Photoelastic 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... 

The fiber fragment length can be measured using a conventional optical microscope for transparent matrix composites, notably those containing thermoset polymer matrices. The photoelastic technique along with polarized optical microscopy allows the spatial distribution of stresses to be evaluated in the matrix around the fiber and near its broken ends. 

These experimental results give only a first insight into the fascinating properties of these very new l.c. elastomers. Further detailed studies have to come to a more profound understanding of the interactions of polymer networks and l.c. order of the mesogenic side chains. The exceptional photoelastic and thermoelastic properties promise to open up a new scientific and technological field of interest. 

Hrouz J, Ilavsky M, Ulbrich K, Kopecek J. The photoelastic behavior of dry and swollen networks of poly(lV,lV-diethylacrylamide) and its copolymer with N-tert butylacrylamide. Eur Polym J 1981 J17 361-366. 

Ilavsky M, Talasova E, Dusek K. The photoelastic behavior of swollen networks of polymethacrylic acid. Eur Polym J 1980 16 191-199. 

An ordered packing of macromolecules may also cause an optical anisotropy and birefringence, which are characteristic, for instance, of polymer spherulites. Because of the radial anisotropy of a spheruHte and the convergence of beams in the spherical structure, the interference picture represents the so-caUed Maltese cross, the center of which is located in the center of spherulite. No calculations are performed using such a picture but the photoelasticity method is very efficient in revealing qualitatively the presence of any spherulites, or a mesomorphic or ordered sate of polymeric chains. 

An independent method of determining stress, especially useful for materials under flow, is from the optical birefringence. Of course, it is limited to transparent materials, which precludes application to filled polymers. Because of the need for transparency, stress birefringence has been used more often for plastics than rubber. Residual anisotropy due to unrelaxed orientation can also be assessed using birefringence this is commonly known as the photoelastic effect. Generally the birefringence is directly proportional to the tme stress... 

It is also useful to be able to study stress distributions experimentally. Some materials, especially glasses and polymers, are photoelastic, i.e., they undergo a change in refractive index under the application of mechanical stress. When viewed under polarized light, meehanical models made from these materials give rise to an optieal pattern that can be related quantitatively to the principal stress distribution in the loaded body. Figure 4.26 shows a phototelastic fringe pattern... 

The study of optico-mechanical properties of one of ladder polyphenyIsesquioxanes in bulk has revealed the complex nature of the photoelasticity effect in the films of this polymer. Ladder polymers are characterized by high equilibrium rigidity of double stranded chains. ... 

PHOTOELASTIC BEHAVIOR OE LIQUID CRYSTALLINE POLYMER NETWORKS... 

Photoelasticity n. Changes in optical properties of isotropic, transparent materials when subjected to stress. Mainly, they become birefringent, a property that has been widely used to detect frozen-in stresses in sheets and other products. Meeten GH (1986) Optical properties of polymers. Springer-Verlag, New York. 

F. De Candia, A. Taglialatela and V. Vittoria, Structure-Property Relationships in Some Composite Systems. Deformation Mechanism, J. Appl. Polym. Sci. 20, 1449 (1976). Simultaneous crosslinking of polybutadiene with polymerization of methacrylic acid or magnesium acrylate. Photoelastic study of hysteresis cycles. 

In recent years, epoxy resins have become the polymer of choice for three-dimensional photoelastic investigations. Further, the phenomena of birefringence has been used to study plasticity and viscoelasticity effects in materials through the use of extensions to the photoelastic method called photoplasticity and photoviscoelasticity (see Brill (1965) and Brinson (1965, 1968), respectively). Brill used polycarbonate, a thermoplastic polymer, as a model material for his work on photoplasticity and Brinson used an epoxy, a thermosetting polymer, as a model material for his work on photoviscoelasticity. Later, it will become clear why thermoplastic materials are used for photoplasticity while thermosetting materials are used for photoviscoelasticity. 

Cross-linked or thermosetting polymers are typically used for photoelastic stress analysis. Thus, it is clear that a certain amount of crystallinity can be induced by stresses in network polymers but the degree of crystallinity is necessarily very small. 

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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