Which statement about synthetic rubber is generally correct in comparison with its behaviour across temperatures and environments?

Introduction / Context:
Synthetic rubbers (e.g., SBR, NBR, CR) span a wide property range. However, a common generalisation in basic exams is their relatively poorer low-temperature flexibility compared with certain natural or specialty elastomers, leading to brittleness in the cold. Understanding temperature effects helps in selecting elastomers for service environments.

Given Data / Assumptions:

• Many general-purpose synthetics (e.g., SBR) stiffen markedly at low temperature and can become brittle below their glass transition.
• At higher temperatures, most rubbers soften rather than harden.
• Chemical resistance and permeability vary widely and cannot be generalised as “highly permeable and readily attacked” for all synthetics.

Concept / Approach:
Low-temperature brittleness is a recognised limitation for several synthetic rubbers because their glass transition temperatures are higher than that of natural rubber or silicone elastomers. Thus, among simplified textbook statements, “brittle at low temperature” is the most broadly correct. The other options are overgeneralised or incorrect.

Step-by-Step Solution:
Consider temperature dependence of modulus near Tg.Note that many synthetics show embrittlement at low temperatures.Select the statement that reflects this behaviour.

Verification / Alternative check:
Data sheets report brittle points and Tg values; many synthetics have brittle points above those of natural rubber or silicone.

Why Other Options Are Wrong:
Permanent deformation at 2x stretch: depends on formulation; not universally true.Harder at higher temperature: elastomers generally soften with heat.Highly permeable/attacked by most chemicals: too broad; many synthetics offer good chemical resistance.Always stronger than natural rubber: not universally true.

Common Pitfalls:
Assuming one synthetic rubber represents all; properties vary significantly among SBR, NBR, EPDM, CR, etc.

Final Answer:
It is brittle at low temperature.