Glotzer Group

All 2009 2008 2007 2006 2005 2004 2003 2002 2001 2000 1999 1998 1997 1996 1995 1994 1993 1992 1991 1990 1988

2003

Glotzer SC

Nanoscale science - Complex rules for soft systems

NATURE MATERIALS 2 (11): 713-714 NOV 2003 [x]
Download PDF
View Abstract

x

The nanometre scale is a brave new world for scientists — mixing materials at such small dimensions can cause all sorts of surprising effects. New studies of experimental systems on the nanoscale further our understanding of these complex phenomena.

Zhang ZL, Horsch MA, Lamm MH, Glotzer SC

Tethered nano building blocks: Toward a conceptual framework for nanoparticle self-assembly

NANO LETTERS 3 (10): 1341-1346 OCT 2003 [x]
Download PDF
View Abstract

x

We perform molecular simulations to study the self-assembly of nanoparticles functionalized with oligomeric tethers attached to specific locations on the nanoparticle surface. We demonstrate that for certain categories of tethered nano building blocks the obtained morphologies may be predicted using concepts from block copolymer microphase separation and liquid-crystal phase ordering, whereas for other categories the unique packing constraints introduced by nanoparticle geometry and by nanoparticle-tether topology lead to structures far richer than those found in conventional block copolymer, surfactant, and liquid-crystal systems. Our results suggest the potential usefulness of considering tethered nano building blocks as a new class of "macromolecule" for assembling novel materials on the nanoscale.

Lacevic N, Starr FW, Schroder TB, Glotzer SC

Spatially heterogeneous dynamics investigated via a time-dependent four-point density correlation function

JOURNAL OF CHEMICAL PHYSICS 119 (14): 7372-7387 OCT 8 2003 [x]
Download PDF
View Abstract

x

Relaxation in supercooled liquids above their glass transition and below the onset temperature of "slow" dynamics involves the correlated motion of neighboring particles. This correlated motion results in the appearance of spatially heterogeneous dynamics or "dynamical heterogeneity." Traditional two-point time-dependent density correlation functions, while providing information about the transient "caging" of particles on cooling, are unable to provide sufficiently detailed information about correlated motion and dynamical heterogeneity. Here, we study a four-point, time-dependent density correlation function g(4)(r,t) and corresponding "structure factor" S-4(q,t) which measure the spatial correlations between the local liquid density at two points in space, each at two different times, and so are sensitive to dynamical heterogeneity. We study g(4)(r,t) and S-4(q,t) via molecular dynamics simulations of a binary Lennard-Jones mixture approaching the mode coupling temperature from above. We find that the correlations between particles measured by g(4)(r,t) and S-4(q,t) become increasingly pronounced on cooling. The corresponding dynamical correlation length xi(4)(t) extracted from the small-q behavior of S-4(q,t) provides an estimate of the range of correlated particle motion. We find that xi(4)(t) has a maximum as a function of time t, and that the value of the maximum of xi(4)(t) increases steadily from less than one particle diameter to a value exceeding nine particle diameters in the temperature range approaching the mode coupling temperature from above. At the maximum, xi(4)(t) and the alpha relaxation time tau(alpha) are related by a power law. We also examine the individual contributions to g(4)(r,t), S-4(q,t), and xi(4)(t), as well as the corresponding order parameter Q(t) and generalized susceptibility chi(4)(t), arising from the self and distinct contributions to Q(t). These contributions elucidate key differences between domains of localized and delocalized particles.

Lacevic N, Glotzer SC

Approach to the glass transition studied by higher order correlation functions

JOURNAL OF PHYSICS-CONDENSED MATTER 15 (31): S2437-S2446 AUG 13 2003 [x]
Download PDF
View Abstract

x

We present a theoretical framework based on a higher order density correlation function, analogous to that used to investigate spin glasses, to describe dynamical heterogeneities in simulated glass-forming liquids. These higher order correlation functions are a four-point, time-dependent density correlation function g(4) (r,t) and a corresponding `structure factor' S-4 (q,t) which measure the spatial correlations between the local liquid density at two points in space, each at two different times. g(4) (r, t) and S-4 (q, t) were extensively studied via molecular dynamics simulations of a binary Lennard-Jones mixture approaching the mode coupling temperature from above in Franz et al (1999 Phil. Mag. B 79 1827), Donati et al (2002 J. Non-Cryst. Solids 307 215), Glotzer et al (2000 J. Client. Phys. 112 509), Lacevic et al (2002 Phys. Rev. E 66 030101), Lacevic et al (2003 J. Chem. Phys. submitted) and Lacevic (2003 Dissertation The Johns Hopkins University). Here, we examine the contribution to g(4)(r, t), S-4(q, t) and the corresponding dynamical correlation length, as well as the corresponding order parameter Q(t) and generalized susceptibility chi(4)(t), from localized particles. We show that the dynamical correlation length xi(4)(ss)(t) of localized particles has a maximum as a function of time t, and the value of the maximum of xi(4)(ss)(t) increases steadily in the temperature range approaching the mode coupling temperature from above.

Lamm MH, Chen T, Glotzer SC

Simulated assembly of nanostructured organic/inorganic networks

NANO LETTERS 3 (8): 989-994 AUG 2003 [x]
Download PDF
View Abstract

x

We perform lattice Monte Carlo simulations to study nanostructured networks formed by linking organically functionalized inorganic nanoscale building blocks. We develop a minimal lattice model of an octafunctional nanoscale building block (NBB) by representing the inorganic core with a rigid cube and the organic linkers with "bead-spring" chains. Using this model, we explore the effect of linker length on network properties including porosity, spatial distribution of NBBs, and extent of cross-linking during assembly. We compare our results with recent experimental data on networks of octafunctional polyhedral oligomeric silsesquioxane (POSS) cubes.

Aichele M, Gebremichael Y, Starr FW, Baschnagel J, Glotzer SC

Polymer-specific effects of bulk relaxation and stringlike correlated motion in the dynamics of a supercooled polymer melt

JOURNAL OF CHEMICAL PHYSICS 119 (10): 5290-5304 SEP 8 2003 [x]
Download PDF
View Abstract

x

We analyze dynamical heterogeneities in a simulated "bead-spring" model of a nonentangled, supercooled polymer melt. We explore the importance of chain connectivity on the spatially heterogeneous motion of the monomers. We find that when monomers move, they tend to follow each other in one-dimensional paths, forming strings as previously reported in atomic liquids and colloidal suspensions. The mean string length is largest at a time close to the peak time of the mean cluster size of mobile monomers. This maximum string length increases, roughly in an exponential fashion, on cooling toward the critical temperature T-MCT of the mode-coupling theory, but generally remains small, although large strings involving ten or more monomers are observed. An important contribution to this replacement comes from directly bonded neighbors in the chain. However, mobility is not concentrated along the backbone of the chains. Thus, a relaxation mechanism in which neighboring mobile monomers along the chain move predominantly along the backbone of the chains, seems unlikely for the system studied.

Starr FW, Douglas JF, Glotzer SC

Origin of particle clustering in a simulated polymer nanocomposite and its impact on rheology

JOURNAL OF CHEMICAL PHYSICS 119 (3): 1777-1788 JUL 15 2003 [x]
Download PDF
View Abstract

x

Many nanoparticles have short-range interactions relative to their size, and these interactions tend to be "patchy" since the interatomic spacing is comparable to the nanoparticle size. For a dispersion of such particles, it is not a priori obvious what mechanism will control the clustering of the nanoparticles, and how the clustering will be affected by tuning various control parameters. To gain insight into these questions, we perform molecular dynamics simulations of polyhedral nanoparticles in a dense bead-spring polymer melt under both quiescent and steady shear conditions. We explore the mechanism that controls nanoparticle clustering and find that the crossover from dispersed to clustered states is consistent with the predictions for equilibrium particle association or equilibrium polymerization, and that the crossover does not appear to match the expectations for first-order phase separation typical for binary mixtures in the region of the phase diagram where we can equilibrate the system. At the same time, we cannot rule out the possibility of phase separation at a lower temperature. Utilizing the existing framework for dynamic clustering transitions offers the possibility of more rationally controlling the dispersion and properties of nanocomposite materials. Finally, we examine how nanocomposite rheology depends on the state of equilibrium clustering. We find that the shear viscosity for dispersed configurations is larger than that for clustered configurations, in contrast to expectations based on macroscopic colloidal dispersions. We explain this result by the alteration of the polymer matrix properties in the vicinity of the nanoparticles. We also show that shear tends to disperse clustered nanoparticle configurations in our system, an effect particularly important for processing.