Morphological Regulation And Controlled Growth For PCL-b-PEO Crystalline Micelles

Crystalline block copolymers can self-assemble into micelles with various morphologies in a selective solvent. The morphologies of crystalline micelles can be regulated by changing the block chain length, crystallization temperature and solvent. Additionally,"living growth" of crystalline micelles can be realized for several block copolymers. In this dissertation, we focused on the poly(ε-caprolactone)-b-poly(ethylene oxide)(PCL-b-PEO) crystalline micelles in aqueous solution. It has been reported that the morphology of the PCL-b-PEO micelles is related to the reduced tethering density of the corona, a. We aimed to explore more methods to regulate the micellar morphology by changing a value. In the meantime, we investigated the possibility of "living growth" of the crystalline cylindrical PCL-b-PEO micelles and the related mechanisms for "living growth".At first, the effects of preparation method, copolymer concentration and original solvent on the morphology of the obtained PCL-b-PEO micelles were investigated. These effects were interpreted by the balance of thermodynamics and crystallization contributions during the formation of crystalline micelles.Since there are many inorganic salts existed and the pH value can’t be constant in the application environment, the effects of inorganic salts and pH value on the morphology of crystalline PCL-b-PEO micelles were studied. It was observed that the inorganic salt could induce sphere-to-rod or sphere-to-lamella transformation of the PCL-b-PEO micelles in aqueous solution. The effect of the inorganic salt on the micellar morphology of PCL-b-PEO block copolymers in aqueous solution was interpreted in terms of the "salting-out" effect. The addition of an inorganic salt leads to a smaller dimension of the soluble PEO block, which decreases the reduced tethering density of the corona in the micelles and drives the transformation of the micellar morphology. The ability of anion to trigger micellar morphological transformation is in the order of SO42->Cl->>SCN-, in accordance with Hofmeister series. The different "salting-out" abilities of anions to PEO chains can be interpreted by Maker/Breaker ion theory.The effect of pH on the morphology of crystalline PCL-b-PEO micelles in aqueous solution was also investigated. Spherical micelles were formed in neutral and acidic solutions. However, the addition of alkali to the neutral micellar solution triggered a sphere-to-cylinder transformation of the micellar morphology. The micelles were stable in both neutral and acidic solutions, but the size of the micelles became gradually larger in the alkali solution. Fourier transform infrared spectroscopy (FT-IR) results showed that the hydrogen bonds of PEO chains and H2O were destroyed by the addition of OH-. So the PEO coils collapsed and the reduced tethering density of the corona in the crystalline micelles was reduced.At last, the "controlled growth" of crystalline cylindrical PCL-b-PEO micelles was explored. The micelles of PCL59-b-PEO113copolymer in different mixed solvents were held at53℃for5min, and seed solutions with different micellar morphologies and amounts of micellar semicrystalline seeds were prepared. The seed solutions were placed at4℃for micellar growth. Micellar growth driven by epitaxial crystallization of PCL chains took place and the length of grown cylindrical micelles increased linearly with time. Two growth modes were observed. One is the growth of unimers (or amorphous spherical micelles) on the active ends of crystalline cylindrical micelles in H2O/DMF (5/1v/v) at the initial growth period. The other is the growth by end-to-end coupling of cylindrical micelles in H2O/DMSO (5/1v/v). The former growth of the cylindrical micelles is much faster than the latter one. The growth rate highly depends on the qulity of solvent for each growth mechanism.