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ABSTRACT: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.

An investigation of the rate behavior of bulk photopolymerizations of methacrylates initiated by p-xylylenebis(N,N-diethyl dithiocarbamate) (XDT) is presented. Kinetic studies of photopolymerizations of monomethacrylates and dimethacrylates were performed by differential scanning calorimetry. A comparison of the polymerization rate behavior between the iniferter polymerizations and the conventional radical polymerizations is presented to propose a mechanism for the kinetics of the iniferter polymerization. The iniferter polymerization mechanism involves two termination pathways:  (1) carbon−carbon radical combination and (2) carbon−dithiocarbamyl (DTC) radical termination. At lower conversions, when the viscosity of the polymerizing system is lower and thus diffusional resistance to termination is low, path 1 is the significant termination mechanism. As the viscosity increases with further conversion, path 2 begins to dominate termination. Therefore, the characteristic autoacceleration effect observed in polymerizations of the mono- and multifunctional methacrylates studied with regular initiators is dramatically reduced or absent in some cases. Additional evidence for preferential cross-termination (i.e., carbon−DTC radical termination) in the presence of an excess of DTC radicals is presented by examining polymerization initiated by a combination of XDT and tetraethylthiuram disulfide (TED). Further, the effects of varying concentration of XDT, intensity of initiating ultraviolet light, and reaction temperature on the rate behavior were examined. The reaction behavior observed in these studies is well explained by the proposed mechanism.

In this work, the structural heterogeneity of copolymers formed by photopolymerizations of diethylene glycol dimethacrylate (DEGDMA) and poly(ethylene glycol 600) dimethacrylate (PEG600DMA) has been characterized as a function of double bond conversion and cross-linking density (as measured by comonomer composition) by dielectric analysis (DEA). The heterogeneity has been characterized by a distribution parameter which represents the breadth of the polymer relaxation spectrum. It was found that the distribution parameter as measured by DEA matched well with those estimated previously by dynamic mechanical analysis. In addition to obtaining fundamental structural information from the α-transition (or glass transition) region, β-transitions (or secondary transitions) were studied in the poly(DEGDMA-co-PEG600DMA) networks studied. These secondary transitions are observed as a result of unreacted pendant as well as monomeric double bonds that are present in the samples. The heights of the damping factor peaks at the secondary transitions increase as the cross-linking density of the copolymer is increased (by increasing the DEGDMA composition). Further, the peak height decreases as the conversion increases in a given sample, strongly suggesting that the residual unsaturation in the copolymer network is responsible for the β-transition. Evidence is also presented that precludes the possibility that the initiator or its fragments cause the secondary transitions.

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