Computational and Theoretical Polymer Science最新文献

In order to simulate a gelation process that occurs by condensation-polymerization reactions in solution, the cluster–cluster aggregation (CCA) model is modified by introducing four- and two-functional units as monomers. The modified CCA model shows a clear dependence of the fractal dimension of the resulting gel structure on the ratio between the numbers of the four- and two-functional units. The model should be applicable to various real systems. For example, it explains the behavior of experimentally found fractal dimension that depends on the amount of water added in the sol–gel aggregation process of SiO₂ systems.

An integral equation theory is investigated for the surface segregation from a blend of star and linear polymers. The molecules of both components are modeled as freely jointed tangent hard sphere molecules, and differ only in their topology, i.e. how the beads are connected. The surface is a hard wall impenetrable to the centers of the beads. The wall polymer reference interaction site model theory is used to study the surface segregation from this blend. The linear polymers are always in excess in the immediate vicinity of the surface as is expected from packing arguments. In most cases, the star polymers segregate to the surface if one looks at the integrated excess of star polymers over the linear polymers. This entropic segregation of the star polymers increases in magnitude if the functionality or arm length is increased.

Several types of substituted carbosilane-based dendrimers are studied in comparison with polyamidoamine (PAMAM), using molecular mechanics approach, to evaluate the shape and steric interactions when the generation number (G) increases. A scaled van der Waals energy parameter: the scaled steric energy, is defined, and used, to compare the steric repulsion in these dendrimers. Our calculations indicate that the steric repulsions, between the end groups at the surface of dendrimers, do not increase for higher generations of such macromolecules. Density calculations show that this property decreases with the increase of G. The moment of inertia calculations show that the shape of the considered dendrimers is asymmetrical for lower generations and becomes spherical at higher generations. The shape of the carbosilane dendrimers is more spherical than PAMAM. The results show that higher generations can afford the increased number of terminal groups at the surface of the macromolecules, without increase of the density in this region, therefor these factors (steric repulsion between the end groups at the surface, or high density) would not impede the chemistry to build higher generations of completely branched dendrimers.

After reviewing our recently proposed analogy between filled phantom rubbers and the electrostatics of conductors in a dielectric medium, we describe a general and open-ended algorithm for the evaluation of the force constant (=capacity) matrix between spherical filler particles. The method is based on the multipolar expansion of a charge distribution and of its electrostatic potential. Its performance is assessed by applying it to randomly dispersed hard spheres at volume fractions between 0.1 and 0.3.

Molecular dynamics simulations of polypeptides at high dilution near a fully hydrated bilayer membrane have been performed. In contrast to previous theoretical predictions, Monte Carlo simulations and conclusions from experiments a spontaneous insertion of amphiphatic or hydrophobic proteins into a membrane is not observed. Rather it is found that an amphiphatic chain has the tendency to remain in proximity to the membrane surface, whereas the location of a hydrophobic chain is more unbound. This is shown using two proteins, melittin and polyleucine. The conformation of the proteins and their orientation with respect to the membrane surface are discussed.

Computer modeling is applied to discuss hot-tube effects in melt spinning from crystallizing polymers. The set of spinning equations used in the model accounts for stress-induced crystallization, crystallinity-dependent melt viscosity and heat of crystallization. Example computations are performed for polyethylene terephthalate assuming temperature-dependent Newtonian viscosity, strongly modified by crystallization. The consequence of coupling of stress-induced crystallization and crystallinity-controlled solidification is limited range of spinning speeds, and multiple solutions of the dynamic equations of spinning. The range of admissible spinning speeds and multiple (amorphous and crystalline) solutions is strongly affected by the hot-tube temperature.

It is predicted that zone heating, with temperatures above glass transition (hot tube), results in considerable increase of amorphous orientation factor for moderate take-up speeds. In the high speed spinning range, the orientation effects saturate and does not exceed the values predicted for high-speed room-temperature spinning. Application of the hot tube is also predicted to reduce considerably take-up stress.

Available experimental data on amorphous orientation in PET fibers spun by hot-tube technique are in qualitative agreement with the model predictions.

This paper presents a procedure for simulating the anisotropic small-strain mechanical properties of oriented amorphous poly(ethylene terephthalate) (PET) starting from an atomistic level. A technique for producing oriented amorphous simulation cells of glassy PET has been developed and closely examined against related structural and property measurement data. The simulated elastic constants of these cells, derived by energy minimisation and molecular dynamics strain fluctuation methods, show encouraging agreement with experimental data.

A method of calculating interaction parameters used in phase equilibrium calculations has been extended for predicting solvent activities of polymer solutions. A pair of interaction parameters are determined by calculating interaction energies between all pairs of molecules in the solution of interest with a molecular mechanics method known as the consistent force field (CFF). The conformational space of a pair of molecules is sampled with a Monte Carlo algorithm followed by energy minimizations. In this paper, the method is used to calculate interaction parameters and solvent activities for the diisobutylketone/polyisobutylene system, using 2,2,4-trimethylpentane as a model compound for the polymer molecule.

The behavior of polymeric globule in a solution containing surfactants is analyzed within the framework of the Flory lattice theory. The amphiphilic structure of surfactant molecule is modeled as two adjacent lattice cells with opposite interaction parameters. Therefore, model surfactants prefer the globule–solvent interface rather than regions inside and outside the globule. It is found that at surfactant concentrations below some Critical Solubilization Concentration (CSC), surfactant molecules are adsorbed on the globular surface of almost invariable spherical shape. The coverage of the globular surface by surfactants takes place in rather broad region of concentrations. If the surfactant concentration exceeds the CSC, the globule sharply transforms to a coil. The transition is found to take place in a very narrow surfactant concentration region, i.e. polymer chain bypasses intermediate globular shapes and can be approximately regarded as a chain of spherical blobs. In this case, the solvent quality and the surfactant concentration are two main factors inducing globule–coil transition. Reducing of the transition point with increasing surfactant concentration as well as the steepness of the transition is in qualitative agreement with the experimental data.

Initial configurations suitable for molecular dynamics runs are usually assembled according to random values for the torsional angles of the molecules, and thus representing unrealistic conformations of the polymeric chains. In general, this would be acceptable if the system is allowed to run for periods of time long enough for the molecule to fully relax. However, in the current state of molecular dynamics runs, the 3D-periodic systems are usually allowed to run for 100 ps, which is too short for the polymeric system to relax. Alternatively, traditional rotational isomeric state approximation (RIS) could be used to generate the initial configurations of the polymeric chains. Unfortunately, RIS does not take into account the possible segment–segment overlap between atoms comprising the polymeric chains. In this work, we investigate the possibility of using the rotational isomeric state approximation to properly construct the initial configuration of 3D-periodic systems, without allowing any segment–segment overlap. In order to ensure that these configurations represent realistically the polymeric system, attempts were made to test the configurational properties of these systems against those determined experimentally. Further, these configurations were used to perform subsequent molecular dynamics runs in order to elucidate the effect of the molecular weight of poly(vinyl chloride) and temperature on some of the important thermodynamic properties such as self-diffusion coefficient, thermal pressure coefficient, heat capacity and dielectric constant.