My Science School

          The hard outer shells of crabs and lobsters are made of a polymer called chitin. Chitin is a polysaccharide which is hard and insoluble, but somehow flexible. It is found in the shells of shrimps and other crustaceans as well.

          Have you observed the shells of crabs? Does it get wet? Don’t be surprised to learn that chitin is waterproof. It is non-toxic, biodegradable and structurally similar to cellulose. However, it is a major source of surface pollution in coastal areas.

           Chitin is a promising biomaterial and a considerable amount of research about chitin is being conducted worldwide now. Researchers have tried hard to synthesize synthetic polymers with the same properties as chitin. Thanks to their efforts, it is commercially produced in some parts of the world today.

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          The aquatic world is a vast repository of natural polymers. Aquatic mammals have several proteins in them. Whales are no exception. The teeth of a whale are rich in baleen, a natural polymer.

          Baleen is made up of keratin, a type of protein. It is strong, durable and flexible. Keratin is generally of two types - hard and soft. Hard keratin is usually found in human fingernails, hair and claws of birds. Soft keratin is found in whale bone. Another prominent polymer obtained from a whale is leather. Leather is processed from its skin and has a variety of applications.

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           Starch and cellulose are very different from each other, but they share a striking similarity. Both are made of glucose monomers and hence called polysaccharides. Though made of glucose, both are bonded in different ways which give them different properties.

         Starch dissolves in water while cellulose doesn’t. Cotton is made up of cellulose; it has a crystalline structure and stretches out. Just imagine what would happen to our cotton clothes if cellulose could be dissolved in water? It would disappear in the very first wash! Cellulose and its derivatives are widely used to make clothing and other things like paper, while starch is used for food.

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          Yes, there are polymers that can be eaten. In fact, we consume a lot of polymers daily without even thinking about it. Don’t be surprised. Think of a chicken burger. Chicken contains a lot of protein and the bun contains starch. Both protein and starch are polymers.

          Proteins make up a major chunk of our body; it constitutes organs, muscles and fingernails. Milk, eggs, meat, seeds and nuts are protein rich foods. Starch is a polysaccharide with high molecular weight. Our body breaks down starch into glucose which gives us energy to work and play. In short, polymers give us energy to do everything. Foods like corn, potatoes and bread are rich in starch.

          Cellulose is also present in vegetables and grains, but we cannot digest it.

          You won’t be wrong if you say that life on Earth won’t be possible without polymers. Wonder why? Look at what our body is made of. Our body contains proteins, peptides and enzymes. These wouldn’t be possible if not for RNA, which is a polymer. RNA makes possible the synthesis of proteins and enzymes. Our hair is made of keratin, a protein and skin is constituted of polymerized collagen. DNA contains the basic programming of our body. DNA too is a polymer.

          Polymers make up not only the human body, but also plants and animals. A rhino’s horn is made of keratin while cell walls of plants are made of cellulose. The list does not end here. Several other polymers like chitin are found in organisms like crabs. Let us now take a close look at different natured polymers.

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           Biodegradable polymers are decomposed by microorganisms like bacteria. While decomposing, they produce natural by-products like gases, water and inorganic salts. Wood is a natural biodegradable polymer.

          Are natural polymers the only biodegradable ones? Not really. Some synthetic polymers are also biodegradable and they do not have any adverse effect on the environment. Remember vitamin E capsules? The shell of the capsule is made of a biodegradable polymer. Surgical bands and sutures also make use of biodegradable polymers. Polylactic acid used for packaging is yet another example. 

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          Think of hospital equipment that is used for processes like sterilization. Any idea about the kind of polymers used in them as they need to withstand high temperature?

          Some polymers are stable at very high temperatures. These polymers have strong force of attraction between the molecules. They also have special network structure. Some additives are added to some of these polymers which enhance their thermal resistance.

          These polymers are usually known as high temperature polymers and can withstand a service temperature greater than 150 degrees. They can even withstand temperatures higher than 250 degrees for a short span. They are very expensive and hence are used in small amounts.

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          Polymers can be arranged in two formations. If the molecules do not have a long-range order, the polymer chain has an amorphous arrangement. Amorphous polymers are generally transparent.

          Contact lenses, plastic windows and transparent food wraps are generally made of amorphous polymers.

          What about polymers those are not transparent? Translucent and opaque polymers have crystalline arrangement of molecules. Here, the atoms and ions are arranged in definite patterns. Generally, the amount of light that can pass through a polymer decreases with increasing amounts of crystallinity. So, the opaqueness of a polymer is an indicator of the degree of crystallinity.

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                    Think of the human body. What is it that gives us our shape? The skeleton of course. Without the skeleton, we would be a lump of flesh. Similarly all polymers have a skeleton which gives them their structure. Organic polymers have a carbon backbone. Their monomers are simple organic compounds.

           On the other hand, the backbone of an inorganic polymer is constituted by elements other than carbon, say phosphorous or sulphur. Most of the organic polymers have simple structures while inorganic polymers are highly branched with complex structures.

           Though they have carbon bonding, graphite and diamond are inorganic polymers. This is because they are constituted by carbon atoms alone which crosslink to give their structure.

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           Nylon is a synthetic fibre which is commonly used in the textile industry. The word fibre will give you an image of interlinked threads. Synthetic fibres are man-made fibres that are derived from several chemical processes. They can be easily woven because of their thread-like nature. The force of attraction between the monomers of a fibre is very strong; the monomers crosslink to form a strong hydrogen bond.

          Nylon is also used for making fishing nets, ropes and parachutes because it is strong and long lasting. Like nylon, polyester and rayon too are synthetic fibres. They are non-toxic substances and can be dyed.

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          Unlike thermoplastics, thermosetting plastics cannot be remoulded or reshaped. It assumes a permanent hard shape once heated.

          Initially, this is a liquid or a soft solid. Chemical reactions take place when it is heated and the long polymer chains crosslink to form a three dimensional structure. You might have bright coloured melamine dinnerware at home.  Melamine is a thermosetting plastic. Bakelite, which is used for insulating electric wires, also belongs to this category.

          Thermoplastics can be remelted quickly, but thermosetting plastics can withstand high temperatures. It cannot be recycled for the same reason.

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           Do you know how a plastic bottle is made? At first, the raw material used for making the bottle is heated and blown. It is then passed through another mould which gives it shape.

          The production of a plastic bottle is possible because the raw material can be moulded into any shape at a certain temperature. It then solidifies after cooling. Polymers of this kind are known as thermoplastics. Thermoplastics soften up on heating and harden when they are allowed to cool down They can be melted and recast into any possible shape. PET bottles and toys are has usually made of thermoplastics.

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          You might have tried stretching a rubber band or squeezed a rubber toy. Have you noticed that they regain their original shape once they are released? This is possible because they are elastomers.

          Elastomers are rubber-like solid polymers with elastic properties. That is, they can be easily stretched and can regain their original shape no matter how much you squeeze or stretch them. The force of attraction between the molecules of an elastomer is weak. Remember vulcanized rubber that we discussed earlier? That too is an elastomer. Vulcanized rubber has a cross linked chain which is helps it remain its original form.

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