Effects of NCO/OH molar ratio on structure and properties of graft-interpenetrating polymer networks from polyurethane and nitrolignin

A series of graft-interpenetrating polymer networks (graft-IPNs) with different NCO/OH molar ratio was synthesized from castor oil-based polyurethane (PU) and nitrolignin (NL) by changing the content of 1,4-butanediol as chain-extender. The effects of NCO/OH molar ratio on the morphology and properties of the PU–NL films (UL) were investigated by infrared, wide-angle X-ray diffraction (WAXD), differential scanning calorimetry, dynamic mechanical analysis, and tensile test. WAXD patterns showed that all the UL films were nearly amorphous. With an increase in NCO/OH molar ratio, the glass transition temperature (Tg), the stiffness and tensile strength (σb) increased, while the breaking elongation (εb) and strain recoverability (Re) decreased. The UL films with 1.20 of NCO/OH molar ratio exhibited the maximum value of tensile strength (σb=30.2 MPa; εb=152%). 2.8 wt% NL plays an important role in the simultaneous enhancement of σb and εb of the UL films. Solid-state 13C nuclear magnetic resonance (13C NMR) showed that the introduction of NL into PU restricted the motion of PU molecules, indicating a relatively high cross-link network structure in the UL films. Therefore, a relatively great network structure like a star by grafting with multi-PU networks on a NL molecule as the graft-IPNs model was deduced in this work.