Study of Structure Formation in Side-Chain Liquid Crystal Copolymers by Variable Temperature Fourier Transform Infrared Spectroscopy

The formation of smectic phases in side-chain liquid crystal copolymers, SCLCPs, containing sulfonic acid-based nonmesogenic units, has been investigated using variable temperature FT-IR microscopy. Two copolymers have been characterized, namely, the poly[10-(4-methoxy-4'-oxy-azobenzene) decyl methacrylate]-copoly[2-acrylamido-2-methyl-1-propanesulfonic acid]s, the X-MeOAzB/AMPS copolymers, containing X = 0.71 and 0.56 mol fraction of mesogenic side-chains, respectively. For comparative purposes the corresponding side chain liquid crystal homopolymer, poly[10-(4-methoxy-4'-oxy-azobenzene) decyl methacrylate], MeOAzB, has also been characterized. The 0.56-MeOAzB/AMPS copolymer exhibits a bilayer smectic A phase, in which the mesogenic side chains constitute one layer with a SmA(1) packing arrangement and the sulfonic acid groups another; whereas in the smectic A phase shown by the 0.71-MeOAzB/AMPS copolymer, the acid groups are located within the smectic layers giving a partially interdigated SmA(d) phase and reducing side chain packing efficiency. Smectic stabilization is attributed to a combination of stronger interactions involving the ester groups, as reflected in changes to the C=O stretching band at nu similar to 1730 cm(-1), and hydrogen bonding between the amide groups within the acid-based layers, as inferred by changes to the NH stretching band at nu similar to 3320 cm(-1). The temperature response observed for groups with different chemical environments permits the mapping of the short-range interactions between the various structural components in SCLCPs with a view to controlling the functionality of the materials.