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[[File:Polypropylen.png|thumb|right|The repeating unit of the polymer [[polypropylene]]]]
A '''polymer''' (from Greek πολύ-ς /po΄li-s/ much, many and μέρος /΄meros/ part) is a large [[molecule]] ([[macromolecule]]) composed of repeating [[structural unit]]s typically connected by [[covalent]] [[chemical bond]]s. While ''polymer'' in popular usage suggests [[plastic]], the term actually refers to a large class of natural and synthetic materials with a variety of properties.

Well-known examples of polymers include [[plastic]]s and [[protein]]s. A simple example is [[polypropylene]], whose repeating unit structure is shown at the right. However, polymers are not just limited to having predominantly carbon backbones, elements such as [[silicon]] form familiar materials such as [[silicone]]s, examples being [[silly putty]] and waterproof plumbing sealant. The backbone of [[DNA]] is in fact based on a [[phosphodiester bond]].

Natural polymer materials such as [[shellac]] and [[amber]] have been in use for centuries. [[Biopolymers]] such as [[proteins]] and [[nucleic acids]] play crucial roles in biological processes. A variety of other natural polymers exist, such as [[cellulose]], which is the main constituent of wood and paper.

The [[list of synthetic polymers]] includes [[Bakelite]], [[neoprene]], [[nylon]], [[PVC]], [[polystyrene]], [[polyacrylonitrile]], [[Polyvinyl butyral|PVB]], [[silicone]], and many more.

Polymers are studied in the fields of [[polymer chemistry]], [[polymer physics]], and [[polymer science]].

==Etymology==
The word ''polymer'' is derived from the Greek words ''πολυ '' (''poly''), meaning "many"; and ''μέρος'' (''meros''), meaning "part". The term was coined in 1833 by [[Jöns Jakob Berzelius]], although his definition of a polymer was quite different from the modern definition. (see [[Jöns Jakob Berzelius#New chemical terms]])

==Historical development==
Starting in 1811, [[Henri Braconnot]] did pioneering work in derivative cellulose compounds, perhaps the earliest important work in polymer science. The development of [[vulcanization]] later in the nineteenth century improved the durability of the natural polymer [[rubber]], signifying the first popularized semi-synthetic polymer. In 1907, [[Leo Baekeland]] created the first completely synthetic polymer, [[Bakelite]], by reacting phenol and formaldehyde at precisely controlled temperature and pressure. Bakelite was then publicly introduced in 1909.

Despite significant advances in synthesis and characterization of polymers, a correct understanding of polymer molecular structure did not emerge until the 1920s. Before then, scientists believed that polymers were clusters of small molecules (called [[colloid]]s), without definite molecular weights, held together by an unknown force, a concept known as [[association theory]]. In 1922, [[Hermann Staudinger]] proposed that polymers consisted of long chains of atoms held together by covalent bonds, an idea which did not gain wide acceptance for over a decade and for which Staudinger was ultimately awarded the [[Nobel Prize]]. Work by [[Wallace Carothers]] in the 1920s also demonstrated that polymers could be synthesized rationally from their constituent monomers. An important contribution to synthetic polymer science was made by the Italian chemist [[Giulio Natta]] and the German chemist [[Karl Ziegler]], who won the Nobel Prize in Chemistry in 1963 for the development of the Ziegler-Natta catalyst. Further recognition of the importance of polymers came with the award of the Nobel Prize in Chemistry in 1974 to [[Paul Flory]], whose extensive work on polymers included the [[chemical kinetics|kinetics]] of [[step-growth polymerization]] and of [[addition polymerization]], [[chain transfer]], [[excluded volume]], the [[Flory-Huggins solution theory]], and the [[Flory convention]].

Synthetic polymer materials such as [[nylon]], [[polyethylene]], [[polytetrafluoroethylene|Teflon]], and [[silicone]] have formed the basis for a burgeoning polymer industry. These years have also shown significant developments in rational polymer synthesis. Most commercially important polymers today are entirely synthetic and produced in high volume on appropriately scaled organic synthetic techniques. Synthetic polymers today find application in nearly every industry and area of life. Polymers are widely used as adhesives and lubricants, as well as structural components for products ranging from children's toys to aircraft. They have been employed in a variety of biomedical applications ranging from [[Implant (medicine)|implantable devices]] to controlled [[drug delivery]]. Polymers such as poly (methyl methacrylate) find application as [[photoresist]] materials used in [[semiconductor]] manufacturing and [[low-k]] dielectrics for use in high-performance [[microprocessors]]. Recently, polymers have also been employed as flexible substrates in the development of [[organic light-emitting diode]]s for electronic displays.

==Polymer synthesis==
{{main|Polymerization}}

Polymerization is the process of combining many small molecules known as monomers into a covalently bonded chain. During the polymerization process, some chemical groups may be lost from each monomer. The distinct piece of each monomer that is incorporated into the polymer is known as a repeat unit or monomer residue.

=== Laboratory synthesis ===

Laboratory synthetic methods are generally divided into two categories, [[condensation polymerization]] and [[addition polymerization]]. However, some newer methods such as [[plasma polymerization]] do not fit neatly into either category. Synthetic polymerization reactions may be carried out with or without a [[catalyst]]. Efforts towards rational synthesis of biopolymers via laboratory synthetic methods, especially artificial [[peptide synthesis|synthesis of proteins]], is an area of intense research.

===Biological synthesis===

{{main|Biopolymer}}

There are three main classes of biopolymers: [[polysaccharides]], [[polypeptides]], and [[polynucleotide]]s.
In living cells, they may be synthesized by enzyme-mediated processes, such as the formation of [[DNA]] catalyzed by [[DNA polymerase]]. The [[Protein biosynthesis|synthesis of proteins]] involves multiple enzyme-mediated processes to [[Transcription (genetics)|transcribe]] genetic information from the DNA and subsequently [[Translation (biology)|translate]] that information to synthesize the specified protein from [[amino acid]]s. The protein may be [[Posttranslational modification|modified further]] following translation in order to provide appropriate structure and functioning.

===Modification of natural polymers===
Many commercially important polymers are synthesized by chemical modification of naturally occurring polymers. Prominent examples include the reaction of nitric acid and cellulose to form [[nitrocellulose]] and the formation of [[vulcanized rubber]] by heating natural rubber in the presence of sulphur.

==Polymer structure==

The structural properties of a polymer relate to the physical arrangement of monomer residues along the backbone of the chain. Structure has a strong influence on the other properties of a polymer.
For example, a linear chain polymer may be soluble or insoluble in water depending on whether it is composed of polar monomers (such as ethylene oxide) or nonpolar monomers (such as styrene). On the other hand, two samples of natural rubber may exhibit different durability, even though their molecules comprise the same monomers. Polymer scientists have developed terminology to describe precisely both the nature of the monomers as well as their relative arrangement.

===Monomer / Repeat Unit===
The identity of the monomers comprising the polymer is generally the first and most important attribute of a polymer. Polymer nomenclature is generally based upon the type of monomers comprising the polymer. Polymers that contain only a single type of monomer are known as homopolymers, while polymers containing a mixture of monomers are known as [[copolymers]]. Poly(styrene), for example, is composed only of styrene monomers, and is therefore classified as a homopolymer. [[Ethylene-vinyl acetate]], on the other hand, contains more than one variety of monomer and is thus a [[copolymer]]. Some biological polymers are composed of a variety of different but structurally related monomers, such as [[polynucleotide]]s composed of [[nucleotide]] subunits.

The [[repeat unit]] is the constantly repeated unit of the chain and is also characteristic of the polymer. The repeating unit of the polymer may be different from the starting monomer(s), for example in [[condensation polymerization]]. A simple example is [[polyethylene terephthalate|PET polyester]]. The monomers are [[terephthalic acid]] (HOOC-C<sub>6</sub>H<sub>4</sub>-COOH) and [[ethylene glycol]] (HO-CH<sub>2</sub>-CH<sub>2</sub>-OH) but the repeating unit is -OC-C<sub>6</sub>H<sub>4</sub>-COO-CH<sub>2</sub>-CH<sub>2</sub>-O-, which corresponds to the combination of the two monomers with the loss of two water molecules.

A polymer molecule containing ionizable subunits is known as a [[polyelectrolyte]]. An [[ionomer]] is a subclass of polyelectrolyte with a low fraction of ionizable subunit.

====Functionality====
This characteristic of a [[monomer]] determines whether a particular monomer can form a polymer or not.<ref>{{cite book | last = Campbell | first = Neil A. | authorlink = | coauthors = Brad Williamson; Robin J. Heyden | title = Biology: Exploring Life | publisher = Pearson Prentice Hall | date = 2006 | location = Boston, Massachusetts | pages = | url = http://www.phschool.com/el_marketing.html | doi = | id = | isbn = 0-13-250882-6 }}</ref> It is defined as the number of reaction sites availabe on the monomer that may form chemical covalent bonds. The basic functionality required for a linear chain is two bonding sites. Higher functionality yields branched or even crosslinked polymer chains.

===Tacticity===
{{main|Tacticity}}
Tacticity describes the relative [[stereochemistry]] of [[chirality (chemistry)|chiral]] centers in neighboring structural units within a macromolecule. There are three types: [[isotactic]] (all substituents on the same side), [[atactic]] (random placement of substituents), and [[syndiotactic]] (alternating placement of substituents).

===Chain linearity===
[[Image:Single Polymer Chains AFM.jpg|right|thumb|175px|Appearance of real linear polymer chains as recorded using an [[atomic force microscope]] on surface under liquid medium. Chain [[contour length]] for this polymer is ~204&nbsp;nm; thickness is ~0.4&nbsp;nm.<ref>Y. Roiter and S. Minko, [http://dx.doi.org/10.1021/ja0558239 AFM Single Molecule Experiments at the Solid-Liquid Interface: In Situ Conformation of Adsorbed Flexible Polyelectrolyte Chains], Journal of the American Chemical Society, vol. 127, iss. 45, pp. 15688-15689 (2005)</ref>]]
The simplest form of polymer molecule is a straight chain or linear' polymer, composed of a single main chain. The flexibility of an unbranched chain polymer is characterized by its [[persistence length]]. A [[branching (chemistry)|branched polymer]] molecule is composed of a main chain with one or more substituent side chains or branches. Special types of branched polymers include star polymers, comb polymers, and brush polymers. If the polymer contains a side chain that has a different composition or configuration than the main chain, the polymer is called a [[graft copolymer|graft or grafted polymer]]. A cross-link suggests a branch point from which four or more distinct chains emanate. A polymer molecule with a high degree of crosslinking is referred to as a polymer network.<ref>IUPAC. "Glossary of Basic Terms in Polymer Science". Pure Appl. Chem. 1996, 68, 2287-2311.</ref> Sufficiently high crosslink concentrations may lead to the formation of an infinite network, also known as a gel, in which networks of chains are of unlimited extent — essentially all chains have linked into one molecule.<ref>Painter, P and Coleman, M. "Fundamentals of Polymer Science". 1997, 96-100.</ref>

===Chain length===

Polymer bulk properties may be strongly dependent on the size of the polymer chain. Like any molecule, a polymer molecule's size may be described in terms of [[molecular weight]] or mass. In polymers, the molecular mass may further be expressed in terms of [[degree of polymerization]], essentially the number of monomer units which comprise the polymer. For synthetic polymers, the molecular weight is expressed statistically to describe the distribution of molecular weights in the sample. This is because almost all industrial processes produce a distribution of polymer chain sizes. Examples of such statistics include the [[number average molecular weight]] and [[weight average molecular weight]]. The ratio of these two values is the [[polydispersity index]], commonly used to express the "width" of the molecular weight distribution.

The maximum length of a polymer chain is its [[contour length]].

===Monomer arrangement in copolymers===
{{main|copolymer}}

Monomers within a copolymer may be organized along the backbone in a variety of ways.
* '''Alternating copolymers''' possess regularly alternating monomer residues.
* '''Periodic copolymers''' have monomer residue types arranged in a repeating sequence.
* '''Random copolymers''' have a random sequence of monomer residue types.
* '''Statistical copolymers''' have monomer residues arranged according to a known statistical rule.
* '''Block copolymers''' have two or more homopolymer subunits linked by covalent bonds. Block copolymers with two or three distinct blocks are called diblock copolymers and triblock copolymers, respectively.

==Polymer properties==
Types of polymer properties can be broadly divided into several categories based upon scale. At the nano-micro scale there are properties that directly describe the chain itself, and can be thought of as polymer structure. At an intermediate mesoscopic level there are properties that describe the morphology of the polymer matrix in space. At the macroscopic level properties describe the bulk behavior of the polymer.

The bulk properties of a polymer are those most often of end-use interest. These are the properties that dictate how the polymer actually behaves on a macroscopic scale.

=== Relationship between chain length and polymer properties ===

Polymer bulk properties are strongly dependent upon their structure and mesoscopic behavior. A number of qualitative relationships between structure and properties are known.

Increasing chain length tends to decrease chain mobility, increase strength and toughness, and increase the glass transition temperature (T<sub>g</sub>). This is a result of the increase in chain interactions such as Van der Waals attractions and entanglements that come with increased chain length. These interactions tend to fix the individual chains more strongly in position and resist deformations and matrix breakup, both at higher stresses and higher temperatures. Chain length is related to melt viscosity roughly as 1:10<sup>3.2</sup>, so that a tenfold increase in polymer chain length results in a viscosity increase of over 1000 times.

===Crystallinity===
When applied to polymers, the term ''crystalline'' has a somewhat ambiguous usage. In some cases, the term ''crystalline'' finds identical usage to that used in conventional [[crystallography]]. For example, the structure of a crystalline protein or polynucleotide, such as a sample prepared for [[x-ray crystallography]], may be defined in terms of a conventional unit cell composed of one or more polymer molecules with cell dimensions of hundreds of angstroms or more.

A synthetic polymer may be lightly described as crystalline if it contains regions of three-dimensional ordering on atomic (rather than macromolecular) length scales, usually arising from intramolecular folding and/or stacking of adjacent chains. Synthetic polymers may consist of both crystalline and amorphous regions; the degree of crystallinity may be expressed in terms of a weight fraction or volume fraction of crystalline material. Few synthetic polymers are entirely crystalline.<ref>{{cite web|url=http://www.iupac.org/publications/books/pbook/PurpleBook-C4.pdf|title=IUPAC Purple Book: Definition of terms relating to crystalline polymers (1988) See Sec.1.3 Degree of Crystallinity|format=PDF}}</ref>

The crystallinity of polymers is characterized by their degree of crystallinity, ranging from zero for a completely noncrystalline polymer to one for a theoretical completely crystalline polymer. Increasing degree of crystallinity tends to make a polymer more rigid. It can also lead to greater brittleness. Polymers with a degree of crystallinity approaching zero or one will tend to be transparent, while polymers with intermediate degrees of crystallinity will tend to be opaque due to light scattering by crystalline or glassy regions.

===Tensile strength===
The [[tensile strength]] of a material quantifies how much stress the material will endure before failing.<ref>[[M. F. Ashby|Ashby, Michael]] and Jones, David. Engineering Materials. p. 191-195. Oxford: Butterworth-Heinermann, 1996. Ed. 2.</ref><ref>Meyers and Chawla. Mechanical Behavior of Materials. pg. 41. Prentice Hall, Inc. 1999.</ref> This is very important in applications that rely upon a polymer's physical strength or durability. For example, a rubber band with a higher tensile strength will hold a greater weight before snapping. In general tensile strength increases with polymer chain length and crosslinking of polymer chains.

===Young's modulus of elasticity ===
[[Young's Modulus]] quantifies the elasticity of the polymer. It is defined, for small strains, as the ratio of rate of change of stress to strain. Like tensile strength, this is highly relevant in polymer applications involving the physical properties of polymers, such as rubber bands. The modulus is strongly dependent on temperature.

===Transport properties ===
Transport properties such as diffusivity relate to how rapidly molecules move through the polymer matrix. These are very important in many applications of polymers for films and membranes.

===Melting point===
The term ''[[melting point]]'', when applied to polymers, suggests not a solid-liquid phase transition but a transition from a crystalline or semi-crystalline phase to a solid amorphous phase. Though abbreviated as simply ''T<sub>m</sub>'', the property in question is more properly called the crystalline melting temperature. Among synthetic polymers, crystalline melting is only discussed with regards to [[thermoplastics]], as [[thermoset]]ting polymers will decompose at high temperatures rather than melt.

===Boiling point===
The [[boiling point]] of a polymer substance is never defined because polymers will decompose before reaching theoretical boiling temperatures.

===Glass transition temperature===
A parameter of particular interest in synthetic polymer manufacturing is the [[glass transition temperature]] (T<sub>g</sub>), which describes the temperature at which amorphous polymers undergo a second-order phase transition from a rubbery, viscous amorphous solid, or from a crystalline solid (depending on the degree of crystallization) to a brittle, glassy amorphous solid. The glass transition temperature may be engineered by altering the degree of branching or crosslinking in the polymer or by the addition of [[plasticizer]].<ref> Brandrup, J.; Immergut, E.H.; Grulke, E.A.; ''eds'' Polymer Handbook 4th Ed. New York: Wiley-Interscience, 1999. </ref>

=== Mixing behavior ===

In general, polymeric mixtures are far less miscible than mixtures of small molecule materials. This effect results from the driving force for mixing is usually entropics, not energetics. In other words, miscible materials usually form a solution not because their interaction with each other is more favorable than their self-interaction, but because of an increase in entropy and hence free energy associated with increasing the amount of volume available to each component. This increase in entropy scales with the number of particles (or moles) being mixed. Since polymeric molecules are much larger and hence generally have much higher specific volumes than small molecules, the number of molecules involved in a polymeric mixture are far less than the number in a small molecule mixture of equal volume. The energetics of mixing, on the other hand, are comparable on a per volume basis for polymeric and small molecule mixtures. This tends to increase the free energy of mixing for polymer solutions and thus make solvation less favorable. Thus, concentrated solutions of polymers are far rarer than those of small molecules.

In dilute solution, the properties of the polymer are characterized by the interaction between the solvent and the polymer. In a good solvent, the polymer appears swollen and occupies a large volume. In this scenario, intermolecular forces between the solvent and monomer subunits dominate over intramolecular interactions. In a bad solvent or poor solvent, intramolecular forces dominate and the chain contracts. In the theta solvent, or the state of the polymer solution where the value of the second virial coefficient becomes 0, the intermolecular polymer-solvent repulsion balances exactly the intramolecular monomer-monomer attraction. Under the theta condition (also called the [[Paul J. Flory|Flory]] condition), the polymer behaves like an ideal [[random coil]].

===Chain conformation===
The space occupied by a polymer molecule is generally expressed in terms of [[radius of gyration]], which is an average distance from the center of mass of the chain to the chain itself. Alternatively, it may be expressed in terms of [[pervaded volume]], which is the volume of solution spanned by the polymer chain and scales with the cube of the radius of gyration.<ref>Rubinstein, M and Colby, R. "Polymer Physics". 2003, 13.</ref>

===Branching===
Branching of polymer chains changes the properties of a polymer by affecting the ability of chains to slide past one another and by altering intermolecular forces. It also affects the bulk properties of polymers. Long chain branches may increase polymer strength, toughness, and T<sub>g</sub> due to an increase in the number of entanglements per chain. Random length and atactic short chains, on the other hand, may reduce polymer strength due to disruption of organization. Short side chains may likewise reduce crystallinity due to disruption of the crystal structure. Reduced crystallinity may also be associated with increased transparency due to light scattering by small crystalline regions. A good example of this effect is related to the range of physical attributes of [[polyethylene]]. High-density polyethylene ([[HDPE]]) has a very low degree of branching, is quite stiff, and is used in applications such as milk jugs. Low-density polyethylene ([[LDPE]]), on the other hand, has significant numbers of both long and short branches, is quite flexible, and is used in applications such as plastic films. The [[Branching (chemistry)#Branching index|branching index]] of the polymer is a parameter that characterizes the effect of long-chain branches on the size of a branched macromolecule in solution. [[Dendrimers]] are a special case of polymer where every monomer unit is branched. This tends to reduce intermolecular chain entanglement and crystallization. Alternatively, dendritic polymers are not perfectly branched but share similar properties to dendrimers due to their high degree of branching.

===Chemical crosslinking===
Crosslinking tends to increase T<sub>g</sub> and increase strength and toughness. Crosslinking consists of the formation of chemical bonds between chains. Among other applications, this process is used to strengthen rubbers in a process known as [[vulcanization]], which is based on crosslinking by sulphur. Car tires, for example, are highly crosslinked in order to reduce the leaking of air out of the tire and to toughen their durability. Eraser rubber, on the other hand, is not crosslinked to allow flaking of the rubber and prevent damage to the paper.

===Inclusion of plasticizers===
[[Image:Polymer Polarity Chart.jpg|bottom|thumb|600px|Polymer/Polarity Chart - Illustration on the varying plasticisers recommended based on Polarity of a given Polymer <ref>http://www.hallstar.com/techdocs/New_Line_of_Modifiers_for_LP_Polymers.pdf</ref>]]

Inclusion of [[plasticizer]]s tends to lower T<sub>g</sub> and increase polymer flexibility. [[Plasticizer]]s are generally small molecules that are chemically similar to the polymer and create gaps between polymer chains for greater mobility and reduced interchain interactions. A good example of the action of plasticizers is related to polyvinylchlorides or PVCs. A uPVC, or unplasticized polyvinylchloride, is used for things such as pipes. A pipe has no plasticizers in it, because it needs to remain strong and heat-resistant. Plasticized PVC is used for clothing for a flexible quality. Plasticizers are also put in some types of cling film to make the polymer more flexible.

==Standardized polymer nomenclature==

There are multiple conventions for naming polymer substances. Many commonly used polymers, such as those found in consumer products, are referred to by a common or trivial name. The trivial name is assigned based on historical precedent or popular usage rather than a standardized naming convention. Both the [[American Chemical Society]]<ref>CAS: Index Guide, Appendix IV (© 1998). </ref> and [[IUPAC]]<ref>IUPAC. "Nomenclature of Regular Single-Strand Organic Polymers". Pure Appl. Chem. 1976, 48, 373-385. </ref> have proposed standardized naming conventions; the [[American Chemical Society|ACS]] and IUPAC conventions are similar but not identical.<ref>{{cite web|url=http://www.polyacs.org/nomcl/mnn18.html|title=Macromolecular Nomenclature Note No. 18<!-- Bot generated title -->}}</ref> Examples of the differences between the various naming conventions are given in the table below:

{| class="wikitable"
|-
! Common Name
! ACS Name
! IUPAC Name
|-
| Poly (ethylene oxide) or (PEO)
| poly(oxyethylene)
| poly(oxyethene)
|-
| Poly (ethylene terephthalate) or (PET)
| poly (oxy-1,2-ethanediyloxycarbonyl -1,4-phenylenecarbonyl)
| poly (oxyetheneoxyterephth= aloyl)
|-
| Nylon
| poly[amino(1-oxo-1,6-hexanediyl)]
| poly[amino(1-oxohexan-1,6-diyl)]
|}

In both standardized conventions, the polymers' names are intended to reflect the monomer(s) from which they are synthesized rather than the precise nature of the repeating subunit. For example, the polymer synthesized from the simple alkene [[ethene]] is called [[polyethylene]], retaining the ''-ene'' suffix even though the double bond is removed during the polymerization process:

[[Image:Ethene polymerization.png]]

[[Image:polyethene monomer.png]]

== Chemical properties of polymers ==
The attractive forces between polymer chains play a large part in determining a polymer's properties. Because polymer chains are so long, these interchain forces are amplified far beyond the attractions between conventional molecules. Different side groups on the polymer can lend the polymer to [[ionic bonding]] or [[hydrogen bonding]] between its own chains. These stronger forces typically result in higher tensile strength and higher crystaline melting points.

The intermolecular forces in polymers can be affected by [[dipole]]s in the monomer units. Polymers containing [[amide]] or [[carbonyl]] groups can form [[hydrogen bond]]s between adjacent chains; the partially positively charged hydrogen atoms in N-H groups of one chain are strongly attracted to the partially negatively charged oxygen atoms in C=O groups on another. These strong hydrogen bonds, for example, result in the high tensile strength and melting point of polymers containing [[urethane]] or [[urea]] linkages. [[Polyester]]s have [[intermolecular force#Dipole-dipole interactions|dipole-dipole bonding]] between the oxygen atoms in C=O groups and the hydrogen atoms in H-C groups. Dipole bonding is not as strong as hydrogen bonding, so a polyester's melting point and strength are lower than [[Kevlar]]'s ([[Twaron]]), but polyesters have greater flexibility.

Ethene, however, has no permanent dipole. The attractive forces between polyethylene chains arise from weak [[van der Waals force]]s. Molecules can be thought of as being surrounded by a cloud of negative electrons. As two polymer chains approach, their electron clouds repel one another. This has the effect of lowering the electron density on one side of a polymer chain, creating a slight positive dipole on this side. This charge is enough to attract the second polymer chain. Van der Waals forces are quite weak, however, so polyethene can have a lower melting temperature compared to other polymers.

== Polymer characterization ==
The characterization of a polymer requires several parameters which need to be specified. This is because a polymer actually consists of a [[statistics|statistical]] distribution of chains of varying lengths, and each chain consists of monomer residues which affect its properties.

A variety of lab techniques are used to determine the properties of polymers. Techniques such as [[wide angle X-ray scattering]], [[Small angle X-ray scattering (SAXS)|small angle X-ray scattering]], and [[Small angle neutron scattering (SANS)|small angle neutron scattering]] are used to determine the crystalline structure of polymers. [[Gel permeation chromatography]] is used to determine the [[number average molecular weight]], [[weight average molecular weight]], and [[polydispersity]]. [[FTIR]], [[Raman spectroscopy|Raman]] and [[Nuclear magnetic resonance|NMR]] can be used to determine composition. Thermal properties such as the [[glass transition temperature]] and [[melting point]] can be determined by [[differential scanning calorimetry]] and [[dynamic mechanical analysis]]. [[Pyrolysis]] followed by analysis of the fragments is one more technique for determining the possible structure of the polymer. [[Thermogravimetry]] is a useful technique to evaluate the thermal stability of the polymer. Detailed analyses of TG curves also allow us to know a bit of the phase segregation in polymers. Rheological properties are also commonly used to help determine molecular architecture (molecular weight, molecular weight distribution and branching)as well as to understand how the polymer will process, through measurements of the polymer in the melt phase.

== Polymer degradation ==
[[Image:PlasticDamage.JPG|right|thumb|250px|A plastic item with thirty years of exposure to heat and cold, brake fluid, and sunlight. Notice the discoloration, swollen dimensions, and tiny splits running through the material]]
[[Polymer degradation]] is a change in the properties—[[tensile strength]], [[colour]], shape, etc.—of a polymer or polymer-based product under the influence of one or more environmental factors, such as [[heat]], [[light]] or [[chemical]]s. It is often due to the [[hydrolysis]] of the bonds connecting the polymer chain, which in turn leads to a decrease in the molecular mass of the polymer. These changes may be undesirable, such as changes during use, or desirable, as in [[biodegradation]] or deliberately lowering the molecular mass of a polymer. Such changes occur primarily because of the effect of these factors on the chemical composition of the polymer. [[Ozone cracking]] and [[UV degradation]] are specific [[failure mode]]s for certain polymers.

The degradation of polymers to form smaller molecules may proceed by random scission or specific scission. The degradation of polyethylene occurs by random scission—a random breakage of the linkages (bonds) that hold the [[atoms]] of the polymer together. When heated above 450&nbsp;°C it degrades to form a mixture of hydrocarbons. Other polymers—like polyalphamethylstyrene—undergo specific chain scission with breakage occurring only at the ends. They literally unzip or depolymerize to become the constituent [[monomer]].

However, the degradation process can be useful from the viewpoints of understanding the structure of a polymer or [[recycling]]/reusing the polymer waste to prevent or reduce environmental [[pollution]]. [[Polylactic acid]] and [[polyglycolic acid]], for example, are two polymers that are useful for their ability to degrade under aqueous conditions. A copolymer of these polymers is used for biomedical applications, such as hydrolysable stitches that degrade over time after they are applied to a wound. These materials can also be used for plastics that will degrade over time after they are used and will therefore not remain as litter.
===Product failure===

[[Image:Chlorine attack1.jpg|thumb|left|200px|Chlorine attack of acetal resin plumbing joint]]
In a finished product, such a change is to be prevented or delayed. Failure of [[safety-critical]] polymer components can cause serious accidents, such as fire in the case of cracked and degraded polymer [[fuel line]]s. Chlorine-induced cracking of [[acetal resin]] plumbing joints and [[polybutylene]] pipes has caused many serious floods in domestic properties, especially in the USA in the 1990s. Traces of [[chlorine]] in the water supply attacked vulnerable polymers in the plastic plumbing, a problem which occurs faster if any of the parts have been poorly [[extruded]] or [[injection moulded]]. Attack of the acetal joint occurred because of faulty moulding leading to cracking along the threads of the fitting, which are serious [[stress concentration]]s.
[[Image:Ozone cracks in tube1.jpg|thumb|right|200px|Ozone cracking in [[natural rubber]] tubing]]
Polymer oxidation leads to cracking and failure of the parts affected and has caused accidents involving [[medical device]]s. One of the oldest known failure modes is [[ozone cracking]] caused by chain scission when [[ozone]] gas attacks susceptible [[elastomer]]s such as [[natural rubber]] and [[nitrile rubber]]. They possess double bonds in their [[repeat unit]]s which are cleaved during [[ozonolysis]]. Cracks in [[fuel line]]s can penetrate the bore of the tube and cause fuel leakage. If cracking occurs in the engine compartment, electric sparks can ignite the [[gasoline]] and can cause a serious fire.

Fuel lines can also be attacked by another form of degradation: hydrolysis. [[Nylon 6,6]] is susceptible to [[acid hydrolysis]], and in one accident, a fractured fuel line led to a spillage of diesel into the road. If [[diesel fuel]] leaks onto the road, accidents to following cars can be caused by the slippery nature of the deposit, which is like [[black ice]].

==References==
<references/>
==Bibliography==
* Allcock, Harry R.; Lampe, Frederick W.; and Mark, James E. ''Contemporary Polymer Chemistry'', Pearson Education, 3rd edition (2003).
* Cowie, J.M.G. ''Polymers: Chemistry and Physics of Modern Materials'', Blackie (in USA: Chapman and Hall), 2nd edition (1991).
* Ezrin, Meyer. ''Plastics Failure Guide: Cause and Prevention'', Hanser-SPE (1996).
* Lewis, Peter Rhys; Reynolds, K.; and Gagg, C. ''Forensic Materials Engineering: Case studies'', CRC Press (2004).
* Wright, David C. ''Environmental Stress Cracking of Plastics'', RAPRA (2001).

==See also==
* [[Biopolymer]]
* [[Copolymer]]
* [[Electroactive polymers]]
* [[Forensic polymer engineering]]
* [[Glass transition temperature]]
* [[List of important publications in chemistry#Polymer chemistry|Important publications in polymer chemistry]]
* [[Monomer]]
* [[Nurdle]]
* [[Polyanhydrides]]
* [[Polymerization]]
* [[Polymer classes]]
* [[Polymer degradation]]
* [[Polymer engineering]]
* [[Polymer science]]
* [[Polymersome]]
* [[Shape memory polymer]]
* [[Smart materials]]
* [[Emulsion dispersion]]

==External links==
* [http://www.polymerchemistryhypertext.com/ Polymer Chemistry Hypertext, Educational resource]
* [http://www.pslc.ws/macrog/index.htm The Macrogalleria - a cyberwonderland of polymer fun!]
* [http://www.campoly.com/application_notes.html Application notes on the characterization of polymers]
* [http://www3.open.ac.uk/courses/bin/p12.dll?C01T838 Distance learning course in polymers]
* [http://openlearn.open.ac.uk/mod/resource/view.php?id=196631 Polymer Structures]
* [http://www.theotherpages.org/abbrev.html Glossary of Polymer Abbreviations]
* [http://www.sigmaaldrich.com/img/assets/3900/Glossary.pdf Sigma-Aldrich Polymer Glossary]

[[Category:Polymers|Polymers]]
[[Category:Soft matter]]

[[ar:بوليمر]]
[[bg:Полимер]]
[[ca:Polímer]]
[[cs:Polymer]]
[[da:Polymer]]
[[de:Polymer]]
[[et:Polümeerid]]
[[el:Πολυμερές]]
[[es:Polímero]]
[[eo:Polimero]]
[[eu:Polimero]]
[[fa:بسپار]]
[[fr:Polymère]]
[[fy:Polymear]]
[[gl:Polímero]]
[[ko:중합체]]
[[hi:बहुलक]]
[[hr:Polimer]]
[[io:Polimero]]
[[id:Polimer]]
[[it:Polimero]]
[[he:פולימר]]
[[jv:Polimèr]]
[[la:Polymerum]]
[[lt:Polimeras]]
[[lmo:Pulimer]]
[[hu:Polimer]]
[[mk:Полимер]]
[[ms:Polimer]]
[[nl:Polymeer]]
[[ja:重合体]]
[[no:Polymer]]
[[nn:Polymer]]
[[pl:Polimery]]
[[pt:Polímero]]
[[ro:Polimer]]
[[qu:Tawqa iñuwa]]
[[ru:Полимеры]]
[[sq:Polimeri]]
[[simple:Polymer]]
[[sk:Polymér]]
[[sl:Polimer]]
[[sr:Полимер]]
[[su:Polimér]]
[[fi:Polymeeri]]
[[sv:Polymer]]
[[te:అణుపుంజము]]
[[tl:Polimero]]
[[th:พอลิเมอร์]]
[[vi:Polyme]]
[[tr:Polimer]]
[[uk:Полімер]]
[[zh:聚合物]]

Revision as of 20:58, 13 April 2009

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