In this article we will discuss about:- 1. Introduction to Polymers 2. Size of a Polymer 3. Mechanism of Polymerisation 4. Depolymerisation 5. Ionic Polymerisation 6. Molecular Structure of Polymers 7. Crosslinking of Polymers 8. Branching of Polymers 9. Properties of Polymers 10. Effect of Polymer Structure on Properties 11. Functionality of Polymers 12. Bond Strength of Polymers and Other Details.
Contents:
- Introduction to Polymers
- Size of a Polymer
- Mechanism of Polymerisation
- Depolymerisation
- Ionic Polymerisation
- Molecular Structure of Polymers
- Crosslinking of Polymers
- Branching of Polymers
- Properties of Polymers
- Effect of Polymer Structure on Properties
- Functionality of Polymers
- Bond Strength of Polymers
- Deformation of Polymers
- Advanced Polymeric Materials
- Behaviour of Polymers
1. Introduction to Polymers:
Polymers are organic materials having carbon as the common element in their make-up. Organic materials are those materials which are derived directly from carbon and consist of carbon chemically combined with hydrogen, oxygen or other non-metallic substances; invariably having a complex structure. Examples being – wood, animal fibers, natural rubber, coal etc., having biological origin and synthetic fibers, plastics, soaps, cutting oils etc.
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The polymers are composed of a large number of repeating units (small molecules) called “monomers.” A polymer is, therefore, made up of thousands of monomers joined together to form a large molecule of colloidal dimension, called “macromolecule.”
The unique feature of a polymer is that each molecule is either a long chain or network of repeating units all covalently bonded together. In some cases, molecules are held together by secondary bonds.
2. Size of a Polymer:
The size of a molecule is determined by dividing the molecular weight by the mer weights. The number is called degree of polymerisation, DP.
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DP = Molecular weight / Mer weight
At DP values of above 10 to 20 mers per molecule, the substance formed is light oil. As the DP increases, the substance becomes greasy, then waxy, and finally at a value of DP of about 1000 the substance becomes a solid and is then, a true polymer. Natural DP is almost unlimited—it may increase to around 100000 or so on.
3. Mechanism of Polymerisation:
The process of linking together of monomers is called “polymerisation.” The need to start with the process of polymerisation lies on the necessity of breaking the double bond (C=C) of the monomers. This requires considerable energy.
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Polymerisation mechanisms may be of the following two types:
1. Addition polymerisation.
2. Condensation polymerisation.
1. Addition Polymerisation:
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This polymerisation process is of simplest form. When a large number of simple molecules are chemically added together to increase the average molecule size without wastage, process of addition polymerisation takes place.
Such a polymerisation takes places by three steps namely:
(i) Initiation,
(ii) Chain propagation, and
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(iii) Termination.
Example- Addition polymerisation of ethylene (Fig. 9.1).
Once polymerisation process is initiated, it does not continue indefinitely since it is impossible to link all the monomers in a plastic one long continuous chain. The polymerisation is terminated by collusion between the active ends of two chains or by addition of a terminator, such as free radicals from the catalyst.
Copolymerisation:
It is another type of addition polymerisation.
Copolymerisation is the addition polymerisation of two or more chemically different monomers. Many monomers will not polymerise with themselves but will copolymerise with other compounds.
Copolymerisation of vinyl chloride and vinyl acetate is shown in Fig. 9.2. This is comparable to a solid solution in metallic ceramic crystals.
Copolymerisation has been applied extensively, in the artificial rubbers.
Copolymers may take several forms, alternating, block and graft types.
Block Polymers:
They contain relatively long chains of a particular chemical composition separated either by a chain of different composition or a low molecular weight composition.
Graft Polymers:
The graft polymers consist of a main stem of one polymer with monomer stems off the sides. Grafting can be accomplished by the ionizing radiation that produces free radicals which initiate polymerisation of branches on the main chains.
2. Condensation Polymerisation:
“Condensation polymerisation” is defined as the process of linking together of unlike monomers accompanied by splitting off a small molecule. This process usually requires a catalyst.
In comparison to addition reaction in which a simple molecular summation occurs, condensation reactions result in splitting out of simple monopolymerisable molecules, e.g., water which are considered to be by-products of the process. Thus when a phenol and formaldehyde monomers are polymerised, water is released, and the resulting product is polymerised phenol formaldehyde, more commonly known as Bakelite (Fig. 9.5).
Comparison between Addition and Condensation Polymerisation:
Addition Polymerisation:
1. It requires molecules which are unsaturated.
2. It does not yield a by-product.
3. Reaction is very fast and may take 10-2 to 10-6 sec.
4. It is kinetic chain reaction.
5. Polymer formed is a thermoplastic type.
Condensation Polymerisation:
1. It requires two unlike molecules.
2. It yields a by-products.
3. Reaction normally takes hours and days to complete.
4. It involves intermolecular reaction.
5. Polymers formed is thermosetting type.
4. Depolymerisation:
During polymerisation process the reaction proceeds only in one direction under controlled conditions. A series of reactions may occur which cause depolymerisation.
A reversal of the polymerisation reaction is known as depolymerisation. Depolymerisation may also take place in plastics formed at high temperature due to thermal vibrations which may disrupt the inter-molecular bonds within the molecules.
This can be represented as below:
nR ← (-R -) n
Examples of Depolymerisation:
(i) Depolymerisation may occur with the area formaldehyde plastic if it is used for extended periods of time with steam.
(ii) Depolymerisation may also occur in any plastic being formed at high temperatures, since thermal vibrations may disrupt the inter-molecular bonds within the molecules.
Depolymerisation is not always harmful, it is useful as well.
Examples:
a. Depolymerisation is commercially used for cracking petroleum into more combustible, light molecules.
b. Charring of carbohydrates (toast) and cellulose (charcoal) are similar examples of depolymerisation.
5. Ionic Polymerisation:
Depending upon the charge of the ion formed, polymerisation may be cationic or anionic.
Cationic polymerisation (carbonium polymerisation) takes place with the formation of carbonium ion which is a polar compound with a tri-covalent carbon atom and carries a positive charge as shown below:
The catalysts used in cationic polymerisation are- AlCl3, SuCl2, SnCl4, TiCl4 and boron fluoride.
Anionic polymerisation (carbonion polymerisation) involves the formation of carbonion, a compound with a trivalent carbon atom having a negative charge. This type of polymerisation takes place in the presence of catalysts (such as sodium or potassium amide, triphenyl methyl sodium alkali, metals and alkyl alkalies) which readily yield electrons.
6. Molecular Structure of Polymers:
The physical characteristics of a polymer depends not only on its molecular weight and shape, but also on differences in the structure of the molecular chains. Various types of structures are observed in the polymers.
These are:
1. Linear and framework structure.
2. Branched chain structure.
3. Cross-linked structure.
4. Crystalline structure.
1. Linear and Framework Structure:
The polymer in which the units are joined together end to end in single chains are called “linear polymers”
Examples- Polyethylene, polystyrene, polyvinyl chloride, nylon etc.
A linear polymer like polyethylene is simple and uniform.
The units are held together by relatively weak secondary bonds. It can be represented as under:
R — R — R — R – – – –
However, certain linear polymers have large lumps along their chain instead of straight chain like structure, e.g., polyvinyl chloride (PVC). The movement between the molecules is restricted due to lumps. There are stronger forces due to polymerisation within the molecules. It is because of this reason that PVC is stronger than polyethylene.
Framework structure polymers are formed by tri-functional and tetra-functional monomers, e.g., phenol formaldehyde. It is hard and strong plastic having high melting point.
2. Branched Chain Structure:
In this case the macromolecules of polymer are branched, instead of linear.
The polymer is stronger and less plastic due to the interlock with each other.
Branching is generally achieved by removing a side atom from the main chain followed by introduction of another C-C bond.
The chain packing efficiency is reduced with the formation of side branches, which results in a lowering of the polymer density. Those polymers that form linear structures may also be branched.
3. Crosslinked Structure:
This type of structure can be represented as shown in Fig. 9.8.
Here the movement of individual molecular chain is restricted as the cross-linking anchors the adjacent chains together.
Crosslinking is done to increase strength and reduce the plasticity.
The process of crosslinking is achieved either during synthesis or by a nonreversible chemical reaction that is usually carried out at an elevated temperature. Often, this crosslinking is accomplished by additive atoms or molecules that are covalently bonded to the chains.
Many of the rubber elastic materials are cross-linked.
4. Crystalline Structure:
A crystalline structure can be represented as shown in Fig. 9.9.
Most of the polymers have a tendency to ‘exist in crystalline state but only few are perfectly crystalline due to weak van der Waal’s forces available for aligning the molecule.
Linear polymers can have crystalline as well as non-polymers structure. Linear polymers have a high degree of crystallinity compared to framework polymers. Framework or crosslinked polymers are considered amorphous.
Degree of crystallinity is determined by the geometry of the polymer chain.
It is important in the linear polymers, it leads to stiffer, stronger materials and generally makes an amorphous material translucent or opaque because light scatters at grain boundaries.
5. Network Polymers:
A three-dimensional network comprises of tri-functional mer units, having three active covalent bonds. Polymers composed of tri-functional units are termed as network polymers.
These materials have distinctive mechanical and thermal properties.
The epoxies and phenol-formaldehyde belong to this group of polymers.
7. Crosslinking of Polymers:
Crosslinks are used to join separate linear or branched polymer chains together along the chains.
Common examples of cross linked structures are those found in the elastomers.
Certain polymers are termed elastomers because they are capable of high (500 to 1000 percent) reversible extensions. In contrast, the elastic extensibility of typical hard solids is about one percent. Extensions and contractions are possible only if the polymer has an amorphous structure and it is above glass transition temperature (Tg).
Elastomers possess high local mobility of the chain segments but slipping past each other must be prevented or the original dimensions will not be regained. The restriction of gross mobility is obtained by crosslinking.
The crosslinks must be relatively few and widely separated so that large extensions are possible without breaking the chains. Permanent deformations occur with “slip” between adjacent molecules.
Mechanism:
Crosslinking usually involves the introduction of covalent type of link between polymer chains or their segments. An initially small amount of crosslinking causes formation of some branched molecules that are still soluble but, on further reaction, gelatin sets in.
This stage is characterised by the presence of insoluble gel and the soluble gel, which can be extracted from the gel. On further crosslinking, a giant three-dimensional network is formed that imparts rigidity, infusibility, insolubility and improved heat resistance to the polymer.
In case the crosslinks are short and densely located, hard and strong polymers are obtained that exhibit little elongation and high moduli.
In order that crosslinking may be extensive, there must be a number of unsaturated carbon atoms within the chain in its normal polymerised condition, since it is through these connecting points that linkage takes place.
8. Branching of Polymers:
Besides crosslinking it is also possible under certain conditions to develop another form of three-dimensional molecules from chain polymers by the method called branching; here main chain is bifurcated into two chains.
The controlled branching of linear molecules is a relatively new and important achievement in the production of plastics. Its importance lies in the fact that if branching is extensive it will effectively restrict movement between adjacent molecules by the simple mechanism of interangling.
Branching of linear molecules is not a spontaneous reaction, since it results in an increase in net energy.
Branching is achieved by removing a side atom from the main chain and introducing another C-C bond. This reaction, although not spontaneous, occurs somewhat more readily than the crosslinking reaction, since taking one side atom from the main chain is easier than simultaneously removing two atoms from adjacent locations on two chains.
9. Properties of Polymers:
The properties of polymers vary with the molecular structure, degree of polymerisation etc., some of the properties are described below:
1. Specific Gravity:
Plastics are generally light having specific gravity between 0.9 to 3.0 compared to 3.0 to 12.00 for metals. However, their strength/weight ratio compares favourably with many light alloys.
The tensile strength of typical thermo-plastics is given in Table 9.1.
2. Specific Heat:
Specific heat is the heat necessary to raise the temperature of 1 kg substance by 1 K.
The specific heat of plastics varies between 200 to 800 J/kg/K as against 400 J/kg/K for steel.
3. Thermal Conductivity:
Plastics have comparatively low thermal conductivity, hence they are good thermal insulating materials particularly expanded or cellular polymers.
4. Thermal Expansion:
Thermal expansion of plastics is very high (approx. five times thermal expansion of aluminium and other metals) which is the main disadvantages associated with the plastics.
5. Electrical Properties:
Plastics are good electrical insulators. However, their usefulness is limited by their low heat resistance and softness.
Also they can acquire electrostatic charges and may cause sparking which is potential fire hazard.
6. Corrosion Resistance:
Plastics are generally resistant to most inorganic chemicals, weathering and soil. However, no plastics become brittle when exposed to sunlight for long duration.
Plastics are resistant to attack by oils and greases, hence superior to rubber.
7. Combustibility:
Plastics are combustible because of presence of carbon. The maximum service temperature is about 100°C.
8. Rigidity:
Plastics have low rigidity compared to other materials, since they have low modulus of rigidity. However, this property can be improved by addition of fillers such as glass fibers.
10. Effect of Polymer Structure on Properties:
1. Effect of Polymer Shape:
The shape of the polymer molecules is one of the factors which affect the resistance to slip and deformation.
Example:
Polyethylene molecule is relatively simple and uniform and so there is only limited restriction of movement of one molecule by another (Fig. 9.11). On the other hand, a polyvinyl chloride molecule has large lumps periodically along its chain (Fig. 9.12). Due to this reason there is much restricted movement between molecules and stronger van der Waals’ force as a result of polymerisation within the molecule.
2. Effect of Degree of Polymerisation:
When DP value is to the tune of 10 to 20 mers per molecule the substance formed is lightest oil. As the DP increases the substance becomes greasy, then waxy and finally at a value of DP of about 1000 the substance becomes a solid and is then a true polymer.
3. Effect of Polymer Arrangement:
The symmetric terephthalate structure is the linear polymer. The asymmetric orthophthalate isomer produces a hard material. Another example which may be quoted is that of rubbers. Polyisoperene (natural rubber) and Gutta percha (an isomer of isoperene) have minor difference between forms but have major difference in properties, the former being an elastic rubber and latter non-elastic plastic.
4. Effect of Branching:
A branched polymer is a polymer which has a molecular structure of secondary chains that extend from the primary main chains. In other words, in branching, main chain is bifurcated into two chains.
Extensive branching restricts the movement between adjacent molecules by interlocking, and consequently polymer’s strength is improved.
5. Effect of Molecular Orientation:
Due to initial polymerisation nearly random orientation is produced. The physical properties of linear polymers can be improved through molecular orientation by extrusion through a die.
6. Effect of Crystallization:
In case of polymers, crystallinity is the state where a periodic and repeating atomic arrangement is achieved by molecular chain alignment. Polymers may range from wholly amorphous to highly crystalline structure.
Crystallisation is more easily achieved in small molecules than in large molecules. The degree of crystallinity of a polymer depends on the rate of cooling during solidification as well as on the chain configuration.
The physical properties of polymeric materials are influenced by the degree of crystallinity to some extent.
Crystalline polymers are usually stronger and more resistant to dissolution and softening by heat.
11. Functionality of Polymers:
The number of bonds the monomer has is called the “functionality.”
Example:
Ethylene monomer is bifunctional as it has the C—C bond which can react with two adjacent monomers.
The monomers must be at least bifunctional for polymerisation.
A network of thermosetting polymers can be formed when the monomers are trifunctional or greater.
12. Bond Strength of Polymers:
In polymers there are van der Waals’ bonds between molecules in addition to the covalent bonds within a molecule. They are very important, even though these are much lower in strength than the covalent bonds, because in most cases, the stress required for fracture is related to the force needed to separate molecules rather than to that required to break bonds within the molecules.
The hydrogen bond deserves special attention because it is very strong in a number of cases, specially in cellulose and polyamides.
13. Deformation of Polymers:
Polymers undergo both elastic and plastic deformation just as metals do. As a class polymers have lower elastic moduli than metals. Also, as a class, plastics undergo permanent deformation more readily than the average metal.
The deformation of plastics is affected by the structure of the polymer. Permanent deformation occurs as slip between adjacent molecules, because the attractive forces are smaller between them within the molecules.
“Thermoplastic” resins (plastics) deform easily under pressure and at high temperature, since the weak van der Waals’ forces are easily overcome. Consequently such a plastic when heated and injected into a mould under pressure takes on the shape of that mould easily. On cooling the plastic becomes rigid again.
In “thermosetting” resins (plastics) there is a continuous framework structure (developed by polymerisation) and slip cannot take place between the molecules. The whole of the structure is one large molecule, since primary covalent bonds are present throughout the structure. These plastics do not increase in plasticity with increased temperature and take on a permanent set.
They are widely used in processes requiring high temperatures. They are used for electrical outlets, telephone receivers and appliance handles whose processing makes use of the electrical or chemical insulating properties of the covalently bonded organic materials. Thermoplastic plastics, on the other hand, have better formability and are economical because scrap resin can be recycled frequently.
14. Advanced Polymeric Materials
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Some of the advanced polymeric materials are:
1. Ultra High Molecular Weight Polyethylene (UHMWPE)
2. Liquid Crystal Polymers (LCPs).
1. Ultra High Molecular Weight Polyethylene:
It is a linear polyethylene and its molecular weight is extremely high (nearly ten times greater than that of high density polyethylene).
(i) It is electrically insulating and possesses excellent dielectric properties.
(ii) It possesses very good chemical resistance.
(iii) It offers outstanding resistance to wear and abrasion.
(iv) Its low-temperature properties are excellent.
(v) Its impact resistance is extremely high.
(vi) It provides a self-lubricating and non-stick surface.
(vii) It offers a very low coefficient of friction.
(viii) It assimilates outstanding damping and energy absorption characteristics.
(ix) Due to its relatively low melting temperature, its mechanical properties diminish rapidly with increasing temperature.
Uses:
(i) Blood filters
(ii) Marking pen nibs
(iii) Pump impellers
(iv) Value gaskets
(v) Bullet proof vest
(vi) Golf ball cores, and
(vii) Ice skating rink surfaces.
2. Liquid Crystal Polymers:
These polymers belong to a group of chemically complex and structurally distinct materials and possess unique properties.
They are composed of extended, rod shaped, and rigid molecules.
In the melt; unlike other polymers (which are randomly oriented) LCP molecules become alighted in highly ordered configurations.
Some of the liquid crystal polymers are rigid solids at room temperatures and exhibit the following properties/behaviour:
(i) High impact strengths, which are retained upon cooling to relatively low temperatures.
(ii) Excellent thermal stability.
(iii) Stiff and strong (tensile strength: 120 to 250 MN/m2; tensile strength: 10 to 24 GN/m2).
(iv) Chemically inert to wide variety of acids, solvents etc.
(v) Inherent flame resistance.
(vi) Combustion products are relatively nontoxic.
Uses:
1. These polymers are mainly used in liquid crystal displays (LCDs), on digital watches, laptop computers and other digital displays.
2. Interconnect devices, relays and capacitor housings etc.
3. Photocopiers and fibrotic components.
15. Behaviour of Polymers
:
The point of difference between thermoplastic (linear) and thermosetting (framework) polymers is that thermoplastic polymers actually melt and lose all semblance of crystallinity at high temperatures. Furthermore, thermosetting plastics, after completion of polymerisation, can eventually lose strength if they are exposed to high temperatures, due to degradation that takes place.
The major effect of increased temperature is an increased rate of chemical reactions. The temperature at which polymers become highly susceptible to chemical reaction corresponds closely to the temperature at which mechanical strength drops off. This is below the melting temperature in linear polymers, and corresponds to the start of degradation in framework polymers.
Mechanical Behaviour:
A polymer in molten state is amorphous and possesses random chain orientation. This structure of the molecules at higher temperatures, can be prevented at lower temperatures through a quench. Tension on such a mass produces most of the initial deformation and improves the alignment of the molecules. As a result, the stress-strain relationships are not like those of metals, because the modulus of elasticity is increased when the stress is applied directly against the polymer chain after alignment has occurred.
Rate of Deformation:
When the metals are exposed to stresses below their yield strength for extended periods of time, they may undergo creep. Atoms are moved locally at points of stress concentration. Whether only elastic deformation occurs, or whether both elastic and plastic deformation develop, depend on the factors namely time and temperature. More time or higher temperatures provide more opportunity for the atoms to establish new positions under applied stresses.
Polymers are subject to the same time-dependent phenomenon as metals are, and although molecular movements are more complicated than atom movements, simply because of size, the bonding forces are generally weaker so that high creep rates are developed.
Instantaneous stressing reveals only elastic responses, while prolonged exposure to even weak stresses allows plastic deformation to occur.
Stress Relaxation:
Stresses are relaxed, with time, in those applications where they are initially developed from elastic elongation. The time required for adjustment of stresses is called the “relaxation time”.
As the relaxation of stresses is a continuous phenomenon, the relaxation time is defined mathematically as the time it takes for the stress to be reduced by 1/e of its original value.
Anelasticity:
It has been observed that in most engineering materials a time-dependent elastic strain component exists (that is, elastic deformation will continue after the stress application and upon load release, some finite time is required for complete recovery). This time- dependent elastic behaviour is known as anelasticity.
Anelasticity may arise because of the following:
(i) Diffusion of thermal energy of interstitial atoms or substitutional solute atoms;
(ii) Grain boundary effects, dislocations and thermal currents within the crystals.
This effect is also a result of retarded elasticity and is called the “elastic aftereffect”.
The anelastic component for metals is normally small and often neglected. However, for some polymeric materials its magnitude is significant, in this case it is termed viscoelastic behaviour.
Viscoelasticity is a third kind of deformation (other two being elastic deformation and plastic deformation), which is found in many polymers, such as plastics. Here, after an applied stress that is within the material’s elastic range is removed, the material will not completely recover to its original size.
An amorphous polymer may behave like a glass at low temperatures, a rubbery solid at intermediate temperatures, and a viscous liquid as the temperature in further raised. At intermediate temperatures, the condition is termed viscoelasticity.
The viscoelastic behaviour of polymeric materials is dependent on both time and temperature. This behaviour can be measured and quantified by several experimental techniques. Stress relaxation measurements represent one possibility.
A specimen, in these tests, is initially strained rapidly in tension to a predetermined and relatively low strain level. The stress necessary to maintain this strain is measured as a function of time, while temperature is held constant. Stress is found to decrease with time due to molecular relaxation processes that take place within the polymer.
Relaxation modulus, a time-dependent elastic modulus for viscoelastic polymers, is defined as:
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The magnitude of the relaxation modulus is a function of temperature; and to more fully characterise the viscoelastic behaviour of a polymer, isothermal stress relaxation measurements must be conducted over a range of temperatures.
It has been observed that most of the polymers exhibit both an elastic as well as a viscous response to the external forces. Thus the models for such viscoelastic behaviour comprise of viscous as well as elastic elements. The spring illustrates purely elastic behaviour and the dashpot purely viscous behaviour. If the spring is Hookean and the dashpot is Newtonian, the connections of these two elements in series and in parallel give two models namely Maxwell model and Kelvin or Voigt model respectively.
Stress relaxation occurs with time if the stresses are developed by elastic deformation. The time required for the adjustment of stresses is called the relaxation time. Relaxation time is taken as the time at which the stress gets reduced to 1/e (i.e., 1/2.718) of its original value. Under conditions of constant strain-
