Although natural polymers have been used by mankind for many centuries, the use of polymers has exploded with the development of synthetic polymers within the last 100 years. Due to satisfactory properties, ease of production, and lower costs, synthetic polymers have replaced many metal, wood, rubber, and fiber parts in many materials applications. In this lesson, we look at the molecular structures of polymers and the development of numerous polymers that are synthesized from small organic molecules. Several different types of end uses of polymers in materials applications including plastics, fibers, coatings, adhesives, films, foams, and advanced materials will be discussed.
By the end of this lesson, you should be able to:
Lesson 8 will take us 1 week to complete. Please refer to Canvas for specific due dates.
To Read |
Read pp 215-231 (Ch. 11) in Introduction to Materials ebook Read pp 232-245 (Ch. 12) in Introduction to Materials ebook |
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To Watch | Plastic: The Secret Life of Materials |
To Do | Lesson 8 Quiz |
If you have general questions about the course content or structure, please post them to the General Questions and Discussion forum in Canvas. If your question is of a more personal nature, feel free to send a message to the instructor through Canvas email. The instructor will check daily to respond.
In this lesson, we will introduce the structure, history of development, and properties of polymers. The roots of the word polymer are actually very descriptive of a polymer. The root ‘mer’ means unit, and poly means many. Taken together, the word polymer can be deconstructed as many units. Typically, ‘mer’ is referred to as a monomer. ‘Mono’, which is the root for one, literally translates as one 'mer'. A commonly used definition of polymer is a material that is composed of many monomers (from 10s to 1000s) all linked together to form chains. A monomer can be composed of one to many atoms which form the base unit which is repeated to form a polymer, as represented in the figure below.
We will also study how chains of polymers are constructed. Polymers can resemble spaghetti noodles (linear), ladders (cross-linked), long chains with smaller chains hanging off the main chain (backbone) known as branched polymers, elaborately complex structures (network), or a mixture of some or all of these basic types. Other polymers, known as copolymers, are constructed from two distinctly different starting monomers and are classified as random, alternating, block, or graft polymers.
Now, watch this TED-Ed video titled, “From DNA to Silly Putty, the Diverse World of Polymers” (4:59), before proceeding on to the next section of our lesson.
DNA (deoxyribonucleic acid), proteins, sugar, starches, and carbohydrates are some examples of natural polymers used by plants and animals. The corresponding monomers for these polymers are listed in the table below. In addition to these important to life polymers, natural polymers derived from plants and animals have been used by humans for many centuries. These include wood, cotton, leather, rubber, wool, and silk. One of the oldest known uses of polymers is depicted in the picture below. The Incas of South America used rubber balls in some of their competitions. In the next sections, we will begin to discuss human-made polymers known as plastics.
Polymer | Monomer |
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DNA (deoxyribonucleic acid) | nucleotides |
Proteins | amino acids |
Sugar, Starches, Carbohydrates | glucose |
Plastics are polymers derived from petroleum products. They are a perfect example of designing better, cheaper, and completely human-made materials. Plastics are inspired by nature, i.e., natural polymers, but are completely synthetic.
Most polymers are made up of carbon and hydrogen atoms, and many plastics are as well. Polymers that contain hydrogen and carbon atoms are called hydrocarbons. Carbon atoms can form single bonds to four other atoms. If the carbon atoms in a polymer are bound to four other atoms the polymer is referred to as a saturated hydrocarbon. If on the other hand, the carbon atom is not bound to four other atoms it will typically form double or triple bonds, as needed, with another carbon atom. In this case, the polymer is referred to as an unsaturated hydrocarbon. This distinction is important as unsaturated polymers are generally unstable and more reactive than their saturated cousins.
Around 1850 billiards was becoming increasingly popular, but there was a problem. The balls were made of ivory, which is in very limited supply and is, thus, very expensive. Not to mention it requires the killing of elephants to obtain. In 1856, the first human-made plastic (Parkesine) was patented by Alexander Parkes from Birmingham, England. Often called synthetic ivory it was composed of nitrocellulose – cellulose treated with nitric acid and a solvent. It was the first thermoplastic, but it failed as a commercial product due to poor product quality control. The following 10-minute video discusses the development of polymers to replace ivory billiard balls, the science behind some of the most-used plastics, and some examples of thermoplastic and thermosetting polymers.
After watching this video, please proceed to the first (of two) reading assignments for this lesson.
As you do the first reading for this lesson, use the following questions to guide your learning. Remember to keep the learning objectives listed on the overview page for this lesson in mind as you learn from this text.
Read pp 215-231 (Ch. 11) in Introduction to Materials ebook
Polymers are formed by two main ways called addition and condensation polymerization. In addition, polymerization, an initiator (or catalyst) reacts with a starting monomer. The result of this initiation reaction is a monomer attached to the initiator with an unsatisfied bond. The unsatisfied bond is free to react with another monomer, thus adding to the chain. The process repeats over and over again until two chains combine or another initiator binds to the end of the chain, both of which will terminate the chain. In condensation polymerization, a monomer with an exposed H (hydrogen) atom binds with a monomer with exposed OH (oxygen-hydrogen) atoms. During the reaction, water is released (compensated) as the H and OH combine to form H2O (water). The following 4-minute video discusses addition and condensation polymerization.
Now that we have reviewed how polymers are formed, let’s discuss one of the possible ways to classify polymers, as thermoplastic or thermosetting.
In the last lesson on ceramics, we saw that one way to classify ceramics is by their uses (refractories, glass, clay products, abrasives, etc.). Other possible classification categories might include crystal structure and whether they are crystalline or non-crystalline. For polymers, one useful classification is whether they are thermoplastic or thermosetting polymers. As you read in the last reading assignment, thermoplastics soften when heated and harden when cooled. This is totally reversible and repeatable. Most linear polymers and branched structure polymers with flexible chains are thermoplastics. This is in contrast to thermosetting polymers, which do not soften when heated due to strong covalent crosslinks. Thermoset polymers are generally harder and stronger than thermoplastics and have better dimensional stability.
For more information about thermoplastic (here referred to as thermo-softening) and thermosetting polymers watch this video (4:40):
Now that we have discussed thermoplastic and thermosetting polymers let us review the different basic structures that polymers form and how that structure can determine whether the polymers are thermoplastic or thermosetting.
There are four basic polymer structures which are shown in the figure below. In practice, some polymers might contain a mixture of the various basic structures. The four basic polymer structures are linear, branched, crosslinked, and networked.
Linear polymers resemble ‘spaghetti’ with long chains. The long chains are typically held together by the weaker van der Waals or hydrogen bonding. Since these bonding types are relatively easy to break with heat, linear polymers are typically thermoplastic. Heat breaks the bonds between the long chains allowing the chains to flow past each other, allowing the material to be remolded. Upon cooling the bonds between the long chains reform, i.e., the polymer hardens.
Branched polymers resemble linear polymers with the addition of shorter chains hanging from the spaghetti backbone. Since these shorter chains can interfere with efficient packing of the polymers, branched polymers tend to be less dense than similar linear polymers. Since the short chains do not bridge from one longer backbone to another, heat will typically break the bonds between the branched polymer chains and allow the polymer to be a thermoplastic, although there are some very complex branched polymers that resist this ‘melting’ and thus break up (becoming hard in the process) before softening, i.e., they are thermosetting.
Crosslinked polymers resemble ladders. The chains link from one backbone to another. So, unlike linear polymers which are held together by weaker van der Waals forces, crosslinked polymers are tied together via covalent bonding. This much stronger bond makes most crosslinked polymers thermosetting, with only a few exceptions to the rule: crosslinked polymers that happen to break their crosslinks at relatively low temperatures.
Networked polymers are complex polymers that are heavily linked to form a complex network of three-dimensional linkages. These polymers are nearly impossible to soften when heating without degrading the underlying polymer structure and are thus thermosetting polymers.
Monomers do not have to be of a single atom type, but when referring to a specific monomer it is understood to be of the same composition structure. When building a polymer from two distinct monomers, those polymers are referred to as copolymers. Next, we will look at how copolymers are classified.
If a chemist is synthesizing a polymer utilizing two distinct starting monomers there are several possible structures, as shown in the figure below. The four basic structures are random, alternating, block, and graft. If the two monomers are randomly ordered then the copolymer is, not surprisingly, referred to as a random copolymer. In an alternating copolymer, each monomer is alternated with the other to form an ABABABA… pattern. In block copolymers, more complex repeating structures are possible, for example AAABBBAAABBBAAA… Graft copolymers are created by attaching chains of a second type of monomer on the backbone chain of a first monomer type.
Before we move on to the many uses of polymers, watch this four-minute video which will introduce the uses of polymers.
Now that you have watched this video, please proceed to the second (of two) reading assignments for this lesson.
As you read the second chapter for this lesson, use the following question to guide your learning. Remember to keep the learning objectives listed on the overview page for this lesson in mind as you learn from this text.
Read pp 232-245 (Ch. 12) in Introduction to Materials ebook
Now that you have read about the classical usages of polymers let us take a look at two short videos that discuss two areas that scientists are working in to improve or increase the usage of polymers in our daily lives.
The first is a video about designer polymers (3:45).
The second video (4:28) is about research into how to make flexible and lightweight electronics.
Now that you have finished these videos please proceed to the next page of our course which will introduce the video for this lesson, Plastics: The Secret Life of Materials. This video will tie together the history, concepts, and usages of polymers that we have been discussing in this lesson, as well as, highlight some possible future usages of plastic.
Now that you have read the text and thought about the questions I posed, go to Lesson 8 in Canvas and watch "Plastic: The Secret Life of Materials" (51 minutes) about the manmade and artificial materials which have changed how we live. In "Plastic: The Secret Life of Materials," materials scientist Dr. Mark Miodownik explains how we have turned our backs on nature and began to create our own better and cheaper materials.
Go to Lesson 8 in Canvas and watch the Plastic: The Secret Life of Material video. You will be quizzed on the content of this video.
Polymers are composed of repeating units which are repeated in four possible chain structures: linear, branched, crosslinked, and network. In this lesson, we discussed how the chemical and structural characteristics affect the properties and behavior of polymers. The seven basic end uses for polymers (plastics, fibers, coatings, adhesives, films, foams, and advanced materials) were introduced. Most polymers are not biodegradable which coupled with their heavy use in today’s society leads to a major source of waste at the end of a polymer's usage.
In the next lesson, we will look at how composites are formed from two or more distinct materials to achieve the best of two worlds.
You have reached the end of Lesson 8! Double-check the to-do list on the Overview page to make sure you have completed all of the activities listed there before you begin Lesson 9.