Structural Investigation of Cellulose-Hemicellulose Composites

Plant cell walls are comprised of three polymers: cellulose, hemicellulose and lignin. The spatial arrangement of these components and changes among them on pretreatment of biomass are not clear leading to poor saccharification of cellulose. This is due to the complex and polymeric nature of plant cell wall structure. To understand the interactions of the plant cell wall components before and after pretreatment, we employed a bottom-up approach. In this approach, composite materials comprising of only two of the three plant cell wall components were prepared by synthesizing bacterial cellulose from Acetobacter xylinus in presence of hemicellulose. Small angle neutron scattering (SANS) was used to study composite morphology and changes on pretreatment. Deuterated bacterial cellulose provided contrast between the cellulose and hemicellulose. The scattering signal from hemicellulose was minimized making it possible to extract size and shape information for cellulose.  In-situ SANS measurements were carried out to observe changes in real time in the composite material during acid pretreatment. Structural changes in cellulose microfibril in presence of hemicellulose (xyloglucan or glucomannan) were observed during the temperature rise from 120 to 170 ˚C. The scattering profile remained the same when temperature was maintained at 170 ˚C for 5 mins and during the cool down phase. This nanoscale observation of the structural fate of the cellulose over the course of pretreatment has provided fundamental insights to cellulose accessibility and crystallinity. Such information will enable further improvement of pretreatment processes by providing details about the events that happen during the course of pretreatment.