Effects of Volume Fraction and Composition on the Rheological Properties of Polyacrylic Acid/Polyacrylamide Hydrogels
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Effects of Volume Fraction and Composition on the Rheological Properties of Polyacrylic Acid/Polyacrylamide Hydrogels Bryan Baker1, Rebecca Murff2, and Valeria T. Milam1 1 School of Materials Science and Engineering, Georgia Institute of Technology, 771 Ferst Drive, Atlanta, GA, 30332 2 Department of Biomedical Engineering, Georgia Institute of Technology, Atlanta, GA, 30332 ABSTRACT Polyacrylamide-based hydrogels are popular materials that have been extensively studied for their applications in the field of biomaterials due to their permeability and biological compatibility. A major limitation of this polymeric material, however, in biological applications stems from its limited mechanical stiffness. The current study examines the mechanical properties of polyacrylamide-polyacrylic acid-based hydrogels at varying volume fractions and compositions using oscillatory rheology. For a fixed chemical cross-linker concentration, an increase in the volume fraction of hydrogel resulted in an increase in the shear elastic modulus of the hydrogel. Additionally, varying the relative ratio of polyacrylamide to polyacrylic acid resulted in modest changes in the shear elastic modulus. INTRODUCTION Hydrogels are water-swollen interconnected polymer networks that can be covalently or physically cross-linked [1]. Polyacrylamide-based hydrogels have been extensively studied for biomaterials applications such as cellular substrates [2-4]. While their high liquid content mimics the permeability of the extracellular matrix surrounding a cell, the liquid component of hydrogels also results in low mechanical stiffness values (modulus on the order of kPa [2]) compared to that of biological tissues (modulus of MPa [5] ñ GPa [6]). A number of methods have been employed to increase the modulus of hydrogel systems, including varying polymer block composition [7] and increasing crosslinker concentration [8]. In the present study we examine the effects of volume fraction and composition on the shear storage and loss moduli for a hydrogel system comprised of polyacrylamide copolymerized with polyacrylic acid. Here, oscillatory rheology was the characterization tool to monitor shear modulus as a function of strain and frequency. While we expect the polyacrylamide to maintain a neutral charge, the polyacrylic acid is expected to impart a net negative charge to the hydrogel via its carboxylic acid groups.
EXPERIMENT Hydrogel Synthesis All chemicals for this study were purchased from Sigma (St. Louis, MO). Polyacrylamide-co-polyacrylic acid hydrogels were prepared using distilled acrylic acid and a 30wt% solution of acrylamide monomer. While the ratio of each monomer and total polymer volume fraction was varied in this study, the methylene-bisacrylamide (Bis) cross-linker was fixed at a value of 0.24 volume fraction (based on total hydrogel volume). For the polymerization step, 5 v% ammonium persulfate and 0.2 v% TEMED were added as initiator and accelerator respectively. (Note: Ammonium persulfate was reduced to 1 % for hydrogels of
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