FUNDAMENTALS OF POLYMER ENGINEERING doc

Plastics Waste Recovery of Economic Value, Jacob Letdner 2 Polyester Molding Compounds, Robert Burns 3 Carbon Black-Polymer Composites The Physics of Electrically Conducting Composites,

West Virginia University

Morgantown, West Virginia, U.S.A.

M A R C E L

MARCEL DEKKER, INC NEW YORK • BASEL

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ISBN: 0-8247-0867-9

The first edition was published as Fundamentals of Polymers by McGraw-Hill, 1997

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PRINTED IN THE UNITED STATES OF AMERICA

Founding Editor

Donald E Hudgin

ProfessorClemson UniversityClemson, South Carolina

1 Plastics Waste Recovery of Economic Value, Jacob Letdner

2 Polyester Molding Compounds, Robert Burns

3 Carbon Black-Polymer Composites The Physics of Electrically Conducting

Composites, edited by Enid Keil Sichel

4 The Strength and Stiffness of Polymers, edited byAnagnostis £ Zachanades

and RogerS Porter

5 Selecting Thermoplastics for Engineering Applications, Charles P Dermott

Mac-6 Engineering with Rigid PVC Processabihty and Applications, edited by I Luis

Gomez

7 Computer-Aided Design of Polymers and Composites, D H Kaelble

8 Engineering Thermoplastics Properties and Applications, edited by James

M Margolis

9 Structural Foam A Purchasing and Design Guide, Bruce C Wendle

10 Plastics in Architecture A Guide to Acrylic and Polycarbonate, Ralph

Montella

11 Metal-Filled Polymers Properties and Applications, edited by Swapan K

Bhattacharya

12 Plastics Technology Handbook, Manas Chanda and Salil K Roy

13 Reaction Injection Molding Machinery and Processes, F Melvin Sweeney

14 Practical Thermoforming Principles and Applications, John Flonan

15 Injection and Compression Molding Fundamentals, edited by Avraam I

Isayev

16 Polymer Mixing and Extrusion Technology, Nicholas P Cheremismoff

17 High Modulus Polymers Approaches to Design and Development, edited by

Anagnostis E Zachanades and Roger S Porter

18 Corrosion-Resistant Plastic Composites in Chemical Plant Design, John H

Mallinson

19 Handbook of Elastomers New Developments and Technology, edited by Anil

K Bhowmick and Howard L Stephens

20 Rubber Compounding Principles, Materials, and Techniques, Fred W

Barlow

21 Thermoplastic Polymer Additives Theory and Practice, edited by John T

Lutz, Jr

22 Emulsion Polymer Technology, Robert D Athey, Jr

23 Mixing in Polymer Processing, edited by Chns Rauwendaal

24 Handbook of Polymer Synthesis, Parts A and B, edited by Hans R

Kncheldorf

26 Plastics Technology Handbook: Second Edition, Revised and Expanded,

Manas Chanda and Salil K Roy

27 Prediction of Polymer Properties, Jozef Bicerano

28 Ferroelectric Polymers: Chemistry, Physics, and Applications, edited by Hari

Singh Nalwa

29 Degradable Polymers, Recycling, and Plastics Waste Management, edited

by Ann-Christine Albertsson and Samuel J Huang

30 Polymer Toughening, edited by Charles B Arends

31 Handbook of Applied Polymer Processing Technology, edited by Nicholas P.

Cheremisinoff and Paul N Cheremisinoff

32 Diffusion in Polymers, edited by P Neogi

33 Polymer Devolatilization, edited by Ramon J Albalak

34 Anionic Polymerization: Principles and Practical Applications, Henry L Hsieh

and Roderic P Quirk

35 Cationic Polymerizations: Mechanisms, Synthesis, and Applications, edited

by Krzysztof Matyjaszewski

36 Polyimides: Fundamentals and Applications, edited by Malay K Ghosh and

K L Mittal

37 Thermoplastic Melt Rheology and Processing, A V Shenoy and D R Saini

38 Prediction of Polymer Properties: Second Edition, Revised and Expanded,

Jozef Bicerano

39 Practical Thermoforming: Principles and Applications, Second Edition,

Revised and Expanded, John Florian

40 Macromolecular Design of Polymeric Materials, edited by Koichi Hatada,

Tatsuki Kitayama, and Otto Vogl

41 Handbook of Thermoplastics, edited by Olagoke Olabisi

42 Selecting Thermoplastics for Engineering Applications: Second Edition,

Revised and Expanded, Charles P MacDermott and Aroon V Shenoy

43 Metallized Plastics: Fundamentals and Applications, edited by K L Mittal

44 Oligomer Technology and Applications, Constantin V Uglea

45 Electrical and Optical Polymer Systems: Fundamentals, Methods, and

Applications, edited by Donald L Wise, Gary E Wnek, Debra J Trantolo,

Thomas M Cooper, and Joseph D Gresser

46 Structure and Properties of Multiphase Polymeric Materials, edited by Takeo

Araki, Qui Tran-Cong, and Mitsuhiro Shibayama

47 Plastics Technology Handbook: Third Edition, Revised and Expanded,

Manas Chanda and Salil K Roy

48 Handbook of Radical Vinyl Polymerization, Munmaya K Mishra and Yusuf

Yagci

49 Photonic Polymer Systems: Fundamentals, Methods, and Applications,

edited by Donald L Wise, Gary E Wnek, Debra J Trantolo, Thomas M Cooper, and Joseph D Gresser

50 Handbook of Polymer Testing: Physical Methods, edited by Roger Brown

51 Handbook of Polypropylene and Polypropylene Composites, edited by

Ricardo Diaz-Calleja, Margarita G Prolongo, Rosa M Masegosa, and alma Salom

Cat-56 Handbook of Polycarbonate Science and Technology, edited by Donald G

LeGrand and John T Bendler

57 Handbook of Polyethylene Structures, Properties, and Applications, Andrew

61 Handbook of Elastomers Second Edition, Revised and Expanded, edited

by Anil K Bhowmick and Howard L Stephens

62 Polymer Modifiers and Additives, edited by John T Lutz, Jr, and Richard F

Grossman

63 Practical Injection Molding, Bernie A Olmstea and Martin E Davis

64 Thermosetting Polymers, Jean-Pierre Pascault, Henry Sautereau, Jacques

Verdu, and Roberto J J Williams

65 Prediction of Polymer Properties Third Edition, Revised and Expanded, Jozef

Bicerano

66 Fundamentals of Polymer Engineering Second Edition, Revised and

Expanded, Anil Kumar and Rakesh K Gupta

Additional Volumes in Preparation

Handbook of Plastics Analysis, edited by Hubert Lobo and Jose Bonilla Metallocene Catalysts in Plastics Technology, Anand Kumar Kulshreshtha

Anil Kumar

To the memory of my father Rakesh Gupta

Preface to the Second Edition

The objectives and organization of the second edition remain essentiallyunchanged The major difference from the first edition is the inclusion ofnew material on topics such as dendrimers, polymer recycling, Hansensolubility parameters, nanocomposites, creep in glassy polymers, and twin-screw extrusion New examples have been introduced throughout the book,additional problems appear at the end of each chapter, and references to theliterature have been updated Additional text and figures have also been added.The first edition has been successfully used in universities around theworld, and we have received many encouraging comments We hope thesecond edition will also find favor with our colleagues, and be useful to futuregenerations of students of polymer science and engineering

Anil KumarRakesh K Gupta

v

Preface to the First Edition

Synthetic polymers have considerable commercial importance and are known

by several common names, such as plastics, macromolecules, and resins.These materials have become such an integral part of our daily existence that

an introductory polymer course is now included in the curriculum of moststudents of science and engineering We have written this book as the maintext for an introductory course on polymers for advanced undergraduates andgraduate students The intent is to provide a systematic coverage of theessentials of polymers

After an introduction to polymers as materials in the first two chapters,the mechanisms of polymerization and their effect on the engineering design

of reactors are elucidated The succeeding chapters consider polymer acterization, polymer thermodynamics, and the behavior of polymers asmelts, solutions, and solids both above and below the glass transitiontemperature Also examined are crystallization, diffusion of and throughpolymers, and polymer processing Each chapter can, for the most part, be

char-vii

read independently of the others, and this should allow an instructor to designthe course to his or her own liking Note that the problems given at the end ofeach chapter also serve to complement the main text Some of these problemscite references to the literature where alternative viewpoints are introduced Wehave been teaching polymer science for a long time, and we have changed thecourse content from year to year by adopting and expanding on ideas of thekind embodied in these problems.

Since polymer science is an extremely vast area, the decision to include

or exclude a given subject matter in the text has been a difficult one In thisendeavor, although our own biases will show in places, we have been guided

by how indispensable a particular topic is to proper understanding We haveattempted to keep the treatment simple without losing the essential features;for depth of coverage, the reader is referred to the pertinent technical literature.Keeping the student in mind, we have provided intermediate steps in mostderivations For the instructor, lecturing becomes easy since all that iscontained in the book can be put on the board The future will tell to whatextent we have succeeded in our chosen objectives

We have benefited from the comments of several friends and colleagueswho read different parts of the book in draft form Our special thanks go toAshok Khanna, Raj Chhabra, Deepak Doraiswamy, Hota V S GangaRao,Dave Kofke, Mike Ryan, and Joe Shaeiwitz Professor Khanna has used theproblem sets of the first seven chapters in his class for several years.After finishing my Ph.D from Carnegie-Mellon University, I (AnilKumar) joined the Department of Chemical Engineering at the Indian Institute

of Technology, Kanpur, India, in 1972 My experience at this place has beenrich and complete, and I decided to stay here for the rest of my life I amfortunate to have a good set of students from year to year with whom I havebeen able to experiment in teaching various facets of polymer science andmodify portions of this book continuously

Rakesh Gupta would like to thank Professor Santosh Gupta for cing polymer science to him when he was an undergraduate student Thisinterest in polymers was nurtured by Professor Art Metzner and Dr K F.Wissbrun, who were his Ph.D thesis advisors Rakesh learned even more fromthe many graduate students who chose to work with him, and their contribu-tions to this book are obvious Kurt Wissbrun reviewed the entire manuscriptand provided invaluable help and encouragement during the final phases ofwriting Progress on the book was also aided by the enthusiastic support ofGene Cilento, the Department Chairman at West Virginia University Rakeshadds that these efforts would have come to nought without the determined help

introdu-of his wife, Gunjan, who guarded his spare time and allowed him to devote it

entirely to this project According to Rakesh, ‘‘She believed me when I toldher it would take two years; seven years later she still believes me!’’

I doubt that this book would ever have been completed without theconstant support of my wife, Renu During this time there have been severalanxious moments, primarily because our children, Chetna and Pushkar, weretrying to choose their careers and settle down In taking care of them, my rolewas merely helping her, and she allowed me to divide my attention betweenhome and work Thank you, Renu

Anil KumarRakesh Gupta

xi

References 39

Appendix 3.1: The Solution of MWD Through the

Generating Function Technique in Step-Growth

4.2 Analysis of Semibatch Reactors 156

Appendix 4.1: Similarity Solution of Step-Growth

6.4 Recycling and Degradation of Polymers 285

Appendix 6.1: Solution of Equations Describing

9.2 Criteria for Polymer Solubility 376

12 Mechanical Properties 487

14.9 Constitutive Behavior of Dilute Polymer Solutions 605

In fact, exploitation of many of these unique properties has made polymersextremely useful to mankind They are used extensively in food packaging,clothing, home furnishing, transportation, medical devices, information technol-ogy, and so forth Natural fibers such as silk, wool, and cotton are polymers and

TABLE1.1 Some Common Polymers

have been used for thousands of years Within this century, they have beensupplemented and, in some instances, replaced by synthetic fibers such as rayon,nylon, and acrylics Indeed, rayon itself is a modification of a naturally occurringpolymer, cellulose, which in other modified forms have served for years ascommercial plastics and films Synthetic polymers (some common ones are listed

find extensive applications as plastics, films, adhesives, and protective coatings Itmay be added that biological materials such as proteins, deoxyribonucleic acid(DNA), and mucopolysaccharides are also polymers Polymers are worth study-ing because their behavior as materials is different from that of metals and otherlow-molecular-weight materials As a result, a large percentage of chemists andengineers are engaged in work involving polymers, which necessitates a formalcourse in polymer science

Biomaterials [3] are defined as materials used within human bodies either

as artificial organs, bone cements, dental cements, ligaments, pacemakers, orcontact lenses The human body consists of biological tissues (e.g., blood, cell,proteins, etc.) and they have the ability to reject materials which are ‘‘incompa-tible’’ either with the blood or with the tissues For such applications, polymericmaterials, which are derived from animals or plants, are natural candidates andsome of these are cellulosics, chitin (or chitosan), dextran, agarose, and collagen.Among synthetic materials, polysiloxane, polyurethane, polymethyl methacry-

late, polyacrylamide, polyester, and polyethylene oxides are commonly employedbecause they are inert within the body Sometimes, due to the requirements ofmechanical strength, selective permeation, adhesion, and=or degradation, evennoncompatible polymeric materials have been put to use, but before they areutilized, they are surface modified by biological molecules (such as, heparin,biological receptors, enzymes, and so forth) Some of these concepts will bedeveloped in this and subsequent chapters.

This chapter will mainly focus on the classification of polymers; quent chapters deal with engineering problems of manufacturing, characteriza-tion, and the behavior of polymer solutions, melts, and solids

subse-1.2 CLASSIFICATION OF POLYMERS AND SOME

FUNDAMENTAL CONCEPTS

One of the oldest ways of classifying polymers is based on their response to heat

In this system, there are two types of polymers: thermoplastics and thermosets Inthe former, polymers ‘‘melt’’ on heating and solidify on cooling The heating andcooling cycles can be applied several times without affecting the properties.Thermoset polymers, on the other hand, melt only the first time they are heated.During the initial heating, the polymer is ‘‘cured’’; thereafter, it does not melt onreheating, but degrades

A more important classification of polymers is based on molecularstructure According to this system, the polymer could be one of the following:

1 Linear-chain polymer

2 Branched-chain polymer

3 Network or gel polymer

It has already been observed that, in order to form polymers, monomers musthave reactive functional groups, or double or triple bonds The functionality of agiven monomer is defined to be the number of these functional groups; doublebonds are regarded as equivalent to a functionality of 2, whereas a triple bond has

a functionality of 4 In order to form a polymer, the monomer must be at leastbifunctional; when it is bifunctional, the polymer chains are always linear It ispointed out that all thermoplastic polymers are essentially linear molecules,which can be understood as follows

In linear chains, the repeat units are held by strong covalent bonds, whiledifferent molecules are held together by weaker secondary forces When thermalenergy is supplied to the polymer, it increases the random motion of themolecules, which tries to overcome the secondary forces When all forces areovercome, the molecules become free to move around and the polymer melts,which explains the thermoplastic nature of polymers

Branched polymers contain molecules having a linear backbone withbranches emanating randomly from it In order to form this class of material,the monomer must have a capability of growing in more than two directions,which implies that the starting monomer must have a functionality greater than 2.For example, consider the polymerization of phthalic anhydride with glycerol,where the latter is tri-functional:

COCO

O

CHOH

C

O

C

OO

CH CH2

(1.2.1)

CH2OH

CH2OH

The branched chains shown are formed only for low conversions of monomers.This implies that the polymer formed in Eq (1.2.1) is definitely of low molecularweight In order to form branched polymers of high molecular weight, we mustuse special techniques, which will be discussed later If allowed to react up tolarge conversions in Eq (1.2.1), the polymer becomes a three-dimensionalnetwork called a gel, as follows:

CH2O

O

O

COO

O

CO

(1.2.2)

O

COO

In fact, whenever a multifunctional monomer is polymerized, the polymer evolvesthrough a collection of linear chains to a collection of branched chains, whichultimately forms a network (or a gel) polymer Evidently, the gel polymer doesnot dissolve in any solvent, but it swells by incorporating molecules of the solventinto its own matrix.

Generally, any chemical process can be subdivided into three stages [viz.chemical reaction, separation (or purification) and identification] Among thethree stages, the most difficult in terms of time and resources is separation Wewill discuss in Section 1.7 that polymer gels have gained considerable importance

in heterogeneous catalysis because it does not dissolve in any medium and theseparation step reduces to the simple removal of various reacting fluids In recenttimes, a new phase called the fluorous phase, has been discovered which isimmiscible to both organic and aqueous phases [4,5] However, due to the highcosts of their synthesis, they are, at present, only a laboratory curiosity Thisapproach is conceptually similar to solid-phase separation, except that fluorousmaterials are in liquid state

In dendrimer separation, the substrates are chemically attached to thebranches of the hyper branched polymer (called dendrimers) In these polymers,

(A) CH2 CHCO2Me(B) NH2CH2CH2NH2(Excess)

Repeat steps (A) and (B)

NH2N

Initiatorcore

‘Dendrimers’

Generations

Dendrimerrepeating units

the extent of branching is controlled to make them barely soluble in the reactionmedium Dendrimers [6] possess a globular structure characterized by a centralcore, branching units, and terminal units They are prepared by repetitive reactionsteps from a central initiator core, with each subsequent growth creating a newgeneration of polymers Synthesis of polyamidoamine (PAMAM) dendrimers aredone by reacting acrylamide with core ammonia in the presence of excessethylene diamine.

Dendrimers have a hollow interior and densely packed surfaces They have

a high degree of molecular uniformity and shape These have been used asmembrane materials and as filters for calibrating analytical instruments, andnewer paints based on it give better bonding capacity and wear resistance Itssticking nature has given rise to newer adhesives and they have been used ascatalysts for rate enhancement Environmental pollution control is the other field

in which dendrimers have found utility A new class of chemical sensors based onthese molecules have been developed for detection of a variety of volatile organicpollutants

In all cases, when the polymer is examined at the molecular level, it isfound to consist of covalently bonded chains made up of one or more repeat units.The name given to any polymer species usually depends on the chemical structure

of the repeating groups and does not reflect the details of structure (i.e., linearmolecule, gel, etc.) For example, polystyrene is formed from chains of the repeatunit:

is polystyrene, it would continue to be represented by

CH2 CH nCH

values The end chemical groups X and Y could be the same or different, andwhat they are depends on the chemical reactions initiating the polymer formation.

Up to this point, it has been assumed that all of the repeat units that make

up the body of the polymer (linear, branched, or completely cross-linked networkmolecules) are all the same However, if two or more different repeat units make

up this chainlike structure, it is known as a copolymer If the various repeat unitsoccur randomly along the chainlike structure, the polymer is called a randomcopolymer When repeat units of each kind appear in blocks, it is called a blockcopolymer For example, if linear chains are synthesized from repeat units A and

B, a polymer in which A and B are arranged as

is called an AB block copolymer, and one of the type

is called an ABA block copolymer This type of notation is used regardless of themolecular-weight distribution of the A and B blocks [7]

The synthesis of block copolymers can be easily carried out if functionalgroups such as acid chloride ( COCl), amines ( NH2), or alcohols ( OH) arepresent at chain ends This way, a polymer of one kind (say, polystyrene orpolybutadiene) with dicarboxylic acid chloride (ClCO COCl) terminal groupscan react with a hydroxy-terminated polymer (OH OH) of the other kind (say,polybutadiene or polystyrene), resulting in an AB type block copolymer, asfollows:

functional groups Another common way of preparing block copolymers is toutilize organolithium initiators As an example, sec-butyl chloride with lithiumgives rise to the butyl lithium complex,

which reacts quickly with a suitable monomer (say, styrene) to give the followingpolystyryl anion:

Graft copolymers are formed when chains of one kind are attached to thebackbone of a different polymer A graft copolymer has the following generalstructure:

Here
(A)n constitutes the backbone molecule, whereas polymer (B)n israndomly distributed on it Graft copolymers are normally named poly(A)-g-poly(B), and the properties of the resultant material are normally extremelydifferent from those of the constituent polymers Graft copolymers can begenerally synthesized by one of the following schemes [1]:

The ‘‘grafting-from’’ technique In this scheme, a polymer carrying activesites is used to initiate the polymerization of a second monomer Depending onthe nature of the initiator, the sites created on the backbone can be free-radical,anion, or Ziegler–Natta type The method of grafting-from relies heavily on thefact that the backbone is made first and the grafts are created on it in a secondpolymerization step, as follows:

CH2CH

(1.2.13)

In this case, grafting does not involve a chain reaction and is best carried out in

a common solvent homogeneously An advantage of this technique is that itallows structural characterization of the graft copolymer formed because thebackbone and the pendant graft are both synthesized separately If the molecularweight of each of these chains and their overall compositions are known, it ispossible to determine the number of grafts per chain and the average distancebetween two successive grafts on the backbone

The ‘‘grafting-through’’ scheme In this scheme, polymerization with amacromer is involved A macromer is a low-molecular-weight polymer chainwith unsaturation on at least one end The formation of macromers has recentlybeen reviewed and the techniques for the maximization of macromer amount

discussed therein [4] A growing polymer chain can react with such anunsaturated site, resulting in the graft copolymer in the following way:

This type of grafting can introduce linkages between individual molecules ifthe growing sites happen to react with an unsaturated site belonging to two ormore different backbones As a result, cross-linked structures are also likely to beformed, and measures must be taken to avoid gel formation

There are several industrial applications (e.g., paints) that require us

to prepare colloidal dispersions of a polymer [5] These dispersions are in aparticle size range from 0.01 to 10mm; otherwise, they are not stable and, over aperiod of time, they sediment If the polymer to be dispersed is already available

in bulk, one of the means of dispersion is to grind it in a suitable organic fluid Inpractice, however, the mechanical energy required to reduce the particle sizebelow 10mm is very large, and the heat evolved during grinding may, at times,melt the polymer on its surface The molten surface of these particles may causeagglomeration, and the particles in colloidal suspensions may grow and subse-quently precipitate this way, leading to colloidal instability As a variation of this,

it is also possible to suspend the monomer in the organic medium and carry outthe polymerization We will discuss these methods in considerable detail in

problem of agglomeration of particles exists even in these techniques

Polymer colloids are basically of two types: lyophobic and lyophilic Inlyophilic colloids, polymer particles interact with the continuous fluid and withother particles in such a way that the forces of interaction between two particleslead to their aggregation and, ultimately, their settling Such emulsions areunstable in nature Now, suppose there exists a thermodynamic or steric barrierbetween two polymer particles, in which case they would not be able to comeclose to each other and would not be able to agglomerate Such colloids arelyophobic in nature and can be stable for long periods of time In the technology

of polymer colloids, we use special materials that produce these barriers to givethe stabilization of the colloid; these materials are called stabilizers If we wanted

to prepare colloids in water instead of an organic solvent, then we could use soap(commonly used for over a century) as a stabilizer The activity of soap is due toits lyophobic and lyophilic ends, which give rise to the necessary barrier for theformation of stable colloids

In several recent applications, it has been desired to prepare colloids inmedia other than water There is a constant need to synthesize new stabilizers for

a specific polymer and organic liquid system Recent works have shown that theblock and graft copolymers [in Eqs (1.2.5) and (1.2.11)] give rise to the neededstability It is assumed that the A block is compatible with the polymer to besuspended and does not dissolve in the organic medium, whereas the B blockdissolves in the organic medium and repulses polymer particles as in Figure 1.1.Because of the compatibility, the section of the chain consisting of A-repeat unitsgets adsorbed on the polymer particle, whereas the section of the chain havingB-repeat units projects outward, thus resisting coalescence

Example 1.1: Micellar or ampliphilic polymers (having hydrophobic as well ashydrophilic fragments in water) have the property of self-organization What arethese and how are they synthesized?

Solution: Micellar polymers have properties similar to surfactant molecules, andbecause of their attractive properties, they are used as protective colloids,emulsifiers, wetting agents, lubricants, viscosity modifiers, antifoaming agents,pharmaceutical and cosmetic formulating ingredients, catalysts, and so forth [8]

FIGURE1.1 Stabilizing effect of graft and block copolymers

Micellar polymers can have six types of molecular architecture, and in thefollowing, hydrophobic and hydrophilic portions are shown by a chain and acircle, respectively, exactly as it is done for ordinary surfactant (i.e., tail and headportions).

(a) Block copolymer

(d) Dendrimer

(e) Segmented block copolymer

N+(CH2)16

n

Example 1.2: Describe polymers as dental restorative materials and theirrequirements

Solution: The dental restorative polymers must be nontoxic and exhibit term stability in the presence of water, enzymes, and various oral fluids Inaddition, it should withstand thermal and load cycles and the materials should beeasy to work with at the time of application The first polyacrylolyte material usedfor dental restoration was zinc polycarboxylate To form this, one uses zinc oxidepowder which is mixed with a solution of polyacrylic acid The zinc ions cross-link the polyacid chains and the cross-linked chains form the cement.

long-Another composition used for dental restoration is glass ionomer cement(GIC) The glass used is fluoroaluminosilicate glass, which has a typicalcomposition of 25–25 mol% SiO2, 14–20 wt% Al2O3, 13–35 wt% CaF2, 4–

6 wt% AlF3, 10–25% AlPO4, and 5–20% Na3AlF6 In the reaction with acrylic acid, the latter degrades the glass, causing the release of calcium andaluminum ions which cross-link the polyacid chains The cement sets around theunreacted glass particles to form a reaction-bonded composite The fluorinepresent in the glass disrupts the glass network for better acid degradation.Completely polymeric material used for dental restoration is a polymer ofmethyl methacrylate (MMA), bisphenol-A, glycidyl methacrylate (bis GMA),and triethylene glycol dimethacrylate (TEGDMA) The network thus formed hasboth hydrophilic as well as hydrophobic groups and can react with teeth as well,giving a good adhesion In order to further improve the adhesion by interpene-tration and entanglements into dental surfaces, sometimes additives like 4-META(4-methoxyethyl trimellitic anhydride), phenyl-P (2-methacryloxy ethyl phenylhydrogen phosphate), or phenyl-P derivatives are added

poly-Example 1.3: Anticancer compounds used in chemotherapy are weight compounds, and on its ingestion, it is not site-specific to the canceroustissues leading to considerable toxicity How can polymer help reduce toxicity?How does this happen? Give a few examples

low-molecular-Solution: Macromolecules are used as carriers, on whose backbone both theanticancerous compounds as well as the targeting moieties are chemically bound

As a result of this, the drug tends to concentrate near the cancerous tissues Thetargeting moieties are invariably complementary to cell surface receptors orantigens, and as a result of this, the carrier macromolecule can recognize (orbiorecognize) cancerous tissues The polymer-mediated drug now has a consider-ably altered rate of uptake by body cells as well as distribution of the drug withinthe body

Some of the synthetic polymers used as drug carriers are HPMA (poly hydroxy propyl methacrylamide), PGA (poly L-glutamic acid), poly(L-lysine),and Block (polyethylene glycol coaspartic acid) Using HPMA, the followingdrugs have been synthesized [9]:

2-Drug Targeting moietyAbriamycin GalactosamineDuanomycin Anti-IakantibodiesChlorin e6 anti-Thy 1.2 antibody

By putting the targeting moiety to the polymer, one has created an ability inthe polymer to differentiate between different biological cells and recognizetumour cells [10] This property is sometimes called molecular recognition andthis technique can also be used for separating nondesirable components fromfoods or fluids (particularly biological ones)

The general technique of creating molecular recognition (having like activity) is called molecular imprinting Templates are defined as biologicalmacromolecules, micro-organisms, or whole crystals When functional mono-mers are brought in contact with the templates, they adhere to it largely because

antibody-of noncovalent bonding These could now be linked using a suitable linking agent If the templates are destroyed, the resulting cross-link polymercould have a mirror-image cavity of the template, functioning exactly like anantibody

cross-1.3 CHEMICAL CLASSIFICATION OF POLYMERS

BASED ON POLYMERIZATION MECHANISMS

In older literature, it was suggested that all polymers could be assigned to one ofthe two following classes, depending on the reaction mechanism by which theyare synthesized

1.3.1 Addition Polymers

These polymers are formed by sequential addition of one bifunctional orpolyfunctional monomer to growing polymer chains (say, Pn) without theelimination of any part of the monomer molecule With the subscript nrepresenting the chain length, the polymerization can be schematically repre-sented as follows:

M represents a monomer molecule; this chain growth step is usually very fast

The classic example of addition polymerization is the preparation of vinylpolymers Vinyl monomers are unsaturated organic compounds having thefollowing structure:

Ring-opening reactions, such as the polymerization of ethylene oxide to givepoly(ethylene oxide), offer another example of the formation of additionpolymers:

1.3.2 Condensation Polymers

These polymers are formed from bifunctional or polyfunctional monomers withthe elimination of a small molecular species This reaction can occur between anytwo growing polymer molecules and can be represented by

where Pmand Pn are polymer chains and W is the condensation product

Polyesterification is a good example of condensation polymerization In thesynthesis of poly(ethylene terephthalate), ethylene glycol reacts with terephthalicacid according to the following scheme:

COOH

OH + COOH

CH2

CH2OH

OO

O n + H2O

(1.3.6)

As indicated by the double arrow, polyesterification is a reversible reaction.Polyamides (sometimes called nylons) are an important class of condensationpolymers that are formed by reaction between amine and acid groups, as in

NH2 (CH2)6 NH2 + COOH (CH2)4 COOH

Hexamethylene diamine

Adipic acid

As researchers learned more about polymerization chemistry, it becameapparent that the notion of classifying polymers this way was somehow incon-sistent Certain polymer molecules could be prepared by more than onemechanism For example, polyethylene can be synthesized by either of the twomechanisms:

CH210

mBr

The latter is neither addition nor condensation polymerization Likewise, thefollowing reaction, which is a typical addition polymerization, gives the samepolyamide as reaction (1.3.7b):

(CH2)4

NHε-Caprolactam

(CH2)5 CO

Nylon 6

(1.3.9)

Similarly, the polymerization of polyurethane does not involve the evolution of

a condensation product, even though its kinetics can be described by that ofcondensation polymerization Clearly, it is not correct to classify polymersaccording to the scheme discussed earlier It is now established that there aretwo classes of polymerization mechanisms:

1 Chain-growth polymerization: an alternative, but more chemicallyconsistent name for addition polymerization

2 Step-growth polymerization: mechanisms that have kinetics of this typeexhibited by condensation polymerization but include reactions such asthat in (1.3.9), in which no small molecular species are eliminated.This terminology for discussing polymerization will be used in this textbook

In chain-growth polymerization, it is found that individual molecules startgrowing, grow rapidly, and then suddenly stop At any time, therefore, thereaction mass consists of mainly monomer molecules, nongrowing polymermolecules, and only a small number of rapidly growing polymer molecules Instep-growth polymerization, on the other hand, the monomer molecules react witheach other at the beginning to form low-molecular-weight polymer, and themonomer is exhausted very quickly They initially form low-molecular-weightpolymer molecules then continue to react with each other to form continuallygrowing chains The polymers formed from these distinct mechanisms haveentirely different properties due to differences in molecular-weight distribution,which is discussed in the following section

1.4 MOLECULAR-WEIGHT DISTRIBUTIONS

All commercial polymers have a molecular-weight distribution (MWD) In

of polymerization and reactor design In Chapter 8, we give some importantexperimental techniques to determine the molecular-weight distribution and itsaverages, and in view of the importance of this topic, we give some of the basicconcepts here The chain length n represents the number of repeat units in a givenpolymer molecule, including units at chain ends and at branch points (eventhough these units have a somewhat different chemical structure than the rest of

the repeat units) For chain molecules with molecular weights high enough to beclassified as true polymer molecules, there are at least one order of magnitudemore repeat units than units at chain ends and branch points It is thereforepossible to write (with negligible error)

In this representation, Wn* is the weight of a species of degree of polymerization nsuch that

Wt¼ Total weight of polymer

¼P1 n¼1

By definition, the weight-average molecular weight, Mw, is given by

Mw¼

P1 n¼1

Although Eqs (1.4.1)–(1.4.3) serve as the starting point for this discussion,

it is more useful to define a weight distribution of degrees of polymerization Wn

mw¼Mw

M0 ¼ Pt n¼1

It is thus seen that mw is just the first moment of the weight distribution of thedegree of polymerization.

There is an alternative but equivalent method of describing distributions ofmolecular weight If Nn* is the total number of moles of a polymer of chain lengthequal to n in a given sample, one can write

1.5 CONFIGURATIONS AND CRYSTALLINITY OF

POLYMERIC MATERIALS

So far, we have examined the broader aspects of molecular architecture in like molecules, along with the relationship between the polymerization mechan-

chain-ism and the repeat units making up the chain We have introduced the concept ofdistribution of molecular weights and molecular-weight averages.

As expected, the architectural features (branching, extent of cross-linking,nature of the copolymer) and the distribution of molecular weight play animportant role in determining the physical properties of polymers In addition,the geometric details of how each repeat unit adds to the growing chain is animportant factor in determining the properties of a polymer These geometricfeatures associated with the placement of successive repeat units into the chainare called the configurational features of the molecules, or, simply, chainconfiguration Let us consider the chain polymerization of vinyl monomers as

an example In principle, this reaction can be regarded as the successive addition

of repeat units of the type

CH2 CHR

(1.5.1)

where the double bond in the vinyl compound has been opened during reactionwith the previously added repeat unit There are clearly three ways that twocontiguous repeat units can be coupled

The head of the vinyl molecule is defined as the end bearing the organic group R.All three linkages might appear in a single molecule, and, indeed, the distribution

of occurrence of the three types of linkage would be one way of characterizing themolecular structure In the polymerization of vinyl monomers, head-to-tailplacement is favored, and this structural feature normally dominates

A more subtle structural feature of polymer chains, called stereoregularity,plays an important factor in determining polymer properties and is explained asfollows In a polymer molecule, there is usually a backbone of carbon atomslinked by covalent bonds A certain amount of rotation is possible around any ofthese backbone covalent bonds and, as a result, a polymer molecule can takeseveral shapes.Figure 1.3ashows three possible arrangements of the substituents