The miscibility behavior of three binary mixtures, solvent with solvent, polymer with solvent, and polymer with polymer, was studied by use of a combination of the Flory-Huggins theory and molecular simulation techniques. Fundamental parameters in the Flory-Huggins theory, including the heat of mixing associated with pairwise interactions (Awn) and the number of possible interaction partners, i.e., coordination number, z, are calculated from molecular simulations. The pair energies (wn, w22, wu) are obtained by averaging a large number of configurations generated by a Monte Carlo approach which includes the constraints associated with excluded volume. The temperature dependence of the interaction parameter x is obtained with the formalism developed in this study. In all cases, the calculated upper critical solution temperatures compare favorably with experimental values. This approach provides an opportunity to test the Flory-Huggins theory for a number of model binary systems and to characterize their miscibility behavior. This combined approach also facilitates study of the thermodynamic behavior of a binary mixture without possessing specific knowledge or experimental data of the system under investigation.
Molecular dynamics has been used to study the wetting of model polymer surfaces, the crystal surfaces of polyethylene ͑PE͒, poly͑tetrafluoroethylene͒ ͑PTFE͒, and poly͑ethylene terephthalate͒ ͑PET͒ by water and methylene iodide. In the simulation a liquid droplet is placed on a model surface and constant temperature, rigid body molecular dynamics is carried out while the model surface is kept fixed. A generally defined microscopic contact angle between a liquid droplet and a solid surface is quantitatively calculated from the volume of the droplet and the interfacial area between the droplet and the surface. The simulation results agree with the trend in experimental data for both water and methylene iodide. The shape of the droplets on the surface is analyzed and no obvious anisotropy of the droplets is seen in the surface plane, even though the crystal surfaces are highly oriented. The surface free energies of the model polymer surfaces are estimated from their contact angles with the two different liquid droplets.
Molecular simulations of Bisphenol A polycarbonate were performed using a modified version of the Dreiding force field. In general, the simplicity of this generic force field was maintained. However, a few parameters were optimized by using ab initio calculations in order to generate better backbone torsional potentials. The validity of this modified force field was tested on a model compound similar to polycarbonate, 4,4'-isopropylidenediphenylbis(pheny1 carbonate). The crystallographic data obtained from simulations of the model compound agreed well with the experimental data. The modified force field was then used in simulations of the glassy polymer. Model amorphous structures of Bisphenol A polycarbonate were built and optimized using periodic boundary conditions. The structure factor, S(k), was calculated from pair distribution functions of the model structures. The peak positions in calculated S(k) compare well with those obtained experimentally. The effects of the initial density on chain packing were also studied. Its impacts on the final density and energy are described. A general approach for calculating the stiffness matrix of any shape unit cell (triclinic system) is presented. A yield phenomenon for the model system was observed at about 10% strain. This value is comparable with experimental results ( 6 4 % ) . The yield stress (0.25 GPa), however, was higher than experimental values, as was the Young's modulus at small strain. These results could be partially explained by the fact that the calculated mechanical properties represent an ideal structure and thus provide the upper limit values for the polymer.
SUMMARY:Constant pressure constant temperature molecular dynamics method is employed to investigate the atomistic scale dynamics of a model Bisphenol A polycarbonate in the vicinity of its glass transition temperature. First, the glass transition temperature and the thermal expansion coefficients of the polymer are predicted by performing simulations at different temperatures. To explore the significance of different modes of motion, various types of time correlation functions are utilized in analyzing the trajectories. In these nanosecond scale simulations, the motion of the chain segments is found to be highly localized with little reorientation of the vectors representing these segments. Detailed analysis of trajectories and the correlation functions of the backbone dihedrals and side methyl groups indicates that they exhibit numerous conformational transitions. The activation energies of the conformational transitions obtained from the simulation are generally larger than the potential barriers for the rotations of these dihedrals, however, both show the same trend. We also have estimated the phenylene ring flip activation energy as 12.6 kcaVmol and the flip frequency as 0.77 MHz at 300 K. These values either fall within the range determined by various NMR spectroscopy experiments or slightly out of the range. The study shows that the conformational transitions between the adjacent dihedrals are strongly correlated. Three basic cooperative modes are identified from the simulation. They are: a positive synchronous rotation of two phenylene rings, a negative synchronous rotation of two phenylene rings, and a carbonate group rotation. Above the glass transition temperature, the large scale cooperative motions become much more significant.
A model of an amorphous polysulfone, patterned after Udel (Union Carbide Corp.) polysulfone, was generated using a molecular simulation technique. The pair distribution functions obtained for the model structures showed no evidence of long-range order. The properties of these simulated structures are in good agreement with available experimental data. For example, the average density of the calculated structures is 1.17 ± 0.02, a value which compares favorably with an experimental value of 1.24 ± 0.04. The distributions obtained for the torsional angles along the backbone are consistent with model compounds. Contour maps obtained through molecular mechanics show that the conformational distribution of the torsional angles is widest for torsion about the C-S bonds. The calculations indicate that rotational barriers for C-0 or C-C bonds are higher than those for C-S bonds and suggest that the mechanism for relaxation in the bulk state may be due to cooperative ring-flip motions associated with rotations about the C-S linkages.
The structure of aromatic amorphous polysulfone was simulated and the energy and internal strain minimized by using molecular dynamics and mechanics methods developed earlier. A plot of the total potential energy as a function of volume or density indicates the existence of an optimum state possessing both the lowest energy and the least internal stress. The minimum-energy structure has a computed density of 1.17 ± 0.02 g/cm3, in good agreement with the experimental value. The mechanical and thermal properties of amorphous polysulfone were simulated using these model amorphous structures. The form of the stiffness matrix and the calculated associated elastic constants agree with known values for such an isotropic amorphous polymer. The thermal expansion coefficient calculated using molecular dynamics also agrees with the experimental value.
A combination of a molecular simulation method and the Monte Carlo method has been successfully utilized to calculate phase diagrams of model polyurethanes. In our model, the entropic contribution of the Flory-Huggins expression has been modified to incorporate the contribution arising from orientation of hard segments. The constraint associated with chain rigidity of hard segments has been explicitly considered. In addition, the interaction term has been modified to include the relative packing of hard segments. Phase diagrams of various MDI-PPG model polyurethanes have thus been predicted utilizing these modifications. The effects of soft-and hard-segment lengths have been considered and the actual degree of phase separation calculated. Our predictions have been compared to experimental values. Additionally, the contribution of hydrogen bonding to the miscibility behavior of hard and soft segments needs to be reevaluated.
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