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Configuration The two types of polymer configurations are cis and trans. These structures can not be changed by physical means (e.g. rotation). The cis configuration arises when substituent groups are on the same side of a carbon-carbon double bond. Trans refers to the substituents on opposite sides of the double bond.

Stereoregularity is the term used to describe the configuration of polymer chains. Three distinct structures can be obtained. Isotactic is an arrangement where all substituents are on the same side of the polymer chain. Asyndiotactic polymer chain is composed of alternating groups and atactic is a random combination of the groups. The following diagram shows two of the three stereoisomers of polymer chain. IsotacticSyndiotactic

Conformation If two atoms are joined by a single bond then rotation about that bond is possible since, unlike a double bond, it does not require breaking the bond. The ability of an atom to rotate this way relative to the atoms which it joins is known as an adjustment of the torsional angle. If the two atoms have other atoms or groups attached to them then configurations which vary in torsional angle are known as conformations. Since different conformations represent varying distances between the atoms or groups rotating about the bond, and these distances determine the amount and type of interaction between adjacent atoms or groups, different conformation may represent different potential energies of the molecule. There several possible generalized conformations: Anti (Trans), Eclipsed (Cis), and Gauche (+ or -). The following animation illustrates the differences between them.

Other Chain Structures The geometric arrangement of the bonds is not the only way the structure of a polymer can vary. A branched polymer is formed when there are "side chains" attached to a main chain. A simple example of a branched polymer is shown in the following diagram. There are, however, many ways a branched polymer can be arranged. One of these types is called "star-branching". Star branching results when a polymerization starts with a single monomer and has branches radially outward from this point. Polymers with a high degree of branching are called dendrimers Often in these molecules, branches themselves have branches. This tends to give the molecule an overall spherical shape in three dimensions.

A separate kind of chain structure arises when more that one type of monomer is involved in the synthesis reaction. These polymers that incorporate more than one kind of monomer into their chain are called copolymers. There are three important types of copolymers. A random copolymer contains a random arrangement of the multiple monomers. A block copolymer contains blocks of monomers of the same type. Finally, a graft copolymer contains a main chain polymer consisting of one type of monomer with branches made up of other monomers. The following diagram displays the different types of copolymers. An example of a common copolymer is Nylon. Nylon is an alternating copolymer with 2 monomers, a 6 carbon diacid and a 6 carbon diamine. The following picture shows one monomer of the diacid combined with one monomer of the diamine: Block CopolymerGraft CopolymerRandom Copolymer

 The mechanical properties of a polymer involve its behavior under stress. These properties tell a polymer scientist or engineer many of the things he or she needs to know when considering how a polymer can be used 43.

Stress-Strain Stress-Strain Curves Tensile Strength %Elongation to Break Young Modulus Toughness

 The tensile stress on a material is defined as the force per unit area as the material is stretched. The cross-sectional area may change if the material deforms as it is stretched, so the area used in the calculation is the original undeformed cross-sectional area A o.

 The strain is a measure of the change in length of the sample. The strain commonly is expressed in one of two ways.  elongation:  extension ratio:  The strain is a unitless number.

 A tensile stress-strain curve is a plot of stress on the y-axis vs. strain on the x-axis. In the plot at the right, strain is expressed as elongation. Stress-strain curves are measured with an instrument designed for tensile testing.  We see that as the strain (length) of the material increases, a larger amount of stress (force) is required.  As the elongation is increased the sample eventually breaks.

 The tensile strength is the stress needed to break a sample 45. It is expressed in Pascals or psi (pounds per square inch).  1 MPa = 145 psi  The tensile strength is an important property for polymers that are going to be stretched. Fibers, for instance, must have good tensile strength 46.

 The elongation-to-break is the strain on a sample when it breaks. This usually is expressed as a percent.  The elongation-to-break sometimes is called the ultimate elongation 46.  Fibers have a low elongation-to-break and elastomers have a high elongation-to- break 47.

 Young's modulus is the ratio of stress to strain. It also is called the modulus of elasticity or the tensile modulus 48.  Young's modulus is the slope of a stress-strain curve. Stress-strain curves often are not straight-line plots, indicating that the modulus is changing with the amount of strain. In this case the initial slope usually is used as the modulus, as is illustrated in the diagram at the right.  Rigid materials, such as metals, have a high Young's modulus. In general, fibers have high Young's modulus values, elastomers have low values, and plastics lie somewhere in between 46.

 The toughness of a material is the area under a stress-strain curve. The stress is proportional to the tensile force on the material and the strain is proportional to its length. The area under the curve then is proportional to the integral of the force over the distance the polymer stretches before breaking.

 This integral is the work (energy) required to break the sample. The toughness is a measure of the energy a sample can absorb before it breaks.

 There is a difference between toughness and strength, as is illustrated in the three plots below.  A material that is strong but not tough is said to be brittle. Brittle substances are strong, but cannot deform very much. Polystyrene (PS) is brittle, for example. High impact polystyrene (HIPS), a blend of polystyrene and polybutadiene (a rubbery polymer above its glass transition temperature ) is said to be rubber-toughened. 