Original Crystal Structures of Even–Even Polyamides Made of Pleated and Rippled Sheets

Abstract

Original crystal structures of even−even polyamides are introduced. They are based on twisted conformations at the amide-aliphatic part links and are reminiscent of the pleated and rippled protein sheet structures introduced by Pauling and Corey in the early 1950s. Pleated sheets are chiral, and rippled sheets are made of two conformational isomers. They have specific X-ray diffraction "profiles". Publications on even−even polyamides describe such profiles, indicating that both pleated and rippled sheet variants exist. The pleated sheet structure is the stable crystal form of the whole family of nylons 2,2n. The rippled sheet applies for a high-temperature "alpha phase variant" of nylons with long aliphatic parts and more generally appears to be (one of?) the so-called "high-temperature α phase(s)" of even−even nylons. Pleated and rippled sheet crystal structures appear to be the entropically favored, high-temperature crystal form(s) of nylons. This new family of crystal structures and their possible variants with different sequences of twisted amide-aliphatic part junctions open a possible way for the (re)analysis of rare and unusual crystal morphologies produced at high Tc after partial melting/self-seeding.

Using Successive Self-Nucleation and Annealing to Detect the Solid–Solid Transitions in Poly(hexamethylene carbonate) and Poly(octamethylene carbonate)

“…In a preceding work, it was shown that poly(hexamethylene carbonate) (PC6) and PC8 exhibited, during a nonisothermal test, solid−solid transitions at low temperatures 32 (resembling the above-described Brill transitions 16,17,27 ). Afterward, we demonstrated, through isothermal tests followed by nonisothermal scans, the presence of the denoted α to δ (during cooling) and δ to α (during heating) transitions, at low temperatures, in both PC6 and PC8, besides the Brill-like transition in the PC8.…”

“…The Brill transition has been widely studied in the polyamide family, ,,− and recently, a Brill-like transition has been reported for poly­(octamethylene carbonate) (PC8) and poly­(hexamethylene dithiocarbonate) (PC6-D) . In PC8, Zhao et al reported the convergence, upon heating, of the d 110 = 0.43 nm and d 200 = 0.38 nm, observed at low temperatures (in the denoted α phase), into a single d -spacing at 0.41 nm at ∼42 °C (in the denoted β phase).…”

Molecular Dynamics Simulation of the Stress–Strain Behavior of Polyamide Crystals

“…The conformations of PPTA and PAP5 reflect the two polymer conformations commonly reported in the literature: ,− extended and wavy. Note that a variety of different terms have been used to describe these two conformations in the literature but, here, the terms extended and wavy will be used.…”

Hydration, Refinement, and Dissolution of the Crystalline Phase in Polyamide 6 Polymorphs for Ultimate Thermomechanical Properties

“…A random distribution of the hydrogen bonding planes results in a high-symmetry pseudo-hexagonal phase. Recently, Lotz recognized the distinction of pleated and rippled sheet structures of even-even polyamides with short methylene sequences, 17 enabling a systematic conformational and structural analysis on the origin of the Brill transition. 18 It was concluded that the Brill transition originates in a temperature-induced dynamic interconversion between mirror image conformers, validating the discussion to define the Brill transition on the molecular scale instead of the crystallographic length scale.…”