Anandkumar R. Kannurpatti

A study of the evolution of mechanical properties and structural heterogeneity of polymer networks formed by photopolymerizations of multifunctional (meth)acrylates

A study of the mechanical properties and the structural heterogeneity of crosslinked polymers formed by photopolymerization of multifunctional monomers is described. By using living radical polymerizations, networks with no trapped carbon radicals have been synthesized. These crosslinked networks, which have no trapped free radicals can be heated without inducing further reaction and crosslinking. This feature makes the living radical polymerizations very useful in the characterization of structure and properties during and after the polymerization. In this work, living radical polymerizations have been used to study the mechanical properties of networks formed by homo- and copolymerization of diethyleneglycol dimethacrylate (DEGDMA) and poly(ethyleneglycol 600) dimethacrylate (PEG600DMA) with n-octyl methacrylate (OcMA) using dynamic mechanical analysis. Further, an acrylate copolymer system consisting of n-heptyl acrylate (HepA) and diethylene glycol diacrylate (DEGDA) has also been examined. The glass transition temperature of the copolymers was characterized as a function of composition as well as size of crosslinking agent in these copolymers. By performing frequency scan experiments, the distribution of relaxation times of the crosslinked polymers were characterized. From such analyses, the structural heterogeneity as measured by the width of the distribution of relaxation times of the networks was characterized as a function of the comonomer composition in the copolymers. Evidence that the dependence of the glass transition temperature on the crosslinking density is not straightforward is presented. Also, the results indicate that the structural heterogeneity of the materials increases as the crosslinking density of the copolymer is increased.

Use of “living” radical polymerizations to study the structural evolution and properties of highly crosslinked polymer networks

Crosslinked polymer networks are used in a wide variety of applications. To use these materials effectively, a fundamental understanding of their structural evolution and the relationship between material properties and structure is essential. In this article, a novel technique employing “iniferters,” i.e., living radical polymerizations, to photopolymerize these networks is utilized to study the property and structural evolution of these highly desirable materials. Living radical polymerizations are used in this work since this technique avoids the problem of carbon radical trapping encountered while using conventional initiators. Dynamic mechanical measurements are performed on highly crosslinked methacrylate networks to glean information regarding their structural heterogeneity. By performing these measurements on homopolymerized samples at various stages of the reaction and on copolymerized samples of multifunctional methacrylates, the mechanical properties are characterized as a function of double bond conversion and comonomer composition. From such analyses, with respect to both temperature and frequency, quantitative conclusions regarding the structure of the networks are drawn. This effort is aimed at exploiting the living radical polymerizations initiated by p-xylylene bis(N,N-diethyl dithiocarbamate) (XDT), to study the mechanical property evolution and structural heterogeneity of crosslinked polymers which is nearly impossible otherwise. Polymers examined in this study include networks formed by homopolymerization of diethylene glycol dimethacrylate (DEGDMA) and polyethylene glycol 600 dimethacrylate (PEG600DMA) as well as copolymers of DEGDMA and PEG600DMA.

Effect of comonomer concentration and functionality on photopolymerization rates, mechanical properties and heterogeneity of the polymer

Polymerization rate, glass transition temperature, storage modulus, molecular weight between crosslinks and heterogeneity of the final polymer product were studied as a function of the crosslinking agent concentration for several multifunctional methacrylates. The goal of this study was to determine the effect of crosslinking monomer structure and concentration on the rates of polymerization and mechanical properties of the polymer. A series of copolymers with equal concentrations of crosslinking double bonds was prepared using a monomethacrylate plus a di- or trimethacrylate. The trimethacrylate systems had higher glass transition temperatures, larger storage moduli, and smaller molecular weights between crosslinks than the comparable dimethacrylate systems. However, the heterogeneity of the trimethacrylate systems was found to be much more extensive. Additionally, the maximum rate of polymerization was found to increases with increasing crosslinking agent when octyl methacrylate was used as the monomethacrylate, but passed through a maximum when 2-hydroxyethyl methacrylate was used. The relative rates of polymerization between the trimethacrylate and dimethacrylate systems also depended on the monomethacrylate employed and the concentration of the crosslinking agent.

The Influence of Comonomer Composition on Dimethacrylate Resin Properties for Dental Composites

During the polymerization of multifunctional monomers for dental restorations, typical final double-bond conversions range from 55 to 75%. The low conversion results in a large amount of extractable monomer, reduced adhesion to the filler, and the potential for increased swelling. In this work, the ability to increase the maximum conversion by optimizing the copolymer composition is explored. A series of multi-ethylene glycol dimethacrylate monomers of various lengths was used as a model system to determine how the copolymer composition affects the final conversion, the mechanical properties, and the predicted shrinkage. It was found that the ultimate conversion can be significantly increased, shrinkage decreased, and mechanical properties maintained. It was found that up to 30 wt% of poly(ethylene glycol) 600 dimethacrylate could be added to diethylene glycol dimethacrylate without reducing the strength and increasing the conversion. Results for other comonomer combinations were similar.

Photochemistry of polymers: photopolymerization fundamentals and applications

The interaction of polymers and light as well as the production of polymers from photoinduced reactions is a vast subject area with numerous varied aspects. These areas include polymer formation from photopolymerizations, photocrosslinking reactions, and photodegradation reactions. This article will be restricted to reviewing the fundamental aspects of photopolymerizations as well as a summary of the current applications of photopolymerizations and other photochemical reactions in polymers.