The nanoporous morphology of photopolymerized crosslinked polyacrylamide hydrogels for DNA electrophoresis
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Photopolymerized crosslinked polyacrylamide hydrogels are emerging as an attractive electrophoresis sieving matrix formulation owing to their ideal range of pore size, rapid polymerization times and the potential to locally tailor the gel structure through spatial variation of illumination intensity. This capability is especially important in microfluidic systems, where photopolymerization allows a gel matrix to be precisely positioned within a complex microchannel network. The achievable level of separation performance is directly related to the nanoscale gel pore structure, which is in turn strongly influenced by polymerization kinetics. Unfortunately, detailed studies of the interplay between polymerization kinetics, mechanical properties, and structural morphology are lacking in photopolymerized hydrogel systems. In this paper, we address this issue by performing a series of in-situ dynamic small-amplitude oscillatory shear measurements during photopolymerization of crosslinked polyacrylamide electrophoresis gels to investigate the relationship between rheology and parameters associated with the gelation process including UV intensity, monomer and crosslinker composition, and reaction temperature. In general, we find that the storage modulus G increases with increasing initial monomer concentration, crosslinker concentration, and polymerization temperature. We also find an optimal UV intensity level at which the resulting hydrogels exhibit a maximum value of G. A simple model based on classical rubber elasticity theory is used to obtain estimates of the average gel pore size that are in agreement with corresponding data obtained from analysis of DNA electrophoretic mobility in hydrogels polymerized under the same conditions.
