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7/27/2019 Lecture 7 - Polymers
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1
Molecular Shape (orConformation)
Chain bending and twisting are possible by rotation ofcarbon atoms around their chain bonds
Not necessary to break chain bonds to alter molecular
shape
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2
Molecular Structures for
Polymers
secondarybonding
Linear Polymers- Mer units are joined together end to end
- Polyethylene, polystyrene etc
Branched Polymers
-Side branches are connected to main ones- Lowering density
Cross linked Polymeres
-Adjacent linear chains are joined to one
another at various positions by covalentbonds
Network Polymers
- polymerization of monomers having two or
more functional groups
-may be made by cross-linking the polymers
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Isomerism
Mmolecules with the same chemical formula and
often with the same kinds ofbonds betweenatoms, but in which the atoms are arrangeddifferently
Propanol (C3
H8
O) is an example
Propan-1-ol (n-propyl alcohol) and Propan-2-ol (isopropyl alcohol)
Molecular Configurations for
Polymers
Configurations to change must break bonds
http://en.wikipedia.org/wiki/Chemical_formulahttp://en.wikipedia.org/wiki/Chemical_bondhttp://en.wikipedia.org/wiki/Image:Structural_isomers.pnghttp://en.wikipedia.org/wiki/Chemical_bondhttp://en.wikipedia.org/wiki/Chemical_formula7/27/2019 Lecture 7 - Polymers
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4
Molecular Configurations for
Polymers
Stereo Isomers Bond structure is the same but the geometrical
positioning of atoms and functional groups in spacediffers
EB
A
DC C
D
A
BE
mirror plane
C C
R
HH
H
C C
H
H
H
R
or C C
H
H
H
R
Stereoisomers are mirror images cant superimpose without breaking a
bond
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5
Tacticity
Tacticity stereoregularity or spatial arrangement ofR unitsalong chain
C C
H
H
H
R R
H
H
H
CC
R
H
H
H
CC
R
H
H
H
CC
isotactic all R groups on
same side of chain
C C
H
H
H
R
C C
H
H
H
R
C C
H
H
H
R R
H
H
H
CC
syndiotacticR groups
alternate sides
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Tacticity (cont.)
atacticR groups randomlypositioned
C C
H
H
H
R R
H
H
H
CC
R
H
H
H
CC
R
H
H
H
CC
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cis/trans Isomerism
C C
HCH3
CH2 CH2
C C
CH3
CH2
CH2
H
cis
cis-isoprene
(natural rubber)
H atom and CH3 grouponsame side of chain
trans
trans-isoprene
(gutta percha)
H atom and CH3 group onopposite sides of chain
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VMSE: Stereo and Geometrical
Isomers
8
Manipulate and rotate polymer structures in 3-dimensions
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Copolymers
two or more monomerspolymerized together
random A and B randomlypositioned along chain
alternating A and B
alternate in polymer chain block large blocks of A
units alternate with largeblocks of B units
graft
chains of B unitsgrafted onto A backbone
A B
random
block
graft
Adapted from Fig.
4.9, Callister &
Rethwisch 4e.
alternating
Sketch the repeat structure of Poly(Styrene-butadine)alternate polymer
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Several Repeat Units in Rubbers
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Thermoplastic and
Thermosetting Polymers
Thermoplastic Polymers
Becomes soft and formable when heated
Hardened when cooled significantly below their softening
point reversible process
Readily recycled Produced in one step and then made into products in a
subsequent process
Thermosetting Polymers
Do not soften upon heating cannot be shaped or formed to any great extent
Produced and formed in the same step
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7/27/2019 Lecture 7 - Polymers
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14
Polymer Crystallinity
Crystalline regions thin platelets with chain folds at faces (Lamellae)
Chain foldedstructure
10 nm
Chain-folded model
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Polymer Crystallinity (cont.)
Polymers rarely 100% crystalline
Difficult for all regions of all chains tobecome aligned
Degree of crystallinity
expressed as % crystallinity.-- Some physical properties
depend on % crystallinity.
-- Heat treating causes
crystalline regions to grow
and % crystallinity toincrease.
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.)
crystallineregion
amorphous
region
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Polymer Crystallinity
Packing of molecular chains so as to produce anordered array
Much more complex than metals / ceramics
Not just atomic or molecular arrangements, rather
molecular chains involved
s is the density of a specimen for which % crystallinity is tobe determined
a is the density of the totally amorphous polymer
c is the density of the totally crystalline polymer
100*)(
)(%
acs
ascitycrystallin
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Example