Atomistic Monte Carlo and molecular dynamics simulation of the bulk phase self-assembly of semifluorinated alkanes

Interface and Interphase in Polymer Nanocomposites with Bare and Core-Shell Gold Nanoparticles

Metal nanoparticles are used to modify/enhance the properties of a polymer matrix for a broad range of applications in bio-nanotechnology. Here, we study the properties of polymer/gold nanoparticle (NP) nanocomposites through atomistic molecular dynamics, MD, simulations. We probe the structural, conformational and dynamical properties of polymer chains at the vicinity of a gold (Au) NP and a functionalized (core/shell) Au NP, and compare them against the behavior of bulk polyethylene (PE). The bare Au NPs were constructed via a systematic methodology starting from ab-initio calculations and an atomistic Wulff construction algorithm resulting in the crystal shape with the minimum surface energy. For the functionalized NPs the interactions between gold atoms and chemically adsorbed functional groups change their shape. As a model polymer matrix we consider polyethylene of different molecular lengths, from the oligomer to unentangled Rouse like systems. The PE/Au interaction is parametrized via DFT calculations. By computing the different properties the concept of the interface, and the interphase as well, in polymer nanocomposites with metal NPs are critically examined. Results concerning polymer density profiles, bond order parameter, segmental and terminal dynamics show clearly that the size of the interface/interphase, depends on the actual property under study. In addition, the anchored polymeric chains change the behavior/properties, and especially the chain density profile and the dynamics, of the polymer chain at the vicinity of the Au NP.

Shape segregation in molecular organisation: a combined X-ray scattering and molecular dynamics study of smectic liquid crystals

Temperature-dependent X-ray scattering studies have been carried out on 4-undecyloxy-4'-cyanobiphenyl (11OCB) and 4-(12,12-dimethyltridecyloxy)-4'-cyanobiphenyl (t-Bu-11OCB) in the smectic A phase, from which their layer spacings and translational order parameters were obtained. Marked differences between the layer structures of the two compounds were demonstrated, showing that the addition of the t-Bu group results in a higher translational order parameter and wider layer spacing for t-Bu-11OCB than 11OCB. Fully atomistic MD simulations of both compounds run for >1000 ns demonstrated the spontaneous formation of smectic mesophases from isotropic starting geometries, and experimental trends in order parameters and absolute layer spacings were shown to be replicated well. Further analysis showed that both the aromatic head-groups and the alkyl tail-groups exhibit interdigitation in the simulated smectic phases of both compounds, and the difference in the layer structures between 11OCB and t-Bu-11OCB could be attributed mainly to a shape segregation effect arising from the addition of the bulky t-Bu end-group to the alkyl chain.

Molecular simulation of the high temperature phase behaviour of α-unsubstituted sexithiophene

We examine the high-temperature phase behaviour of α-unsubstituted sexithiophene (α-6T) by means of Molecular Dynamics (MD) and Monte Carlo (MC) simulations using a recently developed state-of-the-art algorithm based on internal bridging moves. In the MD simulations, a realistic fully flexible united-atom model is used. In the MC simulations, a stiffer version of this united-atom model is implemented by restricting atoms on thiophene rings to remain strictly co-planar by employing holonomic constraints for all bond lengths and intra-ring bond bending angles; on the other hand, inter-ring torsion and bond bending angles are considered to be fully flexible subject to suitable potential energy functions. The MD simulations, which are started from the isotropic (Iso) phase at a relatively high temperature (above 700 K) and continued to lower temperatures under isobaric conditions using a very large simulation cell containing 8960 α-6T molecules, show four phase transitions: an isotropic-to-nematic (Iso-to-Nem) at 640 K, a nematic-to-smectic A (Nem-to-SmA) at 630 K, a smectic A-to-smectic C (SmA-to-SmC) at 620 K demonstrating smectic polymorphism, and a SmC-to-crystal-like (SmC-to-Cry) at 600 K. In the corresponding MC simulations, no Nem phase is observed; the system, as it is isobarically cooled down to lower temperatures from its Iso phase, undergoes directly a transition to a SmC phase at 690 K. This is attributed to the stiffer nature of the forcefield employed in these simulations. Both methods (MD and MC) shed light on the type and degree of molecular self-assembly, orientational and positional ordering as a function of temperature, and manifestation of liquid crystalline behaviour of α-6T. We provide a thorough characterization of structural ordering in all mesophases observed, in terms of several measures (radial correlation functions, orientational order parameters and X-ray diffraction patterns). According to the results, at the phase transition temperatures, drastic configurational changes take place driving α-6T molecules to positionally-ordered phases accompanied by self-assembly into characteristic layers which, in turn, are self-organized into macroscopic smectic phases. Our methodology opens up the way to exploring the rich phase behaviour and anisotropic ordering of the condensed phases of several longer (and perhaps more complex) thiophene-based polymers.

Alteration of intermolecular interactions between units of asphaltene dimers exposed to an amide-enriched modifier

The modified performance of bio-modified asphalt-binder is attributed to the alteration of intermolecular interactions between units of asphaltene stacks exposed to the bio-additive.