Polymers Structure and Properties - Online Tutor, Practice Problems & Exam Prep

Polymers exhibit varying degrees of crystallinity, which significantly influence their strength, density, and thermal properties. Crystallinity arises from strong intermolecular forces and specific stereochemistry, with isotactic structures being the most crystalline. Key thermal properties include the glass transition temperature (Tg), where polymers transition from hard to soft, and the crystalline melting temperature (Tm), where they become viscous liquids. Higher crystallinity correlates with increased Tg and Tm, indicating that stronger polymers require higher temperatures to transition states.

Polymers Structure and Properties Concept 1

Hey, everyone. So here we're going to take a look at the Polymer structure and properties. And with that, we're going to look at crystallinity. Now we're going to see that crystallinity determines the strength and thermal properties of a polymer. And we're going to say here, though not crystalline structures, polymers possess what we call crystallites.

And they're small regions of crystalline structure, so they're ordered. So if we take a look at this image, we have our crystallites here. We're going to say the higher the degree of crystallinity, then the higher the density will be, the higher the rigidity will be, and the higher the strength will be. Opposed to this is our amorphous regions. We're going to say polymer chains are in disordered arrangements when we talk about the amorphous regions.

So if we take a look here, we have these organized. They look linear to me here, but then there's this mess here. They're not as organized. This is our amorphous region. So just remember, when we're talking about the structure and properties of these polymers, crystallinity is a vital topic to cover when it comes to the overall density, rigidity, as well as strength of a given polymer.

Polymers Structure and Properties Example 1

Hey everyone. So in this example question, it says, select a true statement that best describes crystalline polymers. So here it says, higher density polymers are softer than amorphous polymers. Right. So here, if we have higher density, this would lead to higher rigidity, higher strength, so they shouldn't be softer.

Higher degree of crystallinity leads to decreased polymer brittleness. Alright. So here we're saying high degrees of crystalline leads to decreased polymer brittleness. Right. So here, we never mentioned this idea of crystallinity and brittleness.

We have higher crystallinity that leads to higher density, which means we're going to have higher rigidity and higher strength of that given polymer. High degree of branching introduces regions of highly ordered chains. No. If we have more branching, there's the greater chance of more disorder, more amorphous regions being created. These are disordered arrangements.

Crystalline regions are more rigid than amorphous regions in a polymer. That's true. There's more organization, there's more ordered arrangements for a crystalline region than an amorphous region, this leads to greater rigidity. Right? So here, out of all the statements, only option d is true.

Polymers Structure and Properties Concept 2

Hey, everyone. Now let's take a look at the factors that can affect crystallinity. Here we're going to say that higher crystallinity results from stronger intermolecular forces and specific stereochemistry. And we're going to say here that the degrees of crystallinity are isotactic is greater than syndiotactic, which is greater than atactic. So just remember to apply these factors when we're talking about crystallinity and trying to basically compare different polymers to one another.

Polymers Structure and Properties Example 2

Hey, everyone. Let's take a look at the following example question. Here it says, by examining the stereochemistry of each, determine which polymer is considered stronger. If we take a look at each of these, we're going to say here we have a, where we have alternating arrangements between the methyl groups. And we can tell the discernible pattern.

So we can say that this is a syndiotactic polymer. In the next one, b, they're all on the same side, so this would be an isotactic polymer. And in c, there's no discernible pattern here, so this would be an atactic polymer. Remember, when we look at the order in terms of crystallinity, we say that isotactic would be the greatest, followed by syndiotactic, and then atactic would be last. Alright.

So if we look at all these, the answer is b because b is the only one that is an isotactic polymer. Again, this is syndiotactic. And then this one here is atactic. So b will be the greatest in terms of strength, overall strength, when we're comparing all three of these different types of polymers.

Polymers Structure and Properties Concept 3

Hey everyone. So now let's take a look at the thermal properties of polymers. With this, we're going to talk about 2 temperatures. We have our glass transition temperature which we're going to say represents T_g. And this is just the temperature above which the polymer transitions from hard to soft and moldable.

Then we have our crystalline melting temperature, which we're going to say is T_m. This is the temperature at which a polymer turns into a viscous liquid. If we take a look at this image, we have a water bottle at different phases. And we're increasing the temperature as we go from left to right. Before we reach our T_g temperature, we're going to be in its hard phase.

But once we pass the T_g, we see that the bottle starts to bend and become soft and moldable. And if we would increase the temperature even more past the T_m, we would see that the bottle starts to look like it's melting. That's because it's resembling a viscous liquid. Now, we're going to say here as our degree of crystallinity increases, so do our T_g and T_m.

Because remember, if you increase your crystallinity, you're increasing the overall strength of your polymer, which means you have to increase the temperature that much more to go from hard to soft moldable, to go from matte to a viscous liquid. Alright. So, just keep in mind when we're talking about thermal properties, we're talking about these two temperatures which can affect the composition and the way we see a given polymer.

Polymers Structure and Properties Example 3

In this example question, it says, 2 polymers are being studied in a lab. Polymer A has a highly ordered region and strong intermolecular forces, while Polymer B has an amorphous region or amorphous structure and weak intermolecular forces. Explain why Polymer A has a higher \( T_g \) and \( T_m \) than Polymer B. Right. So here it says, Polymer A has higher \( T_g \) and \( T_m \) because crystalline structures are more flexible than amorphous.

No. They're not more flexible. They're less flexible because if your crystalline structure is more ordered, you're going to become more rigid. Strong intermolecular forces and higher degrees of crystallinity in Polymer A restrict chain movement resulting in higher \( T_g \) and \( T_m \). That's true.

So we'll say that this is the answer for now. Higher \( T_g \) and \( T_m \) cause polymers to develop stronger intermolecular forces. No. That's not what's going on. It's the stronger forces, the stronger more ordered structure that leads to higher \( T_g \) and \( T_m \).

Crystalline polymers allow for more movement along the chain resulting in higher \( T_g \) and \( T_m \). No. They don't allow for higher movement. They result in lower or less movement and that leads to higher \( T_g \) and \( T_m \). So out of all our options, only option B is a correct statement.

So if you're here, we'd say B would be our final answer.