Hydrogen-Bonding-Mediated Fragmentation and Reversible Self-assembly of Crystalline Micelles of Block Copolymer

Two hydrogen (H)-bond donors, phenol and L-threonine, were added into the aqueous solutions containing crystalline micelles of a poly(epsilon-caprolactone)-b-poly(ethylene oxide) (PCL-b-PEO) block copolymer, respectively. Dynamic light scattering (DLS) characterization showed that the micellar size became smaller after addition of phenol. Transmission electron microscopy (TEM) results revealed that the long crystalline cylindrical micelles formed in the neat aqueous solution were fragmented into short cylinders and even quasi-spherical micelles, as the phenol concentration increased. By contrast, the spherical PCL-b-PEO crystalline micelles could be transformed into short cylinders and then long cylinders after addition of L-threonine. Reversible morphological transformation was realized for the PCL-b-PEO crystalline micelles by adding these two H-bond donors alternately. It is confirmed that both phenol and L-threonine could form H-bonds with PEO. We proposed that, the micellar corona was swollen by phenol, leading to fragmentation of the micellar core, whereas the PEO blocks in the micellar corona was dynamically cross-linked by L-threonine beacuse of its multiple H-bond-donation groups, resulting in a smaller reduced tethering density.