Scanning probe microscopy and enhanced data analysis on lignin and elemental-chlorine-free or oxygen-delignified pine kraft pulp

Fibres from conventional pine kraft pulp (KP), oxygen-delignified KP (KPO) and elemental-chlorine-free (ECF) bleached KP were characterized by scanning probe microscopy (SPM). The surface morphology of lignin isolated from the respective pine KP was studied. The data analysis of the phase-shift data measured by using different tapping mode imaging parameters enabled the study of both the surface amphiphility and stiffness as a function of delignification. The average phase-shift values were observed to correlate to the respective kappa values, when imaging with small tapping amplitude (30 nm), referred to the capability of SPM to follow local amphipilic (adhesion) differences on a wood fibre surface. The roughness (standard deviation) of the average phase-shift values was taken as a measure of the sample homogeneity. The mean morphological roughness was observed to increase with decreasing bulk lignin concentration. The surface of lignin appeared mainly granular but occasionally some linear structures were observed. The surface roughness of different fractions of flow-through residual lignin samples correlated quite well to the molar mass of the respective fraction. Each grain contained hundreds of lignin molecules. Most of the lignin samples were homogeneous as regards friction and phase-contrast images. The surface of the nondelignified pine KP was typically granular. ECF delignification was more effective than oxygen delignification in removing this granular phase. The decrease in the relative amount of the granular phase correlated to a decreasing kappa value of the pulp. The granular phase is thus suggested to correspond to lignin, but it cannot be ruled out that small amounts of hemicellulose, extractives or a mixture of these components are embedded in this phase. The surface concentration of the granular phase of the KPO samples was, according to the phase-contrast images, clearly higher than the bulk lignin concentration, as indicated by the corresponding kappa values.