Application of an Addition-Fragmentation-Chain Transfer Monomer in Di(meth)acrylate Network Formation to Reduce Polymerization Shrinkage Stress

Abstract

A new addition-fragmentation chain transfer (AFT) capable moiety was incorporated into a dimethacrylate monomer that participated readily in network formation by copolymerizing with multifunctional methacrylates or acrylates. The process of AFT occurred simultaneously with photopolymerization of the AFT monomer (AFM) and other (meth)acrylate monomers leading to polymer stress relaxation via network reconfiguration. At low loading levels of the AFM, a significant reduction in shrinkage stress, especially for acrylate monomers, was observed with nominal effects on conversion. At higher loading levels of the AFM, the photopolymerization reaction kinetics and final double bond conversion were significantly lowered along with a delay in the gel-point conversion. Electron paramagnetic resonance studies during polymerization revealed the presence of a distinct radical species that was present in proportional quantities to the AFM content in the system. The lifetime and the character of the persistent radicals were altered due to the presence of the distinctive radical, in turn affecting the polymerization kinetics. With polymerization conducted at higher irradiance, the differential conversion between the control resin and samples with moderate AFM content was minimal, especially for the methacrylate-based formulations.

Figure 1

Structures of model monomers used in the study.

Figure 2

Network rearrangement upon photopolymerization in AFM-based networks – (a) Normal thermoset network lacking AFM functionality forms a network that has fixed linkages and (b) Network with AFM functionality that can rearrange by breaking and reforming bonds

Figure 3

Double bond conversion as a function of time for (a) BisGMA/TEGDMA and (b) BisGA/TEGDA series. Photocuring conditions: laminated 6 × 1 mm disc specimens exposed for 30 s at an incident irradiation intensity of 200 mW/cm² commencing at t = 30 s.

Figure 4

Shrinkage stress development as a function of time for (a) BisGMA/TEGDMA and (b) BisGA/TEGDA series. Irradiation intensity – 200 mW/cm² for 30 s. Light was started at 30 s after start of run.

Figure 5

Comparison of the effect of AFM and TEGDMA addition to BisGA/TEGDA (a) Shrinkage stress and (b) Double bond conversion. Irradiation intensity – 200 mW/cm² for 30 s. Light was started at 30 s after start of run. Sample dimensions – 6 mm diameter disc with 1 mm thickness

Figure 6

Storage and loss moduli as a function of time for (a) BisGMA/TEGDMA and (b) BisGA/TEGDA series. The gel point conversion is estimated from the crossover between the storage and loss moduli. Irradiation intensity – 2 mW/cm² for 30 s. Light was started at 30 s after start of the run. Sample dimensions – 20 mm diameter disc with 0.5 mm thickness

Figure 7

Dynamic mechanical analysis of the BisGMA/TEGDMA series. Tan delta for (a) 1^st heating scan and (b) 2^nd heating scan; (c) Storage modulus at 2^nd heating scan.

Figure 8

Dynamic mechanical analysis of BisGA/TEGDA series. Tan delta for (a) 1st heating scan and (b) 3rd heating scan; (c) Storage modulus at 3rd heating scan.

Figure 9

Propagating radical in (a) methacrylate polymerization and (b) acrylate polymerization

Figure 10

Final EPR spectra for BisGMA/TEGDMA series. Each spectrum is taken at the end of the polymerization run for each corresponding sample.

Figure 11

EPR spectra of AFM (red) and BisGMA/TEGDMA with 20% AFM (black).

Figure 12

Proposed radical structure generated from the fragmentation reaction of the AFM.

Figure 13

Final EPR spectra for BisGA/TEGDA series. Each spectrum is taken at the end of polymerization run for each corresponding sample.

Scheme 1

Mechanism of addition-fragmentation of AFM upon addition of a radical species (R·)