Surface Morphological Changes in Monolayers of Aromatic Polyamides Containing Various N-Alkyl Side Chains

Abstract

We synthesized new aromatic polyamides (poly-(N-alkylated benzamides), abbrev. PABAn) having both a rigid main chain and a flexible side chain with different lengths. We investigated the solid-state structures, that is, the molecular orientation and surface morphology, of organized molecular films of PABAn by performing surface pressure−area (π-A) isotherm, in-plane and out-of plane X-ray diffraction (XRD), polarized infrared spectroscopy, and atomic force microscopy (AFM) measurements. The solid-state structure of poly-(N-methyl benzamide) (PABA1) belonged to the monoclinic system, whereas PABA3, PABA4, and PABA5 showed an orthorhombic packing pattern. PABA7 and PABA8 formed amorphous polymers. In the case of PABA17, a two-dimensional hexagonal lattice was formed as a subcell consisting of side chains. These polymer monolayers were highly condensed on a water surface at 15 °C. Out-of-plane XRD measurement results showed that the PABA1, PABA3, PABA4, and PABA5 multilayers showed large periodicities of 50−60 Å. From AFM observation results, it was found that these aromatic polyamides formed single particle layers of hydrophilic groups localized at the bottom of the particles. On the other hand, PABA7 and PABA8 monolayers showed irregularity and exhibited shapeless morphologies. In addition, an organized molecular film of PABA17 formed a highly ordered layer structure (periodicity of 30 Å) and a giant circular domain (diameter of 20 nm) made of a side chain crystal. The PABA17 monolayer showed a hexagonal packing pattern formed due to van der Waals interaction between the flexible side chains. From these experimental findings, it was concluded that the polymer synthesis method employed in the present study can be directly used to control the crystal structure (the third order structure of polymers), molecular arrangement, and surface morphologies of polymer monolayers.