Thermal properties of lignin-based polycaprolactones

Abstract

Lignin-based polycaprolactone (PCL) graft copolymers with various graft ratios were prepared by ring-opening reaction in order to design new lignin derivatives. Molecular length of PCL side chain was varied in a wide range, i.e. caprolactone/hydroxyl group (CL/OH) ratio was varied from 3 to 100 mol mol−1. The effect of lignin on thermal properties of PCL was examined by differential scanning calorimetry (DSC) and thermogravimetry. In DSC heating curves of lignin-based PCL, glass transition, pre-melt crystallization and melting were observed. From DSC results, the phase diagram of lignin-based PCL was established. It was found that glass transition temperature (Tg) decreased until CL/OH ratio reached 20, and then, Tg was maintained at around 200 K. Tg slightly increased when CL/OH ratio became higher than 20. The above facts suggest that the random structure of lignin affects the molecular motion of copolymers in a limited length of PCL chains, and that with increasing PCL chain length, free molecular motion is restricted by crystallization. The amorphous region of lignin–PCL was reorganized at pre-melt crystallization, and crystallinity increased. When CL/OH ratio increased more than 20, two melting peaks were observed; the low-temperature side is PCL crystallite affected by lignin and the high-temperature side melting peak is attributed to melting PCL crystal. The effect of lignin on thermal decomposition is apparently observed for lignin-based PCL when CL/OH ratio is smaller than 20. The above results suggest that lignin-based PCL graft copolymers have the potential of an applicant of novel nature-friendly polymers.