Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Fast parallel algorithms for short-range molecular dynamics

Luminogenic materials with aggregation-induced emission (AIE) attributes have attracted much interest since the debut of the AIE concept in 2001. In this critical review, recent progress in the area of AIE research is summarized. Typical examples of AIE systems are discussed, from which their structure–property relationships are derived. Through mechanistic decipherment of the photophysical processes, structural design strategies for generating new AIE luminogens are developed. Technological, especially optoelectronic and biological, applications of the AIE systems are exemplified to illustrate how the novel AIE effect can be utilized for high-tech innovations (183 references).

Aggregation-induced emission

This critical review provides a processing-structure-property perspective on recent advances in cellulose nanoparticles and composites produced from them. It summarizes cellulose nanoparticles in terms of particle morphology, crystal structure, and properties. Also described are the self-assembly and rheological properties of cellulose nanoparticle suspensions. The methodology of composite processing and resulting properties are fully covered, with an emphasis on neat and high fraction cellulose composites. Additionally, advances in predictive modeling from molecular dynamic simulations of crystalline cellulose to the continuum modeling of composites made with such particles are reviewed (392 references).

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Cellulose nanomaterials review: structure, properties and nanocomposites

Cellulose constitutes the most abundant renewable polymer resource available today. As a chemical raw material, it is generally well-known that it has been used in the form of fibers or derivatives for nearly 150 years for a wide spectrum of products and materials in daily life. What has not been known until relatively recently is that when cellulose fibers are subjected to acid hydrolysis, the fibers yield defect-free, rod-like crystalline residues. Cellulose nanocrystals (CNs) have garnered in the materials community a tremendous level of attention that does not appear to be relenting. These biopolymeric assemblies warrant such attention not only because of their unsurpassed quintessential physical and chemical properties (as will become evident in the review) but also because of their inherent renewability and sustainability in addition to their abundance. They have been the subject of a wide array of research efforts as reinforcing agents in nanocomposites due to their low cost, availability, renewability, light weight, nanoscale dimension, and unique morphology. Indeed, CNs are the fundamental constitutive polymeric motifs of macroscopic cellulosic-based fibers whose sheer volume dwarfs any known natural or synthetic biomaterial. Biopolymers such as cellulose and lignin and † North Carolina State University. ‡ Helsinki University of Technology. Dr. Youssef Habibi is a research assistant professor at the Department of Forest Biomaterials at North Carolina State University. He received his Ph.D. in 2004 in organic chemistry from Joseph Fourier University (Grenoble, France) jointly with CERMAV (Centre de Recherche sur les Macromolecules Vegetales) and Cadi Ayyad University (Marrakesh, Morocco). During his Ph.D., he worked on the structural characterization of cell wall polysaccharides and also performed surface chemical modification, mainly TEMPO-mediated oxidation, of crystalline polysaccharides, as well as their nanocrystals. Prior to joining NCSU, he worked as assistant professor at the French Engineering School of Paper, Printing and Biomaterials (PAGORA, Grenoble Institute of Technology, France) on the development of biodegradable nanocomposites based on nanocrystalline polysaccharides. He also spent two years as postdoctoral fellow at the French Institute for Agricultural Research, INRA, where he developed new nanostructured thin films based on cellulose nanowiskers. Dr. Habibi’s research interests include the sustainable production of materials from biomass, development of high performance nanocomposites from lignocellulosic materials, biomass conversion technologies, and the application of novel analytical tools in biomass research. Chem. Rev. 2010, 110, 3479–3500 3479

Cellulose nanocrystals: chemistry, self-assembly, and applications.

Detection of chemical and biological agents plays a fundamental role in biomedical, forensic and environmental sciences1–4 as well as in anti bioterrorism applications.5–7 The development of highly sensitive, cost effective, miniature sensors is therefore in high demand which requires advanced technology coupled with fundamental knowledge in chemistry, biology and material sciences.8–13

In general, sensors feature two functional components: a recognition element to provide selective/specific binding with the target analytes and a transducer component for signaling the binding event. An efficient sensor relies heavily on these two essential components for the recognition process in terms of response time, signal to noise (S/N) ratio, selectivity and limits of detection (LOD).14,15 Therefore, designing sensors with higher efficacy depends on the development of novel materials to improve both the recognition and transduction processes. Nanomaterials feature unique physicochemical properties that can be of great utility in creating new recognition and transduction processes for chemical and biological sensors15–27 as well as improving the S/N ratio by miniaturization of the sensor elements.28

Gold nanoparticles (AuNPs) possess distinct physical and chemical attributes that make them excellent scaffolds for the fabrication of novel chemical and biological sensors (Figure 1).29–36 First, AuNPs can be synthesized in a straightforward manner and can be made highly stable. Second, they possess unique optoelectronic properties. Third, they provide high surface-to-volume ratio with excellent biocompatibility using appropriate ligands.30 Fourth, these properties of AuNPs can be readily tuned varying their size, shape and the surrounding chemical environment. For example, the binding event between recognition element and the analyte can alter physicochemical properties of transducer AuNPs, such as plasmon resonance absorption, conductivity, redox behavior, etc. that in turn can generate a detectable response signal. Finally, AuNPs offer a suitable platform for multi-functionalization with a wide range of organic or biological ligands for the selective binding and detection of small molecules and biological targets.30–32,36 Each of these attributes of AuNPs has allowed researchers to develop novel sensing strategies with improved sensitivity, stability and selectivity. In the last decade of research, the advent of AuNP as a sensory element provided us a broad spectrum of innovative approaches for the detection of metal ions, small molecules, proteins, nucleic acids, malignant cells, etc. in a rapid and efficient manner.37



Figure 1

Physical properties of AuNPs and schematic illustration of an AuNP-based detection system.



In this current review, we have highlighted the several synthetic routes and properties of AuNPs that make them excellent probes for different sensing strategies. Furthermore, we will discuss various sensing strategies and major advances in the last two decades of research utilizing AuNPs in the detection of variety of target analytes including metal ions, organic molecules, proteins, nucleic acids, and microorganisms.

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Gold nanoparticles in chemical and biological sensing.

Many cyclophanes have been investigated in dilute solution, where their internal cavities are accessible for supramolecular interactions. However, their photophysical properties in the solid state remain largely unexplored. We here report a new mechano- and thermoresponsive luminescent cyclophane that is comprised of two 9,10-bis(phenylethynyl)anthracene moieties and features two hexaethylene glycol bridges. The compound was found to exhibit a nematic liquid-crystalline phase at elevated temperature. X-ray diffraction patterns confirm that thermal and mechanical treatments induce changes in the molecular assembly, which are the basis for the observed photoluminescent color variations. The stimuli-responsive behavior of the new compound is quite different from that of a previously reported cyclophane with similar structure but shorter bridges. Thus, merely changing the ring size is an effective tool to tailor the stimuli-responsiveness and the phase behaviour of luminescent cyclophanes.

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Tuning the thermo- and mechanoresponsive behavior of luminescent cyclophanes

The synthesis and properties of a new indole-based squaraine dye functionalized with ethylhexyl substituents are reported. The chromophore displays a large two-photon absorption cross-section of ∼1490 GM at a resonance wavelength of 1160 nm. The crystalline material exhibits a melting temperature of 222 °C, which is sufficiently low to allow for melt-processing without significant thermal decomposition. Crystallization of the dye from the melt is exceedingly slow, so that amorphous glasses with a glass transition temperature of 60 °C can easily be produced, even without particularly rapid cooling. This allows the fabrication of thin films and more complex shapes, which offer a maximum of dye content, yet lack the scattering effects associated with organic crystalline materials.

A melt-processable squaraine-based organic glass for nonlinear optics

A premixing method to produce polymer nanocomposites with cellulose nanocrystals (CNCs) is reported. This method involves the dissolution and dispersion of a polymer and CNCs in an organic solvent, co-precipitation into water, drying of the resulting particles, and subsequent melt processing. The key aspect of the method is that it allows the kinetic trapping of well-dispersed CNCs in the polymer. Although the nanocomposite must be dried before subsequent melt-processing, the organic solvent can be removed by extraction in water and recycled, leaving only residual water in the composite, which is easily eliminated. This process presents numerous advantages compared with the time-consuming solvent casting process, which often suffers from incomplete organic solvent evaporation. As a testbed, polyurethane (PU) composites with up to 30% of CNCs were prepared. These materials were either melt-processed as produced or used as a masterbatch, i.e., they were diluted via melt-mixing with neat polymer toward nanocomposites with lower filler content. All nanocomposites prepared using this approach had a homogeneous appearance. They displayed similar mechanical properties as the corresponding reference materials made by solvent casting, and significantly better properties than materials prepared by direct melt mixing. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 45648.

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Polymer nanocomposites with cellulose nanocrystals made by co-precipitation

With the objective to produce processable, high-melting, hydrophobic, and crystalline polymers, we embarked on the synthesis of poly(p-phenylene butylene) (PPPB) as a new representative of the class of polymers that contain only aromatic and aliphatic hydrocarbon units in their backbone. Acyclic diene metathesis (ADMET) polymerization of p-diallylbenzene followed by catalytic reduction of the resulting unsaturated polymer was used as the primary synthetic route to PPPB. For the ADMET polymerizations, Schrock's alkylidene molybdenum complex, Grubbs' benzylidene ruthenium catalyst, and two classical systems (WOCl2(OAr)2/Bu4Sn and WCl6/Bu4Sn) were employed, and different reaction conditions were compared. WOCl2(OAr)2/Bu4Sn in refluxing toluene proved to be the most appropriate catalyst system to produce the crystalline and high-melting poly(p-phenylene but-2-enylene) precursor polymer in high molecular weight and chemically pure form. Catalytic hydrogenation of the latter led to poly(p-phenylene butylene) wi...

Poly(p-phenylene butylene): A New, Processable, High-Melting Hydrophobic Polymer

Relaxation processes in nonlinear optical (NLO) polymers with glass transition temperatures in the range of 125 °C < T g < 176 °C have been studied. The relaxational mechanisms of these side- and main-chain polymers have been investigated above and below the glass transition by second-harmonic decay, dielectric relaxation, and differential scanning calorimetry measurements, and the results obtained have been compared with a variety of nonlinear optical polymer systems cited in the literature. The nonexponential relaxation in both the time and frequency domain was modeled by the Kohlrausch-Williams-Watts function whereas the nonlinear relaxational behavior of these polymers was modeled in terms of the Tool-Narayanaswamy description of the glassy state using the Adam-Gibbs expression for the relaxation time. This procedure allows for the nonlinear extension of the liquid equilibrium state behavior into and below the glass transition region with an accurate prediction of the relaxation times over more than 15 orders of magnitude in time. Time-temperature scaling of the relaxation times with respect to T g /T as the relevant scaling parameter is observed below the glass transition. The effect of annealing was investigated using differential scanning calorimetry with the result that a single set of parameters is sufficient to describe a wide range of thermal histories with as well as without annealing. Optimum annealing temperatures/annealing times for best orientational stability in NLO polymers were calculated according to the same relaxation scheme.

Christoph Weder

Papers

Tuning the thermo- and mechanoresponsive behavior of luminescent cyclophanes

A melt-processable squaraine-based organic glass for nonlinear optics

Polymer nanocomposites with cellulose nanocrystals made by co-precipitation

Poly(p-phenylene butylene): A New, Processable, High-Melting Hydrophobic Polymer

Relaxation Processes in Nonlinear Optical Polymers: A Comparative Study