Synthesis of Cellulose Phosphate

Synthesis of Cellulose Phosphate

The cellulose powder was dried in vacuum in the presence of phosphorous pentaoxide. Samples were swollen for 24 h in hexanol, N,N-dimethyl-formamide (DMF), or 85% orthophosphoric acid, to assess the influence of swelling prior to chemical modification. Hexanol is the solvent of the reaction and H₃PO₄ is the phosphorylating agent,
besides being a well-known cellulose solvent. DMF is widely used in cellulose chemistry and was already reported in several other systems, in conjunction with H 3 PO 4 , for preparing cellulose phosphates. The phosphorylation reaction was carried out in a four-neck round-bottom flask, equipped with a nitrogen inlet, a condenser, a CaCl 2 drying tube, a thermometer, and a mechanical stirrer. The highest degree of phosphorylation was obtained using the following procedure:

to a stirring suspension of cellulose (4 g) in hexanol (29 mL) was added portionwise a solution of phosphorus pentaoxide (50 g) in triethyl phosphate (37 mL) and orthophosphoric acid (42 mL).

The mixture was stirred at 30°C for 72 h and then filtered. The filter cake was washed successively with hexanol and ethanol, and then rinsed repeatedly with water, to wash out the excess H3PO4 . The filtrate was then centrifuged prior to purification. Purification was carried out by Soxhlet extraction with deionized water and ethanol, for at least 24 h, until tests for inorganic phosphate were negative. The gel obtained was finally freeze-dried.

The reaction parameters were investigated and products characterized. In previous studies, 46,47 the correct sequence of addition of reagents was established. The maximum degree of substitution [DS (0 Յ DS Յ 3)] previously obtained (ϳ 1.0) was comparable to the DS obtained by the most widely used methods. An experimental design was then conceived to optimize the maximum DS. Parameters investigated were: temperature (30, 50, and 70°C), reaction time (24 and 72 h), phosphoric acid to cellulose ratio (18 and 28 mL/g), phospho-
rus pentaoxide to phosphoric acid ratio (0.4 and 0.7 g/mL), phosphoric acid to hexanol ratio (1 and 3), triethyl phosphate to hexanol ratio (0.5 and 1.5), and, finally, the swelling agent (hexanol,DMF, and phosphoric acid were tested). These limits were established according to our experience obtained from preliminary tests, by varying the values used in previously reported works. 46,4

Preparation of Amorphous Cellulose

Amorphous cellulose was prepared by anhydrous deacetylation of cellulose triacetate (CTA), according to a previously published method. 58 Briefly, 3.5 g of CTA (Kodak, Japan) was dispersed in ethanol for 10 min, under strong stirring, and filtered. The filtrate was then dissolved in 9 mL of methylene chloride and diluted in 37.5 mL of pyridine. Saponification was carried out under strong stirring, using 0.1M NaOH (30 mL), acetone (45 mL), and 90 mL of a 1.0M ethanolic NaOH solution (50%, v/v), and then left overnight. The precipitate was filtered, washed with anhydrous ethanol, and dried in air.

All dried materials were stored at 23°C and 50% relative humidity, until constant weight was attained, before further examination.