Effect Of Hydroxyl Groups On Cellulose

Three reactive hydroxyl groups (-OH) in each glucose residue (anhydroglucose, AGU unit) of cellulose enable its chemical modification to cel-lulose derivates. Usually, the hydroxyl groups in the C2 and C3 positions act as secondary alcohols and the hydroxyl group in C6 position acts as a primary alcohol. Degree of substitution(DS) is used to express the average number of OH groups per AGU unit in cellulose mole-cule. DS ranges from 0 to 3. In case all of the hydroxyl are substituted per AGU by chemical reaction, the degree of substitution of cellulose is ex-pressed as 3.0 (Wakelyn, 1998). In general, the majority of the cellulose derivates can be considered as “cellulosic random block copolymers” be-cause of the natural heterogeneity of cellulose,

Under heterogenous reaction conditions the reactivity of three OH groups on the cellulose backbone are influenced by different factors such as steric effects produced by the reacting agent, inherent chemical reactivi-ty of OH groups, and steric effects caused by the supramolecular structure of cellulose (Wakelyn, 1998). For example, in the esterification reaction, the hydroxyl groups on the C6 position react 10 times faster than other hy-droxyl groups on the C2 or C3 position (Roy et al.,

Esterification is widely used modification hydrophobization method of the cellulose sur-face due to its simplicity and straightforwardness. In esterification reac-tion, an ester functional group (O-C=O) is introduced onto the cellulose surface by condensation of the reacting agents such as carboxylic acid, acid anhydrides or acyl chlorides with a cellulosic alcohol group. Among esteri-fication reactions, acetylation reaction is one of the most widely explored esterification method for nanocellulose surface modification. In the acety-lation reaction, an acetyl functional group CH3-C(=O)- is introduced onto the surface of cellulose remaining its nanofibrillar structure. This reaction has been employed in the preparation of commercially important cellulose esters such as cellulose acetate. Acetylation reaction follows two routes ‘fibrous process’ or ‘homogeneous pro-cess’ depending on the presence or absence of a non-swelling diluent. In the ‘fibrous process’, the addition of diluents like toluene to the reaction medium keeps the acetylated chains insoluble and a high degree of acety-lation can be achieved without changing the morphology of nanocellulose. The ‘homogeneous process’ is implemented