Synthesis, Micellization And Crystallization Of Symmetric Amphiphilic Block Copolymer

Amphiphilic block copolymers possess a series of attractive properties and can self-assemble to micelle formation by microphase separation in selective solvent, and have been successfully used for biodegradability, surgical structure, biocompatibility, and in tissue engineering. In the dissertation, amphiphilic block copolymers composed of hydrophilic Polyethylene glycol (PEG) and hydrophobic Poly(s-caprolactone)(PCL) or polystyrene (PS) were synthesized by free radical polymerizetion (FRP) and ring open polymerizetion (ROP). The molecular structures and characteristics of the copolymers and their micelles behaviors were investigated as follow:The amphiphilic Poly(s-caprolactone)-b-poly(ethylene glycol)-b-Poly (s-caprolactone)(PCL-b-PEG-b-PCL) triblock copolymers with well-defined structure were synthesized by ring-opening polymerization of s-caprolactone initiated by PEG with low toxic Sn(Oct)2as catalyst. The block copolymers were characterized by GPC, FTIR,’H-NMR and their thermal behaviors were investigated by DSC. The aggregation behavior of these triblock copolymers in aqueous solution were examined by fluorescence excitation spectra of pyrene probe and dynamic light scattering (DLS). The critical micelle concentration (CMC) decreased with increasing PCL block length, increasing hydrophobic PCL block length effectively promoted the micelle formation in aqueous solution. The partition constant Kv of pyrene in the micelle was about105and increased with increasing PCL block length.A triblock copolymer PCL-b-PEG-b-PCL was prepared by ring-opening polymerization of s-caprolactone and initiator PEG. The α-bromoester terminated Br-b-PCL-b-PEG-b-PCL-b-Br macro initiator was obtained in the subsequent esterification of end hydroxyl groups with a-bromopropionyl bromide and was used in the ATRP of styrene (St). The block copolymers were well characterized by GPC, FT-IR,’H-NMR. The value of critical micelle concentration (CMC) of PS-b-PCL-b-PEG-b-PCL-b-PS was0.0012mg/mL which was obtained by fluorescence method using pyrene as a probe. The aggregation behaviors of these triblock copolymers in aqueous solution were examined by fluorescence excitation spectra of pyrene probe and dynamic light scattering (DLS). The average micelle size was about150nm and the spherical morphology of copolymer micelle can be predominantly observed in the images of Atomic Force Microscope (AFM).It can be measured and analyzed through a series of synthesized PCL-b-PEG-b-PCL triblock by X-ray diffraction (XRD) and differential scanning calorimeter (DSC). When the length of PCL blocks was relatively small, the PCL crystals disappear due to confinement of the PEG block. When the length of PCL blocks was relatively large, the PCL crystals appear and restrict the PEG’s crystallization. Fluorescence spectra of solid polystyrene (PS) and desiccative pentablock of PS-b-PCL-b-PEG-b-PCL-b-PS micelles were tested at a fixed excitation wavelength of295nm. The introduction of oxy and PCL chain segment containing the C=O chromophore make the maximum fluorescence emission wavelength of polystyrene (PS) to5nm red shift. The results of the steady-state fluorescence spectrum indicated that the emission intensity of pyrene in the micelle solution presented a linar decreasing during heating, and had a crossover point at56℃which in agreement with the melt temperature of PCL, while the pyrene in the aqueous solution didn’t have. Combined with pentablock, the pentablock micelles DSC melting peak also appeared in56℃. We concluded when pyrene microenvironment in the nucleus in a temperature of56℃that the PCL chain segment has become loose chain from crystallization of the folded chain.