Simple formulas for dichroism analysis. Orientation of solutes in stretched polymer matrices

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Abstract
The multiplicative property of linear dichroism relative to axes of ’’statistical’’ rotation has allowed a simple, general formulation of the reduced dichroism of solutes oriented in uniaxially stretched polymer matrices: LDr=3 Sp ΣiSiiOi, i=x, y, z; where Sp=1/2F0π/2 (3 cos2ϑ−1) df (ϑ) is a factor characterizing the orientation of the polymer. Sp was derived as a function of stretch for the case when f(ϑ) is the distribution of independent directions p (not necessarily attached to polymer chains) obtained in an isotropic‐substrate sphere when this is deformed into a spheroid at constant volume. Sii define two independent local orientation parameters which depend on the solute–polymer interaction; Oi are optical factors given by the moment directions of the absorbing transitions. The model was tested for a few solutes, with known moment directions, in stretched films of polyvinyl alcohol (PVA) and polyethylene (PE). The agreement between predicted and observed LDr, using the model distribution, was excellent for two elongated dyes in PVA but only qualitative in PE. A significant variation of Sii with the degree of stretch indicates that the solute orientation is not due only to interaction with a single neighbor chain, a behavior which is consistent with a repulsive interaction.

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