Jump to content

Polymer

From Wikipedia, the free encyclopedia
This is an old revision of this page, as edited by Sir Nicholas de Mimsy-Porpington (talk | contribs) at 13:16, 25 January 2007 (Reverted edits by 84.199.22.106 (talk) to last version by LakeHMM). The present address (URL) is a permanent link to this revision, which may differ significantly from the current revision.

You must add a |reason= parameter to this Cleanup template – replace it with {{Cleanup|January 2007|reason=<Fill reason here>}}, or remove the Cleanup template.

Polymer is a term used to describe large molecules 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 [1]. A key feature that distinguishes polymers from other molecules is the repetition of many identical, similar, or complementary molecular subunits in these chains. These subunits, the monomers, are small molecules of low-to-moderate molecular mass, and are linked to each other during a chemical reaction called polymerization.

Similar monomers can have various chemical substituents. These differences between monomers can affect properties such as solubility, flexibility, and strength. In proteins, these differences give the polymer the ability to adopt a biologically-active conformation. (See self-assembly.) Identical monomers with nonreactive side groups result in a polymer chain that will tend to adopt a random coil conformation, as described by an ideal chain mathematical model. Although most polymers are organic, with carbon-based monomers, there are also inorganic polymers; for example, the silicones, with a backbone of alternating silicon and oxygen atoms and polyphosphazenes.

Polymer nomenclature

Polymers are typically classified according to four main groups:

Polymer signifies a chain of thousands of monomers that are covalently bonded together usually by the carbon atoms of the polymer backbone, but the backbone can consist of other atoms such as silicon. Examples of polymers include substances anywhere from proteins to stiff, high-strength Kevlar fibres. For example, the formation of poly(ethylene) (also called polyethene) involves thousands of ethene molecules bonded together to form a straight (or branched) chain of repeating -CH2-CH2- units (with a -CH3 at each terminal):

File:Example polymerization.png

Polymers are often named in terms of the monomer from which they are made. Because it is synthesized from ethene in a process during which all the double bonds in the vinyl monomers are lost, poly(ethylene) has a saturated structure:

Proteins are polymers of amino acids. Typically, hundreds of the (nominally) twenty different amino acid monomers make up a protein chain, and the sequence of monomers determines its shape and biological function. (There are also shorter oligopeptides which function as hormones.) But there are active regions, surrounded by, as is believed now (Aug 2003), structural regions, whose sole role is to expose the active regions. (There may be more than one on a given protein.) So the exact sequence of amino acids in certain parts of the chains can vary from species to species, and even given mutations within a species, so long as the active sites are properly accessible. Also, whereas the formation of polyethylene occurs spontaneously under the right conditions, the synthesis of biopolymers such as proteins and nucleic acids requires the help of enzyme catalysts, substances that facilitate and accelerate reactions. Unlike synthetic polymers, these biopolymers have exact sequences and lengths. (This does not include the carbohydrates.) Since the 1950s, catalysts have also revolutionised the development of synthetic polymers. By allowing more careful control over polymerization reactions, polymers with new properties, such as the ability to emit coloured light, have been manufactured.

Physical properties of polymers

Physical properties of polymers include:

Constitution of polymers

Copolymerization with two or more different monomers results in chains with varied properties. There are twenty amino acid monomers whose sequence results in different shapes and functions of protein chains. Copolymerising ethene with small amounts of 1-hexene (or 4-methyl-1-pentene) is one way to form linear low-density polyethene (LLDPE). (See polyethylene.) The C4 branches resulting from the hexene lower the density and prevent large crystalline regions from forming within the polymer, as they do in HDPE. This means that LLDPE can withstand strong tearing forces while maintaining flexibility.

A block copolymer is formed when the reaction is carried out in a stepwise manner, leading to a structure with long sequences or blocks of one monomer alternating with long sequences of the other. There are also graft copolymers, in which entire chains of one kind (e.g., polystyrene) are made to grow out of the sides of chains of another kind (e.g., polybutadiene), resulting in a product that is less brittle and more impact-resistant. Thus, block and graft copolymers can combine the useful properties of both constituents and often behave as quasi-two-phase systems.

The following is an example of step-growth polymerization, or condensation polymerization, in which a molecule of water is given off and nylon is formed. The properties of the nylon are determined by the R and R' groups in the monomers used.

The first commercially successful, completely synthetic polymer was nylon 6,6, with alkane chains R = 4C (adipic acid) and R' = 6C (hexamethylene diamine). Including the two carboxyl carbons, each monomer donates 6 carbons; hence the name. In naming nylons, the number of carbons from the diamine is given first and the number from the diacid second. Kevlar is an aromatic nylon in which both R and R' are benzene rings.

Copolymers illustrate the point that the repeating unit in a polymer, such as a nylon, polyester or polyurethane, is often made up of two (or more) monomers.

Mechanical properties of polymers

Low coefficients of friction - In general coefficients of friction for polymers against polymers, metals or ceramics range from 0.15 to 0.6. Composites also retain the low coefficients of friction while having greater stiffness and strength. [1]

Creep - the application of a constant (time independent) load causes a continuous displacement associated with the diffusion of the atoms or molecules within the material. This response is termed creep, the progressive deformation of a material under a sustained load. In metal alloys, creep becomes a problem at temperatures above approximately 0.6 Tm, where Tm is the melting temperature of the metal. However, in polymers, creep is an even more critical design problem because the polymers can be described as rubbery and viscous at temperatures above the glass transition temperature.

  • Ways to resist creep in polymers: In order to make polymers more resistant to creep at room temperature, increasing the degree of cross-linking within the polymer chain will raise the Tg. Therefore, a higher glass transition temperature allows for more resistance to creep. Furthermore, as molecular mass of the polymer increases, the viscosity, η will increase and effectively reduce the rate of creep. Also, the more crystalline the polymer, the more creep-resistant it is compared to entirely glassy polymers.

J-shaped stress-strain curves - Many biological materials actually display J-shaped stress-strain curves. In other words, the material will initially experience large extensions for small stresses. Then, as the extension gets larger and larger, the material gradually becomes stiffer and more difficult to extend.

A J-shaped stress-strain curve enables biomaterials to be extremely tough. Because the lower part of the curve has large extensions for small stresses, the shear modulus in that region is very low and so any released strain energy can be prevented from contributing to fracture of the material. Furthermore, large extensions of the material require very large stresses, and so these large extensions are likely to occur only infrequently. The J-shaped stress-strain curve does not lead to the elastic instabilities which arise in S-shaped curves.

An example of biological materials with J-shaped stress-strain curves is mammalian skin tissue. If you pinch your earlobe and try to pull it downwards, it is initially easy to stretch, but then at larger extensions it becomes much more difficult to stretch.

S-shaped stress-strain curves - These occur in lightly cross-linked polymers such as rubber. A material that exhibits an S-shaped stress-strain curve is very is applied to the material, a very large stiffness occurs because at these high loads, the polymer chains are mostly aligned with the applied stress. Therefore, applying even more stress will stretch the strong intramolecular bonds.

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 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 it's chains neatly lined up. This means it is stiff and has high heat tolerance and a depression in clarity.

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 (chain you know what i mean???)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.

References

  1. ^ Frictional Characteristics of Metals, Ceramics, and Polymers Sergio Deana, Steven Kotso, Erin McCleave. Retrieved 1 January 2007.

See also

Retrieved from "https://en.wikipedia.org/w/index.php?title=Polymer&oldid=103133256"