Project B1: Synthesis and self-assembly of branched macromolecular nanohybrids

Principal Investigators: R. Mülhaupt (Freiburg) / P. Lutz (Strasbourg)
Collaborators: G. Reiter (Freiburg)
PhD Student: Felix Kirschvink

Current state of the research

In recent years inorganic-organic compounds have emerged as an important category of new materials. The interest in their development results from the fact that they combine the unique behavior of organic polymers with the good physical properties of ceramics [1-3].  Octafunctionalized cubic silsesquioxanes are often used as nanosized building blocks in such materials, leading to novel hybrid nanocomposite structures. However, little is known about the synthesis, self-assembly, and properties of semicrystalline hybrid materials containing polysilsesquioxane and graphenes to which crystallizable polymer segments are connected.

Therefore, this project focuses on the synthesis and crystallization behavior of new nanometer scaled organic/inorganic macromolecular nanohybrids with core/shell-topologies containing inorganic core molecules and crystallizable isotactic PS (iso-PS) shells, which are connected to the core either via covalent or ionic bonds. Core molecules include functionalized graphenes and both rigid and flexible polysilsesquioxanes. The degree of branching will be varied systematically with the topology variation from star-shaped to hyperbranched molecular architectures. An important objective of the study is to elucidate the crystallization behavior of iso-PS at the interfaces in the confined environments of such multiphase materials.

Contributions of the participating groups

The group of R. Mülhaupt has developed recently a new strategy providing access to iso-PS of controlled molecular weight and functionality [4]. The molecular weight of the PS chain could be varied, and advantage of chain transfer reactions was taken to fit the chains with various functions, including vinyl end groups. The group has also pioneered the shear-induced crystallization of olefin-terminated iso-polystyrenes which represent very versatile new intermediates, useful in block copolymer synthesis. In contrast to the well known syndiotactic PS, which crystallizes during polymerization and is highly insoluble, the iso-PS remains in solution during polymerization and can be crystallized during processing. This new approach offers attractive perspectives for this research. The Mülhaupt group has also established a versatile route to functionalized graphenes [5] which will be employed as core materials, too. At the FMF in Freiburg extensive experience exists with respect to scale-up, reaction engineering, polymer characterization, polymer processing, and materials testing.  The group of P. Lutz has a long-standing experience in the synthesis and characterization of various types of complex macromolecular architectures: macromonomers [6] as well as stars or branched polymers, including star-shaped polymers with octafunctional silsesquioxane cores [7]. In recent years, the Mülhaupt and Lutz groups have combined their competences to design and study a new class of amphiphilic hydrid materials with silsesquioxane cores [8] and well-defined poly(ethylene oxide) star-shaped polymers. The characterization of the crystallization of iso-PS in the synthesized multiphase materials will be carried out in collaboration with the group of G. Reiter in Freiburg, who is one of the leading experts in the domain of the crystallization of (co-)polymers.

Research project and collaborations

The main aim of this joint project will be to investigate the synthesis, the self-assembly, and the solid state characterization of new macromolecular nanohybrid materials with core/shell topology and controlled degree of branching, containing polysilsesquioxane and graphene cores, to which are connected, via covalent and ionic bonds, shells comprising crystallizable polymers, like iso-PS. Among the different sphero-silsesquioxanes, preference will be given to the octafunctional compound (Q8M8H). In a further step, the reaction will be extended to compounds of higher functionality.

If an ω-vinyl iso-PS macromonomer is reacted by hydrosilylation with Q8M8H, chemical links are formed between the precursor chains and the sphero-silsesquioxane compound. The former becomes the branch and the latter the core of the star-shaped polymer named Q8M8isoPS. Polycondensation of trialkoxysilane-terminated iso-PS, mediated by acetic anhydride, will lead to novel hyperbranched polysilsesquioxanes and silicones containing iso-PS side chains. In addition to covalent iso-PS attachment, novel iso-PS with cationic imidazolium end groups will be prepared and attached to carboxylate-functionalized graphenes via polyelectrolyte complex formation. Some typical structures to be obtained by covalent and ionic grafting-onto reactions are illustrated in Figure 1. The controlled crystallization of iso-PS in confined environments of such nanophase-separated hybrid polymers and in blends with iso-PS-block copolymers is expected to lead to novel micro- and nanostructured materials including engineering plastics and thermoplastic elastomers.

Figure 1 Typical structures obtained by grafting of iso-PS macromonomers onto octafunctional silsesquioxanes (on the left) and graphene (on the right).

The Mülhaupt group will take care of the synthesis of the iso-PS and the grafting of the iso-PS onto graphenes. The contribution of the Strasbourg group will be the synthesis and dilute solution characterization of hybrid macromolecular complex star-shaped architectures constituted of iso-PS branches and sphero-silsesquioxane cores of various functionalities. The AFM, TEM and Maldi-Tof studies will be made in Freiburg. Dynamic light scattering studies in selective solvents will be made in Strasbourg, and rheological studies in Freiburg.

 

References
[1]Sellinger, A., Weiss, P. M., Nguyen, A., Lu, Y. F., Assink, R. A., Gong, W. L., Brinker, C. J., Nature, 394, 256, 1998.
[2]Huang, J., Lim, P. C., Shen, L., Pallathadka, P. K., Acta Materiala, 53, 2395, 2005.
[3]Laine, R. M., Zhang, C., Sellinger, A., Viculis, L., Appl. Organometal. Chem., 12, 715, 1998.
[4]Gall, B. T., Pelascini, F., Ebeling, H., Beckerle, K., Okuda, J., Mülhaupt, R. Macromolecules, 41, 1627, 2008.
[5]Steurer, P., Wissert, R., Thomann, R., Mülhaupt, R. Macromol. Rapid Commun. 30, 316, 2009.
[6]“Synthesis of Macromonomers and Telechelic Oligomers by Living Polymerizations”. Boutevin, B., Boyer, C., David, G., Lutz, P.-J. In Macromolecular Engineering, Gnanou, Y., Leibler, L.,  Matyjaszewski, K. Eds. Wiley VCH 2007.
[7]Harris, H., et al., Polym. Prep., San Francisco, (USA), 47, 714, 2006.
[8]Knischka, R., Dietsche, F., Hanselmann, R., Frey, H., Mülhaupt, R., Lutz, P. J., Langmuir, 15, 4752, 1999.