STUDIES ON THE TEMPERATURE-DEPENDENCE OF THE ELASTIC-MODULUS OF CRYSTALLINE REGIONS OF POLYMERS .12. TEMPERATURE-DEPENDENCE OF THE ELASTIC-MODULUS OF CRYSTALLINE REGIONS OF POLYETHYLENE WITH DIFFERENT MICROSTRUCTURES - EXPLANATION WITH THE KINKED-CHAIN MODEL

Temperature dependence of the elastic modulus E1 of crystalline regions in the direction parallel to the chain axis has been measured by x-ray diffraction for various kinds of polyethylene (PE) with different microstructures. The E1 value is 235 GPa for all kinds of PE at room temperature. However, in some cases, E1 began to decrease at a certain temperature, in one case at 50-degrees-C and in one at 65-degrees-C. The lattice spacing for the (002) plane also decreased with increasing temperature. The thermal expansion coefficient alpha(c) changed discontinuously around the temperature range where E1 began to decrease. This indicates that axial thermal molecular motion (i.e., incoherent thermal motion) is enhanced at high temperature. These phenomena could well be explained with the kinked- chain model where two gauche conformations are incorporated in the all-trans, fully extended conformation. The kinked chain deforms very easily through the change of internal rotation around the C-C bond, which has a small force constant. So it is considered that the decrement of E1 at high temperature is due to the incorporation of a small amount of contracted portions in the chain molecules. On the other hand, in some cases, the E1 value was constant at 235 GPa up to 110-degrees-C. In these cases, alpha(c) did not change with increasing temperature. Consequently, whether the molecular motion which brings the decrement of E1 occurs or not is the origin of the difference of the temperature dependence of E1 of PE. The E1 value of PE obtained at -155-degrees-C is as high as 254 GPa.