MIT Chen Gang's team realized conjugated polymer film with high thermal conductivity for the first time
Source:  Release date:2020-05-03  Number of views:630
Polymer materials have penetrated into almost every aspect of modern technology. Flexible and lightweight polymer materials are the premise of the development of wearable sensors, soft robots, 3D printing and other advanced technologies. However, the low thermal conductivity (0.2 w / MK) of polymer materials has hindered their applications in electronic equipment, energy and other fields. The current thermal conductors and ceramics are still the main ones. As the discovery of intrinsic conducting polymers, they play an important role in flexible display, solar cells and wearable biosensors. The development of intrinsic conducting polymers will open up a series of important applications in the next generation of electronic, optoelectronic and energy equipment.
 
Structural disorder and weak intermolecular interaction are the main factors leading to the low thermal conductivity of polymer materials. At present, the research in this field is mainly limited to improving the transmission efficiency of phonons along the molecular chain by improving the intramolecular interaction of polymers, or improving the transmission efficiency of phonons between molecular chains by improving the interaction between polymer molecules. These methods need special preparation process, and the heat rate of the material is anisotropic, so it is difficult to ensure the stability and reliability in practical application. Improving both intramolecular and intermolecular interactions through molecular engineering is a long-term challenge to improve the thermal conductivity of polymers and the key to achieving effective isotropic heat transfer performance.
 
Science advances, an international leading journal, reported that MIT Chen Gang's research group realized the high thermal conductivity of conjugated polymer films (poly (3-hexylthiophene)) for the first time through bottom-up oxidative chemical vapor deposition (OCVD) using strong C = C covalent bonds along the polymer elongation chain and strong π - π stacking non covalent bonds between molecular chains. The room temperature thermal conductivity of 2.2 w / MK is achieved by these two interactions, which is 10 times higher than that of traditional polymers. By using this solvent-free OCVD technology, light and flexible polymer film heat conductors can be grown on various substrates, with electrical insulation and corrosion resistance.

Fig. 1. Synthesis process, molecular structure and film morphology of OCVD.

Conjugated polymers have the potential to develop into thermal conductors due to their rigid conjugated backbone and strong intermolecular π - π stacking interactions. The C-C single bond in diamond (~ 2000 W / MK) and drawn polyethylene (~ 104 w / MK) is the key to their ultra-high thermal conductivity. Compared with the C-C single bond, the strength of the conjugated C = C double bond is almost twice as strong as that of the C-C single bond, so it is expected to significantly improve the phonon transport in the polymer chain direction. Moreover, the π - π stacking interaction between molecular chains is 10-100 times of van der Waals force, which can enhance the phonon transport between polymer chains. However, traditional conjugated polymers exhibit similar low thermal conductivity (~ 0.2 w / MK) as non conjugated polymers. It is believed that the strong phonon scattering caused by the twist and entanglement of conjugated polymer chains is the reason for the low thermal conductivity. Chen Gang's team upregulated the synthesis process of controlled poly (3-hexylthiophene) at the molecular level through OCVD. The excess oxidant can be used as a template for the growth of polymer chain, and also can be used to oxidize the main chain of polymer during the chain growth process. The quinone structure in the chain can be significantly stabilized by adsorbing excess oxidant on the low-temperature substrate. This quinone structure is essential for obtaining high thermal conductivity, because the double bond and extended conjugated structure ensure the planar configuration of the molecular chain. This planar molecular chain can ensure multiple ordered self-assembly stacking through π - π interaction. Therefore, the synthesis of poly (3-hexylthiophene) with rigid conjugated skeleton and strong inter chain π - π interaction was realized by OCVD, and the highest thermal conductivity at room temperature was obtained 

Figure 2. Thermal conductivity obtained by tdtr method.

 

Dr. Yanfei Xu and Xiaoxue Wang are co first authors of the paper, and Prof. Karen K. Gleason and Professor Gang Chen are co authors of the paper.
 
Source: functional composites

 

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