Birefringence-based orientation mapping of cellulose nanofibrils in thin films
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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 µm 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°, 0° and + 45° between crossed polarizers using a polarized light microscope. A BOI was developed based on the interference color changes between the two angles (+ 45° and − 45°). 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.
KeywordsCellulose nanomaterials Fibril orientation Polarized light microscopy Image analysis Birefringence
Polarized light microscopy
Birefringence orientation index
The authors would like to thank USDA National Institute of Food and Agriculture McIntire-Stennis Program for financial support.
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Shokoofeh Ghasemi, Parinaz Rahimzadeh-Bajgiran and Mehdi Tajvidi developed the method to produce the orientation index. Shokoofeh Ghasemi prepared and tested CNF film samples, Parinaz Rahimzadeh-Bajgiran performed image analysis. All authors contributed to the analysis and discussions.
This project was funded by the USDA National Institute of Food and Agriculture, McIntire-Stennis project number ME0-41616 through the Maine Agricultural & Forest Experiment Station. Maine Agricultural and Forest Experiment Station Publication Number 3714.
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