Toughening modification of polyester–urethane networks incorporating oligolactide and oligocaprolactone segments by utilizing castor oil as a core molecule

Ring-opening polymerizations of d-lactide, l-lactide and ɛ-caprolactone initiated from hydroxy groups of castor oil (CO) produced three kinds of branched oligomers (CODLAO, COLLAO and COCLO). The reactions of CODLAO, COLLAO and COCLO with hexamethylene diisocyanate (HDI) produced polyester–urethane networks (PUCO-scLAO/CLOs 100/0, 75/25, 50/50, 25/75 and 0/100) with different feed ratios of stereocomplex oligolactide (scLAO, that is a mixture of equal parts of COLLAO and CODLAO) and COCLO. Also, the similar reactions of COLLAO and COCLO with HDI produced homochiral networks (PUCO-LLAO/CLOs). X-ray diffraction and differential scanning calorimetry analyses revealed that stereocomplex crystallites were exclusively formed for all of the PUCO-scLAO/CLOs except for the 0/100 sample, whereas the oligo(l-lactide) segments of PUCO-LLAO/CLOs 100/0 and 75/25 did not homo-crystallize. Scanning electron microscopic analysis revealed that the compatibility for the PUCO-scLAO/CLO and PUCO-LLAO/CLO 75/25–25/75 conetworks slightly decreased with increasing CLO fraction. Dynamic mechanical analysis revealed that the lowering of storage modulus due to glassy-to-rubbery transition for PUCO-scLAO/CLOs 100/0–25/75 was much smaller than that for PUCO-LLAO/CLOs 100/0–25/75. Although the incorporation of CLO segments was effective to increase the elongation at break, the tensile strengths and moduli of the 75/25–25/75 conetworks were considerably lower than those of the 100/0 and 0/100 networks. Consequently, the 100/0 and 0/100 networks exhibited more balanced tensile properties than the 75/25–25/75 conetworks. It is noteworthy that the tensile toughnesses and elongations at break of the CO-modified 100/0 networks are much higher than those of the similar networks using glycerol instead of CO.