ORIGINAL RESEARCH Microstructural differences between Viscose and Lyocell revealed by in-situ studies of wet and dry fibers Aakash Sharma . Shirish Thakre . Guruswamy Kumaraswamy Received: 26 June 2019 / Accepted: 13 November 2019 Ó Springer Nature B.V. 2019 Abstract Regenerated cellulose fibers manufac- tured using Lyocell and Viscose processes exhibit differences in their structural response to mechanical deformation. Here, we study the effect of stretching and stress relaxation on the orientation of crystal and amorphous phases. Our results show that on stretch- ing, orientation in both crystal and amorphous phases increases linearly with strain, correlating with the increase in stress and with the stretching of the crystalline unit cell along the c-axis. On holding after stretching to a particular strain, the stress relaxes logarithmically in time, correlating with a decrease in the strain along the c-axis of the crystal unit cell. The stress relaxation is also correlated with a logarithmic increase in amorphous orientation, while crystalline orientation stays constant. We attribute the stress development during stretching to deformation of the crystal unit cell, while crystal reorientation in the fiber direction results in increase in the crystalline orienta- tion parameter. On holding the fiber at a fixed total strain, the stress relaxes as strain is transferred from crystal to amorphous regions. Thus, the strain on the unit cell c-axis decreases and amorphous orientation increases. There are quantitative differences between the rate of increase in amorphous phase orientation during stress relaxation for Lyocell and Viscose fibers. For dry fibers, Lyocell shows a slower increase in orientation during stress relaxation. On wetting the fibers, their structural response changes qualitatively. We combine wide angle x ray diffraction and birefringence experiments with our model to infer that that on stretching the wet fiber, the crystalline phase is neither strained nor oriented. However, orientation develops in the amorphous phase. During stress relaxation in wet condition, Lyocell fibers shows a faster increase in amorphous orientation than Viscose fibers, in line with the comparison of relax- ation time spectra for wet Viscose and Lyocell fibers. Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10570-019-02863-6) con- tains supplementary material, which is available to authorized users. A. Sharma Á G. Kumaraswamy CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune, Maharashtra 411008, India A. Sharma Á G. Kumaraswamy Academy of Scientific and Innovative Research (AcSIR), Ghaziabad 201002, India S. Thakre Fiber and Textiles, Aditya Birla Science and Technology Company Pvt. Ltd, Plot No. 1 and 1A/1 MIDC Taloja, Taluka Panvel, District Raigad, Navi Mumbai, Maharashtra 410208, India Present Address: G. Kumaraswamy (&) Chemical Engineering Department, Indian Institute of Tech- nology Bombay, Powai, Mumbai, Maharashtra 400076, India e-mail: guruswamy@iitb.ac.in 123 Cellulose https://doi.org/10.1007/s10570-019-02863-6