Surface tension of homologous series of liquids

Thermodynamic Analysis of Brownian Motion-Induced Particle Agglomeration Using the Taylor-Series Expansion Method of Moments

On the basis of binary perfectly inelastic collision theory, the time evolutions of kinetic energy and surface area for a particle agglomerate system, due to Brownian motion, are investigated by using the Taylor series expansion technology. The asymptotic behaviors over a long time period show a significantly negative power function of time. The thermodynamic constraints of this system are then obtained according to the principle of maximum entropy, which establishes a relationship of inequality between the first three particle moments and some physical parameters (i.e., surface tension and temperature). In the thermodynamic equilibrium state, this function provides a new approach for estimating the effect of molecular structure on surface tension of liquid polymers.

Critical Contact Angle to Induce Capillary Rise of Polymers in Nanopores Does Not Depend on Chain Length

We study the effect of physical confinement on the capillary infiltration of polymers into cylindrical nanopores using molecular dynamics simulations. In particular, we probe whether the critical contact angle (θ_c) above which capillary rise infiltration ceases to occur changes for long-chain polymers, possibly due to loss of conformation entropy induced by chain confinement. Surprisingly, θ_c does not strongly depend on the length of polymer chains and stays constant for large N. A free energy model is developed to show that θ_c depends strongly on the size of statistical segments rather than N, which we confirm by performing MD simulations of infiltration with semiflexible polymers. These results could provide guidelines in manufacturing polymer nanostructures and nanocomposites using capillary rise infiltration.

Petascale Simulations of the Morphology and the Molecular Interface of Bulk Heterojunctions

Understanding how additives interact and segregate within bulk heterojunction (BHJ) thin films is critical for exercising control over structure at multiple length scales and delivering improvements in photovoltaic performance. The morphological evolution of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) blends that are commensurate with the size of a BHJ thin film is examined using petascale coarse-grained molecular dynamics simulations. Comparisons between two-component and three-component systems containing short P3HT chains as additives undergoing thermal annealing demonstrate that the short chains alter the morphology in apparently useful ways: they efficiently migrate to the P3HT/PCBM interface, increasing the P3HT domain size and interfacial area. Simulation results agree with depth profiles determined from neutron reflectometry measurements that reveal PCBM enrichment near substrate and air interfaces but a decrease in that PCBM enrichment when a small amount of short P3HT chains are integrated into the BHJ blend. Atomistic simulations of the P3HT/PCBM blend interfaces show a nonmonotonic dependence of the interfacial thickness as a function of number of repeat units in the oligomeric P3HT additive, and the thiophene rings orient parallel to the interfacial plane as they approach the PCBM domain. Using the nanoscale geometries of the P3HT oligomers, LUMO and HOMO energy levels calculated by density functional theory are found to be invariant across the donor/acceptor interface. These connections between additives, processing, and morphology at all length scales are generally useful for efforts to improve device performance.

Optical Properties and van der Waals - London Dispersion Interactions of Polystyrene Determined by Vacuum Ultraviolet Spectroscopy and Spectroscopic Ellipsometry

The interband optical properties of polystyrene in the vacuum ultraviolet (VUV) region have been investigated using combined spectroscopic ellipsometry and VUV spectroscopy. Over the range 1.5–32 eV, the optical properties exhibit electronic transitions we assign to three groupings, E1, E2, and E3, corresponding to a hierarchy of interband transitions of aromatic (π → π*), non-bonding (n → π*, n → σ*), and saturated (σ → σ*) orbitals. In polystyrene there are strong features in the interband transitions arising from the side-chain π bonding of the aromatic ring consisting of a shoulder at 5.8 eV (E1′) and a peak at 6.3 eV (E1), and from the σ bonding of the C–C backbone at 12 eV (E3′) and 17.1 eV (E3). These E3 transitions have characteristic critical point line shapes associated with one-dimensionally delocalized electron states in the polymer backbone. A small shoulder at 9.9 eV (E2) is associated with excitations possibly from residual monomer or impurities. Knowledge of the valence electronic excitations of a material provides the necessary optical properties to calculate the van der Waals–London dispersion interactions using Lifshitz quantum electrodynamics theory and full spectral optical properties. Hamaker constants and the van der Waals–London dispersion component of the surface free energy for polystyrene were determined. These Lifshitz results were compared to the total surface free energy of polystyrene, polarity, and dispersive component of the surface free energy as determined from contact angle measurements with two liquids, and with literature values. The Lifshitz approach, using full spectral Hamaker constants, is a more direct determination of the van der Waals–London dispersion component of the surface free energy of polystyrene than other methods.

A molecular structure based model for predicting surface tension of organic compounds

A Quantitative Structure-Property Relationship (QSPR) model for the prediction of surface tension of organic compounds was derived from a data set of 320 chemicals including N, O, F, Cl, Br, and/or S atoms and covering a range of about 14-45 dyn cm-1. The model, only involving six molecular descriptors obtained solely from the chemical structures, yielded an r2 of 0.96. Its predictive capability was estimated from an external test set containing 55 structures not considered in the training set (r2 = 0.94). It was shown that the selected molecular descriptors presented a physical meaning corresponding to the different intermolecular interactions occurring in the bulk solution. The model is applicable to a wider variety of compounds, includes less parameters and correlates better than other QSPR models reported in literature.