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 ε _xl , that is the fraction of BAAm forming effective crosslinks decreases as the initial monomer concentration C _o is decreased. At C _o =3%, ε _xl was found to be 10^-2-10 ^-3, 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 ξ, that is, the extension of inhomogeneities in the hydrogels is 10¹ nm. The extent of frozen concentration fluctuations in the hydrogels represented by η² 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.