Mechanical and Thermal Characterizations of Crystalline Polymer Micro/nanofibers

Abstract

For crystalline polymers, especially those in micro/nanoscale, the number of defects per unit volume is significantly lower than that in the bulk. With extended and aligned polymer chains, the resulting polymer fibers possess remarkably enhanced mechanical and thermal properties and approach the inherent properties of the carbon backbones which form the polymer chains. Together with other unique properties of polymers, such as, low density, easy processability, good biocompatibility, and electrical insulation, the crystalline fibers in micro/nano scale can be used in a broad range of applications, for example, heat spreaders in electronics, high strength ropes, and personnel armors. This dissertation studies various schemes of polymer crystallization, especially the stress induced crystallization in fiber drawing process. In this work, a two-stage drawing method is finally adopted to produce individual polyethylene (PE) nanofibers. To demonstrate the PE nanofibers possess more enhanced mechanical properties than the commercially available PE nanofibers, an atomic force microscopy (AFM) based force deflection spectroscopy (FDS) technique is explored to characterize the Young's modulus of the PE nanofibers. By attaching a PE nanofiber onto a specially designed micro trench, and deflecting the nanofiber with an AFM cantilever, we are able to deduce the Young's modulus from the geometry of the trench and the level of deflection on the nanofiber based on Bernoulli's beam equations. The experimentally proved Young's modulus of these nanofibers is 312GPa approaching the theoretical limit of the Young's modulus of PE single crystal. To study thermal properties of a polysilsesquioxane (PSQ) hybrid crystal, we apply a micro device based thermal characterization method. The micro device consists of two suspended SiNx membranes with built-on Pt coils; the two membranes serve as heater and thermometer during the measurements. The PSQ micro beam is placed between the two membranes. Due to the Joule heating on the heating membrane, heat transfers through the sample to the sensing membrane. By analyzing the steady state heat transfer model, we are able to calculate the thermal conductivity of the PSQ beam. The experimentally measured thermal conductivities greatly help us to understand the heat transfer mechanism in the PSQ hybrid crystal which is formed by hydrogen bonding in the longitudinal direction. With the same characterization method as used in PSQ thermal characterization, we also measure the thermal conductivity of PE nanofibers. We discover the thermal contact resistance between the nanofiber and the islands is comparable or even bigger than the intrinsic thermal resistance of the PE nanofibers with an assumed thermal conductivity of 20W/mK at room temperature. Cyanoacrylate based super glue and focus ion beam (FIB) assisted Pt deposition are attempted to reduce the thermal contact resistance, however, as demonstrated by the experiments, super glue is likely to lift the nanofiber above the islands which dramatically increases the thermal resistance. While the FIB assisted Pt deposition introduces great crystal damage on the PE fibers, which results in very low measured thermal conductivities (