Question

How does molecular weight effect glass transition temperature?

Answer 1

We know that glass transition temperature is the range of temperature in which there is change of polymer from glassy state (rigid) to a rubbery state which is more pliable. Chemically glass transition temperature is represented by the symbol of ${T_g}$ and it is a property of amorphous solid.

Complete step by step answer:
Let’s discuss the Flory-Fox equation that gives the relation of molecular weight of a polymer to its glass transition temperature. The mathematical representation of the equation is,
${T_g} = {T_{g,\alpha }} - \dfrac{K} {{{M_n}}}$
Here, ${T_{g,\alpha }}$ is the maximum value of ${T_g}$ that can be achieved at theoretical infinite molecular mass, ${M_n}$ is molecular mass of the polymer, K is a parameter that is related to the free volume of the polymer.
Let’s discuss the free volume in detail. It is the measure of a room a polymer chain has in which to move in relation to the other polymer chains around it. A polymer composed of long chains has lower free volume compared to the polymer of short chains.
So, a polymer of low molecular mass gives less value of ${T_g}$ and higher molecular mass results in the approach of ${T_g}$ to ${T_{g,\alpha }}$.
So, we can conclude that ${T_g}$ increases with increase of molecular mass of polymer and asymptotically reaches a maximum value.

Note: It is to be noted that, below ${T_g}$ polymers are hard and brittle like glass because of lack of mobility, above ${T_g}$ polymers are soft and flexible like rubber due to some mobility and above ${T_g}$ mechanical and physical properties of polymers change.