Stress birefringence

At the same time, a decrease in orientation- and stress-birefringence for the modified BPA-PC could be achieved by optimizing the processing conditions (184,187,189,190,193) this lowers birefringence below 20 nm/mm (single path) for injection-printed injection molded CDs and substrate disks (Fig. 24) (191). The typical characteristics of a modified BPA-PC especially developed for CD production are Hsted in Table 6 (187,193). 

Stress birefringence The presence of stress in glass articles may be monitored readily since stressed glass is birefringent. Standard methods exist for these measurements. 

If the filament or ribbon is made of a material showing intense stress birefringence, the birefringence pattern caused by the load W would be compared with that calculated from the strain energy. A reasonable agreement would indicate that W is suspended by the whole volume of the solid, not by its surface layer alone. On the other hand, if the specimen shows no birefringence when stressed by the load W, then Eq. (30) would gain in respectability. 

The theory of strain birefringence is elaborated in terms of the RIS model as applied to vinyl polymer chains. Additivity of the polarizability tensors for constituent groups is assumed. Stress-birefringence coefficients are calculated for PP and for PS. Statistical weight parameters which affect the Incidences of various rotational states are varied over ranges consistent with other evidence. The effects of these variations are explored in detail for isotactic and syndlotact/c chains. 

Stress Birefringence. Confirmation of the multiphase nature of the polyblends described may be obtained by stress-optical measurements. Since this technique apparently has not been used before to demonstrate incompatibility in polyblends, a brief description is given of the rationale behind the method. 

The light emerging from the sample is recollected by a single CaF, or BaF2 lens and focussed onto the detector. It is essential that as few optical elements as possible are used between the sample and the detector, and that these elements are as free of optical stress as possible, since stress birefringence at this stage may induce artifacts. In addition, reflective optics should be avoided. 

The dependence of stress birefringence on the duration of the diffusion processes... 

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... 

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