Starch-g-polycaprolactone copolymerization using diisocyanate intermediates and thermal characteristics of the copolymers

Starch-g-polycaprolactone copolymers were prepared by two-step reactions. The diisocyanate-terminated polycaprolactone (NCO-PCL) was prepared by introducting NCO on both hydroxyl ends of PCL using diisocyanates (DI) at a molar ratio between PCL and DI of 2:3. Then, the NCO-PCL was grafted onto corn starch at a weight ratio between starch and NCO-PCL of 2:1. The chemical structure of NCO-PCL and the starch-g-PCL copolymers were confirmed by using FTIR and ¹³C-NMR spectrometers, and then the thermal characteristics of the copolymers were investigated by DSC and TGA. By introducing NCO to PCL (M_n:1250), the melting temperature (T_m) was reduced from 58 to 45°C. In addition, by grafting the NCO-PCL (35-38%) prepared with 2,4-tolylene diisocyanate (TDI) or 4,4-diphenylmethane diisocyanate (MDI) onto starch, the glass transition temperatures (T_g's) of the copolymers were both 238°C. With hexamethylene diisocyanate (HDI), however, T_g was found to be 195°C. The initial thermal degradation temperature of the starch-g-PCL copolymers were higher than that of unreacted starch (320 versus 290°C) when MDI was used, whereas the copolymers prepared with TDI or HDI underwent little change.