Regulation Of The Morphology Of Semicrystalline Micelles Of PCL-b-PEO Block Copolymer By Hydrogen-bonding

The crystallization-driven self-assembly of crystalline block copolymers in a selective solvent can produce micelles of various morphologies, so recently it has been paid increasing attention due to the good controllability. The morphologies and size of crystalline micelles can be regulated through changing the reduced tethering density of the corona, σ by different methods. In this research, we studied the morphology of poly(ε-caprolactone)-b-poly(ethylene oxide) (PCL-b-PEO) crystalline micelles in aqueous solution with dynamic laser light scattering (DLS) and transmission electron microscopy (TEM). We aimed to regulate the micellar morphology and size by hydrogen-bonding, which can also alter the value of σ. Besides, we explored the possibility of "reversible transition" of the crystalline cylindrical PCL-b-PEO micelles.Firstly, the effect of phenol, which has only one hydrogen-bond donor group, on the morphology and size of the PCL-b-PEO micelles was investigated. It was found that addition of phenol to the micellar solution could trigger a cylinder-to-sphere transformation of the micellar morphology and the micellar size became more uniform. Such a transformation happened immediately when phenol was added. This was interpreted by the increase of a value and Gibbs free energy of micelles because of the complexation between phenol and PEO chains in the corona via hydrogen-bonding. It’s the first time that the phenomenon of "backward transition" is reported for crystalline micelles.When PCL-b-PEO micelles are used as drug carrier, there exist many amino acids in the application environment. As a result, the effect of L-threonine on the morphology and size of crystalline PCL-b-PEO micelles was studied. It was observed that L-threonine could induce sphere-to-rod or sphere-to-lamella transformation of the PCL-b-PEO micelles in aqueous solution. The morphology transformation of the PCL-b-PEO micelles was also interpreted by hydrogen-bonding between L-threonine and PEO chains. L-Threonine has more than one hydrogen-bond donor group, which can form hydrogen-bonds with PEO chains, thus different PEO chains may be physically cross-linked by hydrogen-bonds, leading to decrease of σ value and growth of the crystalline micelles.The morphology of crystalline micelles is usually fixed and irreversible after growth into micelles with large dimension due to the solidification effect of crystallization. Only with external assistance, for example ultrasound or heating, can the micelles be broken. When phenol and L-threonine were added into the micellar solution of PCL-b-PEO in water, the crystalline cylindrical micelles could be broken into spherical ones or the spherical micelles could grow in cylindrical one, respectively. Therefore, "reversible transformation" of PCL-b-PEO micellar morphology was realized, and prepared micelles had uniform size.