Online monitoring of the hygromechanical properties of spruce tracheid cell walls at the nanoscale

AFM PeakForce QNM under controlled relative humidity (RH) was applied to continuously monitor the indentation modulus (IM) of Norway spruce ( Picea abies ) earlywood (EW) and latewood (LW) tracheid cell walls over three sorption/desorption (S/D) cycles. The IM of the different cell wall layers were close between early- and latewoods indicating small differences between their chemical compositions. AFM IR indicate a gradient of lignin and cellulose across the cell wall with an increase of lignin and a decrease of cellulose from the S2 to the S1 and finally to the CC. Earlywood and latewood cell walls display the same hygro–nanomechanical behavior during S/D cycle, i.e., the IM decrease with during sorption up to 85% and re–increase during desorption. Gaussian fits of the IM distribution were narrower for late wood than early wood and vary with the type of layer and with the relative humidity. The responses of the indentation moduli to RH of the cell wall layers in early- and latewood were fitted according to a three-parameter logistic function. Significant differences are observed for the S2 in both EW and LW indicating a higher slope in the response of indentation moduli to RH between 15 and 50% RH in desorption as compared to sorption. For both desorption and sorption, the comparison between EW vs LW reveals differences in CC and S1, with a higher slope in moduli response to RH in LW between 15 and 50% RH and the opposite between 50 to 85% RH.