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Electrospun Cellulose as a Model Substrate for Enzymatic Hydrolysis
Park, Heidi J
Biomass is a potential feedstock for fuels and chemicals, but is primarily composed of cellulose, which is resistant to hydrolysis. It has been hypothesized that the microstructure of cellulose plays an important role in the hydrolysis process; however, current cellulose substrates do not have easily controllable microstructure. The microstructure of cellulose can be controlled by electrospinning nonwoven mats of pure cellulose fibers from solution. The degree of polymerization, degree of crystallinity, and diameter of the fibers can be controlled by varying the binary solvent and processing conditions for eletrospinning. Cellulose with degrees of polymerization (DP) 210, 550, and 1140 were electrospun with two different binary solvents. Fibers electrospun from solutions of cellulose in N-methlymorpholine-N-oxide (NMMO)/water at elevated temperature had mid to high crystallinities (~50 - 80%), whereas solutions of lithium chloride (LiCl)/dimethylacetamide (DMAc) at room temperature gave less crystalline (~30%) cellulose fibers. Varying the infusion rate of the solution or the distance between the nozzle and the collector allowed for varying the fiber diameters, producing submicron- through micron-scale fibers with superficial surface areas on the order of 10 m2/g. Some preliminary results for hydrolysis of electrospun cellulose (ESC) fibers with cellulase enzymes are reported, and demonstrated the potential for kinetics studies with ESC to provide insight into how the microstructure affects the rates of hydrolysis. The conversion and product profiles demonstrate that ESC is hydrolyzed similarly to other insoluble cellulose substrates. However, the results of these preliminary hydrolysis studies with monoaxial ESC revealed interesting effects on the fibers; specifically, loss of long-range fiber connectivity and residual insoluble fractions that consisted of primarily 10 micron fragments at the end of hydrolysis. Coaxial cellulose fibers were investigated to address issues seen during monoaxial electrospinning and hydrolysis. First, fibers with a cellulose core made from low DP cellulose/LiCl/DMAc solutions were electrospun with well-spinnable solutions on the shell, as the low DP cellulose/LiCl/DMAc solutions did not electrospin monoaxially. Solutions of cellulose acetate (CA) were able to entrain the cellulose solutions in the core of the fibers; however, upon chemical removal of the CA shell the fiber morphology was largely lost. While this demonstrated that coaxial electrospinning can be utilized to form fibers from non-spinnable solutions, further work must be done to retain fiber morphology once the non-cellulose shell is removed. Coaxial electrospinning was also used to form fibers with a cellulose shell and a non-hydrolysable core, to prevent fragmentation and loss of long-range fiber connectivity during hydrolysis. Initial studies used cellulose/LiCl/DMAc solutions as the shell and CA as the core. Though transmission electron microscopy (TEM) showed that these fibers have some coaxial nature, hydrolysis produced no soluble products. Scanning electron microscopy (SEM) of the hydrolyzed pellets showed interesting pitting and surface roughening, indicating the potential for coaxial ESC to provide insights into cellulose degradation. Cellulose/NMMO/water solutions were also investigated as the shell in coaxial electrospinning. Due to the heating required for electrospinning the cellulose/NMMO/water solutions, polyacrylonitrile (PAN) was found to be the most suitable core material. Preliminary hydrolysis of cellulose-PAN fibers showed reasonable degradation, and SEM analysis showed evidence of fiber stripping, peeling, and thinning. These things were not seen in previous ESC hydrolysis with monoaxially spun cellulose, and further demonstrate the potential for coaxial ESC to provide new insights into cellulose hydrolysis mechanisms. However, the conditions required for varying the important microstructural features of the cellulose shell must be investigated, and fiber uniformity still needs to be optimized for coaxial ESC. Once this is achieved, ESC and coaxial ESC may reveal novel details of the evolution of the degradation of cellulose by cellulases and the effects of microstructure on this process.
Committee: Yong Lak Joo, Alan Brad Anton, David B. Wilson [proxy for A. Brad Anton]
American Scientific Publishers
Electrospinning; Cellulose; Coaxial Electrospinning; Enzymatic Hydrolysis
Previously Published As
Park, H.J., Dingee, J.W., Fitzgibbon, S.R., Anton, A.B., Joo, Y. L., "Controlling Cellulose Microstructure via Electrospinning with Applications in Enzymatic Hydrolysis", J. Biobased Materials and Bioenergy 1, 245?256 (2007)
dissertation or thesis