Polymers and intermolecular bonds

We have seen that the atoms that make up a molecule are held together by strong intramolecular bonds. But is there any attraction between neighbouring molecules, and anything that holds them together? The answer is yes. We use the term intermolecular forces, or intermolecular bonds or intermolecular attractions to classify them.

Although they are relatively weak compared to the intramolecular bonds their effects are important and are responsible for many of the properties of a substance – for example, solubility, physical state (whether the substance is a solid, liquid, gas), and in the case of polymers their strength and flexibility.

There are different types of intermolecular forces; some are stronger than others. In the following sections we will look at:

• Instantaneous dipole-induced dipole attractions
• Permanent dipole-permanent dipole attractions
• Hydrogen bonding

Instantaneous dipole-induced dipole attractions

Bromine is a liquid at room temperature. It contains small Br₂ molecules in which a strong intramolecular bond holds two bromine atoms together. This is also called a covalent bond (see below for an understanding of the nature of this bond).

There must also be some sort of attractive forces between the neighbouring molecules, otherwise they would move apart and Bromine would exist as a gas at room temperature.

The following diagrams explain the nature of these forces.

The electrons in the covalent bond are evenly shared between the two bromines. In fact, all the electrons are evenly distributed around the molecule.

The electrons in the cloud are in constant motion. What is the electron distribution at a particular instant in time? One end of the molecule has more negative charge than the other. We can represent this by using a delta minus symbol to indicate a slight excess of negative charge and a delta plus symbol to indicate a slight deficiency in negative charge. This molecule now has a dipole, called an instantaneous dipole. Over time, there is random movement of elections so that the density shifts continuously.

What happens to a molecule near to the molecule with the instantaneous dipole?

The negative end repels the electron cloud in the neighbouring molecule. This molecule now possesses an induced dipole. The resulting attraction between the two dipoles is called an instantaneous dipole-induced dipole attraction.

The attraction is not permanent. Electrons are continually moving within each molecule and so different instantaneous dipoles will be set up resulting in different induced dipoles. Also the molecules in this liquid are themselves moving and tumbling relative to each other. Over a period of time however there is a net attraction. It is very weak when compared to the covalent bond between the atoms in the Br₂ molecule, but strong enough to prevent the molecules moving away from each other, and so keeps bromine a liquid at room temperature.

All molecules will have instantaneous dipole–induced dipole attractions between them.

The attractions are a result of the random movement of electrons. Therefore, the more electrons a molecule has, the bigger it is and the greater these attractions. Iodine molecules (I₂) have 36 more electrons than Br₂ molecules. More energy is required to break the stronger instantaneous dipole-induced dipole attractions holding the molecules together, and so at room temperature iodine is a solid.

Sometimes the three-dimensional shape of a molecule might be important when considering the strength of the instantaneous dipole-induced dipole attractions. If a molecule packs together well, like the long unbranched chain of poly(ethene) shown below, there are more chances for intermolecular forces to be set up than in a similarly sized molecule with many branches off the main chain.