Presentation includes classification of polymer blends based on miscibility, phase diagram of polymer blends and thermodynamics polymer blends which includes Gibbs energy theory and Flory-Huggins Theory

1. Submitted by
Abhinand Krishna Km
MSc.Biopolymer Science
CIPET IPT KOCHI
Roll no:02
Miscibility and Thermodynamics of
Polymer Blends

2. Advantages of Polymer Blends
• The capability to reduce material cost with or without little
sacrifice in properties
• Extended service temperature range
• Light weight
• The ability to improve the processability of materials which
are otherwise limited in their ability to be transformed into
finished products
• Increased toughening
• Enhanced ozone resistance
• Improved impact and environmental stress cracking
resistance, etc.

4. Types of Polymer Blends On the basis
of Miscibility
Miscible Polymer Blends
Completely miscible blends has got ΔHm <0 due to specific
interactions. Homogeneity is observed at least on a nanometer scale, if
not on the molecular level. This type of blends exhibits only one glass
transition temperature (Tg), which is in between the glass transition
temperatures of the blend components in a close relation to the blend
composition. A well-known example of a blend, which is miscible over
a very wide temperature range and in all compositions is PS/PPO

5. Immiscible Polymer Blends
Immiscible blends have a coarse morphology, sharp interface
and poor adhesion between the blend phases. So these
blends are of no use without compatibilisation. Thee blends
will exhibit different Tgs corresponding to the Tg of the
component polymers. Examples of fully immiscible blends are
PA/ABS, PA/PPO, PA/EPDM and PA/PP. Now these blends have
become commercially successful, after being efficiently
compatibilised using suitable compatibilisers.

6. Partially Miscible Polymer Blends
In partially miscible blends a small part of one of the blend
component is dissolved in the other part. This type of blend,
which exhibits a fine phase morphology and satisfactory
properties, is referred to as compatible. Both blend phases
are homogeneous, and have their own Tg. Both Tg s are shifted
from the values for the pure blend components towards the Tg
of the blend component. An example is the PC/ABS blends. In
these blends, PC and the SAN phase of ABS partially dissolve in
one another. In this case interface is wide and the interfacial
adhesion is good.

7. Factors affecting miscibility and immiscibility of
Polymer blend
Polarity: Polymers that are similar in structure or more generally
similar in polarity are less likely to repel each other and more likely to
form miscible blends
Specific group interaction: Polymers that are drawn to each
other by hydrogen bonding, acid-base, charge transfer, ion-dipole,
and donor-acceptor adducts or transition metal complexes are less
common, but when such attractions occur they are very likely to
produce miscibility.
Blend Ratio: Even though two polymers appear immiscible at a
fairly equal ratio, it's quite possible that a small amount of one polymer
may be soluble in large ammount of other polymer and vice versa,as
understoodin conventional phase rule.

8. Molecular Weight: Lower molecular weight permits greater
randomisation on mixing and therefore greater gain of entropy, which
flavours miscibility.More surprisingly, polymer of similar molecular
weight are more miscible where polymer of different molecular weight
are may be immiscible, even if they both have same composition.
Crystallinity: When polymer crystallizes,it forms two phase
system.Thus in a polymer blend when a polymer crystallizes, this adds
another phase to the system.If both polymer crystallizes, they will from
two separate crystalline phases:it's quite rare for the two polymers to co
crystallize in a single crystalline phase.

9. Blend Miscibility and Phase Diagram
When polymer are blended they may exists as single or
different phases.When conditions are changes theses phases
also changes.The Phase diagram explains the exact
behaviour of materials at different conditions graphically.It
consists of Temperature and composition of material is
plotted Y and X axis respectbel.
When phase diagram is plotted, curves are obtained, it
separate the phases of mixture.A solid line represents the
mixture behaviour either miscible or immiscible completely.

11. The diagram shows the phase behaviour for binary polymer blend
(temperature versus composition) with illustration of lower critical
solution temperature (LCST) and upper critical solution temperature
(UCST) behavior.
Critical solution temperature is the temperature at which the
temperature at which polymer blend convert from a immiscible blend
to a uniform miscible blend. temperature range between LCST and
UCST polymer blend exists in a single phase system.
The binodal curves represents the point at which phase transition
starts where both immiscible and miscible phases co-exists the are
under the curve is known as metastable state.Spinodal is the area in
which polymer blends exists in different phases.

12. Phase diagram can be of different types with more than
one critical solution temperature or even without critical
solution temperature depending upon the morphology,
structure etc.

13. Thermodynamics of Polymer Blends
If two polymers are mixed, the most frequent result is a system that
exhibits a complete phase separation due to the repulsive interaction
between the components (i.e. the chemical incompatibility between the
polymers).Complete miscibility in a mixture of two polymers requires that
the following condition is fulfilled:
∆Gm = ∆Hm – T∆Sm < 0
where ∆Gm, ∆Hm, and ∆Sm are the Gibb’s free energy, the enthalpy and
entropy of mixing at temperature T, respectively. For a stable one-phase
system, criteria for phase stability of binary mixtures of composition φ at
fixed temperature T and pressure p are:

14. Miscible polymer blend is a polymer blend which is homogeneous
down to the molecular level and associated with the negative value of
the free energy of mixing and the domain size is comparable to the
dimensions of the macromolecular statistical segment.
The value of T∆Sm is always positive since there is an increase in the
entropy on mixing. Therefore, the sign of ∆Gm always depends on
the value of the enthalpy of mixing ∆Hm. The polymer pairs mix to
form a single phase only if the entropic contribution to free energy
exceeds the enthalpic contribution,
∆Hm < T∆Sm

15. Flory-Huggins Theory
firangiry-Huggins model restrictions are no change of volume during mixing
(incompressible model), the entropy of mixing is entirely given by the number
of rearrangements during mixing (combinatorial entropy) and the enthalpy of
mixing is caused by interactions of different segments after the dissolution of
interactions of the same type of segments. It is a mean-field model, i.e. only
average interactions are taken into consideration. The main problem was to
find an expression for the entropy of mixing because it was found
experimentally that polymer solutions show significant deviations from values
expected for ideal solutions.
Assuming random mixing of two polymers and ∆Vm = 0 yields the well-known
expression for the combinatorial entropy of mixing ∆Sm of the Flory-Huggins
theory:

16. where φi is the volume fraction of the component i and ri is the number
of polymer segments, R is the gas constant. It can be seen that the
entropy of mixing decreases with increasing molar mass (ri is
proportional to the degree of polymerization) and vanishes for infinite
molar masses
Applying the concept of regular solutions and assuming all pair interaction
the framework of a mean-field theory yields for the enthalpy of mixing ∆H
For binary systems the Flory-Huggins equation can be expressed in the
following form

17. where χ is the so called Flory-Huggins binary interaction parameter. R
is the universal gas constant, and T is the absolute temperature. The
first two terms of the right hand side in Equation are related to the
entropy of mixing and the third term is originally assigned to the
enthalpy of mixing.
For polymers having infinite molar mass (i.e. ri is infinite) the entropic
contribution is very small and the miscibility or immiscibility of the
system mainly depends on the value of the enthalpy of mixing.
Miscibility can only be achieved when χ is negative. The term
‘parameter’ is widely used to describe χ but it is definitively better
characterized by the term ‘function’, because χ depends on such
quantities as temperature, concentration, pressure, molar mass,
molar mass distribution and even on model parameters as the
coordination number of the lattice and segment length.

18. interaction parameter, χ is given by the formulaas follows
χ= a+ (b/T)
Where a and b are experimentally determined constants. It is usually
assumed that a is due to local entropic factors while b results from
enthalpic factors .Thus for the simplest case wherein χ varies inversely
with T, a and b have values greater than zero. This type of dependence is
far from universal.