Bergstrom Mechanics of Solid Polymers

Theory and Computational Modeling
1. Auflage 2015
ISBN: 978-0-323-32296-6
Verlag: Elsevier Science & Techn.
Format: EPUB
Kopierschutz: 6 - ePub Watermark

E-Book, Englisch, 520 Seiten

Reihe: Plastics Design Library

Very few polymer mechanics problems are solved with only pen and paper today, and virtually all academic research and industrial work relies heavily on finite element simulations and specialized computer software. Introducing and demonstrating the utility of computational tools and simulations, Mechanics of Solid Polymers provides a modern view of how solid polymers behave, how they can be experimentally characterized, and how to predict their behavior in different load environments. Reflecting the significant progress made in the understanding of polymer behaviour over the last two decades, this book will discuss recent developments and compare them to classical theories. The book shows how best to make use of commercially available finite element software to solve polymer mechanics problems, introducing readers to the current state of the art in predicting failure using a combination of experiment and computational techniques. Case studies and example Matlab code are also included. As industry and academia are increasingly reliant on advanced computational mechanics software to implement sophisticated constitutive models - and authoritative information is hard to find in one place - this book provides engineers with what they need to know to make best use of the technology available. - Helps professionals deploy the latest experimental polymer testing methods to assess suitability for applications - Discusses material models for different polymer types - Shows how to best make use of available finite element software to model polymer behaviour, and includes case studies and example code to help engineers and researchers apply it to their work

Autoren/Hrsg.

Bergstrom, Jorgen S

Leseproben

1

Introduction and Overview

Abstract

Polymers are a broad class of materials that include traditional engineering materials such as elastomers (rubbers), thermoplastics, and most types of adhesives. In addition to these man-made materials, many natural and biological substances are also polymers, for example, DNA, protein, skin tissue, hair, and many more. Although these materials clearly behave differently from each other—based on our every-day experience—they have many important features in common with respect to their mechanical response. The goal of this text is to outline these characteristic features, and specify different ways the mechanical response can be predicted using analytical or computational tools.

Before embarking on a detailed discussion of these topics it is useful to have a basic understanding of the history of polymeric materials, and knowledge about the fundamentals of polymer processing and polymer mechanics. This chapter lays the foundation for the analysis that will follow and presents definitions and terminology that are needed for the following discussions.

Keywords

Elastomer

Thermoplastic

Adhesive Covalent bonds

Biopolymer

Polymer history

Polymer mechanics

Chapter Outline

1.1 Introduction   1

1.2 What Is a Polymer?   2

1.3 Types of Polymers   4

1.4 History of Polymers   7

1.5 Polymer Manufacturing and Processing   11

1.6 Polymer Mechanics   11

1.7 Exercises   15

References   16

1.1 Introduction

Polymers are a broad class of materials that include traditional engineering materials such as elastomers (rubbers), thermoplastics, and most types of adhesives. In addition to these man-made materials, many natural and biological substances are also polymers, for example, DNA, protein, skin tissue, hair, and many more. Although these materials clearly behave differently from each other—based on our every-day experience—they have many important features in common with respect to their mechanical response. The goal of this text is to outline these characteristic features, and specify different ways that the mechanical response can be predicted using analytical or computational tools.

Before embarking on a detailed discussion of these topics it is useful to have a basic understanding of the history of polymeric materials, and knowledge about the fundamentals of polymer processing and polymer mechanics. This chapter lays the foundation for the analysis that will follow and presents definitions and terminology that are needed for the following discussions.

1.2 What Is a Polymer?

To answer this question it is enlightening to first ask the complimentary question: what is not a polymer? It turns out that all solid materials can be classified into one of three basic types: metals, ceramics, or polymers. In addition to these basic material types, there are also two groups that can be considered subsets or combinations of these types: composite materials and semi-conductors.

Let us start by considering metals: a metal is a material in which the atoms are held together by metallic bonds (Figure 1.1). It is the delocalized electrons and the strong interaction forces between the positive atom nuclei that give the characteristic response of metals, such as good thermal and electrical conduction. The metallic bonds allow for relative sliding of large groups of atoms, enabling plastic deformation and ductility [1].

Figure 1.1 The atoms in a metal are held together by metallic bonds. The valence electrons are released from the atom nuclei and form an electron cloud.

Similarly, a ceramic material can be defined as a material in which the atoms are held together by ionic bonds created by positive and negatively charged ions (Figure 1.2). Many ceramic materials have excellent stiffness and compressive strength properties, particularly at high temperatures [2]. In ceramic materials, the electrons are tightly held giving poor conduction of heat and electricity.

Figure 1.2 In a ceramic material, the atoms are held together by ionic bonds created by charged ions.

The third major material type is polymers. As an informal definition, a polymer is a material with many different length scales (Figure 1.3).

Figure 1.3 The microstructure of a polymer contains many different length scales. On the most local level, the atoms are arranged into monomer units (a). The monomers are linked into chain molecules (b), which form a macromolecular network (c).

On the most local scale, the atoms are arranged into monomer units and bonded together using covalent bonds. The monomer units are then connected into long chain-like structures. The different macromolecules (sometimes called chain molecules) can be arranged into a network structure by crosslinks or entanglements, and they interact by weak van der Waals forces. The atoms of the polymer backbone are held together by covalent bonds that share electrons between atoms resulting in very strong bonds with very little electron mobility (Figure 1.4). Polymers are therefore typically poor conductors of heat and electricity. The weak bonds between the molecules create very interesting mechanical properties characterized by low stiffness and high ductility. The details of these characteristic behaviors and how they can be modeled are given in the following chapters of this book.

Figure 1.4 The atoms in a polymer molecule are held together by covalent bonds.

1.3 Types of Polymers

Due to the wide variety of polymers and polymer behaviors it is often useful to categorize polymers into different groups. One approach is to distinguish between natural polymers and synthetic polymers (Figure 1.5). Natural polymers, also called biopolymers, include a vast selection of materials. For example, all plants and animals are largely made from biopolymers. Plants are typically made from cellulose (e.g., cotton and wood) or starch (e.g., potatoes and carrots) both of which are polysaccharides. Another common natural polymer is protein, which is formed from amino acids. Other examples of biopolymers are: DNA, RNA, peptides, enzymes, skin, hair, silk, and chitin. Man-made polymers, also called synthetic polymers, include most traditional engineering polymers such as polypropylene (PP), polyethylene (PE), and nitrile rubber.

Figure 1.5 Examples of natural and synthetic polymers.

Another useful approach to categorize polymers is to distinguish between thermoplastics and thermosets (Figure 1.6). A thermoplastic is a polymer that is not permanently crosslinked and that softens and can be reshaped when heated. Thermoplastics can generally be exposed to repeated temperature cycles without undergoing significant degradation, making them suitable for recycling. A thermoset is a polymer that is crosslinked (cured) through the addition of energy, typically in the form of heat or irradiation. During the curing process the macromolecules are crosslinked and permanently included in a molecular network structure. Thermosets are generally stiffer and stronger than thermoplastics, but cannot be reshaped or melted.

Figure 1.6 Examples of thermoplastics and thermosets.

A third way to distinguish polymers is to separate amorphous and semicrystalline polymers (Figure 1.7). The polymer molecules in an amorphous polymer form an entangled network that is characterized by randomness and lack of long-range structure. In a semicrystalline polymer, parts of the molecular structure are crystalline and other parts are amorphous. The crystalline structure is typically considered to consist of layered lamellar crystals (Figure 1.8). One a larger scale, the amorphous and crystalline phases often aggregate to form supramolecular spherulites. When the crystallization occurs in the absence of flow or mechanical deformation, spherulites is the most common form of the crystal structure. The kinetics of polymer crystallization if very complex and still an active area of research. Amorphous and semicrystalline polymers often exhibit different mechanical behavior. Semicrystalline polymers have a true melting temperature (Tm) at which the crystalline domains break up and become disordered. Amorphous polymers do not have a melting temperature but softens significantly above their glass transition temperature (Tg). At temperatures above Tg, large segmental motions are activated and the polymer starts to behave liquid-like.

Figure 1.7 Examples of amorphous and semicrystalline polymers.

Figure 1.8 (a) Crystallizing polymers from layered structures called lamellae; (b) spherulines are supramolecular structures containing both lamellae and amorphous tie regions.

During the last few years it has started to become more important for the polymer industry to provide products that are sustainable and bio-friendly...