Polymers

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Polymers

• Macromolecule that is formed by linking of
  repeating units through covalent bonds in the
  main backbone
• Properties are determined by molecular weight,
  length, backbone structure, side chains,
  crystallinity
• Resulting macromolecules have huge molecular
  weights

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Polymers

• Terminology
• mer a unit
• monomer one unit
• dimer two units
• trimer three units
• tetramer four units
• polymer many units
• pre-polymer growing towards being a polymer
• oligomer few units fixed in size
• homopolymer polymer of fixed mer type

HOMOPOLYMER
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Polymers

• Terminology (contn)
• copolymer polymers of two mer types
• random -B-A-B-A-B-B-A-
• alternating -A-B-A-B-A-B-A-
• block -A-A-A-A-B-B-B-
• heteropolymer polymers of many mer types

COPOLYMER
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Polymers Molecular Weight

• i degree of polymerization ( of monomer units)
• Mi i x Mm
• Mi molar mass of polymer molecule i
• Mm molecular weight of monomer
• Typically all chains are not equally long but
  display a variation
• monodisperse equal chain lengths is specific to
  proteins
• polydisperse unequal length specific to most
  synthetic molecules
• Therefore we need to define an average
  molecular weight
• number average, Mn
• weight average, Mw

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Polymers Molecular Weight

• number average, Mn

• weight average, Mw

Ni of molecules with degree of polymerization
of i Mi molecular weight of i
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Polymers Molecular Weight

• Ratio of Mw to Mn is known as the polydispersity
  index (PI)
• PI is a measure of the breadth of the molecular
  weight
• PI 1 indicates Mw Mn, i.e. all molecules have
  equal length (monodisperse)
• PI 1 is possible for natural proteins whereas
  synthetic polymers have 1.5 lt PI lt 5
• At best PI 1.1 can be attained with special
  techniques
• EXERCISE Draw the molecular weight distribution
  for PI 1, PI 2, and PI 4

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Polymers Molecular Weight

• Biomedical applications 25,000ltMnlt100,000 and
  50,000ltMwlt300,000
• Increasing molecular weight increases physical
  properties however, decreases processibility

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Polymers

• Types of polymers
• Thermoplastic Polymers that flow when heated
  thus, easily reshaped and recycled. This property
  is due to presence of long chains with limited or
  no crosslinks. (polyethylene, polyvinylchloride)
• Thermosetting Decomposed when heated thus, can
  not be reformed or recycled. Presence of
  extensive crosslinks between long chains induce
  decomposition upon heating and renders
  thermosetting polymers brittle. (epoxy and
  polyesters)
• Elastomers Intermediate between thermoplastic
  and thermosetting polymers due to presence of
  some crosslinking. Can undergo extensive elastic
  deformation (natural rubber, silicone)

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Polymers
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Polymers
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Polymers

• Polymers can be either amorphous or
  semi-crystalline
• Tacticity, i.e. arrangements of substituents
  around the backbone, determines the degree of
  crystallinity
• Atactic polymers are amorphous
• Isotactic and syndiotactic may crystallize
• Crytallinity depends on
• size of side groups (smaller, ?crystallinity)
• regularity of chain
• Increased crystallinity enhances mechanical
  properties

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Polymer Synthesis

• Two common methods of polymerization
• Condensation polymerization (or stepwise
  addition)
• Addition reaction (or chain polymerization)
• Condensation Two monomers react to form a
  covalent bond usually with elimination of a small
  molecule such as water, HCl, methanol, or CO2.
  Reaction continues until one type of reactant is
  used up.
• Addition Monomers react through stages of
  initiation, propagation, and termination.
• initiators such as free radicals, cations, anions
  opens the double bond of the monomer which
  becomes active and bonds with other such monomers
• rapid chain reaction propagates in this fashion
• reaction is terminated by another free radical or
  another polymer

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Polymer Synthesis Condensation

• phenol-formaldehyde results in condensation of a
  water molecule
• nylon (polyamide) an organic acid reacts with an
  amine to form an amide. HCl condenses

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Polymers Synthesis Addition

• Termination may occur by
• two radicalized polymers reacting
• another radicalized monomer
• one initiator (alkoxy radical, OR, in this case)

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Condensation vs. Addition

• Addition
• Difficult to control molecular weight
• Undesirable branching products

• Condensation
• Molecular weight closely controlled
• Polydispersity ratios close to unity can be
  obtained

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Polymers Deformation
Stress
I
II
III
IV
Strain
I. Chain unfolding, unwinding, unwrapping or
uncoupling (low energy) II. Chain sliding (low
energy) III. Bond stretching, side group ordering
(high energy) IV. Bond breaking (high energy)
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Polymers Deformation
Ceramics
Metals
Stress
Polymers
Strain

• Lower elastic modulus, yield and ultimate
  properties
• Greater post-yield deformability
• Greater failure strain

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Polymers Viscoelasticity

• Dependency of stress-strain behavior on time and
  loading rate
• Due to mobility of chains with each other
• Crosslinking may affect viscoelastic response

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Polymers Thermal Properties

• In the liquid/melt state enough thermal energy
  for random motion (Brownian motion) of chains
• Motions decrease as the melt is cooled
• Motion ceases at glass transition temperature
• Polymer hard and glassy below Tg, rubbery above Tg

linear amorphous
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Viscoelastic Deformation Glassy Materials and
Tg Rigid Brittle Solid below Tg Viscous
Deformable liquid above Tg
? is the viscosity of the fluid
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Viscoelastic Modulus versus Temperature
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• Fracture of Polymers
• Thermoset and Thermoplastic materials below Tg
  behave as brittle solids and fail by cracking.
  The cracks are sharp.
• Crystalline Thermoplastic Resins above Tg yield
  and undergo ductile failure.
• Noncrystalline Thermoplastic resins above Tg
  undergo Crazing and Cracking.
• Crazing - Orientation of polymer chains across
  the opening of a crack-like feature. The work
  of crazing is 1000 times larger than the surface
  work to create a crack.
• Cracking then occurs down the middle of the
  craze.

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Craze Formation
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Crack Propagation Through Crazed Area
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Cracking Example in Polymers