JCJC SIMI 8 - JCJC - SIMI 8 - Chimie du solide, colloïdes, physicochimie

Supramolecular Janus nanocylinders formed by self-assembly of hydrogen bonding amphiphilic block copolymers – SupraJanus

Cylindrical nanoparticles with two faces formed by guided self-assembly

The objective of this project was to prepare nano-sized particles having a cylindrical shape and two faces of different chemical nature by hierarchical self-assembly of much simpler molecules.

How can complex nano-objects be obtained following a simple process and why do that?

This project focused on the development of so-called «Janus« nanocylinders which are very small cylinders having two faces of different chemical natures (in reference to the Roman god Janus who had two faces).<br />Such structures are very complex to obtain following conventional synthetic routes where the entire nanocylinder corresponds to a single molecule. The approach proposed here was to mimick nature and prepare such complex particles by supramolecular self-assembly; that is by synthesizing simple molecules designed to recognize each other via weak interactions and self-assemble into the desired complex structures.<br />These Janus nanocylinders could be used as emulsion stabilizers. They are indeed anistropic particles which can be made amphiphilic (one side may be hydrophilic and the other hydrophobic); which would make them much more efficient emulsion stabilizers than conventional molecular surfactants.<br />

This project aimed at developing molecules consisting of a core able to form self-complementary hydrogen bonds and which would be decorated on both sides with polymer arms. As shown in Figure 1, the hydrogen bonds lead to a one-dimensional stacking of the molecules into cylinders decorated by polymeric arms. The incompatibility between the two different polymer arms should then transform the cylinders into Janus particles.
Advanced synthesis techniques were used to obtain the self-assembling molecules. Their self-assembly in organic solvent was studied by microscopy and scattering techniques. These techniques can be used to estimate the morphology and dimensions of nano-sized objects. Finally, the Janus character of the cylinders; that is their propensity to have two faces of different chemical natures, was analyzed by NOESY NMR, a technique giving information about the respective positions of atoms at the nano-scale.

This project initiated a new research activity focused on the self-organization of polymers by hydrogen bonds within our research unit and led to the development of new collaborations with 3 academic research teams.
In this context, self-assembling molecules were synthesized using advanced synthetic techniques. It was shown that their self-organization in solution results from the competition between the favorable establishment of hydrogen bonds and the unfavorable hindrance of the polymeric arms, in agreement with theoretical predictions in the literature. Very long cylinders (> 500 nanometers in length for a few nanometers in diameter) were obtained using sufficiently strong hydrogen bonds.
NMR NOESY however revealed that the phase separation of the polymer arms is not perfect and therefore that the Janus character of the cylinders is not as clear-cut as expected. This problem is currently being addressed within the frame of the collaborations initiated by this project.

During this project, nano-cylinders could be obtained by self-assembly of simple molecules although the Janus nature of these cylinders was not as strong as expected. Moreover, the project initiated new national collaborations and allowed the identification of new research directions. These research directions will be followed in order to obtain real Janus particles which should be able to stabilize emulsions very efficiently. Such particles may have applications in the cosmetic field for example.

The results of this project have been disseminated in three international conferences, two national ones and led to the publication of two scientific articles in Macromolecules (journal impact factor = 5.9). These two articles highlighted the parameters that control the formation of long nanocylinders by supramolecular self-assembly. A third article, currently being finalized, will focus on the Janus character of the nanocylinders.

In this project, we aim at designing Janus nanocylinders via supramolecular self-assembly of amphiphilic block copolymers. Janus particles in general are solid colloids consisting of two sides with different chemistry or polarity. Micron-sized spherical Janus particles have attracted much attention recently because they can be prepared in large quantities and could be used for many applications (emulsion stabilizers, probes, sensors, two-colour display panels...). On the contrary, Janus nanocylinders are very difficult to prepare and have little been studied in spite of their high potential. Their anisotropic shape and nanometric dimensions would indeed make them very efficient emulsion stabilizers because large interfaces could be covered with small quantities. Moreover, in a selective solvent for one of their faces, Janus nanocylinders have been observed to form long superstructures which should exhibit interesting rheological properties (shear thickening, yield stress). It is thus relevant to search for efficient methods for the preparation of Janus nanocylinders. Our approach consists in synthesising amphiphilic diblock copolymers containing a hydrogen bonding unit in their middle and subsequently promote their self-assembly into Janus cylinders in solution. This approach is not demanding in terms of synthetic efforts because the only prerequisite is the preparation of the amphiphilic diblock copolymers. Then, directional self-assembly of the hydrogen bonding units will lead to the formation of supramolecular nanocylinders, whereas phase segregation of the two incompatible blocks will turn the cylinders into Janus structures.
The hydrogen-bonding amphiphilic diblocks will be prepared by a combination of controlled radical polymerization and organic chemistry, leading to very well defined molecules. Then, their self-assembly will be studied in different conditions. In a non-selective non-polar solvent, where hydrogen bonds are promoted but solvophobic interactions are limited, the diblocks should form supramolecular cylinders. The structural characteristics of the cylinders will be studied via scattering techniques (dynamic and static light scattering, small angle neutron and X-ray scattering), whereas the thermodynamic parameters of the self-assembly will be investigated via isothermal microcalorimetry, quantitative Fourier Transformed Infrared Spectroscopy or viscosimetry. A key advantage of our strategy comes from the supramolecular nature of the cylinders: the structural characteristics (length, diameter, rigidity, asymmetry...) of the self-assembled cylinders can be controlled in situ in a dynamic way (that is reversibly and rapidly) by playing with various stimuli such as the temperature, polymer concentration, nature of the solvent or addition of other hydrogen bonding molecules. Once the influence of all these parameters will have been studied in a non-selective solvent, the self-assembly will be studied in non polar but selective solvents. Secondary solvophobic interactions will then develop, leading to the formation of large superstructures in solution. The structural characteristics of these superstructures will be compared to those of the individual cylinders and their rheological properties will be investigated in parallel. Finally, the amphiphilic block copolymers will be used to stabilize liquid-air or liquid-liquid interfaces. State-of-the-art techniques developed in our laboratory will be employed to establish how these self-assembling molecules modify interfacial properties.

Project coordination

Olivier Colombani (UNIVERSITE DU MAINE) – olivier.colombani@univ-lemans.fr

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

UMR6120 UNIVERSITE DU MAINE

Help of the ANR 194,449 euros
Beginning and duration of the scientific project: September 2011 - 36 Months

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