Computational and Theoretical Polymer Science最新文献

The feasibility of the different initial reaction pathways in the gas-phase polymerization of labdatriene monomers, important constituents in amber resins, has been analyzed through ab initio electronic structure calculations. Based on the computed values of density functional theory local Fukui functions and softnesses, tendencies towards free radical additions were quantitatively derived. Results indicate those unsaturated carbons at the diene group 12 and 15 positions possess higher reactivity towards addition reactions. This implies that, in the gas phase, 1,4-addition is the most favored in the ground state. A simple model to account for intermolecular forces and their possible effects to modify the predicted reactivity does not suggest that these forces may be significant to indeed modify it. However, our frontier orbitals calculations suggest the initial formation of a triplet excited state in order to prepare the π-conjugated labdane monomer to undergo the experimentally observed 3,4-addition.

The effect of molecular mobility on the stability of free radicals in amorphous polymers was investigated by a Monte-Carlo (MC) method. Crank, crankshaft-like, kink and double kink were used as the various types of movements of submolecular structures. This work introduces the librational motions of these structures and formulates the methodology for their incorporation into the MC method. The results show that the presence of librational motion significantly influences both transfer and the decay of free radicals.

Thermogravimetric curves of virgin- and waste-PVC in an atmosphere of nitrogen and nitrogen saturated with water vapour were recorded at several heating rates to determine degradation kinetics. The degradation curves of virgin-PVC showed two stages of considerable mass loss. Application of the Friedman method to the experimental data proved to be too inaccurate to determine the kinetics unambiguously. Numerical determination of the kinetics parameters resulted in a better description of the data. Two models were fitted to a set of degradation curves of virgin-PVC. Both models showed good reproducibility. The effects of addition of water vapour to the atmosphere were limited to very high temperatures. The degradation curves of rigid PVC-waste showed an additional mass loss between 950 and 1150 K, presumably caused by limestone conversion. The model proposed to describe the degradation of rigid PVC-waste fitted the experimental curves well. Samples from PVC-waste pipes proved to be too heterogeneous to fit the degradation curves with one set of parameters.

Copolymer chain-statistics and other properties including the heat of copolymerization are derived using the joint concepts of the mean-length of the sequences of each species along the copolymer chain and the number of transitions from sequences of one species to the other. The approach originated in papers of Smoluchowski going back to 1915. By systematic use of the approach, classical relationships are derived for both the terminal and the penultimate models of copolymerization. Relationships are given for the heat of copolymerization and the spectroscopic determination of the monomer reactivity ratios. The simultaneous use of the terminal and penultimate models for specific comonomer species is discussed.

Specific interactions, for example hydrogen bonding, dominate in numerous industrially important polymeric systems, both polymer solutions and blends. Typical cases are water-soluble polymers including biopolymers of special interest to biotechnology (e.g. the system polyethyleneglycol/dextran/water). Furthermore, most polymer blends are non-compatible and the requirement for compatible polymer pairs is often the presence of hydrogen-bonding interactions (e.g. polyvinylchloride/chlorinated polyethylene). In this work we give at first a short, comparative evaluation of existing thermodynamic models suitable for polymeric systems that take into account, explicitly, specific interactions like HB. The range of application of the models in terms of phase equilibria and their specific characteristics (accuracy of calculation, degree of complexity) are discussed. Finally, vapor–liquid equilibria (VLE) calculations for a number of polymer+solvent systems (including five different polymers) with a novel and very promising model are presented. This model is in the form of an equation of state that is (in its general formulation) non-cubic with respect to volume and has separate terms for physical and chemical interactions. The model has recently been proposed and has already been successfully applied to non-polymeric hydrogen-bonding systems (alcohol/water/hydrocarbons). This is the first time that it is extended to polymer solutions.

The reverse Monte Carlo (RMC) modelling technique has been applied to the polymeric melt poly(propylene oxide) (PPO). The modelling was based on neutron diffraction experiments on both hydrogenous and deuterated samples. Inter-atomic distances and calculated bond angle distributions for the RMC produced model were found to be in good agreement with reported experimental results, showing the ability of the RMC method to produce realistic structural models of amorphous polymers on the molecular length scale. The strengths and limitations of the RMC models in studies of the intermediate and long range order in polymers are investigated with emphasis on the information content and information quality.

Particularly, we find by studying the RMC model of PPO that the strong first diffraction peak at about 1.45 Å⁻¹ is almost entirely due to weak inter-chain correlations. The local chain conformation was investigated by calculating the partial atomic pair correlation functions for atoms belonging to monomers close in sequence. The results show that the most probable conformation is a “stretched” trans conformation, where two consecutive methyl groups are pointing in almost opposite directions. Although the dihedral and bond angle distributions found in the model are probably somewhat broader than in reality, they show unambiguously interpretable features which support the findings above.

Self-diffusion and solubility coefficients of six gas molecules (He, Ne, O₂, N₂, CH₄, and CO₂) in two poly(dibutoxyphosphazenes)—poly[bis(n-butoxy)phosphazene] (PnBuP) and poly[bis(sec-butoxy)phosphazene] (PsBuP)—have been investigated by means of molecular simulation using the COMPASS molecular mechanics force field. Diffusion coefficients were obtained from molecular dynamics (NVT ensemble) using up to 3 ns simulation time. Solubility coefficients were obtained by means of a Grand Canonical Monte Carlo (GCMC) method. Results of both simulations were in generally good agreement with experimental data with the exception of the simulation results for gas solubility in PsBuP where differences from the data may be attributed to microcrystallinity of the experimental sample. In the case of diffusivity, diffusion coefficients correlated well with the square of the effective diameter of the diffusing gas. Similarly a good correlation was found between the solubility coefficients obtained by GCMC simulation of sorption isotherms and the Lennard-Jones potential well depth parameter, ϵ/k.

The transition-state model of Gusev and Suter was used to determine free volume and free volume distribution for PnBuP, PsBuP, and poly[bis(iso-butoxy)phosphazene] (PiBuP). The diffusion coefficient for a given gas in each polyphosphazene was found to correlate exponentially with its accessible free volume fraction. A model for the distribution of accessible free volume, derived from the Cohen–Turnbull theory for the self diffusion of a liquid of hard spheres, was found to provide excellent fit with the simulation results.