Thermal conduction in an electrical insulating polymer

许多电子技术向着小型化、轻量化和集成化的方向发展，增加了对能够应对新出现的问题的智能材料的需求，例如需要快速散热。发电中的电机和发电机、汽车、太阳能电池板、电池和热交换器也面临同样的问题。金属由于其高导热性和导电性而可以使用，但它们价格昂贵，而且相当重：因此，研究人员正试图用更便宜、更轻的材料来取代金属。一个明显的选择是使用聚合物材料(塑料)，这种材料除了成本和重量较低外，还具有易于加工成各种形状和尺寸的优点。然而，聚合物通常具有非常低的导热系数，并添加适当的填料(最常见的是金属或陶瓷颗粒)以将导热系数提高到所需的水平。复合材料的使用存在与材料进一步加工的需要、添加填料引起的机械性能变化以及与报废处理相关的问题有关的缺点。此外，如果目标是获得一种既导热又电绝缘的材料，填充物的数量应该被仔细控制。原则上，聚合物中的热传导可以通过晶格振动的机制发生：观察到这些材料导电率很低的原因主要与聚合物链的随机取向和纠缠有关。最近已经证明，如果一种简单的聚合物如聚乙烯的高分子链可以排列，那么在排列的方向上就可以获得高的导热系数。为了获得高电导率，还希望具有很长的聚合物链，以最大限度地减少链端带来的晶格缺陷。这就是超高相对分子质量聚乙烯(UHMWPE)的情况：然而，这种材料的链对齐过程相当繁琐，需要使用大量的溶剂来‘解开’很长的链。拟议的研究旨在克服这些问题，建立在我们成功微调UHMWPE的分子特征和改善其加工性的基础上。我们设计了一种合成策略，使我们能够以较少的纠缠数量直接获得UHMWPE。我们已经证明，这种材料可以很容易地加工，不需要任何溶剂，就可以得到高链取向的胶带和细丝。此外，我们的方法提供了前所未有的可能性，通过简单地改变反应或加工条件来定制聚合物的分子量以及链的排列。在这个项目中，我们希望应用我们在“无纠缠UHMWPE”方面的知识来研究分子结构和取向对这种材料导热系数的影响。该项目的成果将使我们能够实现一种重量轻、价格低廉、易于加工和回收的材料，其中热导率可以通过适当修改合成和加工步骤来调节到一系列有用的值。

The drive of many electronic technologies towards miniaturisation, weight reduction and integration has increased the need for smart materials that can cope with new arising issues, such as the need for fast heat dissipation. The same issue is faced in electric motors and generators, automotive, solar panels, batteries, and heat exchangers in power generation. Metals can be used due to their high thermal and electrical conductivity, but they are expensive and rather heavy: for this reason, research is trying to replace metals with cheaper and lighter materials. An obvious choice is to use polymeric materials (plastic) that, in addition to the lower cost and weight, also have the advantage of being easily processable in a variety of shapes and sizes. However, polymers usually have very low thermal conductivities and suitable fillers (metal or ceramic particles being the most common ones) are added to increase the conductivity to the desired levels. The use of composites has drawbacks related to the need of further processing of the material, the change in mechanical properties due to the addition of fillers and the problems related to the end-of-life disposal. Moreover, the amount of fillers should be carefully controlled if the target is to get a material that is both thermally conductive and electrically insulating.In principle, heat conduction could happen in polymers through the mechanism of lattice vibrations: the reason for the very low conductivity observed for these materials is mainly related to the random orientation and entanglement of polymer chains. It has been recently demonstrated that, if the polymer chains of a simple polymers such as polyethylene can be aligned, high thermal conductivity can be achieved in the direction of alignment. In order to achieve high conductivities, it is also desirable to have very long polymer chains, to minimise lattice defects brought by the chain ends. This is the case for Ultra High Molecular Weight Polyethylene (UHMWPE): however, the chain alignment process for this material is rather cumbersome and demands the use of large amounts of solvent to 'disentangle' the very long chains.The proposed research aims to overcome these issues, building on our success at fine tuning the molecular characteristics and improving the processability of UHMWPE. We have devised a synthetic strategy that enables us to directly obtain UHMWPE with a reduced number of entanglements. We have demonstrated that this material can be easily processed, without the need for any solvent, to give tapes and filaments with high chain alignment. Moreover, our method offers the unprecedented possibility to tailor the molecular weight of the polymer as well as the chain alignment, by simply changing the reaction or processing conditions. In this project, we wish to apply the knowledge that we have developed on "disentangled UHMWPE" to study the effects that molecular structure and orientation have on the thermal conductivity of this material. The results coming from this project will enable us to realise a light-weight, cheap, easy to process and to recycle material where the thermal conductivity can be tuned in a range of useful values by suitable modifications of the synthetic and processing steps.