Cellulose Nanofibril Films as Bioinspired Membranes: Capitalizing on Water Interactions: Dissertation

This work represents an effort to exploit the inherent features of nanoscaled cellulose as practical advantages in membrane materials. The approach was to systematically explore the behavior of 2,2,6,6-tetramethylpiperidine-1-oxyl radical oxidized cellulose nanofibrils (TEMPO CNF) with respect to water vapor sorption mechanisms and transport of water, to tune the inherent properties using facile strategies and to expose the materials to performance testing. Surface-sensitive methods were used for revealing molecular scale phenomena directly at interfaces, whereas bulk methods were used to demonstrate their significance in macroscopic scale. Films made from TEMP O CNF were in the main role, complemented by synthetic polymers to introduce new performance features with significance for membrane materials. Water vapor sorption of TEMPO CNF thin films was studied by precise surface-sensitive analytical methods, i.e. quartz crystal microbalance with dissipation monitoring and spectroscopic ellipsometry, and combined with classical physicochemical models. It was established that water vapor sorption into TEMPO CNF thin films occurs through distinct underlying mechanisms: specific sorption below 10% RH, association of Flory-Huggins population of molecules with the films at 10-75% RH and clustering of water molecules above 75% RH. Kinetic parameters defining the transport of water molecules in the TEMPO CNF film structure were determined. The results showed that diffusion of water vapor could be used as a probing tool for elucidating structural details of moisture-responsive materials in the presence of water. Bulk and interfacial chemical modification approaches were applied. Enhancement of wet strength of TEMPO CNF films was achieved by crosslinking, whereby inherent characteristics, such as hydrophilicity, were not compromised. The water stable structure was suitable for further functionalization with a thermoresponsive polymer, poly(NIPAM). Covering mere 8% of the surface with poly(NIPAM) caused drastic changes in the performance of the TEMPO CNF film, as the increment in slope of relative water permeance around the lower critical solution temperature of poly(NIPAM) increased from 18% to 100%, showcasing the efficiency of the interfacial modification approach. CNF films were also subjected to performance testing in tetrahydrofuran and n-hexane, whereby their suitability for organic solvent nanofiltration was demonstrated. This thesis furthers the fundamental understanding of water interactions of cellulosic nanomaterials and other complex moisture-sensitive structures in the biomaterial genre. It also proposes concrete means to tune selected material properties toward desired environments and effects. In the view of this thesis, inherent structure-derived properties are the key for achieving performance features that will carry future biomaterials development beyond conventional
applications.