Highly transparent polyurethane thermosets with tunable properties and enzymatic degradability derived from polyols originating from hemicellulosic sugars

Abstract

Polyols made entirely from biomass could play a substantial role in producing greener polyurethanes (PUs) with lower environmental impact. Here, six fully biobased branched polyester polyols derived from hemicellulosic sugars and dicarboxylic acids were prepared by melt polycondensation and further utilized for the synthesis of twelve highly transparent and malleable PU thermosets. The latter were prepared by solvent-free step growth polymerization under extremely mild reaction conditions, easily applicable industrially. This included a very short reaction time (125-335 s), low operating temperatures (40-65 °C) and excellent yields (100%). In addition, no purification step was required. All the obtained PUs possessed very good thermal stability exceeding 235 °C, Tg (3.6-70.4 °C), and a broad hot-pressing window up to 192 °C above the respective Tg. These amorphous materials demonstrated a wide range of stress-strain behaviors, from hard to ductile, with elongation at break and tensile strength appearing in the 15-188% and 3.3-31.1 MPa range, respectively, comparable or superior to those of common commercially available fossil-based PU thermosets on the market. The enzymatic hydrolysis behaviour of the synthesized PUs was assessed using lipases from Candida rugosa and Aspergillus niger. All PUs showed some susceptibility to enzymatic attack, with a maximum mass loss up to 35% after 30 days. Most importantly, it was found that compositional control by tailoring of the length of the diacid unit in the branched polyol backbone and/or the chemical crosslinking degree of the resulting PU networks can be used as a practical method for effective ‘on demand’ tuning of the thermal properties, mechanical performance and enzymatic degradation rate. Taking advantage of these promising features in combination with the optical transparency, the developed polyurethane thermosets with easily adjustable properties show great potential as innovative materials for a wide application range.