Introduction of FTIR

  FTIR is widely used to identify the chemical structure of compounds, especially the information of chemical bonds and functional groups, due to simple operation, simple sample preparation, various sample state and shape and high repeatability. At the same time, by FTIR, rapid and precise measurement in a wide range can be achieved with high resolution. Therefore, FTIR is developing rapidly to almost replace the traditional infrared spectrum.

  FTIR involves the study of the interactions between molecules and IR light. When a sample is irradiated with IR light, light with a certain wavelength will be absorbed to stimulate vibrational and rotational energy transitions, for which the light intensities will decrease. Various molecular structures exhibit own energy differences, corresponding to different IR wavelengths ^[1]. An IR spectrum is obtained by plotting transmittance or absorbance versus the wavenumber of IR light. The position, intensity, and shape of the peaks reflect the characteristic vibrational frequencies, functional groups or chemical bonds. Therefore, FTIR can be used to qualitatively analyze the chemical bonds or functional groups of a compound ^[1].

Data of FTIR

                                                                                  Figure1: Infrared spectrum of EVA copolymer ^[2].

In this figure, the Y axis represents the absorptivity (%), and the X axis represents 10² wavenumber (cm^-1).

Discussion of FTIR

In the infrared spectrum of EVA copolymer, there are absorption peaks of ethylene and vinyl acetate ^[2].

1. Wavenumber of 2850-2950 cm^-1, the peak is the stretching vibration absorption peak of saturated C-H bond.
2. Wavenumber of 720 cm^-1, the peak is the bending vibration absorption peak of (CH₂)_n (n > 4).
3. Wavenumber of 1735 cm^-1, the peak is the stretching vibration absorption peak of double bond in ester carbonyl.
4. Wavenumber of 1245 cm^-1, the peak is the stretching vibration absorption peak of single bond in ester carbonyl.

These absorption peaks indicate the existence of specific chemical bonds and functional groups, from which the chemical structure of EVA can be qualitatively analyzed as including saturated C-H, (CH₂)_n (n > 4) and ester groups. From the aspect of structure, EVA does not contain unsaturated bond and the active site which is extremely sensitive to the environment, which makes EVA not absorb water, but also has resistance to acid, alkali, sea water, cold and heat. Therefore, EVA is an ideal material for making waterproof board and packaging materials.

Introduction and principle of NMR

  Briefly, NMR is a equipment which detect the physical process in which the nuclear nucleus with non-zero magnetic moment, the spin level under the action of external magnetic field occurs Zeeman splitting, and resonance absorbs a certain frequency of radiofrequency radiation. A spinning nucleus possesses angular momentum P. The magnetic fields of the spinning nuclei will align either with the external field, or against the field. For each ¹H or ¹³C, their chemical environments are different with same standard. Therefore, each proton or atom will have their characteristic peak and by recording the location, split numbers and area of peaks in NMR graph, the structure of chemical compounds can be deduced.

  In order to locate where the peaks are, chemical shift is needed, which is  where is nuclear magnetogyric ratio, h is Planck constant. Usually, TMS is used for standard to determine chemical shift of compounds: δ_TMs=0. Generally, the proton NMR chemical shift is affected by nearness to electronegative atoms (O, N, halogen.) and unsaturated groups (C=C, C=O, aromatic). Electronegative groups and unsaturated groups tend to the downfield (the left, increase in ppm). The number of peaks, the area of each peak, the chemical shift and the splits present respectively the species of magnetic nonequivalent protons in a molecule, the numbers of each type of proton, the chemical environment of each type proton and the number of proton on adjacent carbon. By analyze these peaks, the chemical content of EVA can be obtained.

Data of NMR

The sample was dissolved into 1,2-Dichlorobenzene-d. The testing conditions are: magnetic field strength: 9.40T, nuclear magnetic tube diameter: 5mm, solvent: 1,2-Dichlorobenzene-d, NMR standard: TMS, temperature: 120℃.

                                           Figure 2. The NMR graph of 1H, the 13C NMR graph and the structure of EVA ^[3].

Discussion of NMR

As the structure shows, the H on Carbon 1 which connects with O has the biggest chemical drift which can be regarded as the characteristic peak of vinyl acetate and in the NMR graph, the peak shows in δ_5.3-4.7 present the H on -CH-. Apparently, the content of VA can be calculated by  and the content of ethylene is where A means the area of peak.

Besides, if ¹³C NMR was used, more information can be gained such as the rough distribution of each monomer on backbone. As the NMR graph shows, according to the calculation of chemical drift, the peak on δ=169.6 present the carbon in carboxyl group, δ in range of 70.0-75.0 shows the methine carbon in vinyl acetate and δ in 0-50 shows the methyl carbon. And according to some researches , the peaks follow the binary/ternary sequences such as EV, EVE, VVV and so on. (E means ethylene, V means vinyl acetate) Therefore, according to the amount of each sequence, the distribution of monomers can be roughly obtained. For the graph above, it shows a random copolymer because it shows all these sequences have relatively close proportions of each other.

As for the application, as the analysis above, this material is a random co-polymer which means it has low tendency to crystalline and also it shows good elasticity and resilience which can absorb energy during the impact and slowly release it. Therefore, this material can be used to produce cushioning sole.  

Introduction of DMA

  To measure the characteristic temperature and the viscoelastic mechanical properties of the material, the dynamic thermomechanical analysis (DMA) can be used. A sinusoidal force will be applied to the sample by a probe of DMA tester, the deformation can be measured and complex modulus (E*), storage modulus (E‘), loss modulus (E‘’) and loss factor can be calculated . According to the loss factor, the degree of viscoelasticity and Glass transition temperature Tg can be defined. The principal equations are shown below:

Data of DMA

                                                                           Figure 3: the viscoelastic properties of EVA

Discussion of DMA

For Figure 3 (a) (b) and (c),the Y axis show the storage modulus E', loss modulus E" and loss factor tanσ respectively.

From the Figure 3 (a) and (b), two viscoelastic moduli of the sample under different temperature are shown by the curves, and the viscoelastic properties of different serving temperature can be found.

In the Figure 3 (b) and (c), the curves reach the peak value at around -40℃, for which the glass transition temperature Tg of the EVA sample can be determined for different oscillation frequency.

From the data, it can be found the glass transition temperature Tg of EVA copolymer increases with the increasing oscillation frequency. But it still around -40℃, which is far lower than room temperature. When the temperature below Tg, EVA will perform like a brittle solid, while when temperature is higher than Tg, it will be rubber-like. Based on the extremely low Tg, EVA can keep the elasticity in the room temperature and be used to manufacture elastic film or foam.

[1]: Yang, Rui. Analytical methods for polymer characterization [M]. CRC Press, Taylor & Francis Group, 2018: 117-155.

[2]: Jiang Qingzhe, Xu Qiang, Song Zhaozheng. Molecular Structure Characterization of Ethylene-Vinyl Acetate Copolymer [J]. Journal of Yantai Normal University (NATURAL SCIENCE EDITION), 2005(04):279-283.

[3]: Bu Shaohua. Determination of VA content and sequence structure in EVA resin by NMR. China synthetic resin and plasics. 2017, 34(6):63

[4]: W. Stark, M. Jaunich (29 December 2010), Investigation of Ethylene/Vinyl Acetate Copolymer (EVA) by thermal analysis DSC and DMA, retrieved from https://www.sciencedirect.com/science/article/pii/S0142941810002126