Computational and Theoretical Polymer Science最新文献

We performed molecular dynamics simulations to analyze the initial stage of the thermal degradation of polyethylene, which is dominated by the random scission reaction. The simulations were initiated from structures that were taken from previously equilibrated snapshots of the amorphous polymer and of a free-standing thin film. Isolated chains were also used for comparison. Our systems were coupled to a thermal heat bath, and the effect of different coupling constants was studied. Rate of random scission increases as the strength of the temperature coupling increases. Rates of reaction are almost similar in thin films and the bulk, whereas the rates are much faster in isolated chains. Expansion of the free-standing thin film accompanies degradation, producing fragments of various sizes. Chains of higher molecular weights than the initial chains can be produced due to recombination of fragments during the expansion of thin films. The polydispersity index of the resulting fragments is higher in thin films compared to the bulk. The bonds at the low density portion of the thin films have a higher probability of being broken.

The spanning-tree approximation model is one of the models used for polymerizations with intramolecular reaction regardless of the size of the ring formed. We present a modification of this model that uses more accurate internal estimates of the probabilities of intramolecular reaction. This requires limited Monte Carlo simulations of some molecular structures, resulting in a hybrid probability model (a combined analytic and Monte Carlo model). We then extend the spanning-tree model so that it may be used in the post-gel region. We show three possible extensions of varying degrees of complexity. The resulting models for stepwise A_f homopolymerizations have been coded into programs that run on desktop PCs in a few seconds. The models calculate the amount of intramolecular conversion, the weight-average molecular weight, the gel point, the weight fraction of soluble material, and the weight-average molecular weight of the sol fraction. We discuss the relative merits of the modified spanning-tree model and its three post-gel extensions and show sample calculations for arbitrary homopolymerization systems.

The semi-empirical ZINDO/1 method is used to calculate the electronic structure and thermodynamic properties of aniline trimers adsorbed onto cluster models of the Al(100) and Fe(100) surfaces. The effects of progressive oxidation and protonation of the trimers on corrosion inhibition are interpreted in terms of these calculations.

A new technique for generating statistical properties of chain-molecule conformations is presented. Conditional probability density functions (PDFs) describing the frequency of occurrence of the relative position and orientation of frames of reference affixed to selected backbone atoms serve as the inputs. Ensemble statistical properties of whole chains are generated by performing multiple generalized convolutions of these conditional PDFs. The formulation is shown to include classical theories such as the hindered and freely rotating chains, the Gaussian random walk, and the rotational isomeric state model. The convolution model is modified to include the long-range effects of excluded volume. An analytical example is used to illustrate the procedure. A general algorithm to calculate the ensemble properties of an arbitrary chain macromolecule is presented. In this algorithm, each of the N degrees of freedom (e.g. torsion angles) is assumed to have K discrete states. Using the convolution procedure, a chain is divided into P statistical units. The computational requirement is reduced from an $\text{O}$(K^N) calculation (corresponding to direct enumeration) to one which is $\text{O}\text{(P(C+K}{}^{\mathrm{N/P}}\text{))}$ where C is the computational complexity of the convolution procedure. In the case of a homopolymer, computations are reduced further to $\text{O}\text{(C}\text{log}\text{(P)+K}{}^{\mathrm{N/P}}\text{).}$

The reaction of hydrogen abstraction by hydroxyl radicals on two substrates (poly(propylene) and poly(propylene oxide)) was studied using three quantum semi-empirical methods (MNDO, AM1, PM3). The calculations were performed as a function of the site of abstraction (hydrogen atom on a secondary or tertiary carbon atom), and of the calculation method. In each case, we localised the transition state and showed that this transition state occurs early along the reaction coordinate. The results concerning the activation energies depend on the sites and the calculation methods. The calculated results were compared to experimental ones.

Conformational changes of a simplified model of grafted poly(ethylene oxide) (PEO) chains were simulated using an off-lattice Monte Carlo model. A random-walk scheme was used in our simulations. The initial polymer structure was modeled with molecular mechanics and models of grafted polymer chains were built using programs developed in our laboratory. During the simulation, all bond angles and bond lengths were kept fixed while the dihedral angles of backbones were changed to search for energy-favorite conformations. Torsional energy, van der Waals interaction, and Coulombic interaction were considered. Periodic boundary conditions were implemented. In addition, the solvent quality was simulated implicitly by modifying the Lennard-Jones 12–6 van der Waals expression. Each PEO chain, 50-monomer long, was represented with a united-atom model. Eight series of simulations with varying solvent quality, simulation temperature, and Coulombic interaction were carried out. For each series, nine different initial grafting densities of grafted PEO chains were considered. Five different conformations were simulated at each grafting density. The calculated system energies, scaling properties, and atom density profiles were studied. Changes in solvent quality produced different structural behaviors. As the grafting density increased, there was a mushroom-to-brush transition, and the scaling property of average layer thickness was dependent on the grafting density.

Simple models of the star-branched and linear polymers were studied by means of a Monte Carlo method. The chains were confined on a simple cubic lattice. Star-branched polymers consisted of f=3 arms of equal length. The total number of beads in both types of polymers was varied from N=49 to N=799. The simulations were performed in different solvent qualities—from a good solvent to a collapsed globule regime. The static properties of the chains under consideration were measured as functions of the temperature of the system. It appeared that the ratio of the radius of gyration to the mean end-to-end vector is very sensitive to solvent quality. It shows that the coil-to-globule transition is a complicated phenomenon. The possible explanation of the phenomenon is discussed.

The theoretical study of hydrogen abstraction by hydroxyl radicals on two substrates (copolymers of fluorinated olefins and allyl or vinyl ethers) was carried using the MNDO, AM1 and PM3 quantum semi-empirical methods. This study was performed as a function of the site of hydrogen abstraction and of the computational method. The results of the calculations clearly show that the transition state is early along the reaction coordinate and pinpoint that the reactions are not under enthalpic control. The results provide evidence of the importance of the polar effects due to the fluorine atoms.

A new stochastic model named CORUB has been used to do a phenomenological description of the photopolymerization of furfuryl methacrylate, with the following methodology. Statistical analysis of the model's adequacy shows a good correlation between experimental data and those modelled stochastically. This model was developed using two stochastic variables that represent the random nature of the intermolecular encounters upon which the Collision Theory is sustained. The amounts of random numbers generated simulate time in a new way, based on the standard physical concept, with satisfactory results. Also, calculating the number of times that each individual step of the reaction mechanism takes place, a new possibility for carrying out the sensitivity analysis is presented. In addition to which, the analogy assumed between the frequency of the random number generated and the effective intermolecular collision allows us to model, exclusively and accurately enough, the effect of temperature as well as to estimate the apparent activation energy precisely. The algorithm of CORUB model was coded in Turbo Pascal 6.0, resulting in the CHEMOD-X program, which constitutes a software specifically elaborated as an integrative part of this work, allowing the model to be used.

Group contribution analysis (GCA) has been applied to many of the physical properties of polymers in the past. In this paper, GCA has been applied for the first time to the frequency dependent complex modulus of polymeric materials, which may be described in terms of the Havrialiak–Negami (H–N) equation. This approach has been tested on a set of polyurethanes for which the H–N parameters have been uniquely determined. It has been shown that the dynamic mechanical behaviour of polymers may be described in terms of group additive relationships, at least for the 14 polyurethanes and nine structural groups which were studied here.