Es velozmente fugaz todo lo celestial

Fiesta de la paz (traducción y prólogo de Rafael Gutiérrez Girardot). Friedrich Holderlin. El Ancora Editores, Santafé de Bogotá, 1994, 85 págs

Nonlinear Stress Relaxation of Miscible Polyisoprene/Poly(<i>p</i>-<i>tert</i>-butylstyrene) Blends in Pseudomonodisperse State

For miscible pair of polyisoprene (PI) and poly­(<i>p</i>-<i>tert</i>-butylstyrene) (PtBS), the component molecular weights, composition, and temperature were tuned to prepare PI/PtBS blends in the <i>pseudomonodisperse</i> state where the component PI and PtBS chains had the same terminal relaxation time, τ₁. These pseudomonodisperse blends had the linear viscoelastic moduli indistinguishable from the moduli of entangled monodisperse bulk homopolymers of particular molecular weights, and satisfied the time-strain separability in their nonlinear stress relaxation behavior under large step strains. The damping function <i>h</i>(γ) of those blends was close to <i>h</i>_DE(γ) calculated from the Doi–Edwards model and classified to be the so-called type-A damping function, even though the major component (PI) in the blends had a large entanglement number <i>per</i> chain (<i>N</i> ≥ 50). Highly entangled monodisperse homopolymers having similarly large <i>N</i> are known to exhibit the so-called type-C damping characterized by <i>h</i>(γ) ≪ <i>h</i>_DE(γ), and this damping behavior was indeed confirmed for high-<i>M</i> bulk PI utilized as the blend component. Thus, the nonlinear damping behavior was different for the pseudomonodisperse PI/PtBS blends and high-<i>M</i> bulk PI, despite the similarity in the entanglement number <i>N</i> for PI therein. This difference was discussed within the molecular scenario of Marrucci and Grizzuti in relation to the topological hindrance for PI segments due to PtBS segments having a much larger friction. This hindrance retarded the Rouse equilibration of the PI backbone in the blends, which possibly provided the highly entangled PI with a slow contour length fluctuation mechanism that competed with reptation. Such a competing mechanism smears the elastic instability underlying the type-C damping as suggested from the Marrucci–Grizzuti scenario, which possibly allowed the pseudomonodisperse PI/PtBS blends containing highly entangled PI to exhibit the type-A damping. Furthermore, the type-A damping was observed also for a chemically homogeneous, highly entangled PI/PI blend being free from the topological hindrance for PI segments. In this PI/PI blends, the partial constraint release of the high-<i>M</i> component, activated by the relaxation of the low-<i>M</i> component, appeared to compete with reptation of the high-<i>M</i> component thereby smearing the instability and suppressing the type-C damping. Thus, the smearing of instability could be a rather universal feature occurring irrespective of the detail of the competing mechanisms

Effect of predictive sign of acceleration on heart rate variability in passive translation situation: preliminary evidence using visual and vestibular stimuli in VR environment-1

<p><b>Copyright information:</b></p><p>Taken from "Effect of predictive sign of acceleration on heart rate variability in passive translation situation: preliminary evidence using visual and vestibular stimuli in VR environment"</p><p>http://www.jneuroengrehab.com/content/4/1/36</p><p>Journal of NeuroEngineering and Rehabilitation 2007;4():36-36.</p><p>Published online 29 Sep 2007</p><p>PMCID:PMC2072956.</p><p></p

Effect of Molecular Weight and Salt Concentration on Ion Transport and the Transference Number in Polymer Electrolytes

Transport of ions in polymer electrolytes is of significant practical interest, however, differences in the transport of anions and cations have not been comprehensively addressed. We present measurements of the electrochemical transport properties of lithium bis­(trifluoromethanesulfonyl)­imide (LiTFSI) in poly­(ethylene oxide) (PEO) over a wide range of PEO molecular weights and salt concentrations. Individual self-diffusion coefficients of the Li⁺ and TFSI^– ions, <i>D</i>₊ and <i>D</i>_–, were measured using pulsed-field gradient nuclear magnetic resonance both in the dilute limit and at high salt concentrations. Conductivities calculated from the measured <i>D</i>₊ and <i>D</i>_– values based on the Nernst–Einstein equation were in agreement with experimental measurements reported in the literature, indicating that the salt is fully dissociated in these PEO/LiTFSI mixtures. This enables determination of the molecular weight dependence of the cation transference number in both dilute and concentrated solutions. We introduce a new parameter, <i>s</i>, the number of lithium ions per polymer chain, that allows us to account for both the effect of salt concentration and molecular weight on cation and anion diffusion. Expressing cation and anion diffusion coefficients as functions of <i>s</i> results in a collapse of <i>D</i>₊ and <i>D</i>_– onto a single master curve

Viscoelastic and Orientational Relaxation of Linear and Ring Rouse Chains Undergoing Reversible End-Association and Dissociation

For dilute telechelic linear and ring Rouse chains undergoing reversible end-association and dissociation, the time (<i>t</i>) evolution equation was analytically formulated for the bond vector of the subchain (or segment), <b>u</b>^[c](<i>n</i>,<i>t</i>) with <i>n</i> being the subchain index and the superscript c specifying the chain (c = L and R for the linear and ring chains). The end-association of the linear chain (i.e., ring formation) occurs only when the ends of the linear chain come into close proximity. Because of this constraint for the ring formation, the time evolution equation for <b>u</b>^[L](<i>n</i>,<i>t</i>) of the linear chain was formulated with a conceptually new, two-step expansion method: <b>u</b>^[L](<i>n</i>,<i>t</i>) was first expanded with respect to its sinusoidal Rouse eigenfunction, sin­(<i>p</i>π<i>n</i>/<i>N</i>) with <i>p</i> = integer and <i>N</i> being the number of subchains <i>per</i> chain, and then the series of odd sine modes is re-expanded with respect to cosine eigenfunctions of the ring chain, cos­(2απ<i>n</i>/<i>N</i>) with α = integer, so as to account for that constraint. This formulation allowed analytical calculation of the orientational correlation function, <i>S</i>^[c](<i>n</i>,<i>m</i>,<i>t</i>) = <i>b</i>^–2⟨<i>u</i>_<i>x</i>^[c](<i>n</i>,<i>t</i>)<i>u</i>_<i>y</i>^[c](<i>m</i>,<i>t</i>)⟩ (c = L, R) with <i>b</i> being the subchain step length, and the viscoelastic relaxation function, <i>g</i>^[c](<i>t</i>) ∝ ∫₀^<i>N</i><i>S</i>^[c](<i>n</i>,<i>n</i>,<i>t</i>) d<i>n</i>. It turned out that the terminal relaxation of <i>g</i>^[R](<i>t</i>) and <i>g</i>^[L](<i>t</i>) of the ring and linear chains is retarded and accelerated, respectively, due to the motional coupling of those chains occurring through the reaction. This coupling breaks the ring symmetry (equivalence of all subchains of the ring chain in the absence of reaction), thereby leading to oscillation of the orientational anisotropy <i>S</i>^[R](<i>n</i>,<i>n</i>,<i>t</i>) of the ring chain at long <i>t</i> with the subchain index <i>n</i>. The coupling also reduces a difference of the anisotropy <i>S</i>^[L](<i>n</i>,<i>n</i>,<i>t</i>) of the linear chain at the middle (<i>n</i> ∼ <i>N</i>/2) and end (<i>n</i> ∼ 0)

Effect of predictive sign of acceleration on heart rate variability in passive translation situation: preliminary evidence using visual and vestibular stimuli in VR environment-0

<p><b>Copyright information:</b></p><p>Taken from "Effect of predictive sign of acceleration on heart rate variability in passive translation situation: preliminary evidence using visual and vestibular stimuli in VR environment"</p><p>http://www.jneuroengrehab.com/content/4/1/36</p><p>Journal of NeuroEngineering and Rehabilitation 2007;4():36-36.</p><p>Published online 29 Sep 2007</p><p>PMCID:PMC2072956.</p><p></p

Effect of predictive sign of acceleration on heart rate variability in passive translation situation: preliminary evidence using visual and vestibular stimuli in VR environment-2

<p><b>Copyright information:</b></p><p>Taken from "Effect of predictive sign of acceleration on heart rate variability in passive translation situation: preliminary evidence using visual and vestibular stimuli in VR environment"</p><p>http://www.jneuroengrehab.com/content/4/1/36</p><p>Journal of NeuroEngineering and Rehabilitation 2007;4():36-36.</p><p>Published online 29 Sep 2007</p><p>PMCID:PMC2072956.</p><p></p

Entanglement Length in Miscible Blends of <i>cis</i>-Polyisoprene and Poly(<i>p</i>-<i>tert</i>-butylstyrene)

The entanglement length <i>a</i>, being equivalent to the plateau modulus <i>G</i>_N (∝<i>M</i>_e^–1 ∝ <i>a</i>^–2), is one of the most basic parameters that determine the slow dynamics of high molecular weight (<i>M</i>) polymers. In miscible blends of chemically different chains, the components would/should have the common <i>a</i> value. However, changes of <i>a</i> with the blend composition have not been fully elucidated. For this problem, this study conducted linear viscoelastic tests for miscible blends of high-<i>M cis</i>-polyisoprene (PI) and poly­(<i>p</i>-<i>tert</i>-butylstyrene) (PtBS) and analyzed the storage and loss moduli (<i>G</i>′ and <i>G</i>″) data in a purely empirical way, considering the very basic feature that unentangled and entangled blends having the same composition exhibit the same local relaxation. (From a molecular point of view, this local relaxation reflects the chain motion <i>within</i> the length scale of <i>a</i>.) On the basis of this feature, a series of barely entangled low-<i>M</i> PI/PtBS blends having various component molecular weights and a given composition were utilized as references for well-entangled high-<i>M</i> PI/PtBS blends with the same composition, and the modulus data of the reference were subtracted from the data of the high-<i>M</i> blends. For an optimally chosen reference, the storage modulus of the high-<i>M</i> blends obtained after the subtraction (<i>G</i>_ent′ = <i>G</i>_{high‑<i>M</i> blend}′ – <i>G</i>_ref′) exhibited a clear plateau at high angular frequencies ω. The corresponding loss modulus <i>G</i>_ent″ decreased in proportion to ω^–1 at high ω, which characterized the short-time onset of the global entanglement relaxation: A mischoice of the reference gave no plateau of <i>G</i>_{high‑<i>M</i> blend}′ – <i>G</i>_ref′ and no ω^–1 dependence of <i>G</i>_{high‑<i>M</i> blend}″ – <i>G</i>_ref″ at high ω, but a survey for various low-<i>M</i> PI/PtBS blends allowed us to find the optimum reference. With the aid of such optimum reference, the entanglement plateau modulus <i>G</i>_N of the high-<i>M</i> PI/PtBS blends was accurately obtained as the high-ω plateau value of <i>G</i>_ent′. <i>G</i>_N thus obtained was well described by a linear mixing rule of the entanglement length <i>a</i> with the weighing factor being equated to the number fraction of Kuhn segments of the components, not by the reciprocal mixing rule utilizing the component volume fraction as the weighing factor. This result, not explained by a mean-field picture of entanglement (constant number of entanglement strands in a volume <i>a</i>³), is discussed in relation to local packing efficiency of bulky PtBS chains and skinny PI chains

Experimental Test for Viscoelastic Relaxation of Polyisoprene Undergoing Monofunctional Head-to-Head Association and Dissociation

A viscoelastic test was made for end-carboxylated polyisoprene (PI-COOH) of the molecular weight <i>M</i> = 30.₅ × 10³ that underwent the interchain association and dissociation through hydrogen bonding of the COOH groups at the chain end. As a reference, the test was made also for neat PI unimer (with no COOH group at the chain end) and for PI₂ dimer (with <i>M</i> = 61.0 × 10³), the latter being synthesized through end-coupling of PI^– anions (precursor of the PI-COOH sample). The PI-COOH, neat unimer, and dimer samples were diluted in oligomeric butadiene (oB) to a concentration of 10 wt %. The neat unimer and dimer exhibited nonentangled Rouse behavior at this concentration, as expected from their molecular weights. At low temperatures (<i>T</i> ≤ 0 °C) the PI-COOH sample relaxed slower than the reference unimer but faster than the dimer, whereas the relaxation of PI-COOH approached that of the unimer with increasing <i>T</i> > 0 °C, and this change of the relaxation time of PI-COOH was associated with changes in the angular frequency (ω) dependence of the dynamic modulus. This behavior of PI-COOH was well described by a recently proposed theory considering motional coupling between the end-associating unimer and its dimer at chemical equilibrium. On the basis of this result, an effect of the polymeric character of PI-COOH chain on the viscoelastically detected association/dissociation of the hydrogen bonding of the COOH groups was discussed

Comparison of RNP activities under thermal stress.

<p>(A) Brief protocol and incubation periods are indicated. (B) Representative analyzed polyacrylamide gel (6%) is shown. * represents statistical significance at p<0.05 in a Student's t-test (n = 3). 293T cells expressing influenza RNP were incubated at 37°C for 24 hours as pre-incubation. Pre-incubated cells were additionally incubated at 34, 37 and 42°C for 9 hours, respectively. Then total RNAs were extracted and analyzed by primer extension assay. A/WSN/33, A/Hong Kong/156/97, A/NT/60/68, A/Vietnam/1194/2004 and pandemic H1N1 2009 virus are abbreviated as WSN, HK, NT, VN and SW, respectively. 5s ribosomal RNA (rRNA) is indicated as an internal control. mRNA, cRNA and vRNA are viral messenger RNA, complementary viral RNA and viral RNA, respectively.</p