Most fibre-forming polymers are crystalline or semi-crystalline after drawing. Which one of the following is commonly cited as an exception that is largely amorphous in fibre applications?

Introduction / Context:
Fibre performance often relies on crystalline regions for strength and thermal resistance. However, acrylic fibres (largely PAN-based) are frequently discussed as predominantly amorphous, especially prior to extensive stabilisation or specialty processing.

Given Data / Assumptions:

• Nylon and PET are semi-crystalline after drawing/heat-setting.
• Polypropylene fibres are semi-crystalline.
• PAN-based acrylic fibres are often described as amorphous in basic exam contexts, despite some local order.

Concept / Approach:
While processing can induce orientation and limited ordered regions in PAN, standard teaching materials commonly list polyacrylonitrile (acrylic) as a comparative exception to the predominantly crystalline fibre polymers such as nylons and polyesters.

Step-by-Step Solution:
Compare crystallinity of nylon, PET, PP with acrylic fibre.Note textbook convention: acrylic ≈ largely amorphous.Select polyacrylonitrile.

Verification / Alternative check:
Introductory fibre science texts often classify acrylic fibres as amorphous relative to highly crystalline nylon and PET fibres.

Why Other Options Are Wrong:
Nylon/PET/PP: semi-crystalline in drawn fibre form.UHMWPE: extremely high crystallinity and orientation in fibres.

Common Pitfalls:
Assuming process-induced orientation makes PAN fully crystalline; PAN fibres remain comparatively amorphous.

Final Answer:
Polyacrylonitrile (acrylic fibre)