Studies with Solvent-dispersed Reactive Nanogels

Objective: Nano-scale polymeric particles (nanogels) have been used to significantly reduce polymerization shrinkage and stress in dental composites and enhance dentin bond strengths in adhesive applications.  Rather than adding nanogel to monomer, this study examines the viscosity, reactivity and network-forming potential of nanogels in several inert solvents.

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.