Joseph X. Zheng

Phase behaviors and supra-molecular structures of a series of symmetrically tapered bisamides

A series of symmetrically tapered 1,4-bis[3,4,5-tris(alkan-1-yloxy)benzamido] benzene bisamides (CPhBA, where is the number of carbon atoms in the alkyl chains, = 10, 12 and 16), was synthesized in order to investigate the effect of alkyl chain length on supra-molecular ordered structures induced by hydrogen (H)-bonding and micro-phase separation. These bisamides consist of a rigid aromatic bisamide core with three flexible alkyl chains at each end of the core. Major phase transitions and their origins in CPhBA bisamides were studied with differential scanning calorimetry, one-dimensional (1D) wide angle X-ray diffraction (WAXD), infrared spectroscopy, and solid-state carbon-13 nuclear magnetic resonance experiments. The structures of these compounds in different phases were identified using 2D WAXD from oriented samples and were also confirmed by selected area electron diffractions in transmission electron microscopy from stacked single crystals and by computer simulations. All of the CPhBA bisamides in this series formed a highly ordered oblique columnar () phase and a low-ordered oblique columnar () phase, similar to a recent report on C14PhBA. The two main driving forces in the formation of these two supra-molecular columnar structures were identified: One was the H-bond formation between N-H and C[double bond, length as m-dash]O groups, and the other was the micro-phase separation between the bisamide cores and the alkyl chains. With increasing the length of alkyl tails, the isotropization temperature decreased, while the disordering temperature of the alkyl tails increased. The 2D lattice structures perpendicular to the columnar axis also increasingly deviated from the pseudo-hexagonal packing with increasing the alkyl tail length. However, the alkyl tail length did not have a significant influence on the packing along the columnar axis direction. Utilizing polarized optical microscopy, the phase identifications were also supported by the observation of texture changes and molecular arrangements inside of the micro-sized domains.

Hydrogen bonding effect on micellization and morphological transformations of the polystyrene-block-poly(ethylene oxide) micelles

Diblock copolymer polystyrene-block-poly(ethylene oxide) (PS-b-PEO) forms micelles in a mixed solvent of N,N-dimethylformamide (DMF)–water. Poly(acrylic acid) (PAA) and poly(methyl acrylic acid) (PMAA) were introduced to the solution in order to study the effect of hydrogen bonding (H-bonding) complexation on the micellization process. The H-bonding complexation changes the micellization free energy of PS-b-PEO, and more importantly affects the kinetics of the morphological transformation of the micelles with temperature, which provides helpful insights into the time-dependence of the isothermal relaxation process from non-equilibrium to equilibrium.