Energetics of rotation and translation in hexagonal aggregates of extended chains

Bleha, Tomáš; Gajdoš, Ján

doi:10.1007/bf01412772

Cited by 3 publications

(1 citation statement)

References 29 publications

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“…From detailed analyses of the local and global orientational order, it is found that the formation of the global orientational order as well as the growth of the local ordered regions proceeds stepwise. The crystal structure of the rotator phase of n-alkanes, which are typical of short polymer chains with simple chemical structure, has been extensively studied by experiments [1][2][3][4][5][6][7][8][9], theoretical analyses [10,11], and computer simulations [12][13][14][15][16][17][18]. Several experimental techniques including x-ray diffraction [1][2][3][4], infrared and Raman spectroscopy [5,6], neutron scattering [7,8], and NMR [9] have been used to reveal interesting features of the molecular packing, intramolecular defects, and molecular motions in the rotator phase of n-alkanes.…”

Section: Susumu Fujiwara and Tetsuya Satomentioning

confidence: 99%

Molecular Dynamics Simulation of Structural Formation of Short Polymer Chains

Fujiwara

Sato

1998

Phys. Rev. Lett.

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Computer simulation of the structural formation of polymer chains has recently become the focus of attention in physics, chemistry and material science. We aim at understanding the mechanisms of the structural for-.mation of polymer chains at the molecular level. To this end, we carry out the molecular dynamics simulations of 100 short polymer chains, each of which consists of 20 CH 2 groups, and analyze the formation process of the orientationally ordered structure.The united CH 2 groups interact via the bonded potentials (bond-stretching, bond-bending and torsional potentials) and the non-bonded potential (12-6 LennardJanes potential). The atomic force field used here is the DREIDING potential 1 ). We use the velocity version of the Verlet algorithm and apply the Nose-Hoover method in order to keep the temperature of the system constant. The integration time step and the relaxation constant for the heat bath variable are 0.001 ps and 0.1 ps, respectively. The cutoff distance for the Lennard-Janes potential is 10.5 A. The polymer chains are exposed to vacuum. The total momentum and the total angular momentum are taken to be zero in order to cancel overall translation and rotation of chains. At first, we prepare random configuration of short polymer chains at high temperature (T = 700 K) and then it is quenched to various low temperatures (T = 300, 320, ... , 460 K) 2 ).We show, in Fig. 1 At last they coalesce into a large cluster and a highly ordered monolayer structure is formed [ Fig. 1 (d)].In order to investigate the growth process of the global bond-orientational order, we calculate the global bond-orientational order parameter S, which is defined bywhere N and n are respectively the number of polymer chains and the number of CH 2 groups per polymer chain (N = 100, n = 20) and 1/Ji is the angle between the subbond vector of the m-th chain bf\ which is formed by connecting centers of two adjacent bonds i and i-1 of the m-th chain, and the director (c-axis) of the layer. The parameter S would take a value of 1.0, 0.0 or -0.5, respectively, for polymer chains whose sub-bonds are perfectly parallel, random or perpendicular to the director.We show the time dependence of S at T = 440 K in

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Section: Susumu Fujiwara and Tetsuya Satomentioning

confidence: 99%

Molecular Dynamics Simulation of Structural Formation of Short Polymer Chains

Fujiwara

Sato

1998

Phys. Rev. Lett.

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Temperature-dependent fracture mechanisms in ultra-high strength polyethylene fibers

Dijkstra

Torfs

Pennings

1989

Colloid & Polymer Sci

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Abstract:The influence of the temperature on the mechanical properties of gel-spun hotdrawn ultra-high molecular weight polyethylene fibers has been investigated.From these experiments two different fracture mechanisms could be distinguished.The results indicate that above 20 ~ a stress-induced orthorhombic-hexagonal phase transition is responsible for fiber failure. In the hexagonal or rotator phase the chains can easily slip past one another and fiber fracture is initiated by creep. Below 20 ~ the phase transition cannot be introduced because the stress needed for the phase transition would exceed the covalent-bond strength in the polyethylene chain. The strength temperature data of the low temperature region was treated with Zhurkov's kinetic concept, leading to a bond-fracture activation energy of 160 kJ/mol and an activation volume of 0.01 nm 3. These values, together with the data from irradiation and shrinkage experiments, indicated that in the low temperature region fiber failure might be initiated by the fracture of trapped entanglements instead of that by overstressed, taut tie molecules.

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Monte Carlo simulation of the crystal structure of the rotator phase of n-paraffins. II. Effects of rotation and translation of the rigid molecules

Yamamoto

1988

The Journal of Chemical Physics

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Structures and molecular motions in the rotator phases of n-paraffins are simulated by use of the Monte Carlo method—both rotational and translational degrees of freedom of the molecules being taken into account. Molecules of rigid trans-planar structure are assumed to be placed in a two-dimensional orthorhombic or orthohexagonal lattice with lattice parameters a and b. A model of the nearly continuous molecular motion is adopted in this work; each molecule is assumed to have 36 discrete orientations (in 10° intervals) and 10 discrete translations (in 0.128 Å intervals) around and along the molecular axis, respectively. Molecular packing modes in various lattice systems of different a/b values are examined ranging from that of the ordinary orthorhombic phase (a/b=1.50) to that of the rotator-II (R-II) phase (a/b=31/2). It is found that the crystals in the rotator phases are generally composed of many domains within which molecules are packed more or less in order. In the rotator-I (R-I) phase, fairly large structural changes are observed with the change in a/b value; the structure of the R-I phase reported in our previous MC work should be slightly modified. The structure of the R-II phase, on the other hand, is found to be essentially the same as that of our previous MC work. Modes of the molecular motion in the R-II phase are studied by monitoring the time evolution of the rotational and translational states of all molecules in the system. It is found that the molecules are making fairly active rotational oscillation around one of the six equivalent directions connecting nearest neighbor molecular pairs. The molecules are also making sporadic large rotations to the different directions. The translational motion of the molecules is found to be very active and evidently independent of the rotational motion. The overall molecular motions in the rotator phase are found to be described in terms of the movement of the boundaries of the ordered domains.

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Energetics of rotation and translation in hexagonal aggregates of extended chains

Cited by 3 publications

References 29 publications

Molecular Dynamics Simulation of Structural Formation of Short Polymer Chains

Molecular Dynamics Simulation of Structural Formation of Short Polymer Chains

Temperature-dependent fracture mechanisms in ultra-high strength polyethylene fibers

Monte Carlo simulation of the crystal structure of the rotator phase of n-paraffins. II. Effects of rotation and translation of the rigid molecules

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