Molecular tectonics. Disruption of self-association in melts derived from hydrogen-bonded solids

An effective strategy for making new ordered molecular materials by design is to build them from subunits that associate with their neighbors and thereby hold them in predetermined positions. Such tectons can be made by attaching multiple peripheral sites of association to geometrically suitable cores. In tectons 3-5, for example, simple triaminotriazine groups grafted to a nominally tetrahedral tetraphenylmethyl core form intermolecular hydrogen bonds according to reliable motifs, and crystallization thereby leads to the formation of porous three-dimensional networks. Functionalization of the triaminotriazine groups with alkyl chains gives rise to amorphous tectons, and the increase of the chain length lowers the glass transition temperatures of these materials, allowing their association in the melt to be studied by rheological methods. The rheological properties depend in a characteristic way on the length of the alkyl substituents. In particular, viscosity and T-g both decrease with increasing length of the alkyl chains in the series of tectons 7-10, suggesting that intertectonic hydrogen bonding is increasingly diminished by the steric effects of the substituents. In this way, formation of extended hydrogen-bonded networks becomes gradually unfavorable, and mobility and fluidity are enhanced. For these reasons, melts derived from tectons 7-10 do not behave like conventional linear polymers or dendrimers, but rather form a novel class of materials with unique properties of their own.