Sequence segregation in molten liquid-crystalline random copolymers

Abstract

THE polymerization of two or more types of monomer to form random copolymer molecules yields commercially important thermotropic liquid-crystalline polymeric materials¹. The limited degree of crystallization observed for these random copolymers is thought to occur through a process of segregation: random but similar sequences of the different monomer units on adjacent chains come into register to form a layered structure with crystalline periodicity perpendicular to the chain axes but with aperiodicity parallel to the chains, giving 'non-periodic layer' crystals^2,3. If, as has been believed, the melt is a nematic liquid crystal , the question of how the necessary large-scale molecular rearrangements can occur even when crystallization is rapid is puzzling. Here we report the results of a wide-angle X-ray scattering study of a series of oriented molten samples of a class of these materials known as B–N random copolymers, which show that segregated, layered structures are already present in the melt. In other words, the melt contains smectic-type regions within a nematic matrix. The presence of these regions of sequence matching opens up the possibility of controlling the microstructure, and thus the properties, of thermoplastics through the introduction of specifically synthesized non-random sequences into the molecular chains.