MOLECULAR-WEIGHT DEPENDENCE OF THE PROPERTIES OF ETHYL CELLULOSE AND HYDROXYPROPYL METHYLCELLULOSE FILMS

The molecular weight dependence of the mechanical and other properties of ethylcellulose and hydroxypropyl methylcellulose has been studied using the Flory equation: property = A + B/M where M is the molecular weight and A and B are constants. B values for the mechanical properties were negative and, when corrected for degree of polymerisation, comparable to those for cellulose acetate but an order of magnitude lower than those for polystyrene and polypropylene. B values have also been used to investigate the molecular weight dependence of permeability, release rate and incidence of cracking of film coatings. Use of B values in investigating possible mechanisms for the deleterious effect seen on the addition of titanium dioxide to hydroxypropyl methylcellulose has been proposed.