Complementary-DNA-Strand Cross-Linked Polyacrylamide Hydrogels

We examine networks of complementary-DNA-strand cross-linked polyacrylamide, with and without covalent N,N'-methylene(bis)acrylamide cross-linking, using rheological time-temperature superposition (TTS) to ascertain how temperature and composition influence the microstructure. A higher DNA-cross-linking efficiency is ascribed to the larger cross-linker imparting greater steric hindrance to the formation of self-terminating loops. TTS unifies the rheological spectra of DNA cross-linked and dual cross-linked gels at low frequencies, furnishing the effective activation energy for DNA-cross-link disengagement. Temperature sweeps also show that the temperature dependence of the dynamic moduli is reversible. The activation energy is temperature-independent (approximate to 318 kJ mol(-1)) at low temperatures but decreases significantly and systematically with increasing temperature (and varying cross-linker composition). We interpret the varying activation energy-relative to the low- temperature limit-as a measure of DNA-cross-linker disassociation, and infer from TTS a cooperative relationship between the DNA-cross-linker disengagement and network connectivity. At low temperature, DNA cross-linked samples exhibit hallmarks of star-polymer-melt relaxation, including a superexponential divergence of the longest relaxation time with increasing cross-linker concentration, increasing from approximate to 2 to 20 entanglements per arm. At high temperature, a new "associative-reptation" scaling furnishes a robust interpretation of the network and longest relaxation times.