Low birefringence

The birefringence for phenyl-substituted PC (4) (T = 176 C) is reduced to about 50%, for benzyl substituted PC (5) (T = 138 C) to about 25%, and for four-ring bisphenol PC (6) to 8% of the value for BPA-PC (183,190,195,197,198) on condition of an optimum conformation of the phenyls in the side groups perpendicular to the aromatic rings in the backbone. In reaUty, however, these low birefringence values are not achieved, because the optimum conformation of the phenyl rings cannot be achieved in injection-stamped disks. 

Special, uv-curable epoxy resins (qv) for substrate disks for optical data storage (Sumitomo BakeHte, Toshiba) excel by means of their very low birefringence (<5 nm/mm) and high Young s modulus. Resistance to heat softening and water absorption are similar to BPA-PC, but impact resistance is as low as that of PMMA. 

B. Zimm. Dynamics of polymer molecules in dilute solutions viscoelasticity, low birefringence and dielectric loss. J Chem Phys 24 269-278, 1956. 

Some diamines carrying very bulky substituents like cardo groups can give colorless polyimides. For example, the bis-9,9-(4-aminophenyl)fluorene (FDA) or brominated and acetylenic FDA derivatives react with 6FDA giving copolymer films62 with low birefringence (low difference between in-plane and out-of-plane refraction index) (Fig. 5.8). A new cardo diamine l,l-bis[4-(4-aminophenoxy)phenyl]cyclododecane (Fig. 5.8) reacts with different aromatic dianhydrides with formation of colorless polyimides.63... 

Alicyclic dianhydrides are interesting for electronic applications. The polyimides obtained from them are colorless with high transparency in the visible range, exhibit low birefringence,125 and have a low dielectric constant.126 The reactivity of the polycyclic aliphatic dianhydride has been investigated. For example, bicyclo-[2,2,2]-oct-7-ene tetracarboxylic dianhydride reacts quickly with an aromatic amine because the bicyclo-imide is less strained than the corresponding dianhydride.127... 

Low-antimony lead alloys, 14 770 Low-birefringence polycarbonates, 19 822 Low-blush copolymers, 26 538 Low boiling node, in separating nonideal liquid mixtures, 22 303 Low-calorie beer, 3 577 Low calorie sweeteners, 12 38 Low calorific value (LCV) gas, 26 575—576... 

The presence of the large cyclic olefin units randomly distributed in the backbone leads to an amorphous copolymer combining high transparency, low birefringence, stiffness, barrier properties and the general characteristics of olefins. 

Certain norbornene derivatives, for example, 5-(2-naphthalene-carbonyloxy)bicyclo[2.2.1 ]hept-2-ene or 5-(4-biphenylcarbonyloxy)-bicyclo[2.2.1]hept-2-ene produce a polymer that is effective in producing molded products with excellent transparency, low water absorption, and low birefringence (29). 

If the slow ray of the compensator is parallel to the slow ray of the particle, the retardation of the compensator will be added to the retardation of the particle and a higher order retardation color will result. If the slow ray of the compensator is perpendicular to the slow ray of the particle, subtraction of retardation will occur and a lower order retardation color will result. With asbestos minerals, or other minerals having low birefringence, the two colors seen will be first order yellow with the slow ray of the asbestos perpendicular to the slow ray of the compensator and second order blue with the slow ray of the asbestos parallel to the slow ray of the compensator if the compensator has 550 nm of retardation, i.e. first order red. 

Microscopic examination of these samples show for the PET-rich blends the presence of very small spherulites. The PBT-rich blends show very low birefringence, low turbidity, and no organized structures. The SALS patterns are circularly symmetrical. 

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