Year

2012

Degree Name

Doctor of Philosophy

Department

The Intelligent Polymer Research Institute, Department of Chemistry

Abstract

The development of nanostructured fibres from organic conductors produced via wet-spinning technique is described in this thesis. The main objectives are (1) to develop the fundamental knowledge and understanding of organic conductors such as conducting polymers, single walled carbon nanotubes (SWNTs) and graphene, (2) to create multifunctional nanostructured fibres based on PEDOT:PSS/SWNTs and liquid crystals of graphene oxide (LC GO), (3) to study the effects of spinning formulation on the electrical conductivity, electrochemical performance and mechanical properties of the nanostructured fibres and (4) possible applications of these structures in fibre based energy storage systems.

A novel continuous wet-spinning approach was employed to spin a formulation consisting of an aqueous blend of PEDOT:PSS and low molecular weight poly(ethlylene glycol) (PEG) resulting in a 30-fold conductivity enhancement from 9 to 264 S cm-1 with respect to PEDOT:PSS fibres. This enhancement is attributed to an improved molecular ordering of the PEDOT chains in the direction of the fibre axis and the consequential enrichment of linear (or expanded-coil like) conformation to preferential bipolaronic electronic structures as evidenced by Raman spectroscopy, solid-state electron spin resonance (ESR) and in situ electrochemical ESR studies. Development of PEDOT:PSS/CNT nanostructured fibres are discussed thoroughly in chapter 4. The spinning formulations were prepared by mixing PEDOT:PSS with two types of SWNTs dispersions: (1) surfactant-based CNT dispersions (SDS-CNT) and (2) water soluble PEG functionalized nanotubes (PEG-CNT). Simultaneous increase in the mechanical properties, electrical conductivity and electrochemical performance of PEDOT:PSS/CNT fibres were achieved by using aggregate free and well-exfoliated SWNTs dispersions. The highest reinforcement rates of dY/dVf =417 GPa and dσ/dVf = 4GPa were obtained for PEG-CNT fibres at volume fraction (Vf) of ≤ 0.02. On the other hand, the highest electrical conductivity of 500 Scm-1 was obtained in the case of SDS-CNT fibres with Vf > 0.02. The highest achieved SDS-CNT (Vf = 0.11) and PEG-CNT (Vf = 0.12) loadings led to approximately eight-fold (77 F g-1) and two-fold (22 F g-1) increase in specific capacitance for PEDOT:PSS/CNT fibres, respectively.

In Chapter 5, this thesis demonstrates that LC GO dispersions in both water and organic solvents provide a viable route to continuously wet-spin fundamentally unlimited lengths of multifunctional and flexible pure GO and reduced GO (rGO) fibres. Rheological investigation confirmed a correlation between wet-spinnability and LC GO nematic phase formation. The size of the giant GO sheets (mean diameter = 37 μm) and their polydispersity (σ = 0.63) drives the production of wetspinnable and fully nematic LC GO dispersion at a remarkably dilute concentration of GO ≥ 0.75 mg ml-1. SEM analysis and birefringence of the gel-state fibres confirmed that LC ordered domains are oriented along the fibre axis. Electrical conductivity of ~18 S cm-1 was obtained by annealing GO fibres at 300 °C. rGO fibres with native conductivity of up to 1.4 S cm-1 were produced in a single step using NaOH as the coagulation bath. The temperature dependent thermal conductivities of rGO fibres were around 250-2250 W m-1 K-1. Galvanostatic chargedischarge testing showed reasonably high and stable capacitance of 104 to 430 F g-1 corresponding to practical current densities of 2 to 50 A g-1. Maximum power and energy densities of 100 kW kg-1 and 66 Wh kg-1 for working two-electrode fibre devices suggested promises for alternative flexible energy storage systems.

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