Thermal and mechanical properties of individual polymer nanofibers

Abstract

Polymer nanofibers have garnered significant attention due to their nanoscale size effects.When their diameter is below ~1 μm, mechanical and thermal properties such as Young’s modulus,tensile strength, and thermal conductivity are enhanced by several times to several orders ofmagnitude. This notable enhancement in the material properties coupled with their intrinsicproperties such as low density, chemical resistance, and biocompatibility open applications intissue engineering, sensors, textiles, composite reinforcements, ballistic armors, thermalmanagement and other areas. The objective of this thesis is to study the thermal, mechanical andthermo-mechanical properties of individual nanofibers and couple the properties with theirmolecular structure.Stress-induced crystallization using two-stage tip drawing technique is known to producehighly crystalline and oriented polymer nanofiber. However, it is time-consuming, low yield andlacks consistency. In this thesis, a local stretching technique is developed to produce highlycrystalline and oriented polyethylene nanofibers consistently. Microstructure characterizationusing a transmission electron microscope (TEM) and micro-Raman analysis verified the evolutionof microstructure from semi-crystalline to highly crystalline from microfiber to nanofiber.The thermal transport in PE microfibers and nanofibers was studied using a previouslydemonstrated suspended micro-thermal device. Temperature-dependent thermal conductivity wasmeasured over a broad temperature range from 20 K to 560 K. PE thermal conductivity increasedfrom the bulk to the microfiber and then to the nanofiber form, consistent with an increase incrystallinity and molecular orientation. The PE nanofiber thermal conductivity increased withincreasing temperature following an unusual ~T1 trend below 100 K, peaked around 130–150 Kreaching 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 backbonesince the inter-chain Van der Waals interactions are much weaker than the covalent bonding alongthe backbone. The thermal contact resistance between a PE nanofiber and the suspended thermaldevice was found to be significant. A capillary-induced van der Waal contact method wasdeveloped to enhance grip and thermal contact. The experimentally measured thermal contactresistance was found to be consistent with the thermal contact resistance predicted using a linecontact model.A fully reversible thermal switching was discovered at 430 K in crystalline PE nanofibersdue to a temperature-induced structural phase transition from the orthorhombic to the hexagonallattice structure. The phase transition introduces segmental rotational disorder along the chain andleads to a switching factor (i.e., the ratio between on-state high and off-state low thermalconductance values) as high as 10 before and after the phase transition, which exceeds anypreviously reported experimental values for solid-solid or solid-liquid phase transition ofmaterials. The phase transformation was found to be thermally stable. A high-performancenanoscale thermal diode was fabricated by creating a heterogeneous amorphous-crystalline PEnanofiber junction. A thermal rectification factor of 25 % was achieved, comparable to the existingsolid-state nanoscale thermal diodes based on carbon nanotubes, boron nitride nanotubes, grapheneand VO2 nanobeams.The tensile strength of individual PE nanofiber was tested in tension using amicroelectromechanical system (MEMS) based device with an on-chip actuator. Since thecrystalline polymer is sensitive to high-energy electron beams, an optical metrology based on subpixelpattern matching was employed. In the tensile tests, PE nanofibers could not be firmlygripped 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 wasfabricated to provide additional grip by mechanical locking. The tensile strength of 11.4 ± 1.1 GPawas obtained for the nanofiber with a diameter of 85 nm. To our knowledge, this is the highestmeasured tensile strength for any polymer-based fiber including carbon fiber, Zylon, Kevlar andnylon fibers.Polymer nanofibers exhibit viscoelastic behavior which is both dependent on time andtemperature. A variable stress-based creep measurement technique was developed to remove thenecessity of the feedback to keep a creep stress constant. From the temperature-dependent creepcompliance curves, a master curve spanning 30 years was developed for polyacrylonitrile (PAN)nanofibers. A thin nanofiber (150 nm) exhibited an order of magnitude less creep compared to athick fiber (250 nm) after 30 years at room temperature. The reduction in creep compliance for thethin fiber was attributed to the increased orientation within the core molecules. After removing theorientation of core PAN molecules by the exposure to high energy electron beam, higher creepcompliance than that of the oriented sample was obtained. This was because of the globally lesserorientation of the PAN molecules.