Nano-scale Heterogeneity of Dentin Adhesive Following Environmental Stress

Dental resins have a higher failure rate than traditional amalgam restorations particularly when these materials are used to restore large, complex lesions. Our previous study show nano-scale heterogeneities of cross-linked dentin adhesives cured in the presence of water and this heterogeneity was related to poor mechanical durability in these model adhesives. Objective: The purpose of this study was to use a novel Modulated DSC method to characterize the heterogeneous microstructure of model adhesives subjected to environmental stress. Methods: The model adhesive formulations with/without 12% water consisted of HEMA and BisGMA with a mass ratio of 45/55. 4mm diameter pellet specimens pre- and post-exposure to aqueous, ethanol and enzyme solution for 16 days were tested by a TA Instruments Q100 modulated DSC (New Castle, DE, USA). DSC cell was purged with nitrogen gas at 50 mL/min and the specimens were heated from -85 °C to 200 °C with a modulation period of 60 s.

Model Adhesive		Pre-exposure	Post-exposure
			in buffer	in enzyme solution	in ethanol
Cured without water	Tg1 Tg2	67.8 (2.3) 119.1 (2.0)	67.4 (1.3) 120.3 (1.7)	69.8 (1.6) 121.4 (2.3)	n/a 122.4 (3.3)
Cured with water	Tg1 Tg2	60.7 (2.2) 124.6 (2.6)	80.2 (2.1) 121.3 (2.5)	n/a 120.4 (1.9)	n/a 125.2 (2.7)

Results: Double glass transition temperatures (Tg) were found in reversing heat-flow signals probably due to heterogeneity of cross-link density. The lower Tg (Tg₁) has not been influenced by pre-incubation in buffer but became weaken or disappeared due to the exposure to enzyme solution or ethanol. Conclusion: A single Tg value does not fully describe the heterogeneous structure of cross-linked dentin adhesive particularly at the nano-scale. Loosely crosslinked region was not resistant to environmental stress, while densely crosslinked domains remain relatively resistant as shown by this novel modulated thermal analysis. Supported NIH/NIDCR R01 DE014392, R13 DK069504