Role of composition and molecular weight on the dissolution of cellulosic yarns

In this work we show that it is the molecular weight in a variety of natural and treated plant yarns that is the dominant factor in controlling both the rate of dissolution and the dissolution activation energy in the ionic liquid 1-ethyl-3-methylimidazolium acetate. We have used an alkali treatment (sodium hydroxide) to primarily reduce the molecular weight of three natural plant yarns (hemp, cotton and flax) in order to investigate how dissolution depends on molecular weight, composition and crystallinity. Dissolution experiments were carried out on both the raw and alkali-treated yarns. Chemical composition, crystallinity, and molecular weight were determined for all these six yarns. After dissolution, the partially dissolved yarns were coagulated in water, resulting in a composite material with an undissolved inner core surrounded by a dissolved and coagulated outer skin region. The growth of this dissolved and coagulated fraction was tracked using optical microscopy, showing it to increase with dissolution time and temperature. Time–temperature superposition was found to hold in all cases, allowing a dissolution activation energy to be determined. The width of the outer skin of the coagulated region was found to be directly proportional to the square root of the dissolution time, demonstrating that the limiting factor for dissolution is the diffusion of the ionic liquid. Finally, all the mercerised yarns were found to dissolve faster than their natural versions, suggesting that molecular weight is a contributing factor in affecting the speed of dissolution.