Thermal behavior of engineering polymers: Which listed polymer typically begins to decompose (e.g., dehydrochlorinate) before reaching a clean melting transition under standard processing conditions?

Introduction / Context:
Different polymers exhibit distinct thermal transitions (glass transition, melting) and stability limits. Recognizing which polymers degrade before melting informs safe processing windows and stabilizer selection, especially for halogen-containing plastics.

Given Data / Assumptions:

• Processing occurs in air or inert atmosphere typical for extrusion/molding.
• ‘‘Decomposing before melting’’ refers to chemical breakdown that precedes a neat crystalline melt.
• Commercial stabilizers may extend stability but not alter the fundamental tendency.

Concept / Approach:
PVC’s backbone contains labile C–Cl bonds. On heating, PVC undergoes dehydrochlorination, forming conjugated polyenes and releasing HCl, often before a well-defined melting event for the polymer backbone. This requires heat stabilizers (e.g., organotin, Ca–Zn) to process safely. In contrast, many nylons melt cleanly before significant decomposition, and polystyrene softens and flows below its degradation threshold in normal processing.

Step-by-Step Solution:

Verification / Alternative check:
Thermogravimetric analysis and DSC profiles show PVC mass loss and HCl evolution preceding neat melting; industrial practice mandates stabilizers and careful temperature control.

Why Other Options Are Wrong:

• Nylon: Exhibits melting prior to significant degradation under normal conditions.
• Polystyrene: Processes below its decomposition temperature in standard operations.
• None of these: Incorrect because PVC fits the described behavior.

Common Pitfalls:
Assuming additives eliminate decomposition tendency; stabilizers only delay degradation and broaden the processing window.

Final Answer:
PVC