(179e) Atomistic Simulation Studies of the Enthalpy of Mixing of Systems Containing Aromatic and Aliphatic Hydrocarbon Oligomers

Using a direct molecular dynamics simulation approach to obtain accurate measurements of the enthalpy of mixing in polymer blend systems is appealing because of its simplicity, although in practice a number of obstacles must be overcome.  Specifically, whilst the density of simple small molecule liquid systems is often known accurately, this is seldom the case for systems containing oligomer or high polymer, and must be determined precisely and accurately from the simulation.  This in turn implies that a high accuracy force field should be available.  Moreover, since a variety of chemically distinct polymers will often be of interest, broad coverage of chemical functional groups must also be provided. A second series of potential difficulties involves the efficiency of sampling phase space during the simulation.  Again, for small molecule systems this is generally not a problem, though as chain length increases sampling all representative regions of phase space within a single simulation becomes problematic, in particular because the broad range of energetically accessible conformations cannot be sampled. Consequently contributions to the mixing energy from changes in conformational energy as a function of composition, which may not always be negligible, become progressively more difficult to measure.  Finally, many polymer blend systems of interest contain high polymer of molecular weights too large to be modeled directly. Fortunately, the mixing energy per unit volume tends towards a constant value as the molecular weight increases, and consequently the question that must be addressed in simulations is whether the range of accessible molecular weights is sufficient to reach the high molecular weight limit, or close enough to be obtained by extrapolation of oligomer data.

In view of advances in quality of available force fields and efficiency of readily available highly parallel simulation codes developed in recent years, it is of interest to reassess the accuracy and precision with which enthalpies of mixing in polymer-containing systems can routinely be predicted.  In this work, we accordingly report on a study of aromatic-aliphatic mixture systems for which experimental data is widely available for both low molecular weight mixtures and higher molecular weight polymer.  Specifically, we utilize a recently-refined version of the PCFF force field developed in this laboratory (denoted PCFF+) [1,2] and the Sandia LAMMPS simulation program [3], and consider systems which serve as models for the extensively-studied styrene-butadiene polymer mixture systems, beginning with a series of calculations of enthalpies of mixing in arene-alkane and arene-alkene systems, where results can be compared directly with experiment.  Following this, we construct a series of short oligomer models that mimic the polystyrene-polybutadiene system and compute the (mostly unknown) densities and heats of mixing.  Finally, we consider how the oligomer data can be extrapolated to higher molecular weights and compared with mixing enthalpies deduced from cloud point curves, subject to the assumption that the non-combinatorial contribution to the free energy of mixing can be neglected, as proposed experimentally for these systems.

References

1. see, for example, Sun, H., Mumby, S.J., Maple, J.R. and Hagler, A.T., J. Am. Chem. Soc. 116, 2978 (1994)
2. Materials Design, Inc., Angel Fire, New Mexico, USA.
3. Plimpton, S., J. Comp. Phys. 117, 1 (1995)