intermolecular distribution of chemical composition. Further to say, sigma(TI) was remarkably dependent on the distributions of the crystallinity and the lamella thickness of the fraction; i.e., the narrower the structural distributions on the crystalline level, the higher the sigma(TI). These results could be explained by the concentration of tie molecules and the homogeniety of tie molecular orientation which would be strongly correlated with the distributions of the crystalline structure.