| Abstract: | The structure and thermal behavior of freeze-dried gels of radiation-crosslinked high molecular weight poly(ethylene oxide) (PEO) were investigated by optical and electron microscopy, wide-angle x-ray scattering (WAXS), DTA, TGA, and thermomechanical analysis. The gels are highly porous with thin crystalline walls. Small spherulite and hedrite structures are observed on the walls. A model for gel formation in solution is suggested. A statistically homogenous chemical network is formed as a result of intrachain and interchain crosslinking. Simultaneous grafting of macromolecular fragments formed by chain scission also occurs. On increasing the irradiation dose from 1 to 15 Mrad, the degree of crystallinity determined by x-ray diffraction and the total intensity of diffraction gradually decrease. The temperature and enthalpy of melting diminish steeply up to 5 Mrad, fall only slightly from 5 to 8 Mrad, and do not change from 8 to 15 Mrad. By comparing the x-ray and DTA crystallinity values, this is shown to be due not only to reduced crystallinity at higher network density but also to Tree energy changes of entropic origin in crystalline and amorphous regions. Radiation chemical yields, G(-units), for these dose ranges are 100, 38, and 0, respectively. Thermomechanical analysis was used to determine the elastic modulus of compression as a function of the dose absorbed, and the average molecular weight of network chains was estimated. decreases with doses up to 10 Mrad and does not change with further irradiation. |
