What are Polymers?

• A polymer is a large molecule composed of repeating structural units. These sub-units are typically connected by covalent chemical bonds.

• Because of the extraordinary range of properties of polymeric materials, they play an essential and ubiquitous role in everyday life

Examples:

• Synthetic rubber • Bakelite• Neoprene• Nylon• PVC • Polystyrene

Polymer Degradation• Polymer degradation is a change in the properties

tensile strength, colour, shape, etc.

• Polymer-based product under the influence of one or more environmental factors such as heat, light or chemicals such as acids, alkalis and some salts. These changes are usually undesirable, such as cracking and chemical disintegration of products.

Modes of Degradation

– Chemical

– Thermal

– Biological

– Photo (Light Induced)

Mechanistic Aspects

Single Step Reactions Reaction rate is proportional to the rate of

initiation.

Chain Reactions Self propagation of the processes, once

started initiation reactions yield products that themselves are capable of undergoing spontaneous reactions with intact molecules.

Chemical Degradation: Polymers can be degraded by solvolysis and mainly

hydrolysis to give lower molecular weight molecules. The hydrolysis takes place in the presence of water containing an acid or base.

Polymers are susceptible to attack by atmospheric

oxygen, especially at elevated temperatures encountered during processing to shape.

Chlorine Induced Cracking:

• Another highly reactive gas is chlorine, which will attack susceptible polymers such as acetal resin and polybutylene pipe work.

• There have been many examples of such pipes and acetal fittings failing in properties in the US as a result of chlorine-induced cracking.

Thermal Degradation:• Thermal degradation of polymers is molecular deterioration as a result of

overheating.• It generally involves changes to the molecular weight of the polymer and

typical property changes include reduced ductility and embrittlement, chalking, color changes.

Graph of Weight v/s temperature

Photo Induced Degradation:

• Most polymers can be degraded by photolysis to give lower molecular weight molecules.

• Electromagnetic waves with the energy of visible light or higher, such as ultraviolet light,

X-rays and gamma rays are usually involved in such reactions.

Biological Degradation:

• Biodegradable plastics can be biologically degraded by microorganisms to give lower molecular weight molecules.