Microscopic and Spectroscopic Study on Phase Separation in Highly Crosslinked Biobased Polyurethane Thermosets

In this study a biobased polyurethane (PU) thermoset is investigated due to its turbidity. In contrary to the expectations, the turbidity increases with a higher amount of a low molecular weight crosslinker. Morphological aspects are investigated with SEM imaging and measurement of the effective scattering coefficient mu(s)'. FTIR spectroscopy is applied to study the influence of the chemical structure. This is combined with multivariate data analysis to identify the relevant peaks. SEM images show spherical precipitations with increasing turbidity and a simultaneous increase in the mu'(s) values. FTIR analysis shows a significant amount of unreacted isocyanate-(NCO)groups and a low level of hydrogen bonding. No formation of typical hard and soft segments is detectable. Therefore, it can be concluded that the increase in polarity differences with increasing crosslinker amount disabled the mixture of the polyol and isocyanate components, resulting in the precipitation of the isocyanate. At the same time, the low molecular weight crosslinker (similar to 200 g mol(-1)) can react with the NCO quickly, reducing the mobility of the polymer chain, with remaining, non-reacted isocyanate. A proof for the correlation of the differences in the FTIR and the mu(s)' values was found by a regression analysis with an R-2 of 0.94.