Relationships between Birefringence and Molecular Structure

According to these models the average polarizability parallel (cQ and perpendicular (tto) to the optical axis can be determined from the principal refractive indices (n and respectively). In most cases the approximations reported by Neugebauer [85] or Vuks [66] are used (for the latter, see Eq. 6). Combining Eqs. (7 a) and (7 b), it follows that 

if the orientational order S (e. g. from nuclear magnetic resonance (NMR), electron spin resonance (ESR) or dichroism measurements) is known, the molecular polarizabilities and can be determined [8, 21, 37, 93-95]. In most cases Eq. (16) is used to calculate the orientational order when the molecular polarizability anisotropy - Ot is available. In a few cases a, -has been obtained by refractive index measurements on a solid monocrystal, provided that the molecular long axes are known with respect to the optical axes [6, 19, 22, 67]. Sometimes oCi - has been calculated from bond polarizabilities [24, 29, 78]. In most cases - (has been obtained using the extrapolation procedure first proposed by Haller et al. [33], where log (ag-ao) or log(ag/ao) is plotted against a reduced temperature At some tempera- 

The principal refractive indices and and the birefringence A = - of 40 ne-matogenic compounds (A=589 nm) for a temperature T n-i -7 =10K, are listed in Table 1. It follows from Eq. (8) that at a constant reduced temperature the magnitude of the birefringence is mainly determined by the molecular polarizability anisotropy a,-0[, but also by the molar volume V=M/p. On the other hand, a, - (Zt depends strongly on the structural features of the molecules. As can be seen from Table 1 some general relationships between molecular structure and birefringence can be derived  

Since aromatic rings have a higher polarizability anisotropy than alicyclic rings, the replacement of aromatic rings by alicyclic rings leads to a decrease in ] - a, and thus to the decrease of the birefringence (compare compounds 1-3 6 and 7 15 and 16 18 and 19 21-23). 

The birefringence of nematic side-chain polymers is of the same order of magnitude as that of analogous low molar mass nematics [31,101]. Plate et al. [101] measured the birefringence in the nematic phase of a side-chain polymer A and that of the low molar mass analog B at the same reduced temper- 

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In this paper, the BPDA-PFMB/PEI molecular composites were oriented by means of zone annealing/drawing slightly above the glass transition temperatures of the respective molecular composites (280 - 400 °C). The dependence of draw ratio on tensile modulus, crystal orientation, and birefringence was determined as a function of composition. The relationship between structure (crystal orientation) and tensile property (modulus) of drawn films has been examined by comparing crystal chain orientations with the prediction of affine deformation (12-16). 

Recently, new models were proposed which can indeed be used to characterize the structure of real elastomeric networks in view of their mechanical properties 6, 7 However several parameters are necessary to describe the relationship between molecular and macroscopic deformations and therefore stress-strain measurements are generally not sufficient to conclude without any ambiguity on the validity of these elaborated theories- Another possible test consists in measuring molecular orientation in stretched rubbery networks- With this in view, the photoelastic properties of rubbers have been widely investigated However birefringence data... 

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