Birefringence patterns

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. 

Starch granules, when observed imder polarized light, exhibit an optical birefringence pattern known as a "Maltese cross" (Fig. 5.4), which implies a high degree of molecular order within the granule (Greenwood, 1979). 

The flow birefringence pattern of these flows can be obtained through the use of a pair of flat glass walls. Using image enhancement and the stress-optical law (68),... 

Nematic phase birefringence, which was observed in the cellulose solutions made from the 24.5/75.5 solvent, appeared immediately following dissolution and persisted throughout a two week period. The uniformly dispersed birefringent patterns were the most prevalent ones in the nematic solutions at all DPs studied, but Schlieren and thread-like patterns indicating nematic phases were readily observed in solutions of DP 450 cellulose. 

Figure 15. Change in birefringence patterns with the flow rate (LLDPE Lotrene FC 1010, 205 °C).

We can see in Fig. 21 that, for the LDPE (FN 1010) above a given flow rate, bands of discontinuity appear in the birefringence pattern. These bands are observed parallel to the flow direction, at an intermediate position between centreline and die wall. Such "defects" may also be detected in the converging flow near the die entry (Fig. 22). For these flow conditions, the extrudate begins to exhibit distorded volume shapes, characteristic of melt fracture. By comparison with rheological studies carried out on a capillary rheometer, we may assume that these defects belong to the family referred to as "upstream instablities" in Chapter III.4. 

Experimental birefringence patterns at 205 °C and numerical simulation with a GOB model and a ml model are compared in Fig. 33. The apparent wall shear rate related to the downstream flow is 25 s and the optical stress coefficient was selected to be 2.110 m /N (see Section III-l). 

Both computations provide a good qualitative description of the birefringence patterns, but the mPTT model captures more accurately the shape of the fringes in the downstream region. 

Fig.35 Experimental and computed birefringence patterns s. (a) (JOB model (b) mPTT model.

The flow computations in a short die (here T= 145 °C) obviously imply free surface determination corresponding to the swell phenomenon. It may be seen in Fig. 37 that the general birefringence patterns are similar, even if the results of the mPTT model seem more realistic in the downstream region than those of the GOB model, for which discontinuities appear along the centreline, apparently due to an incorrect determination of the relaxation time at very low shear rate. 

LDPE is a highly branched materied, whose flow behavioTir exhibits some peculiarities, which constitute a good test for a numerical simulation. As presented in Section lll-l, birefringence patterns are perturbed around the re-entrant comer, which leads to the appearance of W-shaped fringes at the entry of the die land. It can be seen in Fig. 39 that the mPTT model ilows on to obtain these characteristic shapes (the computation is related to experiments carried out at 175 °C and 21 s, for an estimated value of the stress optical coefficient of... 

Figure 39. Flow birefringence pattern (mPTT model, T = 175 °C, y = 21 s ).

Figure 11 Birefringence patterns for the flow of PBb into a two-dimensional fluorinated die, 2 mm gap and 20 mm long. AP = 12 105 Pa qjjj = 9,2 g/h.

Crystalline particles exhibit characteristic birefringence patterns when viewed under a polarized light microscope... 

The birefringence patterns seen under crossed polars as a gel resin swells are easily denrtonstratcd using a polarizing microscope and ordinary transmitted light. 

This experiment suggests a way of studying the swelling and crosslinking characteristics of different resins, for example by measuring the decay times for birefringence patterns. 

Figure 2.26 shows one of the reasons why spherical fillers give good performance in compounded materials. The birefringence patterns show stress distribution in the vicinity of various shapes of inclusions - only with a spherical shape and a good adhesion to the matrix, uniform stress distribution is observed. Stress distribution is an essential element of material design. 

The optical system used to assess flow-induced birefringence consisted of a 2-mW polarized He-Ne laser focused by a condenser lens on the center of the flow field. The birefringence patterns are observed between crossed polars by using a quarter wave (X/4) retardation plate as a Senarmont compensator. The polarized laser has an extinction ratio of 100 1 the polarizer and analyzer are Carl Zeiss components with extinction ratios of approximately 10... 

The deformation modes were monitored with a polariscope (Photoelastic Inc.) as a function of stress for epoxies with 37, 47, and 57 phr T403. An attached camera was used to record the birefringence patterns. 

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