Testing birefringence measurement

Transient birefringence measurements were used by Larson et al. [112] to test the validity of the Lodge-Meissner relationship for entangled polymer solutions. This relationship states that the ratio of the first normal stress difference to the shear stress following a step strain is simply Nx/%xy - y, where y is the strain. Those authors found the relationship was valid, except for ultrahigh molecular weight materials. 

For the polymer considered in the previous section the birefringence measurements and the stretching or shrinkage took place at different times the birefringence was measured in the frozen-in state of orientation. It is, however, possible to measure the birefringence of a real rubber when it is still under stress at a temperature above its glass-transition temperature. This provides a simultaneous test of the predictions of the rubber deformation theory for both orientation and stress. 

There are numerous other experimental methods that provide information about the rheological behaviour, including the molecular response to stresses and strains swell and shrinkage tests, flow-birefringence measurements (Chapter 9) and flow-infrared-dichroism measurements (Chapter 9) to mention but a few. 

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]. 

For almost any experiment that involves a light microscope, quantitative data to test a hypothesis may be obtained by microphotometry. Add a photometer and a few accessories to a light microscope, and it may be possible to quantify a cytochemical reaction product, measure the spectrum of a pigment, quantify natural or induced fluorescence, quantify birefringence, or map and analyze an image. But manually collecting thousands of numbers is unbelievably tedious. The solution is obvious. Use a personal computer (PC) to collect the numbers. 

Transparency, gloss, color, refractive index, and reflectance are the properties normally associated with aesthetics of plastic materials. In some areas, changes in optical properties, increases in haze after abrasion testing (285), color differences after weathering, and birefringence analysis of residual stress within a transparent part (286) are all used to measure the effects of applied stresses. Measurements of color, gloss, refractive index, and haze apply to many products beyond plastics and use similar techniques. Reference should be made to this general topic for detailed information (see Color). 

Optical measurements often have a greater sensitivity compared with mechanical measurements. Semidilute polymers, for example, may not be sufficiently viscous to permit reliable transient stress measurements or steady state normal stress measurements. Chow and coworkers [113] used two-color flow birefringence to study semidilute solutions of the semirigid biopolymer, collagen, and used the results to test the Doi and Edwards model discussed in section 7.1.6.4. That work concluded that the model could successfully account for the observed birefringence and orientation angles if modifications to the model proposed by Marrucci and Grizzuti [114] that account for polydispersity, were used. 

Equations (1) and (2) contain the pitch P besides experimentally available quantities, i.e., the selective reflection wavelength A , the optical rotation 0, and the refractive indices Mo and of a nematic sheet. The usually determined cholesteric birefringence can be converted to the nematic one by A nem= 2Awci,. By fitting Eq. (2) to the actual measured data, the pitch is obtained and the validity of Eq. (2) tested. In the limit of A Eq. (2) leads to... 

Relaxation birefringence is meant to imply the measurement of birefringence during stress relaxation of a deformed material. The typical application has concerned rubbers and rubberlike materials where the objective is to test the degree of ideal Gaussian rubber elasticity by utilising the stress optical law given earlier in eqn. (10). 

The polymer network structure can be studied by various means. Optical characterization is particularly versatile, since it can probe the composites directly and test whether, and to what degree, the network is oriented (75, 27, 30, 31), Hot-stage cross polarized light microscopy can be used to test the influence of monomer or polymer on LC phase transitions of these composites. Measurement of the birefringence of the bare polymer network, or of the LC composite in the isotropic state, yields information concerning anisotropy of the polymer network and of the type and strength of interaction between the network and LC matrix (75, 27, 30, 31). 

Phenomena associated with "dielectric relaxation" are often observed in polymers. In polyethylene, for example, if the fiber is pulled and then clamped at constant strain, the birefringence will increase and become asymptotic to a constant value which depends upon the strain at which it was clamped and the temperature. The evolution of birefringence looks like the graph in Fig. 4. The birefringence in tests of this kind is always measured in places where the specimen is deforming homogeneously. 

Generally, from spectroscopic data such as frequency position, band shape, intensity and dichroism of specific absorption bands conclusions can be derived in terms of the applied mechanical stress and the state of order and orientation of the polymer under investigation. An extremely powerful method for the study of transient phenomena in polyuMr deformation and relaxation is rheo-optics which describes the relation between stress, strain and an optical quantity (for example birefringence, infrared absorption, light scattering. X-ray diffraction) measured simultaneously with stress and strain as a function of time In a given rheo-optical method therefore, a mechanical test is combined with one of these various types of optical measurements. 

Other tests for final device evaluation include analysis of complex collagen structures by light and electron microscopy as well as by measurement of the form birefringence. Light and electron microscopy of collagenous materials has been described in detail elsewhere (Doillon et al, 1986 Doillon et at, 1987 Doillon et ai, 1988 Wasserman et ai, 1988). 

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