Objectives: The polymerization shrinkage of resin composites arises mainly from the chemical reaction itself. In the present study we accurately determine the volume contraction and link it to the number of double bonds converted. Secondly, we investigate the potential of hyperbranched polymers (HBP) as additives in neat resins.

Methods: To determine the volume contraction associated with the number of double bonds converted, we analyzed a range of Bis-GMA/ TEGDMA mixtures. To reduce the polymerization shrinkage, it appears to be necessary to reduce the number of double bond converted per unit of volume. HBP are very large molecules with many branches and a high number of functional groups on each molecule. HBP are thus attractive for photocuring because they could link efficiently with the resin with only a few double bonds. By this way, they should allow to decrease the shrinkage. The potential of HBP as additives in these resins was thus investigated by adding three different HBP with different shell chemistry at several weight ratios. The volume contraction was measured by pycnometry and density column. The study of the photocuring process (degree of conversion) has been performed by Raman spectroscopy.

Results: The volume contraction depends on both the degree of conversion and molecular size. But an univocal linear relationship has been found between the volume contraction and the actual number of vinyl double bonds converted into single ones. While literature reports volume contraction of about 23 cm3/mole, a value of 20 cm3/mole was deduced from 27 measurements.

Conclusions: This study accurately determined the volume contraction associated with the number of double bonds converted in a mixture of Bis-GMA/TEGDMA. It also highlighted HBP as a potential performance-enhancing additive in photocured dental composites.

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