Nanogel Molecular Weight Effects on Resin Properties

Objective:  Incorporate compositionally equivalent reactive nanogels of different size and molecular weight into resin to determine the subsequent effects on initial viscosity and polymer properties. The goal is to demonstrate how nanogel design can be used to favorably balance the desired reduction in polymerization stress with the impact of prepolymer on viscosity.

Methods:  A solution of isobornyl methacrylate and ethoxylated bisphenol-A dimethacrylate (80:20 mol) was polymerized at 80°C in toluene using azobisisobuturonitrile as initiator and 15 mol% mercaptoethanol as chain transfer agent (CTA). In order to achieve nanogel molecular weights lower than 51 KDa, an additional 15 mol% dodecanethiol was added to reduce average chain lengths within the nanogel nanoparticles. 2-Isocyanatoethyl methacrylate was added to introduce methacrylate functionality at chain ends. Following isolation by precipitation from hexane, the nanogels were loaded at 30 wt% in a mixture of 60/40 wt% urethane dimethacrylate (UDMA)/triethyleneglycol dimethacrylate (TEGDMA) containing camphorquinone and ethyl 4-dimethylaminobenzoate as photoinitiator. Resin viscosity was determined (viscometer) and polymerizations were conducted with 320-500nm light and real-time conversion measured by near-IR spectroscopy. Flexural strength and modulus were obtained in three-point bending with dynamic stress measurements monitored by tensometer.

Results:  table

Conclusions:  The addition of 30 wt% nanogel, which is above the percolation nanogel threshold, has no significant effect on conversion and little to no effect on mechanical properties. However, even though the interfacial surface area increases dramatically as nanogel size and molecular weight is decreased at a fixed loading level, the corresponding reduction in nanogel internal volume means a lower proportion of monomer is infiltrating the nanogels, which results in greater particle dispersion. This leads to lower resin viscosities for smaller nanogels and over the range examined here, stress reduction potential is not compromised by the nanogel size. This provides a route to nanogel-modified resins with low stress and low viscosity.