Photoelastic modulators , polymer

A more complex but faster and more sensitive approach is polarization modulation (PM) IRLD. For such experiments, a photoelastic modulator is used to modulate the polarization state of the incident radiation at about 100 kHz. The detected signal is the sum of the low-frequency intensity modulation with a high-frequency modulation that depends on the orientation of the sample. After appropriate signal filtering, demodulation, and calibration [41], a dichroic difference spectrum can be directly obtained in a single scan. This improves the time resolution to 400 ms, prevents artifacts due to relaxation between measurements, and improves sensitivity for weakly oriented samples. However, structural information can be lost since individual polarized spectra are not recorded. Pezolet and coworkers have used this approach to study the deformation and relaxation in various homopolymers, copolymers, and polymer blends [15,42,43]. For instance, Figure 7 shows the relaxation curves determined in situ for miscible blends of PS and PVME [42]. The (P2) values were determined... 

Figure 2 Schematic diagram of a spectropolarimeter adapted for linear dichroism measurements. LS, light source M, monochromator P, polarizer PEM, photoelastic modulator producing time-dependent retardation jisin cot, S, sample PM, photomultiplier A, d.c. amplifier LA, lock-in amplifier operating at frequency 2co REC, recorder. Adapted by permission from Michl J and Thulstrup EW (1995) Spectroscopy with Polarized Light Solute Alignment by Photoselection, in Liquid, Polymers, and Membranes. New York Wiley-VCH.

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