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.
Highlights
Aliphatic polyamides, commonly known as nylons, are usually obtained from polycondensation reactions between diamines and dicarboxylic acids or their derivatives
Nylons can be considered as fossil-based plastics, nowadays great efforts have been focused on the production of bio-based polyamides as those based on castor oil as a natural source, and those derived from cadaverine, a natural diamine that can be produced at large scale by biotransformation of lysine using recombinant Escherichia coli.[3,4,5]
Despite the high commercial applications of polyamides and the intensive research work that has been performed about the improvement
Summary
Commonly known as nylons, are usually obtained from polycondensation reactions between diamines and dicarboxylic acids or their derivatives (nylons XY or AA-BB type). Polyamides may be in some cases an exception to this general behavior since the refractive index becomes superior in the direction where hydrogen bonds are established In this way, spherulites of even and even-even nylons having a typical sheet structure with a single hydrogen bonding direction (i.e., the α form) may display a positive or a negative birefringence depending on the disposition of sheets (i.e., positive for radial and negative for tangential).[82,83] Systematic observations indicated that a negative birefringence was only observed when crystallization was performed under a very low undercooling degree,[84] a result that. Non-isothermal experiments showed 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
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