A method is developed for the detailed atomistic modeling of well-relaxed amorphous glassy polymers. Atactic polypropylene at -40 °C is used as an example. The model system is a cube with periodic boundaries, filled with segments from a single "parent" chain. An initial structure is generated by using a modified Markov process, based on rotational isomeric state theory and incorporating long-range interactions. This structure is then "relaxed" by potential energy minimization, using analytical derivatives. Computing time is kept relatively small by stagewise minimization, employing a technique of "blowing up" the atomic radii. Model estimates of the cohesive energy density and the Hildebrand solubility parameter agree very well with experiment. The conformation of the single chains in the relaxed model system closely resembles that of unperturbed chains. Pair distribution functions and bond direction correlation functions show that the predominant structural features are intramolecular and that long-range orientational order is completely absent.

Large numbers of conformations of unperturbed polypropylene chains are generated in Monte Carlo experiments, based on a rotational isomeric state scheme, and the average instantaneous shape in the system of principal axes of gyration is evaluated. Several new shape measures are introduced to characterize the shape anisotropy, asphericity, and acylindricity. Significant differences are found between shortand medium-length chains of different tacticity, while for long chains all shape measures converge to a common limit. The detailed three-dimensional segment density distributions are examined, and they are found to be bimodal along the longest principal axis of gyration. The loci of highest segment density are always two clearly separated domains, not containing the center of gyration, and they lie on the major principal axis separated by ca. 1.3(s2)o1/2-2.0(s2)01/2. The core of the segment distribution of an unperturbed chain is therefore dumbbell-like in shape.

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Structure and reactivity often are dependent on the polarity of chemical bonds. This relationship is reflected by atomic charges in classical (semiempirical) atomistic simulations; however, disagreement between atomic charges from accurate experimental investigations, ab initio methods, and semiempirical methods has not been resolved. Our aim is to improve the basic understanding of the polarity of compounds with a view to make force-field parametrizations more consistent and physically realistic. The concept is based on the relationship between the atomization energies of the elements and the possible strength of covalent bonding and the relationship between the ionization energies/electron affinities of the elements and the possible strength of ionic bonding. Both quantities, energetically, are of the same order of magnitude and influence atomic charges in a compound, which we illustrate by trends across the periodic table. The relationship between the pure elements and a given compound is shown in an extended Born model. We note that the extended Born model can be used to obtain physically justified charge estimates, relative to available reference compounds. This semiempirical concept has a stronger foundation than electronegativity equalization [Rappe, A. K.; Goddard, W. A., III. J. Phys. Chem. 1991, 95, 3358−3363], which is based on isolated gas-phase atoms and does not include covalent bonding contributions. We demonstrate the assignment of atomic charges for SiO2, the aluminosilicates mica and montmorillonite, and tetraalkylammonium ions, including local charge defects by Si → Al-···K+ and Al → Mg-···Na+ substitution. Our estimates of atomic charges correlate well with experimental data. Classical force fields based on these charges exhibit up to 1 order of magnitude less error in reproducing crystal geometries (only ∼0.5% deviation in unit-cell parameters), phase diagrams, and interfacial energies.

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We use molecular dynamics as a tool to understand the structure and phase transitions [Osman, M. A.; et al. J. Phys. Chem. B 2000, 104, 4433-4439. Osman, M. A.; et al. J. Phys. Chem. B 2002, 106, 653-662] in alkylammonium micas. The consistent force field 91 is extended for accurate simulation of mica and related minerals. We investigate mica sheets with 12 octadecyltrimethylammonium (C(18)) ions or 12 dioctadecyldimethylammonium (2C(18)) ions, respectively, as single and layered structures at different temperatures with periodicity in the xy plane by NVT dynamics. The alkylammonium ions reside preferably above the cavities in the mica surface with an aluminum-rich boundary. The nitrogen atoms are 380-390 pm away from the superficial silicon-aluminum plane. With increasing temperature, rearrangements of C(18) ions on the mica surface are found, while 2C(18) ions remain tethered due to geometric restraints. We present basal-plane spacings in the duplicate structures, tilt angles of the alkyl chains, and gauche-trans ratios to analyze the chain conformation. Agreement with experimental data, where available, is quantitative. In C(18)-mica with less than 100% alkali-ion exchange, the disordered C(18) rods in the island structures [Hayes, W. A.; Schwartz, D. K. Langmuir 1998, 14, 5913-5917] break at 40 degrees C. At 60 degrees C, the headgroups of the C(18) alkyl chains rearrange on the mica surface, and the broken chain backbones assume a coillike structure. The C(18)-mica obtained on fast cooling of this phase is metastable due to slow reverse rearrangements of the headgroups. In 2C(18)-mica with 70-80% ion exchange, the alkali ions are interspersed between the alkyl chains, corresponding to a single phase on the surface. The observed phase transition at approximately 53 degrees C involves an increase of chain disorder (partial melting) of the 2C(18) ions without significant rearrangements on the mica surface. We propose a geometric parameter lambda for the saturation of the surface with alkyl chains, which determines the preferred self-assembly pattern, that is, islands, intermediate, or continuous. lambda allows the calculation of tilt angles in continuous layers on mica or other surfaces. The thermal decomposition seems to be a Hofmann elimination with mica as a base-template.

Polyethylene at equilibrium is studied by computer simulation. Configuration space is sampled efficiently by a novel Monte Carlo simulation scheme developed for the study of long molecules at high densities. Simulations are carried out in an isobaric-isothermal statistical-mechanical ensemble which permits calculation of the density of the polymer matrix at specified conditions of pressure and temperature. A systematic study of the polymer at different temperatures indicates a phase transition; in agreement with experiment, at low temperatures, the polyethylene model studied here crystallizes spontaneously. At temperatures above the melting point, the simulated melt is described accurately by the model.