| SUBJECT |
Rubner's group at MIT has been looking at the fundamentals of both electrostatic and hydrogen-bonded layer-by-layer assembly and also at applications of polyelectrolyte multilayers. The group is particularly interested in the assembly of polyelectrolytes that are not fully ionized in solution. "Polyelectrolyte multilayer architecture depends on the degree of ionization of the polyelectrolyte," Rubner explained. "With weak polyelectrolytes, it is possible to tune charge densities by simple changes in solution pH and therefore manipulate structure and properties. For example, using the same two polymers assembled under different pH conditions, we have shown that it is possible to create thin films with dramatically different, and quite useful, properties." SENSITIVE Multilayers are prepared by alternate dipping in anionic and cationic polyelectrolytes. Structure and properties of the films are controlled by pH. Page: 1 | 2 | Next Page. IN RECENT WORK that may have potential for drug delivery applications, Rubner and graduate student Anita J. Chung showed that pH can be used to control the loading and release of methylene blue dye molecules to and from poly(acrylic acid) and poly(allylamine hydrochloride) multilayer films [Langmuir, 18, 1176 (2002)]. "The use of buffered methylene blue dye solutions enabled faster and more uniform dye loading into the multilayers," Chung and Rubner noted. "Because weak polyelectrolytes become less ionized under different pH environments, a pH-sensitive trigger can be used to release the charged molecules." Rubner explained that his group can now use these two polymers to create thin-film coatings that either completely resist or promote the adhesion of mammalian cells. "These films have potential for biosensor applications and for rendering surfaces bioinert," Rubner observed. "They can also be used as antifogging or antireflection coatings and as nanoreactors for growing nanoparticles within specific regions of the multilayer film. And we can modify the wettability of surfaces in a controlled manner and use the films to dramatically increase the efficiency of light-emitting polymer devices." Rubner's group has shown, for example, that polyelectrolyte multilayered films of poly(acrylic acid) and poly(allylamine hydrochloride) can be employed as nanoreactors for preparing silver nanoparticle composites [Langmuir, 18, 3370 (2002)]. The work was carried out in collaboration with MIT chemical engineering professor Robert E. Cohen. The team also demonstrated that the size of the nanoparticles and the overall metal concentration within the films can be systematically controlled by the polyelectrolyte solution pHs and other processing conditions. "One implication of the control over the silver content in the multilayers is the ability to systematically change the optical properties of these nanocomposite films," the authors suggested. Rubner's group has also developed a range of techniques for patterning these films. "These include both subtractive and additive ink-jet printing," Rubner noted. "In this case, water or polyelectrolyte solutions are used as the ink in a standard ink-jet printer to create patterns with features on the order of 50–300 mm." In a recent paper, Rubner and postdoctoral researcher Sung Yun Yang demonstrated that hydrogen-bonded multilayers composed of polyacrylamide and poly- (acrylic acid), a weak polyacid, can be readily stabilized at neutral pH conditions [J. Am. Chem. Soc., 124, 2100 (2002)]. The multilayer thin films can then be patterned using simple water-based subtractive patterning techniques such as ink-jet printing or photolithography. "Basically, we are printing water to remove the film," Rubner explained. |
|
Want more information ? Interested in the hidden information ? Click here and do your request. |