Biopolymer Gels: Nanostructure and Macroscopic Properties

Abstract

Small-angle neutron scattering (SANS) has been used to investigate the effect of salts (NaCl and CaCl₂) on the structure of DNA and polyacrylic acid (PAA) gels. In the absence of salt a distinct correlation peak is observed in the SANS spectra of both systems indicating that electrostatic interactions play an important role in the organization of the polymer chains. When the salt concentration is increased, the peak position shifts to smaller values of the scattering vector q, and progressively vanishes. Osmotic swelling pressure measurements show that Ca ions reduce the swelling pressure and lead to the collapse of these gels. The Ca/Na ion exchange process does not affect the shear modulus of PAA gels. However, the shear modulus of DNA gels decreases with increasing Ca ion concentration at high swelling degrees, and increases at low swelling degrees. The results indicate that changing the ionic composition provides a simple way to control the nanoscale structures and properties in polyelectrolyte gels.