Simulation of melting in crystalline polyethylene

We carry out a molecular dynamics simulation of the first stages of constrained melting in crystalline polyethylene (PE). When heated, the crystal undergoes two structural phase transitions: from the orthorhombic (O) phase to the monoclinic (M) phase, and then to the columnar (C), quasi-hexagonal, phase. The M phase represents the tendency to the parallel packing of planes of PE zigzags, and the C phase proves to be some kind of oriented melt. We follow both the transitions O→M and M→C in real time and establish that, at their beginning, the crystal tries (and fails) to pass into the partially ordered phases similar to the RI and RII phases of linear alkanes, correspondingly. We discuss the molecular mechanisms and driving forces of the observed transitions, as well as the reasons why the M and C phases in PE crystals substitute for the rotator phases in linear alkanes.     

Molecular dynamics in n-alkanes: premelting phenomena and rotator phases

Molecular dynamics simulations of the n-alkanes C18H38, C19H40, and C20H42 are reported for temperatures just below the melting point. Besides thermodynamic and average structural data for the ordered phase, we discuss the molecular motions initiating the rotator phases observed in spontaneous phase transitions in isothermal, isostress simulations. The RI phase of C19H40 is initiated by particular cork-screw-like jumps combining a quarter turn about the long molecular axis and a half-chain-period translation along the axis. This motion occurs between the minimum-energy conformation of the ordered crystal and a secondary minimum. Transient analogs of the RI and RII phases of the odd alkanes are found on melting C18H38 and C20H42. Collective motions within lamellae of molecules are prominent in the dynamics.     

Discrimination in isotropic, nematic, and smectic phases of chiral calamitic molecules: a computer simulation study

Racemic fluids of chiral calamitic molecules are investigated with molecular dynamics simulations. In particular, the phase behavior as a function of density is examined for eight racemates. The relationship between chiral discrimination and orientational order in the phase is explored. We find that the transition from the isotropic phase to a liquid crystal phase is accompanied by an increase in chiral discrimination, as measured by differences in radial distributions. Among ordered phases, discrimination is largest for smectic phases with a significant preference for heterochiral contact within the layers.     

Phase change materials of n-alkane-containing microcapsules: observation of coexistence of ordered and rotator phases

In the present investigation, the crystallization and phase transition behaviours of normal alkane (n-docosane) in microcapsules with a mean diameter of 3.6 μm were studied by the combination of differential scanning calorimetry (DSC), temperature-dependent X-ray diffraction (XRD) and variable-temperature solid-state nuclear magnetic resonance (VT solid-state (13)C NMR). The DSC and VT solid-state (13)C NMR results reveal that a surface freezing monolayer is formed prior to the bulk crystallization of the microencapsulated n-docosane. More interestingly, it is confirmed that after the bulk crystallization, the ordered triclinic phase coexists with the rotator phase I (RI) for the microencapsulated n-docosane. We argue that the reduction of the free energy difference between the two phases, resulting from the microencapsulation process, leads to the coexistence of the ordered triclinic and rotator phases of the normal alkanes.     

Determination of fluid--solid transitions in model protein solutions using the histogram reweighting method and expanded ensemble simulations

Protein crystallization conditions are usually identified by empirical screening methods because of the complexity of the process, such as the existence of nonequilibrium phases and the different crystal forms that may result from changes in solution conditions. Here the crystallization of a model protein is studied using computer simulation. The model consists of spheres that have both an isotropic interaction of short range and anisotropic interactions between patch-antipatch pairs. The free energy of a protein crystal is calculated using expanded ensemble simulations of the Einstein crystal, and NpT-Monte Carlo simulations with histogram reweighting are used to determine the fluid-solid coexistence. The histogram reweighting method is also used to trace out the complete coexistence curve, including multiple crystal phases, with varying reduced temperature, which corresponds to changing solution conditions. At a patch-antipatch interaction strength five times that of the isotropic interaction, the protein molecules form a stable simple cubic structure near room temperature, whereas an orientationally disordered face-centered-cubic structure is favored at higher temperatures. The anisotropic attractions also lead to a weak first-order transition between orientationally disordered and ordered face-centered-cubic structures at low temperature, although this transition is metastable. A complete phase diagram, including a fluid phase, three solid phases, and two triple points, is found for the six-patch protein model. A 12-patch protein model, consistent with the face-centered-cubic structure, leads to greater thermodynamic stability of the ordered phase. Metastable liquid-liquid phase equilibria for isotropic models with varying attraction tails are also predicted from Gibbs ensemble simulations.     

Electrofreezing of confined water

We report results from molecular dynamics simulations of the freezing transition of TIP5P water molecules confined between two parallel plates under the influence of a homogeneous external electric field, with magnitude of 5 V/nm, along the lateral direction. For water confined to a thickness of a trilayer we find two different phases of ice at a temperature of T=280 K. The transformation between the two, proton-ordered, ice phases is found to be a strong first-order transition. The low-density ice phase is built from hexagonal rings parallel to the confining walls and corresponds to the structure of cubic ice. The high-density ice phase has an in-plane rhombic symmetry of the oxygen atoms and larger distortion of hydrogen bond angles. The short-range order of the two ice phases is the same as the local structure of the two bilayer phases of liquid water found recently in the absence of an electric field [J. Chem. Phys. 119, 1694 (2003)]. These high- and low-density phases of water differ in local ordering at the level of the second shell of nearest neighbors. The results reported in this paper, show a close similarity between the local structure of the liquid phase and the short-range order of the corresponding solid phase. This similarity might be enhanced in water due to the deep attractive well characterizing hydrogen bond interactions. We also investigate the low-density ice phase confined to a thickness of 4, 5, and 8 molecular layers under the influence of an electric field at T=300 K. In general, we find that the degree of ordering decreases as the distance between the two confining walls increases.     

Two-phase coexisting state of n-hexatriacontane in the first-order phase transition

The small crystal of n-hexatriacontane was observed by a polarizing microscope in the rotator phase transition temperature region. In the temperature region, the rotator phase coexists with the solid phase (low-temperature ordered phase). The boundaries of two phases move reversibly with the temperature change. The area fractional change of the rotator phase can be described by the Debye relaxation. The relaxation time decreases and the relaxation strength increases as the sample temperature is raised. The relaxation time agrees well with that of the dynamic specific heat, which was measured in the frequency range of 0.0003≤f/Hz≤0.05.     

Orientational melting and reorientational motion in a cubane molecular crystal: a molecular simulation study

Detailed molecular simulations are carried out to investigate the effect of temperature on orientational order in cubane molecular crystal. We report a transition from an orientationally ordered to an orientationally disordered plastic crystalline phase in the temperature range 425-450 K. This is similar to the experimentally reported transition at 395 K. The nature of this transition is first order and is associated with a 4.8% increase in unit cell volume that is comparable to the experimentally reported unit cell volume change of 5.4% (Phys. Rev. Lett. 1997, 78, 4938). An orientational order parameter, eta(T), has been defined in terms of average angle of libration of a molecular 3-fold axis and the orientational melting has been characterized by using eta(T). The orientational melting is associated with an anomaly in specific heat at constant pressure (C(P)) and compressibility (kappa). The enthalpy of transition and entropy of transition associated with this orientational melting are 20.8 J mol(-1) and 0.046 J mol(-1) K(-1), respectively. The structure of crystalline as well as plastic crystalline phases is characterized by using various radial distribution functions and orientational distribution functions. The coefficient of thermal expansion of the plastic crystalline phase is more than twice that of the crystalline phase.     

Calculating the free energy of self-assembled structures by thermodynamic integration

We discuss a method for calculating free energy differences between disordered and ordered phases of self-assembling systems utilizing computer simulations. Applying an external, ordering field, we impose a predefined structure onto the fluid in the disordered phase. The structure in the presence of the external, ordering field closely mimics the structure of the ordered phase (in the absence of an ordering field). Self-consistent field theory or density functional theory provides an accurate estimate for choosing the strength of the ordering field. Subsequently, we gradually switch off the external, ordering field and, in turn, increase the control parameter that drives the self-assembly. The free energy difference along this reversible path connecting the disordered and the ordered state is obtained via thermodynamic integration or expanded ensemble simulation techniques. Utilizing Single-Chain-in-Mean-Field simulations of a symmetric diblock copolymer melt we illustrate the method and calculate the free energy difference between the disordered phase and the lamellar structure at an intermediate incompatibility chiN=20. Evidence for the first-order character of the order-disorder transition at fixed volume is presented. The transition is located at chi(ODT)N=13.65+/-0.10 for an invariant degree of polymerization of N=14 884. The magnitude of the shift of the transition from the mean field prediction qualitatively agrees with other simulations.     

Phase transitions of a single polyelectrolyte in a poor solvent with explicit counterions

Conformational properties of a single flexible polyelectrolyte chain in a poor solvent are studied using constant temperature molecular dynamics simulation. The effects of counterions are explicitly taken in to account. Structural properties of various phases and the transition between these phases are studied by tracking the values of asphericity, radius of gyration, fraction of condensed counterions, number of non-bonded neighbours, and Coulomb interaction energies. From our simulations, we find strong evidence for a first-order phase transition from extended to collapsed phase consistent with earlier theoretical predictions. We also identify a continuous phase transition associated with the condensation of counterions and estimate the critical exponents associated with the transition. Finally, we argue that previous suggestions of existence of an independent intermediate phase between extended and collapsed phases is only a finite size effect.     

Dielectric study of the slow motional processes in the polymorphic States of anhydrous caffeine

Molecular mobility in crystalline anhydrous caffeine was studied by the dielectric technique of thermally stimulated depolarization currents (TSDC). Two relaxational processes were found, one appearing at approximately -10 degrees C that is ascribed to a reorientational glass transition, and a higher temperature one that probably arises from local molecular motions that are precursors of diffusion and sublimation. The experimental results suggest that both crystalline phases II and I of caffeine, that have distinct crystal structures, are solid rotator phases. Furthermore, this dynamic reorientational disorder shows a reorientational glass transition at the same temperature in phase II and in metastable phase I.     

Crystal structure and ionic conductivity of three polymorphic phases of rubidium trifluoromethyl sulfonate, RbSO3CF3

The crystal structures of three polymorphic phases of rubidium trifluoromethyl sulfonate (RbSO3CF3, rubidium 'triflate') were solved from X-ray powder diffraction data. At room temperature, rubidium triflate crystallizes in the monoclinic space group Cm with lattice parameters of a = 19.9611(5) A, b = 23.4913(7) A, c = 5.1514(2) A, beta = 102.758(2) degrees; Z = 16. At T = 321 K, a first-order phase transition occurs toward a monoclinic phase in space group P2(1) with lattice parameters at T = 344 K of a = 10.3434(5) A, b = 5.8283(3) A, c = 5.1982(3) A, beta = 104.278(6) degrees; Z = 2). At T = 461 K, another phase transition, this time of second order, occurs toward an orthorhombic phase in space group Cmcm with lattice parameters at T = 510 K of a = 5.3069(2) A, b = 20.2423(10) A, c = 5.9479(2) A; Z = 4. As a common feature within all three crystal structures of rubidium triflate, the triflate anions are arranged in double layers with the lipophilic CF3 groups facing each other. The rubidium ions are located between the SO3 groups. The general packing is similar to the packing in cesium triflate. Rubidium triflate can be classified as a solid electrolyte with a specific ionic conductivity of sigma = 9.89 x 10(-9) S/cm at T = 384 K and sigma = 3.84 x 10(-6) S/cm at T = 481 K.     

Crystallization of helical oligomers with chirality selection. I. A molecular dynamics simulation for bare helix

Helical polymers often exhibit pronounced chirality recognition during crystallization. By molecular dynamics simulation, we have already shown that the helical polymers crystallize with or without marked chirality selection depending on structural details of the polymer molecules. We have there classified the helical polymers into two categories: the bare helices made of only backbone atoms which show rather tolerant chirality selection, and the general helices with large side groups showing strict chirality recognition. Polymer crystallization is in general largely hampered and retarded by slow dynamics of the entangled chains, and therefore short helical oligomers are very suitable models for studying the chiral crystallization. We here report on molecular simulations of crystallization in the bare helical oligomer molecules by the use of Monte Carlo and molecular dynamics simulations. First we confirm the low temperature chiral crystal phase and the reversible order-disorder transition. We also observe frequent inversions of the helical sense, and the helix reversal defects propagating along the chains. Then we investigate crystallization from the melt into the chiral crystal phase. We find that the crystallization rate depends very sensitively on the degree of undercooling. The crystallization is found to be the first order transition that conforms well to the traditional picture of crystal growth in small molecules. Even when the crystallization directly into the chiral crystal phase is conducted, marked chirality selections are not observed at the early stage of crystallization; the chains adhere to the crystal surfaces selecting their helical senses rather at random resulting in racemic crystallites. The isothermal crystallization for a sufficiently long time, however, yields lamellar crystals composed of well-developed chiral domains, the growth of which seems to be accomplished through the transition back into the ordered chiral crystal phase.     

A brief review of the relationships between monolayer viscosity, phase behavior, surface pressure, and temperature using a simple monolayer viscometer

The two-dimensional surface shear viscosity, eta, of fatty acid monolayers of different chain lengths, measured using a simple magnetic needle viscometer, strongly correlates with the molecular organization in condensed phases and the absolute temperature. eta can increase by orders of magnitude at phase boundaries associated with tilted to untilted molecular order, providing the underlying order is semicrystalline. Hence, untilted, long-range ordered CS phases are the most viscous films. However, despite being untilted, the LS rotator phase is less viscous than certain laterally ordered tilted phases, suggesting a decrease of the van der Waals interactions due to molecular rotation. In certain regions of the L2 phase, eta reaches a maximum before the L2-LS transition, an anomalous behavior correlated with the change in the lattice symmetry of the headgroup. Surface shear viscosity, even when measured with a macroscopic probe, is particularly sensitive to the microscopic organization of monolayers.     

Room-temperature structures of solid hydrogen at high pressures

By employing first-principles metadynamics simulations, we explore the 300 K structures of solid hydrogen over the pressure range 150-300 GPa. At 200 GPa, we find the ambient-pressure disordered hexagonal close-packed (hcp) phase transited into an insulating partially ordered hcp phase (po-hcp), a mixture of ordered graphene-like H(2) layers and the other layers of weakly coupled, disordered H(2) molecules. Within this phase, hydrogen remains in paired states with creation of shorter intra-molecular bonds, which are responsible for the very high experimental Raman peak above 4000 cm(-1). At 275 GPa, our simulations predicted a transformation from po-hcp into the ordered molecular metallic Cmca phase (4 molecules∕cell) that was previously proposed to be stable only above 400 GPa. Gibbs free energy calculations at 300 K confirmed the energetic stabilities of the po-hcp and metallic Cmca phases over all known structures at 220-242 GPa and >242 GPa, respectively. Our simulations highlighted the major role played by temperature in tuning the phase stabilities and provided theoretical support for claimed metallization of solid hydrogen below 300 GPa at 300 K.     

On the high-temperature phase transition of Gd5Si2Ge2

The first-order monoclinic-to-orthorhombic (beta-->gamma) phase transition of the giant magnetocaloric material Gd(5)Si(2)Ge(2) was studied using in situ high-temperature single-crystal X-ray diffraction. A special crystal mounting procedure was developed to avoid crystal contamination by oxygen or nitrogen at high temperatures. The elastic beta-->gamma transformation occurs at 300-320 degrees C during heating, and it is reversible during fast and slow heating and slow cooling but irreversible during rapid cooling. Contrary to theoretical predictions, the macroscopic distribution of the Si and Ge atoms remains the same in both the orthorhombic gamma-polymorph and the monoclinic beta-phase. It appears that interstitial impurities may affect stability of both the monoclinic and orthorhombic phases. In the presence of small amounts of air, the beta-->gamma transformation is complete only at 600 degrees C. The interslab voids, which can accommodate impurity atoms, have been located in the structure, and an effect of partially filling these voids with oxygen or nitrogen atoms on the beta-gamma transition is discussed.