PET thermodynamics properties testing (based on DSC)

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1. What is DSC?

DSC provides heat flow information of a material as the function of temperature, which enables the measurements of thermal transition temperatures, enthalpy change, and crystallinity. The principle of DSC is that during a physical transition process like a phase transition, it needs heat flow to maintain the sample and reference at the same temperature. This heat flow can be measured as the power changes.

2. Results Analysis

To understand the impact of crystalline morphology on the mechanical performance of PET, discussing the transition temperature, crystallization temperature, and melting (or crystallizing) entropy is necessary. There are 3 types of information that can be received from DSC: glass transition, crystallization, and melting, which have a close relationship with crystal morphology. Specifically, melting characterizes the crystalline phase, while the glass transition corresponds to the amorphous phase. The results are shown in Figure.1 and Table 1 ^[5].

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Figure 1. Characterization results curve for PET with glass transition, crystallization, and melting process.^[5] (left) Microscopic morphology of molecular chains during DSC testing. (right)

Crystallinity can reflect other material properties like toughness, clarity, and stability. For example, the lower crystallinity, the less brittle and more light transfer. For the example data, it has crystallinity of 7.3% which is relatively low.

Chemical structure understanding of PET (based on NMR)

1. What is NMR?

Nuclear Magnetic Resonance (NMR) is to characterize the nucleus through the interference of the nucleus in a strong constant magnetic field by a weak oscillating magnetic field (in the near field), which produces an electromagnetic signal with a specific magnetic field frequency.

2. Results Analysis

NMR is an analytical chemistry technique that can be applied to determining the molecular structure of a sample as well as its content and purity. Here, NMR is used to detect the functional group of the sample. The chemical structure of the sample can be estimated through that information.

Figure 1. ¹H NMR spectra of PET (b) and its structural formula (a)

Table 2. NMR characterization result of PET

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.

Functional group exploration of PET (based on FTIR)

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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

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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)

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(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.

PET（Polyethylene terephthalate）

PET（Polyethylene terephthalate）

Characterization of PET

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