Polyethylene united atom model

Attempts were made to include all hydrogen atoms explicitly in the simulations. This computationally demanding explicit-atom model shows (Fig. 1) that the crystal symmetry is orthorhombic, in agreement with the well-known experimental result for polyethylene single crystals, instead of the hexagonal symmetry seen in united-atom model simulations. 

Yoon, Smith, and Matsuda, on the other hand, compared two approaches, using a united-atom model and a fully atomistic model.Stochastic dynamics and MD simulations of w-tridecane (C13H28) were used to study polyethylene. Besides studying the bulk melt, the authors examined confined melts between solid surfaces. Chain conformations, chain packing orientational correlations, and self-diffusion were among the properties studied. In regard to chain confer-... 

In the simulations with the united atom model (CH2 represented by an appropriate atom of mass 14), the crystal expands or shrinks, depending on the starting temperature and then shows a breathing-mode vibration. The density plots in Fig. 3 show such oscillations about the average which is close to the experimental density of polyethylene. At about the same temperature, different initial structures have slightly different density fluctuations in direction and amplitude, but the same frequency ( 3 x 10 Hz). Assuming that one sees the... 

A third, less obvious limitation of sampling methods is that, due to the heavy computational burden involved, simpler interatomic potential models are more prevalent in Monte Carlo and molecular dynamics simulations. For example, polarizability may be an important factor in some polymer crystals. Nevertheless, a model such as the shell model is difficult and time-consuming to implement in Monte Carlo or molecular dynamics simulations and is rarely used. United atom models are quite popular in simulations of amorphous phases due to the reduction in computational requirements for a simulation of a given size. However, united atom models must be used with caution in crystal phase simulations, as the neglect of structural detail in the model may be sufficient to alter completely the symmetry of the crystal phase itself. United atom polyethylene, for example, exhibits a hexagonal unit cell over all temperatures, rather than the experimentally observed orthorhombic unit cell [58,63] such a change of structure could be reflected in the dynamical properties as well. 

FIG. 2 Single chain structure factor for a united atom model of polyethylene calculated for neutron scattering [7],... 

In view of this discussion, it is clear that even the detailed model of eqs (1.1)-(1,3) should not be taken as a faithful description of polyethylene (PE), but rather as a prototypical schematic model of linear polymers. In the context of simulations of lipid monolayers,it has been suggested that it is necessary to shift the center of gravity of the united atom off the position of the carbon atom at the chain backbone (anisotropic united atom model). Very recent work " (see also Chapter 8) suggests that it is more satisfactory to include the hydrogen atoms explicitly, if one wishes to describe PE properly. For the reasons quoted above, such work is restricted to relatively... 

One of the powerful features of PRISM theory is that it can be applied to more realistic polymer models than just the bead-spring model. In fact, to the authors knowledge this is the only theory that has been applied in this manner. Let us first consider the simple case of the united atom model of polyethylene. Like the bead-spring... 

Muller et al. focused on polybead molecules in the united atom approximation as a test system these are chains formed by spherical methylene beads connected by rigid bonds of length 1.53 A. The angle between successive bonds of a chain is also fixed at 112°. The torsion angles around the chain backbone are restricted to three rotational isomeric states, the trans (t) and gauche states (g+ and g ). The three-fold torsional potential energy function introduced [142] in a study of butane was used to calculate the RIS correlation matrix. Second order interactions , reflected in the so-called pentane effect, which almost excludes the consecutive combination of g+g- states (and vice-versa) are taken into account. In analogy to the polyethylene molecule, a standard RIS-model [143] was used to account for the pentane effect. 

Abstract. The interlamellar domain of semicrystalline polyethylene is studied by means of off-lattice Metropolis Monte Carlo simulations using a realistic united atom force field with inclusion of torsional contributions. Both structural as well as thermal and mechanical properties are discussed for systems with the 201 crystal plane parallel to the interface. In so doing, important data is obtained which is useful for modeling semicrystalline polyethylene in terms of multiphase models. Here, we review the main results published previously by us (P.J. in t Veld, M. Hiitter, G.C. Rutledge Macromolecules 39, 439 (2006) M. Hiitter, P.J. in t Veld, G.C. Rutledge Polymer (in press), (2006)]. 

Figure 8a, b shows the specific volume vs pressure predictions from a simulation of 32 chains Of 24 units each (n-tetracosane) and 10 chains of 78 units each ( polyethylene ) at 450 K. Also shown on these plots are experimental data for the two systems [38]. As can be seen, the comparison is quite reasonable. This means that the imited-atom model employed, when solved exactly by simulation, can successfully capture thermodynamic properties across a wide range of chain lengths. 

An example for polyethylene, using the Jorgensen potential [15] to model the potential in Eq. (7), is illustrated in the upper panel of Fig. 4. Here n r, Ar) is the average number of united atom carbon pairs, i.e., pairs of effective methylene groups, divided by N, whose separation is between r — Ar/2 and r + Ar/2. Here N is the total number of methylene groups in the chain. For small r the distribution is discrete. The first pronouneed peak... 

---