Welcome to the series on polymer properties.
Today's topic is polymer crystallization.
Crystallization is a very important property of a polymer as many of the physical properties
of polymers depend on their crystallinity.
This video we will provide brief overview on fundamental concepts of crystallization,
factors affecting crystallinity, experimental methods to determine crystallinity and polymer
properties affected by its crystallinity.
Polymers are integral part of our life.
Every day we use many polymeric products, but we never realize that how and why these
polymeric products are different from each other.
Somethings are transparent, opaque like water bottle milk containers etc.
Whereas other polymeric products like tables, chair, swings are strong.
So, what makes polymer transparent, opaque or strong.
Among the many factors that influence polymer properties, polymer crystallinity is one of
them.
Depending on the degree of crystallinity polymers can be transparent, translucent, opaque, strong
or weak.
How much crystalline polymer would be that depends on tendency of a polymeric chain to
make crystals.
Those who make crystals are called crystalline or semi crystalline polymers and those who
do not make crystals are called amorphous polymers.
In amorphous polymers polymeric chains have random orientation with very irregular structure.
A perfect example to visualize amorphous polymer is boiled noodles.
The way noodles are physically entangled and loosely bound to each other, polymer chains
in amorphous polymers are also present in the similar fashion.
These polymers do not make crystals event if they make crystals their crystallinity
is very low and that is why these polymers are transparent in nature whereas semi crystalline
polymers have both randomly oriented chains as well as highly ordered structure where
polymeric chains arrange in a very specific pattern to make a shape.
These specific shapes are called crystals and the process of formation of crystal is
called crystallization.
If all the polymeric chains are involved in crystal formation such polymers are called
crystalline polymers but in reality, no polymer is 100% crystalline, most of the polymers
are semi crystalline in nature.
Now the question is how do chains form ordered structure or how do crystals form and why
do not all the chains participate in crystal formation.
Suppose here we have polymer in melt state where polymers have polymeric chains randomly
oriented and there is no crystal.
So thermodynamically the best situation to form crystal is that all the chains lineup
straight parallel to each other but this structure is possible only in one condition when all
the chains are very short.
Suppose all the chains are 10 carbon atom long in this case it is possible that all
the changes may line up stretch-out and form this kind of structure but remember that here
we are dealing with polymers, polymers are macromolecules.
One chain of a polymer contains a few 100 to few thousands of repeat units.
In that case it is almost impossible for polymeric chains to line up stretch out.
So instead of forming this kind of structure polymeric chain forms folded chain structure,
due to very long chain length these chains exist as entangled or coiled chain in polymer
melt.
some part of it is straight and some part of it is part of entangled or curved.
To form folded chain structure polymeric chains, stretch out for some distance as it encounter
any entangled or curb part, the chain folds back.
The chains keep folding back and forth to form tightly packed folded chain structure.
Straight part of the chain participates in crystal formation whereas entangled, coiled
or curbed part creates amorphous region.
Chain ends also do not participate in crystal formation because those are physically and
chemically very different from the rest of the chain.
Now this packed folded chain structure may contain one chain or more than one, it depends
on flexibility of chain.
If the chain is flexible enough to re-enter in the same crystal it can, otherwise it can
go to another crystal.
finally, the packed folded structure stacked together and form a new structure called lamella.
Lamella is a fundamental structure of a polymer crystal these Lamella grow radially and form
a spherical shape called spherulite.
Spherulites are three dimensional crystals and their size range from few micrometre to
one centimetre.
In spherulite the Lamellas are join together buy amorphous region which provide flexibility
to the spherulite or crystal.
Whereas crystalline reason makes them strong.
Now let's see what are the conditions that favor crystal formation.
Polymers can be crystallized under three conditions.
First when polymers melt is cooled.
Second when polymers are stretched and third on solvent evaporation.
In melt crystallization when polymer melt is cooled, during the cooling polymer form
crystals and this process is called melt crystallization.
There are 2 types of melt crystallization homogeneous and heterogeneous.
Homogeneous crystallization initiate due to thermal gradient in the polymer melt.
The thermal gradient tells you about where in which direction the temperature is fluctuating.
Since in polymer melt there are entangled and coiled chains so temperature around these
chains changes frequently compared to rest of the polymer melt and these entangled and
coiled chains act as nuclei and chains starts to attach on them and form crystal.
Whereas heterogeneous crystallization occurs in presence of foreign particles.
It can be dust particle or additive added to polymers to modify their properties.
In this case these particles act as pre-existing nuclei and crystals start to grow on them.
Crystals formed by this process are usually small because thousands of nuclei already
exist in the melt and all the crystals starts to grow at the same time.
On the contrary in case of homogeneous crystallization nuclei form from the melt therefore bigger
and less number of crystals form during the process.
In case of stress induced crystallization a polymer is a stretched while it's temperature
is kept above its glass transition temperature.
During the stretching entangled and coil chains present in the amorphous reason of the polymer
starts to unfold and align in order structure.
Using this method amorphous polymers can be transformed to semicrystalline polymers.
Polymers can also be crystallized on solvent evaporation.
This process depends on degree of dilution.
In dilute solution the molecular chains are separated from each other due to presence
of solvent molecules within the chains thus these chains exit as individual chains in
the solution.
Since evaporation takes place at high temperature chains have sufficient mobility.
With increasing concentration on evaporation, interaction between polymeric chains increases
and therefore chain starts to form crystals.
This method may result in the highest degree of polymer crystallinity.
Now let's try to understand why is it that's some polymers are highly crystalline some
polymers are highly amorphous what are the factors that facilitate or hinder the crystallization
process.
Polymer structure is one of the most important factors because different polymers have different
structures like some polymers are linear some are branched some has small side groups, some
has bulky side groups.
So, to understand the effect of these factors, we need to first understand packing of polymeric
chains.
Suppose we have two boxes, and we need to fill identical objects in one box and different
objects in other box.
Now which box will have highest number of objects, obviously the box with identical
objects why? because identical objects can be arranged nicely.
In the same way polymeric chains who can pack in a crystal nicely form crystalline polymers
and those who cannot form amorphous polymers.
Now if we have two polymers one with linear structure and other with the branched structure.
Linear polymer can pack well as compared to branched structure therefore linear polymer
will be more crystalline than branched polymer.
Similarly, if we have two polymers one with small side groups or side chain and other
with bulky side groups or big chain.
Polymer with small side group or side chain will pack better compared to bulky side group
or big chain polymer, therefore crystallinity will be higher for small side group or small
side chain polymer.
Crystallinity is also favored by strong intermolecular forces.
the presence of polar and hydrogen bonding groups favors crystallinity because these
make possible dipole dipole and hydrogen bonding intermolecular force for example in case of
nylon.
In nylon the polar amide group in the backbone are strongly attached to each other due to
hydrogen bonding and therefore nylon is a very highly crystalline polymer.
Intermolecular force also increase glass transition temperatures and melting point of the polymer.
The degree of crystallinity also depends on arrangement of side groups on polymer backbone
that is also called as tacticity.
If all the side groups are attached on left or right side of the chain backbone polymer
is called isotactic.
If side groups are alternatively arranged in chain backbone, it's a syndiotactic polymer
and if side groups are randomly in polymer backbone the polymer is called as atactic
polymer.
Isotactic polymers are usually more crystalline than syndiotactic polymer due to better packing
of chains.
Atactic polymer do not form crystals due to random arrangement of side groups which do
not allow packing of polymeric chains.
Cooling rates also show significant effect on crystallinity.
When polymers are processed industrially polymers are often cooled from melt.
During cooling polymer generates crystallinity.
If the cooling rate is very high polymer doesn't get enough time to crystallize at it's crystallization
temperature because melt starts to cool fast.
If cooling rate is too low polymer needs longer time to crystalize.
Also, formation of stable nuclei at high temperature is also very critical therefore, an optimum
cooling temperature is required to crystallize polymers completely.
Annealing is performed above glass transition and below melting point of a polymer to create
high degree of crystallinity.
In this process polymers are kept isothermally.
Due to high temperature polymer chains have sufficient mobility to form a crystal or become
a part of other crystal.
This process needs to formation leads to formation of bigger and thicker crystals e.g. in these
images we can see that initially polymer crystals have small size and after annealing at 180
degree Celsius for 5 minutes the crystal size increased which led to increase in degree
of crystallinity.
Nucleating agents are often added to polymer matrix to modify their properties.
Adding nucleating agent to polymers provide a surface to start the crystal growth.
basically, these nucleating agents or fillers act as pre-existing nuclei in polymer melt
which leads to faster crystal formation.
For example here cellulose nano crystals were added in PLA matrix.
PLA is amorphous polymer with very low degree of crystallinity.
The degree of crystallinity of PLA increased in presence of these crystals because these
filler provided surface to polymer chain to form crystals.
since thousands of nuclei started to grow altogether therefore Crystallization occurs
very fast.
Polymer crystallization in presence of nucleating agent or reinforcing filler is also known
as heterogeneous crystallization.
Plasticizers always reduce polymer crystallinity because plasticizers reduce or weaken the
intermolecular force between the chains which reduces chain packing.
Now let's see what are the experimental methods to determine the degree of crystallinity.
There are two common techniques to measure degree of crystallinity DSC and XRD.
Differential scanning calorimetry or DSC measures the amount of energy absorbed or released
by a sample when it is heated or cooled.
since glass transition and melting is energy absorbing process therefore these appears
as endotherm whereas crystallization is an exothermic process so crystallization appears
as an exothermic peak.
The heat or energy required to melt the crystals is known as heat of fusion and heat of fusion
is used to calculate degree of crystallinity of polymer.
Suppose for a given polymer the heat of fusion is 141 Joule per gram and heat of fusion for
100% crystalline polymer is 290 Joule per gram then the percent crystallinity of this
polymer would be 49%.
Since in reality no polymer is 100% crystalline therefore heat of fusion for 100% crystalline
polymer is always taken from literature.
X-ray diffraction technique is also very common technique to calculate crystallinity of a
polymers.
XRD spectrum of a polymer gives two types of peaks.
Sharp peaks from crystalline reason and broad peaks from amorphous reason.
If a polymer highly crystalline it gives sharp peaks if a polymer is amorphous it gives a
broad hump or a broad peak.
In case of semi crystalline polymer both sharp and broad peaks appear in their spectrum.
To calculate crystallinity of semicrystalline polymer first amorphous and crystalline peaks
are separated from the spectrum.
Then the area under the crystalline peak and amorphous peak is calculated.
The crystallinity is calculated by dividing the total area under crystalline peak by the
total area under the diffraction curve means total area of crystalline plus amorphous peaks.
Now let’s discuss the effect of crystallinity on optica,l thermal, and mechanical properties
of polymer.
Optical properties generally semi crystalline polymers a translucent or opaque whereas amorphous
polymers are transparent.
In case of semi crystalline polymers crystals act as scattering center and thus they scattered
the light or reflect the light.
Since light cannot pass through the crystals therefore semicrystalline polymers are opaque
whereas amorphous polymers allow light to pass and therefore are transparent.
Thermal properties, semi crystalline polymer possess higher tg as compared to amorphous
polymer because crystal acts as physical cross link which restricts change mobility and also
reduces free volume therefore with increasing crystallinity TG of a polymer increases.
Crystallinity also increases thermal stability of polymer because more energy is required
to melt the crystals therefore thermal stability of a polymer increases with increasing crystallinity.
Mechanical properties, generally semicrystalline polymers have higher modulus, higher strength
but low elongation at break as compared to amorphous polymers.
They have high modulus because crystals act as physical barrier which increases change
rigidity and thus the modulus.
Semi crystalline polymers also possess high strength because the load bearing capacity
of polymer increases with increasing crystallinity.
Whereas elongation at break decreases with increasing crystallinity because crystals
do not stretch.
So, we learnt that depending on polymer crystallinity polymer mimic different properties.
Crystalline phase tends to make polymer strong while amorphous phase is more effective in
providing flexibility.
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