• <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 ?

    Flexible, high sensitive and radiation-resistant pressure-sensing hydrogel

    2022-06-18 03:00:54ZhiwenJingYusongWngGuoqingXuZhuoniJingZhiqingGeMozhenWngXuewuGe
    Chinese Chemical Letters 2022年2期

    Zhiwen Jing, Yusong Wng, Guoqing Xu, Zhuoni Jing, Zhiqing Ge,Mozhen Wng,*, Xuewu Ge,*

    a CAS Key Laboratory of Soft Matter Chemistry, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, China

    b Hefei National Laboratory for Physical Sciences at the Microscale, University of Science and Technology of China, Hefei 230026, China

    ABSTRACT Excellent radiation resistance is a prerequisite for pressure-sensitive hydrogels to be used in high-energy radiation environments.In this work, tannic acid-modified boron nitride nanosheet (BNNS-TA) is first prepared as the radiation-resistant additive by a facile one-step ball milling of hexagonal boron nitride and tannic acid.Then, polyacrylamide (PAAm)-based pressure-sensitive hydrogel doped with BNNS-TA and Fe3+ ions is fabricated.The ternary BNNS-TA/Fe3+/PAAm hydrogel exhibits excellent compressive strength(at least four times the compressive strength of unfilled pure PAAm hydrogel), pressure-sensitive performance (gauge factor is up to 1.4), and performance recovery due to the combination of multiple intermolecular interactions, such as covalent crosslinking, hydrogen bonds, and ion coordination interactions.The BNNS-TA/Fe3+/PAAm hydrogel can be made as a pressure sensor installed in the control circuit or attached on the human body to detect human activities accurately.More importantly, the compressive strength and the pressure-sensitive performance of the BNNS-TA/Fe3+/PAAm hydrogel can be maintained after the hydrogel is irradiated by 60Co gamma-ray at an absorbed dose of 15 kGy.As a comparison, the compressive strength of the unfilled PAAm hydrogel is only a quarter of that before irradiation.This work not only reveals a facile method to achieve the preparation of chemically modified BNNS as a promising radiation-resistant additive but also provides a novel strategy for the development of pressure-sensitive hydrogel devices in radiation environments.

    Keywords:Pressure-sensitive hydrogel Polyacrylamide Radiation resistance Boron nitride nanosheet Tannic acid Motion detection

    The conductive hydrogels composed of polymer hydrogels doped with various conductive materials not only have excellent flexibility and high deformability but also have the ability to convert external pressure stimuli into electrical signals sensitively [1-4].They are considered to be the most promising material for flexible wearable pressure or strain sensors, which can be widely applied in the behavior monitoring of soft robots and human body motion detection [5-8].With the continuous development of the civil nuclear industry and aerospace technology, pressure-sensitive hydrogels applied in the field of robot operations in the nuclear industry and the astronauts’motion perception in the extravehicular activities are exposed to the high-energy radiation field additionally [9-11].On the one hand, the polymer components are prone to undergo cross-linking or chain-scission degradation under highenergy radiation [12-16].On the other hand, HO·, H·and other reactive species generated by theγ-ray radiolysis of water will also induce various reactions of polymer chains [17-20].These radiation chemistry effects will make the hydrogel sensors fail quickly in a high-energy radiation environment [11,20].Therefore, excellent radiation resistance is a prerequisite for the pressure-sensitive hydrogel sensors to be used in the high-energy radiation field.

    Scheme 1.One-step preparation of BNNS-TA by ball milling with h-BN particles and tannic acid crystals.

    There are two ways to improve the radiation resistance of hydrogel materials.One is to design and synthesize new polymers with intrinsic radiation resistance.Generally, the polymer molecular chains containing benzene and other conjugated structure groups exhibit excellent radiation stability [21-23], such as polyether ether ketone (PEEK) [24], polystyrene (PS) [25], polyethylene terephthalate (PET) [26].However, these kinds of polymers are strongly hydrophobic, which is difficult to be applied to prepare hydrogels.The other way is to add specific fillers that can reduce or suppress the undesirable radiation chemistry effects into the matrix [27-29], such as graphene and hexagonal boron nitride (h-BN).Graphene is usually difficult to prepare.Since there are many active oxygen-containing functional groups on the surface of GO, reduction and hydroxyalkylation reactions induced by the attack of the alcohol free radicals produced by the radiolysis of alcohol/water solution underγ-ray radiation can take place simultaneously on the surface of GO nanosheets, which means GO nanosheets may have enough activity to react with polymer matrix underγ-ray radiation in our previous work [30,31].By contrast,h-BN has been found to have excellent radiation stability at similar radiation conditions due to its excellent chemical, thermal stability [32-35], and good neutron absorption performance [36-40].?zdemiret al.[37] discovered that h-BN filled silicone composite rubber is a suitable material for neutron shielding purposes, and the attenuation rate (I/I0) of 60.7% could be achieved for a 6.9 mm thick composite rubber containing 30 wt% of h-BN.In our previous work [39], it is also found that when h-BN is added into the epoxy resin at a mass fraction of more than 0.52%, the composite shows excellentγ-ray radiation resistance.However, h-BN has a poor dispersibility in polymer matrix or water, and the chemical modification of h-BN is also very difficult due to its inherent chemical inertness.These limit the application of h-BN in the preparation of composite materials.

    In this work, a tannic acid-assisted ball milling method was first put forward to prepare tannic acid-modified BNNS(BNNS-TA) in one step.The prepared BNNS-TA can be well dispersed in the aqueous solution of acrylamide (AAm) andN,N’-methylenebis(acrylamide) (MBA) to fabricate a binary BNNSTA/PAAm hydrogel.After soaking the binary hydrogel in Fe3+ions aqueous solution, a novel ternary BNNS-TA/Fe3+/PAAM composite hydrogel was prepared.

    Tannic acid molecules contain abundant hydroxyl groups, which have strong interactions with PAAm and water molecules.In order to improve the dispersibility of h-BN particles in PAAm hydrogel,we first ball milled the micron-sized h-BN particles and tannic acid crystals together in 300 rpm for 10 h to appropriately reduce the size of h-BN particles and modify them, and obtained tannic-acid modified BNNS (BNNS-TA) in one step, as illustrated in Scheme 1.

    The micron-sized raw h-BN particles have a laminated structure formed of stacked thick plates (Fig.S1a in Supporting information).However, BNNS-TA obtained by the ball milling method is in sheet form with a square size of about 200 nm to 300 nm, as shown in Fig.1a.The actual size of the prepared BNNS-TA can be measured by AFM (Fig.1b), and more than 91% of BNNS-TA has a thickness of less than 2 nm and a horizontal length of 100~200 nm (Figs.S1b and c in Supporting information).XRD spectra (Fig.1c) display that the crystal structure of BNNS-TA is identical to that of the raw h-BN.The peaks at 2θof 26.8°, 41.6°, 43.9°, 50.2°, and 55.1° respond to the (002), (100), (101), (102), and (004) planes of h-BN,respectively, according to JCPDS card No.34–0421, but all diffraction peaks of BNNS-TA become weaker and wider than those of the raw h-BN, indicating the ball milling of h-BN can reduce the particle size, but will not change the crystal structure.

    The FT-IR spectrum of BNNS-TA is displayed in Fig.S1d (Supporting information), the absorption peaks at 1726 cm-1and 1203~1038 cm-1assigned to the stretching vibrations of C=O and C–O respectively.Meantime, the UV absorption spectrum of BNNSTA providing further evidence of the combination between tannic acid molecules and BNNS (Fig.S1e in Supporting information).The thermogravimetric (TG) curves of the raw h-BN and BNNS-TA indicated that the mass loss of 5.42% occurring in the range of 210~700 °C should be the decomposition of tannic acid in BNNS-TA(Fig.S1f in Supporting information).2D solid-state11B MQ MAS NMR spectrum of BNNS-TA was performed in order to determine the interaction between tannic acid molecules and BNNS.It is diffi-cult to accurately distinguish some special chemical environments of11B by 1D MAS NMR spectrum due to the highly anisotropic interaction of the nuclear quadrupole moment with the gradient of the surrounding electric field [41,42].As shown in Figs.1d and e, the 1D solid-state11B MAS NMR spectra of the raw h-BN and BNNS-TA are almost indistinguishable.However, some overlapped signals of11B can be effectively discriminated in the projection along F1 dimension in 2D solid-state11B MQ MAS NMR spectra.Compared with raw h-BN, a new signal of boron atom arises at 20–25 ppm in the spectrum of BNNS-TA (Figs.1f and g), which means the chemical environment of some B atoms in BNNS-TA has been changed.According to the 2D NMR spectrum of B2O3and the previous report of edge-hydroxylated boron nitride, the new signal should be assigned to the boron acid ester bond (B-O-R) [43,44],indicating that tannic acid molecules have been covalently bonded on the surface of BNNS.Based on the reported theoretical calculations and experimental results [45,46], it is generally recognized that large thick h-BN flakes are fractured and peeled into small thin nanosheets with active B edges during the ball milling.The active B sites at the edges of BNNS tend to react with the -OH groups of tannic acid, resulting in the chemical immobilization of tannic acid on the surface of BNNS [47].Summarized the above analysis, tannic acid-assisted ball milling can successfully achieve the exfoliation preparation and functionalization of h-BN simultaneously in one step.

    After the preparation of BNNS-TA, the fabrication of ternary BNNS-TA/Fe3+/PAAm hydrogel is shown in Fig.2.BNNS-TA can be dispersed in water homogeneously and stably due to its nano-size and the hydrophilic tannic acid moieties on the surface.They won’t precipitate out of water even if left for 14 days, while h-BN particles and the ball-milled BNNS precipitate after standing and it is difficult for the preparation of homogeneous BNNS/Fe3+/PAAm hydrogel samples (Fig.S2 in Supporting information).After the polymerization and crosslinking reactions of AAm and MBA were initiated with ammonium persulfate at 60 °C for 4 h, binary BNNSTA/PAAm hydrogel was obtained.Then, the binary hydrogel was immersed in the Fe3+ions aqueous solution for 12 h so that Fe3+ions can diffuse into the hydrogel, resulting in the formation of the ternary BNNS-TA/Fe3+/PAAm hydrogel.

    The Raman spectra (Fig.S3a in Supporting information) show that three peaks appearing at 409 cm-1, 572 cm-1and 655 cm-1of BNNS-TA/Fe3+/PAAm hydrogel should be assigned to the absorption of tannic acid molecules, and two peaks at 572 cm-1and 655 cm-1should be originated from the vibration of Fe-O[48,49].This means Fe3+ions in BNNS-TA/Fe3+/PAAm hydrogel are coordinated with -OH groups of TA moieties on BNNS instead of Cl-ions because the Raman shift of [FeCl4]-should be located at 313 nm (Fig.S3b in Supporting information) [50].Meantime,the UV–vis spectrum of BNNS-TA/Fe3+/PAAm hydrogel in Fig.S3c(Supporting information) shows a broad absorption in the range of 300–500 nm, but PAAm hydrogel and TA molecules have no obvious absorption in this range, which attribute to the redshift of the peak of hydroxyl groups caused by coordination structure.The above analysis demonstrates that the molecular interactions in BNNS-TA/Fe3+/PAAm hydrogel include not only the hydrogen bonds formed between hydroxyl groups and water molecules but also the coordination interactions between these phenolic hydroxyl groups and Fe3+.

    Fig.1.(a) SEM and TEM (inset) images, and (b) AFM images of the BNNS-TA.(c) XRD diagrams, (d, e) solid-state 11B MAS NMR spectra and (f, g) 2D solid-state 11B MQ MAS NMR spectra of raw h-BN and BNNS-TA.

    Fig.2.Preparation and intermolecular interactions of the ternary BNNSTA/Fe3+/PAAm hydrogel.

    The compressive stress-strain curves of these hydrogels are shown in Fig.S4a (Supporting information), and the compressive strength and the corresponding strain of pure PAAm hydrogel are 68.1 kPa and 66.9%, respectively.The introduction of BNNS-TA can greatly enhance the compressive strength and flexibility, and the BNNS-TA/PAAm hydrogel will not be broken even when the strain reaches 83.4%, and the corresponding compressive stress is nearly twice that for pure PAAm hydrogel (117.9 kPa), implying a strong affinity between BNNS-TA and polymer chains.However, the compressive modulus of the hydrogel fell with the introduction of BNNS-TA, which indicates that the existence of BNNS-TA may disturb the crosslinking of the polymer chains in the hydrogel, resulting in the decrease of the crosslinking degree.For the ternary BNNS-TA/Fe3+/PAAm hydrogel, it can be seen that the compressive modulus of hydrogel is much higher than the binary hydrogel and the compressive stress increases alarmingly to 217.2 kPa, nearly twice of that for binary BNNS-TA/PAAm hydrogel at a strain of 82.1%.The addition of Fe3+ions enhances the interactions between polymer chains and BNNS-TA through the coordination interactions with hydroxyl groups on BNNS-TA, which is equivalent to increase the crosslinking degree of the polymer chains and resulting in the further improvement of the mechanical property of the composite hydrogel.Moreover, the excellent structure stability of BNNSTA/Fe3+/PAAm hydrogel is also demonstrated that the compressed hydrogel immediately returns to its original state once the stress is removed (Fig.S4b in Supporting information).The above result shows that Fe3+ions play a key role in enhancing the crosslinking network and strengthening the hydrogel matrix.

    Fig.3a shows that the compressive stress-strain behavior of BNNS-TA/Fe3+/PAAm hydrogel is basically unchanged under the cyclic compression conditions.The corresponding compressive stress retention rates at a certain compression strain (70%) (the ratio of the stress at thenthcompression to that at the 1st compression) are also exhibited in Fig.3b, and demonstrate their outstanding flexibility and recovery mechanical properties.To investigate the influence of the content of Fe3+ions on the mechanical properties, three kinds of BNNS-TA/Fe3+/PAAm hydrogels were prepared by treating the binary BNNS-TA/PAAm hydrogel in the aqueous solution of Fe3+ions with different concentrations (0.2 mol/L,0.5 mol/L, 1.0 mol/L).Their compressive stress-strain curves over five compression cycles are exhibited in Fig.3c.Both compressive modulus and strength of the hydrogels are enhanced with the increase of Fe3+concentration, which is consistent with our previous assumption that Fe3+plays a critical part in enhancing the crosslinking network and hydrogel strength.

    To evaluate the pressure-sensitive performance of BNNSTA/Fe3+/PAAm hydrogels, two pieces of copper foils were attached on the top and the bottom of the hydrogels respectively to assemble a pressure sensor.The real-time resistance curves of these hydrogels prepared under different concentrations of Fe3+ions during the compression process are shown in Fig.3d.The results show that the resistance of hydrogels all changes periodically with the compressive strain.The conductivity of hydrogel obviously increases with the concentration of Fe3+, so the initial resistance of the hydrogels decreases and the range of resistance changes also vary accordingly.

    The gauge factors (GF) of BNNS-TA/Fe3+/PAAm hydrogels were calculated to investigate the sensitivity of these pressure-sensitive hydrogels, and their dependence on the strain was plotted in Fig.3e.Usually, soft or highly compressible conductive materials like hydrogels have a GF lower than 0.8 when they are used as pressure-sensitive elements [51,52].However, the GF of the prepared BNNS-TA/Fe3+/PAAm hydrogels is greater than 0.8 when the strain is larger than 8.8%.When the strain exceeds 20%, the GF reaches a maximum of 1.4 at a strain of 30.6%, then slows down slightly to 1.1.For comparison, the compressive stress-strain and resistance curves of BNNS-TA/Li+/PAAm hydrogel prepared in the same method are shown in Figs.S4c and d (Supporting information).Although BNNS-TA/Li+/PAAm hydrogel also shows a good compression property, the GF value of the ternary hydrogels containing Fe3+is much higher than that of hydrogels containing Li+, because Li+ions generally have no strong interactions with the phenolic hydroxyl groups, leading to worse structure stability of BNNS-TA/Li+/PAAm hydrogel.Interestingly, the GF of BNNS-TA/Fe3+/PAAm hydrogels is closely related to the compressive strain.In a low strain range (<30%), GF increases sharply with the strain, however, when the strain is large,e.g.,>30%, GF begins to decrease with the strain instead, which can be attributed to the existence of BNNS-TA.Under low strain conditions, BNNSTA is dispersed scatteredly, Fe3+ions can transport freely in the hydrogel.With the increase of strain, the distance between BNNSTA nanosheets is gradually reduced, which hinders the directional motion of Fe3+ions and resistance variation, as well as the GF.

    Fig.3.(a) The compression-relaxation curves at the 1st, 25th and 50th cycle and (b) the compressive stress retention rate curve during 50 cycles at 70% strain of the pure PAAm, BNNS-TA/PAAm and BNNS-TA/Fe3+/PAAm hydrogels.(c) The compression-relaxation curves and (d) the real-time resistance curves of a series of BNNS-TA/Fe3+/PAAm hydrogel.(e) GF versus strain for BNNS-TA/Fe3+/PAAm and BNNS-TA/Li+/PAAm.(f) On and off of the bulb can be controlled by the pressure on the hydrogel.(g) Wearable BNNS-TA/Fe3+/PAAm hydrogel sensors applied for human motion detection.

    BNNS-TA/Fe3+/PAAm hydrogel prepared at a condition of 1.0 mol/L Fe3+ions solution was selected as a representative to investigate the practice performance of BNNS-TA/Fe3+/PAAm hydrogel sensor.First, BNNS-TA/Fe3+/PAAm hydrogel sensor and a small bulb were connected in series in a closed circuit (Fig.3f).As the deformation magnitude of the hydrogel increased under pressure, the small bulb gradually lit up.As soon as the pressure was removed, the bulb went off.This pressure-induced turn-onoff process of the small bulb could be repeated at will (Video S1 in Supporting information), suggesting that BNNS-TA/Fe3+/PAAm hydrogel can be potentially applied in pressure-sensitive switching elements, pressure-sensing alarm, or 3D bulky pressure sensors.Moreover, the BNNS-TA/Fe3+/PAAm hydrogel was assembled as wearable pressure sensors and attached to five different parts of the human body to detect diverse human activities (Fig.3g).The sensor can reflect a strong signal to various movements of the human body, including blowing, feeding, and walking, and no obvious shifting of the baseline was observed due to good network structure stability and elasticity of the ternary hydrogel.Furthermore, during the above behavior detection procedure, the prepared sensor as a whole keeps the structure unchanged, and the hydrogel does not fall off or disintegrate, indicating that the BNNS-TA/Fe3+/PAAm hydrogel-based sensor has excellent structural stability.All of these results demonstrate that the BNNSTA/Fe3+/PAAm hydrogel-based sensor developed in this study has great potential for use in smart devices for the accurate sensing of human motion or robot, and even small deformations can be detected.

    The BNNS-TA/Fe3+/PAAm hydrogels were exposed to the60Coγ-ray radiation field in the air atmosphere at normal temperature and pressure.When the absorbed dose reached 15 kGy (which is equal to the total ionizing dose in 400 km height of an orbits satellite for 20 years [53]), the compressive strength of PAAm hydrogel is 23.0 kPa, only a quarter of original strength (Fig.4a).While the compressive strength of BNNS-TA/Fe3+/PAAm hydrogel is 84.2 kPa(53.6% of the original strength), basically the same as that of the un-irradiated PAAm hydrogel.It can be seen that the elastic modulus of both the pure PAAm hydrogels and the BNNS-TA/Fe3+/PAAm hydrogel increased (Fig.4b), which indicates the radiation crosslinking of the PAAm molecular chains is the major radiation chemistry effect [12,28,39].The compressive stress retention rates show that the compressive stress of BNNS-TA/Fe3+/PAAm hydrogel is basically unchanged after radiation (Fig.4c), indicating their radiation resistance performance in the mechanical property.

    The pressure-sensitive performance of BNNS-TA/Fe3+/PAAm hydrogel afterγ-ray radiation shows that the resistance of irradiated ternary hydrogel still changes periodically over 50 compression cycles (Fig.4d).The resistance of the hydrogel increases with the cycles of compression, which is probably caused by the orientation of BNNS-TA during cyclic compression progress.Meantime, the GF of the sensor after irradiation is slightly declined to 0.9 (Fig.4e),and the pressure sensor based on BNNS-TA/Fe3+/PAAm hydrogel can still sensitively control the light-on of the bulb in the closedcircuit by repeated compression (Fig.4f, Video S2 in Supporting information).It proves that BNNS-TA/Fe3+/PAAm hydrogel has excellent radiation resistance in specific high-energy radiation environments.

    The radiation resistance is mainly attributed to the following two points: On the one hand, boron nitride has intrinsic structure stability in a radiation environment [36,39].On the other hand,tannic acid moieties are natural free radical scavengers that can partially remove free radicals, such as HO·and H·, generated by theγ-ray radiolysis of water, which can be verified by Fenton-like reaction of Cu2+[54].The real-time fluorescence emission spectra(λex= 315 nm) of the aqueous solution of terephthalic acid (TPA),Cu2+, and H2O2containing BNNS-TA and the raw h-BN at the same content of 5 mg/L were measured (Figs.4g-i), respectively.TPA can react with HO·radicals to form a hydroxylated product (TPA-OH)which has a strong fluorescence emission at 448 nm.The solution containing BNNS-TA has almost no fluorescence emission signal,but the solution containing h-BN shows strong fluorescence, indicating the existence of TPA-OH.It means BNNS-TA can capture the HO·radicals, but h-BN cannot.The results reveal that BNNS-TA can tremendously reduce the concentration of HO·radicals in the system, thereby reduces the possibility of the attack of free radicals to PAAm molecular chains, so as to slow down the damage to the structure and performance of the hydrogels.

    Fig.4.(a) The compressive stress-strain curves of PAAm hydrogel and (b) BNNS-TA/Fe3+/PAAm hydrogel after 60Co γ-ray irradiation.(c) The compressive stress retention rate curve during 50 cycles of BNNS-TA/PAAm and BNNS-TA/Fe3+/PAAm hydrogel after irradiation.(d) The real-time resistance curves over 50 cycles of irradiated BNNSTA/Fe3+/PAAm hydrogel.(e) GF versus strain for BNNS-TA/Fe3+/PAAm hydrogel before and after irradiation.(f) On and off of the bulb can be controlled by the pressure on the hydrogel.Real-time fluorescence emission spectra (λex = 315 nm) of the aqueous solution of Cu2+and H2O2 containing (g) BNNS-TA and (h) h-BN.(i) The relationship between fluorescence intensity and time derivate from (g) and (h).

    In summary, it is found that tannic acid-assisted ball milling of micron-sized raw h-BN particles can successfully achieve the exfoliation and functionalization of h-BN simultaneously, and obtain nano-sized BNNS-TA in one step.Due to multiple interactions including the covalent bond, coordination interaction, and hydrogen bond in the cross-linking network of the hydrogel, the BNNSTA/Fe3+/PAAm hydrogel exhibits excellent compressive strength (at least four times the compressive strength of unfilled pure PAAm hydrogel), performance recovery (mechanical properties and pressure sensitivity remain after 50 times compression), high sensitivity (gauge factor (GF) is up to 1.4), and can be made as a pressure sensor installed in the control circuit or attached on the human body to detect human activities accurately.More encouragingly, with the help of BNNS-TA as a radical scavenger, the compressive strength and the pressure-sensitive performance of BNNSTA/Fe3+/PAAm hydrogel can be maintained after the hydrogel is irradiated by60Co gamma-ray radiation at an absorbed dose of 15 kGy.The results demonstrate that BNNS-TA/Fe3+/PAAm hydrogel has the potential to be applied to prepare the radiation-resistant pressure-sensitive sensor for the safety monitoring and accident handling of nuclear industry production and the extravehicular activities of astronauts.This work provides a novel strategy for the development of pressure-sensitive hydrogel devices in radiation environments.

    Declaration of competing interest

    The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

    Acknowledgement

    This work was supported by the National Natural Science Foundation of China (Nos.51773189 and 51973205), the Joint Laboratory for University of Science and Technology of China and Yanchang Petroleum (No.ES2060200084), and the Fundamental Research Funds for the Central Universities (Nos.WK3450000005,WK3450000006).

    Supplementary materials

    Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2021.06.043.

    啦啦啦视频在线资源免费观看| 好男人视频免费观看在线| 国产精品一二三区在线看| 美女中出高潮动态图| 黄色日韩在线| 欧美精品一区二区免费开放| 色婷婷av一区二区三区视频| 国产一区亚洲一区在线观看| 欧美bdsm另类| 日韩,欧美,国产一区二区三区| 国产又色又爽无遮挡免| 中国美白少妇内射xxxbb| 三上悠亚av全集在线观看 | av在线app专区| 在线观看一区二区三区激情| 午夜影院在线不卡| 午夜视频国产福利| www.av在线官网国产| 亚洲精品aⅴ在线观看| 自拍偷自拍亚洲精品老妇| 国产永久视频网站| 国产色爽女视频免费观看| 国产欧美日韩精品一区二区| 伊人久久精品亚洲午夜| 成年美女黄网站色视频大全免费 | 国产黄频视频在线观看| 深夜a级毛片| 美女主播在线视频| 欧美日韩精品成人综合77777| 美女国产视频在线观看| 久久久久视频综合| 爱豆传媒免费全集在线观看| 亚洲欧洲日产国产| 久久人人爽av亚洲精品天堂| av在线播放精品| 人妻人人澡人人爽人人| 亚洲精品国产av成人精品| 久久久久精品性色| 一级黄片播放器| av黄色大香蕉| 97超碰精品成人国产| 免费观看在线日韩| 五月伊人婷婷丁香| 国产视频首页在线观看| 一本一本综合久久| 中文字幕制服av| 性色av一级| 久久精品夜色国产| 久久人人爽人人片av| 国产伦精品一区二区三区四那| 永久免费av网站大全| 亚洲国产欧美在线一区| 亚洲精品第二区| 国产日韩欧美在线精品| 精品人妻偷拍中文字幕| 欧美性感艳星| 中文字幕亚洲精品专区| 久久国产精品大桥未久av | 97在线人人人人妻| 久久久久久久久久人人人人人人| 国产精品秋霞免费鲁丝片| av卡一久久| 久久久久久人妻| 日韩熟女老妇一区二区性免费视频| 国产欧美日韩综合在线一区二区 | kizo精华| 亚洲精品久久久久久婷婷小说| 丰满乱子伦码专区| 久热久热在线精品观看| 久久鲁丝午夜福利片| 一区在线观看完整版| 亚洲色图综合在线观看| 国产深夜福利视频在线观看| 成人毛片60女人毛片免费| 777米奇影视久久| 色婷婷久久久亚洲欧美| 蜜桃在线观看..| 日日啪夜夜撸| 国产免费视频播放在线视频| 国产精品麻豆人妻色哟哟久久| av.在线天堂| 亚洲精品乱久久久久久| 久久久久国产精品人妻一区二区| 午夜福利,免费看| 亚洲精品,欧美精品| 成人漫画全彩无遮挡| 亚洲精品中文字幕在线视频 | 成人特级av手机在线观看| 久久精品夜色国产| 精品酒店卫生间| 日韩精品有码人妻一区| 日韩亚洲欧美综合| 中文资源天堂在线| 国产伦精品一区二区三区视频9| 精品少妇内射三级| 在线观看免费视频网站a站| 美女大奶头黄色视频| 免费av中文字幕在线| 免费黄色在线免费观看| 欧美人与善性xxx| 哪个播放器可以免费观看大片| 免费观看性生交大片5| 狠狠精品人妻久久久久久综合| 久久久国产精品麻豆| 亚洲av男天堂| 韩国av在线不卡| 国产伦理片在线播放av一区| 日本午夜av视频| 日日爽夜夜爽网站| 国产淫语在线视频| av播播在线观看一区| 校园人妻丝袜中文字幕| 亚洲国产毛片av蜜桃av| 亚洲精品日韩在线中文字幕| av卡一久久| 一个人免费看片子| 精品人妻偷拍中文字幕| 国产综合精华液| 午夜精品国产一区二区电影| 亚洲成人av在线免费| 精品午夜福利在线看| 水蜜桃什么品种好| 99re6热这里在线精品视频| 少妇丰满av| 成人无遮挡网站| 在线观看一区二区三区激情| 日韩一本色道免费dvd| 免费大片18禁| 国产高清三级在线| 在线观看av片永久免费下载| 久久影院123| 美女视频免费永久观看网站| 99精国产麻豆久久婷婷| 午夜激情久久久久久久| 纯流量卡能插随身wifi吗| 欧美最新免费一区二区三区| 五月玫瑰六月丁香| 久久精品夜色国产| 国产一区二区三区综合在线观看 | 纯流量卡能插随身wifi吗| 久久亚洲国产成人精品v| 纵有疾风起免费观看全集完整版| 久久精品久久精品一区二区三区| 精品一区二区三卡| 久久国产乱子免费精品| 一本一本综合久久| 中国美白少妇内射xxxbb| 国产极品粉嫩免费观看在线 | 建设人人有责人人尽责人人享有的| 黄色配什么色好看| 日本猛色少妇xxxxx猛交久久| 国产乱来视频区| 女性生殖器流出的白浆| 丰满乱子伦码专区| 中文在线观看免费www的网站| 纵有疾风起免费观看全集完整版| 天堂8中文在线网| 99久久人妻综合| 中文字幕人妻丝袜制服| 免费av中文字幕在线| 国产精品蜜桃在线观看| 亚洲欧美精品自产自拍| 91在线精品国自产拍蜜月| 欧美丝袜亚洲另类| 精品少妇黑人巨大在线播放| 精品国产露脸久久av麻豆| 亚洲熟女精品中文字幕| 大码成人一级视频| 最近的中文字幕免费完整| 不卡视频在线观看欧美| 欧美精品国产亚洲| 天堂俺去俺来也www色官网| 国产精品一区二区在线观看99| 国产一区亚洲一区在线观看| 如何舔出高潮| 男女免费视频国产| 免费观看无遮挡的男女| 久热这里只有精品99| 日本av免费视频播放| 久久午夜福利片| 日韩电影二区| 婷婷色综合大香蕉| 人妻少妇偷人精品九色| 免费av不卡在线播放| 久久人人爽人人爽人人片va| 亚洲不卡免费看| av网站免费在线观看视频| 免费观看性生交大片5| 国产一区有黄有色的免费视频| 欧美亚洲 丝袜 人妻 在线| 91精品国产国语对白视频| 久久久久久久亚洲中文字幕| 少妇丰满av| 国产精品久久久久久精品电影小说| 一个人看视频在线观看www免费| 丰满人妻一区二区三区视频av| 久久人人爽人人片av| .国产精品久久| 777米奇影视久久| 美女视频免费永久观看网站| 久久免费观看电影| 婷婷色综合www| 日韩在线高清观看一区二区三区| 免费不卡的大黄色大毛片视频在线观看| 国产日韩欧美视频二区| 午夜福利在线观看免费完整高清在| 国产日韩一区二区三区精品不卡 | 成人午夜精彩视频在线观看| 美女国产视频在线观看| 欧美成人精品欧美一级黄| 在线看a的网站| 国精品久久久久久国模美| 国产色婷婷99| 色婷婷av一区二区三区视频| 嘟嘟电影网在线观看| 成年美女黄网站色视频大全免费 | 特大巨黑吊av在线直播| 国内少妇人妻偷人精品xxx网站| 高清不卡的av网站| 日日摸夜夜添夜夜添av毛片| 久久久国产一区二区| 嘟嘟电影网在线观看| 国产成人精品无人区| 国产av国产精品国产| 一级毛片aaaaaa免费看小| 亚洲天堂av无毛| 日韩亚洲欧美综合| 成人亚洲欧美一区二区av| 男女国产视频网站| 国产免费又黄又爽又色| 久久久久久久久大av| 99热这里只有是精品在线观看| 熟女电影av网| 毛片一级片免费看久久久久| 国产成人一区二区在线| 日韩av免费高清视频| 蜜桃久久精品国产亚洲av| 成人毛片a级毛片在线播放| 国产高清有码在线观看视频| 中国美白少妇内射xxxbb| 各种免费的搞黄视频| 极品教师在线视频| 一区二区三区精品91| 午夜福利影视在线免费观看| 久久久国产精品麻豆| 国产精品一区二区在线不卡| 日本欧美视频一区| 草草在线视频免费看| 狂野欧美激情性bbbbbb| 高清黄色对白视频在线免费看 | 欧美三级亚洲精品| 日日爽夜夜爽网站| 在线精品无人区一区二区三| 久久影院123| 免费少妇av软件| 又大又黄又爽视频免费| 国产亚洲一区二区精品| 亚洲一区二区三区欧美精品| 精品一区二区三区视频在线| 美女脱内裤让男人舔精品视频| 午夜免费鲁丝| 熟女av电影| 在线观看三级黄色| 免费黄频网站在线观看国产| 亚洲av成人精品一二三区| 麻豆精品久久久久久蜜桃| 欧美三级亚洲精品| 性色avwww在线观看| 伊人久久精品亚洲午夜| av天堂久久9| 免费少妇av软件| 亚洲欧美成人综合另类久久久| 国产乱来视频区| 午夜福利网站1000一区二区三区| 一本大道久久a久久精品| 大香蕉久久网| 校园人妻丝袜中文字幕| 人妻一区二区av| 免费久久久久久久精品成人欧美视频 | 黄色一级大片看看| 免费大片18禁| 亚洲国产精品999| 2018国产大陆天天弄谢| 日本91视频免费播放| 精华霜和精华液先用哪个| 国产中年淑女户外野战色| 制服丝袜香蕉在线| 婷婷色综合www| 日韩熟女老妇一区二区性免费视频| 丰满迷人的少妇在线观看| 精品一品国产午夜福利视频| 国产精品人妻久久久影院| 久久久久久久国产电影| av福利片在线| 如何舔出高潮| 美女主播在线视频| 久热久热在线精品观看| 亚洲熟女精品中文字幕| 男人狂女人下面高潮的视频| 欧美日韩在线观看h| 下体分泌物呈黄色| 两个人的视频大全免费| 中文乱码字字幕精品一区二区三区| av天堂中文字幕网| 久久午夜福利片| 黄色配什么色好看| 亚洲,一卡二卡三卡| 在线观看一区二区三区激情| 99热国产这里只有精品6| 热re99久久精品国产66热6| 亚洲激情五月婷婷啪啪| 最近的中文字幕免费完整| 国产精品一区二区性色av| av国产久精品久网站免费入址| 国产在线视频一区二区| 综合色丁香网| 精品人妻熟女毛片av久久网站| 欧美另类一区| 大又大粗又爽又黄少妇毛片口| 日日撸夜夜添| 久久久国产欧美日韩av| 亚洲av男天堂| tube8黄色片| 日本vs欧美在线观看视频 | 国产亚洲欧美精品永久| 黄色欧美视频在线观看| 高清av免费在线| 国产成人精品一,二区| 亚洲国产av新网站| 高清毛片免费看| 五月伊人婷婷丁香| 亚洲精品国产av蜜桃| 亚洲精品日韩av片在线观看| 91精品国产九色| 亚洲真实伦在线观看| 在线免费观看不下载黄p国产| av网站免费在线观看视频| 五月天丁香电影| 久久人妻熟女aⅴ| 午夜老司机福利剧场| 亚洲经典国产精华液单| av国产久精品久网站免费入址| 日韩熟女老妇一区二区性免费视频| 91在线精品国自产拍蜜月| 亚洲久久久国产精品| 丰满迷人的少妇在线观看| 免费看不卡的av| 91久久精品国产一区二区三区| 男女无遮挡免费网站观看| 国产欧美另类精品又又久久亚洲欧美| 久久国产精品男人的天堂亚洲 | 一区二区av电影网| 高清在线视频一区二区三区| 一本色道久久久久久精品综合| 亚洲国产精品国产精品| 一级毛片 在线播放| 国产乱人偷精品视频| 国产一区二区在线观看av| 18禁动态无遮挡网站| 日本色播在线视频| 成年av动漫网址| 少妇被粗大猛烈的视频| 高清毛片免费看| 国产精品秋霞免费鲁丝片| 亚洲精品乱码久久久v下载方式| 人人妻人人澡人人看| 极品教师在线视频| 亚洲欧美成人综合另类久久久| 蜜桃久久精品国产亚洲av| 日韩精品有码人妻一区| 国产日韩欧美亚洲二区| 国产精品国产三级国产av玫瑰| 精品亚洲成国产av| 国产男人的电影天堂91| 久久精品熟女亚洲av麻豆精品| 九色成人免费人妻av| 日韩精品免费视频一区二区三区 | 亚洲精品自拍成人| 青春草国产在线视频| 亚洲真实伦在线观看| 久久99热6这里只有精品| 国产精品国产三级专区第一集| 狂野欧美白嫩少妇大欣赏| 亚洲一区二区三区欧美精品| 韩国高清视频一区二区三区| 欧美日韩视频精品一区| 熟妇人妻不卡中文字幕| 九九久久精品国产亚洲av麻豆| 内地一区二区视频在线| 色婷婷av一区二区三区视频| 国产日韩欧美在线精品| 欧美另类一区| 亚洲久久久国产精品| 久久青草综合色| 五月开心婷婷网| 亚洲婷婷狠狠爱综合网| 免费观看av网站的网址| 久久久久精品性色| 欧美亚洲 丝袜 人妻 在线| 久久精品久久久久久久性| 日本av免费视频播放| 欧美成人精品欧美一级黄| 99热国产这里只有精品6| 高清毛片免费看| 一级毛片 在线播放| 亚洲av成人精品一区久久| 国产国拍精品亚洲av在线观看| av专区在线播放| 日韩强制内射视频| 亚洲性久久影院| 亚洲色图综合在线观看| 亚洲经典国产精华液单| 大香蕉97超碰在线| 欧美精品人与动牲交sv欧美| 少妇熟女欧美另类| 少妇人妻一区二区三区视频| 乱人伦中国视频| 18禁裸乳无遮挡动漫免费视频| 日韩视频在线欧美| 亚洲精品国产色婷婷电影| 伦精品一区二区三区| 高清不卡的av网站| 国产av码专区亚洲av| 老司机亚洲免费影院| 成人毛片a级毛片在线播放| 少妇 在线观看| 国产精品无大码| 国产69精品久久久久777片| 最黄视频免费看| 99久久人妻综合| 黑人高潮一二区| 美女中出高潮动态图| 亚洲欧洲精品一区二区精品久久久 | 久久久久久久亚洲中文字幕| 一级毛片电影观看| 国产探花极品一区二区| 欧美日韩国产mv在线观看视频| 亚洲欧美一区二区三区黑人 | 99热国产这里只有精品6| 日日啪夜夜爽| 国产精品三级大全| 国产毛片在线视频| 一二三四中文在线观看免费高清| 国产熟女欧美一区二区| 下体分泌物呈黄色| 少妇人妻一区二区三区视频| 十八禁高潮呻吟视频 | 中国美白少妇内射xxxbb| 免费观看性生交大片5| 夫妻性生交免费视频一级片| 男人爽女人下面视频在线观看| 日韩av不卡免费在线播放| 另类精品久久| 久久久久久久久大av| 国产淫语在线视频| 综合色丁香网| 99视频精品全部免费 在线| 欧美 亚洲 国产 日韩一| 中文天堂在线官网| 男人狂女人下面高潮的视频| 国产成人免费观看mmmm| 成人午夜精彩视频在线观看| 亚洲,一卡二卡三卡| 国产有黄有色有爽视频| 少妇人妻精品综合一区二区| 国产熟女欧美一区二区| 亚洲va在线va天堂va国产| 美女大奶头黄色视频| 热re99久久精品国产66热6| 草草在线视频免费看| 日韩视频在线欧美| 欧美日韩视频高清一区二区三区二| 成人亚洲精品一区在线观看| av不卡在线播放| 亚洲av中文av极速乱| 久久久久久久大尺度免费视频| 少妇人妻久久综合中文| 80岁老熟妇乱子伦牲交| 高清不卡的av网站| 高清黄色对白视频在线免费看 | 色婷婷av一区二区三区视频| 两个人免费观看高清视频 | 久久人妻熟女aⅴ| 午夜激情福利司机影院| 日韩中字成人| 夜夜爽夜夜爽视频| 视频中文字幕在线观看| 高清午夜精品一区二区三区| 少妇熟女欧美另类| 菩萨蛮人人尽说江南好唐韦庄| 成年女人在线观看亚洲视频| 99九九线精品视频在线观看视频| 久久久久久久久久人人人人人人| 五月伊人婷婷丁香| 国产精品麻豆人妻色哟哟久久| 搡老乐熟女国产| 伦理电影大哥的女人| 女性被躁到高潮视频| 黄片无遮挡物在线观看| 老女人水多毛片| 狂野欧美激情性xxxx在线观看| 亚洲中文av在线| 亚洲四区av| av播播在线观看一区| 精品人妻一区二区三区麻豆| 国产高清国产精品国产三级| 人人妻人人澡人人爽人人夜夜| 亚洲av免费高清在线观看| 久久99蜜桃精品久久| 高清在线视频一区二区三区| 亚洲欧洲精品一区二区精品久久久 | 国产午夜精品一二区理论片| av卡一久久| 欧美精品人与动牲交sv欧美| 中文字幕免费在线视频6| 午夜激情久久久久久久| 日韩,欧美,国产一区二区三区| 午夜91福利影院| 男人舔奶头视频| 内地一区二区视频在线| 多毛熟女@视频| 五月开心婷婷网| a 毛片基地| 少妇丰满av| 国产欧美日韩精品一区二区| 国产成人免费无遮挡视频| 交换朋友夫妻互换小说| 如何舔出高潮| 一边亲一边摸免费视频| 岛国毛片在线播放| 女性被躁到高潮视频| 免费观看a级毛片全部| 黄色欧美视频在线观看| 啦啦啦中文免费视频观看日本| 欧美日韩一区二区视频在线观看视频在线| 色视频www国产| 18禁裸乳无遮挡动漫免费视频| 久久av网站| 最近手机中文字幕大全| 欧美精品一区二区免费开放| 在线观看一区二区三区激情| 久久99热这里只频精品6学生| 永久网站在线| 晚上一个人看的免费电影| 中文精品一卡2卡3卡4更新| 久久久久久久久久成人| 男人和女人高潮做爰伦理| 免费观看a级毛片全部| 大又大粗又爽又黄少妇毛片口| 一区二区三区四区激情视频| 人妻夜夜爽99麻豆av| 纯流量卡能插随身wifi吗| 色哟哟·www| 国产成人精品福利久久| 51国产日韩欧美| 日韩制服骚丝袜av| 日韩成人伦理影院| 中文字幕精品免费在线观看视频 | 一级,二级,三级黄色视频| 亚洲av在线观看美女高潮| 亚洲精品国产色婷婷电影| 国产av精品麻豆| 欧美3d第一页| 国产国拍精品亚洲av在线观看| av有码第一页| 最近的中文字幕免费完整| 美女国产视频在线观看| 麻豆成人av视频| 中文乱码字字幕精品一区二区三区| 看非洲黑人一级黄片| av视频免费观看在线观看| 丰满少妇做爰视频| 日本av免费视频播放| 日韩电影二区| 美女内射精品一级片tv| 在线观看免费日韩欧美大片 | 成人毛片60女人毛片免费| 三上悠亚av全集在线观看 | 日韩熟女老妇一区二区性免费视频| 最近的中文字幕免费完整| 我的女老师完整版在线观看| 亚洲久久久国产精品| 曰老女人黄片| www.色视频.com| 久久久久久久久久成人| 亚洲欧美中文字幕日韩二区| 极品人妻少妇av视频| 在线播放无遮挡| 老司机亚洲免费影院| 日韩不卡一区二区三区视频在线| 91精品一卡2卡3卡4卡| 日本欧美国产在线视频| 国产欧美日韩综合在线一区二区 | 欧美精品国产亚洲| 看十八女毛片水多多多| 国产精品一二三区在线看| 欧美精品国产亚洲| 成人亚洲精品一区在线观看| 国产亚洲一区二区精品| 日韩av免费高清视频| 大陆偷拍与自拍| 久热久热在线精品观看| 男女国产视频网站| 日本-黄色视频高清免费观看| 狠狠精品人妻久久久久久综合| 久久ye,这里只有精品| 国产成人精品久久久久久| 亚洲精品乱码久久久v下载方式| 一级二级三级毛片免费看| 边亲边吃奶的免费视频| 十八禁高潮呻吟视频 | 亚洲欧美一区二区三区黑人 | 国产亚洲午夜精品一区二区久久| 老司机影院毛片| 嘟嘟电影网在线观看|