CHAPTERS 1415: POLYMER STRUCTURES, APPLICATIONS,

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CHAPTERS 14/15POLYMER STRUCTURES, APPLICATIONS,
PROCESSING

• Naturally occurring polymers
• Wood, rubber (Hevea brasiliensis, or Ficus
  elastica), cotton, wool, leather, silk, starches,
  etc etc
• Biopolymers RNA, DNA etc etc
• All livings things are made out of biopolymers
• Our brains are essentially bio-polymer processing
  units, instead of the silicon ones in our
  computers
• After WWII, synthetic polymers started taking a
  hold of our daily lives
• Hydrocarbons, Carbon and Hydrogen backbones

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Hydrocarbons (I)

• Saturated hydrocarbons molecules
• All single bonds
• All are gases at RT

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Hydrocarbons (II)

• Unsaturated hydrocarbons molecules
• Double or triple bonds between C atoms
• More active

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Hydrocarbons (III)

• RADICALS
• Other organic groups can be involved in polymer
  molecules. In table adjacent R represents
  radicals Organic groups of atoms that remain as
  a unit and maintain their identity during
  chemical reactions (e.g. CH3, C2H5, C6H5)

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Polymers

• Polymer molecules are very large, gigantic in
  size called sometimes macromolecules
• Within these atoms are covalently bonded to each
  other
• Most polymers consist of long and flexible chains
  with a string of C atoms as a backbone.
• Side-bonding of C atoms to H atoms or radicals
• Double bonds possible in both chain and side
  bonds
• Repeat unit in a polymer chain (unit cell) is a
  mer
• A single mer is called a monomer
• Many mers is a polymer

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Chemistry of Polymerization

• Ethylene (C2H4) is a gas at STP (RT and pressure)
• Ethylene transform to poly-ethylene (solid) by
  forming active mer through reaction with
  initiator or catalytic radical (R.)
• (.) denotes unpaired electron (active site)
• C-C bond length 0.154 nm

At certain T and P with polymerize with the help
of the Radicals
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Polymer Microstructure

• C-C bond is not 180º, but 109º
• Furthermore, chain rotations act as kinks
• Double or triple bonds are rigid

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POLMER MICROSTRUCTURE
Polymer many mers
Adapted from Fig. 14.2, Callister 6e.
Covalent chain configurations and strength
Direction of increasing strength
Adapted from Fig. 14.7, Callister 6e.
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More on Polymer Microstructure
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Molecular Weight (I)

• Very large molecules can be found in polymers
• Final molecular weight (chain length) is
  controlled by relative rates of initiation,
  propagation, termination steps of polymerization
• Not all chains will grow to the same size hence,
  formation of macromolecules during polymerization
  results in distribution of chain lengths and
  molecular weights
• The average molecular weight can be obtained by
  averaging the masses with the fraction of times
  they appear (number-average molecular weight) or
  with the mass fraction of the molecules
  (weight-average molecular weight).

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Molecular Weight (II)

• Melting / softening temperatures increase with
  molecular weight (up to 100,000 g/mol)
• At room temperature, short chain polymers (molar
  weight 100 g/mol) are liquids or gases,
  intermediate length polymers ( 1000 g/mol) are
  waxy solids, solid polymers have molecular
  weights of 104 - 107 g/mol

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Molecular Shape

• Molecular chains can bend, coil and kink due to
  single C-C bonds rotating on each other
• Neighboring chains may intertwine and entangle
• Large elastic extensions of rubbers correspond to
  unraveling of these coiled chains and getting
  straighter
• Mechanical / thermal characteristics depend on
  the ability of chain segments to rotate, remember
  double CC bonds are stiffer.

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Molecular Structure

• Linear polymers Weak or Van der Waals bonding
  between chains. Examples are polyethylene, nylon,
  and PVC.
• Branched polymers Chain packing efficiency is
  reduced compared to linear polymers - lower
  density
• Cross-linked polymers Chains are connected by
  covalent bonds. Often achieved by adding atoms or
  molecules that form covalent links between
  chains. Many rubbers have this structure.
• Network polymers 3D networks made from
  trifunctional mers, covalent bonds. Have distinct
  mechanical and thermal properties Examples
  epoxies, phenolformaldehyde.

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Isomerism

• Hydrocarbon compounds with same composition can
  assume different atomic arrangements.
• Physical properties may also depend on isomeric
  state (e.g. boiling temperature of normal butane
  is -0.5 C, of isobutane -12.3 C)

• There are two main types of isomerism and each
  have sub groups underneath
• Stereoisomerism spatial pattern of atoms or
  functional groups attaching themselves to the
  chain link
• Isotaticgt all at the same side
• Syndiotacticgt alternating sides
• Atacticgt random positions

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Summary
Natural rubber
Inelastic natural latex -used in early golf ball
cores
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Copolymers

• Copolymers, are polymers which has at least two
  different types of mers.
• They can differ in the way the mers are arranged

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Polymer Crystallinity (I)
polyethylene
Atomic arrangement in polymer crystals is more
complex than in metals or ceramics. The unit
cells are typically very large and complex as
molecules or chains replace ions and or atoms in
these structures. Think of it as packing of
molecular chains in a geometrical array
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Polymer Crystallinity (II)

• Molecular substance such as water, methane etc
  solidify as crystals and are totally amorphous in
  liquid phase
• Polymer molecules are often partially crystalline
  (semicrystalline), with crystalline regions
  dispersed within amorphous material. Because, any
  disorder, kink in the long chains induce an
  amorphous region.
• Degree of crystallinity

Factors effecting crystallinity

• Rate of cooling during solidification time is
  necessary for chains to move and align into a
  crystal structure
• Mer complexity crystallization less likely in
  complex structures, simple polymers, such as
  polyethylene, crystallize relatively easily
• Chain configuration linear polymers crystallize
  relatively easily, branches inhibit
  crystallization, network polymers almost
  completely amorphous, crosslinked polymers can be
  both crystalline and amorphous
• Isomerism isotactic, syndiotactic polymers
  crystallize relatively easily - geometrical
  regularity allows chains to fit together, atactic
  difficult to crystallize
• Copolymerism easier to crystallize if mer
  arrangements are more regular - alternating,
  block can crystallize more easily as compared to
  random and graft
• More crystallinity higher density, more
  strength, higher resistance to dissolution and
  softening by heating

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Polymer Crystallinity (III)
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Examples
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MOLECULAR WEIGHT CRYSTALLINITY
Molecular weight, Mw Mass of a mole of
chains.
Tensile strength (TS) --often increases
with Mw. --Why? Longer chains are entangled
(anchored) better.
Crystallinity of material that is
crystalline. --TS and E often increase
with crystallinity. --Annealing causes
crystalline regions to grow.
crystallinity increases.
Adapted from Fig. 14.11, Callister 6e. (Fig.
14.11 is from H.W. Hayden, W.G. Moffatt, and J.
Wulff, The Structure and Properties of Materials,
Vol. III, Mechanical Behavior, John Wiley and
Sons, Inc., 1965.)
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TENSILE RESPONSE BRITTLE PLASTIC
Stress-strain curves adapted from Fig. 15.1,
Callister 6e. Inset figures along plastic
response curve (purple) adapted from Fig. 15.12,
Callister 6e. (Fig. 15.12 is from J.M. Schultz,
Polymer Materials Science, Prentice-Hall, Inc.,
1974, pp. 500-501.)
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Stress-Strain Behavior of Polymers (I)

• Overall similar to metals, but
• The stress-strain behavior can be brittle
  (A),plastic (B), and highly elastic (C)
• Deformation shown by curve C is totally elastic
  (rubberlike elasticity). This class of polymers -
  elastomers

Elastic Modulus same as metals Ductility (EL)
same as metals Yield strength type B curves,
maximum on the curve right after elastic
region Tensile Strength defined as the fracture
strength, can be lower than YS different than
metals
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TENSILE RESPONSE ELASTOMER CASE
Stress-strain curves adapted from Fig. 15.1,
Callister 6e. Inset figures along elastomer
curve (green) adapted from Fig. 15.14, Callister
6e. (Fig. 15.14 is from Z.D. Jastrzebski, The
Nature and Properties of Engineering Materials,
3rd ed., John Wiley and Sons, 1987.)
Compare to responses of other polymers
--brittle response (aligned, cross linked
networked case) --plastic response
(semi-crystalline case)
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Stress-Strain Behavior of Polymers (II)

• Some points with polymers
• Moduli of elasticity are 10 MPa - 4 GPa (metals
  50 - 400 GPa)
• Tensile strengths are 10 - 100 MPa (metals
  100 MPa to 10 GPa)
• Percent elongation can be up to 1000 in some
  cases (lt 100 for metals)
• WHY ?
• Mechanical properties of polymers change
  dramatically with temperature, going from
  glass-like brittle behavior at low temperatures
  to a rubber-like behavior at high temperatures.
• Polymers are also very sensitive to the rate of
  deformation (strain rate). Decreasing rate of
  deformation has the same effect as increasing

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Stress-Strain Behavior of Polymers (III)

• Impact of temperature
• Decrease in elastic modulus
• Reduction in tensile strength
• Increase in ductility

polymethyl methacrylate (PMMA) plexiglass
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Deformation (I)

• Elastic deformation
• Basic mechanism of elastic deformation is
  elongation (straightening) of chain molecules in
  the direction of the applied stress. Elastic
  modulus is defined by elastic properties of
  amorphous and crystalline regions and by the
  microstructure.
• Plastic deformation
• Plastic deformation is defined by the interaction
  between crystalline and amorphous regions, and is
  partially reversible.
• Stages of plastic deformation
• 1. elongation of amorphous chains
• 2. tilting of lamellar crystallites towards the
  tensile axis
• 3. separation of crystalline block segments
• 4. stretching of crystallites and amorphous
  regions along tensile axis
• IMPORTANT where are the dislocations ?

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Deformation (II)

• The macroscopic deformation involves necking.
  Neck region gets stronger since the deformation
  aligns the chains and increases local strength in
  the neck region (up to 2-5 times) Neck will
  expand along the specimen.
• What is different from metals is ? The neck
  region will expand ! Whereas in metals
  deformation will be limited to the neck region !

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Factors that influence mechanical properties (I)

• Temperature and strain rate
• Chain entanglement, strong intermolecular bonding
  (van der Waals, cross-links) increase strength
• Drawing, analog of work hardening in metals,
  corresponds to the neck extension. Is used in
  production of fibers and films. Molecular chains
  become highly oriented
• properties of drawn material are anisotropic
• perpendicular to the chain alignment direction
  strength is reduced
• Heat treatment - changes in crystallite size and
  order
• undrawn material Increasing annealing
  temperature leads to
• increase in elastic modulus
• increase in yield/tensile strength
• decrease in ductility
• drawn material opposite changes (due to
  recrystallization and loss of chain orientation)

Note that these changes are opposite from metals
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Factors that influence mechanical properties (II)

• Tensile strength increases with molecular weight
  effect of entanglement
• Higher degree of crystallinity stronger
  secondary bonding - stronger and more brittle
  material

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SUMMARY
General drawbacks to polymers -- E, sy,
Kc, Tapplication are generally small. --
Deformation is often T and time dependent. --
Result polymers benefit from composite
reinforcement. Thermoplastics (PE, PS, PP,
PC) -- Smaller E, sy, Tapplication --
Larger Kc -- Easier to form and recycle
Elastomers (rubber) -- Large reversible
strains! Thermosets (epoxies, polyesters)
-- Larger E, sy, Tapplication -- Smaller Kc
Table 15.3 Callister 6e Good overview of
applications and trade names of polymers.
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ANNOUNCEMENTS
Reading Chapters 14 and 15
Core Problems
Self-help Problems
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