Encyclopedia
Polymer is a term used to describe
molecules consisting of structural units and a large number of repeating units connected by
covalent chemical bonds. The term is derived from the Greek words:
polys meaning
many, and
meros meaning
parts . The 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 weight, and are linked to each other during a
chemical reaction called polymerization.
Instead of being identical, similar monomers can have various chemical substituents. The 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 in preference to others. 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:
The term polymer is a large, diverse group of molecules, including substances from
proteins to stiff, high-strength
Kevlar fibres. For example, the formation of
polyethene involves thousands of
ethene molecules bonded together to form a straight chain of repeating -CH
2-CH
2- units :
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, polyethene has the unsaturated structure:
Proteins are polymers of
amino acids. Typically, hundreds of the twenty different amino acid monomers make up a protein chain, and the sequence of monomers determines its shape and biological function. But there are
active regions, surrounded by, as is believed now ,
structural regions, whose sole role is to
expose the active regions. 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. 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
- degree of polymerization,
- molar mass distribution, Because synthetic polymer formation is governed by random assembly from the constituent monomers, polymer chains within a solution
In chemistry [i], a
solution is a homogeneous mixture [i] composed of one or more substances, known a ...
or substance are generally not of equal length. This is unlike basic, smaller molecules in which every
atom is stoichiometrically accounted for, and each molecule has a set molecular mass. An ensemble of differing chain lengths, often obeying a
normal distribution, occurs because polymer chains terminate during polymerization after random amounts of chain lengthening .
- crystallinity, as well as the thermal phase transitions:
- Branching During the propagation of polymer chains, branching can occur. In free-radical polymerization, this occurs when a chain curls back and bonds to an earlier part of the chain. When this curl breaks, it leaves small chains sprouting from the main carbon backbone. Branched carbon chains cannot line up as close to each other as unbranched chains can. This causes less contact between atoms of different chains, and fewer opportunities for induced or permanent dipoles
In physics, there are two kinds of
dipoles = double and polos = pivot)....
to occur. A low density results from the chains being further apart. Lower melting points and
tensile strengths are evident, because the intermolecular bonds are weaker and require less energy to break. Besides branching, polymers can have other topologies: linear, network , IPN , comb, or star as well as
dendrimer and hyperbranched structures.
- Stereoregularity or tacticity describes the isomeric arrangement of functional groups on the backbone of carbon chains.
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 is one way to form linear low-density polyethene . The C
4 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 are made to grow out of the sides of chains of another kind , 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 and R' = 6C . 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 monomers.
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. Also, longer chains are more
amorphous . Polymers can be visualised as tangled spaghetti chains - pulling any one spaghetti strand out is a lot harder the more tangled the chains are. These stronger forces typically result in high tensile strength and melting points.
The intermolecular forces in polymers are determined by
dipoles in the monomer units. Polymers containing
amide groups can form
hydrogen bonds between adjacent chains; the positive hydrogen atoms in N-H groups of one chain are strongly attracted to the oxygen atoms in C=O groups on another. These strong hydrogen bonds result in, for example, the high tensile strength and melting point of kevlar.
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 ethene'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 polyethene 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 melts at low temperatures.
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.
Polymer known as polymer substrate is used for everyday
banknotes in
Australia,
Romania,
Papua New Guinea,
Samoa,
Zambia,
Vietnam,
New Zealand and a few others, and the material is also used in commemorative notes in some other countries. The process of polymer substrate creation was developed by the Australia
CSIRO.
See also
- Biopolymer
- Electroactive polymers
- Polymer chemistry
- Polymerization
- Polymer physics
- Important publications in polymer chemistry
External links
