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Investigations of structure-property relationships to enhance the multifunctional properties of PVDF-based polymers.

机译：为了增强PVDF基聚合物的多功能特性，研究了结构-特性关系。

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摘要

Poly(vinylidene fluoride) (PVDF)-based polymers have been some of the most widely researched semicrystalline polymers over the past several decades, due mostly to their ability to exhibit ferroelectric properties not capable in many soft materials. While much has been learned about these properties and much advancement has been made in utilizing them for many applications, we are still only beginning to understand their origins and how they can be enhanced by altering the polymer structure. In this thesis, structure-property relationships are analyzed in a variety of PVDF-based polymers with varying chemical and crystalline structures. The work consists of three parts as distinguished by the property under investigation: electromechanical effects, electrical energy storage, and the electrocaloric effect (ECE). First is the electromechanical effects, where a large converse piezoelectric effect is discovered in P(VDF-HFP) (HFP: hexafluoropropylene) copolymer. The nature of the piezoelectric property is linked to the structure change through a detailed structural analysis to provide explanation to the large and unusual electromechanical strain response. P(VDF-HFP) is further investigated for its use as an energy storage capacitor and various processing methods are utilized to alter the crystalline structure and study the effects on the energy storage characteristics. The results indicate that uniaxial stretching is beneficial in improving energy storage efficiency up to a certain draw ratio (4--5x the original length), but as the draw ratio is increased and the polar ss crystalline phase becomes more prominent, ferroelectric losses become detrimental to the energy storage efficiency. Furthermore, the effects of biaxial stretching on the crystalline structure are examined. The data suggests that biaxial stretching of extruded films to a similar draw ratio as the uniaxially stretched blown films produces a similar composition of crystalline structure. In view of the fact that the structure-property relationships show that the polar crystalline phase contributes to energy losses in the material, electron irradiation is examined as a method of destabilizing the ss-phase in P(VDF-HFP) and P(VDF-CTFE) and reducing ferroelectric energy loss. A structural analysis confirms that the defects introduced into the crystalline structure of both polymers by the irradiation does indeed increase the composition of the nonpolar alpha-phase. Furthermore, analysis of electric displacement-electric field (D-E) hysteresis loops indicates improvements in energy storage efficiency as a result of irradiation. The final portion of this dissertation probes a property previously unstudied in polymeric materials, the ECE. The results show that applying an electrical field to a polar polymer may induce a large change in the dipolar ordering, and if the associated entropy changes are large, they can be explored in cooling applications. With the use of the Maxwell relation between the pyroelectric coefficient and the ECE, it was determined that a large ECE can be realized in the ferroelectric P(VDF-TrFE) (TrFE: trifluoroethylene) copolymer at temperatures above the ferroelectric-paraelectric transition (above 70°C), where an isothermal entropy change of more than 55 J/(kgK) and adiabatic temperature change of more than 12°C were observed. We further show that a similar level of ECE near room temperature can be achieved by working with the relaxor ferroelectric polymer of P(VDF-TrFE-chlorofluoroethylene). Furthermore, the difference in temperature dependence of the electrocaloric effect in P(VDF-TrFE) and P(VDF-TrFE-CFE) suggests different entropy change mechanisms. The contribution to polarization from nanopolar domains in the terpolymer at low temperature is not as effective at generating large entropy change as the paraelectric to ferroelectric transition. Moreover, the adiabatic entropy change DeltaS is proportional to the square of the electric displacement D (DeltaS = -1/2 ssDeltaD2) in both systems and the coefficient ss increases with temperature in the terpolymer as opposed to the copolymer where ss is temperature independent. This temperature dependent behavior of ss is caused by the ferroelectric relaxor nature of the polymer in which the polarization response from the nano-polar regions does not generate much entropy change. Consequently, the ECE effect in the relaxor ferroelectric terpolymer is smaller than that in the normal ferroelectric copolymer. Moreover, a study of quenched terpolymer samples as compared to annealed samples with higher crystallinity shows strong correlation between the ECE and crystallinity, suggesting that the ECE occurs predominantly in the crystalline regions of the semicrystalline polymers.
机译：在过去的几十年中，基于聚偏二氟乙烯（PVDF）的聚合物已经成为研究最广泛的半结晶聚合物，这主要是由于它们具有铁电特性的能力，这在许多软质材料中都不具备。尽管已经对这些性质学到了很多知识，并且在将它们用于许多应用方面已经取得了很大的进步，但我们仍然才刚刚开始理解它们的起源以及如何通过改变聚合物结构来增强它们。在本文中，分析了具有不同化学和晶体结构的各种基于PVDF的聚合物的结构-性质关系。这项工作分为三个部分，按所研究的特性进行区分：机电效应，电能存储和电热效应（ECE）。首先是机电效应，在P（VDF-HFP）（HFP：六氟丙烯）共聚物中发现了大的逆压电效应。压电特性的性质通过详细的结构分析与结构变化相关，从而为大而不同的机电应变响应提供了解释。对P（VDF-HFP）用作储能电容器进行了进一步研究，并采用了各种加工方法来改变晶体结构并研究其对储能特性的影响。结果表明，单轴拉伸有利于在一定拉伸比（原始长度的4--5倍）下提高储能效率，但是随着拉伸比的增加和极性ss晶相的出现，铁电损耗将变得有害来提高储能效率。此外，检查了双轴拉伸对晶体结构的影响。数据表明，将挤出薄膜双轴拉伸至与单轴拉伸吹塑薄膜相似的拉伸比可产生相似的晶体结构组成。鉴于结构-性质关系表明极性晶相会导致材料中的能量损失，电子辐射被视为一种破坏P（VDF-HFP）和P（VDF-S）中ss相稳定的方法。 CTFE）和减少铁电能量损失。结构分析证实，通过辐射引入两种聚合物的晶体结构的缺陷确实确实增加了非极性α相的组成。此外，对位移电场（D-E）磁滞回线的分析表明，由于辐照，能量存储效率得到了提高。本论文的最后部分探讨了一种以前在高分子材料中未研究过的性能，即ECE。结果表明，对极性聚合物施加电场可能会引起偶极有序的大变化，并且如果相关的熵变化很大，则可以在冷却应用中进行探索。通过使用热电系数和ECE之间的麦克斯韦关系，可以确定在高于铁电-顺电转变的温度下，铁电P（VDF-TrFE）（TrFE：三氟乙烯）共聚物中可以实现较大的ECE（以上70°C），观察到等温熵变超过55 J /（kgK），绝热温度变化超过12°C。我们进一步表明，通过使用P（VDF-TrFE-氯氟乙烯）的弛豫铁电聚合物可以达到接近室温的ECE水平。此外，P（VDF-TrFE）和P（VDF-TrFE-CFE）中电热效应的温度依赖性温度差异表明，熵变化机制也不同。在低温下，三元共聚物中纳米极性域对极化的贡献不如顺电向铁电转变有效地产生大的熵变。此外，绝热熵变ΔS与两个系统中电位移D的平方成正比（DeltaS = -1/2 ssDeltaD2），并且系数ss随三元共聚物中的温度而增加，这与ss与温度无关的共聚物相反。 ss的这种温度依赖性行为是由聚合物的铁电弛豫特性引起的，其中来自纳米极性区域的极化响应不会产生太大的熵变。因此，驰豫铁电三元共聚物中的ECE效果小于常规铁电共聚物中的ECE效果。此外，与结晶度较高的退火样品相比，淬火三元共聚物样品的研究表明，ECE与结晶度之间具有很强的相关性，这表明ECE主要发生在半结晶聚合物的结晶区域中。

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