MadSci Network: Chemistry

Re: What is the explanation for the melting&chemical corrosion of polymers?

Date: Fri Aug 27 09:43:04 1999
Posted By: Dr. Jeries Barghout, Post-doc/Fellow, Department of Materials, University of Oxford
Area of science: Chemistry
ID: 935210586.Ch

I will do my best to answer all your question regarding polyethelyene (PE),
polypropolyene (PP) and polyvinyl chloride (PVC).

1) Why do the above polymers melt when exposed to microwaves?

For a microwave to heat and subsequently melt an object, the object must be
composed of bonds that are either ionic, polar or conducting in nature. 
Polar bonds can exist in PVC but not in PE or PP.  For PVC the C-Cl bonds
is polar because the electron afinity of chlorine is higher than that of
carbon.  So I suspect that PVC could melt in a microwave but PE and PP
shouldn't melt.

The interaction of the microwaves with the polar bond in PVC causes the
melting.  As there is a dipole in the C-Cl bond, the bond reacts with to
the presence of microwaves due to their electromagnetic nature.  In effect
the bond oscillates accroding to the wave's magnetic field.  The movement
of the C-Cl bond causes an increase in the plastics temperature due to
friction between the oscillating bond and the static surroundings. 
Eventually when the plastic temperature reaches its melting point, it

2) What determines the flexibility/elasticity of a polymer?

There are several factors that can determine the elasticity of a polymer. 
These include molecular weight, degree of crystallinity, degree of
cross-linking and size of side groups.
(a) Lower molecular weights mean shorter polymer chains hence there
is little entanglement between the chains increasing elasticity.
(b) Polymer crystals are stiffer than amorphous polymer because in
the crystals polymer chains are aligned parallel with respect to
one another and packed densely togther.  Therefore the chains cannot
move in the crystal making them stiff and rigid.  Hence elastic
polymer are amorphous and stiff polymers are crystalline.
(c) If a polymer is cross-linked then the polymer chains are linked
together to form a network.  The greater the degree of cross-linking the
larger the number of cross-links between polymer chains and thus less
freedom for movement.
(d) The size of side groups can affect the elasticity of a polymer
where larger side groups cause entaglement thus reducing elasticity.  Also 
some side groups can promote interaction between chains by forming hydrogen
/ electrostatic bonding thus reducing elasticity.  Therefore side groups
can either promote elasticity of stiffness.

Regarding the elasticity of PE, PP and PVC. I would expect PE to be most
elastic then PP and then PVC.  All 3 polymers are very similar in structure
expect for the side groups where the side groups are -H for PE, -CH3 for PP
and -Cl for PVC.  PE has the smallest side group and so it is easiest to
compare PP and PVC to it.  PP has a larger side group to PE and so it will
be stiffer.  PVC has a smaller side group than PP and larger than PE
however chlorine has a high electron affinity and therefore will promote
electrostatic bonding with other hydrogens therefore increasing the
stiffness of the polymer.

3) How do you explain why there is no reaction between Sodium Hydroxide 
   and the polymers?

For NaOH (sodium hydroxide) to react with any polymer it must be
energetically favourable for the react to occur.  Therefore the bond energy
of forming compounds between polymer and NaOH must be greater than those
required to break up side groups / hydrogens / carbons from the polymers. 
Also as NaOH is a liquid I presume that the energy of mixing and solution
must play a part as it must be favourable for the polymer to dissolve into
NaOH too.

4) What determines how hard a polymer is?

Hardness is a measure of a materials resistance to deformation.  Therefore
the stiffer the material the harder it will be to deform it.  This question
is simily the same as question 2.  A stiff polymer is one that has a high
molecular weight, high degree of crystallinity and is highly cross-linked. 
So in this case PVC is the hardest polymer followed by PP and then PE.

5) What determines whether a polymer is stain resistant or not?

I have little experience in the field of stain resistance and therefore I
can't really give you a complete answer and so I can only speculate.  When
a polymer stains means it has absorbed another substance.  For the polymer
to stain therefore it must be energetically favourable to do so.  For
example some polymers form crazes which is what we see when a clear plastic
cup is foggy in appearance.  The crazing occurs because the polymer has
absorbed some detergent because it was energetically favourable.  However
why and how this occur I am afraid I don't know why.  Sorry to let you down
on this one.

Hope I have managed to answer most of you question as fully as possible.

Best Regards,

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