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Toughening of isosorbide methacrylate thermoset network via molecular chain extension by Aza-Michael addition with piperazine
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Toughening of isosorbide methacrylate thermoset network via molecular chain extension by Aza-Michael addition with piperazine

Heather Rose Berensmann
Master of Science (M.S.), Drexel University
Jun 2020
DOI:
https://doi.org/10.17918/00000167
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Abstract

Biopolymers Three-dimensional printing Piperazine Vinyl ester resins
In recent years, biobased polymers have gained increased interest in becoming an appropriate substitute for petroleum-based plastics. One promising biobased compound is isosorbide which is synthesized via esterification to fabricate Isosorbide Methacrylate (IM). IM is a low viscosity (157 cP) vinyl-ester (VE) resin which forms a thermoset network via free radical polymerization. Polymerized on its own, IM has a T_g of approximately 220 °C, making it a suitable candidate for a matrix material in high temperature environments. However, due to its high crosslinking density, free radically polymerized IM thermosets are brittle with a K_[1c] value of roughly 0.32 MPa/[square root of]m. In order to toughen IM networks, molecular chain extension by Aza-Michael addition was carried out using piperazine, a difunctional secondary amine compound. By increasing the molecular spacing between crosslinking junctions of IM with piperazine, the crosslinking density is reduced; thus, causing a potential increase in toughness. However, molecular modifications cause other changes in material properties. Therefore, the processing behavior and thermomechanical properties of chain extended IM we also investigated. We aimed to maintain a viscosity of about 500 cP to allow for the resin to be processed via vat photopolymerization using digital light processing (DLP) 3D-printing and we also aimed to identify to what extent toughness can be increased without a significant reduction in IM's material properties including viscosity, T_g, and modulus. It was found that chain extension increases the viscosity of IM significantly and that the viscosity varies exponentially with number average molecular weight of the chain extended monomer. Using 5 wt% (molar ratio of 0.173) resulted in a viscosity around 1900 cP; however, the material could be adequately printed using DLP after determining the appropriate printing parameters by a working curve analysis. It was also found that after printing and post-curing the chain extended IM with 5 wt% piperazine had a T_g of approximately 160 °C, as measured by the position of the tan [delta] peak from DMA. This is significantly lower than the T_g of pure IM (>220 °C), confirming that chain extension with piperazine does reduces the T_g of IM systems significantly. Nevertheless, the values of T_g are significantly higher than those of commercial vinyl-ester resins that have T_g in the range of 110 to 150 °C. Based on the pronounced effect that chain extension has on T_g, it is expected that a significant toughening effect will also be observed. Unfortunately, during March of 2020 Governor Tom Wolf issued an executive order for all nonessential residents to shelter in place in order to prevent the spread of COVID-19. Therefore, fracture toughness experiments were not conducted and are the subject of future work.

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