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

Deciphering glial evolution: genetic and functional characterization of ancestral glia

Presentation of poster 552C at TAGC 2020 Online. Files include a PDF of the poster (TAGC2020_poster_LS.PDF

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

Number of cases and PPV according to the outcome definition for diabetes.

Number of cases and PPV according to the outcome definition for diabetes.</p

The case identification method.

The case identification method.</p

Examples of code lists.

Examples of code lists.</p

Methods for surveillance.

Methods for surveillance.</p

Viscoelastic and Dielectric Relaxation of Reptating Type-A Chains Affected by Reversible Head-to-Head Association and Dissociation

For entangled linear polymer having type A dipoles and undergoing head-to-head association and dissociation reaction, viscoelastic and dielectric behavior is theoretically analyzed on the basis of the reptation dynamics combined with the reaction kinetics. Specifically, for the dissociated unimer and associated dimer (indexed with j = 1 and 2, respectively), the normalized complex modulus gj*­(ω) and the normalized complex dielectric permittivity ε̃j*­(ω) are analytically calculated via eigenfunction expansion of the orientational anisotropy and orientational memory defined in terms of the bond vectors u of entanglement segments. The reaction activates mutual conformational transfer between the unimer and dimer. Multiple coupling occurs for the anisotropy decay modes of the unimer and dimer due to this transfer, and the viscoelastic g1* and g2* of the unimer and dimer, respectively, exhibit considerably retarded and accelerated relaxation compared to the pure reptation case. In contrast, the memory decay modes of the unimer and dimer are only pairwisely coupled, so that the reaction-induced acceleration and retardation for the dielectric ε̃1* and ε̃2* are much weaker than those seen for the viscoelastic g1* and g2*. The orientational anisotropy is the tensorial, second-moment average of u associated with no cancellation in the conformational transfer, whereas the orientational memory is the vectorial, first-moment average accompanied by partial cancellation, which results in the difference between gj* and ε̃j*. This difference between gj* and ε̃j* is noted also for the associating/dissociating Rouse chains. Nevertheless, the reaction-induced retardation of the viscoelastic relaxation is stronger for the reptating unimer than for the Rouse unimer, whereas the reaction-induced acceleration is similar, in magnitude, for the reptating dimer and Rouse dimer. These features of gj* of the unimer and dimer are discussed in relation to the motional coherence along the chain backbone being present and absent in the reptation and Rouse dynamics

Outcome definition in Method 2.

Outcome definition in Method 2.</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