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Dimensional stability of sparse network microstructures formed by photopolymerization-induced phase separation Part B Polymer physics

Guenthner, Andrew J., Hess, David M., Cash, Jessica J.
Journal of polymer science 2010 v.48 no.4 pp. 396-410
dimensional stability, evaporation, molecular weight, momentum, polymers, solidification, solvents, surface area, viscoelasticity
The morphology of sparse filament networks formed during photopolymerization-induced phase separation of mixtures of NOA81 (a UV-curable thiol-ene adhesive) in mixed cosolvents consisting of water, diglyme, and polyethers of varying molecular weight was investigated as a function of the molecular weight and relative amount of the polyethers used. During photopolymerization (50 mW/cm⁻² of 365 nm radiation for 60 s) of solutions containing 5 wt % NOA81 and a total oligo-ether or polyether to water ratio of 8:1 by weight, viscoelastic phase separation produced a sparse network of interconnected NOA81 filaments. During the subsequent evaporation and/or solidification of the solvents, the network compacted significantly via a collapse process that was curtailed by increasing both the weight fraction and molecular weight of the nonvolatile polyether. The influence of mass and momentum transport processes on the collapse of the phase-separated network and the resultant final morphology was determined with the aid of dimensional analysis, leading to the identification of sedimentation and compaction driven by the motion of the interface as key factors. The networks exhibiting the least collapse combine a high level of interconnectivity and specific surface area with a low occupied volume fraction while being fabricated via a simple, template-free process.