Basic Behavior Polymer Polymer Engineering

1. Fundamentals of Polymer Engineering
Polymer Rheology
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Course Code: PE-2102
Credits: 3-0-3
Teacher: Engr. Asra Tariq
Email Id: asra.tariq@ntu.edu.pk

2. What is Rheology
 Rheology is the study of the flow of matter, primarily in a liquid state, but also
as "soft solids" or solids under conditions in which they respond with plastic
flow rather than deforming elastically in response to an applied force. It is a
branch of physics which deals with the deformation and flow of materials,
both solids and liquids.
 The rheology of the polymer melt is sensitive to small changes in the polymer
structure. Rheology is considered as the most convenient method to
characterize polymers. Even small amounts of a high molecular weight
polymer can affect the processing behaviour and so does the melt rheology.
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3. What is Rheology
 Clearly industrial flows are complex, not only because the geometries
are complex, but also because the constituents are not usually simple.
We have several components in a shampoo, performing various
actions. Molecular weight distribution of a polymer is another level of
complexity, as polymers are rarely synthesised in a sharp monodisperse
population.
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4. Viscosity
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 The viscosity of a fluid is a measure of its resistance to deformation at a
given rate.
 One is often interested in understanding the forces, or stresses, involved in
the deformation of a material. For instance, if the material were a simple
spring, the answer would be given by Hooke's law, which says that the
force experienced by a spring is proportional to the distance displaced from
equilibrium. In other materials, stresses are present which can be attributed
to the rate of change of the deformation over time. These are called viscous
stresses. Viscosity is the material property which relates the viscous stresses
in a material to the rate of change of a deformation (the strain rate).

5. Absolute Viscosity
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 The tangential force per unit area of two parallel planes at unit distance apart when the
space between them is filled with a fluid and one plane moves with unit velocity in its own
plane relative to the other. Also known as coefficient of viscosity.
 Considering two parallel plane of area A [m2] at distance y [m], one fixed and one moving
under a Force F [N], the moving plane will move at velocity u [m/s]. In an ideal laminar
flow, at low speed, the velocity variation would be linear and the shear stress τ will be
proportional to distance.

6. Absolute Viscosity
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 Considering that the conditions described above are difficult to obtain a more realistic condition would be that
the velocity would not increase linearly.
 The measuring unit of dynamic viscosity in SI is the pascal per second [Pa s], that is equal to a poiseuille [PI],
often in lubricant industry the submultiple millipascal per second [mPa s] is used or in alternative the
centipoise [cP] (CGS system) that is equal to 1 mPa s.
1 Pa s = 1 PI
1 cP = 1 mPa.s
In order to understand better the measuring
unit used in viscosity:
The shear stress is defined as

7. Viscoelasticity
 Classical theories of elasticity and viscosity of a body assume steady state
stress, strain and strain rate.
 Viscoelasticity is, the study of the response of polymers (or other materials)
which exhibit some of the features of both elastic and viscous behaviour.
Elastic materials deform instantaneously when a load is applied, and
“remember” their original configuration, returning there instantaneously
when the load is removed. In solids, the relaxation of the structure at the
molecular level is extremely low and, therefore, their response is essentially
elastic. On the other hand, viscous materials do not show such
characteristics, but instead exhibit a time-dependent behaviour. While under
a constant stress, a viscous body strains at a constant rate, and when this
load is removed, the material has “forgotten” its original configuration,
remaining in the deformed state.
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8. Viscoelastic Behavior
Elastic Response Viscous Response Visco-elastic Response
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9. Importance of Rheology Studies
 Rheology (particularly in the study of the viscous flow
of polymers) is very important because it gives a
relationship between the properties, structure and
processing of the materials. Rheology helps describe
the mechanical behaviour of materials as a function of
stresses (shear rate), strain , temperature and pressure
in order to develop materials with the correct
processing behaviour based on their viscosity, elasticity
and time. These parameters can be calculated using a
dynamic mechanical testing method at melt
temperatures.
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10. Types of Fluids
 - Ideal fluid
The viscosity is zero, this is used for the analysis of potential flow
problems such as found in aerodynamics
- Newtonian fluid
A good viscosity model for gases and simple liquids such as water
- Non Newtonian fluid
The viscosity of the fluid changes with shear rate. For some fluids,
it increases with shear rate (dilatant fluids) while for others it
decreases with shear rate (pseudoplastic behaviour). This
pseudoplastic (or shear thinning) behaviour is what we observe for
polymers. Factors affecting viscosity
- pressure
- molecular weight
- temperature
- shear rate
- Time
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11. Rheology
Shearing stress Coiling & entanglement
• Random & Brownian motion in fluids
Alignment & disentanglement
• Shear stress applied to the fluid
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Shearing Stress

12. Measurement of Rheological properties
Rheometer
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 The use of rheometers or viscometers
to determine the rheological
properties of materials with the
measured values based on force,
length and time. Rheometers are
devices use to determine both the
viscous and viscoelastic properties of
materials depending on the force
exerted in both the rotational and
oscillatory test.
While on the other hand viscometers
are devices use to determine the
viscosity depending on the rotation,
temperature and time.

13. Measurement of Rheological properties
Rotational Rheometer
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 In a rotational rheometer the sample is sheared
between two plates or a cone and plate geometry.
The viscosity is calculated as the ratio of the
applied stress and the applied deformation rate
(rotation speed). This instrument in the case of
polymers is use to extract information about the
average molecular weight and molecular weight
distribution via measurement of the viscoelastic
properties and thus determine how this affects the
processing characteristics of the material.

14. Measurement of Rheological properties
Melt Flow Indexer
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 Rheological properties can be measured using either
Melt Flow Indexer:
Actually this instrument is mostly use for quality control.
It does not compute a material property. Its mainly to
compare the relative flow behaviour of different materials
based on the viscosity.

15. Measurement of Rheological properties
Capillary Rheometer
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 Capillary rheometers are mostly used to examine processing behaviour,
rather than just determining the rheological parameters. In a capillary
rheometer the material is forced through a slit or round die. From the
pressure drop and the volume flow rate, a steady state viscosity as a
function of the applied rate (piston speed) or stress (applied pressure) is
measured.
Advantages:
(1) Can operate at high shear rates
(2) May be closer to real processing situation than a rotational rheometer
Disadvantages:
(1) Shear rate is not uniform
(2) Wall slip
(3) Melt fracture
(4) Difficult to clean
In addition we also have gravity driven capillary rheometers also called
viscometers but mostly function for viscous Newtonian fluids.