Science Makes Sense- Week 23: Polymers, plastics.

What did ‘Soul Train’ and ‘ The Brady Bunch’ have in common? All the participants/actors wore polyester pants and shirts that were very popular then! The polyester came in bright shades and stayed the rage for some time in the U.S. But in India, polyester saris are available even today and is the preferred choice especially for working class women since they do not have to iron or starch their clothes every time they wear them.
Polyester is a polymer, an organic molecule made of many repeating units. A polymer can be three, two or one-dimensional. Each repeating unit is the ‘mer’ or basic unit, while ‘poly’ means the repeating unit. The units are often made of carbon,hydrogen (organic compounds) and also oxygen, sulfur, some halogens, nitrogen or silicone.(Ref. 1).
Wool, cotton, silk, wood and leather are examples of natural polymers used from ancient times. This includes bio-polymers such as proteins and carbohydrates:constituents of all living matter. Synthetic polymers (which we shall mostly look at today) generally known as plastics, became significant in the 20th century. Chemists were able to engineer them to yield different properties including strength, stiffness and heat resistance. Plastics have totally changed our way of life today.(Ref.2)
Week 7 we looked at alkenes and the first alkene was ethene or more commonly known as ethylene. This ethylene monomer combines with several ethylene molecules to form polyethylene or polythene which is what plastic bags are made of. (Ref.2) Vinyl chloride is mono-chloro-ethene where one of the hydrogen atoms is replaced by a chlorine atom. This monomer forms a polymer called Poly Vinyl Chloride which is so essential in making PVC pipes.(Ref.2)
Polyester is nothing but a polymer of an ester.(Ref.3) An ester and water are formed when an organic acid reacts with an alcohol; the reaction is similar to an inorganic acid reacting with an inorganic base to form salt and water.)
Polymers are mixtures because their molecular weights cover a range of values. Shapes of these polymer molecules are not straight chains of substituted carbon atoms. Free rotation around the C-C bonds allow long polymer molecules to curl up and tangle like spaghetti. With all these entanglements, there are regions in the polymer that could be crystalline and some that are amorphous (non-crystalline). Shorter chain lengths, less branching could lead to ordered layers; hydrogen bonding between adjacent layers also helps. (Ref.2)
We have discussed only a few polymers here, more will be mentioned under “Nuggets of Information”. Polymers have definitely revolutionized our lives in the 20th and 21st centuries. We have replaced metal and animal/plant product usage more and more with polymers. However, Week 24 we will be discussing the important question: ” Are plastics/ polymers a boon or a bane for society?”

Activities for Middle School Teachers
Let students make models of different kinds of polymers using paper clips or toothpicks with marshmallows as the monomer. Make straight chain or branched chains, even connect them at different points. Also have two or three different sized marshmallows to indicate a polymer consisting of more than one single monomer.
The students could carry out an experiment to create polymerization in the laboratory. Using Elmer’s Glue and sodium borate solution, create a plastic. Study its properties.(Ref.4)
Also study isomerism, number of isomers possible, depending on the chain length of a constructed polymer using the models indicated above.

Nuggets of Information:
The first synthetic polymer was made in 1869 as a substitute for ivory. Billiard balls had been made by ivory till then and the high demand had put a strain in obtaining it from hunting and slaughtering elephants. The problem was that it became unstable at higher temperatures. Polystyrene was the next important polymer from the 1920s used as a substitute for natural rubber as well as making toys; its one disadvantage was that it was brittle.(Ref.5)
One needs to remember that the billiard balls made not using ivory were really manufactured by modifying natural materials like cellulose and camphor. The first truly synthetic polymer was made when Bakelite was created using phenol and formaldehyde. It was invented to replace a scarce natural substance shellac that was used as an electric insulator. (Ref.6)
Thermoplastics and thermosets are two kinds of polymers. Thermoplastics are polymers that melt at a certain high enough temperature. These plastics can be injected into molds to form various shapes or extruded (drawn out) into sheets or fibers. Thermosets are polymers that are highly cross-linked, do not melt at all. It is far more complicated to form molds. Most polymers are thermoplastics, only 20% are thermosets.(Ref.2)
Thermoplastics can be further divided into homoplymers and heteropolymers. When the monomer units are the same, one forms a homo-polymer, but when the monomer units are different hetero-polymers or co-polymers are formed. Polyethylene is an example of a homo-polymer and nylon is an example of a hetero-polymer.(Ref.2)
Thermoplastic polymer structure can vary; one can have branched-chain or straight -chain homo-polymers/heteropolymers. The monomers could be joined end-to-end or cross-linked to form a harder material. If the cross links are fairly long and flexible, adjacent chains can move with respect to each other forming elastomers.(Ref.2)
In a linear polymer like polyethylene, rotations around the C-C bonds allows the chains to bend or curl up in different ways. But if one of the H atoms is replaced, say, by a halogen or a methyl group, the orientations of the monomers become significant. Now add the fact that there is a C=C bond, a double bond in the carbon chain, allowing diastereomers like cis and trans configurations. This small change can lead to profound effects in the properties of the polymer. Natural rubber is mostly cis-polyisoprene, while the trans form known as gutta percha latex has very different and inferior properties.(Ref.2)
Some objects exhibit the property of “handedness”, which means that object and mirror image are similar but not identical, like our left and right hands. This behavior is called chirality. Chiral molecules have two forms which are also called enantiomers. Polymers can exhibit chirality, depending on the arrangement of the differing groups attached. When the monomer units are aligned so that group A is attached on one side and group B is attached on the other side, the polymer is an isotactic polymer. When you have alternating groups A and B on either side it is called a syndiotactic polymer. However, when you have different groups to form the polymer, usually the placement of the two groups is random and it will be an atactic polymer. (Ref.7)
Finally, we look at 4 bio polymers:
1.Hydrocarbons/lipids, homopolymers with hydrocarbon monomer units
2.Polysaccharides,homo or hetero-polymers with sugar monomer units.
3.Proteins, hetero-polymers with amino acid monomer units
4.Polynucleotides,hetero-polymers with nucleotide monomer units. (Ref.8)

References:

1.plastics.americanchemistry.com
2.chem.com/acad/webtext/states/polymers.html
3.britannica.com/science/polyester
4.matse1.matse.illinois.edu
5.courses.sens.buffalo.edu
6.Freinkel,Susan Plastic: A Toxic Love Story(The Text Publishing Company,2011)
7.web.mit-edu/10.491-md/www/CourseNotes/Polymer/Chiral
8.brooklyn.cuny.edu/bc/ahp/SDPS/SD.PS.polymers.html

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