Two-dimensional Fourier transform rheological study on thermosensitivity of poly(N,N-diethylacrylamide) in aqueous solutions

The phase transition of a thermo-responsive polymer, poly(N,N-diethylacrylamide) (PDEA) above its critical overlap concentration (c*) has been studied by two-dimensional Fourier transform (FT) rheology using Large Amplitude Step Shear Oscillation (LASSO). This technique allows the separation of the linear and nonlinear contributions to different relaxation processes and the determination of their time scale and amplitude through the time response of the shear stress after step strain experiments. The interchain interactions increase at the onset of the phase transition at 29 °C, indicated by an increased non-linear contribution at short relaxation times as compared to the single phase condition. During the phase separation of a concentrated solution above the phase transition temperature, the polymer-rich phase can form a transient network created by the hydrophobic interactions between the collapsed polymer chains. The non-linear behavior of a phase-separated system well above the transition temperature (at 33 °C) reflects the stretching of the bridging chain segments between larger aggregated domains and the coalescence of aggregates broken during the step in strain. Relaxation time distributions have been fitted in the LASSO spectra by the nonlinear regularization (NLREG) technique and the relaxation times have been attributed with various linear and non-linear processes below and above the phase transition temperature.