Perforated layer structures in liquid crystalline rod-coil block copolymers

We report a novel observation of the tetragonal perforated layer structures in a series of rod-coil liquid crystalline block copolymers (BCPs), poly(styrene-block-(2,5-bis[4-methoxyphenyl]oxycarbonyl)-styrene) (PS-b-PMPCS). PMPCS forms rigid rods while PS forms the coil block. Differential scanning calorimetry (DSC), polarized light microscopy (PLM), small-angle X-ray scattering (SAXS), wide-angle X-ray diffraction (WAXD), and transmission electron microscopy (TEM) techniques were used to investigate these rod-coil molecules, and a perforated layer structure was observed at f^PMPCS ≈ 0.37 in relatively low molecular weight (M_w) samples and ∼0.5 in high M_w PS-b-PMPCS. This substantial phase boundary shift was attributed to the rod-coil nature of the BCP. The perforation obeys a tetragonal instead of hexagonal symmetry. The "onset" of perforation was also observed in real space in sample PS₂₇₂-b-PMPCS₉₃ (f^PMPCS ≈ 0.52), in which few PS chains punctuate PMPCS layers. A slight increase in f^PS, by blending with PS homopolymer, led to a dramatic change in the BCP morphology, and uniform tetragonal perforations were observed at f^PMPCS ≈ 0.48.