Computational and Theoretical Polymer Science最新文献

Molecular dynamics has been used to determine the glass transition temperature of the amorphous phase of five di-substituted polysilanes from plots of specific volume versus temperature. In each case, good agreement was obtained between the simulation values and the reported DSC results. The effect of amorphous cell dimensions and equilibration time on T_g has been investigated. The use of larger cells provides better agreement with experimental T_g and probably more accurate densities as suggested by earlier studies. The effect of pressure on the T_g of two different polysilanes was also investigated. Although experimental data for comparison is unavailable, values obtained for dT_g/dp are consistent with those reported for other polymers. Vectorial autocorrelation analysis was used to explore the mobility of the polysilane main chains and side groups relative to polyalkanes, polyphosphazenes, and polysiloxanes.

Two simple simulation methods, which will be useful for the prediction of the infrared (IR) spectral features of polymers, are reported. This paper focusses on simple aromatic models of the main aromatic backbones of structural polymeric resins. These methods were normal coordinate analysis (NCA) using the PM3 Hamiltonian implemented under MOPAC6 and a dipole autocorrelation function (DACF) calculated using coordinates and velocities from a series of molecular dynamics runs performed using the universal force-field (UFF) as implemented in the Cerius² modelling package. The semi-empirical NCA approach yielded useful information about the fundamental modes of vibration of the molecules but, as expected, could not be used to predict combination modes for anything but the simplest of molecules. The DACF approach showed potential for the prediction of combination bands but thorough evaluation of the method was found to be extremely difficult, owing to problems with band assignment for all but the simplest of molecules. An estimate of the accuracy of prediction for the different types of vibrational mode is included for each method.

Micellization behavior of diblock copolymers in selective solvent near the critical micelle temperature (c.m.t.) is theoretically studied focusing our attention to that the core must be swollen with the solvent near c.m.t. Supposing a micellar solution of core–corona type spherical micelles with swollen hollow cores, we calculate the association number distribution and the micelle structure at equilibrium in terms of controlling parameters, i.e. intrinsic interfacial tensions (γ_eff0 and γ_AS0) for core/corona and core/solvent interfaces, and core-segment/solvent interaction parameter χ_AS. Infinitely-large micelle region (ILM-Region), where the micelle size is divergent, is found at smaller values of γ_AS0 and χ_AS other than micelle and unimer regions. In the micelle region near ILM-Region, the micelle size and the degree of swelling become extremely large as ILM-Region is approached, while the micelles become very compact far away from ILM-Region. By investigating the micellar behavior with increasing the association strength on a particular trace in the χ_AS−γ_eff0−γ_AS0 space, it is demonstrated that large swollen hollow micelles are easily formed near c.m.t., and then sharply change to be more compact micelles with decreasing solvent quality to core blocks. This is exactly similar to the so-called anomalous micellization experimentally observed near c.m.t.

This study concentrates on the important conducting polymer, polypyrrole. Detailed atomistic molecular models have been developed with the help of ab initio and semi-empirical quantum mechanical calculations.

The vibrational spectra of isolated pyrrole monomers and oligomers from n=1 and 2, where n is the number of structural repeat units used, have been computed using the ab initio 3-21G basis set. The results obtained are compared with data for the case of oligomers with n=2–5 for both neutral benzenoid and quinonoid oligopyrroles, from semi-empirical predictions obtained by AM1 and PM3. The trends in the computed harmonic force fields, vibrational frequencies and intensities are monitored as a function of the chain length. The data are analysed in conjunction with the trends in computed equilibrium geometries.

Also the examination of the heat of formation of these two degenerate forms (quinonoid and benzenoid) has been conducted with respect to increases in the number of rings and the change of methods from AM1 to PM3.

Monte Carlo Modelling of random polymer chains, course grained onto a cubic F lattice, provides the ability to monitor the long range relaxation processes and the dynamic parameters of chains up to 400 units long. The model, described and verified by Haire et al. (Haire KR, Carver TJ, Windle AH. A Monte Carlo model for dense polymer systems and its interlocking with molecular dynamics simulation. Computational and Theoretical Polymer Science 2000; in press), is here applied to the study of molecular parameters in the vicinity of different types of surface and also to the process of polymer welding, whereby adhesion between two adjacent surfaces is achieved by the interpenetration of chains which are across the surface.

The model demonstrates that a surface distorts the conformation of chains adjacent to it to give an oblate molecular envelope, that the concentration of vacant sites and chain ends increases near to the surface and that the density of points representing the centres of mass of the chains increases in the sub-surface regions. These results confirm earlier predictions and provide additional confidence in the model.

Modelling of the welding process leads to the parameter intrinsic weld time, t_w, which is the time from initial perfect contact of the surfaces to the achievement of a weld within which the chain conformation is indistinguishable from the bulk. After the initial period in which the mating surfaces roughen, the welding proceeds according to the t^1/4 law predicted by reptation theory. The time to a given level of interdiffusion across the boundary is proportional to the chain length l, a comparatively weak dependence, while t_w is proportional to l³, a strong dependence. This is the same dependence on length as for the relaxation time of the chain end-to-end vectors. In fact, the agreement between the relaxation time, measured on the model of the bulk, and t_w is surprisingly close, at least for the monodisperse polymers investigated here.

We investigate the density of vibrational states g(ω) for 6000 atom polymer particles involving all 18,000 degrees of freedom. The particles are efficiently generated using a molecular dynamics-based computational algorithm and a molecular mechanics method. The density of states spectrum g(ω) clearly shows two distinguishable frequency regions in the polymer system: high $\text{(760<ω<1240}\text{cm}{}^{\mathrm{-1}}\text{)}$ and low $\text{(0<ω<}\text{350}\text{cm}{}^{\mathrm{-1}}\text{)}$ frequency modes. By calculating the level-spacing distributions, we find the distribution of the low eigenfrequency corresponds to that of a Wigner distribution. In contrast, Poisson behavior is found for the high frequency region. The eigenvectors for the two regions are analyzed by using a random walk method and Stewart's perturbation theory, both indicate random character for the eigenvectors of the low frequency modes. The random character of the eigenvectors should have ramifications to most types of spectroscopy since transformations of the transition operator to random normal coordinates will cause a widespread mixing, i.e., no selection rules.

Calculations were performed to assign defect-resonance patterns observed in solid-state ¹³C NMR spectra obtained from the crystalline regions of isotactic polypropylene. The spectral features of interest are associated with stereo, regio, and comonomer-type defects, which can typically be found in metallocene-synthesized polymers. The calculations were carried out as follows: a model of the crystalline region of defect-free isotactic polypropylene was constructed from available X-ray data corresponding to the α-lattice. A series of irregularities including ethylene comonomer, stereo-mrrm, regio 2,1-erythro, and butylene-comonomer defects were introduced one at a time at various positions in a specific stem occupying a central position in the model crystallite. Low-lying conformations were then obtained from simulated annealing calculations that were initiated from these structures. Finally, quantum mechanical calculations were performed on the representative segments of the defect-containing chains excised from the annealed crystallites and the calculated chemical shifts were compared to the observed resonances. The results of the calculations were used as a basis for interpreting the NMR intensities of defect-related resonances in terms of the partitioning of defects and to help establish the conformational structures of the defect-containing stems.

It was found experimentally that after increasing pressure, the decay of free-radicals in solid polymers is slowed down (Szöcs F, Rostašová O, Tiňo J, Plac̆ek J, European Polym J, 1974;10:725). Since the mechanism for decay is associated with molecular mobility, a Monte Carlo method has been used for studying the effect of the polymer density on molecular mobility and free-radical decay in a model system with the parameters close to those of polyethylene. Increased pressure is correlated with higher density of the polymer system. Rotational motions were found to be considerably limited at increased density (ρ=0.85 g cm⁻³ versus 0.81 g cm⁻³). Consequently, free-radical decay is slowed down at the higher density in accord with the experimental results.

The conformational properties of the bisphenol-A polycarbonate (PC) were studied using the rotational-isomeric-state metropolis Monte Carlo (RMMC) simulations adopting a polymer consistent forcefield (pcff). The conformations of a single PC chain were generated at theta condition in a wide range of molar mass. When the maximum bond number for non-bonded interactions is set to 5, the conformation corresponds to the unperturbed state. The probability that the dihedral bond pairs of diphenyl propane (DPP) groups exist at two minima of energy where the bond angles are (50.5, 50.5) and (−49.5, 129.5) is 3.4×10⁻⁴ at 300 K. The probability of the trans–trans form of diphenyl carbonate (DPC) groups is 6.3×10⁻⁴ at 300 K, and is reduced to 2.1×10⁻⁴ as temperature increases to 600 K. The limiting unperturbed dimension in terms of mean-square end-to-end distance over molar mass (〈r²〉/M) is 1.01 Å² mol/g. From intrinsic viscosity calculated with the radius of gyration (S), it was found that the gyration or the conformation of the PC chain at 300 K is closer to that of dilute solutions in chloroform at 293 K or in p-dioxane at 303 K than in tetrahydrofuran (THF) solution at 297 K. In future studies, the conformation obtained from the RMMC method here will be used as an initial structure to perform the molecular dynamics simulation in order to investigate interfacial properties of the PC bulk.

Polymerization-induced phase separation (PIPS) via spinodal decomposition (SD) under a temperature gradient for the case of a monomer polymerizing in the presence of a non-reactive polymer is studied using high performance computational methods. An initial polymer (A)/monomer (B) one-phase mixture, which has an upper critical solution temperature (UCST) and is maintained under a temperature gradient, phase-separates and evolves to form spatially inhomogeneous microstructures. The space-dependence of the phase-separated structures under the temperature gradient field is determined and characterized using quantitative visualization methods. It is found that a droplet-type phase-separated structure is formed in the high-temperature region, corresponding to the intermediate stage of SD. On the other hand, lamella or interconnected cylinder type of phase-separated structure is observed in the low-temperature region, corresponding to the early stage of SD structure, in the large or small temperature gradient field, respectively. The kinetics of the morphological evolution depends on the magnitude of the temperature gradient field. The non-uniform morphology induced by the temperature gradient is characterized using novel morphological techniques, such as the intensity and scale of segregation. It is found that significant non-uniform structures are formed in a temperature gradient in contrast to the uniform morphology formed under constant temperature.