MME 4733-D2-2025

Classification of polymers according to their thermal behaviours:
1) THERMOPLASTICS
PE
•Thermoplastic polymers can be
softened when heated and hardened
when cooled with little change in their
properties, repeatedly.
•The softening/melting occurs as the
polymer chain move more and more
freely because of absence of cross
link.
PS
PVC
Classification of polymers according to their thermal behaviours:
2) THERMOSETS
EPOXY BINDERS:
Thermosets are generally
prepared
from
low
molecular mass semifluid
substances. When heated
they get highly cross linked
to form rigid and insoluble
products. The cross links
hold the molecule in place
so that heating does not
allow them to move freely.
Ring-opening reaction of epoxy by amine groups
Covalently crosslinked polymer network
....
FIBERS
Carpet Fiber
These polymers which have strong intermolecular forces between
the chains. These forces are either hydrogen bonds or dipoledipole interactions. Because of strong intermolecular forces, the
chains are closely packed giving them high tensile strength and
less elasticity. Therefore, these polymers have sharp melting
points. Therefore, these are used for making fibres.
FIBERS: Kevlar
➢It’s five times stronger than steel,
on an equal weight basis.
Its strength comes from
strong intermolecular
forces (hydrogen
bonding) between
adjacent chains.
Additional strength is
derived from ‘stacking
interactions’ between
the hexagonal aromatic
rings in adjacent chains.
Elastomers: The polymers can be easily stretched by applying small stress
and regains their original shape when the stress is removed. The most
important example of elastomers is natural rubber.
The molecular structure of
elastomers can be imagined as
a 'spaghetti and meatball'
structure, with the meatballs
signifying cross-links.
Cross-linkages ensure that the
elastomer will return to its
original configuration when the
stress is removed.
The principal difference between thermoset elastomers and thermoplastic
elastomers is the type of crosslinking bond in their structures:
Crosslinks can be achieved either chemically or physically.
In thermoset elastomers; there are chemical crosslinks which are covalent
and therefore irreversible.
In thermoplastic elastomers; there are physical crosslinks which are noncovalent and reversible.
Hydrogen bonding and thermoplastic elastomers
Hydrogen bonds show temperature-dependent behavior:
The crosslinks consist of hydrogen bonds (thick red lines) at low
temperatures (left side), which are opened at an increased temperature
(right side).
Thermoplastic elastomer: Spandex
Spandex: DuPont sells under the trade name Lycra
Intermolecular Forces: (non-bonding)
Intermolecular Forces:
Ion-dipole: Interaction of an ion (cation or anion) with a polar molecule
Dipole-dipole: Interaction of polar molecules with other polar molecules
Hydrogen bonding: A special case of dipole-dipole when there is a H bonded to
a N, O, or F.
Dipole-induced dipole: A polar molecule interacting with a nonpolar one.
London (also called instantaneous dipole-induced dipole)
These are the very weak interactions that hold two nonpolar molecules together.
➢ Many general chemistry textbooks say that London forces are always present
because these are forces that are inherent in anything that has mass.
➢ These last two forces are collectively known as Van der Waals forces and are
in general very weak.