WAXS and SAXS investigation of the supermolecular structure of LDPE films irradiated with fast electrons

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Abstract
Low-density polyethylene (LDPE) films irradiated with fast electrons within the range of irradiation doses 0-40 Mrad have been studied by means of WAXS and SAXS. The degree of crystallinity, the total intensity of WAXS, and the dimension of the mosaic blocks in the [110] and [200] directions have been found to be almost independent of the irradiation dose. The SAXS parameters (long period, lamella thickness, intensity of scattering from the diffuse and the discrete regions, total intensity) depend on the dose in the same way. A parallel has been made between the x-ray and the calorimetric degree of crystallinity. The latter shows a threefold greater decrease with the increase of the dose, in comparison with the former. The average dimensions of the scattering particles, as well as the difference between the crystalline and the amorphous density (ρc-ρa), do not change with the increase of the irradiation dose. The data obtained lead us to the conclusion that the irradiation of LDPE films with fast electrons at low doses results in the formation of a nonhomogenous network. The points of crosslinking are localized within the amorphous phase and the surface nonarranged regions of the crystallites. It has been shown that the partial crosslinking of the crystalline phase does not significantly affect the polymer's crystallinity, but does affect the thermodynamic heat of melting. The lower enthalpy of melting of the crosslinked films, whose degree of crystallinity stays almost constant, has been explained by the increase of the surface free energy of the crystallites as a result of crosslinking within the noncrystalline surface regions of the lamellae.

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