Adopted from : Zbigniew D Jastrzebski, “The Nature And

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Polymers

Adopted from : Zbigniew D Jastrzebski, “The Nature And Properties of Engineering Materials”, John Wiley & Sons, ISBN

0-471-63693-2, 1987, CHAPTER 10.

Darpublic

October 2013 – For Students Attending EL3004

Not for commercial

POLYMERS

[Adopted from: Zbigniew D Jastrzebski, “The Nature And Properties of Engineering Materials”, John Wiley & Sons, ISBN 0-471-63693-2, 1987, CHAPTER 10.]

A large group of engineering materials of steadily increasing importance in industrial applications is composed of natural and synthetic organic polymers. Natural polymers such as starch and cellulose are the basic constituents of all plants, while proteins form the basis for all animal life. Advances in our understanding of the relation between the molecular structure of polymers and their chemical and physical properties make it possible to design and produce various polymeric materials of required characteristics for specific engineering applications. Three main types of commercial products are considered: plastics. rubbers, and fibers.

FORMATION OF POLYMERS

High polymers are formed by polymerization reactions that occur by two main mechanisms: addition polymerization and condensation polymerization .The name polymer is formed by adding the prefix “poly” to the monomer generic name, for example, polyethylene. When the monomer has a substituted parent name or a multiword name, the parentheses are used after the prefix “poly.” Thus we can write poly(vinyl chloride), poly(propylene oxide), and poly(chlorotrifluoroethylene). The parentheses are purposely omitted in common usage.

10-1 ADDITION POLYMERIZATION

Addition polymerization is defined as the reaction that yields a product that is an exact multiple of the original monomeric molecule. Such a monomeric molecule usually contains one or more double bonds that, by intermolecular rearrangement, may make the molecule bifunctional. Addition polymerization reactions usually proceed by a chain mechanism involving either free radicals or ionic catalysis. The reaction involves three steps: initiation, chain propagation, and termination (Equation 10-lb, 10-1c, and 10-1d). Initiation involves the dissociation of an initiator or catalyst into two free radicals (R) and addition to a monomer molecule M to form an active radical, R − M*.

Propagation or growth of the polymer chain results from successive addition of other monomers to the active group.

Propagation

Termination may occur because of collision between the active ends of two growing chains resulting either in their combination (coupling) or chain transfer mechanism, or by addition of terminator such as a free radical or another molecular species present in the system.

Termination

Examples are given by polymerization of ethylene, styrene, and vinyl chloride.

In all these reactions only bifunctional monomeric molecules are formed by the intermolecular rearrangement of the double bond present in the original molecule. This can result only in the formation of long chains.

Another kind of addition polymerization is copolymerization. Copolymerization is the simultaneous polymerization of two or more chemically different monomers that react to form a polymer containing both monomers linked in one chain. For example. GRS rubber is the product of copolymerization of butadiene and styrene.

In this long-chain polymer double bonds are still present that, on activation, are able to form crosslinks between the chains in further polymerization reactions.

10-2 CONDENSATION POLYMERIZATION

Condensation polymerization can be defined as the reactions between functional molecules that lead to the formation of a polymer with elimination of some small molecules, usually water. Condensation polymerization reactions proceed by a stepwise intermolecular mechanism. The following reactions between dicarboxylic acid and dihydroxy alcohol, resulting in polyesters, illustrate this process.

The resultant molecule reacts again in the same way with the dihydroxy alcohol molecule; the process repeats itself until linear chains of indefinite length are formed.

R and R’ stand for organic groups such as CH 2 , (CH 2 ) n , and others. The resulting polymer is a linear polymer that, depending on the degree of polymerization, may range from viscous liquids to rigid solids. Because of its relatively symmetrical structure and the presence of numerous polar groups, the polymer is a good fiber-forming material and can be converted into commercial fibers such as Dacron and Terylene.

Another reaction of polycondensation, which results in the formation of long linear chains, is used in the manufacture of nylon.

The resultant molecule can subsequently react with other molecules or either adipic acid or hexamethylenediamine, yielding a linear polyamide, characterized by the linkage —NH ⋅ CO—.

These reactions also show that a compound having two functional groups at the end can only produce long linear molecules.

Still another type of polycondensation reaction can be illustrated by the formation of a linear polycarbonate polymer.

Here, the HCl molecule is evolved as the result of condensation and the carbonate linkage

is formed. Phenol is an aromatic compound consisting of six-membered carbon ring of the benzene type in which one hydrogen atom is replaced by the hydroxyl group.

The accepted schematic designation of the benzene ring is shown here.

10-3 CONFIGURATION OF POLYMER CHAIN

During addition polymerization of a monomer A, the resultant polymer can be either a straight chain

Furthermore, the monomer molecules can react in one or more of the following ways: head-to- head, head-to-tail, and tail-to-tail. This is illustrated by the formation of vinyl polymers, which are obtained from vinyl monomers with the general formula

where X may stand for any atom or group, such as H, Cl. F, CH 3 , etc. The mechanism of polymerization involves adding a free radical R’ to the monomer in two possible ways:

Form I would lead to the formation of a head-to-tail arrangement in which the substituents occur on the alternate carbon atoms as shown by

A combination of forms I and II may lead to the tail-to-tail or head-to-head arrangement:

The possibility of obtaining only a regular head-to-head or tail-to-tail arrangement is relatively remote. Usually a mixture of all these arrangements occurs.

In the case of copolymerization, where two different monomers A and B are added together, or polycondensation reactions, where two different monomers react with the evolution of a small molecule, there is a greater variety of structural forms and, correspondingly, greater differences in polymer properties may result. Depending on the reactivities of monomers A and B on their In the case of copolymerization, where two different monomers A and B are added together, or polycondensation reactions, where two different monomers react with the evolution of a small molecule, there is a greater variety of structural forms and, correspondingly, greater differences in polymer properties may result. Depending on the reactivities of monomers A and B on their

Alternating copolymers are formed when the A and B units are placed alternately along the polymer chain:

Another possibility is the formation of a graft copolymer, which is essentially a branched-chain structure having side chains composed of one type of the monomer unit attached to the backbone chain from another monomer unit:

Such a graft copolymer can be produced either by prepolymerizing monomer B and grafting it onto the main backbone chain consisting of the monomer A.

or by polymerizing in situ where a molecule first attaches to the backbone chain A and forms a “grafted-on” section:

Stereoregular Polymers . Using special stereospecifïc catalysts, like Ziegler-Natta, it is possible to control the stereoregularity of the polymer chain by varying the type of initiator and the polymerization conditions. For example, a polymer chain of the composition contains asymmetric carbon atoms (C*) holding bulky R groups.

The R group may stand for CH 3 ,C 2 H 5 ,C 6 H 5 , and the like. If the R groups are all located on the same side of the plane, above or below, the arrangement is called isotactic; if the groups are alternate regularly from one side of the plane to another a syndiotactic polymer is obtained (Fig. 10-1). Finally, if the groups are oriented at random then the polymer has an atactic arrangement The R group may stand for CH 3 ,C 2 H 5 ,C 6 H 5 , and the like. If the R groups are all located on the same side of the plane, above or below, the arrangement is called isotactic; if the groups are alternate regularly from one side of the plane to another a syndiotactic polymer is obtained (Fig. 10-1). Finally, if the groups are oriented at random then the polymer has an atactic arrangement

FIGURE 10-1 Stereoisomers of a polymer chain having a bulky group R along the backbone chain.

The bonds between the repeat units of a polymer chain are relatively flexible to permit rotation of the groups about the bond. This may result in various helical conformations of the chain to attain their close packing. The distinct rotational states of the groups that stabilize the helical conformation are the trans position and the two gauche positions. The trans and two gauche forms are alternately located along the backbone polymer chain to relieve any steric hindrance that might arise because of the bulky group. For two gauche positions left or right, either a left- hand helix or right-hand helix is formed (Fig. 10-2). A polymer helix is usually characterized by the identity period, I.P., and by the number of monomeric or repeat units in this period. For example, the propylene helix has an identity period of 650 pm and the number of repeat units per turn is 3. If the side groups are very bulky more space is required and the resultant helix may contain more than three repeat units per turn, forming much looser helices.

10-4 MOLECULAR WEIGHT DISTRIBUTION

In straight-chain and branched-chain polymers, the individual chains are held together by intermolecular forces of attraction that increase with molecular weight or chain length of the polymer. Such polymers exhibit mechanical strength only when macromolecules have a chain length greater than about 150 to 200 atoms in line. Shorter chains produce oils or crystalline solids, such as paraffin. The lower molecular weight polymers are quite soft and gummy resins, but they are brittle to impact at fairly low temperatures. The higher molecular weight resins are tougher and more heat resistant. Thus, by controlling the molecular weight or chain length of individual macromolecules, it is possible to vary the properties of the polymer from soft and flexible up to hard, hornlike products.

FIGURE 10.2 Helical conformation of an isotactic vinyl polymer. IP is the identity period of the repeat distance of the helix containing three repeat units. Hydrogen atoms are not shown.

As the molecular weight of the polymer is related to its chain length or to the degree of polymerization, it can be defined as

where M m is the molecular weight of the monomer and DP denotes the degree of polymerization. During the course of the polymerization reactions, the polymer chains grow to different lengths, giving a product consisting of a mixture of macromolecules of wide molecular weight range. This is because the rates governing the growth and termination of polymer chains are subject to the hazards of random encounter of reacting molecules. Thus the resulting distribution of molecular weight of the polymer can, in most cases, be estimated from statistical analysis of kinetics of polymerization reactions. To characterize the polymer, we have to use certain average values of molecular weight. These averages can be obtained by several different methods and are defined below.

Number-average molecular weight, M n , is the weight of a polymer sample divided directly by the number of molecules in this sample. Mathematically this is expressed as

Number-average molecular weights are obtained from such measurements as osmotic pressure, boiling point elevation, and freezing point depression. In all these methods the number of molecules for each fraction is counted in a known mass of the polymer and, through Avogadro’s number, the number-average molecular weight is estimated.

Weight-average molecular weight, M w , is defined as the weight fraction of w i of the polymer chains times their corresponding molecular weight M i divided by the total weight of the polymer

sample investigated. This can be represented by the relation

where w i is the weight of fraction i having mean molecular weight M i , and n i is defined as before. The weight-average molecular weight is usually determined by light scattering, which depends on the size and the mass of the molecule.

Number-average and weight-average molecular weights are most frequently used in characterization of the molecular weight and the molecular weight distribution of the polymer. Another two averages occasionally used are z-average molecular weight, obtained by sedimentation in an ultracentrifuge and defined as

and viscosity-average molecular weight, defined by

where K and a are constants to be determined experimentally. The term [ η ] is specific viscosity of a polymer solution, which is determined from

The curve representing the number and weight fraction of the molecular-weight distribution is given in Fig. 10-3, which shows the number- and weight average molecular weights. For a polymer with all chains of the same length all four averages will be the same:

Frequently, the ratio of the weight-average molecular weight to the number average molecular weight, M w / M n is used to determine the spread of the molecular weight distribution of the

polymer. For a narrow molecular weight distribution the ratio M w / M n is close to one but, for a broad molecular weight distribution, it may be as high as 3 to 10.

Illustrative Problem 10.1

Solution:

FIGURE 10-3 Differential molecular weight distribution. Illustrative problem10-1.

LINEAR POLYMERS

Linear polymers form the largest group of plastics covering a great variety of diversified products used in different forms and applications. Linear polymers can be obtained by either addition polymerization or condensation polymerization of bifunctional monomers. Being thermoplastic, they can be easily worked into required forms and shapes at elevated temperatures.

The structure and properties of linear polymers depend on the chemical nature of the monomer, the geometry of the polymer chain, and the magnitude of the intermolecular forces between the chains. These intermolecular forces depend on the molecular weight or the chain length, the presence of polar groups and their spacing and regular distribution along the backbone chain, the possibility of formation of the hydrogen bond, and the distance between chains. The structural regularity of the chain determines the degree of packing of the chains and its state, either amorphous, crystalline, or semicrystalline.

10-5 DEGREE OF CRYSTALLINITY

Polymers crystallized from melt on cooling of different degrees of perfection consist of individual single crystal lamellae connected to each other through tie molecules, which may meander randomly through the disordered regions before participating in the formation of another chain-folded crystal (Fig. 10-4). We can therefore consider such a crystalline polymer as

a two-phase system consisting of an amorphous phase and an ordered crystalline phase that differ in their density and other physical characteristics. Crystallization causes a denser packing of molecules, increasing the intermolecular forces. The degree of crystallinity may range from 0% for noncrystallizable polymers through intermediate crystallinities such as 20% for poly(vinyl chloride), 50% for branched polyethylene, and up to 95% for polytetrafluoroethylene (TFE) and

a linear polyethylene.

FIGURE 10.4 Schematic representation of the typical crystalline polymer showing randomly arranged crystalline and amorphous regions. A polymer chain may go through

several crystalline and amorphous regions .

Crystallinity in polymers crystallized from the melt develops through spherulitic growth (see Fig. 5-15). Spherulites have different sizes and degrees of perfection, and they completely fill the volume of all well-crystallized polymers. They play a role similar to that of the grain structures in polycrystalline metals. The crystalline phase has a higher and sharper softening point, a greater tensile strength, and a greater density than the amorphous phase of the same chemical nature and molecular weight. For example, the isotactic crystalline polystyrene has a melting point of 230°C (446°F), whereas the normal amorphous polystyrene of similar molecular weight has a softening point of only 90°C (190°F). Similarly, branched-chain polyethylene, which is only up to 60% crystalline, has a density of 910 to 930 kg/m 3 (0.910—0.930 g/cm) and a melting point of 115°C

(240°F). whereas linear polyethylene, considered to be 90 to 95% crystalline, has a density of 940 to 965 kg/rn3 and a melting point of 135°C (275°F).

The density of perfectly crystalline material can be obtained from X-ray measurement, while that of the amorphous material can be easily determined above its melting point and extrapolated to room temperature. Thus the percentage crystallinity can be calculated from the relation

The degree of crystallinity and the amount of amorphous region can be controlled by copolymerization, which lowers the structural symmetry of the polymer chain, thereby decreasing the crystallization tendency. For example, vinylidene chloride is usually copolymenzed with about 10% to 15% vinyl chloride to produce a material of greater flexibility than the pure vinylidene polymer. A copolymerization is a normal procedure in producing rubberlike polymers. A crystalline or amorphous polymer can be made more flexible by adding plasticizers.

Crystallinity of a polymer is also controlled by its cooling rate. At fast cooling rates, even a strongly crystalline polymer will not be able to crystallize and will form an amorphous structure. Many polymers that crystallize more slowly can be quenched to a glassy amorphous solid and, if

their T g is sufficiently high, as in the case of a polyamide (nylon 66) or isotactic polystyrene and poly(ethylene terephthalate), they may remain amorphous at room temperature for an indefinite period of time. On the other hand the rate of crystallization in polyethylene and Teflon is so high that crystallization cannot be prevented by quenching the melt, even in liquid nitrogen.

10-6 EFFECT OF POLAR GROUPS

The presence of a polar group

in the monomer increases the intermolecular forces between the chains, resulting in a higher softening point and a greater stiffness and rigidity of polymer, as exemplified by poly(vinyl chloride) compared with polyethylene. A normal poly(vinyl chloride), although much less crystalline than polyethylene because of the introduction of a chlorine atom, which lowers the symmetry of the chain, is stronger than polyethylene:

However, when polar groups are spaced regularly and symmetrically along the backbone, polymer chain crystallization is enhanced, as exemplified by linear polyesters (Dacron), polyamides (nylons), polyvinylidene chloride, and others. Such structures produce polymers of high melting points, high rigidity, and tenacity. The polyamides and polyterephthalates have a molecular weight only around 20,000. This value is an order of magnitude lower than that of other plastics, such as polystyrene. poly(methyl methacrylate), poly(vinylchloride), and polyethylene, which would have very low strength with such a low molecular weight. This shows clearly that the excellent strength of polyamides is due to the strong secondary valences caused by the presence of the regularly ordered polar groups. Thus, in nylon 66, the However, when polar groups are spaced regularly and symmetrically along the backbone, polymer chain crystallization is enhanced, as exemplified by linear polyesters (Dacron), polyamides (nylons), polyvinylidene chloride, and others. Such structures produce polymers of high melting points, high rigidity, and tenacity. The polyamides and polyterephthalates have a molecular weight only around 20,000. This value is an order of magnitude lower than that of other plastics, such as polystyrene. poly(methyl methacrylate), poly(vinylchloride), and polyethylene, which would have very low strength with such a low molecular weight. This shows clearly that the excellent strength of polyamides is due to the strong secondary valences caused by the presence of the regularly ordered polar groups. Thus, in nylon 66, the

The structure shows three hydroxyl groups on each glucose unit. The hydrogen-bonded structure of cellulose and its molecular weight account for the very strong intermolecular forces between the chains. These give high rigidity and crystallinity to the polymer, preventing it from melting and dissolving below its decomposition temperature. To make the cellulose more amenable for manufacturing operations, it is necessary to decrease these intermolecular forces by reducing the molecular weight or by neutralizing the polar character of the hydroxyl groups.

Regenerated cellulose is essentially the same chemical compound as natural cellulose, but it has a much lower molecular weight, containing only from 300 to 500 glucose units. This lower molecular weight decreases greatly the intermolecular forces between the individual chains, making it possible to obtain cellulose in a solution form. This reduction of the molecular weight is accomplished by special chemical treatments with sodium hydroxide and subsequent treatment with carbon disulfide to produce a heavy viscous solution, known as viscose. The viscose can be spun to give fibers or extruded through a die for film. Then the cellulose is regenerated from this viscous solution by precipitating it in an acid batch as fibers (viscose rayon) or as film (cellophane), If the hydroxyl groups are replaced by acetate groups in the natural cellulose as the result of an esterification reaction with acetic anhydride and glacial acetic acid so that about 2 to 2.5 acetyl groups per glucose residue remain, cellulose acetate is produced. Cellulose acetate is no longer soluble in sodium hydroxide, but it is soluble in acetone. Complete substitution of three acetyl groups per glucose unit, however, makes the polymer insoluble in acetone. Cellulose acetate can be spun from viscous solution in acetone as flbers or used with plasticizers to form various plastic products and films.

Other industrially important cellulosic polymers that are obtained from natural cellulose by modifying its molecular weight and the polarity of the side groups are cellulose nitrate, cellulose acetate—butyrate, cellulose propionate, ethyl and methyl cellulose, and hydroxy- and sodium carboxymethyl cellulose.

The requirements of high tensile strength and high melting point (usually above 200°C (400°F)) require a high cohesion energy associated with a high degree of crystallinity. Thus the characteristic feature of a linear polymer to be a good fiber-making material is a high geometrical symmetry of the polymer chain and high intermolecular forces between the chains. Branching is not desirable because it disrupts the crystalline lattice and lowers the crystallinity.

The crystallization of a polymer can be enhanced by its exposure to a shear gradient, as in stretching or drawing the solution or the melt of the polymer during or after cooling. This is used in the production of synthetic fibers and films. The melt or solution is first squeezed through a thin spinneret or a die, and the resulting fiber or film is stretched while being cooled or the solvent from viscous solution is evaporated.

10-7 TRANSITION TEMPERATURES

Since crystalline polymers are never completely crystalline and contain both ordered (crystalline) regions and unordered amorphous regions, depending on the structure and thermal history of the polymer, they usually exhibit two characteristic temperatures: the melting point, T m , defined as the temperature at which crystalline aggregates disappear, and

the glass transition temperature, T g . The glassy state of polymers is a rigid and brittle noncrystalline state. However, the brittle

characteristics vary widely from polymer to polymer and with temperature. The glass transition temperature arises because of hindered relaxation of the chain molecules as the volume decreases with the temperature. Although the translational movement of the chain segment can be frozen and the rotation of the bulky groups may be stopped, some degree of vibration and local motion of the atoms may occur. Thus many polymers have other secondary or tertiary transitions in addition to the main glass transition. For example, crystalline polytetrafluoroethylene (TFE) shows four different transitions below its melting point of 327°C the main glass transition at + 127°C and the other three glass transitions at +30 −

19, and − 97°C. Polycarbonate has a glass transition of 150°C but it is a strong, rigid polymer having exceptional impact resistance at temperatures much below T g . This appears to be due to the local motion within the carbonate linkage

which persists up to − 110°C and is capable of absorbing energy upon impact loading. Another example can be provided by nylon 66, which has a main glass transition at about 50°C, but retains a crankshaft motion of its methyl groups within the chain up to a temperature of − 120°C.

Polymers may also exhibit the decomposition temperature, T d , at which the molecular structure breaks down and the material decomposes. These three temperatures (T m ,T g , and T d ) define the range of applicability of the polymers in practice. Rubbers are used between T g and T d , whereas amorphous polymers are used below their glass transition temperature, but Polymers may also exhibit the decomposition temperature, T d , at which the molecular structure breaks down and the material decomposes. These three temperatures (T m ,T g , and T d ) define the range of applicability of the polymers in practice. Rubbers are used between T g and T d , whereas amorphous polymers are used below their glass transition temperature, but

Crystalline polymers can be used up to their T m ; they are hot formed and shaped between T m

and T d and cold formed between T g and T m . Because of the complex structure of crystalline

polymers and the many defects in the crystalline regions, their melting temperature is usually

within a few degrees of the thermodynamic melting temperature, T m ==== ∆ H f / ∆ S f .

The heat capacity, coefficient of expansion, diffusion coefficient, and elastic moduli undergo rapid changes in going through the glass transition. The mechanical damping of low-

frequency oscillations exhibits a sharp peak in the region of T g . The glass transition

temperature increases with the increase in intermolecular forces, molecular weight, crosslinking, and bulky and side group substituents that restrict rotation. For crystalline polymers the glass transition temperature is related to the melting point:

0.5, whereas for unsymmetrical polymers such as poly(trifluorochloroethylefle) or propylene, the ratio T g /T m is about 0.67. The glass transition temperature is lowered with the addition of plasticizers.

For symmetric polymers T g /T m ≈

Below T g , the segments of an amorphous polymer undergo vibratory motions around fixed positions, while above T g , the segments exhibit translational and diffusional motions. Above

T g . linear amorphous polymers exhibit time-dependent behavior. At a small constant stress, they show an elastic response, a retarded recoverable response, and finally, a non-Newtonian flow. Amorphous linear (thermoplastic) polymers show the glass transition temperature which may vary from 40 to 150°C (104—300°F) and sometimes higher, depending on the type and the structure of the polymer.

For high molecular weight polymers, the solid is glassy below T g but, as the temperature

gradually increases, it becomes leathery, rubbery and, finally, liquid. For low molecular

weight polymer, the polymer changes from glassy below T g to liquid above T g . The major factor determining the value of T g is the flexibility of the polymer chain. Steric hindrance and bulkiness of the side groups attached to the backbone chain usually cause an increase in T g .

The viscoelastic behavior of a polymer as a function of temperature is illustrated in Fig. 10-5, which shows the changes in modulus elasticity versus temperature for different structural arrangements.

There are five regions of viscoelastic behavior of the amorphous polymer. The glassy state

below T g characterized by a steady value of the modulus of elasticity, the glass transition region T g where the modulus drops rapidly, the rubbery plateau region with a steady value of the modulus, and the rubbery flow and liquid flow regions where the modulus again sharply drops. For a crosslinked polymer (curve D) a plateau is reached and the polymer is infusible. This depends on the degree of crosslinking, and the curve moves up with increasing density of crosslinks of the polymer. For such a highly crosslinked polymer the glass transition temperature does not have any physical meaning.

FlGUR 10-5 Changes of the modulus elasticity with temperature at constant time of 10 s for different structural arrangements. E r (10) is the relaxation tensile modulus during 10 s at a particular temperature. Curve A represents an amorphous polymer of low molecular weight, B is an amorphous polymer of high molecular weight, C is the crystalline polymer, and D is a crosslinked polymer. (After A. V. Tobolsky, Propemes and Structure of Polymers, John Wiley & Sons, Inc.. New York, 1960.)

CROSSLINKING IN POLYMERS

Crosslinking usually involves the introduction of a covalent type of link between polymer chains or their segments. An initially small amount of cross-linking causes formation of some branched molecules that still are soluble but, on further reaction, gelation sets in. This stage is characterized by the presence of insoluble gel and the soluble sol, which can be extracted from the gel. On further crosslinking a giant three-dimensional network is formed that imparts rigidity, infusibility, insolubility, and improved heat resistance to the polymer. If the crosslinks are short and densely located, hard and strong polymers are obtained that exhibit little elongation and high moduli. Crosslinking may occur through (1) functional groups, (2) addition at the double bond of the polymer or to the reactive groups located along the polymer chain, (3) radical formation, and (4) secondary valences, such as vander Waals forces, dipole-dipole interaction, hydrogen bond, and ionic bonding.

10-8 CROSSLINKING THROUGH FUNCTIONAL GROUPS

This mechanism of crosslinking involves a condensation reaction that follows exactly the same path as that of the original polycondensation reactions during the formation of the polymer macromolecule. For this reason the term thermoset is applied to such crosslinked polymers, since their final curing after application is carried out under a pressure adequate to prevent the evolution of water molecules formed during crosslinking from the system.

Phenol-Formaldehyde (Phenolic) Resins. Phenol-formaldehyde resins, simply called phenolic resins, are formed by polycondensation reactions between phenol and formaldehyde in the presence of suitable catalysts by a one- or two-stage process. The initial reaction results in the formation of mono-, di-, and trimethylol phenols, depending on the ratio of phenol to formaldehyde (P/F), the kind of catalyst, the temperature, and the time of reaction. Two quite separate phenolic resins are produced:

With two or three molecules of formaldehyde we obtain

The reaction between methylol phenols and phenol may occur either (1) between the methylol hydroxyl group and the hydrogen in the benzene ring, or (2) between the two methylol hydroxyl groups. In both cases the water molecule is split off:

Further heating produces a linear polymer of varying length:

A bond through the radical —CH2— in Reaction 10-37 is called the methylene linkage; the bond through the radical —CH2—O—CH2— in Reaction 10-38 is called the ether linkage. In the one-stage process, phenol is reacted in the presence of alkaline catalysts with an excess of formaldehyde, so that the phenol-to-formaldehyde ratio (P/F) is less than one. The reaction is stopped when the product is still soluble and fusible, producing either A-stage or B-stage resin. The A-stage resin, called resol, is a relatively short, low molecular weight, linear polymer, which is completely soluble in the alkaline solution present in the reaction vessel. The B stage, called resitol, is a rather long, linear polymer with a slight amount of crosslinking between chains; it is insoluble in alkaline so lutions but readily soluble inorganic solvents and it is fusible. Resin A or B, or the combination of both, is used for adhesives, casting, plastics, and laminates.

In the two-stage process only a part of formaldehyde is introduced so that the P/F ratio is greater than one. The reaction is carried out in the presence of acid catalysts. and it results in

a hard, brittle product, called Novolac resin which, still being a linear chain, is fusible and soluble. Novolac resin is then ground to a powder and mixed with proper ingredients such as fillers, stabilizers. lubricants, and dies. A crosslinking agent (hexamethylenetetramine) is added to provide the source of

—CH 2 — links for subsequent crosslinking reactions. Final curing of either A- or B-stage resin or Novolac, which is carried out after the application of material, gives a three- —CH 2 — links for subsequent crosslinking reactions. Final curing of either A- or B-stage resin or Novolac, which is carried out after the application of material, gives a three-

Phenolic resins find numerous and varied industrial applications as adhesives, casting, coatings, laminates, and structural products with various fillers and fibers where high rigidity, corrosion resistance, and heat resistance are required.

Amino ResIns. Similar mechanisms of reactions occur during polycondensation of urea- formaldehyde and melamine-formaldehyde, producing various amino resins known as urea and melamine resins:

10-9 CROSSLINKING THROUGH ADDITION

Crosslinking through addition may involve reaction of an unsaturated compound between two polymer chains containing a polymerizable double bond, as in unsaturated polyesters or some rubbers. Another addition reaction may occur between a compound (catalyst) and the active end group of prepolymer chains, as exemplified by curing epoxy and polyurethane resins. In the first case the covalent crosslink is formed through the rearrangement of double bonds in the crosslinking agent as well as in the polymer. In the second case the crosslink results from the rearrangement of atoms in the end groups so that a simple addition of the catalyst molecule to both chains becomes possible. In both cases no molecule is evolved as the result of crosslinking reactions. This makes possible easy casting of the prepolymer resin with the catalyst (crosslinking agents) and various additives such as reinforcing agents under normal atmospheric pressure.

Unsaturated Polyesters. Linear, or saturated, polyesters as mentioned before, are typical thermoplastic polymers, and they are not subject to crosslinking. However, unsaturated polyesters and alkyd resins offer another possibility of introducing crosslinks to produce a network polymer. The unsaturated polyesters are obtained by introducing unsaturated dibasic acids such as maleic and fumaric or dibasic unsaturated alcohols. The resulting polymers are usually crosslinked by monomeric compounds such as styrene, resulting in a thermosetting resin, as illustrated by the following reactions:

A vinyl-type compound of general formula

is used for crosslinking of the linear chains, as shown below. The reaction occurs by simple addition. The extent of crosslinking can be controlled by varying the amount of unsaturation in the polyester and the amount and kind of crosslinking agent. For example, an unsaturated polyester crosslinked with styrene is harder and tougher than that crosslinked with methyl methacrylate. Unsaturated polyester resins are widely used as glass-fiber reinforced materials for various structural applications.

Epoxy Resins. Epoxy resins are the combination of bisphenol A and epichlorohydrin. Which leads to the formation of a relatively short chain linear polymer containing two reactive groups, epoxide and hydroxyl:

The reactive epoxide and hydroxyl groups are the points of reaction with catalysts, also called curing agents or hardeners, to form a three-dimensional network. Such catalysts, as various polyamines, react only with an epoxide group, whereas aliphatic and aromatic anhydrides react with both hydroxyl and terminal epoxy groups. Thus the density of the resultant crosslinks in a cured resin is much greater with anhydrides and catalysts, especially with dianhydrides instead of with amines. The example of crosslinking between the epoxide groups of the linear epoxy resin is given here using ethylenediamine as a catalyst. Note that no elimination of a small molecule takes place during crosslinking.

It is also possible to carry out a wide range of crosslinking reactions with other polymeric resins such as amines, phenolics, polyamides, cellulose, and vegetable oil fatty acids, producing a great variety of products of specifically modified properties.

Because the ether linkage is a very stable one, epoxy resins have high chemical resistance to water, various solvents, acids and alkalies, and other chemicals. The reactive groups are comparatively widely spaced, resulting in high flexibility, but, at the same time, the presence of crosslinks accounts for the toughness and heat resistance of the cured polymer. Furthermore, the polar nature of such groups as epoxide and hydroxyl contributes to good adhesion. Epoxy resins are most frequently used as coatings, adhesives, and glass-fiber reinforced plastics.

10-10 CROSSLINKING BY FREE RADICALS

This crosslinking may be accomplished by using free radicals that being a very active species, can attach themselves between the polymer chains, forming covalent links. Such free radicals can

be formed as the result of intensive irradiation or by chemical action on the prepolymer, creating active centers along the backbone chains, or by using highly reactive compounds as organic peroxides and azo compounds. On heating, the latter decompose, giving free radicals.

Crosslinking by Irradiation. Radiation-induced crosslinking occurs as the result of the impact of high-energy radiation on the polymer chains, which causes knocking out of hydrogen atoms and produces secondary free radicals on the polymer chain. This makes possible the formation of covalent links between the polymer chains. Crosslinking is generally found to occur more readily in the amorphous regions between polar crystallites.

High-energy irradiation results in either crosslinking or chain scission, depending on the chemical structure of the polymer, and on the dose of radiation. Polymers that crosslink are polymethylene, polyethylene, polypropylene, polystyrene, polyvinyl chloride, polyamides, polyethylene siloxane, and others. Polymers that disintegrate are polybutylene, polytetratluorethylene, poly(methylmethacrylate), cellulose derivatives, and others. High-energy radiation is also used to initiate polymerization in grafting polymer on another polymer as polystyrene and rubber.

Crosslinking by Organic Peroxide. Organic peroxides are frequently used in the curing of many polymeric products such as unsaturated polyesters and various saturated backbone rubbers. On heating, these peroxides decompose. giving free radicals that, in turn, activate the double bonds of the monomer, effecting polymerization. In the case of saturated backbone polymers such as polyethylene, fluororubber, silicone rubbers, and ethylene—propylene rubbers, crosslinking requires hydrogen abstraction as shown by crosslinking silicone rubber with peroxides:

Free radicals abstract hydrogen atoms from methyl groups of the two polymer chains forming a crosslink between them.

where R* stands for C 6 H 5 COO* or C 6 H 5 *.

10-11 CROSSLINKING THROUGH SECONDARY VALENCES

We can extend the concept of crosslinking through secondary valence forces, especially groups such as —COOH and —CONH 2 which, because of hydrogen bond formation, are especially tight together. Such secondary valence cross-linking does not always have to be caused by hydrogen bonds. In principle all groups with high molar cohesion are capable of secondary

valence cross-linking, especially polar groups such as —COOH and —SO 3 H. Secondary valence crosslinking differs from covalent crosslinking in that it disappears on

heating. This makes the polymer thermoplastic in nature and accounts for its easy processability as compared to completely crosslinked covalent materials. This secondary crosslinking can also

be removed by treatment with suitable polar solvents such as dimethylformamide. After removal of the solvent during wet spinning, the resulting threads behave as vulcanized rubber. Secondary valence crosslinking plays a highly important and decisive role in nature; all proteins are reversibly crosslinked through the —CONH— groups. A similar type of crosslinking can be found in polyelectrolytes, also called ionomers. lonomers are copolymers derived from ethylene and methacrylic acid in which the ionized carboxylic groups form ionic crosslinks in the intermolecular structure:

The ionized groups are neutralized by zinc or sodium ion.

10-12 ELASTOMERS

Elastomers are an important group of polymeric materials that are subject to many crosslinking processes to impart desired properties to the rubber. Any linear polymer can be made a good rubber if it meets the following characteristics.

1. The polymer chain should be very long and geometrically irregular so that thermal agitation will result in a strongly entangled and coiled-type arrangement.

2. The intermolecular forces between the polymer chains should be such that at room temperatures thermal energy is sufficient to maintain them in a state of constant mobility. This is comparable to the statement that the glass transition temperature should be below the working range of the temperature for a rubber.

3. There must be a possibility for introducing crosslinks between the chains so that the required degree of rigidity can be obtained.

To achieve these desired characteristics, synthetic rubbers are usually produced by copolymerization processes, which have a tendency to lower the symmetry and regularity of the chain and give a long chain of relatively weak intermolecular attraction. The presence of polar groups is usually avoided unless special characteristics such as oil resistance and improved heat resistance are required at the expense of flexibility of the polymer chain.

Vulcanization. Vulcanization is the term used in the rubber industry to refer to the variety of crosslinking processes used. Natural and synthetic rubbers such as styrene—butadiene (SBR), polybutadiene, acrylonitrile, and others contain a double bond capable of crosslinking (see Appendix A7). The mechanism of such crosslinking is essentially identical with crosslinking through addition polymerization, as illustrated by the curing of unsaturated polyesters. However, in a typical rubber, there is only about one crosslink to every few hundred chain atoms.

FIGURE 10-6 Vulcanization of natural rubber. (a) Mechanism of crosslinking of isoprene molecules by means of sulfur atoms between two carbon atoms with double bonds (b) Schematic representation of transition from the randomly crosslinked coiled snarls to the oriented state on stretching. The presence of crosslinks causes the chain molecules to return to their previously coiled conformation on relaxation of stress, thereby preventing the permanent

set.

Vulcanization can be accomplished by heating raw rubber with sulfur or some sulfur compounds and accelerating agents. The snarls of the rubber are fastened at certain points by the covalent bonds between sulfur and two carbon atoms associated with the double bond (Fig. 10-6a). The more numerous the points of linkage are, the greater restriction of molecular slip exists and the lower the extensibility is until, finally, in hard rubber (ebonite) the structure becomes similar to that of a completely crosslinked thermosetting resin such as Bakelite (phenol—formaldehyde resin). In the unstretched state the snarls are in a random arrangement, thereby accounting for the amorphous state of the polymer. On stretching, the snarls begin to disentangle and straighten out and the chains become oriented (Fig. 10-6b). This orientation results in crystallization that increases the attraction forces between the chains, causing the material to stiffen. When the force is released, strained bonds are allowed to return to the original random snarl arrangement of the molecules. Such changes in the molecular configuration account for characteristic elongation of rubbers on stretching and their contraction (unloading) on the release of force (see also Fig. 7-9). Some rubbers, especially the natural rubber, crystallize easily on stretching. considerably improving their tensile strength. On the other hand, synthetic rubbers such as styrene—butadiene Vulcanization can be accomplished by heating raw rubber with sulfur or some sulfur compounds and accelerating agents. The snarls of the rubber are fastened at certain points by the covalent bonds between sulfur and two carbon atoms associated with the double bond (Fig. 10-6a). The more numerous the points of linkage are, the greater restriction of molecular slip exists and the lower the extensibility is until, finally, in hard rubber (ebonite) the structure becomes similar to that of a completely crosslinked thermosetting resin such as Bakelite (phenol—formaldehyde resin). In the unstretched state the snarls are in a random arrangement, thereby accounting for the amorphous state of the polymer. On stretching, the snarls begin to disentangle and straighten out and the chains become oriented (Fig. 10-6b). This orientation results in crystallization that increases the attraction forces between the chains, causing the material to stiffen. When the force is released, strained bonds are allowed to return to the original random snarl arrangement of the molecules. Such changes in the molecular configuration account for characteristic elongation of rubbers on stretching and their contraction (unloading) on the release of force (see also Fig. 7-9). Some rubbers, especially the natural rubber, crystallize easily on stretching. considerably improving their tensile strength. On the other hand, synthetic rubbers such as styrene—butadiene