2016 
Li, D., Xu, H., & Wittmer, J. P. (2016). Glass transition of twodimensional 8020 KobAndersen model at constant pressure. J. PhysicsCondensed Matter, 28(4), 045101.
Abstract: We reconsider numerically the twodimensional version of the KobAndersen model (KA2d) with a fraction of 80% of large spheres. A constant moderate pressure is imposed while the temperature T is systematically quenched from the liquid limit through the glass transition at Tg approximate to 0.3 down to very low temperatures. Monodisperse LennardJones (mdLJ) bead systems, forming a crystal phase at low temperatures, are used to highlight several features of the KA2d model. As can be seen, e.g. from the elastic shear modulus G(T), determined using the stressfluctuation formalism, our KA2d model is a good glassformer. A continuous cuspsingularity, G(T) proportional to (1 – T/Tg)(alpha) with alpha approximate to 0.6, is observed in qualitative agreement with other recent numerical and theoretical work, however in striking conflict with the additive jump discontinuity predicted by modecoupling theory.
Keywords: computersimulation,dynamics,elastic moduli,elasticconstants,glass transition,order,simulation,solids,stress fluctuations


Wittmer, J. P., Kriuchevskyi, I., Cavallo, A., Xu, H., & Baschnagel, J. (2016). Shearstress fluctuations in selfassembled transient elastic networks. Phys. Rev. E, 93(6), 11 pp.
Abstract: Focusing on shearstress fluctuations, we investigate numerically a simple generic model for selfassembled transient networks formed by repulsive beads reversibly bridged by ideal springs. With Lambda t being the sampling time and t(star)(f) similar to 1/f the Maxwell relaxation time (set by the spring recombination frequency f), the dimensionless parameter Delta x = Delta t/ t(star) (f) is systematically scanned from the liquid limit (Delta x >> 1) to the solid limit (Delta x << 1) where the network topology is quenched and an ensemble average over mindependent configurations is required. Generalizing previous work on permanent networks, it is shown that the shearstress relaxation modulus G(t) may be efficiently determined for all Delta x using the simpleaverage expression G(t) = mu(A) – h(t) with mu(A) = G(0) characterizing the canonicalaffine shear transformation of the system at t = 0 and h(t) the (rescaled) meansquare displacement of the instantaneous shear stress as a function of time t. This relation is compared to the standard expression G(t) = (c) over tilde (t) using the (rescaled) shearstress autocorrelation function (c) over tilde (t). Lower bounds for the m configurations required by both relations are given.


Wittmer, J. P., Xu, H., & Baschnagel, J. (2016). Simple average expression for shearstress relaxation modulus. Phys. Rev. E, 93(1), 5 pp.
Abstract: Focusing on isotropic elastic networks we propose a simpleaverage expression G(t) = mu(A) – h(t) for the computational determination of the shearstress relaxation modulus G(t) of a classical elastic solid or fluid. Here, mu(A) = G(0) characterizes the shear transformation of the system at t = 0 and h(t) the (rescaled) meansquare displacement of the instantaneous shear stress (tau) over cap (t) as a function of time t. We discuss sampling time and ensemble effects and emphasize possible pitfalls of alternative expressions using the shearstress autocorrelation function. We argue finally that our key relation may be readily adapted for more general linear response functions.


2015 
Wittmer, J. P., Kriuchevskyi, I., Baschnagel, J., & Xu, H. (2015). Shearstrain and shearstress fluctuations in generalized Gaussian ensemble simulations of isotropic elastic networks. European Physical Journal B, 88(9).
Abstract: Shearstrain and shearstress correlations in isotropic elastic bodies are investigated both theoretically and numerically at either imposed mean shearstress tau (lambda – 0) or shearstrain gamma (lambda – 1) and for more general values of a dimensionless parameter. characterizing the generalized Gaussian ensemble. It allows to tune the strain fluctuations mu(gamma gamma) beta V


Wittmer, J. P., Xu, H., & Baschnagel, J. (2015). Shearstress relaxation and ensemble transformation of shearstress autocorrelation functions. Physical Review E, 91(2).
Abstract: We revisit the relation between the shearstress relaxation modulus G(t), computed at finite shear strain 0


Wittmer, J. P., Xu, H., Benzerara, O., & Baschnagel, J. (2015). Fluctuationdissipation relation between shear stress relaxation modulus and shear stress autocorrelation function revisited. Molecular Physics, 113(1718), 2881–2893.
Abstract: The shear stress relaxation modulus G(t) may be determined from the shear stress (tau) over cap (t) after switching on a tiny step strain gamma or by inverse Fourier transformation of the storage modulus G'(omega) or the loss modulus G ''(omega) obtained in a standard oscillatory shear experiment at angular frequency.. It is widely assumed that G(t) is equivalent in general to the equilibrium stress autocorrelation function C(t) = beta V


2013 
Wittmer, J. P., Xu, H., Polinska, P., Gillig, C., Helfferich, J., Weysser, F., et al. (2013). Compressibility and pressure correlations in isotropic solids and fluids. European Physical Journal E, 36(11), 1–17.
Abstract: Presenting simple coarsegrained models of isotropic solids and fluids in d = 1 , 2 and 3 dimensions we investigate the correlations of the instantaneous pressure and its ideal and excess contributions at either imposed pressure (NPTensemble, lambda = 0 or volume (NVTensemble, lambda = 1 and for more general values of the dimensionless parameter lambda characterizing the constantvolume constraint. The stress fluctuation representation of the compression modulus K in the NVTensemble is derived directly (without a microscopic displacement field) using the wellknown thermodynamic transformation rules between conjugated ensembles. The transform is made manifest by computing the Rowlinson functional also in the NPTensemble where with x = P (id)/K being a scaling variable, P (id) the ideal pressure and f (0)(x) = x(2x) a universal function. By gradually increasing lambda by means of an external spring potential, the crossover between both classical ensemble limits is monitored. This demonstrates, e.g., the lever rule FRow vertical bar(lambda) – K[lambda + (1 – lambda)f(0)(x)].


Wittmer, J. P., Xu, H., Polinska, P., Weysser, F., & Baschnagel, J. (2013). Communication: Pressure fluctuations in isotropic solids and fluids. Journal of Chemical Physics, 138(19).
Abstract: Comparing isotropic solids and fluids at either imposed volume or pressure, we investigate various correlations of the instantaneous pressure and its ideal and excess contributions. Focusing on the compression modulus K, it is emphasized that the stress fluctuation representation of the elastic moduli may be obtained directly (without a microscopic displacement field) by comparing the stress fluctuations in conjugated ensembles. This is made manifest by computing the Rowlinson stress fluctuation expression Krow of the compression modulus for NPTensembles. It is shown theoretically and numerically that Krow vertical bar P = Pid(2 – Pid/K) with Pid being the ideal pressure contribution. (C) 2013 AIP Publishing LLC.


Wittmer, J. P., Xu, H., Polinska, P., Weysser, F., & Baschnagel, J. (2013). Shear modulus of simulated glassforming model systems: Effects of boundary condition, temperature, and sampling time. Journal of Chemical Physics, 138(12).
Abstract: The shear modulus G of two glassforming colloidal model systems in d = 3 and d = 2 dimensions is investigated by means of, respectively, molecular dynamics and Monte Carlo simulations. Comparing ensembles where either the shear strain gamma or the conjugated (mean) shear stress tau are imposed, we compute G from the respective stress and strain fluctuations as a function of temperature T while keeping a constant normal pressure P. The choice of the ensemble is seen to be highly relevant for the shear stress fluctuations mu(F)(T) which at constant tau decay monotonously with T following the affine shear elasticity mu(A)(T), i.e., a simple twopoint correlation function. At variance, nonmonotonous behavior with a maximum at the glass transition temperature Tg is demonstrated for mu(F)(T) at constant gamma. The increase of G below Tg is reasonably fitted for both models by a continuous cusp singularity, G(T) proportional to (1 – T/Tg)(1/2), in qualitative agreement with recent theoretical predictions. It is argued, however, that longer sampling times may lead to a sharper transition. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4790137]


2012 
Schulmann, N., Xu, H., Meyer, H., Polinska, P., Baschnagel, J., & Wittmer, J. P. (2012). Strictly twodimensional selfavoiding walks: Thermodynamic properties revisited. European Physical Journal E, 35(9).
Abstract: The density crossover scaling of various thermodynamic properties of solutions and melts of selfavoiding and highly flexible polymer chains without chain intersections confined to strictly two dimensions is investigated by means of molecular dynamics and Monte Carlo simulations of a standard coarsegrained beadspring model. In the semidilute regime we confirm over an order of magnitude of the monomer density rho the expected power law scaling for the interaction energy between different chains e(int) similar to rho(21/8), the total pressure P similar to rho(3) and the dimensionless compressibility g(T) = lim(q) (>0) S(q) similar to 1/rho(2). Various elastic contributions associated to the affine and nonaffine response to an infinitesimal strain are analyzed as functions of density and sampling time. We show how the size xi(rho) of the semidilute blob may be determined experimentally from the total monomer structure factor S(q) characterizing the compressibility of the solution at a given wave vector q. We comment briefly on finite persistence length effects.


Xu, H., Wittmer, J. P., Polinska, P., & Baschnagel, J. (2012). Impulsive correction to the elastic moduli obtained using the stressfluctuation formalism in systems with truncated pair potential. Physical Review E, 86(4).
Abstract: The truncation of a pair potential at a distance r(c) is well known to imply, in general, an impulsive correction to the pressure and other moments of the first derivatives of the potential. That, depending on r(c), the truncation may also be of relevance to higher derivatives is shown theoretically for the Born contributions to the elastic moduli obtained using the stressfluctuation formalism in d dimensions. Focusing on isotropic liquids for which the shear modulus G must vanish by construction, the predicted corrections are tested numerically for binary mixtures and polydisperse LennardJones beads in, respectively, d = 3 and 2 dimensions. Both models being glass formers, we comment briefly on the temperature (T) dependence of the (corrected) shear modulus G( T) around the glass transition temperature Tg.


2011 
Wittmer, J. P., Cavallo, A., Xu, H., Zabel, J. E., Polinska, P., Schulmann, N., et al. (2011). ScaleFree Static and Dynamical Correlations in Melts of Monodisperse and FloryDistributed Homopolymers A Review of Recent BondFluctuation Model Studies. Journal of Statistical Physics, 145(4), 1017–1126.
Abstract: It has been assumed until very recently that all longrange correlations are screened in threedimensional melts of linear homopolymers on distances beyond the correlation length xi characterizing the decay of the density fluctuations. Summarizing simulation results obtained by means of a variant of the bondfluctuation model with finite monomer excluded volume interactions and topology violating local and global Monte Carlo moves, we show that due to an interplay of the chain connectivity and the incompressibility constraint, both static and dynamical correlations arise on distances ra parts per thousand


2010 
Van Horn, R. M., Zheng, J. X., Sun, H. J., Hsiao, M. S., Zhang, W. B., Dong, X. H., et al. (2010). Solution Crystallization Behavior of CrystallineCrystalline Dibloek Copolymers of Poly(ethylene oxide)blockpoly(epsiloncaprolactone). Macromolecules, 43(14), 6113–6119.
Abstract: The crystallization behavior of crystallinecrystalline (CC) diblock copolymers has been shown to be dependent on the crystallization temperature and relative molecular size leach component The behavior of copolymers with similar crystallization temperatures is controlled by the block with the larger weight fraction and solventpolymer interactions Using samples of poly(ethylene oxide)blockpoly (epsiloncaprolacione) (PEObPCL), dilute solution crystallization methods were investigated to determine their role in crystallization of CC copolymers A single crystal of one block, either PEO or PCL, (the first block), crystallized first with the second block segregated to the crystal basal surfaces For the first time, solvent quality and homopolymer seeds were in to manipulate crystallization of the block with the smaller weight fraction to crystallize first and form the lamellar single crystal In addition, subsequent crystallization of the tethered chains (the second block) on the surface was also observed, depending upon the molecular weight of the second block and crystallization conditions These crystallites formed by the second block exhibited preferred orientations on the crystal surface as observed by electron diffraction It is believed that this orientation was induced by “soft” epitaxy between the fold surfaces of the adjacent single crystals of the first block formed by the in crystallization.


2008 
Xu, S., & Giuseppone, N. (2008). Selfduplicating amplification in a dynamic combinatorial library. Journal Of The American Chemical Society, 130(6), 1826–+.
Abstract: A dynamic combinatorial library has been designed to produce a set of constituents among which one is able to selfcomplementarily direct its duplication. The overall molecular distribution in the library evolves along both kinetic and thermodynamic biases, leading to the amplification of the species that reproduces most efficiently and to the depletion of the other competitors.


1996 
Thess, A., Lee, R., Nikolaev, P., Dai, H. J., Petit, P., Robert, J., et al. (1996). Crystalline ropes of metallic carbon nanotubes. Science, 273(5274), 483–487.
Abstract: Fullerene singlewall nanotubes (SWNTs) were produced in yields of more than 70 percent by condensation of a laservaporized carbonnickelcobalt mixture at 1200 degrees C. Xray diffraction and electron microscopy showed that these SWNTs are nearly uniform in diameter and thai they selforganize into ''ropes,'' which consist of 100 to 500 SWNTs in a twodimensional triangular lattice with a lattice constant of 17 angstroms. The xray form factor is consistent with that of uniformly charged cylinders 13.8 +/ 0.2 angstroms in diameter. The ropes were metallic, with a singlerope resistivity of <10(4) ohmcentimeters at 300 kelvin. The uniformity of SWNT diameter is attributed to the efficient annealing of an initial fullerene tubelet kept open by a few metal atoms; the optimum diameter is determined by competition between the strain energy of curvature of the graphene sheet and the danglingbond energy of the open edge, where growth occurs. These factors strongly favor the metallic (10,10) tube with C5v symmetry and an open edge stabilized by triple bonds.


1989 
Xu, H., Martinho, J., Winnik, M., & Beinert, G. (1989). Cyclization Dynamics Of Polymers .24. Transient Effects In Diffusion Controlled Reactions .7. Transient Effects In DiffusionControlled Polymer Cyclization. Makromolekulare ChemieMacromolecular Chemistry And Physics, 190(6), 1333–1343.

