Chemical modification of cellulose-new possibilities of some classical routes
Abstract: Owning to its unique structure, along with the inexhaustible renewability, cellulose has been a subject of scientific and commercial interest for over 150 years. However, given attractive structural properties, such as stiffness, hydrophilicity, stereoregularity, potential for chemical modifications and ability to form superstructures, utilization of this biopolymer is far below its potential. The prospect of improving it is closely connected with chemical modification possibilities. The research presented in this thesis explores these possibilities with the emphasis on reactive groups employed for substitution of cellulose backbone in different modification systems. Under homogeneous conditions, in the system dimethyl sulfoxide/tetrabutylammonium fluoride four new esterfication agents for in situ activation of carboxylic acids were successfully employed in preparation of cellulose esters. In heterogeneous aqueous systems, focus was on readily quantified cationizations of cellulose accomplished by oxirane mediated etherifications. Reactions of two new etherification agents, 2-oxiranylpyridine and N-oxiranylmethyl-N-methylmorpholinium chloride were explored. Etherification with the former provided cellulose with reactive pyridine moieties that could be utilized in further functionalizations, i.e. quaternizations of the pyridine nitrogen yielding cationic celluloses. Further, etherification with N-oxiranylmethyl-N-methylmorpholinium chloride introduced good leaving groups, N-methylmorpholine moieties, which could be employed in subsequent self-crosslinking reactions. Obtained crosslinked materials exhibited remarkably altered structure accessibility, highly defined by choice of crosslinking conditions. Oxirane mediated cationization was further applied on surface cationzation of cellulose nanocrystals. Reaction with N-oxiranylmethyltrimethylammonium chloride yielded sufficient surface cationization required to provide electrostatic colloidal stabilization of their aqueous suspensions. Interestingly, these suspensions exhibited thixotropic gelling along with the typical tendencies to self-order. Another cationization method applied on cellulose nanocrystals was studied, as well. It was based on intermediate esterification of cellulose nanocrystals with chloroacetylchloride in a non-aqueous system, followed by subsequent substitution with a tertiary amine. This procedure yielded highly cationized cellulose nanocrystals with varying extent of surface and bulk modification. In spite of overall high surface cationization the modified nanocrystals lacked colloidal stabilization in aqueous suspensions, which is likely an effect of strong interactions between introduced groups.
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