Lecture 13
Structure of Solids
Recap
Long range arrangements in ionic solids:
NaCl, CsCl, ZnS
Crystalline and non-crystalline Silica
Role of modifiers (e.g. Na2O, CaO) in
lowering the viscosity of glass
Introduction of polymers
Classification of Polymers
Linear polymer/chain polymer
Arrangement of monomers in curvilinear
fashion
Network polymers
Arrangement of monomers in cross network
in 3 dimensions
Classification of Polymers
Thermoplast and thermosets:
Based on the behaviour with
temperature
Thermoplast
The polymers whose shape can be changed by the application
of heat and pressure is known as thermoplast.
Thermoplast: Long chain molecules held together by
secondary bonds
These soften on heating while harden on cooling
It can be easily pliable at high temperature, can be
reshaped easily
Examples: polyethylene, PVC (e.g. tubings, pipes, gramophone
record), PP (e.g. bumpers and dash boards of cars ), PMMA
(e.g. plexi glass, wind shield)
Thermosets
The polymers which after curing resist strongly to any
change of its shape by application of heat and pressure
Polymerization gets completed while heating which results in
hardened structure
Once having cured (or hardened) by a chemical reaction, will
not soften or melt when subsequently heated, cannot be
reshaped easily.
They degrade before melting
Examples: Bakelite, Melamine, Araldite
These materials are sold in market as partially polymerised
material
On the basis of arrangement of
long chains in space we will
talk about plastics and
elastomers in detail in the next
few slides
Also very common engineering
materials
Long chain polymers
Plastics: chains are placed in zig-zag manner, the bond along
the chain is strong covalent bond while the bond between the
chain is weak Vander Wall type
Fibres: chains of different lengths are aligned parallel to
one and other, the bond along the chain is strong covalent
while the bond between the chain is weak hydrogen type.
Very strong along the axis of fibers.
Elastomers: Long chain polymers with few cross links across
the chains. Importantly the cross links are of primary bond
between chains. Chains have translation mobility at RT .
Plastics
Structure of Long Chain Polymers
Degree of Polymerization:
No. of repeating monomers in a chain
On the basis of monomer unit other plastics
can be described
Monomer Unit
Side groups can be labelled as R1, R2, R3 and R4
Common Plastics
Name
R1 R2
R3
R4
Uses
PE
Containers, bags
PVC*
Cl
Piping's, tubing
PP
CH3
PMMA
CH3
COOCH3
Dash boards, bumpers
of cars, buckets
Plexiglass, window
pane
Polystyrene H
C6H5
PTFE
Styrofoam, packaging
material
Sliding and rotating
bearings
* Plasticisers are often added to make it soft
Common Plastics
Elastomers
Exhibit rubber like elasticity
Long chain molecules with some cross-link
Crosslinks are present at a distance of few hundred
monomers
Natural tendency for bending
and coiling
Mobile segments
Chain segments have
translational mobility
at room temperature
Cross-link points
The conditions listed in previous slide can
be found satisfied in the next few slides
Poly-isoprene (Natural Rubber)
Liquid natural
rubber (latex)
being collected
from the rubber
tree
Liquid state:
Translation point
confirmed
Isoprene molecule
commons.wikimedia.org
H H3C H
C=C-C=C
HH
CH3 H
CC
CC
Isoprene
molecule
Polymerization
CH3 H
CC
H
Polyisoprene
mer
Liquid
(Latex)
Bigger side group requires more space, since rotational
movement is difficult because of double bond it starts
bending, this provides natural tendency of bending and coiling
A natural rubber in the liquid state cannot
be used as rubber band, truck tyre or eraser
Then what we should do??
Provide few crosslinks (possible because of
double bond)
The process is known as Vulcanization
Vulcanisation
Heat the liquid with Sulphur
CC
H
CC
+ 2S
CC
H
CH3 H
CC
CH3 H
Weak
van der
Waals
bond
Vulcanisation
CC
CC
H
CH3 H
CC
H
H
Crosslinks
CC
CH3 H
Now this becomes solid mass!!
Effect of cross-linking on
polyisoprene
Natural
rubber
liquid
not
x-linked
Elastomer
Ebonite
Elastic
solid (soft
rubber)
Hard &
brittle
lightly
x-linked
heavily
x-linked
You may have seen tyres are more rigid
than tube. Why this is so because of
degree of vulcanization.
Note there are variety of rubbers.
Summary of key points
Note that by playing with degree of the
vulcanization one can generate various
possibilities.
After 100% crosslinking we call this as
ebonite.
Cross link after every few 100 monomers
gives soft rubber.
Mechanical behaviour of Elastomer
Mechanical behavior of
Elastomers
1. A few hundred percent of temporary
(recoverable) deformation on stretching.
2. The deformation is non linear, Stress strain
3. The stretched elastomers contracts on
heating
Elastomer sample
under tension
Elastomer
sample
heat
Tensile force
straight
chains
Coiled chains
Higher
entropy
Lower
entropy
Still lower
entropy
Contracts on
heating
Elastomers have ve thermal
expansion coefficient, i.e., they
CONTRACT on heating!!
EXPERIMENT after Minor-I
Section 10.3 of the textbook
