Introduction to Polymer Science and Technology

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INTRODUCTION TO POLYMER SCIENCE AND TECHNOLOGY

CHARLES E. CARRAHER, JR.

Department of Chemistry and Biochemistry, Florida Atlantic University, Boca Raton, FL 33431 and Florida Center for Environmental Studies, Palm Beach Gardens, FL 33410

Introduction Polymer Synthesis Synthetic Routes Polymer Testing Polymer Companies Polymer Structure-Property Relationships Solubility and Flexibility Size Polymer Shape Polymer Structures

Introduction

The term polymer is derived from the Greek's poly and mers meaning many parts. Some prefer the term macromolecule or large molecule.

There are many ways to measure the Importance of a specific discipline. One is to consider its pervasiveness. Polymers serve as the basis of life in the form of nucleic acids, proteins and polysaccharides. They permit replication, energy transformation, transmission of foods within plants and animals, act as essential natural building materials, ... Polymers have served as the very building blocks of society-clays for jars, wood for fuel, hides for clothing, vegetation for food and shelter,... They are present in a variety of forms-as fibers and cloths, paper, lumber, elastomers, plastics, coatings, adhesives, ceramics, enzymes, DNA, concretes, and are major ingredients in soils and plant life.

The basic concepts of polymer science apply equally to natural and synthetic polymers and to inorganic and organic polymers, and as such are important in medicine, nutrition, engineering, biology, physics, mathematics, computers, environment, space, ecology, health,...

Today, synthetic polymers are produced at the annual rate of over 200 pounds for every person in the USA. Paper products account for another over 200 pounds per person. This does not include the inorganic polymers such as graphite, glass and diamonds; natural polymers such as polysaccharides, llgnin, and proteins; and

22 C.E. Carraher, Jr.

regenerated materials such as rayon and rayon acetate. Polymers are involved in all of the major new technologies including synthetic blood

and skin; computer chips, CDs, liquid crystals, and circuit boards-information visualization, storage and retrieval; energy creation, storage, and transmission(portable electrical power (batteries), efficient, light, and low-emission transportation; high temperature superconductors; medicines, targeting and control of drug delivery, and synthetic limbs and other replacement parts; transportation; space craft; solar and nuclear energy; and photonics (optical fibers).

Science, in its broadest sense is our search to understand what is about us. This quest is marked by observation, testing, inquiring, gathering data, explaining, questioning, predicting, ... Technology had been described as simply applied science. The line dividing technology and science is often non-existent, though technology almost always follows science. The term "scientific method" is actually one that both scientists and each of us use everyday. As we bake cookies, we may vary the ratios and actual ingredients-testing (eating) the cookies and then drawing conclusion-and the next time using those changes we felt made a "better" cookie and laying aside those changes that were not positive. This is the process of inquiry, observation, testing, etc. noted above that allows us to understand, test, develop, and produce materials for the 21st century.

While the "technology" of polymers is ancient, the science of polymers Is relatively new. This polymer science waited until an understanding of basic scientific principles and concepts such as atoms, electrons, periodic table, balanced equations and chemical structures were in place. Even then, the concept of very large molecules was at times debated to 1950s, even after the birth of the polymer industry producing plastic products that replaced metal products and other materials. The Importance of polymeric materials is today accepted as basic chemistry within the chemistry curriculum of most chemistry courses.

Polvmer Svnthesis

Carothers brought together many of the polymer terms and concepts we use today. He noted a correlation between so-called vinyl polymers, polymers generally derived from the reaction with vinyl monomers such as vinyl chloride and styrene and the chain type of kinetics. He also noted a correlation between condensation polymers such as nylons and polyesters and step-wise kinetics. While these associations, condensation polymers/step-wise kinetics and vinyl polymers/chain kinetics, are generally accurate, there are exceptions.

Chain Type Kinetics. Chain type kinetics are probably the most studied and best understood kinetic system. With vinyl reactants, the reaction can be understood in terms of three main reactions. Using the free radical process employing an initiator,!, monomer M and free radical with a 'v' we can deschbe these three groupings.

Initiation I—>2R* (1) R# + M —> R.M» or simply M* (2)

Introduction to Polymer Science and Technology 23

Since the activation energy for decomposition is higher than that for addition of R* to M, the first step is the rate determining step with an associated specific rate constant kd so that the rate of formation of initiator radical chains is

Rate, = Ml ] (3)

This expression is generally modified to reflect that two active free radicals are created for each decomposition of I by inclusion of a "2". Further, all R* 's do not create growing polymer-producing free radical chains so that some factor, f, is added that Is the fraction of chains that do successfully begin chain growth. Thus the rate of radical formation is generally given as

Rate, = 2k,f[l] (4)

Propagation M-+M—>M-M- (5) M-M'+M—>M-M-.M-M-M-M- + M —> M-M-M-M* Etc.

Since it is experimentally found that the rates of addition of the various monomers to the growing free radical chain are similar, it is customary to treat the entire propagation sequence as follows.

M- + M —> M' (6)

with an associated rate expression for chain propagation (p)

Ratep = kp [M-][M] (7)

Termination

Termination is generally via two mechanisms combination M* + M* —> M-M (8)

and disproportionation M* + M* —> M + M (9)

The associated rate expression for termination (tr) is

Rate,, = 2k,,[M-][M-] (10)

The "2" is added to recognize the fact that two growing chains are involved and are destroyed each time termination occurs.


The rate of monomer use can be described as

Rate = kp [M*][M] + k| [Ri[M] (11)

where kj is the specific rate constant for the monomer-consuming initiation step.

Since most of the monomer consumption occurs in the growth or propagation stage the second term can be neglected giving

Ratep = kp [M1[M] (12)

While this is a simple rate expression, the concentration of free radical monomer is not easily measured. Thus, the typical kinetic treatment seeks a way to describe the reaction rate in terms of more easily measurable values.

Experimentally it is found that the total number of "live" or growing free radical chains is generally about constant during the polymerization process so that the rate of initiation and propagation are the same.

ki [R1[M] = 2Kr[M*][IVh] (13)

Further, there is no change in R* with time so that the two rate expressions for the initiation reactions involving the creation and demise of R* are equal to one another.

2k,f[l] = k,.[Ri[M] (14)

Through manipulation of equations 13 and 14, it is possible to get an expression for [M«] that contains easily measurable quantities that can be substituted In for [M*].

Ratep = kp [M-][M] = kp [M] (KmiKt (^5)

The kinetic expressions for condensation reactions are similar to those of most other Lewis acid base reactions. For the formation of polyesters from the reaction of a diacid, A, and a diol, B, in the absence of added catalysts it is found that the rate expression Is

Rate = k [A][B] and where [A] = [B] the expression becomes Rate = k[A]2 (16)

Given that "p" is the extent of reaction so that "1-p" is the fraction of functional groups that have not reacted, we have on integration and subsequent substitution

kt = (1/[AJ).(1/[A,]) (17)

and that


[M = [Ao](1-p) (18)

giving after rearrangement

[A,]kt = (1/1-p)-1 (19)

Thus, a linear relationship of 1/1-p with time exists. Polymer molecular weight Is quite sensitive to the presence of impurities in step

wise processes. This is demonstrated in the following calculation. The degree of polymerization, DP, or number of repeat units is defined as

[Ao]/[M (20)

Multiplying Equation 19 by [AJ and rearranging gives

DP = [AJkt + 1 (21)

Combining this with Equation 18 gives

DP = 1/1-p (22)

Thus, high purity and high extent of reaction is necessary to achieve high molecular weight polymers.

Free Radical Copolvmerization. While the mechanism of copolymerization is similar to that described for homo-polymerizations, the reactivities of monomers may differ when more than one monomer is present in the reaction giving polymer chains with amounts varying with the co-monomer concentrations. The difference In the reactivity of monomers can be expressed with reactivity ratios, r.

The copolymer equation, which expresses the composition of growing chains at any reaction time t, was developed in the 1930s by a group of investigators including Wall, Mayo, Simha, Alfrey, Dorstal, and Lewis.

Four chain extension reactions are possible when monomers A and B are present in a polymerization reaction mixture. Two of these steps are self-propagation steps and two are cross-propagation steps. The difference in the reactivity of the monomers can be expressed in terms of reactivity ratios that are the ratios of the propagating steps. The difJFerence in the reactivity of the monomers can be expressed in terms of reactivity ratios that are the ratios of the propagating rate constants where r1 = k̂ g /K bb and r2 = k bb/k ba-

A* + A — > AA • Rate = k33[A ]̂[A] (23)

A • + B -—> AB • Rate = k3b[A*][B] (24)

B •+ B -—> BB • Rate = kbb[B •][B] (25)


B • + A — > BA • Rate = kba[B •][A] (26)

Experimentally, as in the case of chain polymerization, the specific rate constants are found to be approximately independent of chain length, with monomer addition primarily dependent only on the adding monomer unit and the growing end. Thus, the four equations above are sufficient to describe the monomer consumption. The rate of monomer consumption can be described by the following:

Rate of A consumption = -d[A]/dt = \^ [A*][A] + k ba[B ̂ [A] and (27)

Rate of B consumption = -d[B]/dt = k J B ^][B] + k3b[A*][B] (28)

The relative rate of monomer consumption is found by simply dividing the two expressions describing the rate of monomer consumption.

Again, experimentally it is found that the number of growing chains remains essentially constant giving a steady state concentration of monomer radical. Further, the concentrations of A •and B* are found to be the same and the rate of conversion of A* to B • is equal to the rate of conversion of B* to A*. Thus,

kJB*][A] = k3b[A*][B] (29)

Solving for [A*] gives [A*] = k ba[B*][A]/k JJS\ (30)

Substitution of this expression for [A»] into the expression describing the relative rate of monomer consumption and rearrangement gives

d[A]/d[B] = ([A]/[B])(r1 [A] + [B]/[A] + r2[B]) (31)

which gives the copolymer composition at any monomer concentration without the need to know any free radical concentrations.

From knowledge of the r values it is possible to predict the type of copolymer composition that is being formed. For instance, when the r values are both less than 1, meaning growing chain ends tend to add monomer that are unlike the growing end, gives alternating polymer structures; when both r values are larger than one meaning growing chains will add monomer that is like that of the growing chain end, block copolymers are formed; when both r values are near one, a polymer chain formed from random addition of the two monomers is formed; etc.

Step-Wise Kinetics. Step-wise reactions generally require heating whereas chain processes are generally run below room temperature and normally require cooling. Chain polymerizations are generally exothermic and more rapid than the step-wise reactions. The difference in the reaction speed is a result of the differences in activation energy for the controlling step. For chain polymerizations, the rate determining step is the initial initiation step, but the polymer forming step has a low activation energy, generally between 2 to 10 kcal/mole. This allows ready polymer formation anytime free radicals are


present. Thus, during most of the polymerization, high polymer is formed and the most abundant species are polymers and monomers.

The activation energy for the step-wise polymerization is generally much higher being on the order of 30 kcal/mole. During the polymerization sequence, high polymer is formed only near the completion of the reaction with molecular weight steadily increasing as the reaction progresses.

To illustrate typical step-wise polymerization kinetics, we will look at the formation of polyesters from reaction of a diol and a dicarboxylic acid. For uncatalyzed reactions where the dicarboxylic acid and diol are present in equal molar amounts it is found that one diacid molecule is used as a catalyst. This leads to the following kinetic expression.

Rate of polycondensation = -d[A]/dt = k[A][A][D] = k[A]^ (32)

where [A] is the diacid concentration, [D] is the diol concentration and [A] = [D].

Rearrangement gives -d[A]/[A]^ = kdt (33)

Integration gives

2kt = 1/[Atf - 1/[Aof = 1/[At]2 + Constant (34)

It is convenient to express this equation in terms of extent of reaction, p, where p is the fraction of functional groups that have reacted at a time "t". Thus, 1 -p is the fraction of unreacted groups and

At = Ao(1-p) (35)

Substitution of the expression for At and rearrangement gives

2Ao2kt = 1/(1-p)2 + Constant (36)

A plot of 1/(1-p)̂ as a function of time should be linear with a slope of 2Ao2k from which k is determined. Determination of k as a function of temperature allows the calculation of the activation energy.

The number average degree of polymerization, DP, can be expressed as

DP = number of original molecules/number of molecules at time "f = No/N = Ao/At = Ao/Ao(1-p) = 1/(1-p) (37)

This relationship is called the Carothers equation. Because the value of k at any temperature can be determined from the slope of the line (2Ao2k) when 1/(1-p)̂ is plotted against t, DP can be determined at any time from the expression

DP2 = 2kt[Aop + Constant (38)


Synthetic Routes

Step-Wise Condensation Polymerizations. Following is a brief discussion of the three major techniques utilized to synthesize condensation polymers.

The melt synthetic technique is also referred to by other names including high melt, bulk melt, bulk or neat. The melt process is an equilibrium-controlled process in which polymer is formed by driving the reaction toward completion, usually through removal of the by-product or condensate. Thus, in the reaction of a diacid and a diol to form a polyester, the water Is removed causing the reaction to proceed towards polymer formation.

The reactants are added to the reaction vessel along with any other needed material such as catalysts. Heat is applied to melt the reactants, permitting them to come into contact with one another. Additional heat can be added and the pressure reduced. These reactions typically take several hours to days before the desired polymer is formed. The product yield is necessarily high.

Solution polymerizations are also equilibrium processes, with the reaction also often driven by removal of the small by-product. Because the reaction is often run at a lower temperature, more reactive reactants are generally required. Solvent entrapment, recovery and reuse are problems. Counter, the use of lower temperature provides an energy savings and minimizes thermally induced side reactions and rearrangements.

The interfacial polycondensation reactions are heterophasic, with two fast-reacting reactants dissolved in a pair of immiscible liquids, one of which is usually water. Reaction occurs near the interface under somewhat non-equilibrium conditions and unlike typically step-wise processes, high polymer is formed throughout the reaction. These latter conditions are due to the removal of the forming polymer from at least one phase and the use of reactive reactants where the energy of reaction is lower that for reactant usually employed in the solution and melt processes. Because reaction can occur at or below room temperature, thermally induced side-reactions are minimized. The reactions are often completed within a matter of seconds and minutes also minimizing side-reactions.

Chain-Reaction Polymerizations. Most free radical, and ionic polymerizations employing vinyl reactants can be run at or below room temperature. Heating Is normally employed only when melting Is required or to decompose initiators. Most are rapid occurring within a matter of minutes and hours. The principle methods employed in free radical processes are bulk, solution, suspension, and emulsion polymerizations. The bulk process can be carried out in a batch or continuous process. In the bulk process, the reactants are employed "neat" (without solvent). Heat control is important since most of these reactions are exothemnic. In the solution processes, the reactant(s) is dissolved and the product may be recovered from the reaction system through addition of the reaction liquid to a non-solvent, removal of the solvent or through direct precipitation of the polymer from the reaction system.

Water-insoluble monomers can be polymerized as suspended droplets in a process called suspension polymerization. Coalescing of the droplets is prevented by use of small amounts of water-soluble polymers such as poly(vinyl alcohol). It allows good heat control and easy removal of discrete polymer particles.


The emulsion process differs from the suspension polymerization in the size of the suspended particles and in the mechanism. While the particles in the suspension systems vary from about 10 to 1000 nm, those in the emulsion process range from about 0.05 to 5 nm. The small beads produced in the suspension process may be separated by filtering, but the latex produced in the emulsion systems is normally stable where the charged particles cannot be removed by ordinary separation procedures. A typical "recipe" for emulsion polymerizations includes monomer, water, a surfactant (normally a "soap"), and an initiator such as potassium persulfate. When the concentration of soap exceeds some critical micelle concentration, the molecules are present in micelles where the hydrophilic ends of the molecules are oriented toward the water-micelle Interface, and the lyophilic portions of the molecules are oriented towards the center of the micelle. Since the initiation occurs in the aqueous phase, little polymerization occurs in the globules. Thus, the globules generally serve as reservoirs for the monomer that is supplied to the micelles as monomer is converted to polymer.

Polymer Testing

As with non-polymeric materials, the use and acceptance of polymer-containing products Is generally based on testing. Many of the tests are standardized and included as standard tests in the American Society for Testing and Materials (ASTM) procedures. The ASTM cooperates on a world-wide basis with other similar organizations including the International Standards Organization (ISO), American National Standards Institute (ANSI), British Standards Institute (BSI), and the Deutsche Normenausschuss (DNA).

Here we will concentrate on ASTM standardized tests because they are the ones used in America and they are generally accepted internationally. The ASTM, through a group of committees that emphasize specific materials (for Instance Committee D-1 is concerned with paints and Committee D-20 with plastics), manage "accepted" tests. Tests are continually being developed and submitted to the appropriate ASTM committee. After adequate verification through "round robin" testing, tests are accepted by consensus as "standard tests". Some tests are developed within a company to measure a certain property peculiar to that particular company and product. These tests may or may not be submitted to the ASTM. In order to insure, as well as possible, that different companies are using similar test conditions, the ASTM tests specify as many possible variables as believed to influence the test results. These variables may Include temperature, atmosphere, rate of addition of stress, humidity, etc. It must be remembered that for many tests, It is not always clear what particular property is being measured since a number of properties are related and dependent on one another. Even so, the results from such tests form the basis for product reliability and reproducibility.

Rhelogy is the branch of science related to the study of deformation and flow of materials. Rheology includes two quite varied branches called fluid and solid mechanics. Many polymers are viscoelastic and thus can act as both solids and fluids.

The elastic component is dominant in solids with the basic mechanical properties described by Hooke's law which states that the applied stress, S, is proportional to the resultant strain but is independent of the rate of this strain, d(strain)/dt). Crystalline solids generally obey Hooke's law and amorphous materials, below the glass transition

30 C.E. Carrahen Jr.

temperature exhibit some Hooke-like behavior.

S = G X Strain (39) where G is the Shear Modulus

The fluid or viscous component is dominant in liquids and are thus described using Newton's law that states that the applied stress S is proportional to the rate of strain, d(strain)/dt but is independent of the strain or applied velocity gradient.

S = Viscosity x d(Strain)/dt (40)

Since many polymers have both amorphous and crystalline regions, they can be described using combinations of both Hooke and Newton laws.

Hookean behavior is believed to describe bond bending while Newtonian behavior is believed to describe chain movement and slippage. Thus, effort is made in rheology to describe polymer stress-strain behavior in terms of modes employing components that "mimic" Hookean and components that "mimic" Newtonian behavior in an attempt to understand the importance and/or effect of the various polymer components on particular properties.

Temperature is important when discussing polymer behavior. Thus, plastics that are measured at temperatures below their glass transition temperature will have a larger component of their stress-strain behavior be Hookean-like while plastics that are measured at temperatures above their glass transition temperature will have more of their behavior be Newtonian-like. Since one desired plastic property is flexibility, the use-temperature of plastics must be above the glass phase transition temperature of that particular plastic material.

Polymer characterization generally involves both property characterization and structural characterization. Property characterization may include thermal and electrical behavior, chemical behavior, stress-strain-related properties (flexural strength, tensile strength, compression strength, impact resistance, shear strength), hardness (Rockwell, scratch), fatigue or endurance, etc. Structural characterization typically involves instrumentation that is also applied to smaller molecules including Infrared spectroscopy, molecularweight (and molecular weight distribution), NMR, UV-VIS-NIR, scanning electron microscopy. X-ray spectroscopy, electron paramagnetic resonance, Auger electron spectroscopy. X-ray photoelectron spectroscopy, etc. Generally, with specific information about how a polymer might behave differently from a small molecule, scientists can directly apply knowledge gained from small molecule experiments to polymers.

Polymer Companies

About 10,000 American companies are active in the general area of synthetic polymers. These companies can be divided into three groupings as follows-Manufacturers-Over 200 companies produce the "bulk" polymers that are used by the other two groupings of companies. While most of these produce the bulk polymers in large quantities, some produce what are called "specialty polymers", those polymers that are


used in special applications on a small scale volume wise. Processors-While some companies produce their own polymers, most purchase the raw polymer material from one of the 200 manufacturing companies. Processors may specialize in the use of selected polymers, such a polypropylenes, polyethylenes, nylons; or on a particular mode of processing; or on the production of particular markets such as films, sheets, laminates, adhesives and coatings. Fabricators and finishers-The large majority of companies are involved in the fabrication and finishing of polymer-containing products. Fabrication can be divided into three broad areas-

*machining *forming, and *fashioning.

Polvmer Structure-Propertv Relationships

The properties of polymers are dependent on many factors including Inter and intrachain bonding, nature of the backbone, processing events, presence/absence of additives including other polymers, chain size and geometry, and molecular weight distribution.

Interchain and Intrachain Forces. The forces present in molecules can be divided into primary (generally greater than 50 kcal/mole of interactions) and secondary forces (generally less than 10 kcal/mole of interactions). Most synthetic polymers are connected by covalent bonds. These bonds are directional with bond lengths on the order of 9 to 20 nm.

Secondary forces, frequently called van der Waals forces, interact over longer distances-generally on the order of 25 to 50 nm. The force of these interactions is inversely proportional to some power of r, generally 2 or greater, and thus is dependent on the distance between the interacting molecules.

Atoms of individual polymer molecules are joined to each other through primary covalent bonds. Polymer molecules, as with small molecules, are also attracted to one other through a variety of secondary forces. These intermolecular forces include London dispersion forces, induced permanent forces, and dipolar forces including hydrogen bonding. Non-polar molecules, such as ethane and polyethylene, are attracted to one another by weak London or dispersion forces. The transient dipoles are due to instantaneous fluctuations in the electron cloud density and amount to about 2 kcal for each mole of methylene units. Thus, a polyethylene chain of about 1000 units long with have a London attractive force of about 2000 kcal/mole. This London force is sufficient to render the polyethylene non-volatile so that chain degradation occurs prior to volatilization of the polyethylene chain. Polar molecules such as ethyl chloride and poly(vinyl chloride), PVC, are attracted to each other by both the London forces and through dipole-dipole interaction. The combined attraction forces are of the order of about 5 kcal per (mole) repeat unit. Thus, for a PVC of 1000 units, the attractive forces are of the order of 5 kcal/(mole)chain. Again, this attractive force is sufficient to render PVA nonvolatile.


These secondary forces, combined with structural considerations, are majorfactors that determine the shape of polymer chains both in the solid and mobile (melt or solution) states.

Crvstalline-Amorphous Structures. A three-dimensional crystalline common synthetic polymer such as polyethylene is often described as a fringed micelle (where the chains are packed as a sheaf of grain) or as a folded chain. Regions where the polymer chains exist in an ordered array are called crystalline domains. Imperfections in polymer crystalline domains are more frequent than found for smaller molecules. Regions where the polymer chains exist in a more non-ordered fashion are referred to as amorphous regions.

For the same polymer composition, generally the more crystalline the polymer the higher is the density, higher the melting point, greater is the resistance to swelling and dissolution, higher is the moduli of rigidity (more stiff), and greater is the resistance to gas and solvent (including a decreased flow of materials through it).

The amount and kind of crystallinity depends on both the polymer structure and on its treatment (pre-history). This dependence on treatment is much greater than for small molecules. For instance, the proportion of crystallinity can be controlled by controlling the rate of formation of crystalline areas. Thus, polypropylene can be heated above its melting range and cooled quickly (quenched) to produce a product that has only a moderate amount of crystalline areas because the chains were not given enough time to rearrange themselves in crystalline regions. Cooling at a slower rate allows the chains to fold, etc. giving a product with a higher degree of crystallinity.

Following are a number of factors that influence the shape a polymer can take. Chain flexibility is related to the activation energies required to initiate rotational and vibrational segmental motions. For some polymers, as flexibility increases, the tendency towards crystalline formation increases. Polymers containing regularly spaced single C-C, C-N, and C-0 bonds allow rapid conformational changes that contribute to the flexibility of the polymer chain and to the tendency towards crystalline formation. Yet, chain stiffness can also enhance crystalline formation by permitting or encouraging only certain "well-ordered" conformations to occur within the polymer chain. Thus p-polyphenylene is a rigid linear polymer that is highly crystalline.

Crystallization is favored by the presence of regularly spaced units that permit strong inter and intra-chain associations. This is reenforced by the presence of regularly spaced polar groups that can form secondary dipole-dipole interactions such as Is present in polycarbonates, polyesters and polyamides (nylons).

Structural regularity and the absence of large substltuents enhance the tendency for crystallization. The precise effect of substltuents depends on a number of factors including location, size, shape, and mutual interactions. For instance, the presence of ethyl to hexyl substltuents tends to lower the tendency for crystallization because their major contribution is to increase the average distance between chains and thus decrease the secondary bonding forces. When the substltuents become longer (from 12 to 18 carbons) and remain linear, a new phenomenon may occur-the tendency of the side chains to form crystalline domains of their own.


End Uses As Related to Structure. The usefulness of polymers depends not only on their properties, but also on their abundance and availability and on cost including the cost of manufacture, shipping, machining, fabrication and finishing. Even so, polymer properties is an essential element in their usefulness. Polymer properties are related to a number of factors including molecular weight, distribution of chain sizes, previous treatment (history), nature of the polymer, additives, etc. These properties, in turn, are reflected in polymer properties-flex life, strength, biological response, weatherability, resistance to chemical attack, degradation (syntheticand biological), electrical resistance, flammability, dyeability, machinability, comfort, hardness,....

Today, polymers are used in may ways that defy easy classification. Even so, we will look at a few of these major polymer divisions.

Elastomers (or rubbers)-are polymeric material that can be (relatively) shaped through application of force (stress), and when the force is released, the material will return to its original shape. This return to original shape is called "memory" and it is a result of the presence of crosslinks (either physical or more normally chemical). Further, the driving force for the return to the original shape is entropy. Products with low amounts of crossllnking will be easily deformable and "stretchable". As the amount of crosslinking increases, the material becomes harder. In order for elastomers to easily stretch, the attractions between chains should be low and the material should be above the temperature where local segmental mobility occurs. These properties are found in many hydrocarbon-intense polymers such as those listed in Table 1 In the normal, unstretched state the polymer is amorphous with a relatively high entropy or level of disorder. When stretched, the material should be more crystalline with a greater degree of order.

Table 1. Synthetic elastomers.

Polychloroprene Epichlorhydrin Copolymers Styrene-Butadiene, SBR Polybutadiene Nitrile Ethylene-Propylene Neoprene Polyfluorocarbon Silicon Polyurethane (Segmented) Polyisoprene Butadiene-Acrylonitrile Styrene-lsoprene « « _ _ - _ - _ _

Fibers-are polymeric materials that have high tensile strength and high modulus (high stress for small strains). Fibers are usually drawn (oriented) in one direction producing high mechanical properties in that direction. They are typically symmetrical allowing for the chains to more closely approach one another, increasing the bonding strength between the various chain segments. The attractive forces between the chains are, relative to elastomers, high. These materials should have no local mobility and only small amounts of crosslinking is introduced after fabrication to lock In a preferred structure. Table 2 contains a listing of some of the more popular fiber families.


Table 2. Industrially important synthetic fibers

Acrylic Modacryllc Polyester Polyurethane Nylon Rayon (Rayon) Acetate (Rayon) Triacetate Fibrous Glass Olefins

Plastics-are materials that have "mid-way" properties between fibers and elastomers. Thus, it is expected that many of the polymers that are either fibers or elastomers are also plastics. Thus, crystalline nylon is a good fiber, whereas less crystalline nylon material is a plastic. It is unusual for a polymer to be a plastic, fiber and an elastomer. Table 3 contains a listing of plastics.

Table 3. Industrially important plastics

Epoxies Polyesters Urea-Formaldehydes Melamine-Formaldehydes Phenolics (Phenol-Formaldehydes) Polyethylenes Polypropylene Styrene-Acrylonitriles Polystyrene Polyamides Poly(vinyl chloride) and Co-polymers Polytetrafluoroethylene Poly(methyl methacrylate) Polycarbonates Silicons Polysulphone Poly(phenylene oxide) Polyimides

Coatings and adhesives are generally derived from polymers that "fit" into the plastics grouping though there are major groups that do not. For instance, silicone rubbers are elastomers but also can be used as adhesives. Coatings, or coverings, are generally highly viscous (low flowing) materials. The major purposes of coatings are to protect and to decorate. Coatings protect much that is about us from wear and tear and degradation from the "elements" including oils, oxidative chemical agents, effects of temperature and temperature change, rain, snow, and ionizing radiation. They protect our stoves, chairs, cabinets, cars, planes, bridges, homes (inside and outside), etc. Coatings must adhere to the surface they are applied to. Coatings are typically a mixture of a liquid (vehicle or binder (adhesive) and one or more colorants (pigments). Coatings often also contain a number of so-called additives that can furnish added protection against ionizing radiation. Increased rate of drying and/or curing (crossllnking), microorganisms, etc. Coatings are specially formulated for specific purposes and locations.

Coatings of today and tomorrow may fulfill purposes beyond those of beauty and protection. They may also serve as energy collective devices and burglar alarm systems.

In contrast to coatings that must adhere to only one surface, adhesives are used to join two surfaces together (Table 4). Adhesion for both adhesives and coatings can occur through a number of mechanisms including physical interlocking, chemical adhesion


where primary bonding occurs between the adhesive and the surfaces being joined, secondary bonding where hydrogen bonding or polar bonding occurs and viscosity adhesion where movement is restricted because of the viscous nature of the adhesive material.

The combination of an adhesive and adherent is a laminate. Commercial laminates are produced, on a large scale, with wood as the adherent and phenolic, urea, epoxy, resorcinol, or polyester resins as the adhesives. Plywood is one such laminate. Laminates of paper or textile include Formica (TM) and Micarta (TM). Laminates of phenolic, nylon, or silicone resins with cotton, asbestos, paper, or glass textiles are used as mechanical, electrical, and general purpose structural materials.

Table 4. Synthetic polymeric adhesives.

Aromatic Polyamides Acrylonitrile-Butadiene Copolymers Butyl Rubber Epoxy Resins Polychloroprene Polyurethane Resins Poly(vinyl Acetate) Polyethylene Poly(alkyl Cyanacrylates) Silicone Polymers Unsaturated Polyester Resins

Acrylic Acid & Acrylic Ester Polymers

Cellulose Derivatives Phenol-Formaldehyde Polyisobutylene Poly(vinyl Alcohol) Polyamides Poly(vinyl Butyral) Resorcinol-Formaldehyde Styrene-Butadiene Copolymers Vinyl Acetate-Ethylene Copolymers

Sealants and caulks provide a barrier to the passage of gases, liquids and solids, maintain pressure differences, moderate mechanical and thermal shock, etc. While adhesives are used for "load transfer", requiring high tensile and shear strengths, sealants act as insulators and shock attenuators and do not require high tensile and shear strengths.

Films are two dimensional forms of plastic, thick enough to be coherent, but thin enough to be flexed, creased or folded without cracking. Sheeting is two dimensional forms of plastic that are thicker (generally >250 um) than films and generally they are not easily flexed, creased or folded without cracking.

Composites are materials that contain strong fibers, reinforcement, embedded in a continuous phase called the matrix. Today's composites are often called "space-age" or "advanced materials" composites. They are found in the new jet fighters such as the stealth fighters and bombers. In the "reusable" space shuttle, graphite (a composite material) golf clubs, as synthetic human body parts, and for many years in marine craft (fibrous glass).

Polyblends are made by mixing components together in extruders or mixers, on mill rolls, etc. Most are heterogeneous systems consisting of a polymeric matrix in which another polymer is imbedded. The components of polyblends adhere through secondary bonding forces.

36 c.E. Carraher, Jr.

Liquid crystals, LCs, are materials that undergo physical reorganization where at least one of the rearranged structures involve molecular alignment along a preferred direction causing the material to exhibit non-isotropic behavior and associated molecular birefringent properties, ie. molecular asymmetry.

The term ceramic comes from the Greek word keramos which means "potter's clay" or "burnt stuff'. Most ceramics contain large amounts of inorganic polymers. While traditional ceramics were largely based on natural clays, the ceramic of today generally contains synthetic materials. Ceramics are generally brittle, strong; resistent to chemicals such as acids, bases, salts, and reducing agents; and they are high melting.

The term cement covers a large grouping of materials. Some of these have bulk as their commonality, such as Portland cement (concrete), while others are termed cements and are actually performing as an adhesive, such as dental cements.

Solubilitv and Flexibilitv

Most linear and some two-dimensional polymers are soluble. Polymers with moderate to high degrees of crosslinking are insoluble.

The large size of polymers makes their solubility, in comparison to smaller molecules, poorer. This is due to both kinetic (motion) and thermodynamic factors. With respect to kinetic factors, solvent molecules must be able to come Into contact with the materials that they are to dissolve. With polymers, this means that solution occurs "one layer" at a time with the solvent molecules having to penetrate outer layers before contact to inner layers is possible. Thus, solubilization of polymers may take hours to days and even weeks with the approach to solubilization often passing through what is referred to as a gel state where the solvent molecules have become entrenched in the polymer network, but the concentration of solvent molecules is not great enough as to dissolve the polymer.

In thermodynamic considerations, solubility can be described in terms of free energy relationships such as

AG=A H-A(TS) (41)

where G = Gibbs Free Energy, H = Enthalpy or Energy term, T = (Absolute) Temperature and S = Entropy or Order term.

At constant T, the free energy relationship becomes

A G =A H - TA S (42)

Since "like-likes-like" (or a material will have its greatest solubility In itself) better than anything else, the energy (A H) term will be against solution occurring. Thus, it is the entropy, A S, term that generally is the "driving force" for solubility to occur. (Nature tends to go from ordered systems to disordered systems so that systems go from greater order to less order "naturally".)


The number of geometric arrangements of connected polymer segments in a chain are much less than if the segments were free to act as individual units. Thus, the increase in randomness, the A S term, is much lower for polymers in comparison to small molecules making them less soluble and soluble in a lower number of liquids, in general, in comparison to small molecules.

As noted above, the energy term acts against solution occurring. Thus, there is an effort to "match" the solvent and polymer such that their "energy patterns" (shape, size and polarity, solubility parameter and cohesive energy density) are similar so that while the energy term will be against solution occurring, the entropy term is more likely to overcome the energy term allowing solubility to be achieved.

Many polymers are themselves brittle at room temperature. For these polymers to become more pliable, additives called plasticizers, that allow segmental solubility, and consequently segmental flexibility, are added. The proteins and nucleic acids in our bodies are actually inflexible in themselves, but flexibility is essential for them to carry out their functions and for replication to occurs. In this case, water acts as the plasticizer allowing the natural macromolecules to remain "solid" yet be flexible.

As noted above, flexibility is critical for many applications. Thus, elastomers and plastics must be flexible to allow them to be bent and reshaped. Polymer morphology or shape can be divided into two general groupings-amorphous or disordered and crystalline or ordered. Thus, elastomers in their "rest" state have a high degree of amorphous character (that is the entropy or disorder is high), yet in the stretched state, crystallization occurs. The amorphous areas of a polymer contribute to the flexibility of the polymer while the crystalline regions contribute to the strength of the polymer. Polymers, unlike small molecules, undergo two primary solid state transitions. As a polymer is heated, energy is added. At a given temperature, segmental motion begins. The temperature where segmental motion begins is called the glass transition temperature, Tg. Because polymer chains are present in many different orientations and because the addition of temperature is generally done in a relatively rapid manner (even increases of 0.1 C/ minute will contribute to Tg ranges), the Tg is generally reported as a range rather than as a specific temperature. As heating continues, there is a temperature, again actually a temperature range, where wholesale entire chain motion occurs. This temperature is called the melting point or melting range, Tm. Amorphous regions of a polymer exhibit Tg values while crystalline regions exhibit Tm values. Since most polymers have both amorphous and crystalline regions, they have both Tg and Tm values.

Polymers, such as elastomers, that have a moderate to high degree of crosslinking, exhibit only Tg since whole-sale movement of the polymer chain is not possible because of the presence of the crosslinks. Further, in order for plastics and elastomers to exhibit the essential property of flexibility, the "use" temperature must be above the Tg.

The inflexible regions of a polymer are often referred to as the "hard" regions while the flexible regions of a polymer are referred to as "soft" regions. This combination of hard and soft can be illustrated with so-called segmented polyurethanes. Here, the urethane portion of the polymer is involved In hydrogen bonding and is considered as "hard" while the polyether portion is considered "soft". These segmented polyurethane are sold under a number of trade names including Spandex.


-(-0-CH2-CH2-),-(-C-NH-R-NH-C-)y-Soft Hard

Segmented Polyurethane

Size

Unlike small molecules where there is a single molecular weight, polymers often are produced where there is a range of chain lengths. Because there is a range of molecular weights, the particular molecular weight average is dependent on the type of measurement used to determine the molecular weight, which in turn is dependent on the particular mathematical relationship that relates the measured polymer property and molecular weight. Polymer chemists mainly use two types of molecular weights referred to as number average molecular weight and weight average molecular weight.

The number average molecular weight, Mn, is measured by any technique that "counts" the molecules. These techniques include vapor phase and membrane osmometry, boiling point elevation, end-group analysis, and freezing point lowering. Mn can be described using a jar filled with plastic capsules such as those in the circuses that contain tiny prizes. Each capsule is the same size and contains one polymer chain, regardless to the size of the polymer chain. Capsules are then withdrawn, opened, and the individual chain length determined and recorded. The probability of drawing a capsule containing a chain with a specific length is dependent on the fraction of capsules containing such a chain and independent of the length of the chain. The most probable value is the number average molecular weight.

The weight average molecular weight, Mw, is measured by any technique that is dependent on the size of the polymer chain such as light scattering photometry. Using the capsule scenario described above except where the size of the capsule is directly proportional to the chain size, the capsules can be withdrawn, opened, and the individual chain length determined and recorded. The probability of drawing a capsule containing a chain with a specific length is dependent on both the fraction of capsules containing such a chain and on the size of the capsule. The most probable value is the weight average molecular weight.

The ratio between Mw and Mn is referred to as the polydispersity index. The closer to one this ratio is, the less disperse or heterogeneous are the polymer chain lengths. Natural polymers that perform specific functions such as DNA and enzymes have a dispersity index of 1 so that all the chains are of the same size.

The major polymer properties are related to size. This large size allows polymers to behave in a more or less coherent manner. Such factors that influence one part of a polymer chain can be "felt" further along the polymer chain.

One prominent polymer property is the increase in viscosity of polymer melts and dilute solutions containing polymers. Polymers that are heated so that they can flow under applied pressure or simply through gravity effects are referred to as polymer melts.


Since many polymers must have some mobility (often under high pressure and when heated) to be processed, the energy that must be applied to process the polymer is dependent on the polymer molecular weight. There is often a polymer molecular weight where the desired polymeric property (such as strength) and molecular weight is such that further molecular weight increases result in minimal, and unnecessary property increases, but does result in increased effort needed to process the material. This "favorable" molecular weight range is called the commercial molecular weight.

Since polymer chains are long, they can reside in several "flow planes" acting to retard the flow of polymer-containing solutions. This resistance to polymer flow is called viscosity.

Polvmer Shape

Two terms, configuration and conformation, are often confused. Configuration refers to arrangements fixed by chemical bonding that cannot be altered except by breakage of primary bonds. Terms such as heat-to-tail, d- and /-, cis, and trans refer to the configuration of a chemical species. Conformation refers to arrangements around primary bonds. Polymers in solution or in melts continuously undergo conformational changes.

Natural polymers utilize a combination of primary and secondary forces to form polymer structures with both long-range (supra or multi-macromolecular) and short-range structures with both structures critical for the "proper" functioning of the macromolecule. While most synthetic chemists focus on what is referred to as primary and secondary (short-order structural control) structures, work has just begun on developing the appropriate structural control to allow tertiary and quaternary structural (long-range) control.

Here we will focus on primary and secondary polymer shapes. Overall, flexible linear polymer chains will have some tendency to minimize size constraints while keeping "like" polymer moieties together and they will utilize strong (polar and hydrogen bonding) secondary bonding. Most polymers exist as some form of helix with the amount, extent, of helical nature varied. The second most common secondary polymer form is similar to that of a pleated skirt where the hydrogen or dipolar bonding is taken advantage of.

Unsymmetrical reactants, such as substituted vinyl monomers, react almost exclusively to give what is called "head-to-tail" products where the substitutants occur on alternative carbon atoms.

-CH2-CHX-CH2-CHX-CH2-CHX-CH2-CHX-

Occasionally a heat-to-head, tail-to-tail configuration occurs. For most vinyl polymers this structure occurs less than 1 % of the time in a random manner throughout the chain.

-CHo-CHX-CHX-CH^-CH,-

40 C.E. Carrahen Jr.

Even with the head-to-tail configuration, a variety of structures are possible. These include simply a linear polymer structure

-M-M-M-M-M-M-M-M-M-M-M-M-M-M-M-M-M-

and branched structures with varying amounts and lengths of branching.

-M-M-M-M-M-M-M-M-M-M-M-M-M-M-M-M-M-M M M M M M M M M M M M M

Copolymers, polymers derived from two different monomers, M and N, are also important polymer groups, some of the more important ones are as follows.

-M-N-M-N-M-N-M-N-M-N-M-N-M-N- -N-N-M-N-M-M-N-M-N-N-M-N-M-M-N-Alternating Random

-M-N-M-M-M-M-M-M-M-M-M-M-M-N- -N-N-N-N-N-N-M-M-M-M-M-M-M-N-N-Block in "M" Block in both "M" & "N"

-M-M-M-M-M-M-M-M-M-M-M-M-M-M-N N N N Graft N N N N N N

Crosslinked or network (three-dimensional) polymers offer a wide variety of structures dependent on extent and type of crosslinking, type of polymer, type, functionality, and amount of reactants, etc.

Polymers and associated polymer properties also vary with the structural regularity with respect to the substituted carbon atoms in the polymer chain. Each substituted carbon atom is a chiral site with different geometries possible. There are three main geometries or configurations. When the chiral carbons contained within a polymer chain are present in a random fashion the geometry is referred to as atactic. When the pendant group is attached so that the geometry on the carbon atom alternates, the polymer geometry is said to be syndiotactic. When the geometry about the various chiral carbons is all alike, the polymer chain is said to be isotactic.


Linear crystalline polymers often form spherulites. For linear polyethylene the initial structure formed is a single crystal with folded-chain lamellae. These then form sheaf-like structures called axialites or hedrites. As gro\A^h continues, the lamellae develop on either side of a central plane. The lamellae continue to fan out, occupying increasing volume sections through the formation of additional lamellae at appropriate branch points. The end result is the formation of three-dimensional spherulites.


Polymer Structures

Following are structures of a number of the more important synthetic polymers.

Acrylonitrile-Butadiene-Styrene Terpolymer, ABS

-|-CH2CHCH2CH===CHCH2CH2CH-

CN

Butyl Rubber

:H3

CH3 CH3

Ethylene-Methacrylic Acid lonomer

-j—CH 2 CH 2 CH 2 Y

ioo©

Melamine-Formaldehyde Resin Structure

-CH2—N— _N—CH2—

Cn, H X CH2 CH

\ ^ I

N

- C H 2 - N J < S . J I -N-CH2-

€ H ,

'N' CHj I

Introduction to Polymer Science and Teciinology 43

Nitrile Rubber, NBR

:H2CH"

Nylon'6

f CN

CHjCH^CHCHj^ -hNH(CH2)5—C

O

Nylon 6,6

O II

o 4-NH-<CH2)6-NH-C-(CH2)^-C-k

/

OH OH OH

'"• ' •"• bf"'-^'"' TO: OH

/

CHj

.OH Phenol-Formaldehyde Resin Structure ^^^ | ^

Polyacrylonitrile Polybenzimidazole

- e C H j - C H ^

CN

Poly(butylene Terephthalate), PBT

O O

_(_|L_/Qy_|l:_o-CH2-CH2-CH2-CH2-CHi


Polycarbonate, PC

Polychloroprene

Tt H,—0=CH—CHjT-

Poly(dimethyl CH, Siloxane) '

4-Si-(Hr

Polyethylene, PE

- f -CHjCH^i r

Poly(ethylene Glycol), PEG; Poly(ethylene Oxide), PEO

-E-OCH^CH^aF

Poly(ethylene Terephthalate), PET

O

^l^rh-o

Polyimide

C—O—CH,—CH2—(Hi

4-N

L O

N—R-4-


Polyisobutyiene

CH3 I H-CH,—C-I CH3

4CH2C-Polyisoprenes

CH,

CH II CH2

1,2

+ —CH,CH-I

CH3—C II CH2 3,4

+ \ /

c=c / \

.CH3 H CIS-1,4

-HCH,

+

H \ /

C=C / \

CH3 CH2^~ rrans-1,4.

Poly(methyl Acrylate)

f H^—CH =]-

CO^CH.i

Poiyoxymethylene

Poly{ methyl Methacrylate), PMMA

CH,

4€H COOCH3

Poly(phenylene Oxide), PPO

CH3


Poly(phenylene Sulfide), PPS

\o J "

Polypropylene, PP

rer Polytetrafluoroethylene

Poly(vlnyl Acetate) -fCH; ,—CH-OCOCH3

Poly(vinyl Butyral)

-fCHj—CH CH

CH I

(CH^hCH,

Polyphosphazene

OR I

4N=P— I OR

Polystyrene, PS

-fcHj—CH-

Polyurethane, PU

OH HO II I I II

-fO—R—O—C—N—R—N—C^ Poly(vinyl Alcohol), PVA or PVAI

m Poiy(vinyl Chloride), PVC

-j-CHzCH-

CI


Poly(vinylidene Chloride) Poly(vinyl Isobutylether)

-f-CHj—CH-3-

O I

CH, I

CH(CH3)2

Poly(v!nyl Pyridine)

-CCHj—CH

Poly(vinyl Pyrrolidone

-ECH^-CH-^j

(V Styrene-Acrylonitrile Copolymer, SAN Styrene-Butadlene rubber, SBR

(1,4-addition of butadiene)

-PCHXH—CH.CH

CN

-j-CH 2 CH==CHCH 2' CH2CH

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Introduction to Polymer Science and Technology