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  • 1
    Electronic Resource
    Electronic Resource
    On the role of hydrodynamic interactions in block copolymer microphase separation (1999)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 110 (1999), S. 9739-9749 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A melt of linear diblock copolymers (AnBm) can form a diverse range of microphase separated structures. The detailed morphology of the microstructure depends on the length of the polymer blocks An and Bm and their mutual solubility. In this paper, the role of hydrodynamic forces in microphase formation is studied. The microphase separation of block copolymer melts is simulated using two continuum methods: dissipative particle dynamics (DPD) and Brownian dynamics (BD). Although both methods produce the correct equilibrium distribution of polymer chains, the BD simulation does not include hydrodynamic interactions, whereas the DPD method correctly simulates the (compressible) Navier Stokes behavior of the melt. To quantify the mesophase structure, we introduce a new order parameter that goes beyond the usual local segregation parameter and is sensitive to the morphology of the system. In the DPD simulation, a melt of asymmetric block copolymers rapidly evolves towards the hexagonal structure that is predicted by mean-field theory, and that is observed in experiments. In contrast, the BD simulation remains in a metastable state consisting of interconnected tubes, and fails to reach equilibrium on a reasonable time scale. This demonstrates that the hydrodynamic forces play a critical part in the kinetics of microphase separation into the hexagonal phase. For symmetric block copolymers, hydrodynamics appears not to be crucial for the evolution. Consequently, the lamellar phase forms an order of magnitude faster than the hexagonal phase does, and thus it would be reasonable to infer a higher viscosity for the hexagonal phase than for the lamellar phase. The simulations suggest that the underlying cause of this difference is that the hexagonal phase forms via a metastable gyroid-like structure, and therefore forms via a nucleation-and-growth mechanism, whereas the lamellar phase is formed via spinodal decomposition. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.478939
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  • 2
    Electronic Resource
    Electronic Resource
    Passive Q-switch at 1.53 μm using divalent uranium ions in calcium fluoride (1995)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 78 (1995), S. 2959-2961 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Using a U2+:CaF2 passive Q-switch for the Er:glass laser, 20 ns, 3 mJ pulses were obtained. This is the shortest duration passive Q-switched pulse obtained for this laser without intracavity focusing and without optical damage. The measured U2+:CaF2 absorption cross section and relaxation lifetime are compared with those of U2+:SrF2 and U2+:BaF2. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.360042
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  • 3
    Electronic Resource
    Electronic Resource
    Grain size effect on ferro-antiferromagnetic coupling of NiFe/FeMn systems (1996)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 80 (1996), S. 4528-4533 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In order to investigate grain size effects on ferro–antiferromagnetic coupling in NiFe/FeMn systems, samples of glass/Cu zA(ring)/NiFe 75 A(ring)/FeMn 150 A(ring)/Ta 30 A(ring) were made and the grain size of FeMn was changed by changing the Cu thickness z. Increasing z from 0 to 315 A(ring), the mean grain size increases from 18 to 50 A(ring). The blocking temperature, at which the coupling field NiFe/FeMn disappears, increases from 330 to 430 K with the increase of the mean grain size. The curve shape of the temperature dependence of the coupling field changes from a concave type to a convex type with the increase of the mean grain size. Temperature dependence of the coupling field were calculated using a thermal fluctuation model of antiferromagnet. The calculated curve of the temperature dependence of the coupling field agrees with the experiment qualitatively. However, the calculated curves of the temperature dependence of coercivity does not agree with the experiment. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.363433
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  • 4
    Electronic Resource
    Electronic Resource
    Aluminum and nickel contact metallizations on thin film diamond (1995)
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 78 (1995), S. 2877-2879 
    Details
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Aluminum and nickel contact metallizations have been investigated on polycrystalline, randomly oriented diamond films of varying dopant concentrations. Hall measurements have been used to characterize the diamond films to indicate good control of dopant incorporation with carrier mobility comparable with those of the highest reported in similar films. Rectifying characteristics have been observed for both Al and Ni contacts provided the sheet resistance of the films is greater than 1200 Ω/sq. The thermal stability of these contacts have been investigated to 400 °C and Al diodes have been found to be electrically stable to such treatments. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.360096
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  • 5
    Electronic Resource
    Electronic Resource
    Dynamic simulation of diblock copolymer microphase separation (1998)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 108 (1998), S. 8713-8724 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The dissipative particle dynamics (DPD) simulation method has been used to study mesophase formation of linear (AmBn) diblock copolymer melts. The polymers are represented by relatively short strings of soft spheres, connected by harmonic springs. These melts spontaneously form a mesocopically ordered structure, depending on the length ratio of the two blocks and on the Flory–Huggins χ-parameter. The main emphasis here is on validation of the method and model by comparing the predicted equilibrium phases to existing mean-field theory and to experimental results. The real strength of the DPD method, however, lies in its capability to predict the dynamical pathway along which a block copolymer melt finds its equilibrium structure after a temperature quench. The present work has led to the following results: (1) As the polymer becomes more asymmetric, we qualitatively find the order of the equilibrium structures as lamellar, perforated lamellar, hexagonal rods, micelles. Qualitatively this is in agreement with experiments and existing mean-field theory. After taking fluctuation corrections to the mean field theory into account, a quantitative match for the locations of the phase transitions is found. (2) Where mean-field theory predicts the gyroid phase to be stable, the simulations evolve toward the hexagonally perforated lamellar phase. (3) When a melt is quenched the stable structure emerges via a nontrivial pathway, where a series of metastable phases can be formed before equilibrium is reached. The pathway to equilibrium involves a percolation of the minority phase into a network of tubes, which is destabilized by a nematic or smectic transition. (4) We conclude that either hydrodynamic interactions, or the precise form of the Onsager kinetic coefficient play an important role in the evolution of the mesophases.© 1998 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.476300
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  • 6
    Electronic Resource
    Electronic Resource
    Quantum and semiclassical calculations of H2-He vibrational line-shape parameters at high temperature (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 3845-3852 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Quantum close-coupling (CC) and semiclassical (SC) calculations of broadening and shifting coefficients of Q(j) rovibrational lines have been performed using an ab initio potential energy surface. The agreement between the theoretical results and experimental data available up to 1000 K is very good for the broadening coefficient γ and reasonable for the shift δ. The main interest is to test the validity of the semiclassical method versus CC calculation on a wide range of temperature to allow confident application of the SC method to more complex systems. The agreement is very satisfactory. Further the SC model permits a detailed analysis of the mechanisms involved in the temperature dependence of the molecular parameters γ and δ. © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.474768
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  • 7
    Electronic Resource
    Electronic Resource
    Dissipative particle dynamics: Bridging the gap between atomistic and mesoscopic simulation (1997)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 107 (1997), S. 4423-4435 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: We critically review dissipative particle dynamics (DPD) as a mesoscopic simulation method. We have established useful parameter ranges for simulations, and have made a link between these parameters and χ-parameters in Flory-Huggins-type models. This is possible because the equation of state of the DPD fluid is essentially quadratic in density. This link opens the way to do large scale simulations, effectively describing millions of atoms, by firstly performing simulations of molecular fragments retaining all atomistic details to derive χ-parameters, then secondly using these results as input to a DPD simulation to study the formation of micelles, networks, mesophases and so forth. As an example application, we have calculated the interfacial tension σ between homopolymer melts as a function of χ and N and have found a universal scaling collapse when σ/ρkBTχ0.4 is plotted against χN for N〉1. We also discuss the use of DPD to simulate the dynamics of mesoscopic systems, and indicate a possible problem with the timescale separation between particle diffusion and momentum diffusion (viscosity). © 1997 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.474784
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  • 8
    Electronic Resource
    Electronic Resource
    Molecular theory of strain hardening of a polymer gel: Application to gelatin (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 9202-9219 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: The elasticity of gelatin gels at large deformation has been measured for various experimental conditions. The general pattern is that stress increases with strain in a nonlinear way up to the point where the gel fails. To interpret this nonlinear stress increase, we studied a number of molecular models by Monte Carlo simulation and by mean-field methods. The effect of finite polymer length is studied via the FENE model (finite extensible nonlinear polymer connections) and via the exact statistics of Kramers' model (chains of freely rotating stiff rods) for a small number of elements per chain. To investigate the effect of fractal connections, the end-point distribution that comes forward from scaling theory has been generalized to arbitrary fractal dimension. Finally we studied a heterogeneous network model: connections formed by rods and coils. We also discuss the consequence of microphase separation. Combining experiment and theory we conclude the following: (i) The elastically active network connections in gelatin are most certainly not Gaussian. (ii) Strain hardening in gelatin can be attributed to either: (a) finite polymer length (the chain length between connection points should be some 2.5 times the persistence length), or (b) a fractal structure of the polymer strands (the fractal dimension should be roughly df=1.3–1.5), or (c) the presence of both stiff rods and flexible coils (the length of the rods should be 1.4–4.4 times the radius of gyration of the coils). (iii) Models b and c describe the experimental data significantly better than model a. From a single parameter (the fractal dimension) the fractal model correctly describes (1) the nonlinearity of the stress–strain curve, (2) the scaling of Young's modulus with polymer concentration, (3) the scaling of neutron scattering intensity with wave number, and (4) it predicts the scaling exponent of the linear dynamic modulus with frequency in the glassy transition zone (no experimental data available). The experimental parameters found for the rod+coils model suggest a Rouse diffusion controlled growth mechanism for the rods. Although the theory presented here is applied to gelatin, its formulation is quite general, and its implications are also relevant for other strain hardening polymer gels. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471611
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  • 9
    Electronic Resource
    Electronic Resource
    Molecular theory of the yield behavior of a polymer gel: Application to gelatin (1996)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 104 (1996), S. 9220-9233 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: A microscopic model for the endpoint separation dependent scission rate of a polymer connection in a network is developed. The predicted dissociation rate is proportional to the exponential of the bond force, which is in line with experiments. This atomistic description is thereupon incorporated in a mesoscopic theory to describe strain hardening and failure of physical gels. The resulting theory has been analyzed by a new numerical algorithm, which is some 100 to 1000 times faster than the algorithm described in the literature. We applied this theory to gelatin. To arrive at the correct nonlinear rheologic behavior of the gel, the non-Gaussian nature of the polymer endpoint distribution has to be taken into account. There are four important physical quantities that describe the nonlinear rheology of gelatin. For relatively small shear strain (0〈γ〈1), stress increases nonlinearly with strain when a gel is deformed (strain hardening). The strain at which the gel ruptures (the yield strain γy) increases quite slowly with shear rate: γy∝γ(overdot)0.05. When experiments are carried out at different shear rate, we find a linear correlation between yield stress and yield strain. Finally it is observed that the yield strain decreases with increasing strain hardening. These four observations are all covered by the theory up to quantitative accuracy. The interpretation that comes forward from this work is that the nonlinearity of the stress–strain curve for γ〈1 is correlated to the strain at which the gel yields. The reason for this correlation is that both effects are dominated by a non-Hookean force–distance relationship of the polymer connections. On the one hand side, this function directly causes the upturn of the stress–strain curve. On the other hand, the rate by which polymer connections break is proportional to the exponential of this force. Therefore a nonlinear force–distance relationship leads both to a nonlinear stress–strain relation and to an early and sudden yield behavior. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.471612
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  • 10
    Electronic Resource
    Electronic Resource
    Torsion-inversion coupling in the S1 state of acetaldehyde (1995)
    College Park, Md. : American Institute of Physics (AIP)
    The Journal of Chemical Physics 102 (1995), S. 623-632 
    Details
    ISSN: 1089-7690
    Source: AIP Digital Archive
    Topics: Physics , Chemistry and Pharmacology
    Notes: Coupling between the methyl torsion (υ15) and inversion (υ14) modes is used to model the singlet π*←n spectrum of jet-cooled acetaldehyde and its deuterated analogs. The anomalies observed in the two torsion progressions built onto the lowest inversion doublet are accurately reproduced by using a potential of the form 1/2V3[1−cos3(τ−kq)], where k indicates the proportionality of interaction between the torsion and inversion motions. The coupling also results in intensity to torsion–inversion combination levels that, in the absence of coupling, would not be observed. Consequently, many of the observed transitions are simultaneously Franck–Condon allowed and vibronically induced. The weak origin transition and long progression in the methyl torsion mode is also reproduced in the intensity calculation, indicating that the equilibrium position of the methyl rotor has changed from the S0 eclipsed position to a nearly staggered geometry in the S1 state. The calculated potential energy surface and Franck–Condon intensity calculations predict an S1 equilibrium geometry in which the methyl rotor is staggered with respect to the CO bond by 54° and where the acetyl hydrogen is bent out of the CCO plane by 35°. © 1995 American Institute of Physics.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1063/1.469461
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