In this article we will discuss about:- 1. Introduction to Polymers 2. Chemistry of Polymer Molecules 3. Molecular Weight 4. Molecular Structure 5. Copolymers 6. Polymer Crystallinity 7. Behaviour of Polymers under Different Situations 8. Fibres Polymers 9. Applications 10. Polymer Additions 11. Degradation.
Introduction to Polymers:
They are gigantic molecules and are often referred as macromolecules. Within each molecule the atoms are bound together by covalent bonds. For most molecules are in the form of long flexible chains. The backbone of each chain is a string of carbon atoms.
Remaining two valence electrons of each carbon atom may be involved in side bonding with atom or radical.
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These long molecules are composed of structural entities called “Mer” units, which are repeated in a chain. The term polymer is composed of many mers. e.g.
Chemistry of Polymer Molecules:
When the repeating units along a chain are of same type, the resulting polymer is homopolymer.
While those composed of two or more different Mer units are termed as copolymers. Almost all polymers are formed when an active ‘mer’ is formed by the reaction between an initiator or catalyst species.
Thus the active sets, or unpaired electron is transferred to each successive end monomer as it is linked to the chain.
Molecular Weight of Polymers:
During the polymerization process not all polymers chains grow to the same length. This results in a distribution of molecular weights. Thus an average weight is specified for a polymer.
The average molecular weight (Mn) is obtained by dividing the chains into a series of size range.
Where, Mi is the mean of size range and xL in fraction of total no. of chains of a particular size range.
Where, Mi is again mean molecular weight of a size range and wi is the weight fraction of molecules within same size interval.
Average chain size of a polymer is also expressed by degree of polymerization (DOP).
Molecular Shape of Polymers:
Single chain bonds are capable of rotating and bending in three dimensions. In the shown figure the third carbon atom can take any position on the cone of revolution and still maintain an included angle of 109°.
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As chain atoms are rotated, the chain gets twisted and bended. A single chain molecule composed of many chain atoms might have a large number of links, twists and bends as shown in figure where (r) represents the end to end distance.
In a polymer this bending, linking and coiling of many chains leads to extensive inter twining and entanglement of neighbouring chain molecules. These random entanglement are responsible for number of important characteristics of polymer such as large elastic extensions, their abilities to experience rotation in response to applied stresses or thermal vibrations.
Double and triple bonds are rotationally rigid as compared to single bonds. Also, introduction of a bulky or large side group of atoms restricts rotational movement. For e.g. Polystyrene molecules which have a phenyl side group are more resistant to rotation then polyethylene chains.
Molecular Structure of Polymers:
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Physical characteristics of polymer depend not only on its molecular weight and shape but also on difference in the structure of the molecular chains.
(a) Linear Polymers:
In which mer units are joined together end to end in single chains. These long chains are flexible.
For linear polymer these are extensive vanderwals and hydrogen bonding between the chains.
Common example are polyethylene, PVC, polystyrenes, PMMA, Nylon.
(b) Branched Polymers:
Here side branches are connected to the main branch.
These branches occur as a result of side reaction that occur during synthesis of the polymer. The Chain packing efficiency is reduced which results in lowering of polymer density.
(c) Cross Linked Polymers:
Here adjacent chains are joined to one another at various positions by covalent bonds.
The cross linking process in either achieved during synthesis or by a non-reversible chemical reaction that is carried out at elevated temperature often this is accomplished by additive atoms or molecules that are covalently bonded to the chains, e.g. is vulcanization of rubber.
(d) Network Polymers:
A Mer unit having there active covalent bonds forms a 3-D network called network polymers.
A polymer that is highly cross linked may be called network polymers. These materials have distinctive mechanical and thermal properties.
Examples are epoxy and phenol formaldehyde.
Copolymers:
Synthesised for obtaining better properties compared to homopolymers. Various sequencing arrangements along the polymer chains are possible.
They are:
i. Random
ii. Alternating
iii. Block
Identical mers are illustrated in blocks along the chain
iv. Grafted:
When homopolymer side branches of one mer chain is grafted to the main chain of the second homopolymer.
Polymer Crystallinity:
The unit cell of polyethylene molecules has an orthorhombic geometry. As a result to their large size and complexity, polymer molecules are only partially crystalline, having crystalline regions dispersed within the remaining amorphous material. Any chain disorder will result in an amorphous region since twisting and bending prevent strict ordering of chains.
Also crystallization is highly favoured i.e. polymers having simple Mer structures as compared to polymer with complex Mer units. Linear polymer are easily crystallined as compared to branched polymers, as branches interfere in crystallization with regard to stereo-isomerism. Atactic polymers are difficult to crystallize because of random configuration.
For copolymers, as a general rule, the more irregular and random the mer arrangements, the greater is non crystallinity.
Alternating and block polymer are somewhat similar to crystallization on the other hand graft and random copolymers are amorphous.
Behaviour of Polymers under Different Situations:
Thermosets have low shelf (or pot) life which necessitates that they should be consumed within this time. Therefore, they are unsuitable for high volume (i.e. huge quantity) production. Thermoplasts are suitable for high volume production that helps in lowering their costs.
Polymers in mixture and alloys forms are suitable for high performance applications because of improved toughness in them. Important amongst these are poly ether ketone (PEEK), poly phenylene sulfide (PPS), Poly ether sulfone (PES), Acrylonitrile butadiene styrene (ABS), Poly ethylene terephthalate (PET), Poly phenyloxide (PPO), Poly carbonate, Poly ether imide (PEI), Polytetrafluoroethylene (PTEE), Poly pathalamide.
Thermal Behaviour of Polymers:
In the glassy region (up to 80°C), a polymer is hard and brittle. In this region, strain occurs by stretching of bonds within and between molecular chains. Behaviour of polymers in this region resembles like the deformation of a spring.
i. Leathery Region:
The modulus drops rapidly with increasing temperature. Reversible sliding becomes possible in the chain as the small sections move causing neighboring sections to move.
ii. Rubbery Region:
Viscoelastic behaviour of polymer dominates deformation. As the temperature increases above the glass-transition temperature Tg, the molecular segments slide reversibly past one another and tend to strengthen out.
iii. Rubbery Flow and Liquid Regions:
Permanent sliding molecules dominate the deformation process. As temperature increases, the viscosity decreases and at high temperatures the polymer is essentially a liquid.
iv. Glass Transition Temperature (Tg):
The temperature at which the polymer experiences the transition from rubbery to rigid state is called as glass transition temperature. Normally Tg lies between 0.5 to 0.8 times of melting temperature.
Fibres Polymers:
Fiber polymers are capable of being drawn into long filaments having at least a 100: 1 length to diameter ratio.
They have applications in textile industries, being woven or knit into cloth or fabric, during application fibers may be subjected to stretching, twisting, shearing and abrasion. So, they must have high tensile strength and high modules of elasticity and have high abrasion resistance.
Molecular weight and degree of crystallinity of the fiber material should be high as tensile strength increases with effect of these two parameters.
Furthermore, they must exhibit chemical stability in variety of environments including acids, bases, bleaches etc. They should be non-flammable.
Polymeric Fibers:
Polymeric fibres are long chains of molecules aligned in longitudinal direction. This furnish directional properties to the fibre.
The elastic modulus and strength are much higher in the longitudinal direction than in the other transverse directions. These fibres may be further sub-grouped as follows.
Natural fibres such as wool, cellulose, cotton, silk etc.
Synthetic fibres such as nylon, terylene (Dacron), rayon, orlon, Kevlar etc.
Cellulose fibres are flexible and strong in tension. Cotton clothes shrink due to the presence of wrinkled molecules. Sulfurized cotton clothes are non-shrinking as long chains are made to align in such clothes.
i. Polyester Fibres:
Dimensional synthetic polymeric fibres belong to the family of both, the thermosets and the thermoplastics. Polyester fibres (a thermoset) such as terylene, is defined by
linkage. Oxygen provides flexibility to polyester. Becomes soft more easily with increasing temperature these are durable, uniform dimension.
ii. Nylon Fibres:
Nylon 66 (a thermoplast) which is a polyamide fibre does not soften easily with increasing temperatures. The polyamide fibres are characterized by
Nylon 6 is polyimide, SP, gravity 1.14, tensile strength 820 MPa ans used for tire cords.
Different Types of Processed Natural Rubber:
i. Chlorinated rubber is used in the production of adhesives and protective coatings.
ii. Packing of delicate equipment is done by rubber hydrochloride.
iii. Cyclised rubber is used in manufacturing of papers with the use of paraffin wax (i.e. solid alkanes).
iv. Synthetic rubbers are superior to natural rubbers in many respects. Synthetic rubber are manufactured from raw materials like coal tar, petroleum, coke, limestone, natural gas, alcohol, ammonia, salt and sulphur etc. These are superior to natural rubber.
v. Polychloroprenes (Neoprene) are highly resistance to temperature, oil, grease and ageing, used as oil seals, gaskets, adhesives, tank lining, and low voltage insulation.
vi. Butyl are highly resistant to oxidizing agents, and impermeable to gases. Used as tyres, tubes insulations for cables and wires.
vii. Butadiene are highly abrasion and weather used resistant. Used as belts, soles of shoes, flooring.
viii. Nitrile (Buna-N) are excellent resistant to solvent, grease and oily situations. Used as conveyor belts, tank lining.
ix. Polyurethane are high strength and highly abrasion resistant, poor to acidic and alkaline attacks. These are used as expanded foams, tyres, conveyor belts.
x. Silicone are mechanically weak, costlier, temperature stability upto about 600 K. Used as coatings, packaging tube, gaskets, cable insulation.
General Applications of Polymers:
(i) Coatings:
a. To protect the item from environmental damage such as corrosion.
b. To improve items appearance
c. To provide electrical insulation. The coating fall into many categories: Paint, enamel, shellac etc.
(ii) Adhesive:
a. Used to join surfaces of two solid materials to produce a joint with a high shear strength.
b. The bonding forces between adhesive and solid material are electrostatic in nature.
c. A strong joint is produced by adhesive layer.
d. If a good joint is formed, the adhesive material may fracture before the adhesive.
e. The primary drawback is the service temperature. Its adhesive strength decreases rapidly as service temperature increases, e.g. cyanoacrylate araldite.
(iii) Films:
a. Films have thickness between 0.025 – 0.125 mm. Used for packing foods, beverages etc.
b. Important characteristics of materials used for film production are low density, high degree of flexibility, high tensile and tearing strengths, resistance to attack by moisture and other chemicals. Examples are polythene, cello-phase, polypropylene and cellulose Acetate.
(iv) Foams:
a. These are plastic materials that contain high percentage of volume of small pores.
b. Both thermoplast and thermoset are used as foams.
c. These includes polythene, rubber, polystyrene and PVC.
d. Common use of foams are as automobile cushions and furniture as well as in packaging and thermal insulation.
e. Foaming process is carried out by incorporating into the batch of material a blowing agent that upon heating decomposes with the liberation of a gas.
f. These gas bubbles remain as pores upon cooling and give rise to a spongy structure.
g. Some effect can also be produced by bubbling an inert gas through a material while it is in molten state.
h. e.g. polymethane, formed polymethane as insulator used.
Addition Polymerization:
Addition polymerization also called chain reaction polymerization, there bifunctional monomer units are attached one at a time in chain like fashion to form a linear macromolecule.
Thus the resultant product is an exact multiple of the original reactant monomer.
(i) Initiation:
Here an active center is formed by a reaction between initiator and monomer unit.
(ii) Propagation:
It involves linear growth of the molecules as monomer unit attach are at a time to produce a chain structure-
This a very rapid process as 1000 mer units add up in 10-2 to 10-3s.
(iii) Termination:
It can be done in two ways to form a non-reactive molecules.
(iv) Condensation Polymerization:
It is also called step reaction polymerization. It is a process whereby one or more than one monomer units combine stepwise and as a result of reaction by products are condensed out.
This stepwise process is repeated to produce a linear molecule. Reaction times of condensation are generally longer than for addition polymerization.
Condensation polymerization often produces monomer capable of forming cross linked or network polymers. All thermosets nylon and polycarbonates are produced by condensation polymerization.
Polymer Additions:
(i) Filler Materials:
These are added to polymers to improve tensile and compressive strength, abrasion resistance, toughness, dimensional stability etc.
Materials used as fillers are wood flour (saw dust), silica flour and sand, glass, clay, talc, limestone.
Particle sizes range all the way to macroscopic dimensions.
Because these are inexpensive materials replace some volume of the more expensive polymer, the cost of final product is reduced.
(ii) Plasticizers:
Here the flexibility, ductility and toughness of polymers are improved by addition of plasticizers.
But hardness and stiffness are also reduced. Plasticizers are generally liquids having low vapour pressure and low molecular weight.
Small plasticizer molecules occupy positions between the large polymer chains, effectively increasing the interchain distance and reduction in secondary intermolecular bonding forces.
There applications include thin sheets or films, raincoat, tubing and curtains.
(iii) Stabilizers:
Some polymers even under normal environmental conditions are rapidly deteriorated in terms of mechanical integrity of polymers.
UV causes severance of covalent bonds along the molecular chain.
Common stabilizers are carbon black (soot) as it is a good absorber of UV rays.
Oxidation also has detrimental effect on polymers and antioxidant coatings are used to prevent oxidation of polymers.
(iv) Colorants:
They impart colour to a polymer.
They are dyes or pigments. The molecules of dye dissolve and become part of the molecular structure of polymer.
On the other hand pigments do not dissolve and remain as a separate phase inside the polymers.
(v) Flame Retardants:
Most polymers are flammable in their pure form.
Exceptions are PVC and PTFE. These compounds retard flammability of polymers.
These retardants either interfere in the combustion process through the gas phase or initiate a chemical reaction that cools the combustion region.
Thermoplastic Elastomers:
Polymeric materials that exhibit elastomeric (rubbery) behavior at room temperature. Most of the elastomers are thermostat as they are cross-linked by vulcanization.
Most common example of TPE is styrenebutadiens. Where hard and rigid thermoplastic mer occupies the molecular chain ends, while soft butadiene mers occupy center positions.
At ambient temperature, the soft, amorphous central segments impart the rubbery behaviours to the material. Whereas at temperature below Tm, hard component forms the rigid domain regions. These hard ends act as anchor points so as to restrict soft chain segment motions.
The useful temperature range lies between Tg of soft components and Tm of the hard component.
The main advantage of TPEs over thermoset elastomers is that upon heating above Tm of the hard component, they emit and therefore, may be processed by conventional thermoplastic forming technique.
Typical application are automotive bumpers, gaskets, shoe soles and heals, sporting goods, protective coating etc.
Vulcanization:
Cross linking process in elastomers is called vulcanization, which is achieved by non-reversible chemical reaction, carried out at elevated temperature. In this process sulphur compounds are added to the elastomer.
Sulphur atoms bond with adjacent polymer backbone chains and cross link them.
Main reaction sites are C—C double bond. Modulus of elasticity, tensile strength and resistance to degradation by oxidation are all enhanced by vulcanization. Vulcanized elastomeric materials are thermosetting in nature.
Degradation of Polymers:
Plastic are organic and can are prone to physiochemical attacks.
Degradation of plastic is not termed as corrosion as it is of physiochemical in nature in opposite to electrochemical corrosion of metals
Polymeric materials degrade by non-corrosive processes.
Degradation of polymers, thus involves a wide variety of reactions and results like absorption and swelling, dissolution, bond rupture due to heat, chemical effects, or radiation; weathering, etc.
Degradation of polymers may be due to exposure to light (specially UV), humidity oxygen bacteria, heat and external loads.
i. Swelling and Dissolution:
Polymers, when exposed to liquids, they get swelling as a result of solute diffusion and absorption of solute. Because of swelling i.e. separation of chains, secondary bonds become weaker. It results in the material becomes softer and more ductile.
Swelling and dissolution effects are influenced by temperature because of their physiochemical nature.
ii. Bond Rupture:
Bond rupture in polymers due to degradation is known as scission Polymers bond may get rupture due to radiation, heat energy, or chemical reactions.
At elevated temperatures, bonds in polymers may get weakened, leading to deterioration of polymers.
Some chemical elements like oxygen, ozone may alter the chain scission rate as a result of chemical reactions. This is especially accelerate in vulcanized rubbers.
A degradation of polymers due to outdoor circumstances is termed as weathering, which in fact is a combination of several different process.
Examples:
i. Deterioration of acrylic painting and pieces of art
ii. Decomposition of photographic films
iii. Decolorization of plastics pieces preserved in museums
Solutions to Polymer Degradation:
Tailor made scavengers such as activated charcoal or Ageless help to create a low oxygen environment. Ageless is a reactive powdered iron and is normally used to prolong the shelf-life of dry foods by absorbing oxygen. Epoxidised soya bean oil (ESBO), has also been tested with encouraging results as an acid absorbing coating on degrading on cellulose nitrate.