Figure 6. The procedures for fabricating PDMS stamps. Reprinted with permission from , Y. Xia, Ann. Rev. Mater. Sci. 28, 153 (1998). © 1998, Annual Reviews.
immersion of a substrate in the solution containing a ligand reactive toward the surface, or by exposure of the substrate to the vapor of a reactive species. The self-assembled multilayer can be formed by electrostatic layer-by-layer assembly process via the alternate adsorption of oppositely charged polyelectrolytes, as first demonstrated by Decher and Hong [162-165].
An important difference between traditional SAMs and the multilayer surfaces is that polyelectrolyte multilayers can be tuned during the layer-by-layer assembly process to vary surface charge density, density of functional groups at the surface, film thickness, and wetting properties [166-169].
The Hammond group at MIT has been investigating the use of this f CP technique on the self-assembly monolayer or multilayer for the formation of various patterns.
Their early studies were still on the Au substrate. The substrate was patterned by the chemically modified PDMS stamp. A polyelectrolyte multilayer film has been deposited on the chemically patterned surface via alternate adsorption of a polyanion and polycation. The polyelectrolyte used here include poly(diallyldimethylammonium chloride) (PDAC), a strong polycation and sulfonated polystyrene (SPS), a strong polyanion. Weak polyelectrolyte multilayers were formed from linear poly(ethylene imine) (LPEI), a weak polycation, and PAA, a weak polyanion. Five-and-one-half polyelectrolyte pairs were deposited for each multilayer, such that each stack is terminated by the polycationic species. A net positive surface charge is thus presented to the microspheres. Solution pH and ionic strength were adjusted to achieve optimal selective deposition of the polyelectrolyte films. For the LPEI/PAA system, the optimum selectivity of polyelectrolyte adsorption onto the patterned SAM surface occurs at pH = 4.8 for the weak polyelectrolytes. For the PDAC/PSS system, the addition of NaCl electrolyte causes partial screening of charges for the strong polyelectrolyte, resulting in thick, loopy films and optimal selective adsorption onto the COOH regions.
The polyelectrolyte is subsequently immersed in an aqueous colloidal suspension of bare SiO2 microspheres or functionalized polystyrene latex particles. The colloids self-organized at the surface, are driven by the spatially varied electrostatic and secondary interactions between the colloid and the substrate. The polyelectrolyte multilayer platform provides a strong bond to the colloids and introduces the functionality into the underlying layers .
Three mechanisms have been demonstrated to control the adsorption—pH of the colloid suspension, ionic strength of the suspension, and concentration of added surfactant. By adjusting the pH of the colloidal slurry, it is possible to alter the strength of the electrostatic attraction between the colloid and the surface in weak polyelectrolyte systems. Adjusting the pH alters the equilibrium balance of the ionization reaction which occurs on the polyelectrolyte backbone. For the PDAC/SPS strong polyelectrolyte system, adjustment of electrolyte concentration in the colloid slurry results in varying strengths of the adsorption. When there is no electrolyte used, a strong electrostatic attraction exists between the polyelectrolyte surface and the SiO2 spheres, and results in the adsorption selectivity. When a small amount of NaCl is added, the counterions form a double layer that screen surface charges from the colloids. The double-layer screening also reduces interparticle repulsion, so that the spheres are more likely to form multiple layers. As the electrostatic repulsion between spheres decreases, the relatively neutral surface also becomes a more likely site for the deposition of particles on the basis of the secondary interaction. The net result is decreased selectivity. Further increasing the salt concentration will provide additional surface charge screening and then result in further reduced selectivity. However, the suspension aggregation limit will be approached upon increasing electrolyte. So manipulation of the pH appears to be a more effective way of controlling the selectivity and density of adsorption. The degree of ionization of the surface can be altered almost independently of SiO2 surface charge, and the stability of the colloidal suspension is maintained.
On the basis of f CP and LBL assembly techniques, a polymer on polymer stamping (POPS) process was developed to create the chemical pattern by direct stamping of functional polymers onto a surface containing complementary functional groups, which is illustrated in Figure 7. A polymer is transferred directly to the top surface of a polyelectrolyte multilayer thin film, based on covalent and/or noncovalent interactions. For example, a polyanion can be stamped directly onto a charged polycation top multiplayer surface to create alternating regions of plus and minus
Flexible PDMS Stamp
Surface functional groups
Surface functional groups
Pattern transfer by stamping of copolymers, dyes, or polyions
Polyelectrolyte multilayer platform
Substrate can be glass, silicon, plastic
Figure 7. Schematic illustrating the transfer of a functional polymer to a surface with complementary functional using polymer-on-polymer stamping. Reprinted with permission from , X. Jiang et al., Lang-muir 18, 2607 (2002). © 2002, American Chemical Society.
charge. On the other hand, blocks and graft copolymers can present both a reactive functional group for attachment to the surface and a second functional group to modify the surface. In either case, the goal is to create a stable functional surface, which is either charged or contains reactive functional groups. This approach can be applied to various functional polymer and substrate systems as well as different thin film deposition technique. The concept of polymer on polymer stamping was first illustrated by stamping a functional graft polymer onto a polyelectrolyte multiplayer film , which utilized the polymers with andride groups that can react with the polymer surface.
Copolymers contain two different types of functional groups—one functional group that can attach itself to the polymer surface and a second functionality acting as the desired surface modifier onto polyelectrolyte multilayer surfaces. Such multifunctional molecules include a block and random copolymer, a graft copolymer, and some surfactants. The patterning of polyelectrolyte multilayer surfaces can be a stepping point to the creation of composite, multilevel structures .
To create templates for layer-by-layer adsorption, it is necessary to form surface regions that resist polyion adsorption. A random graft copolymer of oligoethylene oxide allylether and maleic anhydride (EO-MAL) was used as a multifunctional polymer. It is demonstrated that EO-MAL can be used to direct selective adsorption on various substrates, including plastic surfaces, and to create three-dimensional complex microstructures of multilayered polymer films. pH adjustment was used to control the effectiveness of EO-MAL templates.
Recently, this technique was further broadened to transfer surface chemistry to different regions of a substrate using a block copolymer and a common polyelectrolyte. A broad range of substrates, including the polymeric surfaces, has been explored and the complex multiple level heterostruc-tures via stamping atop continuous or patterned polymer thin film was created. The use of functionalized polymer, polyions, and low molar mass substituents, which can be stamped directly onto other surfaces, particularly plastic substrates and multiplayer films, has been investigated . And the ability to create multilevel microstructures with layer-by-layer self-assembly broadens the area by allowing the construction of devices such as transistors, diodes, sensors, and other optical and electrical components. The patterning of the top surfaces of the multiplayer films also brings new opportunities to incorporate other materials onto multilayer films; the use of such patterns to direct material deposition can lead to the patterning of metal electrodes, the placement of colloidal particles, or the directed deposition of other polymer films atop layer-by-layer functional thin film. The Hammond's group created patterns on a polyelectrolyte surface. By this way, any surface that can be covered with at least one surface layer of polyion is able to be modified.
Many new functional surfaces can be achieved through the use of a block copolymer or a graft copolymer, because a number of existing polymers may be used for this technique. Examination of the effectiveness and stability of polymer-on-polymer requires an understanding of the interaction between the polymer to be transferred and the underlying surface. Conditions must be achieved that allow for effective contact of the desired functional group with the chemical functional group on the surface, and both the polymer "ink" and the surface functional groups must be in a form that encourages covalent, ionic, or hydrogen bonding. The influence of factors such as temperature and pH, as well as the stability of polymer-on polymer-stamping on model and multiplayer surfaces, were investigated by this group . Another important aspect of this technique is the optimization of the underlying polymer surface to achieve a high density of desired functional groups. In Hammond's group, direct stamping of polysterene-poly (acrylic acid) block copolymer (PSS-PAA) has been utilized to create the alternating hydrophobic/hydrophilic regions and poly-electrolytes to generate positively and negatively charged regions.
By stamping onto the top polyamine surface. When a patterned polyelectrolyte film is used as the base layer or substrate, a second pattern can be stamped atop the original pattern array. Subsequent selective adsorption of a polymer yields a second level of microstructures. In addition, a chemical pattern can be achieved by the direct stamping of functional polymers onto a surface containing complementary functional groups. The resulting pattern is then used as a template for the further deposition of the materials on the surface.
Two kinds of new patterns were generated using polyelec-troyte as inks and surface-treated PDMS stamps to create a patterned "plus/minus(+/-)" polyelectrolyte surface. Before using hydrophilic materials, such as charged polymers or proteins as ink, PDMS stamps are often treated with oxygen or air plasma [172, 175-178]. Air plasma is used to make the PDMS surface hydrophilic through the introduction of charged groups, making it wettable with aqueous solutions of charged polymers. It has been indicated that when the PDMS stamp is treated with air plasma for 20 s or more, the PDMS surface becomes completely wettable by aqueous solutions (a 0-5° contact angle with water) . To transfer a negative image of the original PDMS stamp's pattern
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