Question

Assertion A: Below glass transition temperature polymers behave as a glass and above glass transition temperature polymers behave as an elastomer.
Reason R: Below glass transition temperature polymer chain becomes rigid and behaves like a glass; above glass transition temperature polymer chain segments becomes flexible as a result polymers behave like elastomer.
In the light of the above statements, choose the most appropriate answer:

• A. Both A and R are correct and R is the correct explanation of A
• B. Both A and R are correct but R is NOT the correct explanation of A
• C. A is correct but R is not correct
• D. A is not correct but R is correct

Hint:

$T_g$ is like the "thaw point" for polymer chains. Below $T_g$, the chains are "frozen" in place; above $T_g$, they "melt" enough to start wiggling!

Answer 1

The Correct Option is A

Concept: The glass transition temperature ($T_g$) is a critical point for amorphous and semi-crystalline polymers, marking the transition from a hard, brittle state to a soft, rubbery state.

Step 1: Evaluate Assertion A.
Below $T_g$, polymers are in a "glassy" state—hard, rigid, and brittle. Above $T_g$, they enter a "rubbery" or elastomeric state where they are flexible and can be easily deformed. Thus, A is correct.

Step 2: Evaluate Reason R.
The transition at $T_g$ is related to the onset of long-range segmental motion of the polymer chains. Below $T_g$, the thermal energy is too low for chains to slide, making them rigid. Above $T_g$, enough energy exists for segments of 10-50 atoms to move, creating flexibility. Thus, R is correct.

Step 3: Conclusion.
Reason R provides the exact molecular-level explanation for the macroscopic behavior described in Assertion A. Therefore, R is the correct explanation for A.