Thermal properties of lignin-based polycaprolactones

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 (T-g) decreased until CL/OH ratio reached 20, and then,T(g)was maintained at around 200 K.T(g)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.

Automated summary

Lignin-based polycaprolactone (PCL) graft copolymers with various graft ratios were prepared to investigate the thermal properties of PCL influenced by lignin. The study found that as the length of the PCL chains increased, crystallization constraints were enhanced, affecting the thermal properties of the copolymers.