A Novel Route to Calculate the Length Scale for the Glass Transition in Polymers

The occurrence of glass transition is believed to be associated to cooperative motion with a growing length scale with decreasing temperature. We provide a novel route to calculate the size of cooperatively rearranging regions CRR of glass-forming polymers combining the Adam-Gibbs theory of the glass transition with the self-concentration concept. To do so we explore the dynamics of glass-forming polymers in different environments. The material specific parameter $\alpha$ connecting the size of the CRR to the configurational entropy is obtained in this way. Thereby, the size of CRR can be precisely quantified in absolute values. This size results to be in the range 1 $\div$ 3 nm at the glass transition temperature depending on the glass-forming polymer

Atomic motions in the αβ\alpha\beta-region of glass-forming polymers: Molecular versus Mode Coupling Theory approach

We present fully atomistic Molecular Dynamics simulation results on a main-chain polymer, 1,4-Polybutadiene, in the merging region of the $\alpha$- and $beta$-relaxations. A real space analysis reveals the occurrence of localized motions (``$\beta$-like'') in addition to the diffusive structural relaxation. A molecular approach provides a direct connection between the local conformational changes reflected in the atomic motions and the secondary relaxations in this polymer. Such local processes occur just in the time window where the $\beta$-process of the Mode Coupling Theory is expected. We show that the application of this theory is still possible, and yields an unusually large value of the exponent parameter. This result might originate from the competition between two mechanisms for dynamic arrest: intermolecular packing and intramolecular barriers for local conformational changes (``$\beta$-like'').Comment: 10 pages, 6 figure

Methyl group dynamics in a confined glass

We present a neutron scattering investigation on methyl group dynamics in glassy toluene confined in mesoporous silicates of different pore sizes. The experimental results have been analysed in terms of a barrier distribution model, such a distribution following from the structural disorder in the glassy state. Confinement results in a strong decreasing of the average rotational barrier in comparison to the bulk state. We have roughly separated the distribution for the confined state in a bulk-like and a surface-like contribution, corresponding to rotors at a distance from the pore wall respectively larger and smaller than the spatial range of the interactions which contribute to the rotational potential for the methyl groups. We have estimated a distance of 7 Amstrong as a lower limit of the interaction range, beyond the typical nearest-neighbour distance between centers-of-mass (4.7 Amstrong).Comment: 5 pages, 3 figures. To be published in European Physical Journal E Direct. Proceedings of the 2nd International Workshop on Dynamics in Confinemen

Effect of Chain Stiffness on the Structure of Single-Chain Polymer Nanoparticles

Polymeric single-chain nanoparticles (SCNPs) are soft nano-objects synthesized by purely intramolecular cross-linking of single polymer chains. By means of computer simulations, we investigate the conformational properties of SCNPs as a function of the bending stiffness of their linear polymer precursors. We investigate a broad range of characteristic ratios from the fully flexible case to those typical of bulky synthetic polymers. Increasing stiffness hinders bonding of groups separated by short contour distances and increases looping over longer distances, leading to more compact nanoparticles with a structure of highly interconnected loops. This feature is reflected in a crossover in the scaling behaviour of several structural observables. The scaling exponents change from those characteristic for Gaussian chains or rings in $\theta$-solvents in the fully flexible limit, to values resembling fractal or `crumpled' globular behaviour for very stiff SCNPs. We characterize domains in the SCNPs. These are weakly deformable regions that can be seen as disordered analogues of domains in disordered proteins. Increasing stiffness leads to bigger and less deformable domains. Surprisingly, the scaling behaviour of the domains is in all cases similar to that of Gaussian chains or rings, irrespective of the stiffness and degree of cross-linking. It is the spatial arrangement of the domains which determines the global structure of the SCNP (sparse Gaussian-like object or crumpled globule). Since intramolecular stiffness can be varied through the specific chemistry of the precursor or by introducing bulky side groups in its backbone, our results propose a new strategy to tune the global structure of SCNPs.Comment: 20 pages, 17 figure

SIZE-WEIGHT RELATIONSHIPS OF THE BIGEYE TUNA (THUNNUS OBESUS) FROM NORTH ATLANTIC AREAS USING LINEAR AND NON-LINEAR FITS

Linear and non-linear fits of 1501 observations of size (FL cm) and live-weight (RW kg) of bigeye (Thunnus obesus) obtained in the NE Atlantic over recent years were tested. Results were compared among themselves and with those considered as a reference for this species-stock. The equations obtained from linear and non-linear fits (RW=5.29919E-05 * FL 2.8211264 and RW=6.0568-05 * FL 2.79379, respectively) showed a minor difference in predicting individual weight by size class. The non-linear fit parameters slightly increase the predicted mean weight of the whole sample in around +0.2% compared to the linear fit parameters suggesting that the type of fit would have a negligible impact on both the predictive individual mean weight and the whole sample mean weights. The comparison of results obtained with both types of fits versus the equation considered as a reference also showed minor differences in the estimated average individual weight from size. However, the average weight of the whole sample analysed would increase by over 4% in relation to that obtained when using the reference equation. A review of the literature on size-weight relationships for this species is also included suggesting a considerable diversity of results probably due to diverse factors that are discussed.En prens

Tests of mode coupling theory in a simple model for two-component miscible polymer blends

We present molecular dynamics simulations on the structural relaxation of a simple bead-spring model for polymer blends. The introduction of a different monomer size induces a large time scale separation for the dynamics of the two components. Simulation results for a large set of observables probing density correlations, Rouse modes, and orientations of bond and chain end-to-end vectors, are analyzed within the framework of the Mode Coupling Theory (MCT). An unusually large value of the exponent parameter is obtained. This feature suggests the possibility of an underlying higher-order MCT scenario for dynamic arrest.Comment: Revised version. Additional figures and citation

On the spin modulated circular polarization from the intermediate polars NY Lup and IGRJ1509-6649

We report on high time resolution, high signal/noise, photo-polarimetry of the intermediate polars NY Lup and IGRJ1509-6649. Our observations confirm the detection and colour dependence of circular polarization from NY Lup and additionally show a clear white dwarf, spin modulated signal. From our new high signal/noise photometry we have unambiguously detected wavelength dependent spin and beat periods and harmonics thereof. IGRJ1509-6649 is discovered to also have a particularly strong spin modulated circularly polarized signal. It appears double peaked through the I filter and single peaked through the B filter, consistent with cyclotron emission from a white dwarf with a relatively strong magnetic field. We discuss the implied accretion geometries in these two systems and any bearing this may have on the possible relationship with the connection between polars and soft X-ray-emitting IPs. The relatively strong magnetic fields is also suggestive of them being polar progenitors.Comment: 8 pages, 6 figures and 1 table. Accepted for publication in MNRA

Nuevas citas de Parasquilla ferussaci (Roux, 1830) (Crustacea, Stomatopoda) en el Atlántico oriental y Mediterráneo occidental

En esta nota se informa sobre la presencia del estoma­tópodo Parasquilla ferussaci en las costas de la península ibérica. La documentación está basada en tres especímenes capturados, respectivamente, en Isla Cristina (Huelva) en el Golfo de Cádiz, Fuengirola (Málaga) en el Mar de Alborán y en Gavà (Barcelona) en el Me­diterráneo noroccidental. Se amplia la distribución conocida de la especie completando así el vacío existente hasta el momento entre las citas atlánticas y las del Mediterráneo central. Palabras clave: Parasquilla ferussaci, Estomatópodo, Mediterráneo occidental, Golfo de Cádiz.We report the occurrence of the little known stomatopod Parasquilla ferussaci on the Atlantic and Mediterranean coasts of the Iberian peninsula. Documentation is based on three specimens captured off Isla Cristina (Huelva) in the Gulf of Cadiz, off Fuengirola (Málaga) in the Alboran Sea and off Gavà (Barcelona) in the North-Western Mediterranean. These reports fill the distribution gap between Eastern Central Atlantic reports and previous Mediterranean reports east of the Balearic Islands. Key words: Parasquilla ferussaci, Stomatopoda, Western Mediterranean, Gulf of Cadiz.En esta nota se informa sobre la presencia del estoma­tópodo Parasquilla ferussaci en las costas de la península ibérica. La documentación está basada en tres especímenes capturados, respectivamente, en Isla Cristina (Huelva) en el Golfo de Cádiz, Fuengirola (Málaga) en el Mar de Alborán y en Gavà (Barcelona) en el Me­diterráneo noroccidental. Se amplia la distribución conocida de la especie completando así el vacío existente hasta el momento entre las citas atlánticas y las del Mediterráneo central. Palabras clave: Parasquilla ferussaci, Estomatópodo, Mediterráneo occidental, Golfo de Cádiz