Diffusional self-organization in exponential layer-by-layer films with micro- and nanoscale periodicity

The layer-by-layer (LBL) assembly technique is currently one of the most widely utilized methods for the preparation of nanostructured, multilayered thin films. The structure of LBL films is typically controlled by varying the deposition sequence of adsorbed layers, leading to stratified assemblies. For specific, non-spherical inorganic LBL components, such as sheets, or axial nanocolloids, such as nanotubes, nanowires, nanowiskers, or nanorods, the structure of the films can also be controlled by their orientation. As such, clay nanosheets spontaneously adsorb almost exclusively in the orientation parallel to the substrate, whilst assembly of axial nanocolloids under conditions of shear or dewetting results in partial alignment of the fibrous components. Morphological or structural control of the multilayers can also be imparted by the choice of the assembly method (e.g. spin coating versus dip coating), the assembly conditions, or post-assembly processing of the assembly. The shape and surface morphology of the assemblies can also be tailored by the structure or shape of the substrate, as has been shown in the preparation of hollow capsules or sculptured/perforated membranes. Both polymers and nanoparticles exhibit strong tendencies toward self-organization. This effect has not been utilized in the LBL assemblies, except for the recent observation by Yoo et al. of the organization of rod-shaped viruses on the surface of a film consisting of a few bilayers. Overall, the need for more sophisticated degrees of structural organization is quite extensive and commensurate with the increasingly complex applications for which they are being prepared. Importantly, this control must be possible on a nanometer and a micrometer scale. In principle, the LBL approach does allow such a broad-scale control, but microscale films require deposition of a great number of layers in traditional LBL. It would be exceptionally advantageous to design a method that can lead to well-organized materials combining fast deposition and hierarchical nano-, micro-, and macroscopic levels of organization. To achieve this aim, a degree of smartness and the presence of elements of selforganization in the film will most likely be required. Layered systems with alternating microand nanostrata of a stiff and an elastic nature might be particularly interesting because of mechanical properties associated with the distribution of stress in hierarchical structures and predicted theoretically unique mechanical properties. Exponentially grown LBL (e-LBL) films are multilayers in which polymer chains retain their mobility and diffuse through the deposited strata. The degree of mobility makes possible to observe self-organization phenomena in such structures. Herein, we show that a system with alternating nanometerand micrometer-scale layers of predominantly inorganic (stiff) and polymeric (elastic) layers forms upon LBL deposition of poly(diallyldimethylammonium chloride) (PDDA), poly(acrylic acid) (PAA), and sodium montmorillonite clay nanosheet (MTM) multilayers. Despite the expectations of fairly homogeneous coatings in the framework of both traditional and exponential LBL deposition, the deposition sequence (PDDA/PAA/PDDA/MTM)n (n is the number of deposition cycles), results in well-defined indexing of the films after the first few cycles, with a periodicity of (1.7 0.4) mm for 10 min deposition, and superimposed organization of MTM sheets at the interfaces with 0–10 nm spacing (Figure 1). The indexing can be further controlled by varying the deposition times for polyelectrolytes (Supporting Information, Figure S1). A typical assembly included alternate immersion of a glass slide into solutions of the polycation PDDA and an [*] Prof. N. A. Kotov Departments of Chemical Engineering, Materials Science and Engineering, and Biomedical Engineering University of Michigan, Ann Arbor, MI 48109 (USA) Fax: (+1)734-764-7453 E-mail: kotov@umich.edu