CHAPTER 4: POLYMER STRUCTURES

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CHAPTER 4POLYMER STRUCTURES
Spherulite, rubber specimen. Chain-folded
lamellar crystallites, 10 nm thick, 30,000
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4.1 Structures of Polymers

• Introduction and Motivation
• Polymers are extremely important materials (i.e.
  plastics)
• Have been known since ancient times cellulose,
  wood, rubber, etc..
• Biopolymers proteins, enzymes, DNA
• Last 50 years tremendous advances in synthetic
  polymers
• Just like for metals and ceramics, the properties
  of polymers
• Thermal stability
• Mechanical properties
• Are intimately related to their molecular
  structure

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4.1 Ancient Polymers

• Originally natural
• polymers were used
• Wood
• Rubber
• Cotton
• Wool
• Leather
• Silk

Oldest known use Rubber balls used by Incas Noah
used pitch (a natural polymer) for the ark
Noah's pitch Genesis 614 "...and cover it inside
and outside with pitch."
gum based resins extracted from pine trees
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4.2 Polymer Composition

• Most polymers are hydrocarbons
• i.e., made up of H and C
• Saturated hydrocarbons
• Each carbon singly bonded to four other atoms
• Example
• Ethane, C2H6

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4.2 Unsaturated Hydrocarbons

• Double triple bonds somewhat unstable
• Thus, can form new bonds
• Double bond found in ethylene or ethene - C2H4
• Triple bond found in acetylene or ethyne - C2H2
  

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4.2 Structures of Polymers

• about hydrocarbons
• Why? Most polymers are hydrocarbon (e.g. C, H)
  based
• Bonding is highly covalent in hydrocarbons
• Carbon has four electrons that can participate in
  bonding, hydrogen has only one
• Saturated versus unsaturated

• Unsaturated species contain carbon-carbon
  double/triple bonds
• Possible to substitute another atom on the carbon
• Saturated carbons have four atoms attached
• Cannot substitute another atom on the carbon

Saturated
Unsaturated
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4.2 Hydrocarbon Molecules
Acetylene Ethyne
Ethylene Ethene
Hydrocarbons have strong chemical bonds, but
interact only weakly with one another (van der
Waals forces)
(normal) butane
isobutane
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4.2 Isomerism

• compounds with same chemical formula can have
  quite different structures
• for example C8H18
• normal-octane

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Isomerism compounds of the same chemical
composition but different atomic arrangements
(i.e. bonding connectivity)
2,4-dimethylhexane
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4.2
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4.3 Polymer Molecules
Molecules are gigantic Macromolecules Repeat
units Monomer
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4.3 Polymers

• Polymer molecules
• what is a polymer?
• Polymers are molecules (often called
  macromolecules) formed from a series of building
  units (monomers) that repeat over and over again

• polymers can have a range of molecular weights
• There are many monomers
• Can make polymers with different monomers, etc..

n is often a very large number! e.g. can make
polyethylene with MW gt 100,000! 3600 mers 7200
carbons
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Chemistry of polymer molecules

• Example ethylene
• Gas at STP
• To polymerize ethylene, typically increase T, P
  and/or add an initiator

Initiation
Propagation
After many additions of monomer to the growing
chain
R initiator activates the monomer to begin
chain growth
Initiator example - benzoyl peroxide
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4.4 Polymer chemistry

• Polymers are chain molecules. They are built up
  from simple units called monomers.
• E.g. polyethylene is built from ethylene units
  which are assembled into long chains

Polyethylene or polythene (IUPAC name
poly(ethene)) is a thermoplastic commodity
heavily used in consumer products (notably the
plastic shopping bag). Over 60 million tons of
the material are produced worldwide every year.
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Tetrafluoroethylene monomer polymerize to form
PTFE or polytetrafluoroethylene
poly(tetrafluoroethene) or poly(tetrafluoroethylen
e) (PTFE) is a synthetic fluoropolymer. PTFE is
the DuPont brand name Teflon. Melting 327C
Vinyl chloride monomer leads to poly(vinyl
chloride) or PVC
PVC manufacturing toys, packaging, coating,
parts in motor vehicles, office supplies,
insulation, adhesive tapes, furniture, etc.
Consumers shoe soles, children's toys, handbags,
luggage, seat coverings, etc. Industrial
sectors conveyor belts,
printing rollers. Electric and electronic
equipment circuit boards, cables, electrical
boxes, computer housing.
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Chemistry and Structure of Polyethylene
Adapted from Fig. 4.1, Callister Rethwisch 3e.
Note polyethylene is a long-chain hydrocarbon -
paraffin wax for candles is short polyethylene
Polymer many mers
Adapted from Fig. 14.2, Callister 6e.
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Polymer chemistry

• In polyethylene (PE) synthesis, the monomer is
  ethylene
• Turns out one can use many different monomers
• Different functional groups/chemical composition
  polymers have very different properties!

Monomers
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Homopolymer and Copolymer

• Polymer chemistry
• If formed from one monomer (all the repeat units
  are the same type) this is called a homopolymer
  
• If formed from multiple types of monomers (all
  the repeat units are not the same type) this is
  called a copolymer
• Also note the monomers shown before are
  referred to as bifunctional
• Why? The reactive bond that leads to
  polymerization (the CC double bond in ethylene)
  can react with two other units
• Other monomers react with more than two other
  units e.g. trifunctional monomers

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The Top 10 Bulk or Commodity
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4.5 MOLECULAR WEIGHT
Molecular weight, M Mass of a mole of chains.
high M
Not all chains in a polymer are of the same
length i.e., there is a distribution of
molecular weights
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Molecular weight

• The properties of a polymer depend on its length
• synthesis yields polymer distribution of lengths
• Define average molecular weight
• Two approaches are typically taken
• Number average molecular weight (Mn)
• Weight-average molecular weight (Mw)

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MOLECULAR WEIGHT DISTRIBUTION
Adapted from Fig. 4.4, Callister Rethwisch 3e.
Mi mean (middle) molecular weight of size
range i
xi number fraction of chains in size range i
wi weight fraction of chains in size range i
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Molecular weight

• Are the two different? Yes, one is essentially
  based on mole fractions, and the other on weight
  fractions
• They will be the same if all the chains are
  exactly of the same MW! If not Mw gt Mn

Get Mn from this
Get Mw from this
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Molecular weight

• Other ways to define polymer MW
• Degree of polymerization
• Represents the average number of mers in a chain.
  The number and weight average degrees of
  polymerization are

m is the mer MW in both cases. In the case of a
copolymer (something with two or more mer units),
m is determined by
fj and mj are the chain fraction and molecular
weight of mer j
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Example Problem 4.1

• Given the following data determine the
• Number average MW
• Number average degree of polymerization
• Weight average MW

• How to find Mn?
• Calculate xiMi
• Sum these!

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Example Problem 4.1

• Number average degree of polymerization
• (MW of H2CCHCl is 62.50 g/mol)

• How to find Mw?
• Calculate wiMi
• Sum these!

Weight average molecular weight (Mw)
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Degree of Polymerization, DP

• DP average number of repeat units per chain

DP 6
mol. wt of repeat unit i
Chain fraction
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4.6 Polymers Molecular Shape

• Molecular Shape (or Conformation) chain bending
  and twisting are possible by rotation of carbon
  atoms around their chain bonds
• note not necessary to break chain bonds to alter
  molecular shape

Adapted from Fig. 4.5, Callister Rethwisch 3e.

• C-C bonds are typically 109 (tetrahedral, sp3
  carbon)
• If you have a macromolecule with hundreds of C-C
  bonds, this will lead to bent chains

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Structures of Polymers

• Molecular shape
• Taking this idea further, can also have rotations
  about bonds
• Leads to kinks, twists
• the end-to-end distance of a polymer chain in
  the solid state (or in solution) is usually much
  less than the distance of the fully extended
  chain!
• This is not even taking into account that you
  have numerous chains that can become entangled!

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• 4.7 Molecular structure
• Physical properties of polymers depend not only
  on their molecular weight/shape, but also on the
  difference in the chain structure
• Four main structures
• Linear polymers
• Branched polymers
• Crosslinked polymers
• Network polymers

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4.7 Molecular Structures for Polymers
Adapted from Fig. 4.7, Callister Rethwisch 3e.
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Linear polymers

• polymers in which the mer units are connected
  end-to-end along the whole length of the chain
• These types of polymers are often quite flexible
• Van der waals forces and H-bonding are the two
  main types of interactions between chains
• Some examples polyethylene, teflon, PVC,
  polypropylene

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Branched polymers

• Polymer chains can branch
• Or the fibers may aligned parallel, as in fibers
  and some plastic sheets.
• chains off the main chain (backbone)
• This leads to inability of chains to pack very
  closely together
• These polymers often have lower densities
• These branches are usually a result of
  side-reactions during the polymerization of the
  main chain
• Most linear polymers can also be made in branched
  forms

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Crosslinked polymers

• Molecular structure
• adjacent chains attached via covalent bonds
• Carried out during polymerization or by a
  non-reversible reaction after synthesis (referred
  to as crosslinking)
• Materials often behave very differently from
  linear polymers
• Many rubbery polymers are crosslinked to modify
  their mechanical properties in that case it is
  often called vulcanization
• Generally, amorphous polymers are weak and
  cross-linking adds strength vulcanized rubber is
  polyisoprene with sulphur cross-links

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Network polymers

• polymers that are trifunctional instead of
  bifunctional
• There are three points on the mer that can react
• This leads to three-dimensional connectivity of
  the polymer backbone
• Highly crosslinked polymers can also be
  classified as network polymers
• Examples epoxies, phenol-formaldehyde polymers

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POLYMER MICROSTRUCTURE
Covalent chain configurations and strength
Direction of increasing strength
Adapted from Fig. 14.7, Callister 6e.
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4.8 Molecular configurations
Classification scheme for the characteristics of
polymer molecules
isomerism different molecular configurations
for molecules (polymers) of the same
composition Stereoisomerism Geometrical Isomerism
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4.8 Molecular Configurations Repeat unit R Cl,
CH3, etc
Configurations to change must break bonds
Stereoisomers are mirror images cant
superimpose without breaking a bond
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Head to-tail
Typically the head-to-tail configuration dominates
Head to-head
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Structures of Polymers

• Stereoisomerism
• Denotes when the mers are linked together in the
  same way (e.g. head-to-tail), but differ in their
  spatial arrangement
• This really focuses on the 3D arrangement of the
  side-chain groups
• Three configurations most prevalent
• Isotactic
• Syndiotactic
• Atactic

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ISOTACTIC

• Stereoisomerism
• Isotactic polymers
• All of the R groups are on the same side of the
  chain

Isotactic configuration

• Note All the R groups are head-to-tail
• All of the R groups are on the same side of the
  chain
• Projecting out of the plane of the slide
• This shows the need for 3D representation to
  understand stereochemistry!

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SYNDIOTACTIC

• Stereoisomerism
• Syndiotactic polymers
• The R groups occupies alternate sides of the chain

Syndiotactic configuration

• Note The R groups are still head-to-tail
• R groups alternate one of out of the plane, one
  into the plane

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ATACTIC

• Stereoisomerism
• Atactic polymers
• The R groups are random

Atactic configuration

• R groups are both into and out of the plane, no
  real registry
• Two additional points
• Cannot readily interconvert between stereoisomers
  bonds must be broken
• Most polymers are a mix of stereoisomers, often
  one will predominate

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StereoisomerismHead-to-tail
isotactic configuration
Syndiotactic conformation
Atactic conformation
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cis/trans Isomerism
cis cis-isoprene (natural rubber) H atom and CH3
group on same side of chain
trans trans-isoprene (gutta percha) H atom and
CH3 group on opposite sides of chain
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Geometrical Isomerism
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4.9 Plastics

• variety of properties due to their rich chemical
  makeup
• They are inexpensive to produce, and easy to
  mold, cast, or machine.
• Their properties can be expanded even further in
  composites with other materials.

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Glass-rubber-liquid

• Amorphous plastics have a complex thermal profile
  with 3 typical states

Glass phase (hard plastic)
Leathery phase
Log(stiffness)Pa
Rubber phase (elastomer)
Liquid
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THERMOPLASTICS

• Thermosetting and thermoplastic polymers
• Another way to categorize polymers how do they
  respond to elevated temperatures?
• Thermoplastics these materials soften when
  heated, and harden when cooled this process is
  totally reversible
• This is due to the reduction of secondary forces
  between polymer chains as the temperature is
  increased
• Most linear polymers and some branched polymers
  are thermoplastics

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THERMOSETS

• Thermosetting and thermoplastic polymers
• Thermosets these materials harden the first
  time they are heated, and do not soften after
  subsequent heating
• During the initial heat treatment, covalent
  linkages are formed between chains (i.e. the
  chains become cross-linked)
• Polymer wont melt with heating heat high
  enough it will degrade
• Network/crosslinked polymers are typically
  thermosets

• Polymers which irreversibly change when heated
  are called thermosets.
• Most often, the change involves cross-linking
  which strengthens the polymer (setting).
• Thermosets will not melt, and have good heat
  resistance.
• They are often made from multi-part compounds and
  formed before setting (e.g. epoxy resin).
• Setting accelerates with heat, or for some
  polymers with UV light.

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Thermoplastics

• Polymers which melt and solidify without chemical
  change are called thermoplastics.
• They support hot-forming methods such as
  injection-molding and FDM.

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THERMOPLASTICS VS THERMOSETS
Thermoplastics --little cross linking
--ductile --soften w/heating
--polyethylene (2) polypropylene (5)
polycarbonate polystyrene (6)
Thermosets --large cross linking
(10 to 50 of mers) --hard and brittle
--do NOT soften w/heating --vulcanized
rubber, epoxies, polyester resin,
phenolic resin
Adapted from Fig. 15.18, Callister 6e. (Fig.
15.18 is from F.W. Billmeyer, Jr., Textbook of
Polymer Science, 3rd ed., John Wiley and Sons,
Inc., 1984.)
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4.10 Structures of Polymers

• Copolymers
• Idea polymer that contains more than one mer
  unit
• Why? If polymer A has interesting properties,
  and polymer B has (different) interesting
  properties, making a mixture of polymer should
  lead to a superior polymer

Random copolymer exactly what it sounds like
Alternating copolymer ABABABA
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Structures of Polymers

• Copolymers
• Idea polymer that contains more than one mer
  unit
• Why? If polymer A has interesting properties,
  and polymer B has (different) interesting
  properties, making a mixture of polymer should
  lead to a superior polymer

Block copolymers. Domains of pure mers
Graft copolymers. One mer forms backbone,
another mer is attached to backbone and is a
sidechain (it is grafted to the other polymer)
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Copolymers
Adapted from Fig. 4.9, Callister Rethwisch 3e.

• two or more monomers polymerized together
• random A and B randomly positioned along chain
• alternating A and B alternate in polymer chain
• block large blocks of A units alternate with
  large blocks of B units
• graft chains of B units grafted onto A backbone
• A B

random
alternating
block
graft
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Copolymers

• Polymers often have two different monomers along
  the chain they are called copolymers.
• With three different units, we get a terpolymer.
  This gives us an enormous design space

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4.11 Polymer structure

• The polymer chain layout determines a lot of
  material properties
• Amorphous
• Crystalline

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Crystallinity in Polymers
Adapted from Fig. 4.10, Callister Rethwisch 3e.

• Ordered atomic arrangements involving molecular
  chains
• Crystal structures in terms of unit cells
• Example shown
• polyethylene unit cell

• Polymers can be crystalline (i.e. have long
  range order)
• However, given these are large molecules as
  compared to atoms/ions (i.e. metals/ceramics) the
  crystal structures/packing will be much more
  complex

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Structures of Polymers

• Polymer crystallinity
• (One of the) differences between small molecules
  and polymers
• Small molecules can either totally crystallize or
  become an amorphous solid
• Polymers often are only partially crystalline
• Why? Molecules are very large
• Have crystalline regions dispersed within the
  remaining amorphous materials
• Polymers are often referred to as semicrystalline

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Structures of Polymers

• Polymer crystallinity
• Another way to think about it is that these are
  two phase materials (crystalline, amorphous)
• Need to estimate degree of crystallinity many
  ways
• One is from the density

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Structures of Polymers

• 4.11 Polymer crystallinity
• What influences the degree of crystallinity
• Rate of cooling during solidification
• Molecular chemistry structure matters
• Polyisoprene hard to crystallize
• Polyethylene hard not to crystallize
• Linear polymers are easier to crystallize
• Side chains interfere with crystallization
• Stereoisomers atactic hard to crystallize
  (why?) isotactic, syndiotactic easier to
  crystallize
• Copolymers more random harder to crystallize

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4.11 Polymer Crystallinity (cont.)

• Polymers rarely 100 crystalline
• Difficult for all regions of all chains to become
  aligned

crystalline
region
Degree of crystallinity expressed as
crystallinity. -- Some physical properties
depend on crystallinity. -- Heat
treating causes crystalline regions to
grow and crystallinity to
increase.
amorphous
region
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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4.11 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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4.12 Polymer Crystallinity

• 4.12 Polymer crystals
• Chain folded-model
• Many polymers crystallize as very thin platelets
  (or lamellae)
• Idea the chain folds back and forth within an
  individual plate (chain folded model)

• Crystalline regions
• thin platelets with chain folds at faces
• Chain folded structure

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4.12 Single Crystals

• Electron micrograph multilayered single
  crystals (chain-folded layers) of polyethylene
• Single crystals only for slow and carefully
  controlled growth rates

Adapted from Fig. 4.11, Callister Rethwisch 3e.
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4.12 Semicrystalline Polymers

• Some semicrystalline polymers form spherulite
  structures
• Alternating chain-folder crystallites and
  amorphous regions
• Spherulite structure for relatively rapid growth
  rates

Spherulite surface
Adapted from Fig. 4.13, Callister Rethwisch 3e.
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Structures of Polymers

• Polymer crystals
• More commonly, many polymers that crystallize
  from a melt form spherulites
• One way to think of these the chain folded
  lamellae have amorphous tie domains between
  them
• These plates pack into a spherical shape
• Polymer analogues of grains in polycrystalline
  metals/ceramics

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Photomicrograph Spherulites in Polyethylene
Cross-polarized light used -- a maltese cross
appears in each spherulite
Adapted from Fig. 4.14, Callister Rethwisch 3e.
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END of chapter 4