Accurate Measurements of Nonlinear Dielectric Relaxation Spectra of Polymers

我们研制了一种精确测量非线性介电弛豫谱的装置。在样品上施加一个失真最小的正弦电场，通过数字傅里叶变换求出基波和高次谐波的电介质位移。在0.01 Hz-10 kHz范围内，我们得到了线性介电常数和三阶非线性介电常数与频率的函数关系。利用该装置，我们研究了三种聚合物：偏二亚甲基氰化物共聚物、聚乙酸乙烯酯和离子导电聚合物。偏二亚甲基氰化物共聚物，特别是与乙酸乙烯酯的共聚物，由于其非常大的线性介电常数，在过去的几年里引起了人们的兴趣。非线性介电测量表明，第三种非线性介电常数与频率有关，符合唯象理论的公式：epsilon_3^*=Delta epsilon_3/(1 I Omega Tau_3)^3。结合关于Delta epsilon_1和Delta epsilon_3的知识可以估计…等效偶极子的个数和力矩更大。亚乙基氰化物聚合物的高介电常数归因于非常大的等效偶极矩，这表明它们的分子运动中存在很强的协作性。聚乙酸乙烯酯在电致伸缩作用下表现出形变的非线性光谱。弹性柔度的额外测量允许减去这一贡献，从而得到与观察到的亚乙叉氰化物共聚物一致的本征非线性光谱。类似的测量已经扩展到包括聚氧乙烷及其衍生物在内的离子导电聚合物。在这些结果的基础上，我们证实了非线性介电谱与其线性介电谱相结合提供了关于聚合物体系中分子相互作用和动力学的有用信息。较少

We have developed an apparatus for making accurate measurements of non-linear dielectric relaxation spectra. A sinusoidal electric field with minimal distorsion is applied to a sample and fundamental and higher harmonic dielectric displacements are ditected by means of digital Fourier transform. We obtained linear and third non-linear permittivities as a function of frequency in a 0.01Hz-10kHz range. Using this apparatus, we investigated three kinds of polymers ; vinylidene cyanide copolymers, polyvinyl acetate, and ion-conducting polymers.1. Vinylidene cyanide copolymers, especially with vinyl acetate, have attracted interest past few years because of their very large linear permittivity. Nonlinear didelctric measurements have shown that the third nonlinear permittivity depends upon frequency according to, epsilon_3^*=DELTA epsilon_3/ (1+i omega tau_3) ^3 in accordance with phenomenological prediction. Combined knowledge about DELTA epsilon_1 and DELTA epsilon_3 allow estimates of the … More number and moment of equivalent dipoles. The large permittivities of vinylidene cyanide opolymers are attributed to very large equivalent dipole moments suggesting an existence of strong cooperativity in their molecular motions.2. Polyvinyl acetate is shown to exhibit deformed nonlinear spectra due to the contribution of electrostriction. Additional measurements of the elastic compliance allow subtraction of this contribution to yield the intrinsic nonlinear spectra which are consistent with those observed for vinylidene cyanide coplymers.3. Analogous measurements have been extended to ion-conducting polymers including polyethylene oxide and its derivatives. The results are immediately converted into non-linear conductive spectra to enable discussion on conduction mechanisms of such polymers.On the basis of these results, we have confirmed that the non-linear dielectric spectroscopy in combination with its linear counterpart provides useful information about molecular interaction and dynamics in polymeric systems. Less