Conformational and Structural Changes with Increasing Methylene Segment Length in Aromatic-Aliphatic Polyamides

The synthesis and structural characterization of various aromatic-aliphatic polyamides are reported in this study. The polymers are obtained by solution polymerization of p-phenylenediamine with various aliphatic diacid chlorides. The resulting polyamides are labeled PA P-X, where X varies between 5 and 10 and corresponds to the number of carbon atoms of the dicarboxylic acid monomers used in the synthesis. The polyamides are obtained with Mn values of 10 kg/mol or higher, as determined by solution NMR spectroscopy and gel permeation chromatography (GPC). The polymers PA P-5 to PA P-8 degrade prior to melting, whereas only PA P-10 shows melting on heating. The structural changes in the polymers, with increasing methylene segments, are investigated by X-ray diffraction and molecular modeling. Conformational changes as a function of temperature have been studied by solid-state NMR spectroscopy. These studies have been illustrative in following the phase transformations in the aromatic-aliphatic polymers. For the crystal packing of the polymer based on the odd acid (PA P-5) a sheetlike structure, similar to that of the aromatic polyamide PPTA, is observed. Despite the presence of the odd spacer, PA P-5 exhibits a hydrogen bonding length very similar to that of PPTA, whereas the intersheet distance increases and the interchain distance decreases. As a result, the crystal structure of PA P-5 is distinctively different from that of the aliphatic polyamides having the same odd diacid, e.g. PA 65. In contrast, the crystal packing of PA P-6 with even diacid is similar to that of the α form of PA 46. The change of the chemical shift of the carbonyl groups with increasing number of methylene units suggests a weakening in the hydrogen bonding with respect to PPTA. For PA P-10 this weakening ultimately translates to melting of the polymer prior to degradation.