In my previous video I talked about polymers and how do they differ from each other. In
this video I’m going to talk about the typical properties of polymers. The properties I am
going to discuss today are thermal, mechanical, rheological, optical and electrical properties.
A good understanding of these properties help to select the right polymer for right application
and also the right conditions to make those products. So let’s start with the thermal properties.
Thermal properties are very important because thermal behavior of polymer provides
temperature limit in which it can be safely used. It also help to determine their processing conditions.
Polymer typically shows three transition points. First one is glass transition temperature.
Which is also known as Tg. This is a temperature at which a polymer transition
from brittle glass like material to a rubbery material
which can be stretched or bend.
Why it is so?
because below Tg polymer only have vibrational motion as a result when we try to bend or
deform them we have to break all the chains. Above Tg polymeric materials gain segmental
mobility which means a small segment of the chain can bent or rotate
so polymer can be bent or deform above Tg.
Second important transition is crystallization temperature which is also known as Tc.
At Tc the random polymer chain in amorphous state form highly ordered crystalline phase.
Tc is a very important factor during product manufacturing
As it helps to decide mold temperature and cooling time. Third important transition temperature
is melting temperature or Tm. At Tm crystals melt and form randomly oriented free flowing polymer melt.
Tm of a polymer helps to decide the processing temp. Typically Tm + 50 °C
is used as processing temperature. The thermal transition temperatures Tg, Tm and Tc depend
on chain flexibility which is related to monomer type and percentage crystallinity.
To know more about glass transition and crystallization please watch our videos on these topics.
Now coming to the mechanical property which is the most important property of polymers
for its different applications. Mechanical properties of polymers are typically represented by three
parameters tensile strength, modulus and toughness. The strength of a material is a stress value at which it breaks
whereas modulus is the ratio between stress and strain and typically
represents materials ability to resist deformation. And toughness is the amount of energy consumed
by the material before it breaks. It is mainly measured by area under the stress strain curve.
Polymers can be broadly classified into three categories brittle, plastic and elastomer.
From the graph it very clear that brittle polymer show very high strength but very little flexibility.
Highly cross link polymer usually show this kind of behavior whereas elastomers
are like rubber, it shows very high deformation at significantly lower stress. Plastics are
regular polymer which we use in our daily life. They show very unique deformation behavior.
During initial stage very high stress is required to deform them but after reaching yield point
it shows less resistance to deformation and after reaching certain level of deformation
it shows more resistance to deformation and finally breaks at higher stress value than yield point.
Next is the rheological property.
Rheological property of polymer significantly differ from metal and common fluids. Flow behavior
of polymer both in solid and melt state depends on percentage strain which means how much
material has been stretched and on time which means how long load have been applied. Most
polymer melt are non-Newtonian fluid because they do not show linear co-relationship between
stress and strain. The reason for this is viscosity of polymeric materials changes with
change in strain level. In the right graph we can see that viscosity of polymer changes
from Newtonian behavior to very low viscosity at high strain rate. The change in viscosity
is the result of chain disentanglement and orientation in the stress direction. Majority
of the polymers show either shear thinning behavior which means decrease in viscosity
at higher strain rate or shear thickening behavior which means increase in viscosity
at higher strain rate. Depending upon application like during polymer processing shear thinning
behavior is preferred as it reduces the amount of energy required to process the material.
But in case of paints shear thickening behavior is preferred so that paints do not drop or flow normally.
Optical properties of polymers are very important
for consumer goods like glasses, water bottle and packing films etc. Optical properties of polymers
can be controlled by product manufacturing process because during a process we can select
a condition to have fully amorphous or a semi crystalline polymer. As amorphous polymers
are homogeneous materials. They allow light to pass through without significant loss in intensity
Whereas in semi-crystalline polymers majority of light is scattered by the crystals.
So they appear opaque. Depending upon the end application we can modify the optical
properties of polymers.
Polymeric materials significantly differ from metals in their electric properties.
Unlike metals majority of polymers are insulators that is they do not allow transfer of electrons.
Due to their insulating nature polymers have wide range of applications. However for certain applications like electronics, wire and cable,
EMI shielding, polymeric materials need to have certain amount of conductivity typically
in the range of 10-9 to 10-2 S/cm. This range of conductivity can be easily achieved by
mixing conducting fillers in the polymers matrix. There are also a class of polymer
known as electroactive or conducting polymers which possess inherent conductivity.
However because of their high cost and processing issue they have limited applications.