Title: POLYMERS
1POLYMERS
2Introduction
- Polymers play important role in a range of
technologies, eg. lubrication, adhesion,
stabilization, flocculation, enhanced oil
recovery. - When particles not fully covered and polymer
chain is long enough, adsorption may occur on
two or more particles ? bridging flocculation.
Basis of separation in water (drinking and
wastewater) treatment, minerals processing, paper
manufacturing. - For particles fully covered with polymer,
repulsion may - occur ? steric stabilization as in paints and
coatings, foods, drugs. - Natural polymers (eg. protein, DNA) are critical
to life biological processes. App. in medicine,
food industry.
3Introduction
- Polymer - from Greek poly (many) and mer
(part). - IUPAC definition A polymer is a substance
composed of molecules characterized by the
multiple repetition of one or more species of
atoms or groups of atoms (constitutional
repeating units) linked to each other in amounts
sufficient to provide a set of properties that do
not vary markedly with the addition of one or a
few of the constitutional repeating units.
4Introduction
Monomer - chemical species from which a polymer
is made. MW of typical monomer, Mo ? 50 - 100 MW
of polymer, M nMo ? 1,000-1x106 or more where n
is number of repeating units
5Common Polymers
6Common Polymers
Israelachvili Intermolecular and Surface Forces
2 Ed. p. 290
7Solution Properties
- In solution, polymer segments cannot overlap due
to volume exclusion ? segment repulsion (also
electrostatic repulsion for polyelectrolytes). - Segment-segment attraction possible, eg. van der
Waals and hydrophobic forces. - Solvent affects intra-molecular segment-segment
interactions ? different polymer configurations
possible depending upon solvent - Solvent where segment-segment interactions very
weak or zero ? polymer adopts randomly
fluctuating 3-D structure or random coil
configuration. Such a solvent is an ideal solvent.
8Solution Properties
Rg
- Length of unperturbed coil defined by radius of
gyration, Rg (rms distance from center of mass) - where l effective segment length
- n number of segments
- Equation is used for ideal solvent case, ie. no
interactions (repulsive or attractive) between
segments.
Atkins, P. W., Physical Chemistry, 3rd Ed. p. 624
(1988).
9Polymer at the Solid-Liquid Interface
- Polymer adsorption and small molecule adsorption
(eg. surfactants, ions) properties are very
different. This is due to the many configurations
that a polymer can assume in liquid and at
surface. - For flexible polymers, entropy loss per molecule
greater than for small molecules (opposes
adsorption). However, decrease in DGo is greater
due to multiple attachments / chain (favors
adsorption). - Polymer adsorption is generally a slower process
than low MW species such as surfactants.
10Solution Properties
- Energy change (kT) to transfer segment from pure
polymer to pure solvent (or vice-versa) given by
Flory-Huggins parameter, c. - Volume excluded by polymer (Flory and DeGennes,
etc.) defined by - vl3
- where v is dimensionless excluded volume
parameter and is given by - v 1 - 2?
- ? 0.5 ? v 0, and segments behave ideally
(ideal solvent). This condition is known as the
?-point. Polymer size defined by Rg.
11Solution Properties
- In non ideal solvents, polymer can be larger or
smaller than Rg. Flory Radius, RF defined by - RF aRg
- where a intra-molecular expansion factor.
- a 1 for ideal solvent (or Q-solvent).
- If solvent and segment have same polarizability
- ? c 0. Polymer chain then expands (swells)
due to - volume exclusion. Such a solvent is referred
to - as a good solvent.
- For good solvents, c lt 0.5
12Polymer Adsorption
- A key parameter is the determination of the
amount - of polymer adsorbed and the conformation of the
- polymer on the surface.
- Amount of polymer adsorbed generally represented
- in form of an adsorption isotherm.
Scheutjens, Fleer, Cohen Stuart, Cosgrove,
Vincent, Polymers at Interfaces, (1993).
13Polymer Adsorption
Effect of molecular weight
14Polymer Adsorption
- Polymer adsorption isotherms generally have the
following characteristics - (1) high affinity character. However, the lower
MW, the lower the high affinity character
(isotherm becomes more rounded). - (2) Plateau level is of the order of a few mg/m2.
- (3) Adsorption (G) increases with decreasing
solvent - quality.
- (4) In good solvents (c lt 0.5), G increases with
MW for low MWs. For high MW, plateau is
independent of MW.
15Polymer Adsorption
- (5) In q-solvents, G keeps increasing with MW.
- For same MW, G higher in q-solvents than in
- good solvents, particularly for higher MW.
- Adsorption energy depends on both the nature of
the - solvent and the surface, as well as competition
between - polymer and solvent for binding sites.
- Polymer adsorption can be accompanied by a
change - in conformation compared to that occupied in
the bulk. - Nature of the adsorbed layer (conformation,
- thickness, surface coverage) and its
interaction with - the solvent determine a suspensions properties.
16Segment/Surface Interaction
- Polymer/solvent interactions conveniently
described by Flory-Huggins c parameter. - For free energy change in bringing polymer
segment from solvent to surface ? analogous
parameter is cs.. - ?s is an exchange free energy ? segment
attachment is accompanied by displacement of
solvent molecules. - where Gads is adsorption free energy solvent and
segment.
17Segment/Surface Interaction
- -cskT is difference in free energy between
segment / surface and solvent/surface contact. - For positive cs ? polymer adsorbs. For cs ? 0 ?
- polymer will not adsorb.
- If cs is only slightly positive, polymer will
not adsorb due to entropic restrictions, ie. loss
of conformational entropy upon adsorption must be
outweighed by decrease in DGo through formation
of polymer/surface contacts. - cs must ?exceed critical value, csc, before
polymer can adsorb. - csc is generally a few tenths of kT.
18Segment/Surface Interaction
- For cs gt csc, adsorption increases sharply with
increasing cs. - For small molecules such as surfactants,
adsorption will be possible for all positive cs. - If solvent/surface contact has very low free
energy, cs lt csc and polymer adsorption will be
impossible ? some solvents can inhibit
adsorption. - csc can be estimated by adding displacer solvent
to a - system such that desorption of adsorbed polymer
occurs at a critical (mixed) solvent composition.
19Adsorbed Conformation
As bound fraction increases, adsorbed polymer
adopts flatter conformation. Such a conformation
is referred to as a pancake. In this case, cs gt
csc. For limiting case of infinite chain length,
layer thickness will be of the order of segment
size. As bound fraction decreases, layer
thickness increases. Limiting case is the
mushroom conformation, where polymer is adsorbed
(or grafted) onto surface by one segment. In this
case, cs lt csc and the chain is entropically
repelled from surface. Layer thickness will ? be
2Rg.
20Adsorbed Conformation
In some cases, as amount of adsorbed polymer
increases, lateral displacement of adsorbed
segments may occur in order to accommodate more
polymer. When adsorbed density reaches point of
chain overlap, conformation of adsorbed layer
changes dramatically. Polymer chains undergo
stretching perpendicular to the surface and adopt
a brush conformation. In the limiting case,
adsorbed layer thickness will be proportional to
polymer chain length.
21Adsorbed Conformation (contd.)
Israelachvili Intermolecular and Surface Forces
2 Ed. p. 291
22Polymeric Dispersants -- Good Solvents --
23Interparticle Forces
24Background
- Stabilization (dispersion) or aggregation of
particles using polymers widely exploited, eg.
paint, glue, detergent, ink and drug emulsions - Also used in biological systems, eg. blood, milk
- Use of polymers to stabilize colloidal
dispersions known since 2,500 B.C. (Egypt,
China) to prepare ink. - Lamp (carbon) black mixed with natural polymer
solution, eg. Casein (milk), albumin (eggs),
rolled into rod, dried and stored. Rod immersed
in water ? spontaneous redispersion
(characteristic feature of colloids stabilized by
polymers).
25Background
- Faraday (1857) aggregation of gold sols by NaCl
addition prevented by adding gelatin ? particles
coated with envelopes of that animal substance. - Zsigmondy (1901) measured relative effectiveness
of polymers in preventing aggregation of gold sol
with salt ? Gold number (mg polymer just
preventing aggregation of 10 cm3 of gold sol when
1cm3 10 NaCl added).
More effective stabilizing agent
(Hunter, Foundations of Colloid Science, p. 452,
(1993)
26Background
- This type of protective action is referred to as
steric repulsion or steric stabilization. - Conversely, when particles are aggregated, must
define two distinct processes - Coagulation - aggregation induced by van der
Waals attraction or electrostatic forces between
particles. Usually compact agglomerates. - Flocculation - aggregation induced by polymers.
- Generally more open agglomerates.
27Dispersant Selection Criteria for Steric
Stabilization
1. Adsorption (?s) Dispersant must adsorb to
surface under given process conditions 2.
Solubility (?) Interacting moieties of
dispersants adsorbed on separate surfaces
must not phase separate 3. Barrier Magnitude and
Extent Extent of polymer layer and magnitude of
steric force must be great enough to prevent
agglomeration under given process conditions
28Steric Stabilization
- Suspension of uncharged particles will coagulate
due to van der Waals attraction between
particles. - Force is long range (5-10 nm). Stabilization ?
requires repulsive interaction equal or greater
than (in magnitude and range) van der Waals. - Adsorbed polymers extend into solution over
distances similar to or larger than van der Waals
attractive forces. - Uncharged polymer segments are mutually repulsive
in good solvents ? can be used as stabilizing
agents.
29Steric Stabilization
Comparison of electrical double layer thickness
for various electrolyte concentrations and
extension of adsorbed polymer into solution as
function of MW.
(Adapted from Hunter, Foundations of Colloid
Science, p. 453, (1993)
30Steric Stabilization
- Advantages of Steric stabilization over
electrostatics - Insensitivity to added salt, i.e. little variance
on extension of polymer into solution with salt. - Contrasts with length of e.d.l. Above 10-2 M
salt e.d.l. can shrink such that electrostatic
repulsion no longer dominant force ? van der
Waals attraction and coagulation. - Effective in aqueous and non-aqueous media.
- Electrostatic stabilization only effective in
polar solvents. - Effective at high and low volume fraction (f) of
particles. Low viscosity at high f. Electrostatic
stabilization effective for low f.
31Forces between Surfaces with Grafted Polymer
Often used as an approximation for strongly
adsorbed polymers (fitting equation) Scaling
theory approach of de Gennes. For H lt 2L
Wflt/flt Energy between flat surfaces (J/m2) k
Boltzmanns constant (J/K) T temperature
(K) L Polymer layer thickness (m) S Polymer
end to end distance parking area diameter
(m)
S
S
L
L
32Steric Stabilizers
- Block or graft copolymers often used to
stabilize particles (Barrett, 1975). - One type of moiety is insoluble in solvent ?
segments - adsorb onto surface. Anchor moieties.
- Other moieties very soluble in solvent ? chains
expand away from surface into solvent, providing
maximum steric repulsion. Stabilizing moieties.
33Steric Stabilizers
- Stress relief by migration (desorption) of
stabilizing segments away from surface.
Anchoring moieties prevent such desorption. - Alternative stress relief mechanism by lateral
movement of stabilizing segments. Prevented by
complete surface coverage. - Full surface coverage also prevents polymer
attached to one particle adsorbing to bare
surface on another - ? bridging flocculation.
34Electrosteric Stabilization
- For adsorption of polyelectrolyte, can have
combination of steric and electrostatic
stabilization ? electrostatic stabilization. - Electrostatic stabilization common in biological
systems, e.g. proteins - Effectiveness dependent on pH and Ionic Strength
35Depletion Flocculation
- Asakura and Oosawa (1954) flocculation caused by
free polymer in solution referred to as depletion
flocculation. - For two approaching particles, separation
distance is such that polymer chains are excluded
from interparticle (interaction) zone ? occurs
when interparticle separation is approximately
less than Rg.
36Depletion Flocculation
- As polymer is excluded from interaction zone the
local - concentration becomes lower than in the bulk.
- The concentration gradient creates an osmotic
pressure - difference that attracts the two polymer coated
surfaces - together.
37Depletion Stabilization
- Similar but not identical to solvation forces
- Energy required bringing particles from infinite
separation to distance less than polymer
diameter. This repulsive potential energy barrier
opposes approach and provides stability
? depletion stabilization.
38Summary
(Hunter, Foundations of Colloid Science, p. 492,
1993)