A Study of Shrinkage Stress Reduction and Mechanical Properties of Nanogel-Modified Resin Systems

Abstract

A series of nanogel compositions were prepared from urethane dimethacrylate (UDMA) and isobornyl methacrylate (IBMA) in the presence of a thiol chain transfer agent. The linear oligomer of IBMA was synthesized by a similar solution polymerization technique. The nanogels were prepared with different crosslinker concentrations to achieve varied branching densities and molecular weights. The prepolymers were dispersed in triethylene glycol dimethacrylate at loading levels ranging from 10 wt% to 50 wt%. Photopolymerization reaction kinetics of all prepolymer modified systems were enhanced relative to the nanogel-free control during early stage polymerization while limiting conversion was similar for most samples. Volumetric polymerization shrinkage was reduced proportionally with the prepolymer content while the corresponding decrease in polymerization stress was potentially greater than an additive linear behavior. Flexural strength for inert linear polymer-modified systems decreased significantly with the increase in the prepolymer content; however, with an increase in the crosslinker concentration within the nanogel additives, and an increase in the concentration of residual pendant reactive sites, flexural strength was maintained or improved regardless of the nanogel loading level. This demonstrates that covalent attachment rather than just physical entanglement with the polymer matrix is important for effective polymer mechanical reinforcement by nanogel additives. Reactive nanogel additives can be considered as a practical, generic means to achieve substantial reductions in polymerization shrinkage and shrinkage stress in common polymers.

Figure 1

Molecular structures for methacrylate monomers and schematic interpretation of nanogels with different branching densities.

Figure 2

GPC – RI traces for PIBMA, N19, N37 and N55 prepolymers.

Figure 3

¹H NMR spectra for N19, N37 and N55.

Figure 4

Viscosity data for PIBMA, N19, N37 and N55 prepolymer in TEGDMA at various loading levels from 0 to 50 wt%. Note: error bars are included but are generally smaller than the symbols.

Figure 5

Volumetric shrinkage results for N37 and N55 nanogel systems at 0 - 50 wt% addition values.

Figure 6

Tan δ behaviors for the N19 and N55 modified systems at different loading levels as a function of temperature from 0 °C to 220 °C with f=1 Hz and a scan rate of 2 °C/min. Samples were preheated to 160 °C for overnight to prevent continued thermal polymerization during the DMA testing.

Figure 7

Photopolymerization kinetics profiles for control as well as PIBMA, N19, N37, and N55 nanogel-modified systems at 0 - 50 wt% addition values in the early stage of polymerization. All composites were irradiated with 365 (± 10) nm UV light at an intensity of 10 mW/cm² starting at 0.5 min with 0.5 wt% DMPA (relative to TEGDMA) as photoinitiator.

Figure 7

Photopolymerization kinetics profiles for control as well as PIBMA, N19, N37, and N55 nanogel-modified systems at 0 - 50 wt% addition values in the early stage of polymerization. All composites were irradiated with 365 (± 10) nm UV light at an intensity of 10 mW/cm² starting at 0.5 min with 0.5 wt% DMPA (relative to TEGDMA) as photoinitiator.