Crystalline Melting Point: Definition, Factors Affecting and Determination

What is Crystalline Melting Point ?

When a polymer is treated beyond its glass transition temperature (T𝘨), it passes from a glassy state to a rubbery state and further heating causes melting of the polymer and starts flowing. 

So we can say that the temperature below which the polymer is in a rubbery state and above which it is a liquid is called the crystalline melting point (T𝘮) of the polymer.

Factors affecting crystalline melting point (T𝘮)

Following are the factors which affect the crystalline melting point (T𝘮).

1. Chain Flexibility

Chain flexibility and crystalline melting point (T𝘮) are inversely proportional to each other that is higher the flexibility, the lower will be the Tm. Also, the presence of double bond and aromatic group in the polymeric backbone lowers the flexibility which increases the crystalline melting point (T𝘮).

2. Size and type of side groups

Size and type of size groups greatly influence the crystalline melting point (T𝘮). Bulky or large side groups tend to restrict the molecular rotation which increases the crystalline melting point (T𝘮).

3. Presence of Polar groups

The presence of polar groups like CN, OH, Cl in the main chain and the hydrogen bonding within the crystal increases the crystalline melting point (T𝘮).

4. Degree of Branching

Side branching introduces defects into the crystalline materials which decrease the crystalline melting point (T𝘮).

Determination of crystalline melting point (T𝘮)

crystalline melting point (Tm) can be determined by various methods like differential scanning calorimetry (DSC), Dialometry, Dynamic Mechanical Analysis (DMA), Broad-line NMR, Dielectric loss method.

The Differential Scanning Calorimetry (DSC) method is the most common method to calculate crystalline melting point (T𝘮).

Differential Scanning Calorimetry (DSC)

Differential scanning calorimetry (DSC) is a technique used to investigate the response of polymers to heating. DSC can be used to study the Crystalline melting point (T𝘮) as well as Glass transition temperature (T𝘨).

A given in the above figure, the DSC setup is composed of a measurement chamber and a computer. Two pans are heated in the measurement chamber. The sample pan contains the material being investigated whereas the second pan which is typically empty is used as a reference pan.

Differential Scanning Calorimetry (DSC)

The computer is used to monitor the temperature and regulate the rate at which the temperature of the pan changes. A typical heating rate is around 10 ℃ / min.

The rate of temperature for a given amount of heat will differ between the two pans. This difference depends on the composition of the pan contents and physical changes like phase changes. 

For that heat flux, DSC varies the heat provided to one of the pans to keep the temperature of both pans the same. Finally, the difference in heat output of the two heaters is recorded and the result is a plot of the difference in heat (q) versus temperature (T).

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