Degree of vinyl conversion, polymerization shrinkage and stress development in experimental endodontic composite

Abstract

This study explores degree of vinyl conversion (DVC), polymerization shrinkage (PS) and shrinkage stress (PSS) of the experimental amorphous calcium phosphate (ACP) composites intended for use as an endodontic sealer. Light-cure (LC), chemical cure (CC) or dual-cure (DC; combined light and chemical cure) resins comprised urethane dimethacrylate (UDMA), 2-hydroxyethyl methacrylate (HEMA), methacryloyloxyethyl phthalate (MEP) and a high molecular mass oligomeric co-monomer, poly(ethyleneglycol)-extended UDMA (PEG-U) (designated UPHM resin). To fabricate composites, a mass fraction of 60 % UPHM resin was blended with a mass fraction of 40 % as-made (am-ACP) or ground ACP (g-ACP). DVC values of copolymer (unfilled UPHM resin) and composite specimens were determined by infrared spectroscopy. Glass-filled composites were used as controls. PS and PSS of composites were determined by dilatometry and tensometry, respectively. LC copolymers attained extraordinary high DVC values at 24 h post-cure (95.7 %), compared to CC (52 %) and DC (79.3 %) copolymer specimens. While the DVC values of LC and DC am-ACP composites were reduced between 5 and 10 %, DVC values of DC g-ACP composites increased almost 8 % compared to the corresponding copolymers. High DVC attained in LC composites was, expectedly, accompanied with high PS values (on average 7 vol%). However, PSS developed in LC and especially DC composites did not exceed PSS values seen in other UDMA-based composites. Based on this initial evaluation, it is concluded that, DC, g-ACP filled UPHM composite shows promise as an endodontic sealer. However, further physicochemical evaluations, including water sorption, mechanical stability and ion release as well as a leachability studies need to be performed before this experimental material is tested for cellular responses and, eventually recommended for clinical utility.

Fig. 1

Schema of the dilatometer used to measure polymerization shrinkage.

Fig. 2

Drawing of the tensometer used to measure stress developed in composites upon polymerization (light-curing of the specimen is shown in part a).

Fig. 3

FTIR spectrum (a) and X-ray diffraction pattern (b) typical of am- and g-ACP filler.

Fig. 4

Volume particle size distributions and the corresponding SEM images of am-ACP and g-ACP fillers used to formulate UPHM/ACP composites.

Fig. 5

Polymerization shrinkage of light-cure composites. Indicated are mean values + one standard deviation (n ≥ 3/group).

Fig. 6

Polymerization stress developed in light-cure (LC) and dual-cure (DC) composites. Indicated are mean values + one standard deviation (n ≥ 3/group).