Nanostructured Polymer Films with Metal-like Thermal Conductivity (1708.06416v1)

Abstract: Thermally conductive polymers are of fundamental interest and can also be exploited in thermal management applications. Recent studies have shown stretched polymers can achieve high thermal conductivity. However, the transport mechanisms of heat in thermally conductive polymers have yet to be elucidated. Here we report a method for scalable fabrication of polyethylene films with a high thermal conductivity of 62 W/m-K. The achieved thermal conductivity is over two orders-of-magnitude greater than that of typical polymers (~0.1 W/m-K), and exceeds those of many metals and ceramics used as traditional heat conductors. Careful structural studies are carried out and reveal that the film consists of nanofibers with crystalline and amorphous regions. Contrary to conventional wisdom, we reveal the importance of the amorphous morphology in achieving such high thermal conductivity, rather than simply from enhancements in the degree of crystallinity and crystallite alignment. The amorphous phase reaches a remarkably high thermal conductivity of ~16 W/m-K. Even still, we identify that the presence of this amorphous phase is the dominant factor as the film thermal conductivity is still much lower than the predicted values for bulk single-crystal polyethylene (237 K/m-K). This work lays the foundation for the rational design and synthesis of thermally conductive polymers, and opens up new opportunities for advanced heat management, particularly when flexible, lightweight, chemically inert and electrically insulating thermal conductors are desired.

• The paper presents a method to achieve polyethylene films with a breakthrough thermal conductivity of 62 W/m-K, far exceeding typical polymer values.
• It details a unique process using Couette-flow extrusion and liquid nitrogen-cooled substrates to precisely align and disentangle polymer chains.
• The study reveals that engineered amorphous regions significantly enhance heat transfer, challenging conventional views on polymer thermal transport.

Nanostructured Polymer Films with Metal-like Thermal Conductivity: An Evaluation

The research article titled "Nanostructured Polymer Films with Metal-like Thermal Conductivity" presents a method for fabricating polyethylene films with unprecedented thermal conductivity, achieving values of 62 W/m-K. This development represents a significant enhancement over the thermal conductivities of typical polymers, which hover around 0.1 W/m-K, and even surpasses various metals and ceramics traditionally employed in thermal management applications.

Experimental Insights and Methodologies

The authors embarked on a sophisticated process to attain these films, beginning with the dissolution of commercial semi-crystalline polyethylene powders, which were subsequently melted and extruded through a custom-built Couette-flow system. Following this, coupling methodologies involving liquid nitrogen-cooled substrates and roll-to-roll systems facilitated the alignment and disentanglement of polymer chains. This precise engineering yielded nanofiber structures composed of both crystalline and amorphous regions, a dichotomy instrumental in achieving high thermal conductivity.

A groundbreaking finding of this paper lies in the reevaluation of the amorphous phase's contribution to thermal conductivity. Traditionally, the crystalline phase has been emphasized; however, this research illustrates that controlling the amorphous morphology plays a critical role. The amorphous regions registered a surprising thermal conductivity of ~16 W/m-K, challenging conventional beliefs regarding thermal transport in polymers and bridging the gap between empirical and theoretical predictions.

Structural and Thermal Characterization

A comprehensive suite of characterization techniques were employed, including high-resolution synchrotron X-ray scattering and time-domain thermoreflectance (TDTR), revealing intricate details about the spatial ordering and heat transport within the films. These investigations elucidate the alignment and size of crystallites, with synchrotron X-ray analysis indicating that while crystallinity increases with draw ratio, the orientation enhancement contributed immensely at lower draw ratios.

Importantly, the thermal conductivity improvements exhibited no saturation even at high draw ratios, suggesting potential for further enhancements. The authors attribute this to the amorphous phase's alignment and extension, enabling heat transfer in a manner akin to the crystalline domain. This contrasts with historical studies, intensifying the discourse on polymer structure-property relationships.

Implications and Future Directions

This research has profound implications for the design of thermally conductive polymers. By investigating the intricate balance between morphology, crystallinity, and thermal transport, the authors lay a framework for synthesizing polymers akin to metals in terms of thermal efficiency. The exploration of the amorphous domain, often overlooked, poses new theoretical and practical pathways for developing advanced materials for flexible, lightweight, and electrically insulating applications, including electronics, heat sinks, and consumer goods.

Looking forward, future work might delve into optimizing the orientation and distribution of the amorphous chains within different polymer matrices or considering alternative polymers beyond polyethylene. Achieving higher thermal conductivity may involve tailoring these structures at a molecular level to maximize the existing trade-offs between crystallinity and amorphous contributions.

In summation, this paper provides vital insights that are not only foundational but could very well steer the next wave of research towards the advanced integration of engineered polymers within thermal management and beyond. The potential applications of these films are vast, marking an exciting era for polymer science and material engineering.