Mai / Yu Polymer Nanocomposites

1. Auflage 2006
ISBN: 978-1-84569-112-7
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: Adobe DRM (»Systemvoraussetzungen)

E-Book, Englisch, 608 Seiten

Reihe: Woodhead Publishing Series in Composites Science and Engineering

Polymer nanocomposites are polymer matrices reinforced with nano-scale fillers. This new class of composite materials has shown enhanced optical, electrical and dielectric properties. This important book begins by examining the characteristics of the main types of polymer nanocomposites and then reviews their diverse applications.Part one focuses on polymer/nanoparticle composites, their synthesis, optical properties and electrical conductivity. Part two describes the electrical, dielectric and thermal behaviour of polymer/nanoplatelet composites, whilst polymer/nanotube composites are the subject of Part three. The processing and industrial applications of these nanocomposite materials are discussed in Part four, including uses in fuel cells, bioimaging and sensors as well as the manufacture and applications of electrospun polymer nanocomposite fibers, nanostructured transition metal oxides, clay nanofiller/epoxy nanocomposites, hybrid epoxy-silica-rubber nanocomposites and other rubber-based nanocomposites.Polymer nanocomposites: physical properties and applications is a valuable reference tool for both the research community and industry professionals wanting to learn about these materials and their applications in such areas as fuel cell, sensor and biomedical technology. - Gives a comprehensive review of polymer nanocomposites and their properties - A standard reference on this area - Written by distinguished editors and a international team of authors

Dr Zhong-Zhen Yu is Professor of Physics and Materials at the City University of Hong Kong.

Autoren/Hrsg.

Mai, Yiu-Wing

Yu, Zhong-Zhen

Leseproben

2 Epoxy nanocomposites based on layered silicates and other nanostructured fillers
O. Becker; G.P. Simon    Monash University, Australia 2.1 Introduction
In the late 1930s Pierre Castan of Switzerland and Sylvian Greenlee from the United States independently synthesized the first bisphenol-A epichlorohydrin-based resin material. A few years later in 1946, the first industrially-produced epoxy resins were introduced to the market. Since then, the use of thermosetting polymers has steadily increased. The wide variety of epoxy resin applications include: coatings, electrical, automotive, marine, aerospace and civil infrastructure as well as tool fabrication and pipes and vessels in the chemical industry. Due to their low density of around 1.3 g/cm2 and good adhesive and mechanical properties, epoxy resins became a promising material for high performance applications in the transportation industry, usually in the form of composite materials such as fibre composites or in honeycomb structures. In the aerospace industry, epoxy-composite materials can be found in various parts of the body and structure of military and civil aircrafts, with the number of applications on the rise. A recent approach to improve and diversify polymer properties in the aerospace industry is through the dispersion of nanometer-scaled fillers in the polymer matrix.1 A significant number of academic and industrial projects have investigated the possibility to further improve epoxy resins (and in some cases composites or other binary systems) through the strategy of producing nanocomposites. This chapter reviews the published work on the use of layered silicates and other nanofillers to improve epoxy resin systems. The term ‘epoxy resin’ refers to both the prepolymer and its cured resin/hardener system. The former is a low molecular weight oligomer that contains one or more epoxy groups per molecule (more than one unit per molecule is required if the resultant material is to be crosslinked). The characteristic group, a three-membered ring known as the epoxy, epoxide, oxirane, glycidyl or ethoxyline group is highly strained and therefore very reactive. Epoxy resins can be cross-linked through a polymerization reaction with a hardener at room temperature or at elevated temperatures (latent reaction). Curing agents used for room temperature cure are usually aliphatic amines, whilst commonly-used higher temperature, higher performance hardeners are aromatic amines and acid anhydrides. However, an increasing number of specialized curing agents, such as polyfunctional amines, polybasic carboxylic acids, mercaptans and inorganic hardeners are also used. All of these result in different, tailored properties of the final polymer matrix. In general, the higher temperature cured resin systems have improved properties, such as higher glass transition temperatures, strength and stiffness, compared to those cured at room temperature. Figure 2.1 illustrates the simplified cure reaction of an epoxy resin with an amine hardener.2 The two different functional groups react during the initial conversion (Reaction I) and form a linear or branched polymer. The addition of the primary amine to an epoxide group leads to the formation of a hydroxyl group and a secondary amine, which continues until the primary amine groups are exhausted. Reaction II illustrates the crosslinking through the addition of secondary amines with epoxy groups, where the macromolecules develop a three-dimensional network. One of the most common side reactions is etherification (Reaction III), where a hydroxyl group reacts with an epoxide group, forming an ether linkage and a further hydroxyl group. The extent to which etherification takes place during cure depends on the structure and chemistry of the resin and hardener, as well as the cure conditions. When the branched structures extend throughout the whole system, the gel point is reached. At this characteristic point, the crosslinked resin does not dissolve in a suitable solvent of the parent resin, although a soluble (sol) fraction may still be extractable. Further, diffusion-controlled cure is required to increase the degree of crosslinking and to finally produce a structural material with a mechanical modulus of a vitrified or glassy solid material. The point at which the glass transition temperature of the growing network reaches the cure temperature is known as vitrification. 2.1 The three possible main reactions during cure of an epoxy resin with an amine – (I) primary amine-epoxy addition, (II) secondary amine-epoxy addition, (III) etherification2. (from Chiao, L., (1990) Macromolecules 23: 1286) 2.2 Epoxy-layered silicate nanocomposites
The use of nanostructured fillers in epoxy systems has gained significant importance in the development of thermosetting composites. One of the more widely studied nanocomposite strategies is the incorporation of layered silicates into the epoxy matrix. In comparison to other nanoparticles to be discussed later in this review, layered silicates belong to a unique group of nanofillers with only one dimension on the nanometer scale. The individual platelets of this filler are slightly below 1 nm in thickness, and the diameter of the platelets varies between 200 and 600 nm, these fillers being distinguished from other nanoscaled additives by their high aspect ratio. Layered silicate minerals belong to the structural group of swelling phyllosilicates or smectites. This group of minerals consists of periodic stacks of layers, which form tactoids between 0.1 and 1 µm.3 The crystal lattice of the individual silicate platelets is composed of two tetrahedral silica sheets that are merged at the tip to a central octahedral plane of alumina or silica4,5. Due to this repeating structure, these minerals are also often referred to as 2:1 phyllosilicates. Determined by isomorphous substitution of central anions of lower valences in both the tetrahedral and the octahedral plane, the layered silicates have a negative charge on their surfaces. This negative charge is counterbalanced by inorganic cations located in the interlayers or galleries. Further details about the crystallography of layered silicates can be found in the literature.4,5 2.2.1 Layered silicate surface modification
The untreated smectite mineral is strongly hydrophilic and hence is not suitable for the absorption of most organic molecules. It is the exchange reaction of the interlayer ions with organophilic ions that modify and tailor the layered silicates for the use as polymer filler. The key parameter for the modification of a layered silicate is its charge density, which determines the concentration of exchangeable ions in the galleries. Studies by Lan et al.6 and Kornmann et al.7 showed that layered silicate minerals with cation exchange capacities (CEC) of 60–100 mol-equivalent/100 g of the mineral (such as montmorillonite and hectorite minerals) gave better exfoliation after modification and cure compared to other clay minerals with higher CEC values. It has been theorized, that the differences in the degree of separation are related to the space available to the epoxy within the ion-populated silicate layers. Further details about the absorption of organics through cation exchanged layered silicates8,9 and the particular modification process of layered silicates for epoxy nanocomposite applications10–17 can be found in the literature. Another determining factor for the control of organoclay-epoxy interaction during in-situ polymerization is the nature of the interlayer exchanged ion. The number and the structure of these ions determine the initial space available and hence the accessibility of the resin/hardener monomers to the layered silicate galleries. Lan et al.6 varied the alkylammonium ion chain length of the layered silicate modification from 4 to 18 units. Investigation of the d-spacing of the organoclay showed that both the swelling of the clay by the resin before cure and the intercalation during cure were affected by the chain length of the interlayer exchanged ion. A minimum of eight methylene units was required to achieve nanocomposite formation in the final structure. Wang and Pinnavaia13 showed that the acidity and therefore the ability of the interlayer exchanged ion to act as a catalyser for the cure (and homopolymerization reaction) plays a key role in the nanocomposite formation. In their work it was shown for a series of primary to quaternary octadecylammonium modified layered silicates that the higher Brönsted-acid catalytic effect of the primary and secondary alkyl-ammonium ions gave larger interlayer increases during the cure process. A general overview of the intercalation process before and during cure is provided in Section 2.2.3. 2.2.2 Rheology of epoxy layered silicate network precursors
Whilst layered silicates are a relatively new type of filler to improve the materials performance of crosslinked thermosets, these minerals have been widely used for flow modification of coatings and paints, primarily to induce thixotropic behaviour. Organophilic layered silicates are widely known for their ability to swell in organic fluids and form a thixotropic gel. Mardis18 related thixotropy to the dispersed silicate platelets forming a three-dimensional network in the fluid via interplate hydrogen bonds of low bond energies....