【作者】 王珊；

【导师】 杨伟；

【作者基本信息】 四川大学 ， 材料加工工程， 2021， 硕士

【摘要】 随着人工智能与信息技术等的高速发展,电子皮肤等柔性电子器件受到广泛的关注。压电材料因其具有自供能、可及时响应、电信号可与压力成线性关系等独特优势,适用于制备电子皮肤类的柔性传感器。压电材料可在外力作用下产生电势差,从而将机械能转化为电能。目前压电材料在生活中已经有了广泛的应用,而在自供能传感器和纳米发电机等领域也具有广阔的应用前景。相比于陶瓷压电材料,聚合物压电材料不仅具有更优异的加工性能,还具有良好的柔性、无毒性等特点。在诸多聚合物压电材料中,聚偏氟乙烯(PVDF)及其共聚物具有优异的柔性、可加工性、电活性和稳定性,是最具前景的聚合物压电材料之一。目前,聚合物压电材料在机械刺激下产生的压电响应仍然较小,在柔性传感器和驱动器件方面应用受到了限制,因此大量研究均主要针对于提高聚合物压电材料的压电性能。对于PVDF及其共聚物,提高其压电性能的主要措施包括提高具有铁电性的β晶的含量以及提高材料的极化程度。为了使压电材料中的偶极子定向排列,从而表现出压电性能,在压电材料制备过程中施加较强的电场(～100 MV/m)使材料极化,是有效的手段之一。静电纺丝法使用的高强度电场可以在纺丝过程中实现材料的原位极化,因此静电纺丝法是一种便捷的制备聚合物压电材料的方法。本论文以PVDF及其共聚物PVDF-TrFE为基体,采用静电纺丝的加工方法制备了聚合物压电材料,探究了静电纺丝过程中界面辅助收集(相分离法)、高速收集等方法对PVDF及PVDF-TrFE纤维膜形貌、结晶结构、力学性能(压缩模量)、压电输出性能等的作用,引入MXene进一步改善PVDF-TrFE压电材料的压电性能,并将制备的压电材料用作自供能传感器监测人体的日常活动。主要研究内容和结论如下:(1)采用静电纺丝法制备PVDF纤维膜时,高强度电场对PVDF分子链的拉伸作用和原位极化作用使PVDF纤维膜具有良好的压电性能。PVDF溶液(DMF作为溶剂)在接触水面时会发生相分离过程,通过极性水分子与PVDF之间的作用促进PVDF生成β晶相。用水介质作为静电纺丝的接收介质结合了静电纺丝和相分离的优势,促进了PVDF纤维膜的结晶和极化过程,提高PVDF纤维膜的压电输出性能。在此基础上,调控纺丝液浓度、接收介质中的盐浓度、温度等条件也可以调控PVDF纤维膜的压电性能。使用浓度为15 wt.%的PVDF纺丝液,采用0°C,20 wt.%的Na Cl溶液作为接收介质制备的PVDF纤维膜在1 Hz和20 N的压力下可输出约2.3 V的开路电压,是普通铝箔接收制备的PVDF纤维膜的7.7倍(0.3 V)。(2)溶液加工中,PVDF-TrFE比PVDF更容易生成具有电活性的β晶。采用静电纺丝-高速收集法制备PVDF-TrFE基纤维膜时,PVDF-TrFE分子链受到电场的拉伸作用和高速旋转辊筒的牵伸作用,同时在MXene片层与PVDF-TrFE分子链之间的界面相互作用下,更容易生成高含量的β晶相,从而提高了PVDF-TrFE基纤维膜的压电性能。PT/M 2.0静电纺丝复合纤维膜(纺丝液中MXene浓度为2.0 wt.%)在1 Hz和20 N的压力下可输出1.58 V的开路电压和实现约3.64 m W/m~2的瞬时输出功率密度。此外,这种PVDF-TrFE/MXene静电纺丝复合纤维膜可用于监测人体日常活动及环境的湿度变化,在多功能电子皮肤中具有潜在的应用前景。(3)采用静电纺丝-低温相分离法制备PVDF-TrFE/MXene复合纤维膜。PVDF-TrFE分子比PVDF分子更容易形成β晶,且与MXene之间有良好的相互作用。在纺丝过程中,MXene的导电性和亲水性促进PVDF-TrFE分子在电场和相分离的双重作用下定向排列,同时低温也有利于保持偶极子的极化状态。所制备的PVDF-TrFE/MXene静电纺丝复合纤维膜表面形成了一定尺度的褶皱结构,表现出良好的压电性能,PT/M2-W复合纤维膜在1 Hz和20 N作用力下可产生约3.4 V开路电压和约48.1 m W/m~2的瞬时输出功率密度,与静电纺丝-高速收集法制备的PVDF-TrFE纤维膜(0.5 V)和PT/M 2.0纤维膜(1.58 V)相比,开路电压分别提高了5.8倍和1.2倍。更多还原

【Abstract】 Currently,the booming of artificial intelligence and internet of things highlights the significance of fabricating flexible and portable electronic devices for realizing human-machine interface.As a kind of materials can not only converse mechanical stimulus to electrical signal,but also have instant and linear response as well as good signal stability,organic piezoelectric materials have attracted extensive attention for the fabrication of flexible and wearable electronics thanks to low density,good biocompatibility,flexibility,the ability to self-power,as well as ease of processing if compared with inorganic piezoelectrics.Among the existing organic piezoelectric materials,polyvinylidene fluoride(PVDF)and its copolymers are the most widely used ones owing to their relatively high piezoelectric coefficient.However,by comparison with piezoelectric ceramics,their piezoelectric coefficient is still much lower,which hinders their self-powered sensor application in practice.In order to improve piezoelectric properties of PVDF and its copolymers,two main kinds of methods,i.e.increasing the piezoelectric phase content and improving remnant polarization,have been reported in literatures.As the piezoelectric response of PVDF and its copolymers extremely relies on the dipole alignment,a high electric field(～100 MV/m)is usually applied to polarize such piezoelectric materials to achieve optimal piezoelectric effect.Unlike conventional processing methods,electrospinning provides strong elongational force to generate nano/micro-scale fibers through a high electrical field between the nozzle and collector.Intriguingly,it also offers in-situ polarization of dipoles in polymer chains,thus it is a convenient method to prepare piezoelectric fibrous mats of PVDF and its copolymers.In this work,water media assisted electrospinning and orientation electrospinning methods are used to fabricate PVDF and PVDF-TrFE piezoelectric fibers.Besides,MXene is used to improve the piezoelectricity of PVDF-TrFE films.The impact of different electrospinning methods on morphology,crystallization,mechanical properties(modulus of compression)and piezoelectric output performance of PVDF and PVDF-TrFE fibers are also explored.Furthermore,the prepared films can be used as self-powered sensors to monitor the daily activities of human body.The main work contents and conclusions are as follows:1.In electrospinning process,the elongation and the in-situ polarization effects of strong electric field on PVDF molecules endow the PVDF films good piezoelectricity.While PVDF solution meets water,the phase inversion occurs,in which the solvent enters into the water and the PVDF is separated out on the water surface.In such a process,the interaction between polar water molecules and PVDF molecules can promote the formation ofβ-crystal phase of PVDF.Therefore,water media assisted electrospinning methods can promote the crystallization and polarization process of PVDF fibers and improve the piezoelectric output performance of PVDF films by combining the advantages of electrospinning and phase inversion.In addition,the piezoelectric properties of PVDF fibers can also be controlled by the concentration of the electrospinning solution,the temperature and ionic concentration of the collecting water.For example,the PVDF film prepared with 15 wt.%PVDF electrospinning solution and collected by 0°C,20 wt.%Na Cl solution can output an open-circuit voltage of about 2.3 V under the compression of 20 N,which is 7.7 times of that of the PVDF film collected by ordinary aluminum foil(0.3 V).2.In electrospinning process,PVDF-TrFE molecules are able to formβphases due to the stretching of the electrical field force and the effect of steric hindrance.Meanwhile,the conductive MXene sheets generate induced charge in the electric field,and the adjacent sheets increase the local electric field intensity,making the dipoles ofβ-phase PVDF-TrFE crystals orient easily.Besides of the electrical polarization of PVDF-TrFE molecules during electrospinning,mechanical stretching during the nanofiber collecting at high speed enables further alignment of polymer chains,as well as the alignment dipoles.Therefore,the piezoelectric properties of PVDF-TrFE films are improved.For example,the composite film PT/M 2.0 can output an open-circuit voltage of 1.58 V and achieve an instantaneous output power density of～3.64 m W/m~2under the pressure of 20 N with the frequency of 1 Hz.Such piezoelectric PVDFTrFE/MXene films are capable of sensing body motion for healthcare.Furthermore,the hydrophilicity of MXene allows the film pressure sensor to monitor humidity change,thus the composite film sensor demonstrates potential application in multifunctional electronic skins.3.PVDF-TrFE/MXene composite films are prepared by 0°C water assisted electrospinning.In electrospinning process,the conductivity and hydrophilicity of MXene promote the orientation of PVDF-TrFE molecules in the electric field and phase inversion process,and the low temperature field is conducive to maintaining the polarization state of the dipoles.Therefore,the PT/M2-W composite film can generate an open-circuit voltage of about 3.4 V and an instantaneous output power density of～48.1 m W/m~2 under the pressure of 20 N with the frequency of 1 Hz.更多还原