Polymers
Ok. I got it.
You are here to learn about polymers. I will teach you polymers for sure. It is just that this article might blow your mind by the end of it. Hence I request you to wear your helmets and get ready for a roller coaster ride. The article begins from the next paragraph. NEET Aspirants can directly jump here
"This made me tremble as a doctor; I felt there might be a common reason behind these cases," Zhang told a Chinese newspaper
In late June 2008, a urologist named Zhang Wei found something alarming and dangerous among some newly born babies in China.
These newborn babies had developed Kidney stones.
Kidney stones at such an age are rare but, if formed, cause unbearable pain. Dr Zhang treated four such babies in a short span of 10 days. However, the increasing frequency of such complaints pointed towards a problem that was the tip of the iceberg. A hidden national disaster that would affect some 3,00,000 newborn babies in china.
But how is this disaster linked to the chapter on “Polymers”? Well, because those kidney stones were the result of contaminated baby formula. A chemical was added to the baby milk formula to fool the quality inspectors. What was this chemical, and why was it added to the baby milk formula? How did the Chinese government deal with the scandal? And what happened to the culprits? To get all these answers, continue reading these notes. Apart from this Chinese food scandal, we have some more stories to be learnt in this chapter which would make you fall in love with the topic. Here they are:
A popular emulsion of polymer that we widely use in our households could power our homes in future. What are these polymers, and how do they achieve their magical feats?
Read the article to get the answers to these puzzles. We already have self-healing polymers that might revolutionise our world. Polymers have come a long way in serving humanity. They are now being used to keep the babies dry by absorbing their urine to make bio-printed hearts that soon might be found beating inside us. Let’s invite our guests tonight, who could teach us about the beautiful world of polymers to give the love and respect this chapter from NCERT deserves
Mr. N: We are grateful you agreed to participate in our interview series Mr Polymer.
Polymer: I am so glad to be here. I would be delighted if this interview could help students understand the “Polymers” chapter to some extent.
Mr. N: That is so nice of you. Let us start with your origin. How exactly was the idea of Polymers conceptualised by humans? When did they start taking you (Polymers) seriously?
Polymer: Sure. I would love to talk about the history of polymers and how they changed our world forever. However, start with the video below. Video on Polymer:
Polymer: Any idea that challenges the long-held belief of humans has always found difficulty in getting accepted by the public. We have so many examples that prove this point. Ludwig Boltzmann, one of the most respected physicists of the 20th century, is speculated to have committed suicide because the world did not take the idea of his “Atoms’ seriously. Can you even believe that?
Mr. N: Believe what?
Polymers: Can you believe that there was a time we did not think that atoms were real? Our entire modern material development results from this realisation that stuff around us is made of atoms. This one fact single-handedly revolutionised humanity forever. The year before Boltzmann’s suicide, Einstein published a paper that provided a solid mathematical proof for the existence of atoms. In today’s time, a significant science discovery reaches every corner of the world in seconds via the internet. However, the information had its limited means of spreading during the time of Einstein and Boltzmann. Had Boltzmann been aware of Einstein’s work of atoms, he would have never taken that extreme step.
Mr. N: Wait, wait, did you say that Einstein was the guy who provided mathematical proof for atoms? I thought all his life, he worked on relativity.
Polymer: In that case, let me quote “Max Born” here.
"Einstein would be one of the greatest theoretical physicists of all time even if he had not written a single line on relativity." ― Max Born
Mr. N: Why would Born say that?
Polymer: Well, because Einstein was much more than relativity. Although both special and general relativity were among the most significant works, he made other seminal contributions too. And this includes the mathematical proof for atoms and the particle nature of light.
Image details: An engaging Einstein portrait I got on Pixabay for free.
The Light Quanta Hypothesis
Einstein considered the “light quanta” hypothesis his most original work, not relativity. In fact, for almost 15 years, the world thought that Einstein was a fool for proposing the particle nature of light. However, Einstein’s conviction and the work of experimentalists paid off. Einstein received the Nobel prize for his work on the particle nature of light.
Robert Millikan, who almost worked for decades to prove Einstein wrong and intensely disliked the idea of light quanta, admitted that all his experimental work did validate Einstein’s light quanta hypothesis. Einstein again had the last laugh.
Einstein considered the “light quanta” hypothesis his most original work, not relativity. In fact, for almost 15 years, the world thought that Einstein was a fool for proposing the particle nature of light. However, Einstein’s conviction and the work of experimentalists paid off. Einstein received the Nobel prize for his work on the particle nature of light. Robert Millikan, who almost worked for decades to prove Einstein wrong and intensely
disliked the idea of light quanta, admitted that all his experimental work did validate Einstein’s light quanta hypothesis. Einstein again had the last laugh.
Mr. N: Whoaa, this is so interesting. The world has consistently shown strong opposition to new and bold ideas in the past. The torchbearers of ultimate truth have always faced strong resistance from dogma lobbyists. We have many examples to prove this point. From the Nobel prize-winning work on QUASICRYSTALS to bizarre BLACK HOLES, new ideas took much time to get wider acceptance within the scientific community.
Image details: These are computer-generated Quasicrystal designs
Before the students get angry, we should start talking about polymers. The very reason for them reading this piece.
Polymers: Oh yes. I am here to talk about polymers. But it is essential to set a context. The idea that there could be molecules as heavy and long as polymers were also not accepted readily by the scientific community
Mr. N: Indeed. I am not at all surprised. Please tell us how it all started.
Polymer: Before we talk about the invention, we must learn about the inventor. This helps us know the path of discovery better. It would also help the budding scientists understand how discovery happens and how sometimes science is all about your bravery. Your self-belief and conviction.
Nr. N: I believe it is high time you introduce us to our hero.
Polymer: German scientist Hermann Staudinger. Yes, Hermann Staudinger was the first to conceptualise that small molecules can hold their hand to form massively long chains. It is like millions of humans continually holding each other’s hands to form a single unit. Staudinger was born in Germany. He initially wanted to be a botanist but later switched to chemistry.
Mr.N: Thank God he was in Germany. In India he would have had taken M.B.B.S under societal pressure.
Polymer: Indeed. I completely agree with you on that point.
Staudinger’s early work in chemistry is related to “Ketenes”. Ketenes would later find their use in making bacteria-killing weapons we know as Penicillin. Staudinger got interested in polymer chemistry while he was at ETH, Zurich, Switzerland. Chemistry researchers working with rubber had found that molecules present in natural rubber were super heavy. A value that was too high to believe easily. Let me give you an example to explain how weird and surprising this idea of superheavy molecules was. It is like on a new planet; you have only found tiny bacteria so far. But suddenly, someone announces to have seen Dinosaurs wandering around. How would that feel? All these years, you had seen only tiny microbes through your microscope, and there was no sign of macro life like insects and animals. Suddenly someone announces that he has seen Dinosaurs roaming around.
Mr. N: I would feel strange, of course.
Polymer: Exactly. This is how the scientific community reacted to the discovery of molecules weighing hundreds of thousands of times more than the smaller molecules they were familiar with. In the world they thought was made of microorganisms, they had detected “Dinosaurs”. However, people were still waiting to believe in the reality of large, heavy molecules. They thought it could be large number of small molecules that formed aggregates. An aggregate is like a large crowd of people accumulated in one place. This is different from same people arranging themselves in a line by holding each other’s hands. It is like those colourful beads kept in a box. They are aggregates. There is no real connection between beads. In another you have a jewellery from that bead where a thread commonly connects all the bead in one structure. Here the single structure is the jewellery and not the bead. Staudinger was talking about the jewellery model while the rest of them saw each bead as separate entity in the aggregate
However, Staudinger proposed in his landmark paper that macromolecules are polymers formed when hundreds of thousands of single molecules join together via covalent bonds. This one idea marked the beginning of polymer chemistry. Be it the keyboard that I am using to type this interview or the frying pan that your mom hit on your head for low test marks, all modern material development we owe to Staudinger’s deep insight about the nature of bonding in polymers. He published his landmark work on the model of Polymers titled “Uber Polymerisation”. It was not mere speculation. It was genuine scientific work backed up by solid research data. The work of other scientists like Mark and Carothers proved that polymers are real. Staudinger was a visionary and realised the potential of his work quite early. “It is not improbable,” Staudinger commented in 193,
“that sooner or later a way will be discovered to prepare artificial fibres from synthetic high-molecular products because the strength and elasticity of natural fibres depend exclusively on their macro-molecular structure – i.e., on their long thread-shaped molecules.”
Staudinger’s work should have been taken more seriously. The work of scientists like Mark and Carothers convinced the world that Staudinger was right. Macromolecules are not imaginary entities but a hardcore reality. Staudinger had his last laugh, too, as he received the Nobel prize in Chemistry in 1953. In addition, the American Chemical Society and Gesellschaft Deutscher Chemiker – the leading chemical society of Germany, designated Staudinger’s work as an International Historic Chemical Landmark in 1999.
Mr. N: This is an incredible story of a man’s belief in his ideas and his relentless quest for truth. This is how a model scientist should be, and every student reading this chapter should strive to develop such an attitude. Anyways, could you explain the meaning of the word “Polymer”?
Polymer: Let me first share the official definition.
As per wikipedia “A polymer (/ˈpɒlɪmər/;Greek poly-, “many” + -mer, “part”) is a substance or material consisting of very large molecules, or macromolecules, composed of many repeating subunits.” When millions of ethene molecules join hands to form one megastructure, polyethylene or polyethene is formed – A Polymer. Similarly, proteins are created when hundreds of amino acids join each other via their unique peptide bond. So, in short, Polymers are macromolecules formed due to the repetition of small molecular amino acid units.
Mr. N: ohk. I ultimately got the definition of polymers. What is the classification of Polymers? I mean, both Nylon and Proteins are polyamides; how are they different from each other?
Watch this interesting TED Talk on the history of polymers here:
CLASSIFICATION OF POLYMERS
Polymers have their classification and technical terms. The first classification of the Polymers is based on their source. If the polymer has a natural origin, it is called a natural polymer. Silk, Cotton, Proteins, Carbohydrates, DNA, and Cellulose are all examples of natural polymers. Interestingly glycogen, starch and cellulose all are natural polymers made from the same repeating unit, i.e. glucose.
Mr. N: If glycogen, starch and cellulose are made of glucose, why don’t we eat grass like cows? We can digest starch, a glucose polymer, so we should also be able to digest cellulose, a polymer of glucose.
Polymer: I wish it were true. If we can ever digest cellulose, we would be served pizza with toppings of grass. We might have grass laden with mayonnaise sauce in sandwiches. However, things are more complex than you think. You see, the digestion of every natural polymer in your body requires certain enzymes. These enzymes are designed to cut some particular bonds. For example, even though glycogen, starch and cellulose are made from glucose, they have different types of linkage formed by joining different carbon atoms. This makes it impossible for one enzyme to digest other sugars even though they are made up of the same repeating unit. This is why you need cellulase Enzyme to digest cellulose and amylase for the digestion of starch.
Enzymes are like molecular scissors. Have you ever seen a barber cutting someone’s hair using the scissor used by a gardener? Similarly, the tailors have their scissors to do their job. A tailor’s scissor is designed to do a particular job, and so is the scissor of a barber. They are designed to cut different things. This is the exact case for the enzymes. They are biological scissors designed to cut particular bonds. The biological scissor designed to cut the cellulose will not cut the bonds of starch.
Mr. N: Ohh, interesting. Enzymes and hormones are everything to the biological world. These are the natural tools gifted to the micro and macro life by natural selection. Because of these tools, life has survived and thrived in the harshest conditions on this planet for billions of years.
Polymer: You are right. Every species has unique set of enzymes that help them tackle specific challenges thrown by its surroundings. For example, the fish living in oceans where the temperature reaches below zero produce antifreeze proteins. These antifreeze proteins help this species of fish to safeguard themselves from the deadly effects of ice crystals that would otherwise form in their blood due to freezing. In short, these antifreeze proteins decrease the freezing point of their blood. So if blood were freezing at zero degrees, it would freeze even at a lower temperature due to the antifreeze proteins. Similarly, humans prevent petrol from freezing in countries that experience sub-zero temperature. The chemical “Ethylene Glycol” is used as Antifreeze. The addition of the “Ethylene Glycol” decreases the freezing point of the petrol.
Mr. N: Now, let us continue with our classification of the polymers. We had already discussed natural polymers.
Polymer: Yeah, so the first classification was based on the sources. Polymers, like cotton, wool, silk, or starch, could be completely natural, while polymers like nylon, polystyrene, PVC, etc., are man-made or synthetic. Synthetic polymers are entirely created in the factories and designed for the first time in the research labs. There is also a category called “Semi-synthetic” polymers. You take a natural polymer and modify it by adding some chemicals to get completely new desired properties. The best examples are cellulose nitrate obtained by the nitration of cellulose. You could also do the acylation of cellulose to create cellulose acetate. Cellulose acetate is called Rayon. Vulcanised rubber is also an example of a semi-synthetic polymer. A rubber prepared by adding sulphur to natural rubber results in a much stronger rubber. Interestingly, the new material retains its elastic property.
Mr. N: Cool. So based on the sources, we have 1. Natural Polymers 2. Synthetic Polymers 3.Semi-Synthetic Polymers
Polymer: Yes. There is also a classification based on the structure of the polymers. If the repeating units of the monomers are arranged in a straight chain without any branching, the polymer is called the Linear Polymer. Polyethene, PVC etc., are examples of such Polymers. The polymer would be classified as branched if they are branches within the linear chain. An example of a branched polymer is low-density polyethene. The last category is the cross-linked Polymers, where, unlike the branching in the main chain, the different primary chains are linked via covalent bonds. Examples of cross-linked polymers are bakelite, melamine and vulcanised Rubber.
Mr. N: Interesting. Now let us discuss polymerisation based on the mode of polymerisation.
Polymer: Sure. The most important thing during polymerisation is the linking of repeating units that we call monomers. You could create a very long chain of such monomers in different ways. One of the ways through which the monomers get connected is the “addition reactions”.
Mr. N: Ok, the unsaturated compounds show the addition reaction – the molecules with pi-bonds. For example, the polymerisation of ethene happens via the addition polymerisation reaction. The unsaturated ethene molecules connect with other ethene via the addition reaction. I remember this lesson.
Polymer: Yeah. It is via the addition reaction that the unsaturated compounds convert to saturated ones. The triple bond compounds are also unsaturated; therefore, they could also show addition polymerisation. It is just that they won’t convert into saturated compounds immediately. Since the degree of unsaturation for the ethyne is 2, they would need another addition reaction to get completely saturated. e.g., the formation of polythene from ethene and polypropene from propene.
Mr. N: Could you tell us something about the homopolymers?
Polymer: Homo is a prefix that means same. If your polymer is made up of one single molecule that acts as a monomer – we would call such polymer to be a homopolymer. A homopolymer is formed when hundreds of thousands of monomer units made up of a single small molecule are connected through covalent bonds. Ethene monomers join each other to form polyethylene, while vinyl chlorides join to form polyvinyl chloride. Since you know about homopolymers, it would not be difficult to understand the “co-polymers”. In co-polymers, instead of one, we have two different molecules coming together to form the repeating unit. When two different types of molecules come together to create a new monomer unit that gets repeated in the polymer, the category of such polymers is called “Copolymer”. For example, in the case of the Styrene Butadiene rubber, the monomer unit is formed by the reaction of 1,3- Butadiene and Styrene. These molecules react via the addition reaction. The word addition indicates that the monomer units during the polymerisation are connected via addition reaction. Addition polymers, therefore, could be homopolymers as well as copolymers.
Mr. N: What are condensation polymers?
Polymer: Addition is one of many reactions that connects two monomer units during polymerisation. There is also an alternate way that connects two different monomer units. In this reaction, two other functional groups from the different molecules react to release a small molecule. These small molecules could be water, HCl, ammonia or methanol. The monomer units get connected with the release of these small molecules. We call this reaction as condensation. Condensation polymers are always co-polymers.
Polymers like Terylene (Dacron), Nylon-6,6, and Nylon-2,6 are examples of condensation polymers. The Nylon-6,6 is a condensation polymer of hexamethylene diamine and adipic acid, while ethylene glycol and terephthalic acid make “terylene”.
Mr. N: And is there a specific reason behind those numbers of Nylon like Nylon-2,6, Nylon-6,6 and Nylon-6?
Polymer: Of course, there is a reason behind those numbers. The numbers usually indicate the no. of carbon atoms in the molecules that make the repeating unit of Nylon. So, for example, the first six indicate the six carbon atoms present in hexamethylene diamine, and the other six show the six carbon atoms of adipic acid.
Mr. N: Interesting. Nylon-6 indicates the six carbon atoms in the monomer of Nylon-6 called caprolactam.
Polymer: Indeed. The six in the case of Nylon-6 indicates the six carbon atoms of caprolactam. Now let us discuss the classification of Polymer-based on the type of molecular forces that make up the polymer.
Polymer: Sure. I would love to answer those questions. However, I am feeling hungry. Could you do something about that?
Mr. N: Of course, I can. Tell me, what would you like to eat?
Polymer: Well, this depends on what you have in the menu.
Mr. N: In that case, let me call Ramu-2,3
Polymer: Sorry, who?
Mr. N: Ramu-2,3?
Polymer: Interesting. The polymers like Nylon-2,6 and Nylon-6,6 inspire this name. But what does Ramu-2,3 signify here?
Mr. N: Ramu-2,3 is our chef. You either vomit twice or thrice when you eat the food he made. Hence his name is Ramu-2,3.
Polymer: What? Seriously. In that case, I would prefer to be hungry. But hey I thought you guys also served the “Neoprene Dhosa” with the coconut chutney.
Mr. N: Oh yes. The Neoprene Dhosa are rubbery unlike the regular dhoshas. Most of the consumers complained of digestion issues and complained of severe constipation for months. We have stopped that product too sir. I am really sorry.
Polymer: I am glad you do not serve it anymore. What about the “Rubbery Lassi” made from the milky latex of the rubber free. You guys used to add cardamom flavour to it.
Mr. N: Oh yes. We stopped making that product too. You can still eat our new biodegradable vadapavs that would dissolve after days once ingested.
Polymer: No. I am totally fine.
Mr. N: Your wish is our command, sir. Let us continue with our discussion. Please tell us something about the forces between the monomer units of the polymer. How do they differ from each other?
Polymer: You see the mechanical properties of the polymer, like their toughness depends on the type of forces that hold the monomers together. If the forces acting between the monomer units are not quite strong, then such polymer turns out to be stretchable. We call such polymers elastomers. It is these weak interactions that give them the elastic property. The best example of elastomers is rubber. However, in some instances, the bonds between the monomer units are strong. These forces that keep the monomers tightly bonded to each other are hydrogen bonds. These Polymers formed due to the strong hydrogen bonds are called fibres. The strong hydrogen bonds are responsible for specific polymers’ high tensile strength and modulus properties.
Mr. N: I have heard of the tensile strength, but this modulus property is new. What is the modulus property?
Polymer: Modulus is the opposite of elasticity. Modulus is a measure of a material’s tendency to avoid deformation—a property expected from plastics. Now there is another classification on whether your polymer could be melted and given a new shape or recycled, for that matter. The polymer that could melt on heating and be given new shapes is called “thermoplastics.” These are the linear or slightly branched long-chain molecules capable of repeatedly softening on heating and hardening on cooling. These polymers possess intermolecular forces of attraction that lie between the elastomers and fibres. Some common thermoplastics are polyethene, polystyrene, polyvinyl, etc.
Mr. N: I know this. There is also another category called a thermosetting polymer. Thermosetting polymers are cross-linked or highly branched. Due to the three dimensional network of bonds (crosslinking), thermosetting plastics last longer than thermoplastic materials. They also sustain high-temperature applications up to the decomposition temperature. The resistance towards the higher temperature comes from the fact that they can maintain their shape because of strong covalent bonds between polymer chains. All of these properties emerge from the extensive cross-linking within the structure. A thermoset polymer’s resistance to chemical and heat attacks increases with increasing crosslink density and aromatic content.
Polymer: Indeed, you got the thermosets right.
Mr. N: Is there any classification that we shall discuss further?
Polymer: There is one last point I want to make regarding the classification of polymers. The addition and condensation polymers are now called chain-growth polymers and step-growth polymers. This depends on the polymerisation mechanism they undergo during their formation. Before I discuss the technical details of polymers further, I would like to share something interesting.
Mr. N: Sure. This sounds fun. Please go ahead
Polymer: Some research labs are working on a new technology around paints. These would be paints capable of generating electricity for your household.
Mr. N: What?
Polymer: Yes, welcome to the world of solar paints.
Mr. N: Cool man. I had heard of roads that could convert electricity from the vibrations of heavy vehicles. But paints generate electricity? This is something new man. I would like to know more.
Polymer: Yeah. These are particular paints made from the “Quantum Dot” technology.
What are condensation polymers?
Polymer: Addition is one of many reactions that connects two monomer units during polymerisation. There is also an alternate way that connects two different monomer units. In this reaction, two other functional groups from the different molecules react to release a small molecule. These small molecules could be water, HCl, ammonia or methanol. The monomer units get connected with the release of these small molecules. We call this reaction as condensation. Condensation polymers are always co-polymers. Polymers like Terylene (Dacron), Nylon-6,6, and Nylon-2,6 are examples of condensation polymers. The Nylon-6,6 is a condensation polymer of hexamethylene diamine and adipic acid, while ethylene glycol and terephthalic acid make “terylene”.
Image source: Digitimes
What is "Quantum dot" technology?
Polymer: I would suggest you google the “Quantum dot”. It is better that you do some self-study here.
Mr. N: So cool. I am sure that students would love all this information. Let us continue with the chapter. Please tell us something about the addition or the chain growth polymerisation.
Polymer: In Chemistry, we can guess the reaction mechanism by focusing on the language. Our following topics are simple and could be assumed from the language too.
Polymerisation fundamentally forms a vast chain of repeating units called monomers. You know that. This is a stepwise process where some monomer units become a large chain. They grow to become a vast chain starting with the basic monomer unit. If the monomers are unsaturated alkenes and alkadienes, the monomer units connect via an addition reaction, with the chain multiplying with millions to billions of monomer units. Free radicals usually initiate this process. They work as initiators to kickstart the chain growth polymerisation process. In short, free radical-governed addition or chain growth polymerisation is the most common mode. However, many polymerisation processes involve other reactive intermediates like cations and anions.
Mr. N: Could you share the details of the chain growth polymerisation?
Polymer: Sure. Why not?
The most commonly used source of free radicals required to initiate polymerisation is benzoyl peroxide, acetyl peroxide, and tert-butyl peroxide. For example, the polymerisation of ethene to polythene consists of heating or exposing a mixture of ethene to light with a small amount of benzoyl peroxide initiator. The process starts with adding phenyl free radicals formed by the peroxide to the ethene double bond, thus generating a new and more prominent free radical. This step is called the chain initiation step. Then, another more giant-sized radical forms as this radical reacts with another ethene molecule. The repetition of this sequence with new and more significant radicals carries the reaction forward, and the step is called chain propagation. Ultimately, at some stage, the product radical thus formed reacts with another radical to form the polymerised product. This step is called the chain-terminating step.
Mr. N: Now let us talk about some polymers we widely use in our lives.
Polymers: Let us start with polyethene, which has become a menace today. Before discussing the plastic pollution issue, let’s discuss its synthesis. Polyethene is generally of two types. There are low-density polyethylenes and high-density polyethylenes. Let us discuss the low-density polyethene first: The polymerisation of low-density polyethene happens under high pressure of 1000 to 2000 atmospheres at a temperature of 350 K to 570 K—the traces of dioxygen or a peroxide initiator act as catalysts for the process. The low-density polyethene (LDP) obtained through the free radical addition, and H-atom abstraction has a highly branched structure. Low-density polyethene is chemically inert and rigid but flexible and a poor conductor of electricity. Hence, low-density polyethene finds its use in insulating electricity-carrying wires and manufacturing squeeze bottles, toys, and flexible pipes.
Mr. N: Could you tell us why most polymers are inert? What makes them non-reactive?
Polymer: The inert nature of polyethenes or other polymers is linked to their strong bonds and saturated nature. The carbon-carbon bonds in polymers are non-reactive compared to polar carbon bonds. The other reason that makes the polymers is the strength of the carbon-carbon bonds. Materials with stronger carbon-carbon bonds are less reactive than those with weak bonds. Nitrogen’s strong bonds make it non-reactive, whereas Fluorine is among the most reactive gases. It takes a lot of energy to break those three Nitrogen bonds. In contrast, the Fluorine bond breaks down quickly due to higher interelectronic repulsion within the dense and small p orbitals.
Mr. N: Thank you so much for this information. I have been wondering about the inert nature of plastics for a long time. I finally got the answers to the questions that had been haunting me for a long.
Polymer: I am so glad I could answer those questions for you.
Mr. N: I am glad too. Please tell us something about the high-density polymers now.
Polymer: Sure. The polymerisation of the high-density polyethene requires the following
conditions:
1. The polymerisation happens via the addition of a polymerisation
2. It takes place in the presence of the hydrocarbon solvent
3. Pressure is between 6-7 atmospheres
4. Temperature requirements are between 333 K and 343 K. 5. Triethylaluminium and
titanium tetrachloride (Ziegler-Natta) are catalysts. High-density polyethene (HDP), thus produced, consists of linear molecules and has a high density due to close packing. It is also chemically inert and tougher as well as hard. The polymer is used to manufacture dustbins, buckets, pipes, bottles, etc.
Mr. N: You also said something about the menace created due to global plastic pollution. Innocent marine animals are paying the highest price for anthropogenic pollution.
Polymer: Indeed, marine animals have been widely affected due to ocean plastic pollution. Let me quote some statistics to make the readers realise the issue’s urgency here.
“As per UNESCO, over 1 million marine animals die yearly due to ocean plastic pollution. The report also claims that more than 100 million tons of plastic could be in the ocean globally.”
I would also like to talk about the devastating effects of other polymers or the chemicals used in producing these polymers.
Mr. N: Our readers would love to read this. Please share the stories that you believe are worth telling.
Polymer: I am indebted for that encouragement. Let me start with “Teflon” in that case. Teflon has been a controversial polymer from the beginning. I strongly suggest you watch this recently released film called “Dark Waters” to learn about one of American history’s greatest cover-ups. Watch Dark Waters trailer here:
Mr. N: “Dark Water”? Seems like a movie on water contamination.
Polymer: It is indeed about water contamination with a cancer-causing chemical. The interesting point is that the antagonist in the movie is Du-Pont, the chemical giant that invented TEFLON.
Mr. N: Oh. Please tell us some more details. I would, however, watch this movie for sure.
Polymer: It was the year 1998. A corporate lawyer named Robert Billot visited his grandmother in Parkersburg, West Virginia, USA. During his trip, he was approached by a local farmer Wilbur Tennant. Billot was bewildered to learn about the deaths of Tenant’s 190 cattle. Wilbur Tenant had a strong hunch that the contamination of their land from the nearby chemical factory could be the reason behind those cattle deaths. Wilbur Tennant showed some videos of dead cattle and how their internal organs had blackened. The videos and the photographs hinted towards something that the chemical giant in the town covered up. The cases were not limited to cattle. A lady had reported that her granddaughter’s teeth had turned black. There were cases of young boys who had testicular cancer.
Mr. N: What was the chemical linked to these deadly side effects? What was this cover-up all about? And who was trying to cover up what?
Polymer: Do you have non-stick cookware in your home?
Mr. N: Yes I do. How is this related to non-stick cookware?
Polymer: What chemical would you associate with non-stick cookware?
Mr. N: Teflon I guess.
Polymer: And which company invent Teflon?
Mr. N: Du-Pont.
Polymer: Now you know the main culprit.
Mr. N: You are implying that the villain here was Du-Pont, which is related to their Teflon production unit.
Polymer: Exactly. But before I reveal the exact details, I would like to talk about the exciting history of Teflon.
Mr. N: Sure.
Polymer: Du-Pont had no plans to synthesise Teflon in the first place.
Mr. N: What do you mean by no plan? Are you trying to say that they discovered Teflon accidentally?
Polymer: Exactly. Du-Pont did discover Teflon accidentally. In fact Roy J. Plunkett, who accidentally synthesised Teflon, was trying to make a chemical to cool things. A refrigerant that we use in Air conditioners and Refrigerators. A new type of chlorofluorocarbon. Instead of a new chlorofluorocarbon, the tetrafluoroethylene gas polymerised in the presence of iron at high pressure. This is how “Polytetrafluoroethylene”, aka Teflon, was born. This newly made wonder material was patented by Kinetic Chemicals – a firm owned by the chemical giant Du-Pont. The trademark Teflon was registered in 1945.
Mr. N: Wow. I never knew this story.
Polymer: By 1948, Du-Pont had produced 907185 kg of Teflon in Parkersburg, West Virginia.
Mr. N: Wait. What did you say? Parkersburg, West Virginia? Isn’t it the same place where that farmer Wilbur had reported cattle deaths and cancer cases?
Polymer: Yes. You got it right. This is the same place where DuPont “dumped, poured and released” at least 1.7 million pounds of PFOA between 1951 and 2003.
Mr. N: What is PFOA?
Polymer: That’s the villain chemical. PFOA stands for Perfluorooctanoic acid – a chemical needed to manufacture Teflon. Du-Pont originally bought this chemical from a company called 3M. 3M had clear
instructions on how this hazardous chemical should be disposed of. However, Du-Pont ignored everything, dumped the chemical in the surroundings, and ended up entering the water supply in lethal amounts. It is estimated that approximately 70,000 people in the localities living around the Du-Pont plant have been exposed to dangerous levels of PFOA. But there is something more alarming that our brave hero Rob Bilott discovered in his investigations.
Mr. N: What?
Polymer: Bilott had access to documents amounting to some 110,000 pages. He was surprised that Du-Pont had conducted secret research about the health issues associated with PFOA exposure. They knew that the chemical caused abnormalities among the foetus and was responsible for testicular, pancreatic and liver cancers.
Mr. N: And yet they dumped tons of PFOA for decades in the surroundings?
Polymer: Indeed. I don’t know how many innocent animals and human lives succumbed to cancer caused by PFOA released by Du-Pont. And hence I salute heroes like Rob Bilott and Wilbur Tennant, who fought for the justice of not only humans but animals too. Wilbur Tennant also developed cancer but died of a heart attack. Wilbur’s wife died of cancer two years after Wilbur’s death.
Mr. N: Did Wilbur’s sacrifice and Bilott’s struggle pay off? Did it bring any change?
Polymer: Of course it did. DuPont stopped the production and use of PFOA in 2013. Du-Pont and five other chemical companies in the world had to phase out production. Some 200 scientists from different disciplines signed “The Madrid statement”. The statement expresses concern about producing all fluorochemicals, or PFASs, including those that have replaced PFOA.
Image details: Exposure: Poisoned Water, Corporate Greed, and One Lawyer’s Twenty-Year Battle against DuPont is his first book.
In 2017, Bilott won a $671 million settlement on behalf of more than 3,500 victims of DuPont’s toxic PFOA exposure.
Mr. N: Wow. I am so glad about this development. Truth always triumphs. It is so interesting to know how the conviction of a lawyer became a nightmare for a conglomerate as giant as Du-Pont.
Polymer: Apart from the movie titled “Dark Waters” I would also recommend the readers to watch “The Devil we know”. Robert Bilott’s book “Exposure” is also a must-read for all who want to dive deeper into the topic.
Watch the “The devil we know” trailer here:
Mr N: Cool. I guess it is high time we switch back to NCERT. Let us start with Teflon again.
Polymer: Sure. Why not? Polytetrafluoroethylene or Teflon is manufactured by heating tetrafluoroethylene with a free radical or persulphate catalyst at high pressures. It is chemically inert and resistant to attack by corrosive reagents. It is used to make oil seals and gaskets for non-stick surface-coated utensils.
Mr. N: The next Polymer in our list is Polyacrylonitrile. Polymer: The addition polymerisation of acrylonitrile in the presence of a peroxide catalyst leads to the formation of polyacrylonitrile. Polyacrylonitrile is used to substitute wool for commercial fibres like orlon or acrilan.
Mr. N: The next category of polymers belongs to condensation Polymers. Something we discussed already. Now we will talk about specific examples.
Polymer: What category are we going to start with?
Mr. N: We gotta start with the “Polyamides”. Polymer: Polyamide is a polymer where you would see the repetition of the “amide’ bonds. Protein is among the natural polymers that belong to this class. Amide bonds are formed from the condensation reaction of Amino acids. You see, the acid part of one amino acid combines with the amine part of another. When these groups combine, they release a molecule of water and —CONH—– or amide bond is formed. As so many amino acids combine in a long chain, you would see the amide bond regularly repeating in the chain. This is why polymers like Nylon are called Polyamide. Nylons have amide linkage, and they are essential among the fibres. The general preparation method for “Nylon” involves the condensation polymerisation of diamines with dicarboxylic acids and amino acids and their lactams.
Mr. N: Let us talk about Nylon 6,6 Polymer: Nylon 6,6 – The condensation polymerisation of hexamethylenediamine prepares it with adipic acid under high pressure and temperatures. Nylon 6, 6 is used to make sheets, bristles for brushes, and the textile industry. (ii) Nylon 6: It is obtained by heating caprolactam with water at a high temperature Nylon 6 is used to manufacture tyre cords, fabrics and ropes.
Mr. N: The next polymers category in our list is “Polyesters”. Polymers: It should not be difficult for the students to guess that Polyesters would be made of repeating “ester” bond units. Esters are compounds that form when compounds with alcoholic and carboxylic groups react with each other. The polyesters would be polycondensation products of dicarboxylic acids and diols. Dacron or terylene is the best-known example of polyester. It is manufactured by heating a mixture of ethylene glycol and terephthalic acid at 420 to 460 K in the presence of a zinc acetate antimony trioxide catalyst. Dacron fibre (terylene) is crease-resistant and is used in blending with cotton and wool fibres and also as glass reinforcing materials in safety helmets, etc
Mr. N: Interesting. The next Polymer is Bakelite which is also called as Phenol-Formaldehyde resin. Phenol-formaldehyde polymers are the oldest synthetic polymers. It is clear from the category itself ke they are made from Phenol and Formaldehyde. It is to be noted that acid or a base catalyst is used in the manufacturing process. The reaction starts with the initial formation of o-and/or p-hydroxymethyl phenol derivatives, which further react with phenol to form compounds having rings joined to each other through –CH2 groups. The initial product could be linear – Novolac used in paints. Novolac, on heating with formaldehyde, undergoes cross-linking to form an infusible solid mass called bakelite. It is used for making combs, phonograph records, electrical switches and handles for various utensils.
Image details: Bakelite was widely used in the production of phonograph records.
Mr. N: The next Polymer in the category is melamine.
Polymer: I hope you remember the first few lines of this interview.
Mr. N: Of course. I have been dying to know what chemical the milk powder contained in that Chinese baby powder scandal.
Polymer: You just shared the name of the chemical.
Mr. N: You mean that villain in the Chinese milk powder scandal was “Melamine”? Polymer: Of course, the chemicals are innocent. It is the people who misuse them for their gains. They are the villains here and not the chemicals. The farmers, in this case, added melamine to fool the quality inspectors. Let me share the details of that story once again. This is the story of a scandal that permanently shattered the trust of Chinese people in the baby foods manufactured in their own country. More than ten years have passed since the scandal came to light, and people in China still find it hard to trust state-owned food suppliers.
Mr. N: Sure. The readers are listening too. Please go ahead. On 16 July, Sixteen babies in Gansu province were diagnosed with Kidney Stones. There were some 54,000 newborns who were hospitalised later, and three of them died. Doctors like Zhang Wie were one of the first to treat these babies. Most of these babies came up with severe pain linked to kidney stones. These stones were supposed to be rare at such an age. The rising frequency of such cases pointed towards something significant and alarming. The Doctors successfully linked those cases of kidney stones to the contaminated baby formula supplied by the local Chinese companies. The brave doctors complained to the local health authorities. New Zealand’s Fonterra, one of the international business partners with the local baby formula suppliers, brought the issue to the national health authorities. This resulted in an intense investigation which found some 22 companies involved in the scandal. China’s Sanlu and Yili turned out to be the primary culprits of the entire issue. The hospitalised babies were fed infant formula produced by the Sanlu group based on Shijiazhuang province in China. Sanlu was one of China’s leading infant formula producers and partners with New Zealand’s Fonterra.
Mr. N: That was very sad. But what has this to do with melamine?
Image details: This is the monomer of the “Melamine” polymer.
Mr. N: This is something unfortunate. I, however, wonder why someone would add melamine to baby food formula. Was it a deliberate move or an accident?
Polymer: This was a scandal and not an accident. Melamine was added to baby food formula with malicious intent. You see, the most critical parameter to check milk quality is by measuring its protein content. The proteins are made up of amino acids, and amino acids contain Nitrogen. In short, by measuring the nitrogen content of milk, you are directly estimating the protein content of the milk. The other components of milk, like lactose, do not contain Nitrogen. Sanlu found that the local farmers were adding melamine to their milk to boost the apparent protein levels. This helped them to pass the nutritional testing. The level of melamine in the powder was found to be as high as 2,560 mg/kg, whereas the tolerable daily intake as per USFDA for the chemical compound is 0.63 mg/kg of body weight.
Mr. N: Ohh. Since melamine also contains Nitrogen, it could be added to milk to fool the authorities. Low-quality milk with lots of melamine would pass the quality test.
Polymer: Exactly, Sherlock. Melamine was added to fool the Milk testing authorities. Let me quote wikipedia again. Of an “estimated 300,000 victims, 6 babies died from kidney stones, and other kidney damages and an estimated 54,000 babies were hospitalised.” The statement below captures the sentiment of victims suffering to date because of the Chinese baby formula scandal.
“Why did you bring me into the world to suffer?”
Some teenagers still have to pay the price for what happened to them as infants. Many must undergo regular bouts of dialysis to keep their kidneys alive. Many among them feel like not living anymore due to the pain the tragedy has accrued on them so far
Image details: The city of Shanghai remains one of the modern symbols of China’s growth story in recent times. It is believed that the strong demand for milk in prosperous China contributed to this melamine tragedy.
Mr. N: What about the culprits? Were they punished?
Polymer: Of course, they got the deserving punishment. The chairwoman of Sanlu got a life sentence. She was held responsible for failing to stop the production and distribution of contaminated milk. Some other executives of Sanlu got prison sentences from 5 to 15 years. A dairy farmer in late 2009 got capital punishment for deliberately contaminating the milk supply with the deadly melamine.
Sanlu Group executives Tian Wenhua, Wang Yuliang, Hang Zhiqi and Wu Jusheng, wearing yellow vests, stand trial on 31 December 2008. Image credit: Xinhua/AP Photo by Ding Lixin – daylife.com Wikipedia link: https://en.wikipedia.org/wiki/Sanlu_Group#/media/File:Sanlu_show_trial.jpg
Mr. N: Well, I am happy they got the punishment they deserved. I feel sad for those babies. We need stringent laws regarding the quality control of food and related products. You never know who would trade their conscience for millions of dollars. “You never know who would trade their conscience for millions of dollars.”
Polymer: Indeed. Now let us complete the textual part of melamine. Melamine formaldehyde polymer forms from the condensation polymerisation of melamine and formaldehyde. The unbreakable crockery is made from melamine.
Mr. N: Cool. The following polymer we have is a copolymer.
Polymer: Ok. Copolymers are something I already explained. They are formed from the combinations of two different molecules. Unlike “Homopolymers” made from a single repeating molecule, the repeating unit of copolymers is made from two other molecules. The copolymer can be made not only by chain growth polymerisation but by step-growth polymerisation also. It contains multiple units of each monomer used in the same polymeric chain. For example, a mixture of 1, 3 – butadiene and styrene can form a copolymer. Copolymers have properties quite different from homopolymers. For example, the butadiene-styrene copolymer is quite tough and is a good substitute for Natural Rubber. This polymer is used in manufacturing auto tyres, floor tiles, footwear components, cable insulation, etc.
Mr. N: Let us talk about the rubber in detail.
Polymer: Rubber, unlike money, grows on trees. When cut within their branches, Rubber trees release milky white latex. This latex is a white, sticky, milky colloidal sol in an aqueous medium. This latex is further processed with chemicals to get the rubber we use in different forms. Rubber, as you know, belongs to elastomers. Elastomers are stretchable polymers with weak intermolecular forces. I suggest you watch this video on natural rubber and its processing. Amazing Asia Natural Rubber Farm – Rubber Harvesting and Processing This latex is obtained from the bark of rubber trees. You can find rubber trees in India, Srilanka, Indonesia, Malaysia and South America. Natural Rubber may be considered a linear polymer of isoprene (2-methyl-1, 3-butadiene), called cis – 1, 4 – polyisoprene.
The cis-polyisoprene molecule consists of various chains held together by weak van der Waals interactions and helical structures. Thus, it can be stretched like a spring and exhibits elastic properties.
Mr. N: What about the vulcanised rubber?
Polymer: have you heard of this company called “Goodyear tyres”.
Mr. N: Yes, I have seen its ads on TV. Polymer: Goodyear comes from Charles Goodyear, who invented the vulcanisation process and gave us synthetic rubber. It was Charles Goodyear who invented the process of vulcanisation. Adding sulphur to natural rubber results in all the properties that are so useful to us. However, quantity is the key here. An extra amount of sulphur could completely change the properties of the rubber.
Mr. N: Interesting. Polymer: Let me dive a bit deeper into the topic of vulcanisation. Natural Rubber becomes soft at high temperatures (>335 K) and brittle at low temperatures (<283 K) and shows high water absorption capacity. Natural rubber is not resistant to oxidising agents’ attack and is soluble in non-polar solvents. Vulcanisation improves upon these physical properties of natural rubber. A mixture of raw rubber, sulphur, and an appropriate additive is heated throughout the process at a temperature between 373 K and 415 K. Sulphur atoms create cross-links at the reactive regions of double bonds during vulcanisation, which stiffens the rubber. The most suitable concentration of sulphur that improves the property of rubber is 5%.
Mr. N: What about the synthetic rubbers? And how different or similar are they to the vulcanised rubber? Polymer: Synthetic rubber is any vulcanised rubber-like polymer that can stretch to twice its length. However, when the external stretching force is released, it returns to its original shape and size. Thus, synthetic rubbers are either homopolymers of 1, 3 – butadiene derivatives or copolymers of 1, 3 – butadiene or its derivatives with another unsaturated monomer. Neoprene and Buta-N are some examples of Synthetic Rubber. Neoprene: Neoprene or polychloroprene forms due to the free radical polymerisation of chloroprene. It has superior resistance to vegetable and mineral oils. Conveyor belts, gaskets and hoses are made from neoprene. Buna – N You have already studied Buna-S in Section 15.1.3. Buna –N is obtained by the copolymerisation of 1, 3 – butadiene and acrylonitrile in the presence of a peroxide catalyst It is resistant to the action of petrol, lubricating oil and organic solvents. Therefore, one of the significant applications of this polymer is to manufacture oil seals and tank lining.
Mr. N: I have an essential question to ask.
Polymer: Sure. Please go ahead.
Mr. N: As we have discussed, the milky latex secreted by the trees is white. Since the raw rubber is white, why are the tyres primarily black?
Polymer: Interesting question, for sure. The tyres were not always black. However, the chemical that gives them black colour adds some properties to highly desirable tyres.
Mr. N: What material are we talking about here?
Polymer: The chemical responsible for the black colour of the tyre is “Carbon black”. The carbon black protects the tyre from the harmful effects of U.V radiation and Ozone. Carbon black also prevents the excessive heating of tyres. The black-coloured chemical also increases the overall life of tyres by reducing wear and tear. In short, the colour of the tyres is black due to the addition of carbon black.
Mr. N: Could you tell us something about the Molecular mass of the Polymers?
Polymer: At the beginning of this interview, I discussed how large molecular masses of Polymers are determined using methods like Raoult’s law. Osmotic pressure made even the best minds in Chemistry scratch their head. Only Hermann Staudinger came up with the first viable polymer model after extensive research on rubber. For the rest of the other details, you could read Wikipedia too. The following are the exact lines from NCERT. Polymer properties are closely related to their molecular mass, size and structure. The polymer chain’s growth during synthesis depends upon the monomers’ availability in the reaction mixture. Thus, the polymer sample contains chains of varying lengths; its molecular mass is always expressed as an average. Chemical and physical methods can determine the molecular mass of polymers.
Mr. N: Cool. We have finally come to our last topic of Biodegradable polymers. They are different from conventional polymers that are chemically and biologically inert. Decomposer and transformer species of bacteria cannot naturally degrade them. I wanted to know the fundamental difference between biodegradable and non-biodegradable polymers that leads to such differences.
Polymer: You see, the bacteria that decompose organic stuff have evolved for billions of years to break specific types of bonds. They can usually decompose lignin, carbohydrates, proteins and lipids. The bacteria can break the bonds present in these natural polymers due to their natural scissors called enzymes. But if you look at plastics, they are a modern phenomenon. The enzymes in the bacteria cannot recognize the bonds of synthetic polymers and hence they cannot degrade them.
Mr. N: Interesting. Very Interesting. Now, what about biodegradable polymers?
Polymer: As far as biodegradable polymers are concerned we have tried to fool the microbes.
Mr. N: Fool the microbes? What do you mean by that?
Polymer: We are trying to mimic the bonds present in natural polymers like cellulose, proteins and others. According to Wikipedia ester, amide, or ether linkages are frequently found in biodegradable polymers. Based on their production and structure, biodegradable polymers can be divided into two main categories. Agropolymers, or those made from biomass, are one of these categories. The second category is made up of biopolyesters, which are created artificially from naturally occurring or synthetic monomers or from microorganisms. The most commonly made biodegradable polymers are aliphatic polyesters. Aliphatic polyesters are one of the critical classes of biodegradable polymers. Some noteworthy examples are given below: 1. Poly β-hydroxybutyrate – co-β-hydroxy valerate (PHBV) It is obtained by the copolymerisation of 3-hydroxybutyric acid and 3 – hydroxypentanoic acid. PHBV is used in speciality packaging, orthopaedic devices, and the controlled release of drugs. PHBV undergoes bacterial degradation in the environment 2. Nylon 2–Nylon 6 It is an alternating polyamide copolymer of glycine and aminocaproic acid and is biodegradable.
Mr. N: Hello mate, before we say bye won’t you answer those questions about polymer trade linked to Aurangzeb?
Polymer: Sure. Aurangzeb made a fortune by trading silk, linen, cotton and other textile products from India. Of course, the spices were involved too. The polymers here were the natural ones used in the textile industry, mainly Silk. There was a time when 95% of the textile in Britain was imported from India. There are people who see problems and there are ones who see solutions. I would like to share a story of a group that is trying to make use of plastic waste to make life better. A group of innovative folks have started a revolution to enlighten the life of people who have been living in darkness using plastic bottles. How plastic waste is lighting the life of people in darkness:
Mr. N: Interesting. Very Interesting. Now, what about biodegradable polymers?
Polymer: As far as biodegradable polymers are concerned we have tried to fool the microbes.
Mr. N: Fool the microbes? What do you mean by that?
Polymer: We are trying to mimic the bonds present in natural polymers like cellulose, proteins and others. According to Wikipedia ester, amide, or ether linkages are frequently found in biodegradable polymers. Based on their production and structure, biodegradable polymers can be divided into two main categories. Agropolymers, or those made from biomass, are one of these categories. The second category is made up of biopolyesters, which are created artificially from naturally occurring or synthetic monomers or from microorganisms. The most commonly made biodegradable polymers are aliphatic polyesters. Aliphatic polyesters are one of the critical classes of biodegradable polymers. Some noteworthy examples are given below: 1. Poly β-hydroxybutyrate – co-β-hydroxy valerate (PHBV) It is obtained by the copolymerisation of 3-hydroxybutyric acid and 3 – hydroxypentanoic acid. PHBV is used in speciality packaging, orthopaedic devices, and the controlled release of drugs. PHBV undergoes bacterial degradation in the environment 2. Nylon 2–Nylon 6 It is an alternating polyamide copolymer of glycine and aminocaproic acid and is biodegradable.
I cannot thank you enough for reading this article. It was written for you so that you could feel the awe of science. I would love to know about the things you liked from this piece.
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Further reading: Einstein’s other important works including relativity are discussed in detail in this book by John Gribbin:
https://www.simonandschuster.com/books/Einsteins-Masterwork/John-Gribbin/97 81681775289
The 2008 Chinese melamine scandal:
https://qz.com/why-rich-indians-cant-get-rid-of-hazardous-plastic 1849697887
Du Pont Teflon cover-up:
https://www.nytimes.com/2016/01/10/magazine/the-lawyer-who-became-duponts -worst-nightmare.html
Cellulose digestion by the cattles:
https://www.nature.com/articles/ismej20132
Why are tyres black?
http://www.hugtheroads.com/why-are-tyres-black/
Solar paints and quantum dot technology:
https://observer.com/2017/06/solar-paint-is-the-next-new-source-of-sustainable-energy/
Hermann Staudinger: The father of macromolecular chemistry
https://www.acs.org/content/acs/en/education/whatischemistry/landmarks/staudingerpolymerscience.html
Biodegradable polymers: