Aliphatic polyamides (nylons): Interplay between hydrogen bonds and crystalline structures, polymorphic transitions and crystallization

Funding information Generalitat de Catalunya, Grant/Award Number: 2017SGR373; Spanish Ministry of Economy and Competitiveness, Grant/Award Number: RTI2018-101827-B-I00 Abstract Aliphatic polyamides (nylons) constitute a family of polymers with outstanding properties and multiple applications. Despite the intensive research studies carried out with nylons, there are still multiple unsolved questions concerning crystallization processes, crystalline structures, polymorphic transitions, and crystalline morphologies. Constrains imposed by the strong intermolecular interactions affect the amorphous state, the rigid amorphous phase, the molecular folding, the morphology and obviously the crystalline structure. Some of these relevant points are discussed in the present work.


| INTRODUCTION
Aliphatic polyamides, commonly known as nylons, are usually obtained from polycondensation reactions between diamines and dicarboxylic acids or their derivatives (nylons XY or AA-BB type).
Nylons can also be obtained by both the polycondensation of ω-amino acids and the ring opening polymerization of lactams (nylons X or AB-type).
Polyamides are semicrystalline thermoplastics with outstanding mechanical properties (e.g., mechanical strength, flexibility, toughness, and durability) and wide applications in distinct areas (e.g., automotive industry, textile sector where nylon was the world's first synthetic fiber, sportwear products and even medical sector). 1 Thus, nearly 10% of plastics used in modern vehicles correspond to different polyamides. For example, polyamides with more than 10 CH 2 groups between adjacent amide groups have excellent properties like toughness, low water absorption, and good dimensional stability to be employed as automobile hoses. 2 In general, nylons can be considered as fossil-based plastics, although nowadays great efforts have been focused on the production of bio-based polyamides as those based on castor oil as a natural source (e.g., nylon 11, Rilsan ® ), and those derived from cadaverine (e.g., nylons 54, 56 and 510), a natural diamine that can also be produced at large scale by biotransformation of lysine using recombinant Escherichia coli. [3][4][5] Capability of amide groups to form strong intermolecular hydrogen bonding interactions is the main reason of the high thermomechanical performance of polyamides that contrasts with the weaker properties found for example in polyesters having similar repeat units (i.e., those derived by the simple change of the NHCO amide group by the OCO ester group). The high level of intermolecular cohesion displayed by polyamides is unique and cannot be attained when only dipolar electrostatic interactions exist.
Establishment of intermolecular hydrogen bonds is the main driving force that determines the peculiar crystalline structures and morphologies of polyamides and even their complex structural polymorphism. Seguela 6 has recently pointed out and discussed the relative difficulty of polyamides to render high degrees of crystallinity despite their high chain regularity, the establishment of strong intermolecular interactions and their usually limited molecular weights (i.e., in the case of polycondensation polymers).
Another specific phenomenon of polyamides resulting from the presence of H-bonds in the molten state is the so called memory effect upon crystallization. [6][7][8][9] Hydrogen bonds are weaker in the molten state, but some organized domains may persist. 10 Therefore, a complete fusion usually requires keeping the sample for a significant time at some degrees above the melting point. Furthermore, the indicated domains can behave as homogeneous nuclei promoting crystallization upon cooling 11 and giving also rise to a low melting entropy. 12

| POLYMORPHISM OF CONVENTIONAL NYLONS
First proposed crystalline structures of aliphatic polyamides correspond to nylons 66, 610 and 6 and were given by Bunn and his group. 13,14 These structures were based on a sheet arrangement of hydrogen bonded molecular chains that adopted an all trans minimum energy conformation. Hydrogen bonds were formed along a single direction since a correct geometry between NH and CO groups of neighboring chains could be established. This structure, α-form, was characterized by X-ray fiber diffraction patterns showing two equatorial reflections at spacings close to 0.440 and 0.380 nm, which were associated to intrasheet and intersheet spacings, respectively. Sheets held together by weak van der Waals interactions and showed a progressive (triclinic unit cell) or recuperative (monoclinic unit cell) shift along the chain axis to minimize dipolar interactions between close amide groups. Accurate experimental information concerning molecular conformation, setting orientation of molecules and relative shifts between neighboring chains could be derived from the crystallographic resolution of small model compounds as performed for example in those constituted by suberamide or sebacamide units ( Figure 1). 15 Obviously, structures having this type of unidirectional hydrogen bonds display highly anisotropic mechanical properties. 16 The named γ-form is a second well-known structure of nylons that is also characterized by the arrangement of hydrogen bonds along a single direction. 17 This structure was initially postulated for odd-odd nylons (i.e., those derived from diamines and dicarboxylic acids having an odd number of carbon atoms), but is also frequent in polyamides with long polymethylene sequences and even it is a second polymorphic form of nylon 6. 18 This structure can be considered as the result of a near 60 rotation of amide groups from the typical sheet arrangement. Hydrogen bonds can be well established between parallel chains (when the polymer is directional) giving rise to a pseudohexagonal packing (equatorial reflection close to 0.415 nm) and a slightly shorter chain repeat length. CO and NH groups are nearly collinear in the γ-form in contrast with the slight deviation of linearity postulated for the α-form (i.e., see that amide groups do not lie exactly in the zig-zag plane of molecular chains in Figure 1). 14 Therefore, it has been suggested that the γ-form has stronger hydrogen bonds than the α-form, a feature that is in agreement with FTIR and NMR observations 19 but that is controversial when computation data are considered. 20 In general, thermodynamic stability between α and γ forms is similar (i.e., nylon 6 21 ) and consequently one or the other form can be obtained depending on specific solvent (e.g., exposition to iodine solutions 18 ), thermal or mechanical treatments, processing conditions (e.g., spinning rate and postdrawing treatments 22 ), incorporation of fillers and nucleating agents, 23 mixing with other polymers, 24 and logically on the selected crystallization conditions (e.g., solvent, substrate and temperature for solution, epitaxial and melt crystallizations). Note also that the γ-form lacks a truly hexagonal symmetry since for example hydrogen bonds are established along a single direction and not along three directions at 60 . In addition, parallel and antiparallel chains exist in the unit cell and the previously indicated reflection can be split in two close reflections at spacings slightly higher and lower than 0.415 nm.
A mesomorphic form highly similar to the γ-form is also commonly found in nylons. This form has been described with different names in the literature (e.g., α 0 , δ, γ*) and usually appears when the F I G U R E 1 Crystalline structure of the N,N 0 -dipropylsuberamide model compound: (A) View perpendicular to the amide planes that shows the sheet arrangement and reflects the close to one bond shear between chains having an extended molecular conformation. (B) View showing the packing between sheets that is characterized by a shears along the chain axis that is, (close to three bonds) that interlock neighboring methylene groups. (C) View along a parallel direction to the chain axis that reveals a slight twist between the plane defined by the methylene carbons and the hydrogen bonding direction (close to 15 ), and the shear of neighboring sheets along the hydrogen bonding direction. Copyright 2000. Reproduced with permission from John Wiley & Sons, Inc polymer is fast cooled from the melt state. In this case, hydrogen bonds seem to be completely established but randomly distributed along the chain axis and according to three different crystallographic planes that gives rise to a hexagonal packing. 25 From an experimental point of view, it is difficult to distinguish between γ and α 0 forms since both are characterized by X-ray reflections around 0.415 nm. Quantification of the ratio between the two polymorphs is problematic since depends on the appropriate choice of the deconvolution parameters. Nevertheless, the metastable α 0 form usually experiments a transition on heating towards the α form at moderate temperatures (e.g., 100-120 C for nylon 6 26,27 ), which is not characteristic for the γ form. This transition could even be detected in the corresponding DSC heating runs. 26 γ and α 0 forms of nylon 6 have clearly been distinguished from well differentiated FTIR bands 27,28 and also from confocal Raman microspectroscopy. 29 Thus, main distinction is obtained from the local scale conformation data derived from vibrational spectroscopy and not from the long-range order deduced from diffraction profiles.
Probably, one of the most problematic points concerning the polymorphism of nylons corresponds to the prediction of the conditions at which structural transitions take place. This point is aggravated by the multiple names that the different crystalline structures have received along the time, the great diversity of conditions tested, and by the great variability of observed behaviors that probably needs to be properly categorized in order to discard non-representative differences. In general, a treatment with aqueous iodine-potassium iodide solutions favors the conversion of the α form into the γ form, 30 a process that could be reversed by exposition to phenol in the case of nylon 6. For this polymer, it has also been described that the weight fraction of the γ form decreased with increasing temperature and crystallization time. 31 Transitions of nylon 12 have also extensively studied. This polymer mainly crystallizes in the γ form but the sheet structure can also be obtained by drawing at high temperature and high-pressure crystallization. 32 Polyamides having α or γ structures crystallize in solution given rise to lath-like morphologies, which preferential growth direction becomes aligned to the single hydrogen bonding direction. This feature is clearly distinctive from the rhombic crystals usually obtained with polyesters having similar repeat units but displaying only dipolar interactions. 33 Lamellar chain folding could take place between antiparallel chains belonging to the polymethylene sheets (i.e., between hydrogen bonded and non-hydrogen bonded chains for the α and γ structures, respectively. Note that only in the first case hydrogen bonding and folding directions coincide). 34 Interestingly the thickness of the lamellae of polyamides can be driven by the topology of hydrogen bonds as postulated by Dreyfuss et al., 35 although folds implying hydrogen bonds have also been proposed. 36 It should also be pointed out that the low crystallinity of polyamides obtained from quiescent melt requires a high thickness of the amorphous layers in the lamellar stacks. Therefore, chains emerging from lamellar surfaces should follow a quite large trajectory before to have an opportunity to reenter in the lamellae, a feature that has low probability to occur just in the neighboring chain segment. 6 3 | STRUCTURES WITH DIFFERENT HYDROGEN BOND DIRECTIONS Peculiar crystalline structures have been determined for nylons when they incorporate special chemical units where a single methylene group becomes placed between two amide groups. This is the case of nylons derived from monomethylendiamine, malonic acid, and glycine units due to their conformational preferences. In the first case, the monomethylene bisamide residue adopts a gauche conformation and, as a consequence, its two NH groups point out at 180 instead of the 0 expected for an all trans conformation. Structures with different hydrogen bonding directions were postulated depending on the number of methylene groups in the dicarboxylic unit (i.e., a single direction for an even number and three directions for a low odd number). 37,38 More interestingly, hydrogen bonds were distributed along three directions in copolyamides with regularly alternating glycine and ω-amino acid units. [39][40][41] The final molecular arrangement is related with the typical polyglycine II (PGII) structure, 42  with nylon 92 (note that is another example of an odd-even sequence) may be the clearest evidence of the indicated structures with two hydrogen bonding directions. 60 This geometry was also extrapolated to oddodd nylons (e.g., nylon 55) where the rhombic lamellar morphology (Figure 2A, bottom) could be well explained. 61 Finally, it should be pointed out that similar models have also been formulated for copolyamides based on diaminobutane and odd dicarboxylic acids with highly different lengths (e.g., glutaric and azelaic acids). 53

| BRILL TRANSITION: RECENT ADVANCES
Polyamides can show on heating and just before fusion a solid state crystalline transition, which is known as the Brill transition. 62 nylons (e.g., nylon 46 78 ) when they were submitted to a mechanical stretching. In this case, the single reflection associated to an initial γ-form was split in the two characteristic reflections of the sheet structure. A conformational change that leads to the extended molecular conformation seems logical, but uncertainties concerning disruption and reforming of hydrogen bonds when amide groups became untwisted require explanations at a molecular level.
Interestingly, the transformation of even-even nylons has recently been described as a two-step process with an intermediate state based on a pleated/rippled sheet structure (i.e., similar structures to those postulated by Pauling and Corey 79,80 for protein sheet structures). 69 Note that the rippled sheet arrangement is characteristic of the named PGI structure of polyglycine (i.e., nylon 2). 81 These two steps appear merged in most nylons, being consequently difficult to experimentally separate the two events.
It should be pointed out that highly complex transitions have also been described with nylons having the two-hydrogen bonding structure. Specifically, in this case, intermediate structures ( Figure 3) have been observed before to reach T B . A gradual evolution of the twisting between amide groups was suggested. 53 In fact, great uncertainties exist between the integrity of hydrogen bonds during the heating process, being less and less accepted the initial hypothesis based on a disruption of initial bonds and the establishment of new random bonding distributions along three directions. 63,74 New hypotheses about the Brill transition of even-even and even nylons indicate that the intrasheet hydrogen bonds are basically pre- c. New interpretations concerning negatively birefringent spherulites from even-even polyamides must be considered. These hypotheses formulated by Lotz et al. 86 indicate the existence of scrolled lamellae that lead to hydrogen bonds established along an oblique direction to the spherulite radius. Moreover, spherulites seem to be complex since two crystalline entities with competitive properties (e.g., birefringence and unit cell orientation) are involved: the scrolled lamellae that are firstly formed and the subsequently crystallized in-filling material. 86 d. Odd-even nylons display a peculiar temperature dependence of the birefringence sign since it changed in the sequence positivenegative-positive when crystallization temperature was decreased ( Figure 4). 58 By contrast, conventional nylons showed a change from negative to positive birefringence with decreasing temperature. Furthermore, some reversibility of the birefringence sign with temperature has also been described for some even-odd nylons. 52 Non-isothermal experiments showed also changes on birefringence during cooling and heating rates that were dependent on the corresponding rate. 50,52 These features are peculiar and may be consequence of the constrains imposed by the two-hydrogen bonding geometry or even by the existence of complex structures as pointed out in the previous point. Efforts are also needed to unify the denominations of the different crystalline structures, trying also to discern the conditions (e.g., undercooling degree, cooling rate, annealing treatment) that favor a determined structure in function of the characteristics of the repeat unit (e.g., parity and length of the polymethylene sequences).

| PERSPECTIVE AVENUES AND CONCLUSIONS
Thermal transitions between polymorphs require greater attention and specially the role of mesophases.
The crystallographic resolution of small model compounds has provided useful information about new structures with three and two hydrogen bonding directions. These appear in polyamides having particular residues (e.g., glycine and malonamide) and also on polyamides with constrains derived from the peculiar hydrogen bonding geometry. The study of the morphology of lamellar crystals appears also a fundamental tool to discern about the intermolecular interactions. In summary, nylons are a highly attractive family of polymers where a great task is still needed to understand the crystallization behavior and the interplay between morphology and the establishment of strong intermolecular hydrogen bonds.

ACKNOWLEDGMENTS
This work is financially supported by the Spanish Ministry of Economy and Competitiveness (project RTI2018-101827-B-I00) and the Generalitat de Catalunya (grant 2017SGR373).
Open access funding enabled and organized by Projekt DEAL.