Network structure of cellulose ethers used in pharmaceutical applications during swelling and at equilibrium

Purpose. The purpose of this work was to investigate the swelling behavior of four cellulose ethers that differ in their type and degree of substitution and to elucidate the network structure of the swollen matrices under dynamic and equilibrium conditions. Methods. Dynamic vapor sorption was performed to assess the ability of polymer chains and water molecules to interact. Dynamic and equilibrium swelling studies were performed to calculate molecular parameters of swollen polymers using the Flory-Rehner theory. Results. We determined the volume-swelling ratio of the polymer matrices and observed that it was dependent on their hydrophilicity. We determined molecular parameters that characterize the swollen hydrogels of cellulose derivatives, such as the polymer volume fraction in the swollen state, upsilon(2,S), the effective molecular weight of the polymer chain between physical entanglements, (M) over bar (e), the number of repeating units between two entanglements, u, and the number of entanglements per chain, e. The (M) over bar (e) of the cellulose derivatives studied varied significantly depending on the type of cellulose ether and on the swelling time. Conclusions. The order of mesh size, an important parameter for predicting drug diffusion and release, taking into account all determined parameters, is: hydroxypropyl cellulose < hydroxyethyl cellulose < hydroxypropyl methyl cellulose K100M < hydroxypropyl methyl cellulose K4M.