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Accelerated radiation polymerization of vinyl-divinyl comonomer systems Micko, Michal

Abstract

Gel-effect (Ge) acceleration in radiation polymerization of methyl methacrylate (MMA) -divinyl monomer (DVM) systems and properties of resulting polymer products have been investigated. Four methacrylate esters with variable molecular bridge length between the double bonds, i.e., ethylene glycol dimethacrylate (EGDMA), diethylene glycol dimethacrylate (DEGDMA), triethylene glycol dimethacrylate (TrEGDMA) and tetraethylene glycol dimethacry-late (TEGDMA), were used as DVM accelerators and crosslinking agents. The course of polymerization was followed by temperature (T)-time(t) polymerization exotherm curves. A new technique was developed to determine the "Gel-Effect Point" (GEP) and individual polymerization parameters associated with gelation in crosslinked network, i.e., polymerization rate coefficient (PRC), curing time (t[MAX]), overall curing rate (l/t[MAX]), and overall acceleration constant (K). The overall curing rate was found to be proportional to the volume concentration of crosslinking agent only up to 15 to 20% DVM in the system. Within this concentration interval the overall acceleration constants increased with molecular distance between DVM double bonds in the following order: EGDMA (1.1 x 10⁻³), DEGDMA (1.5 x 10⁻³), TrEGDMA (1.8 x 10⁻³), and TEGDMA (2.4 x 10⁻³ min⁻¹ conc.⁻¹). Half-time concentration values, i.e., concentration of divinyl monomer required to reduce the curing time to one-half of that for pure MMA, increased in reverse order. Numerically, half-time concentration values for TEGDMA, TrEGDMA, DEGDMA, and EGDMA were 3, 4, 5, and 7% respectively. The calculated overall acceleration constant allowed prediction and calculation of curing times for individual comonorner mixtures. The agreement between predicted and experimentally measured values was within 5% error. The derived empirical equation for prediction of curing time was also applicable to the heat-catalyst polymerization system. All divinyl monomers studied were found to be efficient crosslinking agents and improved the thermomechanical and strength properties of the resulting copolymers. The compression stress, strain and toughness exhibited well defined maxima within 5-10% of divinyl monomer in the mixture. Within this concentration interval both maximal acceleration and superior mechanical properties of copolymers were obtained. The numerical value of copolymer connection number (CN[sub co]) was found to be a useful structural parameter relating mechanical and thermomechanical properties with copolymer crosslinking density.

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