Figure. 1 shows the NMR spectrum of PET. Due to the highly symmetrical structure of PET (Figure. 1. a), the protons on PET can be divided into two groups of equivalent protons H1 and H2. The different chemical shifts come from various electron cloud densities, which is due to the electron-induced effect and conjugation effect. Based on these, the neighbor structure of different protons can be determined.[3] The chemical shift of the H1 proton is 8.11 ppm, and the chemical shift of H2 proton is 4.67 ppm.[2] Combined with the intensity of H1, the benzene ring can be identified in terms of the value of chemical shift. However, the measured value differs from the theoretical value of the benzene group (7.27 ppm [3]), which may be affected by the neighbor group (ester group). Furthermore, the NMR can also be employed to analyze the components of the sample quantitatively based on the proportional relationship between the intensity of the integrated signal and the number of resonant nuclei.[3]

Figure 2. The 1H-NMR spectra of PET, PET-3, PET-4 and PET-5

In this experiment, benzene ring and ester group have been identified. The benzene ring provides extra strength for the material and improves its performance at relatively high temperatures. Furthermore, NMR can be used to identify the end group and another segment in the material system that affects the properties of the sample (as shown in Figure 2.), which can be used in conjunction with DSC for further analysis.

1. What is FTIR

Fourier transform infrared spectroscopy (FTIR) is based on the principle that the bonds between different elements selectively absorb light at different frequencies which makes bands vibrate and rotate to obtain the information of  PET functional groups or chemical bonds. The whole FTIR spectra of PET is shown in the Fig.1

2. Results analysis 

Figure 1.  FTIR spectra of PET^[1]

From Fig.1, it seems that the relatively effective and important group in PET sample are  -CH₂ and C=O. Therefore, it is necessary to mainly explore the absorbance change of these two functional groups to detect the durability and thermal stability of PET in the liquid from the microstructure aspect. For the preparation, PET samples were set under 90℃ in pure water. Then, FTIR was used to characterize these treated samples after 1 day, 5 days, and 8 days. Ultimately, the changes of the functional groups of PET, including -CH₂ and C=O groups, are obtained from the FTIR images which are shown in Fig.2 respectively.

(a)

(b)

Figure 2. Functional group of PET treated in pure water at 90℃ (a) CH₂ (left); (b) C=O (right).^[2]

Based on Fig. 2, the absorbance data is recorded in Tab.1.

Data listed in Tab.1 show the absorbance changes of CH₂ and C=O in PET under different time treatments. When PET samples are heated at 90 °C in pure water, the absorbance of CH₂ and ester group C=O as the main chain structure of PET did not change significantly especially in 24 hours. It seems that it is not until more than five days have passed under such conditions that the main chain content of PET cannot be observed to decrease significantly, which may mainly due to the hydrolysis reaction. 

Specifically, after 5 days, water molecules cause PET molecular chain cracking in the PET hydrolysis to produce -COOH. The -COOH group further ionizes hydrogen ions and catalyzes PET further degradation, so that the entire PET hydrolysis reaction exhibits self-catalytic characteristics, and significantly reducing the surface finish, intensity, and other apparent physical properties of the material in a short time. Therefore, these results fully demonstrate that PET has considerable short-term thermal stability.

Based on the short-term thermal stability, the disposable mineral water bottle is the most possible potential application of PET due to the short lifecycle of water bottles. Meanwhile, in the production of PET bottles, it is necessary to dry the PET chips to prevent the degradation of raw materials and the high-temperature environment should be avoided in the process of using PET water bottles.