Correlation between crosslinking efficiency and spatial inhomogeneity in poly(acrylamide) hydrogels

Elasticity and light scattering measurements were carried out on poly(acrylamide) (PAAm) hydrogels prepared from acrylamide (AAm) and N,N'-methylenebisacrylamide (BAAm) monomers under various reaction conditions. Elasticity tests showed that the crosslinking efficiency of BAAm epsilon(xl), that is the fraction of BAAm forming effective crosslinks decreases as the initial monomer concentration C-o is decreased. At C-o=3%, epsilon(xl) was found to be 10(-2)-10(-3)supercript stop, indicating that 99 to 99.9% of BAAm used in the hydrogel preparation are wasted in elastically ineffective links. Debye-Bueche analysis of the light scattering data showed that, irrespective of the gel synthesis conditions, the correlation length xi, that is, the extension of inhomogeneities in the hydrogels is 10(1) nm. The extent of frozen concentration fluctuations in the hydrogels represented by decreases with increasing crosslinking efficiency of BAAm. The combination of the light scattering and the elasticity data of gels shows a direct correlation between the fraction of wasted crosslinker molecules during gelation and the spatial gel inhomogeneity.