Cellulose

Chrystalline cellulose is partailly soluble in water with some swelling [3].

δ=14.5-16.5 [2].

♦ Xyloglucans adhere strongly to cellulose microfibrils through hydrogen bonds between the xyloglucan backbone and cellulose. The binding is quantitatively to both types of cellulose [1]. The binding to cellulose I is almost the same at pH 5-12 and 1M NaCl as in destilled water [1]. At pH 13 the binding diminishes 14% [1].

♦ Scizophyllan binds quantitatively to cellulose I, but only sparingly to cellulose II [1]. The binding to cellulose I is almost the same at pH 5-11 and 1M NaCl as in destilled water [1]. At pH 12 and 13 the binding diminishes 15 and 35% respectively [1]. The binding mechanism is presumed to be different from binding of other polymers, depending on the ability to form triple-helices, which is only possible for (1→3)-β-D-glucans at high molecular weight [1].

♦ Locust bean gum and barley β-D-glucan can bind quantitatively to both cellulose I and II [1].

♦ Chitosan dissolved in acetate buffer adheres strongly to both cellulose I and II [1]. Tests on chitobiose and chitotriose which are water soluble, confirms that the binding of chitosan is a property of the chitosan itself [1].

♦ Specific adhesion to cellulose was not observed for pullulan, cyclosophoran, pustulan, yeast mannan, arabinogalactan, xanthan gum, succinoglycan and carboxymethyl cellulose [1].

♦ Polystyrene can be grafted onto cellulose using an initiator of ceric ammonium nitrate with nitric acid. The cellulose/polystyrene is thermally more stabel than cellulose and polystyrene and is biodegradeable [9].

♦ Order of reactivity of the hydroxyl groups toward methylation and carboxymethylation: 2-OH>6-OH>3-OH. There are experimental results showing that 6-OH is more reactive than 2-OH [5].

♦ Many substitutions can be made by an alkali cellulose intermediate, followed by treatment with a halogenated substituent (Williamson ether synthesis) [16,17]

Cellulose derivatives can be cross linked with oxalylchloride in p-dioxane solution containing a tertiary amine as the catalyst [8].

Oxidizing cellulose poses a series of problems in regards to producing material which is homogenous in chemical and physical properties. Several oxidation agents are not selective, and many methods are topochemical. Also physical degradation of the fibres are a problem, giving a friable material that powders easily [12].

♦ A non-specific oxidation of cellulose can be obtained by nitrogen dioxide, giving a fibrous material which is not friable [12]. The method is relative simple exposing the cellulose to NO₂ until the desired DS and rinsing with destilled water until the water is no longer acidic [12]. Max DS by this method is 0.25 [13]. There are two versions of this method called 'cyclic method' and 'static method' refering to the way the cellulose fibres are exposed to NO₂ [12]. The method has a preference for oxidation of primary alcohols i.e. C(6) [13]. Cellulose derivatives from nitrogen dioxide are yellow to brown in color, and are highly degraded [11].



♦ Selective oxidation of C(6) in the glucose unit (DS>90%) is done by oxidation with phosphoric acid and sodium bromate, sodium chorate and sodium chlorite as oxidant. Of these, sodium bromate was the best, giving a DS=96%, and a lesser ring opening degradation of the chains, while sodium chorate and sodium chlorite severly degraded and ring opened the cellulose and only had a DS of 90 and 95% respectively [10]:



♦ Selective oxidation of C(6) in the glucose unit is also possible (DS>95%) by oxidation with phosphoric acid and sodium nitrite or sodium nitrate. Cellulose is dissolved in phosphoric acid and treated with sodium nitrite or sodium nitrate. The oxidations at C(2) and C(3) are reduced by treatment with sodium borohydride. When using nitrate, only a small excess of nitrate is needed (4 nitrate per 3 glucose), while using nitrite an excess of 4 nitrite per glucose unit is needed. Both reactions will reduce the molecular weight of the polymer. The reaction time is decreased by increasing the temperature, but the degradation of the polymer increases significantly at elevated temperatures. The reaction has been done at 4 - 20°C. The oxidation and subsequent reduction of the glucose units gives some epimerization. Experiments on the same reaction on ß-cyclodextrin show that the reaction is autocatalytic [14]:



♦ Other oxidation methods are oxidation by periodate [11]. The periodat oxidation was an attempt to selectively oxidize C(6). The reaction however was not specific, giving an overoxidation, and DS at C(6) was only 0.63. The low oxidation of C(6) by periodate is suggested to be due to sterical hindrance, as analogue oxidation on alginate only gave a DS = 0.44 while xylan had a DS= 1.0 [11].

Crystallographic data [7].