Viscoelasticity: Application of rheology Flashcards
What is a melt flow indexer?
Method of characterizing polymer flow
steps:
1. load polymer
2. remove bubbles
3. set weight atop polymer
4. pull plug on bottom and let the polymer flow for 5-10 minutes
5. measure flowrate to get viscosity
**however shear rate is constant so would need to do multiple experiments
What is a capillary rheometer?
- add pressure transducer right before capillary
- have a mechanically driven piston used at different rates to see how pressure/viscosity
*use Poiselle equation to get viscosity
what is a torque rheometer?
looks at the melt/flow for combinations of polymers
- add polymers together and mix them together and look at how torque changes over time
*after long periods of time, torque may increase (due to cross linking) or decrease (due to chain cission)
**Has issue in that its complex geometry makes it hard to get Poiselle flow for viscosity to be measured
What is oscillatory shear rheology?
- currently the most common method
- looks at very small flows
- and is a dynamic mechanic analysis
Move the polymer a very small amount very quickly by applying a torque back and forth for only a small angle
**if rotate too much, might get edge fracture where polymer collapses in on itself (get bad data), so instead use a very small amplitude of shear
What is the Cox-Mertz Rule?
An empirical relationship that establishes an equivalence between shear viscosity as a function of shear rate and complex viscosity as a function of angular frequency
n(g) = n(w)
What is the Cox-Mertz rule important?
Allows us to use small amplitude oscillatory shear and relate it to high shear rates (thus avoiding edge fracture)
What does the phase angle mean in oscillatory shear experiments?
The lag of the observed stress from the applied strain
A low delta indicates more solid-polymer like behaviour (90 degree is where system is elastic) - G is only storage modulus
A high delta indicates liquid-like behaviour (0 degrees means total liquid) - G is only loss modulus
What are the different oscillatory geometries?
- cone and plate (truncated (flat) tip) - can get different shear rates with cones of different angles
- cup and bob
- parallel plate (results in larger strain amplitude with smaller gap) + (inside fluid gets relatively less deformed than fluid at edge (whereas angle in other two causes same deformation everywhere)
*each has different geometry based values of Ka and kg used to calculate the modulus/viscosity
When is the Cox-Mertz rule not valid?
Past a certain strain magnitude the response is no longer linear
Once modulus is not longer a straight line with oscillation frequency, Cox-Mertz rule no longer applies
What happens to G’ and G’’ in a frequency sweep?
They vary differently with respect to shear!
At low frequency (slow shear rate) liquid is less viscoelastic - looses more E
while at high frequencies (high shear rate) acts more viscoelastic - retains more E
*material is viscoelastic polymer, viscosity decreases like a viscoelastic material at high shear rates, making the storage modulus greater
What are time sweep experiments and what are they used for?
oscillation at constant amplitude and frequency observed over a long time may result in change in rheology properties as the sample structure changes (crosslinking or chain scission) if changes occur
*this is referred to as chemorheology
**they can also be performed under different atmospheres to observe the impact of inert atmospheres on thermal stability (PEEK see less stable and change under air as opposed to N2 atmosphere)
What are temperature sweep experiments and what are they used for?
oscillation at constant amplitude and frequency observed at different temperatures to observe how the rheology properties change as the structure undergoes temperature change (can see clear differences between crystal, crystal growth, and melt stages)
What happens to G’ and G’’ during crosslinking?
They crossover. As Mw increases (due to cross linking), we get 3 changes:
1. decreasing G’’
2. increase G’
3. increasing viscosity, thus increasing overall G (G*)
**this cossover point where tan theta = 1, often indicates point of gelation
What is the gel point?
At the gel point theta becomes independent of shear rate. Thus is is found by repeated multiple experiments at different shear rates and finding where they overlap
**looks at multiple time sweep experiments at different shear rates to find gel point(here it is not where tan theta = 1 but where they all overlap)
*Typically, gelation is when tan theta = 1
How is elongation flow different than shear flow?
Shear is very easy to pull chains apart
- elongation is pulling chains along (harder to pull apart)
- polymer’s viscoelastic response to elongation flow is sensitive to architecture, especially crosslinking and branching
