School of Polymer, Textile and Fiber Engineering.

PAN/SWNT Composite Fibers

Carbon Nanotube Research

Introduction

"In the early nineteen twenties, Herman Staudinger began to publish articles asserting that polymeric molecules are practically endless chains held together by ordinary chemical bonds" (D. A. Hounshell and J. K. Smith Jr., "Science and Corporate Strategy: DuPont R & D, 1902-1980", Cambridge University Press, 1988, p. 231).  Even though this idea of practically endless chains appeared foreign to most chemists of that time, these concepts ultimately led to the development of synthetic polymer and fiber industry, resulting in revolutionary developments in the field of polymeric materials. Developments in nano-scale science and technology and its applications to polymers and fibers suggest that we are at the initial stages of another revolutionary period, which will bring about polymeric and fibrous materials with properties not yet seen. The field of polymer/carbon nanotube composites alone is expected to open up new vistas in materials properties. Despite the fact that nearly 500 research papers have appeared on polymer carbon nanotube composites in 2002-2003 alone, the field is still in its infancy and is at the same state where carbon fiber based composites were in 1960s when carbon fibers were just being developed. The carbon fibers and its composites   manufacturing technology after 40 years of development has now matured. In last 40 years, carbon fiber cost has also come down dramatically. From almost no composites in aerospace structures in 1960s, we have now reached the stage that 50% weight of the new Boeing 787 will be composite materials. In 20+ years, carbon nanotube enabled materials may reach similar application levels, with the added difference that the carbon nanotube based materials will be much more functional than the carbon fiber  composites.

 

Supercapacitors

 

Supercapacitor is a kind of energy storage device, which is also known as electrical double layer capacitor, ultracapacitor and electrochemical capacitor. The general requirements for electrodes of supercapacitos are porous materials with a pore size distributions of 2 – 50nm, high specific surface area of  a few hundreds to thousands m2/g, good electrical conductivity, and chemical stability.  Carbonization and/or activation of polymer/SWNT composite films enable formation of film electrodes possessing these properties.

 

Polymer/SWNT composite films are carbonized to create a porous structure and these films are used to from the electrode for the supercapacitor.  Carbonization leads to an increase in pore volume and this subsequently leads to an increase in the specific surface area of the electrode.

 

Electrospinning

 

Electrospinning is a fiber forming process that generates nanoscale fibers.  In the electrospinning process, a droplet of polymer solution or melt is suspended at the tip of a needle or capillary due to the surface tension.  When a high voltage is applied to the tip, an electrical force is generated.    Once the electrical force is sufficient to overcome the surface tension of droplet, a jet will be ejected.  This jet will be collected on a grounded target as nanoscale fibers.   After evaporating the solvent from the fibers, an interconnected fiber mat is collected.  Since the jet is drawn and stretched by electrical force, thin fibers with diameter of several hundred nanometers can be obtained.

 

In recent years, electrospinning has attracted attention with the progress of nanotechnology.  Electrospinning may be an effective method to bridge the dimensional gap between nanoscopic and macroscopic structures.   For carbon nanotubes with small dimensions and superior properties, electrospinning is a good approach to incorporate and align carbon nanotubes into a polymer matrix to utilize their properties.

 

Electro-spun polymer/SWNT nano fibers where the polymer forms the outer layer on the otherwise oriented SWNT can find numerous applications including in electronic packaging (reduce interconnect size), filtration where core of the nano fiber is electrically conducting, as well as porous carbon coating on swnt or swnt bundle for supercapacitor application.

 

SWNT/Polymer Composites

 

Materials:

Films:

SWNT/polymer composite films are used in supercapacitor applications.  Composite films also allows for the utilization of the excellent optical properties of the SWNT.  Polymer/SWNT as well as pure SWNT films and coatings are being developed and studied for mechanical, electrical, thermal, and optical properties.

 

Fibers:

SWNTs are excellent candidates for producing composite fibers due to the unique combination of properties: high modulus, high tensile strength, large aspect ratio, flexibility and resilience, and chemical stability.  SWNT/polymer composite fibers have potential applications in structural composites used in civil, automobile and aerospace industry, electrical shielding device, flat panel display, and heat dissipation management, and optical devices. Therefore the processing and property study of SWNT/polymer composite fiber is very meaningful.

 

Bulk Composites:

Various materials (bulk composites, fibers, films, coatings) are being characterized for mechanical, electrical, optical, thermal and other properties and structure as appropriate.

 

Polymer Composites:
Polymer composites are being processed using thermoset as well thermoplastic polymers. Emphasis is  to process materials using fully exfoliated SWNTs. Attempts have been  made to develop SWNT dispersion/exfoliation
techniques both using solution and melt processing. Thermosets include epoxy and phenolic resins, Thermoplastics include both commodity as well as high performance thermoplastic resin.

 

Processing:

SWNT orientation:
SWNT orientation in various systems is being achieved using flow field, electrical field, as well as magnetic field.

 

Exfoliation:
It is recognized that to achieve full SWNT potential in a polymer matrix, they must be fully exfoliated, i.e. be dispersed as individual nanotube. Therefore the research focus is on approaches that are likely to lead to exfoliation in practically feasible way.

 

Properties and Characterization:

Solution Behavior of SWNT:

SWNT can be dispersed in aqueous media using surfactant, organic solvents, as well as in strong acids. A given batch of SWNT contains about 50 different types of tubes with varying diameter and chirality. The mixture includes metallic as well as semi-conducting tubes. SWNT dispersion can also be assisted by polymer. Both, binary (SWNT/solvent) and ternary systems (polymer/swnt/solvent) are being  studied. The emphasis is on identifying specific polymer and/or solvent interactions with a given type of tube as well as to develop conditions for nanotube exfoliation in various polymers. Both, experimental and theoretical approaches are being  considered.

Phase behavior is  looked into both from the point of view of thermodynamic equilibrium as well as from the point of view of forming kinetically stable dispersion.  

 

Structure and Property Study, Characterization:
Polymer crystallization in the presence of SWNT as well as polymer SWNT interaction is being  studied. The enhanced solvent resistance of the polymer in the presence of carbon nanotube is being further studied. As nanotube separation techniques develop, study of polymer and or solvent interaction with the specific type SWNT will also become feasible. Characterization tools may be developed as necessary.

 

Research Topic Continued: Page 2