Method: A nanogel was prepared by solution polymerization of isobornyl methacrylate and UDMA (80:20 molar ratio) with 15 mol% mercaptoethanol (ME) as chain transfer agent. Following the polymerization, isocyanatoethyl methacrylate (IEM) was attached via the ME-based chain ends. The nanogel was dispersed in xylene, toluene, 2-butanone, tetrahydrofuran or dimethylsulfoxide in various concentrations and viscosities were determined. With added initiator, either 10 or 50 wt% nanogel dispersions were photopolymerized while conversion was monitored by near-infrared spectroscopy.
Result: Only small increases were observed in normalized viscosity for nanogel loadings up to 20-30 wt% with DMSO-dispersions providing significantly lower viscosity change (p<0.05) over this range. Alternatively, at the 50 wt% nanogel loading level, where particle-particle interactions dominate over particle-solvent interactions, the DMSO solution yielded the highest normalized viscosity. To illustrate differences in standardized photopolymerization reactivity, the nanogel dispersed in toluene or xylene at 10 wt% loading (mainly intra-particle reactions) reached conversion values of 18.9±9.3% or 67.3±8.7%, respectively, compared with the 50 wt% loading level (both inter- and intra-particle reactions) with conversions of 95.0±2.2% or 88.2±7.7%, respectively. At the higher nanogel concentrations, polymerization produced clear, solid polymer gels.
Conclusion: The viscosity and conversion data suggests that at either end of the solvent polarity spectrum, partially collapsed nanogel structures with enhanced intra-particle reactivity are present. Based on the solubility parameter match between a nanogel and resin, either more monomer swollen or more condensed nanogel morphology can be designed and this along with the nanogel loading can affect viscosity and the ultimate network structure/properties of nanogel-modified resins.
Keywords: Biomaterials, Polymerization and Polymers
See more of: Dental Materials 6: Polymer-based Materials-Chemistry and Composition