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Anionic polymerization: Arborescent and other graft polymers, telechelic polymers, block copolymers. Stable free-radical polymerization reactions. Emulsion polymerization. Ion-containing polymers: Ionomers, polyelectrolytes. Copolymerization and post-polymerization modification reactions. Polymer characterization: Applications of laser light scattering techniques, size-exclusion chromatography. Rheological properties and mechanical testing. Solution properties.


Arborescent Polymers

Our research efforts are currently focused on the synthesis and physical characterization of a new class of branched macromolecules, called arborescent polymers. The term arborescent (from the latin arbor = tree) refers to the tree-like or dendritic structure of the molecules. These compounds are obtained in successive reaction cycles of anionic polymerization and grafting. For example, grafting linear polystyrene side chains onto a linear polystyrene substrate yields a comb-branched (generation G=0) structure. Repetition of the grafting reaction leads to higher generation arborescent polymers of generations G=1, 2, etc. (M. Gauthier and M. Möller 1991, Macromolecules 24, 4548):

Arborescent polymer synthesis

Schematic representation of the synthesis of an arborescent polymer

Different types of polymer segments (other than polystyrene) can also be grafted on the substrate to yield copolymers with a layered or "core-shell" morphology (see for example M. Gauthier, L. Tichagwa, J.S. Downey and S. Gao 1996Macromolecules 29, 519; R.A. Kee and M. Gauthier 1997, Polym. Mater. Sci. Eng. 77, 114). This approach provides further control over the chemical and physical properties of arborescent polymers.


Arborescent copolymer synthesis leading to a layered morphology

The highly branched structure of arborescent polymers leads to very unusual properties in solution, in the solid state and in the molten state, making them interesting for many specialty applications. For example, the intrinsic viscosity [h ] of arborescent polymers is very low, in spite of their high molecular weight. Arborescent polymers in solution thus have a viscosity that is much lower that for linear polymers of comparable molecular weight. The plot below shows that while the viscosity of linear polymers increases exponentially with molecular weight, it remains relatively constant for arborescent polymers. This effect, discussed in more details in the original paper (M. Gauthier, W. Li and L. Tichagwa 1997, Polymer 38, 6363-6370), shows that arborescent polymers have a very rigid or hard sphere-like structure.

Intrinsic viscosity

Intrinsic viscosity of arborescent polystyrenes of successive generations. The curves, from top to bottom, are for polymers with side chains of 30 000 g/mol dissolved in toluene and in cyclohexane, and for side chains of 5 000 g/mol in toluene and in cyclohexane. The two straight lines are for linear polystyrene dissolved in the same solvents.

Another spectacular demonstration of the stiffness of arborescent polymers was found in atomic force microscopy (AFM) measurements of arborescent polymer monolayers deposited on mica (S. Sheiko, M. Gauthier and M. Möller 1997, Macromolecules 30, 2343-2349). Films prepared from linear polymers have an essentially flat surface, because of the very flexible structure of the molecules. Films obtained from arborescent polymers are shown below. A G=1 polymer with 190 side chains yields a granular surface (left), but it is difficult to distinguish the individual molecules because they are sufficiently flexible to interpenetrate each other. A film prepared from a G=3 polymer with 5600 side chains, in contrast, displays long range ordering characteristic of hard spheres of uniform diameter. The individual molecules are clearly visible in the close-packed array. These results show that it is possible to control the structural rigidity of arborescent polymers by design.


Atomic force microscopy pictures of monomolecular films of generation G1 (left) and G3 (right) arborescent polystyrenes. The width of each picture is 500 nm.

These examples illustrate some of the physical properties unique to arborescent polymers that make them a fascinating research topic. Our interest in these materials is threefold. Firstly, we need to develop synthetic methods allowing the synthesis of materials based on a range of monomers, using different grafting techniques. Secondly, structure-property relations are needed for arborescent polymers. This is useful not only to achieve a better understanding of the effects of branching on the physical properties of polymers, but also to allow the design of arborescent polymers with specific properties for targeted applications. Finally, we are also involved in the development of applications for these materials in a broad range of areas including microencapsulation, viscosity control additives, smart (pH-sensitive) gels, catalyst supports, etc.