Polymer Rheology

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Chapter III Optical Characterizations
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Topics in Each Section

• 3-1 Introduction to Rheo-Optics Method
• 3-1.1. Introduction Review of Optical Phenomena
• 3-1.2. Characteristic Dimension Optical Range
• 3-2 Typical Experimental Set-ups
• 3-2.1 Flow Dichroism and Birefringence
  Measurements for Case Study
  1-2
• 3-2.2 Combined Rheo-Optcial Measurements
  (including Rheo-SALS) for Case Study 3
• 3-3 Information Retrieval in Individual
  Measurements
• Case Study 1 Flow Dichroism and Birefringence of
  Polymers
• Case Study 2 Dynamics of Multicomponent Polymer
  Melts
• Case Study 3 Combined Rheo-Optcial Measurements
• References

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3-1. Introduction to Rheo-Optics Method

• 3-1.1 Introduction Review of Optical Phenomena
• A rheological measurement entails the measurement
  of
• Force (related to the stress)
• Displacement (related to the strain)
• In a rheo-optical experiment, both the force and
  optical properties of the sample are measured

Table A comparison of some important features in
optical and mechanical measurements
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• When incident electromagnetic radiation interacts
  with matter, three broad classes of phenomena are
  of interest
• Transmission of Light
• The light can propagate through the material with
  no change in direction or energy, but with a
  change in its state of polarization
• Birefringence Dichroism Turbidity
• Scattering Radiation
• The radiation can be scattered (change in
  direction) with either no change in energy
  (elastic) or a measruable change in energy
  (inelastic)
• Static Light, X-Ray, and Neutron Scattering
  Dynamic Light Scattering
• Absorption and Emission Spectroscopies
• Energy can be absorbed with the possible
  subsequent emission of some or all of the energy
• Fluorescence Phosphorescence

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• 3-1.2 Characteristic Dimension Optical Range
• Typical levels of structures in
  polymeric systems are listed below

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The general order of structural levels
that can be studied by some of the different
techniques is given below
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3-2. Typical Experimental Set-ups

• 3-2.1. Flow Dichroism and Birefringence of
  Polymers in Shear Flows
• Basic Concepts

Turbidity
The Lambert-Beers Law
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Dichriosm
EX 1 Polariod Sun Glasses (A daily Eexample of
dichrism resulting from absorption)
FIG. Representation of a Polaroid sheet. Light
with a polarisation direction parallel to the
aligned polymers is absorbed more strongly as
compared to light with a polarisation direction
perpendicular to the polymers
EX 2 Colloids under Shear Flow
The total amount of scattered light depends on
the polarisation direction due to the anisotropic
nature of the microstructure under shear flow
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Birefringence
FIG. Linear polarised light is generally
transmitted as elliptically polarized light
through a birefringent material
More specifically, the polarisation direction of
the light can be decomposed into a component
parallel to the direction where the refractive
index is large and a component parallel to the
direction where the refractive index is
small After having traversed the sample, there
is a relative phase shift of the two
field components and the sum of the two fields is
generally elliptically polarised
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• Basic Polarimeter Design

PSG
PSA
Light source
Sample
Detector
FIG. Basic polarimeter schematic
A Polarization State Generator, PSG, defines the
polarization of the light prior to
transmission through the sample A Polarization
State Analyzer, PSA, determines the state of
polarization of the existing light
In transmission exps, one is normally concerned
with the measurement of light polarization
The Stokes Vector Entering the Detector is
The specific Mueller matrix components (optical
properties) of the sample can be identified
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• Example The Crossed Polarizer System

FIG. Birefringent and dichroic sample sandwiched
between corssed polarizers
The Measured Intensity I for a Sample with
Coaxial Birefringence and Dichroism oriented at
an Angle ? is
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• Typical Arrangement for Flow Birefringence and
  Dichroism Measurements

FIG. Schematic of the experimental set-up for
dichroism and birefringence measurements

• A somewhat simpler set-up can be sued
• Dichroism only removal of R2, P2, and D2
• Turbidity only removal of R1, R2, P2, and D2

BS,BS Beam splitter D1-D3 Detectors P1,P2
Polarizers R1,R2 Rotating quarter wave platelets
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• Optical Train Mounted on a Rheometer

FIG A combination of rheomechanical and
rheo-optical measurements
FIG Experimental apparatus for determination of
flow birefringence and flow dichroism
http//www.chemie.uni-hamburg.de/tmc/kulicke/rheol
ogy/rheo2.htm
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• 3-2.2. Combined Rheo-Optical Measurements
• Optical Setup for Shear-Small-Angle-Light-Scatteri
  ng (SALS) Mesurements Kume et al. (1997)

FIG. Schematic diagram of the experimental setup
for shear-light scattering one-dimensional
detector (photodiode array), cone-and-plate type
shear cell to generate Couette flow, and
coordinate system used in this study
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• Optical Setup for Shear-Dichroism,
  Shear-Birefringence, and Rheological Measurements
  Kume et al. (1997)

FIG. Schematic diagram of the experimental setup
for shear-dichroism, shear-birefringence, and
rheology (a) Schematic diagram of the combined
mechanical rheometer and optical train. (b) Shear
cell and optical train
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3-3. Information Retrieval in Individual
Measurements

• CASE STUDY 1 Flow Dichroism and Birefringence of
  Polymers in Shear Flows
• A Rheo-Optical Study of Shearing Thickening and
  Structure Formation in Polymer Solutions
  Kishbaugh and Muhugh (1993) Figs. Reproduced
  from Sondergaard and Lyngaae-Jorgensen (1995)

FIG. Schematic of photoelastic modulation
rheo-optical device. Optical elements in the
alignment configuration
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Note that only data for the case of Mw1.54 x
106 is shown in the following 3 pages
One-to-one correlation between the onset of
shear thickening and the occurrence of a maximum
in the dichroism
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The viscosity and dichroism patterns for the
lowest concentration are similar to those
exhibited by a lower molecular weight
sample (Mw4.3 x 105). Namely, the dichroism
rises to a plateau, while viscosity undergoes a
monotonic drop with shear rate to an eventual
Newtonian plateau
Dichroism
At higher concentrations, a dramatic
and distinctive pattern emerges. One sees a
shaper rise in the dichroism to an eventual
maximum, while the viscosity simultaneously drops
to a minimum. This is followed by a region of
shear thickening in which the viscosity
continuously rises, while the dichroism decreases
and eventually turns negative
Viscosity
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This figure shows that, in this range,
the orientation angle dropped to a constant
near-alignment with the flow axis
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Throughout the entire flow curve, the
birefringence exhibits a steady monotonic
increase with shear rate
These data offer strong evidence that the
overall orientation of the chain segments is
independent of the structuring processes, which
may take place as indicated in the dichroism
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• CASE STUDY 2 Dynamics of Multicomponent Polymer
  Melts
• Infrared Dichroism Measurements of Molecular
  Relaxation in Binary Blend Melt Rheology
  Kornfield et al. (1989)

1. Chains are identical in chemical composition,
but differ in M.W.. Isotopic labeling with
deuterium (D) can be used to distinguish one M.W.
component from another 2. At 2,180 cm-1 the C-D
bond absorbs but the C-H bond does not
3. The most interesting result is that the
longest relaxation time of the the shorter chains
is a strongly increasing function of the
volume fraction of longer chains. This contrasts
with the predictions of the basic reptation
model
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• CASE STUDY 3 Combined Rheo-Optical Measurements
• Rheo-Optical Studies of Shear-Induced Structures
  in Semidilute Polystyrene Solutions Kume et al.
  (1997)

1. Shear-induced structure formation in
semidilute solutions of high molecular weight
polystyrene was investigated using a wide range
of rheo-optical techniques 2. The effects of
shear on the semidilute polymer solutions could
be classified into some regimes w.r.t. shear
rate
FIG. A complete picture of the shear-induced
phase separation and structure formation from a
wide range of techniques on the same polymer
solutions
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• Continued

Homogeneous solution
Strong butterfly-type LS pattern
Streaklike LS pattern
Oblate-ellipsoidal structures
Long stringlike structures
Shear-microscopy results
Change of the sign
Due to the stringlike structures oriented
parallel to the flow dir.
Chains weakly orient along the flow dir.
Chains in the strings with their end-to-end
vectors parallel to the flow dir.
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• Continued

Comparisons with Mechanical Characterizations
Mechanical
Notice that the behavior of the shear viscosity
is also classified into three regimes
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References