Question

The Glass Transition Temperature (Tg) of a polymer is a critical parameter. Which of the following statements correctly describes the behaviour of a polymer below Tg?

• A. The polymer chains are in a liquid-like, flowable state
• B. The polymer is in a "rubbery" state with high molecular mobility
• C. The long-range segmental motion of polymer chains is "frozen," making the material brittle
• D. The polymer undergoes a first-order phase transition from liquid to solid

Hint:

Think of polymers below $T_g$ as behaving like window glass (hard, rigid, brittle), and above $T_g$ as behaving like rubber or soft plastics.
The glass transition temperature is a second-order transition, meaning there is a change in heat capacity ($C_p$) but no latent heat.

Answer 1

The Correct Option is C

Step 1: Understanding the Question:
The question asks for the physical behavior and mechanical state of an amorphous polymer when its temperature is lowered below its glass transition temperature ($T_g$).

Step 2: Detailed Explanation:

• What is Glass Transition ($T_g$)?
The glass transition is a second-order thermodynamic transition characteristic of amorphous or semi-crystalline polymers.
It marks the transition from a hard, glassy, brittle state to a soft, flexible, rubbery state as temperature increases.

• Behavior BELOW $T_g$:
At temperatures below $T_g$, the thermal energy ($k_B T$) is insufficient to overcome the energy barriers for cooperative, long-range segmental motion (typically groups of 10 to 50 carbon atoms) of the polymer backbone.
These long-range movements are effectively "frozen" or locked into a disordered, glassy state.
Only local vibrational and short-range rotational motions of side-groups are possible.
As a result, the polymer behaves as a rigid, hard, and brittle solid with high modulus and low impact resistance.

• Behavior ABOVE $T_g$:
Above $T_g$, the long-range segmental motions are activated, allowing polymer chains to slide and slide past one another. This gives rise to high molecular mobility and rubbery/flexible behavior.

• Thermodynamic Classification:
The glass transition is a kinetic and relaxation phenomenon (a second-order transition), not a first-order phase transition like melting or crystallization, which involves a latent heat.

Step 3: Final Answer:
Below $T_g$, the long-range segmental motion of polymer chains is "frozen," making the material brittle.