Effects of Isocyanate-to-Polyols (NCO/OH) Ratio on Bio-based Polyurethane Film from Palm Kernel Oil based Monoester Polyols (PKO-p) for Polymer Electrolytes Application

Abstract

This study investigated the synthesis of bio-based polyurethane (PU) film from palm kernel oil-based polyols (PKO-p) at four different isocyanate-to-polyols (NCO/OH) ratios comprising 100/100, 100/150, 100/200, and 100/250. The PU film was prepared by mixing 2,4-methylene diphenyl diisocyanate (MDI) and PKO-p in acetone using the pre-polymerization technique under a nitrogen atmosphere at room temperature. The effect of the NCO/OH ratio on the properties of the PU films was analyzed through infra-red spectroscopy, molecular weight distribution, thermal behavior, and impedance spectroscopy. According to the infra-red spectroscopic analysis, the formation of urethane linkages (–NHCOO–) after the polymerization was indicated through the disappearance of isocyanate (N=C=O) peak and the appearances of secondary amine, carbonyl, carbamate, ether, and ester groups in the PU chain. In addition, the Gel Permeation Chromatography (GPC) showed that the weight of average molecular weight (M_W) increased with increasing NCO/OH ratio up to 1.23 × 10⁶ g mol^-1. Nevertheless, higher content of PKO-p resulted in a low crosslink PU (4 % crosslink) and poor physical properties such as soft, sticky, and easy to tear due to the non-hydrogen bonded urethane. Moreover, the glass-transition temperature (T_g) reduced from 67 °C to 30 °C with the increase in PKO-p content. Despite all samples experienced the same thermal stability at 187 °C, a significant difference was observed in the mass loss of NCO/OH ratios. The highest thermal degradation was found at (100/200) NCO/OH ratio with T_max = 444 °C. Furthermore, the piezoelectricity characteristic of pristine PU recorded the bulk resistant of up to ~10⁷ – 10⁵ Ω which reduced further after the presence of lithium iodide (LiI) salts as the charge carrier with the calculated ionic conductivity around 10^-5 S cm^-1. Thus, this study demonstrated the promising physicochemical properties of PU film from PKO-p for polymer electrolyte application.