双折射性是各种光学材料的重要性能之一，具有高双折射率的光学材料在诸多研究及工业领域的应用越来越广泛.然而，作为常用的光学薄膜及光波导材料之一的聚合物材料的双折射性通常却很弱，只能通过实验对其双折射率进行大致的表征，缺乏对其双折射率的系统性理论计算，从而限制了提高聚合物双折射性的研究.本文建立了从聚合物的单体分子结构到多分子链的系统性的双折射率理论计算方法，并借助此方法研究了导致聚合物弱双折射性的限制因素.以聚甲基丙烯酸甲酯(PMMA)为研究对象，运用密度泛函理论研究了其本征双折射率，这里的本征双折射率是指分子链完全取向时其单体单元的双折射率.计算结果表明其本征双折射率高达0.0738左右，并且通过计算给出了PMMA单体单元的平均双折射率色散曲线.采用分子动力学方法研究了该聚合物(包含20个分子链)的材料双折射率.理论计算结果表明，尽管该聚合物具有较大的本征双折射率，但是由于聚合物中分子链取向度极低，聚合物材料最终表现出来的双折射率只有0.00052.本文建立的研究方法及研究结果为研究增强聚合物材料双折射性提供了理论依据.%The birefringence is one of the most important properties of all kinds of optical materials. and is widely used in many basic researches and industrial fields. By utilizing high birefringent materials or waveguides, a variety of unique and interesting optical features or functions can be achieved, such as in manipulating the polarization of an optical beam in a miniaturized way and providing the organic electro-luminescence display. Crystals, liquid crystals, semiconductor, silicon, ferroelectric material and polymer can exhibit their birefringences. While polymer materials are commonly used to fabricate optical films and waveguides, most polymer materials show relatively weak birefringences, and thus they are restricted in realizing novel functional photonics devices. In the past, such a weak birefringence has been roughly characterized in experiment. There is a lack of systematic method to theoretically calculate the birefringences of polymer materials, especially at a molecular level. This restricts the research on enhancing the birefringences of polymer materials. To study the birefringences in fluorinated polymers and find the way to enhance them, the origin of the birefringence in fluorinated polymer should be investigated in depth and the birefringence should be exactly calculated. In this paper, a theoretical method is established to calculate the birefringence of polymer systematically from the monomer unit to the molecular chain. Based on this method, the limiting factor that leads to a weak birefringence in polymer material is investigated. Taking the polymethyl methacrylate (PMMA) for example, the density functional theory (DFT) is first used to study the intrinsic birefringence of PMMA, where the intrinsic birefringence value is indeed the birefringence of the monomer unit and is also a maximum birefringence of the polymer material when the molecular chains are fully oriented. In the DFT, a stable structure of the PMMA monomer unit is constructed, and the intrinsic birefringence of this PMMA monomer unit structure is calculated. The calculation result shows that the intrinsic birefringence of PMMA monomer unit can reach up to 0.0738, the dispersion curve of the average birefringence of the monomer unit is also given. Furthermore, the molecular dynamics is used to study the material birefringence of the PMMA material consisting of 20 molecular chains. The calculation results show that although the intrinsic birefringence is much larger, the material birefringence of the PMMA is only 0.00052, due to the low degree of orientation of molecular chain in the PMMA. It is found that the molecular structure and the molecular orientation of the polymer are the two main factors influencing the birefringence. The theoretical method established in this work and the calculation results provide a research basis for enhancing the birefringences of polymer materials.
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