Glass transition temperature of low molecular weight poly(3-aminopropyl methyl siloxane). A molecular dynamics study

The average specific volume of the model poly(3-aminopropyl methyl siloxane) as a function of temperature near the glass transition was computed from molecular dynamics simulations. The glass transition temperature was defined as the slop intersection around 210 K, a value similar to that of the experimental result. Globular polymer shaped chains were observed where the chain is closed upon itself. Three amino groups of amino propylene chains were located in the center and the rest of the amino groups were situated outside the main chain. The glass transition temperature of this low molecular weight polymer strongly depends on the binding energies between chains. The intersection of binding energy slopes defines a temperature of 213 K near the glass transition temperature. The most important contributions to the glass transition changes were the electrostatic binding contributions. The Van der Waals contributions in the volume changes were less important. The chain mobility was evaluated by the transition between angles for the states trans, g⁺ and g⁻. The glass transition temperature observed experimentally, 208±2 K, is due to cooperative movements of two different torsion angles, (O–Si) and (Si–C) of the main chain and the lateral chain, respectively, and its rotational mobility. Self-diffusion constant variation for all polymer atoms with the temperature is a probe that the polymer chain cooperative movement had started at temperatures around the glass transition temperature.This work was supported by the CAM through Grant 07N/0002/1998

Curing of polymer matrix composites. Fluorescence study of dansyl fluorophore labelled to glass fibres and DGEBA-ethyelenediamine epoxy resin

Curing process of diglycidyl ether of bisphenol A/ethylenediamine mixture in the presence of glass fibers was monitored by fluorescence in two ways: first, using dansyl labeled glass fibers and diglycidyl ether of bisphenol A (DGEBA)/ethylenediamine mixture and second, using unlabeled glass fibers and DGBA-dansyl labeled/ethylenediamine mixture. Integral fluorescence intensity was analyzed as a function of time. Results allow comparison between the curing process inside the bulk of the resin and at the glass fiber interface. It was concluded that for the system DGEBA/ethylenediamine the polymer matrix viscosity increases with the curing time faster inside the bulk than at the glass fiber interface.Authors wish to thank CAM (07N/0002/1998) for suppor

Conformations and mobility of polyethylene and trans-polyacethylene chains confined in α-cyclodextrins channels

Conformational properties and mobility of two polymeric chains containing 30 repeating units of either polyethylene (PE) or trans-polyacethylene (PA) confined into the channel formed by eight α-cyclodextrins (αCDs) are studied by Molecular Dynamics simulations performed at 500 K and compared with the behavior exhibited by the same chains when they stand alone in vacuum. The rotaxane structure (i.e. polymeric chains threaded into the αCDs channel) is stabilized with respect to the separated chain and αCDs mostly because of van der Waals interactions. As it might be expected, large differences are observed in the molecular characteristics of the isolated chains as compared to their confined counterparts. The differences are in the sense of decreasing the conformational mobility in favor of extended conformations in the case of confined chains. Comparison of the results obtained for confined PE and PA chains indicates a noticeably larger mobility of the PA chain. Molecular dimensions obtained for the isolated PE chain agree with the results published in the literature.This work was supported by the DGESIC (Dirección General de Enseñanza Superior e Investigación Cientı́fica) through Grant PB97-0778

Molecular Mechanics Study of the Complexes of β-Cyclodextrin with 4-(dimethylamino)benzonitrile and Benzonitril

Molecular Mechanics calculations with the Tripos Force Field were employed to study the complexation of 4-(dimethylamino)benzonitrile (DMABN) and/or benzonitrile (BN) with β-cyclodextrin (βCD). The systems studied have 1 : 1 (DMABN : βCD and BN : βCD), 2 : 2 (DMABN : βCD) and 1 : 1 : 2 (DMABN : BN : βCD) stoichiometries. Evidence for the formation of such complexes, binding constants and other thermodynamic parameters were extracted from the analysis of the steady state fluorescence measurements performed in a previous work. The Molecular Mechanics study, based on the energy changes upon guest-host approaching, was performed in vacuo and in the presence of water as a solvent. Results show that the driving forces for 1 : 1 complexation are mainly dominated by non-bonded van der Waals host : guest interactions. However, the driving forces for association between 1 : 1 complexes to give 2 : 2 homo- or 1 : 1 : 2 heterodimers are dominated by non-bonded electrostatic interactions. Head-to-head electrostatic interactions between βCDs, which are presumably due to the hydrogen bonding formation between secondary hydroxyl groups of CDs, are responsible for most of the stability of the dimers.This research was supported by CICYT grant PB94-0364. We wish to express our thanks to M.L. Heijnen for assistance with the preparation of the manuscript

Inclusion Complexes of Chain Molecules with Cycloamyloses. 2. Molecular Dynamics Simulations of Polyrotaxanes Formed by Poly(ethylene glycol) and α-Cyclodextrins

Molecular dynamics (MD) simulations were performed for “channel type” polyrotaxanes with α-cyclodextrins (α-CDs) threaded onto monodisperse chains of poly(ethylene glycol) (PEG). The polymer captures as much α-CD as its length permits, forming a close-packed structure from the one end to the other. The van der Waals interactions are the main source of the stabilization of these polyrotaxanes. Hydrogen bonds between successive α-CDs slightly favor head-to-head, tail-to-tail sequences over head-to-tail sequences. The α-CDs in polyrotaxanes are more symmetric and less distorted than the isolated α-CDs. The PEG in the polyrotaxane is more extended than an unperturbed chain, because it has a larger population of trans states at internal bondsThis research was supported by UAH-017/95 and DGICYT PB94-0364 and by National Science Foundation grant DMR 9523278

Intramolecular excimer formation in naphthalene-containig polyesters. Bichromophoric model compounds derived from phathalic, siccinic or malonic acid and 2-hydroxynaphthalene or 2-hydroxy-methylnaphthalene.

Steady state fluorescence measurements in dilute solutions are performed for bis(2-naphthyl) phthalate, bis(2-naphthylmethyl) phthalate, bis(2-naphthyl) succinate, bis(2-naphthylmethyl) succinate, bis(2-naphthyl) malonate and bis(2-naphthylmethyl) malonate, as models for polyesters containing naphthalene in their rigid units and flexible spacers that impose differences in the types of interactions between successive naphthalene units. The amount of intramolecular excimer in dilute solution depends on the type of flexible spacer, and also whether the compound is derived from 2-naphthol or 2-hydroxymethylnaphthalene. No excimer is detected from the compounds derived from 2-naphthol, but all compounds derived from 2-hydroxymethylnaphthalene showed excimer formation. These results are rationalized with a theoretical analysis of the conformations of the flexible spacers.This research was supported by DGICYT PB91-0166 (FM) and by National Science Foundation grant DMR 9220369 (WLM)

High porosity scaffold composites of graphene and carbon nanotubes as microwave absorbing materials

The design of microwave absorbing materials requires low reflection and high absorption of radiation simultaneously. Low reflection of electromagnetic waves can be achieved inducing porous faces which minimize the impedance mismatch. High absorption can be obtained by increasing the conductivity of the material. We report the preparation of highly porous scaffolds from a combination of graphene and carbon nanotubes. The bimodal porous structure was controlled making use of the surface properties of graphene oxide that are able to stabilize hexane droplets in aqueous dispersions of graphene oxide and carbon nanotubes. After hydrothermal and two step freeze-drying processes, macro-(220 microns) and mesoporous (10 microns) structures, due to hexane droplets and freeze drying, respectively, were obtained. DC conductivities of 8.2, 14.7, 33.2, and 60.7 S m(-1) were obtained for graphene scaffolds containing 0, 10, 20 and 40% of carbon nanotubes respectively. An electromagnetic characterization was performed on scaffolds infiltrated with epoxy resin; using appropriate models, the electromagnetic properties of the conducting phase have been obtained. Scaffolds with a thickness of 9 mm were able to absorb up to 80% of the incident radiation keeping the reflection as low as 20%.This work was supported by grant NANONARQ (MAT2014-57557-R) from the Spanish Ministerio de Economía y Competitividad

Magnetic silica: epoxy composites with a nano- and micro-scale control

Multiscale composites with magnetic properties were prepared by incorporating Cu-Ni nanoferrites filled silica microparticles in an epoxy matrix. The size of the nanoferrites was controlled both by the structure of the silica template and the annealing temperature (700 and 900 °C) used during the synthesis procedure. The ferrite:silica particles prepared at 700 °C showed a narrow size distribution close to 8.3 nm with a superparamagnetic behaviour. A less symmetric size distribution was obtained when annealing was performed at 900 °C, with diameters ranging from 15 to 80 nm. The reinforcement incorporation increased up to 7 °C the glass transition temperature and 30 °C the decomposition temperatures of the composites. The proposed strategy permits the nanoscale control, by the trapping effect of the silica on the magnetic nanoparticles, as well as the control of the micro-scale distribution through a simple protocol. These composites could have potential applicability as EMI shielding materials, owing to their magnetic nature, lightweight and enhanced thermal stability.This work was supported by Nacopan (MAT2007-31173-E) and Nanomod (MAT2010-17091) from the Spanish Ministerio de Ciencia e Innovación

Molecular Dynamics Simulations of Proton Conduction in Sulfonated Poly(phenyl sulfone)s

Full molecular dynamics was used to simulate separately the diffusion of naked protons and H₃O⁺ hydrated protons across sulfonated poly(phenyl sulfone)s. Simulations were carried out for wet membranes with the following characteristics:  ion-exchange capacity, 1.8 mequiv/g of dry membrane; water uptake, 10−30%; temperature range, 300−360 K. The diffusion coefficient of naked protons is nearly 1 order of magnitude higher than that of the hydrated protons for the membranes with the lower water uptake (10%). For the membranes with higher water uptake the ratio between the diffusion coefficients of the two particles reduces to about half an order of magnitude. The conductivity of the naked protons increases from 21.4 × 10⁻³ to 52.5 × 10⁻³ S/cm when the water uptake increases from 10% to 30%. For hydrated protons the conductivity increases from 1.54 × 10⁻³ to 7.57 × 10⁻³ S/cm. The conductivities obtained through simulations carried out at 300 K for the hydrated proton across membranes with water uptake 18% and 30% are roughly similar to those experimentally measured for a membrane with ion exchange capacity = 1.8 mequiv/g and water uptake = 24.3%. Simulated conductivities of both naked protons and hydrated protons follow Arrhenius behavior.This work was supported by the Comunidad de Madrid through the grant interfaces S-505/MAT-0227, Fondos Europeos de Desarrollo (FEDER), and Fondo Social Europeo (FSE). Financial support was also supplied by the Dirección General de Investigación Científica y Técnica (DGICYT) (grant MAT 2005-05648-C02-02)

Intramolecular energy transfer in naphthalene-containing polyesters: Experiment and simulation for model compounds derived from five aliphatic dicarboxylic acids and 2-hydroxynaphthalene

Steady-state fluorescence depolarization measurements and molecular dynamics simulations have been used to study the efficiency of nonradiative intramolecular singlet energy transfer between 2-naphthoxy groups, denoted N, in model compounds for polyesters derived from 2,6-dihydroxy-naphthalene and aliphatic dicarboxylic acids. The five bichromophoric compounds studied are the diesters abbreviated as N-OOC-(CH₂)n-COO-N;n = 2–6, which are condensation products obtained from 2-naphthol and aliphatic dicarboxylic acids. The anisotropy of the fluorescence of these compounds dispersed in a solid matrix of glassy poly(methyl methacrylate) indicates that there is nonradiative singlet energy transfer between naphthoxy groups. The efficiency of this transfer depends on. A theoretical treatment using molecular dynamics simulations for the conformations of the five model compounds has been performed in order to evaluate the parameters related to the efficiency of the transfer. The experimental and theoretical variation of such parameters withn is consistent with the estimated Förster radius of 9–10 Å for this system.This research was supported by Grants DGICYT PB94-0364 and UAH 017/95 (A.S.-C., J.P., and F.M.) and by NSF Grant DMR 9523278 (W.L.M.