Polybutadiene Vitrimers with Tunable Epoxy Ratios: Preparation and Properties

Traditional crosslinked diene rubber has excellent thermal–mechanical properties and solvent resistance, yet it is incapable of being recycled via universal molding or injecting. Vitrimers, a new class of covalently crosslinked polymer networks, can be topologically rearranged with the associative exchange mechanism, endowing them with thermoplasticity. Introducing the concept of vitrimers into crosslinked networks for the recycling of rubbers is currently an attractive research topic. However, designing tailored rubber vitrimers still remains a challenge. Herein, polybutadiene (PB) vitrimers with different structures were prepared via partial epoxidation of double bonds and ring-opening esterification reactions. Their mechanical and relaxation properties were investigated. It was found that the increasing crosslinking density can increase tensile strength and activation energy for altering the network topology. The influence of side-group effects on their relaxation properties shows that an increase in the number of epoxy groups on the polybutadiene chain can increase the chance of an effective exchange of disulfide units. This work provides a simple network design which can tune vitrimer properties via altering the crosslinking density and side-group effects.

Effect of modification on the physical, mechanical and relaxation properties of the polymer - nanodispersed graphite system

The paper analyzes the physical, mechanical and relaxation properties of the polychlortrifluoroethylene (PCTFE) - modified nanodispersed graphite (TEG) system. It has been shown that the modification of the surface of conducting carbon nanoplates with ultradispersed dielectric silicon dioxide (SiO2) (30%) leads to a nontrivial effect - an increase in the electrical conductivity of the PCTFE - 2.5% TEG/30% SiO2 composite by more than two orders of magnitude in comparison with the PCTFE - TEG composites, containing unmodified carbon nanoplates. The functionalization of carbon nanoplates was carried out by treating the particle surface with an active solution of chlorosilane in an organic solvent, that in case of hydrophobization of the filler surface, leads to an increase in the system percolation threshold. It is shown that the method of hydrolyzation of the filler surface can be highly effective due to a directed change in some, in particular, shielding, properties of polymer composites based on nanocarbon. It is established that the modification of the nanofiller (dispersed thermoexpanded graphite) increases the intermolecular interaction of the filler-matrix. Depending on the concentration of the filler, the structure of the matrix and the system as a whole demonstrates dynamic transformations in the size of the heterogeneity of the structure. Ultrasonic studies of composites have shown that the smallest size of structure inhomogeneity is achieved in the range of concentrations of percolation development, and the change in the size of system structure inhomogeneity is associated with the transition from inhomogeneity as the size of crystallites activated by nanofiller at low concentrations associated with coagulation of nanoparticles at concentrations exceeding the percolation threshold. Thus, in the case of nanofillers, it is impractical to use concentrations that significantly exceed the percolation threshold, as this leads to coagulation of the filler particles and the relative loosening of the matrix.

Mutual Information in Molecular and Macromolecular Systems

The relaxation properties of viscous liquids close to their glass transition (GT) have been widely characterised by the statistical tool of time correlation functions. However, the strong influence of ubiquitous non-linearities calls for new, alternative tools of analysis. In this respect, information theory-based observables and, more specifically, mutual information (MI) are gaining increasing interest. Here, we report on novel, deeper insight provided by MI-based analysis of molecular dynamics simulations of molecular and macromolecular glass-formers on two distinct aspects of transport and relaxation close to GT, namely dynamical heterogeneity (DH) and secondary Johari–Goldstein (JG) relaxation processes. In a model molecular liquid with significant DH, MI reveals two populations of particles organised in clusters having either filamentous or compact globular structures that exhibit different mobility and relaxation properties. In a model polymer melt, MI provides clearer evidence of JG secondary relaxation and sharper insight into its DH. It is found that both DH and MI between the orientation and the displacement of the bonds reach (local) maxima at the time scales of the primary and JG secondary relaxation. This suggests that, in (macro)molecular systems, the mechanistic explanation of both DH and relaxation must involve rotation/translation coupling.