THERMAL EXPANSIVITY OF HIGHLY-STRETCHED L INEAR POLYETHYLENE WITH EXTENDED CHAINS IRRADIATED WITH DIFFERENT DOSES OF GAMMA-RAYS

Temperature behavior of crystal lattice parameters of highly-stretched samples of linear polyethylene with extended chains irradiated with d ifferent doses of gamma-rays was studied. It was found that transverse vibrations of macromolecular chains are excited at irradiation doses D greater-than-or-equal-to 500 Mrad and temperatures close to the melt ing temperature of the crystallites. These vibrations cause a sharp in crease in the lattice parameter a. As a result, no phase transition to the hexagonal packing occurs although the condition a : b = square-ro ot 3, which is necessary for the existence of this phase, is fulfilled . It was shown that the thermal expansivity of the lattice parameter c changes its sign at high irradiation doses. These results are explain ed by the presence in the crystallites of samples irradiated with larg e doses of a considerable number of intermolecular chemical bonds.