Soft, Nanoscale Janus Particles by Macromolecular Engineering and Molecular Self-assembly

ANDREAS WALTHER AND AXEL H. E. MÜLLER

1.1 Introduction

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...