Classification Of Polymers
There are many ways to classify polymers. Some of the classifications is specific to the use of the polymer, others deal with the synthesis of a polymer. These classifications are pertinent as this makes the study of polymers easier. Before classifying polymers, let’s classify materials.
We are done with the classification of materials, we will now select the section of the polymer from the realm of materials and classify them in different ways, to make it easy for different people to study it.
There are different criteria on which the polymers are classified. Some of them are:
- Intermolecular Interaction
- Skeletal Structure
- Type of mer involved
- Reaction mechanism
Intermolecular Interaction
Based on intermolecular forces, polymers are divided into three; i.e.
- Thermoplastics
- Thermosets
- Elastomers
Thermoplastics are those polymers which soften and ultimately melt upon heating. These polymers show a rational behavior (as one may think) as common solids, i.e. their molecular motion increases as heat is added to them and after a certain amplitude of motion, the molecules are allowed to freely slide over each other (at melting point). The polymeric chains have stronger bonds and restricted motion at normal temperatures, when subjected to heat, the amplitude of the molecular motion increases resulting in lowering of intermolecular forces as a consequence of which polymer softens and ultimately melts.
Thermosets are those polymers which harden or cure upon heating. To explain this behaviour one needs to know about the term cross-linking. Cross-linking is the chemical bonding of two different chains of a polymer. In thermosets, as the polymer is heated, the kinetic energy of the chains increase and the potential sites in two different chains of the polymer reacts at this higher energy, forming new chemical bonds and thus hardens the polymer due to restricted movement of the polymeric chains.
Elastomers are polymers which are relatively flexible if compared to thermosets, and harder if compared to thermoplastics. They are three to ten times stretchable. They have lower cross-linking density, hence the flexible characteristic. These polymers have limited sites to crosslink hence the flexibility.
Skeletal Structure
Depending on the structure of the polymer, it is divided into three types:
- Straight Polymers
- Branched Polymers
- Network Polymers
Monomers react with each other in end to end fashion such that they form linear or straight chains of polymers. These monomers are known as straight polymers
When monomers react in a manner that branches are formed in the chains of the polymers, then those polymers are known as branched polymers.
When the monomers react to form a network, a 3-D structure, then those polymers are known as Network polymers.
Types of mer Involved
On the basis of monomer involved in the polymer, it is divided into three:
- Homopolymer
- Copolymer
- Blending Polymer
Homopolymers are those polymers which are formed by the same type of monomer. For example polyethylene. The entire polymer is made up of a single type of monomer i.e. ethylene.
Blending polymers are those polymers which are made of more than one type of monomers, but they are not chemically bonded. These kinds of polymers are used to enhance the strength of a polymeric membrane to withstand operating conditions.
Copolymers are those polymers which consist of more than one type of monomer, for example, polystyrene-butadiene rubber. Copolymers are further divided into four based on the arrangement of the monomeric units. The types are:
- Random Copolymer
- Alternate Copolymer
- Block Copolymer
- Graft Copolymer
In random copolymer, there is no order of arrangement of the attachment of two polymers, e.g. A-A-B-B-B-B-A-B-B-A-B-A-B-A-A-A
Alternate copolymers are those in which the order of the monomer alternates e.g.
A-B-A-B-A-B-A-B-A-B-A-B-A-B-A-B
Block copolymers are those in which one block of monomers join to other block of monomers e.g.
A-A-A-A-A-B-B-B-A-A-A-A-B-B-B-A-A-A-A-B-B-B
Graft copolymers are those in which the backbone chain is made of one type of monomer and side chain from another type of monomer.
Reaction Mechanism
Polymers are divided into two, based on the reaction mechanism, i.e.:
- Step Growth Polymer
- Chain Growth Polymer
In step-growth polymerization, the polymer grows in a stepwise fashion. A bifunctional monomer is required in step growth polymerization, e.g. formation of Nylon.
In chain growth polymerization, the polymer grows via a chain reaction mechanism where an initiator is needed to initiate the reaction, forming a free radical which would propagate the reaction to form chains of polymers, examples include polyethylene, polypropylene etc.
