Under the theta condition also called the Flory condition the polymer behaves like an ideal random coil. 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 Tg. 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 3. Branching of polymer chains also affect the bulk properties of polymers. Long chain branches may increase polymer strength, toughness, and Tg 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 short branches, is quite flexible, and is used in applications such as plastic films. 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 tends to increase T g and increase strength and toughness. Cross linking 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 cross linking by sulfur.
Car tires, for example, are highly cross linked 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 cross linked to allow flaking of the rubber and prevent damage to the paper. Inclusion of plasticizers tends to lower Tg and increase polymer flexibility. Plasticizers 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 unplastiscized 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. Increasing degree of crystallinity tends to make a polymer more rigid. It can also lead to greater brittlness.
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. 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:. 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. These strong hydrogen bonds, for example, result in the high tensile strength and melting point of polymers containing urethane or urea linkages. 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 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. 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. 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 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 molecules 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 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. Cracking refers to thermally or otherwise degrading the polymer to recover either monomers or oligomers.
In a finished product such a change is to be prevented or delayed. 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. Today there are primarily six commodity polymers in use, namely polyethylene , polypropylene , polyvinyl chloride , polyethylene terephthalate , polystyrene and polycarbonate. Examples of polymers are rubber, plastics, and nylon.
The properties of a polymer are affected by the structure, type of monomer units from which polymers are formed, and other factors. Polymers have different physical and chemical properties, which are listed below:. Tensile Strength — The strength of a polymer to elongate without breaking is its tensile strength. Physical strength and durability depend on this property of polymers. Melting Point and Boiling Point — Polymers have a high melting point and boiling point.
Greater the intermolecular forces, longer the chains, and hence higher the melting point and boiling point. Hardness — Hard polymers resist the penetration of hard substances into them. They withstand wear and tear, scratches and are used in the manufacturing of constructing devices. Density- Polymers are classified into high-density polymers and low-density polymers based on the density differences.
The stiffness of molecules decides whether a polymer is a good conductor of heat. Thermal Expansion — The extent to which a polymer expands or contracts when subjected to heat or cold is measured by this property.
Crystallinity — Polymers with less crystallinity are more useful as they are brittle. This property is based on the type of arrangement of polymeric chains. Elasticity — Polymers with weak intermolecular bonds stretch to a greater extent and are more elastic. Permeability — This is the tendency of particles to pass through the polymers. For example, polyethylene is less permeable to air, so it is used to pack food items.
Refractive Index — The extent through which the light bends as it passes the polymer is measured as its refractive index. Polymers use this property in spectroscopy. Resistance to Electric current — Most of the polymers are bad conductors of electricity. Nowadays, conductive polymers are used in semiconductor devices. Their conductivity arises due to conjugated carbon-carbon double bonds. Bonding and reactivity — The strong covalent bond and other weak forces such as hydrogen bonding between the particles of polymers determine its property like reactivity.
Generally, polymers are resistant to chemicals due to their low reactivity. Interaction between the reactive groups — Intermolecular forces among the monomers is decided by their dipole. The carbonyl group amide group present at the side chains of the monomers is responsible for the formation of the hydrogen bond. Adhesion of polymers on the surface, its interaction with coating, and the external environment also affects their quality, like paints.
Biodegradability — Polymers can degrade by the action of decomposers. Natural polymers like rubber are biodegradable, while synthetic polymers are non-biodegradable. But there are also many polymers that occur in nature. The starches found in corn and potatoes are polysaccharides sugar polymers. Silk and hair are polymers known as polypeptides. Cellulose, which makes up the cell wall of plants, is another natural polymer. DNA is a naturally occuring polymer. The first man-made polymers were actually modified versions of these natural polymers.
Celluloid, the stuff from which silent-movie film was made, was a plastic created from chemically modified cellulose. The first completely synthetic polymer that is, made by people through chemical synthesis , invented in the early years of the twentieth century, was Bakelite: a plastic made by reacting phenol and formaldehyde under pressure at high temperatures. It was discovered when its inventor, Leo Baekeland, was trying to find a replacement for shellac, a natural polymer made from the shells of Asian lac beetles.
Different combinations of monomers in these long polymer chains result in polymers with different properties more about how and why a bit later on , so polymers can be created depending on what kind of characteristics you need your material to have—strength, durability, flexibility, and so on. In other words, they can be moulded—using heat, for example. Many plastics are synthesised from hydrocarbon-containing oil or petroleum though not all plastics are: bioplastics , for example, can be made from plants or even bacteria.
The process by which oil is turned into plastic typically goes something like this. First, an oil refinery cracks the oil into small hydrocarbons GLOSSARY hydrocarbons an organic compound made up of only hydrogen and carbon the monomers. Finally, the polymers, in the form of a resin a mass of polymer chains go to a plastics factory, where additives give the plastic the desired properties. In addition polymerisation—you guessed it—monomers are simply added together in a repeating pattern.
This results in no other, additional, substance being created. The other way in which polymers can be created is called condensation polymerisation. In this process, when each monomer is added to the chain, an additional, small molecule—such as water—is created as a by-product. Nylon and polyester are made this way. Addition polymerisation relies on a monomer with a double bond connecting two carbon atoms. A molecule called a free radical is introduced, which causes the double bond to open up and link with the next monomer molecule.
The polymer chain forms when the same basic unit is repeated over and over in a regular chain structure. This means that polymers can be made faster, cheaper, cleaner and with greater control of the final product.
Polyethylene is the simplest synthetic polymer. Other polymers can be made of two or more different monomers. Polyethylene is formed when many thousands of ethylene molecules are joined end to end. This causes it to cleave in two, creating a free radical. A free radical is a molecule with a single unpaired electron. Or, to get technical, a molecule with an unpaired electron in its outermost valence shell is an unstable molecule.
Either way, the lone electron is going to want to pair up with another electron.
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