Define glass transition temperature of a polymer and describe what type of molecular motion is enabled around glass transition temperature (Tg).
- Glass transition temperature is a characteristic temperature at which polymers behaviour changes between rigid glassy solid and rubbery material.
- The spatial arrangement of atoms can change by rotation around the chain. This molecular motion is not possible when the temperature is below Tg due to insufficient internal energy in the material. (amorphous molecules only).
Explain why glass transition temperature is important to polymer-based applications from stiffness perspective and what implications this has for structural design using polymers.
- Stiffness, and other physical properties, of polymers typically undergoes a significant change around Tg and may be viewed as upper limit temperature for service.
- For loading bearing design, significant stiffness drop can result in dimension instability and excessive deformation due to creep.
Explain what material model you may consider to describe stressstrain behaviour when
(a) T < Tg, (b) T = Tg, (c) T > Tg, and (d) T»_space; Tg
(a) Glassy, Linear elastic model / Hooke’s law
(b) Leathery, Viscoelastic model (e.g. Kelvin Voigt
model)
(c) Rubbery Flow, Viscoelastic model (e.g. Maxwell model)
(d) Viscous flow (e.g. Newton’s law)
How do you calculate the instantaneous strain in the maxwell model?
strain[0] = stress[0] / E
How do you calculate the strain after 100 seconds in the maxwell model?
strain[100] = stress[100] / E + (stress[100] / n)*t
How do you calculate the stress after 150 seconds if the strain is kept constant from the value calculated at 100 seconds in the maxwell model?
stress[150] = stress[0] * exp (-t/tow)
tow = n/E
What is the equation used for strain using the Kelvin-Voigt Model?
strain[t] = stress[0] / E * [1- exp(-E*t/n)]
What is the bending moment formula
stress[0] = M*Y/I
where I = b*d^3 / 12
What factors impact the glass transition temperature (Tg).
1. Chain stiffness • Chemical nature of backbone • Bulky side-groups 2. Intermolecular interactions • Hydrogen bonding (polar group) • Ionic interactions (charged groups) • Covalent bonds (crosslinking) 3. Molar mass 4. Additives (e.g. fillers) 5. Moisture (swelling)
