Characterizing the pH-responsive behavior of thin films of diblock copolymer micelles at the silica/aqueous solution interface

Sakai, Kenichi, Smith, Emelyn G., Webber, Grant B., Baker, Murray, Wanless, Erica J., Butun, Vural, Armes, Steven P. and Biggs, Simon (2006) Characterizing the pH-responsive behavior of thin films of diblock copolymer micelles at the silica/aqueous solution interface. Langmuir, 22 20: 8435-8442. doi:10.1021/la061708f


Author Sakai, Kenichi
Smith, Emelyn G.
Webber, Grant B.
Baker, Murray
Wanless, Erica J.
Butun, Vural
Armes, Steven P.
Biggs, Simon
Title Characterizing the pH-responsive behavior of thin films of diblock copolymer micelles at the silica/aqueous solution interface
Journal name Langmuir   Check publisher's open access policy
ISSN 0743-7463
1520-5827
Publication date 2006-09-26
Year available 2006
Sub-type Article (original research)
DOI 10.1021/la061708f
Volume 22
Issue 20
Start page 8435
End page 8442
Total pages 8
Place of publication Washington, DC United States
Publisher American Chemical Society
Language eng
Abstract The pH-responsive behavior of cationic diblock poly(2-(dimethylamino)ethyl methacrylate)-block-poly(2-(diethylamino)ethyl methacrylate) copolymer micelles adsorbed at the silica/aqueous solution interface has been characterized. The micellar morphology of this copolymer, initially adsorbed at pH 9, can be dramatically altered by lowering the solution pH. The original micelle-like morphology of the adsorbed copolymer chains at pH 9 completely disappears as the pH is decreased to 4, and a brush-like layer structure is produced. This change results from protonation of the copolymer chains: the subsequent electrostatic repulsions within the film drive the copolymer chains to expand into the aqueous phase. Returning the solution pH from 4 to 9 causes this brush-like layer to collapse, with atomic force microscopy images suggesting degradation of the film. Hence, the pH-responsive behavior of the copolymer film exhibits irreversible morphological changes. Measurements of the adsorbed/desorbed amounts of the copolymer film were conducted using both a quartz crystal microbalance with dissipation monitoring (QCM-D) and optical reflectometry (OR). After an initial rinse at both pH values, the OR adsorbed mass becomes almost constant during subsequent pH cycling, whereas the corresponding QCM-D adsorbed mass changes significantly but reversibly in response to the solution pH. Since the QCM-D measures a bound mass that moves in tandem with the surface, the discrepancy with the OR data is due to changes in the amount of bound water in the copolymer film as a result of the pH-induced changes in surface morphology. The larger effective mass observed at pH 4 suggests that the brush-like layer contains much more entrapped water than the micellar films at pH 9. The pH dependence of the contact angle of the adsorbed film is consistent with the changes observed using the other techniques, regardless of whether the solution pH is altered in situ or the aqueous solution is completely replaced. In fact, comparison of these two approaches provides direct evidence of the exposure of adsorbed micelle core blocks to the solution during pH cycling and the concomitant impact upon all the other measurements.
Q-Index Code C1
Q-Index Status Provisional Code
Institutional Status Non-UQ

Document type: Journal Article
Sub-type: Article (original research)
Collection: Faculty of Engineering, Architecture and Information Technology Publications
 
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