Document Type

Article

Publication Date

2006

Journal or Book Title

Langmuir

Volume

22

Issue

4

First Page

1469

Last Page

1473

DOI

10.1021/la0527691

Abstract

A unique pH-dependent phase behavior from a copolytner micellar solution to a collapsed hydrogel with micelles ordered in a hexagonal phase was observed. Small-angle neutron scattering (SANS) was used to follow the pH-dependent structural evolution of micelles formed in a solution of a pentablock copolymer consisting of poly((diethylaminoethyl methacrylate)-b-(ethylene oxide)-b-(propylene oxide)-b-(ethylene oxide)-b-(diethylaminoethyl methacrylate)) (PDEAEM 25-b-PEO 100-b-PPO 65-b-PEO 100-b-PDEAEM 25). Between pH 3.0 and pH 7.4, we observed the presence of charged spherical micelles. Increasing the pH of the micelle solution above pH 7.4 resulted in increasing the size of the micelles due to the increasing hydrophobicity of the PDEAEM blocks above their pK a of 7.6. The increase in size of the spherical micelles resulted in a transition to a cylindrical micelle morphology in the pH range 8.1-10.5, and at pH > 11, the copolymer solution undergoes macroscopic phase separation. Indeed, the phase separated copolymer sediments and coalesces into a hydrogel structure that consists of 25-35 wt% water. Small-angle X-ray scattering (SAXS) clearly indicated that the hydrogel has a hexagonal ordered phase. Interestingly, the process is reversible, as lowering of the pH below 7.0 leads to rapid dissolution of the solid into homogeneous solution. We believe that the hexagonal structure in the hydrogel is a result of the organization of the cylindrical micelles due to the increased hydrophobic interactions between the micelles at 70°C and pH 11. Thus we have developed a pH-/ temperature-dependent, reversible hierarchically self-assembling block copolymer system with structures spanning nano- to microscale dimensions.

Comments

Reprinted (adapted) with permission from Langmuir, 22 (4): pp. 1469-1473, doi: 10.1021/la0527691. Copyright 2006 American Chemical Society.

Copyright Owner

American Chemical Society

Language

en

File Format

application/pdf

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