On the stretching of alkyl chains by nematics

When a semiflexible chain molecule is subjected to an anistropic solvent, e.g., leads to a partial elongation and alignment of the molecule. The ordering of the CC bonds as a funciton of position along the chain was treated in an earlier communication by solving a lattice version of the rotational isomer state model in the presence of an external field. In the present paper, we extend this approach to the explicit calculation of solvent-induced size and anisotropy effects. Specifically, we determine the increase in 〈Z _N²〉^1/2 as a function of chain conformation (i.e., gauche vs trans) energy ε and of monomer-solvent coupling strength f. Here 〈Z_N²〉^1/2 is the root-mean-square value of the Z component of the end-to-end vector for an N-bond molecule (the space-fixed Z axis being taken to lie along the nematic direction). The elongation of the molecule is described in terms of the deformation of a sphere with radius R₀=(3〈Z_N ²〉_f=0)^1/2 into an ellipsoid of revolution with eccentricity determined by 〈Z_N²〉 _f>0.