Molecular Weight of Polymers

Molecular weight of Polymers

Molecular weight of a polymer is defined as sum of the atomic weight of each of the atoms in the molecules, which is present in the polymer.

A complete description of the molecular weight distribution of a polymer is necessary in order to understand its physical, rheo-logical, and mechanical properties. Plastic processing can also be affected by molecular weight distribution; in some process narrow distribution of molecular weight is preferred and in some case wider ranges is preferred.

Influence of Molecular weight of Polymers

The influence of molecular weight on the bulk properties of polyolefin's, an increase in the molecular weight leads to

Increase in:

1. Melt viscosity

2. Impact strength

Lowers in:

1. Hardness

2. Stiffness

3. Softening point

4. Brittle point

High molecular weight polymer does not crystallize so easily as lower molecular weight material and that affect the bulk properties.

A high molecular weight polymer increases the mechanical properties. Higher molecular weight implies longer polymer chains and a longer polymer chain implies more entanglement thereby they resist sliding over each other.

Increasing the molecular weight and the chain length of the polymer increases impact strength.

Thermal properties can also improved by increasing the molecular weight.

Polydispersity Index for Molecular weight of Polymers

Polydispersity is a very important parameter and it gives an idea of lowest and the highest molecular weight species as well as the distribution pattern of the intermediate molecular weight species. Plastics processing are affected by the molecular weight distribution.

Polydispersity Index for Molecular weight of Polymers

To bring into sharper focus the effect of molecular weight on physical properties, a more generalized form of representation is given in figure, mechanical strength is plotted against 'DP .

The useful range of DP is from 200 to 2000, which corresponds to its molecular weight 20000 to 200000

Polydispersity Index for Molecular weight of Polymers

The nature of polymerization process for UHMWPE results in a narrow MWD that is why this material is not useful for the extrusion and other process except injection molding.

Polymerization

1. Two major types of polymerisation methods are used to convert small molecules (monomers) into polymers.

These methods were originally referred to as

1. Addition polymerisation and is also called as chain-, chain growth-, or chain reaction polymerisation

2. Condensation polymerisation and is also described under headings such as step-, step growth-, or step reaction polymerisation and sometimes polycondensation.

For example, polyethylene and polypropylene are addition polymers (as the monomers undergo addition polymerisation) whereas Nylons, PET etc. are condensation polymers (as the monomer pairs undergo condensation).

Chain Growth Polymerisation

1. The monomers contain at least a double bond that participates in the polymerisation reaction.

2. The initiators for chain polymerisation may be different depending upon the nature of initiation i.e. free radical, cation and anion.

3. The carbon-carbon double bond in vinyl monomers and the carbon-oxygen double bond in aldehydes and ketones are the two main types of linkages that undergo chain polymerisation.

4. Important characteristics of chain polymerisation

5. Once the initiation occurs, the polymer chains form very quickly i.e. in the time scale of 10-1 to 10-6 s,

6. The catalyst concentration needed is very low and that means during the course of polymerisation only monomers and polymer are present,

7. The process is exothermic.

8. High polymers with molecular weights of 10,000 to 10 million can be obtained,)