Physical Review Letters, 111(9), 095701] that purportedly showed unusual irregular responses in the isothermal structural recovery behavior of glasses because the observation challenges the general view of the dynamics being smooth functions of time as observed in extensive dilatometric experiments by Kovacs [Kovacs, A. J. (1964). In Fortschritte der Hochpolymeren-Forschung, 3, 394-507. Springer, Berlin, Heidelberg.] in the 1960s. The reported data show an apparent two mechanism structural (enthalpy) recovery at aging temperatures ranging from 6 to 17 K below T g which is also controversial as it could not be reproduced in work from Koh and Simon [Koh, Y. P., & Simon, S. L. (2013). Macromolecules, 46(14), 5815-5821.] where a second plateau observed in the enthalpy loss curve was not reproduced in nearly 1-year-long experiments at 15 K below T g . Therefore, it is important to determine what might be possible reasons for the apparent non-smooth relaxations. Here, we examine the possibility that the poor temperature control of a typical vacuum oven could explain the anomalous results. We used the Tool-Narayanaswamy-Moynihan (TNM) model of structural recovery to calculate the effect of typical vacuum oven temperature variation on the structural recovery of polystyrene and find that we can reproduce the reported experimental results. The issue of temperature control using a vacuum oven is discussed, and data from thermocouple measurements of temperature at various locations inside a vacuum oven are shown.
Eight unprecedented Ln-NDC coordination
polymers with BBI as templates
{[HBBI][Ln(NDC)2(H2O)]·H2O [Ln
= La (1), Pr (2)], [HBBI]2[Sm(NDC)2(H2O)]2·1/2H2O (3), and [HBBI][Ln(NDC)2(H2O)] [Ln =
Eu (4), Gd (5), Tb (6), Dy
(7), Er (8)] [Ln = lanthanide, H2NDC = 1,4-naphthalenedicarboxylic acid, BBI = bis(2,2′-biimidazole)]}
have been hydrothermally synthesized and structurally characterized
by elemental analyses, IR spectra, and single-crystal X-ray diffraction.
Complexes 1–8 crystallize in the
monoclinic space group P21/c and display similar (8,8)-connected 3-D frameworks with different
dinuclear Ln secondary building units due to the effect of Ln contraction
and diverse coordination modes of NDC2– ligands.
As the ionic radii of Ln ions decrease, the coordination numbers of
Ln ions decrease from 10, to 9, to 8. The variable-temperature magnetic
properties of 2–8 have been investigated.
The strong fluorescent emissions of 4 demonstrate that
ligand-to-EuIII energy transfer is efficient. In addition,
thermogravimetric analyses and optical diffuse reflectance spectra
of these compounds are also described.
reported a set of interesting results for the viscosity of unentangled polystyrene of two different molecular weights determined from capillary infiltration into a confining matrix of silica nanoparticles. They found that the viscosity and glass transition temperature (T g ) increase as the pore size of the matrix decreases, that is, the confinement effect becomes stronger. They attributed the change in viscosity to a change in segmental relaxation, that is, an increase in T g . However, their interpretation did not consider that a changing T g leads to a change in the monomeric friction coefficient and viscosity-molecular weight relations should be compared at constant friction factor, that is, at a constant distance from the glass transition temperature. We reanalyzed the results at constant friction factor under a framework of changing chain dynamics as confinement strength increases rather than being due only to the changing glass transition temperature. Our reanalysis of the data from Hor et al. as well as data from other literature shows that as unentangled polymers become increasingly confined, the chain dynamics change from Rouse-like to those of an entropic barrier regime. For entangled polymers, the chain dynamics change from those of a reptation-like regime to those of an entropic barrier regime.
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