Cellulose is an unbranched polysaccharide composed of β(1→4)-linked D-glucose units, each containing three hydroxyl groups. A commercial derivative, methylcellulose, is prepared by substituting approximately 30% of the hydroxyl groups with methoxy (−OCH3) groups.
The following thermal data are obtained:
Tg(cellulose)=225∘CTg(methylcellulose)=150∘C
Neither polymer possesses covalent cross-links.
(a) Identify the dominant noncovalent interactions that render native cellulose insoluble in water and confer high mechanical rigidity.
(b) Explain how replacing hydroxyl groups with methoxy groups alters both the magnitude and spatial distribution of the interactions named in part (a).
(c) Account for the 75∘C decrease in glass-transition temperature (Tg) observed upon partial methylation.
(d) Predict qualitatively how the water solubility of methylcellulose compares with that of native cellulose and justify your prediction on the basis of intermolecular forces.