Schizophrenic Behavior of a Thermoresponsive Double Hydrophilic Diblock Copolymer at the Air−Water Interface

• Published in: Langmuir Vol. 26; no. 3; pp. 1807 - 1815
• Main Authors: Romão, Rute I. S, Beija, Mariana, Charreyre, Marie-Thérèse, Farinha, José Paulo S, Gonçalves da Silva, Amélia M. P. S, Martinho, José M. G
• Format: Journal Article
• Language: English
• Published: Washington, DC American Chemical Society 02.02.2010
• Subjects: Acrylamides - chemistry; Air; Chemistry; Colloidal state and disperse state; Exact sciences and technology; General and physical chemistry; Hydrophobic and Hydrophilic Interactions; Interfaces: Adsorption, Reactions, Films, Forces; Langmuir blodgett films; Microscopy, Confocal; Pressure; Rhodamines - chemistry; Surface physical chemistry; Surface Properties; Temperature; Water - chemistry; Gas liquid interface; Critical property; Monolayer; Rhodamine; Composition; Glass; Temperature effect; Confocal microscopy; Air water interface; Substrate; Laser; Diblock copolymer; Isotherm; Covalent bond; Surface pressure; Langmuir Blodgett layer; Homopolymer; Critical temperature; Aggregate; Block copolymer; Fluorescence microscopy; Low temperature
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la902510q

The thermoresponsive behavior of the rhodamine B end-labeled double hydrophilic block copolymer (DHBC) poly(N,N-dimethylacrylamide)-b-poly(N,N-diethylacrylamide) (RhB-PDMA207-b-PDEA177) and the 1:1 segmental mixture of PDEA and rhodamine B end-labeled PDMA homopolymers was studied over the range of 10−40 °C at the air−water interface. The increase in collapse surface pressure (second plateau regime) of the DHBC with temperature confirms the thermoresponsiveness of PDEA at the interface. The sum of the π−A isotherms of the two single homopolymers weighted by composition closely follows the π−A isotherm of the DHBC, suggesting that the behavior of each block of the DHBC is not influenced by the presence of the other block. Langmuir−Blodgett monolayers of DHBC deposited on glass substrates were analyzed by laser scanning confocal fluorescence microscopy (LSCFM), showing schizophrenic behavior: at low temperature, the RhB-PDMA block dominates the inside of bright (core) microdomains, switching to the outside (shell) at temperatures above the lower critical solution temperature (LCST) of PDEA. This core−shell inversion triggered by the temperature increase was not detected in the homopolymer mixture. The present results suggest that both the covalent bond between the two blocks of the DHBC and the tendency of rhodamine B to aggregate play a role in the formation of the bright cores at low temperature whereas PDEA thermoaggregation is responsible for the formation of the dark cores above the LCST of PDEA.