Polymers: Polyelectrolytes, Viscosity & Rheology Flashcards
What are polyelectrolytes?
Polymers which have ionisable functional groups (charged)
What are the 3 different types of polyelectrolyte?
- Cationic (+) polyelectrolyte = polybases
- Anionic (-) polyelectrolyte = polyacids
- Amphoteric (+/-)polyelectrolyte = polyampholytes
How do polyelectrolytes become anionic?
Ionisation
e.g. COOH functional group of chain becomes COO- + H+
How do cationic molecules become soluble in water?
They become ionised/protonated
e.g. NH2 +H+ = NH3+
Why are cationic polyelectrolytes insoluble in water when uncharged?
They are slightly crystalline due to a large umber of hydrogen bonds within the molecule
What is the isoelectric point (IEP)?
The pH at which the effective net charge on a macromolecule is 0
What is the charge of a polyampholyte affected by?
pH
Define: Viscosity
The measure of a material’s resistance to flow
It is a result of the internal friction of the material’s molecules
List the 4 conformations (shapes) of macromolecules in solution
- Globule
- Statistical coil
- Rigid rod
- Helix
What does a macromolecule’s conformation depend on?
The chain flexibility and polymer-solvent interactions
Describe the statistical coil conformation
Very swollen
Lots of solvent molecules inside
Describe the rigid rod conformation
When the macromolecule is fully stretched
When is the statistical coil conformation formed?
Only when the polymer is in a certain solvent = thermodynamically ideal solvent
When does an unfolded chain conformation form?
In a thermodynamically good solvent
When is the globule conformation formed?
In a thermodynamically poor solvent
When does an aggregate of globules form?
In a non-solvent
When is a solution at its most viscous?
When its polymers are in an unfolded chain conformation
It is at its least viscous when its polymers form an aggregate of globules
Define: Rheology
The study of the flow and deformation properties of matter
Define Newton’s Law
γ = σ/η
What do the symbols in Newton’s Law represent?
γ = shear rate (s-1) σ = applied stress (N) η = dynamic (absolute) viscosity (Nm-2s)
What is a Newtonian fluid?
A fluid in which the viscous stresses arising from its flow, at every point, are linearly proportional to the local strain rate = the rate of change of its deformation over time
i.e. Shear rate is linearly proportional to shear stress
What is a non-Newtonian fluid?
A fluid whose properties differ in any way from a Newtonian fluid
List the 3 types of solution viscosity
- Relative viscosity
- Specific viscosity
- Reduced viscosity
How does pH affect the viscosity of a polyanion?
Increased pH = reduced viscosity
How does pH affect the viscosity of a polycation?
Increased pH = increased viscosity
How does pH affect the viscosity of polyampholyte?
Most viscous at IEP = isoelectric point = overall charge on molecule is 0
Describe the properties of alginates
- Sodium and potassium alginates are soluble in water
- They spontaneously form physical gels upon contact with divalent metal ions (e.g. calcium, zinc, etc)
What is an alginate?
An anionic polysaccharide
How does Gaviscon work?
- By forming a gel raft (thick layer) on top of the stomach contents soon after it has come into contact with the acid
- The raft acts as a strong physical barrier and helps keep all the components the stomach, not letting them seep back up the oesophagus where they cause discomfort
List 3 types of non-Newtonian flow
- Dilatant flow
- Simple pseudoplastic flow
- Simple plastic flow
Describe plastic flow
Plastic flow is associated with the presence of flocculated particles in concentrated suspensions
Describe pseudoplastic flow
Typically exhibited by polymers in relatively concentrated solutions
Describe dilatant flow
Dilatant flow is exhibited by certain suspensions with a high percentage of dispersed solids
