Rigid can be flexible – Prediction and Control conjugated polymer has a glass transition temperature
Over the past decade, most working on the molecular structure of a conjugated polymer type receptor in the field of photoelectric materials (such as organic photovoltaic devices (OPV) and organic field effect transistors (the OTFT) received great attention. the main study modified in order to obtain a higher electrical performance, however, is still very limited for improve its mechanical properties. this makes the conjugated polymer used in many advanced technology in (an organism of wearable / portable electronics equipment, heat stabilizers, solar cells, etc.) used in aerospace has been extremely limited. Therefore, the study of conjugated polymers structure transition temperature (Tg) and is essential in order to influence the mechanical properties of their glass. recent , University of Southern Mississippi United States (University of Southern Mississippi) polymer Science and Engineering Professor Gu Xiaodan team and North Dakota State University (North Dakota State University) civil and environmental engineering professor Xia Wenjie team to reports, the conjugated polymer molecular chain flexible its glass transition temperature, and the relationship between the Young\’s modulus can be predicted from a linear empirical formula. the doctoral research to raise the first sheet of order \”Toward the prediction and control of glass transition temperature for donor -acceptor polymers \”was published in the journal Advanced Functional Materials.
1. conjugated polymer mechanical and thermal performance test
get mechanics and thermal properties of conjugated polymers experimentally very difficult. first, the conjugated polymer film (< 100纳米）的力学性能很难直接通过拉伸实验测试，传统上使用聚二甲基硅氧烷PDMS作为基底测试薄膜力学性能的方法，由于拉伸速度得不到控制，基底与薄膜之间作用力的影响不得知等问题，得到的杨氏模量在很大程度上并不具有可参考性。作者通过使用自制的伪独立式拉伸测试仪（pseudo-free-standing tensile tester），将共轭高聚物薄膜浮在水面上，进行单向拉伸即可获得其应力-应变曲线。
另外，给受体共轭高聚物的Tg并无法简单地通过常见设备（如差示扫描量热仪DSC, 动态热机械分析 DMA，旋转流变仪等）而测得。其原因有：一，每次合成得到的产量只有几十微克；二，共轭高聚物链刚性大，结晶性强，导致其Tg前后的热容差特别小。因此，这里作者将共轭高聚物溶液滴涂在玻璃纤维上，只需要5-10毫克样品即可使用DMA进行本体Tg的测试。
作者以给受体共轭高聚物，吡咯并吡咯二酮（diketopyrrolo, DPP）为主要研究对象，发现随着其支链长度的增加，Tg与弹性模量会同时降低，而Tg与弹性模量之间呈现出线性关系。同时，通过粗粒化分子动力学模拟 （CG-MD simulation），将侧链的长度简化为球状模型的数量，从而测试分子链对温度与外界应力的反应，可以得到与实验相似的结果。
2. empirical prediction a Tg of the polymer conjugate [123 ] increasing the chain length of the branched polymer conjugate influence Tg of the polymer can be directlyLifting chain flexibility linked. According Di Marzio and mass-per-flexible bond Schnider proposed theoretical model, the flexible molecular repeating units allocated to each of a covalent bond (0 being completely restricted, a representative may freely) coefficient from 0 to 1, It can be obtained mass-per-flexible bond a repeating unit. On using the model, polythiophene conventional conjugated polymers, for example, successfully demonstrated a linear relationship between the Tg and the flexible polymer chain. Further analysis showed that this model can also be used in complex conjugate to the receptor in the DPP polymer, and the other two can successfully predict the same main chain and side chain length DPP polymers of different Tg. Next, the authors once again test the predictive ability of the model to different DPP Tg polymer main chain, found that rigid backbone upgrade will lead to increased Tg, while dominated by a side chain length of the linear relationship still exists.
in addition to the main chain and side chain length stiffness, the amount of side branches linked conjugated polymer of Tg also has an important influence. Because of the difficulty of the synthesis of new materials, tested by experiment Tg it would be very time-consuming and laborious, and by molecular dynamics simulation is more relaxed. Simulation results showed that, regardless of the number or length of the side chain results in decrease of the polymer Tg increases are conjugated, and tends to decrease with a specific amount of the graft, and there is no linear relationship between the length of the side chains. When the large side chain content, there will be a Tg of plateau, and side chain length / number of the corresponding number is directly determined by the free volume of the
Figure 5. coarsen Effect of molecular dynamics simulation pendant side chain length and degree of branching and the kinetics of Tg