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Atomistically detailed models of free-standing thin films of random poly(ethylene-co-propylene) are produced with the help of a method recently applied to atactic polystyrene. The findings indicate that the raw experimental data obtained with an increase in ethylene fraction can be explained by the changes in the number of methyl and methylene groups on the surface rather than the increase in surface excess of the methyl groups.

The surface free energy calculated from the chemical potential and the contribution of the internal energy to the surface energy are compared for a molecular model of a free-standing thin film of n-tetracontane. The implementation of the chain increment method for calculating chemical potential becomes unreliable when crystals appear in thin film.

Research was conducted to examine the effect of chain end segregation on the surface characteristics of thin films by comparing two different films formed by linear and cyclic polyethylene (PE) thin chains. The simulation of the PE thins films was conducted on a high coordination lattice which is developed by connecting every other site on a diamond lattice. Results demonstrate that both films experience similar increases in mobility in the interfacial region up to the point where the bulk density decreases to rho(sub B)/3.

A study was conducted to analyze the inclusion complexes of 2-methylnaphthoate (MN) with gamma-cyclodextrin. Steady-state fluorescence and molecular mechanics computations were utilized to carry out the analysis. MN was determined from 2-naphthoyul cloride and methanol in chloroform during the presence of triethylamine. Experimental results indicated that MN:gamma-cyclodextrin complexes are thermodynamically stable.

We demonstrate the application of a modified form of the configurational-bias algorithm for the simulation of chain molecules on the second-nearest-neighbor-diamond lattice. Using polyethylene and poly(ethylene-oxide) as model systems we show that the present configurational-bias algorithm can increase the speed of the equilibration by at least a factor of 2–3 or more as compared to the previous method of using a combination of single-bead and pivot moves along with the Metropolis sampling scheme [N. Metropolis, A. W. Rosenbluth, M. N. Rosenbluth, A. H. Teller, and E. Teller, J. Chem. Phys. 21, 1087 (1953)]. The increase in the speed of the equilibration is found to be dependent on the interactions (i.e., the polymer being simulated) and the molecular weight of the chains. In addition, other factors not considered, such as the density, would also have a significant effect. The algorithm is an extension of the conventional configurational-bias method adapted to the regrowth of interior segments of chain molecules. Appropriate biasing probabilities for the trial moves as outlined by Jain and de Pablo for the configurational-bias scheme of chain ends, suitably modified for the interior segments, are utilized [T. S. Jain and J. J. de Pablo, in Simulation Methods for Polymers, edited by M. Kotelyanskii and D. N. Theodorou (Marcel Dekker, New York, 2004), pp. 223–255]. The biasing scheme satisfies the condition of detailed balance and produces efficient sampling with the correct equilibrium probability distribution of states. The method of interior regrowth overcomes the limitations of the original configurational-bias scheme and allows for the simulation of polymers of higher molecular weight linear chains and ring polymers which lack chain ends. [ABSTRACT FROM AUTHOR]

For a model system of polyethylene of chain lengths 40 and 100 carbon atoms, we calculated the pressure at different densities and compared them with the experimental values. The simulation was conducted on the second nearest neighbor diamond lattice, and the pressure was calculated using the virtual-volume-variation method after the system was reverse mapped to its fully atomistic form in continuous space and energy minimized. In addition, the pressure was also calculated from the virial route by conducting a short molecular dynamics simulation starting from the energy minimized structure. We show that the pressure obtained from our simulations is quite reasonable in the length of simulation time (in Monte Carlo steps) normally employed in our group. These results provide additional evidence for the equilibration of our model systems, and methodology to calculate the pressure in our lattice models. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

An amorphous 50/50 atactic polypropylene (aPP)/isotactic polypropylene (iPP) mixture at 125 °C was simulated using a second nearest neighbor diamond lattice and a three states rotational isometric state model. The result suggests that at the liquidlike density that corresponds to the atmospheric pressure, aPP prefers to interact with other aPP chains rather than with iPP chains. The result is consistent with the inference of Keith and Padden [J. Appl. Phys. 35, 1286 (1964)] that aPP and iPP will tend to separate from one another in their melt at 125 °C, before the onset of crystallization of iPP. The tendency for immiscibility of the amorphous aPP/iPP blend is likely attributed to the presence of short syndiotactic sequences in the aPP chains adopting all-trans conformations. The attractive intermolecular interaction of pairs of such subchains at 125 °C promotes the separation of aPP from iPP. This interaction is weakened at higher temperature, where aPP and iPP become miscible. The result also shows that miscibility of the blend increases with increasing pressure. However, the origin of the pressure effect is not clear. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Recently we described a coarse-grained model of poly(ethylene oxide) and then employed that model to study the amount of spontaneous threading of cyclic molecules by linear chains in the melt [C. A. Helfer, G. Xu, W. L. Mattice, and C. Pugh, Macromolecules 36, 10071 (2003)]. Since the amount of statistical threading at equilibrium is small, there is interest in identifying physical changes in the system that will increase the threading. We now use that coarse-grained model to investigate the effect on threading of various hypothetical (but feasible) modifications to the two-component system of macrocycles and linear chains in the melt, and different confinement geometries, that can bring about correlations in the arrangement of the rings. Our work follows on the concept of an amphiphilic approach [C. Pugh, J.-Y. Bae, J. R. Scott, and C. L. Wilkins, Macromolecules 30, 8139 (1997)] for increasing the statistical threading in homopolyrotaxane melts. We investigate whether introducing such correlations in the macrocycles can increase the spontaneous threading. This paper shows that some of our modifications can yield more than double the amount of threading seen in purely statistical mixing. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The configuration of thin films of short polyethylene chains during the crystallization has been investigated using a recently developed Monte Carlo method on a high coordination lattice, which bridges the coarse-grained and the fully atomistic simulations. Thin films select their own crystal configurations, in which the chains have different orientations with respect to the surface normal. Two kinds of crystal structures, with and without grain boundary in the center of the thin films, in which all the chains are parallel to the free surface, have been found after a deep quench from an equilibrated thin film above the melting temperature in the previous simulations [J. Chem. Phys. 116, 2277 (2002)]. However, another crystal configuration shows up after a quench from the thin film formed by increasing the periodic boundary conditions in one direction from a homogeneous melt. In this configuration all the chains are parallel with each other and to the z axis. This configuration was found both in the experiments and in the previous molecular dynamics simulation of a free-standing thin film of a short alkane. It is demonstrated here that the initial configuration does affect the final structure in the simulation of the crystallization of the free-standing thin film. The current configuration has slightly lower enthalpic and entropic energies and higher melting point than the previous ones. The similarities and differences between three configurations during the crystallization, annealing, and melting processes will be discussed. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A recently developed high coordination lattice based Monte Carlo method is applied to investigate the glass transition of free-standing atactic polypropylene thin films. In this method the short-range interactions are described by a rotational isomeric state model and the long-range interactions are obtained from a discretized form of a Lennard-Jones function. Simulations of five thin films of C[sub 150]H[sub 302] with thickness of 20-100 Å at T[sub g] are performed by a stepwise cooling from the melts to the glass states. The temperature dependence of the thickness of the thin films is obtained from the fitting of the density profile by a hyperbolic function. The glass transition temperatures of the thin films are evaluated from the change in thermal expansion coefficients at T[sub g]. The T[sub g] decreases as the films become thinner. For all but the thinnest film, 1/T[sub g], is inversely proportional to the thickness of the film. The proportionality constant is within the range cited in experimental studies of other systems. The depression of the glass transition temperature arises mainly from the fast dynamics in the surface region. [ABSTRACT FROM AUTHOR]

Thin films of a mixture of two kinds of short polyethylene chains, n-tricontane and n-tetracontane, are simulated both at high and low temperatures by a dynamic Monte Carlo method on a high coordination lattice, which bridges the coarse-grained and the fully atomistic simulations. Films are obtained from an equilibrated model for the melt by increasing one of the three periodic boundaries to a very large value. The melting temperatures (T[sub m]) of the two pure components in the simulation are obtained from a sharp transition of various parameters, such as the probability of trans conformation of C-C bonds, orientation order parameters, energies, etc. At high temperature, 420 K (above T[sub m] of the two pure components), the chains have a trend to separate with each other. The shorter chains are enriched on the free surface due to their larger fraction of chain ends. At low temperature (below T[sub m] of the two pure components), two situations exist, which depend on the prior history of the mixture. If the quench takes place from a homogeneous mixture the chains crystallize together with almost the same density profile except for a large fluctuation in the bulk region of the thin film. On the other hand, if the quench takes place from the equilibrated thin film at 420 K, the shorter chains remain enriched on the surface, and the segregation of the shorter chain is enhanced. [ABSTRACT FROM AUTHOR]

The radial local dynamics in poly(1,4-trans-butadiene) melts is studied using molecular dynamics simulations. In this work, the hopping peak is observed in the Van Hove space-time correlation function G[sub s],(r,t) for the first time in polymeric systems. The hopping motion, observed only for methine hydrogen, is also identified in the mean-square displacement 〈Λr²(t)) and the intermediate scattering function F[sub s],(k,t) through the relative dynamics of the hopping methine hydrogen and the nonhopping methylene hydrogen. The hopping motion is found to cause an unusual broadening of the dispersion width in the dynamic structure factor S[sub inc](k, ω). Active free volume is proposed in terms of G[sub s],(r, t) at a short time, which offers a consistency to the relationship between free volume and local dynamics. Fast counterrotation at a pair of CH-CH[sub 2] bonds across a CH=CH bond is found responsible for the hopping peak in this polymer, and a new hopping criterion modified for polymeric liquids is proposed. [ABSTRACT FROM AUTHOR]

A freestanding thin film of n-tetracontane chains is simulated by a Monte Carlo (MC) method on a high coordination lattice. The coarse-grained chains, represented by 20 beads each, can be reverse-mapped into the fully atomistic description, C[sub 40]H[sub 82]. The Hamiltonian includes a short-range interaction based on a rotational isomeric state model and a long-range interaction obtained from a Lennard-Jones potential energy function. When the melt is instantaneously quenched from 473 to 298 K, crystallization initiates in the surface region and propagates into the interior of the film, as was found in a prior molecular dynamics simulation of a united atom model of polyethylene [M. Ito, M. Matsumoto, and M. Doi, Fluid Phase Equilibria, 144, 395 (1998)]. Several repetitions of the MC simulation, starting from different configurations of the melt at 473 K, reveal that two distinctly different structures can be obtained. Usually the independently initiated crystals at the two surfaces of the thin film produce a disordered grain boundary when they impinge on one another as a consequence of propagation into the interior of the film. This grain boundary was also observed by Ito et al. However, if the MC simulation is repeated many times, there are a few instances in which the independently initiated crystals happen to have a similar orientation, and then crystallization propagates completely through the thin film without producing a grain boundary in the interior. A well-defined melting phenomenon is observed at about 390 K when the film without the grain boundary is heated. Annealing at 380 K of the film with the grain boundary causes growth of one crystal at the expense of the other. This growth eventually leads to a completely crystalline film, with elimination of the grain boundary. Therefore, the stable structure of the thin film is the one that is completely crystalline, with no grain boundary in the interior, even though rapid quenching is more likely t... [ABSTRACT FROM AUTHOR]

Recently Clancy et al. reported a Monte Carlo simulation that reproduces the tendency, observed by Mu¨lhaupt and co-workers, for demixing of melts of isotactic polypropylene (iPP) and syndiotactic polypropylene (sPP). The short-range intermolecular interactions in the coarse-grained chains in the simulation are controlled by a rotational isomeric state model for PP that was described by Suter et al. The intermolecular interactions of the beads are controlled by a discretized representation of a continuous Lennard-Jones (LJ) potential that describes the interaction of two molecules of propane. The work reported here reveals that the outcome is sensitive to the truncation of this LJ potential. A weak tendency for demixing of the 50:50 iPP:sPP melt is seen if the LJ potential is truncated so that only its repulsive part is used in the simulation. Inhomogeneity is enhanced if the truncation of the LJ potential is shifted to larger distances, so that the attractive branch of the LJ potential is incorporated in the simulation. This observation in the simulation leads to conclusive identification of the mechanism responsible for demixing of the 50:50 melt. It also implies that the same mechanism may affect the miscibility of melts of other vinyl polymers in which the chains differ in stereochemical composition. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Studies the detection of the onset of demixing in simulations of polypropylene melts in which the chains differ only in stereochemical composition. Mean square dimensions of the chains; Cohesive energy; Pair correlation functions.

The local motions of bulk amorphous polybutadienes (PBDs) have been studied by a molecular dynamics simulation. Fully atomistic models of PBDs have been obtained at a realistic bulk density under periodic boundary conditions. Local chain motions are examined for poly(cis-1,4-butadiene) (cis-PBD) and poly(trans-1,4-butadiene) (trans-PBD) well above the glass transition temperature, where the only structural difference is imposed by the different conformational states at the double bond. The local motions are found to be strikingly different in the two polymers. Intramolecular cooperative conformational transitions occur at second-neighbor bond pairs of CH–CH2 in both PBDs. In trans-PBD, both CH2–CH2 and CH=CH triplets undergo cooperative counterrotations. In cis-PBD, CH2–CH2 triplets undergo counterrotations while CH=CH triplets undergo corotations. Intermolecular cooperativity, probed by the local free volume, has little influence on local relaxation processes. The effect of libration on the shape of autocorrelation functions (ACFs) is significant over the entire time range. A universality is found in the time evolution of the Kohlrausch–Williams–Watts exponent regardless of the type and order of the ACF. On the other hand, the single exponential decay regarded as common at short times is only due to the discreteness of conformational states. [ABSTRACT FROM AUTHOR]