Transitions of monodisperse aPS studied by model-based 2D correlation infrared spectroscopy incorporating S-shaped function

• Boltzmann sigmoid function was incorporated into the model-based 2D correlation IR.
• Model-based 2D IR was used to study the mechanism of Tg and Tll of monodisperse aPS.
• Flip motions of the phenyl rings are earlier than main chains creep of aPS during Tg.
• Tll is a consequence of the melts motions and the slippage of chain entanglements.

In this paper, a new S-shaped function (model) called Boltzmann sigmoid was incorporated into the model-based 2D correlation infrared spectroscopy. Boltzmann sigmoid is a typical transition function, and it was found from the curve fitting of the IR spectral intensity variations of monodisperse aPS at heating. The glass transition and the liquid–liquid transition of aPS were determined by MW2D IR spectrum. The model-based 2D correlation IR spectroscopy incorporating Boltzmann sigmoid was successfully employed to investigate the mechanism of these two transitions. We found that the flip motion of the side benzene rings is earlier than the creep of the main chains of monodisperse aPS during the glass transition. Moreover, the difference of the sequential order is precisely defined as ΘE = 4.5°. The movements of the main chains and the side benzene rings are completely simultaneous without a difference during the liquid–liquid transition of aPS, and the difference of the sequential order is ΘE = 0°. It represents the whole movement of macromolecular chains of aPS. The liquid–liquid transition is probably a consequence of the onset of melts motions, and the slippage of chain entanglements. The difference of the effective phase angle in the model-based 2D spectra is proportional to the difference of the transition temperature at the different bands in MW2D spectra.