Thermally induced structural transitions in cotton fiber revealed by a finite mixture model of fiber tenacity distribution

Authors: Nam, Sunghyun; ALHASSAN, DANIEL - University Of New Orleans; Condon, Brian; French, Alfred - Al; LING, ZHE - Beijing Forestry University

Submitted to: ACS Sustainable Chemistry & Engineering
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/9/2018
Publication Date: 4/9/2018
Citation: Nam, S., Alhassan, D.A., Condon, B.D., French, A.D., Ling, Z. 2018. Thermally induced structural transitions in cotton fiber revealed by a finite mixture model of fiber tenacity distribution. ACS Sustainable Chemistry & Engineering. 6:7420-7431. https://doi.org/10.1021/acssuschemeng.7b04919.
DOI: https://doi.org/10.1021/acssuschemeng.7b04919

Interpretive Summary: Because of the complexity of the structure of cotton, identifying its structural transformation under heat treatments is not a trivial task. In particular, the thermal transition of the amorphous phase has been a controversial topic. In this study, the stepwise response of amorphous cellulose to elevated temperatures was revealed by parameterizing the periodic pattern of bimodality in the tenacity distribution using the mixed Weibull model. Analyzing variations of the five parameters identified the gradual glass transition, which could not be detected by the simple summary statistics reported in the literature, as well as two subsequent responses at higher temperatures. The calculation of the X-ray diffraction patterns using the Rietveld refinement method provided information on three distinctive thermal crystallization processes and the decomposition behavior of the crystalline cellulose. The results of this study demonstrate that the mixture distribution model can not only fully describe the complex statistical behavior of the tenacity of heated cotton fibers but also provide clues regarding the thermal responses of the amorphous segments and their properties. Identification of such structural alterations induced by heat would contribute to controlling or enhancing the thermal processing quality and efficiency of cotton fiber. The determined glass transition could be also advantageously employed in heat treatments such as annealing to improve the physical properties of cotton fiber.

Technical Abstract: Much processing of cotton fibrous materials accompanies heat treatments. Despite their critical influence on the properties of the material, the structural responses of cotton fiber to elevated temperatures remain uncertain. This study demonstrated that modeling the temperature dependence of the fiber tenacity distribution was a new approach to uncovering the details of the thermally induced structural transitions. As temperature increased, the tenacity probability density developed a unique pattern—periodic evolution/degeneration of bimodality—which was successfully parameterized by the mixed Weibull model. Interpretation of the variations of the model’s five parameters indicates that the amorphous cellulose underwent the following sequential transitions: glass transition at 160-220°C, thermal weakening (cellulose dehydration) at 240-260°C, and chain scission at 280-300°C. The crystallographic and thermogravimetric analyses showed the coexistence of thermal crystallization at 160-260°C. The decomposition of the crystalline cellulose, which occurred at 340-500°C, was predominant along the fiber axis, preserving the lateral crystalline structure in the remains even after a 90% weight loss.