Chemical Modification of Electrospun Cellulose Nanofibers
Doctoral thesis, 2015
The forest industry is a large part of the Swedish economy and the export of pulp, paper and wood products constitutes 11% of the total Swedish export of goods. The main product of a pulp mill is purified cellulose, which has many great properties like biodegradability and non-toxicity. Cellulose pulps are mainly used for paper and board products whereas speciality pulp such as dissolving pulp is used for regenerated cellulose in fiber and cellophane production. Cellulose is difficult to dissolve and process, but some ionic liquids have the ability to dissolve cellulose. Ionic liquids also have low vapor pressure and low flammability which make them suitable for large scale processes.
In this thesis sub-micron scale cellulose nanofibers were created by electrospinning of cellulose from ionic liquid. Solution properties were studied and correlated to nanofiber formation. It was found that co-solvents could decrease viscosity and surface tension. As a consequence the rheological properties of the electrospun cellulose solutions could be linked to fiber formation. Successfully electrospun solutions had a high zero shear viscosity were highly shear thinning.
Electrospun cellulose nanofibers were thereafter further functionalized towards different applications. They were made conductive by synthesis of a polypyrrole layer on the nanofiber surface. It was shown that polypyrrole adhered to the nanofiber surface and increased the surface roughness of the nanofibers. The non-toxic property of cellulose was retained after polypyrrole synthesis and neural cell culture experiment indicated that polypyrrole enhanced cell adhesion to the nanofibers.
Electrospun cellulose nanofibers were chemically modified to give carboxylate rich surfaces. These anionic fibers were used as templates for synthesis of nano-porous structures of metal-organic framework (MOF). Carboxymethylated nanofibers had an even distribution of MOFs over the nanofiber surface. The MOF functionalized cellulose nanofibers had good adsorption properties and the surface area was greatly increased.
The concept of synthesizing MOFs on cellulose was transferred to another type of nano structured cellulose, cellulose nanofibrils (CNF), which currently can be produced in larger amounts than electrospun cellulose. A zeolitic imidazolate framework (ZIF) was synthesized on CNF in aqueous medium. Highly porous novel ZIF/CNF hybrid materials were created.
Zeolitic Imidazolate Framework