Conversion-dependent Mechanical Stress Relaxation Behavior in Crosslinked Polymers

A significant problem associated with current polymer-based composite materials is stress development at the restorative-tooth interface during polymerization.  Extensive work has been directed towards new lower stress materials and dynamic stress measurement but little attention has been focused on improving the fundamental understanding of stress development in polymers, which includes stress relaxation as an integral component.  Objective: To examine how the stress relaxation rate varies with the extent of polymer formation.

Methods: Bis-GMA/TEGDMA polymer specimens were obtained by UV irradiation with exposure intervals of 15-330 s.  A photo-iniferter was used to produce stable polymers spanning a broad range of conversion, which was determined by near-infrared spectroscopy prior to evaluation of polymer modulus and stress relaxation kinetics at a specified deflection on a universal testing machine.

Results: Photopolymer specimens with conversion ranging from <10% to >50% were obtained.  Both modulus and peak stress corresponding with polymer deflection increased in an exponential manner with increasing conversion.  The normalized residual stress values at 1, 10 and 100 s were 99, 93 and 80%, respectively, for the high conversion, high modulus polymers compared with residual stress values of 93, 78 and 53% for the low conversion material held over the same time intervals.  A first-order dependence was observed between polymer modulus and normalized residual stress with r² correlations of 0.77, 0.87 and 0.90, respectively, for data taken at 1, 10 and 100 s. 

Conclusions: While minimal stress levels are supported in low conversion polymers, substantial stress percentages are retained over extended time periods even for polymers only slightly beyond gel point.  Despite higher absolute stress levels possible in high conversion polymers, normalized stress relaxation decreases considerably with increasing polymerization.  This information will assist in understanding the complex dynamic stress evolution process in bonded composites.    

Supported by NIH/NIDCR 2R01DE14227 and a gift from Septodont.