Relaxation mechanisms in low-stress polymer networks with alternative chemistries

Low-stress resin-based composites (RBCs) are available to the clinician, some using stress relaxation mechanisms on the basis of network reconfiguration, modulated photopolymerization, or chain transfer reactions. This study investigated those materials in terms of their overall stress relaxation and their relationship with polymerization kinetics and compared them with an experimental low-stress thiourethane (TU) material.

Experimental composites (bisphenol-A-diglycidyl dimethacrylate, urethane dimethacrylate, and triethylene glycol dimethacrylate [50:30:20 mass ratio]; 70% barium aluminosilicate filler; camphoroquinone, ethyl-4-dimethylaminobenzoate, and 2,6-di-tert-butyl-4-methylphenol [0.2:0.8:0.2% by mass]) with or without TU oligomer (synthesized in-house) and commercial composites (SureFil SDR Flow+ Posterior Bulk Fill Flowable Base [SDR Flow+] [Dentsply Sirona], Filtek Bulk Fill Posterior Restorative [3M ESPE], and Filtek Supreme Ultra Universal Restorative [3M ESPE]) were tested. Polymerization kinetics (near-infrared) and polymerization stress (Bioman) were evaluated during light-emitting diode photoactivation at 100 mW/cm² for 20 seconds. Stress relaxation was assessed using dynamic mechanical analysis. Data were analyzed with a 1-way analysis of variance and Tukey test (α = 0.05).

The kinetic profiles of all materials differed substantially, including more than a 2-fold difference in the rate of polymerization between TU-modified composites and SDR Flow+. TU-modified RBCs also showed more than a 2-fold higher conversion at the onset of deceleration vs the experimental control and commercial materials. RBCs that used stress reduction mechanisms showed at least a 34% reduction in polymerization stress compared with the controls and significantly reduced the amount of early-onset stress buildup. SDR Flow+ and the TU-modified RBCs showed the greatest amount of viscoelastic stress relaxation postpolymerization.

The novel TU-modified materials showed similar or improved performance compared with commercial low-stress RBCs, showing that chain transfer may be a promising strategy for stress reduction, both during polymerization and after polymerization.