Advances in Polymer Nanocomposites

Types and Applications

Fengge Gao

• Publisher: Woodhead Publishing
• Year: 2012
• Print ISBN:
  9781845699406
• eBook ISBN:
  9780857096241
• Topic: Technology & Engineering
• Subtopic: Materials Science
• Index: Technology & Engineering

The addition of nanoparticles to polymer composites has led to a new generation of composite materials with enhanced and novel properties. Advances in polymer nanocomposites reviews the main types of polymer nanocomposites and their applications.Part one reviews types of polymer nanocomposites according to fillers. Processing of carbon nanotube-based nanocomposites, layered double hydroxides (LDHs) and cellulose nanoparticles as functional fillers and reinforcement are discussed, alongside calcium carbonate and metal-polymer nanocomposites. Part two focuses on types of polymer nanocomposites according to matrix polymer, with polyolefin-based, (PVC)-based, nylon-based, (PET)-based and thermoplastic polyurethane (TPU)-based polymer nanocomposites discussed. Soft, gel and biodegradable polymer nanocomposites are also considered. Part three goes on to investigate key applications, including fuel cells, aerospace applications, optical applications, coatings and flame-retardant polymer nanocomposites.With its distinguished editor and international team of expert contributors, Advances in polymer nanocomposites is an essential guide for professionals and academics involved in all aspects of the design, development and application of polymer nanocomposites. - Reviews the main types of polymer nanocomposites and their applications - Discusses processing of carbon nanotube-based nanocomposites, layered double hydroxides (LDHs) and cellulose nanoparticles as functional fillers and reinforcement - Discusses polyolefin-based, (PVC)-based, nylon-based, (PET)-based and thermoplastic polyurethane (TPU)-based polymer nanocomposites

Part I

Types of polymer nanocomposites according to fillers

1

Processing of nanotube-based nanocomposites

V.M. Karbhari, University of Alabama in Huntsville, USA

C.T. Love, U.S. Naval Research Laboratory, USA

Abstract:

With the continuing developments in materials synthesis and characterization at the nanoscale the potential of true materials tailoring has been enhanced substantially. In general this class of materials involves structures that have at least one dimension at the nanometer scale (usually taken to be up to 100 nanometers). The utilization of nanoscale fibers in polymer composites not only enables the development of uniquely created structures but also provides a means for the development of unique properties and functionalities at levels not possible with conventional fiber reinforced composites. It is the goal of this chapter to introduce the advances in carbon nanotube (CNT) nanocomposite research with specific emphasis on processing routes used to disperse, align and fabricate carbon nanotube reinforced polymer nanocomposites for enhanced physical and mechanical properties. Acknowledging that the potential for CNTs as reinforcement and conductive media has not yet been fully realized, a discussion on future trends is also given.

Key words

carbon nanotubes

composites

alignment

processing

performance attributes

1.1 Introduction

1.1.1 Nanoscience and composites

With the continuing developments in materials synthesis and characterization at the nanoscale the potential of true materials tailoring has been enhanced substantially. In general this class of materials involves structures that have at least one dimension at the nanometer scale (usually taken to be up to 100 nanometers). Remarkable progress has been made regarding the use of carbon nanotubes (CNTs) to reinforce polymer matrices since the helical tube geometry of carbon nanotubes was first discovered by Iijima in 1991 (Iijima, 1991). As the name implies, CNTs are cylindrical tube structures of varying lengths made of carbon atoms. They exist as either single-walled carbon nanotubes (SWCNT) or multi-walled carbon nanotubes (MWCNT) where individual tubes are nested concentrically inside one another like tree rings. These unique nanostructures are considered one-dimensional due to their high aspect ratio (length-to-diameter) leading to superior mechanical and electrical properties. The diameter and chirality of the CNTs produce either metallic (conducting) or semi-conducting nanotubes. Additionally, their anisotropic nature makes CNTs interesting reinforcing fibers for multi-functional ultra-light, high strength and stiffness composite materials and devices. Compared with carbon fibers, the very high modulus versions of which have elastic moduli of over 750 GPa, the elastic moduli of CNTs have been measured in the range of 1–4.7 TPa (Cooper et al., 2002; Lau et al., 2006). The tensile strength is also approximately two orders of magnitude higher than that shown by currently available carbon fibers (Cooper et al., 2002). In addition, the change in diameter of these materials also results in a significant increase in surface area for the same volume fraction in a composite, leading to a variety of very interesting characteristics. For example, a 30 nm diameter nanotube has 150 times more surface area than a 5 μm diameter carbon fiber for the same filler volume fraction (Eitan et al., 2006).

The utilization of these fibers in polymer composites not only enables the development of uniquely created structures but also provides a means for the development of unique properties and functionalities at levels not possible with conventional fiber reinforced composites. Several comprehensive reviews of carbon nanotube reinforced polymer composites have been published over the last few years (Bal and Samal, 2007; Coleman et al., 2006; Hussain et al., 2006; Thostenson et al., 2001, 2005; Gibson et al., 2007; Xie et al., 2005; Wang et al, 2004). It is the goal of this chapter to introduce the advances in CNT nanocomposite research since the publication of these reviews with specific emphasis on processing routes used to disperse, align and fabricate carbon nanotube reinforced polymer nanocomposites for enhanced physical and mechanical properties.

1.1.2 Aspects for consideration

Just as on the macroscale, the properties of the nanocomposite are dictated by the distribution, orientation and fiber/matrix interactions. Because nanotubes tend to form clusters and bundles, the biggest challenges on the nanoscale are to fully disperse individual nanotubes in the matrices and achieve good interfacial adhesion between them and polymer for load transfer capabilities. The tendency for the reinforcement to agglomerate persists unless high shear forces are applied by vigorous mixing of the polymer. However, the mixing intensity must be controlled since overmixing often damages CNT structures, compromising their properties. Another issue is that the polymer-nanotube mixtures are highly viscous (due to the large surface area of nanotubes). This creates process-related problems, because the composites do not flow easily and are hence more difficult to mold. Viscosities of nanotube-filled polymers are known to show abrupt increases above fairly low loading thresholds following a Schulz-Blaschke type response. Processing is also hindered by the poor compatibility of nanotubes with most solvents and polymers. Nevertheless, several approaches have been successfully adopted to obtain intimate mixing of nanotubes with polymer phases, including dry powder mixing, melt mixing, polymerization of monomers onto and surrounding CNT surfaces, and surfactant-assisted mixing. More creative processing techniques are still needed. It should be noted that the potential of the mechanical, electrical, and thermal properties offered by nanotubes has not fully been realized and is mostly limited by processing methods. It is in this context that the current chapter provides a state-of-the-art review of the topic, highlighting important advances in a variety of processing routes, and ending with a brief identification of potential future directions.

1.2 Structure of carbon nanotubes

1.2.1 Carbon nanotube morphology

There are two primary types of CNTs available. SWCNTs consist of a single graphene sheet seamlessly wrapped into a cylindrical tube. The one-dimensional nature of the CNTs means that they exhibit electrical conductivity as high as copper, thermal conductivity as good as diamond, and strength levels as much as 100 times greater than steel at a fraction of the weight. The structure of MWCNTs can be thought of as concentrically nested SWCNTs where dimensions such as inner and outer tube diameter are important for strength and conduction. In most cases tube diameter is linearly proportional to tube thickness (due to concentric tube layering). MWCNTs offer higher stiffness than SWCNTs, especially in compression, due to the reinforcing efforts of centrically aligned tubes.

Single-walled carbon nanotubes can exist in three distinct structures: armchair, zigzag and chiral. The packing of the carbon hexagons in the graphitic sheets defines a chiral vector (m,n) and angle. The indices of the vector determine the morphology of the CNT (Hussain et al., 2006). Zigzag structures are identified by (n,0) type indices, while armchair are (m,n), and chiral are (n,m) where n ≠ m. When (m-n)/3 is an integer, the resulting structure is conductive and is termed ‘metallic’. Other variations have semiconductive properties. Therefore all armchair and one-third of zigzag CNTs are metallic, having a continuous conduction band (Hussain et al., 2006). The remaining two-thirds of the zigzag and chiral CNTs are semiconducting, having an energy gap in the conduction band (Hussain et al., 2006).

1.2.2 Synthesis of carbon nanotubes

The mechanisms by which CNTs may be synthesized can be grouped into two categories: ablation of graphite or decomposition of carbon-containing compounds. The main methods of CNT synthesis using graphite sublimation are direct-current arc discharge and l...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Contributor contact details
• Preface
• Part I: Types of polymer nanocomposites according to fillers
• Part II: Types of polymer nanocomposites according to base
• Part III: Applications of polymer nanocomposites
• Index