Compared to thermoplastic polymers, thermosetting resins are generally characterised by which of the following behaviours or properties?

Introduction / Context:
Thermosets and thermoplastics differ structurally: thermosets form three-dimensional crosslinked networks during cure, whereas thermoplastics are linear or lightly branched and soften on heating. Recognising the practical implications of this structural distinction is essential for materials selection.

Given Data / Assumptions:

• Thermosets do not melt upon reheating; they char or degrade.
• Crosslinking restricts chain mobility, enhancing modulus but reducing ductility and impact strength.
• Solubility is generally low for crosslinked networks; they may swell but not dissolve.

Concept / Approach:
Crosslinks lock chains into a rigid network, increasing heat resistance and dimensional stability but making the material more brittle. In contrast, thermoplastics can be remelted and reshaped. Thermosets can be made through step-growth (e.g., epoxies, phenolics, unsaturated polyesters); they are not limited to addition polymerisation.

Step-by-Step Solution:
Compare structural features: network vs. linear chains.Relate network formation to brittleness and insolubility.Select the statement describing increased brittleness.

Verification / Alternative check:
Mechanical testing shows higher modulus but lower elongation at break for cured thermosets compared with many thermoplastics.

Why Other Options Are Wrong:
Soluble in all solvents: networks are typically insoluble.Only by addition polymerisation: epoxies/phenolics are step-growth; the claim is false.Easily reshaped: thermosets cannot be remelted.Always lower Tg: not generally true; some thermosets have very high Tg.

Common Pitfalls:
Assuming all brittle plastics are thermosets; some thermoplastics are also brittle depending on temperature and formulation.

Final Answer:
They are more brittle in the cured state.