Structure and Morphology of Cellulose Films Coagulated from Novel Cellulose/Aqueous Sodium Hydroxide Solutions by Using Aqueous Sulfuric Acid with Various Concentrations

Abstract

Structure and morphology of the cellulose films coagulated from novel cellulose/9 wt% aqueous (aq.) sodium hydroxide (NaOH) systems (polymer concentration CP≤5.6 wt%) by using aq. sulfuric acid (H2SO4) with various concentration (Csa=20—80 wt%) as coagulants were investigated. For this purpose two types of alkali-soluble celluloses with either crystal form of cellulose-I (Cell-I; steam exploded spruce pulp) or cellulose-II (Cell-II; regenerated from cotton/cuprammonium solution) were utilized. SEM observation on the lyophilized coagulated cellulose films revealed that all the films have basically porous structure more or less, constituted by collision of secondary particles. Coagulation from two types of cellulose solutions underwent in a quite different way as a function of Csa of coagulant: (1) For alkali-soluble Cell-II system, the existence of secondary particles was evident in the range of Csa≥20 wt% and the most dense structure was given when Csa=60—65 wt%, (2) For alkali-soluble Cell-I system, the secondary particles became detectable at Csa≥40 wt% and the coagulant with Csa=70 wt% gave the most dense structure of the film, and (3) the size of particles constituting the most dense films is smaller for Cell-II system than Cell-I system. The coagulant with Csa≥60 wt% proved to act as strong dehydrant from cellulose solutions by Raman spectroscopy and the neutralization rate of Cell-II system was much higher than Cell-I system. CP/MAS 13C NMR analysis showed that both densely coagulated films developed practically no intramolecular hydrogen bond at C3 position.