Birefringence measurement copolymers

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... 

An interesting feature of polarized IR spectroscopy is that rapid measurements can be performed while preserving molecular information (in contrast with birefringence) and without the need for a synchrotron source (X-ray diffraction). Time-resolved IRLD studies are almost exclusively realized in transmission because of its compatibility with various types of tensile testing devices. In the simplest implementation, p- and s-polarized spectra are sequentially acquired while the sample is deformed and/or relaxing. The time resolution is generally limited to several seconds per spectrum by the acquisition time of two spectra and by the speed at which the polarizer can be rotated. Siesler et al. have used such a rheo-optical technique to study the dynamics of multiple polymers and copolymers [40]. 

The selective reflection of circularly polarized light on radiation of normal light is also exhibited by the cholesteric polymers. Like the 1-l.c. systems, the Grandjean-texture is formed spontaneously, if the polymer is sandwiched between glass plates as shown in Fig. 23. The measurements of indicate no difference to low molar mass systems. In Fig. 24 VHT) is shown for the induced cholesteric polymers, whose birefringence was discussed above (refer to Table 8, copolymers No. 4, m = 6). The different curves refer to different mole fractions of the chiral comonomer. With... 

Recently, the orientation behavior of poly-(IV-5) was compared to both poly-(IV-4) and a statistical copolymer poly-IV-5-COEN (COEN=cyclooctene) [51]. The experiments shotved that, compared to poly-(IV-5), samples of poly-(IV-5-COEN) were easier to orientate, a finding that was attributed to the more flexible chain. The dependence of the magnetic field and the temperature on the degree of orientation, measured by the birefringence and the optical transparency, is shown in Fig. 6. 

Until now we have considered the basic origin of birefringence and some of the general techniques used for determining this optical parameter. It is necessary, however, to discuss certain limitations when interpreting this parameter. Until now no mention has been made of two or multiphase systems such as semicrystalline polymers, amorphous block copolymers or even plasticized or filled polymers. In such systems the measured birefringence can be expressed as... 

As an alternative to holographic experiments, the photoinduced birefringence can also be measured by the response of the material to polarized light. Its value depends on the concentration of the photoactive chromophores, which decreases with increasing amount of the homopolymer. Normalizing the results to the azobenzene content yields very similar results for each series of blends in which the same block copolymer is used (see Fig. 37). This also confirms that the photoresponse is independent of the homopolymer content. 

The sonic technique suffers from one of the same difficulties as the birefringence method in that only an average orientation of the total system is obtained. Thus, if one has a multicomponent system (e.g. semicrystalline, block copolymer. etc.X one cannot separate component orientation by this technique alone. It is noteworthy that the orientation measured by the sonic method can be correlated directly with birefringence data on the same material.This ease of correlation has been suggested to lead to values of the intrinsic birefringence by extrapolation of the sonic data to perfect orientation. 

The following is the summary of the results of the studies (31) on the effect of stereoregularity on stress-induced crystallization and mechanical properties of the vulcanlzates of propylene oxide copolymers. In the studies on stress-induced crystallization, stress (O ) and birefringence ( ) were measured as a function of temperature using an Instron tester fitted with a thermostatted Insulated chamber and an optical system. In an amorphous rubber, the quantity (T Vo) (where T is absolute temperature) is almost constant. Stress-induced crystallization leads to the formation of crystals oriented in the stretching direction which substantially Increase the birefringence. 

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