Frequency and temperature dependence of poly(N-isopropylacrylamide) gel rheology



Poly(N-isopropylacrylamide) or p-NIPAAM gels undergo a prominent deswelling transition near physiological temperatures. Using passive microrheology, we have investigated the viscoelastic response of p-NIPAAM gels over a frequency range not accessible to bulk rheological measurements. Overall, NIPAAM gels moderately shear stiffen with increasing frequency. More intriguingly, sample viscosity rapidly declines with increasing frequency before leveling off near the solvent viscosity. The frequency for this crossover coincides with the emergence of fast gel modes seen in dynamic light scattering (DLS) from the gel. Furthermore, we monitored viscoelastic responses on approach to the deswelling transition. Intrinsic light scattering indicates that experimental conditions are not near the critical point and that the deswelling transition is second order in nature. Nevertheless, the corresponding elastic and viscous moduli of p-NIPAAM displayed power-law decreases with temperature. These changes with temperature were independent of probe frequency. Power law exponents, however, are sensitive to details of the sample preparation suggesting that these viscoelastic responses vary with gel structure. Correlating our microrheological measurements with DLS from the gel matrix itself, we find that several of the observed microrheological features are closely related to the intrinsic dynamics of the p-NIPAAM gels. In particular, the transition from gel- to solvent dominated dissipation coincides with a transition from fast to slow gel modes. Combining microrheology with intrinsic light scattering, therefore, provides a compelling approach to probe rheological responses and correlate them to the underlying network dynamics. © 2012 Wiley Periodicals, Inc. J Appl Polym Sci., 2013