Study Of The Synthesis, Characterization And Micelle Behavior Of Amphiphilic Block Copolymer

The aggregation and micelle behavior of amphiphilic block copolymers are not only a theoretic topic, but also important in many industrial applications. From the theoretical point of view, this topic concerns the chain conformation, dissolution and solution properties of block copolymers. In practices, the aggregation behavior fo block copolymer is important in the stabilization of emulsion and the preparation of dye and pigment in printing industry; the polymer micelle can be used as micro-reactor, drug delivery-system and encapsulation materials. Therefore, many scientists have paid their attention on the study of aggregation and micelle behavior of amphiphilic block copolymers.In this thesis, we synthesized a series of double thermo-sensitive PNPAm-PEO-PPO-PEO-PNIPAm pentablock copolymer with narrow polydispersity by atomic transfer radical polymerization (ATRP) method with N-isopropylacrylamide (NIPAm) as the monomer and modified poly(ethylene oxide)-poly(propyleneoxide)-poly (ethyleneoxide) (PEO-PPO-PEO) as the macroinitiator. Micro-differential scanning calorimetry (Micro-DSC) results showed that the pentablock copolymer exhibited two low critical solution temperatures (LCSTs), which can be attributed to the thermal phase transition of the PPO block and the poly(N-isopropylacrylamide)(PNTPAm) blocks, respectively. What's more, the LCSTs are relative with the ratio of the PNIPAm and PPO blocks. The aggregation and micelle behavior of the pentablock copolymer in aqueous solution were then studied in details by using a combination of static and dynamic light scattering (SLS & DLS). The SLS & DLS results indicated that the loose "associates" and single coil chains coexisted in the aqueous solution of pentablock copolymer at the low temperature, where the PEO, PPO, and PNIPAm blocks were soluble in water. These phenomena were in consistent with those observed in other PEO-containing block copolymer systems. At the high temperature above the LCSTs of PPO and PNIPAm blocks (38℃-60℃), the pentablock coplymer chains formed large and stable core-shell micelles with collapsed PPO and PNIPAm cores and swollen PEO shells. The normal and cryogenic transmission electron microscopy (normal and cryo-TEM) experiments provided visual images, confirming the formation of loose "associates" at 21℃and large stable micelles at 38℃. Increase of concentration hindered the formation of "associate" at low temperature but the micelles formed at high temperature were almost independent of the solution concentration investigated. We also investigated the micelle behavior of [poly(N-isopropylacrylamide)-b-poly(tert-butylacrylate)-b-poly(N-isopropylacrylamide)]m (PNIPAm-PtBA-PNIPAm)m multiblock copolymer in aqueous solution by means of normal TEM, cryo-TEM, static and dynamic light scattering. The morphology, size, and size distribution of (PNIPAm-PtBA-PNIPAm)m micelles were found to be dependent on the initial concentration of multiblock copolymer in THF. Spherical micelles, associated aggregates of spherical micelles, cage-like micelles, layered structures, and vesicular micelles were experimentally observed, which were in good agreement with the prediction of theory and simulations on linear amphiphilic multiblock copolymer in selective solvent. The (PNIPAm-PtBA-PNPAm)m micelles also exhibited reversible thermo-sensitive behavior in aqueous solution because of the PNIPAm blocks.In last part of thesis, the micelle formation and micelle morphologies of the poly(DL-lactide)-b-poly(ethylene glycol) (PLAx-PEG44) copolymers in selective solvents were investigated by using cryo-TEM and light scattering techniques. PLAX-PEG44 diblock copolymers in aqueous solution formed various micelle structures when increasing x from 56 to 212 in order to minimize the overall free energy of the systems. The micelles transformed from worm-like micelles for PLA56-PEG44 into vesicles for PLA212-PEG44. Interestingly, vesicular structures with various morphologies, such as large polydisperse vesicles, entrapped vesicles, hollow concentric vesicles, ellipsoidal vesicles, open bending lamellae, vesicles with irregular shapes, etc., were found to be coexisting in PLA212-PEG44 THF/H2O and PLA212-PEG44 dioxane/H2O mixtures with 30 wt% and 40 wt% water contents. Toroid micelles with new morphologies were also observed. These observations indicated that the vesicular micelles of amphiphilic block copolymers in mixed solvents fluctuate from time to time and are able to kinetically form different shapes of morphologies in the solutions. The membrane fluctuation of PLA212-PEG44 vesicles in mixed solvent was verified by dynamic light scattering.