Mobility of small molecules in PEO-PPO-PEO triblock copolymer (F127 and P104) hydrogels




Hosseini Nejad, Heliasadat

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Pluronics are triblock copolymers of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) available in different molecular weights and PPO/PEO ratios. Pluronic hydrogels are able to dissolve hydrophobic compounds and they have application in different areas including drug delivery systems and oil recovery. The structure of Pluronic polymers can be designed for specific application by changing the size and ratio of the PPO and PEO blocks. In aqueous environments, the PPO blocks of different unimers form aggregates as they are more hydrophobic compared to the PEO blocks, and in the aggregates the PPOs have less exposure to water. The PEO blocks that are still hydrophilic remain soluble in water and form a shell around the PPO aggregated core. Moreover, some of the Pluronic copolymer aqueous solutions can form hydrogels at elevated temperatures. The aim of this thesis is to study the microheterogeneity of two different Pluronic hydrogels using singlet excited state probes and also study the mobility of small molecules in Pluronic hydrogels using triplet excited state probes. In the first project, the properties of different microenvironments in Pluronic F127 (PEO99PPO65PEO99) were characterized. The quenching of singlet excited state probes was used to determine the number and characteristics of solubilization sites in F127 hydrogels. This method was used to gain information on the accessibility of different quenchers to singlet excited molecules bound to the micellar structures. Singlet excited states are short lived, and these excited states do not move within the gel before their decay to the ground state. The techniques used for these studies were steady-state fluorescence and time-resolved fluorescence spectroscopies. My results showed that there are different solubilization sites in F127 micelles and the accessibility of quenchers to the singlet excited molecules bound to the micellar structure depends on the nature of the quencher and the size of the excited molecules. In the second project, the different microenvironments in Pluronic P104 (PEO27PPO61PEO27) were characterized, and these results were compared with those obtained for the Pluronic F127. Pluronic P104 has similar units of PPO blocks as F127 but different units of PEO blocks which results in different properties between these two Pluronic copolymers. My results showed that the solubilization sites inside Pluronic micelles changes with the change in PEO/PPO ratio. In the third project, I studied the mobility of different small molecules between aqueous and micellar environments in the F127 hydrogel by quenching triplet excited state probes. Excited triplet states are suitable for such studies because their lifetimes are longer than the lifetimes for singlet excited states. The laser flash photolysis technique was used for this aim. The results showed that the exit from the micellar environment is slow and depend on the size and hydrophobicity of the probe molecules.



Pluronic, hydrogel, fluorescence, singlet excited state, triplet excited state, quenching, supramolecular dynamics