Hierarchical Self-assembly of Polyamide Helical Fibers

Herein, we reported the synthesis and self-assembly of a novel temperature-responsive polyamide. The temperature- responsive polyamides (APA) was synthesized by forming-salts and solution-melt polycondensation based on the hexanedioic acid and temperature-responsive poly(propylene glycol) bis(2-aminopropyl ether). The polyamide structures of as-prepared polyamides were ascertained by Fourier transform infrared spectroscopy (FT-IR) measurement. And the gel permeation chromatography (GPC) curve showed that the as-prepared temperature-responsive polyamide possessed a distribution with a number-average molecular weight of 17800 Da and a polydispersity of 2.91. The micro differential scanning calorimetry (Micro-DSC) and UV results showed that the APA possessed a LCST of 33 degrees C Then a direct hydration method was used to induce the self-assembly of APA by putting polymers into deionized water with a concentration of 1.0 mg/mL at room temperature. On the other hand, we also prepared the other self-assemblies through the same direct hydration method while the temperature is at 60 degrees C which is beyond the LCST of as-prepared APA. The transmission electron microscope (TEM) images ascertained that the as-prepared APA should self-assemble into normal fibers at room temperature (c=1.0 mg/mL). And the formation of these long fibers are attributed to the aggregation and fusion of the primary polyamide micelles. However, very interesting, when the temperature was increased to 60 degrees C the atomic force microscope (AFM) and TEM results showed that the original fibers have transformed into helical fibers with more than 5 mu m length. And the average helix pitch was about 35 nm. This fascinating morphology transformation should be attributed to the solubility change of PPG segments in APA fibers. When the temperature was increased to 60 degrees C beyond the LCST of APA, the solubility of PPG segments would decrease, and the PPG segments would also collapse. And what's more, due to an alternating hydrophilic/hydrophobic structure of APA, and the PPG segments on the surface of the APA fibers should twist. Finally, the helical APA fibers have been obtained. The present work represents a new progress for macromolecular self-assembly.