Skeletal Structure

Skeletal Structure mxw142

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?

diagram of various skeletal structures described in the text
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 (Tm). 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 (Tg) to characterize their softening point. Tg 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.

The term polymer is used to describe a macromolecule made of many monomers — or repeating units. The properties of these polymers depend on a variety of factors: the monomer unit, the linkages between each monomer, and the intermolecular and intramolecular forces that exist between polymer chains.

In this lesson, we’ll learn about two main classes of polymers: thermosoftening polymers and thermosetting polymers. We’ll also explore their properties — and why they behave the way they do. 

The term plastics refers to a wide range of polymers made from monomers derived from products obtained through the fractional distillation of crude oil. You may already be familiar with common examples like polyethylene, polypropylene, and polyvinyl chloride (PVC). You can learn more about the structure of these polymers, how they’re made, and their real-life applications in other videos on our channel.

Here, we’ll focus specifically on how these polymers respond to heat — and why. 

Thermosoftening Polymers

Polyethene, polypropylene, and polyvinyl chloride are all thermosoftening polymers. This means they soften when heated. In their softened, liquid state, they can be molded into many different shapes.

These plastics are used to make countless everyday items — such as window and door frames, pipes, wiring insulation, and waterproof clothing.

This behavior is possible because the polymer chains are not chemically linked together. Think of it like a bowl of noodles: although the noodles are coiled and tangled, they aren’t bonded. Similarly, thermosoftening polymer chains can slide over one another, making the resulting materials soft and flexible.

In fact, these polymers are held together only by weak intermolecular forces — which means they can be separated relatively easily when heated. This gives them relatively low melting points. 

Other examples of thermosoftening polymers include polystyrene and polytetrafluoroethylene (PTFE).

Thermosetting Polymers

In contrast, thermosetting polymers do not soften when heated. 

Unlike thermosoftening polymers, thermosetting polymers are cross-linked — meaning their chains are chemically bonded together. Can you think about how this might affect their properties? (Pause, think, and continue when ready.)

The presence of cross-links hardens the overall structure.

A classic example is vulcanized rubber. Natural rubber, tapped from para rubber trees, is a polymer made of isoprene monomers. In its raw form, it’s a runny liquid that can be processed to make latex gloves, erasers, and party balloons.

But to make car or bicycle tires, it must first be vulcanized. In this process, sulfur is added — forming disulfide bridges that link the polymer chains together. These cross-linkages greatly increase the material’s strength and prevent it from softening easily when heated.

Think about it: no matter how fast you ride your bike, the tires don’t change shape! Other examples of thermosetting polymers include materials used to make old TV casings and certain types of strong industrial glue.

Summary

Thermosoftening plastics are soft and melt when heated. Thermosetting plastics are hard and do not soften or change shape when heated.

Understanding these differences helps us choose the right polymer for the right application — whether we need flexibility or durability.

Source: FuseSchool-Global Education. "What is Thermosetting and Thermosoftening Polymers?" February 29, 2016.

PROBLEM 1


If a polymer becomes more flexible, then how do you think Tg will change?

  1. Increase
  2. Decrease
  3. Stay the same

ANSWER


B. Decrease

PROBLEM 2


If intermolecular interactions between polymers increase, then how will Tg change?

  1. Increase
  2. Decrease
  3. Stay the same

ANSWER 2


A. Increase