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Research Guenet

Making of polymer/self-assembled system hybrid materials

We have developed the making and characterization of new molecular architectures consisting of covalent polymers and functional fibrillar organogels [1]. As a rule, these organogels cannot be processed into materials on account of their poor mechanical properties as their fibrillar structure is chiefly obtained at rather low concentrations. The use of a covalent polymer matrix can be an appropriate choice for retaining the original organogel fibrillar structure while allowing a better tractability of the system. The originality of our approach consists in using thermoreversible gels of co-valent polymers together with physical processes for achieving the fine, mesoscopic dispersion of the orga¬nogels into the polymer network. This is made possible because these covalent polymer gels also possess a similar fibrillar morphology, particularly an average mesh size in the micrometer range. While these novel molecular architectures may be relevant for applications in functional materials the primary goal of these investigations is to establish a proof of concept.

Two systems have been considered : intermingled gels where the organogel grows within the polymer gel with little perturbation ; and “sheathed” fibrils where nanotubes encapsulate polymer gel fibrils.

Figure 1 : Sketch of the two types of hybrid systems.

The feasibility of intermingled gels has been investigated as a function of polymer, organogel concentration and temperature, by differential scanning calorimetry (DSC), optical microscopy, and AFM [2]. The organogel was made up from π-conjugated oligo (phenylene vinylene) (OPV) while the polymer gel was prepared from isotactic polystyrene (iPS) or syndiotactic polystyrene (sPS). OPV organogels change colour at the SOL-GEL transition (yellow to green).
We have observed that a mesoscopic dispersion of one gel into another could be achieved[1] provided the mesh size of either gel is in the micrometer range (see Figure 2). For smaller mesh size, microscopic phase separation occurs. These results were confirmed by SAXS and SANS. Basically, we can keep the organogel concentration rather low (0.004 g/cm3), and so retaining the optical property while increasing dramatically the elastic modulus of the ternary gel. This process is totally reversible as the organogel can be melted and then reformed by cooling without altering the polymer gel structure.

Figure 2 : AFM image of an intermingled gel [1]. Organogel= large cross-section fibrils, polymer gel= small cross-section fibrils.

The fact that the organogel structure depends hgihly on the end groups of the OPV molecules and the solvent type opens up new horizons for the making of taylor-made materials [3,4].
Sheathing polymer fibrils, namely encapsulation of polymer gel fibrils by nanotubes (synthesis by P. Mésini [5] see below) has been investigated by DSC, AFM and scattering techniques, SANS and SAXS. These nanotubules are of very low size polydispersity as oscillations are observed in the scattering curve that can be asigned to the scattering by a hollow cylinder.

Figure 3 : left : AFM image of an array of nanotubules. The formation of the nanotubules arises from the wrapping of ribbons, which produces this ringlets aspect. ; right : SAXS curve with a fit obtained by considering a hollow cylinder and a structure maximum.

We have observed by DSC that the formation exotherm and the melting endotherm of the nanotubes vanished when incorporated in a ternary system with iPS. Although the nanotube structure was still present in the ternary gel as ascertained by SANS, their formation was clearly concurrent with the polymer gel formation. We have concluded that nanotube formation is nucleated by iPS gel fibrils. AFM experiments have shown that the iPS fibril cross-section increased under these conditions, which is consistent with a sheathing process. SANS experiments have shown that only those fibrils with a diameter in register with the inner diameter of the nanotubes are sheathed [6].
This sheathing process is a low-energy path for physically modifying fibrils surface. This may have applications whenever finely dispersed media are required (e.g. catalysis, filtering membranes for capture of pollutants at the molecular level,.. ;).
In these two systems, the solvent used for gel formation is extracted by supercritical CO2, a process which preserves the original morphology and opens the way for preparing solid functional materials.

[1] J.-M. Guenet
Nouveaux types de matériaux hybrides : polymère/système auto-assemblé
Matériaux et Techniques 2011 98, 329

[2] D. Dasgupta, S. Srinivasan, C. Rochas, A. Ajayaghosh, J. M. Guenet
Hybrid thermoreversible gels from covalent polymers and organogels -Langmuir 2009 25 8593

[3] D. Dasgupta, S. Srinivasan, C. Rochas, A. Thierry, A. Schröder, A. Ajayaghosh, J. M. Guenet
Insight into the gelation habit of oligo(para-phenylene vinylene) derivatives : effect of end-groups
Soft Matter, 2011, 7, 2797

[4] D. Dasgupta, S. Srinivasan, C. Rochas, A. Ajayaghosh, J.M. Guenet
Solvent-mediated fiber growth in organogels
Soft Matter 2011, 7, 9311.

[5] N. Díaz, F.-X. Simon, M. Schmutz, M. Rawiso, G. Decher, J. Jestin and P. Mésini
Angew. Chem., Int. Ed. Engl. 2005, 44, 3260

[6] D. Dasgupta, Z. Kamar, C. Rochas, M. Dahmani, Ph. Mésini, J.M. Guenet
Design of hybrid networks by sheathing polymer fibrils with self-assembled nanotubules
Soft Matter 2010 6 3576

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