A Unique Pleated-Phase Crystal Structure in Nylon-6,12 Induced by Cylindrical Nanoconfinement

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By applying geometric boundary conditions under nanoconfinement, polymer crystallization can access structural states that are inaccessible in bulk systems. The pleated-phase crystal structure, characterized by twisted-chain conformations, represents a distinct structural motif of even–even nylons recently defined by Bernard Lotz (2021). Previous studies reported that pleated-phase crystals appear only as transient intermediates during heating or cooling. Herein, using even–even nylon-6,12 as a model system, we demonstrate that pleated-phase crystals can be stabilized under ambient conditions through cylindrical confinement. The crystal structure was identified by one- and two-dimensional wide-angle X-ray diffraction (WAXD) using “vertical” and “parallel” geometries, where the incident beam is perpendicular or parallel to the rod long-axis. In the vertical geometry, besides the common α-phase reflections (d = 4.41 and 3.68 Å), two additional reflections (d = 4.17 and 4.00 Å) appear from the [001]p zone of the pleated phase. In the parallel geometry, a reflection with a d-spacing of 11.18 Å corresponding to the cp-axis of the pleated-phase crystal emerges from the [hk0]p zone. Three characteristic reflections agree well with the simulated 1D WAXD pattern of the tentatively proposed pleated-phase crystal model, confirming the formation of pleated phase under cylindrical confinement. The relative fraction of pleated-phase crystals decreases as the rod diameter decreases, indicating that structural selection under confinement results from the combined effects of confinement size and interfacial curvature. Overall, our findings show that geometric confinement provides a route for stabilizing crystal polymorphs that are otherwise inaccessible in bulk systems.