Crosslinking PMMA: Molecular dynamics investigation of the shear response

Crosslinking can fundamentally change the mechanical properties of a linear glassy polymer. It has been experimentally observed that when lightly crosslinked, poly(methyl‐methacrylate) (PMMA) has a characteristically more ductile response to mechanical loading than does linear PMMA despite having a higher glass transition temperature. Here, molecular dynamics (MD) simulations are used to investigate conformational and energetic differences between linear PMMA and lightly crosslinked PMMA under shear deformation. As consistent with experiments, crosslinked PMMA is found to have a reduced yield stress relative to linear PMMA. Using the probing capabilities of our explicit atom MD approach, it is observed that while the crosslinks have a minimal direct energy contribution to the total system, they can alter how the main chains conform to macroscopic loading. In crosslinked PMMA, the backbone aligns more with the direction of external loading, thereby reducing the force applied to (and associated deformation of) the polymer bonds. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 444–449     

ACID DEPENDENT FLUORESCENCE QUANTUM YIELD AND TETRAMETHYL- 1,2-DIOXETANE CHEMI-ENERGIZED FLUORESCENCE OF ERGOSTATETRAENONE*

Abstract. –In acidified acetonitrile, ergostatetraenone (E) efficiently quenches the tetramethyl-1,2-dioxetane (TMD) chemiluminescence at 410 nm, with the appearance of a new chemiluminescence at ca. 530 nm due to ergostatetraenone fluorescence. Control experiments exclude energization by triplet acetone and the trivial process (emission and reabsorption), establishing singlet-singlet energy transfer between excited singlet acetone (A) and ground state ergostatetraenone as major pathway. From the kinetics the rate constant for singlet-singlet energy transfer ( K ^ss_A,1,) was estimated to be ca. 10^{1 2}S^-1. The chemiluminescence yield (φ^CL) for the TMD/ergostatetraenone system was determined to be ca. 12 × 10^-8. The fluorescence efficiency of ergostatetraenone (φ) in acidified acetonitrile is a function of acid concentration, ranging between 0.06-0.82. The efficiency of singlet acetone production (α) was found to be 500-fold lower than in neutral medium due to a competing acid-catalysed, dark decomposition of TMD. A 60-fold light enhancement has been established for this binary chemiluminescent system.     

Enhancement of the photocatalytic activity of modified ZnO nanoparticles with manganese additive

ZnO nanoparticles were synthesized under mild hydrothermal conditions (T = 150 °C, P = autogenous, experimental duration = 18 h). Manganese was added as an additive to ZnO nanoparticles in different molar percentages. In situ surface-modification was successfully carried out for these manganese-added ZnO nanoparticles using n-butylamine as a surface modifier. The modified manganese-added ZnO nanoparticulates are hydrophilic in nature and are well dispersed in various solvents. The modified nanoparticles were characterized using powder XRD, FTIR, SEM, Zeta potential, and UV?CVis spectrophotometry. The characterization results indicated tailoring of the morphology and size of the nanoparticles, and changing the surface chemistry of the nanoparticles synthesized. The SEM results show that the surface modified manganese-added ZnO nanoparticles have a very thin layer of organic coverage around the inorganic nanoparticles, thus, giving rise to hybrid nanoparticles. The photodegradation of Brilliant Blue dye under sunlight showed the higher efficiency of the modified manganese-doped ZnO nanoparticles compared to the reagent-grade ZnO.     

Novel oxadiazole functionalized pyridopyrimidine derivatives; their anticancer activity and molecular docking studies

A series of novel oxadiazole functionalized pyridopyrimidine derivatives prepared starting from 6-methyl/ethyl-2-oxo-4-(trifluoromethyl)-1,2-dihydropyridine-3-carbonitrile 1 . This compound 1 on reaction with sulfuric acid obtained compound 2 , further compound 2 on reaction with chloroacetamide followed by reaction with ethoxy methylene malonic diethyl ester coupling and further cyclization to obtain compound 5 . Compound 5 on reaction with hydrazide hydrate obtained hydrazide derivatives 6 . Compound 6 on reaction with diverse substituted aromatic acids to get oxadiazole derivatives 7a–l . All the final compounds 7a–l evaluated for anticancer activity against four human cancer cell lines such as HeLa—cervical cancer (CCL-2); COLO 205—colon cancer (CCL-222); HepG2—liver cancer (HB-8065); and MCF7—breast cancer (HTB-22) and promising compounds 7d and 7k have been identified and evaluated for molecular docking interactions.     

Pore-Scale Modeling of Electrokinetics in Geomaterials

Pore-scale finite-volume continuum models of electrokinetic processes are used to predict the Debye lengths, velocity, and potential profiles for two-dimensional arrays of circles, ellipses and squares with different orientations. The pore-scale continuum model solves the coupled Navier–Stokes, Poisson, and Nernst–Planck equations to characterize the electro-osmotic pressure and streaming potentials developed on the application of an external voltage and pressure difference, respectively. This model is used to predict the macroscale permeabilities of geomaterials via the widely used Carmen–Kozeny equation and through the electrokinetic coupling coefficients. The permeability results for a two-dimensional X-ray tomography-derived sand microstructure are within the same order of magnitude as the experimentally calculated values. The effect of the particle aspect ratio and orientation on the electrokinetic coupling coefficients and subsequently the electrical and hydraulic tortuosity of the porous media has been determined. These calculations suggest a highly tortuous geomaterial can be efficient for applications like decontamination and desalination.

     

Microwave‐Assisted Synthesis of N‐Substituted Maleimide Derivatives as Exogenous Antioxidant Agents

A series of N‐substituted maleimide derivatives have been developed via acetic acid‐mediated microwave reaction pathway, which was identified as the incomparable method for this maleimide compounds. All the synthesized compounds were tested for antioxidant activity by DPPH and H₂O₂ methods. Compounds 5h and 5m were displayed with higher antioxidant activity in two methods. The structure–activity relationship demonstrated that the compounds having electron releasing substitutions 5h and 5m generally show beneficial activity than electron capture substitution cores. Thus, compounds 5h and 5m may be useful as an exogenous antioxidant.     

Effect of quantum partial charges on the structure and dynamics of water in single-walled carbon nanotubes

In this work, using quantum partial charges, computed from 6-31G(**)B3LYP density functional theory, in molecular dynamics simulations, we found that water inside (6,6) and (10,0) single-walled carbon nanotubes with similar diameters but with different chiralities has remarkably different structural and dynamical properties. Density functional calculations indicate that tubes with different chiralities have significantly different partial charges at the ends of tubes. The partial charges at the ends of a (10,0) tube are around 4.5 times higher than those of a (6,6) tube. Molecular dynamics simulations with the partial charges show different water dipole orientations. In the (10,0) tube, dipole vectors of water molecules at the end of the tube point towards the water reservoir resulting in the formation of an L defect in the center region. This is not observed in the (6,6) tube where dipole vectors of all the water molecules inside the tube point towards either the top or the bottom water reservoir. The water diffusion coefficient is found to increase in the presence of the partial charges. Water in the partially charged (10,0) tube has a lower diffusion coefficient compared to that of in the partially charged (6,6) tube.     

Self-assembly of graphenes

Given the technological significance of graphene, various aspects of graphene have been recently explored. Here, we demonstrate the self-assembly of graphene fragments in water using molecular dynamics simulation. We observe that graphene fragments dispersed in water are assembled into a single aggregate. The assembly process is investigated by using the potential of mean force analysis and the significance of the enthalpic and entropic contributions is described. Other fundamental quantities such as hydrogen bonding and excluded volume are also examined. We anticipate that the fundamental finding in this paper can be extended to design the assembly process of complex carbon-based structures.     

Influence of ageing on the low cycle fatigue behaviour of an Al–Mg–Si alloy

Abstract

The low cycle fatigue (LCF) performance of AA6063 Al–Mg–Si alloy at under-aged (UA), peak-aged (PA) and over-aged (OA) conditions has been examined to understand the micromechanism of fatigue and the associated dynamic structural changes in this alloy. The LCF behaviour of the differently aged AA6063 alloys has been studied at strain amplitudes ranging between 0.2 and 1.0% under strain control mode. The UA state exhibits pronounced cyclic hardening unlike the PA and the OA states at strain amplitudes greater than 0.4%. The PA and the OA states show hardening only for a few cycles followed by prolonged softening. Characterisations of the micro- and the sub-structural alterations due to LCF establish that the phenomenon of dynamic precipitation results in cyclic hardening the UA alloy. The softening of PA alloy occurs due to shearing of precipitates and that in the OA alloy takes place owing to reversibility of slip by the formation and annihilation of the Orowan loops around the β (Mg₂Si) precipitates. Analyses of the hysteresis loops reveal Masing, nearly-Masing and non-Masing behaviour in the UA, OA and PA states, respectively. Analyses of the asymmetry factor of the hysteresis loops assist to infer that the Masing behaviour in the UA alloy is due to dislocation–dislocation interactions, whereas the nearly-Masing behaviour in the OA alloy and the non-Masing behaviour in the PA alloy are the consequence of varying degrees of dislocation–precipitate interactions associated with inhomogeneous deformation.