Orientation birefringence measurements

The compensation birefringence measurement is very easily coupled to optical microscopy in the transmission and reflection modes, thus allowing characterizing orientation with a spatial resolution of a few hundreds of nanometers [14]. Polarizing microscopes are widely available and are often used for birefringence studies even if spatial resolution is not required. Objectives specifically designed for cross-polarized microscopy are necessary to avoid artifacts. 

The polarizing microscope can be used to observe morphological features of all crystalline polymers that can be prepared as thin films. Birefringence measurement is applicable to all polymers in which orientation or anisotropy can be induced. 

The evolution of microstructural orientation in block copolymers was studied in real time by Kornfield and co-workers using birefringence measurements on diblocks sheared in situ (Gupta et al. 1995 Kannan and Kornfield 1994). The birefringence of the polymer in two orthogonal planes was measured using laser light... 

In general, this configuration is not preferred for birefringence measurements except if the orientation angle is known a priori. In that case the sample orientation should be set to 0 = 0 to produce the simplest result. 

The optical apparatus used in this work was described in section 8.6 and has the capability of providing both Raman scattering and birefringence measurements simultaneously. The Fourier expansion of the overall Raman scattering signal is given by equation (8.51), and the coefficients are given by equations (8.52) to (8.54). In these expression, a simple, uniaxial form for the Raman tensor was assumed. From these coefficients, the anisotropies in the second and fourth moments of the orientation distribution can be solved as... 

To illustrate the usefulness of birefringence measurements in orientation studies, we now briefly discuss two simple models of orientation leading to different expressions of the second moment of the orientation function the affine deformation model for rubbers and the pseudo-affine model more frequently used for semi-crystalline polymers. 

In regime III, the flow field is very strong and shear-induced molecular orientation becomes important. According to birefringence measurements for anisotropic HPC/H2O solutions and HPC/ m-cresol solutions, the molecular orientation is a monotoni-cally increasing function of the steady state shear rate. 

Tricot M, Houssier C. Electro-optical studies on sodium polystyrene-sulfonate). 1. Electric polarizability and orientation function from electric birefringence measurements. Macromolecules 1982 15 854-865. 

Other Analyses. Birefringence measurements using a Babinet compensator were made on the SBS films to detect changes in the orientation of the molecules. 

The strain-indueed orientation is analyzed with the help of birefringence measurements [107, 115, 120, 123] (realized in the isotropic state, close to the transition), measurements of the IR dichroism [124, 126], and X-ray experiments [93, 108,120, 122, 125]. It was observed that the orientational order parameter, labeled P2 on Fig. 32, obtained from X-ray experiments [122], quiekly inereases with strain at the beginning, then saturates at a P2 value of about 0.4-0.6 for a sample synthesized in an isotropic state [99, 107, 121, 122]. On condition that they were realized above the gel point [122], the networks display reproducible and reversible behavior independent of the sample history. 

When the two phases are sufficiently different, methods such as measurement of infrared dichroism or Raman scattering can sometimes be used to characterise the orientation of the two phases independently, which will usually give more accurate results. Nevertheless, birefringence measurements are often used as a cross-check on the results, because all methods of characterising orientation are subject to errors that are sometimes difficult to quantify and it is usually desirable to obtain as much data as possible by various methods. In practice, the maximum birefringences of the various phases are not always well known, so large uncertainties can arise from using equation (10.31). 

For the polymer considered in the previous section the birefringence measurements and the stretching or shrinkage took place at different times the birefringence was measured in the frozen-in state of orientation. It is, however, possible to measure the birefringence of a real rubber when it is still under stress at a temperature above its glass-transition temperature. This provides a simultaneous test of the predictions of the rubber deformation theory for both orientation and stress. 

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