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(+91) 9686 448899
Kruger Industries E 42 & 43 2nd Main 2nd Phase, Peenya Ind Area, Bengaluru - 560058, India
An introduction to the Basics of polymers
Importance of polymers: wherever we look around us, we see polymer plastics. The tires of the cars that we travel in, the rubber tips on our earphones, the material of our jackets, everything around us is made of polymers. So much so that you should realise that you are looking at polymers even when you are taking a look at yourself in the mirror since your hair and nails are made of keratin. The DNA present in the cells of your body is also a polymer.
Polymers may be defined as huge molecules having high to the very high molecular weight which is made by chemically bonding two or more building blocks. These building blocks are termed monomers which have covalent bonds between them and may or may not be identical in structure.
These monomers tend to be linked together in a straight line in the form of links in a chain. That is not to say that branching or cross-linking does not occur, but we will go over that later. Monomers can consist of any number of atoms and their structure depends on the number of atoms in the chain.
The word ‘polymer’ comes from a combination of two Greek words: poly which means ‘many’ and meres which means ‘parts’. Polymers are present in abundance all around us, and they can either be human-made (artificial) or found in nature (natural). Plastics, elastomers, fibres and other materials having similar characteristics are all artificially produced polymers. To give a better understanding, the toothbrush that you use for brushing your teeth is made of a polymer (plastic) and even its bristles are made of a polymer (fibres).
It is also known that human, as well as animal bodies, are essentially made of protein which, in itself, is a polymer having amino acid units linked together with the help of peptide bonds. Some natural polymers include silk, cellulose and the aforementioned proteins. That is not to say that they cannot be prepared artificially. Chemists can also choose to create polymers using compounds prepared within the lab.
Before we take a gander at the detailed chemistry of polymers, let us first go over a few landmarks in the history of polymers. Polymers have been with us forever but only after the onset of the 20th century is when we began understanding the true nature of polymers. And if we look around us now, it is impossible to imagine a world where we are not surrounded by polymeric materials both naturally obtained and artificially prepared.
Listed below are some of the important works of prolific scientists in the field of synthetic polymers science which have also been worked on and developed upon by other scientists:
|1839||Polystyrene was found by Eduard Simon|
|1843||Vulcanisation of rubber is done by mixing it with sulphur. This was developed by Hancock in England and Goodyear in the USA.|
|1862||In an International Exhibition in London, Alexander Parkes showcased Parkesine, which is developed using cellulose nitrate.|
|1868||Cellulose nitrate was mixed with camphor to produce celluloid by the Hyatt brothers in America. Celluloid was found to be unstable which led to the formation of cellulose acetate. Along with that, several first plastics mass production techniques were also formulated. Some of these are blow moulding, extrusion and compression moulding.|
|1869||The rights to produce Parkesine in England were taken by Daniel Spill. He then also produced Xylonite and Ivoride after establishing the Xylonite Company.|
|1872||Polyvinyl chloride (PVC) was invented by Eugen Baumann, among others.|
|1907||The first synthetic thermoplastic, bakelite or phenol-formaldehyde was manufactured by Leo Baekeland. It is now famously known as the phenolic resin and it is cast to have pigments similar to those in onyx, jade, marble and amber.|
|1910||Cellulose acetate lacquers and plastic film were modified and made perfect by the Dreyfus brothers.|
|1912||Polyvinyl acetate was found by Fritz Klatte. He also developed a new manufacturing process for PVC and patented it.|
|1930||Synthetic rubber, Nylon and Teflon were produced by DuPont in the USA.|
|1933||Polyethylene was discovered by ICI.|
|1937||Polyurethane was patented by Otto Bayer.|
|1948||Acrylonitrile butadiene styrene (ABS)|
|1951||High-density polyethylene and crystalline polypropylene were discovered by Paul Hogan and Robert Banks of Phillips.|
|1953||The polymerisation of polyethylene was done at low pressure with the help of Ziegler catalysts.|
|1954||Giulio Natta discovered polypropylene.|
|1958||Mass manufacture of polycarbonate was started.|
|1987||Polyacetylene, a polymer having two times the electrical conductivity of copper, was produced by BASF in Germany.|
Materials and energy are the foundations for products, machinery and construction all the same. The type of material used depends heavily on the cost, availability, and the requirement for purpose. We observed how metal replaced wood but with the onset of plastics, we found it to be a cheaper alternative to metal.
After World War II, when plastic was found to be produced in a cheaper manner, the price of casting metals observed a steep rise. And for this purpose, most products that were traditionally made using metal have been replaced by plastics. Determining the kind of material to be used requires expertise and experience for good judgement. Since the 1960s, the plastics industry has grown over 20 times.
Most plastics require additives and fillers to meet their ideal performance characteristics. Different materials, forms and additives create possibilities for a large range of options. Over time, plastic manufacturers have learnt about these variations and how they take into effect, but they are still dependent on polymer companies to acquire the exact formulations.
The chain links are often identical to their neighbours in the case of artificial polymers. However, in the case of proteins, DNA, wool, cotton, silk, and other such natural polymers the links in a chain are usually different from their neighbours. Sometimes, polymers tend to form branching links instead of single chains. Irrespective of the shape of the polymers, the molecules are unusually big.
Due to the huge size of these molecules, they are classified by scientists as macromolecules. The length of the polymer chain also determines its weight alongside increasing the viscosity (or resistance to the flow of a liquid). Greater surface area results in a greater area to which molecules can stick and hence, a higher viscosity.
Let us take an example. The sugar glucose monomer is used to make both starch and cellulose. Starch dissolves in water and can be easily digested. However, cellulose has opposite properties, it does not dissolve in water and cannot be digested by humans, either. This difference in properties is solely due to how the glucose monomers have been linked together.
A polymer structure can consist of two different constituents. The initial point is usually a simple chain made of chemically bonded links which may also be referred to as the backbone. Some of these polymers may have secondary components hanging on any (or all) links of the main chain. These can either be as simple as a simple atom or complex groups of atoms called pendant groups.
And since these secondary components are more in contact with the environment as compared to atoms in the main chain, they more or less decide how a polymer may react with itself or other things in the surroundings.
In some cases, these hanging pendant groups, instead of hanging from the main carbon chain tend to connect two chains and these links are named by scientists as crosslinks. These crosslinks provide increased strength within the materials prepared using such polymers. They also contribute to the hardness of the material and make it difficult to melt. The length of these crosslinks determines the flexibility of the material.
A chemical bond is responsible for holding atoms together in a molecule or some crystals. Theoretically, any atom that is capable of forming two chemical bonds can make a chain. This can be better understood if you think of our two hands joined with other people’s hands to form a circle.
That means atoms which usually form only two chemical bonds are not capable of forming long chains similar to those in polymers. This is because as soon as these atoms form two bonds, they are stable. Or as in the case of our example, both your hands are busy holding someone else’s hands so you cannot hold any more hands.
The elements that are usually observed in the polymer chains are those which become stable after forming four chemical bonds, for example, carbon and silicon. This is because there is always at least one pendant group on the atoms that are part of the polymer backbone (or main chain). Additionally, some polymers are flexible, and some are rigid. This can also be observed by taking into account the different kinds of plastics. Some of them, like cold drink bottle, is very flexible and on the other hand, some plastics, like the pipes made using PVC (Poly Vinyl Chloride) are extremely stiff and hard to break.
In some cases, scientists choose to add certain external agents named plasticizers to their polymers to increase their flexibility. Plasticizers use up the space between the polymer chains and allow each chain to slide over one another more easily. Over time, the plasticizers in these plastics are lost to the environment and some of them may also react with the environment. These changes explain the reason behind certain materials being flexible during the start but later becoming stiff or brittle.
A traditional plastic resin will be a combination of two or more polymers along with additives and fillers. These are added to help increase a certain property of the material which may include processability, thermal stability and mechanical characteristics.
Polymers do not have any defined length, nor do they form any crystals. They also do not have a definite melting point at which they perform phase change immediately. When heated, materials or plastics made using polymers tend to deform or soften before melting completely.
There was not a lot of data and knowledge about the molecular theory of polymers during the early advancements in polymer technology. Emil Fischer gave the idea that polymers present in nature have a structure that could support research and the possibility of manufacturing plastics, in 1901. He also found out that linked chains of molecules were what natural polymers were made of.
The process of creating polymers is termed polymerisation. The polymers obtained after the process are further processed to produce plastic products of various kinds. Essentially, monomers (or the building blocks) combine forming chemical bonds eventually forming a macromolecule or large molecule. A polymer is formed by the combination of hundreds of macromolecules.
To obtain polymers for specific purposes and applications, different polymerization techniques (or processes) are employed. For instance, for the formation of polyethene polymer, hundreds of ethylene monomers are polymerised. The polyethene polymer is used to make everyday objects like milk jugs, trash barrels, food packers, storage containers and other such application-specific products.
Certain things like temperature, pressure and the presence of a catalyst are necessary for the polymerisation of monomers into a polymer. Catalysts are agents that help initiate or fasten a reaction while making no net change to the reaction itself. Another necessity for the polymerisation process is that the building blocks or monomers need to have the capability to bond or combine to make a polymer.
Polymerisation is also known as polymer synthesis. The smaller molecules that are repeatedly bonded together chemically to form a polymer are termed as repeating units. The repeating units can either be all the same or have different components specific to the required characteristics of the plastic product.
There are different mechanisms which can be used to control the bonding of a repeating unit with its neighbouring molecules. There are two main types of polymerization processes, namely polymerization of addition (chain-growth) and polymerization of condensation (step-growth). The chain growth polymerization acts in a chain-like process as it proceeds in only one direction. In the condensation reaction, the growth of the chain is not immediate or spontaneous and occurs gradually and slowly. To produce polymers having high average molecular weights, long reaction times are a must.
Irrespective of the mechanism used in polymerisation, the molecular structure of a polymer is dependent on the functionality of its monomers. The number of bonds that a monomer is capable of making during the polymerisation reaction is known as the functionality of that monomer. The different types of functionalities that have been observed in monomers are:
Bifunctional monomers – These are capable of forming linear polymer chains by combining.
Trifunctional monomers – As the name suggests, these kinds of monomers can link up with three neighbouring monomers to form a three-dimensional network. Other monomers having higher functionality also tend to form networks.
Mixtures of bifunctional monomers – These kinds of monomers tend to produce copolymers having linear (or straight) chains.
A bifunctional monomer is mixed with a small amount of trifunctional monomer – This kind of combination will form loose network polymers.
It can be said that if you need polymers having closely packed networks and high crosslink density, increasing trifunctional monomer proportions is highly recommended.
There are certain prerequisites for the polymerisation reaction to take place which include a large number of monomers and an appropriate catalyst or initiator to initiate the reaction which forms the polymer molecules. These molecules consist of hundreds to thousands of monomers combined. The medium in which the reaction takes place is a good way to classify polymerisation processes, namely, bulk, suspension., slurry, gas, solution and emulsion.
Bulk polymerisation is known as the polymerisation of a pure liquid or gaseous monomer. Its general applications are in the preparation of free-radical polymers along with some condensation polymers. Some key highlights of this polymerisation process are:
In bulk polymerisation, during the reaction, there are no additives or fillers required and hence, the final product obtained is pure. The only things that are present during the reaction include the monomer, the polymer and the initiator.
The polymerisation process takes place rapidly and there is high heat removal during the reaction thereby making it exothermic. This can be risky because there is a steep rise in temperature along with runaway reactions. Additionally, excessively high temperatures can cause branching and crosslinking in polymers and can also result in the formation of gels.
The process is also known to produce polymers which are highly transparent for example, PS and PMMA.
The major constituents of this type of polymerization process is an inert solvent with a boiling point that fits the temperature of the polymerization and a monomer is applied to this inert solvent. Some key highlights of this polymerisation process are:
Since the boiling point is reached, some of the solvents are lost due to evaporation during the polymerisation process. This removed solvent helps to eradicate the heat of polymerisation.
A constant polymerisation temperature is maintained because the boiling point of the solvent is constant.
Once the polymerisation process is completed, the removal of the residual solvent from the polymer can be a tedious and difficult task.
However, since there is more heat capacity of solvent and lower viscosity in this process, the temperature control in solution polymerisation is extremely easier as compared to that in bulk polymerisation.
In the case of droplets, bulk polymerisation takes place in an aqueous solution which often involves a scattered monomer.
The precipitates of these polymers are often observed in the form of fine spherical particles. Some key highlights of this polymerisation process are:
The reaction is initiated with the help of radical initiators in the monomer droplets.
To prevent particle coagulation and to obtain uniform polymer particles, protective colloids are added.
The heat of polymerisation is absorbed by water.
PVC and PS manufacturing are the most widely used examples.
The residual additives which are obtained during the process are required to be eliminated separately.
The main use of this process is in the manufacture of polyolefins. A liquid monomer or a liquid diluent with a dissolved monomer is taken and the catalyst is dissolved in it. Similar to emulsion polymerisation, precipitation takes place as the polymer is not soluble in the reaction medium. This precipitate keeps forming on the catalyst eventually resulting in slurry.
The process involved in Suspension polymerisation is similar to emulsion polymerisation. A monomer is also dispersed in water but the only difference is the size of the droplets. Emulsifiers, like soap, are used to produce these significantly smaller droplets. The soapy solution keeps the chemicals from binding to each other, such as the monomer droplets and the polymer molecules formed and thus avoids forming a blob. Since there is minimal change in viscosity with conversion, temperature control is also increasingly easier. Moreover, organic solvents have thermal conductivity and specific heat much lower than that in the case of water.
However, some of these emulsifiers tend to stick to each other eventually forming micelles while others tend to form droplets by isolating the nearby monomer particles by surrounding them. These micelles contain hydrophilic groups filled with water on the outside and their hydrophobic groups are surrounded by other hydrophobic monomers on the inside. When the procedure is started, due to the increase in temperature the formation of free radicals is increased. They draw monomers from these free radicals and gradually move out of the droplet due to high energy and temperature and continue to add themselves to the free radical site in the micelle, resulting in polymer molecules trapped in the centre.
At the end of the polymerisation process, some residual monomers, initiators, and free radicals that are formed may not have responded due to steric hindrances in the process. Residues of surfactants and coagulates are difficult to extract and account for an increase in impurity levels as well. Polyvinyl acetate, PMMA, and PVC are some of the polymers developed using this technique.
The reacting monomer should be in the gaseous state. A heterogeneous catalyst for synchronization, such as the catalyst for Ziegler Natta, is often used. In the case of gas-phase and slurry polymerisation, the Ziegler Natta catalyst requires support on another added substance such as SiO2, whereas in the case of solution polymerisation they can be added directly. To reach the active site, a fresh monomer needs to diffuse through the polymer particle. The catalyst is supported with the help of mechanical stirring or fluidization in the case of reactors.
Processes of gas-phase and slurry polymerization are widely used in the production of polyolefins such as HDPE.
The introduction of coordination catalysts has made the development of polyolefins into the commercially important thermoplastics that they are today. Philips catalysts, Ziegler Natta and other single-site catalysts such as constraint geometry and metallocene are the numerous forms of coordination catalysts.
The discovery of Ziegler and Natta was the first and, ultimately, the most important step in developing crystalline polyolefins. The German chemist Karl Ziegler (1898-1973) discovered in 1952 that a highly active heterogeneous catalyst for the polymerization of ethylene at atmospheric temperature was released when TiCl2 and triethyl aluminium were allowed to react with each other to form a mixture.
The Ziegler-Natta catalysts are used all around the globe to produce the below-listed classes of polymers:
Polyethylene; HDPE, linear low-density polyethene LLDPE, and ultra-high molecular weight polyethene (UHMWPE)
Polypropylene; homopolymer, random copolymer and high-impact copolymers
Thermoplastic polyolefins (TPO)
Ethylene propylene diene monomer polymers (EPDM)
The polymerisation is said to occur due to the integration of a double bond repeatedly from its monomer to a previously formed Ti-C bond. If the catalyst is impregnated on a solid support, the efficiency of the heterogeneous catalyst can be improved majorly.
Homogenous and single-site catalysts (SSC) are called metallocene catalysts. Every molecule of the catalyst provides more or less the same activity and accessibility to monomers. Metallocene, individually, are not active during or for the polymerisation process. Usually, the co-catalyst is used to activate the metallocene.
The metallocene catalysts which are Ne activated can be utilized for the polymerisation of polyolefins. Metallocene polymerisation has had a significant impact on the polymer business. One of the most cherished things about metallocene catalysts is that the Ziegler-Natta catalysts can generate polyethylene with a much higher molecular weight than possible.
The Ziegler-Natta catalysts possess numerous advantages when compared to the metallocene catalysts. Some of them are enlisted as follows:
Catalytic activities are extremely high
Ability to polymerise different kinds of polyolefins which was not possible with Ziegler-Natta catalysts
The inclusion of unsaturated chain ends in the main chain termination mechanisms; extra flexibility is added.
These catalysts not just enable control over short and long-chain branches but also allow control over having equal distance between the branches and chains alongside having uniform chain length distribution. All of these things directly affect the rheological properties thereby affecting processing as well.
Greater uniformity is produced in microstructural morphology.
The Structure of Polymers
The polymers that are commonly used in engineering consist of natural materials such as natural rubber and synthetic materials like plastics and elastomers. The structure of polymers is essential for the production of polymers because it can be modified and created to create materials:
Having a large variety of mechanical properties
Having a wide spectrum of colours
Having different transparent properties.
The main chain in every organic material is made of carbon atoms. A polymer is also an organic material with carbon in its main chain. There are four carbon atoms present in the valence shell of the carbon atom and these electrons are capable of forming a covalent bond with another carbon or a foreign atom. Some of the most commonly found elements in polymers are H, F, Cl, Br, I (all having one valence electron), O, S (having two valence electrons), N (having three valence electrons), C and Si (having four valence electrons).
The ability to form large chains is crucial for the production of polymers. The polymer chain is usually represented in two dimensions, but it is important to know that they have a three-dimensional structure. That is to say that the angle between its bonds is 109 degrees and hence, the backbone chain (or carbon chain) extends through space. On the application of stress, these polymer chains get stretched and the elongation in these chains is a thousand times more than it is in the case of its crystalline structures.
The length of the carbon chain plays an important role in defining the phase of the polymer. The material will tend to become a waxy solid after going through the liquid state as the number of carbon atoms in the carbon chain is increased beyond several hundred. This phase change is observed due to the total binding force between the molecules increasing as the length of the polymer chain and hence, molecules increase. It is also important to know that molecules are not usually straight in structure but are tangled masses.
The binding forces are acquired because of the Van der Waals forces between the molecules and the polymer chains’ mechanical entanglement. When heat is applied to thermoplastics, there is a rise in the temperature as well as the energy of molecules. This results in the movement of molecules which makes it easier to break the bonds between the molecules. The ability of thermoplastics to be emitted can be explained with the help of this reasoning.
There exists another group of polymers in which there is the formation of one large network rather than the production of several molecules during polymerisation. Since polymerisation is started by heating the raw materials and then binding them together, these groups of polymers are referred to as thermosetting polymers or plastics.
And the mass is set once they are combined since it is essentially one huge molecule and so, there is no molecular movement between the molecules. Thermosetting polymers are more rigid and have more strength compared to thermoplastic polymers. Since there is no possibility of molecular movement in thermosetting polymers, the conversion to plastic on the application of heat is also not possible.
(b) Basic Polymer Structure
Now, let us discuss the four basic polymer structures which are, linear, branched, crosslinked, and networked. However, a polymer can have a mixture of various basic structures.
These polymers are straight long-chained polymers. Van der Waals forces or hydrogen bonding usually help keep the long chains together. These are essentially thermoplastic since the bonds between the molecules are easy to break on the application of heat.
This heat, for a sustained period of time, allows the polymer chains to flow past one another eventually allowing the material to be remoulded. These polymer chains can also be hardened by cooling.
The structure of these kinds of polymers more or less ensembles that of linear polymers along with the addition of other long chains hanging from the carbon main chain (or backbone). In comparison, the branched-chain polymers are less dense and since the short chains do not jump from one carbon backbone to another, the application of heat can break the bonds between them easily, therefore making the polymer a thermoplastic.
However, there also exist some extremely complex branched polymers that offer resistance to this melting and usually become hard before softening. These extremely complex branched-chain polymers fall under the thermosetting polymers group.
The structure of crosslinked polymers resembles that of a ladder. There are no hanging branches in this type of polymer and carbon chains are linked from one backbone to another. So, instead of the weaker Van der Waals forces, crosslinked polymers are bonded by covalent bonding. This makes most of the crosslinked polymers fall under the thermosetting group of polymers.
However, some of the crosslinked polymers tend to break their crosslinks at relatively low temperatures.
Networked polymers are complex polymers that form a heavily linked network of three-dimensional linkages. There is no way to maintain the polymer structure while heating and is hence, nearly impossible to soften. This makes them fall under the thermosetting group of polymers.
Whenever a chemist tries to synthesize a polymer with help of two different monomers, there can be several possible structures of the resulting polymer. Four basic structures include random, alternating, block, and graft. If monomers are randomly ordered within the polymer, then the copolymer is referred to as a random copolymer. If an ABABABAB.. pattern is followed and each monomer is alternated with the other, then the copolymer is termed an alternating copolymer. In the case of block copolymers, the monomers are arranged in a more complex and repetitive form and the pattern can be AAABBBAABBBAAA… Finally, graft monomers are prepared by attaching chains of the second type of monomer to the carbon chain of the first monomer type.
Properties of Polymers
There are essentially three kinds of properties that we study when we talk about polymers, namely, physical, thermal and mechanical. Let us go over each type in detail.
Molecular weight, molar volume, density, degree of polymerization, and the material’s crystallinity are some of the physical properties of polymers. Let us define them one by one.
Degree of Polymerization and Molecular Weight
The degree of polymerization is defined as the number of repeated units or the number of monomers in a polymer molecule polymer chain. The molecular weight of the polymer molecule results in the degree of polymerization and the monomer’s molecular weight.
The polymer molecules do not contain the same repeating units throughout. There are varying degrees of polymerization and different molecular weights of these various repeated units. the average values of the molecular weights in the case of polymers are taken into account.
Now, the average values of the molecular weights can be calculated in the following three ways:
Molecular Weight Averages
Number-Average Molecular Weight
Weight-Average Molecular Weight
Polydispersity Index or Heterogeneity Index
Due to their extremely large size, polymer chains are found in the polymer in the following two forms: A shape in which the chains fold and create a consistently positioned lamellar structure known as the crystalline Lamellar form and the amorphous in form in which the chains are irregularly arranged.
The general crystallinity range can be classified as amorphous (0 per cent) to highly crystalline (> 90 per cent). The polymers with clear structural chains will show strong crystallinity along with a slow cooling rate.
Slow cooling provides enough time for crystallisation to take place. Polymers that possess a high degree of crystallinity are observed to rigidly possess a high melting point and their impact resistance is observed significantly less. In the case of amorphous polymers, they are soft and have lower melting points.
Some examples of amorphous polymers: are polystyrene, poly (methyl methacrylate)
Some examples of crystalline polymers: are polyethene, and PET polyester.
At lower temperatures which is the amorphous region of polymers, the molecules of the polymer are more or less frozen and so in this state, the molecules can undergo minimal vibration, but any kind of significant movement is rendered impossible. This state is called the glassy state.
The properties that describe the polymer best during this state are rigid, brittle and hard which are quite similar to those observed in glass, hence the name. On the application of heat to the polymer, the polymer chains are allowed to wiggle, and, much like rubber, the polymer becomes elastic and flexible. The rubbery state is referred to as this state.
At a certain temperature, polymer changes from the glassy state to the rubbery state are known as the glass transition temperature.
The glass transition temperature is specific to only the amorphous region as there is a melting point in the crystalline regions of the polymer. There are two types of transitions in thermodynamics, namely, first-order and second-order transitions.
In this scenario, the glass transition temperature is the second-order transition, where the first stage transition is the melting point. The glass state is not in balance, and the value of the glass transition temperature is also not specific. Some of the factors on which the glass transition temperature depends are:
Pendant groups (bulky pendant groups and flexible pendant groups)
It is highly useful and smart to know about the mechanical properties of the material before it is applied in any sphere or industry. Some of these important mechanical properties in the case of polymers are discussed below.
Strength – The stress required to break the polymer is known to be strong. There are a lot of different kinds of strength which include tensile (or stretching), compressional, flexural (or bending), torsional (or twisting), impact (or hammering) and many more. The strength of the polymers is influenced by some of the factors as follows:
Per cent Elongation to Break (Ultimate Elongation) – The strain-induced, during its breakage on the material, is known as the ultimate elongation. It is measured before the damage occurred by calculating the per cent difference in the length of the material. It is also used for measuring the ductility of that particular material.
Young’s Modulus (or Modulus of Elasticity or Tensile Modulus) -The ratio of the stress to the strain in the linearly elastic region is known as Young’s modulus of that substance. It is an indicator of the material’s stiffness.
Toughness – The area under the stress-strain curve is known as the toughness of the material. It measures the absorbed energy within the material before a fracture occurs. The breakage of the polymer is related to the stress caused by tensile strength. There is a significant effect of temperature on the mechanical properties of materials. The eclectic module and the tensile strength are decreased as the temperature increases, but the ductility of the material is increased.
Viscoelasticity – Deformations are essential for two types: elastic and viscous. The strain is induced right where the steady load is applied to inelastic deformation, and this strain is sustained as long as the material is loaded. The substance returns to its normal form entirely as soon as the load is withdrawn. This implies that the origin of the elongation is absolutely reversible inside the material.
However, in the case of viscous deformations, the induced strain is not instantaneous and depends on time. With time and constant application of load, the strain keeps increasing and the recovery process when the load is removed is delayed. It should also be noticed that the material does not entirely return to its original shape after the load is removed, and so this sort of deformation is permanent.
In general, polymers tend to show a combination of the behaviours observed in plastic and elastic deformations which depends on the strain rate and temperature. If we consider high temperature and low strain rate, we observe viscous behaviour and Elastic behaviour is found in cases of low temperatures and high strain speeds. Therefore, we consider intermediate values of temperature and strain rate to observe the plastic and elastic behaviours in polymers. This kind of behaviour is known as viscoelasticity and the polymer is known as viscoelastic.