the evaluation of polymers

1. EVALUATIONOFPOLYMERS
SUBMITTEDTO:
DR.JAVED ALI
Dept . Of Pharmaceutics
Faculty of Pharmacy.
Jamia Hamdard
SUBMITTED BY:
DIVYAVERMA
M.pharm
pharmaceutics-2nd
semester.

2. WHAT IS A POLYMER?
A polymer is a substance composed of molecules
characterized by the multiple repetition of one or more
species of atoms or groups of atoms (constitutional
repeating units) linked to each other in amounts
sufficient to provide a set of properties that do not vary
markedly with the addition of one or a few of the
constitutional repeating units.”

3. PHYSICOCHEMICAL
PROPERTIES

4. Stretch
Linear Polymer
The chains can be stretched, which causes
them to flow past each other. When released,
the polymer will not return to its original form.
Stretch
Cross-Linked Polymer
The cross-links hold the chains together.
When released, the polymer will return to it's
original form.
Relax

5. Molecular weight, M: Mass of a mole of chains. Not all
chains in a polymer are of the same length
i.e., there is a distribution of molecular weights
 The properties of a polymer depend on its length.
 The molecular weight distribution in a polymer describes the
relationship between the number of moles of each polymer
species and the molar mass of that species.
low M
MOLECULAR WEIGHT

6. iiw
iin
MwM
MxM


Mi = mean (middle) molecular weight of size range I
xi = number fraction of chains in size range i
wi = weight fraction of chains in size range i
The polydispersity index is given by Mw /Mn

7. Polymer chain length
Many polymer properties are affected by the length of the
polymer chains. For example, the melting temperature
increases with increasing molecular weight.
At room temp, polymers with very short chains (roughly
100 g/mol) will exist as liquids.
Those with weights of 1000 g/mol are typically waxy
solids and soft resins.
Solid polymers range between 10,000 and several million
g/mol.
The molecular weight affects the polymer’s properties
(examples: elastic modulus & strength).

8. Molecular Shape (or Conformation)
Chain bending and twisting are possible by rotation of carbon atoms
around their chain bonds
(note: not necessary to break chain bonds to alter molecular
shape)
In some of the polymer mechanical and thermal characteristics are a
function of the chain segment rotation in response to applied
stresses or thermal vibrations.
Straight chain Twisted chain

9. Stereo regularity (tacticity)
It describes the isomeric arrangement of functional
groups on the backbone of carbon chains.
I
Istatic chains
are defined as
having
substituent
groups aligned
in one direction.
This enables
them to line up
close to each
other, creating
crystalline areas
and resulting in
highly rigid
Atatic chains have
randomly aligned
substituent
groups. The
chains do not fit
together well and
the intermolecular
forces are low.
This leads to a
low density and
tensile strength,
but a high degree
of flexibility.
syndiatactic
substituent groups
alternate regularly in
opposite directions.
Because of this
regularity, syndiotactic
chains can position
themselves close to
each other, though no
as close as istatic but
have better impact
strength than isotactic
polymers because of
the higher flexibility
resulting from their
weaker intermolecular
forces.

10. BRANCHING
During the propagation of polymer chains, branching can occur. In
free-radical polymerization, this occurs when a chain curls back
and bonds to an earlier part of the chain. When this curl breaks, it
leaves small chains sprouting from the main carbon backbone.

11. Chain End-to-End Distance, r
Representation of a
single polymer chain
molecule that has
numerous random
kinks and coils
produced by chain
bond rotations; it is
very similar to a
heavily tangled
fishing line.
“r” is the end to end
distance of the
polymer chain which
is much smaller than
the total chain length.

12. Molecular structure
 Physical properties of polymers depend not only on their
molecular weight/shape, but also on the difference in the
chain structure
 Four main structures
• Linear polymers
• Branched polymers
• Cross linked polymers
• Network polymers
Branched Cross-Linked NetworkLinear
secondary
bonding

14. Crystallinity in Polymers
The crystalline state may
exist in polymeric
materials.
However, since it involves
molecules instead of just
atoms or ions, as with
metals or ceramics, the
atomic arrangement will
be more complex for
polymers.
There are ordered atomic
arrangements involving
molecular chains.
Example shown is a
polyethylene unit cell

15. % crystallinity depends on several factors:
 Rate of cooling (faster cooling – less crystallinity)
 Type of polymer; (simple structures – more crystallinity)
Linear polymers more easily form crystals
The higher % Crystallinity → higher strength

16. The presence of crystallinity has a significant effect on polymer
properties because crystalline regions act as cross links for the
regions and for this reason stiffen and toughen the polymer and
reduce swelling in solvents.
Furthermore, because crystalline regions are impermeable to
diffusing molecules, an enhancement of crystallinity results in a
decrease in polymer permeability.
Crystalline regions are also essentially impermeable to water, so
the rate of polymer hydrolysis in crystalline regions is significantly
reduced.

17. Fe3C – iron carbide –
orthorhombic crystal
structure
Some physical properties
depend on % crystallinity.
-- Heat treating causes
crystalline regions to grow
and % crystallinity to
increase.

18. Material characteristic time
Physical behavior of a material can be predicted
from the
1. Characteristic time of the material (λ)
2. The process time scale (θ)
1. Characteristic time- is intrinsic property which
reflects the distribution of polymer chain lengths.
Characteristic time can be determined
experimentally from the Rheological techniques
2. The process time scale-is a duration of the
process, such as diffusion or a mechanical
deformation.

19. Deborah Number De = λ/θ
De << 1 the response is
termed as “viscous”
According to the
nomenclature in fluid and
solid mechanics
De >>1 the response is
termed as “elastic"
polymer diffusion
Polymer diffusion takes
place in glassy polymer
In both cases amt of penetrant absorbed or desorbed is
α √T
Rheological measurements provide a means to
determine the MW rheological measurement , also
determine material characteristic of time
Instrument used : cone and plate , parallel disk
rotational viscometer

20. hydrophobicity
When a polymer is placed in an aqueous environment,
it will gradually absorb water, and the amount of
absorbed water is determined by the polymer
structure.
According to the nature of polymer-water interactions,
polymers can be broadly classified into ;
1. Hydrophobic polymers: water impermeable and when
placed in aqueous environment will absorb very little
water ( less than 5 wt % water)

21. Structure parameters that contribute to polymer
hydrophobicity are :
Chain stiffness
High degree of crystallinity
Presence of highly hydrophobic groups where C-H
bonds have been replaced by C-F bonds.
2. Hydrophilic polymers :
absorb more than 5 wt % water
Structural parameters that contribute
To polymer hydrophilicity are :
Chain flexibility
Absence of crystallinity
Presence of certain groups such as amino,hydroxyl

22. 3.Water soluble polymers :
freely water soluble
4. Hydrogels :
Hydrophilic or water soluble polymers that has been
cross linked by means of covalent bonds and due to
covalent cross links can not dissolve in water

23. Melting point
The (Tm) when applied to polymers suggests not a solid-
liquid phase transition, but a transition from a
crystalline phase to a solid amorphous phase.
Crystalline melting is only discussed with
thermoplastics, as thermosets will decompose at high
temperatures rather than melt.
Tm
1st order transition
characterized by sharp change
in specific volume.
Highly crystalline polymers do
not melt very often degradation
occur before melting

24. Glass TRANSITION
TEMPERATURE
The glass transition temperature (Tg) describes the temperature at which
amorphous polymers undergo a second order phase transition from a
rubbery, viscous amorphous solid to a brittle, glassy amorphous solid.
The hard,brittle state is known as Glassy state.
The soft flexible state is known as Rubbery or Viscoelastic state.
The polymer when on further heating becomes highly Viscous & starts
flowing, is termed as visco fluid state
The Temperature at which visco fluid state arises ,is termed as Flow
temp.[Tf]

25. As a consequence of this transition, the polymer undergoes an
abrupt change in properties.
Among these are :
coefficient of expansion
Permeability
Refractive index
Hardness.
The glass transition temperature, also known as second-order
transition, is a characteristic of a particular polymer structure,
and its value is closely related to intermolecular forces and chain
stiffness. So polymers with strong intermolecular interactions will
tend to have high glass transition temperature.

26. Determination OF GLASS TRANSITION
TEMPERATURE
Below the glass transition temperature , the
available polymer motions are limited, but above
the glass transition, a motion that starts with one
atom can pass through the chain and cause an
effect 50 atoms away.
Tg can be measured by techniques such as :
DIFFERENTIAL SCANNING CALORIMETRY
(DSC)
DYNAMIC MECHANICAL THERMAL ANALYSIS
(DMTA or DMA)

27. In general
Polymers whose
Tg is above the service temperature ------ are strong,
stiff and sometimes brittle e.g. Polystyrene (cheap, clear
plastic drink cups)
Tg is below the service temperature ------ are
weaker, less rigid, and more ductile Polyethylene (milk
jugs)

28. TENSILE STRENGTH
The tensile strength of a material quantifies how
much stress the material will endure before failing.
In general tensile strength increases with polymer
chain length.

29. Tensile Response: Brittle & Plastic
29
brittle failure
plastic failure
s (MPa)
e
x
x
crystalline
regions
slide
fibrillar
structure
near
failure
crystalline
regions align
onset of
necking
Initial
Near Failure
semi-
crystalline
case
aligned,
cross-
linked
case
networked
case
amorphous
regions
elongate
unload/reload
Stress-strain curves adapted from Fig. 15.1, Callister 7e. Inset figures along plastic response curve adapted from
Figs. 15.12 & 15.13, Callister 7e. (Figs. 15.12 & 15.13 are from J.M. Schultz, Polymer Materials Science, Prentice-
Hall, Inc., 1974, pp. 500-501.)

30. 30
TensileResponse: Elastomer Case
• Compare to responses of other polymers:
-- brittle response (aligned, cross linked & networked polymer)
-- plastic response (semi-crystalline polymers)
s(MPa)
e
initial: amorphous chains are
, cross-linked.
x
final: chains
are straight,
still
cross-linked
elastomer
Deformation
is reversible!
brittle failure
plastic failure
x
x

31. MechanicalProperties
Stress-strain behaviour of polymer
31
brittle polymer
plastic
elastomer
Strains – deformations > 1000% possible
(for metals, maximum strain ca. 10% or less)
elastic modulus
– less than metal

32. CHARACTERISATION AND
EVALUATION

33. Molecular Weight
Although in principle the measurement of any
colligative property of a solution ( such as
freezing point depression, elevation of
boiling point, or osmotic pressure) can be
used to determine the molecular weight of a
dissolved solute, only osmotic pressure is
sensitive enough to measure the high molecular
weights characteristic of polymeric substances.

34. TechniquestodetermineMOLECULAR
Weight
Methods Measured
Parameter
M.Wei
ght
Measu
red
Upper Limit
(g per mole
Membrane
osmometry
Osmotic pressure of polymer
solvent
Mn 5x10⁴
Light
scattering
(LS)
Intensity of light scattered by
dilute
polymer solutions
Mw 1x10⁸
Gel
permeation
chromatogr
aphy (GPC)
Elution volume of the
polymer
solution through a GPC
column
packed with porous
microparticles
Mn , Mw 1 x 108
Viscometry
Flow time of polymer solution
through a capillary
M v 1 x 108

35. OSMOMETRY
Osmotic measurements use a semipermeable
membrane through which the solvent can freely
pass but which excludes polymer molecules.
If this membrane separates two compartments,
one filled with pure solvent and the other with a
polymer solution, the activity of the solvent in the
two compartments is different.

36. Osmotic pressure is a colligative property, which means that it is
proportional to the concentration of solute. The van’t Hoff
equation is often presented in introductory chemistry for
calculating osmotic pressure (Π) from the moles of solute (nsolute)
that occupy a given volume (V) and the absolute temperature (T)
of the solution
∏= nRT/ V
According to equation, the
molecular weight of a solute can
be obtained by plotting osmotic
pressure divided by c versus
concentration and extrapolating
the data back to c = 0.

37. Light Scattering
Scattering of light by liquids can be related to local
fluctuations in density due to thermal motions of
molecules.
From measurements of light scattering of dilute polymer
solutions it is possible to derive the weight average
molecular weight.
It is measured by applying Lord Rayleigh’s
electromagnetic theory, which shows that the intensity of
scattering is proportional to the square root of particle
mass

38. Viscometry
Unlike osmometry and light scattering
which are absolute methods in that they allow molecular
weight determinations of unknown polymers, viscometry
is a relative method and requires calibration with
samples of polymer of known molecular weights.
Determination of polymer molecular weight by
measurement of the viscosity of polymer solutions is
based on the fact that, as polymer molecular weight
increases, so does the viscosity of its solutions. The
viscosity is measured by timing flow of the solution
between two marks in various viscometers.

39. GEL PERMEATION CHROMATOGRAPHY
This ia a procedure whereby polymer molecules are
separated according to their size. This method, also a
relative method, is capable of measuring not only
molecular weight, but also molecular weight distribution.
MOLECULARWEIGHT IS
DETERMINED ONLY IFTHE
METHOD IS FIRST CALIBRATED
WITH POLYMER SAMPLES OF
KNOWNWEIGHTS ANDA PLOT
OF MOL.WEIGHTVS
RETENTIONTIME IS
CONSTRUCTED.

40. THERMAL ANALYSIS
A true workhorse for polymer characterization is thermal
analysis, particularly
DSC-Differential scanning calorimetry
TGA-Thermogravimetric Analysis
Narrow peaks are indicative of 1st order transitions such
as melt temperature
2nd order transition like Tg occurs at inflection points
Chemical reaction are indicated by broad peaks

41. Thermogravimetric analysis
This method uses a thermobalance that is
capable of measuring the weight of a sample
contained in a pan.
The pan is placed in a furnace and the
temperature of the furnace is slowly raised,
usually at 5 to 10 degree Celsius/ min.
The technique is used to determine thermal
stability of polymers and the upper limit of
thermal stability is usually taken as the
temperature at which loss of the sample begins.

42. It is unable to detect chain cleavage that produce
degradation fragments that are too large for
volatilization.

43. Differential scanning
calorimetry (DSC)
measures the energy
necessary to establish a
zero temperature
difference between the
sample and an inert
substance
DIFFERENTIAL SCANNING
COLORIMETER
This is useful technique for measuring
 glass transition temperature,
 crystalline melting points,
 heats of fusion
 heats of crystallization.

44. The sample and a reference substance, which does not
undergo a thermal transition in the temperature range of
interest, are placed in 2 small metal containers and
heated by individual electric heaters.
The temperature of both samples, are monitored by
thermocouples, is then gradually raised in such a manner
that the temperature of sample and reference remain the
same.
In this way, transition temperatures can be very
accurately measured by monitoring the electric current
going to the heaters.

45. THERMOMECHANICAL ANALYSIS
This measures deformation of a substance under a non-oscillatory
load as a function of the temperature of the sample, which is
placed on a platform and contacted with a probe.
It can conveniently measure transitions from a glassy to a
rubbery polymer and can also measure softening temperature

46. MECHANICAL PROPERTIES
It is determined by measuring their stress-strain
relationship.
Stress is the stretching force applied to the sample and
strain is the elongation of the sample under a given
stress.
Here the specimen is clamped in a tester that is capable
of extending the specimen at a chosen constant rate
and measuring the force that the specimen exerts on
a load cell.
In the initial phase, application of stress causes a
moderate elongation to the yield point, after which
significant elongation takes place without greatly
increased stress. Elongation then continues until the
specimen breaks.

47. CHARACTERISTIC STRESS STRAIN CURVES FOR FIVE
DIFFERENTTYPES OF POLYMERIC MATERIALS

48. reference
The Controlled Drug Delivery, 2nd edition
revised and expanded edited by Robinson,
J.R., & Lee ,V. H , Marcel Dekker, page- 164-
176
Polymer in drug delivery by Tayler and
Francis group
Hetch G. Remington : the science and
practice of pharmacy, vol 2, page no 832-
833.
http://en.wikipedia.org

49. 3/28/2015 49
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