Polymer blends, birefringence

For substrates of WORM and EOD(PCR) disks the industry in the future wants polymers that have a markedly improved resistance to heat softening compared to BPA-PC and, if possible, a lower water absorption and lower birefringence, but otherwise maintain the good characteristics in toughness, production, and cost (194). This goal is being approached in different ways further modification of BPA-PC, newly developed polymers, improvement of the processing characteristics of uv-curable cross-linked polymers, and development of special copolymers and polymer blends, eg,... 

The main limitation of birefringence is that it only provides an averaged orientation value without any discrimination between amorphous and crystalline phases, or between the components in polymer blends, copolymers, and... 

The final chapter on applications of optical rheometric methods brings together examples of their use to solve a wide variety of physical problems. A partial list includes the use of birefringence to measure spatially resolved stress fields in non-Newtonian flows, the isolation of component dynamics in polymer/polymer blends using spectroscopic methods, the measurement of the structure factor in systems subject to field-induced phase separation, the measurement of structure in dense colloidal dispersions, and the dynamics of liquid crystals under flow. 

Originally. TOA was devised and used for analyses of chain mobility in polymers (7) and polymer blends (8.91 by monitoring birefringence disappearance during programmed heating in scratches scribed on film surfaces. Lenz (10.Ill has reported the use of TOA in the study of liquid crystal polymers, and recently we have used the technique in determining melt- and isotropization temperatures for thermotropic fully aromatic liquid crystal polymers (4.51. 

Among these techniques, the addition of small molecules is relatively easier than the others due to good miscibility with polymers. In contrast, the combination of species is limited for the polymer blend and copolymerization technique. Therefore, addition of small molecules is becoming a good candidate these days. Addition of needle-shaped solid particles having strong optical anisotropy also modifies the orientation birefringence and its... 

Saito, H., Inoue, T. (1987). Chain orientation and intrinsic anisotropy in birefringence-free polymer blends,25, 1629-1636. 

Kubo3rama, K., Kuroda, T., and Ougizawa, T. (2007). Control of wavelength dispersion of birefringence by miscible polymer blends, MacromoI.SvmD.. 249-250.641-646. 

Inoue, T. and Kyu, T. (2000) Optical characterization Light scattering, birefringence, and ellipsometry, in Polymer Blends (eds D.R. Paul and C.B. Budoitdl), Wiley Sons, Ltd, Chichester, pp. 319-347. 

Polarizing light microscopy employs crossed polarizers to view the sample. With isotropic specimens, the field of view is dark, while anisotropic, birefringent samples or areas of a sample will appear bright. Polarizing microscopy is employed to view spherulitic structure [66-68] and deformation morphologies (crazes, shear banding) [69] in polymer blends. Samples for... 

Anthracene labelled polystyrene chains (called PAP) with a number average molecular weight, M = 287,000, have been incorporated (at 1 % w/w concentration) in a polystyrene matrix (PS 200) with a narrow M.W. distribution (M = 191,000, M = 207,000). The samples moulded under pressure and annealed to obtain transparent bubbleless samples and to avoid any birefringence. Sample size was 8 cm long, 2 cm wide and 0.2 thicko Tg of the resultant polymer blend measured by differential scanning calorimeter at a heating rate 10 C min was 107.5 C. 

Dichroism is a much more powerful technique than birefringence. The latter measures mean values over all the components and phases present, whereas dichroism is specific to particular, absorption bands. It can thus be applied to different groups on the same molecule, to different segments in a copolymer (102,104), or to different components of a polymer blend (103,105). 

It is well known that birefringence of polymers in processing is determined by the solidification stress, the processing stress when the polymers begin to solidify [9]. For polymer blends, the birefringence of individual component is determined by its own solidification stress. Thus, it is important for us to analyze the stress distribution of the blends. At solidification of any continuous phase, the polymer blends do not flow anymore. The average stress of the fibers, the measured one, can be given based on a simple model as... 

Figure 13.5 Birefringence as a function of wind-up speed (a) A, PET control (b) A, PET containing 3% copolyester of 1,4-phenyleneterephthalate and p-oxybenzoate (c) O, PET containing 3% copolymer of 6-oxy-2-naphthalene and p-oxybenzoate [17]. From Orientation suppression in fibers spun from melt blends, Brody, H., J. Appl. Polym. Sci., 31, 2753 (1986), copyright (1986 John Wiley Sons, Inc.). Reprinted by permission of John Wiley Sons, Inc.

The anisotropy of polarizability can be positive (eg, polycarbonate) as well as negative (eg, polystyrene). This offers the possibility of minimizing birefringence by copolymerization or blending of suitable polymers with the right mixture ratio, eg, blends of poly(phenylene ether) (PPE) and polystyrene (PS). The magnitude of birefringence of axial-symmetrically oriented polymers vs their molecule orientation has been described (182). 

Breakdown of the stress-optic rule can also occur in multiphase or multicomponent mixtures, as well as in melts with crystalline domains. However, as in glassy polymers, for miscible blends, a revised stress-optic law can sometimes be recovered by breaking the stress and birefringence tensors into two components, one for each component in the blend (Zawada et al. 1994 Kannan and Komfield 1994). 

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