Synthesis And Aggregation Behavior Of Double Hydrophilic Triblock Copolymer Of Poly (acrylic Acid) And Poly (ethylene Glycol) In Aqueous Solution

Two kinds of PEG-based macrointiators (Br-PEG-Br) were successfully synthesized through quantitative esterification of hydroxyl-capped poly (ethylene glycol) (HO-PEG-OH, the molecular weight of 10000 and 20000) with 2-bromoisobutyryl bromide. A double hydrophilic triblock copolymer poly (acrylic acid)-b-poly (ethylene glycol)-b-poly (acrylic acid) (PAA-b-PEG-b-PAA) with well-defined structure and narrow molecular weight distribution (Mw/Mn < 1.7) was prepared by atom transfer radical polymerization (ATRP) of t-butyl acrylate (tBA) initiated by the PEG macroinitiator, followed by selective acidolysis of the PtBA block. The pH-sensitive behavior of double hydrophilic triblock copolymer in aqueous solution and the micellization of PAA-b-PEG-b-PAA and oppositely charged surfactant dodecyltrimethyl ammonium bromide (DTAB) were investigated by potentiometric titration, fluorescence spectrum, dynamic light scattering and zeta potential.The pKa values of PAA-b-PEG-b-PAA were 6.6 and 6.0 in deionized water and in 0.1 M NaCl solution. The triblock copolymer formed a micelle composed of a weak hydrophobic core of complexed PAA and PEG and a hydrophilic PEG coronal induced by the hydrogen bonding between protons of carbocylic acid in PAA block and ether oxygens in PEG block at pH < 5.5. At pH < 4.5, steady and narrow distributed micelles were formed. Increasing pH to 5.0, unsteady and broad distributed micelles were observed. At pH > 5.5, the micelle was destroyed owing to the ionization of the PAA blocks. The length of PAA has played an important role on the micellization of the triblock copolymer, and the hydrodynamic radius Rh,app of the micelle strongly depends on the length of PAA and PEG blocks.The novel water dispersible nanoparticles from PAA-b-PEG-b-PAA triblock copolymer and oppositely charged surfactant dodecyltrimethyl ammonium bromide (DTAB) were prepared by mixing the individual aqueous solutions. The neutralization of the anionic PAA blocks with cationic DTAB accompanied with the hydrophobic interaction of alkyl tails of DTAB led to formation of core-shell nanoparticles with the core of the DTAB neutralized PAA blocks and the shell of the looped PEG blocks. The water dispersible nanoparticles with negativeζ-potential were obtained over the DN range from 0.4 to 2.0 and their sizes depended on the DN. The looped PEG blocks hindered the further neutralization of the PAA blocks with cationic DTAB, resulting in existence of some negative charged PAA-b-PEG-b-PAA backbones even when DN > 1.0.