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Polymer

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 A polymer is a substance composed of molecules with large molecular mass consisting of repeating structural units, or monomers, connected by covalent chemical bonds. The term is derived from the Greek words: polys meaning many, and meros meaning parts [2]. The individual molecules which comprise a polymer are referred to as polymer molecules, where the word "polymer" functions as an adjective.[1]

Overview

While the term polymer in popular usage suggests "plastic", polymers comprise a large class of natural and synthetic materials with a variety of properties and purposes. Natural polymer materials such as shellac and amber have been in use for centuries. Paper is The term polymer was coined in 1833, around the same time as Henri Braconnot's 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. The first wholly synthetic polymer, Bakelite, was introduced in 1909.

Despite significant advances in synthesis and characterization of polymers, a proper understanding of polymer molecular structure did not come until the 1920s. Before that, scientists believed that polymers were clusters of small molecules (called colloids), 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. In the intervening century, synthetic polymer materials such as Nylon, polyethylene, Teflon, and silicone have formed the basis for a burgeoning polymer industry.

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 childrens' toys to aircraft. 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. Future applications include flexible polymer-based substrates for electronic displays and improved time-released and targeted drug delivery.

Polymer science

Main article: Polymer science

Most polymer research may be categorized as polymer science, a sub-discipline of materials science which includes researchers in chemistry (especially organic chemistry), physics, and engineering. Polymer science may be roughly divided into two subdisciplines:

The field of polymer science is generally concerned with synthetic polymers, such as plastics, or chemical treatment and modification of natural polymers.

The study of biological polymers, their structure, function, and method of synthesis is generally the purview of biology, biochemistry, and biophysics. These disciplines share some of the terminology familiar to polymer science, especially when describing the synthesis of biopolymers such as DNA or polysaccharides. However, usage differences persist, such as the practice of using the term macromolecule to describe large non-polymer molecules and complexes of multiple molecular components, such as hemoglobin. Substances with distinct biological function are rarely described in the terminology of polymer science. For example, a protein is rarely referred to as a copolymer.

Polymer synthesis

Polymers are synthesized by three primary methods: organic synthesis in a laboratory or factory, biological synthesis in living cells and organisms, or by chemical modification of naturally occurring polymers.

Organic synthesis

Main article: Polymerization

In 1907, Leo Baekeland created the first completely synthetic polymer, Bakelite, by reacting phenol and formaldehyde at precisely controlled temperature and pressure. Subsequent work by Wallace Carothers in the 1920s demonstrated that polymers could be synthesized rationally from their constituent monomers. The intervening years have shown significant developments in rational polymer synthesis. Most commercially important polymers today are entirely synthetic, produced in high volume on appropriately scaled organic synthetic techniques.

Laboratory synthetic methods are generally divided into two categories, chain-growth polymerization and addition polymerization though some newer methods, such as plasma polymerization do not neatly fit 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 synthesis of proteins, is an area of intense research.

Biological synthesis

Natural polymers and biopolymers formed in living cells may be synthesized by enzyme-mediated processes, such as the formation of DNA catalyzed by DNA polymerase. The synthesis of proteins involves multiple enzyme-mediated processes to transcribe genetic information from the DNA and subsequently translate that information to synthesize the specified protein. The protein may be modified further following translation in order to provide appropriate structure and function.

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 sulfur.


Describing polymers

The properties of a polymer depends both on what kinds of monomers make up the molecule, and how those monomers are arranged. 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 are comprised of the same monomers. Polymer scientists have developed terminology to precisely describe both the nature of the monomers as well as their relative arrangement:

Types of monomers

The type of monomer(s) which compose a polymer molecule may generally be determined by the name of the polymer. Poly(styrene), for example, is composed of styrene monomers. Since there is no internal variety to the monomers, poly(styrene) is classifed as a homopolymer. Polymers with more than one variety of monomer are called copolymers, such as ethylene-vinyl acetate. Some biological polymers are composed of a variety of different but structurally related monomers, such as polynucleotides composed of nucleotide subunits.

A polyelectrolyte molecule is a polymer molecule comprised of primarily ionizable repeating subunits. An ionomer molecule is also ionizable, but to a lesser degree.

Arrangement of monomers

The simplest form of polymer molecule is a straight chain or linear polymer, composed of a single main chain. A 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 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.[2]

Polymer molecules may also be described in terms of how the neighboring structural units are arranged relative to each other, a property referred to as tacticity.

Physical properties of polymers

The properties of polymers vary dramatically depending on the nature, number, and arrangement of the constituent subunits. The same terminology used to describe the properties of non-polymer substances or molecules may be applied to polymers. For example, a polymer molecule may be described as polar, non-polar, or amphiphilic just as any other molecule. There are several cases, however, where a particu

Expressions of mass or size

Like any molecule, a polymer molecule may be described in terms of molecular weight or mass. In homopolymers or block copolymers, however, the molecular mass may be expressed in terms of degree of polymerization, essentially the number of monomer units which comprise the polymer or block. For synthetic polymers, the molecular weight is expressed statistically to describe the distribution of molecular weights in the sample. 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.

The space occupied by a polymer molecule is generally expressed in terms of radius of gyration or excluded volume.

Expressions of 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 comprised of one or more polymer molecules with cell dimensions of hundreds of angstroms or more.

A synthetic polymer may be 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.[3]

Phase transitions

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 "Tm", 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 thermosetting polymers will decompose at high temperatures rather than melt.

The boiling point of a polymer substance is never defined, in that polymers will decompose before reaching assumed boiling temperatures.

A parameter of particular interest in synthetic polymer manufacturing is the glass transition temperature (Tg), which describes the temperature at which amorphous polymers undergo a second order phase transition from a rubbery, viscous amorphous solid to a brittle, glassy amorphous solid. The glass transition temperature may be engineered by altering the degree of branching or cross-linking in the polymer or by the addition of plasticizer.[4]


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[5] and IUPAC[6] have proposed standardized naming conventions; the ACS and IUPAC conventions are similar but not identical.[7] Examples of the difference between the various naming conventions are given in the table below:

Common Name ACS Name IUPAC Name
Poly(ethylene oxide) or (PEO) poly(oxyethylene) poly(oxyethylene)
Poly(ethylene terephthalate) or (PET) poly(oxy-1,2-ethanediyloxycarbonyl -1,4-phenylenecarbonyl) poly(oxyethyleneoxyterephth= aloyl)
Nylon poly[imino(1-oxo-1,6-hexanediyl)] poly[imino(1-oxohexane-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:

File:Example polymerization.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 melting points.

The intermolecular forces in polymers can be affected by dipoles in the monomer units. Polymers containing amide or carbonyl groups can form hydrogen bonds 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 containg urethane or urea linkages. Polyesters have 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, but polyesters have greater flexibility.

Ethene, however, has no permanent dipole. The attractive forces between polyethylene chains arise from weak van der Waals forces. 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 actually 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.

Polymers in solution

In dilute solution, the properties of the polymer are defined 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 are stronger than 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 Flory condition) the polymer behaves like an ideal random coil.

Polymer characterization

The characterization of a polymer requires several parameters which need to be specified. This is because a polymer actually consists of a 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, and 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 and 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.

Manipulating Polymers

Polymers can be manipulated in many ways. This can change their strength and flexibility. The four main ways are:

  • Branching
  • Cross-linking
  • Inclusion of plastisizer
  • Chain Length

Branching

Branching of polymer chains also affect the strength and durability of the chain. An amorphously created polymer has random arrangements of chain, with each of varying length. This means there is no construction, leaving gaps in the atomic structure. This means the product is clear and has a low density. A polymer chain with no branching is highly arranged. This is high density and is translucent to opaque. This is a crystalline. For example, a carrier bag, made of polythene, has a random arrangement of polymer chains. The ethene monomers have been polymerized under pressure and the polymer chains have not lined up neatly. This bag is less durable and can rip very easily. On the other hand, a plastic milk carton has its chains neatly lined up. This means it is stiff and has high heat tolerance and a depression in clarity. The 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.

Cross-linking

Cross-linking is mainly used to strengthen rubbers and vary their strength to be used for different purposes. The cross linking makes the bond between two chains stronger. The process of doing this, by adding sulfur to rubber, is called Vulcanisation. This is supposed to make the rubber more resistant to heat and wear. In an eraser, the polymers are not cross linked with this sulfur. This is to make the rubber weak enough for the paper to not tear. This is a good property of the eraser as it needs to 'flake off' and erase the lead from the pencil. On the tyre of an automobile, the rubber has been cross-linked. The polymer chains are the same but the material is now stronger and the bonds between the polymers are virtually unbreakable.

Inclusion of plasticizer

Plasticizers are oily substances, which make the polymer chains slide upon each other. It is mainly related to polyvinylchloride or PVC. A uPVC or unplastisized polyvinylchloride is used for such things as pipes. A pipe has no plasticizer in it because it needs to remain strong and heat resistant. The chains lie close together meaning it is stronger due to the higher intermolecular force. Normal PVC is used for clothing. It has added plasticizer to it. This means that the chains are separated giving them a flexible quality. The chains can slide past each other, meaning that the material is more comfortable to wear.

Chain Length

The chain length affects the strength and durability of a polymer. A polymer with a short chain of bonded monomers has a low intermolecular force. It's this force that keeps the chains bound together. In a candle, the chain length is quite short, making the wax weak and brittle. In a plastic milk carton, the chain length is longer so the intermolecular force is higher. This makes it hard to rip or break the material. As the force is higher, the amount of heat required to separate these chains, will also be higher.

Polymer degradation

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 chemicals. 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.

The degradation of polymers to form smaller moleculars may proceed by random scission or specific scission. The degradation of polyethylene occurs by random scission - that is by a random breakage of the linkages (bonds) that hold the atoms of the polymer together. When heated above 450 Celsius 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.

In a finished product such a change is to be prevented or delayed. However the degradation process can be useful from the view points 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.

Industry

Today there are primarily six commodity polymers in use, namely polyethylene, polypropylene, polyvinyl chloride, polyethylene terephthalate, polystyrene and polycarbonate. These make up nearly 98% of all polymers and plastics encountered in daily life.

Each of these polymers has its own characteristic modes of degradation and resistances to heat, light and chemicals.

Cracking of Polymers

Cracking is the process by which a polymer is divided into its subcomponents or monomers. The resulting subcomponents are more viscous than the original polymer.

References

  1. ^ IUPAC. "Glossary of Basic Terms in Polymer Science". Pure Appl. Chem. 1996, 68, 2287-2311.
  2. ^ IUPAC. "Glossary of Basic Terms in Polymer Science". Pure Appl. Chem. 1996, 68, 2287-2311.
  3. ^ http://www.iupac.org/publications/books/pbook/PurpleBook-C4.pdf
  4. ^ Brandrup, J.; Immergut, E.H.; Grulke, E.A.; eds Polymer Handbook 4th Ed. New York: Wiley-Interscience, 1999.
  5. ^ CAS: Index Guide, Appendix IV (© 1998).
  6. ^ IUPAC. "Nomenclature of Regular Single-Strand Organic Polymers". Pure Appl. Chem. 1976, 48, 373-385.
  7. ^ [1]
  • Ashby, Michael and Jones, David. Engineering Materials. p. 191-195. Oxford: Butterworth-Heinermann, 1996. Ed. 2.
  • Meyers and Chawla. Mechanical Behavior of Materials. pg. 41. Prentice Hall, Inc. 1999.

See also

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