Influence of polymer structure on the rheological behavior of hydroxypropyl-methylcellulose-sodium carboxymethylcellulose dispersions

Aqueous dispersions of mixtures of hydroxypropylmethylcellulose (HPMC) and sodium carboxymethylcellulose (NaCMC) were prepared in accordance with a tw o-component simplex lattice design, using polymer varieties with different molecular weights and substitution characteristics. The resulting systems w ere characterized theologically by capillary viscometry, flow rheometry, an d oscillatory shear techniques, for the determination of kinematic viscosit y, index of consistency, index of fluidity, elastic modulus, and viscous mo dulus. The values obtained for these parameters were fitted with appropriat e canonical models, which revealed synergistic effects for some polymer pro portions. Maximum synergy was observed when polymer proportions were optima l for the establishment of between-polymer interactions. The synergistic ef fects on viscosity and elasticity are attributable to the establishment of hydrophobic interactions and hydrogen bonds between HPMC and NaCMC chains, as revealed by IR spectroscopy and modifications in the cloud-point tempera ture. The observed among-mixture differences in the polymer proportions at which maximum synergy occurs, and the degree of this synergy, are explained by differences in molecular weights and substitution characteristics, and indeed the degree of synergy (as measured by interaction parameters from th e fitted canonical models) showed strong dependence on these variables. Mic roviscosity values, derived from theophylline diffusion data for some of th e mixtures, show that the crossover and chain expansion of the polymers in the mixtures (i.e. increased viscosity and elasticity) give rise to a three -dimensional network with greater mesh size and a more hydrophilic microenv ironment, favoring solute mobility.