Polymer Processing III

Question 1

How does concentration impact polymer viscosity in solutions?

Answer 1

viscosity is highly dependent on concentration
- analogous to the relationship between viscosity and molecular weight in polymer melts

see 3 distinct phases:
- to C* is dilute and increases slightly

• C* to Ce is semidilute, unentangled - where the hydrodynamic volumes start to overlap
• Ce to C** and up is semi-dilute entangled, where the polymer lines are entangled and causes significant increase in viscosity

And then above C** is concentrated, where may start to get phase separations

Question 2

Increasing polymer solution concentration and viscosity change with shear rate

Answer 2

• increasing concentration starts at a higher initial viscosity, but all end up with similar slope when it enter shear thinning

Question 3

What is solvent casting?

Answer 3

• not often done on industrial scale, but most basic form
• polymer is blended with a volatile solvent (chi < 0.5) then cast onto a smooth surface. Solvent is removed through evaporation to create a dry film (drying may be at ambient conditions or assisted by a vacuum oven)

Question 4

What is spray coating?

Answer 4

can be used at scale for ultrathin coatings

• spray out polymer through a nozzle to get fine dispersion (this becomes more difficult if polymer has high entanglement/viscoelastic effects)

Question 5

What is spin coating?

Answer 5

• often used in organic polymers and to produce uniform layers
• rotation is used to evenly spread a polymer solution across a prepared substrate (solvent evaporates as its spun - spinning forces solvent to evaporate very quickly)

Question 6

What is dip coating?

Answer 6

use a large bath of polymers, dip a substrate in and get polymer to attach to side (put on polymer coatings - desired to get certain surface qualities) solvent evaporates once pulled out of bath

qualities:
- corrosion resistance
- waterproofing
-antifouling
- modifying friction
- electrical insulation

*an inexpensive technique that allows complete surface coverage

Question 7

What is fibre spinning?

Answer 7

uses extensional flow of a polymer solution to produce highly oriented continuous fibres with controlled diameters

• similar to nanofibrillation, a spinnet (extruder like) where the fibres are pulled into a spindel

*can be done wet spinning or dry spinning (where hot air evaporates the solvent)

Question 8

What is electrospinning?

Answer 8

uses a current to draw extremely fine filaments from a polymer solution. The polymer narrows into a taylor cone as its pulled and then as its pulled even more away from the spinneret, it starts to coil

• this is an extremely finicky process and sensitive to process variables. It has a very low throughput and challenging to scale, but capable of producing fibres with nanoscale diameters

Question 9

How is fibre spinning affected by polymer concentration?

Answer 9

• spinnability and fibre morphology are highly dependent on polymer concentration in solution
• C < C* = electrospray (get droplets)
• Ce > C > C* = beaded fibres (not desired)
• Cmax > C > Ce = homogeneous fibres
• C > Cmax = needle clogging (high fiber diameters)