In chemically crosslinked, glassy epoxy thermosets, the effective chain between crosslinks is a vital molecular parameter, contributing significantly to its mechanical properties. In order to determine the effect of the volume of the effective chain on the mechanical properties of glassy thermosets, we have examined in detail, SU-8, a glassy epoxy thermoset, widely used for mask fabrication in the microelectronics industry, in microfluidics and in microelectromechanical systems (MEMS). We have performed conformational analyses, based on a Boltzmann search method employing, the Metropolis Monte Carlo algorithm, on different epoxy polymers to determine the rms length of this effective chain. The experimentally calculated volumes of the effective chains and the orientation of chains in the network, yield the lengths of such chains. For all the epoxies, the lengths of the effective chains, based on densities of the bulk materials, have been compared to the rms chain lengths, obtained using these conformer analyses. We have proposed a structure of a dense, 3D, crosslinked molecular network of SU-8. The rms length of SU-8 is compared to the chain length obtained via the experimentally calculated bulk density, considering the chain orientations in the proposed network model. We have employed photolithography to fabricate microcompression pillar samples of SU-8 on silicon wafers. The microcompression experiments have been using a flat-punch nanoindenter at different strain-rates, to determine the Eyring activation volume. A plot of the stress-strain curves for one such set of microcompression experiments is shown in the figure. A clear correlation has been observed between the experimentally obtained activation volume and the volume of the chain between crosslinks. We propose the molecular description of the Eyring activation volume, based on this effective chain volume.

Molecular basis of deformation behavior of chemically crosslinked epoxy thermosets

249th National Meeting (2015)

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