Overview

When you think of the word “polymer”, what do you envision — what comes to mind? My guess is that you immediately think “plastics”. Indeed, the materials we call plastics are polymers, but not all polymers are plastics, not by a long shot! Through this course, we will discover how diverse polymer materials really are, in terms of both their chemistry and structure and explore some of the unique properties that make polymers so useful in our daily lives.

Take a moment and look around you — can you identify some materials that are made of polymers? Perhaps you are wearing a shirt that has cotton (a natural polymer, cellulose) or pants that are stretchy because they have Spandex (a synthetic polymer)? Are you wearing contacts or glasses? Both are polymers — contact lenses are made of crosslinked 2-hydroxyethylmethacrylate and the lenses of most glasses are polycarbonate. Do you have hair and fingernails? (I hope so!) They are made of keratin, another polymer. You are “you” because of your DNA — yet another example of a polymer! In fact, it may be hard for you to find materials around you that aren’t made of polymers, at least in part. Polymer materials have revolutionized our world — don’t you want to know what they are and why they’re so special?

Figure 1.1: Common ploymers

Learning Outcomes

By the end of this lesson, you should be able to:

• Provide examples of polymer materials in daily life.
• Define monomer, dimer, trimer, oligomer, polymer.
• Describe the difference between monomer and repeat unit.
• Define and compare homopolymer, copolymer, and blend.
• Draw skeletal structures of linear polymers, branched polymers, network polymer.
• Compare statistical copolymer, random copolymer, alternating copolymer, block copolymer, graft copolymer, and draw schematics of each.
• Compare and contrast the properties and structure of thermoplastics, elastomers, thermosets.
• Define glass transition temperature and melting temperature, compare and contrast.
• Calculate the degree of polymerization
• Calculate the mean repeat unit molar mass
• Calculate and contrast number average molar mass, weight-average molar mass
• Define and calculate  molar mass dispersity

Lesson Checklist

Please refer to the Canvas Calendar for specific timeframes.

Questions?

If you have questions, please feel free to post them to the General Questions and Discussion forum. While you are there, feel free to post your own responses if you, too, are able to help a classmate.

The name of this course is “Introduction to Polymer Materials”. Let’s break it down.

Introduction:
I assume that this is the first class you have ever had teaches you what a polymer is – no background knowledge about polymers specifically is needed for this course. That being said, this course builds upon basic principles in general chemistry, organic chemistry, and math that you will need to know.

Polymer:
What is it? You’ll find out soon!

Materials:
This class is not a polymer “physics” course, nor is it a polymer “chemistry” course – it’s a polymer materials course. To me, “materials” means that we are going to emphasize an understanding of how the molecular, atomic level structure of a polymer affects its macroscale properties as a material. This structure – functional relationship, as you will see, is especially important and fascinating for polymers. And, we are going to emphasize not just the fundamental science, but also delve into some applications and explore how these unique properties of polymers are useful.

Figure 1.2: Materials Thought Flow Chart

Source: Lauren Zarzar

Within the “materials” world, you may hear the distinction “soft” versus “hard” materials. Typically, one would lump inorganic materials like metals or ceramics into “hard” materials that are strong and tough, while polymers and other organic materials evoke thoughts of “soft” materials – materials that are squishy, flexible, and weak. And while maybe this categorization is applicable to many materials, there are numerous cases in which these assumptions fail. For example, mercury is a metal – but it’s a liquid (a soft material). Kevlar is a polymer used for making many products including bullet proof vests, but it is extremely strong – stronger than steel in fact. Diamond, one of the hardest materials known to man, is organic and made entirely of carbon. So dispel any preconceived notions about the physical properties of polymers – in fact, one of the unique aspects of polymers that makes them so useful is how tunable and diverse the physical properties are. We will discuss in detail how the physical properties of a polymer are related to the chemistry and molecular structure of a polymer.

Given how ubiquitous polymers are in our lives today, it’s rather incredible to think that the concept of a polymer did not exist until relatively recently. People used to think that polymer materials were actually colloids – they described the material properties they observed by saying that there were “lots of small molecules that were interacting strongly with each other”. If you told scientists 100 years ago that there were actually really long, huge, molecules in there that were strung together with covalent bonds, they would have said you were crazy. The concept of such “macro molecules” just did not exist. So even though people have been using polymers for millennia (wood, silk etc.) and even more recently (do you know those really old telephones made of Bakelite?) – the concept of a polymer did not exist until Staudinger came along in the 1920s – it’s that recent! So it also just goes to show you that a lot of materials science, in terms of applications – you can get pretty far sometimes without understanding the fundamentals, but once people understood polymers on a molecular level, both research and applications exploded.

Figure 1.3: Bakelite

Source: Telephone image (pxhere)

As stated previously, natural polymers have been used for ages – wood and cotton, for example, are made of natural polymer. But the earliest examples of actual polymer chemistry really start in the 1830s, when people began experimenting with reactions of cotton – cotton, of course, being cellulose. This work led to the discovery of various nitrated cellulose products. Nitrocellulose, also called Celluloid, was used as an early replacement material for billiard balls. At the time, billiard balls were made from elephant ivory, and it was getting really expensive (and environmentally unsustainable), so there was a push to find a suitable substitute. The problem was that the “sound” billiard balls made when they hit each other was really important, and difficult to replicate in other materials. John Wesley Hyatt used one of these modified cellulose polymers to create billiard balls, and it was quite successful! However, there was a slight problem with nitrated cellulose polymers in that there were quite flammable and legend has it that occasionally the billiard balls exploded! So clearly, more work needed to be done. Along came Bakelite – which is the first example of a truly synthetic polymer (the rest were just modifications of cellulose). Bakelite, formed from the condensation reaction of phenol with formaldehyde, became hugely popular – perhaps you have seen products with Bakelite, such as those old telephones. Keep in mind that all this time, people still didn’t really know what a polymer was.

Hermann Staudinger was the first to suggest in 1920 that a polymer is actually a very large molecule, a macromolecule, where the atoms in the molecule were held together by covalent bonds. Recall from general chemistry that a covalent bond is made when atoms share their electrons – this is distinct from a simple attraction between molecules, which is what up until this point, was what everyone thought was what was giving these materials their unique properties. The general consensus was that the materials were simply colloids, where the particles were small molecules held together by attractive intermolecular forces. So this idea that there was actually a giant molecule in there was at the time incomprehensible. It took over a decade for Staudinger’s ideas to fully catch on, and he received the Nobel Prize for his contributions in 1953.

There have been lots of amazing discoveries about polymers since then. For example, even more recently, conductive polymers were discovered – again, at the time, it was difficult for people to believe that an organic material like a polymer could be conductive like a metal, but it's true! Polymers have found even more uses than you could imagine. They are replacing traditional metals and semiconductors, they are being used in solar cells and electronics. There are polymer formulations that are being used in composites for building materials, medicine, drug delivery, adhesives, paints, packaging, clothing. Everywhere.

As just mentioned, a polymer is a large macromolecule. The word “polymer” can be broken down into the Greek components: poly (many) + mer (part). “One part” would then be called a monomer, “two parts” would be a dimer, “three parts” a trimer, “a few parts” is an oligomer. Polymers can be made of a single repeating unit, over and over, there can be many different monomers in a polymer, you can have ordered repeats, random repeats, there are infinite combinations, and we will talk about the structure of the polymer in detail.

Figure 1.4: Linking monomers together to create polymers

Source: Lauren Zarzar

To create a polymer, imagine linking together a bunch of monomer units by sequentially reacting each monomer together. Table 1.1 in the textbook shows examples of chemical structures of common monomers and the polymers that result from their polymerization. Notice the nomenclature for how we name the polymer from the monomer, also pay attention to the notation used for describing the repeat unit of the polymer, which is the structure appearing in the brackets [ ]. Let’s use styrene as an example (Figure 1.5).

Figure 1.5: Styrene

Source: Lauren Zarzar

Styrene is the name of the monomer, but the repeat unit is the part that is “repeated” (in the brackets) that make up the polymer. The repeat unit is not necessarily the same as the monomer; a repeat unit can be made from multiple monomers for example. If the number of atoms, and hence the molecular weight, of the monomer is the same as the repeat unit, then we call this a polyaddition polymerization. However, the number of atoms in the repeat unit is not always the same as the monomer, if there are chemical byproducts of the polymerization reaction (and we would call this a polycondensation polymerization). A good example of this would be nylon 6,6 produced from the polymerization of hexamethylenediamine and adipic acid (Figure 1.6). Notice that the repeat unit of nylon 6,6 is a combination of both monomers, and that some atoms were lost during the polymerization so that the chemical structure of the repeat unit is not just the simple addition of both monomers; we have lost H₂O in this reaction. Can you identify the bond formed between the hexamethylenediamine and the adipic acid in the nylon polymer?

Figure 1.6: Hexamethylenediamine combines with adipic acid to form nylon 6,6

Source: Lauren Zarzar

Depending on how many different monomers are combined into a polymer, and in what order and structure, we have different ways of generally classifying polymers (Table 1.2 in text). If the polymer is made up of a single monomer, we call it a homopolymer (Figure 1.7) (e.g., if our monomer is A, the polymer would be poly(A)). If a polymer is derived from the polymerization of multiple different monomers, we call it a copolymer (Figure 1.8) (e.g., if our monomers are A and B, our polymer would be poly(A-co-B). There are many varieties of copolymers. If the positions of monomers is randomly distributed within the polymer, then we call that poly(A-ran-B). Here, “random” does not mean the same thing as “I didn’t intentionally put the monomers in a specific order”; to be random, there truly has to be a random distribution over the entire length of the polymer, which in many cases does not happen even if you do not intentionally control monomer order. More often, “statistical” is correct; since each monomer may have a different reactivity, one monomer may be more readily incorporated in the polymer than the other and so the distribution of each monomer along the polymer MAY be different. So in the cases where the monomers follow such statistical distributions (that are not random) we call them statistical copolymers, poly(A-stat-B). Figure 1.8 shows drawings of alternating copolymers poly(A-alt-B), block copolymers poly(A-block-B), and graft copolymers polyA-graft-polyB. A blend is a mixture of different polymers.

Figure 1.7: Homopolymer

Source: Lauren Zarzar

Figure 1.8: Copolymers

Source: Lauren Zarzar

So now we know that monomers can be ‘strung’ together to form a long molecule called a polymer. Perhaps you are imaging that it looks like a tiny piece of string or spaghetti – and in many cases, we can simplify the drawing of the polymer by just drawing the skeletal structure as a squiggly line. In such drawings of skeletal structures, like those shown in Figure 1.9, we don’t draw out the specific chemical structure, but the lines are supposed to represent the polymer backbone and can help us visualize higher order structure. Because a polymer doesn’t just have to be a linear – it can be much more complicated than that. It can be branched – like a tree – or it can be network, where all the strings are connected to each other at linking points called crosslinks. The skeletal structure of a polymer significantly affects its properties. For example, network polymers tend to hold their 3D shape much better than linear polymers; can you imagine trying to build a sculpture out of spaghetti?

Figure 1.9: Skeletal structures

Source: Based on figures from Young, Robert J., and Peter A. Lovell.
Introduction to Polymers, Third Edition, CRC Press, 2011.

In addition to classifying polymers by their chemical structure, we also classify them based on their physical properties (Figure 1.3 in the text). There are three main types: thermoplastics, elastomers, and thermosets. Elastomers are stretchy – think “elastic”, like a rubber band. They can be stretched and deformed and return to their original shape because their 3D structure is held together by crosslinks (i.e. most elastomers are network polymers). Their unique properties are a function of their 3D network structure. Things like crosslink density affect their macroscale material properties. Thermosets are rigid that usually have a very high degree of crosslinking. When they are heated, they don’t often flow or soften, they usually just degrade (i.e., the bonds in the polymer are broken). This is in contrast to thermoplastics or thermosoftening polymers, which do flow upon heating. Thermoplastics are typically linear or branched and do not have that network structure to hold their shape (hence they flow when heated). Most commercial polymers are thermoplastics. They can be crystalline, semi-crystalline, or amorphous. Crystalline phases have a melting temperature (T_m). Amorphous phases can’t really “melt” because they are already amorphous (it’s not considered a phase transition), so we use the term glass transition temperature (T_g) to characterize their softening point. T_g might be a range of temperature over which the transition occurs.

Some polymers have characteristics of more than one of these classes. In a sense, it’s a continuum. For example, some elastomers can also be characterized either as a thermoplastic or thermoset.

Usually, "short" polymers are called oligomers. When we start getting to tens, hundreds, thousands of repeat units or more, we have polymers. They can be huge. Remember back to general chemistry, where you learned how to calculate the molar mass of a molecule? Similarly, we can describe the "size" of a polymer using molar mass (or molecular weight) which is typically defined in units of g/mol and abbreviated as "M". Another way of describing the size of a polymer is by its degree of polymerization. Degree of polymerization (described by "x") is the number of repeat units in a polymer. If we know the molar mass of a polymer, how can we figure out the degree of polymerization? Similarly, if we know degree of polymerization, how can we figure out molar mass? All we need to know is the molar mass of the repeat unit!

Let us set up this relationship, where the new variable M₀ is the molar mass of the repeat unit:

$$M=x{\overline{M}}_0$$

Where:

• $M$ = molar mass
• $x$ = degree of polymerization
• $M_0$ = molar mass of the repeat unit

We see that the total molar mass of the polymer is just a function of the degree of polymerization and the molar mass of the repeat unit.

PROBLEM 2

A polymer with molar mass of 35,200 g/mol. has a degree of polymerization of 800. Which polymer, of the ones shown below, could it be?

Click here for the answer.

ANSWER 2

D. poly(vinyl alcohol)

PROBLEM 3

What is the molar mass of polyethylene (shown below) which has a degree of polymerization of 100?

Click here for the answer.

ANSWER 3

$$M=x{\overline{M}}_0= 100*\frac{28g}{mol}=\frac{2,800g}{mol}$$

PROBLEM 4

The following monomer is polymerized (condensation polymerization with loss of water). If the degree of polymerization is 100, what is the molar mass of the polymer?

Click here for the answer.

ANSWER 4

$M$ = 7,200 g/mol

1. First, draw the polymer in terms of the repeat unit:
   
2. Determine the molecular formula of the repeat unit:
   3 Carbon, 2 Oxygen, 4 Hydrogen
3. Calculate the molar mass of the repeat unit, ${\overline{M}}_0$:
   3 (12 g/mol) + 2 (16 g/mol) + 4 (1 g/mol) = 72 g/mol
4. Calculate polymer molar mass, $M=x{\overline{M}}_0$:
   $M=100\ast \frac{72\text{g}}{\text{mol}}=\text{7,200 g/mol}$

What if our polymer has more than one monomer, or more than one type of repeat unit? In that case, we define the mean ${\overline{M}}_0$ of the copolymer as just a weighted average of the repeat unit, where the weights are the mole fraction (X) of each type of repeat unit.

$${\overline{M}}_0={{\displaystyle \sum }}^{\text{}}{X}_j{M}_0^j$$

Where:

• $M$ = molar mass
• $x$ = degree of polymerization
• $M_0$ = molar mass of the repeat unit

Example:

What is the mean molar mass of the repeat unit for a copolymer comprised of 20 mol% styrene, 30 mol% methyl methacrylate, and 50% vinyl chloride? The chemical structures of the monomers and the resulting copolymer are shown below.

$${\overline{M}}_0=0.2*\frac{104g}{mol}+0.3*\frac{100g}{mol}+0.5*\frac{62.5g}{mol}=\frac{82.05g}{mol}$$

When you first learned about the molar mass of molecules, you learned that the molar mass is linked to the identity of the compound; for example, H₂O always had a molar mass of 18 g/mol. If the molar mass wasn't 18g/mol, it couldn't be water! The situation is very different for polymers. Take polypropylene, for example:

Polypropylene

Source: Lauren Zarzar

The molar mass of this polymer could be 420 g/mol (if degree of polymerization is 10) or 21,000 g/mol (if the degree of polymerization is 500). Although vastly different in molar mass, both of these molecules are polypropylene. For polymers, there is almost always a molar mass distribution. An example distribution is given in Figure 1.10. Although this curve looks continuous, we know that in fact it cannot be - the mass of the polymer does change in discreet units, depending on the size of the repeat unit. However, we do typically draw the distributions as continuous function.

Figure 1.10: A typical weight-fraction molar mass distribution curve for a polymer with the most probable distribution of molar mass and a repeat unit molar mass 100 g mol^-1.

Source: Based on figure 1.4 from Young, Robert J., and Peter A. Lovell.
Introduction to Polymers, Third Edition, CRC Press, 2011.

Because there is a distribution in molar mass, we have a choice as to how to actually define a characteristic molar mass for a sample. There are three general approaches for calculating the molar mass from a distribution, giving us three different values: ${\overline{M}}_n$(number average molar mass), ${\overline{M}}_w$, (weight average molar mass) and ${\overline{M}}_z$(z-average molar mass). You'll notice from Figure 1.10 that $M_n<M_w<M_z$. Each value of molar mass is defined differently.

$M_n$ is defined as the sum of the products of the molar mass of each size of polymer multiplied by its mole fraction (X). Recall from general chemistry that a mole fraction is equal to the ratio of number of moles (or molecules) of a type of polymer (N) divided by the total number of moles (or molecules). Basically, this is the same as your "average" arithmetic mean!

$${\overline{M}}_n={{\displaystyle \sum }}^{\text{}}{X}_i{M}_i=\frac{{{\displaystyle \sum }}^{\text{}}{N}_i{M}_i}{{{\displaystyle \sum }}^{\text{}}{N}_i}$$

Sometimes it's easier for us to work in weight fractions ($w_i$) rather than mole fractions, since mass is often easier to measure. The weight fraction $w_i$ is defined as the mass of molecules of molar mass $M_i$ divided by the total mass of all the molecules present:

$$w_i=\frac{N_i{M}_i}{{{\displaystyle \sum }}^{\text{}}{N}_i{M}_i}$$

Thus, we can define $M_w$:

$${\overline{M}}_w={{\displaystyle \sum }}^{\text{}}{w}_i{M}_i=\frac{{{\displaystyle \sum }}^{\text{}}{N}_i{M}_i^2}{{{\displaystyle \sum }}^{\text{}}{N}_i{M}_i}$$

Compare ${\overline{M}}_w$ to ${\overline{M}}_n$ — do you notice how ${\overline{M}}_w$ is a function if $M_i$ squared? Therefore, bigger polymers have a greater influence on ${\overline{M}}_w$ than they do on ${\overline{M}}_n$, skewing the value of ${\overline{M}}_w$ to be larger than ${\overline{M}}_n$.

The dispersity is an indication of the breadth of the molar mass distribution. Consider the two mass distributions shown below in Figure 1.11. The orange curve is broader than the blue, hence we would say that the orange polymer sample has greater dispersity.

Figure 1.11 - Sample Molar Mass Distributions

Source: Lauren Zarzar

We define the dispersity as the ratio of $M_w$ and $M_n$:

$$\mathrm{dispersity=\; Đ } =\frac{{\overline{M}}_w}{{\overline{M}}_n}$$

If your polymer is completely uniform, and every polymer molecule is exactly the same size, your dispersity would be 1. If there is any distribution in molar mass, then dispersity will be greater than 1 because $M_w$ is always greater than $M_n$.

Earlier in the lesson, we learned about degree of polymerization. Well, if there is a distribution in polymer molar mass, then there must also be a distribution of degree of polymerization. So to describe the degree of polymerization for a polydisperse polymer we use degree of polymerization averages, and similarly to molar mass distributions, we have both a number average and a weight average for degree of polymerization.

$${\overline{x}}_n=\frac{{\overline{M}}_n}{{\overline{M}}_0}$$

$${\overline{x}}_w=\frac{{\overline{M}}_w}{{\overline{M}}_0}$$

Summary

In Lesson One you have learned the very basics of polymers - how we describe the composition (monomer, repeat unit, homopolymer, copolymer, etc.), the skeletal structure (linear, branched, network), and the polymer size (degree of polymerization, number and weight average molar mass). You will have hopefully noticed how different polymer molecules are than molecules you are most familiar with. In particular, the polymer molar mass does not define its composition unlike other molecules, because a polymer have any degree of polymerization. Polymers are also so much higher molar mass than other molecules and as we will see in upcoming lessons, this gives polymers some unique material properties. Next, we will be learning about what is necessary to make a polymer in the first place, because not all molecules are monomers. How do we link together these monomers, and what determines the skeletal structure?

Reminder - Complete all of the Lesson 1 tasks!

You have reached the end of Lesson 1! Review the checklist on the Lesson 1 Overview / Checklist page to make sure you have completed all of the activities listed there before you begin Lesson 2.