A Comparison between Predictions of the Miller-Macosko Theory, Estimates from Molecular Dynamics Simulations, and Long-Standing Experimental Data of the Shear Modulus of End-Linked Polymer Networks.

Long-standing experimental data on the elastic modulus of end-linked poly(dimethylsiloxane) (PDMS) networks are employed to corroborate the validity of the Miller-Macosko theory (MMT). The validity of MMT is also confirmed by molecular dynamics (MD) simulations that mimic the experimentally realized networks. It becomes apparent that for a network formed from bulk, where the fractions of the loops are small, it is sufficient to account for the topological details of a reference tree-like network, i.e., for its degree of completion, junction functionalities, and trapped entanglements, in order to practically predict the modulus. However, a mismatch is identified between the MMT and MD simulations in relating the fraction of the soluble material to the extent of reaction. A large contribution of entanglements to the modulus of PDMS networks prepared with short precursor chains is presented, suggesting that the elastic modulus of commonly used end-linked PDMS networks is in fact entanglement-dominated.

Templated Assembly of Pore-forming Peptides in Lipid Membranes.

Pore-forming peptides are of interest due to their antimicrobial activity and ability to form gateways through lipid membranes. Chemical modification of these peptides makes it possible to arrange several peptide monomers into well-defined pore-forming structures using various templating strategies. These templated super-structures can exert antimicrobial activity at significantly lower total peptide concentration than their untemplated equivalents. In addition, the chemical moieties used for templating may be functionalized to interact specifically with targeted membranes such as those of pathogens or cancer cells. A range of molecular templates has been explored, including dimerization of pore-forming monomers, their covalent attachment to cyclodextrin, porphyrin or fullerene scaffolds as well as attachment of amino acid linkers or nucleic acid constructs to generate assemblies of 4 to 26 peptides or proteins. Compared to free peptide monomers, templated pore assemblies showed increased membrane affinity, prolonged open-state lifetimes of the pores and more frequent pore formation due to higher local concentration. These constructs are useful model systems for biophysical studies to understand porin and ion channel proteins and their mechanisms of insertion into lipid membranes. Recently designed DNA-templates are expanding the usefulness of templated pore assemblies beyond applications of cell killing and may include targeted drug delivery and accelerate the emerging field of single-molecule detection and characterization of biomolecules by nanopore-based resistive pulse sensing.

Multispeed models in off-lattice Boltzmann simulations.

The lattice Boltzmann method is a highly promising approach to the simulation of complex flows. Here, we realize recently proposed multispeed lattice Boltzmann models [S. Chikatamarla, Phys. Rev. Lett. 97 190601 (2006)] by exploiting the flexibility offered by off-lattice Boltzmann methods. The approach is based on the general characteristic-based algorithm for off-lattice Boltzmann simulations that preserves all appealing properties of the standard lattice Boltzmann method while extending the method to unstructured grids. We show that the use of multispeed models indeed gives rise to major improvements in accuracy. The suggested approach thus renders truly large-scale off-lattice Boltzmann computations practical.

Finite element mapping for spring network representations of the mechanics of solids.

We present a general finite element mapping procedure for defining spring network representations of solid mechanics. The procedure is rigorous and equally suitable for setting regular and unstructured spring network models of generally anisotropic solids. We use the procedure to define close-packed triangular and simple cubic lattice spring models of isotropic 2D and 3D elastic media, respectively. We extend the study to heterogeneous solids and show that the mapped spring network approach constitutes an appealing route for incorporating subelement level constitutive equations.