Polarized Raman spectroscopy for determining the orientation of cellulose microfibrils in wood cell wall

It is well established that wood cellulose has highly ordered architecture at scales from a few nanometres to several microns in wood cell wall. Its preferred orientation could be in-situ investigated by polarized Raman spectroscopy. However, the technique is currently underused due to the challenges in sample preparation, instrumentation access and availability of prediction models. Here, a novel strategy is proposed to analyse the microfibril orientation without requiring prior knowledge of the fiber alignment. We derive the mathematical dependence between the incident light polarization angle and the intensity of Raman signal. An updated prediction model of microfibril orientation was thus constructed to replace the previous empirical functions. The problems of degradation and burning were addressed by adjusting the laser power and shortening the acquisition time. The orientation direction of cellulose microfibrils can be determined by the polarization-dependent 1096 cm⁻¹ intensity without rotating sample. The strategy was successfully used to study the variations in molecular orientation throughout a cell wall of balsa wood. It was found that the cellulose microfibrils are arranged spirally to form the cell wall.