Time–Temperature Superposition (TTS) is widely used to construct viscoelastic master curves under the assumption of thermorheological simplicity. However, amorphous polymers frequently violate this assumption in the temperature interval immediately above the glass transition temperature, T_g, where multiple relaxation mechanisms with distinct temperature sensitivities may coexist. In this work, the breakdown of TTS near T_g is investigated in a broad molecular weight distribution polystyrene using small-amplitude oscillatory shear measurements.

Conventional frequency sweeps performed between 110 °C and 160 °C fail to superpose in the onset region of the α-relaxation, particularly in the storage modulus G', as revealed by Gurp-Palmen representations. This behavior reflects temperature-dependent distortions of the relaxation spectrum rather than simple horizontal or vertical shifts.
To overcome these limitations, broadband CHIRP excitation was performed using an ARES-G3 rotational rheometer, whose strain-controlled architecture, high dynamic precision, and sensitive torque measurement make it particularly well suited for rapid broadband oscillatory testing. This approach enabled the viscoelastic response near T_g to be probed over more than five decades in frequency within a significantly reduced measurement time. The CHIRP-derived spectra provide a cleaner and more continuous description of the α-relaxation, allowing reliable analysis using the Havriliak–Negami model. These results demonstrate that CHIRP rheometry, implemented on a high-performance platform such as the ARES-G3, offers an effective alternative to conventional TTS for characterizing polymer dynamics near the glass transition.