Effects of Thermal Aging on the Adhesion Forces of Biopolymers of Wood Cell Walls

Abstract

Wood is the most important, industrially used renewable resource on the planet, but the aging mechanism of biopolymers on cell walls is poorly understood. Adhesion properties are of critical importance for wood and many other lignocellulosic materials. We used atomic force microscopy and defined the jump-off force ratio in the retract force–displacement curve to study the adhesion force phenomenon and the effects of heat treatments. Here, we identified two sigmoidal curves describing the shift of the adhesion force and the jump-off force ratio: the first curve was attributed to the movement of extractives and the second to the degradation of the hemicellulose–lignin matrix. We confirmed the hypothesis formulated in this paper by the cell wall surface topography, the analysis of the treated samples by Fourier transform infrared spectroscopy, and the analysis of volatiles during heating by headspace gas chromatography-mass spectrometry.

The immediate and short-term degradation of the wood surface in a cement environment measured by AFM

“…6a). In the previous paper [27], we found two sigmoidal curves (the 1st and 2nd curves in Fig. 1c) to describe shifting of data points (adhesion force, jump-off force ratio) after thermal treatments.…”

“…The cellulose aggregates turn into a loose structure without the cohesive matrix. The pit border is an over-arched secondary cell wall, and the cellulose aggregates are not aligned the same as in the unarched part [27] where regular parallel lines were shown. Therefore, we believe that the particles on the surface are clusters of cellulose aggregates after the removal of the extractives and the hemicelluloseslignin matrix.Therefore, we conclude that the shifts in the adhesion forces and jump-off force ratios represent the degradation process of the border surface chemical components: the removal of the non-structural component extractives, the degradation of the hemicelluloses-lignin matrix and the exposure of the cellulose fibrils.…”

“…We hypothesize that the removed substance is extractives and the hemicelluloses-lignin matrix, and the particles left on the surface are clusters of cellulose aggregates. We reported the cellulose aggregates to be in the width of 40-60 nm on wood cell walls, which were exposed after the extractives and the hemicelluloses-lignin matrix were removed by heating [27]. More explanations will be given with the data on the change of adhesion forces in the latter part of the paper.…”

“…It has been reported that wood extractives are released and might retard the cement hydration process [31][32][33]. ab The 1st and 2nd sigmoidal curves in the previous paper [27] We assume that the alkaline solutions remove the extractives the pit border surface: This causes the data points to shift from the stage I to the stage II. The morphologies (Fig.…”

“…Correspondingly, the surface modulus decreased to 0.20-0.23 of the untreated samples (the 95% CI, Fieller's theorem [25]) after 648-h treatments. We assume that the shift from the stage II (264h treatments) to the stage III (648-h treatments) indicates that the hemicelluloses-lignin matrix is degraded and the underlying cellulose aggregates are uncovered [27]. The cellulose aggregates turn into a loose structure without the cohesive matrix.…”