Birefringence, strain-induced

An example of a relevant optical property is the birefringence of a deformed polymer network [246]. This strain-induced birefringence can be used to characterize segmental orientation, both Gaussian and non-Gaussian elasticity, and to obtain new insights into the network chain orientation (see Chapter 8) necessary for strain-induced crystallization [4,16,85,247,248]. 

Figure 18 shows the percent crystallinity obtained by birefringence measurements for NR at various elongations as a function of temperature. The relative shapes of the curves in this Figure show the pronounced temperature and strain dependence on the strain induced crystallization of NR. Of particular importance is the relatively high amounts of crystallinity that develop at room temperature. 

Such curves, illustrated in Figure 7, show no deviations from linearity which could be attributed to strain-induced crystallization. Similarly, birefringence-temperature measurements also carried out at a > au show no deviations from linearity that could be attributed to crystallization, or to other inter-molecular orderings of the network chains. Typical results of this type are shown in Figure 8 (16),... 

PLZT can be exploited for image storage in a variety of ways the first of the following two examples makes use of strain-induced birefringence and the second light scattering (Fig. 8.14). 

Based on a study of the temperature dependence of strain-induced birefringence in amorphous polymers, Raha and Bowden [12] suggested that the thermal dissociation of entanglements can be described by... 

A final caveat is with respect to any polarized experiments (linear or circular) that are contemplated on solid solutions. Many solid solutions such as polymer films or glassed solutions are prone to strain-induced birefringence. Birefringence modifies the polarization of the incident beam. This can seriously degrade the precision of a measurement, and furthermore the effect is neither spatially uniform nor necessarily constant during an experiment. A useful visual check is to view fhe sample between crossed polarizers. If there are no obvious bright areas in fhe field of the sample then useful polarized experiments can probably be performed. 

Methods to study crystallization of deformed elastomers include x-ray diffraction [207,208,263-265], optical birefringence [266,267], infrared or Raman spectroscopy, electron microscopy [268], dilatometry [269, 270], NMR [271], and mechanical measurements [193,262,272]. Strain-induced crystallization is manifested in the latter by both greater hysteresis (Fig. 23) and a longer time for stress decay (Fig. 24). However, the shape of the stress-strain curve during extension does not obviously reveal the onset of crystallization [207,208,262]. 

In some cases, double networks have shown increases in orientability and strain-induced crystallization, as well as improved fatigue resis-tance. ° In fact, some results show that there maybe less of a compromise between failure properties in general and the modulus, which may be due in part to the decreased hysteresis observed for some of these elastomers. There have even been reports of improved thermal stabil-ity, although it is hard to visualize how this would occur. Finally, electrical resistivity is more sensitive to strain in carbon-black reinforced double networks. Better molecular understanding of these observations is being sought with, for example, extensive studies of residual strains and birefringence. ... 

The strain-induced birefringence in the amorphous region of a semicrystalline polymer disappears at the glass-transition temperature, as the chains assume isotropic configurations. 

BirGfring6nC6 of Polymer Networks. Elastomeric polymer networks deserve special mention because their cross-linked structure gives them unique physical properties, unlike those of other polymers. Covalently bonded networks are insoluble in any solvent, even in those that dissolve their precursor polymers. Optical techniques such as strain-induced birefringence allow the development of structure-property relationships as well as the study of their optical properties. Here we review some of the classical theories as well as some of the latest developments in the field. 

The speed of sound through a medium is a function of applied strain. The relationship is governed by the acoustoelastic coefficient. Stress states with unequal principle stresses (i.e., nonhydrostatic) have the effect of introducing anisotropic acoustic behavior to otherwise isotropic materials. This effect is similar to strain-induced optical birefringence (covered in the next section). The advantage is that ultrasonic birefringence can be measured in optically opaque materials. 

Strain-induced optical birefringence is a rapid and powerful technique for transparent isotropic materials. Not only is it possible for one to see residual strains introduced as a result of processing, but inclusions that are invisible to the naked eye can be seen by their associated strain fields. This technique is extremely fast. The product can be conveyed between two crossed polarizers, allowing a single inspector to inspect the output of a small factory. This technique is capable of resolving stresses of as little as 55 MPa (8 kpsi) in Pyrex glass. 

In the case of an FBG sensor under torsion, both shear strain-induced birefringence and UV-induced birefringence are considered ... 

There are UV-induced linear birefringence and shear strain-induced circular birefringence in the fibre. 

The transverse strain-induced wavelength sensitivity of FBGs in silica optical fibres is low. For instance, in lateral compression, the changes in fibre birefringence are smaller than 10 This level of birefringence corresponds to wavelength separations much smaller than the typical bandwidth of an FBG. However, we have shown in Section 10.3.1 that the sensitivity factor is a function of the two principal transverse strains and axial strain, which are normally unknown. The needs are apparent for multi-axial measurements of strain and temperature. 

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