Janus Triad - Janus particles inspired by an ancient Roman god

in technology

The two-faced Roman god Janus has inspired a class of unique particles that possess two different sides or surfaces of different chemistry or polarity. Researchers now report the synthesis of three different Janus topologies from one single specific triblock copolymer.

These so-called Janus particles display unusual properties as they combine features known from molecules (directionality, side-selective reactivity) with a solid particle character familiar from (nano)particles. For instance, it is possible to combine different functions within one particle and render one side water-soluble and the other one oil-soluble, hence furnishing particulate amphiphiles.
 

Janus cylinder -janus triad from copolymer
Janus cylinder with the asymmetric biphasic character visible. Image provided to ScienceDebate.com by Dr. Andreas Walther 

In fact, Janus particles are ideal for stabilizing emulsions and dispersions and are much better at this job than usual emulsifiers. Furthermore, they find use as highly specific biosensors, in which nanoscopic sensing and macroscopic detection are spatially separated, or for self-propelling carriers upon immobilization of catalytically active centers on one face.

Janus particles -Janus triad nanoparticles
Janus particles, a rendering.
Image credit: Dr. Andreas Walther

Despite progress in the synthesis of Janus particles, it has remained a real challenge to produce truly nanoscopic ones, in particular with non-spherical architectures and high homogeneity. A team of Bayreuth and RWTH Aachen Universities has now demonstrated a new and versatile pathway. Simply by exploiting their knowledge on the manipulation of nanostructured ABC triblock copolymer phases and the selective crosslinking of “B middle blocks” at the interface of A and C, it has become possible to synthesize a triad of non-spherical Janus particles from one single triblock copolymer: Janus sheets, Janus cylinders and Janus tapes.

Their research was recently published in the peer-reviewed journal Macromolecules.

As Dr. Walther, a corresponding author, adds: “It has never been easier for us to tailor the architecture of the Janus particles by just modifying the conditions for the crosslinking reactions. Earlier this required to precisely adjusting the triblock copolymer weight fractions by synthesizing different polymers. Now, this can be overcome and we can quickly produce a whole range of nanoscale Janus topologies with simplest means on a multigram scale. This enables us to look further into the application possibilities of these asymmetric particles and understand the effect of different geometries on the overall properties”.

Moreover, the authors could also demonstrate exciting images for proving the existence and stability of phase-segregation of nanoscale, polymeric Janus particles by imaging Janus cylinders with quasi in-situ cryogenic-TEM (an electron microscopy technique imaging thin vitrified specimens of colloidal dispersions). A suitable change of the contrast conditions for one of the corona sides using ions exclusively coordinating to one charged polyanion hemicylinder, allowed to selectively improve the contrast and visualize the partitioning within Janus cylinders.

These results show that the clever combination of advanced macromolecular engineering and the understanding of physics of self-assembly of polymer nanostructures can be used to drastically simplify the production of anisotropic soft nanoobjects with desirable properties.  “Research on further applications and on upscaling into the kilogram range is ongoing. For example, larger amounts are necessary to improve the mechanical properties of polymer blends by compatibilizing two immiscible polymers,” Prof. Müller adds. The scale-up is being pursued in his labs in Bayreuth.

Science Story Reference: 

Janus Triad: Three Types of Nonspherical, Nanoscale Janus Particles from One Single Triblock Terpolymer. Andrea Wolf, Andreas Walther, and Axel H. E. Müller. Macromolecules. DOI: 10.1021/ma2020408: Published online: November 3, 2011.

Additional Sources: 

Dr. Andreas Walther

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