Effect of Crosslink Density and Concentration on Rheological Properties of Crosslinked Acrylic Acid Homopolymers

Nova Southeastern University, Davie, Florida, USA

Objective. In this study, using gel strength analysis we measured the effect of polymer crosslink density and its concentration on rheological behavior of poly(acrylic acid) gelling agents in aqueous solutions. Background. Crosslinked acrylic acid homopolymers are commercially available in different types and grades suitable for wide range of pharmaceutical applications, in particular in semi-solid dosage forms. At pH 5-8, these polymers thicken, forming viscous gels in aqueous or hydroalcoholic solutions. Although the effect of structural and environmental factors have widely been reported in terms of solution viscosity, the gel strength analysis was found to be very informative in characterizing flow behavior of such formulations. Methods. Different polymer (lightly and highly crosslinked) solutions were prepared in water at 0.1%, 0.3%, 0.5%, 0.7%, and 1.0%w/w concentrations. The pH of the solutions was adjusted to 5-6, and the gel strength was measured using a Texture Analyzer before and after neutralization. Results. The gel strength of the neutralized lightly and highly crosslinked polymer solutions at 0.1-1.0% w/w concentrations ranged from 28-56 mN, and 18-196 mN, respectively. Conclusion. Degree of crosslinking affected the rigidity and uncoiling behavior of the polymer in solutions. Lightly crosslinked structures are less rigid and become easily uncoiled in solutions, resulting in greater interaction with the solvent and thus higher gel strength at low concentrations. On the other hand, highly crosslinked structures, despite their lower uncoiling and solvent interaction, provide higher gel strengths when used at higher polymer concentrations. Grants. This study was supported by NSU Grant 335081