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POLYMERS

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Title: POLYMERS


1
POLYMERS
2
Introduction
  • 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.

3
Introduction
  • 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.

4
Introduction
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
5
Common Polymers
6
Common Polymers
Israelachvili Intermolecular and Surface Forces
2 Ed. p. 290
7
Solution 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.

8
Solution 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).
9
Polymer 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.

10
Solution 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.

11
Solution 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

12
Polymer 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).
13
Polymer Adsorption
Effect of molecular weight
14
Polymer 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.

15
Polymer 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.

16
Segment/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.

17
Segment/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.

18
Segment/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.

19
Adsorbed 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.
20
Adsorbed 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.
21
Adsorbed Conformation (contd.)
Israelachvili Intermolecular and Surface Forces
2 Ed. p. 291
22
Polymeric Dispersants -- Good Solvents --
23
Interparticle Forces
24
Background
  • 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).

25
Background
  • 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)
26
Background
  • 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.

27
Dispersant 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
28
Steric 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.

29
Steric 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)
30
Steric 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.

31
Forces 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
32
Steric 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.

33
Steric 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.

34
Electrosteric 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

35
Depletion 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.

36
Depletion 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.

37
Depletion 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.

38
Summary
(Hunter, Foundations of Colloid Science, p. 492,
1993)
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