GATE 2021 

Glass Transition Temperature | Melting Point | Density 

Refer to the table below and start mugging up for a lot of sequential questions of 2 marks these are easy marks.


The values are given of all polymers but mug up all regular polymer values. as GATE exam doesn't take the recent questions hence focus on density and melting point range they are having odds of coming in exam.

Glass Transition Temperature Vs Melting Temperature


At the molecular level, at Tg, the chains in amorphous (i.e., disordered) regions of the polymer gain enough thermal energy to begin sliding past one another at a noticeable rate. The temperature where the entire chain movement occurs is called the melting point (Tm) and is greater than the Tg

  1. Glass Transition is a property of the amorphous region while melting is the property of the crystalline region
  2. Below Tg, there exists a disordered amorphous solid where chain motion is frozen and molecules start wiggling around above Tg. The more immobile the chain, the higher the value of Tg.
  3. While below Tm it is an ordered crystalline solid which becomes disordered melt above Tm

The operating temperature of polymers is defined by transition temperatures




Factors Affecting Tg


Chemical Structure

  • Molecular Weight – In straight-chain polymers, an increase in MW leads to a decrease in chain-end concentration resulting in decreases in the free volume at the end group region – and an increase in Tg
  • Molecular Structure - Insertion of bulky, inflexible side group increases Tg of material due to a decrease in mobility,
  • Chemical cross-linking - Increase in cross-linking decreases mobility leads to a decrease in free volume and increase in Tg
  • Polar groups - The presence of polar groups increases intermolecular forces; interchain attraction and cohesion leading to a decrease in free volume resulting in an increase in Tg.

Addition of Plasticizers

The addition of plasticizer increases the free volume in polymer structure (Plasticizer gets in between the polymer chains and spaces them apart from each other)

This results in polymer chains sliding past each other more easily. As a result, the polymer chains can move around at lower temperatures resulting in a decrease in Tg of a polymer

Water or moisture content

An increase in moisture content leads to the formation of hydrogen bonds with polymeric chains increasing the distance between polymeric chains. And, hence increases the free volume and decreases Tg.

Effect of entropy and enthalpy

The value of entropy for amorphous material is higher and low for crystalline material. If the value of entropy is high, then the value of Tg is also high.

Pressure and free volume

An increase in pressure of the surrounding leads to a decrease in free volume and ultimately high Tg.

Other factors like branching, alkyl chain length, bond interaction, the flexibility of polymer chain, film thickness, etc. also have a significant impact on the glass transition temperature of polymers.


DENSITY OF ALL POLYMERS 


Density of Polymers

Density measures the mass per unit volume. It is calculated by dividing the mass of the material by the volume and is normally expressed in g/cm3.

The density of a plastic sample may change due to change in crystallinity, loss of plasticizers, absorption of solvent, etc. It is important to note that density varies with temperature.

Therefore, to determine the exact density of plastic, it is important to:

   »  Identify the Material
   »  Follow the physical changes in the sample
   »  Indicate the uniformity among different sampling specimens
   »  Indicate the average density of a large item, and
   »  Calculate the strength weight and cost-weight ratios

Density can be converted to specific gravity by dividing the Density value (in g/cm3) by 0.9975.