Computational and Theoretical Polymer Science最新文献

In recent studies, as well as the results presented herein, it has been shown that the classical dynamics of macromolecular systems exhibit chaos far below the zero-point energy consequently resulting in the loss of a qualitative correspondence to quantum behavior. Mechanisms responsible for this undesirable and unrealistic dynamics have been shown to be due to the flow of energy out of the high frequency modes into the low frequency, large amplitude modes. A very powerful tool for eliminating the high frequency modes in macromolecular systems has been the development of internal coordinate molecular dynamics. This method only integrates the chosen degrees of freedom that determine the overall structure of the molecular system (the torsion). In this paper, we have used this technique with the appropriate analysis from semi-classical theory and nonlinear dynamics to study the trajectories generated for some simple polymer fragments. The method does effectively eliminate most of the problems associated with zero-point energy flow and the resulting phase space structure exhibits a high degree of stable quasiperiodic motion. However, the semi-classical relationships to quantum mechanics in this quasiperiodic regime require that initial conditions be chosen carefully in order for the resulting trajectory to have any quantum relevance.

The paper reports the development of a Monte Carlo lattice model (cubic F) of polymer chains which is able to access times where the diffusion of the centre-of-mass of the chains is the dominant process, even though the chain lengths are well above that for entanglement. The volume of the model is large when compared with the volume of gyration of the individual molecules. The model incorporates an algorithm, which allows for the possibility of co-operative motions over sections of the chains and increases the time efficiency of the simulation. Both the model and the modifying algorithm have been tested against the known scaling laws.

The model, for shorter chains, is ‘reverse mapped’ into full atomic detail as polyethylene and the shorter time processes simulated using molecular dynamics (MD). The MD model is tested against experimental diffusion data for polyethylene, of the same molecular weight and at the same temperature, and then used to time-calibrate the lattice model.

Both the fine grained MD model and the coarse grained MC model are thus interlocked to cover a time range from the individual atomic motions of MD up to the order of a microsecond, a range of six orders of magnitude.

The combination of the thermodynamic-integration approach and Widom's particle-insertion method is shown to be appropriate to determine the excess chemical potential of water in dense, amorphous polymer microstructures from MD simulation at an atomistically detailed level. The two-step method is applied to bisphenol-A–polycarbonate (BPA–PC) and polyvinylalcohol (PVA). The results are compared to previously published calculations on water sorption of various polyamides and show the applicability of the two-step method for the calculation of the excess chemical potential of water in a variety of polymer materials to obtain an estimate of their water sorption behavior.

A model for the swelling of polyelectrolyte gels in salt solutions is developed and solved numerically. The model accounts for the effect of network stress, osmotic pressure, and electrical potential on the species diffusive flux. The osmotic pressure and the network stress are derived from the Helmholtz free energy of the system that is the sum of mixing, elastic, and electrostatic components. One- and two-dimensional swelling in unconstrained and constrained geometries are simulated for a salt–solvent–polymer system. The transient and equilibrium fields of electrical potential, concentrations, deformation, and stress are obtained. Transient overshoots and non-uniformities in the residual profiles are predicted.

In this work we present, for the first time, a mathematical development of the moments of the molecular weight distribution in terpolymerization systems where donor–acceptor complexes are formed and the propagating reactions are carried out according to the complex participation model. The resulting set of equations is applied to the system formed by vinyl chloride (VC), vinyl acetate (VAc) and maleic anhydride (MA), in order to show the use of the equations and the type of information that might be obtained from them.

The model of electrical degradation describes the observed growth of damage in polymers quantitatively. It is shown that the fractal approach can be used to explain the non-monotonic behaviour of voltage versus scale for electrical degradation.

Polyamides have many desirable properties such as high melting temperatures, chemical resistance and superior mechanical properties. However, its crystalline morphology can limit its applications. It is the specific interaction, hydrogen bonding that gives rise to the crystalline structure of polyamides. This interaction is strong and important when blending on the final morphology and mechanical properties. Polyurethane contains polar functionality that can also interact with the polar component of polyamide. Hence, it is important to study the interaction between such a blend as polyurethane can enhance the toughness of polyamide due to its elastic properties. This study is an insight into the specific interaction between two polar polymers in a simulation whereby the interaction is maximised. Hydrogen bonding has been observed between molecules of either polyamide–polyurethane and polyamide–polyamide, and it is sufficiently strong to cause the polymer chains to distort rather than disrupt the hydrogen bonds. When groups of like polarity, such as carbonyl groups, come into proximity, the polymer chains again distort from their regular conformation because of mutual repulsion.

Glass transition temperature is the most important descriptor of the properties of amorphous polymers. In this study, molecular dynamics (MD) simulation is used to generate volume-temperature (VT) data at constant pressure for poly(vinylchloride) (PVC) over a temperature range that includes the experimental glass transition temperature (T_g) to study the validity of MD simulation in predicting T_g of amorphous polar polymers. PVC contains a polar group (chloride) which induces a partial charge distribution on all atomic sites of the polymer repeat unit. Two types of MD simulation were conducted. In the first type, all atomic sites were explicitly represented in the polymer chain model. In the second type of simulation, the CH₂ and CHCl groups were modeled as rigid units to minimize the computational effort. The T_g values obtained from the MD VT curves were slightly displaced upward relative to the experimental T_g. The rigid unit model tends to under estimate the liquid density compared with the explicit atom model. MD simulation seems to be a practical procedure for predicting the T_g of polar polymers. The rigid unit model provides substantial saving in the computational effort without loss of accuracy.

A generalized method for calculating diffusion rates in polydisperse systems, valid in the concentrated regime, is outlined. In the formulation of the method the discrete variable that describes the molecular size, is replaced by a continuous variable in the same range. This replacement diminishes the number of degrees of freedom but keeping the essential physics of the original statement. The effects of monomeric friction coefficient, Flory-Huggins thermodynamic interaction parameter, individual species molecular weights, local molecular weights distribution and local T_g are consistently included in the model.

The method is used to calculate concentration distribution profiles generated by diffusion of polydisperse polymers blends, and experimentally tested. For this purpose polystyrene with discrete (bimodal and tetramodal) molecular weight distributions and polystyrene with wide and continuous molecular weight distributions were used to simulate polydisperse systems. They are allowed to diffuse in a blend of polyphenylene oxide and polystyrene.

The simulated concentration profiles are compared with results obtained by using two experimental techniques based on independent physical properties, which give complementary information: Raman spectroscopy and DMA. The total PS concentration profiles calculated using the proposed method agree well with Raman spectroscopy results. Simulated DMA results—which are sensitive to the PS species molecular weight distribution—obtained using the concentration profiles calculated for each PS molecular weight species, agree well with the experimental DMA results. Calculations based on average molecular weights on the other hand, give incorrect results.