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Hydrocarbon Molecules
Unsaturated: Double and triple bonds
eg.,
CnH2n
CnH2n-2
Ethylene
Acethylene
CH2=CH2
CHCH
C2 H 4
C2 H 2
Chapter 14: Polymer Structures
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Hydrocarbon Molecules
Saturated: single bonds
eg., CH4,C2H6, C3H8
CnH2n+2
Isomerism: n-butane
Straight chain
Chapter 14: Polymer Structures
Isobutane
Branched chain
3
Hydrocarbon Molecules
R-COOH
R-CHO
R-C
6
H
5
Source: William Callister 7th edition, chapter 14, page 493, table 14.2
Chapter 14: Polymer Structures
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Polymer Molecules
Gigantic: Macromolecules
Monomer: One unit
Polymer Many units
eg., one unit
Chapter 14: Polymer Structures
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Polymer Molecules continue…
PTFE: TEFLON
Polytetrafluoro ethylene
Mer
Chapter 14: Polymer Structures
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Polymer Molecules continue…
PVC: Vinyl
Polyvinyl chloride
Mer
Polypropylene:
Mer
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Polymer molecules
Homopolymer:
Repeating units of the chain are of the
same type
Co-polymer: Two or more different mer units.
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Polymer molecules continue
Bifunctional:
Two (2) active bonds
Trifunctional:
Three (3) active bonds
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Molecular weight
•Large macromolecules synthesized from molecules
•Not all polymer chains grow to the same length
•Average molecular weight is determined by measuring
viscosity and osmotic pressure
•The chain is divided into size ranges
•No. of moles (or fraction) of each size range is
determined
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Molecular weight continue…
Number average molecular weight, M n =xiMi
Where
Mi=Mean molecular weight within a size range
xi=Fraction of number of chains within the corresponding
(same) size range
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Molecular weight continue…
Weight Average Molecular weight, M w =wiMi
Where,
Mi=Mean molecular weight within a size range
wi=weight fraction of molecules within the same size
range
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Molecular weight continue…
Notice the shift
Source: William Callister 7th edition, chapter 14, page 498, figure 14.3
Chapter 14: Polymer Structures
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Molecular weight continue…
Source: William Callister 7th edition, chapter 14, page 498, figure 14.4
Chapter 14: Polymer Structures
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Molecular weight continue…
Degree of polymerization (n):
n= Average no of mer units in a chain
nn=Number average degree of polymerization
nw=Weight average degree of polymerization
Mn
nn 
m
m  Mer molecular weight
Mw
nw 
m
Chapter 14: Polymer Structures
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Molecular weight continue…
For a copolymer (i.e., two or more mer units),
m  f j m j
Where,
fj= chain fraction of mer
mj=molecular weight of mer
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Problem 14.1:
Computations of Average Molecular Weights and Degree
of Polymerization
Assume that the molecular weight distributions shown in
Figure 14.3 are for poly(vinyl chloride). For this material,
compute:
(a) the number-average molecular weight,
(b) the degree of polymerization, and
(c) the weight-average molecular weight.
Chapter 14: Polymer Structures
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Problem 14.1: continue…
xiMi=21,150
Where, xi: Fraction of total no. of chain within the corresponding size change
M n : Number average molecular weight
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Problem 14.1: continue…
xiMi=23,200
Where, M w : weight average molecular weight
Chapter 14: Polymer Structures
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Problem 14.1: continue…
PVC: C2H3Cl
C
Atomic weight (g/mol) 12.01
H
Cl
1.01
35.45
m  2(12.01) 3(1.01) 35.45
m  62.50g/mol
Chapter 14: Polymer Structures
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Problem 14.1: continue…
Number average degree of polymerization,
M n 21 ,150
nn 

 338
m
62.50
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Molecular weight of polymers
•Melting point increases with molecular weight (for M
up to 100,000 g/mol). i.e. increased intermolecular forces
•Long chain increased bonding between molecules.
(Van der Waals or hydrogen bond)
•At room temperature,
Short chains: Molecular weight: 100 g/mol – liquids/gases
(1000 g/mol: waxes, soft resins)
High polymers: 10,000 to several million g/mol – solids
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Molecular shape
•Single chain bonds can rotate like a cone/bend in three
dimensions
•Bends, twists, kinks, in single chain molecules
CC: rigid (rotationally)
bulky or large side group: restricted rotation
Benzene ring: restricted rotation
Chapter 14: Polymer Structures
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Molecular structure
Linear
Mer units end-to-end
in chains
Source: William Callister 7th edition, chapter
14, page 502, figure 14.7(a)
e.g.,
Polyethylene
Chapter 14: Polymer Structures
PVC
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Molecular structure continue….
Polystyrene
PMMA
Poly(methyl methacrylate)
Chapter 14: Polymer Structures
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Molecular structure continue….
Branched Polymers
•Side-branch chains
•Less packing efficiency; lower density
Source: William Callister 7th
edition, chapter 14, page 502,
figure 14.7 (b)
Chapter 14: Polymer Structures
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Molecular structure continue….
Cross-linked Polymers
•Formed by non-reversible chemical reaction
•Additives covalently bonded to chains
e.g., sulfur in vulcanizing
Source: William Callister 7th
edition, chapter 14, page 502,
figure 14.7 (c)
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Molecular structure continue….
Net-work polymer
•Three active covalent bonds
•Highly cross-linked
Source: William Callister 7th edition,
chapter 14, page 502, figure 14.7 (d)
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Molecular configurations
Head-to-tail configuration
Bonded to alternate carbons
on the same side
Source: William Callister 7th edition, chapter 14, page 503
Where, R: Alkyl radical
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Molecular configurations continue….
Head-to-head configuration
Bonded to adjacent carbon
atoms
Source: William Callister 7th edition, chapter 14, page 503
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Molecular configurations continue….
Stereoisomerism
Isotactic configuration
R groups are situated
on the same side of
the chain
Source: William Callister 7th edition, chapter 14, page 504
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Molecular configurations continue….
Syndiotactic
On alternate sides
Source: William Callister 7th edition, chapter 14, page 504
Chapter 14: Polymer Structures
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Molecular configurations continue….
Atactic
At random position
Source: William Callister 7th edition, chapter 14, page 504
Conversion from to another is only by severing branches
and through new reaction
Chapter 14: Polymer Structures
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Molecular configurations continue….
Geometric Isomerism
CIS-Isoprene
eg., Natural rubber
Attacked by acids/alkalis
TRANS-Isoprene
eg., Gutta Percha
Highly resistant to acid/alkalis
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Molecular configurations continue….
Geometric Isomerism continue…
TRANS- isoprene
e.g., Gutta Percha
–Highly resistant to acids/alkalis
Chapter 14: Polymer Structures
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Molecular configurations continue….
Chapter 14: Polymer Structures
Source: William Callister 7th edition, chapter 14, page 506, figure 14.8
36
Copolymers (different types of mers)
Random
Source: William Callister 7th
edition, chapter 14, page 508,
figure 14.9(a)
Alternate
Source: William Callister 7th
edition, chapter 14, page 508,
figure 14.9(b)
Chapter 14: Polymer Structures
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Copolymers continue…
Block
Source: William Callister 7th
edition, chapter 14, page 508,
figure 14.9(c)
Styrene butadiene rubber (SBR), (Random copolymer):
Automobile tires.
Nitrile butadiene rubber (NBR),
Random copolymer): Gasoline hose
Chapter 14: Polymer Structures
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Polymer Crystallinity
Crystallinity: Packing of chains to produce ordered atomic
array.
Crystalline
Crystalline
+
or
noncrystalline
noncrystalline
Total
Chapter 14: Polymer Structures
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Polymer Crystallinity continue…
ρ c (ρ s ρ a )
%Crystalli nity 
100
ρ s (ρ c ρ a )
Where,
s=Density of specimen
a=Density of totally amorphous polymer
c=Density of perfectly crystalline polymer
Chapter 14: Polymer Structures
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Polymer Crystallinity continue…
Crystallinity characteristics
•Degree of crystallinity depends on rate of cooling; need
sufficient time to result in ordered configuration.
•Amorphous (No crystallinity) if chemically complex
microstructure. Crystalline if chemically simple polymer.
e.g., polyethylene, PTFE, even if rapidly cooled
Chapter 14: Polymer Structures
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Polymer Crystallinity continue…
•Amorphous if network polymer. Crystalline if linear
polymer (no restrictions to prevent chain alignment)
•Amorphous: Atactic stereoisomer. Crystalline: Isotactic
or Syndiotactic stereoisomer
•Amorphous: If bulky/large side-bonded group.
Crystalline: Simple straight chain
Chapter 14: Polymer Structures
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Polymer Crystallinity continue…
•Amorphous: Most copolymers (and more irregular/
random mers)
Crystalline: Alternating or block polymers
•Amorphous: Random or graft polymers
•Crystalline: Strong, more resistant to dissolution by
softening by heat
Chapter 14: Polymer Structures
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Polymer crystals
Fringed micelle model
•Aligned small crystalline regions (crystallites or
micelles)
•Amorphous regions in-between platelets of crystals (1020 nm thick) (10m long)
Chapter 14: Polymer Structures
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Polymer crystals continue…
Fringed micelle model continue…
So, multilayered structure
Chain-fold model: amorphous molecular chains within
platelets; back and forth
Chapter 14: Polymer Structures
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Polymer crystals continue…
Spherulite model
•Bulk polymers solidify as small spheres (Spherulites)
•Within each such sphere, folded crystallites (lamellae),
~10 nm thick form
•Adjacent spherulites impinge on each other forming
planar boundaries
e.g., Polyethylene, Polypropylene, PVC, PTFE, Nylon
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Polymers: summary
•Large molecules of polymers
•Mers, homopolymers, copolymers
•Molecular weight
–Number-Average
–Weight-Average
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Polymers: summary continue…
•Isomerism
Isotactic
Syndiotactic
Atactic
•Crystallinity: Degree of crystallinity
•Polymer crystals
Chapter 14: Polymer Structures
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Thermosetting and Thermoplastic
Polymers
Determined by mechanical behavior upon heating to high
temperatures
Thermosetting
Thermoplastic
•Thermosets
•Thermoplasts
Become permanently hard Soften upon heating;
upon heating. Do not
harden upon cooling. It is
soften upon subsequent
reversible
heating
Fabricated by applying
heat and pressure
Chapter 14: Polymer Structures
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Thermosetting and Thermoplastic
Polymers continue…
Thermosetting
Initial Heating:
Covalent crosslink form and
link adjacent molecular
chains. Chains are anchored;
no vibrational or rotational
chain motions, 10-50% of
chain mer units are crosslinked
Chapter 14: Polymer Structures
Thermoplastic
As Temperature is
increased
Secondary bonds break
(due to molecular
motion). So when stress
is applied, adjacent
chains move
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Thermosetting and Thermoplastic
Polymers continue…
Thermosetting
Further heating:
Severance (breaking) of
crosslink bonds and polymer
degradation
Chapter 14: Polymer Structures
Thermoplastic
Irreversible
degradation upon
further heating:
Violent molecular
vibrations break
primary covalent
bonds
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Thermosetting and Thermoplastic
Polymers continue…
Thermosetting
Thermoset polymers are harder,
stronger and brittle
Better dimensional stability
e.g., Cross linked and network
polymers
Vulcanized rubbers, epoxies and
phenolic and some polyester resins
Chapter 14: Polymer Structures
Thermoplastic
Soft and Ductile
Most linear
polymers and
polymers with
branched structures
with flexible
chains
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