Filler free technology for enhanced thermally conductive optically transparent polymeric materials using low thermally conductive organic linkers

Abstract

This work presents for the first time a non-conventional technology for higher thermally conductive optically transparent polymeric material by employing lower thermally conductive organic molecules. In last couple of decades, practically all work on thermally conductive polymer based materials has been mainly confined to conventional polymer-filler composites which comes with various challenges like high loading, expensive fillers, etc., in addition to pronounced phonon scattering at filler-polymer interface. In this work, we have thrown light on alternating yet promising strategy for thermally conductive polymer based materials without incorporating any traditional metallic, ceramic or carbon fillers. One of the fascinating fact is the achievement of effective higher thermal conductivity in spite of using constituent components with lower thermal conductivity. This was achieved by rationally designed polymer-organic molecules system where thermal conduction was induced by engineering intermolecular interaction within the polymer chain. Additionally, these materials were optically transparent. Apart from presenting these new classes of materials, this study generates some of the prominent fundamental understanding on the heat transport of polymers. Short chains of diethylene glycol (DEG) were incorporated into base polymer matrix of polyvinyl alcohol (PVA) which lead to remarkable thermal conductivity enhancement of 260% and 175% than DEG and neat PVA respectively. Phonon transport was driven by the “thermal bridges” designed through intermolecular hydrogen bonding between PVA and DEG. A further impact of the length of “thermal bridges” and its functional groups (hydroxyl, carboxyl, carbonyl) at terminal position were investigated. It was found that shorter organic molecules are more efficient in driving thermal conduction owing to less inter-chain resistance. Interestingly, these materials were found to have inverse thermal conductivity–crystallinity relationship which is usually an opposite trend to the present belief. Overall, it was found that size, molecular geometry, terminal functional group of the thermal bridging chain in host polymer are very crucial factors in determining thermal conductance. Overall, this work demonstrates an intriguing tale of engineering intermolecular interaction in polymer based material for the development of thermally conductive and optically transparent materials for thermal management applications.