In the realm of organic chemistry, the mechanical properties of rubber seals generally decline significantly after swelling. Organic solvents can have an affinity for high - molecular organic compounds. However, some high - molecular organic compounds are not easily soluble. They will adsorb solvent molecules, causing their volume to expand. Hydrophilic high - molecular substances will also absorb water molecules and expand in volume. This is what is known as the swelling property of polar substances.

It can be seen that the swelling property of rubber materials can also be explained by the principle of "like dissolves like". When they come into contact or under certain pressure and temperature conditions, they have a mutual solubility effect, which has nothing to do with intermolecular forces. Swelling is one of the common properties of rubber or polymers.

In some solvents, cross - linked rubber or other polymers generally do not dissolve. However, solvent molecules will enter the gaps between the polymer chains, increasing the volume between the chain segments. As a result, the volume of the polymer expands due to swelling.

The swelling property of rubber seals manifests in two ways:
(1) Infinite swelling: Linear polymers can absorb solvents without limit when dissolved in a good solvent until they form a homogeneous solution. Therefore, dissolution can also be regarded as the result of infinite swelling of polymers. For example, NR rubber swells in gasoline.
(2) For cross - linked polymers and linear polymers in poor solvents, swelling can only proceed to a certain extent. After that, no matter how long they are in contact with the solvent, the amount of absorbed solvent will not increase, and an equilibrium state will be reached. The system always remains in a two - phase state. For example, the material of nitrile rubber seals (a synthetic rubber) can swell in a liquefied dimethyl ether organic solution.

The swelling mechanism of nitrile rubber in a liquefied dimethyl ether organic solution can be roughly understood as follows: When the valve of the gas cylinder is opened, the liquefied dimethyl ether in the cylinder comes into contact with the nitrile rubber seal in the valve, and the nitrile rubber will swell. When the valve of the gas cylinder is closed, the inside of the valve gradually "dries", and the swelling property of the nitrile rubber gradually fades, causing the rubber to shrink in volume. As the number of times the valve is opened increases and the content of dimethyl ether blended in the liquefied petroleum gas increases, the nitrile rubber undergoes multiple stress cycles of "swelling - shrinking". The stress of the rubber decreases, the aging process accelerates, and finally, the elasticity of the rubber fails, reducing the performance of the seal and leading to gas leakage from the valve.

Therefore, swelling is a very important property of rubber. Hence, rubber should be kept away from solvents with similar polarities as much as possible.