Chemistry 112 - Supplementary Reading

In this part of the course we depart from the "normal" Chem 112 curriculum and talk a little about the chemistry of materials, specifically polymers, metals, and alloys. When we talk about solid materials, we will find that their important properties depend not only on what they are made of, but how they are made and how they are processed as well. This is particularly important for engineers, who rarely have to deal with problems involving pure molecular compounds, but who often have to worry about the properties of composite solid materials. Remember, if the bridge falls down, there is no partial credit! In this part of the course, we will learn how the microscopic composition and structure of solid materials determines if they are strong, brittle, crystalline, amorphous, ductile, electrically conducting, insulating, semiconducting, opaque, or transparent.


Polymers


Polymers. We have already seen a couple of examples of polymerization reactions. Polymer chemistry is devoted to the study of large molecules - macromolecules. Polymers are ubiquitous in our society, finding applications as films, foams, paints, fibers, and structural materials. The microelectronics industry utilizes organic polymers in the fabrication of semiconductor devices and as dielectrics between layers of semiconductors in advanced computer chips. All modern technologies in some way use polymeric materials, and new polymers are being discovered with unique sets of properties that may create entirely new technologies. They can be derived form a single atoms, such as sulfur, or from numerous and different building blocks arranged in a specific sequence, as in proteins or nucleic acids.

The materials properties of a polymer are intimately related to its molecular structure. For example, many common polymers (polystyrene, Lucite, Lexan) are glassy and are therefore transparent like window glass. They do not crystallize readily for reasons we will discuss. Other polymers such as polyethylene (Gladwrap) and poly(ethylene terephthalate) (Mylar) are partially crystalline, having both crystalline and amorphous regions. These materials are typically translucent because their crystallites scatter light.

Polymers are macromolecules, i.e., very large molecules. Large molecules entangle with each other and lead to properties, such as the ability to form fibers and elastomers, which can never be achieved with small molecules. Many polymer molecules are on the order of hundreds of Angstroms in size, but others can be exceptionally large. Some DNA chains, for example, can approach a few cm. in length if stretched from end to end. Polymer chains are typically linear, branched, or crosslinked, although other less common topologies are known. In a crosslinked polymer (also referred to as a network or thermoset polymer), covalent bonds connect different chains, and quite literally the molecular weight of such a material approaches infinity. A typical rubber band, a collection of flexible polymer chains crosslinked with sulfur, is really a single molecule!





Polymers that have covalent crosslinks can either be soft (like a rubber band) or hard (like cured epoxy). Crosslinked polymers are called thermosets because they cannot be re-processed into different shapes upon heating without permanent chemical degradation. Linear and branched polymers can be re-processed upon heating (or by dissolving them in a suitable solvent), and are termed thermoplastics.

Polymer synthesis and composition. The basic building blocks of polymers are called monomers. These monomer units are bonded together and repeat over and over again, normally in covalently linked chains. There are three basic types of reaction by which these monomers are joined together to make polymers:

1. Addition reaction (sometimes called free radical addition, because an extra electron is added to the monomer to initiate the reaction). There table below shows the structures of some of the most important of these polymers (such as teflon, polyethylene, PVC, polystyrene, natural and synthetic rubber). The monomer usually contains a double bond, e.g.,









This kind of reaction yields linear polymers without cross-links, i.e. thermoplastic polymers. Cross linking agents for addition polymers are molecules that contain two or more double bonds per molecule, for example divinylbenzene (see drawing below). Cross-linking of linear chain polymers causes them to stiffen and retain their shape, and also makes them insoluble in solvents that dissolve the linear chain polymer. A demonstration was done showing how a styrofoam cup (composed of linear chain - i.e., not cross-linked - polystyrene) dissolves in a polar solvent (acetone). The styrofoam cup is mostly air, which is blown into the polymer as it forms. Materials that contain lots of air pockets are good thermal insulators, which is why your hand doesn't burn when you hold a styrofoam cup full of hot coffee.

Note that the repeat unit of addition polymers, such as polyethylene, contains the same number of atoms as the monomer. What happens in the reaction is that a carbon-carbon double bond turns into a single bond, and one new carbon-carbon single bond (per monomer unit) is formed. The molecular weights of addition polymers can be very high, and 105 is not an uncommon average MW.

Note that all the addition polymers pictured above have very similar structures, that is, they are all derivatives of polyethylene. The different substituents (e.g., the benzene ring in polystyrene, the chlorine atom in PVC, etc.) added to the linear polymer backbone have a profound effect on the physical properties of these materials, which is why polyethylene is a flexible polymer whereas PMMA and PVC are rather stiff.





2. Condensation polymerization. Two different monomer units, each containing two functional groups, react by eliminating a small molecule like water. Examples are polyamides, made by reacting a diacid with a diamine, and polyesters, made by reacting a diacid with a diol (a diol is a molecule containing two terminal OH groups).








The synthesis of Nylon 66 was demonstrated in class, using a solution of adipoyl chloride (an activated form of adipic acid) in hexane and hexanediamine in water. The hexane solution floats on top of the aqueous solution, and the two reagents meet at the interface between the two solutions, forming the linear polymer. As the polymer is pulled out, the interfacial reaction continues, and a nylon "rope" can be pulled continuously as it forms from the liquid-liquid interface.

3. Ring-Opening polymerization. The monomer is a cyclic molecule that opens up and forms a linear chain polymer, e.g., the isomerization of S8 (crystalline yellow sulfur, a monomer) to linear polymeric sulfur chains:





Polymer Structure: Crystallization, Melting, and the Glass Transition. We know that small molecules like to form crystalline solids when they get cold. A familiar example is water, which makes ice crystals if cooled slowly enough from the vapor or liquid state (the symmetry of snowflakes, for example, shows the hexagonal symmetry of water molecules in crystals). Likewise, sugar will crystallize from honey if it gets cold and is allowed to sit around long enough. Long polymer chains can have a hard time crystallizing, however, since the individual chains get tangled up and need to untangle to make a regular crystalline array. This is particularly true if the polymer melt is very viscous (which means its individual chains do not flow very easily), or if there are substituents on the chain that do not pack very well in the solid state.

Crystalline polymers usually contain regions of well-packed chains separated by amorphous (liquid-like) regions. Pulling a fiber of a linear polymer causes the chains to line up, and can induce crystallization. This phenomenon occurs, for example, when you pull slowly on a polyethylene sixpack harness (the polymer necks down and becomes much harder to break, because the chains align along the pulling direction). Snapping the plastic abruptly apart (before stretching it) is easier, because the chains don't have time to orient.





Besides viscosity, there are other factors that influence the ability of a polymer to crystallize. One of them is the nature of the side groups on the polymer chains. With very bulky side groups, or side groups that vary in an irregular way, the chains have a hard time organizing into an ordered, crystalline solid. This effect is important, because crystalline polymers tend to be much stiffer, harder, and more dense than amorphous polymers. A good example of this phenomenon is polypropylene, which can be made in either atactic, isotactic, or syndiotactic forms. The atactic form doesn't pack well and therefore is normally amorphous, and is used in making garbage bags and other applications where flexible plastics are needed. The isotactic form is crystalline at ordinary temperatures (its melting point is 160o), is translucent and much stiffer, and is used to make jars, tupperware, etc.









While small molecules pretty much always form crystals upon cooling, polymers have another choice, namely, they can form glasses. Crystallization requires quite a bit of chain re-orientation (into a regular, ordered array), and if the chains are tangled enough or viscous enough, then they will not "find" the crystalline arrangement before solidifying. The result is a glassy solid in which the structure looks like the liquid, but the chains are no longer mobile.

Glasses (either organic polymers or inorganic glasses based on SiO2) have a volume-temperature curve like that shown below. Above Tm, the melting point, the chains are fluid. At Tm, they would like to crystallize, since the crystalline form has lower molar volume (higher density), but can't find the right orientation. At the glass transition temperature Tg, the chains become frozen into a glass. Between Tm and Tg, the polymer is a metastable viscous liquid, in which the chains can undergo segmental motion. In the macroscopic sense, the polymer will be elastomeric above Tg and stiff below Tg. A demonstration was done in class, showing how freezing a rubber squash ball (i.e., making it into a glass) causes it to shatter rather than bounce on impact. The same effect was responsible for the tragic space shuttle disaster in 1986: because the launch took place in very cold weather, some elastomeric O-rings were cooled below Tg, became brittle, and ruptured.



Note that the thermal behavior of a glass-forming liquid depends on the cooling rate, and that there is a range of temperature (centered about Tg) where the glass can be formed.

There are several factors that influence the value of Tg, and determine therefore the temperature range over which a polymer will be elastomeric or brittle. One of the most imporant is the flexibility of the polymer backbone, since chain motions generally require flexing of the backbone and rotation about intrachain bonds. For example, the silicone polymers, of which poly(dimethylsiloxane) is an example, have very



low Tg values (in this case -123oC) because the Si-O-Si linkage is very flexible and deformable. This polymer happens to be what Silly Putty is made of. Polyethers like polyethylene oxide (-CH2CH2O-)x also have flexible backbones and low Tg. Benzene rings either in the side-groups or the backbone have a stiffening effect on the polymer, and increase Tg. For this reason, polystyrene (Styrofoam, Tg = 100oC) and polyethylene terephthalate (Dacron, Tg = 70oC) are rather stiff, glassy polymers.

Intrachain forces - covalent and non-covalent. One of the most important factors controlling polymer properties are the forces between polymer chains. We have seen that covalent crosslinking makes a thermoplastic polymer into a thermoset one. With light crosslinking (e.g., sulfur in natural rubber (polyneoprene), which makes C-S-C linkages between chains during vulcanization) the polymer is rubbery and flexible, but with heavy crosslinking it is stiff, like an epoxy resin or cured silicone. There are also three kinds of non-covalent interactions between chains that are important:

1. The strongest non-covalent forces are hydrogen bonds, which involve a positively charged hydrogen interacting with an electronegative element. Only hydrogen that is bonded to nitrogen, oxygen, or fluorine can do it, e.g. in OH...O, OH...N, NH...O, OH...F, etc. interactions. Take, for example, the nylon polymer. There are NH groups that can make hydrogen bonds to the C=O groups of another chain. Each of these hydrogen bonds is worth only about 15-20 kJ/mole, compared to 300-400 kJ/mole for a covalent bond. Nevertheless, lots of interchain hydrogen bonds add up, making the nylon polymer rather stiff and giving it a Tg of 57oC. The corresponding polyester, which cannot make hydrogen bonds, has a Tg of -40oC. Hydrogen bonds (in combination with London interactions, see below) hold the DNA double helix together, and are the things that make complementary nucleotides "recognize" each other, so that DNA chains can replicate. Without these interactions, there could be no molecular basis for heredity.







2. The next strongest interactions are dipole-dipole, like we get between C-Cl dipoles in polyvinyl chloride. The magnitude of these interactions depends on the electronegativity difference of the two atoms involved in the polar bond. These are weaker than hydrogen bonds, typically only about 5 kJ/mole, but again add up so that PVC has a Tg of 81oC, whereas that of polyethylene is -125oC.

3. The weakest interchain interactions are called London forces, or van der Waals forces, which are small attractive forces between all atoms, regardless of whether or not there is a dipole moment. While they are weak, they are very important because they hold all nonpolar liquids and solids together (otherwise polyethylene would be a gas!).


Viscoelasticity. There is a property that distinguishes polymers from all other types of materials. This property is termed viscoelasticity, and is most easily appreciated by considering the behavior of Silly Putty (polydimethylsiloxane), or the slime (polyvinylalcohol, PVA) we made in class. The silicone polymer contains a -Si-O-Si-O- backbone that terminates in Si-OH groups, and these chain ends are crosslinked by making hydrogen bonds with boric acid, B(OH)3. The linear PVA chains were crosslinked the same way. Unlike covalent bonds, hydrogen bonds can form, break up, and then form again many times in the liquid state. Without these weak crosslinks, Silly Putty could only be a very viscous liquid - it wouldn't bounce. When Silly Putty is thrown at a wall it bounces back, a behavior characteristic of an elastic solid. However, under (slower) tensile or compressive stress, like pulling or squeezing, it deforms like a viscous liquid. How can a material be both an elastic solid and a viscous liquid? The key is the timescale of the deformation. Under application of rapid stress, like bouncing it against a wall, the hydrogen bonds in the polymer don't have time to break and re-form; therefore the deformation is elastic. Slow pulling results in plastic deformation, because there is sufficient time to break and re-form these non-covalent bonds. Recall that the slime we made had the same properties - it deformed like a liquid under the force of gravity, but only slowly - it felt solid enough if somebody threw a hunk of it at you. The phenomenon of viscoelasticity is therefore time-dependent.

Many liquid and glassy polymers, and to some extent inorganic glasses as well, show this phenomenon of viscoelasticity, that is, they deform in a plastic manner under stress on slow timescales, but are elastic under more rapid stress.