Here, we demonstrate that water, in the superheated state, is a solvent for polyamide 4,6 (PA4,6) and that the water molecules can strongly influence hydrogen bonding. In the presence of superheated water, the melting temperature of PA4,6 can be suppressed by nearly 100 degrees C. The depression in the melting temperature follows the Flory-Huggins principle. The instantaneous dissolution of the polymer hardly influences the molar mass of the polymer. However, if the polymer is retained in solution above the dissolution temperature for more than 10 min, hydrolysis occurs. These findings suggest that the dissolution of the aliphatic polymer in superheated water is mainly a physical process as opposed to a chemical process. Time resolved X-ray studies show that the dissolution occurs prior to the Brill transition temperature, as reported earlier. Crystals grown from the water solution show a lath-like morphology with interchain and intersheet distances that are similar to the distances obtained for crystals grown from other known solvents. Electron diffraction further confirmed that the crystals grown from superheated water are single crystals, where the chains are perpendicular to the ab-plane. SAXS performed on dried sedimented water grown single crystals showed a lamellar thickness of 6 nm. The lamellar thickness is in accordance with other reported studies on PA4,6, confirming that the single crystals incorporate four repeat units between re-entrant folds with an amide group incorporated in the tight fold. Solid state NMR studies performed on mats of these single crystals showed two different mobilities of the proton associated with the amide groups: a higher mobility linked to the amide protons in the fold and a reduced mobility of the hydrogen bonded amide protons within the crystal. Additionally, the solid state NMR studies on the dried water crystallized single crystals show the presence of water molecule(s) in the vicinity of the amide groups. This was confirmed by infrared studies that conclusively demonstrated the appearance of two new bands arising due to the binding of a water molecule in the vicinity of the amide group (i.e., NH3(+) and COO(-) bands that disappear upon heating at approximately 200 degrees C). Additionally, DSC traces of the water crystallized PA4,6 show an exothermic event in the same temperature region (i.e., in the vicinity of the Brill transition temperature, where the bound water exits from the lattice). Furthermore, this event was corroborated by TGA data.

Copolyamides, based on 1,12-dodecanedicarboxylic acid and different ratios of 1,2-ethylenediamine and piperazine, i.e., PA2,14-co-pip,14 as well as the homopolymers PA2,14 and PApip,14 are studied. Incorporation of the piperazine component in the homopolymer PA2,14 reduces the number of hydrogen bonds. This provides a unique opportunity to investigate the influence of hydrogen bonding on the origin of the Brill transition and chain mobility within polymer crystals. Time-resolved conformational, structural, and morphological changes during heating are followed by FTIR spectroscopy, WAXD, and SAXS. The findings are that from 0 to 62 mol % of piperazine the Brill transition occurs in the same temperature region. The transformation is triggered by the conformational changes in the methylene sequences of the main chain, followed by twisting in the methylene sequences next to the amide group. This results in enhanced chain mobility along the c-axis, causing lamellar thickening. For 80 mol % of piperazine and higher, no Brill transition is observed. However, conformational changes in the methylene sequences of the main chain occurs, triggering lamellar thickening.

Introduction. The room temperature, ambient pressure crystalline structure of most nylons is formed from hydrogen-bonded -sheets linked by weak van der Waals interactions. It is this hydrogen bonding that plays a prominent role in both the crystallization and melting behavior of these materials and also lends nylons their ubiquitous strength and intractability. However, Fourier transform infrared (FTIR) studies on nylon-10,10 1 have shown that with increasing temperature there is a greater conformational disorder in the methylene segments and a weakening of the hydrogen bonding above the so-called Brill transition temperature. The Brill transition occurs in several nylons 2 and has been shown by time-resolved wide-angle X-ray diffraction (WAXD) to be a crystalline transformation from a triclinic unit cell to a pseudohexagonal phase, the (100) reflection related to the interchain/intrasheet distance merging into the (010)/(110) intersheet reflection at this temperature. 3-6 The Brill transition and melting temperatures in nylons show a strong dependence on the molecular structure and molecular weight: in nylon-4,6 the Brill transition occurs much below the melting temperature of ∼295°C, 7 whereas in nylon-10,10 the transition occurs just below the melting temperature of 197°C. 8,9 It should be noted, however, the crystallization conditions and the molecular weight can affect the Brill transition temperature and lead to some discrepancies between published temperatures, e.g., nylon-4,6 in the range 180-250°C. 3,10 However, it can be supposed that since nylon-4,6 exhibits such a high Brill transition temperature compared to those of several other nylons, e.g., nylon-6 with a Brill transition temperature of ∼150°C, 11 the hydrogen bonding is likely to be more affected at the Brill transition temperature in this polymer. Furthermore, it is known that the triclinic interchain and intersheet distances are strongly affected by the crystal perfection, the conventional spacings of 0.44 and 0.37 nm, respectively, only being achievable after annealing at elevated temperatures or upon solution crystallization. 10 Initially on cooling below the Brill transition, various authors 3,10,12 show that nylon-4,6 transforms into a high-temperature triclinic (monoclinic) phase in which the interchain and intersheet distances are closer together (0.41-0.42 and ∼0.40 nm, respectively), followed by a transition to a room temperature triclinic phase typical of other nylons.Pressure is an important component that is present during the processing of polymers. While in most materials pressure is known to increase the melting temperature, its influence on hydrogen bonding is not