Effects of adhesive, base and diluent monomers on water sorption and conversion of experimental resins

To establish the relationship of resin composition and resin hydrophilicity (indicated by solubility parameters and logP) to water sorption (WS), solubility, and degree of double bond conversion (DC) of resin mixtures designed for adhesive restoratives by varying the concentration of pyromellitic glycerol dimethacrylate (PMGDM) and various co-monomers.

Sixteen resin mixtures were prepared with (30-70) mass fraction % PMGDM. At given PMGDM concentrations there were up to five compositions with increasing logP. Polymer disks (13 mm x 0.7 mm) were exposed to 96% relative humidity (RH) to determine water sorption in humid atmosphere (WSH) and subsequently immersed in water for immersion water sorption (WSI) and solubility. DC was assessed by near infrared spectroscopy.

WSI was somewhat higher than WSH, which ranged from (2.1 to 5.3) mass fraction %. Both data were positively correlated to PMGDM concentrations [Pearson correlation, p<0.02; R(2)=0.74, 0.73 (WSI)] and solubility (R(2)=0.64), but not to logP. When grouped by structural similarities, i.e., base resins with bisphenol A core (Group B), Group O containing diluent monomers, or Group U containing urethane dimethacrylate, WS within each group was inversely correlated to logP with R(2)=0.98, 0.81, 0.95, and WS/solubility correlation improved with R(2)=0.88, 0.92 and 0.75, respectively. Solubility ranging from 0.3% to 2.3% was inversely related to DC (r=-0.872). Conversion ranging from 41% to 81% was lower for resins with high base monomer concentrations and highest in mixtures with UDMA.

LogP was a good predictor of WS after grouping the resins according to functional, compositional and structural similarities. WS and conversion were reasonably well predicted from Hoy's solubility parameters and other physical resin properties.