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 two-component simplex lattice design, using polymer varieties with different molecular weights and substitution characteristics. The resulting systems were characterized theologically by capillary viscometry, flow rheometry, and oscillatory shear techniques, for the determination of kinematic viscosity, index of consistency, index of fluidity, elastic modulus, and viscous modulus. The values obtained for these parameters were fitted with appropriate canonical models, which revealed synergistic effects for some polymer proportions. Maximum synergy was observed when polymer proportions were optimal for the establishment of between-polymer interactions. The synergistic effects 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 temperature. 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 the fitted canonical models) showed strong dependence on these variables. Microviscosity values, derived from theophylline diffusion data for some of the 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 microenvironment, favoring solute mobility.