Linear and nonlinear rheological properties of self-assembling tectons in polypropylene matrices

The supramolecular assembling of small, equimolar amounts of octyl-substituted complementary tectons based upon barbituric acid (BA) and 2,4,6-triaminopyrimidine (TP) in an isotactic and, particularly, in an atactic polypropylene matrix was studied by means of dynamic-mechanical spectroscopy. Self-organization of these tectons within the matrix depends on temperature and concentration of tectons and leads to network-like behavior, which is accompanied by drastic changes of the melt rheology of these compounds in comparison to the pure matrix. In addition to external parameters such as temperature, the formation of a tecton network which sets in for a concentration of 2 wt % and higher is determined by the enthalpic interactions between matrix and tectons. It turned out that none of the thermorheological and viscoelastic properties of the compounds depend on tacticity. Strain-sweep experiments reveal the extreme shear sensitivity of the formed supramolecular structures. Moreover, these experiments show that in all samples a certain amount of tecton (approximately 1%) remains dissolved and thus constitutes an "effective matrix" in which the network formation occurs. An estimation of the critical deformation, which is on the order of magnitude of 10(-4), was achieved by means of creep experiments.