The chemical structure of a polymer usually is represented by that of the repeat unit enclosed by brackets. Table 1. The naming of polymers or envisaging the chemical structure of a polymer from its name is often an area of difficulty. At least in part this is because most polymers have more than one correct name, the situation being further complicated by the variety of trade-names which also are used to describe certain polymers.
The approach adopted here is to use narnes which most clearly and simply indicate the chemical structures of the polymers under discussion. The names given to the polymers in Table 1. Thus sourcc-based nomenclature places thc prcix 'poly' bcforc thc namc of the monomer, the monomer's name being containcd within parentheses unless it is a simple single word. In structurc-based nomenclature the prefix poly is followed in parentheses by words which describe the chemical structure of the repeat unit.
This type of nomenclature is used for polymers nine and ten in Table 1. However, in accordance with use of the word homopolymer, it is common practice to use a structure-based definition. Thus the word copolymer more commonly is used to describe polymers whose molecules contain two or more different types of repeat unit.
Hence polymers nine and ten in Table 1. There are several categories of copolymer, each being characterized by a particular form of arrangement of the repeat units along the polymer chain. For simplicity, the representation of these categories will be illustrated by copolymers containing only two different types of repeat unit A and B.
Statistical copolymers are copolymers in which the sequential distribution of the repeat units obeys known statistical laws e. Random copolymers are a special type of statistical copolymer in which the distribution of repeat units is truly random sorne words of caution are necessary here because older textbooks and scientific papers often use the term random copolymer to describe both random and non-random statistical copolymers.
Statistical, random and alternating copolyrners generally have properties which are intermediate to those of the corresponding homopolymers. Thus by preparing such copolymers it is possible to combine the desirable properties of the homopolymers into a single material. This is not normally possible by blending because most homopolymers are immiscible with each other. Block copolymers are linear copolymers in which the repeat units exist onlyin long sequences, or blocks, of the same type.
Two common block. Graft copolymers are branched polymers in which the branches have a different chemical structure to that of the main chain. In their simplest form they consist of a main homopolymer chain with branches of a different homopolymer BBB8BB 1. In distinct contrast to the types of copolymer described earlier, block and graft copolymers usually show properties characteristic of each of the constituent homopolymers.
They also have sorne unique properties arising from the chemical linkage s between the homopolymer sequences preventing them from acting entirely independently of each other. The current principies of nomenclature for copolymers are indicated in Table 1. Thus a statistical copolymer of ethylene and propylene. In certain cases, additional square brackets are requircd. Thermoplastics are then further separated into those which are crystalline and those which are amorphous i.
This method of classification has an advantage in comparison to others since it is based essentially upon the underlying molecular structure of the polymers. Thermoplastics, often referred to justas plastics, are linear or branched polymers which can be melted upon the application of heat. They can be moulded and remoulded into virtually any shape using processing techniques such as injection moulding and extrusion, and now constitute by far the largest proportion of the polymers used in industry.
Generally, thermoplastics do not crystallize easily upon cooling to the solid state because this requires considerable ordering of the highly coiled and entangled macromolecules present in the liquid state. Those which do crystallize invariably do not form perfectly crystalline materials but instead are semi-crystalline with both crystalline and amorphous regions. The crystalline phases of such polymers are characterized by their melting temperature T,,,. Man y thermoplastics are, however, completely amorphous and incapable of crystallization, even upon annealing.
Amorphous polymers and amorphous phases of semi-crystalline polymers are characterized by their glass transition temperature T,J, the temperature at which they transform abruptly from the glassy state hard to the rubbery state soft. This transition corresponds to the onset of chain motion; below T,: the polymer chains are unable to move and are 'frozen' in position. Both T,,, and T,: increase with increasing chain stiffness and increasing forces of intermolecular attraction.
Elustomcrs are crosslinkcd rubbcry polymcrs i. This extremely important and uscful property is a reflection of their molecular structure in which the network is of low crosslink density. The rubbery polymer chains become extended upon deformation but are prevented from permanent flow by the crosslinks, and driven by entropy, spring back to their original positions on removal of the stress.
The word rubber, often used in place of elastomer, preferably should be used for describing rubbery polymers which are not crosslinked. Thermosets normally are rigid materials and are network polymers in which chain motion is greatly restricted by a high degree of crosslinking.
As for elastomers, they are intractable once formed and degrade rathcr than melt upon the application of heat. This normally is done by measuring the molar mass M of a polymer which is simply the mass of 1 mole of the polymer and usually is quoted in units of g mo or kg The term 'molecular weight' is still often used instead of molar mass, but is not preferred because it can be somewhat misleading.
By multiplying the numerical value of molecular weight by the specific units g it can be converted into the equivalent value of molar mass. For example, a molecular weight of is equivalent to a molar mass of g which in turn is equivalent to a molar mass of kg mol-,1 For network polymers the only meaningful molar mass is that of the polymer chains existing between junction points i. For copolymers the sum of the products xM0 for each type of repeat unit is required to define the molar mass.
Since the molar mass changes in intervals of M0, the distribution of molar mass is discontinuous. However, for most polymers these intervals are extremely small in comparison to the total range of molar mass and the distribution can be assumed to be continuous, as exemplified in Fig. These usually are defined by considering the discontinuous nature of the distribution in which the macromolecules exist in discrete fractions i containing N; molecules of molar mass M;.
The number-average molar mass M ,, is defined as 'the sum of the products of the molar mass of each fraction multiplied by its mole fraction'. Therefore it follows that. The weight fraction w; is defined as the mass of molecules of molar mass M1 divided by the total mass of ali the molecules present. The weight-average molar mass M is defined as 'the sum of the products of the molar mass of each fraction multiplied by its weight fraction'.
Higher molar mass averages sometimes are quoted. For example , certain methods of molar mass measurement e. In addition, more complex exponent averages can be obtained e. Degree of polymerization averages are of more importance than molar mass averages in the theoretical treatrnent of polymers and polymerization, as will be highlighed in the su9sequent chapters. For homopolymers they may be obtained simply by dividing the corresponding molar mass average by M0 Thus the number-average and weight-average degrees of polymerization are given by.
Further reading Billmeyer, F. Cowie, J. Elias, H-G. Jenkins, A. Booth and C. Price , Pergamon Press, Oxford. Kaufrnan, M. Mandelkern, L. Mark , H. Morawetz, H. Treloar, L. Given this relatively simple requirement, there are a multitude of chemical reactions and associated monomer typcs that can be used to effect polymerization.
To discuss each of these individually would be a major task which fortunately is not necessary since it is possible to place most polymerization reactions in one of two classes, each having distinctive characteristics. The classification used in the formative years of polymer science was due to Carothers and is based upon comparison of the molecular formula of a polymer with that of the monomer s from which it was formccl.
This usually arises from chemical reactions which involve the elimination of a small molecule e. H20, HCl.
Addition polymerizations are those which yield polymers with repeat units having identical molecular formulae to those of the monomers from which they are formed. Table I. Carothers' method of classification was found to be unsatisfactory when it was recognized that certain condensation polymerizations have the characteristic features of typical addition polymerizations and that sorne addition polymerizations have features characteristic of typical condensation polymerizations.
A better basis for classification is provided by considering the underlying polymerization mechanisms, of which there are two general types. Polymerizations in which the polymer chains grow step-wise by reactions that can occur between any two molecular species are known as step-growth polymerizations.
Polymerizations in which a polymer chain grows only by reaction of monomer with a reactive end-group on the growing chain are known as chain-growth polymerizations, and usually require an initial reaction between the monomer and an initiator to start the growth of the chain. There has been a tendency in recent years to change these names to step potymerization and chuin polymerization, and this practice will be used here. The essential. Open navigation menu. Close suggestions Search Search.
User Settings. Skip carousel. Carousel Previous. Carousel Next. What is Scribd? Young, P. Original Title R. Lovell-Introduction to Polymers 2nd Printing of 2nd Ed. Did you find this document useful? Is this content inappropriate? Report this Document. Flag for inappropriate content. Download now. Save Save R. Lovell-Introduction to Polymers For Later. Original Title: R. Related titles.
Carousel Previous Carousel Next. Jump to Page. Search inside document. Introduction to Polymers Second Edition R. Characterization 3. Structure Polymer crystals 4. The presentation of sorne theories and Preface to the First Edition experimental results has been changed from the original approach for the sake of clarity and consistency of style.
The polymeric materials described so far are serni-synthetic since they lntroduction to Polymcrs are produccd rom natural polymcrs. Whilst Polymer Science is now considered to be a mature subject, its breadth is ever increasing and there are many demanding challenges awaiting scientists who venture into this fascinating multidisciplinary science. Branched polymers have side chains, or branches, of significant length which are bonded to the main chain at branch points also known as Linear Fig 1.
V, ::s QJ :e 'o :e :;. Introduction 11 Elustomcrs are crosslinkcd rubbcry polymcrs i. Therefore it follows that M,. The weight fraction w; is defined as the mass of molecules of molar mass M1 divided by the total mass of ali the molecules present r.
I"' ,,wM 1. Documents Similar To R. Maggie Antu. Jin Zhang. Ws Lim. Polymer science and engineering is a major international academic and industrial discipline. The current importance of this field, as well as its development over the last six decades, is reflected in the introduction to polymers third edition solutions manual pdf, and also the fact that there exist at least fifty volumes dedicated to polymer science and technology. Of those, more than thirty cover the basic fundamentals, whereas only four cover polymers from a more life-sciences perspective.
On the other hand, applications-oriented texts such as Polymer Science and Technology: An Introduction Fourth Edition place less emphasis on. The focus here is instead on how the properties are derived from the polymer structure. New chapters have been added on polymers in biological systems, emerging technologies in polymer science, supramolecular polymers, defects in macromolecules and polymer defects.
Thoroughly updated, Introduction to Polymers, Third Edition presents the science underpinning the synthesis, characterization and properties of polymers. The material has been completely reorganized and expanded to include important new topics and provide a coherent platform for teaching and learning the fundamental aspects of contemporary polymer science. Polymerization mechanisms have been made more explicit by showing electron movements.
Part II In this part, the authors have added new topics on diffusion, solution behaviour of polyelectrolytes and field-flow fractionation methods. In addition, the Flory—Huggins theory for polymer solutions and their phase separation is treated more rigorously.
Part III A completely new, major topic in this section is multicomponent polymer systems. The book also incorporates new material on macromolecular dynamics and reptation, liquid crystalline polymers and thermal analysis.
Many of the diagrams and micrographs have been updated to more clearly highlight features of polymer morphology. Part IV The last part of the book contains major new sections on polymer composites, such as nanocomposites, and electrical properties of polymers.
Other new topics include effects of chain entanglements, swelling of elastomers, polymer fibres, impact behaviour and ductile fracture. Coverage of rubber-toughening of brittle plastics has also been revised and expanded.
0コメント