Birefringence-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),... 

Figure 8. Representative birefringence-temperature relations for the 220-18,500 PDMS networks. Filled circles locate results obtained to check for reversibility (16).

The idea that the critical behaviour of LCEs may be influenced by varying the geometry and density of the crosslinkers was introduced in an early theoretical work [23]. Indications that an increased crosslinking density leads to a more gradual (more supercritical-like) thermoelastic response and birefringence temperature dependence may be found in various publications [7, 18]. Recently, the first systematic and dedicated experimental investigation, by means of H-NMR and high-resolution calorimetry [4,5], demonstrated that the increase in the crosslinkers... 

A common measurement usehil in predicting threadline behavior is fiber tension, frequentiy misnamed spinline stress. It is normally measured after the crystallization point in the threadline when the steady state is reached and the threadline is no longer deformed. Fiber tension increases as take-up velocity increases (38) and molecular weight increases. Tension decreases as temperature increases (41). Crystallinity increases slightiy as fiber tension is increased (38). At low tension, the birefringence increases as tension is increased, leveling off at a spinline tension of 10 MPa (1450 psi) (38). 

Fig. 25. Influence of blend composition on birefringence (207). Blend is PC and modified PS. Temperatures noted are mol ding temperatures.

More recently, test products were created of a blend of PMMA with a phenyl-substituted methacrylate these products have a glass-transition temperature of around 125°C, a significantly reduced water absorption compared to pure PMMA of about 0.32%, but also a higher birefringence (a stress-optic coefficient of 5.2 X 10 , compared with 0.3 X 10 for PMMA and 6.8 x 10 for BPA-PC). 

The TPX experimental product of Mitsubishi Petrochemical Ind. (221) is an amorphous, transparent polyolefin with very low water absorption (0.01%) and a glass-transition temperature comparable to that of BPA-PC (ca 150°C). Birefringence (<20 nm/mm), flexural modulus, and elongation at break are on the same level as PMMA (221). The vacuum time, the time in minutes to reach a pressure of 0.13 mPa (10 torr), is similarly short like that of cychc polyolefins. Typical values of TPX are fisted in Table 11. A commercial application of TPX is not known as of this writing. 

Fig. 26. Qualitative compatison of substrate materials for optical disks (187) An = birefringence IS = impact strength BM = bending modulus HDT = heat distortion temperature Met = metallizability WA = water absorption Proc = processibility. The materials are bisphenol A—polycarbonate (BPA-PC), copolymer (20 80) of BPA-PC and trimethylcyclohexane—polycarbonate (TMC-PC), poly(methyl methacrylate) (PMMA), uv-curable cross-linked polymer (uv-DM), cycHc polyolefins (CPO), and, for comparison, glass.

Gelatinization is accompanied by a loss of birefringence. The temperature at which this occurs is called the gelatinization temperature. 

Anisotropic behaviour is also exhibited in optical properties and orientation effects can be observed and to some extent measured by birefringence methods. In such oriented materials the molecules are in effect frozen in an unstable state and they will normally endeavour to take up a more coiled conformation due to rotation about the single bonds. If an oriented sample is heated up the molecules will start to coil as soon as they possess sufficient energy and the mass will often distort. Because of this oriented materials usually have a lower heat distortion temperature than non-oriented polymers. 

Annealing temperature rc) Annealing time (min) Birefringence (An) Anid Volume crystallinity (%) TTM fraction Critical dissolve time (s) Amorphous orientation function (/ )... 

The dependence of birefringence on the temperature seems to be correlated to the temperature dependence of the lattice parameters ratio c/a shown in Fig. 102. 

Fig. 9. Plot of amorphous orientation average fa, obtained from birefringence, etc., versus f, obtained from im etc- Triangles, draw temperature 80 °C circles draw temperature 85 °C open symbols, single-stage draw full symbols, two-stage draw. Reproduced from Polymer by permission of the publishers, Butterworth Co (Publishers) Ltd. (C)...

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