Automated Birefringence-Based Orientation Mapping of Cellulose Nanofibril Films for High-Performance Sustainable Packaging Applications
Published: 2026
Publication Name: ACS Applied Nano Materials
Publication URL: https://pubs-acs-org.wv-o-ursus-proxy02.ursus.maine.edu/doi/full/10.1021/acsanm.6c00095
Abstract:
The nanoscale dimensions of cellulose nanofibrils (CNFs) impart significantly higher barrier and mechanical properties to CNF films compared with their original cellulosic form, making them a viable choice for food packaging applications. These properties are also highly affected by the orientation of CNFs in thin films. A fast and accurate method for CNF film quality control in terms of fibril orientation is still needed despite the existence of some current approaches. In this work, we introduce the improved birefringence orientation index (iBOI) based on polarized light microscopy through an automated process for calculating the BOI. Unlike the traditional BOI approach, which compares only two polarized images taken at −45° and +45° using differences in the blue channel digital numbers, the iBOI uses images captured from −90° to +90° in 15° increments or even smaller increments, leading to improved performance and thematic resolution. The iBOI also results in a vectorized map that represents the fibril orientation angles. The iBOI was first validated on polyvinyl chloride samples, which have a more homogeneous structure and can be easily oriented by stretching. Then, oriented and nonoriented CNF films were prepared and evaluated using the iBOI. Mechanical tensile tests and scanning electron microscopy images confirmed the classification, with oriented samples showing higher tensile modulus and lower toughness as well as a clear, near-normal orientation distribution centered around the dominant orientation angle as compared with the nonoriented samples. The results were also in agreement with calculated Herman’s orientation parameter obtained through X-ray diffraction. We also showed that iBOI is independent of film thickness, and its thematic resolution can be improved by reducing imaging angle increments. The automation and fine angular resolution of the iBOI provide an efficient and reliable tool for CNF orientation quantification and mapping.