The ability to control the structure of the wood-pulp fiber cell wall is an attractive means to obtain increased accessibility to the fiber interior, providing routes for functionalization of the fibers that support further processing and novel material concepts, e.g. improved degree of polymerization, nanofiltration as demonstrated in previous studies. It has been proposed that dynamic compression and decompression of the cellulose pulp fibers in the wet state make it possible to modify the cell wall significantly. We hypothesize that hydrostatic pressure exerted on fibers fully submerged in water will increase the accessibility of the fiber wall by penetrating the fiber through weak spots in the cell wall. To pursue this, we have developed an experimental facility that can subject wet cellulose pulp samples to a pressure pulse -10 ms long and with a peak pressure of -300 MPa. The experiment is thus specifically designed to elucidate the effect of a rapid high-pressure pulse passing through the cellulose sample and enables studies of changes in structural properties over different size ranges. Different characterization techniques, including Scanning electron microscopy, X-ray diffraction, and wide- and small-angle X-ray scattering, have been used to evaluate the material exposed to pulsed pressure. The mechanism of pressure build-up is estimated computationally to complement the results. Key findings from the experiments consider a decrease in crystallinity and changes in the surface morphology of the cellulose sample.

Studies of molecular changes in cellulose structure as a response to different physical conditions are essential for understanding the fundamental mechanisms that can be used to optimise processing conditions and contribute to improved sustainability. Cellulose strongly interacts with water, raising the question of whether it is possible to change its molecular structure by changing the physical structure and properties of the surrounding water. Previous studies have established that the hyperbaric treatment of bio-materials permanently affects molecular structure in terms of crystallinity and accessibility. The present study shows the changes in cellulose-rich pulps on the molecular level in response to static extreme-pressure conditions. This is achieved by statically compressing the pulp-water mixture up to 3 GPa pressure using a resistive-heated di-amond anvil cell (DAC). During compression, the water transforms through a phase transformation from liquid to ice VI and ice VII, inducing a permanent increase in the crystallinity of the pulp. High-pressure, cellulose, X-ray diffraction, diamond anvil cell, crystallinity, morphological changes

The ability to modify the structure of the wood-pulp fibre cell wall structure is an attractive means to obtain increased accessibility to the fibre interior and enable functionalization such as controlled drug delivery, interpenetrated networks, and selective removal of metal ions from aqueous mixtures just to mention a few examples. By changing the physical state of water, it should be possible to significantly alter the structure of the wet fibre wall, providing the possibility to perform cell wall modifications under extreme conditions. To address this challenge, we have focussed on investigating the structural development of the wet softwood kraft pulp fibre wall under high pressure (HP) conditions (up to 2 GPa). The experiments aim to clarify the effect of the HP conditions on the porosity and the accessibility of the fibre wall for treated and untreated fibres. The second goal is to observe the changes in the crystalline structure of cellulose due to HP conditions. Different characterization techniques, including Electron microscopy, X-ray diffraction, Small and wide-angle X-ray scattering, and Cross-polarized/magic angle spinning 13C-NMR, are used to characterize material that has been exposed to HP. Key findings from the experiments relate to changes in crystallinity, specific surface area, bound water content and surface morphology.