Severin, , Nikolai: Molecular Dynamics Simulations of Polymers and Micelles at Interfaces
Molecular Dynamics Simulations of Polymers and Micelles at Interfaces
Dissertation

zur Erlangung des akademischen Grades
doctor rerum naturalium
(Dr. rer. nat.)

im Fach Physik

eingereicht an der
Mathematisch-Naturwissenschaftlichen Fakultät I
der Humboldt-Universität zu Berlin

von (Diplom-Physiker) Nikolai Severin, ,
geb. 4.1.1971 in Moskau

Präsident der Humboldt-Universität zu Berlin
Prof. Dr. Dr. h.c. H.Meyer

Dekan der Mathematisch-Naturwissenschaftlichen Fakultät I
Prof. Dr. J.P.Rabe

Gutachter:
Prof. Dr. J.P. Rabe
Prof. Dr. E.W. Knapp
Prof. Dr. L. Schimansky-Geier

Tag der mündlichen Prüfung: 8.7.99

Abstrakt

Molekulardynamik (MD) Simulationen wurden an zwei verschiedenen Systemen durchgeführt: 1. Grenzfläche zwischen Polyethylen und isotaktischem Polypropylen (PE-iPP) und 2. Zylindrische Mizellen, bestehend aus Tetradecyltrimethylammoniumbromid (C14TAB), in wässriger Lösung und an Fest-Flüssig-Grenzflächen.

Die allgemeinen Schwierigkeiten bei der Simulation von Grenzflächen kristalliner Polymere wurden diskutiert und eine Methode für solche Simulationen vorgeschlagen. Diese Methode wurde zur epitaxialen Kristallisation von PE auf iPP benutzt. Experimentelle Ergebnisse der epitaxialen Kristallisation konnten durch die Simulation bestätigt werden. Ferner konnte vorhergesagt werden, dass PE bevorzugt auf einer iPP-Oberfläche mit hoher Methylgruppenkonzentration kristallisiert. Ebenso wurde durch die MD Simulation vorhergesagt, dass PE in der Grenzflächenregion von einer orthorhombischen zur monoklinischen Kristallstruktur wechselt.

Die Simulationsdauer für die Mizellen betrug einige Nanosekunden. Die Ergebnisse für die Mizellen in wässriger Lösung stehen hierbei in guter Übereinstimmung mit experimentellen Werten. Im Widerspruch zur allgemein üblichen Vorstellung führte die Simulation der Mizellen zur Ausbildung eines Hohlraums in ihrer Mitte sowie zu einer inhomogenen Dichte des hydrophoben Mizellkerns. Dies wurde zum Teil der inhomogenen Verteilung der terminalen Methylgruppen im Mizellkern zugeschrieben. Zylindrische und halbzylindrische Mizellen wurden an den Paraffin/Wasser- und Gold/Wasser-Grenzflächen simuliert.

Keywords:
Molekulardynamik Simulation, Polymer Grenzfläche, Mizellen

Abstract

Molecular Dynamic (MD) simulation of two different systems was performed: 1) Polyethylene- isotactic Polypropylene (PE-iPP) interfaces and 2) cylindrical micelles formed by tetradecyl trimethylammonium bromide (C14TAB) molecules in aqueous solution and at solid liquid interfaces.

The general difficulties of simulation of polymer crystalline interfaces were discussed and one method was proposed for such simulations. Thise method was used to simulate epitaxial crystallisation of PE on iPP. The experimental results on epitaxial crystallisation were confirmed by MD simulation and in addition epitaxial crystallisation of PE on iPP surface with high dencity of methyl groups was predicted. MD simulation also predicted that PE should change at the interfacial region from the orthorhombic to monoclinic crystalline structure.

Several nanoseconds of life of cylindrical micelles were simulated. The simulation results for the micelle in aqueous solution were favourably compared with experimental results. In contradiction to the standard picture of an ionic micelle the simulated micelle formed hole in its centre and the density of the hydrophobic micelle core was inhomogeneous. This effect partially was explained by the inhomogeneous distribution of the terminal methyl groups in the micelle core. Cylindrical and half cylindrical micelles of C14TAB molecules were simulated at the paraffin- and gold-aqueous interfaces.

Keywords:
molecular dynamic simulation, polymer interfaces, micelles


Seiten: [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59] [60] [61] [62] [63] [64] [65] [66] [67] [68] [69] [70] [71] [72] [73] [74] [75] [76] [77] [78] [79] [80] [81] [82] [83] [84]

Inhaltsverzeichnis

TitelseiteMolecular Dynamics Simulations of Polymers and Micelles at Interfaces
1 General introduction
2 Molecular dynamics (MD) Simulation
3 MD simulation of Polyethylene/isotactic Polypropylene (PE/iPP) interfaces.
3.1.Introduction to the simulation of PE - iPP interfaces
3.2.MD simulations
3.2.1.Polyethylene, Polypropylene and their blends
3.2.2.Model for the PE/iPP interface
3.2.3.Results
3.2.4.Discussion
3.3.Conclusions.
4 MD simulation of cylindrical and half-cylindrical micelles in water and at solid liquid interfaces.
4.1.Introductory remarks.
4.2.MD simulation
4.2.1.Amphiphilic molecules
4.2.2.Model
4.2.3.Results
4.2.4.Discussion
4.3.Conclusions
5 Summary
6 Zusammenfassung
Bibliographie Literature
Bibliographie Teil 1
Part 1
Bibliographie Teil 2
Part 2
Lebenslauf
Selbständigkeitserklärung

Abbildungsverzeichnis

Figure 1 Graphic illustration of the pcff force-filed terms.
Figure 2 The all trans conformation (zigzag) of PE. Side and end-on view. The carbon atoms are large and hydrogen atoms are small. The ellipse is drawn to facilitate the display of PE crystalline forms shown in the figure 3. The shading is chosen in agreement with Figure 3 in order to outline the difference in height between sides 1 and 2.
Figure 3 Orthorhombic and monoclinic crystalline forms of polyethylene. Cut through ab - plane with macromolecular long axes perpendicular to it. The ellipses are just a guide for the eyes and show the orientation of PE zigzags. It is important to notice that in the monoclinic form all ellipses have the same orientation while in the orthorhombic cell they have different orientation. The shading is consistent with Figure 2.
Figure 4 The helical form of iPP macromolecule. On the side view two helical turns are presented. Arrows show three methyl groups along one helix.
Figure 5 Schematic structure of a spherulite. Lamella crystals grow from the centre of the spherulite. The enlarged part shows several lamellae and an amorphous regions in-between lamellae. The amorphous part is formed by chains that fold back to the same lamella where they emerged from or by chains that reach into the neighbouring lamella.
Figure 6 Bridging mechanism established by epitaxial crystallisation. It was found that lamellae of PE form an angle of about 50° with iPP lamellae. This figure shows the bridging of several iPP lamellae by PE lamellae.
Figure 7 The two (010) faces of iPP (alpha-modification). The face with the higher density of methyl groups (surface of type A) results in rows of methyl groups parallel to and with inter-row distances of 5.05 Å and 4.25 Å respectively. Faces with the lower density of methyl groups (surface of type B) have less pronounced rows of methyl groups but exhibit the same symmetry as the faces with the higher density of methyl groups.
Figure 8 A) Interface of crystalline iPP and crystalline PE with low density of methyl groups on the PP contacting surface. End on view on PE molecules, iPP molecules are rotated about 47° to the point of view. Arrows point out the methyl groups on the surface of iPP which were initially oriented upwards. B) Top view on orthorhombic crystalline polyethylene.
Figure 9 Interface of crystalline iPP and crystalline PE with high density of methyl groups on the crystalline iPP contacting surface. End on view on PE molecules, iPP molecules are rotated about 47° with respect to the point of view.
Figure 10 Examples of structures formed by amphiphilic molecules.
Figure 11 Starting structure for cylindrical micelle in water environment.
Figure 12 Starting structure for half cylindrical micelle on gold substrate in water environment.
Figure 13 Cylindrical micelle in water environment after 2 ns of MD simulation.
Figure 14 Density distribution functions in cylindrical micelle in water plotted vs the distance from the micelle centre: a) full-square : density of the hydrophobic core composed of alkyl chains; b) full-delta : head groups ( ), c) - density of the Br- ions; d) • - density of the water and e) --- - density of the total molecular system of the cylindrical micelle of TAB molecules in water.
Figure 15 Pair distribution functions of the Br- and N+ ions in cylindrical micelle. a) solid line: pair distribution function for initial configuration of the subset of Br- and N+ ions (Br - Br, N - N and Br - N pair distribution functions), the first maximum is the Br - N distance in the dipole and second is the Br - Br or N - N distance, hence inter-dipole distance; b)-e) pair distribution functions after 2 ns: b) dotted line: Br - N pair distribution function; c) dashed line: N - N pair distribution function; e) dashed-dotted line: Br - Br pair distribution function.
Figure 16 Correlation functions of the terminal methyl groups that are picked: a) from the centre of the micelle and b) from the micelle surface. The dashed line is the best fit of the curve a) with decay law , and realxation time tau 2.4±0.2 ps.
Figure 18 Relaxation times. Filled squares (full-square): correlation times for MD simulated cylindrical micelle of TAB molecules. Open circles (o): experimental relaxation times for PPALM lamella ( ). Open up-triangles (Delta): experimental relaxation times for PPALM spherical micelle.
Figure 19 Order parameters S open symbols - experiments, closed symbols - simulated data. Filled squares (full-square): order parameter from MD simulation of cylindrical micelle of TAB molecules. The dashed line is a line guide for eyes. Open circles (o): experimental order parameter for the lamella phase of SOBS molecules . Open triangles (Delta): experimental order parameter for the micelle phase of CTAC molecules
Figure 20 Modified order parameter Sm for a cylindrical micelle of TAB molecules plotted vs the distance from an edge of the micelle.
Figure 21 Radial scattering density distribution function for a cylindrical micelle of TAB molecules simulated in water. a) rhomb - micelle with hydrogens, b) full-square - all hydrogens were substituted by deuteriums. Both curves where scaled so that curve b) would be unity at its maximum.
Figure 22 a) rhomb- radial scattering density for MD simulated cylindrical micelle, b) •- radial distribution of the terminal methyl groups for MD simulated cylindrical micelle and c) - - radial scattering density deduced from neutron scattering from cylindrical micelles formed by egg yolk lecitin14. All profiles were scaled in order to fit to one graph.
Figure 23 Volume weighted probability density P(r) versus the distance from micelle centre r. Full circles connected by the dotted line are the data of a MD simulation of a cylindrical micelle of TAB molecules. The solid line is the FFT smoothing.
Figure 24 Volume weighted probability density P(r). • - MD data corrected to polydispersity add to shape. ···· - FFT smoothing of MD data. - - the experimental data for micelles of LDS molecules ( ) with the radius of the hydrophobic core 18.9 Å. (For comparison the radius of the hydrophobic core of the MD simulated cylindrical micelle is roughly 20 Å.)
Figure 25 Snapshot of a half cylindrical micelle of TAB molecules on a gold substrate in water environment after 1.5 ns of MD simulation.
Figure 26 Density distribution functions for a half cylindrical micelle on a gold substrate plotted vs the distance from the geometrical centre of the first adsorbed layer of TAB molecules: a) full-square - density of the hydrophobic core composed of alkyl chains, b) full-delta - head groups ( ), c) - Br- ions, d) • - water and e) --- - total density of the system half cylindrical micelle of TAB molecules on gold substrate and water.
Figure 27 Pair distribution functions of Br- and N+ ions in half cylindrical micelle on the gold substrate. a) solid line: initial configuration of the subset of Br- and N+ ions (Br - Br, N - N and Br - N pair distribution functions), the first maximum reflects the Br - N distance in the dipole and the second the Br - Br or N - N distance, hence an inter-dipole distance; b) dotted line: Br - N pair distribution function; c) dashed line: N - N pair distribution function; e) dashed-dotted line: Br - Br pair distribution function.
Figure 28 Order parameter Sm/Smax for the half cylindrical micelle of TAB molecules on the gold substrate. The error bars denote the standard deviation from the mean. It is more important to display the standard deviation instead of the standard error of the mean determination as the standard deviation defines actual fluctuations in the system. The error of the mean determination is about four times lower than the fluctuations.
Figure 29 Order parameter Ss for a half cylindrical micelle formed by TAB molecules on a gold substrate.
Figure 30 Snapshot of the half cylindrical micelle of TAB molecules on the paraffin substrate after 1.2 ns of MD simulation
Figure 31 Density distribution functions in half cylindrical micelle on paraffin substrate plotted vs the distance from the geometrical centre of the first adsrobed layer of TAB molecules: a) full-square - density of the hydrophobic core composed of alkyl chains, b) full-delta - density of the head groups ( ), c) nabla - density of the Br- ions, d) • - density of the water and e) --- - total density of half cylindrical micelle of TAB molecules on paraffin substrate and water.
Figure 32 Pair distribution functions of the Br- and N+ ions in a half cylindrical micelle on a paraffin substrate. a) solid line: pair distribution function for initial configuration of the subset of Br- and N+ ions (Br - Br, N - N and Br - N pair distribution functions); the first maximum is the Br - N distance in the dipol and the second is the Br - Br or N - N distance, hence an inter-dipole distance; b) dotted line: Br - N pair distribution function; c) dashed line: N - N pair distribution function; e) dashed-dotted line: Br - Br pair distribution function.
Figure 33 Ration of order parameter to the maximum order parameter for the half cylindrical micelle of TAB molecules on the paraffin substrate. The error bars show the standard deviation from the mean. The error of the mean determination is about four times lower than the deviations.
Figure 34 Order parameter Ss for half cylindrical micelle formed by TAB molecules on paraffin substrate.
Figure 35 Snapshot of a cylindrical micelle of TAB molecules on a gold substrate in water environment after 1 ns of MD simulation.
Figure 36 Density distribution functions for cylindrical micelle on a gold substrate plotted vs the distance from the micelle center of mass a) full-square - density of the hydrophobic core composed of alkyl chains, b) full-delta - density of the head groups ( ), c) nabla - density of the Br- ions, d) • - density of the water and e) --- - total density of the system half cylindrical micelle of TAB molecules on the paraffin substrate and water.
Figure 37 Pair distribution functions of the Br- and N+ ions in a cylindrical micelle on a gold substrate. a) solid line - pair distribution function for initial configuration of the subset of Br- and N+ ions (Br - Br, N - N and Br - N pair distribution functions), the first maximum is the Br - N distance in the dipol and the second is the Br - Br or N - N distance, hence inter-dipole distance; b) dotted line - Br - N pair distribution function; c) dashed line - N - N pair distribution function; e) dashed-dotted line is the Br - Br pair distribution function.
Figure 38 Order parameter for the cylindrical micelle on the gold substrate. The error bars show the standard deviation from the mean.

[Titelseite] [1] [2] [3] [4] [5] [6] [Bibliographie] [Lebenslauf] [Selbständigkeitserklärung]

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