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Isotactic polypropylene (iPP) exhibits polymorphism in the crystalline state that has long been leveraged to tune the material properties. It has been established that even the most prevailing α-iPP can exhibit significant variability with two distinct melting points Tmoften attributed to α1and α2forms. Several structural models with specific ordering in chain orientations and correlations along the crystals have been proposed. Precise structural investigation relies on inverse modeling of diffraction patterns, but the impact of thermal history, a potential transition between the two forms, and any existence of multiple solutions challenge characterization. Recent detailed atomistic simulations offered a direct route to link postulated structural arrangements to melting transitions with studies focusing on the α1and β forms of iPP. In this study, we employ atomistic molecular dynamics to examine models of α1and α2with variable levels of ordering in terms of chain axis orientation. At rapid heating rates, we find high melting points that can differ by up to 30 K depending on the fraction of the material that follows the P21/cstructural arrangement. To contrast systems in the absence of superheating, we assembled configurations with free surfaces as well as models of semicrystalline iPP. We report melting kinetics above Tmfor the P21/cand Cccrystals and provide Tmestimates in agreement with literature data. In addition, our study provides valuable microscopic information about the dynamics of melting of α-iPP along specific directions.

The achievement of sufficient dispersion of vulcanization accelerators is critical to tailoring superior cross-linked elastomers. Modern recipes rely on multicomponent formulations with silica particles covered by coupling agents. We study the molecular properties of select accelerators in polyisoprene melts and their affinity for functionalized surfaces via extensive all-atom molecular dynamics simulations. We focus on the common (N-cyclohexyl)-2-benzothiazole sulfenamide (CBS), 1,6-bis(N,N-dibenzylthiocarbamoyldithio)hexane (DBTH), and diphenyl guanidine (DPG) molecules and their mixing characteristics at curing temperatures. Our results support a low self-association affinity for CBS and DBTH within polyisoprene, whereas DPG forms small hydrogen-bonded aggregates. Subsequently, we examine systems in contact with silica interfaces, bare or grafted with (3-mercaptopropyl)triethoxysilane (MPTES), (3-octanoylthio) 1-propyl-triethoxysilane (NXT), and bis[3-(triethoxysilyl)propyl]disulfide (TESPD). Accelerator-substrate affinity is first assessed at infinite dilution using free energy calculations and subsequently at finite concentrations. Accelerators exhibit high substrate affinity (DPG > CBS > DBTH) irrespective of functionalization. However, coupling agents are able to displace from the surface a significant amount that increases with the grafting density and the size of the coupling agent. Finally, we investigate the behavior of DPG in binary DPG-CBS formulations, where the former can act as a covering agent that solubilizes CBS into the bulk polymer.

The conformations of polymer chains in poly(ethylene oxide)/silica nanoparticles, PEO/SiO2, nanohybrids have been investigated through a combined approach that involves molecular dynamics (MD) simulations and attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR) measurements. Systems with different polymer molecular weights, nanoparticle radii, and concentrations have been employed to investigate the effect of the confinement on polymer conformations across a variety of different conditions. Qualitatively similar behavior between experimental and simulation results is observed since in both cases an increase of gauchepopulation for the OCCO angle is attained, in comparison to the respective of the bulk. This increase becomes larger as the degree of confinement becomes higher. More specifically, both simulations and experiments indicate a corresponding progressive increase with the degree of confinement. On the contrary, the conformations of the C–O bond (COCC angle) seem to remain unaffected by the confinement, at least in the range of degrees of confinement covered computationally. In addition, chain dimensions in the nanocomposite are found to be slightly decreased compared to bulk, especially at low temperatures. This results in a reduced effective confinement that allows the polymer matrix to accommodate larger nanoparticle fractions.