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Polymers are the substances containing a large number of structural units joined by the same type of linkage. These substances also form into a chain like structure. Polymers have a range of applications that far exceeds that of any other class of material available to man. Current applications extend from adhesives, coatings, foams, and packaging materials to textile and industrial fibres, optical fibres etc. The major applications ---- Structural polymers and composites,
Packaging materials and coatings,
Transparent and optical materials,
Biological and medical materials ,
Fluid modifiers ans suspension stabilizers etc.
A polymer is a large molecule composed of repeating structural units connected by covalent chemical bonds. The word polymer is derived from the Greek words poly - meaning "many", and meros - meaning "part". The term was coined in 1833 by Jons Jacob Berzelius. Because of the extraordinary range of properties of polymeric materials, they play an essential role in every day life, ranging from familiar synthetic plastics and elastomers to natural bipolymers such as nucleic acids and proteins that are essential for life. Natural polymers are being in use from centuries. Bipolymers such as proteins and nucleic acids play crucial part in boilogical processes. Natural polymeric materials such as shellac, amber, and natural rubber have been used for centuries. A variety of other natural polymers exist, such as cellose, which is the main constituent of wood and paper. The list of synthetic polymers includes synthetic rubber, bakelite, neoprene, nylon, PVC, polystryene, polyethylene, polypropylene, polyacrylonitrile, PVB, silicone, and many more. Most commonly, the continuously linked backbone of a polymer used for the preparation of plastics consists mainly of carbon atoms. A simple example is polyethylene, whose repeating unit is based on ethylene monomer. However, other structures do exist; for example, elements such as silicon form familiar materials such as silicones, examples being silli putty and waterproof plumbing sealant. Oxygen is also commonly present in polymer backbones, such as those of polyethylene glycol, polysaccharides, and DNA.
In 1811, Henri Braconnot did a fantastic work in derivative cellulose compounds which too was the earliest and the important work in the polymeric science. Later in the nineteenth century, the development of vulcanisation improved the durability of natural polymer rubber, signifying the first popularised semi - synthetic polymer. In 1907, Leo Bakeland 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 colliods, without definite molecular weights, and held together by an unknown force. Then 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. Also in 1920 Wallace Corothers demonstrated that polymers could be synthesized rationally from their constituent monomers. Then an important contribution to synthetic polymer science was made by the Italian chemist Giulio Nattta and the German chemist Karl Ziegler, who won the Nobel Prize in Chemistry in 1963 for the development of the Ziegler-Natta catalyst. Then in 1974, Paul Flory got the nobel prize for his extensive work on polymers included the kinetics of step growth polymeriastion and of addition polmerisation, chain transfer, excluded volume, the Flory Huggins solution theory, and the Flory convention.Synthetic polymer materials such as nylon, polyethylene, 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. The synthetic polymers today are used in almost every industry and the area of life.
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.
For example:-Take the example of PET polyester. The monomers are terephthalic acid (HOOC-C6H4-COOH) and ethylene glycol (HO-CH2-CH2-OH) but the repeating unit is -OC-C6H4-COO-CH2-CH2-O-, which corresponds to the combination of the two monomers with the loss of two water molecules. The distinct pieces of each monomer that is incorporated into the polymer is known as a repeat unit or monomer residue.
Laboratory synthetic methods are generally divided into two categories, step-growth polymerization and chain-growth polymerization. Growth polymerisation is the main difference between the step growth and the chain growth polymeristion. Monomers are added to the chain one at a time only in the chain growth polymerisation, whereas in step-growth polymerization chains of monomers may combine with one another directly. 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. Laboratory synthesis of biopolymers, especially of protein, is an area of intensive research.
There are three main classes of biopolymers, and these are polysaccharides, polypeptides and polynucleotides. In the living cells, they may be synthesized by enzyme-mediated processes,which is as such in the formation of DNA catalyzed by DNA polymerase
(microstructure of a part of DNA )
Synthesis of proteins involves multiple enzyme-mediated processes to transcribe genetic information from the DNA to RNA and subsequently translate that information to synthesize the specified protein from amino acids. In order to provide the appropriate structure of and functuioning the protein has to be modified further.
Modification of natural polymers
Many commercially important polymers are synthesized by chemical modification of naturally occurring polymers. Prominent examples of the modification of natural polymers include the reaction of nitric acid and cellulose to form nitrocellulose and the formation of vulcanized rubber in the presence of sulfur by heating natural rubber.
Polymer properties are broadly divided into several classes based on the scale at which the property is defined as well as upon its physical state/ basis. The most and basic property of a polymer is the identity of its constituent monomers. A second set of properties, known as microstructure, essentially describe the arrangement of these monomers within the polymer at the scale of a single chain. These basic structural properties play a major role in determining the bulk physical properties of the polymer, which describe how the polymer behaves as a continuous macroscopic material. Chemical properties, at the nano-scale, describe how the chains interact through various physical forces. At the macro-scale, they describe how the bulk polymer interacts with other chemicals and solvents. It can be also shown as :
Identity of constituent monomers
Monomers and repeating units
The first and the most important attribute is to identify the repeating unit i.e. its monomer residues The identity of the monomer residues (repeat units) comprising a polymer is its first and most important attribute. Polymer nomenclature is generally based upon the type of monomer residues comprising the polymer. Polymers that contain only a single type of repeat unit are known as homopolymers, while polymers containing a mixture of repeat units are known as copolymers. Ethylene-vinyl acetate, on the other hand, contains more than one variety of repeat unit and is thus a copolymer. A polymer molecule containing ionizable sub units is known as a polyelectrolyte or ionomer.
The microstructure of a polymer relates to the physical arrangement of monomer which residues along the backbone of the chain and these are the elements of polymer structure which requires the breaking of a covalent bond in order to change it. Structure has a strong influence on the other properties of a polymers also. For example, two samples of natural rubber may exhibit different durability, even though their molecules comprise the same monomers.
The simplest polymer architecture is a linear chain i.e. a single backbone with no branches.Where as the related unbranching architecture is called a ring polymer. 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, brush polymers, dendronized polymers, ladders, and dendrimers. Branching of polymer chains affects the ability of chains to slide past one another by altering intermolecular forces, in turn affecting bulk physical polymer properties. The long chain branches can increase the polymer strength, toughness, and the glass transition temperature (Tg) which is due to an increase in the number of entanglements per chain. The effect of such long-chain branches on the size of the polymer in solution is characterized by the branching index. Random length and atactic short chains, on the other side, may reduce polymer strength and due to disruption of organization and may likewise reduce the crystallinity of the polymer.
(branch point in a polymer)
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. The structure of the polymer is often physically determined by the functionality of the monomers from which it is formed. This property of a monomer is defined as the number of reaction sites at which may form chemical covalent bonds. The basic functionality required for forming even a linear chain is two bonding sites.The higher functionality yields branched or even crosslinked or networked polymer chains. An effect related to branching is chemical crosslinking - the formation of covalent bonds between chains. Crosslinking tends to increase Tg and increase strength and toughness. Among other applications, this process is used to strengthen rubbers in a process known as vulcanization, which is based on crosslinking by sulfur.
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.
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. 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.
The physical properties of a polymer are strongly dependent on the size or length of the polymer chain. For example, as chain length is increased, melting and boiling temperatures increase quickly . Impact resistance also tends to increase with chain length, as does the viscosity, or resistance to flow, of the polymer in its melt state. Chain length is related to melt viscosity roughly as 1:103.2, so that a tenfold increase in polymer chain length results in a viscosity increase of over 1000 times. Increasing chain length furthermore 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.The flexibility of an unbranched chain polymer is characterized by its persistence length.
Tacticity describes the relative stereochemistry of 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).
Polymer morphology generally describes the arrangement and microscale ordering of polymer chains in space.
Crystalline has almost a ambiguous usage. In some of the 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 i.e rather than macromolecular length scales, usually arising from intramolecular folding and/or stacking of adjacent chains. Synthetic polymers can 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. 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. Thus for many polymers, reduced crystallinity may also be associated with increased transparency.
The space that is occupied by a polymer molecule is generally expressed in terms 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, that is the volume of solution spanned by the polymer chain and scales with the cube of the radius of gyration.
Most of the 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.
Tensile strength of a material quantifies that how much stress the material will take before suffering the permanent defrmation. 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.
(a polethylene sample necking under tension)
Young's modulus of elasticity
Young's Modulus shows the elasticity of the polymer. It is only defined, for small strains, as to the ratio of rate of change of stress to strain. As that of tensile strength, this is highly relevant in polymer applications involving the physical properties of polymers, such as rubber bands. The young's modulus of elasticity is strongly dependent on temperature.
Transport properties shows how rapidly molecules move through the polymer matrix. These are very very important in many applications of polymers. For example:- Transport properties are used in case of films and membranes.
Melting ponint in polymers suggests it not a solid-liquid phase transition but a transition from a crystalline or semi-crystalline phase to a solid amorphous phase.In case of synthethic polmers, thermosetting polymers will decompose at high temperatures rather than melt.
Glass transition temperature
Glass transition is the temperature in which a parameter of particular interest in synthetic polymer manufacturing describes the temperature at which amorphous polymers undergo a transition from a rubbery, viscous amorphous solid, to a brittle, glassy amorphous solid. The glass transition temperature may be calculated by altering the degree of branching or crosslinking in the polymer or by the addition of plasticizer.
Polymeric mixtures are far less miscible than mixtures of small molecule materials. This effect results from the fact that the driving force for mixing is usually entropy, not interaction energy. 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 is far smaller than the number in a small molecule mixture of equal volume. The energetics of mixing, on the other hand, is 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.
(Phase diagram of the typical mixing behavior of weakly interacting polymer solutions)
Thus, concentrated solutions of polymers are far rarer than those of small molecules.The phase behavior of polymer solutions and mixtures is more complex than that of small molecule mixtures. Whereas most small molecule solutions exhibit only an upper critical solution temperature phase transition, at which phase separation occurs with cooling, polymer mixtures commonly exhibit a lower critical solution temperature phase transition, at which phase separation occurs with heating. 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.
Inclusion of plasticizers
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 PVC's. 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.
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 crystalline melting points. Van der Waals forces are quite weak, however, so polyethylene 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. 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. 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. Another Polymer characterization technique is Automatic Continuous Online Monitoring of Polymerization Reactions(ACOMP) which provides real-time characterization of polymerization reactions. It can be used as an analytical method in R&D, as a tool for reaction optimization at the bench and pilot plant level and, eventually, for feedback control of full-scale reactors. ACOMP measures in a model-independent fashion the evolution of average molar mass and intrinsic viscosity, monomer conversion kinetics and, in the case of copolymers, also the average composition drift and distribution. It is applicable in the areas of free radical and controlled radical homo- and copolymerization, polyelectrolyte synthesis, heterogeneous phase reactions, including emulsion polymerization, adaptation to batch and continuous reactors, and modifications of polymers.
Polymer degradation is a change in the properties i.e tensile strength, color, shape, molecular weight, etc.. It is often due to the scission of polymer chain bonds via hydrolysis, leading to a decrease in the molecular mass of the polymer.Polymer degradation is the change in polmer or polymer-based product under the influence of one or more environmental factors, such as heat, light, chemicals and, in some cases, galvanic action. Although such changes are frequently undesirable, in some cases, such as biodegradation and recycling, they may be intended to prevent environmental pollution. Degradation can also be useful in biomedical settings. For example, a copolymer of Polylactic acid and polyglycolic acid is employed in hydrolysable stitches that slowly degrade after they are applied to a wound.
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 linkage bonds that hold the atoms of the polymer together. When heated above 450 Â°C it degrades to form a mixture of hydrocarbons.
Biopolymers are polymers produced by living organisms. Biopolymers (also called renewable polymers) are generally produced from biomass which comes from crops such as sugar beet, potatoes or wheat. Some biopolymers are biodegradable. That is why they are broken down into CO2 and water by micro - organisms. In addition some of these biodegradable polymers are compostable. Some pf the examples of the biopolymers are cellulose, starch, proteins, peptides, DNA, RNA, etc. Cellulose is the most common biopolymer.
( Microstructure of cellulose the most common biopolymer)
Applications Of Polymers
Macromolecular science has a major impact on our daily lives. It is difficult to find even a single part of our life which is not affected by polymers. Just about 50 years ago, materials we now take for granted were non-existent. With further advancement in the study of polymers, and with new applications being researched, there is no reason to believe that the revolution will stop any time soon. There are some of the common applications of the polymer introduced in the section on structural polymers. These are the cross section of the ways in which polymers are used in industries.
Rubber is the most important of all elastomers. Natural rubber is the polymer having isoprene as repeating unit. This material which is obtained from the bark of the rubber tree, has been used by humans for centuries. It was not until 1823, however, that rubber became the valuable material we know today. In that year, Charles Goodyear succeeded in "vulcanizing" natural rubber by heating it with sulfur. In the process of valcanizing, sulfur chain fragments attack the polymer chains and lead to cross linking. The term vulcanization is often used now to describe the cross-linking of all elastomers.
Much of the rubber used in the United States today is a synthetic variety called styrene-butadiene rubber (SBR). Initial attempts to produce synthetic rubber revolved around isoprene because of its presence in natural rubber. Researchers eventually found success using butadiene and styrene with sodium metal as the initiator. This rubber was called Buna-S -- "Bu" from butadiene, "na" from the symbol for sodium, and "S" from styrene. During World War II, hundreds of thousands of tons of synthetic rubber were produced in government controlled factories. After the war, private industry took over and changed the name to styrene-butadiene rubber. Today, the United States consumes on the order of a million tons of SBR each year. Natural and other synthetic rubber materials are quite important.
About 60 billion pounds of plastic is consumed by only Americans per year. The two main types of plastics are Thermoplastics and Thermosets. Thermoplastic is that type of plastic which soften on heating and harden on cooling while thermosets, on heating, flow and cross-link to form rigid material which does not soften on future heating. Thermoplastics account for the majority of commercial usage. Among the most important and versatile of all plastics is polyethylene. Polyethylene is used in a wide variety of applications because, based on its structure, it can be produced in many different forms. The first type to be commercially exploited was called low density polyethylene (LDPE) or branched polyethylene. This polymer is characterized by a large degree of branching, forcing the molecules to be packed rather loosely forming a low density material. Low Density Polyethylene (LDPE) is soft and pliable and has applications ranging from plastic bags, containers, textiles, and electrical insulation, to coatings for packaging materials. Another form of polyethylene differing from low density polyethylene only in structure is high density polyethylene (HDPE) or linear polyethylene. This form demonstrates little or no branching, enabling the molecules to be tightly packed. HDPE is much more rigid than branched polyethylene and is used in applications where rigidity is important. Major uses of HDPE are plastic tubing, bottles, and bottle caps.
Other forms of this material include high and ultra-high molecular weight polyethylenes. HMW and UHMW, as they are known. These are used in applications where extremely tough and resilient materials are needed.
Fibers is a very important application of polymeric materials which includes many examples from the categories of plastics and elastomers. Natural fibers like cotton, wool and silk have been used by humans for many centuries. In 1885, artificial silk was patented and launched the modern fiber industry. Man-made fibers include materials such as nylon, polyester, rayon, and acrylic. The combination of strength, weight, and durability have made these materials very important in modern industry. Generally speaking, fibers are at least 100 times longer than they are wide. Typical natural and artificial fibers can have axial ratios (ratio of length to diameter) of 3000 or more. Synthetic polymers have been developed that posess desirable characteristics, such as a high softening point to allow for ironing, high tensile strength, adequate stiffness, and desirable fabric qualities. These polymers are then formed into fibers with various characteristics. Nylon (a generic term for polyamides) was developed in the 1930's and used for parachutes in World War II. This synthetic fiber, known for its strength, elasticity, toughness, and resistance to abrasion, has commercial applications including clothing and carpeting. Nylon has special properties which distinguish it from other materials. One such property is the elasticity. Nylon is very elastic, however after elastic limit has been exceeded the material will not return to its original shape. Like other synthetic fibers, Nylon has a large electrical resistance. This is the cause for the build-up of static charges in some articles of clothing and carpets. From textiles to bullet-proof vests, fibers have become very important in modern life. As the technology of fiber processing expands, new generations of strong and light weight materials will be produced.
Once a polymer with the right properties is produced, it must be manipulated into some useful shape or object. Various methods are used in industry to do this. Injection molding and extrusion are widely used to process plastics while spinning is the process used to produce fibers.
One of the most widely used forms of plastic processing is injection molding. Basically, a plastic is heated above its glass transition temperature (enough so that it will flow) and then is forced under high pressure to fill the contents of a mold. The molten plastic in usually "squeezed" into the mold by a ram or a reciprocating screw. The plastic is allowed to cool and is then removed from the mold in its final form. The advantage of injection molding is speed; this process can be performed many times each second.
Extrusion is similar to injection molding except that the plastic is forced through a die rather than into a mold. However, the disadvantage of extrusion is that the objects made must have the same cross-sectional shape. Plastic tubing and hose is produced in this manner.
The process of producing fibers is called spinning. There are three main types of spinning: melt, dry, and wet. Melt spinning is used for polymers that can be melted easily. Dry spinning involves dissolving the polymer into a solution that can be evaporated. Wet spinning is used when the solvent cannot be evaporated and must be removed by chemical means. All types of spinning use the same principle, so it is convenient to just describe just one. In melt spinning, a mass of polymer is heated until it will flow. The molten polymer is pumped to the face of a metal disk containing many small holes, called the spinneret. Tiny streams of polymer that emerge from these holes (called filaments) are wound together as they solidify, forming a long fiber. Speeds of up to 2500 feet/minute can be employed in spinning. Following the spinning process, as noted in the section on polymer morphology, fibers are stretched substantially - from 3 to 8 or more times their original length to produce increased chain alignment and enhanced crystallinity in order to yield improved strength.