EFFECT OF HYDROPHOBIC BLOCKS ON THE AGGREGATE BEHAVIOR OF AMPHIPHILIC TRIBLOCK COPOLYMERS IN AQUEOUS SOLUTION

Abstract

The effect of hydrophobic blocks on the formation of micelle and gel for amphiphilic triblock copolymers of poly(methyl methacrylate)-b-poly(ethylene glycol)-b-poly(methyl methacrylate)(PMMA-b-PEG-b-PMMA) and polystyrene-b-poly(ethylene glycol)-b-polystyrene(PS-b-PEG-b-PS) was examined by fluorescence excitation spectra of pyrene probe,dynamic light scattering(DSL),atomic force microscopy(AFM),transmission electron microscopy(TEM) and rheology measurements.The core-shell micelle with hydrophobic PS or PMMA core and hydrophilic PEG loop chain corona formed in aqueous solution.The critical micelle concentration(CMC) was less than 0.01 mg/mL for all samples and decreased with increasing PS and PMMA block length,while the PEG block length had less effect on the CMC.The hydrodynamic radius R_(h,app) ranged from 15.3 to 24.3 nm and increased with increasing PEG block length.Increasing hydrophobic PS and PMMA block length effectively promoted the micelle formation in aqueous solution.The spherical shape of the micelle of the triblock copolymers was observed by AFM and TEM.The physical gel of PS-b-PEG-b-PS formed when the concentration was higher than 4.5 wt%.The gel formed by PMMA-b-PEG-b-PMMA when its concentration was higher than 7.5 wt%.The storage modulus G′ and loss modulus G″ increased with increasing of triblock copolymer concentration and the length of PS or PMMA blocks.The equilibrium modulus G_e was 10~3 Pa for PS-b-PEG-b-PS at 7.5 wt%,while less than 10 Pa for PMMA-b-PEGb-PMMA at 15.0 wt%.The stronger hydrophobicity of PS blocks comparison with PMMA block resulted in the stronger hydrophobic interaction and formation of stronger gels.