Enormously Wide Range Cylinder Phase of Liquid Crystalline PEO-b-PMA(Az) Block Copolymer

A series of amphiphilic liquid crystalline diblock copolymers, PEOm-b-PMA(Az)(n), consisting of hydrophilic poly(ethylene oxide) and hydrophobic poly(methacrylate) moieties with side chains containing liquid crystalline (LC) azobenzene moieties, produced highly ordered microphase-separated films with PEO cylinders aligned perpendicular to the smectic LC layer of azobenzene in the PMA(Az) matrix. In this paper, morphological phase diagrams of PEOm-b-PMA(Az)(n), diblock copolymers above and below the isotropic transition temperature of LC azobenzene (T-iso) are presented. The diagrams are based on small-angle X-ray scattering (SAXS) measurements of approximately 70 kinds of polymers with varying degrees of polymerization in each block. An asymmetric phase diagram described against the volume fraction of PEO (f(PEO)) was obtained at temperatures above and below T-iso. The lamellar phase appears in the f(PEO) window 0.52 <= f(PEO) <= 0.78 above and below T-iso. Besides, the wide window, 0.087 <= f(PEO) < 0.52, allows the PEO cylinder phase to form below T-iso. In particular, the PEO sphere phase, observed above T-iso, was completely eliminated through an order-order transition (OOT) to the PEO cylinder phase in the window 0.087 <= f(PEO) <= 0.23. Such a large expansion in the PEO-cylinder-phase window could be attributed to the main chain of LC PMA(Az) being shorter than that of the flexible PEO chain, and LC azobenzene forming a smectic layer in the microphase separated system.