Welcome to our third lesson in our introduction to material structure course. Today we'll
be focusing on polymer materials, specifically we'll
be defining the elements of basic polymeric
chains and talking about how these chains and side groups arrange themselves in our
bulk material. Polymers can be split into two general categories; the first is thermoplastics,
which soften upon heateing and harden upon cooling.
An example of this is polyethylene, which makes up milk bottles.
The second are thermosets which have covalent bonds or crosslinks between our polymeric
chain materials.
Now this changes a number of things about our polymer, but one major difference is now
thermsets simply burn when heated up. Think of an epoxy resin-- once hardened and cured,
it doesn't soften upon heating.
Let's start with basic elements of polymer chains. I have a schematic of polymer chain
shown here. We can see our backbone, made of carbon atoms that are bonded together by
covalent bonds. These carbon atoms have side groups, notated
here by the letter R. These side groups can be atoms or
molecules-- we'll see some examples in a minute.
Now the word polymer means " many mers". The word mers is derived from the greek
word meros meaning parts; so our polymer is
made of many parts.This is a monomer. This is the small organic molecule that's
the basis for our polymer. These monomers are reacted together, using different techniques
we won't cover in this course, and then we get our polymer. So, our polymer is made of
repeating units, like the one shown here.
This might be easier to understand with a real world
example. Let's take polyethylene, that we
talked about in the intro to our course. The monomer for polyethylene is the hydrocarbon
ethylene, with a chemical composition of C2H4. This is reacted together in the proper
conditions to give us this-- our polyethylene chain. Our backbone is our carbon and our
side groups are single hydrogen atoms. Therefore our repeating group looks like this. Sometimes
a polymer
chain might be written out like this, where our repeating unit is in brackets with a subscript
n. This n indicates the length of our chain. Now remember, that this simple drawing
that does not take into account our bonding constraints. Our covalent hybridized carbon-carbon
bonds
are directional, which means that we don't actually have this 180
degree angle. Instead it's closer to 109 degrees, like this.
Now we said we can change our side groups. We also
talked about Teflon in our introduction. We
can see the structure and its repeating unit here. We've swapped our hydrogens for
fluorines. Now for both polyethylene and Teflon, the repeating units has two carbons, both
with the same side groups. Why do we bother showing two carbons? Well some systems, like
polyvinyl chloride or PVC, have uneven side groups. The structure for
PVC is shown here; we have three hydrogen side groups and one chlorine side group per
repeat unit.
Side groups don't have to be just atoms. For example, polypropylene has an ethylene side
group along with some hydrogens. Now these side groups can have multiple configurations
depending on how they arrange themselves along the chain--which is called
stereoisomerism-- or twhat side of the chain they arrange themselves on--
which is called geometric isomerism. Unfortunately we don't have time to discuss these today,
but since we're dealing with three-dimensional
bulk materials, we need to mindful of how these side groups arrange themselves.
We also need to be mindful of how our polymers arrange
themselves in the bulk material. We have the linear polymer structure where our chains
resemble what's often referred to as cooked spaghetti. Here we find our Van der Waals
or hydrogen bonds from Lesson 1 between the chains, weakly binding them to one another.
When we have linear polymeric chains, we can have a high degree of chain stacking, leading
to what's called crystallites. These areas lead to polymers that are semi-crystalline
in nature, which alters our polymeric properties. We can also have branched polymers,
which again are bonded together by our secondary bonds. The difference is the branch polymers
can't pack as closely to one another compared to our linear polymers. Not only does this
lead to minimal crystallization, but it also reduces the density of our polymer. For example,
our plastic bags from the introduction are made of a low density
polyethylene that has this branched structure.
High density polyethylene, made with a linear structure, is used to reduce wear and friction
in total knee replacements. Quite a different application with a change in structure.
We can also have covalent bonds between our polymer chains. These are called cross links.
We mentioned crosslinks at the beginning of the lesson
as a difference between thermosets and thermoplastics. Our thermosets don't soften because of these
covalent bonds. We need just as much energy to break the covalent bonds between the chains
as we do within the chain so they all break at roughly the same time. The degree of crosslinking
between our chains can vary from low to high. As we get to a high
degree of crosslinks, we have what's called
a network polymer. An example of a polymer with a low degree of crosslinking is rubber,
which I can stretch and manipulate as an elastomer. A polymer with a high degree of cross-linking
is epoxy; I can't stretch epoxy in the same way I can stretch rubber, yet they're both
cross-linked polymers. So our degree of cross-linking dramatically impacts our performance. And
with that, we've come to the end of a very terminology heavy lesson. I know we've talked
about
a lot of terms but since polymers can be so varying in their structure, their composition,
how their side groups and chains are arranged within the bulk polymer, we need a lot of
different terms in order to describe the differences between them. I highly encourage
you to use the note taking sheet from our course website in order to help you keep
the terms straight. And with that we're down to
our final lesson. This one is focused on composite materials,
so stay tuned to learn how composites are slightly different
than the other material families we've discussed so far.