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 a 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.