PS-b-PEO Reverse Micelle And Morphology Transition

Amphiphilic diblock copolymers can self-assemble into micelles in a selective solvent. Micelles with excellent thermostability are found to have various morphologies which can be tuned easily by several factors. The formation of diblock copolymer micelles, the pathway of morphological transiton and factors controlling micelle morphology have been extensively investigated due to their potential applications in drug delivery, inorganic nanoreactor, emulsion and separation systems. This thesis focuses on the reverse micelle of PS-b-PEO in organic solvent, developing a simple and effective way to control and tune micelle morphology, investigating the effect of solvent composition and temperature on the morphology of reverse micells and revealing the key factor driving morphological transitions.A series of PS164-b-PEO827reverse micelles composed of a PEO core and PS coronas were prepared by adding cyclohexane into PS164-b-PEO827/dioxane solution. Stable miclles with various morphologies as sphere, cylinder, and lamellar was observed. The influences of selective solvent weight fraction and PS164-b-PEO827concentration were discussed. A phase diagram was constructed to indicate the regions of stable morphology and the boundaries corresponding to morphological transtions. On the basis of observation of morphological reversibility, the phase diagram was proven to be in the thermodynamic equilibrium and the pathway of morphological transition was brought up.The streching of the core, the surface tension and surface area between the core and solvent, the repulsion between coronas were discussed for micelles with different morphologies. Free energies of the core, the interface, the corona and the total micelle free energy was calculated to understand the driving force for morphological transition. The competition between the free energy of interface and the free energy of corona was found to be the reason for morphological transiton of micelle in dioxane/cyclohexane system.Temperature induced morphological transitions were observed by cooling. On decreasing temperature from room temperature, transitions from sphere to cylinder to vesicle were observed. The repulsion between PS coronas was found to be decreased by comparing micellization behaviors of three samples containg different cyclohexane weight fraction. The pathway of morphological evolution was discussed based on the time-dependent morphological transition during cooling. Micelle free energies were calculated for samples at different temperatures and the competition between the free energy of interface and the free energy of corona was found to determine micelle morphology during cooling.