Computational and Theoretical Polymer Science最新文献

Molecular dynamics simulations for 4-n-pentyl-4′-cyanobiphenyl (5CB) with as many as 944 molecules are reported. The order-N fast multipole method (FMM) is used to treat the long-range interactions. For a droplet of 944 molecules, the simulation shows a correlation between the droplet shape and the nematic order and a strong surface effect; little nematic order is found in a 118 molecule droplet. Simulations of the bulk system result in similar order parameters for both the 118 and 944 molecular ensembles. Although the nematic–isotropic transition was not observed at temperatures as high as 400 K using the CHARMM force field, a modification of the force field using ab initio determined partial atomic charges lowers the order parameters.

Lattice Boltzmann simulations are used to explore the behavior of liquid crystals subject to Poiseuille flow. In the nematic regime at low shear rates we find two possible steady-state configurations of the director field. The selected state depends on both the shear rate and the history of the sample. For both director configurations there is clear evidence of shear thinning, a decrease in the viscosity with increasing shear rate. Moreover, at very high shear rates or when the order parameter is large, the system transforms to a ‘log-rolling state’ with boundary layers that may exhibit oscillatory behavior.

Nacre (mother-of-pearl), the inner layer of seashells is a ceramic laminated biocomposite with exceptional mechanical properties of fracture toughness and strength. The organic layers in the composite play a significant role in the mechanical response of nacre to stresses. In this work, three dimensional finite element models of nacre (constructed in our previous work to design ‘brick and mortar’ micro-architecture of nacre) were used to study influence of nonlinear response of organic component. In this work, nonlinear elasto-plastic models for organic component are applied to model the mechanical response of nacre. Nanoscale material parameters (elastic modulus and hardness) were obtained using nanoindentation experiments. The yield stress of the organic was maintained at 40×10⁻⁶, 50×10⁻⁶, 60×10⁻⁶, 80×10⁻⁶, 120×10⁻⁶, 240×10⁻⁶, 320×10⁻⁶, and $\text{400×10}{}^{\mathrm{-6}}\text{N/μm}{}^2$ (40–400 MPa). The choice of initial value of yield stress of organic phase is the onset of nonlinearity in nacre response at that value. Tensile tests were simulated for each of these values of yield stress of organic phase under identical loading conditions of $\text{0–60×10}{}^{\mathrm{-6}}\text{N/μm}{}^2$ in increments of $\text{2.5×10}{}^{\mathrm{-6}}\text{N/μm}{}^2\text{.}$ For each value of organic phase yield stress stress–strain response of nacre is plotted. The resulting yield stress of nacre was compared to experimentally obtained value. This indicates that a much higher yield stress of organic is necessary to obtain the experimentally obtained yield stress of nacre. Microstructural implications of this result are suggested.

Dendrimers and hyperbranched polymers represent a novel class of structurally controlled macromolecules derived from a branches-upon-branches structural motif. The synthetic procedures developed for dendrimer preparation permit nearly complete control over the critical molecular design parameters, such as size, shape, surface/interior chemistry, flexibility, and topology. Dendrimers are well defined, highly branched macromolecules that radiate from a central core and are synthesized through a stepwise, repetitive reaction sequence that guarantees complete shells for each generation, leading to polymers that are mono-disperse. This property of dendrimers makes it particularly natural to coarsen interactions in order to simulate dynamic processes occurring at larger length and longer time scales. In this paper, we describe methods to construct 3-dimensional molecular structures of dendrimers (Continuous Configuration Boltzmann Biased direct Monte Carlo, CCBB MC) and methods towards coarse graining dendrimer interactions (NEIMO and hierarchical NEIMO methods) and representation of solvent dendrimer interactions through continuum solvation theories, Poisson–Boltzmann (PB) and Surface Generalized Born (SGB) methods. We will describe applications to PAMAM, stimuli response hybrid star-dendrimer polymers, and supra molecular assemblies crystallizing to A15 colloidal structure or Pm6m liquid crystals.

Particulate fillers used to reinforce polymers need not be spherical; some experiments have in fact been carried out on prolate ellipsoidal particles. These experimental results encouraged Monte Carlo simulations on prolate particles in amorphous polyethylene described in the present report. The particles were placed on a cubic lattice, and were oriented in a way consistent with their orientation in the composites experimentally investigated. Rotational isomeric state representations of the chains were then generated for the polymer matrix, with the discarding of spatial configurations that involved chains impinging on any of the particles. The chain end-to-end distributions were found to be non-Gaussian, and their dependences on the excluded volumes of the filler particles were documented. There were found to be particle-induced deformations which corresponded to decreased chain dimensions and radii of gyrations upon insertion of spherical particles amongst the chains, which is consistent with earlier simulations and with recent neutron scattering results. The decreases in dimensions and radii, however, were subsequently followed by increases upon increasing the axial ratios to distort the spherical particles into prolate shapes. The chain dimensions also became anisotropic, with significant differences parallel and perpendicular to the direction of the particle axes. Use of these distributions in the standard three-chain model of rubber-like elasticity gave the corresponding elongation moduli. The stress–strain isotherms thus obtained were found to be dependent on the sizes, numbers and axial ratios of the particles, as expected. In particular, the reinforcement from the prolate particles was found to be greatest in the parallel direction, and the changes were in at least qualitative agreement with the corresponding experimental results.

Molecular simulations of poly(vinyl phenol) were performed to study the effect of hydrogen bonds. Three conformations were constructed and their structure was validated in terms of the solubility parameter and gyration radius. Amorphousness was confirmed by calculating the X-ray pattern and pair correlation function. Isotropy of the structure was verified using the bond-orientational correlation function for backbone, phenyl rings, and O–H groups forming hydrogen bonds. Glass transition temperature was calculated using a stepwise change on temperature at constant pressure. The values were found to be comparable to experimental data and were consistent with poly(styrene) simulations published in the literature. The percentage of hydrogen bonds found in the model, 63%, was in good agreement with previous semi-quantitative evaluation by FTIR spectroscopy.

A kinetic model simulating the glass transition and enthalpic relaxation in poly(3-hydroxybutyrate) is introduced. The model is based on the concept that enthalpic relaxation or physical ageing is a continuation of the glass forming process and uses the KWW function to describe the glass formation process and the subsequent ageing of the glass. Non-linearity is introduced by incorporating a dependence of the relaxation time on the fictive temperature. The effects of non-linearity on the distribution of relaxation times and the physical ageing process are investigated together with the development of the endothermic ageing peak at the glass transition with increasing extents of ageing.

Molecular simulation techniques have been applied to previously synthesised liquid crystalline polymers containing azobenzene and diphenyl mesogenic groups within the chain. Single chains and amorphous unit cells of aromatic polymers with a degree of polymerisation of 4–16 and containing propylene and diethyletheric (oxydiethylene) spacers were used. The energy was minimised and then molecular dynamics were performed for 1000 ps at seven temperatures between 10 and 600 K. The axial ratio or coefficient of asymmetry was calculated from computer-generated structures. The predictive capability of the orientational order parameter was used to estimate the degree of orientation and the liquid crystalline–isotropic transition temperature of the polymers. The simulated results for the monotropic polymers agreed very well with Maier–Saupe mean field theory and experimental data, though the enantiotropic polymer did not show a good agreement. The predicted glass transition and decomposition temperatures of the simulated polymers are also reported.

A method described in the statistical literature for the numerical inversion of Laplace Transforms of real functions has been adapted for the derivation of molecular weight distributions from calculations of the generating function for a polymeric system. It has been shown that a third-order refinement is sufficiently accurate for molecular weight distributors broader than 4.0. This allows the calculations to be carried out with a precision of 16 decimal digits which is commonly used in Fortran. Where higher precision is available, the treatment is applicable to narrower MWDs.