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Chapter III Optical Characterizations

Chapter III Optical Characterizations. 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

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Chapter III Optical Characterizations

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  1. Chapter III Optical Characterizations

  2. 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

  3. 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

  4. 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

  5. 3-1.2 Characteristic Dimension & Optical Range Typical levels of structures in polymeric systems are listed below

  6. The general order of structural levels that can be studied by some of the different techniques is given below

  7. 3-2. Typical Experimental Set-ups 3-2.1. Flow Dichroism and Birefringence of Polymers in Shear Flows • Basic Concepts: Turbidity The Lambert-Beer’s Law:

  8. 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

  9. 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

  10. PSG PSA • Basic Polarimeter Design: 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

  11. 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 θ is:

  12. Typical Arrangement for Flow Birefringence and Dichroism Measurements: FIG.Schematic of the experimental set-up for dichroism and birefringence measurements BS’,BS: Beam splitter D1-D3: Detectors P1,P2: Polarizers R1,R2: Rotating quarter wave platelets • 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

  13. Optical Train Mounted on a Rheometer FIG Experimental apparatus for determination of flow birefringence and flow dichroism FIG A combination of rheomechanical and rheo-optical measurements http://www.chemie.uni-hamburg.de/tmc/kulicke/rheology/rheo2.htm

  14. 3-2.2. Combined Rheo-Optical Measurements • Optical Setup for Shear-Small-Angle-Light-Scattering (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

  15. 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

  16. 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

  17. Note that only data for the case of Mw=1.54 x 106is 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

  18. • The viscosity and dichroism patterns for the lowestconcentration are similar to those exhibited by a lower molecular weight sample (Mw=4.3 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

  19. • This figure shows that, in this range, the orientation angle dropped to a constant near-alignment with the flow axis

  20. • 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

  21. 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 CASE STUDY 2: Dynamics of Multicomponent Polymer Melts • Infrared Dichroism Measurements of Molecular Relaxation in Binary Blend Melt Rheology [Kornfield et al. (1989)] 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

  22. 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

  23. 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.

  24. Continued Comparisons with Mechanical Characterizations: Mechanical Notice that the behavior of the shear viscosity is also classified into three regimes

  25. References

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