SEC-MALLS analysis of softwood kraft pulp using LiCl/1,3-dimethyl-2-imidazolidinone as an eluent

Various cellulose and pulp samples including softwood bleached kraft pulp ( SBKP) were dissolved in 8% lithium chloride/ 1,3-dimethyl-2-imidazolidinone ( LiCl/DMI) and 8% LiCl/ N, N-dimethylacetamide ( DMAc), and the obtained solutions were subjected to size-exclusion chromatographic analysis with multiangle light scattering detection ( SEC-MALLS). Although SBKP was not completely soluble in 8% LiCl/DMAc, 1% LiCl/DMI always gave a clear solution of SBKP without centrifugation. Molecular mass ( MM) and MM distribution measurements using 1% LiCl/DMI as an eluent for the SEC-MALLS analysis revealed that SBKP had MM higher than those of the other cellulose and pulp samples at the same elution volume. The slope of the conformation plots for SBKP showed an anomalously low value of 0.41, while those for other cellulose and pulp samples were in the range of 0.57 - 0.59, which corresponds to the normal random coil conformations. These results indicate that some compact structures like branches or cross-linkages other than molecular-dispersed states are present in the high MM region of SBKP, which can be detected when the LiCl/DMI solvent system is used as the solvent as well as the eluent for the SEC-MALLS analysis. On the other hand, no such structures were observed for the other cellulose and pulp samples including softwood bleached sulfite pulps. Thus, the compact structures present in SBKP are likely to be susceptible to acid treatments.