Cellulose Nanocrystals: Characteristics, Crystalline Modification, and New Preparation Routes

This thesis focused on how the properties of cellulose nanocrystals (CNCs) are affected by crystal lattice modification and crystal surface modification. In addition, developing new production methods were examined from the point of view of the limitations of the common production methods and the effect of the native environment and characteristics of cellulose. In order to investigate new production methods, hurdles in the current production methods, mechanical treatment and CNC isolation, were probed by omitting the hydrolysis step. Microcrystalline cellulose (MCC) was disintegrated to CNC via introduction of charged units by TEMPO-mediated oxidation and mechanical forces caused by sonication. However, the yields for the product fraction containing majority of the CNCs, alongside with some larger particles, remained low at 10-20% unlike what has been reported for nanofibrillated cellulose (NFC). The possible reason for the low yield of disintegration could be the limitations of the sufficient particle size upon sonication. On the other hand, the introduction of carboxylic groups on the particle surface could have resulted in additional hydrogen bonding between the particles since carboxylic groups are capable of forming more hydrogen bonds than the hydroxyl group. Additionally, TEMPO-mediated oxidation resulted in intriguing properties for the smaller particles that were produced and isolated. The porosity of the smaller particles, showed a distinct lack of larger pores that were present in larger TEMPO-oxidized particles and the untreated MCC. A possible explanation was suspected to be the hydrogen bonds created between the particles in combination of particle cleavage at the sites of the larger pores during disintegration. The bonding between CNCs was also examined by hydrophobization of the CNCs by silylation. The CNCs were first produced via acid vapour hydrolysis, which allows the CNC to remain in the fibrillar matrix of the hydrolysed cellulose microfibril (CMF) until dispersed into a medium. The silylated CNCs were disintegrated into a hydrophobic medium, toluene, and examined with TEM, which revealed that the end-to-end interactions of the CNCs were undisturbed by the hydrophobic medium, and the silylated CNCs were observed to stay in nanowire-like formations similar to the native CMF shape. However, the degree of polymerisation (DP) had reached the levelling-off DP, and therefore the formations were considered to consist of end-to-end associated CNCs. Furthermore, the effect of polymorph transition from cellulose I to III induced by complexation with ethylenediamine to CNC morphology was examined. When the polymorph transition was performed in dispersion, the dimensions of the CNCs remained unchanged. By contrast, when the polymorph transition was performed on CNCs immobilized on a surface, the crystal width decreased by half. It was hypothesized that the bonding between the CNC and the substrate caused frustration in the crystal lattice causing the outermost layer to be exfoliated.