Computational and Theoretical Polymer Science最新文献

Three representative members of the family of poly(α-alkyl β-l-aspartate)s (PAALA-12, -16 and -18) have been investigated using Monte Carlo simulation of an atomistically explicit model. At the temperatures investigated, the main chains (helices) are arranged in highly regular layers, while the space between layers is filled by the alkyl side chains. The molecular structure of the inter-layer region is found to be very approximately liquid-like. The simulations are also able to predict the anomalous (densification) behavior of the PAALAs as the temperature is raised through the transition value.

We report preliminary simulations of anisotropic scattering from aggregates of small hard spherical particles embedded in an elastic polymer matrix, using simple geometrical methods. First we build several types of aggregates in three dimensions: crystalline, amorphous compact, fractals, of different numbers of particles and varying polydispersity. We then turn to the spectra of deformed samples simulated in two dimensions. We impose an affine displacement inside the matrix to the fillers, which can be isolated particles or small aggregates, and account for the collisions which arise due to lateral shrinking of the material. The two-dimensional scattering spectra are shown and discussed. They reproduce experimentally observed isointensity curves: ellipses, banana-shaped maxima and splitting of these maxima in four spots. Finally, we explore the consequences of the reduction to two dimensions via statistics of the number of collisions. It is found that even if collisions are more important in 3 dimensions, the behavior is qualitatively similar in two and three dimensions.

A new Monte Carlo method is proposed for the simulation of bulk systems of atomistically detailed polymers. Each move consists of a configurational rearrangement of the atoms in a specified region of the material, rather than a specified molecule. Thus atoms within different chains may be displaced cooperatively in each Monte Carlo move. Here, the method is implemented for the case of melts of linear chains, where the bond lengths and bond angles are held constant during the move. The performance of the algorithm is examined for linear polyethylene systems with chain lengths of 100 and 1000 backbone atoms, under a range of conditions. The method shows a considerable potential as a very general and flexible tool for simulating realistic polymer materials, subject to a number of performances limiting factors which are described in detail.

Using an extension of a previously developed methodology, the intrinsic viscosity of mixed hyperbranched–linear polymers is calculated as a function of molecular weight, relative reactivity of the functional groups on the brancher AB₂-molecules and the amount of linear AB-comonomer used. It is shown that using even relatively high amounts of linear AB-comonomers does not increase the intrinsic viscosity too much. For a ratio 4:1 of linear to brancher monomers the intrinsic viscosity only increases by a factor of 2 compared to a hyperbranched polymer originating from brancher AB₂ monomers only. The intrinsic viscosity of hyperbranched (co)polymers as a function of degree of branching follows a master curve not depending on how the polymer (using AB₂ or AB-monomers) was formed. This relation only fails at very high and very low degrees of branching where the architecture of the polymer chains becomes a very important factor.

Some considerations are made concerning the question of how predictions of the intrinsic viscosity can be extended to predictions of the viscosity at high concentration or in the melt. The possible use of the polymer–reference interaction site model (PRISM) and dissipative particle dynamics is discussed.

We present a theory of hysteresis friction of sliding bulk rubber networks using the dynamic Rouse model for the rubber–glass transition region. The hard substrate is described by a self-affine rough surface that is a good representative for real surfaces having asperities within different length scales of different orders of magnitude. We find a general solution of the friction coefficient as a function of sliding velocity and typical surface parameters (e.g. surface fractal dimension, correlation lengths of surface profile). Further, we show the correlation with the viscoelastic loss modulus of the bulk rubber and the applicability of the Williams–Landel–Ferry transform to the velocity and temperature dependence of the frictional force as found experimentally. We demonstrate how the succesive inclusion of relaxation Rouse modes p=1,2,3,… into the final expression for the frictional force leads to a superposition of the contributions of the different modes and, as a consequence, to a broad, bell-shaped frictional curve as observed in the pioneering experiments of Grosch. We show how the theory simplifies for the special case of a Rouse slider interacting with a Brownian surface.

Two forcefields were evaluated for possible use in polysiloxanes containing aromatic phenyl or phenylene groups, either as side chains or as part of the chain backbone. The criterion was reproducing results on the crystal structure of the cyclic diphenylsiloxane trimer, and both forcefields were satisfactory. The better of the two was used to obtain conformational energies for estimating some of the statistical properties of the corresponding polymer poly(diphenylsiloxane) (PDPS) [Si(C₆H₅)₂–O–]. The calculations were based almost entirely on the rotational isomeric state (RIS) theory, but the possibility of using a Metropolis Monte Carlo method was also considered. Since the stiffness of the chains was of primary interest, the quantities calculated were the unperturbed dimensions and its temperature coefficient and the radii of gyration. Degrees of polymerization ranged from 40 to 400, and temperatures from 300 to 1000 K. The “characteristic ratio” (of the mean-square unperturbed dimensions to those of the corresponding freely-jointed chain) was found to be approximately 65 in the limit of very long chains. This is an order of magnitude larger than that of the much studied and very flexible poly(dimethylsiloxane), and the associated chain stiffness this indicates for PDPS seems to be consistent with its high transition temperatures and other properties.

The birefringence of uniaxially stretched, long-chain polyethylene (PE) melts is predicted through detailed atomistic simulations by employing the end-bridging Monte Carlo method. The method involves two steps: First, a large number of well-equilibrated, uniaxially stretched polymer configurations are sampled by invoking the methodology developed in our recent work on the simulation of polymer melt elasticity. A key feature in this step is the tensorial field a_xx which orients and, under certain conditions, deforms the polymer chains in the x direction, inducing anisotropy in the melt. Second, the structural characteristics of the oriented polymer configurations are analyzed and a description of their anisotropy at the monomer level is obtained. By transforming the polarizability tensor of each individual skeletal bond (or united atom group) from the coordinate frame of its principal axes to the laboratory frame, the ensemble average polarizability tensor per methylene group 〈α〉 of the uniaxially stretched polymer melt is calculated as a function of the segment order parameter S_x. The anisotropic melt refractive index Δn(≡n_xx−n_yy) is obtained from 〈α〉 by using the Clausius–Mossoti and Lorentz–Lorenz relationships. Results obtained for two linear PE melts (average chain length C₇₈ and C₂₀₀) verify the validity of the stress optical law for small enough imposed elongational flow rates a_xx. The calculated stress optical law coefficient C is found to be equal to (3.15±0.20)×10⁻⁹ Pa⁻¹ for the C₇₈ melt and equal to (2.35±0.10)×10⁻⁹ Pa⁻¹ for the C₂₀₀ melt. The experimentally measured value for high-molecular weight, linear, high-density PE melts is 2.20×10⁻⁹ Pa⁻¹.

Perylene containing main-chain polyimides are candidate materials for the photogenerator layer of electrophotographic devices. These polymers are semi-crystalline as synthesized. Annealing improves the crystallinity while causing subtle changes in the X-ray diffraction pattern and a red-shift in the UV–vis absorption spectrum. Molecular dynamics simulations of linear chains of …Pe–C₁₂–Pe… led to chain folded collapsed structures. It is known that a minimum chain length of 150 CH₂ units is required, in the case of linear alkanes, for stable folding to occur. However, the strong attractive π interaction between the perylene units causes the chain to fold, with as few as 12 CH₂ spacer units. This shows that both intra and intermolecular nematic order can occur in these systems, the former caused by chain folding.

The microscopic structure of a semi-crystalline polymer interphase has been investigated by off-lattice Monte Carlo techniques for polyethylene-like flexible chains. In this approach, the conformational space consisting of chain populations of loops, bridges and tails is explored by robust cutting and splicing moves in real space. The simulations capture the most probable equilibrium distributions. The populations of loops and tails follow a truncated exponential distribution and the population of bridges shows a maximum as a function of chain length. For simulations of flexible chains, 40–45% of the chains form adjacent entry folds. The effect of molecular weight has been investigated. The bridge population is found to increase from 5 to 10%, for the interphase thicknesses studied, as the molecular weight of material simulated increases from ∼8000 gm/gmol to ∼30 000 gm/gmol. A Gaussian model for the interphase has been developed and compared with simulations of non-interacting phantom chains. The distributions match well at long chain lengths and deviate at short lengths.

The ordering in polymer brushes formed by macromolecules with mesogenic groups in the main chain is investigated. The numerical method of self-consistent field approximation was used. The existence of two different liquid crystalline nematic states is shown: homeotropic (HLC) and planar (PLC) states. The free energy of the HLC state is always less than that of the PLC state. However, with the increase of energy of anisotropic interactions, (with decrease in temperature) our numerical procedure leads us to either one or another state depending on the grafting density. The results obtained show that both brush surfaces, play an essential role in establishing the concrete LC state structure. The grafting surface and the external surface force the planar order.