Improved order parameter (alignment) determination in cellulose nanocrystal (CNC) films by a simple optical birefringence method

Birefringence, based on the crystal orientation of a cellulose nanocrystal (CNC) film, has been investigated for the determination of crystal orientation. The prime focus of our study is to establish a simple and low-cost experimental technique using a standard UV-Vis spectrometer to determine the order parameter (S) for both isotropic and anisotropic configurations. Self-standing CNC films of various order parameters (S: 0-0.95) were prepared using shear flow with varying shear rates. The transmitted light intensity of CNC films between cross polarizers for the bright and dark fields were measured to determine the linear dichroitic ratio, which were used to calculate the order parameters of different crystalline arrangements. Two-dimensional X-ray diffraction results were compared to the birefringence technique to explore the presence of short-range amorphous order, which is significant in higher order parameter (S > 0.60) specimens. Thus, the new birefringence technique was shown to obtain a more accurate measurement of order parameter with an easier method using more common and lower cost equipment.