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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.
What Polymer is and which types of polymers exist.(definition and an insight in the chemistry involved)
A polymer is a large molecule which is composed of repeating structural units typically connected by covalent chemical bonds. Usually what first comes to one's mind is 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.Chains of polymers are usually made of carbon atoms. However, there are other structures present; for example, elements such as silicon form materials such as silicones. 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, Henri Braconnot derivated cellulose compounds,first and the most important work which led to development of other branch of chemistry - polymer science. Only nearly 100 years later, Leo Baekeland created the first manmade synthetic polymer, Bakelite, through the reaction of phenol and formaldehyde at precisely controlled temperature and pressure. Bakelite was then publicly introduced in and various goods started being produced.
The science of polymers started its official development in 1920s when various theories were proposed.
Scientists believed that polymers were clusters of small molecules (called colloids), without definite molecular weights, held together by an unknown force, a concept known as association theory.Hermann Staudinger proposed that polymers consisted of long chains of atoms held together by covalent bonds; however this proposition was not accepted by scientific community for a long period of time. Work by Wallace Carothers in the 1920s also demonstrated that polymers could be synthesized rationally from their constituent monomers.
Synthetic polymers are widely used in nearly every industry and area of life. Polymers can also be used as adhesives and lubricants, as well as structural components for products ranging from pc cases to racing cars.
Polymerization is the process of combining many small molecules (monomers) into a covalently bonded chain containing many monomers. During the polymerization process, some chemical groups may be lost from each monomer. This is the case, for example, in the polymerization of PET polyester. The common monomers used to demonstrate this fact 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 piece of each monomer that is incorporated into the polymer is known as a repeat unit or monomer residue.
Synthesis in laboratory
Laboratory synthetic methods are divided into two categories, chain-growth polymerization and step-growth polymerization . There is an essential difference between the two ways 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 a 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:
Properties of Polymers
Polymer properties are difficult to define and can be 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 its monomer composition. Another property is microstructure, the way monomers are arranged in a chain .These properties play a major role in determining physical properties of the polymer, which describe how the polymer behaves as a finished product. Chemical properties of polymers, describe how the chains interact through various physical forces(which reactions occur). At the bigger scale, they describe how the polymer reacts with other chemicals and solvents.
Monomers and repeat units
The identity of the monomer residues (repeat units) which make the polymer are vital in order to determine how polymer will behave.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 various repeat units are known as copolymers. Polyethane, for example, is composed only of styrene monomer residues, and is therefore classified as a homopolymer. Chloromethane, on the other hand, contains more than one variety of repeat units and is thus a copolymer. Some biological polymers are composed of a variety of different but structurally related monomer residues; for example, polynucleotides such as DNA are composed of a variety of nucleotide subunits.
A polymer molecule containing ionizable subunits is known as a polyelectrolyte or ionomer.
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.
The tensile strength of a material shows how much stress the material will take before braking.This is very important in applications that rely upon a polymer's physical strength or durability for example when designing aircrafts. For example, a rubber band with a higher tensile strength will hold a greater weight before braking compared to the one with lower tensile strength. In general, tensile strength increases with polymer chain length and crosslinking of polymer chains.
Young's modulus of elasticity
Young's Modulus 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 define how fast molecules can pass through given thickness of material. These are very important in many applications of polymers for films and membranes.
The term melting point, when applied to polymers, is not a solid-liquid phase transition like in metals but a transition from a crystalline or semi-crystalline phase to a solid amorphous phase. Though abbreviated as simply Tm, the property in question is more properly called the crystalline melting temperature. Among synthetic polymers, crystalline melting is only discussed with regards to thermoplastics, as thermosetting polymers will decompose at high temperatures rather than melt.
The boiling point of a polymer is nearly solely dependent on chain length. However polymers with long chain of atoms exhibit do not achieve theoretical boiling point due to the fact that they decompose before reaching theoretical boiling temperatures. Other than that it is possible to observe this trend for shorter chains. The general trend is as the chain length increases so does the boiling point.
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.
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. 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 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.
This essay also contains experimental part which will occupy nearly half of the required word count(approximately 2000 words). However I will only be able to perform those experiments later in september and do the wright up for them. I planned to do a series of experiments involving single monomer and apply various initiators.When the sufficient number of products will be obtained the following experiments will be done:
By doing those experiments it would be possible to create matrix of results which appears to be a suitable way of representing information and dealing with obtained information. Then the information will be analyzed and conclusion of our investigation published. It appears when scanned by the anti-plagiarism scanner the percentage of plagiarism is high the only explanation that I have is that in science there are not many ways by which one can express them.I tried to offer as many references as possible to decrease the extent of this problem.