Application of temperature modulation to FTIR spectroscopy: an analysis of equilibrium and non-equilibrium conformational transitions of poly(ethylene terephthalate) in glassy and liquid states

Abstract

In this paper, the application of a temperature modulation to the temperature-resolved FTIR analysis is reported. The advantage offered by the spectroscopic investigation, able to follow the micro-structural and conformational sample modification involved in sample thermal transformation, was merged to that of temperature modulation, related to the possibility to separate the reversing (in-equilibrium within the experimental condition) to the non-reversing (non-equilibrium) processes. The potentiality of the technique (modulated temperature FTIR, MTFTIR) is highlighted through the study of the thermal transitions of amorphous poly(ethylene terephthalate) from 50 °C to the cold-crystallization. After the presentation of the theoretical framework and the experimental conditions, a step-by-step description of acquired data elaboration is given. The total variation of a selected band intensity as function of mean temperature as well as its reversing and non-reversing components are obtained. The evolution of the bands at 1340 and 971 cm−1, assigned to the trans conformation of the ethylenic unit and to the all-trans conformation of the repeating unit, respectively, are investigated. As expected, the glass transition is observed in the reversing components meanwhile the recovery of the glass relaxation and cold crystallization in non-reversing ones. Particularly interesting resulted the behaviour of the sample in the supercooled liquid state, between the glass transition and the cold-crystallization onset, in which the results show that the ethylenic conformers are in-equilibrium while the all-trans sequences are not. MTFTIR is confirmed to be a technique particularly suitable for the characterization of non-equilibrium conformational states of polymers.

The Initial Molecular Interactions in the Course of Enthalpy Relaxation and Nucleation in Polyethylene Terephthalate (PET) as Monitored by Combined Nanocalorimetry and FTIR Spectroscopy

“…Nonetheless, conformational changes dominate the transition from amorphous to crystalline PET owing to the possibility of many rotational degrees of freedom in PET backbone. [25] The aromatic units and the ethylene glycol moiety (O-CH 2 ) exhibit both trans and gauche conformers where the trans-content is high in the crystalline state compared to the gauche rich amorphous phase. [23,25,26] (Figure 4g-j).…”

“…[25] The aromatic units and the ethylene glycol moiety (O-CH 2 ) exhibit both trans and gauche conformers where the trans-content is high in the crystalline state compared to the gauche rich amorphous phase. [23,25,26] (Figure 4g-j). The IR absorption data for different molecular moieties show that the intensity of the IR peaks assigned to trans conformation in PET increases during crystal growth.…”