Thermal Drawing of Polymer Nano-composites: Fluid Dynamic Analysis and Application to Novel Functional Fibers

The thermal drawing process is emerging as a versatile platform for the development of multimaterial fiber devices and advanced textiles for sensing, energy harvesting or medical applications, owing to the possibility to integrate insulators, conductors and various types of functional materials together in complex architectures. This field of research remains relatively recent and still has unexploited potential and unanswered questions. In particular, the lack of transparent conducting material compatible with thermal drawing has so far limited the development of optoelectronic fiber devices. It has indeed hindered the ability to fabricate the types of optoelectronic devices that require a transparent electrode, most notably photovoltaic cells. Furthermore, conductive polymer composites based on carbon black have been utilized for years as an alternative to crystalline metals that can maintain finer geometries, crystalline metals being limited by capillary instabilities due to their low viscosity in the melted state during thermal drawing. However, the observation that the conductivity of composites depends on drawing conditions has thus far not been explained through an understanding of the underlying mechanism. In this thesis, we investigate the production and thermal drawing of thermoplastic nanocomposites based on carbon black, but also on high aspect ratio conductive fillers such as silver nanowires and carbon nanotubes. The latter are indeed suitable to fabricate transparent conducting films, and we show that we can find appropriate compositions and drawing conditions to integrate them in multimaterial fibers. After looking into the thermal drawing of such composites in more details, we propose an advanced fluid dynamic analysis that accurately models the thermal drawing process, with new insights on the radial dependency of the axial velocity. We design a way to visualize the deformation in the neckdown region and show that our model compares well with the experimental observation. We use this model to further develop the theoretical approach to account for the influence of thermal drawing on the conductivity of nanocomposites. The impact of both the draw ratio and the radial position observed experimentally is explained quantitatively by the results of the model. Our approach can potentially be applied to other types of materials whose properties change under the deformation due to thermal drawing. From these findings, we demonstrate a variety of novel composite-based fiber devices. We fabricated a functional photodetecting fiber using a transparent electrode made of a carbon nanotube composite, paving the way towards the development of novel optoelectronic fiber devices that integrate transparent electrodes. Moreover, we present a new type of touch sensing fiber device that relies on a freely moving conductive domain made of a carbon black nanocomposite. Such type of electronic fiber device is capable of both detecting a pressure applied and localizing it along its entire length. We can envision applications in soft electronics or healthcare with individual fiber devices or by embedding them in smart textiles.