L. I. Manevich

High-temperature quasi-hexagonal phase in the simplest model of a polymer crystal

ISSN 0012-5016, Doklady Physical Chemistry, 2008, Vol. 418, Part 2, pp. 15–18.

Coarse-grained polyethylene: Including cross terms in bonded interactions and introducing anisotropy into the model for the orthorhombic crystal

Our previous paper (E.A. Zubova, I.A. Strelnikov, N.K. Balabaev, A.V. Savin, M.A. Mazo, and L.I. Manevich, Polym. Sci., Ser. A 59 (1) (2017)) addressed the simplest coarse-grained model of polyethylene and alkanes. The CH2 group in this united-atom model is replaced with a bead. In the framework of the model, nonbonded interactions are described by the Lennard-Jones potential (6–12), whereas the potential for bonded interactions accounts for the bonds between beads and for coarse grained angles and dihedral angles but not for the cross terms between them. We found the area of geometrical parameters of the model where all the three known crystalline phases of polyethylene are stable at low temperatures. We parametrized the force field of the model using the dynamic properties of the system, namely, the inelastic neutron scattering spectrum of the orthorhombic phase of polyethylene. However, the simplest model underestimates the value of the elastic modulus along the chains of the crystal by a factor of two. The derived setting angle of the molecules also differs appreciably from the experimental data. Moreover, the acoustic dispersion curves for the modes with the wave vector directed along the chain axis deviate from the experimental data at low frequencies. In the present study, we included the cross terms into the bonded interactions. This made it possible to reproduce the experimental elastic modulus along the chains of the crystal and to decrease the frequency range for the optical skeletal dispersion curve to proper values. As for the beads, we separated force centers for the bonded and nonbonded interactions, which enabled us to reproduce the optical skeletal curve and to bring the anisotropy of interchain interactions in line with the experiment. However, the model fails to reproduce the balance of interactions between the neighboring chains in the crystal. For this, a different form of the potential energy of van der Waals interactions seems to be needed.

Features of the phase behavior of block copolymers related to their polydispersity

To calculate cloud points of a melt of block copolymers, at which its microphase separation occurs, for the first time in the theory of polymers, a spectral method is used in a model that takes into account the compressibility of the melt and the polydispersity of its macromolecules. A particular case of this calculation for a copolymer whose macromolecules consist of two blocks is described, and the polydispersity of each of them is defined through the exponential Flory distribution.

Thermophysics of a crack propagating in poly(methyl methacrylate)

Temperature kinetics along the crack propagation path in the bulk of PMMA at 20°C and under impact loading is explained. The initial experimental data are taken from [1, 2]. The kinetic temperature-time curve includes six different regions. In this study, the sequence of regions in the temperature kinetics is interpreted as a sequence of stages corresponding to the extension and breakdown of polymer chains in a microregion with dimensions of ≈130 μm located on the trajectory of crack propagation. At the first stage with a duration of ≈8 μs, the temperature increases by ≈10°C. This temperature rise that is related to the arrival of a loading wave front. At the second stage with a duration of ≈5 μs, temperature decreases by ≈8°C. This process is controlled by the thermal elasticity of polymer chains. At the third stage, temperature increases up to ≈90°C owing to the plastic flow of the material. At this stage, ahead of a crack, a craze is formed. Rearrangement of the isotropic polymer structure into an anisotropic polymer structure proceeds within ≈7 μs. At the fourth stage, temperature strongly decreases (nearly down to −150°C). This temperature reduction is provided by thermoelastic deformation of valence angles and chemical C-C bonds of the polymer backbone. At the fifth stage, the material heats up owing to the dissipation of the stored elastic energy due to the breakdown of extended polymer chains. This stage of the temperature rise on polymer chains spans over ≈18 μs. This phenomenon indicates the marked spatial continuity (diffuse character) of a crack tip and is related to a broad length distribution of ruptured chains. During the sixth stage, temperature remains at a constant level of ≈100°C for more than 100 μs. This stage reflects the emergence of exothermic processes on the newly formed fractured surface.

Low-frequency localized oscillations of the DNA double strand

The results of the molecular dynamic study on the low-energy elementary linear and nonlinear excitations of DNA macromolecule obtained within the framework of the coarse-grain model of the DNA double strand are presented. The characteristics of the basic states of the model agree well with the experimental parameters of both A and B conformations of the DNA double strand. The correlation between the directly calculated dispersion curves and density distribution of the frequency spectrum obtained in the simulation experiments is found. Special attention is focused on the soliton type of nonlinear localized excitations (breathers). These excitations are shown to exist in several frequency gaps in which the propagation of harmonic linear waves is prohibited. The types of motions corresponding to all calculated breathers are identified. The correlation between the two types of breathers and their analogs studied in terms of unidimensional models and treated as elementary excitations responsible for the initial stage of the opening of the DNA double strand is established.

Phase behavior of compressible melts of multiblock polydisperse copolymers

In terms of the previously proposed model, specific features of the phase behavior of Markovian polydisperse copolymers with allowance for their compressibility have been investigated via bifurcation analysis followed by continuation with respect to a parameter that characterizes the deviation of the temperature of the system from its value on the spinodal. These features above all include competition between microphase separation and macrophase separation under conditions when the local instability of the homogeneous state appearing at the spinodal corresponds to the macrophase separation only. Nevertheless, it was shown that depending on the structural parameters, the global instability characterized by a cloud-point hypersurface can result in either macrophase or microphase separation, with the microphase separation occurring in the vicinity of the critical point. In this case, the results are consistent with the conclusions of the Landau theory of phase transitions, whose applicability limits with respect to deviation from the critical point have been evaluated in this study. Outside the range of applicability of this theory, cloud-point curves that correspond to macrophase separation and microphase separation are very similar. These conclusions remain valid over a wide range of compressibility whose influence has been assessed for the first time. It has been found that the type of copolymers under consideration has a characteristic feature that was not noticed previously: Namely, the distribution of density in the nucleus of a new phase in this case will look like a spatially localized solitonlike profile.

Criterion for neck-propagation stability in polymers

The conditions for the occurrence of instability during neck propagation in polymers are theoretically studied. The mechanism behind this phenomenon is concerned with the thermomechanical instability of neck propagation, which is provided by the transformation of a sample’s stored energy of elastic deformation into heat during tensile drawing of the plastic polymer. An analytical criterion for the occurrence of instability is formulated. The critical length of the sample, below which self-oscillations are not excited, is inversely proportional to strain rate and directly proportional to the coefficient of thermal conductivity and the elastic modulus of the material. Two principal distinctions between thermomechanical and mechanical instabilities are revealed. The first distinction is the existence of elastic energy (length), below which oscillations do not occur; the second is the fact that the interval of thermomechanical instability is wider than that of mechanical instability.

Molecular dynamics simulation of thermo-mechanical properties of montmorillonite crystal

The clay phyllosilicate mineral montmorillonite, which is composed of 1-nm lamellar crystals (lamellas), attracts more and more attention due to its significant role in many industrial applications, such as petroleum and civil engineering, the food and cosmetic industry, heterogeneous catalysis, waste storage (including radioactive), etc. Special interest in these mineral appeared after it was disclosed that nanoclay fillers can significantly improve the mechanical and thermal properties of polymer composites and decrease their humidity and gas permeability. Such nanocomposites are now used as barrier films in packaging, as fire retardant coatings, in aerospace and automotive parts, and they have potential applications in aviation, medicine, and other industries. Calculating the elastic properties of the composite requires accurate knowledge of the elastic modules and the behavioral properties of the components, including clay particles; however, their experimental study is a rather complicated problem because montmorillonite is not perfect crystal. In this situation, computer simulations have become very useful. In this review, alongside single with clay lamellae, we also deal with a dynamics simulation of the structure and thermo-mechanical properties of montmorillonite crystals intercalated by water or polyethylene oxide.

Rupture of thin polymer films

Abstract A study has been made of the deformation and rupture of thin polymer films with and without notches. It is shown that the deformation of linear and densely cross-linked polymer films is accompanied by the formation of a plasticity region in front of the notch. The length of the region is perpendicular to the stretching direction and obeys the Dugdale-Barenblatt equation. In the case of densely crosslinked polymers the size of the plasticity region along the stretching axis is close to the film thickness, a fact that allowed formulation of a rupturing criterion. The effective energy of rupture for thin (∼ 100 μm) polymer films greatly exceeds the corresponding energy for thick specimens.

Origin of “cold” water molecules in the composition of low-molecular-mass products of the mechanical fracture of poly(methyl methacrylate)

A hypothesis on the correlation between the origin of “cold” (∼53 K) water molecules released by a growing crack and a low (∼123 K) temperature of stretched and then thermoelastically cooled polymer chains that ruptured at the crack top is advanced. A mechanism behind the formation of “cold” water molecules is suggested. It includes their “soft” desorption due to mechanical action onto thermoelastically cooled side groups with adsorbed cooled water molecules from an unloading wave that is induced by the rupture of the main chain and that travels along it.