Wang Bo, Li Xiangwu, Wang Zhifen, Qi Ning, Gong Wei, Yu Dahai, Lu Chengliang School of Physical Science and Technology, Wuhan University, Wuhan, 430072e-mail: bwangpositron.whu.edu.CN Positron is an antiparticle of electrons and is the earliest knowledge of human beings Antimatter. Compared with electrons, except that the sign of the charge is positron, other properties (including mass child, spin, charge quantity, etc.) are the same as positrons. After the positron enters the substance, it is first lost through various non-elastic scattering processes The energy is slowed down to thermal energy, and the slowed positrons continue to diffuse in the medium. When encountering electrons on the diffusion path, a relativistic mass-energy conversion process will occur. That is, the positron and electrons are annihilated and emit Y rays. Nuclear spectroscopy records the information of annihilated photons, which can give the microstructure information of the polymer of interest. In the molecular materials, positrons can not only directly annihilate with electrons, but also form a metastable atom with electrons. ——Positron (referred to as Psh according to the spin state of the positron and the electron, PS atoms can be divided into spin triplet oPs and spin singlet P "Ps. Ps atoms preferentially form subannihilation in free volume, Therefore, the annihilation parameters of oPs can be measured to obtain the size and concentration distribution of nano-scale free volume in the polymer. According to the calculation of quantum mechanics principle, the following quantitative relationship between Ps annihilation life T and free volume radius R has been obtained ： In recent years, the international development of the continuum spectrum analysis program MELT based on the principle of maximum entropy positron annihilation life. Using the MELT program of positron life analysis can directly obtain the free volume size, concentration and distribution map (1) and figure (2 ) Shows the changes in the size and distribution of PS life and free volume in PET with temperature. From this, the two secondary relaxation transition temperatures (0 transition and Y transition) and the expansion coefficient of free volume can be determined.
We measured the change of free volume fraction and conductivity with temperature in PUE-LiC104 polymer solid electrolyte, using the WLF equation for the first time in the world to obtain a direct quantitative relationship between conductivity and free volume fraction: the above formula can reasonably explain Li ion transport The physical aging of the free-volume mechanism polymer materials is an important problem that must be overcome during use. Therefore, it is of great significance to study the microstructure changes during the aging process. We cooperated with the University of Science and Technology of China and Fudan University to study the aging effect of polymers. By measuring the change of the positron annihilation parameter with the aging time, and using the KWW equation: the relaxation function of the free volume is calculated, and it is found that the free volume decreases due to structural relaxation and segment rearrangement during the aging process.
By studying the annihilation intensity of oPs in PP, PB, HDPE and LDPE as a function of temperature, we found that the scale behavior of free volume void concentration (characterized by -Ps intensity I) in these linear polymers with linear chain structure: (r-: Tg / index P can be used to characterize the degree of branching of the chain.
Recently, we collaborated with East China University of Science and Technology, Beijing University of Chemical Technology and Hong Kong Polytechnic University to study the free volume mechanism of gas permeability in polymers using positron method. Relevant research results have been accepted. Member.Sci, Mater.Chem.Phys, EaroPolynU and the College of Chemistry Journal published and accepted for publication.
Due to space limitations, this article only covers part of our research. Internationally, positron probes have been widely used in studying the microstructure of polymers, such as. Compatibility of blend copolymers, interfacial properties of polymer composites, irradiation effects, dynamic deformation processes, pressure effects, doping effects, crystallization effects, temperature effects, short-range structure of segments, etc. In summary, due to the in-situ, non-destructive and high sensitivity characteristics of the positron probe, it has broad application prospects in the research of polymer microstructures.
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