Small-angle x-ray scattering (SAXS) was used to determine the structural changes in polyethylene induced by radiation. The changes in densities of the crystalline and amorphous phases, pc and Pa, were calculated after direct determination of the 9 9 2 ...... mean square denszty fluctuatzon < 7? >. pa increases with increasing radiation dose for both linear and branched polyethylene. This accounts for the serious discrepancy between crystallinities determined from wide-angle x-ray scattering and density measurements. This study confirms our previous proposal that crosslinks occur primarily in the noncrystalline phase, most likely at the defects in the lateral grain boundary regions.

Branched polyethylene irradiated (0-400 Mrad) with a Co 6~ source at room temperature under vacuum was studied by density, wide-and small-angle X-ray scattering (WAXS and SAXS) measurements. The radiation effects on the structure of bulk, branched polyethylene are quite similar to those observed by others on single crystals or oriented preparations. These effects include changes in bulk density Q, crystallinity (we or vc) and dl00 and d200 spacings as a function of irradiation. A decrease in crystallinity is seen to begin at radiation dose ~ 100 Mrad whereas lattice expansion indicating onset of an orthorhombic-hexagonal transition can begin as low as 10 Mrads. The decrease in crystallinity can be attributed to additional lattice distortions primarily introduced by the crosslinks occurring at the lateral grain boundaries, while lattice expansion can be associated with the same crosslinking mechanism which begins at the defects both within the crystals as well as those outside the crystals at the lateral grain boundaries. Strong evidence for a primary crosslinking-at-the-defects mechanism has also come from 0c and 0a data obtained in this study as a function of radiation dose. The same data have also led to an excellent correspondence between the measured density crystallinity vc and the measured WAXS crystallinity we. Without consideration of the effects of crosslinks on 0c and Qa one would have obtained a divergence of the two crystallinities, especially at radiation doses greater than 100 Mrads.

A hydrophobic restorative composite based on a fluorocarbon analog of an alkyl methacrylate and a bisphenol adduct was formulated into a one-paste system, which polymerized in the presence of blue light. Physical, mechanical and water-related properties were determined. High contact angles and low water sorption were shown by the experimental composite. Capillary penetration of oral fluids around restorations, therefore, could be prevented in the presence of this highly hydrophobic surface. The physical and mechanical properties of the experimental composite were either comparable to or somewhat less favorable than commercial Bis-GMA composites.

Small-angle x-ray scattering (SAXS) was used to determine density fluctuation in radiation-induced crosslinked polyethylene of varying degrees of crystaUinity. Density fluctuation FL decreases with increasing crystallinity, while it increases linearly with increasing radiation dose or degree of crosslinking. By means of extrapolation, density fluctuations in the crystalline and the amorphous phases FLc and FG were obtained. At a given dose, Faa is greater than FLc. The increase in FLa with radiation is found to be much greater than that of FLc compared with the initial values at 0 Mrad, FLc showing only a negligible increase even at 312 Mrad. The present findings suggest that crosslinks are not introduced within the crystalline phase; they take place pmnarlly in the noncrystalline phase, in agreement with the conclusions reached previously on the basis of changes in crystalline and amorphous densities in irradiated polyethylene.