Synthesis and characterization of POSS-(PAA)8 star copolymers and GICs for dental applications

Long-Term Water Balance Evaluation in Glass Ionomer Restorative Materials

The complex role of water in glass ionomer cement (polyalkenoate) dental restorative materials has been studied, but much of the present understanding concerning water balance within these materials is based on very early studies and short-term experiments. This study evaluated the nature of the water species of six conventional and four resin modified glass ionomer restorative materials over 3 years using thermogravimetric analysis techniques. Materials were prepared, placed in crucibles, and stored in physiologic phosphate buffered saline and evaluated at 24 h, 1 week, and then at 1, 3, 6, 9, 12, 18, 24, 30 and 36 months. All materials demonstrated a significant increase in unbound water percentage content but except for the resin modified materials, the enthalpy required to remove the unbound water species did not significantly change over 36 months. Also, bound water content percentage and removal enthalpy was established at 24 h, as no significant increase was noted in both bound water content and removal enthalpy over the course of this evaluation. This study suggests that unbound water species may increase with time and is loosely held except for the resin modified materials. Protective coatings placement and re-evaluation are prudent to prevent unbound water loss.

Modified POSS nano-structures as novel co-initiator-crosslinker: Synthesis and characterization

OBJECTIVE

To synthesize an amine-modified polyhedral oligomeric silsesquioxane (POSS) nano-structure as a novel co-initiator-crosslinker (co-Ini-Linker) and to determine the effect of the co-Ini-linker on the physical and mechanical behavior of an experimental dental composite.

METHODS

The amine-methacrylate POSS nano-structures (AMA-POSS) were chemically synthesized by anchoring a tertiary amine functionality on the methacrylate POSS (MA-POSS) branches. Three types of AMA-POSS, having different amine branches in their structures, were synthesized through the Aza Michael reaction. The chemical structure of AMA-POSSs were evaluated by¹H-NMR spectroscopy. Afterward, the AMA-POSS was incorporated into a dental resin system composed of Bis-GMA, TEGDMA, and photo-initiator. Three resin systems with different AMA-POSS types were then prepared, and their properties were compared with a resin containing DMAEMA as a conventional co-initiator. The degree of conversion evaluated by FTIR spectroscopy and the shrinkage kinetics of the resins were determined through the bonded-disk technique. The flexural properties of the photopolymerized resins were also investigated. The distribution of nano-structures in the matrix resin was analyzed using EDX analysis.

RESULTS

The modified POSS structure and the number of amine branches were confirmed with¹H-NMR spectroscopy. The resin containing 8 amine branches (P8) showed the same degree of conversion (DC%) as the resin containing DMAEMA (P > 0.05). Decreasing the amine branches in the POSS structure, however, revealed an increasing trend in DC%. The resin containing P8 showed the lowest shrinkage strain. By incorporating AMA-POSS into the resin system, the water sorption significantly decreased (P < 0.05). The flexural strength and modulus increased by adding P3 into the resin system (P < 0.05). EDX Si-map revealed that the co-Ini-linker was well dispersed in the resin matrix.

SIGNIFICANCE

The synthesized novel amine-methacrylate POSS nanostructures not only act as an amine co-initiator but also work as a reinforcing filler and a cross-linking agent.

Effect of poly(acrylic acid) architecture on setting and mechanical properties of glass ionomer cements

OBJECTIVE

This work focuses on the influence of poly(acrylic acid) (PAA) architecture (linear or branched) on setting behavior and compressive strength of glass ionomer cements (GICs).

METHODS

Branched and linear poly(acrylic acid)s were synthesized according to the Strathclyde methodology or by free radical polymerization. They were characterized by ¹H-NMR spectroscopy and size exclusion chromatography to determine their molecular weight and size distribution. GIC setting was characterized by oscillating rheometry and time-dependent FTIR spectroscopy. In addition, compressive strength was tested on cylindrical samples (6 × 4 mm; n = 8/cement composition) after storage in deionized water at 37 °C for one day.

RESULTS

We used two different routes to prepare PAA. One direct route in order to provide straightforward access to branched PAA and a two-step approach in order to get more control about the PAA molecular weight using tert-butyl acrylate (tBA) for polymerization with subsequent deprotection. Using the second approach we obtained several linear PAA of which a mixture was used in order to mimic the molecular weight and size distribution of branched PAA. This allowed the direct comparison of properties relying only on the polymer architecture. Comparing linear PAA to branched samples in general led to faster setting but at the same time decreased the compressive strength. Increasing molecular weight of branched PAA resulted in even faster GIC setting while increasing compressive strength and this correlates well with the trends reported for linear PAA in literature. Mixing of branched and linear PAA, however, turned out to be an effective way of tailoring GIC properties.

SIGNIFICANCE

our results suggest that both molecular weight and dispersity need to be considered when choosing suitable PAA architecture for obtaining specific GIC properties.

A star-shaped POSS-containing polymer for cleaner leather processing

A water-based silsesquioxane (POSS)-containing polymer, POSS-PAA, was synthesized by using octavinyl-POSS (V-POSS) and acrylic acid (AA) via interfacial polymerization. The TEM of POSS-PAA showed that the polymer formed a core-shell structure in aqueous solution and was well-dispersed. The star-shaped POSS-PAA and linear PAA were both tanned with 3.5% chromium tanning agent, and leather hide was tanned with 3.5% chromium tanning agent as a control. The results showed that the shrinkage temperature of wet-blue leather treated by POSS-PAA was increased by 3.5 °C than that of the control. The thickening rate of the POSS-PAA treated wet-blue leather samples was increased by 21% and 96% than the linear PAA-treated leather and the control leather, respectively. The EDS results suggested that the POSS-PAA pre-treated leather had a higher chromium content than the others, and the chromium distribution from the leather flesh side to the grain side was uniform. Moreover, the Cr₂O₃ content in both the POSS-PAA and the PAA pre-treated tanning waste water was reduced by about 50%, compared to the control waste. The COD and BOD of the POSS-PAA pre-tanning waste were decreased compared to the others. Therefore, POSS-PAA appeared to be promising for promoting the development of cleaner leather production.