Estimation of the Cellulose Microfibril Angle in Acacia mangium Wood Using Small-Angle X-Ray Scattering

Abstract

The term microfibril angle, MFA in wood science refers to the angle between the direction of the helicalwindings of cellulose microfibrils in the secondary cell wall, S2 layer of fibres and the long axis of the cell wall.A method is presented for the measurement, using small-angle X-ray scattering (SAXS), of the microfibril angle,(MFA) and the associated standard deviation for the cellulose microfibrils in the S2 layer of the cell walls ofAcacia mangium wood. The length and orientation of the microfibrils of the cell walls in the irradiated volume ofthe thin samples are measured using SAXS and scanning electron microscope, (SEM). The undeterminedparameters in the analysis are the MFA, (M) and the standard deviation (σ?) of the intensity distribution arisingfrom the wandering of the fibril orientation about the mean value. Nine separate pairs of values are determinedfor nine different values of the angle of the incidence of the X-ray beam relative to the normal to the radialdirection in the sample. The results show good agreement. The curve distribution of scattered intensity for thereal cell wall structure is compared with that calculated with that assembly of rectangular cells with the sameratio of transverse to radial cell wall length. It is demonstrated that for β = 45° (where β is the angle between theplane face of the wood samples and the radial direction) the peaks in the curve intensity distribution for the realand the rectangular cells coincide. If this peak position is ?45, Then the MFA can be determined from the relationM = tan-1 (tan ?45 / cos 45°), which is precise for rectangular cells.