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2.
Langmuir ; 23(8): 4589-98, 2007 Apr 10.
Artigo em Inglês | MEDLINE | ID: mdl-17371057

RESUMO

Mesostructured films of alkyltrimethylammonium bromides or cetylpyridinium bromide and polyethylenimines that spontaneously self-assemble at the air/water interface have been examined using a range of surface sensitive techniques. These films are unusual in that they can be micrometers thick and are relatively robust. Here we show that the films can be cross-linked and thus removed from the liquid surface where they form, as solid, mesostructured polymer-surfactant membranes. Cross-linking causes little change in the structure of the films but freezes in the metastable mesostructures, enhancing the potential of these films for future applications. Cross-linked films, dried after removal from the solution surface, retain the ordered nanoscale structure within the film. We also report grazing incidence X-ray diffraction (GID), which shows that most films display scattering consistent with 2D-hexagonal phase crystallites of rodlike surfactant micelles encased in polymer. Polymer branching makes little difference to the film structures; however, polymer molecular weight has a significant effect. Films with lower polymer MW are generally thinner and more ordered, while higher polymer MW films were thicker and less ordered. Increased pH causes formation of thicker films and improves the ordering in low MW films, while high MW films lose order. To rationalize these results, we propose a model for the film formation process that relates the kinetic and thermodynamic limits of phase separation and mesophase ordering to the structures observed.


Assuntos
Polímeros/química , Tensoativos/química , Cátions , Físico-Química/métodos , Reagentes para Ligações Cruzadas/química , Reagentes para Ligações Cruzadas/farmacologia , Concentração de Íons de Hidrogênio , Cinética , Micelas , Modelos Químicos , Estrutura Molecular , Nêutrons , Polietilenoimina/química , Hidróxido de Sódio/química , Propriedades de Superfície , Termodinâmica
3.
J Phys Chem B ; 110(11): 5330-6, 2006 Mar 23.
Artigo em Inglês | MEDLINE | ID: mdl-16539465

RESUMO

Surfactant-templated polymer films prepared from polyethylenimine (PEI), cetyltrimethylammonium bromide (CTAB), and octaethylene glycol monohexadecyl ether (C(16)E(8)) were examined and the effect of increasing the percentage of nonionic surfactant in the micelles measured using both surface and bulk-sensitive techniques. It was found that there is a strong interaction between CTAB and C(16)E(8), although no interaction between the C(16)E(8) and PEI was observed. Generally, increasing the percentage of C(16)E(8) in the micelles decreases both the thickness and degree of order in the films; however, it was observed, depending on the conditions, that films could still be formed with as little as 20% cationic surfactant. Experiments on the CTAB/Brij56/PEI system were also performed and these indicate that it is similar to the CTAB/C(16)E(8)/PEI system.

4.
Langmuir ; 20(24): 10679-84, 2004 Nov 23.
Artigo em Inglês | MEDLINE | ID: mdl-15544401

RESUMO

Off-specular X-ray reflectivity measurements were carried out to follow the in situ development of surfactant-templated silica thin films at the air-water interface under conditions of controlled relative humidity and temperature, using an enclosed sample cell designed for this purpose. The results suggest a strong dependence of formation time and growth mechanism on ambient conditions. Thin films were synthesized at the air-water interface using cetyltrimethylammonium bromide (CTAB, 0.075 M) and a silica precursor, tetramethoxysilane (TMOS, 0.29-0.80 M) in an acidic medium. The studied humidity range was from 50 to 100%, the temperature was between 25 and 40 degrees C, and the TMOS/CTAB molar ratio was between 3.3 and 10.7. We observed that high humidity slows down the growth process due to lack of evaporation. However, increasing the temperature results in a decrease in the film-formation time. We proposed a formation mechanism for film growth as a consequence of phase separation, organic array assembly, and silica polymerization.


Assuntos
Compostos de Cetrimônio/química , Umidade , Membranas Artificiais , Silicatos/química , Temperatura , Ar , Cetrimônio , Porosidade , Fatores de Tempo , Água/química , Difração de Raios X
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