• <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
    午夜两性在线视频| 亚洲乱码一区二区免费版| 日韩欧美三级三区| svipshipincom国产片| 欧美一区二区精品小视频在线| 国产成+人综合+亚洲专区| 国产激情欧美一区二区| 精品久久久久久久久久久久久| bbb黄色大片| 精品无人区乱码1区二区| 99热这里只有精品一区 | 深夜精品福利| 亚洲九九香蕉| 十八禁人妻一区二区| 熟女人妻精品中文字幕| 久久伊人香网站| 一级毛片精品| 亚洲精品在线美女| 国产精品av视频在线免费观看| 国产欧美日韩精品亚洲av| 香蕉丝袜av| 禁无遮挡网站| 美女高潮的动态| 欧美日韩亚洲国产一区二区在线观看| 精品国产乱子伦一区二区三区| 亚洲国产日韩欧美精品在线观看 | 又紧又爽又黄一区二区| 亚洲成人久久性| 五月伊人婷婷丁香| 欧美性猛交黑人性爽| 少妇裸体淫交视频免费看高清| 久久亚洲真实| 成人鲁丝片一二三区免费| xxx96com| 老熟妇乱子伦视频在线观看| 亚洲成人精品中文字幕电影| 九九热线精品视视频播放| 国产精品久久视频播放| 可以在线观看的亚洲视频| 在线观看日韩欧美| 51午夜福利影视在线观看| 老熟妇乱子伦视频在线观看| 亚洲五月婷婷丁香| 美女高潮喷水抽搐中文字幕| 亚洲人成网站在线播放欧美日韩| 欧美一级a爱片免费观看看| 国产野战对白在线观看| 免费av不卡在线播放| 人妻丰满熟妇av一区二区三区| 国产真实乱freesex| 视频区欧美日本亚洲| 亚洲精品一区av在线观看| 夜夜看夜夜爽夜夜摸| 中文亚洲av片在线观看爽| 麻豆成人av在线观看| 色综合欧美亚洲国产小说| 国产99白浆流出| 九色国产91popny在线| 国产成人aa在线观看| 精品乱码久久久久久99久播| 可以在线观看毛片的网站| 午夜免费激情av| 国产一区二区激情短视频| 精品不卡国产一区二区三区| 午夜福利成人在线免费观看| 免费电影在线观看免费观看| 老熟妇仑乱视频hdxx| 特级一级黄色大片| e午夜精品久久久久久久| 美女 人体艺术 gogo| 精品熟女少妇八av免费久了| 真人一进一出gif抽搐免费| 麻豆国产av国片精品| 亚洲成人久久爱视频| 老司机午夜福利在线观看视频| 国产高清三级在线| 亚洲午夜精品一区,二区,三区| 两个人的视频大全免费| www.999成人在线观看| 69av精品久久久久久| 中文字幕精品亚洲无线码一区| 黑人欧美特级aaaaaa片| 欧美日韩精品网址| 在线观看66精品国产| 国产亚洲精品一区二区www| 国产成人影院久久av| 99热只有精品国产| 精品国产超薄肉色丝袜足j| 国产午夜精品论理片| 精品久久久久久久久久免费视频| 国产在线精品亚洲第一网站| 中出人妻视频一区二区| 69av精品久久久久久| 国产成人精品无人区| 欧美日本亚洲视频在线播放| 91在线观看av| 亚洲色图 男人天堂 中文字幕| 美女免费视频网站| 亚洲第一电影网av| 长腿黑丝高跟| 中文字幕久久专区| 欧美成人一区二区免费高清观看 | 欧美成人免费av一区二区三区| 日本与韩国留学比较| 久久精品国产99精品国产亚洲性色| 国产精品影院久久| 久久久久性生活片| 两个人视频免费观看高清| www.999成人在线观看| 午夜福利在线观看免费完整高清在 | 亚洲精品粉嫩美女一区| 欧美性猛交黑人性爽| 90打野战视频偷拍视频| 日本免费a在线| 国产一区二区激情短视频| 国产免费av片在线观看野外av| 国产毛片a区久久久久| 99久久综合精品五月天人人| 亚洲人与动物交配视频| 岛国在线免费视频观看| 欧美午夜高清在线| av天堂中文字幕网| 丁香六月欧美| 性欧美人与动物交配| 黄色视频,在线免费观看| 在线观看一区二区三区| 小说图片视频综合网站| 国产高清三级在线| 久久天躁狠狠躁夜夜2o2o| 亚洲精品粉嫩美女一区| 国产真人三级小视频在线观看| av天堂在线播放| 亚洲美女黄片视频| 国产不卡一卡二| av欧美777| 91麻豆精品激情在线观看国产| 在线免费观看的www视频| 男女床上黄色一级片免费看| 俺也久久电影网| 欧美中文综合在线视频| 久久精品国产综合久久久| 两个人的视频大全免费| 欧美日韩中文字幕国产精品一区二区三区| 亚洲人成电影免费在线| 人人妻人人看人人澡| 黄色 视频免费看| 香蕉av资源在线| 一夜夜www| 亚洲人成伊人成综合网2020| 亚洲国产欧美网| 三级国产精品欧美在线观看 | 嫩草影院入口| 精品久久蜜臀av无| 亚洲一区二区三区不卡视频| 亚洲专区字幕在线| 欧美日本视频| 制服人妻中文乱码| 两性夫妻黄色片| 午夜福利在线观看吧| 两个人的视频大全免费| 国产成人精品久久二区二区免费| 国产真实乱freesex| 亚洲欧美日韩卡通动漫| 在线观看一区二区三区| 午夜视频精品福利| 99热精品在线国产| 99精品久久久久人妻精品| 91麻豆精品激情在线观看国产| 亚洲最大成人中文| 色尼玛亚洲综合影院| 女警被强在线播放| 久久精品91无色码中文字幕| 国产亚洲欧美在线一区二区| 99久久精品热视频| 国产精品日韩av在线免费观看| 巨乳人妻的诱惑在线观看| 岛国在线免费视频观看| 国产精品98久久久久久宅男小说| 两人在一起打扑克的视频| 亚洲av电影在线进入| 88av欧美| 亚洲 国产 在线| 中文字幕久久专区| 国产 一区 欧美 日韩| 性欧美人与动物交配| 国产成人欧美在线观看| 18禁黄网站禁片午夜丰满| 亚洲男人的天堂狠狠| 国产av麻豆久久久久久久| 一本综合久久免费| 免费无遮挡裸体视频| 99国产精品一区二区三区| 在线a可以看的网站| 舔av片在线| 精品一区二区三区视频在线观看免费| 1000部很黄的大片| 婷婷六月久久综合丁香| 国产亚洲欧美在线一区二区| 欧美在线一区亚洲| www.www免费av| 免费看日本二区| 人妻丰满熟妇av一区二区三区| 欧美日本视频| 成年版毛片免费区| 国产精品1区2区在线观看.| 美女午夜性视频免费| 国产伦精品一区二区三区视频9 | 亚洲精品国产精品久久久不卡| 美女大奶头视频| 每晚都被弄得嗷嗷叫到高潮| 成人特级黄色片久久久久久久| 丰满人妻熟妇乱又伦精品不卡| 国产精品野战在线观看| 熟妇人妻久久中文字幕3abv| 脱女人内裤的视频| 免费电影在线观看免费观看| 此物有八面人人有两片| 变态另类成人亚洲欧美熟女| 俺也久久电影网| 男人舔女人下体高潮全视频| 亚洲国产日韩欧美精品在线观看 | 国产精品影院久久| 最好的美女福利视频网| 婷婷精品国产亚洲av在线| 午夜影院日韩av| 久久99热这里只有精品18| 欧美又色又爽又黄视频| 97人妻精品一区二区三区麻豆| 91av网一区二区| 国产三级在线视频| 亚洲美女视频黄频| 亚洲熟妇熟女久久| 久久精品aⅴ一区二区三区四区| 九九久久精品国产亚洲av麻豆 | 桃红色精品国产亚洲av| 麻豆成人午夜福利视频| 日本a在线网址| 午夜精品在线福利| 99在线视频只有这里精品首页| 亚洲人成电影免费在线| 国内精品久久久久久久电影| 精品午夜福利视频在线观看一区| 午夜激情福利司机影院| 首页视频小说图片口味搜索| 亚洲国产精品久久男人天堂| 久久热在线av| 99国产精品99久久久久| 国产精品98久久久久久宅男小说| 夜夜看夜夜爽夜夜摸| 日韩免费av在线播放| 欧美在线黄色| 麻豆国产97在线/欧美| 免费观看人在逋| 日韩高清综合在线| 熟女人妻精品中文字幕| 国产v大片淫在线免费观看| svipshipincom国产片| 国产人伦9x9x在线观看| 天堂网av新在线| 国产亚洲精品av在线| av天堂中文字幕网| 欧美av亚洲av综合av国产av| 欧美在线黄色| 国产精品免费一区二区三区在线| 免费大片18禁| av女优亚洲男人天堂 | 女同久久另类99精品国产91| 国产爱豆传媒在线观看| 久久欧美精品欧美久久欧美| 国产私拍福利视频在线观看| 国产一区二区三区在线臀色熟女| 日韩欧美免费精品| 麻豆成人午夜福利视频| 色尼玛亚洲综合影院| 日日摸夜夜添夜夜添小说| av天堂中文字幕网| 亚洲美女黄片视频| 久久人人精品亚洲av| 草草在线视频免费看| 国产成年人精品一区二区| 色综合站精品国产| 成人午夜高清在线视频| 狠狠狠狠99中文字幕| 麻豆成人午夜福利视频| 欧美成人免费av一区二区三区| 国产av一区在线观看免费| 丁香六月欧美| 日本成人三级电影网站| 亚洲成人久久性| 久久亚洲精品不卡| 91av网站免费观看| 国产欧美日韩一区二区精品| 久久伊人香网站| 亚洲va日本ⅴa欧美va伊人久久| 亚洲成人久久性| 国产精品免费一区二区三区在线| 日韩av在线大香蕉| 99热6这里只有精品| 中文字幕最新亚洲高清| 国产伦一二天堂av在线观看| 99久久久亚洲精品蜜臀av| 一区福利在线观看| 九色成人免费人妻av| 波多野结衣巨乳人妻| 中出人妻视频一区二区| 国产成人av激情在线播放| 特级一级黄色大片| 日本与韩国留学比较| 校园春色视频在线观看| svipshipincom国产片| 两性午夜刺激爽爽歪歪视频在线观看| 999久久久国产精品视频| 男女下面进入的视频免费午夜| 观看美女的网站| 亚洲人与动物交配视频| 九色国产91popny在线| 国产亚洲精品综合一区在线观看| av在线蜜桃| 亚洲精品粉嫩美女一区| 午夜亚洲福利在线播放| 男人的好看免费观看在线视频| 熟女少妇亚洲综合色aaa.| 99热这里只有精品一区 | 两个人视频免费观看高清| 麻豆国产av国片精品| 国产aⅴ精品一区二区三区波| 激情在线观看视频在线高清| 日本免费一区二区三区高清不卡| 91麻豆精品激情在线观看国产| 夜夜爽天天搞| 操出白浆在线播放| 国产精品永久免费网站| 高潮久久久久久久久久久不卡| 网址你懂的国产日韩在线| 人人妻人人澡欧美一区二区| 精品国产美女av久久久久小说| 日本一二三区视频观看| 观看免费一级毛片| 日本成人三级电影网站| 美女午夜性视频免费| 国产一区二区三区在线臀色熟女| 99热6这里只有精品| 国产成人精品久久二区二区免费| 一区二区三区高清视频在线| 亚洲精品粉嫩美女一区| 欧美zozozo另类| 狂野欧美激情性xxxx| 白带黄色成豆腐渣| 日韩欧美精品v在线| 中文字幕熟女人妻在线| 精品日产1卡2卡| 免费观看精品视频网站| 天天一区二区日本电影三级| 亚洲 欧美 日韩 在线 免费| 男女做爰动态图高潮gif福利片| 丝袜人妻中文字幕| 怎么达到女性高潮| 国内少妇人妻偷人精品xxx网站 | 亚洲欧美激情综合另类| 少妇的逼水好多| 亚洲乱码一区二区免费版| 久久中文看片网| 久久久久九九精品影院| 久久久久久久久中文| 精品国产亚洲在线| 一进一出好大好爽视频| 给我免费播放毛片高清在线观看| 淫妇啪啪啪对白视频| 黄色日韩在线| 天堂动漫精品| 国产亚洲精品久久久com| 久久国产精品人妻蜜桃| 在线观看免费视频日本深夜| 男女下面进入的视频免费午夜| 成年人黄色毛片网站| 国产1区2区3区精品| avwww免费| 毛片女人毛片| 美女被艹到高潮喷水动态| 国产精品香港三级国产av潘金莲| 俺也久久电影网| 男女那种视频在线观看| 欧美日本视频| 日韩免费av在线播放| 叶爱在线成人免费视频播放| e午夜精品久久久久久久| 婷婷亚洲欧美| 国产精品久久久人人做人人爽| 精品欧美国产一区二区三| 制服人妻中文乱码| 日韩精品青青久久久久久| 亚洲中文av在线| 非洲黑人性xxxx精品又粗又长| 成人永久免费在线观看视频| 国产真人三级小视频在线观看| 少妇裸体淫交视频免费看高清| 最近最新中文字幕大全电影3| 国产精品一区二区精品视频观看| 色在线成人网| 亚洲成av人片免费观看| 日本一本二区三区精品| 91字幕亚洲| 亚洲无线在线观看| 国产成人av教育| 色老头精品视频在线观看| 好男人在线观看高清免费视频| 亚洲欧美精品综合一区二区三区| 午夜两性在线视频| 国产v大片淫在线免费观看| 国产亚洲精品av在线| 人人妻,人人澡人人爽秒播| 色综合站精品国产| 欧美一级毛片孕妇| av福利片在线观看| 99精品欧美一区二区三区四区| 99精品久久久久人妻精品| 波多野结衣巨乳人妻| 亚洲美女黄片视频| 久久久久九九精品影院| 成人欧美大片| www.自偷自拍.com| 亚洲黑人精品在线| 一级毛片女人18水好多| 国产不卡一卡二| 精品久久久久久久末码| 国产高清videossex| 好男人电影高清在线观看| 男女之事视频高清在线观看| 久久人妻av系列| 免费在线观看影片大全网站| 琪琪午夜伦伦电影理论片6080| 亚洲精品久久国产高清桃花| 国产综合懂色| 国产伦一二天堂av在线观看| 日韩欧美精品v在线| 久久久久九九精品影院| 成人欧美大片| 亚洲自拍偷在线| 久久性视频一级片| 人人妻人人看人人澡| 男女下面进入的视频免费午夜| 久久久久九九精品影院| 91在线观看av| 丰满的人妻完整版| 在线观看免费午夜福利视频| 天堂av国产一区二区熟女人妻| 日韩 欧美 亚洲 中文字幕| 国产亚洲精品久久久com| 精品一区二区三区av网在线观看| 午夜影院日韩av| cao死你这个sao货| 九色国产91popny在线| 午夜影院日韩av| 中亚洲国语对白在线视频| 99久久99久久久精品蜜桃| 村上凉子中文字幕在线| 欧美日韩综合久久久久久 | 老汉色av国产亚洲站长工具| 青草久久国产| 日本熟妇午夜| 久久久成人免费电影| 色尼玛亚洲综合影院| 天堂√8在线中文| 国产成人av教育| 超碰成人久久| 久久精品影院6| 首页视频小说图片口味搜索| 日韩中文字幕欧美一区二区| 在线观看美女被高潮喷水网站 | 九九在线视频观看精品| 亚洲成人精品中文字幕电影| 桃红色精品国产亚洲av| 在线观看舔阴道视频| 色吧在线观看| www国产在线视频色| 一级黄色大片毛片| 天天添夜夜摸| 日日夜夜操网爽| 亚洲成人中文字幕在线播放| 麻豆av在线久日| 亚洲一区二区三区不卡视频| 婷婷六月久久综合丁香| 俺也久久电影网| 国产黄片美女视频| 岛国视频午夜一区免费看| 一本一本综合久久| 久久久久免费精品人妻一区二区| 在线视频色国产色| 毛片女人毛片| 天天一区二区日本电影三级| 色综合站精品国产| 操出白浆在线播放| 亚洲欧美精品综合一区二区三区| 久久中文字幕人妻熟女| 亚洲精品乱码久久久v下载方式 | 久99久视频精品免费| 三级男女做爰猛烈吃奶摸视频| 国产av不卡久久| 欧美另类亚洲清纯唯美| 91av网一区二区| 欧美黄色淫秽网站| 神马国产精品三级电影在线观看| 免费观看的影片在线观看| 在线观看舔阴道视频| 亚洲国产欧美人成| 精品久久久久久久久久免费视频| 精品国产美女av久久久久小说| 亚洲avbb在线观看| 2021天堂中文幕一二区在线观| 女警被强在线播放| ponron亚洲| 国产真实乱freesex| 三级国产精品欧美在线观看 | 国产成+人综合+亚洲专区| 噜噜噜噜噜久久久久久91| 国产aⅴ精品一区二区三区波| 91麻豆精品激情在线观看国产| 国产精品久久电影中文字幕| 精华霜和精华液先用哪个| e午夜精品久久久久久久| 国产男靠女视频免费网站| 老司机午夜十八禁免费视频| 国产男靠女视频免费网站| aaaaa片日本免费| 我的老师免费观看完整版| 亚洲国产色片| 日本黄色视频三级网站网址| 性色avwww在线观看| 国产精品1区2区在线观看.| 老鸭窝网址在线观看| av天堂中文字幕网| 久久久久国内视频| 日本成人三级电影网站| 99riav亚洲国产免费| 中文亚洲av片在线观看爽| 在线观看午夜福利视频| 亚洲中文字幕日韩| 久久久色成人| 国产极品精品免费视频能看的| 给我免费播放毛片高清在线观看| 精品国产超薄肉色丝袜足j| 中亚洲国语对白在线视频| 免费无遮挡裸体视频| 午夜福利在线观看吧| 精品不卡国产一区二区三区| 国产成人影院久久av| 亚洲国产高清在线一区二区三| tocl精华| 欧美一区二区精品小视频在线| 国产淫片久久久久久久久 | 免费观看人在逋| 无限看片的www在线观看| 在线观看美女被高潮喷水网站 | 99精品欧美一区二区三区四区| 国产高清三级在线| 香蕉av资源在线| 最近最新中文字幕大全电影3| 久久久成人免费电影| 久久午夜综合久久蜜桃| 欧美午夜高清在线| 伦理电影免费视频| 午夜免费观看网址| 最新美女视频免费是黄的| 亚洲国产欧洲综合997久久,| 午夜精品一区二区三区免费看| 亚洲熟妇熟女久久| 免费一级毛片在线播放高清视频| 床上黄色一级片| 日日夜夜操网爽| 视频区欧美日本亚洲| 亚洲av成人一区二区三| 亚洲九九香蕉| 国产成+人综合+亚洲专区| 免费大片18禁| 国内精品一区二区在线观看| 免费人成视频x8x8入口观看| av天堂中文字幕网| 毛片女人毛片| 黄色 视频免费看| 久久久国产成人免费| 在线观看免费视频日本深夜| 这个男人来自地球电影免费观看| 精品国产亚洲在线| 欧美日韩一级在线毛片| 国产精品久久久久久亚洲av鲁大| 欧美色欧美亚洲另类二区| 国产三级在线视频| 色av中文字幕| 久久精品国产亚洲av香蕉五月| 两个人看的免费小视频| 中文字幕人妻丝袜一区二区| 日日摸夜夜添夜夜添小说| 听说在线观看完整版免费高清| 国产伦在线观看视频一区| 天天添夜夜摸| 国产精品国产高清国产av| 在线观看午夜福利视频| 国产极品精品免费视频能看的| 日韩有码中文字幕| 亚洲真实伦在线观看| 无限看片的www在线观看| or卡值多少钱| 超碰成人久久| 久久天躁狠狠躁夜夜2o2o| 不卡一级毛片| 最近最新免费中文字幕在线| 亚洲国产高清在线一区二区三| 久久久国产成人免费| 最好的美女福利视频网| 精品99又大又爽又粗少妇毛片 | 日本免费a在线| 精品日产1卡2卡| 一级a爱片免费观看的视频| www.精华液| 亚洲自拍偷在线|