Characterization of properties and structural heterogeneity of crosslinked polymers formed by living radical photopolymerizations.

摘要

Polymer networks formed by radical photopolymerizations of multifunctional monomers are finding use in an increasing number of applications. To meet this increasing demand, it is important to tailor these materials and their properties to suit the application requirements. However, to achieve this goal, an understanding of the underlying polymerization-structure-property relationships of these networks is necessary. This work focuses on understanding the effect of polymerization conditions and the evolution of material properties and structural heterogeneity in crosslinked polymers.;While photopolymerizing these multifunctional monomers, microgels, unreacted double bonds (monomeric and pendant), and trapped radicals are features that have been observed by several researchers. Also, the resultant structure of the crosslinked polymers is extremely heterogeneous. Previously, examining the material properties of such networks as a function of temperature has been very difficult because the unreacted double bonds and trapped radicals continue to react as the temperature approaches the glass transition temperature of the material. Therefore, while studying the properties of the sample, the structure and properties are altered. In this work, "living" radical polymerizations are used to avoid radical trapping. As a result, for the first time the properties and structural heterogeneity of the polymers are studied as a function of temperature at various double bond conversions and crosslinking densities.;To enable the use of "living" radical photopolymerizations in the characterization of polymer networks, it was important to understand the mechanism of the "living" radical polymerizations. Therefore, a study of the kinetics and mechanisms of the "living" radical polymerizations was also undertaken. Experimental and modeling studies were performed to understand the mechanism of these reactions.;By performing dynamic mechanical and dielectric measurements on the polymer samples, modulus relaxation and dielectric relaxation spectra were obtained over a wide range of relaxation times. Such analyses provide useful information regarding the structural heterogeneity of the polymer networks because the distribution of relaxation times is a measure of the inhomogeneity. It was observed that by increasing crosslinking density, the structural heterogeneity increases. Also, decreasing the functionality of the comonomer results in a more homogeneous sample while copolymerizing multifunctional (meth)acrylates.;Finally, the mechanical strengths, glass transition temperatures and structural heterogeneity of photocured dimethacrylate dental resins have been characterized as a function of double bond conversion as well as composition of comonomers.