Thermosetting materials exhibit which of the following structural and behavioural features after cure?
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AAre cross-linked molecules
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BSoften readily on heating and can be reshaped
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CAre generally soluble in common solvents
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DNone of these
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EAlways elastomeric regardless of formulation
Answer
Correct Answer: Are cross-linked molecules
Explanation
Introduction / Context:Thermosets like phenolics, epoxies, and melamines cure into networks. Recognising this architecture helps explain their heat, solvent, and mechanical responses.
Given Data / Assumptions:
- Network formation restricts chain mobility.
- Crosslinked polymers do not melt upon reheating and are typically insoluble.
- Elastomeric behaviour is formulation-dependent and not universal for thermosets.
Concept / Approach:The key identifier of a thermoset is a three-dimensional crosslinked network after cure. This structure underlies heat resistance and chemical resistance, in contrast to thermoplastics which soften with heat.
Step-by-Step Solution:Link “thermoset” to “crosslinked network.”Eliminate claims of easy softening/solubility.Select “Are cross-linked molecules.”
Verification / Alternative check:Thermal analysis lacks melting endotherm for cured thermosets; instead, decomposition dominates at high temperatures.
Why Other Options Are Wrong:Soften and reshape: thermoplastic trait.Solvent soluble: networks generally only swell.Always elastomeric: many thermosets are rigid structural materials.
Common Pitfalls:Confusing uncured resin behaviour with cured network behaviour.
Final Answer:Are cross-linked molecules