THE EFFECT OF ADDED POLYMERS ON DISPERSION STABILITY

1
Lecture 3

• THE EFFECT OF ADDED POLYMERS ON DISPERSION
  STABILITY
• polymers at interfaces
• steric interaction
• bridging interaction
• poor conditions for the adsorbed polymer
• depletion interaction

2
? polymers at interfaces

• it is very important to establish the polymer
  adsorption isotherm, for the system being studied
  (by direct surface ananlysis or by analysing the
  equilibrium solution)
• it is also useful to determine the adsorbed
  layer thickness (e.g. DLS)

adsorbed amount, ?
conformation
adsorbed layer thickness, ?
C
D
?
?
C
D
at B
B
?
at C
A
B
A
equil. polymer conc.
initial polymer conc.
3
? steric interaction
this is most effective closest to point C in the
adsorption isotherm

• where
• ? is the Flory polymer/solvent interaction
  parameter
• ? lt 0.5 good solvent VS repulsive (the
  case shown below)
• ? gt 0.5 poor solvent VS attractive

VS
h 2?
h
4
? requirements for efficient steric stabilization

• (1) high ? (around point C)
• (2) high ?
• (3) strong adsorption of polymer to the surface
• (4) good solvent environment for the polymeric
  stabilizer
• not too high a free polymer concentration
• Note (3) (4) are somewhat incompatible for a
  homopolymer
• Therefore graft or block copolymers are better
• (a non-ionic surfactant is effectively an
  oligomeric block copolymer)

A anchor B bouy
B
A
B
A
5
? relaxation of the conditions for steric
stabilization

• If any of the above conditions is relaxed
  then aggregation may result, i.e.
• ? too low (i.e. between A and B on the adsorption
  isotherm)
• bridging flocculation can occur

made use of in, e.g. , water and wine/beer
purification, mineral recovery, bacterial
harvesting
6
? relaxation of the conditions ( cont.)

• (2) ? not large enough
• weak, reversible flocculation may occur
  into Vmin

reduce ?
Vtot
VS
Vtot VS VA
h
Vmin
VA
(3) if the polymer is only weakly adsorbed, then
it may be displaced from the surface during a
particle collision. This would result in
(irreversible) coagulation, as the two particles
come into primary contact (h0).
7
? relaxation of the conditions ( cont.)
(4) poor solvent environment for stabilising
chains (? lt 0.5) also leads to weak
reversible flocculation into Vmin
VS eventually becomes repulsive again at small h,
due to elastic repulsion on squashing the
adsorbed chains.
VS

Vmin
h

• lt 0.5 in previous equ. for VS

Note VA omitted for clarity
8
? relaxation of the conditions ( cont.)

• (5) too high a free polymer concentration, after
  adsorption equilibrium attained (region D of the
  adsorption isotherm)
• Leads again to weak, reversible flocculation
  into Vmin, i.e.
• depletion flocculation can occur.

adsorbed polymer sheath
?
free polymer chains cannot enter the gap between
the particles
solvent tries to leave the gap (osmotic
pressure), pulling the two particles together
depletion attraction ? another form of Vmin
9
depletion interaction

• also occurs for non-adsorbing polymers

?p
?pb
effective depletion layer, free of polymer (ie
only solvent)
h
?
? ?
z
2?
 
h
z
Vmin
Vdep
? is a function of Mp and ?pb ? decreases
as ?pb increases beyond ?pb hence, Vmin goes
through a maximum with increasing ?pb

10
example of system showing bridging and
depletion aggregation
PEO grafted
poly(acrylic acid)
PS core
Cawdery and Vincent, 1995