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Chapter 6
Noncrystalline and Semicrystalline Materials
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Introduction
The emphasis thus far has been on crystalline materials.
There are numerous engineering materials that lack the long rangetranslational periodicity of a crystalline material.
These non-crystalline materials are referred to as either- amorphous,glassy, or super-cooled liquids.
Theoretically, any material can form an amorphous structure if thecooling rate from the melt is sufficiently rapidto suppress crystalformation.
This chapter will emphasize the structural considerations thatfacilitate the development of an amorphous structure.
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N-th Order Phase Transition
The order of a phase transition can be understood by investigating the
behavior of derivatives of the Gibbs free energy (G).
An n-th order transition is the one in which discontinuities appear only in the
n-th and higher derivatives of G with respect to T and p.
The ordinary first-order transition has discontinuities in
Tp
GV
pT
GS
The ordinary second-order transition has discontinuities in
pTp p
GTVT
VV
11
TT
TpG
VpV
V
2
211
VpV
VT
G
TT
T
STC
pp
pT
GT
T
STC
2
2
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Glass Transition Temperature
3. Glassy State
A state of material in the absence of long-range order below the glass
transition temperaturelarge scale mobility is frozenatomic movement
requires time.
(Liquid-like structure and molecular mobility is frozen)
Window glass vs. Rubber bandWhat if you hammer them?
Glass transition temperature: the temperature below which the physical
properties of amorphous materials vary in a manner similar to those of a
solid phase (glassy state), and above which amorphous materials behave
like liquids (rubbery state).
2. Rubbery State
A state of material in the absence of long-range order above the glass
transition temperatureatomic movement takes shorter time.
(Liquid-like structure and liquid-like molecular mobility)
1. Liquid State
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Specific Volume for a Variety of Materials
Liquid to glass solid transformationin a
pure substance. The glass transition
temperature, Tg, is not an equilibrium
transformation temperature.
Liquid to crystalline solid transformation
for a pure substance. Tmis an
equilibrium transformation temperature
Glass Transition
The slope normalized by the volume V is the volumetric thermal expansion
coefficient (v)dTdV
Vv 1
Decrease of specific volume of liquid with decreasing temperature
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Glass Transition
Below Tm, material tends to crystallize.
The crystal formation (crystallization)
occurs over a period of time becausethe establishment of long-range order
(LRO) requires atomic rearrangement
by diffusion.
It is possible to avoid crystallization
by cooling at a sufficiently high rate
so as to suppress the diffusion
necessary to establish LRO in thecrystal.
The volume of the collection of
atoms continues to decrease with
the slope characteristic of the liquid
below the melting temperature,
forming a super-cooled liquid.
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The Effect of Cooling Rate
on the Glass Transition Temperature, Tg
SpecificVo
lume
Tg1Tg2 Tm
Temperature
T1
.
T2
.
Liquid
Solid
GlassSuper-cooled liquid
T1
.T2
.>
Glass Transition
Kinetic aspect of Tg
T1
.
T2
.
Super fast fast
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Liquidto Semi-crystallineSolidTransformation
Tm
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Structure and Properties of Amorphous and
Semicrystalline Polymers
Ethylene
Basic building
block
Poly (ethylene)
monomer
Poly (ethylene)
chain
Poly (ethylene) chains pack well
because the side groups are only
hydrogen
Formation (polymerization) of Poly (ethylene)from a Basic
Chemical Unit of C2H4
C=C
HH
H H
C=C
HH
H H
C C
H H
H H
.. C C
H H
H H
C C
H H
H H
.. C C
H H
H H
C C
H H
H H
C C
H H
H H
.. C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
..
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
C C
H H
H H
Glass transition temperature: -78 C.
Melting temperature: 100 C.
Amorphous density at 25 C: 0.855 g/cm3.
Crystalline density at 25 C: 1.00 g/cm3.
Molecular weight of repeat unit: 28.05 g/mol
used in making translucent, lightweight, and
tough plastics, films, containers, insulation,
etc
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C
H
HC
H
HC C C C
H
H
H
H
H
H
H
H... ... C
H
HC
H
Hn
Degree of Polymerization (DP)
n: the number of monomeric unit
~10,000 to ~1,000,000
The degree of polymerization
C C
H
H
H
Cl
C C
H
H
H
Cl
C
C
C
C
C
C
C
C
C
C
Cl Cl Cl Cl Cl
H H H H HH H H H H
H H H H H
n
Another Example: Poly (vinyl chloride)
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Poly (ethylene terephthalate), PET or PETE, one of the polyesters
Film, fibers, clothing, drink bottle
Density 1370 kg/m3
Young modulus (E) 28003100 MPa
Tensile strength (t) 5575 MPa
Glass temperature 75 C
melting point 260 C
Poly (styrene), PS
Density 1050 kg/m
Specific Gravity 1.05
Young's modulus (E) 3000-3600 MPa
Tensile strength (st) 4660 MPa
Glass temperature 95 C
Melting point 240 C
Containers and toys
http://upload.wikimedia.org/wikipedia/commons/1/10/Polystyrene_formation.PNGhttp://upload.wikimedia.org/wikipedia/commons/5/5f/PET.png -
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CH2
C
H2
CH2
C
H2
CH2
C
H2
NH
NH
C
O
CH2
C
H2
CH2
C
H2
C
O
n
Nylon 66, one of the polyamides
Carpet fiber, apparel, airbags, tires,
ropes, conveyor belts, and hoses
Poly (p-phenyleneterephthalamide), PPTA or Kevlar
Fibers and bulletproof vests
One of the most strong polymers
C
F
F
C
F
F
n
Poly (tetrafluoroethylene), PTFEbearings, bushings, gears, slide plates
One of the most hydrophobic polymers
With the lowest frictional coefficient
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semi-crystalline structure of polymer amorphous structure of polymer
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Thermoplastic Polymer and Thermoset Polymer
Thermoplastic polymer: capable of softening or fusing (melting) whenheated and of hardening again when cooled
e.g.) various linear polymers (no chemical crosslinking)poly (ethylene), poly (propylene), and Poly (ethylene terephthalate)
Thermoset polymer: not capable of softening or fusing (melting) whenheated and of hardening again when cooledThe curing (crosslinking)
process makes three-dimensional network structure in polymeric material
e.g.) various polymers with chemical crosslinkingVulcanized rubber
Bakelite, a Phenol Formaldehyde Resin (used in electrical insulators and plastic wear)Urea-formaldehyde foam (used in plywood, particleboard and medium-density fibreboard)
Melamine resin (used on worktop surfaces)
Polyester Resin (used in glass-reinforced plastics/fibreglass (GRP))
Epoxy Resin
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Thermoplastic Polymer and Thermoset Polymer
Polyester with saturated
bonds along the chain
Polyester with unsaturated
bonds along the chain
Crosslinking with a
polystyrene monomer
Thermoplastic
Thermoset polymerPET
PET
PET-based thermosetpolymer
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Structure of Cross-linked Rubber
Thermoplastic Polymer andThermoset Polymer
C C
R H
H
C C
H H
H
C C
R H
H
C C
H H
H
C C
R
H
H
C C
H
HH
C C
R
H
H
C C
H
HH
Doublebonds
C C
R H
H
C C
H H
H
C C
R H
H
C C
H H
H
C C
R H
H
C C
H H
H
C C
R H
H
C C
H H
H
C C
R
H
H
C C
H
HH
C C
R
H
H
C C
H
HH
Doublebonds
C C
R H
H
C C
H H
H
C C
R H
H
C C
H H
H
C C
R
H
H
C C
H
HH
C C
S H
H
C C
H S
H
SSCross-linkedPrimary Bonds
H H
C C
R H
H
C C
H H
H
C C
R H
H
C C
H H
H
C C
R
H
H
C C
H
HH
C C
S H
H
C C
H S
H
SSCross-linkedPrimary Bonds
H H
Unsaturated bonds are used to form cross-links with cross-linker.
Vulcanized rubber
Thermoset polymers do not crystallize well. Why?
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Factors Affecting Crystallinity in Polymers
The size of the side groups
The extent of chain branching
Tacticity
The complexity of the repeat unit
The degree of secondary bonding
Factors influencing the efficiency of packing polymer chains
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Tacticity
crystallizable
The complexity of the repeat unit
C
H
H
C
H
H
n
V.S.
The degree of secondary bonding
CH2
CHn
OH
C
H
H
C
H
H
n
CH3
V.S.
C
O
O CH2
CH2
C
O
On
Not crystallizable
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Rubbers
CH
2
C
CH3
CCH
2
n
H
Cis-1,4-poly (isoprene)
Natural Rubber
Synthetic Rubber
CH2 CH CH CH2 n
CH2
CH CH CH2 n
CH2
CH CH CH2
CH2
CH CH CH2CH2 CH CH CH2
S S
S S
CH2
CH CH CH2Poly (butadiene)
Sulfur bridge
Sulfur cross-link
Poly(isoprene) : Tg=~-70C
Poly(butadiene) : Tg=~-70C
http://en.wikipedia.org/wiki/Image:20070104rubberlatex.jpghttp://en.wikipedia.org/wiki/Image:Ceylon_rubber.jpg