In modern world it is difficult to imagine life without various polymers which allow us to manufacture various products to satisfy our needs. There is a great number of various polymers and therefore in this essay it would be possible to investigate only a limited number out of many. As the science developed it became possible for scientists to develop polymers with required properties .Conditions under which polymers manufactured often determine what properties the product will have. The starting reactants can be the same but the final will be determined by such factors as which catalyst is being used pressure and temperature. It is important to understand what those conditions are in order to be able to produce the product with required characteristics. These conditions often cannot be derived from theoretical part of chemistry but can be found experimentally. Through the course of this work several experiments will be performed in order to investigate these conditions using particular examples such as making nylon e.t.c. The results of each of the experiment will be recorded, processed and analyzed. But at the beginning term polymer must be defined.
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What Polymer is and which types of polymers exist.(definition and an insight in the chemistry involved)
AÂ polymerÂ is a large molecule composed of repeatingÂ structural unitsÂ typically connected byÂ covalentÂ chemical bonds. Usually what first comes to one's mind isÂ plastic, but this term can actually refer to a large group of materials with a wide variety of properties. For example a chain of monomers form DNA which is present in every cell of a living organism and there are also natural occurring ones-rubber. An example of commonly used polymer is polyethane which is produced from ethene. Most commonly, as in this example, the continuously linked backbone of a polymer used for the preparation of plastics consists mainly of carbon atoms. However, other structures may exist; for example, elements such asÂ siliconÂ form familiar materials such asÂ silicones.Â Another common polymer which most people have encountered is paper which consists of cellulose. The list can also contain various synthetically made polymers such as Bakelite, PVC e.t.c. This work will be mainly focused on artificially synthesized ones rather than natural occurring polymers.
Starting in 1811,Â Henri BraconnotÂ did pioneering work in derivative cellulose compounds, perhaps the earliest important work in polymer science. The development ofÂ vulcanizationÂ later in the nineteenth century improved the durability of the natural polymerÂ rubber, signifying the first popularized semi-synthetic polymer. In 1907,Â Leo BaekelandÂ 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 (calledÂ colloids), without definite molecular weights, held together by an unknown force, a concept known asÂ association theory. In 1922,Â Hermann StaudingerÂ 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 theÂ Nobel Prize. Work byÂ Wallace CarothersÂ in the 1920s also demonstrated that polymers could be synthesized rationally from their constituent monomers. An important contribution to synthetic polymer science was made by the Italian chemistÂ Giulio NattaÂ and the German chemistÂ Karl Ziegler, who won the Nobel Prize in Chemistry in 1963 for the development of the Ziegler-Natta catalyst. Further recognition of the importance of polymers came with the award of the Nobel Prize in Chemistry in 1974 toÂ Paul Flory, whose extensive work on polymers included the kineticsÂ ofÂ step-growth polymerizationÂ and ofÂ addition polymerization,Â chain transfer,Â excluded volume, theÂ Flory-Huggins solution theory, and theÂ Flory convention.
. 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 children's toys to aircraft.
Polymerization is the process of combining manyÂ small moleculesÂ known as monomers into a covalently bonded chain. During the polymerization process, some chemical groups may be lost from each monomer. This is the case, for example, in the polymerization ofÂ PET polyester. The monomers areÂ terephthalic acidÂ (HOOC-C6H4-COOH) andÂ ethylene glycolÂ (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 piece of each monomer that is incorporated into the polymer is known as aÂ repeat unit or monomer residue.
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Laboratory synthetic methods are generally divided into two categories,Â step-growth polymerizationÂ andÂ chain-growth polymerization. The essential difference between the two is that in chain growth polymerization, monomers are added to the chain one at a time only, whereas in step-growth polymerization chains of monomers may combine with one another directly. Synthetic polymerization reactions may be carried out with or without aÂ catalyst but generally various catalysts are used in order to lower activation energy. In this investigation chain polymerization and in particular free radical polymerisation will be mainly discussed.
Three phases of chain (free radical) polymerisation.
The monomers used in chain polymerisation are unsaturated referred to as vinyl monomers. In order for the reaction to occur small trace of an initiator material is required. The initiators readily fragment into free radicals either when heated or when irradiated with electromagnetic radiation. The two most common initiators used are benzoyl peroxide and azobisisobutyronitrile.(AIBN)
Reactions of Benzoyl Peroxide
In addition to heat and light generation of free radicals can be accomplished by using y rays, X-rays or through electrochemical means.
When free radical is produced it reacts rapidly with a molecule of monomer to yield a new species that is a still free radical. Shown below
The efficiency of the initiator is a measure of the extent to which the number of radicals formed reflects the number of polymer chains formed. Typical initiator efficiency is 0.6-1.0.
This is the name given to the series of reactions in which the free radical unit at the end of the growing polymer molecule reacts with monomer to increase still further the length of the polymer chain. This is represented by the following general reaction.
Polymerisation does not continue until all of the monomer is used because the free radicals are so reactive and as a result they lose their radical activity. The two methods of termination are: combination and disproportionation.
Combination occurs when two radical species react together to form a single bond and one reaction product:
Two radicals can interact via hydrogen abstraction, leading to the formation of two reaction products, one which is saturated and another one is unsaturated. Shown below:
Polymer properties are broadly divided into several classes based on the scale at which the property is defined as well as upon its physical basis. The most basic property of a polymer is the identity of its constituent monomers. A second set of properties, known as microstructure, is the way monomers are located in a chain .These basic structural properties play a major role in determining bulk physical properties of the polymer, which describe how the polymer behaves as a finished product. 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.
Monomers and repeat units
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 asÂ copolymers. Poly(styrene), for example, is composed only of styrene monomer residues, and is therefore classified as a homopolymer.Â Ethylene-vinyl acetate, on the other hand, contains more than one variety of repeat unit and is thus aÂ copolymer. Some biological polymers are composed of a variety of different but structurally related monomer residues; for example,Â polynucleotidesÂ such as DNA are composed of a variety ofÂ nucleotideÂ subunits.
A polymer molecule containing ionizable subunits is known as aÂ polyelectrolyteÂ orÂ ionomer.
The microstructure of a polymer (sometimes called configuration) relates to the physical arrangement of monomer residues along the backbone of the chain. These are the elements of polymer structure that require the breaking of a covalent bond in order to change. Structure has a strong influence on the other properties of a polymer. For example, two samples of natural rubber may exhibit different durability, even though their molecules comprise the same monomers.
Branch point in a polymer
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An important microstructural feature determining polymer properties is the polymer architecture.Â The simplest polymer architecture is aÂ linearÂ chain: a single backbone with no branches. A related unbranching architecture is aÂ ringÂ polymer. AÂ branched polymerÂ molecule is composed of a main chain with one or more substituent side chains or branches. Special types of branched polymers includeÂ star polymers,Â comb polymers,Â brush polymers,Â dendronized polymers,ladders, andÂ 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. Long chain branches may increase polymer strength, toughness, and theÂ glass transition temperatureÂ (Tg) 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 theÂ branching index. Random length and atactic short chains, on the other hand, may reduce polymer strength due to disruption of organization and may likewise reduce the crystallinity of the polymer.
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 both long and short branches, is quite flexible, and is used in applications such as plastic films.
Dendrimer and dendron
DendrimersÂ 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 architecture of the polymer is often physically determined by theÂ functionalityÂ 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. 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 Tgand increase strength and toughness. Among other applications, this process is used to strengthen rubbers in a process known asvulcanization, 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 networkSufficiently 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 theÂ 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.
A common means of expressing the length of a chain is theÂ degree of polymerization, which quantifies the number of monomers incorporated into the chain.As with other molecules, a polymer's size may also be expressed in terms ofÂ molecular weight. Since synthetic polymerization techniques typically yield a polymer product including a range of molecular weights, the weight is often expressed statistically to describe the distribution of chain lengths present in the same. Common examples are theÂ number average molecular weightÂ andÂ weight average molecular weight.The ratio of these two values is theÂ polydispersity index, commonly used to express the "width" of the molecular weight distribution.Â A final measurement isÂ contour length, which can be understood as the length of the chain backbone in its fully extended state
The flexibility of an unbranched chain polymer is characterized by itsÂ persistence length.
Monomer arrangement in copolymers
Monomers within a copolymer may be organized along the backbone in a variety of ways.
Alternating copolymersÂ possess regularly alternating monomer residues:]Â [AB...]nÂ (2).
Periodic copolymersÂ have monomer residue types arranged in a repeating sequence: [AnBm...] m being different from n .
Statistical copolymersÂ have monomer residues arranged according to a known statistical rule. A statistical copolymer in which the probability of finding a particular type of monomer residue at an particular point in the chain is independent of the types of surrounding monomer residue may be referred to as a trulyÂ random copolymer.
Block copolymersÂ have two or more homopolymer subunits linked by covalent bonds . Polymers with two or three blocks of two distinct chemical species (e.g., A and B) are called diblock copolymers and triblock copolymers, respectively. Polymers with three blocks, each of a different chemical species (e.g., A, B, and C) are termed triblock terpolymers.
Graft or grafted copolymersÂ contain side chains that have a different composition or configuration than the main chain.
The bulk properties of a polymer are those most often of end-use interest. These are the properties that indicate how the polymer actually behaves on a macroscopic scale.
TheÂ tensile strengthÂ of a material quantifies how much stress the material will endure before suffering permanent deformation.Â 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.
Young's modulus of elasticity
Young's ModulusÂ quantifies the elasticity of the polymer. It is defined, for small strains, as the ratio of rate of change of stress to strain. Like tensile strength, this is highly relevant in polymer applications involving the physical properties of polymers, such as rubber bands. The modulus is strongly dependent on temperature.
Transport properties such as diffusivity relate to how rapidly molecules move through the polymer matrix. These are very important in many applications of polymers for films and membranes.
The termÂ 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 simplyÂ Tm, the property in question is more properly called the crystalline melting temperature. Among synthetic polymers, crystalline melting is only discussed with regards tothermoplastics, asÂ thermosettingÂ polymers will decompose at high temperatures rather than melt.
TheÂ boiling pointÂ of a polymeric material is strongly dependent on chain length. High polymers with a large degree of polymerization do not exhibit a boiling point because they decompose before reaching theoretical boiling temperatures. For shorterÂ oligomers, a boiling transition may be observed and will generally increase rapidly as chain length is increased.
Inclusion of plasticizers
Inclusion ofÂ plasticizersÂ tends to lower TgÂ and increase polymer flexibility.Â PlasticizersÂ are generallyÂ small moleculesÂ 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 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 toÂ ionic bondingÂ orÂ hydrogen bondingÂ between its own chains. These stronger forces typically result in higher tensile strength and higher crystalline melting points.
The intermolecular forces in polymers can be affected byÂ dipolesÂ in the monomer units. Polymers containingÂ amideÂ orÂ carbonylÂ groups can formÂ hydrogen bondsÂ 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 containingÂ urethaneÂ orÂ ureaÂ linkages.Â PolyestersÂ haveÂ dipole-dipole bondingÂ 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 thanÂ Kevlar's (Twaron), but polyesters have greater flexibility.
Ethene, however, has no permanent dipole. The attractive forces between polyethylene chains arise from weakÂ 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 attract the second polymer chain. 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 aÂ statisticalÂ distribution of chains of varying lengths, and each chain consists of monomer residues which affect its properties.