Macromolecular Theory and Simulations最新文献

Front Cover: Symmetry break is introduced by a novel non-twin kneading element. The self-cleaning still remains where one-tip rotor engaged with two-tip one replaces two identical two-tip rotors of conventional twin screws. The numerical simulation is performed using finite element method and mesh superposition technique. A Lagrangian statistics method reveals that the non-twin geometry offers equivalent distributive mixing and better dispersive mixing. This is reported by Baiping Xu, Ruifeng Liang, Shuping Xiao, Lingcao Tan, Jian Song, and Huiwen Yu in article number 2200052.

A three-site metallocene catalyst is used in a gas-phase semi-batch reactor to produce ethylene/hexene copolymers. At the end of each batch, polyethylene (PE) is collected and analyzed to determine the carbon-13 nuclear magnetic resonance (¹³C-NMR) triad sequence distribution. Joint molecular weight (MW) and composition distribution data are obtained using gel permeation chromatography with an infrared detector (GPC-IR). Data from ten experimental runs are used for kinetic parameter estimation. Using a mean-squared error (MSE) selection methodology, 23 of the 36 model parameters are selected for estimation using the available polymerization rate and PE characterization data. The remaining parameters are held at initial guesses to avoid overfitting. Addition of the triad data to the parameter estimation problem allows for one additional parameter to be estimated and results in improved parameter estimates. Standard deviations of all but one of the estimated parameters decreased due to inclusion of triad data. The updated parameter estimates result in good fits for the triad data and for joint MW and composition data. The model accurately predicts four validation data sets not used for parameter estimation. The new model and its updated parameter estimates will be valuable for scaling up new polymer grades from laboratory-scale to commercial-scale.

The effects of N-isopropyl-N′-phenyl-phenylenediamine (4010NA) content on the damping and aging properties of hydrogenated nitrile butadiene rubber (HNBR)/nitrile butadiene rubber (NBR) (abbreviated as H-NBR) matrix are studied via molecular simulation and experiments. The effects of 4010NA addition on the damping and aging properties of H-NBR are analyzed by molecular simulation using solubility parameters (δ), hydrogen bonds, free volume fraction (FFV), binding energy (E_binding), hydrogen bond dissociation energy (ΔG), and mean square displacement (MSD). The damping, mechanical, and thermo-oxygen aging properties of the 4010NA/H-NBR composites are studied experimentally using infrared (FTIR) spectroscopy, X-ray diffraction (XRD), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA). The results indicate that 4010NA has good compatibility with HNBR and NBR, and the addition of 4010NA can effectively improve the damping properties of H-NBR. When 4010NA is added at 32 phr, the composite has better damping properties, mechanical properties, and aging resistance, which provides a new insight for the construction of high performance elastomer composites.

The effect of topology on the collapse transition and instantaneous shape of an energy polydisperse polymer (EPP), a model heteropolymer is studied by means of computer simulations. In particular, three different chain topologies, namely, linear (L), ring (R), and trefoil knot (T), are considered. The heteropolymer is modeled by assigning each monomer an interaction parameter, ε_i, drawn randomly from a Gaussian distribution. Through chain size scaling, the transition temperature, θ, is located and compared among the chains of different topologies. The influence of topology is reflected in the value of θ and observed that θ(L) > θ(R) > θ(T) in a similar fashion to that of the homopolymer counterpart. Also studied chain size distributions, and the shape changes across the transition temperature characterized through shape parameters based on the eigenvalues of the gyration tensor. It is observed that, for the model heteropolymer, in addition to chain topology, the θ-temperature also depends on energy polydispersity.

Cross-linking and branching of primary polymer molecules are investigated using the Galton–Watson (GW) process. Starting with the probability generating function (pgf) of the primary molecular weight distribution (MWD), analytical expressions are derived for the bivariate pgfs g(n_br, s) of branched polymers which depend also on the number of branch points n_br. The bivariate MWDs n(n_br, i) (i: number of molecular units) are then derived as Taylor expansions in s. All three cases of random branching: X-shaped (cross-linking), T-shaped (only one end takes part in the branching process), and H-shaped (both ends can take part in the branching process) are treated. An extension of the formalism does not require the construction of the pgf and allows the direct use of the MWD of the primary chains. However, using pgfs allows to go past the gel point and to determine the MWD and content of the sol. Explicit expressions are given for special distributions: the mono modal, the most probable, the Schulz-Zimm, the Poisson, and the Catalan distribution for the cases of X-shaped and T-shaped branching.

In this work, the tensile strength of polyimide/silica composites with the covalently bonded interface (bonded PI/SiO₂) is investigated by molecular dynamic simulation. It is found that the nanofiller with smaller size can bring out a larger number of hydrogen bonds and interfacial non-bond energy in the composites, resulting in higher tensile strength. As the immobilization of the PI chains in the vicinity of SiO₂, the covalently bonded interface is found to offer a greater reinforcing effect than the unbonded interface that is confirmed by the self-diffusion coefficient. The tensile strength of 9 wt.% bonded PI/SiO₂ composites is 11.34% higher than that of the unbounded composites. The tensile strength of PI/SiO₂ composites is enhanced with the increase of SiO₂ concentration up to critical mass percent (X_c), beyond which it will be decreased. To quantitatively predict X_c of PI/SiO₂ composites, an empirical equation based on the non-bond energy of the composites is proposed. The empirical equation showed that the X_c of PI/SiO₂ composites ranged from 8.03 to 10.36 wt.%, which is consistent with experimental values. These results provided the understanding of size-dependent covalently bonded interface structure, which would be beneficial to the design of nanocomposites with excellent mechanical performances.

The effects of copolymerized monomer vinyl acetate (VAc) on processing properties and thermal stability of poly(vinyl chloride-co-vinyl acetate) (PVCA) are investigated via experiment and molecular dynamics simulation. Experimental results showed that PVCA with higher VAc content has larger loss tangent (tanδ), lower complex viscosity (η*), and glass transition temperature (Tg), which improved the processing properties of PVCA. A series of PVCA models are constructed to study the microstructure on the processing properties of PVCA, and the results showed the PVCA with higher VAc content exhibits larger molecular chain mobility and free volume fraction (FFV), smaller intermolecular interactions, and the mean square end-to-end distance (<Ree²>). Furthermore, the IR spectra of gas products indicated that thermal degradation of PVCA mainly generated hydrogen chloride (HCl), carboxylic acid, and aliphatic hydrocarbons between 200 and 500 °C, and the removal of HCl and carboxylic acid is almost simultaneous. The degradation models of PVCA chains demonstrated the CCl bond in vinyl chloride (VC) and CO bond in VAc have similar thermal stability, which corresponded to the experimental results. In a word, the work provides a promising technique to study the structure and property of PVCA at molecular dynamic level.

Confidence contours in parameter space are a helpful tool to compare and classify determined estimators. For more intricate parameter estimations of nonlinear nature or complex error structures, the procedure of determining confidence contours is a statistically complex task. For polymer chemists, such particular cases are encountered in determination of reactivity ratios in copolymerization. Hereby, determination of reactivity ratios in copolymerization requires nonlinear parameter estimation. Additionally, data may possess (possibly correlated) errors in both dependent and independent variables. A common approach for such nonlinear estimations is the error-in-variables model yielding statistically unbiased estimators. Regarding reactivity ratios, to date published procedures neglect the non-Gaussian structure of the error estimates that is a consequence of the nonlinearity of the model. In this publication, this issue is addressed by employing a Bayesian hierarchical model, which correctly propagates the errors of all variables. The statistical procedure is discussed in chemist friendly language to encourage confident usage of the tool. The approach is based on a Python program requiring minimal installation effort. A detailed manual of the code is included in the appendix of this work, in an effort to make this procedure available to all interested polymer chemists.

Front Cover: The effects of the primary structure of reactive polymers on the structure and mechanical properties of gels are studied by a molecular dynamics simulation. The figure illustrates the improvement of structure uniformity by changing the arrangement of functional groups from a random one (top) to a periodic one (bottom), where red surfaces show low crosslinking density regions. This is reported by Tsutomu Furuya and Tsuyoshi Koga in article number 2200044.