Polymer materials have high strength and easy to be processed and molded, so they can replace traditional metals and inorganic materials and are used in many fields. Because of its high molecular weight, polydispersity, non-volatile, no fixed structure, random coil and intertwined molecular chain, it is difficult to accurately identify its structure. The structure of the material determines the performance, and the relationship between structure and performance can be known only through the accurate characterization of its composition and structure. It is helpful to study the microstructure change and mechanism of polymer and its composites in various stages of synthesis, modification, compounding, molding and processing, and to develop new functional materials. The progress of instrumental analysis has promoted the research and development of polymer materials. This Application Note summarizes the practical application of modern spectral technology in the structural characterization of polymer materials, and expounds its analytical principle, experimental means, application range and characteristics.

Application of Modern Spectral Technology in Structural Characterization of Polymer

Molecular Spectroscopy Analysis Technology
Molecular spectrum analysis is based on the quantum energy level transition in the material when the matter molecules interact with electromagnetic radiation, and the wavelength and intensity of the reflected, absorbed or scattered radiation are measured.

Fourier Transform Infrared Spectrometer (FTIR)

Infrared spectroscopy has distinct characteristics in the qualitative analysis of polymer materials. Including chain composition, arrangement, configuration, conformation, branching, cross-linking, crystallinity, orientation change analysis. If there are some polar groups such as esters, acids, amides and imides in the polymer molecules, the band has a significant characteristic peak, which reflects the structure and existence of the polymer.

Laser Raman Scattering Spectroscopy

Raman spectrum is a kind of scattering spectrum, which is particularly sensitive to the vibration of carbon-carbon, sulfur-sulfur, nitrogen-nitrogen single bond and multiple bonds. it is used to study the chemical composition, carbon chain skeleton, length, conformation, isomerization, unsaturation and conjugation of polymers. Compared with infrared, there are many similarities. For example, some absorption peaks of the compounds are identical in infrared wave number and Raman shift, which can be verified by each other. The difference between the two is that the infrared Abscissa is wavenumber, and its absorption peak is caused by the change of molecular dipole moment or charge distribution caused by vibration, and the Raman Abscissa is Raman shift. Its scattering is caused by the instantaneous polarization caused by the instantaneous deformation of the electron cloud distribution on the bond, resulting in induced dipole scattering when returning to the ground state, which is most suitable for the study of non-polar bonds and symmetrical molecules of polymers composed of the same atoms. The two are mutually exclusive and complementary to each other and are used for structural identification.

Ultraviolet Absorption Spectrum

Ultraviolet-visible absorption spectroscopy utilizes the chromophore of a substance to absorb radiation in the 200-800nm spectral region to generate molecular valence electrons, causing n→π* and π→π* energy level transitions, and structure identification is carried out accordingly. In the research of polymer materials, it can be used to identify the structure of characteristic functional groups in polymers and additives, such as olefin, alkyne, benzene ring, carbonyl, carboxyl, azo, nitro, nitroso, nitrate, amide groups and so on, which have unsaturated bonds; the changes before and after the polymerization are detected to explore the polymerization mechanism; the composition of the copolymer and the kinetics of polymerization were studied through the change of absorption intensity and displacement caused by the concentration change of polymer and trace substance.

Fluorescence Spectrum

Molecular fluorescence analysis is that the ground state molecule is excited to the excited state by the excited light source, and different wavelengths of fluorescence are produced when it is returned to the ground state. By measuring the fluorescence intensity, the fluorescence spectrum is obtained. Because the molecular structure of polymer materials is different, the absorption wavelength of ultraviolet light is different, and the fluorescence wavelength emitted when returning to the ground state is also different, this characteristic can be used for qualitative analysis. Quantitative analysis can be carried out based on the linear relationship between fluorescence intensity and concentration produced by dilute polymer solution. Fluorescence spectroscopy is useful for studying polymer solution morphology transformation, polymer blend compatibility and phase separation, polymer degradation and aging, polymer luminescent material properties, etc.

Nuclear Magnetic Resonance Spectroscopy (NMR)

Nuclear magnetic resonance spectroscopy is a powerful tool for analyzing how the functional groups in polymers are connected. It is divided into 1H-NMR and 13C-NMR. Solid-state nuclear magnetic technology is often used for the structure exploration of crystals, microcrystalline powders, colloids, membrane proteins, protein fibers and polymers.

X-ray Analysis Technology (X-ray)
X-ray analysis is a type of analysis method that uses X-rays as the radiation source. The sample to be tested can be powder, block, film, fiber. Different analysis purposes have different sample preparation methods.

X-ray Fluorescence Analysis (XRF)

Choosing appropriate wavelength x-rays to irradiate different substances will excite the secondary characteristic x-ray spectra of different wavelengths, and qualitative and quantitative analysis can be carried out by measuring the fluorescence wavelength and intensity. This technology has become a rapid and accurate analysis method for the element composition, three-dimensional structure and phase structure of compounds and polymer materials, and has been used in many fields, such as petrochemical industry, material science, life science, environmental science and so on.

X-ray Diffraction(XRD)

X-ray diffraction analysis is mainly used for phase analysis, structure analysis and structure identification. In the analysis of polymer materials, the x-ray diffraction data obtained can be compared with the standard CJPDS card, which can be used not only for phase analysis, but also for the determination of aggregation structure parameters, such as crystallinity, orientation, grain and micropore size.

X-ray Absorption Fine Structure(XAFS)

XAFS technology is sensitive to the local structure around the absorption atom and the chemical environment. It can give the microstructure information of several adjacent coordination shells around an element at the atomic scale, including the type of coordination atom, the length of coordination bond, the coordination number, the degree of disorder, etc., and does not require the sample to have long-range ordered structure, which is suitable for the characterization of specific atoms in solid and liquid materials or the local structure of polymer materials.

X-ray Photoelectron Spectroscopy (XPS)

X-ray photoelectron spectroscopy uses x-rays to irradiate the sample to cause the inner electrons or valence electrons of atoms or molecules to be excited by photons. By measuring the energy distribution of photoelectrons, the photoelectron energy spectrum can be obtained, thereby obtaining the structure composition of the test object. For metals and their oxides, the X-ray photoelectron mean free path is only 0.5~2.5nm, for organic and polymer materials, it is 4~10nm, so this method is a surface analysis method.

Mass Spectrometry (MS)
Mass spectrometry uses high-energy ion beams to bombard the vapor molecules of the sample to knock out the valence electrons in the molecules to form positively charged ions, and then collect and record them in the order of mass-to-charge ratio (m/z) to obtain a mass spectrum. According to the mass spectrogram, the information of sample composition, structure, molecular weight and fragmentation law can be obtained. Combined with chromatography, nuclear magnetic resonance spectroscopy and infrared spectroscopy, it can be used to analyze and identify the structure of complex compounds.

Pyrolysis Gas Chromatography (PGC)
Pyrolysis gas chromatography is a combination of pyrolysis technology and gas chromatography, which extends the application of gas chromatography to non-volatile polymer materials for composition, structure analysis and thermal performance characterization. Because the composition and relative content of pyrolysis products have a certain corresponding relationship with the structure and composition of the tested substances, fingerprint pyrolysis spectra can be used as a qualitative and quantitative basis. It has been applied in the structural identification of polymer materials, the study of thermal decomposition mechanism and thermal processing process, food chemistry, environmental resources, life science, medicine, and cultural relic identification.