Tacticity Effect on Mesogen-Free Liquid Crystalline Self-assembly Induced by Strong Dipole-Dipole Interactions

Strong dipole-dipole interactions in mesogen-free aliphatic comblike polymers can induce unique liquid crystalline (LC) self-assembly. By comparing atactic and isotactic poly(oxypropylene)s with n-alkylsulfonyl side groups (aPOP-SO2Cn and iPOP-SO2Cn), the effect of main-chain tacticity on the ordered phase behavior was studied in this work. For aPOP-SO2Cn, LC self-assemblies were observed when n is >= 3. Two types of LC phases were formed depending on the side chain length n: (i) a three-dimensional (3D) high-order LC structure (possibly crystal B or E) for n = 3-5 and (ii) a smectic A (SmA) structure for n = 6-12. For iPOP-SO2Cn, crystallization was favored as a result of regular main-chain configuration. Only when n >= 10, the isotactic comblike polymers exhibited a monotropic phase behavior with the SmA phase appearing during cooling from the isotropic melt. Regardless of atactic or isotactic main chains, these ordered LC and crystalline phases featured a double-layer structure, which was dictated by the optimal dipole-dipole interaction distance (i.e., 0.40-0.44 nm) between sulfonyl groups in the neighboring side chains. This study showed that the chiral centers incorporated in the backbone could not induce chiral smectic self-assembly for the isotactic polymers. As such, we are currently implementing chiral centers in the side chains for the formation of ferroelectric structures for these mesogen-free aliphatic LC polymers.