Difficulty: Easy
Correct Answer: Viscosity decreases with increase in temperature.
Explanation:
Introduction / Context:
Processing polymers requires understanding how viscosity responds to key variables. Melt extrusion, injection moulding, and solution spinning rely on predictable rheology as functions of temperature and molecular weight.
Given Data / Assumptions:
Concept / Approach:
For polymer melts, zero-shear viscosity scales roughly as MW^a (a ≈ 3.4 above entanglement), so increasing molecular weight raises viscosity dramatically. Temperature provides thermal energy for chain mobility, thus decreasing viscosity (Arrhenius or WLF dependence, depending on reference temperature relative to glass transition).
Step-by-Step Solution:
Check MW effect: higher MW → higher viscosity (so (a) is false).Check temperature effect: higher T → lower viscosity (so (b) is true).Options (c) and (d) contradict general rheological behaviour.
Verification / Alternative check:
Capillary rheometry and DSC-supported processing windows consistently show decreased viscosity at elevated temperatures until degradation begins.
Why Other Options Are Wrong:
(a) opposes established scaling; (c) and (d) ignore temperature dependence of polymer mobility.
Common Pitfalls:
Overheating can induce degradation, increasing apparent viscosity irregularly or causing gelation; always stay within safe thermal windows.
Final Answer:
Viscosity decreases with increase in temperature.
Discussion & Comments