Thermal properties of freezing bound water restrained by sodium lignosulfonate-based polyurethane hydrogels

Abstract

In order to develop a new functional product from lignin, sodium lignosulfonate (LS)-based polyurethane (LSPU) hydrogels were prepared from LS and hexamethylene diisocyanate (HDI) derivatives in water. Isocyanate/hydroxyl group ratio (NCO/OH ratio) was varied from 0.05 to 0.8 mol mol−1, and water content (Wc = mass of water/mass of dry sample) of the obtained LSPU hydrogels was varied from 0 to 3.0 g g−1. Phase transition behavior of hydrogels with various Wc’s was investigated by differential scanning calorimetry (DSC) and thermogravimetry (TG). In DSC heating curve of LSPU hydrogels, glass transition, cold crystallization, melting and liquid crystallization were observed. Cold crystallization, two melting peaks and variation of melting enthalpy indicate that three kinds of water, i.e., non-freezing water, freezing bound water and free water, exist in LSPU hydrogel. Glass transition temperature (Tg) decreased from 230 to 190 K in a Wc range where non-freezing water was formed in the hydrogel. Tg increased when freezing bound water was formed in the system. Tg leveled off in a Wc range where normal ice was formed. The effect of NCO/OH ratio on molecular motion of LSPU hydrogel is examined based on Tg and heat capacity difference at Tg (ΔCp). Water vaporization curve measured by TG also indicates the presence of bound water which evaporates at a temperature higher than ca. 410 K. By atomic force microscopic observation, the size of molecular bundle of LSPU hydrogel is calculated and compared with that of LS-water system. By cross-linking, the height of molecular bundle decreased from ca. 3–1 nm and lignin molecules extend in a flat structure.