• <tr id="yyy80"></tr>
  • <sup id="yyy80"></sup>
  • <tfoot id="yyy80"><noscript id="yyy80"></noscript></tfoot>
  • 99热精品在线国产_美女午夜性视频免费_国产精品国产高清国产av_av欧美777_自拍偷自拍亚洲精品老妇_亚洲熟女精品中文字幕_www日本黄色视频网_国产精品野战在线观看 ?

    Thermal and mechanical properties and micro-mechanism of SiO2/epoxy nanodielectrics?

    2021-12-22 06:50:58TianYuWang王天宇GuiXinZhang張貴新andDaYuLi李大雨
    Chinese Physics B 2021年12期
    關(guān)鍵詞:天宇大雨

    Tian-Yu Wang(王天宇), Gui-Xin Zhang(張貴新), and Da-Yu Li(李大雨)

    Department of Electrical Engineering,Tsinghua University,Beijing 100084,China

    Keywords: nanodielectric,surface grafting treatment,molecular dynamics simulation,interface properties

    1. Introduction

    Epoxy resin (EP) is widely used in the field of insulation due to its excellent insulating properties, chemical stability, good mechanical properties, and low cost. However,with the increasing demand for electricity and the trend towards the miniaturization and integration of electrical equipment,ever-greater requirements for the various aspects of EP performance are being raised. Doping the insulating material with nanoparticles can effectively improve the insulating properties,thermal properties,and mechanical properties.[1–5]However,there are a large number of hydroxyl groups and unsaturated bonds on the surface of common nanoparticles(such as SiO2), and so the nanoparticles are in a highly chemically active and unstable state. Thus, they can easily agglomerate with other nanoparticles during the preparation of nanodielectrics, resulting in poor affinity with the polymer matrix(such as EP).[6]Some researchers have grafted a silane coupling agent to the nanoparticle surface to reduce the number of surface hydroxyl groups, and found that inhibiting the accumulation of surface charge and increasing the volume resistivity is more effective than pure SiO2nanoparticle doping.[6,7]Therefore,applying this surface grafting treatment to nanoparticles to reduce the surface hydroxyl content and then doping EP is a promising means of improving performance. However, to date, there have been few studies of the thermal and mechanical properties of doped surface-grafted nanoparticles in polymers. These properties of nanodielectrics are also very important for practical applications. Additionally, more importantly, owing to the limitations of the current experimental conditions,the change in the micro-mechanism of nanodielectrics after the grafting treatment remains unclear. Therefore,in this article,we will try to solve the above problems.

    With the rapid development of computer science,molecular dynamics (MD) simulation technology is being widely used for new material design and polymer synthesis.[8–10]At the same time, the MD simulations can analyze materials on an atomic scale, allowing the macro performance and micro structure of the material to be combined.In recent years,some scholars have studied the properties of EPs and their composite materials through MD simulations,[11–15]and the results obtained by MD simulation are basically the same as the actual experimental results,[11–21]indicating that the results of MD simulation have high reliability. At present, MD simulation has gradually become one of the important scientific research methods.[22]For example, Wanget al.[23]and Shenet al.[24]conducted the MD simulations of epoxy resin doped with C60and graphene, and they found that the relative permittivity of the system was reduced. Shenet al.performed MD simulations on graphene/epoxy resin composites and studied the influence of the number of layers and the size of the graphene on the thermal conductivity of the polymer.[11]Wanget al.analyzed the covalent bonds of the SiO2/epoxy nanocomposite interface through the molecular simulations and believed that the covalent bonds formed at the interface are the reason for the increase in the glass transition temperature of the polymer and the reduction in the mobility of the molecular segments.[12]Fasanelia and Sundararaghavan simulated the thermal conductivity of single-walled carbon nanotube/epoxy resin composites and studied the changes in thermal conductivity of composites doped with different types of single-walled carbon nanotube.[13]Gouet al.studied the interactions at the interface between single-walled carbon nanotubes and epoxy resin.[14]Junget al.simulated the doping of nitrogen-modified carbon nanotubes into epoxy resin and found that the mechanical properties of the polymers were improved.[15]

    In our previous work, we found experimentally that the epoxy resin doped with SiO2nanoparticles grafted with hexamethyldisilazane (HMDS) on the surface can effectively inhibit the surface charge accumulation,increase the volume resistivity, and improve the insulation performance.[6]Thus, in this study,the MD simulations are used to investigate the doping of EP with traditional SiO2nanoparticles and with SiO2nanoparticles grafted with hexamethyldisilazane (HMDS) at surface grafting rates of 10%and 20%,respectively. The thermal and mechanical properties of these nanodielectrics and the microscopic mechanisms are studied in depth,and the conclusions obtained will provide guidance for the analysis and design of nanodielectrics in the future.

    2. Methods

    2.1. Model construction

    2.1.1. Model structure and MD simulation

    All MD simulations described in this work were conducted using Materials Studio 8.0. The EP molecule constructed in this study was bisphenol-A epoxy resin(DGEBA).The curing agent was an amine curing agent(593 curing agent,the adducts of diethylene triamine and butyl glycidyl ether).Diagrams of the molecular structures are shown in Fig. 1(a).The DGEBA molecule (degree of polymerization is 1) has epoxy groups on both ends,and the curing agent molecule has primary and secondary amine groups at one end. Spherical SiO2nanoparticles with a radius of 1 nm were constructed.Owing to a large number of unsaturated bonds on the surface of the constructed SiO2, hydroxyl groups were added to the surface to ensure consistency with the actual situation. The SiO2nanoparticles are shown in Fig. 1(b). The reason why SiO2nanoparticles with a smaller particle size are selected for the simulation in this paper is that if the size of the nanoparticles is normal (such as tens of nanometers), then the entire system will be very large. Limited by the computing power of the computer, each simulation experiment lasted a very long time. Since this work is devoted mainly to the study of physical properties and microstructure,if the entire system is scaled up or down,these properties will not be changed. These SiO2nanoparticles were then surface grafted with HMDS. The reaction equation for the surface grafting treatment is shown in Fig.2(a).The SiO2nanoparticles with surface grafting rates of 10%and 20%were constructed(that is,10%and 20%of the hydroxyl groups on the surface were replaced by HMDS).The surface-modified SiO2nanoparticles are shown in Fig.1(c).

    A three-dimensional amorphous unit cell of each model was then constructed. Four models were constructed in this study, namely pure EP, EP doped with SiO2nanoparticles(SiO2/EP), SiO2nanoparticles with a surface grafting rate of 10% (10%-SiO2/EP), and SiO2nanoparticles with a surface grafting rate of 20% (20%-SiO2/EP). Table 1 shows the nanoparticle number and mass percentage of each molecule.

    Fig. 1. Schematic diagram of molecular structure of model, showing (a) epoxy resin molecules (top) and curing agent molecules (bottom), (b) SiO2 nanoparticles with a radius of 1 nm, (c) SiO2 nanoparticles after surface grafting with HMDS (surface grafting rate 20%), (d) EP and (e) nanocomposite doped with SiO2 nanoparticles (this panel shows 20%-SiO2/EP), with white, gray, red, blue, and yellow balls representing H, C, O, N, and Si atoms,respectively.

    Table 1. Amorphous unit cell composition of composite system.

    The initial density was set to be 0.6 g/cm3. With the subsequent structural optimization and other steps,the density of the system gradually approached to the actual density. All simulations described in this paper used the COMPASS force field. Atom-based method and the Eward method were used to calculate the van der Waals interaction and the electrostatic interaction,respectively,and the simulation quality was set to be medium.

    2.1.2. Optmization of model structure

    The structure of the three-dimensional amorphous unit cell was optimized to find the structure with the lowest energy. First, the steepest descent algorithm was used to optimize 3000 steps, and then the conjugate gradient algorithm was adopted to optimize 3000 steps. The model was then optimized for energy minimization. TheNVTensemble(an ensemble of particles with a constant number of particlesN, a constant volumeV, and a constant temperatureT) was used to balance 200 ps, and then theNPT(a constant pressureP)ensemble was used to balance 300 ps. The simulated temperature was 300 K, the MD simulation step was 1 fs, and the pressure was 1 atm (1 atm=1.01325×105Pa); the Andersen temperature control mode and the Berendsen pressure control mode were used.

    2.1.3. Build epoxy resin cross-linking model

    The crosslinking reaction process of DGEBA molecules and curing agent molecules are simulated in the subsubsection. The main reaction equation of DGEBA molecules with the amine curing agent is shown in Fig.2(b). The principle of simulating the crosslinking reaction is to first select the carbon atoms in the DGEBA molecule that can undergo the crosslinking reaction,and then search for the nitrogen atoms on the curing agent molecular around a certain cut-off range.A chemical bond forms between the carbon atom and the nitrogen atom.In the process of forming carbon–nitrogen bond,the breaking of carbon–oxygen bond and the forming of carbon–nitrogen bond correspond to the opening of the ring of epoxy molecule and the crosslinking between epoxy molecule and curing agent molecule,respectively. The criterion for whether a crosslinking reaction can occur is the cut-off distance. First,3.5 ?A was set to be the cut-off distance, and all nitrogen atoms within 3.5 ?A that can crosslink with carbon atoms were identified.As the degree of crosslinking increases,the formation of new chemical bonds becomes increasingly difficult. To accelerate the crosslinking reaction to the preset crosslinking degree,the cut-off distance was gradually increased to 8 ?A in steps of 0.5 ?A. Ideally, the crosslinking degree within the cut-off distance is 100%,but this is difficult to achieve in practice,so the crosslinking degree was set to be 90% and the reaction temperature was set to be 400 K. The process of simulating the crosslinking reaction was completed in the Perl language.

    To make the model structure stable and closer to the real material, the crosslinked model was relaxed and quenched to eliminate the local internal stress of the model and make the density of the system close to the true value. The simulation for the case of running 100 ps at 500 K was conducted with theNVTensemble,and then theNPTensemble(an ensemble of particles with a constant number of particlesN, a constant pressureP, and a constant temperatureT) was used to simulate the case of running 500 ps at 500 K until the density and energy fluctuated are both in a very small range. TheNVTandNPTensembles were then used to cool the system from 500 K to 300 K at a cooling rate of 25 K per 400 ps(that is,every time the temperature drops by 25 K,theNVTensemble is used to simulate the case of running 100 ps,and then theNPTensemble is adopted to simulate the case of running 300 ps).Finally,theNPTensemble was used to run 500 ps at 300 K to obtain the final model.

    Fig.2. (a)Reaction equation for surface grafting treatment of SiO2 nanoparticles with HMDS. (b) Main reaction equation of DGEBA molecule and amine curing agent.

    2.2. Simulation principle of thermal performance

    In this study,the changes in the thermal properties of materials in terms of the glass transition temperature (Tg), coefficient of thermal expansion(CTE),and thermal conductivity,are mainly considered. The density and volume of EP change with temperature. When the temperature is lower than theTgof EP, the EP is in a glassy state, and when the temperature is higher than theTgof EP, the EP is in a rubbery state. The value ofTgcan be obtained from the curve of specific volume(reciprocal of density)versustemperature. The intersection of the fitting line of temperature with the fitting line of specific volume refers to theTgof EP when the temperature is higher and lower thanTg.[25–27]

    The CTE is used to analyze the thermal expansion and contraction of a material as the temperature changes. The formula for calculating CTE is[26,27]

    whereV0is the initial volume of the system andVis the volume of the system when the temperature isT. The CTE is related to the slope of the fitted line of specific volume versus temperature when the temperature is higher or lower thanTg. When calculating CTE,the value ofV0when the material temperature is lower or higher thanTgis taken as the volume when the temperature is 300 K or 425 K,respectively.

    The nonequilibrium molecular dynamics (NEMD)method was used to calculate the thermal conductivity of the material. The NEMD method constructed a stable temperature gradient under a fixed heat flux,and then the thermal conductivity of the material was obtained by using Fourier’s law.[28–30]In this study,the unit cell was divided into 20 parts in one direction,and the temperature of the material was taken as the average temperature of the model. The two edge layers were high-temperature layers and the middle two layers were low-temperature layers. The exchange of energy was achieved by exchanging the hottest particles in the cold layer with the coldest particles in the hot layer.

    2.3. Principles of mechanical performance simulation

    Mechanical properties are important indicators in the production and use of EP.This study simulated the changes in the bulk modulus, shear modulus, and Young’s modulus of different materials at different temperatures. The static constant strain method was used to calculate the mechanical properties of the system, and a small strain was applied to the equilibrium EP system. Through the response of the system to strain,a simplified stiffness matrixCi jcould be obtained for isotropic amorphous materials as follows:[31,32]

    Fig. 3. Thermal performance simulation results, showing (a) change in specific volume with temperature, with vertical line denoting glass transition temperature,(b)thermal expansion coefficients of different materials in glassy state and rubbery state,and(c)thermal conductivities of different materials at 300 K.

    By analyzing the changes in mean square displacement(MSD)with simulation time at different temperatures, the molecular chain-segment motion of the system could be determined.The greater the slope of the curve,the stronger the mobility of the molecular segment of the material is,which is a key factor affecting the mechanical properties of EP. The MSD is defined as[33,34]

    whereβis the MSD of the system,Nis the total number of atoms in the system, andRj(t) andRj(0) are the displacement vector of any atomjin the system at timetand at the initial time,respectively. The angle brackets denote averaging over all chosen time origins.

    The movement of molecular chains under the thermal field depends on the size of the free space. The polymer free volume ratio is given by

    whereV0is the volume occupied by the molecules that constitute the substance andVfis the volume that accommodates the free movement of molecular segments in the system.

    3. Results and discussion

    3.1. Thermal properties

    The simulation results for the thermal performance of different models are shown in Fig. 3. Figure 3(a) shows theTgvalues of the different models. Although doping with different types of SiO2nanoparticles gives a slight increase inTg, it is not much different from the undoped case. TheTgvalues of all models lie in a range of 402 K–410 K.

    Figure 3(b)shows the CTE values of different models in the glassy state and the rubbery state. The doping of SiO2nanoparticles reduces the CTE of the system. If SiO2is surface grafted,this suppression effect is more significant,and a higher grafting rate produces a lower CTE value. When the surface grafting ratio is 20%,the CET value in the glassy state and rubbery state are only 83.2% and 54.9% of those for undoped EP,respectively. This shows that the thermal expansion and contraction of EP are both suppressed.This may be related to the small CTE of SiO2nanoparticles and the strong interaction between SiO2and EP molecules. The effect of a higher grafting rate producing a lower CTE value may be due to the increased affinity of SiO2molecules and EP molecules after surface grafting, which makes the interaction between SiO2and EP molecules more intense. The specific analysis is described in more detail in the EP molecular density distribution map around the SiO2nanoparticles in next subsection.

    Figure 3(c) shows the changes in thermal conductivity of different models at room temperature. After doping SiO2nanoparticles,the thermal conductivity is slightly higher than that of the undoped system. According to the Maxwell–Eucken model, the thermal conductivity of a composite material is given by the following equation:[23]

    Fig.4. Curves of MSD versus time at(a)300 K,(b),400 K,and(c)500 K;(d)curves of free volume ratio versus temperature.

    whereλ,λp, andλfare the thermal conductivity of the composite, polymer, and filler, respectively, andVfis the volume fraction of the filler. The thermal conductivity of EP is generally 0.2 W/(m·K),and that of SiO2is 1.4 W/(m·K).According to the above formula, the thermal conductivity of SiO2/EP is 0.235 W/(m·K).The thermal conductivity of EP is close to the simulation result,but the thermal conductivity of doped SiO2is slightly lower than the simulation result. This is mainly because the model ignores the influence of the formation of thermally conductive chains, so the thermal conductivity of the compound calculated by the model is slightly lower than that of the actual material.[25]After the surface grafting treatment of SiO2nanoparticles,the thermal conductivity of the system further increases,but not significantly. The reason for this can be analyzed by using the Agari model[35,36]

    whereC1is a free factor for forming a thermally conductive chain andC2is a coefficient for the degree of difficulty of the filler forming a thermally conductive chain. This shows that after the surface grafting treatment of SiO2nanoparticles,the thermally conductive chains can form more easily,thereby improving the thermal conductivity of the system.

    3.2. Mechanical properties

    The MSD values of different models at different temperatures are shown in Fig.4.

    Fig.5. Simulation results of mechanical properties: temperature-dependent(a)Young’s modulus,(b)bulk modulus,and(c)shear modulus.

    Fig.6. Behaviors of EP molecules in the interface area,showing(a)normalized EP molecular density in the interface area,and(b)temperature-dependent thickness of the van der Waals excluded region.

    At different temperatures, the MSD value generally conforms to the following order: EP>SiO2/EP>10%-SiO2/EP>20%-SiO2/EP. This shows that the doping of SiO2nanoparticles effectively inhibits the movement of molecular chains in the system, especially the surface-grafted SiO2nanoparticles. Figure 4(d) shows the free volume ratios at different temperatures. From this figure, it is apparent that the free volume ratio exhibits similar properties. This is because the degree of molecular chain motion mainly depends on the proportion of free volume. At room temperature, the free volume ratio after doping with 20%surface-grafted SiO2nanoparticles is only 85.48%of that of EP.As the temperature increases,the MSD value and the free volume ratio gradually increase,indicating that a rising temperature enhances the degree of molecular chain movement.

    Figure 5 shows the simulation results for the Young’s modulus,bulk modulus,and shear modulus.After doping with SiO2nanoparticles,especially surface-grafted SiO2nanoparticles,these values are improved,indicating that the mechanical properties have been improved. The reason for this is that in addition to the inhibition of the molecular segment movement mentioned above, there is stronger interaction between SiO2and EP molecules.

    Figure 6(a) shows the density distribution of EP molecules around SiO2nanoparticles. After doping with SiO2nanoparticles, an interface region forms around the surface.In this interface area,the molecular density of EP is very low when it is less than 2.5 ?A from the surface of the SiO2nanoparticles. This area is also called the van der Waals excluded region. The thickness of this area is defined as follows:[37]

    whereδis the region thickness,rNPis the radius of the nanoparticle,ρris the radial density from the surface of the nanoparticle to a given distancer,andρ0is the polymer bulk density of the system. At 300 K, after doping with SiO2nanoparticles, the thickness of the van der Waals excluded region is 2.5 ?A, whereas after doping with 10% and 20%surface-grafted SiO2nanoparticles,the thickness values of the van der Waals repelling zone become 2.4 ?A and 2.1 ?A, respectively. This shows that chemical modification of the SiO2nanoparticles weakens the molecular repulsion force between SiO2nanoparticles and EP, which is consistent with the understanding that the compatibility between SiO2nanoparticles and polymers can be improved after grafting treatment. Figure 6(b)shows the variations ofδwith temperature at different temperatures. It can be seen that at any temperature,δfor the grafted SiO2nanoparticles is lower than that in the nongrafted case, and a higher grafting rate produces a lower value ofδ, which means that there is a higher density of EP molecules in the interface region. A higher molecular density corresponds to the better elastic properties, that is, better mechanical properties. This is also why the mechanical properties of the materials runs in the order 20%-SiO2/EP>10%-SiO2/EP>SiO2/EP.This is true at different temperatures,indicating that this material modification method has good thermal stability.

    4. Conclusions

    This study presents the results of MD simulations in which the SiO2nanoparticles and SiO2nanoparticles grafted with HMDS at 10%and 20%surface grafting rates are doped into epoxy resin. The thermal and mechanical properties of these nanodielectrics and the microscopic mechanisms were studied in depth. The main conclusions are as follows.

    (i)After doping with SiO2nanoparticles,the glass transition temperature does not change significantly. However, the thermal expansion and contraction of the system and the thermal conductivity are improved,especially by the surface grafting treatment of SiO2nanoparticles. The main reason may be that after the surface grafting treatment,the thermal chains can form more easily.

    (ii) After doping with SiO2nanoparticles, the mechanical properties of the system are significantly improved. Of the systems considered in this study,the improvement in mechanical properties can be ordered as follows:20%-SiO2/EP>10%-SiO2/EP>SiO2/EP>EP.We think,this is mainly related to two factors. First, the doping of SiO2nanoparticles inhibits the degree of movement of the molecular chains in the system,so SiO2/EP>EP.Second, after the surface grafting treatment,the molecular repulsion between SiO2and EP is weakened,and the van der Waals excluded region becomes thinner, that is,the compatibility between SiO2nanoparticles and polymer is improved by the grafting treatment. Thus, the mechanical properties are further improved,leading to the ordering 20%-SiO2/EP>10%-SiO2/EP>SiO2/EP.

    Based on the analysis in this article, we believe that the surface grafting treatment of SiO2nanoparticles with HMDS and then doping into epoxy resin can effectively improve the thermal and mechanical properties of the system, and has strong potential practical applications in future.

    猜你喜歡
    天宇大雨
    快樂的小草
    小主人報(2022年18期)2022-11-17 02:19:52
    大雨
    Instructional Design Is A System
    青年生活(2020年19期)2020-10-14 21:54:16
    你最珍貴
    大雨
    Galloping Horse Treading on a Flying Swallow and Its Influence in Modern Advertising
    大雨
    一場大雨
    要命,大雨一直不停地下
    大雨
    快樂語文(2016年12期)2016-11-07 09:45:49
    好男人在线观看高清免费视频| 老司机深夜福利视频在线观看| 99久久99久久久精品蜜桃| 亚洲av不卡在线观看| 亚洲在线自拍视频| 天天一区二区日本电影三级| 久久香蕉精品热| 亚洲 国产 在线| 搡女人真爽免费视频火全软件 | 狂野欧美白嫩少妇大欣赏| aaaaa片日本免费| 亚洲天堂国产精品一区在线| 怎么达到女性高潮| 中文字幕av在线有码专区| 免费av不卡在线播放| 99国产极品粉嫩在线观看| 日韩欧美一区二区三区在线观看| 性色av乱码一区二区三区2| 日韩欧美精品免费久久 | 国产精品久久久久久久久免 | 日本在线视频免费播放| 在线十欧美十亚洲十日本专区| 我的老师免费观看完整版| av欧美777| 国产欧美日韩一区二区三| 亚洲熟妇中文字幕五十中出| 男插女下体视频免费在线播放| 夜夜夜夜夜久久久久| 国产色婷婷99| 全区人妻精品视频| 97人妻精品一区二区三区麻豆| 国产黄a三级三级三级人| 亚洲黑人精品在线| www国产在线视频色| 日韩欧美国产在线观看| 国产黄片美女视频| 一a级毛片在线观看| 亚洲国产欧美人成| 精品欧美国产一区二区三| 99热这里只有精品一区| 亚洲最大成人中文| 男女做爰动态图高潮gif福利片| 亚洲精品久久国产高清桃花| 丰满乱子伦码专区| 少妇的逼好多水| 大型黄色视频在线免费观看| 欧美日韩国产亚洲二区| 免费av观看视频| 搞女人的毛片| 搞女人的毛片| 在线播放无遮挡| 中出人妻视频一区二区| 中文字幕人妻熟人妻熟丝袜美 | 一区二区三区免费毛片| 成年人黄色毛片网站| 变态另类丝袜制服| 三级男女做爰猛烈吃奶摸视频| 18+在线观看网站| 国产高清三级在线| 国产午夜精品论理片| 91在线观看av| 日韩国内少妇激情av| 日韩国内少妇激情av| 国产黄片美女视频| 国产高清视频在线播放一区| av国产免费在线观看| 88av欧美| 亚洲性夜色夜夜综合| 国产精品 国内视频| 毛片女人毛片| 国产69精品久久久久777片| 国产久久久一区二区三区| 怎么达到女性高潮| 国产精品亚洲美女久久久| 三级男女做爰猛烈吃奶摸视频| 欧美一区二区精品小视频在线| 久久久久久人人人人人| 男人舔女人下体高潮全视频| 99国产综合亚洲精品| 母亲3免费完整高清在线观看| 18禁裸乳无遮挡免费网站照片| 狂野欧美白嫩少妇大欣赏| 日本在线视频免费播放| 91九色精品人成在线观看| av天堂中文字幕网| 亚洲国产精品sss在线观看| 美女黄网站色视频| 一个人免费在线观看电影| 18禁美女被吸乳视频| 99riav亚洲国产免费| 少妇人妻精品综合一区二区 | 两人在一起打扑克的视频| 成人鲁丝片一二三区免费| 日本三级黄在线观看| 内地一区二区视频在线| 一级黄色大片毛片| 午夜福利欧美成人| 少妇裸体淫交视频免费看高清| 男插女下体视频免费在线播放| 青草久久国产| 国产精品久久久久久人妻精品电影| 久久精品夜夜夜夜夜久久蜜豆| 嫩草影视91久久| 热99在线观看视频| 日韩欧美免费精品| 亚洲人与动物交配视频| 日本在线视频免费播放| 岛国在线观看网站| 亚洲av成人不卡在线观看播放网| 国产探花极品一区二区| 国产不卡一卡二| 国产午夜精品久久久久久一区二区三区 | 亚洲片人在线观看| 90打野战视频偷拍视频| 黄片小视频在线播放| 精品久久久久久久毛片微露脸| 亚洲黑人精品在线| xxxwww97欧美| 免费在线观看日本一区| 婷婷精品国产亚洲av| 亚洲欧美日韩东京热| 亚洲成人久久性| 色在线成人网| 热99在线观看视频| 天天添夜夜摸| 国产精品美女特级片免费视频播放器| 一卡2卡三卡四卡精品乱码亚洲| av黄色大香蕉| 校园春色视频在线观看| 欧美日本视频| 每晚都被弄得嗷嗷叫到高潮| 精品国产超薄肉色丝袜足j| 男女午夜视频在线观看| 亚洲美女黄片视频| 丰满乱子伦码专区| 久久午夜亚洲精品久久| 法律面前人人平等表现在哪些方面| 国产av麻豆久久久久久久| 伊人久久精品亚洲午夜| 成人av一区二区三区在线看| 国产精品一及| 美女黄网站色视频| 国产一区二区激情短视频| 亚洲性夜色夜夜综合| 久久九九热精品免费| 桃色一区二区三区在线观看| 精品人妻一区二区三区麻豆 | 国产精品久久久久久亚洲av鲁大| 国产精品久久久久久精品电影| 国产精品爽爽va在线观看网站| 久久天躁狠狠躁夜夜2o2o| 桃色一区二区三区在线观看| 一区二区三区国产精品乱码| 一进一出抽搐gif免费好疼| 九九久久精品国产亚洲av麻豆| 久久久久久久久久黄片| 90打野战视频偷拍视频| 亚洲av电影在线进入| 久久精品91无色码中文字幕| 狂野欧美激情性xxxx| 日本一本二区三区精品| 在线观看66精品国产| 亚洲欧美日韩无卡精品| 舔av片在线| 亚洲aⅴ乱码一区二区在线播放| 欧美成人性av电影在线观看| 日本黄色片子视频| 国产熟女xx| 99久久久亚洲精品蜜臀av| 亚洲片人在线观看| 两个人视频免费观看高清| 性色av乱码一区二区三区2| 精品一区二区三区视频在线 | 午夜福利成人在线免费观看| 国产色婷婷99| 午夜福利在线观看免费完整高清在 | 久久精品91无色码中文字幕| 人妻久久中文字幕网| 日本在线视频免费播放| 男女午夜视频在线观看| 一a级毛片在线观看| 国产三级中文精品| 一个人看视频在线观看www免费 | 国产精华一区二区三区| 久久6这里有精品| 成人特级av手机在线观看| 搡老岳熟女国产| 国产精品99久久99久久久不卡| 可以在线观看的亚洲视频| 国产蜜桃级精品一区二区三区| 12—13女人毛片做爰片一| 国产精品99久久久久久久久| 人妻丰满熟妇av一区二区三区| aaaaa片日本免费| 中亚洲国语对白在线视频| 色综合站精品国产| 少妇丰满av| 久久精品91蜜桃| 亚洲专区国产一区二区| 国产亚洲精品综合一区在线观看| 网址你懂的国产日韩在线| 精品电影一区二区在线| 国产精品女同一区二区软件 | 观看免费一级毛片| 日韩成人在线观看一区二区三区| 亚洲精品久久国产高清桃花| 久久精品亚洲精品国产色婷小说| 色噜噜av男人的天堂激情| 国产精华一区二区三区| 老司机深夜福利视频在线观看| 午夜免费观看网址| 老司机午夜福利在线观看视频| 在线a可以看的网站| 高清日韩中文字幕在线| 国产69精品久久久久777片| 国产视频一区二区在线看| 国产精品日韩av在线免费观看| 久久草成人影院| 国产精品亚洲一级av第二区| 床上黄色一级片| 欧美丝袜亚洲另类 | 午夜福利成人在线免费观看| 欧美不卡视频在线免费观看| 成人特级av手机在线观看| 熟女少妇亚洲综合色aaa.| 精品一区二区三区视频在线观看免费| 亚洲av第一区精品v没综合| 夜夜夜夜夜久久久久| 网址你懂的国产日韩在线| 亚洲av免费在线观看| 国产成人av教育| 99国产精品一区二区三区| 免费av毛片视频| 欧美成狂野欧美在线观看| 欧美乱色亚洲激情| 亚洲熟妇熟女久久| 狂野欧美激情性xxxx| 黄色日韩在线| 久久久久久久午夜电影| 蜜桃久久精品国产亚洲av| 久久香蕉精品热| 国产亚洲欧美在线一区二区| 狂野欧美激情性xxxx| 国产老妇女一区| 亚洲精品粉嫩美女一区| 内地一区二区视频在线| 国产精品综合久久久久久久免费| 岛国在线免费视频观看| 国产色婷婷99| 欧美bdsm另类| 噜噜噜噜噜久久久久久91| 免费看日本二区| 欧美最新免费一区二区三区 | 国产精品亚洲一级av第二区| 久久久精品欧美日韩精品| 女人被狂操c到高潮| 免费看光身美女| 日韩欧美一区二区三区在线观看| 黄色片一级片一级黄色片| 搡老熟女国产l中国老女人| av欧美777| 别揉我奶头~嗯~啊~动态视频| 亚洲国产精品合色在线| 久久久精品欧美日韩精品| 国内久久婷婷六月综合欲色啪| 一区二区三区高清视频在线| 天美传媒精品一区二区| 国产久久久一区二区三区| 国产极品精品免费视频能看的| 嫩草影院精品99| 欧美中文综合在线视频| 大型黄色视频在线免费观看| 国产一区二区三区在线臀色熟女| av黄色大香蕉| 最好的美女福利视频网| 中亚洲国语对白在线视频| 丁香六月欧美| av国产免费在线观看| 女警被强在线播放| 国产一区二区三区视频了| 69人妻影院| 男女那种视频在线观看| eeuss影院久久| 99在线视频只有这里精品首页| 欧美日韩国产亚洲二区| 亚洲,欧美精品.| 在线看三级毛片| 国内精品美女久久久久久| 欧美日韩乱码在线| 精品国内亚洲2022精品成人| 久久婷婷人人爽人人干人人爱| 久99久视频精品免费| 亚洲成人免费电影在线观看| 男女之事视频高清在线观看| 免费一级毛片在线播放高清视频| av欧美777| 国产成人aa在线观看| 亚洲色图av天堂| 国产三级黄色录像| 亚洲精品乱码久久久v下载方式 | 黄色片一级片一级黄色片| 网址你懂的国产日韩在线| 少妇裸体淫交视频免费看高清| 久久人妻av系列| 亚洲自拍偷在线| 亚洲成av人片免费观看| 国产探花极品一区二区| 琪琪午夜伦伦电影理论片6080| 国产成人啪精品午夜网站| 色在线成人网| 亚洲一区高清亚洲精品| 99久久久亚洲精品蜜臀av| 狠狠狠狠99中文字幕| 窝窝影院91人妻| 国产主播在线观看一区二区| 欧美色欧美亚洲另类二区| 国产精品综合久久久久久久免费| 最近在线观看免费完整版| 99久久九九国产精品国产免费| 哪里可以看免费的av片| 国产精品 欧美亚洲| 欧美又色又爽又黄视频| 高清毛片免费观看视频网站| 少妇熟女aⅴ在线视频| 麻豆国产97在线/欧美| 国产aⅴ精品一区二区三区波| 免费看a级黄色片| 免费观看的影片在线观看| 日本熟妇午夜| 国产一区二区亚洲精品在线观看| 国产色爽女视频免费观看| 国产真实伦视频高清在线观看 | 男人舔女人下体高潮全视频| 久久久久性生活片| 免费在线观看影片大全网站| 日韩欧美精品免费久久 | 一级毛片高清免费大全| 在线观看一区二区三区| xxxwww97欧美| 久久久久久九九精品二区国产| 成人午夜高清在线视频| 国产伦一二天堂av在线观看| 久久久精品欧美日韩精品| xxx96com| 18禁国产床啪视频网站| www国产在线视频色| 午夜福利高清视频| 亚洲人成电影免费在线| 亚洲精品在线美女| 黄片大片在线免费观看| www日本在线高清视频| 亚洲精品粉嫩美女一区| a级一级毛片免费在线观看| avwww免费| 久久性视频一级片| 真人做人爱边吃奶动态| 欧美黑人欧美精品刺激| 亚洲不卡免费看| 国内少妇人妻偷人精品xxx网站| 一个人免费在线观看的高清视频| 精品国内亚洲2022精品成人| 97超视频在线观看视频| 亚洲男人的天堂狠狠| 日本免费一区二区三区高清不卡| 男人的好看免费观看在线视频| 婷婷亚洲欧美| 精品免费久久久久久久清纯| 波多野结衣巨乳人妻| 国产精品久久久人人做人人爽| 亚洲成a人片在线一区二区| 亚洲av成人不卡在线观看播放网| 毛片女人毛片| 国产极品精品免费视频能看的| 老鸭窝网址在线观看| 特级一级黄色大片| 日本免费a在线| 中文字幕av在线有码专区| 亚洲美女视频黄频| 国产精品 欧美亚洲| 亚洲美女视频黄频| 日本在线视频免费播放| 啦啦啦免费观看视频1| 国产中年淑女户外野战色| 精品一区二区三区人妻视频| 久久久国产成人免费| 九九在线视频观看精品| 亚洲aⅴ乱码一区二区在线播放| 午夜日韩欧美国产| 天堂√8在线中文| 国产精品电影一区二区三区| www.999成人在线观看| 亚洲精品乱码久久久v下载方式 | 国产中年淑女户外野战色| 亚洲国产高清在线一区二区三| 国产精品自产拍在线观看55亚洲| 麻豆一二三区av精品| 美女黄网站色视频| 中文字幕人妻熟人妻熟丝袜美 | 精品久久久久久成人av| 色在线成人网| 最新在线观看一区二区三区| 在线观看av片永久免费下载| 给我免费播放毛片高清在线观看| 国产精品久久久久久久电影 | 久久精品亚洲精品国产色婷小说| 狠狠狠狠99中文字幕| 手机成人av网站| 久久天躁狠狠躁夜夜2o2o| 成人国产综合亚洲| 嫩草影院精品99| 欧美日韩国产亚洲二区| 69人妻影院| 精品福利观看| 欧美黑人巨大hd| 熟女电影av网| 亚洲国产色片| 成人国产一区最新在线观看| 又爽又黄无遮挡网站| 久久精品国产自在天天线| 国产久久久一区二区三区| 国产欧美日韩一区二区三| 黑人欧美特级aaaaaa片| 亚洲熟妇熟女久久| 欧美日本视频| 搡老妇女老女人老熟妇| 国产一区二区三区在线臀色熟女| 91九色精品人成在线观看| 国产黄色小视频在线观看| 亚洲va日本ⅴa欧美va伊人久久| 亚洲国产精品成人综合色| 波野结衣二区三区在线 | 国产伦一二天堂av在线观看| 日本a在线网址| 又黄又爽又免费观看的视频| 最后的刺客免费高清国语| 亚洲中文字幕日韩| 黄色片一级片一级黄色片| 亚洲精品一区av在线观看| 中出人妻视频一区二区| av欧美777| 最近最新中文字幕大全电影3| 久久精品影院6| 午夜日韩欧美国产| 亚洲欧美日韩卡通动漫| 日韩中文字幕欧美一区二区| 国产成人av教育| 日韩欧美精品v在线| 国产成人啪精品午夜网站| 免费电影在线观看免费观看| 99热精品在线国产| 免费观看人在逋| 国产乱人伦免费视频| 成人亚洲精品av一区二区| 91av网一区二区| 欧洲精品卡2卡3卡4卡5卡区| 亚洲av成人av| 中文资源天堂在线| 亚洲国产精品成人综合色| 亚洲中文字幕日韩| 国产麻豆成人av免费视频| 亚洲自拍偷在线| 色综合欧美亚洲国产小说| 精品国产美女av久久久久小说| 观看免费一级毛片| 久久精品国产清高在天天线| 亚洲不卡免费看| 亚洲,欧美精品.| 日韩大尺度精品在线看网址| 深夜精品福利| 精品一区二区三区av网在线观看| 亚洲国产欧美人成| 少妇熟女aⅴ在线视频| 日韩欧美精品免费久久 | 麻豆国产97在线/欧美| 国产伦一二天堂av在线观看| 一区二区三区免费毛片| 欧美一区二区精品小视频在线| 国产97色在线日韩免费| 国产精品久久久久久精品电影| 精品一区二区三区av网在线观看| 乱人视频在线观看| 久久久久国产精品人妻aⅴ院| 一本久久中文字幕| 人人妻,人人澡人人爽秒播| 蜜桃亚洲精品一区二区三区| 宅男免费午夜| 在线观看舔阴道视频| 一个人观看的视频www高清免费观看| 亚洲人成网站在线播放欧美日韩| 午夜激情欧美在线| 精品国产三级普通话版| 啦啦啦韩国在线观看视频| 超碰av人人做人人爽久久 | 亚洲性夜色夜夜综合| 白带黄色成豆腐渣| 别揉我奶头~嗯~啊~动态视频| 国产真实伦视频高清在线观看 | 国产69精品久久久久777片| 婷婷六月久久综合丁香| 天堂网av新在线| 18禁在线播放成人免费| 麻豆成人av在线观看| 别揉我奶头~嗯~啊~动态视频| 亚洲成人精品中文字幕电影| av黄色大香蕉| 露出奶头的视频| www日本在线高清视频| 欧美乱妇无乱码| 国产野战对白在线观看| 国产av一区在线观看免费| 色综合站精品国产| 网址你懂的国产日韩在线| 成人性生交大片免费视频hd| 午夜福利欧美成人| 一级毛片女人18水好多| 两个人视频免费观看高清| 国产极品精品免费视频能看的| 色视频www国产| 精品国产美女av久久久久小说| 免费一级毛片在线播放高清视频| 亚洲七黄色美女视频| 国产极品精品免费视频能看的| 亚洲一区高清亚洲精品| 亚洲,欧美精品.| 亚洲无线观看免费| 桃红色精品国产亚洲av| 国产av在哪里看| 亚洲天堂国产精品一区在线| 听说在线观看完整版免费高清| 99久久九九国产精品国产免费| 99精品久久久久人妻精品| 亚洲精品日韩av片在线观看 | 中文字幕人成人乱码亚洲影| 99热这里只有是精品50| 一个人免费在线观看电影| 在线a可以看的网站| 免费在线观看成人毛片| 久久这里只有精品中国| 一级毛片高清免费大全| 日本 欧美在线| 久久久久久九九精品二区国产| 欧美极品一区二区三区四区| 国产成人福利小说| 婷婷亚洲欧美| 国产美女午夜福利| 久久精品国产亚洲av香蕉五月| 少妇的逼水好多| 一个人观看的视频www高清免费观看| 欧美一区二区精品小视频在线| 亚洲男人的天堂狠狠| 亚洲国产欧美人成| 国产精品98久久久久久宅男小说| 非洲黑人性xxxx精品又粗又长| 国产免费av片在线观看野外av| 9191精品国产免费久久| 嫁个100分男人电影在线观看| 最近在线观看免费完整版| 日韩 欧美 亚洲 中文字幕| 精品国内亚洲2022精品成人| 啪啪无遮挡十八禁网站| 精品人妻偷拍中文字幕| 亚洲国产中文字幕在线视频| 久久久国产精品麻豆| 三级男女做爰猛烈吃奶摸视频| 97超级碰碰碰精品色视频在线观看| 久久中文看片网| 国产亚洲精品一区二区www| 一边摸一边抽搐一进一小说| 亚洲精品在线观看二区| 最近最新中文字幕大全电影3| 亚洲av一区综合| 两个人的视频大全免费| 亚洲国产中文字幕在线视频| 久久久久久久午夜电影| 欧美日韩综合久久久久久 | 嫩草影院精品99| 久久精品91蜜桃| 老司机午夜十八禁免费视频| 国产真人三级小视频在线观看| 看片在线看免费视频| 18美女黄网站色大片免费观看| 久久欧美精品欧美久久欧美| 国产又黄又爽又无遮挡在线| 超碰av人人做人人爽久久 | 精品久久久久久久末码| 国产高清视频在线播放一区| 法律面前人人平等表现在哪些方面| 亚洲熟妇熟女久久| 免费av不卡在线播放| 亚洲一区高清亚洲精品| 99在线人妻在线中文字幕| 欧美最新免费一区二区三区 | 尤物成人国产欧美一区二区三区| 欧美性感艳星| 国产一级毛片七仙女欲春2| 欧美极品一区二区三区四区| 成人特级av手机在线观看| 日韩中文字幕欧美一区二区| 特大巨黑吊av在线直播| 90打野战视频偷拍视频| 日本黄色视频三级网站网址| 欧美日韩亚洲国产一区二区在线观看| 午夜福利免费观看在线| 亚洲成人免费电影在线观看| 午夜福利成人在线免费观看| 露出奶头的视频| 身体一侧抽搐| svipshipincom国产片| 亚洲av一区综合| ponron亚洲| 中文在线观看免费www的网站| 午夜亚洲福利在线播放| 国内少妇人妻偷人精品xxx网站| 制服人妻中文乱码| 成人午夜高清在线视频|