Thermal and mechanical properties of individual polymer nanofibers

2018-08-24T00:00:00Z (GMT) by Ramesh Shrestha
Polymer nanofibers have garnered significant attention due to their nanoscale size effects.<br>When their diameter is below ~1 μm, mechanical and thermal properties such as Young’s modulus,<br>tensile strength, and thermal conductivity are enhanced by several times to several orders of<br>magnitude. This notable enhancement in the material properties coupled with their intrinsic<br>properties such as low density, chemical resistance, and biocompatibility open applications in<br>tissue engineering, sensors, textiles, composite reinforcements, ballistic armors, thermal<br>management and other areas. The objective of this thesis is to study the thermal, mechanical and<br>thermo-mechanical properties of individual nanofibers and couple the properties with their<br>molecular structure.<br>Stress-induced crystallization using two-stage tip drawing technique is known to produce<br>highly crystalline and oriented polymer nanofiber. However, it is time-consuming, low yield and<br>lacks consistency. In this thesis, a local stretching technique is developed to produce highly<br>crystalline and oriented polyethylene nanofibers consistently. Microstructure characterization<br>using a transmission electron microscope (TEM) and micro-Raman analysis verified the evolution<br>of microstructure from semi-crystalline to highly crystalline from microfiber to nanofiber.<br>The thermal transport in PE microfibers and nanofibers was studied using a previously<br>demonstrated suspended micro-thermal device. Temperature-dependent thermal conductivity was<br>measured over a broad temperature range from 20 K to 560 K. PE thermal conductivity increased<br>from the bulk to the microfiber and then to the nanofiber form, consistent with an increase in<br>crystallinity and molecular orientation. The PE nanofiber thermal conductivity increased with<br>increasing temperature following an unusual ~T1 trend below 100 K, peaked around 130–150 K<br>reaching a metal-like value of 90 W m-1 K-1, and then decayed as T-1. It was found that thermal transport in aligned PE chain bundles is highly anisotropic and is dominated by the chain backbone<br>since the inter-chain Van der Waals interactions are much weaker than the covalent bonding along<br>the backbone. The thermal contact resistance between a PE nanofiber and the suspended thermal<br>device was found to be significant. A capillary-induced van der Waal contact method was<br>developed to enhance grip and thermal contact. The experimentally measured thermal contact<br>resistance was found to be consistent with the thermal contact resistance predicted using a line<br>contact model.<br>A fully reversible thermal switching was discovered at 430 K in crystalline PE nanofibers<br>due to a temperature-induced structural phase transition from the orthorhombic to the hexagonal<br>lattice structure. The phase transition introduces segmental rotational disorder along the chain and<br>leads to a switching factor (i.e., the ratio between on-state high and off-state low thermal<br>conductance values) as high as 10 before and after the phase transition, which exceeds any<br>previously reported experimental values for solid-solid or solid-liquid phase transition of<br>materials. The phase transformation was found to be thermally stable. A high-performance<br>nanoscale thermal diode was fabricated by creating a heterogeneous amorphous-crystalline PE<br>nanofiber junction. A thermal rectification factor of 25 % was achieved, comparable to the existing<br>solid-state nanoscale thermal diodes based on carbon nanotubes, boron nitride nanotubes, graphene<br>and VO2 nanobeams.<br>The tensile strength of individual PE nanofiber was tested in tension using a<br>microelectromechanical system (MEMS) based device with an on-chip actuator. Since the<br>crystalline polymer is sensitive to high-energy electron beams, an optical metrology based on subpixel<br>pattern matching was employed. In the tensile tests, PE nanofibers could not be firmly<br>gripped using a variety of adhesives because of the low surface energy of PE. Instead, slip occurred before they were tested to failure. A microscale dog bone shape on a PE nanofiber was<br>fabricated to provide additional grip by mechanical locking. The tensile strength of 11.4 ± 1.1 GPa<br>was obtained for the nanofiber with a diameter of 85 nm. To our knowledge, this is the highest<br>measured tensile strength for any polymer-based fiber including carbon fiber, Zylon, Kevlar and<br>nylon fibers.<br>Polymer nanofibers exhibit viscoelastic behavior which is both dependent on time and<br>temperature. A variable stress-based creep measurement technique was developed to remove the<br>necessity of the feedback to keep a creep stress constant. From the temperature-dependent creep<br>compliance curves, a master curve spanning 30 years was developed for polyacrylonitrile (PAN)<br>nanofibers. A thin nanofiber (150 nm) exhibited an order of magnitude less creep compared to a<br>thick fiber (250 nm) after 30 years at room temperature. The reduction in creep compliance for the<br>thin fiber was attributed to the increased orientation within the core molecules. After removing the<br>orientation of core PAN molecules by the exposure to high energy electron beam, higher creep<br>compliance than that of the oriented sample was obtained. This was because of the globally lesser<br>orientation of the PAN molecules. <br>