Basics of Polymers
An introduction to polymers
Wherever we look around ourselves, we see polymers. The tires of the cars that we travel in, the rubber tips on your earphones, the material of your jackets, everything around us is made of polymers. So much 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 a very high molecular weight which is made by chemically bonding two or more building blocks. These building blocks are termed as 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.
Photo credit: https://www. thoughtco .com/what-is-a-polymer-820536
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 natural polymers cannot be prepared artificially. Chemists can also choose to create polymers using compounds prepared within the lab.
Historical Development - Basics of Polymers
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 since 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 polymer science which have also been worked and developed upon by other scientists:
Polystyrene was found by Eduard Simon
Vulcanisation of rubber is done by mixing it with sulphur. This was developed by Hancock in England and Goodyear in the USA.
In an International Exhibition in London, Alexander Parkes showcased Parkesine, which is developed using cellulose nitrate.
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.
The rights to produce Parkesine in England were taken by Daniel Spill. He then also produced Xylonite and Ivoride after establishing the Xylonite Company.
Polyvinyl chloride (PVC) was invented by Eugen Baumann, among others.
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.
Cellulose acetate lacquers and plastic film were modified and made perfect by the Dreyfus brothers.
Polyvinyl acetate was found by Fritz Klatte. He also developed a new manufacturing process for PVC and patented it.
Synthetic rubber, Nylon and Teflon were produced by DuPont in the USA.
Polyethylene was discovered by ICI.
Polyurethane was patented by Otto Bayer.
Acrylonitrile butadiene styrene (ABS)
High-density polyethylene and crystalline polypropylene were discovered by Paul Hogan and Robert Banks of Phillips.
The polymerisation of polyethylene was done at low pressure with the help of Ziegler catalysts.
Giulio Natta discovered polypropylene.
Mass manufacture of polycarbonate was started.
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, where 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.
Photo credits: https://www.grida.no/resources/6923
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 the polymer companies to acquire the exact formulations.
Photo credits: https://quizlet.com/256760440/311-monomers-and-polymers-diagram/
The chain links are often identical to its neighbours in case of artificial polymers. However, in case of proteins, DNA, wool, cotton, silk, and other such natural polymers the links in a chain are usually different from its 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.
Basics of Polymers - Anatomy
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 since 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 a 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.
Photo credits: https://www.mdpi.com/2073-4360/10/9/1036
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, neither 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 manufacture of 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 as 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 process) 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.
Source: http: //preparatorychemistry.info /Bishop_Addition_Polymers.htm
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.
The 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.
- Polymerisation mechanisms
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.
Addition (Chain-growth) polymerisation
Unsaturated molecules are those molecules which consist of carbon-carbon double bonds, for example, ethylene or styrene and are termed as vinyl polymers. Long-chain molecules in case of such monomers are produced by addition polymerisation. There are three mechanisms for addition polymerisation which include radical, anionic, and cationic. All of these mechanisms are carried out in the same three stages: initiation, propagation and termination.
There are no by-products formed as a result of these reactions, however, the materials that you begin with need to be unadulterated and if polymers with high molecular weight are required, then the reaction needs to be carried out without oxygen. The polymerisation process is stopped in case the monomer is removed from the system. Some common examples of polymers that make use of addition polymerisation are polystyrene, polyethene, polypropylene, polyvinyl chloride (PVC).
A free radical is required to initiate the polymerisation reaction. This free radical has the function of breaking up the carbon-carbon double bond by combining with one side of the monomer. This permits the other monomers to react with open monomers present on their side. Eventually, polymer chains are formed. Some common examples of initiators are benzoyl peroxide, azo-bis(iso-butyronitrile) (AIBN)free radicals formed when the heat is added to hydrogen peroxide.
Source: http ://www.materials .unsw.edu.au/sites/default/files/initiation.png
This process involves the formation of even longer chains due to addition of monomers thereby causing an increase in number of free radical chains. A vinyl polymer is asymmetrical in regards to its ends, i.e., a head and a tail. The R group attached to the carbon group is referred to as the head, and when the R group is absent in the carbon group, it is referred to as the tail. This process takes place due to a reduction in the average energy of the system. In simple words, there exists a driving force in the reaction such that the total energy of the produced polymer chains is much lesser than the total energy of the monomers separately.
Source: http://www .materials.unsw.edu.au/sites/default/files/propagation.png
In polymerisation, the chain transfer to polymer can also take place during the propagation step. This relates to the manner in which a growing chain radical is then moved to another polymer chain, forming branches on the polymer chains. These branches are responsible for reduced melting point and mechanical strength of the polymer. We can observe extensive branching whenever the high-pressure radical polymerisation of ethylene. This ethylene is used in LDPE polymerisation.
The termination step includes the reaction between any two free radicals done either by the process of combination or disproportionation. In other words, this step can be carried out either by the addition of a terminating free radical chain or by the combination of two free radicals.
Source: http://www.materials .unsw.edu.au/sites/default/files/termination.png
Whenever a Ziegler-Natta or any other catalyst is present, the metal atom of the catalyst gains a growing polymer chain and the monomer addition taking place is coordinated with the metal atom. This type of polymerisation is known as coordination polymerisation. The monomer is required to be in a certain specific position for reaction to be possible. This reaction enables us to produce stereoregular polymers.
- Condensation (step-growth) polymerisation
Monomers having different functions are produced in steps to eventually form polymer molecules. During polymerisation, we go from dimers, trimers, tetramers, oligomers to finally polymer molecules. The process also involves the production of a by-product (usually H2O) created from the reactions. This is essentially why this type of polymerisation is known as condensation polymerisation.
There is an exponential growth in the overall molecular size. In order to obtain commercially viable high-quality polymers with high molecular weight, the materials that we are starting with are pure along with a specific balance of monomers.
In case of elimination condensation, a small molecule (usually water) is removed and that slows the formation of the long-chain molecules. On the other hand, rearrangement condensation does not involve the formation of any by-product. Some examples of elimination condensation include phenol-formaldehyde, urea-formaldehyde, melamine-formaldehyde, polyesters, polyamides and some examples of rearrangement condensation reaction include epoxides and polyurethanes.
Photo credits: https://www.onlinemathlearning.com/polymers-condensation.html
- Functionality of Monomers
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.
- Polymerisation Processes
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
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.
Photo credits: https://pubs.rsc.org/en/Content/ArticleLanding/PY/2016/C6PY00590J#!divAbstract
- Solution Polymerisation
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.
- Suspension 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.
- Slurry Polymerisation
The main use of this process is in the manufacturer 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.
- Emulsion Polymerisation
The process involved in Suspension polymerisation is similar to emulsion polymerisation. A monomer is also dispersed in water but the only difference being 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, the organic solvents have thermal conductivity and specific heat much lower than that in 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 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 center.
Photo credits: https://en.wikipedia.org/wiki/Emulsion_polymerization
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.
- Gaseous-phase Polymerisation
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 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 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.
- Ziegler-Natta 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)
- polybutene (PB).
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.
- Metallocene Catalysts
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, 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 they 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 consists 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.
- Polymer Chains
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 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 to beyond several hundred. This phase change is observed due to the total binding force between the molecules increases 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 a tangled mass.
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 as 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.
Photo credits: https://www.e-education.psu.edu/matse81/sites/www.e-education.psu.edu.matse81/files/images/lesson08/polymer_structures.png
- Linear Polymers
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.
- Branched Polymers
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.
- Crosslinked Polymers
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
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 are 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 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 as 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 about 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
- Polymer Crystallinity
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 sn 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 the crystallisation to take place. Polymers that possess a high degree of crystallinity are observed to be rigidly possessing 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 of the Examples of amorphous polymers: polystyrene, poly (methyl methacrylate)
Some of the Examples of crystalline polymers: polyethene, 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 the 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:
- Intermolecular forces
- Chain stiffness
- Pendant groups (bulky pendant groups and flexible pendant groups)
- Molecular weight
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:
- Molecular weight
- 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 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 inelastic deformation, and this strain is sustained as long as the material is being 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.