Dynamic mechanical analysis of two crosslinked copolymer systems.

This research investigated the dynamic mechanical properties of two model dental restorative copolymers to elucidate the influence of pendent group length and addition of crosslinking agent. Two different monomethacrylate resins [(methyl methacrylate (MMA) or 2-ethoxyethylmethacrylate (EEM)] were added to various molar fractions (0, 5, 10, 20, 40, 60, 80 and 100%) of a difunctional monomer [triethylene glycol dimethacrylate (TEGDMA)], resulting in two different copolymer systems (TMMA and TEEM). Bar-shaped specimens were heat-cured and tested for their dynamic mechanical properties at a fixed frequency (1Hz): flexural storage modulus and loss factor. Transition temperatures associated with peaks were determined using tan delta, phase angle and loss factor. The properties of one specimen were also examined using a range of applied frequencies. Below the Tg, all resin systems had similar storage moduli. An increase in pendent group length lowered transition temperatures and demonstrated greater influence on temperature changes than did a change in crosslinker content. With increasing crosslinker, the transition temperatures increased. The values ascribed to transition temperatures were dependent upon the analysis method used: tan delta gave highest values, followed by phase angle, with loss factor values the lowest. Storage and loss moduli values and their ratio increased with increasing frequency of applied signal. Loss factor analysis provided the most elucidation of sub-Tg transitions. Restorative polymers with specific desired physical and mechanical properties can be achieved by both judicious selection of monomer structure as well as by the controlled addition of the crosslinking agent.