Segmental relaxation in semicrystalline polymers: a mean field model for the distribution of relaxation times in confined regimes

The effect of confinement in the segmental relaxation of polymers is considered. On the basis of a thermodynamic model we discuss the emerging relevance of the fast degrees of freedom in stimulating the much slower segmental relaxation, as an effect of the constraints at the walls of the amorphous regions. In the case that confinement is due to the presence of crystalline domains, a quasi-poissonian distribution of local constraining conditions is derived as a result of thermodynamic equilibrium. This implies that the average free energy barrier $\Delta F$ for conformational rearrangement is of the same order of the dispersion of the barrier heights, $\delta (\Delta F)$, around $\Delta F$. As an example, we apply the results to the analysis of the $\alpha$-relaxation as observed by dielectric broad band spectroscopy in semicrystalline poly(ethylene terephthalate) cold-crystallized from either an isotropic or an oriented glass. It is found that in the latter case the regions of cooperative rearrangement are significantly larger than in the former.Comment: 10 pages, 4 figures .ep

Ergebnisse neuerer Untersuchungen zur Würmlöß-Gliederung in Hessen

Die über dem letztinterglazialen Boden (Parabraunerde bzw. pseudovergleyte Parabraunerde) folgenden würmzeitlichen äolischen Sedimente können durch kennzeichnende Bodenhorizonte in 3 Abschnitte (Alt-, Mittel- und Jungwürm) gegliedert werden. Charakteristisch für das Altwürm sind Humuszonen, für das Mittelwürm neben einigen „Naßböden" braune Verlehmungszonen bis zu einer Mächtigkeit von 1,1 m und für das Jungwürm mehrere schwach ausgebildete, geringmächtige Verbraunungszonen und „Naßböden". Den wichtigsten Leithorizont des Jungwürms bildet das Kärlicher Tuffbändchen, das in jüngster Zeit auch in Nordhessen aufgefunden wurde. Abschließend wird das in Hessen aufgestellte Gliederungsschema mit den Würmlöß-Gliederungen in anderen Teilen Europas verglichen und eine Parallelisierung versucht.researc

Kinetics of the glass transition of styrene-butadiene-rubber : Dielectric spectroscopy and fast differential scanning calorimetry

The glass transition is relevant for performance definition in rubber products. For extrapolation to high-frequency behavior, time–temperature superposition is usually assumed, although most complex rubber compounds might be outside of its area of validity. Fast differential scanning calorimetry (FDSC) with cooling rates up to 1500 K/s and broadband dielectric spectroscopy (BDS) with frequencies up to 20 MHz are applied here to directly access both kinetics and dynamics of glass formation in a wide frequency range. For the first-time, the relation between the thermal vitrification and the dielectric relaxation is studied on vulcanized styrene-butadiene rubber, showing that both cooling rate and frequency dependence of its glass transition can be described by one single Vogel-Fulcher-Tammann-Hesse equation. The results indicate the validity of the Frenkel-Kobeko-Reiner equation. Another focus is the sample preparation of vulcanized elastomers for FDSC and BDS as well as the temperature calibration below 0°C. © 2020 The Authors. Journal of Applied Polymer Science published by Wiley Periodicals LLC

Metastable Dynamics above the Glass Transition

The element of metastability is incorporated in the fluctuating nonlinear hydrodynamic description of the mode coupling theory (MCT) of the liquid-glass transition. This is achieved through the introduction of the defect density variable $n$ into the set of slow variables with the mass density $\rho$ and the momentum density ${\bf g}$. As a first approximation, we consider the case where motions associated with $n$ are much slower than those associated with $\rho$. Self-consistently, assuming one is near a critical surface in the MCT sense, we find that the observed slowing down of the dynamics corresponds to a certain limit of a very shallow metastable well and a weak coupling between $\rho$ and $n$. The metastability parameters as well as the exponents describing the observed sequence of time relaxations are given as smooth functions of the temperature without any evidence for a special temperature. We then investigate the case where the defect dynamics is included. We find that the slowing down of the dynamics corresponds to the system arranging itself such that the kinetic coefficient $\gamma_v$ governing the diffusion of the defects approaches from above a small temperature-dependent value $\gamma^c_v$.Comment: 38 pages, 14 figures (6 figs. are included as a uuencoded tar- compressed file. The rest is available upon request.), RevTEX3.0+eps

Low frequency vibrations and diffusion in disordered polymers bearing an intrinsic microporosity as revealed by neutron scattering

The microscopic diffusion and the low frequency density of states (VDOS) of PIM-EA-TB(CH3) are investigated by inelastic and quasi-elastic neutron scattering where also the demethylated counterpart of PIM-EA-TB(H2) is considered. These intrinsic microporous polymers are characterized by large BET surface area values of several hundred m2/g and pore sizes between 0.5 and 2 nm. Detailed comparison is made to the archetype of polymers of intrinsic microporosity, PIM-1, and polynorbornenes also bearing a microporosity. Due to the wavelength of neutrons, the diffusion and vibrations can be addressed on microscopic length and time scales. From the inelastic neutron scattering experiments the low frequency density of states (VDOS) is estimated which shows excess contributions to the Debye-type VDOS known as Boson peak. It was found that the maximum frequency of the Boson peak decreases with increasing microporosity characterized by the BET surface area. However, besides the BET surface area, additional factors such as the backbone stiffness govern the maximum frequency of the Boson peak. Further the mean squared displacement related to microscopic motions was estimated from elastic fixed window scans. At temperatures above 175 K, the mean squared displacement PIM-EA-TB(CH3) is higher than that for the demethylated counterpart PIM-EA-TB(H2). The additional contribution found for PIM-EA-TB(CH3) is ascribed to the rotation of the methyl group in this polymer because the only difference between the two structures is that PIM-EA-TB(CH3) has methyl groups where PIM-EA-TB(H2) has none. A detailed comparison of the molecular dynamics is also made to that of PIM-1 and the microporous polynorbornene PTCNSi1. The manuscript focuses on the importance of vibrations and the localized molecular mobility characterized by the microscopic diffusion on the gas transport in polymeric separation membranes. In the frame of the random gate model localized fluctuations can open or close bottlenecks between pores to enable the diffusion of gas molecules

Microscopic molecular mobility of high-performance polymers of intrinsic microporosity revealed by neutron scattering - bend fluctuations and signature of methyl group rotation

Polymers of intrinsic microporosity exhibit a combination of high gas permeability and reasonable permselectivity, which makes them attractive candidates for gas separation membrane materials. The diffusional selective gas transport properties are connected to the molecular mobility of these polymers in the condensed state. Incoherent quasielastic neutron scattering was carried out on two polymers of intrinsic microporosity, PIM-EA-TB(CH3) and its demethylated counterpart PIM-EA-TB(H2), which have high Brunauer-Emmett-Teller surface area values of 1030 m2 g-1 and 836 m2 g-1, respectively. As these two polymers only differ in the presence of two methyl groups at the ethanoanthracene unit, the effect of methyl group rotation can be investigated solely. To cover a broad dynamic range, neutron time-of-flight was combined with neutron backscattering. The demethylated PIM-EA-TB(H2) exhibits a relaxation process with a weak intensity at short times. As the backbone is rigid and stiff this process was assigned to bend- and-flex fluctuations. This process was also observed for the PIM-EA-TB(CH3). A further relaxation process is found for PIM-EA-TB(CH3), which is the methyl group rotation. It was analyzed by a jump-diffusion in a three-fold potential considering also the fact that only a fraction of the present hydrogens in PIM-EA-TB(CH3) participate in the methyl group rotation. This analysis can quantitatively describe the q dependence of the elastic incoherent structure factor. Furthermore, a relaxation time for the methyl group rotation can be extracted. A high activation energy of 35 kJ mol-1 was deduced. This high activation energy evidences a strong hindrance of the methyl group rotation in the bridged PIM-EA-TB(CH3) structure.</p

Broadband Dielectric Spectroscopy on Glass-Forming Propylene Carbonate

Dielectric spectroscopy covering more than 18 decades of frequency has been performed on propylene carbonate in its liquid and supercooled-liquid state. Using quasi-optic submillimeter and far-infrared spectroscopy the dielectric response was investigated up to frequencies well into the microscopic regime. We discuss the alpha-process whose characteristic timescale is observed over 14 decades of frequency and the excess wing showing up at frequencies some three decades above the peak frequency. Special attention is given to the high-frequency response of the dielectric loss in the crossover regime between alpha-peak and boson-peak. Similar to our previous results in other glass forming materials we find evidence for additional processes in the crossover regime. However, significant differences concerning the spectral form at high frequencies are found. We compare our results to the susceptibilities obtained from light scattering and to the predictions of various models of the glass transition.Comment: 13 pages, 9 figures, submitted to Phys. Rev.

Molecular dynamics and physical stability of amorphous nimesulide drug and its binary drug-polymer systems

yesIn this paper we study the effectiveness of three well known polymers: inulin, Soluplus and PVP in stabilizing amorphous form of nimesulide (NMS) drug. The re-crystallization tendency of pure drug as well as measured drug-polymer systems were examined at isothermal conditions by using broadband dielectric spectroscopy (BDS), and at non-isothermal conditions by differential scanning calorimetry (DSC). Our investigation has shown that the crystallization half-life time of pure NMS at 328 K is equal to 33 minutes. We found that this time can be prolonged to 40 years after adding to NMS 20% of PVP polymer. This polymer proved to be the best NMS’s stabilizer, while the worst stabilization effect was found after adding the inulin to NMS. Additionally, our DSC, BDS and FTIR studies indicate that for suppression of NMS’s re-crystallization in NMS-PVP system, the two mechanisms are responsible: the polymeric steric hindrances as well as the antiplastization effect excerted by the excipient.The authors J.K., Z.W., K.G. and M.P., are grateful for the financial support received within the Project No. 2015/16/W/NZ7/00404 (SYMFONIA 3) from the National Science Centre, Poland. H.M. and L.T. are supported by Science Foundation Ireland under grant No. 12/RC/2275 (Synthesis and Solid State Pharmaceuticals Centre)

Slow dynamics near glass transitions in thin polymer films

The $\alpha$-process (segmental motion) of thin polystyrene films supported on glass substrate has been investigated in a wider frequency range from 10$^{-3}$ Hz to 10$^4$ Hz using dielectric relaxation spectroscopy and thermal expansion spectroscopy. The relaxation rate of the $\alpha$-process increases with decreasing film thickness at a given temperature above the glass transition. This increase in the relaxation rate with decreasing film thickness is much more enhanced near the glass transition temperature. The glass transition temperature determined as the temperature at which the relaxation time of the $\alpha$-process becomes a macroscopic time scale shows a distinct molecular weight dependence. It is also found that the Vogel temperature has the thickness dependence, i.e., the Vogel temperature decreases with decreasing film thickness. The expansion coefficient of the free volume $\alpha_f$ is extracted from the temperature dependence of the relaxation time within the free volume theory. The fragility index $m$ is also evaluated as a function of thickness. Both $\alpha_f$ and $m$ are found to decrease with decreasing film thickness.Comment: 9 pages, 7 figures, and 2 table

Glassy dynamics in mono-, di-, and tri-propylene glycol: From the alpha- to the fast beta-relaxation

We present a thorough characterization of the glassy dynamics of three propylene glycols (mono-, di- and trimer) by broadband dielectric spectroscopy. By covering a frequency range of more than 15 decades, we have access to the entire variety of dynamic processes typical for glassy dynamics. These results add three more molecular glass formers to the sparse list of materials for which real broadband spectra, including the region of the fast beta-process, are available. Some first analyses of the various observed dynamic processes are provided