Archive for February, 2016

Science Makes Sense-Week 26: Iron and Steel, alloys.

February 28, 2016

My mother who was born in the early part of the 20th century, was innovative and forward-thinking in many ways. She was the first among her sisters to discard bronze and lead vessels for cooking and start using stainless steel containers on a regular basis. The kitchen used to sparkle with those bright silver-like containers gleaming in the sunshine! I must say my mother loved those new vessels for cooking and storing food. We called them ever-silver, small wonder.
As we already know, iron is a transition metal and we have studied a lot of the characteristics of iron earlier. We cannot forget about the alloys of iron that include the different forms of steel which have revolutionized our lives both in the kitchen and outside. An alloy is basically a mixture of one or more elements added to the main element and melted together. The added metal or metals arrange themselves in between the rows of the main metal.
Stainless steel is an alloy of iron and mainly chromium. Iron alone is prone to oxidation and forms the reddish-brown iron oxide , familiarly known as ‘rust’. When carbon is added, you get stronger iron to use industrially as well as to make cast iron pots and pans. Stainless steel is corrosion resistant and the chromium added gives it a shine.(Ref.1)
Pure iron, like most metals is not particularly strong; the atoms of iron are too free to move around. The addition of other elements allows some of the atoms of iron to be pinned down, but too much addition might make the iron too brittle, too rigid. The trick is to add just the right amount to create a stronger metal. (Ref.2)
The fascinating fact is that this knowledge of forming mixtures with other metals, called alloys has been known to mankind for the longest time. One of the earliest steels made was by Indian metalworkers and it was called ‘wootz”(Ref.2) However, these alloys were made in small quantities and only in the 18th century did we perfect the art of creating alloys on a larger scale.(Ref.2)
Wrought iron was first made in the 18th century using a method called “puddling”: this involved the heating of iron ore and refined iron until impurities like Sulfur, S, and silicon, Si, formed a slag with Oxygen, O. (Wrought iron is a black metal that is used for railings.) After removal of most of the slag by hand, the temperature was raised to let the carbon react with the oxygen and burn. The remaining slag sits among the pure grains of iron to form a material that is stronger and more flexible than the original iron. (Ref.2)
Here are some improvements in the manufacture of alloys of iron in the 20th and 21st centuries:
Basic oxygen process (BOP): The steel is made in a giant egg-shaped container, open at the top, called a basic oxygen furnace. This is like an ordinary blast furnace, only it can rotate to one side to pour off the finished metal. The air draft used in a blast furnace is replaced with an injection of pure oxygen through a pipe called a lance. This furnace is based on the Bessemer process developed by Sir Henry Bessemer in the 1850s. (Ref.3)
Open-hearth process (also called the regenerative open hearth): Reminds one of a giant fireplace in which pig iron, scrap steel, and iron ore are burned with limestone (calcium carbonate) until they fuse together. More pig iron is added, the unwanted carbon combines with oxygen to form the slag that is removed and the iron turns to molten steel. The steel is sampled and the process is continued until the iron has the right carbon content to make the type of steel needed.
Electric-furnace process: The electric furnace, uses electric arcs (effectively giant sparks) to melt pig iron or scrap steel. Since they’re much more controllable, electric furnaces are generally used to make higher-specification alloy, carbon, and tool steels. (Ref.3)
The alloys of iron have become an essential part of our lives; we are beholden to this element,iron for thousands of years. In fact, there is more to talk about this interesting ‘d’ shell element, especially about its magnetic properties. Next time we will delve into this very exciting property of iron.
Activities for Middle School Teachers
Let students compare and contrast the periods of the Bronze Age and the Iron Age. What were the tools created out of bronze and iron mainly used for? What was happening in the world at that period? Consult with Social Studies teachers for this activity.
Nuggets of Information:
The word ‘iron’ comes from a word that means ‘metal from the sky’. Early iron was extracted mostly from meteorites.(Ref.1)
“Puddling” was very popular in the 18th and 19th centuries; one of the famous wrought iron structures built during this period is the Eiffel Tower in Paris, France. (Ref.2)
“Pig iron” is„ raw iron, the immediate product of smelting(a metallurgical process by which any metal is extracted from its ore and heated it to a high enough temperature to melt it(Ref.4)) iron ore with carbon(coke) and calcium carbonate(limestone). It has a high carbon content, around 4-5% and makes it very brittle. (Ref.5)
There are basically two kinds of alloys formed when metals are combined together at high temperatures. Sometimes, the size of the guest metal atom is almost the same as that of the host metal atom and these atoms take the place of the host atom. These alloys are called substitutional alloys like brass where zinc atoms substitute for some copper atoms in the lattice structures.
Then there are interstitial alloys like steel alloys where the manganese, chromium or carbon atoms place themselves in between the spaces around the host atoms, which in this case are iron atoms.(Ref.6)
Carbon steel with the lowest carbon content is typically called wrought iron. The metal is hard, but not brittle and is used for fences, chain links, gates and railings. The low carbon content allows this alloy to be worked into different shapes. The most commonly used carbon steel has a medium carbon content; uses of carbon steel in this category include structural steel to build buildings and bridges. It is also used for making automobile parts and in shipbuilding. High-carbon steel is hard but brittle and less easily worked. This type of carbon steel is used to create springs and high-strength wires. The increased hardness makes this category of steel ideal for cutting tools, punches, dies and industrial knives.
Finally, carbon steel with ultra-high carbon content is commonly referred to as cast iron. This type of cast iron is very hard but highly brittle. It has little to no malleability and cannot be easily welded or tooled. Often, it is used for cast iron pots, hot water radiators and certain types of lamp posts. Industrially, this steel is used for castings.(Ref.7)

References:
1.http://www.chemistryexplained.com/St-Te/Steel.html
2.Trefill, James, A Scientist in the City, p.38 (Doubleday,1994)
3.http://www.explainthatstuff.com/ironsteel.html
4.http://www.thefreedictionary.com/Smelting
5.http://chem.tamu.edu/class/majors/chem470/Steel.pdf
6.http://www.scienceiscool.org/solids/metallic.html
7.http://www.wisegeek.com/what-are-the-uses-of-carbon-steel.htm

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Science Makes Sense-Week 25: Lead and Tin

February 22, 2016

The recently revealed shocking story of lead-poisoned water for the residents of Flint, Michigan for almost a year (Refs.1,2) brought back memories of lead paint poisoning of children living in poor, older inner city neighborhoods.(Ref.3) The tragedy is that this old story of lead paint has resurfaced in many African-American communities in Chicago once again.(Ref.4) Both situations could have been avoided since there is ample evidence to point out that even low level continuous ingestion of lead causes damage in hearing, learning abilities and coordination.(Ref.5)
Why has lead been used from the time of the Greek-Roman civilizations in spite of knowledge regarding its toxicity?(See Nuggets) This is because lead has a high corrosion resistance, it is soft, easy to work with and has a low melting point.(Ref.6) Furthermore, it has been durable for many uses, especially till recent times for lead-based paints and lead pipes.(Ref.5)
The word ‘plumbing’ derived from ‘Plumbum’, gave rise to the chemical symbol Pb for lead since lead was used a lot in the field of plumbing.(Ref.3)
This shiny blue-white soft metal,lead, when exposed to the air containing oxygen and carbon dioxide, becomes coated with a dull grey layer of basic carbonate that adheres to the surface and protects it from further change. A similar kind of protection takes place in the presence of sulfuric acid: it forms a layer of lead sulfate. Because of this property, lead is often used to sheathe cables, to carry tanks of sulfuric acid and to protect roofs from the atmosphere.(Ref.6)
In recent times, it is mainly used in lead-acid storage batteries.
The electron configuration of Pb is 6s2 6p2 which means there are 2 electrons each in the 6s and 6p level respectively. Lead exhibits the oxidation state +2, when it loses the ‘p’ electrons and is generally basic. However, the oxidation state +4 also exists and in it, lead is more acidic. This property where a metal exhibits both acidic and basic characteristics like another metal, aluminium, Al, is called amphoterism. Tin also exhibits this behavior in the +4 oxidation state.(Ref.6)
Lead exists as many oxides, carbonate and chromate and shows brilliant colors that were useful for paint pigments till the 20th century when the toxic nature of ingested lead was documented. The reason it was used for so long is because no one imagined that lead paints would be ingested till there was evidence to show that children were licking lead paint off frames in old homes.
The other area where lead was used extensively was in motor fuel as an anti-knock agent called tetraethyl lead. The addition of lead to the fuel reduced engine knock and so was used for years. Here the lead replaced the carbon in methane and each of the hydrogen atoms was replaced by an ethyl group, fooling the motor fuels into letting it dissolve. In the cylinder, the heat would knock the ethyl radicals off and form a cloud of PbO, lead oxide. The ethyl radicals stopped any explosions and let the fuel charge not detonate and burn smoothly. This permitted the use of less expensive straight-chain hydrocarbons as fuel versus the more expensive branched and aromatic hydrocarbons of higher octane rating.(Higher octane rating improves engine efficiency.) The lead in the air was found to be especially harmful for younger children and so leaded fuel was finally discontinued. It is important to note that most adults working with lead are not unduly affected by it, unless ingested continuously. Its widespread use in paints and in fuels made it dangerous, since children and growing young adults are particularly vulnerable to it; yet lead is not as toxic as mercury.(Ref.6)
Tin is another metal similar to lead in some of its properties, and in the same Group in the Periodic Table. Known with the chemical symbol Sn, today we mainly think it is used with other metals in alloys as well as coating other metals.
Known from the Bronze Age where tin and copper formed the very useful alloy bronze, tin like lead has an ancient history. Just like lead it has a low melting point, but its boiling point is very high. This large range where it is a solid helps in forming alloys without loss in vaporization. (Ref.7)
Tin also exists in two oxidation states having the same outermost electronic configuration like lead. This leads to its amphoteric behavior just like lead. Tin forms chlorides and stannous chloride is a reducing agent. Metallic tin is safe and is not toxic when ingested like lead. (Ref.7)
Organo-tin compounds were used as a biocide, but because of their toxicity, these compounds have been phased out. (Ref.8)
Tin and lead have much in common, both known from ancient times, both are amphoteric, low melting point solids. Lead has been less expensive and even though many of its uses have been phased out, it is still useful in certain specific areas. Tin though is coming back to be used more often than before. (See Nuggets) These two metals cannot be dismissed that easily!
Activities for Middle School Teachers:
What are the other metals in this group besides tin and lead? What are their properties and uses? How are they similar and different in their properties and uses?
Lead is toxic when ingested; what about mercury? Compare and contrast how both lead and mercury are being used in different ways today compared to earlier because of their toxicity.
Nuggets of Information:
The Roman architect, Vitruvius Pollio during the first century BC recognized the dangers of using lead.(Ref.3)
In the mid 19th century lead poisoning was called’painter’s colic’.(Ref.3)
The slightly sweet taste of lead made it a good additive in Roman wine and there is a belief that this use of lead could have been one of the causes for the fall of the Roman Empire.(Ref.3)
The relative higher density of lead, compared to copper and iron, has made it suitable in the manufacture of bullets and sinkers.(Ref.6)
Even though many European countries banned the use of interior lead paints in 1909, the US waited till 1971 to do so.(Ref.3)
Tin was also called white lead, being a soft white metal.(Ref.7)
Tin exists in three allotropic forms:Below 18 degrees C, it exists as alpha tin or grey tin with a diamond structure (like carbon, silicon and germanium)and is a semiconductor. From 18 to 161 degrees C, the stable form is beta-tin or white tin with a slightly higher density and a tetragonal crystal structure. Above 161 degrees C to the melting point is gamma-tin with about the same density but rhombic crystal form. When beta-tin is bent it emits a sound called “tin cry’ from the shearing movement between the crystal grains.(Ref.7)
Apart from being used most often in the alloy, bronze, tin is also used in several other alloy forms like pewter(tin and lead), a superconducting wire alloy of tin and niobium,and Babbitt metal which is a combination of tin,copper and antimony. The superconducting wires are used in the manufacture of very powerful magnets and Babbitt metal is used for bearings. Tin plating which is a coating of tin over steel is still used for canned goods and to prevent corrosion (Refs.8,9)
The biggest use of tin is for soldering especially electrical circuits. Molten glass used in windows is floated on tin to get a flat surface; stannous flouride is used in toothpaste and organo-tin products are needed to stabilize PVC plastics.
(Ref.9)
Indonesia and China are the largest producers of tin.(Ref9)

Chicago Tonight, April 2016, had a story about the presence of lead in service lines that could leach and enter homes and become a crisis like the Flint, Michigan water situation.(Ref.10)

References:

1.http://www.theguardian.com/us-news/2016/feb/17/flint-water-crisis-michigan-contaminated-water-bills-food-water-watch-study
2.http://www.democracynow.org/2016/2/17/thirsty_for_democracy_the_poisoning_of_an
3.http://www.toxipedia.org/display/toxipedia/History+of+Lead+Use
4.http://www.chicagotribune.com/news/watchdog/ct-lead-poisoning-chicago-met-20150501-story.html
5.http://www.dhr.virginia.gov/pdf_files/LeadPaint1.PDF
6.http://mysite.du.edu/~jcalvert/phys/lead.htm
7.http://mysite.du.edu/~jcalvert/phys/tin.htm
8.http://humantouchofchemistry.com/history-of-tin.htm
9.http://wanttoknowit.com/uses-of-tin/
10.http://chicagotonight.wttw.com/2016/02/18/chicago-s-lead-pipes-what-you-need-know

Science Makes Sense-Week 24:Chemistry and Social Justice, plastics in the environment.

February 14, 2016

Several years ago, I used to travel by train in India regularly. When we stopped at a station, we would buy tea from vendors who would serve it in little clay pots that we could throw out of the window and watch it break. Now when I come to India and drink tea, there are no clay pots that are biodegradable, only plastic cups that litter the trash and get into water bodies. Milk no longer comes in glass bottles but in plastic bags. Take-out food is no longer given in leaves stitched together and wrapped up with string; it is now given in little plastic bags! In the U.S., the amount of plastic in packaging in addition to replacing glass with plastic everywhere has led to huge amounts of plastic trash getting into lakes, seas and oceans.
We already know that plastic has been man-made using polymers of different monomers to arrive at the varieties of plastic discussed last week. We will look at how humans and other animals are affected by plastic.
Residual Monomer:
Formation of long polymer chains is never perfectly stoichiometric and can be a random process. This can lead to unreacted monomers. Some monomers like formaldehyde, styrene(from take-out containers),vinyl chloride(from PVC),and bis-phenolA(from polycarbonates) are known carcinogens.(Ref.1)
Plasticizers:
Plasticizers are added to polymers to render them more flexible. Many take up space in between the polymer chain and some are small enough to diffuse and cause health problems.(Ref.1)
Endocrine disrupters:
Many of the monomers have been found to be physiologically active. This is because they mimic the action of hormones or other signaling molecules. This is done by probably fitting and binding on to the specialized receptor sites in tissues. The health effects on adults is still not clear,but there is definite concern of its effects on fetuses.(Ref.1)
Decomposition problems:
Most polymers are not biodegradable, particularly in the anaerobic (absence of free oxygen) conditions of landfills. However, decomposition products here could combine with rainwater and contaminate nearby streams and water supplies.
Plastics with fluorochlorcarbons break down into perfluoro octane which damages aquatic animals.(Ref.1)
Effects on living beings:
Perhaps the most devastating effects has been on animals and birds and sea life. Since we know that most of the plastic trash gets into the water bodies, all life on water and near the shore are adversely affected by plastic. Animals and birds consume the plastic as food and die. Many big fish get entangled in big plastic sheets in the sea and get choked. Add to that the health hazards that animals face just like humans.(Ref.1)
Recycling is one solution, but the United States is dragging its feet to get some federal legislation passed to make it a national movement. Most countries in Europe have a viable recycling plan having bins in the neighborhood for all kinds of plastic packaging to be recycled.
When glass was replaced by plastic most of us were relieved at the light, non-brittle nature of plastic, the convenience of using trash bags that did not leak and the lunches that were so neatly packed in little plastic bags. Little did we worry about how it would impact us and other life forms around us. Today, in developing countries like India, plastic trash is everywhere, choking roadsides and canals of water. In countries like the U.S., though plastic trash is not so visible, its adverse effect on human and animal life is a daily reminder that more should be done to look for alternatives. Meanwhile, let us start cleaning up the mess created by this polymer invasion.

Activities for Middle School Teachers:
Let students go to a supermarket and list the number of products where plastic is used. Are all plastic containers the same? How are they classified? What is recyclable? (Look at Nuggets of Information also for some help)
Students should look at their own neighborhoods to see if plastic trash is around. How about in local water bodies?
Take the students to recycling centers and find out how and which kinds of plastic are recycled. Attend special green festivals where plastic is reused in innovative ways.
Teachers and Students! Do you travel for vacations to beaches in, say, Mexico, Thailand, India? When you stay at hotels/ resorts find out what happens to all the plastic trash used by tourists. Are there viable recycling programs in these countries? How are we as US tourists impacting world use of plastic? What are our ethical responsibilities?
Encourage yourself and students to use cloth bags when shopping anywhere; avoid plastic bags as much as possible. Recycle plastic bags at suitable supermarkets. Use glass instead of plastic to store food and spices.

Nuggets of Information:
Each year 8 million tons/16.6 billion pounds/7.2 billion kilograms of plastic enters the ocean. Each day two Empire Buildings (New York)full of plastic washes into the ocean. A 21-year old Dutch scientist named Boyan Slat is getting ready to clean the Pacific Ocean of its plastic trash. (Ref.2)
In India,you are charged for using plastic bags in several places where you shop.
PDFA, a monomer from which Teflon is made, was the subject of a 2004 lawsuit against DuPont when ground water was contaminated. (Ref.1)
Bis phenol A, commonly known as BPA, (actually two benzene rings with hydroxyl groups connected by a hydrocarbon link) has been found mainly in certain kinds of plastics used to make water bottles, baby bottles, sealants, medical devices and sports equipment to name a few. This kind of plastic has been shown to be endocrine- disruptors; babies and young children are especially vulnerable to this health hazard. (Ref.3)
The European Union(EU) has a far better record at recycling plastic than the United States(US);EU recycles 25%, while the US recycles only 10%.(Ref.4)
There are seven different codes for plastic found as a number and some letters at the bottom of plastic containers:
1. Coded number 1 is polyethylene terphthalate or PET/PETE. This high-impact plastic is used for beverage bottles, food jars and frozen food trays. Recycled PET is reused as bottles, as well as fleece jackets, to name a few uses.
2. Plastic code 2 is made out of high density polyethylene or HDPE which is used in milk/juice/detergent/bleach/cosmetic bottles and is recycled into garden products and buckets.
3.Code 3 is PVC-polyvinyl chloride and is used for bedding, medical equipment, pipes, etc. Recycled it is used as decking, carpet backing and for traffic cones.
4.Code number 4 is used for low density polyethylene, LDPE and is used a lot for garbage bags,bread bags and for squeeze bottles. Recycled it is used for shipping envelopes, garbage can liners, etc.
5.Number 5 plastics is made from polypropylene, or PP for yogurt, margarine, syrup containers as well as medicine bottle and auto parts. It is recycled to make garden rakes, storage bins, brooms and brushes.
6.Polystyrene or PS is coded number 6. These are short shelf-life products like take-out containers, cutlery and packing peanuts for example. This is recycled into insulation, licence plate frames and plastic moldings.
7.Code 7 is layered resins and composite materials that are very difficult to recycle and used for baking bags, ketchup bottles. It also contains BPA that is toxic: number 7 is to be avoided at all costs.
(Ref. 5)
Japan has been most successful in recycling plastics. In 2010, Japan recycled 77% of its plastic waste. Japan recycles 72% of PET bottles, while Europe recycles 48% and US is a dismal third with 29% of PET bottles recycled. (Ref.6)
‘Micro beads’ is the latest scourge in plastic contamination. This is being used mainly by the cosmetic industry and touted as a great face cleanser, but these little plastic pieces are entering our water bodies and contaminating and killing aquatic life in our seas and oceans.(Ref.7)
The latest news from January of 2016 from Germany is that several dead sperm whales washed up on the shores in Germany and a lot of plastic was found in their stomachs.
(Ref.8)

References:
1.chem.com/acad/webtext/states/polymers.html
2.http://environment.about.com/od/earthtalkcolumns/a/recycleplastics.htm
3.http://www.medicalnewstoday.com/articles/221205.php
4.http://www.plastemart.com/upload/Literature/Recycling-of-plastic-packaging-in-Europe-is-ahead-of-USA.asp
5.http://homeguides.sfgate.com/recycle-number-mean-plastic-container-79186.html
6.http://www.theguardian.com/environment/2011/dec/29/japan-leads-field-plastic-recycling
7.http://www.theguardian.com/sustainable-business/microbeads-cosmetics-gyres-plastics-pollution-makeup
8.http://www.bing.com/news/search?q=Plastics+And+Sperm+Whales&qpvt=plastics+and+sperm+whales&FORM=EWRE

Science Makes Sense- Week 23: Polymers, plastics.

February 2, 2016

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