Supramolecular materials via polymerization of mesophases of hydrated amphiphiles.

Abstract

Hydrated amphiphiles form various phases as a function of molecular structure, temperature, concentration, and pressure.1–4 There appears to be a one-to-one correspondence between the structures observed for hydrated amphiphiles and that for block copolymers.5 Amphiphiles are characterized by having a hydrophilic headgroup attached to at least one hydrophobic tail. The unfavorable interfacial enthalpic interaction between the hydrophobic tail(s) of the amphiphile with the polar water molecules induces the former to aggregate with the hydrophobic tail(s) of other amphiphiles.4 The hydrophilic headgroup therefore separates the water from the tail(s), in much the same way that the A–B junction of a diblock AB copolymer separates the two homopolymer blocks A and B.6 Self-organized arrays of non-covalently associated amphiphiles may exist as self-supported lamellar/vesicular, various bicontinuous cubic, or hexagonal/cylindrical phases. Amphiphiles are also frequently studied as supported assemblies, e.g., monolayers at the air–water interface, LB multilayers, or self-assembled monolayers. During the past two decades or so, the understanding of each of these supramolecular assemblies has advanced significantly. This progress is a consequence of fundamental and applied research in many laboratories. The advent of methods to polymerize supramolecular assemblies—first in monolayers in the 1970s, followed by bilayer vesicles in the early 1980s, and more recently in nonlamellar phases, i.e., cubic and hexagonal phases—has led to the creation of new materials, the development of new methods, and a widening perspective on the potential applications of these novel polymeric materials. These uses include the controlled delivery of reagents and drugs, the preparation of biological membrane mimics, the separation and purification of biomolecules, the modification of surfaces, the stabilization of organic zeolites, and the preparation of nanometer colloids, among others. The concept of an area-minimizing surface has been used extensively to describe the morphologies of amphiphile/water systems.2,7 The free energy of the system is described by the topology of the surfaces. In this analysis, a spontaneous curvature term arises purely as a result of the fact that the dimensions of the microdomain are only a few orders of magnitude greater than that of the constituent molecules. This means that the shape of the interface is influenced by the interactions on a molecular level. In order for a system to achieve equilibrium, the various terms in the free energy expression, chief of which is the mean curvature, must be minimized. This theory has been extended to describe the effects of surface charge8 and branched alkyl chains9 on the formation of nonlamellar assemblies. The distribution of a mixture of lipids in nonlamellar phases has also been investigated.10 The identification of the morphology of amphiphile/water systems traditionally relies on methods such as X-ray and neutron diffraction scattering, differential scanning calorimetry (DSC), NMR spectroscopy (e.g., diffusion, 2H, 31P), and transmission and scanning electron microscopy (TEM, SEM). With the exception of the various electron microscopy methods, the characterizations are usually indirect. Though space-group identification and unit cell dimensions are readily obtained with diffraction methods, exactly how the molecules are organized in the unit cell for the amphiphile/water system has only been recently settled.11 In principle, the polymerization of supramolecular assemblies of amphiphiles could be accomplished by at least two strategies: (a) the formation of the hydrated phase from amphiphiles containing a reactive group, followed by either linear or cross-linking polymerization of all or a portion of the organic region of the phase, or (b) the prepolymerization of the amphiphile in isotropic organic media, followed by solvent removal, polymer purification, then hydration of the linear polymer to form the desired phase. This review will emphasize the first strategy. The second approach was successfully employed for the formation of polymerized monolayers at the air–water interface and their transfer to yield LB multilayers but has been infrequently described as a method for the polymerization of self-supported assemblies of amphiphiles.12,13 This review covers the reports, up to late-2000, of the polymerization of self-supported assemblies. The review emphasizes those publications that appeared since the extensive 1988 review by Ringsdorf et al.14 Less attention is given to the numerous studies of reactive amphiphiles in monolayers or multilayers, except in those cases that aid in the understanding of hydrated amphiphilic phases. The following sections review the various methods to polymerize hydrated amphiphiles in bilayer membranes and in nonlamellar phases. In addition, the characteristics of the resultant polymers and the polymeric materials are described, if they are reported by the authors.