No abstract

A versatile approach to the synthesis of hybrid dendrimers composed of a rigid aromatic core and a flexible carbosilane shell is proposed. For this, carbosilane monodendrons of the first, second, and third generations were linked to a Muellen-type polyphenylene dendrimer via the copper-catalyzed azide−alkyne cycloaddition reaction. A series of hybrid dendrimers with a fixed phenylene core size and various carbosilane peripheries were synthesized to clarify the influence of a growing flexible shell on thermal and rheological dendrimer properties. Small-angle X-ray scattering experiments and density functional theory calculations revealed that the dendrimers form a columnar phase with hexagonal (G1, G2) and orthorhombic (G3) types of packing.

A series of carbosilane dendrimers of the 4th, 6th, and 7th generations with a terminal trimethylsilylsiloxane layer was synthesized. Theoretical models of these dendrimers were developed, and equilibrium dendrimer conformations obtained via molecular dynamics simulations were in a good agreement with experimental small-angle X-ray scattering (SAXS) data demonstrating molecule monodispersity and an almost spherical shape. It was confirmed that the glass transition temperature is independent of the dendrimer generation, but is greatly affected by the chemical nature of the dendrimer terminal groups. A sharp increase in the zero-shear viscosity of dendrimer melts was found between the 5th and the 7th dendrimer generations, which was qualitatively identical to that previously reported for polycarbosilane dendrimers with butyl terminal groups. The viscoelastic properties of high-generation dendrimers seem to follow some general trends with an increase in the generation number, which are determined by the regular branching structure of dendrimers.

Realization of self-healing polymer materials cannot rely on the wealth of active repair tools found in living systems but must focus entirely on the structural composition of the material and the properties of its constituents. Current challenges of the search for such compositions include healing of large-scale defects as well as the need for a healing process that is generated by the scission itself. Herein, we describe ionomer–rubber blends from poly­(ethylene-co-methacrylic acid) and peroxide cross-linked ethylene–propylene–diene monomer (EPDM) that combine three types of cross-links: covalent links of a network of EPDM, clusters of aggregated ionic groups, and crystalline domains of longer ethylene sequences in the ionomer. Above the melting point of the latter, the components mix homogeneously, indicated by the clarity of the samples and supported by small-angle X-ray scattering (SAXS) and NMR. At ambient conditions, the samples are hard like a thermoplastic material. Self-healing after mechanical damage is enabled by two types of structural memory related to a hierarchy of deformation- and defect-caused stresses and their relaxation paths. Because of the solid-like character of the materials, damage-caused stress is retained by the micro deformation and rupture of the aggregates on small scales and on large scaleby the macroscopic shape memory effect of the deformed covalent network. When the samples get annealed at an elevated temperature, the former enables mending of fracture-caused surfaces and the lattershape recovery. Based on a careful evaluation of the structural relaxation effects on the blends and their constituents (differential scanning calorimetry, NMR, and wide-angle X-ray scattering/SAXS), we demonstrate the repair of defects in the range of millimeters to centimeters by the defect-caused stresses. It is intrinsic to our concept that it holds only to damages such as scratches, small cuts, and microcracks, whereby the object is not fully fragmented, and that it will require thermal activation.