The research conducted during the second stage of the project
The project is aimed at studying the mechanisms of intermolecular interactions of high-generation dendrimers in melts and concentrated solutions on the basis of a complex approach, including experimental and theoretical studies. At the second stage of the project, in the course of experimental studies of the features of the rheological behavior of melts of hybrid carbosilane dendrimers with a siloxane shell of the fourth (G4), sixth (G6) and seventh (G7) generations, a sharp increase in viscosity was found during the transition from G4 to G6. It is shown that melts of dendrimers G4 are Newtonian liquids, while G6 and G7 exhibit behavior typical of solids. A similar jump in viscosity was observed earlier for melts of polybutylcarbosilane dendrimers. The replacement of butyl groups with siloxane segments did not lead to a qualitative change in the rheological behavior of melts, which may indicate the universality of the phenomenon of liquid-solid transition with an increase in the generation number, which is based on the specific tree-like structure of dendrimer macromolecules. Full-atom models of hybrid carbosilane-siloxane dendrimers were created, computer simulations of single dendrimers of the 4th, 6th and 7th generations were carried out. Good agreement between the simulation data and small-angle X-ray scattering data confirmed the correctness of the developed models.
Computer simulations modeling of the conformational behavior of silicone-containing dendrimers under various external influences, depending on the generation, is continued. In particular, within the framework of full-atom modeling, the mechanisms of adsorption of dendrimers G4-G7 of four homologous series were studied: two types of carbosilane dendrimers with three- and four-functional cores and two types of siloxane dendrimers with different lengths of spacers. It was shown that carbosilane dendrimers above G4 mainly interact with the surface by the outer layers, keeping the conformations of the inner layers unchanged, while siloxane dendrimers of the 6th generation are adsorbed mainly by the inner region, spreading over the surface. Siloxane dendrimers with a short spacer containing only one oxygen atom have a dense molecular structure and only slightly change their size in the range of the studied adsorption energies. The behavior of G7 dendrimers is practically independent of the nature and structure of the dendrimer; a dense molecular structure plays a key role in their adsorption. To study the behavior of carbosilane dendrimers under uniaxial compression, which began at the previous stage of the project in the PUMA package, the system parameters were translated for further modeling in the GROMACS software package for analyzing the system dynamics at large times.
Simulation of 4-3 polybutylcarbosilane dendrimer melts from the 4th to 7th generations in a wide temperature range from 300 to 600 K in a cell with 27 molecules was continued, and new series of computational experiments were performed for dendrimer melts with different initial packing: body-centered cubic (in calculation cell of 16 molecules or 8 crystallographic cells), face-centered cubic (32 molecules or 8 crystallographic cells), hexagonal dense (27 molecules or 27 oblique crystallographic cells). Simulation of 8 ensembles was carried out in the PUMA package, and long trajectories from 150 to 300 ns were obtained in the GROMACS package. It has been shown that dendrimers are able to adapt to the external environment and uniformly fill all available space without leaving cavities.
Based on the results of the work at this stage, two articles were prepared, which were submitted for publication to Polymers.
