Customized sequential designs for random simulation experiments: Kriging metamodeling and bootstrapping

This paper proposes a novel method to select an experimental design for interpolation in random simulation, especially discrete event simulation. (Though the paper focuses on Kriging, this design approach may also apply to other types of metamodels such as non-linear regression models and splines.) Assuming that simulation requires much computer time, it is important to select a design with a small number of observations (or simulation runs). The proposed method is therefore sequential. Its novelty is that it accounts for the specific input/output behavior (or response function) of the particular simulation at hand; i.e., the method is customized or application-driven. A tool for this customization is bootstrapping, which enables the estimation of the variances of predictions for inputs not yet simulated. The method is tested through two classic simulation models, namely the expected steady-state waiting time of the M/M/1 queuing model, and the mean costs of a terminating (s, S) inventory simulation. For these two simulation models the novel design indeed gives better results than a popular alternative design, namely Latin Hypercube Sampling (LHS) with a prefixed sample.     

Molecular visualization of conformation-triggered flow instability

A new type of flow fingering instability was observed in monolayer-thick polymer films as they spread on a solid substrate. Tracing the movement of individual molecules by atomic force microscopy enabled us to follow the development of the flow instability on the molecular level and to understand the underlying physical mechanism. The fingering instability was observed to be triggered by conformational changes of brushlike macromolecules in response to the pressure gradient driving the flow.     

Magnetization and magnetoresistance of amorphous FexB1−x in high magnetic fields

The magnetization and magnetoresistance of ferromagnetic melt-quenched amorphous Fe_xB_1?x has been measured. The magnetization indicates no saturation to fields above 30 tesla. The absolute value of the negative magnetoresistivity increases by a factor of five with decreasing B content from x = 0.25 to 0.15. These results are discussed in terms of different models of the resistivity minimum behaviour of amorphous alloys.     

Remark on the solution given by Muriel for a two-level atom in an arbitrarily intense sinusoidal field

It is shown that the solution recently given by Muriel for a two-level atom in an arbitrary strong sinusoidal field can not be applied directly to the calculation of the time averaged transition rate.     

Block Copolymer PEO‐b‐PHPMA Synthesis Using Controlled Radical Polymerization on a Chip

Summary: Block copolymers of poly(ethylene oxide‐block‐2‐hydroxypropyl methacrylate) (PEO‐b‐PHPMA) with a range of molecular masses of the PHPMA block were obtained by controlled radical polymerization on a chip (CRP chip) using a PEO macroinitiator. A series of well‐controlled polymerizations were carried out at different pumping rates or reaction times with a constant ratio of monomer to initiator. The stoichiometry of the reactants was also adjusted by varying relative flow rates to change the reactant concentrations.

A schematic of a CRP chip and SEC traces of the PEO‐b‐PHPMA produced from different pump rates with a 1:100 ratio of initiator to monomer. The dashed peaks are the macroinitiator, PEO‐Br (left), and monomer, HPMA (right).     

Application-driven sequential designs for simulation experiments: Kriging metamodelling

This paper proposes a novel method to select an experimental design for interpolation in simulation. Although the paper focuses on Kriging in deterministic simulation, the method also applies to other types of metamodels (besides Kriging), and to stochastic simulation. The paper focuses on simulations that require much computer time, so it is important to select a design with a small number of observations. The proposed method is therefore sequential. The novelty of the method is that it accounts for the specific input/output function of the particular simulation model at hand; that is, the method is application-driven or customized. This customization is achieved through cross-validation and jackknifing. The new method is tested through two academic applications, which demonstrate that the method indeed gives better results than either sequential designs based on an approximate Kriging prediction variance formula or designs with prefixed sample sizes.     

Stable and sensitive method for the simultaneous determination of N5-methyltetrahydrofolate, leucovorin, methotrexate and 7-hydroxymethotrexate in biological fluids

A high-performance liquid chromatographic method for the determination of N5-methyltetrahydrofolic acid, leucovorin, methotrexate and 7-hydroxymethotrexate in plasma and liquor samples is presented. Gradient elution is used to increase the sensitivity. Four sample preparation methods were compared with respect to the stability of the injectable sample. Samples can be pretreated with a simple deproteinization method. For enhanced selectivity a solid-phase extraction procedure is described.     

Macromolecular transport through nanostructured and porous hydrogels synthesized using the amphiphilic copolymer, PCL-b-PEO-b-PCL

We have studied macromolecular transport through nanostructured and porous hydrogels. These hydrogels were produced by (1) synthesizing poly(ε-caprolactone-b-ethylene oxide-b-ε-caprolactone) (PCL-PEO-PCL) block copolymers, (2) crosslinking the block copolymer in aqueous solutions at high concentrations, and (3) degrading the PCL blocks by hydrolysis. Lamellar morphologies of the PCL-b-PEO-b-PCL were observed using Small Angle X-ray Scattering (SAXS) and Atomic Force Microscopy (AFM). The regions formerly occupied by PCL domains have no spanning PEO chains and thus should allow fast transport of macromolecules without significant entanglement. This work presents Fluorescence Recovery After Photobleaching (FRAP) data showing that macromolecular diffusivities are higher in these nanostructured materials than in comparable homogeneous PEO hydrogels prepared under the same crosslinking conditions to yield similar crosslink density. These materials offer a route to high macromolecular mobility, and so may be useful in bioengineering areas such as drug delivery and protein separation.     

Proton conduction in materials comprising conducting domains with widths less than 6 nm

We review the literature on proton conductivity and water uptake of composite polymer electrolyte membranes comprising bicontinuous hydrophilic and hydrophobic domains with well-controlled geometries. Both quantities appear to be enhanced when the width of the hydrophilic channels is smaller than 6 nm.