Birefringence-based orientation mapping of cellulose nanofibrils in thin films

Determination of nanofibril orientation is crucial for predicting the properties of films and membranes made from cellulose nanofibrils (CNF) because of their inherent anisotropic nature. A novel method is proposed based on image analysis of the polarized light micrographs to quantify and map nanofibril orientation in the film structure. Thin films (average 30 mu m in thickness) of CNF were produced using a filtration method and were wet-stretched to two extension levels. Randomly-oriented films were also produced as the control without applying stretch. Samples were imaged at - 45 degrees, 0 degrees and + 45 degrees between crossed polarizers using a polarized light microscope. A BOI was developed based on the interference color changes between the two angles (+ 45 degrees and - 45 degrees). The proposed BOI values range between - 1 and + 1 differentiating orientation in perpendicular directions. The index was shown to work successfully for mapping of the fibril orientation in CNF films. Statistical analysis of the tensile test results confirmed significant difference between tensile modulus of CNF films produced using different stretch ratios. This difference was also supported by the good agreement between the tensile properties of the films, the BOI and directionality results obtained from the surface analysis of scanning electron micrographs. The method was validated by applying to single pulp fibers with known orientation as well as un-stretched and stretched polyvinyl chloride films and oriented cellulose nanocrystals. The advantages of the proposed method over other conventional methods used for orientation analysis are discussed.