Polymer Chemistry

1. Crystallization and Crystallinity of Polymers
Presented by
Dr. Prasanta Baishya
1

2. Outline
 Crystallization of polymers
 Factors affecting the Crystallinity of polymers
Methods of evaluating the degree of crystallinity
2

3. 3
Crystallinity refers to the degree of structural order in a solid. In a crystal, the
atoms or molecules are arranged in a regular, periodic manner.
On the basis of the nature of order present in the arrangement of molecular
chain, the polymers are classified as
 Crystalline Polymers: Crystallization of polymers is a process associated
with partial alignment of their molecular chains. Crystalline polymers have a
certain degree of crystallinity in their structures owing to the long range
order arrangement of some segments of the polymer chains.
Examples: Polyethylene (PE), Nylon - 6,6, Kevler, Isotactic polypropylene
(PP), etc.
 Amorphous Polymers: Most of the polymers do not have orderness in their
structure and hence these type of polymers do not have any degree of
crystallinity and these are known as amorphous polymers.
Examples: Butadiene rubber, atactic polypropylene, etc.
 Polymers are never completely crystalline. They contains crystalline
regions with amorphous regions together.

4. 4
 The crystals forms by folding of polymer chains.
 The chains are much longer than the dimension of crystal they belong to.
 A given chain may belongs to both crystalline and amorphous region
 A given crystal may consist of more than one chains.
 Crystallinity defines the degree of long-range order in a material, and
strongly affects its properties. The more crystalline a polymer, the more
regularly aligned its chains.

5. 5
Why it is important?
It is one of the most important property of the polymers. Some physical
properties of polymers depend on their crystallinity.
For example,
 Crystalline polymers possess high density and high melting.
 Crystalline polymers are usually opaque because of light scattering
 Crystalline polymers are more difficult to stain than amorphous ones
 Crystallinity makes polymers strong but lowers their impact resistance.
Crystallization of polymers are broadly classified under
three groups:
(A) Crystallization during polymerization
(B) Crystallization induced by stress and
(C) Crystallization under quiescent condition.

6. 6
Factors affecting the crystallinity
 Length of polymer chain: Long chain (high degree of polymerisation)
are less likely to be crystallise. Long chains are more likely to get
entangled and formed amorphous region.
 Chain Branching: Branch chains are less likely to be crystallise
 Inter chain bonding (Copolymers): On the arrangement of repeating
unit
Crystallinity
Random –A-B-A-A-A-B-B-A-A-B- No
Block -A-A-A-B-B-B-A-A-A- No
Graft No
Alternating -A-B-A-B-A-B-A-B-A YES

7. 7
Differential Scanning Calorimetry (DSC) : Additional energy is released
upon melting a semi-crystalline polymer. This energy can be measured with
differential scanning calorimetry and compared with that released upon
melting of the standard sample of the same material with known
crystallization degree.
Degree of crystallinity = ((Delta
Hf - Delta Hc)/Delta Hf,100%) x
100%
where Delta Hf is the enthalpy of
melting,
Delta Hc is the enthalpy of
crystallization, and
Delta Hf,100% is the enthalpy of melting
for a fully crystalline polymer. Enthalpy
is the area under each peak so to get
enthalpy values for crystallization and
melting
Methods of evaluating the degree of crystallinity:

8. 8
% Crystallinity= Total area of all crystalline peaks X 100% /Total Area of all peaks
 The calculation of crystallinity by XRD is based
on the presumption that the broad peak comes
from amorphous phase, the sharp peak comes
from crystal phase.
 The sharp peaks become slightly broad. This
causes from the small size effect of crystalline
X-ray diffraction (XRD): Regular arrangement of atoms and molecules produce
sharp diffraction peaks whereas amorphous regions result in broad halos. The
diffraction pattern of polymers usually contains a combination of both. Degree of
crystallinity can be estimated by integrating the relative intensities of the peaks and
halos.

9. 9
Infrared spectroscopy (IR): Infrared absorption or reflection spectra from
crystalline polymers contain additional peaks which are absent in amorphous
materials with the same composition. These signals may originate from deformation
vibrations of the regular arrangement of molecular chains. From the analysis of these
bands, the degree of crystallinity can be estimated.
Nuclear magnetic resonance (NMR): Crystalline and amorphous areas differ
by the mobility of protons. The latter can be monitored through the line shape of
NMR signals and used to estimate the degree of crystallinity.

10. 10
The distribution of crystalline and amorphous regions can be visualized with
microscopic techniques, such as
1. Polarized Optical Microscopy (POM) and
2. Transmission Electron Microscopy (TEM)
30 sec 60 sec 120 sec
90 sec 150 sec
180 sec
420 sec
390 sec
330 sec 360 sec
210 sec 240 sec 270 sec 300 sec
450 sec
510 sec 540 sec 570 sec 600 sec
480 sec
POM experiment is
done on these
parameter
Heating cycle
Rate (o/min) = 50
Limit (o) = 190
Holding time (min) =
3
Cooling cycle
Rate (o/min) = 50
Holding temperature =
120co
time = 10 min
Each images taken at
30 second interval
upto 10 min

11. 11