Naphthalene, birefringence

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.

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). 

A few examples will show the value of such considerations. Naphthalene (monoclinic, a = 8 29 A, b = 5-97 A, c = 8 68 A, jS = 122° 42 ) has refractive indices 1-442, 1-775, and 1-932. The birefringence is so strong that we should not be far wrong in assuming that the planes of the two molecules in the cell are roughly parallel and if we associate the three indices with the three vibration directions of the molecule itself, we are led to suppose that the longest axis of the molecule lies parallel to the vibration direction of y (that is, nearly parallel to c), and the intermediate axis along b (the j8 vibration direction). 

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