  Medium or large amplitude oscillatory shear (M/LAOS) is sensitive to polymer
chain structure, yet poses unsolved challenges for 'a priori' structural
characterisation. In this letter, we present a MAOS protocol applied to
near-monodisperse linear polymer melts, from which chain-stretch relaxation, a
key structural feature, is clearly discernible. The third harmonics of MAOS
frequency sweeps are decomposed into real and imaginary components and found to
obey time-temperature superposition. 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. 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 greatly increases the scope of rotational rheometry
for structural analysis of polymers. Although the theory shows good qualitative
agreement with experimental data, 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.
