Questions from class - Lecture 11 Flashcards
How are the moduli split up in oscillatory measurements? What is the physical meaning of these components?
Split up in two parts:
1) Storage modulus: the energy stored elastically (real part)
2) Loss modulus: the energy dissipated (heat, vibration, … ) (imaginary part)
How are the viscoelastic properties of polymeric materials modelled? Which two kinds of viscoelastic materials are distinguished?
- Maxwell model: combination of elastic spring and viscous element in series. The viscoelastic fluid is modelled.
- Voigt-Kelvin model: parallel arrangement of spring and dashpot, models viscoelastic solid.
Which regions are distinguished in the frequency-dependent shear moduli?
There are four regions in the frequency dependent shear modulus:
1) Terminal flow: viscous fluid
2) Rubber plateau: elastic behaviour
3) Transition
4) Glassy region
Which temperature behavior is found for local dynamic processes?
The relaxation time decreases with increasing temperature. Arrhenius behaviour.
Which polymer characteristics are the frequency width and the plateau modulus related to?
Frequency width: dependent on molar mass - because of entanglements.
Level of plateau: dependent on density, temperature and molar mass.
Which two equivalent expressions have been given for the time-temperature superposition principle?
WLF (Williams-Landel-Ferry): a_T = exp[-(c1*(T_1-T-ref))/(T-T_ref+c2)]
a_T is the shift factor that shifts the curve at temperature T to T_ref.
VFT (Vogel-Fulcher-Tammann): eta = eta_0 * exp|E_A/R(T-T_VFT))
VFT law describes the viscosity in T-region above glass transition temperature.
What is rubber elasticity due to? Why does the simple description fail for high deformations?
The elasticity of rubber is due to the entropy loss upon stretching the polymer.
Since the simple description makes the assumption that the chain is Gaussian and some other simplifications, the model breaks down at high deformations.
