Abstract
Photo-activated resin composites are widely used in industry and medicine. Despite extensive chemical characterisation, the micro-scale pattern of resin matrix reactive group conversion between filler particles is not fully understood. Using an advanced synchrotron-based wide-field IR imaging system and state-of-the-art Mie scattering corrections, we observe how the presence of monodispersed silica filler particles in a methacrylate based resin reduces local conversion and chemical bond strain in the polymer phase. Here we show that heterogeneity originates from a lower converted and reduced bond strain boundary layer encapsulating each particle, whilst at larger inter-particulate distances light attenuation and monomer mobility predominantly influence conversion. Increased conversion corresponds to greater bond strain, however, strain generation appears sensitive to differences in conversion rate and implies subtle distinctions in the final polymer structure. We expect these findings to inform current predictive models of mechanical behaviour in polymer-composite materials, particularly at the resin-filler interface.
Highlights
Previous Fourier transform infrared (FTIR) spectroscopy based studies by several groups have quantified degree of conversion (DC) in similar composite systems[18,22,31,32], but to date have only been able to provide bulk measurements where the spatial resolution of the FTIR probe is significantly greater than inter-particulate distances, effectively averaging over values of conversion arising from resin within the immediate vicinity of filler particles and the inter-particulate resin matrix
Employing the brightness of a multi-beam synchrotron source coupled with a diffraction-limited IR wide-field imaging instrument and spatial oversampling enabled us to discern subtle differences in the micro-scale structure of composites at inter-particulate length scales
As each filler particle is viewed in 2D, the measured values of conversion and strain in a pixel represent the combined contribution of the values arising from the boundary layer and the inter-particulate resin matrix
Summary
These composites combine a resin matrix with an inorganic filler phase and can be demand-set using light to excite a photo-initiator species dispersed within the matrix to initiate free radical polymerisation. The composite microstructure with respect to the pattern of conversion at interparticulate distances is unknown This is important as the microstructure, at the interface between the resin and filler phase, will dictate final physico-mechanical properties and material performance.
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