Engineering plastics from lignin. VII. Structure property relationships of poly(butadiene glycol)‐containing polyurethane networks

Hydroxypropyl lignin‐based thermosetting polyurethanes containing polybutadiene (PBD) glycol soft segments (Mn of 2800 g M−1) were synthesized with excess hexamethylene diisocyanate (HDI) and tolylene diisocyanate (TDI) by solution casting. Miscibility of the glycol with the lignin derivative was found to be poor as expected, and phase separation between the two polyol components in polyurethanes was detected by thermal and mechanical analysis, and by electron microscopy. This study examines the effect of concentration of polybutadiene glycol on the thermal and mechanical properties of the polyurethanes. The two‐phase network system displayed significantly different properties than either the poly(ethylene glycol)‐containing polyurethanes or their soft segment‐free counterparts described previously. Macrophase separation was observed at nearly all degrees of mixing and was found to affect thermal and mechanical properties. The glass transition temperature (Tg) of the lignin phase in the TDI‐based networks increased with poly(butadiene glycol) content rising from 3.6 to 71.4% of polyurethane, and this was attributed to the employment of a constant diisocyanate weight fraction which gave rise to a variable NCO/OH ratio and crosslink density. Distinct phase separation was evidenced by scanning electron microscopy (SEM) at above 3.6 and 7.1% glycol content for HDI‐ and TDI‐based films, respectively. The polyurethane films behaved like rubber‐toughened lignin networks when PBD was the discrete phase, and like lignin‐reinforced rubber when the lignin derivative was discrete. This behavior was evidenced by the Young's modulus decreasing from 2000 to 50 MPa and ultimate strain rising from 6 to greater than 150%, with soft segment content increasing from 0 to 71.4%.