Investigation of stability of branched structures in softwood cellulose using SEC/MALLS/RI/UV and sugar composition analyses

Size-exclusion chromatography using multi-angle laser-light scattering, refractive index, and ultraviolet absorption (SEC/MALLS/RI/UV) detection was applied to Japanese cedar (JC) and eucalyptus (E) powders after delignification (D), extraction with 4% NaOH (H), and acid hydrolysis (A), with different sequences of the D, H, and A treatments. All the delignified wood samples were dissolved in 8% (w/w) LiCl/N,N-dimethylacetamide (DMAc) after ethylenediamine pretreatment. The wood sample/LiCl/DMAc solutions were diluted to 1% (w/v) LiCl/DMAc and subjected to SEC/MALLS/RI/UV analysis. The cellulose molecules in the high-molar-mass (HMM) fractions of the eucalyptus samples, i.e., E-D, E-DHA, and E-ADH, were linear polymers with random coil conformations. In contrast, the cellulose molecules in the HMM fractions of the Japanese cedar samples, i.e., JC-D, JC-AD, JC-DHA, and JC-D-α-cellulose (prepared from JC-D by soaking in 17.5% NaOH), had branched structures. However, the JC-ADH HMM fraction contained no branched structures. The results of SEC/MALLS/RI/UV and neutral sugar composition analyses of the Japanese cedar samples showed the presence of chemical linkages between cellulose and glucomannan molecules through lignin or lignin fragments. The stability of the cellulose/glucomannan linkages in the Japanese cedar holocellulose was investigated based on the results for samples prepared using different sequences. The obtained results were consistent with those of SEC/MALLS/RI analysis of softwood acid-sulfite and kraft pulps; softwood kraft pulp has branched structures in the HMM fraction, whereas softwood acid-sulfite pulp has no such branched structures.