Molecular dynamics simulations of monodisperse/bidisperse polymer melt crystallization

We use large scale coarse‐grained molecular dynamics simulations to study the kinetics of polymer melt crystallization. For monodisperse polymer melts of several chain lengths under various cooling protocols, we show that short chains have a higher terminal crystallinity value compared to longer ones. They align at the early stages and then cease evolving. Long chains, however, align, fold into lamella structures and then slowly optimize their dangling ends for the remaining simulation time. We then identify the mechanism behind bidisperse blend crystallization. To this end, we introduce a new algorithm (called Individual Chain Crystallinity) that allows the calculation of the crystallinity separately for short and long chains in the blend. We find that, in general, bidispersity hinders crystallization significantly. At first the crystallinity of the long chain components exceeds that of the monodisperse melt, but subsequently falls below the corresponding monodisperse melt curve after a certain “crossover time.” The time of the crossover can be attributed to the time required for the full crystallization of the short chains. This indicates that at the early stages the short chains are helping long chains to crystallize. However, after all short chains have crystallized they start to hinder the crystallization of the long chains by obstructing their motion. Lastly, polymer crystallization upon various thermodynamic protocols is studied. Slower cooling is found to increase the crystallinity value. Upon an instantaneous deep quench and subsequent isothermal relaxation, the crystallinity grows rapidly with time at early stages and subsequently saturates. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 2318–2326     

Bead-spring models of entangled polymer melts: Comparison of hard-core and soft-core potentials

Two bead-spring models of flexible chains for generic coarse graining of entangled polymer melts, the excluded volume Kremer–Grest (KG) model and the modified segmental repulsive potential (mSRP) combined with a weakly repulsive potential, are compared. For chains containing an equivalent number of entanglements, we compare the chain characteristics of the KG and mSRP polymer models by determining the ratios of the entanglement lengths , the required total number of particles to capture comparable entanglement phenomena , and the time scaling ratios τ^mSRP/τ^KG. Our findings show that systems using the mSRP polymer model require half the number of particles and relax four times faster compared to the KG polymer model. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

Influences of three kinds of springs on the retraction of a polymer ellipsoid in dissipative particle dynamics simulation

The impacts of Rouse spring, Fraenkel spring, and one kind of finitely extensible nonlinear elastic spring (FENE‐PM spring) on the surface tension‐induced retraction of a polymer ellipsoid in a matrix were compared using dissipative particle dynamics. Using the same spring constant, obvious differences among the three kinds of springs were found. A fast retraction process was observed from the hard Fraenkel spring, a slow process from the soft Rouse spring, and an intermediate process from the FENE‐PM spring. The effects of varying the spring constant and the chain length were also investigated. The results indicate that the influence of increasing spring hardness for a given spring type was significant; whereas, the influence of chain length was minor after five bonds were reached. The effects of varying the FENE‐PM r_m parameter were also studied to provide a reliable value for this study. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

Identification of the precursor cluster in thermolysin crystallization solution by molecular dynamics methods

An analysis of the crystal structure of thermolysin revealed four possible precursor clusters (hexamers) of its crystal. Using the method of molecular dynamics and plots of root mean square fluctuation, root mean square deviation and radius of gyration, the most stable hexamer, which is a precursor cluster,was determined. The importance of the established structure of the thermolysin precursor cluster for determining the mechanism of crystal formation is shown.     

Molecular fractionation in isotactic polypropylene during isothermal crystallization

The nucleation frequency of isotactic polypropylene shows for certain molecular weight distributions during isothermal crystallization a “stepwise” development of the nuclei as a function of time. The resulting curve can be fitted by a superposition of exponential functions assuming a separate nucleation density for each nucleation step. The multimodal nucleation is discussed as a consequence of molecular fractionation effects during crystallization. Received: 23 October 1997 Accepted: 12 May 1998     

Molecular dynamics simulations of semicrystalline polymers: Crystallization,melting, and reorganization

Large‐scale molecular dynamics (MD) simulations of semicrystalline entangled polymers are carried out to explore crystallization and melting processes. Semicrystalline polymers are obtained from disordered melts via homogeneous nucleation. In the early stage of the crystallization process, the collective scattering does not show the emergence of nuclei seeds. Although the crystallization process is thermodynamically simple, the melting process is complex resulting in multiple‐peaked melting endotherms. The molecular origin is found to be the different thermal stabilities of microcrystalline domains (MCDs). Coexistence of melting and growth of different MCDs during sufficiently slow heating enlarges the difference of their thermal stabilities. An increase of stem length close to the melting point is assisted by disorder effects in particular in the surface regions of the MCDs. The number of trans–trans states is decreasing, which increases the flexibility and mobility of the crystalline stems. We have also investigated self‐seeding processes, and we show how these can be used to obtain single lamellar crystals in MD simulations. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2010     

聚乙烯在石墨表面结晶的分子动力学模拟

采用分子动力学方法模拟了聚乙烯在石墨(001)表面的吸附和结晶过程;直观的给出了聚乙烯链被石墨(001)面吸附并诱导形成有序的片层晶体的过程;发现结晶温度对得到的有序结构中的聚乙烯链相对石墨表面的特定取向有影响(300 K和600 K时的取向方向不同);表面覆盖率影响聚乙烯吸附层的厚度,对取向的方向无影响.     

Hairpin polymer unfolding in square nanochannels

The unfolding dynamics of a flexible hairpin polymer inserted in a square nanochannel is studied using Brownian dynamics simulations of the bead‐spring model. Because the hairpin polymer is not an equilibrium configuration, the molecule starts unfolding until it reaches a stretched configuration inside the tube. We study the effect of varying the channel height and width D, and the number of monomers N in the folded arm on the unfolding times. We show that for square nanochannels, the unfolding time scales as DN2, for small values of D. The unfolding relaxation dynamics obeys similar mechanisms described in the escaping dynamics of partially inserted polymers in cylindrical nanotubes. We also show that the velocity of the polymer center of mass scales as D^?1, in agreement with DNA unfolding experiments in solid‐state nanochannels and recent computational simulations. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 1411–1418     

Computer modeling of the crystallization process of single‐chain ethylene/1‐hexene copolymers from dilute solutions

Langevin molecular dynamics (LMD) simulations have been performed to understand the role of the short chain branches (SCB) on the formation of ordered domains by cooling dilute solutions of ethylene/α‐olefins copolymer models. Three different long single‐chain models (C₂₀₀₀) with 0, 5, and 10 branches each 1000 carbons were selected. These models were equilibrated at high reduced temperature (T* = 13.3) and cooling in steps of 0.45 until the final temperature (T* = 6.2) by running a total of 35 × 10⁶ LMD steps. During the cooling process, global order parameter, torsion distribution, position of the branches, and local‐bond order parameter were calculated and monitored. The peaks of crystallization for each model were calculated by differentiating the global order parameter with temperature. The T_c (crystallization temperature) decreases as the number of branches increases as has been experimentally reported. The formation of order in the copolymers is affected by the amount of the SCB in the backbone of the polymer chain. Initially, the SCB move to the folding surface. Once the SCB are located near the folding surface the order starts to grow. In all cases here shown, the C₄ branches are excluded from the ordered domains. To take into account, the influence of the branch distribution, a different branch distribution model has been considered for the two‐branched systems. The crystallization fraction (α) and the density of the amorphous and ordered fractions was defined using the local‐bond order parameter. Both magnitudes decrease as the number of branches increases. These facts fairly agree with experimental literature data. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011     

Nanoconfinement as a tool to study early stages of polymer crystallization

This article promotes the idea that crystallization experiments under confinement can be an interesting tool to learn more about the early stages of polymer crystallization in bulk systems. Based on various results from the recent literature, it is demonstrated that crystalline forms that are metastable, transient, or inaccessible in bulk samples can be easily obtained and studied in nanoscopic compartments. This is interesting in the light of the recent discussion about thermodynamic reasons for the temporary occurrence of a mesophase at the growth front of crystals in bulk polymers, in particular, about a hexagonally packed mesophase in polyethylene. The experimental findings for nanoconfined methylene sequences seem to support indirectly thermodynamic approaches explaining the occurrence of a mesophase based on the small thickness of the crystal at the growth front. A first estimate for the critical crystal thickness d_mc defining the transition from hexagonal to orthorhombic packing in case of confined methylene sequences is provided based on results for side chain polymers. Further perspectives of crystallization experiments on confined systems are discussed. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 1556–1561, 2008     

Effect of polymer chain stiffness on initial stages of crystallization of polyetherimides: Coarse‐grained computer simulation

We use dissipative particle dynamics simulation to study the role of the intramolecular stiffness in the crystallization process of aromatic polyetherimides. We have developed and parameterized a coarse‐grained model for polyimides R‐BAPB and R‐BAPS, which have similar chemical structures but different macroscopic properties. The former one is known as semicrystalline, while the latter one is amorphous. In our model, the only difference between these two polyimides is the intramolecular stiffness. We show that this model can reasonably reproduce the structure formation in polyimide melts. We observe initial stages of crystallization of polyimide R–BAPB while R‐BAPS stays amorphous. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1254–1265     

Molecular dynamics study of crystalline water ices

The behavior of structures of H₂O crystalline ices Ih, Ic, XI, VII, VIII, VI is studied in molecular dynamics experiment using the potential offered by Poltev and Malenkov. The behavior of the system consisting of one of the two identical interpenetrating, but without any common hydrogen bonds, water frameworks comprising the ice VI structure is also simulated. As a result of simulations, the ice VII structure has collapsed, whereas other systems proved to be stable. The reasons of instability of the ice VII and previously studied ice IV structures in molecular dynamics experiments are discussed. Based on the simulation results of the above-mentioned ices and previous simulation of ices II, III, IX, IV, and XII, the general regularities of dynamic properties of water molecules in crystalline water ices are formulated. Unreliability of results obtained by molecular dynamics in the investigation of self-organizing processes in aqueous systems is shown.     

Enhancing heterogenous crystallization resistance in a bead‐spring polymer model by modifying bond length

Linear bead‐spring chain models have been widely employed to study the glass‐formation of polymer melts due to their apparent resistance to homogenous crystallization. In this article, we present simulation data demonstrating that, in contrast to their bulk behavior, widely used bead‐spring models are subject to rapid heterogenous nucleation and crystallization when exposed to rigid crystalline walls. We propose a modified bead‐spring model, employing the finitely extensible nonlinear elastic bond, and show that it exhibits robust glass‐formation in the presence of crystalline walls. This new model will enable study of supercooled polymers in the presence of rigid, atomistically structured walls, with application to study of nanoscale confinement effects on the glass transition. © 2013 Wiley Periodicals, Inc. J. Polym. Sci. Part B: Polym. Phys. 2013 , 52, 134–140     

Molecular dynamics study of ferrocene topology under various temperatures

We present the first quantum mechanical Atom-Centered Density-Matrix Propagation molecular dynamic (MD) study to investigate ferrocene (Fc) conformation in gas phase. The MD simulations were performed at several temperatures (7, 18, 80, 120, 180, 293, and 500 K) for a period of 10 ps. It is found that, at very low temperatures (≤18 K), ferrocene prefers eclipsed-like conformation. At higher temperatures (>18 K), the cyclopentadienyl rings (Cp) of ferrocene exhibit apparent fluxional rotations, leading to configurations with the rotational angle δ distributing within a range of 0° (eclipsed) to 18° (approximately half of 36° for the staggered conformation), accompanied by the cyclopentadienyl ring tilt up to approximately 12° at 500 K. The simulated mean inferred (IR) spectrum of ferrocene at 7 K is clearly dominant by a doublet-splitting band of eclipsed-like Fc features in the region of 400 to 600 cm⁻¹, in agreement with previous IR studies. The animation obtained from the MD simulations indicates that, at room temperature, the Fe-C distances in ferrocene are in fact not strictly congruent but 2:2:1-fold.     

Spinodal patterns indicating unstable regime of polymer crystallization

It has been considered that crystallization of polymer from melt proceeds via the coexistence of molten matrix and growing crystals that have once overcome a nucleation barrier to a critical size. The nucleation process has often been explained analogously with so-called nucleation and growth (NG) behavior of the phase separation of a binary mixture in metastable conditions, although the crystallization in one-component polymer is not a real component separation but a phase transition. Among the mechanisms of polymer crystallization, the topic is whether a liquid–liquid transition between states of different densities within one-component polymers takes place before the aforementioned nucleation process. The liquid–liquid transition between states, which is probably driven by chain orientation, is also categorized into NG and the controversial spinodal decomposition (SD) type processes depending on the quenching depth. This article provides the optical microscopic observations that favor the occurrence of the SD-like process when a one-component polymer melt is very rapidly quenched below a stability limit, including a drastic morphological change from a spherulitic to a spinodal pattern at the critical (or spinodal) temperature. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1817–1822, 2004     

Non-iterative and exact method for constraining particles in a linear geometry

We present a practical numerical method for evaluating the Lagrange multipliers necessary for maintaining a constrained linear geometry of particles in dynamical simulations. The method involves no iterations and is limited in accuracy only by the numerical methods for solving small systems of linear equations. As a result of the non-iterative and exact (within numerical accuracy) nature of the procedure, there is no drift in the constrained geometry, and the method is therefore readily applied to molecular dynamics simulations of, for example, rigid linear molecules or materials of non-spherical grains. We illustrate the approach through implementation in the commonly used second-order velocity-explicit Verlet method. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 911-916, 2005     

Transient and athermal effects in the crystallization of polymers. II. Nonisothermal crystallization

Nonisothermal crystallization of several polymers was investigated with differential scanning calorimetry and optical microscopy. The results indicated that as in the case of isothermal processes, crystallization starts with nucleation on noncompletely melted crystalline residues. It is assumed that if the crystalline residues are subcritical at melting temperatures, they can become stable by an athermal mechanism during cooling. There is also some contribution of nucleation on heterogeneities. The next mechanism of nucleation is a classical homogeneous process occurring by thermal fluctuations. The results showed the non‐steady‐state character of the nonisothermal crystallization of polymers. In the investigated range of cooling rates, the non‐steady‐state character of nonisothermal crystallization of polymers is dominated by the transient thermal effects. In the range of high temperatures, the transient homogeneous nucleation can be interpreted with the Ziabicki model, and the steady‐state rate determined from nonisothermal experiments coincides with the rate determined in isothermal crystallization. The athermal nucleation occurring at the beginning of crystallization from noncompletely melted aggregates seems to be independent of the applied cooling rate. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 68–79, 2003     

Dissipative particle dynamics simulation study of the bilayer-vesicle transition

A bilayer structure is an important immediate for the vesicle formation. However,the mechanism for the bilayer-vesicle transition remains unclear. In this work,a dissipative particle dynamics(DPD) simulation method was employed to study the mechanism of the bilayer-vesicle transition. A coarse-grained model was built based on a lipid molecule termed dimyristoylphosphatidylcholine(DMPC). Simulations were performed from two different initial configurations:a random dispersed solution and a tensionless bilayer. It was found that the bilayer-vesicle transition was driven by the minimization of the water-tail hydrophobic interaction energy,and was accompanied with the increase of the position entropy due to the redistribution of water molecules. The bulk pressure was reduced during the bilayer-vesicle transition,suggesting the evolved vesicle morphology was at the relatively low free energy state. The membrane in the product vesicle was a two-dimensional fluid. It can be concluded that the membrane of a vesicle is not interdigitated and most of the bonds in lipid chains are inclined to orient along the radical axis of the vesicle.     

Simulations of polymer crystallization under high pressure

High pressure crystallization of polypropylene was studied by means of PVT measurements and computer simulations. The isothermal crystallization behaviour were described by using a model which takes into account the effect of pressure on the temperature dependence of nucleation rate and linear growth rate. The agreement between the simulation and the experiments was seen in the tendency that the crystallization was accelerated by the high pressure. The non-isothermal crystallization behavior was also simulated by applying a generalized Avrami equation. The simulation curves well reproduced the experimental values below relative crystallinity 0.5 and below 100 MPa.     

Molecular dynamics study of the cations,water molecules,and polymer chains in Nafion type membranes

The structure and dynamics of the hydrated cationic complexes in Nafion type membrane pores has been studied by the molecular dynamics approach. The mechanism of the cationic transport has been examined. The dependence of the cationic transport coefficients on temperature and the number of water molecules has been investigated.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1233–1236, July, 1995.The present work was made possible in part by financial support of the International Science Foundation (Grant MPG000) and the Russian Foundation for Basic Research (Grant 93-03-4205).