Direct Access to Stable, Freestanding Polymer Membranes by Layer‐by‐Layer Assembly of Polyelectrolytes

Abstract

The alternating, layer-by-layer assembly of oppositely charged polyelectrolytes, often referred to as electrostatic self-assembly (ESA), has been well-established in the past decade for the preparation of ultrathin polymer films with precise thicknesses of between 1 and 1000 nm. [1,2] The method is based on the formation of water-insoluble, nearly stoichiometric complexes of polycations and polyanions (so-called ªsymplexesº) at an interface. Characteristically, the preparation of ESA films involves the use of an appropriate substrate onto which the polymers are adsorbed layer by layer. Consequently, the ESA method results, first of all, in the surface modification of a given substrate. However, ultrathin polymer films are often desirable as freestanding films. [3,4] In fact, numerous examples have been reported in which the substrate was removed after film preparation in order to prepare substrate-free films, particularly in cases where colloidal supports have been used. [5,6] In these cases, hollow polyelectrolyte capsules dispersed in liquid are produced by the dissolution or decomposition of the substrate. [5,7] Following the same strategy, planar films without a supporting substrate have also been prepared in exceptional cases, either by removing the entire support or by dissolving a thin buffer coating between the primary substrate and the stable ESA film. [3,8±14] In all these examples, the preparation of freestanding, ultrathin polymer films required two steps: first, film growth on a substrate and second, the removal of the substrate. This twostep approach is not only cumbersome, but may also damage the ultrathin polymer films as a result of, for example, chemical and mechanical stresses during the removal of the substrate. In fact, nearly all examples of freestanding films have been made using charged colloidal particles, which have been used to improve the mechanical stability of the films. [3,10±13] Therefore, direct access to freestanding ESA films, which is suited for organic, flexible polyelectrolytes, appears very attractive. In this communication, we present a strategy to prepare ultrathin, freestanding ESA films directlyÐi.e., without using a substrateÐwith a thickness/width ratio of up to 1:10 000, by appropriately choosing the polyelectrolyte pair and conditions for self-assembly. We also investigate the response of the freestanding films to chemical stimuli before and after their stabilization by chemical crosslinking.In order to prepare freestanding films, we used electron microscopy (EM) grids, with typical hole sizes of 100 lm 100 lm, as supports. Although the formation of films which successfully span pores has been reported occasionally using supports with very small pores (< 50 nm), [15,16] so far, the use of supports with larger pores has resulted in coating of the pore walls.[8,9,17±19] Indeed, when we used the classical experimental conditions during the dipping cyclesÐ namely, washing between subsequent dipping steps and drying in ambient atmosphereÐall our attempts to obtain f...

Freestanding Nanostructures via Layer‐by‐Layer Assembly

“…Mallwitz and Laschewsky have been able to arrive at a solution to this problem. [43] Unlike other preparation processes, which require the growth of a film and its removal from a substrate, Laschewsky and his group have devised a strategy for the direct assembly of ultrathin freestanding LbL films into large openings. The initial PAH/PAA LbL films spanning the large openings of a TEM grid are formed by three dipping cycles without the usual rinsing steps.…”

“…[31] Planar, ultrathin, freestanding LbL films have been developed by several groups. [28,29,42,43] Fery and co-workers have developed a novel approach for making arrays of planar LbL multilayer films by using sacrificial layers. [29] Laschewsky and his group have reported a strategy to directly prepare ultrathin freestanding arrays of LbL films without sacrificial precursor sublayers.…”

“…The obtained freestanding films are stable under ambient conditions and can be used for further surface-modification steps. [43] This method provides the opportunity to directly synthesize molecularly thin LbL structures for various applications.…”

“…[29] Laschewsky and his group have reported a strategy to directly prepare ultrathin freestanding arrays of LbL films without sacrificial precursor sublayers. [43] Alternatively, 2D polymer films have been obtained by a peeling-off procedure. [44] We have fabricated freestanding LbL multilayer films with functional nanoparticles and explored their unique micromechanical properties.…”

Fabrication of Bio-friendly Polymer Nanosheets for Biomedical Applications

“…The nanosheets represent unique properties such as high transparency and flexibility due to their dimensions (centimeter size but only tens-of-nanometers thickness) [1]. The techniques that have been adopted to prepare these sheets include the utilization of polymers and/or inorganic materials such as layer-by-layer nanosheets of polyelectrolytes [2][3][4], assemblies of triblock copolymers [5], cross-linked self-assembled monolayers [6], amphiphilic Langmuir-Blodgett nanosheets [7,8], sol/gel interpenetrating network nanomembranes [9], and graphene nanosheets dissociated from graphite [10]. These nanosheets have been developed for various research fields, e.g.…”