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Janus Particle Synthesis, Self-Assembly and Applications
By Shan Jiang, Steve Granick
The Royal Society of ChemistryCopyright © 2012 The Royal Society of Chemistry
All rights reserved.
Soft, Nanoscale Janus Particles by Macromolecular Engineering and Molecular Self-assembly
ANDREAS WALTHER AND AXEL H. E. MÜLLER
Macromolecular engineering has evolved into a powerful toolbox for the preparation of complex polymer topologies with remarkable control over both the architecture and the distribution of monomer sequences into, e.g., block-type structures or well-defined branched macromolecules. The rapid advances in controlled/living polymerization techniques during the last two decades have greatly facilitated this development. In the context of Janus particles, macromolecular engineering is interesting not only for the direct synthesis of phase-segregated unimolecular objects, but also for harnessing the self-assembly capabilities of tailor-made polymers owing to mutually incompatible polymer blocks, solvophobic effects or specific molecular interactions. Indeed, self-assembly of block copolymers has proven to be a remarkably elegant strategy to generate polymer-based nano-objects, where we have seen progress to increasingly sophisticated soft nanoparticles, from simple diblock copolymer micelles and vesicles, to multicompartment micelles (MCMs) with increasingly complex geometries. Still, directly breaking the symmetry into biphasic Janus (Figure 1.1) particles or micelles has remained a considerable challenge.
Polymer-based Janus particles formed by direct synthesis or self-assembly of block copolymers are unique among this class of non-centrosymmetric colloids. First, truly nanoscale dimensions (i.e. <100 nm) can be approached that are very difficult to tackle by, e.g., common desymmetrization reactions at interfaces or phase separation processes in emulsions, microfluidics or electrohydrodynamic jetting. Second, smart polymer segments, able to respond to environmental changes by phase transitions, impart a large-scale responsiveness to trigger superstructure formation or create strongly amphiphilic particles relevant for surface nanostructuring and the stabilization of interfaces. These properties render them a key building block for switchable materials. As a third criterion, polymers are also the crucial soft materials to communicate with the environment and to mediate interactions with cells, proteins and other living matter when approaching the biological interface with synthetic materials. Consequently, they are a valuable material class in the multitude of Janus particles available nowadays.
In this chapter, we review and discuss recent developments towards polymeric Janus particles. We place an emphasis on strategies specifically involving advanced polymer synthesis to create unimolecular objects and on methodologies utilizing self-assembly as well as post-transformations of self-assembled structures to create biphasic particles. Thereby, we focus on the small size regime and discuss particle architectures with different dimension alities, in which at least one dimension is truly nanoscale (i.e.<100 nm). It may be noted that there are other approaches towards polymer-based Janus particles on the (sub)micron scale, such as phase separation in emulsion droplets, lithographic approaches in microfluidic channels and electrohydrodynamic co-jetting, which are, however, beyond the focus of this contribution and are discussed in other chapters. This chapter is grouped into four topics. The first three are (a) Janus particles via direct macromolecular engineering, (b) Janus particles via direct self-assembly and/or transformations in solution and (c) Janus particles via transformation of self-assembled triblock terpolymer bulk structures. We finally discuss (d) self-assembly properties of the synthesized Janus particles and highlight some potential applications that have already been realized.
1.2 Janus Particles via Direct Macromolecular Engineering
The rapid advances in synthetic tools available to polymer chemists have triggered significant interest in the preparation of Janus particles. One of the earliest strategies involved the attachment or growth of different polymer chains to/from a single focal point or to/from a focal line with the aim of preparing spherical or cylindrical Janus particles, also known as heterografted star-shaped and cylindrical brush polymers. The resulting structures are outlined in Figure 1.2, which also highlights one of the major challenges for such nanoscale objects with high dynamics of the polymer chains. Phase separation of the chemically different polymer arms is required to realize a true Janus particle character in solution. However, phase separation for polymer arms emanating from a single focal point or from a dynamic micellar core – as will be discussed later – is not self-evident. It is governed by the interplay between entropy, favoring mixing of the polymer chains, and the enthalpic force of polymer chains to phase separate. In solution, the latter is drastically reduced compared with the bulk state and it has proven a challenging task to design systems that allow a freely occurring phase separation. In this context, it is also important to point to another major obstacle, namely the difficulty of providing solid in situ proof for corona segregation of polymer nano-objects in solution. The nanoscale dimensions and the often weak natural contrast of different organic parts for imaging are the main complications. This challenge can be best met by 2D 1H–1H NOESY NMR (NOE = nuclear Overhauser effect), an NMR technique probing intermolecular distances via through-space coupling, or by direct (cryogenic) transmission electron microscope (TEM) imaging using suitable staining methods to augment the natural contrast.
Some of the first evidence that a phase separation in heteroarm star polymers can indeed occur was delivered by Kiriy et al. In their atomic force microscopy (AFM) investigations on a system of heteroarm star polymers composed of seven arms of polystyrene (PS) and seven arms of poly(2-vinylpyridine) (PS7–P2VP7), it was observed that different topologies of the molecules result upon deposition from different solvents on to mica (Figure 1.3). Chloroform (CHCl3) led to a hat-shaped appearance, whereas tetrahydrofuran (THF) yielded a more globular shape. The dissimilar shapes were attributed to the adsorption of Janus-type conformations in the case of CHCl3, whereas a mixed conformation was suggested for the molecules deposited from THF. These observations were supported by calculations of the solubility parameters, which confirmed that CHCl3 is a more selective solvent for P2VP and thus can enforce intramolecular phase segregation and a Janus-type conformation in solution. One uncertainty related to the imaging of deposited molecules of dynamic species always lies in the unclear effect of the surface properties on the adsorption behavior, i.e. selective adsorption due to preferred adhesion – a problem hard to come by with ex situ techniques.
An improved focal point design was suggested by Ge et al., who reported the synthesis and stimuli-responsive self-assembly of double-hydrophilic Janus-type A7B14 heteroarm star copolymers with two types of water-soluble polymer arms, poly(N-isopropylacrylamide) (PNIPAAm) and poly(2(diethy-lamino)ethyl methacrylate) (PDEAMA), emanating from the two opposing sides of a rigid toroidal β-cyclodextrin (β-CD) core. Owing to the pre-encoded phase separation within the focal point, an enhanced tendency for phase separation of the two arms can be expected. The authors found an interesting schizophrenic self-assembly behavior. Depending on the conditions for triggering the solubility to insolubility phase transitions of the two polymer arms, which are high temperature for PNIPAAm and high pH for PDEAMA, it was possible to switch between two vesicle states by inverting the membrane structure. Such a vesicle inversion procedure is a highly unlikely scenario for simple coil–coil diblock copolymers and can serve as indirect evidence for the Janus character of these stars.
Zhu and co-workers described a facile and large-scale synthesis of possibly the smallest unimolecular Janus nanoparticles by intramolecular crosslinking of the inner P2VP block of a polystyrene-block-poly(2-vinylpyridine)-block-poly(ethylene oxide) (PS-b-P2VP-b-PEO or SVEO) triblock terpolymer (Figure 1.4). Simple addition of an α, ω-dibromoalkane in a common solvent, dimethylformamide (DMF), resulted in nanoscale Janus particles with exactly one polymer arm of each end block attached to the central core. Interestingly, these Janus particles showed a concentration-dependent self-assembly behavior into supermicelles upon reaching a critical aggregation concentration, cac, of ~2 mg mL-1. Strikingly, this aggregation also took place in a good solvent (DMF) for both end blocks, PS and PEO, which is a first example of the unusual and intuitively unexpected self-assembly behavior of polymeric Janus particles in good solvents for both corona hemispheres.
Moving from these 3D systems with overall spherical character to 2D systems with cylindrical architectures, one can identify various synthetic efforts targeting different types of copolymer bottle brushes. Various groups have reported cylindrical copolymer brushes of poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) and poly(e-caprolactone) (PCL), PS and polylactide (PLA) or PS and PEO with a statistical distribution of side-chains along the backbone (Figure 1.2b). Although these molecules showed clustering into some irregular aggregates upon exposure to selective solvents, there are no conclusive data and discussion on the potential Janus character of such structures. Simulations by de Jong and ten Brinke demonstrated that complete phase separation may only occur at very high incompatibilities of the two grafted polymers (similar to hetero-arm star-shaped polymers), as expressed by a large Flory–Huggins parameter, λ. It was further suggested that a well-defined Janus cylinder may only be reached at theta conditions for both blocks and for rigid backbones (Figure 1.5). For good solvents and highly flexible backbones, common to most synthetic comb-shaped polymers, the extent of phase separation is reduced and the molecules undergo bending into different shapes.
Schmidt and co-workers found differently bent shapes for cylindrical brushes composed of P2VP and poly(methyl methacrylate) (PMMA), when imaged with AFM after deposition from different solvents. After quaternization of parts of the P2VP segments and deposition from CHCl3 and H2O, strongly bent horseshoe and multiply bent, spiral/meander-type conformations were observed. The strong bending within the horseshoe structures was attributed to a basically quantitative phase separation along the main axis, whereas a patchy structure was ascribed to the meander-type patterns. Ishizu and co-workers reported imaging data on the side-by-side aggregation of cylindrical brushes obtained by polymerization of PS and PEO macromonomers during a very slow evaporation process of 1 week, starting from a THF–water solution. These observations indicated that a reorientation can occur and that a biphasic character can develop if the correct solvent conditions (selectivity to drive self-assembly), concentration regime, and time scale are provided. These experimental systems and simulations, however, point to some limitations of this strategy when aiming at robustly phase-segregated two-dimensional cylindrical Janus brushes with corona segregation along the main axis. Further below, we will describe how to create similar Janus cylinders with very high precision using the controlled crosslinking of triblock terpolymer bulk phases.
Advances in polymer synthesis, however, have allowed the synthesis of a different type of cylindrical Janus particles. These are characterized by a phase separation perpendicular to the main axis into a cylindrical AB-type diblock brush. Such structures are typically obtained by consecutive block polymerization of macromonomers or by the polymerization of AB diblock copolymers with orthogonally reactive moieties in both blocks permitting side-by-side polymer-analogous attachment of preformed polymer chains (grafting to) or the growth of polymers via (parallel) grafting from reactions. A variety of AB-type Janus cylinders have been reported with widely different physical properties. For instance, Rzyaev et al. described in great depth the synthesis and structure formation of PLAcomb-b-PScomb AB-type Janus brushes with total molecular weights exceeding 1 MDa. Since the dimensions of the resulting lamellar bulk phases of the stiff AB cylinders approached the wavelength of visible light, a photonic bandgap behavior and an opalescent appearance could be observed for solid samples. Direct visualization by AFM was reported by Matyjaszweski and Sheiko and co-workers for their poly(n-butyl acrylate)comb-block-PCLcomb (PnBAcomb-b-PCLcomb) AB diblock brushes (Figure 1.6) with amorphous and crystalline side-chains.
In addition, Deffieux et al. employed a multi-step reaction scheme to create PScomb-block-polyisoprenecomb (PScomb-b-PIcomb) with glassy and liquid-like side-chains, thus further broadening the property range. Exposure to selective solvents such as heptane induced self-assembly into homogeneous and nearly spherical micelles. Similar self-assembly behavior was also reported for AB diblock brushes composed of PS and poly(acrylic acid) (PAA) side-chains.
In addition to the stepwise synthesis of unimolecular polymer objects via classical polymer chemistry, the desymmetrization of particles at interfaces has also reached the field of polymer-based nanoscale Janus particles. In general, toposelective modifications of immobilized particles are widely used to break the symmetry of inorganic particles and have greatly impacted the synthesis of Janus objects.
In the context of soft nano-objects, Chen and co-workers reported a simple one-pot process, in which they used PEO-b-P4VP-stabilized yttrium hydroxide nanotubes (YNTs, diameter ~200 nm and length 3–4 µm) as the interface for symmetry breaking. After initial formation of the polymer-coated YNT rods due to the adsorption of P4VP segments on the YNT surfaces via hydrogen bonding, a mixture of a radical initiator [azobisisobutyronitrile (AIBN)] in divinylbenzene (DVB) and additional NIPAAm was added to the dispersion (Figure 1.7). Because of the solubility characteristics of the various compounds, AIBN and DVB accumulated in the P4VP phase, whereas NIPAAm remained dissolved in the continuous phase. Subsequent heating induced polymerization of the DVB in the confined space on the YNTs and nanoscopic, crosslinked polydivinylbenzene (PDVB) beads were formed due to the increased incompatibility of P4VP and PDVB polymers developing during the DVB polymerization. The radicals reaching the outer surface also initiated the NIPAAm polymerization, which led to the side-selective growth of PNIPAAm grafts, equaling an in situ desymmetrization. The collapsed state of the PNIPAAm during the thermal polymerization prevented the dissolution of the modified PDVB beads from the YNTs. After resting at room temperature, the PNIPAAm/PDVB Janus particles separated from the polymer-coated inorganic rods. Dynamic light scattering (DLS) and TEM revealed flower-like aggregates of the strongly amphiphilic Janus particles with a hydrodynamic radius <Rh>z = 320 nm, which could be dissociated by the addition of excess surfactant to yield isolated Janus particles with an average radius of 80 nm. Interestingly, the process might be suitable for different monomers and could potentially be cycled using the same polymer-coated YNTs.
Excerpted from Janus Particle Synthesis, Self-Assembly and Applications by Shan Jiang, Steve Granick. Copyright © 2012 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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