POLYMERS

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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.

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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
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Common Polymers
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Common Polymers
Israelachvili Intermolecular and Surface Forces
2 Ed. p. 290
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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.

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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).
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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.

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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.

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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

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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).
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Polymer Adsorption
Effect of molecular weight
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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.

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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.

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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.

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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.

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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.

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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.
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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.
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Adsorbed Conformation (contd.)
Israelachvili Intermolecular and Surface Forces
2 Ed. p. 291
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Polymeric Dispersants -- Good Solvents --
23
Interparticle Forces
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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).

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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)
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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.

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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
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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.

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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)
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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.

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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
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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.

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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.

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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

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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.

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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.

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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.

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