1 Introduction 1.1 DEFINING POLYMERS Polymers are materials of very high molecular weight that are found to have multifarious applications in our modern society. They usually consist of several structural units bound together by covalent bonds [1,2]. For example, polyethy- lene is a long-chain polymer and is represented by ÀCH 2 CH 2 CH 2 À or ½ÀCH 2 CH 2 À n ð1:1:1Þ where the structural (or repeat) unit is ÀCH 2 ÀCH 2 À and n represents the chain length of the polymer. Polymers are obtained through the chemical reaction of small molecular compounds called monomers. For example, polyethylene in Eq. (1.1.1) is formed from the monomer ethylene. In order to form polymers, monomers either have reactive functional groups or double (or triple) bonds whose reaction provides the necessary linkages between repeat units. Polymeric materials usually have high strength, possess a glass transition temperature, exhibit rubber elasticity, and have high viscosity as melts and solutions. In fact, exploitation of many of these unique properties has made polymers extremely 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 1 Copyright © 2003 Marcel Dekker, Inc. TABLE 1.1 Some Common Polymers Commodity thermoplastics Polyethylene Polystyrene Polypropylene Polyvinyl chloride Polymers in electronic applications Polyacetylene Poly(p-phenylene vinylene) Polythiophene Polyphenylene sulfide Polyanilines Biomedical applications Polycarbonate (diphenyl carbonate) Polymethyl methacrylate Silicone polymers 2Chapter1 Copyright © 2003 Marcel Dekker, Inc. havebeenusedforthousandsofyears.Withinthiscentury,theyhavebeen supplementedand,insomeinstances,replacedbysyntheticfiberssuchasrayon, nylon,andacrylics.Indeed,rayonitselfisamodificationofanaturallyoccurring polymer,cellulose,whichinothermodifiedformshaveservedforyearsas commercialplasticsandfilms.Syntheticpolymers(somecommononesarelisted inTable1.1)suchaspolyolefins,polyesters,acrylics,nylons,andepoxyresins find extensive applications as plastics, films, adhesives, and protective coatings. It may 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 other low-molecular-weight materials. As a result, a large percentage of chemists and engineers are engaged in work involving polymers, which necessitates a formal course in polymer science. Biomaterials [3] are defined as materials used within human bodies either as artificial organs, bone cements, dental cements, ligaments, pacemakers, or contact 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, polymeric materials, which are derived from animals or plants, are natural candidates and some of these are cellulosics, chitin (or chitosan), dextran, agarose, and collagen. Among synthetic materials, polysiloxane, polyurethane, polymethyl methacry- Specialty polymers Polyvinylidene chloride Polyindene Polyvinyl pyrrolidone Coumarone polymer Introduction 3 Copyright © 2003 Marcel Dekker, Inc. late, polyacrylamide, polyester, and polyethylene oxides are commonly employed because they are inert within the body. Sometimes, due to the requi rements of mechanical strength, selective permeation, adhesion, and=or degradation, even noncompatible polymeric materials have been put to use, but before they are utilized, they are surface modified by biological molecules (such as, heparin, biological receptors, enzymes, and so forth). Some of these concepts will be developed in this and subsequent chapters. This chapter will mainly focus on the classification of polymers; subse- quent chapters deal with engineering problems of manufacturing, characteriza- tion, and the behavior of polymer solutions, melts, and solids. 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. In the former, polymers ‘‘melt’’ on heating and solidify on cooling. The heating and cooling 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 on reheating, but degrades. A more important classification of polymers is based on molecular structure. 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 must have reactive functional groups, or double or triple bonds. The functionality of a given monomer is defined to be the number of these functional groups; double bonds 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 least bifunctional; when it is bifunctional, the polymer chains are always linear. It is pointed 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, while different molecules are held together by weaker secondary forces. When thermal energy is supplied to the polymer, it increases the random motion of the molecules, which tries to overcome the secondary forces. When all forces are overcome, the molecules become free to move around and the polymer melts, which explains the ther moplastic nature of polymers. 4Chapter1 Copyright © 2003 Marcel Dekker, Inc. Branched polymers contain molecules having a linear backbone with branches 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: C O C O O CH OH CH 2 OH + CH CH 2 O OH C O C O OCH 2 C O C O O CH CH 2 (1.2.1) CH 2 OH CH 2 OH 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 molecular weight. In order to form branched polymers of high molecular weight, we must use special techniques, which will be discussed later. If allowed to react up to large conversions in Eq. (1.2.1), the polymer becomes a three-dimensional network called a gel, as follows: OC O C O CH 2 CH CH 2 O O O CO C O O OC O C O CH 2 CH CH 2 OO CH 2 CH CH 2 OC O C O CH 2 CH CH 2 OC O C O (1.2.2) O CO C O O Introduction 5 Copyright © 2003 Marcel Dekker, Inc. In fact, whenever a multifunctional monomer is polymerized, the polymer evolves through a collection of linear chains to a collection of branched chains, which ultimately forms a network (or a gel) polymer. Evidently, the gel polymer does not dissolve in any solvent, but it swells by incorporating molecules of the solvent into its own mat rix. Generally, any chemical process can be subdivided into three stages [viz. chemical reaction, separation (or purification) and identification]. Among the three stages, the most difficult in terms of time and resources is separation. We will discuss in Section 1.7 that polymer gels have gained considerable importance in heterogeneous catalysis because it does not dissolve in any medium and the separation step reduces to the simple removal of various reacting fluids. In recent times, a new phase called the fluorous phase, has been discovered which is immiscible to both organic and aqueous phases [4,5]. However, due to the high costs of their synthesis, they are, at present, only a laboratory curiosity. This approach is conceptually similar to solid-phase separation, except that fluorous materials are in liquid state. In dendrimer separation, the substrates are chemically attached to the branches of the hyper branched polymer (called dendrimers). In these polymers, (A) CH 2 CHCO 2 Me (B) NH 2 CH 2 CH 2 NH 2 (Excess) Repeat steps (A) and (B) NH 2 N N N NH 2 N H 2 NNH 2 NH 2 H 2 N (Generation = 1.0) (etc) (Generation = 0) N N N N N N NN NN H 2 N H 2 NNH 2 NH 2 NH 2 NH 2 NH 2 NH 2 H 2 N H 2 N H 2 N H 2 N Terminal groups Initiator core ‘Dendrimers’ Generations Dendrimer repeating units 0 1 2 (1.2.3a) N NHC CNH CNH O O O NH 2 H 2 N NH 2 H 2 N N NH 2 NH 2 NH 3 6Chapter1 Copyright © 2003 Marcel Dekker, Inc. the extent of branching is controlled to make them barely soluble in the reaction medium. Dendrimers [6] possess a globular structure characterized by a central core, branching units, and terminal units. They are prepared by repetitive reaction steps from a central initiator core, with each subsequent growth creating a new generation of polymers. Synthesis of polyamidoamine (PAMAM) dendrimers are done by reacting acrylamide with core ammonia in the presence of excess ethylene diamine. Dendrimers have a hollow interior and densely packed surfaces. They have a high degree of molecular uniformity and shape. These have been used as membrane materials and as filters for calibrating analytical instruments, and newer paints based on it give better bonding capacity and wear resistance. Its sticking nature has given rise to newer adhesives and they have been used as catalysts for rate enhancement. Environmental pollution control is the other field in which dendrimers have found utility. A new class of chemical sensors based on these molecules have been developed for detection of a variety of volatile organic pollutants. In all cases, when the polymer is examined at the molecular level, it is found 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., linear molecule, gel, etc.). For example, polystyrene is formed from chains of the repeat unit: CH CH 2 (1.2.3b) Such a polymer derives it name from the monomer from which it is usually manufactured. An idealized sample of polymer would consist of chains all having identical molecular weight. Such systems are called monodisperse polymers.In practice, however, all polymers are made up of molecules with molecular weights that vary over a range of values (i.e., have a distribution of molecular weights) and are said to be polydisperse. Whether monodisperse or polydisperse, the chemical formula of the polymer remains the same. For example, if the polymer is polystyrene, it would continue to be represented by CH 2 CH n CH CH 2 XCH 2 CH Y (1.2.4) For a monodisperse sample, n has a single value for all molecules in the system, whereas for a polydisperse sample, n would be characterized by distribution of Introduction 7 Copyright © 2003 Marcel Dekker, Inc. values.TheendchemicalgroupsXandYcouldbethesameordifferent,and whattheyaredependsonthechemicalreactionsinitiatingthepolymerformation. Uptothispoint,ithasbeenassumedthatalloftherepeatunitsthatmake upthebodyofthepolymer(linear,branched,orcompletelycross-linkednetwork molecules)areallthesame.However,iftwoormoredifferentrepeatunitsmake upthischainlikestructure,itisknownasacopolymer.Ifthevariousrepeatunits occurrandomlyalongthechainlikestructure,thepolymeriscalledarandom copolymer.Whenrepeatunitsofeachkindappearinblocks,itiscalledablock copolymer.Forexample,iflinearchainsaresynthesizedfromrepeatunitsAand B,apolymerinwhichAandBarearrangedas iscalledanABblockcopolymer,andoneofthetype iscalledanABAblockcopolymer.Thistypeofnotationisusedregardlessofthe molecular-weightdistributionoftheAandBblocks[7]. Thesynthesisofblockcopolymerscanbeeasilycarriedoutiffunctional groupssuchasacidchloride( COCl),amines( NH 2 ),oralcohols( OH)are presentatchainends.Thisway,apolymerofonekind(say,polystyreneor polybutadiene)withdicarboxylicacidchloride(ClCO COCl)terminalgroups canreactwithahydroxy-terminatedpolymer(OH OH)oftheotherkind(say, polybutadieneorpolystyrene),resultinginanABtypeblockcopolymer,as follows: ClCCCl + OHOH OO CC OO OO n H Cl (1.2.7) InChapter2,wewilldiscussinmoredetailthedifferenttechniquesofproducing functionalgroups.Anothercommonwayofpreparingblockcopolymersisto utilizeorganolithiuminitiators.Asanexample,sec-butylchloridewithlithium givesrisetothebutyllithiumcomplex, CH 3 CH CH 3 CH 2 Cl + LiCH 3 CH CH 3 CH 2 Li + . . . Cl – (1.2.8) 8Chapter1 Copyright © 2003 Marcel Dekker, Inc. which react s quickly with a suitable monomer (say, styrene) to give the following polystyryl anion: . . . Cl – + n 1 CH 2 CH 3 CH CH 3 CH 2 Li + CH 2 . . . Cl – Li + CH CH 2 1 CHCH 2 CH 3 (1.2.9) CH 3 n This is relatively stable and maintains its activity throughout the polymerization. Because of this activity, the polystyryl anion is sometimes called a living anion; it will polymerize with another monomer (say, butadiene) after all of the styrene is exhausted: CH 3 CH CH 3 CH 2 CH . . . Cl – Li + CH CH CH 3 CH 3 (1.2.10) Cl – + n 2 CH 2 Li + CH 2 . . . CH CH CH 2 CH 2 CH 2 CH 2 HC CH CH 1 n 1 n 2 n In this way, we can conveniently form an AB-type copolymer. In fact, this technique of polymerizing with a living anion lays the foundation for modifying molecular structure. Graft copolymers are formed when chains of one kind are attached to the backbone of a different polymer. A graft copolymer has the following general structure: (1.2.11) AAAAAAA BB BB BB . . . . . . . . . . . . Introduction 9 Copyright © 2003 Marcel Dekker, Inc. Here À(A) n constitutes the backbone molecule, whereas polymer (B) n is randomly distributed on it. Graft copolymers are normally named poly(A)-g- poly(B), and the properties of the resultant material are normally extremely different from those of the constituent polymers. Graft copolymers can be generally synthesized by one of the following schemes [1]: The ‘‘grafting-from’’ technique. In this scheme, a polymer car rying active sites is used to initiate the polymerization of a second monomer. Depending on the 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 the fact that the backbone is made first and the grafts are created on it in a second polymerization step, as follows: CH ∗ + nCH 2 R CH 2 CHCH 2 CH RR (1.2.12) This process is efficient, but it has the disadvantage that it is usually not possible to predict the molecular structure of the graft copolymer and the number of grafts formed. In addition, the length of the graft may vary, and the graft copolymer often carries a fair amount of homopolymer. The ‘‘graft-onto’’ scheme. In this scheme, the polymer backbone carried a randomly distributed reactive functional group X. This reacts with another polymer molecule carrying functional groups Y, located selectively at the chain ends, as follows: CH 2 CH CH 2 R X + Y CH 2 CH R CH 2 CH R (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 it allows structural characterization of the graft copolymer formed because the backbone and the pendant graft are both synthesized separately. If the molecular weight of each of these chains and their overall compositions are known, it is possible to determine the number of grafts per chain and the average distance between two successive grafts on the backbone. The ‘‘grafting-through’’ scheme. In this scheme, polymerization with a macromer is involved. A macromer is a low-molecular-weight polymer chain with unsaturation on at least one end. The formation of macromers has recently been reviewed and the techniques for the maximization of macromer amount 10 Chapter 1 Copyright © 2003 Marcel Dekker, Inc. [...]... copolymer CH3 OH (b) (CH2CH2O) m (CH CH2 O)n Star copolymer COOP POOC COOP where (c) Graft polymer CH2CH CH CH2 n CH CH CHCH2 NH CH2CH2 Copyright © 20 03 Marcel Dekker, Inc x m 14 Chapter 1 (d) Dendrimer (e) Segmented block copolymer CH3 H3C (CH2)16 N+ Br– (f) n Polysoap CH2 CH n CH2 (CH2)17 COO– Na+ Example 1 .2: Describe polymers as dental restorative materials and their requirements Copyright © 20 03... simplified form nCH2 CH CH2 Cl CH (1.3.3) n Cl Ring-opening reactions, such as the polymerization of ethylene oxide to give poly(ethylene oxide), offer another example of the formation of addition polymers: CH2 nCH2 CH2 O CH2 O n (1.3.4) The correct method of naming an addition polymer is to write poly( ), where the name of the monomer goes into the parentheses If ÀR in compound (1.3 .2) is an aliphatic... class of condensation polymers that are formed by reaction between amine and acid groups, as in NH2 (CH2)6 NH2 Hexamethylene diamine + COOH (CH2)4 COOH Adipic acid NH (CH2)6 (1.3.7a) NHCO (CH2)6 + H2O Nylon 66 (CH2)5 COOH ω-Aminocaproic acid NH2 + H2O NH (CH2)5 CO n (1.3.7b) Nylon 6 Both of these polymers are classified as polyamides because the repeat units contain the À[COÀNH]À amide group Naming of. .. histogram of the degree of polymerization It is thus seen that mw is just the first moment of the weight distribution of the degree of polymerization There is an alternative but equivalent method of describing distributions of molecular weight If Nn is the total number of moles of a polymer of chain length * equal to n in a given sample, one can write t Nn ¼ W* n Mn ð1:4:6Þ The total number of moles of polymer, ... must be equal to some whole number The product of a given polymerization reaction can be thought of as having a distribution of the degrees of polymerization (DPs), which is given by a histogram, as shown in Figure 1 .2 In this representation, Wn is the weight of a species of degree of polymerization n * such that Wt ¼ Total weight of polymer 1 P ¼ Wn * ð1:4 :2 n¼1 By definition, the weight-average molecular... adipamide part is associated with the amide unit in the backbone As researchers learned more about polymerization chemistry, it became apparent that the notion of classifying polymers this way was somehow inconsistent Certain polymer molecules could be prepared by more than one mechanism For example, polyethylene can be synthesized by either of the two mechanisms: CH2 CH2 CH2 n (1.3.8a) Br + 2mH2 CH2 CH2 5m... ratio of mw and mn by the following relation: Q¼ mw l2 l0 ¼ 2 mn l1 ð1:4:15Þ The polydispersity index is a measure of the breadth of mole fraction (or molecular weight) distribution For a monodisperse polymer, Q is unity; commercial polymers may have a value of Q lying anywhere between 2 and 20 1.5 CONFIGURATIONS AND CRYSTALLINITY OF POLYMERIC MATERIALS So far, we have examined the broader aspects of. .. distribution of the degree of polymerization (DP) Nn as Nn ¼ N* n Nt Copyright © 20 03 Marcel Dekker, Inc ð1:4:9Þ 22 Chapter 1 such that 1 P Nn ¼ 1 ð1:4:10Þ n¼1 Because Nn is also the fraction of the molecules of polymer of DP equal to n or molecular weight of nM0 , Eq (1.4.8) then becomes Mn ¼ 1 P ð1:4:11Þ Mn N n n¼1 which gives the number-average chain length, mn , as 1 Mn P ¼ nN M0 n¼1 n mn ¼ ð1:4: 12 and,... method of characterizing a polymer is by its extent of stereoregularity, or tacticity Copyright © 20 03 Marcel Dekker, Inc 26 Chapter 1 Further, when a diene is polymerized, it can react in the two following ways by the use of the appropriate catalyst: The 1 ,2 polymerization leads to the formation of substituted polymers and gives rise to stereoregularity, as discussed earlier (Fig 1.5) The 1,4 polymerization,... satisfied, polymers can, indeed, form highly crystalline domains in the solid state and in concentrated solution There is even evidence of the formation of microcrystalline F IGURE 1.5 Spatial arrangement of diene polymers Copyright © 20 03 Marcel Dekker, Inc Introduction 27 regions in moderately dilute solutions of a highly tactic polymer Formation of highly crystalline domains in a solid polymer has a profound . 0) N N N N N N NN NN H 2 N H 2 NNH 2 NH 2 NH 2 NH 2 NH 2 NH 2 H 2 N H 2 N H 2 N H 2 N Terminal groups Initiator core ‘Dendrimers’ Generations Dendrimer repeating units 0 1 2 (1 .2. 3a) N NHC CNH CNH O O O NH 2 H 2 N NH 2 H 2 N N NH 2 NH 2 NH 3 6Chapter1 Copyright. the branches of the hyper branched polymer (called dendrimers). In these polymers, (A) CH 2 CHCO 2 Me (B) NH 2 CH 2 CH 2 NH 2 (Excess) Repeat steps (A) and (B) NH 2 N N N NH 2 N H 2 NNH 2 NH 2 H 2 N (Generation. Block copolymer OH (CH 2 CH 2 O) (CH CH 2 CH 3 O) n m (b) Star copolymer COOP COOPPOOC where (c) Graft polymer CH 2 CH CH CH 2 CH n CH CHCH 2 NH CH 2 CH 2 m x Introduction 13 Copyright © 20 03 Marcel