POLYMER
`SCIENCE and
`TECHNOLOGY
`
`Robert D. Ebewele
`
`Faculty of Engineering
`University of Benin
`Benin City, Nigeria
`
`CRC Press
`Boca Raton London New York Washington, D.C.
`
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`Library of Congress Cataloging-in-Publication Data
`
`Ebewele, Robert Oboigbaotor.
`Polymer science and technology I Robert 0. Ebewele.
`p. em.
`Includes bibliographical references (p.
`ISBN 0-8493-8939-9 (alk. paper)
`1. Polymerization. 2. Polymers.
`TP156.P6E24 1996
`668.9--dc20
`
`I. Title.
`
`) and index.
`
`95-32995
`CIP
`
`This book contains information obtained from authentic and highly regarded sources. Reprinted material is
`quoted with permission, and sources are indicated. A wide variety of references are listed. Reasonable efforts
`have been made to publish reliable data and information, but the author and the publisher cannot assume
`responsibility for the validity of all materials or for the consequences of their use.
`
`Neither this book nor any part may be reproduced or transmitted in any form or by any means, electronic or
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`
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`
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`
`Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used
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`
`© 2000 by CRC Press LLC
`
`No claim to original U.S. Government works
`International Standard Book Number 0-0849-8939-9
`Library of Congress Card Number 95-32995
`Printed in the United States of America 1 2 3 4 5 6 7 8 9 0
`Printed on acid-free paper
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`Chapter 1
`
`Introduction
`
`I. HISTORICAL DEVELOPMENT
`
`Before we go into details of the chemistry of polymers it is appropriate to briefly outline a few landmarks
`in the historical development of what we now know as polymers. Polymers have been with us from the
`all classes of
`beginning of time; they form the very basis (building blocks) of life. Animals, plants -
`living organisms- are composed of polymers. However, it was not until the middle of the 20th century
`that we began to understand the true nature of polymers. This understanding came with the development
`of plastics, which are true man-made materials that are the ultimate tribute to man's creativity and
`ingenuity. As we shall see in subsequent discussions, the use of polymeric materials has permeated every
`facet of our lives. It is hard to visualize today's world with all its luxury and comfort without man-made
`polymeric materials.
`The plastics industry is recognized as having its beginnings in 1868 with the synthesis of cellulose
`nitrate. It all started with the shortage of ivory from which billiard balls were made. The manufacturer
`of these balls, seeking another production method, sponsored a competition. John Wesley Hyatt (in the
`U.S.) mixed pyroxin made from cotton (a natural polymer) and nitric acid with camphor. The result was
`cellulose nitrate, which he called celluloid. It is on record, however, that Alexander Parkes, seeking a
`better insulating material for the electrical industry, had in fact discovered that camphor was an efficient
`plasticizer for cellulose nitrate in 1862. Hyatt, whose independent discovery of celluloid came later, was
`the first to take out patents for this discovery.
`Cellulose nitrate is derived from cellulose, a natural polymer. The first truly man-made plastic came
`41 years later (in 1909) when Dr. Leo Hendrick Baekeland developed phenol-formaldehyde plastics
`(phenolics), the source of such diverse materials as electric iron and cookware handles, grinding wheels,
`and electrical plugs. Other polymers- cellulose acetate (toothbrushes, combs, cutlery handles, eyeglass
`frames); urea-formaldehyde (buttons, electrical accessories); poly(vinyl chloride) (flooring, upholstery,
`wire and cable insulation, shower curtains); and nylon (toothbrush bristles, stockings, surgical sutures)(cid:173)
`followed in the 1920s.
`Table 1.1 gives a list of some plastics, their year of introduction, and some of their applications. It
`is obvious that the pace of development of plastics, which was painfully slow up to the 1920s, picked
`up considerable momentum in the 1930s and the 1940s. The first generation of man-made polymers was
`the result of empirical activities; the main focus was on chemical composition with virtually no attention
`paid to structure. However, during the first half of the 20th century, extensive organic and physical
`developments led to the first understanding of the structural concept of polymers -
`long chains or a
`network of covalently bonded molecules. In this regard the classic work of the German chemist Hermann
`Staudinger on polyoxymethylene and rubber and of the American chemists W. T. Carothers on nylon
`stand out clearly. Staudinger first proposed the theory that polymers were composed of giant molecules,
`and he coined the word macromolecule to describe them. Carothers discovered nylon, and his funda(cid:173)
`mental research (through which nylon was actually discovered) contributed considerably to the elucida(cid:173)
`tion of the nature of polymers. His classification of polymers as condensation or addition polymers
`persists today.
`Following a better uqderstanding of the nature of polymers, there was a phenomenal growth in the
`numbers of polymeric products that achieved commercial success in the period between 1925 and 1950.
`In the 1930s, acrylic resins (signs and glazing); polystyrene (toys, packaging and housewares industries);
`and melamine resins (dishware, kitchen countertops, paints) were introduced.
`The search for materials to aid in the defense effort during World War II resulted in a profound
`impetus for research into new plastics. Polyethylene, now one of the most important plastics in the
`world, was developed because of the wartime need for better-quality insulating materials for such
`applications as radar cable. Thermosetting polyester resins (now used for boatbuilding) were developed
`for military use. The terpolymer acrylonitrile-butadiene-styrene (ABS), (telephone handsets, luggage,
`
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`Table 1.1
`
`Introduction of Plastics Materials
`
`Date
`
`Material
`
`Typical Use
`
`1868
`Cellulose nitrate
`1909
`Phenol-formaldehyde
`1919
`Casein
`1926
`Alkyds
`1927
`Cellulose acetate
`1927
`Poly(vinyl chloride)
`1929
`Urea-formaldehyde
`1935
`Ethyl cellulose
`1936
`Polyacrylonitrile
`1936
`Poly(vinyl acetate)
`1938
`Cellulose acetate butyrate
`1938
`Polystyrene
`1938
`Nylon (polyamide)
`1938
`Poly(vinyl acetal)
`1939
`Poly(vinylidene chloride)
`1939 Melamine-formaldehyde
`1942
`Polyester (cross-linkable)
`1942
`Polyethylene (low density)
`1943
`Fluoropolymers
`1943
`Silicone
`1945
`Cellulose propionate
`1947
`Epoxies
`1948
`Acrylonitrile-butadiene-styrene copolymer
`1949
`Allylic
`1954
`Polyurethane
`1956
`Acetal resin
`1957
`Polypropylene
`1957
`Polycarbonate
`1959
`Chlorinated polyether
`1962
`Phenoxy resin
`1962
`Polyallomer
`1964
`lonomer resins
`1964
`Polyphenylene oxide
`1964
`Polyimide
`1964
`Ethylene-vinyl acetate
`1965
`Polybutene
`1965
`Polysulfone
`1970
`Thermoplastic polyester
`1971
`Hydroxy acrylates
`1973
`Polybutylene .
`1974
`Aromatic polyamides
`1975
`Nitrile barrier resins
`
`Eyeglass frames
`Telephone handsets, knobs, handles
`Knitting needles
`Electrical insulators
`Toothbrushes, packaging
`Raincoats, flooring
`Lighting fixtures, electrical switches
`Flashlight cases
`Brush backs, displays
`Flashbulb lining, adhesives
`Irrigation pipe
`Kitchenwares, toys
`Gears, fibers, films
`Safety glass interlayer
`Auto seat covers, films, paper, coatings
`Tableware
`Boat hulls
`Squeezable bottles
`Industrial gaskets, slip coatings
`Rubber goods
`Automatic pens and pencils
`Tools and jigs
`Luggage, radio and television cabinets
`Electrical connectors
`Foam cushions
`Automotive parts
`Safety helmets, carpet fiber
`Appliance parts
`Valves and fittings
`Adhesives, coatings
`Typewriter cases
`Skin packages, moldings
`Battery cases, high temperature moldings
`Bearings, high temperature films and wire coatings
`Heavy gauge flexible sheeting
`Films
`Electrical/electronic parts
`Electrical/electronic parts
`Contact lenses
`Piping
`High-strength tire cord
`Containers
`
`safety helmets, etc.) owes its origins to research work emanating from the wartime crash program on
`large-scale production of synthetic rubber.
`The years following World War II (1950s) witnessed great strides in the growth of established plastics
`and the development of new ones. The Nobel-prize-winning development of stereo-specific catalysts by
`Professors Karl Ziegler of Germany and Giulio Natta of Italy led to the ability of polymer chemists to
`"order" the molecular structure of polymers. As a consequence, a measure of control over polymer
`properties now exists; polymers can be tailor-made for specific purposes.
`The 1950s also saw the development of two families of plastics- acetal and polycarbonates. Together
`with nylon, phenoxy, polyimide, poly(phenylene oxide), and polysulfone they belong to the group of
`plastics known as the engineering thermoplastics. They have outstanding impact strength and thermal
`properties that place them in direct competition with more conventional
`and dimensional stability -
`materials like metals.
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`3
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`The 1960s and 1970s witnessed the introduction of new plastics: thermoplastic polyesters (exterior
`automotive parts, bottles); high-barrier nitrile resins; and the so-called high-temperature plastics, includ(cid:173)
`ing such materials as polyphenylene sulfide, polyether sulfone, etc. The high-temperature plastics were
`initially developed to meet the demands of the aerospace and aircraft industries. Today, however, they
`have moved into commercial areas that require their ability to operate continuously at high temperatures.
`In recent years, as a result of better understanding of polymer structure-property relationships, intro(cid:173)
`duction of new polymerization techniques, and availability of new and low-cost monomers, the concept
`of a truly tailor-made polymer has become a reality. Today, it is possible to create polymers from different
`elements with almost any quality desired in an end product. Some polymers are similar to existing
`conventional materials but with greater economic values, some represent significant improvements over
`existing materials, and some can only be described as unique materials with characteristics unlike any
`previously known to man. Polymer materials can be produced in the form of solid plastics, fibers,
`elastomers, or foams. They may be hard or soft or may be films, coatings, or adhesives. They can be
`made porous or nonporous or can melt with heat or set with heat. The possibilities are almost endless
`and their applications fascinating. For example, ablation is the word customarily used by the astronomers
`and astrophysicists to describe the erosion and disintegration of meteors entering the atmosphere. In this
`sense, long-range missiles and space vehicles reentering the atmosphere may be considered man-made
`meteors. Although plastic materials are generally thermally unstable, ablation of some organic polymers
`occurs at extremely high temperatures. Consequently, selected plastics are used to shield reentry vehicles
`from the severe heat generated by air friction and to protect rocket motor parts from hot exhaust gases,
`based on the concept known as ablation plastics. Also, there is a "plastic armor" that can stop a bullet,
`even shell fragments. (These are known to be compulsory attire for top government and company officials
`in politically troubled countries.) In addition, there are flexible plastics films that are used to wrap your
`favorite bread, while others are sufficiently rigid and rugged to serve as supporting members in a building.
`In the years ahead, polymers will continue to grow. The growth, from all indications, will be not
`only from the development of new polymers, but also from the chemical and physical modification of
`existing ones. Besides, improved fabrication techniques will result in low-cost products. Today the
`challenges of recycling posed by environmental problems have led to further developments involving
`alloying and blending of plastics to produce a diversity of usable materials from what have hitherto been
`considered wastes.
`
`II. BASIC CONCEPTS AND DEFINITIONS
`
`The word polymer is derived from classical Greek poly meaning "many" and meres meaning "parts."
`Thus a polymer is a large molecule (macromolecule) built up by the repetition of small chemical units.
`To illustrate this, Equation 1.1 shows the formation of the polymer polystyrene.
`
`•CH@
`
`styrene (monomer)
`
`rH~
`
`n
`polystyrene (polymer)
`
`(1.1)
`
`(I)
`
`(2)
`
`The styrene molecule (1) contains a double bond. Chemists have devised methods of opening this double
`bond so that literally thousands of styrene molecules become linked together. The resulting structure,
`enclosed in square brackets, is the polymer polystyrene (2). Styrene itself is referred to as a mo1wme1;
`which is defined as any molecule that can be converted to a polymer by combining with other molecules
`of the same or different type. The unit in square brackets is called the repeating unit. Notice that the
`structure of the repeating unit is not exactly the same as that of the monomer even though both possess
`identical atoms occupying similar relative positions. The conversion of the monomer to the polymer
`involves a rearrangement of electrons. The residue from the monomer employed in the preparation of a
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`POLYMER SCIENCE AND "TECHNOLOGY
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`polymer is referred to as the structural unit. In the case of polystyrene, the polymer is derived from a
`single monomer (styrene) and, consequently, the structural unit of the polystyrene chain is the same as its
`repeating unit. Other examples of polymers of this type are polyethylene, polyacrylonitrile, and polypro(cid:173)
`pylene. However, some polymers are derived from the mutual reaction of two or more monomers that
`are chemically similar but not identical. For example, poly(hexamethylene adipamide) or nylon 6,6 (5)
`is made from the reaction of hexamethylenediamine (3) and adipic acid (4) (Equation 1.2).
`
`hexamethylenediamine
`
`adipic acid
`
`poly(hexamethylene adipamide)
`
`(3)
`
`(4)
`
`(5)
`
`H
`H
`The repeating unit in this case consists of two structural units: -~-(CH2)6-~-. the residue from hexam-
`ethylenediamine; and -~-(CH2)c~-. the residue from adipic acid. Other polymers that have repeating
`
`units with more than one structural unit include poly(ethyleneterephthalate) and proteins. As we shall
`see later, the constitution of a polymer is usually described in terms of its structural units.
`The subscript designation, n, in Equations 1.1 and 1.2 indicates the number of repeating units strung
`together in the polymer chain (molecule). This is known as the degree of polymerization (DP). It specifies
`the length of the polymer molecule. Polymerization occurs by the sequential reactions of monomers, which
`means that a successive series of reactions occurs as the repeating units are linked together. This can proceed
`by the reaction of monomers to form a dime1; which in tum reacts with another monomer to form a trimer
`and so on. Reaction may also be between dimers, trimers, or any molecular species within the reaction
`mixture to form a progressively larger molecule. In either case, a series of linkages is built between the
`repeating units, and the resulting polymer molecule is often called a polymer chain, a description which
`emphasizes its physical similarity to the links in a chain. Low-molecular-weight polymerization products
`such as dimers, ttimers, tetramers, etc., are referred to as oligomers. They generally possess undesirable
`thermal and mechanical properties. A high degree of polymerization is normally required for a material to
`develop useful properties and before it can be appropriately described as a polymer. Polystyrene, with a
`degree of polymerization of 7, is a viscous liquid (not of much use), whereas commercial grade polystyrene
`is a solid and the DP is typically in excess of 1000. It must be emphasized, however, that no clear
`demarcation has been established between the sizes of oligomers and polymers.
`The degree of polymerization represents one way of quantifying the molecular length or size of a
`polymer. This can also be done by use of the term molecular weight (MW). By definition, MW(Polymer) =
`DP x MW(Repeat Unit). To illustrate this let us go back to polystyrene (2). There are eight carbon atoms
`and eight hydrogen atoms in the repeating unit. Thus, the molecular weight of the repeating unit is 104
`(8 x 12 + 1 x 8). If, as we stated above, we are considering commercial grade polystyrene, we will be
`dealing with a DP of 1000. Consequently, the molecular weight of this type of polystyrene is 104,000.
`As we shall see later, molecular weight has a profound effect on the properties of a polymer.
`
`Example 1.1: What is the molecular weight of polypropylene (PP), with a degree of polymerization of
`3 X 104?
`
`Solution: Structure of the · repeating unit for PP
`
`Molecular weight of repeat unit= (3 x 12 + 6 x 1) = 42
`Molecular weight of polypropylene= 3 x 104 x 42 = 1.26 x 106
`
`(Str. 1)
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`INTRODUCTION
`
`5
`
`Mn
`
`...
`(1j
`E
`>
`0 c.
`
`... 0 .. ~
`
`:I
`0
`E
`<{
`
`totolecular weight
`
`Figure 1.1 Molecular weight distribution curve.
`
`So far, we have been discussing a single polymer molecule. However, a given polymer sample (like
`a piece of polystyrene from your kitchenware) is actually composed of millions of polymer molecules.
`For almost all synthetic polymers irrespective of the method of polymerization (formation), the length
`of a polymer chain is determined by purely random events. Consequently, any given polymeric sample
`contains a mixture of molecules having different chain lengths (except for some biological polymers
`like proteins, which have a single, well-defined molecular weight [monodisperse]). This means that a
`distribution of molecular weight exists for synthetic polymers. A typical molecular weight distribution
`curve for a polymer is shown in Figure 1.1.
`The existence of a distribution of molecular weights in a polymer sample implies that any experimental
`measurement of molecular weight in the given sample gives only an average value. Two types of
`molecular weight averages are most commonly considered: the number-average molecular weight rep(cid:173)
`resented by M., and the weight-average molecular weight Mw. The number-average molecular weight
`is derived from measurements that, in effect, count the number of molecules in the given sample. On
`the other hand, the weight-average molecular weight is based on methods in which the contribution of
`each molecule to the observed effect depends on its size.
`In addition to the information on the size of molecules given by the molecular weights Mw and M.,
`their ratio M,JM. is an indication of just how broad the differences in the chain lengths of the constituent
`polymer molecules in a given sample are. That is, this ratio is a measure of polydispersity, and conse(cid:173)
`quently it is often referred to as the heterogeneity index. In an ideal polymer such as a protein, all the
`polymer molecules are of the same size (Mw = M. or M,JM. = 1 ). This is not true for synthetic polymers(cid:173)
`the numerical value of Mw is always greater than that of M •. Thus as the ratio MjM11 increases, the
`molecular weight distribution is broader.
`
`Example 1.2: Nylon 11 has the following structure
`
`0
`H
`II
`I
`[
`-N-(CH) -C-
`2 10
`
`]
`
`11
`
`(Str. 2)
`
`If the number-average degree of polymerization, x., for nylon is 100 and Mw = 120,000, what is its
`polydispersity?
`Solution: We note that x. and n(DP) define the same quantity for two slightly different entities. The
`degree of polymerization for a single molecule is n. But a polymer mass is composed of millions of
`molecules, each of which has a certain degree of polymerization. x. is the average of these. Thus,
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`where N = total number of molecules in the polymer mass
`M, = molecular weight of repeating unit
`n; = DP of molecule i.
`Now Mn = XnMr = 100 (15 + 14 X 10 + 28)
`= 18,300
`
`.
`.
`o y Ispersity =
`P I d
`
`120,000
`M
`w =
`Mn
`18,300
`
`= .
`6 56
`
`Ill. CLASSIFICATION OF POLYMERS
`
`Polymers can be classified in many different ways. The most obvious classification is based on the origin
`of the polymer, i.e., natural vs. synthetic. Other classifications are based on the polymer structure,
`polymerization mechanism, preparative techniques, or thermal behavior.
`
`A. NATURAL VS. SYNTHETIC
`Polymers may either be naturally occurring or purely synthetic. All the conversion processes occurring
`in our body (e.g., generation of energy from our food intake) are due to the presence of enzymes. Life
`itself may cease if there is a deficiency of these enzymes. Enzymes, nucleic acids, and proteins are
`polymers of biological origin. Their structures, which are normally very complex, were not understood
`until very recently. Starch- a staple food in most cultures- cellulose, and natural rubber, on the other
`hand, are examples of polymers of plant origin and have relatively simpler structures than those of
`enzymes or proteins. There are a large number of synthetic (man-made) polymers consisting of various
`families: fibers, elastomers, plastics, adhesives, etc. Each family itself has subgroups.
`
`B. POLYMER STRUCTURE
`1. Linear, Branched or Cross-linked, Ladder vs. Functionality
`As we stated earlier, a polymer is formed when a very large number of structural units (repeating units,
`monomers) are made to link up by covalent bonds under appropriate conditions. Certainly even if the
`conditions are "right" not all simple (small) organic molecules possess the ability to form polymers. In
`order to understand the type of molecules that can form a polymer, let us introduce the termfunctionality.
`The functionality of a molecule is simply its interlinking capacity, or the number of sites it has available
`for bonding with other molecules under the specific polymerization conditions. A molecule may be
`classified as monofunctional, bifunctional, or polyfunctional depending on whether it has one, two, or
`greater than two sites available for linking with other molecules. For example, the extra pair of electrons
`in the double bond in the styrene molecules endows it with the ability to enter into the formation of two
`bonds. Styrene is therefore bifunctional. The presence of two condensable groups in both hexamethyl(cid:173)
`enediamine (-NH2) and adipic acid (-COOH) makes each of these monomers bifunctional. However,
`functionality as defined here differs from the conventional terminology of organic chemistry where, for
`example, the double bond in styrene represents a single functional group. Besides, even though the
`interlinking capacity of a monomer is ordinarily apparent from its structure, functionality as used in
`polymerization reactions is specific for a given reaction. A few examples will illustrate this.
`A diamine like hexamethylenediamine has a functionality of 2 in amide-forming reactions such as
`that shown in Equation 1.2. However, in esterification reactions a diamine has a functionality of zero.
`Butadiene has the following structure:
`
`CH2=CH-CH=CH2
`1
`2
`3
`4
`(6)
`
`(Str. 3)
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`7
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`From our discussion about the polymerization of styrene, the presence of two double bonds on the
`structure of butadiene would be expected to prescribe a functionality of 4 for this molecule. Butadiene
`may indeed be tetrafunctional, but it can also have a functionality of 2 depending on the reaction
`conditions (Equation 1.3).
`
`. n CH2 =CH-CH=CH2 ----1....
`
`rCH
`
`I '2 or
`3.4
`
`I
`
`2
`
`3
`
`4
`
`- ~H l
`
`2
`
`CH
`II
`CH2
`
`n
`
`(7)
`
`1·4 • tCH,-CH~ CH-CH,i,
`
`(1.3)
`
`(8)
`
`Since there is no way of making a distinction between the 1,2 and 3,4 double bonds, the reaction of
`either double bond is the same. If either of these double bonds is involved in the polymerization reaction,
`the residual or unreacted double bond is on the structure attached to the main chain [i.e., part of the
`pendant group (7)]. In 1,4 polymerization, the residual double bond shifts to the 2,3 position along the
`main chain. In either case, the residual double bond is inert and is generally incapable of additional
`polymerization under the conditions leading to the formation of the polymer. In this case, butadiene has
`a functionality of 2. However, under appropriate reaction conditions such as high temperature or cross(cid:173)
`linking reactions, the residual unsaturation either on the pendant group or on the backbone can undergo
`additional reaction. In that case, butadiene has a total functionality of 4 even though all the reactive sites
`may not be activated under the same conditions. Monomers containing functional groups that react under
`different conditions are said to possess latent functionality.
`Now let us consider the reaction between two monofunctional monomers such as in im esterification
`reaction (Equation 1.4 ).
`
`R-COOH + R'-OH
`
`acid
`(9)
`
`alcohol
`(10)
`
`0
`II
`R-C-0-R'
`
`ester
`(II)
`
`(1.4)
`
`You will observe that the reactive groups on the acid and alcohol are used up completely and that the
`product ester (11) is incapable of further esterification reaction. But what happens when two bifunctional
`molecules react? Let us use esterification once again to illustrate the principle (Equation 1.5).
`
`0
`II
`HOOC-R-COOH + HO-R'OH----i .... HOOC-R-C-0-R'-OH
`
`(1.5)
`
`bifunctional
`(12)
`
`bifunctional
`(13)
`
`bifunctional
`(14)
`
`The ester (14) resulting from this reaction is itself bifunctional, being terminated on either side by
`groups that are capable of further reaction. In other words, this process can be repeated almost indefinitely.
`The same argument holds for poly functional molecules. It is thus obvious that the generation of a polymer
`through the repetition of one or a few elementary units requires that the molecule(s) must be at least
`bifunctional.
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`Linear
`
`Branched
`
`Figure 1.2 Linear, branched, and cross-linked polymers.
`
`Cross linked
`
`The structural units resulting from the reaction of monomers may in principle be linked together in
`any conceivable pattern. Bifunctional structural units can enter into two and only two linkages with other
`structural units. This means that the sequence of linkages between bifunctional units is necessarily linear.
`The resulting polymer is said to be linear. However, the reaction between polyfunctional molecules
`results in structural units that may be linked so as to form nonlinear structures. In some cases the side
`growth of each polymer chain may be terminated before the chain has a chance to link up with another
`chain. The resulting polymer molecules are said to be branched. In other cases, growing polymer chains
`become chemically linked to each other, resulting in a cross-linked system (Figure 1.2).
`The formation of a cross-linked polymer is exemplified by the reaction of epoxy polymers, which
`have been used traditionally as adhesives and coatings and, more recently, as the most common matrix
`in aerospace composite materials. Epoxies exist at ordinary temperatures as low-molecular-weight
`viscous liquids or prepolymers. The most widely used prepolymer is diglycidyl ether of bisphenol A
`(DGEBA), as shown below (15):
`
`0
`.
`0
`CH -CH-CH-0- /j ~ ~ lj ~ 0-CH -CH-CH
`OCHD
`2 _ I_
`CH3
`
`/
`
`""
`2
`
`2
`
`/ " "
`
`2
`
`(Str. 4)
`
`diglycidyl ether of bisphenol A (DGEBA)
`(15)
`
`The transformation of this viscous liquid into a hard, cross-linked three-dimensional molecular
`network involves the reaction of the prepolymer with reagents such as amines or Lewis acids. This
`reaction is referred to as curing. The curing of epoxies with a primary amine such as hexamethylene(cid:173)
`diamine involves the reaction of the amine with the epoxide. It proceeds essentially in two steps:
`
`1. The attack of an epoxide group by the primary amine
`
`N-R-NH
`
`H
`
`2
`
`+ CH2 -CH- _____.. H
`
`2
`
`2
`
`OH
`H
`I
`I
`N-R-N-CH2-CH-
`
`(1.6)
`
`I 0 amine
`I 0 amine
`(16)
`
`epoxide
`(17)
`
`l 0 amine
`
`2°amine
`(18)
`
`RBP_TEVA05017897
`
`DRL - EXHIBIT 1036
`DRL010
`
`

`
`INTRODUCTION
`
`9
`
`2. The combination of the resulting secondary amine with a second epoxy group to form a branch
`point (19) .
`
`l 0 amine
`
`epoxide
`
`branch point
`
`(19)
`
`The presence of these branch points ultimately leads to a cross-linked infusible and insoluble polymer
`with structures such as (20).
`
`CH-OH
`I
`OH
`CH2
`OH
`I
`I
`I
`-CH-CH -N-R-N-CH -CH-
`I
`2
`2
`CH2
`I
`CH-OH
`I
`
`(20)
`
`(Str. 5)
`
`In this reaction, the stoichiometric ratio requires one epoxy group per amine hydrogen. Consequently,
`an amine such as hexamethylenediamine has a functionality of 4. Recall, however, that in the reaction
`of hexamethylenediamine with adipic acid, the amine has a functionality of 2. In this reaction DGEBA
`is bifunctional since the hydroxyl groups generated in the reaction do not participate in the reaction.
`But when the curing of epoxies involves the use of a Lewis acid such as BF3, the functionality of each
`epoxy group is 2; that is, the functionality of DGEBA is 4. Thus the curing reactions of epoxies further
`illustrate the point made earlier that the functionality of a given molecule is defined for a specific reaction.
`By employing different reactants or varying the stoichiometry of reactants, different structures can be
`produced and, consequently, the properties of the final polymer can also be varied.
`Polystyrene (2), polyethylene (21), polyacrylonitrile (22), poly(methyl methacrylate) (23), and
`poly(vinyl chloride) (24) are typical examples of linear polymers.
`
`tCH,-CH,1"
`
`tcH,-Tu1
`
`CN
`
`n
`
`(21)
`
`(22)
`
`CH3
`I
`CH -C
`I
`2
`C=O
`I
`0
`I
`CH 3
`
`(23)
`
`tcH,-T"1
`
`Cl
`
`n
`
`(24)
`
`(Str. 6)
`
`0
`II
`Substituent groups such as -CH3, -O-C-CH3, -Cl, and -CN that are attached to the main chain of
`skeletal atoms are known as pendant groups. Their structure and chemical nature can confer unique
`properties on a polymer. For example, linear and branched polymers are usually soluble in some solvent
`at normal temperatures. But the presence of polar pendant groups can considerably reduce room tem(cid:173)
`perature solubility. Since cross-linked polymers are chemically tied together and solubility essentially
`
`RBP_TEVA05017898
`
`DRL - EXHIBIT 1036
`DRL011
`
`

`
`10
`
`POLYMER SCIENCE AND TECHNOLOGY
`
`involves the separation of solute molecules by solvent molecules, cross-linked polymers do not dissolve,
`but can only be swelled by liquids. The presence of cross-linking confers stability on polymers. Highly .
`cross-linked polymers are generally rigid and high-melting. Cross-links occur randomly in a cross-linked
`polymer. Consequently, it can be broken down into smaller molecules by random chain scission. Ladder
`polymers constitute a group of polymers with a regular sequence of cross-links. A ladder polymer, as
`the name implies, consists of two parallel linear strands of molecules with a regular sequence of cross(cid:173)
`links. Ladder polymers have only condensed cyclic units in the chain; they are also commonly referred
`to as double-chain or double-strand polymers. A typical example is poly(imidazopyrrolone) (27), which
`is obtained by the polymerization of aromatic dianhydrides such as pyromellitic dianhydride (25) or
`aromatic tetracarboxylic acids with ortho-aromatic tetramines like 1,2,4,5-tetraaminobenzene (26):
`
`0
`II
`
`0
`II
`
`o""'
`II
`0
`
`/cXXc""'
`c c
`
`1 ~ /o +
`
`II
`0
`
`(25)
`
`(26)
`
`(Str. 7)
`
`(27)
`
`The molecular structure of ladder polymers is more rigid than that of conventional linear polymers.
`Numerous members of this family of polymers display exception