Polarizability Measurements for Salicylic Acid Embedded in Polymer Matrix Using Terahertz Time-Domain Spectroscopy
ZHANG Tian-yao1,2, ZHANG Zhao-hui1*, ZHAO Xiao-yan1, WEI Qing-yang1, CAO Can1, YU Yang1, LI Ying1, LI Xing-yue1
1. Beijing Engineering Research Center of Industrial Spectrum Imaging, School of Automation and Electrical Engineering, University of Science and Technology Beijing, Beijing 100083, China
2. Department of Materials Physics, School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing 100083, China
摘要： 极化率作为材料的重要微观属性，与其呈现出的多种宏观属性等紧密相关，在新型药物及功能材料的设计中扮演着关键角色。对于微观结构存在复杂分子间作用的物质体系，获取其太赫兹波段的动态极化率具有重要意义。极化率的实验测定建立在其本征介电常数的准确测定之上，太赫兹时域光谱系统凭借其相干探测属性，可直接提供太赫兹波在穿透样品后的相位变化信息，用于精确测定样品的介电常数。当样品由待测物纯物质组成时，实验测得的介电常数可直接用于计算极化率。但对于大多数的有机晶体，其自身对太赫兹波的特征吸收以及与颗粒尺寸与波长相当造成的散射，使其纯样品的太赫兹时域光谱表征受到系统信噪比的限制，通常需要将待测物均匀分布于对太赫兹波透明且可塑性极强的聚合物载体中。聚合物载体的使用导致实验直接测定的混合物介电常数中也包含了聚合物的介电影响，无法直接用于计算待测物质的极化率。针对这一问题，将嵌于聚合物载体中的有机晶体整体等效为聚合物、空气气隙和待测样品三组份混合介质模型，结合各组份体积占比以Landau, Lifshitz and Looyenga有效介质模型从实验测得的混合物介电常数中移除聚合物和空气气隙的影响，获取待测物的太赫兹本征介电常数，从而进一步结合Clausius-Mossotti关系式计算其太赫兹极化率。选取水杨酸晶体为研究对象，利用所提出的理论方案，从聚乙烯和聚四氟乙烯两种聚合物载体三种浓度混合物的原始介电常数中，提取出了具有高度一致性的本征介电常数和太赫兹极化率，验证了方法的有效性，并利用实验结果首次报道了水杨酸晶体的太赫兹极化率，在聚乙烯和聚四氟乙烯载体中的测定值分别为(17.2±0.2)和(17.6±1.0)Å3。
关键词：极化率；太赫兹时域光谱技术；有效介质理论；聚合物载体；水杨酸
Abstract：Polarizability acts as an essential property in the design of novel materials. It is of great significance to obtain the dynamic polarizability over terahertz frequencies for substance containing complex intermolecular interactions. The experimental access of the polarizability is based on the accurate determination of its intrinsic dielectric constant. Meanwhile, thecoherent detection scheme of terahertz time-domain spectroscopy (THz-TDS) makes it an ideal alternative for dielectric constants measurements. However, most organic crystalline materials occupy strong absorption features over terahertz frequencies, and samples for THz characterization are commonly prepared by homogeneously dispersing the analyte within the THz transparent polymer matrix. The application of the dilution matrix makes the dielectric constants of mixture samples measured by the THz-TDS cannot be directly utilized to calculate the polarizability. We proposed in this work a ternary model based on the Landau, Lifshitz and Looyenga (LLL) theory to extract the intrinsic dielectric constants of analyte by removing the dielectric contribution of trapped air and polymer background. The extracted intrinsic dielectric constants can be further used to calculate polarizability combining with the Clausius-Mossotti equation. The proposed methodology is exemplified in detail with the polarizability determination of crystalline salicylic acid (SA) embedded in twopolymer matrixpolyethylene (PE) and polytetrafluoroethylene (PTFE), respectively. Mixture pellets at three concentrations are designed for THz-TDS measurement for each polymer matrix. The application of the proposed method on the original dielectric spectra of mixture pellets provides intrinsic dielectric constants and polarizability values of salicylic acid with promising consistency. The results reported in this work also represent the first experimentally measured polarizability values for crystalline salicylic acid, which is (17.2±0.2) and (17.6±1.0) Å3 for analyte embedded in PE and PTFE respectively.
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