Jeannine E. Elliott

The effect of cure rate on the mechanical properties of dental resins

OBJECTIVE This study investigates the effect of cure rate on the mechanical properties of a common dimethacrylate dental resin formulation (75/25 wt% bis-GMA/TEGDMA). METHODS The polymerization rate and final conversion of the exact specimens subsequently used for mechanical testing were monitored by near-infrared (near-IR) spectroscopy. The glass transition temperature (T(g)) and modulus, as a function of temperature, were determined by dynamic mechanical analysis (DMA). Iniferter initiating systems were used to create partially cured networks that did not contain any trapped radicals. By the elimination of trapped radicals from the system, the formed networks can be characterized as a function of both temperature and double bond conversion without inducing additional thermal cure during testing. RESULTS Copolymer specimens were cured with UV and visible light initiating systems, UV light intensities that varied by over four orders of magnitude, and cure temperatures that differed by 60 degrees C. Even though the polymerization rates for these resins were vastly different, similar T(g) and modulus were measured for specimens cured to the same final double bond conversion. SIGNIFICANCE This study shows that highly cross-linked dimethacrylate systems, such as bis-GMA/TEGDMA, exhibit similar network structure and properties as a function of double bond conversion, regardless of the method or rate of cure.

Primary cyclization in the polymerization of bis-GMA and TEGDMA: a modeling approach to understanding the cure of dental resins.

An optimal dental restorative polymeric material would have a homogeneous cross-linking density giving it consistent mechanical strength throughout the material. When multifunctional monomers are polymerized, a pendant double bond can react intramolecularly with the radical on its propagating chain to form a loop, which results in a primary cyclization reaction. Primary cyclization does not contribute to overall network structure, causes microgel formation, and leads to heterogeneity in the polymer. Knowledge of how cure conditions control the degree of primary cyclization and cross-linking in the polymer is important in developing better dental materials. To gain more understanding about the evolving polymer network, the photopolymerization of a typical dental resin (75/25 wt% bis-GMA/TEGDMA) is modeled using a first principals approach. The overall polymerization rate behavior of 75/25 wt% bis-GMA/TEGDMA is predicted using experimentally obtained propagation and termination kinetic rate constants. The effect of chain stiffness and light intensity on the polymerization kinetics is also explored. Furthermore, the model predicts the extent of cross-linking and primary cyclization in the growing polymer network. At 45% conversion, the fraction of bis-GMA and TEGDMA pendant double bonds created that have cycled is 11 and 33%, respectively. The model shows that using a stiff monomer, like bis-GMA, in dental resins diminishes the extent of cyclization and increases the cross-linking density of the polymer. Therefore, better mechanical properties are obtained than if more flexible monomers were used.

Kinetic modeling of the effect of solvent concentration on primary cyclization during polymerization of multifunctional monomers

Controlling the swelling ratio, diffusion rate, and mechanical properties of a crosslinked polymer is important in hydrogel design for biomedical applications. Each of these factors depends strongly on the degree of crosslinking. Primary cyclization, where a propagating radical reacts intramolecularly with a pendant double bond on the same chain, decreases the crosslinking density and increases the molecular weight between crosslinks. Processing conditions, specifically the solvent concentration, strongly affect the extent of primary cyclization. In this work the effects of solvent concentration and comonomer composition on primary cyclization are investigated using a novel kinetic model and experimental measurement of mechanical properties. Two divinyl crosslinking agents were investigated, diethyleneglycol dimethacrylate (DEGDMA) and polyethyleneglycol 600 dimethacrylate (PEG(600)DMA), and each was copolymerized with hydroxyethyl methacrylate (HEMA) and octyl methacrylate (OcMA). The model is further used to predict the gel point conversion and swelling ratio of PAA hydrogels polymerized in the presence of varying amounts of water. Model results show how increasing the solvent concentration during the polymerization increases the molecular weight between crosslinks by nearly a factor of three and more than doubles the swelling ratio. Where possible, experimental results provide quantitative agreement with model predictions.

The effect of primary cyclization on free radical polymerization kinetics: experimental characterization

Abstract Free radical polymerization kinetics are influenced by many factors including solvent concentration during polymerization, monomer structure, and comonomer composition. This study isolates the effects of the balance between primary cyclization and crosslinking on the polymerization kinetics. Isomeric crosslinking agents, 1,2-cyclohexanediol dimethacrylate (1,2-CHDDMA), 1,3-cyclohexanediol dimethacrylate (1,3-CHDDMA), and 1,4-cyclohexanediol dimethacrylate (1,4-CHDDMA), were utilized for the study because they have differing cyclization rates due to the conformation of the two methacrylate groups in the monomer, but they are otherwise similar. In copolymerizations of 1, 2, 5, and 10% crosslinking agents with 2-methoxyethyl methacrylate (MEMA), the 1,4-CHDDMA samples were always found to have an earlier onset of autoacceleration than 1,2-CHHDMA samples. 1,3-CHDDMA copolymerized with MEMA had a polymerization rate between the 1,2-CHDDMA and 1,4-CHDDMA, as expected. Mechanical property data showed that copolymer samples made with the 1,4-CHDDMA crosslinking agent exhibited a lower M c and higher T g than the analogous 1,2-CHDDMA copolymers. It is concluded that reduced mobility from greater crosslinking than cyclization causes the earlier onset of autoacceleration in the 1,4-CHDDMA copolymers.

PREDICTING NETWORK FORMATION OF FREE RADICAL POLYMERIZATION OF MULTIFUNCTIONAL MONOMERS

Free radical polymerization of multifunctional monomers leads to the formation of crosslinked polymer materials. These crosslinked polymers are heterogeneous networks caused by varying pendant reactivity and primary cyclization reactions. A kinetic model was utilized to probe the manner in which pendant double bonds react throughout the polymerization and to characterize the cycle size. The effects of crosslinking agent size, copolymerization, and solution polymerization were each examined. Increasing crosslinking agent size causes fewer small cycles to form and diminishes the total number of cycles that are created. Increasing the solvent concentration during polymerization has the converse effects of increasing the number of cycles that form within the first couple of repeat units and increasing the total fraction of pendants that form cycles. The model was further used to predict the molecular weight between crosslinks as a function of polymerization conditions.