Materials and Properties
ZHOU Danyan, HUANG Hanxiong
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392 )
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Flexible piezoresistive pressure sensors were prepared through assembling the surface⁃microstructural and the flat thermoplastic polyurethane sensing substrates. The surface of the microstructural substrate with a size of 10 mm×10 mm was sprayed with diffe⁃rent masses of multi⁃walled carbon nanotubes （MWCNTs） （0.02， 0.05 and 0.1 g）. The surface morphology of the microstructural flexible sensing substrate and the performance of the sensors were characterized and analyzed. The results demonstrated that there was a carbon nanotube layer with a certain thickness formed on the top surfaces of the microcolumns on the microstructural substrate. The sensor based on the microstructural sensing substrate sprayed with a greater mass of MWCNTs exhibited a higher sensitivity and a lower detection limit. This may be attributed to greater increments in the pressure⁃induced overlap degree of the MWCNTs network and the contact area between the sensing substrates. The sensor based on the microstructural sensing substrate sprayed with 0.1 g MWCNTs presented a higher sensitivity of 0.143 kPa-1 （0~3 kPa）， a lower detection limit of 100 Pa， together with a certain piezoresistive response in the wide pressure range of 3~200 kPa. Moreover， the sensor maintained a stable piezoresistive response during the 4 000⁃cycle compression/release test with a peak pressure of 200 kPa， and had an accurate detection capability toward the piezoresistive responses generated by typical human motions. The sensor exhibits great application potential for intelligent wearables.
WU Jingjing, ZHANG Wenming, ZHOU Qinpeng, ZHANG Xinqing, LU Chong
Abstract (
307 )
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Poly（ethylene glycol monomethyl ether⁃co⁃poly lactide） （mPEG⁃PLA） was prepared through ring opening polymerization of methoxy poly（ethylene glycol） and D， L⁃lactide， and then the mPEG⁃PLA/paclitaxel drug⁃loaded micelles （DMs） was prepared. The degradation characteristics of mPEG⁃PLA and drug⁃loaded micelles were studied under different conditions. The results indicated that mPEG⁃PLA and DMs kept stable at a low temperature of 4 ºC even for a long period of 24 months. The degradation rates of mPEG⁃PLA and DMs improved with an increase in temperature， but their degradation mechanisms were different， especially at temperature above 40 ºC. The degradation of mPEG⁃PLA was dominated by the random fracture inside the segment， resulting in a significant decrease in molecular weight. However， there was no obvious increase in the contents of lactide and lactic acid. The degradation mechanism of DMs was more possibly attributed to the “chain⁃end scission rule” at a high temperature of 60 ºC. This means the depolymerization at the terminal， leading to the production of more amounts of lactide and lactic acid.
YANG Xu, FANG Jian, QIN Min, YU Lei
Abstract (
369 )
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Chitosan/gellan gum bilayer films were prepared using a solution flow⁃casting method and a layer⁃by⁃layer self⁃assembly technology， their optimum preparation conditions were determined by single factor and response surface tests， and their microstructure， chemical structure， mechanical properties， optical properties， water resistance and oxidation resistance of the bilayer films were evaluated. The results indicated that the mass fractions of chitosan and gellan gum were determined to be 1.7 and 1.5 wt%， respectively， for the bilayer films. Moreover， the volume ratio of chitosan to gellan gum was 4/6， and the mass fraction of glycerol was 25 wt% in the bilayer films. Under these conditions， the bilayer films presented tensile strength of 38.83 MPa and moisture permeability of 750.76 g/（m2·d）. Compared to the single film， the bilayer films show a smooth surface and a compact cross section. Fourier⁃transform infrared spectroscopic results indicated that the bilayer films had good compatibility and their mechanical properties and water resistance were improved effectively compared to pure chitosan film and pure gellan gum film.
WANG Fang, LI Yan, GUO Xintao, DOU Peng
Abstract (
295 )
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A series of ternary polysulfone copolymers with different contents of bisphenol fluorene were synthesized through polycondensation of bisphenol A， bisphenol fluorine， and 4，4⁃dichlorodiphenyl sulfone. The effects of reaction solvent， rea⁃ction time and salt forming agent on the polymerization were studied. The structure and properties of ternary polysulfone copolymers containing bisphenol fluorene were characterized using gel permeation chromatograph， Fourier⁃transform infrared spectrometer， and differential scanning calorimeter. The results indicated that the ternary copolymers with a number⁃ave⁃rage molecular weight higher than 2×104 were successfully synthesized at a reaction time of 4 h using N⁃methyl pyrrolidone as a reaction solvent and potassium carbonate as a salt forming agent. The heat resistance of the copolymers increased gradually with an increase in the content of bisphenol fluorene， and the decomposition temperature of the main chain was higher than 510 ℃. The copolymers presented a glass transition temperature of 254 ℃ at a bisphenol fluorene content of 80 mol%， and their transmittance was 84 % in the visible light range. This can meet the requirements of temperature resistance for aircrafts at high Mach number. This study establishes the basis of application for highly heat⁃resistant transparent materials.
GU Jianguo, SUN Jiankai, HOU Peipei, HE Hao
Abstract (
427 )
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Biaxially oriented polyvinyl chloride （PVC⁃O） pipes were prepared through one⁃step extrusion based on a pure formula using polyvinyl chloride resin as a main raw material. PVC⁃O pipes were prepared using the same formula through axial tensile at stretching temperatures of 80， 85， 88， 90 ºC and a radial stretching ratio to 1.8/1 （the outside dia⁃meter of the feedstock pipe： 63 mm and wall thickness： 5.2 mm） and 1.9/1 （the outside diameter of the feedstock pipe： 60 mm and wall thickness： 6.0 mm）. The mechanical properties of the pipes produced through different processes were characterized through hydrostatic， falling⁃weight impact， tensile， and ring stiffness tests. The results indicated that the optimal tensile temperature of the PVC⁃O pipe extrusion was determined to be 85 ºC， and its optimal radial tensile ratio was 1.9/1.
ZHANG Zhaoyang, TANG Gan, KE Xue, WANG Xin, JIANG Xueliang, LIU Fangjun, YOU Feng
Abstract (
354 )
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Glycidyl methacrylate （GMA）⁃grafted calcium sulfate whiskers （CSW） was synthesized via chemical modification of CSW with GMA， and its structure and GMA loading were analyzed using Fourier⁃transform infrared spectroscopy and thermogravimetric analysis （TGA）. The CSW and GMA⁃grafted CSW were melt⁃blended with high⁃density polyethylene （PE⁃HD） and polyamide 6 （PA6） to form the PE⁃HD/PA6/CSW composites. The composites were investigated using scanning electron microscopy， differential scanning calorimetry， TGA， and universal testing machine. The results indicated that the PE⁃HD/PA6/GMA⁃grafted CSW composite with a bridge structure exhibited better crystallinity， thermal stability， and mechanical properties. As the length of CSW was greater than the diameter of PA6， the CSW penetrated the PA6 phase to insert into the PE⁃HD phase in the composites， resulting in the formation of the bridge structure. The GMA⁃grafted CSW generated a heterogeneous nucleation effect on the composites， promoting the crystallization process of PA6 and improving its crystallinity accordingly.
LI Wenting, LI Mingpeng, CHEN Jihe, YUAN Zhitong, CHENG Haitao
Abstract (
313 )
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Flexible polyurethane foam （FPUF）/twisted plant fiber （TPF） composites were prepared using a multidimensional implanting method. The effects of the implantation direction， hairiness rate， and volume fraction of TPF on the interface bon⁃ding， mechanical properties， and fatigue resistance of FPUF/TPF were investigated. The results indicated that the mechanical properties and fatigue resistance of the composites were improved after the TPF was planted. When the volume fraction of TPF implantation was set between 0.35 % and 0.7 %， the composites obtained a better enhancement in performance together with a lightweight. The enhancement of mechanical properties after planting bars reflected an increase in indentation hardness， which increased by up to 89.69 %. The indentation ratio was generally improved up to 3.56， resulting in an increase by 37.98 %. The composites also showed an improvement in support performance. The transversely planted reinforcement samples generally presented a decrease in hysteresis loss rate， improving their comfortability when used as a cushion material.The longitudinally plan⁃ted reinforcement samples generally presented an increase in hysteresis loss rate， leading to an improvement in buffering performance. The enhancement of fatigue resistance of the composites after planting bars reflected an improvement in creep resistance and deformation resistance， and the deformation was smaller after long⁃term use. After bar implantation， the 40% indentation hardness loss rate was reduced to a minimum of 11.01 %， showing a decrease by 38.59 %. The cycle times after bar implantation reached 4.1 times than that of the blank sample. A comparative study indicated that the transversely plan⁃ted samples had a better interface than the longitudinally planted ones. The TPF with less hairiness had a better interface with polyurethane， and too much hairiness might affect the size and shape of the cells to a certain extent.
LI Guili, YU Qiuran, HAO Mingliang, LI Haimei
Abstract (
285 )
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154
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The effect of surface treatment parameters of ramie fibers （RF） on the crystallization behavior and tensile proper⁃ties of molded polylactic acid （PLA）/RF composites were investigated using polarized light microscopy， differential scanning calorimetry， and scanning electron microscopy. The results indicated that the crystallization behavior of PLA was greatly affected by the alkaline treatment time of RF surface. After the RF surface was treated for 3 h， PLA presented an increase in transcrystalline density. Its cold⁃crystallization temperature was reduced， but its relative degree of crystallinity was improved. When the treatment time was increased to 6 h， the crystallization of PLA was hindered by RF. The Mo’s method was adopted to study the non⁃isothermal crystallization kinetics of PLA quantitatively. The results indicated that the change of the fiber morphology did not cause a change in the crystallization mechanism of PLA. The tensile test results showed that the optimal strengthening and toughening effects were achieved for the PLA/RF composites at an alkaline treatment time of 6 h. In this case， the composites exhibited an increase in tensile strength and elongation by 19.6 % and 23.9 %， respectively.
CHEN Congbo, FAN Shuo, ZHANG Rui, LI Hui, LUO Siqi, REN Yijin
Abstract (
552 )
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The non⁃isothermal crystallization behavior and crystal structure of poly（vinylidene fluoride） （PVDF）/poly（methyl methacrylate） （PMMA） blends were investigated at different cooling rates using differential scanning calorimeter and X⁃ray diffractometer， and their non⁃isothermal crystallization kinetics were studied using the Jeziorny’s equation and Mo’s method. The results indicated that a high mass ratio of PVDF to PMMA （6/4 and 5/5） completely inhibited the crystallization of PVDF. For the crystallizable blending samples at mass ratios of PVDF to PMMA of 9/1， 8/2， and 7/3， their crystallization temperature increased at first and then tended to decrease with an increase in the proportion of PMMA. However， their crystallization rate increased slightly at first and then decreased. A small amount of PMMA could generate a heterogeneous nucleation to promote the crystallization of the PVDF domain in the blends， whereas a large amount of PMMA evidently hindered the molecular rearrangement of PVDF and inhibited its crystallization. The addition of PMMA promoted the crystal structure transform of the PVDF phase to lower dimensions. The crystal structure of the PVDF phase was mainly the α⁃form crystal and was not influenced by the increase of PMMA content. This resulted in an increase at first and then a decrease in the degree of crystallinity.
ZHANG Kehong, CHU Chengxiang, LIU Xiaolong
Abstract (
483 )
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Cellulose nanofibers （CNF） were prepared using wood powders as a raw material， and then the epoxy （EP）⁃based composites with the glycidyl methacrylate （GMA）⁃modified CNF were prepared using a solution casting method. The effect of CNF and GMA contents on the properties and mechanisms of the composites were investigated through characterizing their mechanical properties， light transmission properties， hydrophilic properties， thermal stability， and microstructure. The results indicated that the EP/CNF composites exhibited an increase at first and then a decrease in tensile strength， elongation at break， and light transmittance with an increase in the CNF content， and their hydrophilic property increased with the CNF content. The EP/CNF composites obtained optimal performance at a CNF content of 0.6 wt%， resulting in tensile strength of 32.166 MPa， elongation at break of 20.995 %， and light transmittance of 79.8 % at 600 nm， and a contact angle of 77.34°. The compatibility between the GMA⁃modified CNF and EP was improved， leading to an enhancement in the mechanical properties and thermal stability of the composites. Their tensile strength， elongation at break， light transmittance， and hydrophilic property varied with an increase in the GMA content. The EP/GMA⁃modified CNF composites exhibited optimal performance at a GMA content of 4.8 wt%， resulting in tensile strength of 57.933 MPa， elongation at break of 18.762 %， light transmittance of 86.3 % at 600 nm， and a contact angle of 81.42°.
WANG Pei, NIU Lili, LI Jingyu, GENG Hongmei
Abstract (
271 )
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102
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Polycaprolactone （PCL）/curcumin （CUR） porous scaffold materials were prepared using a solution⁃casting method， and their porosity and drug loading and sustained release behaviors were investigated using scanning electron microscopy and thermogravimetric analysis. The in vitro drug release model of the scaffold materials was also analyzed. The results indicated that the drug release model of the scaffold materials conformed to the first⁃order equation at a CUR content of 2 wt%， a PCL concentration of 10 wt% in acetic acid， and a chitosan content of 2.86 wt% in the scaffold materials. The scaffold materials presented porosity of over 95 %， resulting in a drug loading of 1.63 %. In the phosphate buffer solution， the cumulative release rate of CUR for 90 h was 76.2 %. For other formulations， the scaffold materials showed porosity of above 95 %， and their cumulative slow⁃release rate of CUR for 90 h was between 60 % and 87 %. This demonstrated that the developed scaffold materials had ideal porosity and obvious sustained⁃release effect of CUR drug， exhibiting a good application prospect in the tissue⁃engineering field.
Processing and Application
MA Xiuqing, HAN Shuntao, LIU Xiaohong
Abstract (
344 )
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163
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An extensional flow static mixer was designed for enhancing the mixing effect in the polymer process. Taking the high⁃density polyethylene/polystyrene mixture as a mixing system， combining with the SEM images of the mixing system and the average particle size of the dispersed phase， the mixing performance of the extensional flow static mixer with different plate structures and gaps （0.125~2.0 mm） was investigated. The results indicated that there was little influence on the mixing performance from the plate inlet structure of the extensional flow static mixer. A larger fillet radius at the top of the plate resulted in a better the mixing effect. As the plate gap increased， the mixing performance presented a decreasing⁃increasing⁃decreasing trend.
ZHAO Wenwen, XU Shuangping, JIA Hongge, WANG Xing, XU Jingyu
Abstract (
271 )
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In this paper， the research progress in the hyperbranched polymer⁃based gas⁃separation membrane materials was reviewed. Different types of hyperbranched polymers were introduced， and their synthetic technologies were described， such as traditional hyperbranched polymers and large π⁃conjugated hyperbranched polymers. The mechanisms and functions of azo⁃hyperbranched polymers， polyamide⁃hyperbranched polymers， and blend hyperbranched polymers in the preparation of separation membranes were introduced in detail. The future research directions of hyperbranched polymers in the field of gas⁃separation membranes were prospected.
GAO Ge, LI Yang, XU Jie, WANG Weifeng, LI Zhe
Abstract (
246 )
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160
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An intensive investigation was carried out using thermogravimetric⁃infrared spectroscopy to evaluate the thermal degradation characteristic variation of poly（vinyl chloride） （PVC）⁃encapsulated copper wire after suffering from overloading compared to normal wire insulation materials. The activation energy of insulation materials were calculated using the Flynn⁃Wall⁃Ozawa method. The types of gases escaping from the pyrolysis of insulation materials were determined based on the infrared spectroscopy to explore the influence of overloaded failure on the thermal degradation characteristics of PVC insulation materials. The results indicated that the initial temperature of oxidation exothermic reaction of overloaded wire insulation material （408 ℃） was lower than that of normal wire insulation materials （424 ℃）. In addition， the activation energy required by the oxidation exothermic reaction of the insulation materials for the overloaded wire （142.18 kJ/mol） was far lower than that of the normal wire （231.54 kJ/mol）. This suggested that the overloaded wire insulation materials were prone to oxidation exothermic reaction compared to the normal wire insulation materials. The HCl removal reaction of PVC⁃based insulation materials during the failure⁃overloading process might result in a significant reduction in the corrosive HCl gas generated if the wire was burning. Meanwhile， the temperature range of the oxidation exothermic reaction became narrow.
LIN Xiang, ZHANG Jun
Abstract (
317 )
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The rheological properties of a highly filled solid propellant stimulant were studied using capillary and rotational rheometers under the steady shear and dynamic shear conditions. The effects of melt pressure and shear amplitude on the shear viscosity of the solid propellant stimulant were investigated in the temperature ranges of 100~120 ℃ and 50~100 ℃， respectively. The dependency of shear viscosity on the melt pressure was evaluated through a pressure coefficient， correcting the effect of pressure on viscosity. The results indicated that the pressure coefficient was distributed between 4.81 and 9.65 GPa-1 at a pressure between 12 and 121 MPa. The shear viscosity of the solid propellant stimulant increased as a power⁃law trend， indicating a steady capillary shear. In addition， dynamic shear test results indicated that the viscosity of solid propellant stimulant increased two times when the normal force rose from 15 to 42 N. Furthermore， the effect of shear amplitude on the rheological properties of the highly filled solid propellant stimulant was found to be sensitive. A slip occurred between the sample and plate at a relatively low temperature and large shear amplitude. This should be avoided in practical use.
XU Bin, CHEN Zhiqiang, CHEN Yanfei, MA Ning
Abstract (
591 )
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263
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This paper introduced the application of twin⁃screw extruder for the high⁃proportion solvent polyolefin elastomer （POE） devolatilization and granulation. Using an experiment plate， the “controlled stepwise devolatilization process” was investigated through setting a suitable temperature and configuration of twin screw under the different process requirement of different section and controlling different vacuum of every devolatilization section. A high proportion of solvent （75 wt%） in the POE fluid was devolatilized up to 50~200 ppm through the twin⁃screw extruder. Meanwhile， a key technical point in devolatilization process was described， which provided a design support in the mega⁃extrusion and devolatilization unit.
LI Wanlong, YANG Weimin, LAN Tianjie, LI Haoyi, DING Yumei, QIU Yonghong
Abstract (
463 )
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315
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This paper introduced the principle and characteristics of the supercritical fluid （SCF） technology and its application progress in plastic processing. The applications of SCF in plastic microfoaming， plastic degradation， plastic modification， anhydrous dyeing， auxiliary atomization， extraction， and green plasticizing were discussed in detail. Based on the current research progress， the SCF technology can be used to resolve many problems that can hardly be overcome using the traditional technology in plastic processing. Therefore， the SCF technology exhibits a broad application prospect in the new plastic processing field. Through the utilization of the SCF technology for plastic processing， a new approach can be acquired for the green and high⁃performance development of plastic processing.
Additive
PEI Yu, LYU Chunsheng, WANG Jun
Abstract (
487 )
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331
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This paper introduced the fours type of most studied antioxidants for polymers， including loaded antioxidants， high molecular weight antioxidants， biological antioxidants， and multifunctional antioxidants， and explained their structure， synthetic process， antioxidant properties， and application effects on polymers. Finally， the future development direction of antioxidants for polymers was prospected.
Plastic and Environment
YANG Shangshan, SHANG Pengpeng, XU Jing, XIE Jiazhuo, ZHANG Kun, ZHANG Lili
Abstract (
445 )
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195
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The garbage bags based on poly（butylene adipic acid⁃co⁃terephthalate） （PBAT）/poly（lactic acid） （PLA）/biomass complex powder composites at a mass ratio 74.26/4.95/19.80 were prepared through granulation and extrusion blowing processes. PBAT and PLA were melt blended with bamboo， lignin， and straw complex powders. The microstructure， composition， heat resistance， tensile properties and leakage resistance of the garbage bags were characterized using scanning electron microscopy， Fourier⁃transform infrared spectroscopy， X⁃ray diffraction， thermogravimetric analysis， and differential scanning calorimetry， and their practicability was also evaluated. The results indicated that the dispersibility of three types of biomass powders in the PBAT/PLA matrix had little effect on the film structure and thermal properties. Compared to the garbage bags containing bamboo and lignin powders， the garbage bags containing straw powders presented an increase in strength by over 40 %. This work exhibited an important significance for reducing the production cost of PBAT/PLA garbage bags and promoting the industrial application of biodegradable materials.
Machinery and Mould
ZHAO Liping, ZHANG Yanhua, YU Weibin, ZHANG Weihe
Abstract (
401 )
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350
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This paper analyzed the design process of an injection mold for the dust box cover of a sweeping robot in terms of the characteristics of the product and injection mold using antistatic acrylonitrile butadiene styrene copolymer （ABS） as a raw material. To meet the requirements of appearance， structure， and production capacity for the dust box cover of the sweeping robot， the mold was designed by adopting the layout of the first mock examination and two holes. The pouring system of converting the hot runner to the cold runner of the single nozzle mold and the side gate feeding form were also employed for the mold. This improved the molding yield and appearance quality of the product. This mold was designed with an inclined top core pulling mechanism of the front mold and a secondary core pulling mechanism of the slider to take off the reverses in two directions simultaneously. An inclined top core pulling mechanism of the rear mold combined with the ordinary slider core pulling mechanism was also adopted to complete the demolding， solving the demolding problem of the hole position reverses in different directions of the product and the sticking force problem of the ring deep rib position of the product. Through the actual production verification， the structural design of the mold was reasonable， and the product molding was stable， which met the requirement of production.
HE Hezhi, GAO Qi, ZHANG Tao
Abstract (
606 )
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With an increase in demand for large⁃scale injection⁃molding products in the fields of aerospace， automobile， national defense and medical treatment in modern production and life， high⁃efficiency， energy⁃saving and intelligent large⁃scale injection⁃molding machine equipment and its manufacturing technology have gained rapid development. This paper reviewed the current development trend in large⁃scale injection⁃molding machines at home and abroad and summarized the development process of large⁃scale injection⁃molding machines with clamping device as the main line in the direction of light weight. In addition， the paper introduced the energy⁃saving direction of large⁃scale injection⁃molding machines with a power⁃driving system as a main approach. The development trend in intelligent injection⁃molding machines was also summarized with a combination of the development environment of Industry 4.0.
Review
QU Yanqing, ZHA Yuxin, JIA Hongge, XU Shuangping, ZHANG Mingyu
Abstract (
483 )
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369
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Interfacial polymerization technology was widely used in the preparation of separation membranes due to its excellent performance， such as mild reaction conditions， strong controllability， self⁃sealing and self⁃termination， etc. This paper introduced the principle of interfacial polymerization technology and the applications of interfacial polymerization in membrane separation， including nanofiltration membrane， reverse osmosis membrane， and gas separation membrane. The application status of interfacial polymerization technology in microcapsules， conductive polymers， and fiber materials were discussed. The research direction of the preparation of separation membranes using an interfacial polymerization technology was prospected.
ZHU Wenqin, ZHANG Liyang, CUI Liang, WANG Yisen, ZHANG Fengbo, ZHU Kaige, GAO Kejing, LI Shuanhong
Abstract (
386 )
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266
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This article introduced the latest research progress in the preparation of micro/nano laminated polymer systems as well as the technological process of micro/nano lamination. The improvement of micro/nano lamination on the electrical properties， barrier performance， mechanical properties， optics， and shape memory of composite materials was summarized. The future development direction of the micro⁃nano stacking technology was prospected.
YU Wenxia, DANG Chunlei, HE Yilin, WANG Yaomin, ZHANG Yan’e, LIU Qian, TIAN Huafeng
Abstract (
520 )
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765
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This paper introduced the preparation methods， structure， and properties of poly（vinyl alcohol） （PVA） blending films， including PVA blends with natural polymers such as starch， chitosan， and protein， synthetic biodegradable polymers such as polylactic acid， polycaprolactone and polyethylene glycol， and synthetic non⁃biodegradable polymers such as polyaniline， low density polyethylene， and polyurethane. The application of PVA blending films in the fields of medical treatment， separation， food packaging， and agriculture was introduced， and their future research direction was prospected.
SONG Jianing, DU Bin, ZHANG Zhenli, GAO Ganggang, LI Shuanhong, ZHU Wenqin, CAO Wenbin, CHEN Shangtao
Abstract (
445 )
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933
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This paper introduced the basic requirements of direct current （DC） cable insulation as well as the problems exi⁃sting in mechanical properties， aging resistance， insulation properties and thermal conductivity when using polypropylene as a raw material for DC cable insulation. The research progress in the copolymerization modification， nanoparticle modification， blend modification and graft modification of polypropylene was reviewed. The future research direction of polypropylene⁃based materials for DC cable insulation was prospected.
LIU Jinyu, JIA Yongxing, WEN Bianying, QIU Munan
Abstract (
449 )
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292
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This paper introduced the utilization status of corn straw， rice straw， and wheat straw. The surface treatment methods of straw fiber were discussed， which included physical treatment method， chemical treatment method and other treatment methods. The main varieties and properties of fully biodegradable polyester/straw fiber composites were reviewed， and their future research and development directions were prospected.
