The stiffness modulus of polyethylene as a function of temperature and structure



The stiffness moduli of polyethylenes covering a wide range of degrees of branching and molecular weights have been investigated from −150°C. up to the melting points. Qualitatively, the stiffness temperature curves can be broken into three regions. The first of these is the very low temperature region which extends from −150 up to about −50°C. In this temperature region, all of the polymers have the same stiffness at a given temperature, although they do not have the same density. The second region extends from about −50° up to the melting region. In this region, it is generally true that the stiffness is a function of the density alone. This is the case even though the temperatures at which polymers have equal densities are different. There are exceptions, however. It has been found that extremely high molecular weight polymers have higher stiffness than would be expected from their densities. Presumably, this arises directly from the very high molecular weight. It has further been found that highly crystalline polyethylenes increase in density upon annealing, but show little increase in stiffness or degree of crystallinity as measured by x-rays. This may well arise from the removal of small voids or flaws on i Annealing, the removal of which would increase density but not affect stiffness or crystallinity. The third region of the stiffness-temperature curves is the melting region. As expected, the stiffness curves reflect the higher melting points of the more linear resins. The stiffness curves also qualitatively indicate the randomness of branching. Polymers which are nonrandomly branched suffer a more gradual decrease of stiffness with temperature, reflecting a broader melting range than the more randomlybranched polymers. This effect probably arises from a broader distribution of crystallite sizes in the nonrandomly branched polymers. Unlike any other crystalline polymer which has been examined, the stiffness-temperature curves for polyethylene do not exhibit evidence of a sharp glass transition temperature. Rather, polethylene appears to undergo a gradual “thawing” from about −50°C. up to within a few degrees of the melting point.