Abstract
Nematic phase transition in thermotropic semi-flexible polymers is considered. A general theory for polydispersity effects in the system is proposed. It is shown that nematic transition occurs in a finite-temperature gap, δT, where the system separates into lower-(isotropic) and higher-molecular-weight (anisotropic) phases. The width of the gap depends on the polydispersity index, δ; for a given δ, it is inverse proportional to the average degree of polymerization.