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Polymeric Materials-Structure, Properties, and Applications

C.V. Pious,  and Sabu Thomas     International and Interuniversity Center for Nanoscience and Nanotechnology, Mahatma Gandhi University, Kottayam, India

Abstract

Polymers dominate all other classes of materials due to their wide spectrum of properties and possibility of further modification by the addition of ingredients. This chapter gives a brief introduction to polymeric materials, followed by a discussion on their structure, properties, and applications. The structural features of polymeric chains at the molecular level and ways to control polymer architecture are discussed. Different properties of polymers such as thermal, electrical, and mechanical are presented in brief. Properties and applications of some important polymers and some specialty polymers are also included.

Keywords

Application of polymers; Electrical properties; Mechanical properties; Polymer structure; Thermal properties

Outline

2.1 Introduction 21

2.2 Structure of a Polymer 25

Configuration 25

Conformation 26

2.2.1 Classification of Polymers 26

2.2.1.1 Crystalline and Amorphous Polymers 26

2.2.1.2 Thermoplastics and Thermosetting Plastics 26

2.2.1.3 Homopolymers and Copolymers 27

2.2.2 Designing the Structure of Polymers 27

2.3 Properties of Polymers 28

2.3.1 Thermal Properties 28

2.3.1.1 Thermal Transitions of Polymers 28

2.3.1.2 Thermal Stability of Polymers 29

2.3.1.3 Coefficient of Thermal Expansion and Thermal Conductivity 29

2.3.2 Mechanical Properties 31

2.3.3 Electrical Properties of Polymers 32

2.3.4 Polymer Blends 32

2.3.5 Polymer Composites 33

2.3.6 Polymer Nanocomposites 33

2.4 Application of Polymers 33

2.4.1 Application of Commodity Polymers 33

2.4.1.1 Polyethylene 34

2.4.1.2 Polypropylene 34

2.4.1.3 Polyvinylchloride 35

2.4.1.4 Polystyrene 35

2.4.2 Applications of Engineering Polymers 35

2.4.2.1 Polyamides 35

2.4.2.2 Polybutylene Terephthalate 35

2.4.2.3 Acrylonitrile Butadiene Styrene 36

2.4.2.4 Polyoxymethylene or Polyacetals 36

2.4.3 Polymers for Specialty Applications 36

2.4.3.1 Polymers in Electronic Applications 37

2.4.3.2 Biomedical Applications 37

2.4.3.3 Polymers in Sensor Applications 37

2.5 Conclusion 37

References 37

2.1. Introduction

Polymers are materials of the twentieth century. They have substituted other materials in most of the applications because of their wide property spectrum. Nowadays, most of the articles that we use in our day-to-day lives, such as carry bags, packing materials, pens, beverage bottles, containers, cloths, furniture, adhesives, and syringes, are made of polymers. Polymers also have found application in engineering components such as gears and structural components. Their properties can be easily tuned by the selection of a proper synthetic route or conditions and incorporation of additives during product manufacturing. Most polymers can be processed very easily in comparison with other classes of materials. This enables the manufacturer to produce a larger number of goods with less energy consumption. Even though many concerns are arising on the disposal of polymeric waste, it is impossible to find a class of materials that can substitute them. The term polymer is derived from two Greek words: poly means many, and meros means parts. As the name suggests, polymers are large molecules formed by the chemical reaction of smaller molecules or repeating units. These repeating units are termed monomers. The size of the molecule or molar mass can vary from thousands to millions, depending on the monomer- and polymer-synthesizing method. Animal and plant bodies contain many polymeric materials such as cellulose, polypeptides, and starch. People were using these materials for various applications like construction works, shelter, apparels, and household articles before the invention of synthetic polymers and these are still in use. The era of synthetic polymers started in the late-nineteenth century. Initially, these polymers appeared as by-products of organic reactions. Scientists considered them as colloids until the introduction of the macromolecular hypothesis to the scientific world in 1920 by Staudinger (Flory, 1953). The synthesis and science of polymeric materials evolved enormously in the twentieth century. Some of the milestones in polymer science are listed below: 1846-Synthesis of cellulose nitrate, 1844-Invention of vulcanization of natural rubber, 1866-Discovery of polystyrene by Berthelot, 1907-Manufacture of the first synthetic rubbers by polymerization of conjugated dienes, 1920-Formulation of the macromolecular hypothesis, 1942-Propounding of the Flory Huggins theory of polymeric solutions, 1953-Invention of the Zeigler Natta Catalyst, 1955-Establishment of a relationship between the relaxation time of chains and the deviation from the glass transition temperature, 1974-Development of aromatic polyamides, 1991-Awarding of the Nobel Prize to Pierre-Gilles de Gennes for discovering that methods developed for studying order phenomena in simple systems can be generalized to polymers, 1994-Development of atom transfer radical polymerization (ATRP), 2000-Awarding of the Nobel Prize to H. Shirakawa, A.J. Heeger, and A.G. McDiarmid for the invention of intrinsic conductive polymers, 2005-Awarding of the Nobel Prize in Chemistry to Y. Chauvin, R. Grubbs, and R. Schrock, for their work on the metathesis reaction and its application to polymers. Polymers are synthesized by a polymerization process, wherein individual units or monomers react together to form bigger polymer molecules. A molecule with two or more reactive sites can be a monomer unit, so that it can form a long chain or a crosslinked network. The number of reactive sites in a monomer is termed its functionality. It is one of the important parameters that govern polymer structures. Polymerization reactions are generally classified as condensation or step polymerizations and addition polymerizations. Normally, in condensation polymerization, monomers with functional groups can undergo condensation reactions to form a polymer chain. Examples for such condensation reactions are esterification, amidation, and formation of urethanes and aromatic substitution. Other than these reactions, condensation polymers can also be prepared by ring opening polymerization. Preparation of Nylon 66 from hexamethylenediamine and adipic acid is an example of a condensation polymerization reaction. The condensation polymerization reaction proceeds in a stepwise manner. During polymerization, the monomer units first react together to form oligomers that have a lower molecular weight than that of the polymer. These oligomers react together to form a high-molecular-weight polymer at the end of the polymerization reaction. Chain growth polymerizations are characterized by the occurrence of activated species or active centers that can polymerize monomer units. Unsaturated monomers can be polymerized through this method. Activated centers are either free radicals generated by the homolytic scission of an organic compound termed initiators or ions/highly polarized molecules. These active centers can attack the double bond of a monomer followed by the generation of a new active center. This process is called initiation. Newly generated active centers can attack monomer units, thereby generating new active centers. This process is termed propagation. Other addition reactions include ionic polymerization wherein the initiation and propagation proceed via the attack of a double bond by ions instead of attack by a free radical. Propagation continues till the termination or death of the active center. Polymerization of propylene (CH2CH(CH3)) to polypropylene (-[CH2-CH(CH3)]n-) is an example for chain growth polymerization. Unlike in step polymerization, in chain growth mechanism, the molecular weight increases with the polymerization reaction time (Odian, 2004). Some examples for polymers and their chemical structures are given in Table 2.1.

Table 2.1

Chemical Structures of Different Polymers

Polyethylene Polyethylene glycol Poly(ethylene terephthalate) Polyvinylchloride (PVC) Polytetrafluoroethylene Poly(acrylic acid) Poly(methyl methacrylate) Polyacrylonitrile Poly(ethyl...