Chain-stretch relaxation from low-frequency Fourier transform rheology

Medium- or large-amplitude oscillatory shear (MAOS and LAOS, respectively) is sensitive to polymer chain structure, yet poses unsolved challenges for a priori structural characterization. We present a MAOS protocol applied to near-monodisperse linear polymer melts, from which chain-stretch relaxation, a key structural feature, is discernible. The third harmonics of MAOS frequency sweeps are decomposed into real and imaginary components and found to obey time-temperature superposition. Significantly, these third harmonic features occur at low frequency and are readily accessible with standard rheometers. For materials where phase transitions restrict the use of time temperature superposition, this method has potential to greatly increase the scope of rotational rheometry for structural analysis of polymers. However, the relationship between MAOS data and characteristic relaxation times is complex, and to elucidate this, a modeling approach is required. The GLaMM molecular tube-based model of linear entangled melt rheology and structure, which has no free parameters, closely follows the form of our experimental results for the third harmonics and contains discriminatory features which depend only on the polymer's chain stretch relaxation time. However, we find fundamental differences in magnitude and the frequency dependence of the third harmonics which must be resolved in order to fully understand the molecular basis of the stress response and quantitatively study chain stretch.