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The method for producing polyvinyl alcohol (PVA) by the solution polymerization of vinyl acetate first and then alcoholysis strategy is industrial. Where solvent plays a vital role in control over stereoregularity, molecular weight and its distribution, crystallinity and so on. In this report, when methyl acetate was used as a solvent in the polymerization of vinyl acetate instead of methanol, prepared PVA was characterized by Fourier-transform infrared spectroscopy (FT-IR), nuclear magnetic resonance (NMR), gel permeation chromatography (GPC), thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC). The results showed that PVA obtained in methyl acetate has a higher content of syndiotactic structure, crystallinity melting temperature and enhanced thermal stability compared with PVA obtained in methanol due to the formation of [PI] - [PI] complex between methyl acetate and vinyl acetate and the enhanced steric hindrance effect during polymerization. Additionally, methyl acetate has a smaller chain transfer constant (1.31 * [10.sup.-4]), which benefits to decrease the branching degree and molecular weight distribution index of PVA. This research provides more ideas for the choice of solvents in the industrial production of polyvinyl alcohol. KEYWORDS branching degree, molecular weight distribution, polyvinyl alcohol, tacticity, vinyl acetate, 1 | INTRODUCTION Polyvinyl alcohol (PVA) is a biodegradable synthetic polymer with excellent characteristics including transparency, gas barrier property, water-solubility, mechanical properties, which has been widely used in various fields [...]

Aiming at the problems of more blind monitoring areas and unable to show the distribution of gas flow field in single point monitoring mode of gas in mining face, based on edge computing technology, a method of constructing gas concentration field in mining face, visual display of gas flow field and monitoring and early warning of abnormal gas emission is proposed. The platform revolutionized the traditional methane monitoring model, replaced single-point monitoring with area monitoring, replaced methane substations with edge computing gateways, and realized global monitoring of methane in coal mining faces, built methane data fields underground, displayed methane cloud diagrams, and captured methane emission abnormally. The specific ideas are as follows: the distribution characteristics of “step rise” and “gradual rise” of methane in mining face are obtained by field measured data. Based on this, a full coverage monitoring scheme of methane is designed, and the full coverage monitoring layout system of methane is completed by using a new MEMS methane sensor; the monitoring data are transmitted to the edge computing gateway by wireless or wired means; edge computing gateway is located in mining roadway, embedded data processing model, single point methane concentration abnormal jump recognition model and periodic abnormal fluctuation discrimination model. The data processing model to build methane digital field, methane dynamic cloud diagrams, the jump model of single point methane concentration anomaly are used to judge the fluctuation of single point methane concentration anomaly, the discrimination by region mode to complete the mining face methane abnormal fluctuation behavior capture. Finally, the macroscopic display of methane concentration field cloud diagram and microscopic capture of abnormal fluctuation of methane concentration are realized.

Starting from the requirement of environmental safety monitoring and early warning in intelligent working face, the monitoring and early warning of five kinds of disasters including water disasters, fire disasters, gas disasters, mine pressure disasters and dust disasters in working face is taken as the main line. By means of information technology, the real-time monitoring and early warning realization of five disasters in intelligent working face are studied. Based on the data source of basic analysis, disaster analysis method and monitoring analysis results, the environmental safety monitoring and early warning of intelligent working face is analyzed.

In view of goaf spontaneous combustion prevention of 12401 fully mechanized working face with large mining height in Shangwan Coal Mine, CO is determined as prediction index gas of coal spontaneous combustion and C2H4 is used as auxiliary index gas to master coal spontaneous combustion through temperature programmed oxidation experiment. Gas in goaf is monitored by laying beam tubes on air return side in goaf of the working face, and spontaneous combustion 'three zones' in goaf were divided according to monitoring results. Heat dissipation zone is 0-32 m away from the working face, spontaneous combustion zone is 32-225 m away from the working face, and suffocation zone is 225 m away from the working face. According to total length of heat dissipation zone and spontaneous combustion zone and the minimum coal spontaneous combustion period, the minimum safe advancing speed of the working face is about 6.4 m/d. The research results can provide reliable basis for formulation of fire prevention measures on the working face.

Changing rule of composite gas indexes under different dry air flow and oxygen concentration was analyzed through coal temperature-programmed experiment. The experimental results show that φ(O2)/(φ(CO)+φ(CO2)) should be selected as gas indexes for coal spontaneous combustion prediction in the case of uncertain dry air flow. φ(O2)/(φ(CO)+φ(CO2)) can be used as gas indexes for coal spontaneous combustion prediction when coal temperature is below 100 °C. φ(C2H4)/φ(CH4) and φ(CO)/φ(CO2) can be used as gas indexes for coal spontaneous combustion prediction when coal temperature exceeds 100 °C. φ(C2H4)/φ(CO) can also be used as gas indexes for coal spontaneous combustion when coal temperature exceeds 160 °C and oxygen volume fraction is approximately 5%.

Attackers exploit vulnerabilities to change RET gadget chains and control the order of program execution, threatening the security of software implementations. For example, memory overflow attacks and remote shellcode attacks will lead to the leak of information. The previous key agreement protocols have not considered the security of protocol implementations at the source code level. As a result, when used on the networks (like wireless sensor networks, SAE/LTE networks, IOT networks, and cloud computing environment), they often suffer from vulnerabilities. To solve these problems, a novel key agreement protocol is proposed on the base of the RET gadget chain in this paper. The novel key agreement protocol not only considers the security of cryptographic techniques and the integrity of the control flow when executing programs, they can also prevent the attacks from the vulnerabilities during implementation at the source code level. At the same time, its security analysis and experiments conducted are presented in this paper. It has been shown in the experiments that the novel key agreement protocol can prevent code reuse attacks in the process of implementation at the source code level, and its performances (like time overheads and security) are better than those of other common key agreement protocols.

A real 3D model of mine ventilation system was built by VENTSIM 3D ventilation dynamic simulation system taking a coal mine as research object. On the basis of obtaining reliable basic ventilation parameters, an accurate simulation of optimal regulation and control scheme of the ventilation system was made. The actual measurement results are in good agreement with the simulation results, and error is very small, so it verified the reliability of optimal regulation and control scheme of ventilation system based on 3D model.

In view of problem of big power consumption, low degree of automation, low accuracy and real-time performance existed in traditional and artificial control method of fireproof and extinguishing by even air pressure, an automatic frequency-conversion control system of fireproof and extinguishing by even air pressure was designed, and composition, operation mode， control method and logic implementation process of the system were introduced. The system adopts theory of local dynamic air pressure balance of multilayer goaf flow field, uses method of automatic frequency-conversion to control local ventilator to change pressure of working face according to differential pressure of working face and upper adjacent goaf, so as to reach the purpose of pressure equalizing of working face and ambient, which realizes automatic control of fireproof and extinguishing by even air pressure of interconnected mining working faces of multilayer goaf.

Claims millions of middle school graduates are unable to assist in meeting the nation's needs in economic development. States that educational and the economic sectors are not working together to solve the urgent task facing rural education. Provides plan for curriculum development with the stress on vocational education. (NL)

The Jian'an [TEXT NOT REPRODUCIBLE IN ASCII] (196-220) period witnessed a flourishing of 'presentation and reply' poetry (zengda shi [TEXT NOT REPRODUCIBLE IN ASCII]). (1) Many of these poems were [...]