The effects of moisture on the dielectric relaxation of urethane dimethacrylate polymer and composites.

Dielectric characterization evaluated the influence of sorbed moisture on urethane dimethacrylate (UDMA) polymer matrix and unsilanated and silanated zirconia-silica (ZS)-filled UDMA composites. The moisture content (MC) of samples tested ranged from 0 to 7.32% with maximum values for UDMA, silanated and unsilanated ZS-filled UDMA were 3.72, 6.0 and 7.32%, respectively. Dielectric spectra were measured from 0.053 to 100 kHz and from -180 to 180;C. Small MC stiffened the polymers, which was shown by a shift of beta-peaks to higher temperatures, a decrease in the half width of bands and an increase in the change of activation energies. Higher MC resulted in water plasticizing the polymer by shifting the beta-peaks to lower temperatures. Maximum water sorption restricted the motion of the polymer chains and the b-peaks shifted to higher temperatures. Further, increasing MC shifted the a-relaxation to lower temperatures in both UDMA and composites. However, in composites with unsilanated ZS, a new relaxation occurred at high temperatures and low frequencies which was attributed to Maxwell-Wagner-Sillars (MWS) polarization. Thus, in UDMA, water occurs as fragments that form non-freezing clusters. However, in unsilanated ZS-UDMA composites, bulk water can exist at the polymer-filler interface and form disordered structures of ice at low temperatures which can evaporate at high temperatures. Silanated ZS-UDMA composites function similarly to UDMA at low MC but are more plasticized at high temperatures.