Temperature modulated optical refractometry

Kraft lignin has been widely proposed as a renewable raw material for bio-based polyurethane (PU) synthesis. Drawbacks related to direct use of unmodified lignin for this purpose motivated considerable efforts for lignin chemical modifications, including oxypropylation, resulting in high-performance PUs. A molecular perspective on curing kinetic characterization of reactive PUs might shed some light on the relationship between PU molecular organization and phase formation during curing. This type of investigation requires the monitoring of specific thermodynamic susceptibility, i.e. volume expansion coefficient, which allows accessing molecular organization and cohesion. A novel experimental technique, temperature-modulated optical refractometry (TMOR) has in many cases the capability to identify and differentiate static, kinetic and dynamic molecular contributions to volume expansion coefficient changes. In our work, TMOR was applied to monitor the curing kinetic of bio-based reactive PU, obtained from oxypropylated kraft lignin. PUs were synthesized under three NCO-to-OH ratios and cured at 25 °C. Results revealed chemically induced glass transition of PUs and pointed out different curing rates and mass densities, as a consequence of NCO content.