Thermal conduction in an electrical insulating polymer

许多电子技术朝着小型化、轻量化和集成化的方向发展，增加了对智能材料的需求，这些材料可以科普新出现的问题，例如对快速散热的需求。电动机和发电机、汽车、太阳能电池板、电池和发电中的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，但它们昂贵且相当重：出于这个原因，研究正在试图用更便宜和更轻的材料取代金属。一个明显的选择是使用聚合物材料（塑料），除了较低的成本和重量，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热导率，并且添加合适的填料（金属或陶瓷颗粒是最常见的填料）以将电导率增加到所需水平。复合材料的使用具有与材料的进一步加工的需要、由于添加填料而导致的机械性能的变化以及与寿命终止处理相关的问题相关的缺点。此外，如果要获得导热和电绝缘的材料，则应仔细控制填料的量。原则上，热传导可以通过晶格振动的机制在聚合物中发生：这些材料观察到的非常低的电导率的原因主要与聚合物链的随机取向和缠结有关。最近已经证明，如果简单聚合物如聚乙烯的聚合物链可以对齐，则可以在对齐方向上实现高热导率。为了实现高电导率，还期望具有非常长的聚合物链，以最小化由链端带来的晶格缺陷。超高分子量聚乙烯（UHMWPE）就是这种情况：然而，这种材料的链排列过程相当繁琐，需要使用大量溶剂来“解开”非常长的链。拟议的研究旨在克服这些问题，建立在我们成功微调分子特性和改善UHMWPE加工性能的基础上。我们设计了一种合成策略，使我们能够直接获得具有较少缠结的UHMWPE。我们已经证明，这种材料可以很容易地加工，而不需要任何溶剂，以得到具有高链排列的带和长丝。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排列。在这个项目中，我们希望应用我们在“解缠结UHMWPE”上开发的知识来研究分子结构和取向对这种材料的导热性的影响。来自该项目的结果将使我们能够实现重量轻，价格便宜，易于加工和回收的材料，其中热导率可以通过合成和加工步骤的适当修改在一系列有用的值中进行调整。

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.