聚合物及其混合物的太赫兹介电性质测定与分析方法

聚合物材料因其对太赫兹波的高透过率以及良好的塑形能力在太赫兹研究中扮演了重要的角色，由于材料的介电特性直接关系着折射率、极化率等重要性质，不同的应用场合通常需要材料呈现出特殊的太赫兹介电响应，一方面可以选取不同的聚合物材料，另一方面可以通过多种聚合物的混合实现材料介电性质的调制。聚合物材料合理的选取和设计建立在材料太赫兹介电精确表征之上，然而利用太赫兹时域光谱系统(THz-TDS)对聚合物材料进行透射式太赫兹介电表征时，材料内部空气孔隙的存在会影响表征结果的复现性，同时也会影响共混聚合物介电性质的分析和预测。因此以Landau, Lifshitz, Looyenga(LLL)模型为基础提出了考虑空气影响的介电分析模型，并选取了在太赫兹研究中广泛使用的聚合物材料聚乙烯(PE)和聚四氟乙烯(PTFE)对模型的有效性和稳定性加以验证，展开了两种材料单质和混合物两方面的介电分析。在太赫兹波透射样品之后的相位变化信息中提取出样品的介电常数，同一种物质制备的样片间太赫兹介电谱存在明显差异，使用包含气隙影响的LLL模型处理实验数据后，样品介电常数中空气的介电影响被移除，从而得到了两种材料的本征介电常数，在此基础上，使用测得的本征介电常数和混合物样品中两种材料的体积占比信息代入包含气隙影响的LLL模型计算得到了不同配比混合物的太赫兹介电常数的模拟值，并与THz-TDS实验获取的实验值进行了对比。利用所提出的有效介质模型，聚乙烯和聚四氟乙烯在10～40 cm-1波段内移除空气影响后的平均介电常数为2.315±0.003（±0.13%）和2.109±0.003（±0.14%），在不同重量、不同厚度的单组份样品间模型测定的聚合物介电常数保持了良好的重复性，在对混合物的太赫兹介电性质测定与分析中，利用模型计算的混合物介电常数模拟值与THz-TDS测定的介电常数实验值保持了高度线性相关，其相关系数为0.964 3，全局相对误差为1.08%，体现了模型的可靠性。提出的介电分析模型可以扩展到更多的高分子材料单质及其混合物的太赫兹介电性质表征中，对太赫兹波段的共混聚合物材料设计具有参考价值。

Dielectric Analysis of Polymeric Materials and Mixtures Using Terahertz Time Domain Spectroscopy

Due to the transparency to the terahertz waves, many polymeric materials have been demonstrated various important applications as optical components as well as dilution matrix for terahertz research. Certain dielectric responses are usually preferred considering different application circumstances. The adjustment of dielectric property could be achieved by proper selection of polymer as well as design of polymer blend by physically mixing. Methodology is well described in this study for the terahertz dielectric analysis of polymeric materials and their mixtures in a quantitative manner, as exemplified by experiments with polyethylene (PE) and polytetrafluoroethylene (PTFE) for their widespread applications in terahertz research. Following the regular procedure of measuring the THz dielectric property of single component pellets with the phase delay information provided by the coherent detection of terahertz time domain spectroscopy, the intrinsic dielectric constants of PE and PTFE were determined as 2.315±0.003 (±0.13%) and 2.109±0.003 (±0.14%) by removing the influence of trapped air utilizing effective medium theory (EMT). These measured values were then used to simulate the THz dielectric constants of mixture pellets consisting of PE and PTFE at several mass ratios combing with EMT. The comparison between simulated values and experimental values of mixture samples presents a well linear correlation with R2=0.964 3 while the overall relative difference is 1.08%.