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Polymer StructureAlthough the fundamental property of bulk polymers is the degreeofpolymerization, the physical structure of the chain is also an important factor thatdetermines the macroscopic properties.

The terms configurationand conformationare used to describe the geometricstructure of a polymer and are often confused. Configurationrefers to the orderthat is determined by chemical bonds. The configuration of a polymer cannot bealtered unless chemical bonds are broken and reformed. Conformationrefers toorder that arises from the rotation of molecules about the singlebonds. Thesetwo structures are studied below.

Configuration

The two types of polymer configurations are cisand trans. These structures cannot be changed by physical means (e.g. rotation). The cisconfiguration arises

when substituent groups are on the same side of a carbon-carbon double bond.Transrefers 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. Isotacticis an arrangement where allsubstituents are on the same side of the polymer chain. A syndiotacticpolymer

chain is composed of alternating groups and atacticis a random combination ofthe groups. The following diagram shows two of the three stereoisomersofpolymer chain.

Isotactic Syndiotactic

Conformation

f two atoms are !oined by a singlebond then rotation about that bond is possiblesince, unlike a double bond, it does not re"uire breaking the bond.

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The ability of an atom to rotate this way relati#e to the atoms which it !oins isknown as an ad!ustment of the torsionalangle. f the two atoms ha#e other atomsor groups attached to them then configurations which #ary in torsional angle areknown as conformations. Since different conformations represent #aryingdistances between the atoms or groups rotating about the bond, and thesedistances determine the amount and type of interaction between ad!acent atomsor groups, different conformation may represent different potential energies of themolecule. There se#eral possible generalized conformations$ Anti (Trans),%clipsed (&is), and 'auche ( or -). The following animation illustrates thedifferences between them.

Conformation Lattice Simulation

ike the polymer growth simulation, the conformation lattice simulation takes astatistical approach to the study of polymers. *robabilities of the differentconformations are assigned which produces a polymer chain with many possibleshapes. &lick the icon to enter the #irtual laboratory.

Polymer Growth Lattice

Other Chain Structures

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The geometric arrangement of the bonds is not the only way the structure of apolymer can #ary. A branched polymeris formed when there are +side chains+attached to a main chain. A simple eample of a branched polymer is shown inthe following diagram.

There are, howe#er, many ways a branched polymer can be arranged. ne ofthese types is called +star-branching+. Star branching results when apolymerization starts with a singlemonomer and has branches radially outwardfrom this point. *olymers with a high degreeof branching are called dendrimersften in these molecules, branches themsel#es ha#e branches. This tends togi#e the molecule an o#erall spherical shape in three dimensions.

A separate kind of chain structure arises when more that one type of monomer isin#ol#ed in the synthesis reaction. These polymers that incorporatemore thanone kind of monomer into their chain are called copolymers. There are threeimportant types of copolymers. A random copolymercontains a randomarrangement of the multiple monomers. A block copolymercontains blocks ofmonomers of the same type. inally, a graft copolymercontains a main chainpolymer consisting of one type of monomer with branches made up of othermonomers. The following diagram displays the different types of copolymers.

Block Copolymer Graft Copolymer Random Copolymer

An eample of a common copolymer is /ylon. /ylon is an alternating copolymerwith 0 monomers, a 1 carbon diacid and a 1 carbon diamine. The followingpicture shows one monomer of the diacid combined with one monomer of thediamine$

Cross-Linking

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n addition to the bonds which hold monomers together in a polymer chain, manypolymers form bonds between neighboring chains. These bonds can be formeddirectly between the neighboring chains, or two chains may bond to a thirdcommon molecule. Though not as strong or rigid as the bonds within the chain,these cross-linksha#e an important effect on the polymer. *olymers with a high

enough degreeof cross-linking ha#e +memory.+ 2hen the polymer is stretched,the cross-links pre#ent the indi#idual chains from sliding past each other. Thechains may straighten out, but once the stress is remo#ed they return to theiroriginal position and the ob!ect returns to its original shape.ne eample of cross-linking is vulcanization. n #ulcanization, a series of cross-links are introduced into an elastomerto gi#e it strength. This techni"ue iscommonly used to strengthen rubber.

Classes of Polymers

*olymer science is a broad field that includes many types of materials which

incorporatelong chain structure of many repeat units as discussed abo#e. Thetwo ma!or polymer classes are described here.Elastomers,or rubbery materials, ha#e a loose cross-linked structure. This type ofchain structure causes elastomers to possess memory. Typically, about 3 in 344molecules are cross-linked on a#erage. 2hen the a#erage number of cross-linksrises to about 3 in 54 the material becomes more rigid and brittle. /atural andsynthetic rubbers are both common eamples of elastomers. Plasticsarepolymers which, under appropriate conditions of temperature and pressure, canbe molded or shaped (such as blowing to form a film). n contrast to elastomers,plastics ha#e a greater stiffness and lack re#ersible elasticity. All plastics arepolymers but not all polymers are plastics. &ellulose is an eample of a polymeric

material which must be substantially modified before processing with the usualmethods used for plastics. Some plastics, such as nylon and cellulose acetate,are formed into fibers (which are regarded by some as a separate class ofpolymers in spite of a considerable o#erlap with plastics). As we shall see in thesection on li"uid crystals, some of the main chain polymer li"uid crystals also arethe constituents of important fibers. %#ery day plastics such as polyethylene andpoly(#inyl chloride) ha#e replaced traditional materials like paper and copper for awide #ariety of applications. The section on *olymer Applicationswill go intogreater detail about the special properties of the many types of polymers.

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