Rikard Bergman

The merging of the dielectric α- and β-relaxations in poly-(methyl methacrylate)

Using broad-band dielectric spectroscopy (10−2–109 Hz) the merging of the α- and β-relaxations has been investigated for a polymer, syndiotactic poly(methyl methacrylate) (PMMA), in which the dielectric losses are dominated by a strong β-relaxation. The asymmetrically shaped β-relaxation cannot be described by a Cole–Cole function, not even at low temperatures where the α- and β-relaxations are well separated in frequency. At higher temperatures close to Tg (=404 K), the weak α-relaxation enters our dynamic window and rapidly merges with the β-relaxation. To investigate this merging process we first used a simple addition of two Havriliak–Negami (HN) equations to fit the spectra. The obtained relaxation time for the β-relaxation then displays a kink in its temperature dependence close to Tg. To gain further understanding of the merging, the data were analyzed by means of a regularization method in order to calculate the corresponding distributions of relaxation times directly from the actual measurements ...

Quasielastic neutron scattering of two-dimensional water in a vermiculite clay

A well-characterized Na–vermiculite clay, containing zero, one, or two molecular layers of water between the clay platelets, has been studied by quasielastic neutron scattering (QENS). Experiments were carried out at a temperature of 300 K in two different scattering geometries; the clay platelets being at 45 and 135° angles to the incident beam in order to make the elastic Q-vector perpendicular and parallel, respectively, to the clay platelets for a scattering angle of 90° (Q≈1.33 A−1). The resulting QENS spectra show that almost no hydrogen motion occurs perpendicular to the clay platelets on the experimental time scale (about 2–40 ps). The two-H2O layer vermiculite exhibits a planar rotational motion of water molecules, forming hydration shells around the Na ions, and a basically two-dimensional translational jump-diffusion motion. The translational motion was modeled using the Gaussian jump-length distribution model, resulting in a mean jump length of 1.1 A and an average residence time of 2.3 ps. Us...

A neutron spin-echo study of confined water

We have investigated the dynamics of confined water in a fully hydrated Na-vermiculite clay using the neutron spin-echo (NSE) technique. NSE measures the intermediate self-scattering function, i.e., the dynamics directly in the time domain. In the present experiments we performed measurements, in the time range 3–3000 ps and temperature ranging from 254 to 323 K, on the essentially two-dimensional water with a layer thickness corresponding to only two molecular layers. The data can be described by the Kohlrausch–Williams–Watts (KWW) stretched exponential function, probably indicating a broad distribution of relaxation times. The reason for the very stretched behavior of the intermediate self-scattering function IS(Q,t), particularly in the supercooled regime, is most likely that the water molecules have widely different local environments. Some water molecules are strongly interacting with the clay surfaces or the intercalated Na+ ions, whereas the remaining molecules are interacting only with other water...

Investigating hydration dependence of dynamics of confined water: Monolayer, hydration water and Maxwell-Wagner processes

The dynamics of water confined in silica matrices MCM-41 C10 and C18, with pore diameter of 21 and 36 A, respectively, is examined by broadband dielectric spectroscopy (10(-2)-10(9) Hz) and differential scanning calorimetry for a wide temperature interval (110-340 K). The dynamics from capillary condensed hydration water and surface monolayer of water are separated in the analysis. Contrary to previous reports, the rotational dynamics are shown to be virtually independent on the hydration level and pore size. Moreover, a third process, also reported for other systems, and exhibiting a saddlelike temperature dependence is investigated. We argue that this process is due to a Maxwell-Wagner process and not to strongly bound surface water as previously suggested in the literature. The dynamics of this process is strongly dependent on the amount of hydration water in the pores. The anomalous temperature dependence can then easily be explained by a loss of hydration water at high temperatures in contradiction to previous explanations.

Role of solvent for the dynamics and the glass transition of proteins.

For the first time, a systematic investigation of the glass transition and its related dynamics of myoglobin in water-glycerol solvent mixtures of different water contents is presented. By a combination of broadband dielectric spectroscopy and differential scanning calorimetry (DSC), we have studied the relation between the protein and solvent dynamics with the aim to better understand the calorimetric glass transition, T(g), of proteins and the role of solvent for protein dynamics. The results show that both the viscosity related α-relaxation in the solvent as well as several different protein relaxations are involved in the calorimetric glass transition, and that the broadness (ΔT(g)) of the transition depends strongly on the total amount of solvent. The main reason for this seems to be that the protein relaxation processes become more separated in time with decreasing solvent level. The results are compared to that of hydrated myoglobin where the hydration water does not give any direct contribution to the calorimetric T(g). However, the large-scale α-like relaxation in the hydration water is still responsible for the protein dynamics that freeze-in at T(g). Finally, the dielectric data show clearly that the protein relaxation processes exhibit similar temperature dependences as the α-relaxation in the solvent, as suggested for solvent-slaved protein motions.

Water dynamics in n-propylene glycol aqueous solutions

The relaxation dynamics of dipropylene glycol and tripropylene glycol (nPG-n=2,3) water solutions on the nPG-rich side has been studied by broadband dielectric spectroscopy and differential scanning calorimetry in the temperature range of 130-280 K. Two relaxation processes are observed for all the hydration levels; the slower process (I) is related to the alpha relaxation of the solution whereas the faster one (II) is associated with the reorientation of water molecules in the mixture. Dielectric data for process (II) at temperatures between 150 and 200 K indicate the existence of a critical water concentration (x(c)) below which water mobility is highly restricted. Below x(c), nPG-water domains drive the dielectric signal whereas above x(c), water-water domains dominate the dielectric response at low temperatures. The results also show that process (II) at low temperatures is due to local motions of water molecules in the glassy frozen matrix. Additionally, we will show that the glass transition temperatures (T(g)) for aqueous PG, 2PG, and 3PG solutions do not extrapolate to approximately 136 K, regardless of the extrapolation method. Instead, we find that the extrapolated T(g) value for water from these solutions lies in the neighborhood of 165 K.

Dielectric relaxation in PMMA revisited

Broad-band dielectric spectroscopy (10−2–109 Hz) has been performed on a mainly syndiotactic sample of poly(methyl methacrylate) (PMMA) of molecular weight (Mw) 50 000. The syndiotactic (85%) PMMA was chosen to study the merging of the α and β relaxations in a system where the dielectric loss is dominated by a β (or Johari–Goldstein) relaxation. Contrary to other polymers the measured β relaxation has an asymmetrical shape that cannot be described by a Cole–Cole function, not even at low temperatures where the α and β relaxations are well separated in frequency. Therefore the more general Havriliak–Negami equation (HN) was used to fit the β-loss. At temperatures close to Tg(=404K) the influence of the α relaxation complicates the analysis. To investigate this merging of the α and β processes the data were analyzed in different ways. Both directly in frequency domain and by means of a regularization method also in the time domain. We have also analyzed the data adopting the Williams ansatz, i.e. instead of a simple addition of the α and β relaxations the processes are convoluted in the frequency domain.

Dielectric study of supercooled 2D water in a vermiculite clay

We report results of dielectric spectroscopy on water confined in a two-dimensional layer-structured Na-vermiculite clay. Several relaxation processes of different origin can be discerned in the spectra. A strong dielectric loss peak is observed in the clay with two layers of water molecules between the rigid clay platelets at temperatures between 125 and 215 K. This loss peak is neither observed in bulk ice, nor in the dry clay or clay with only one water layer between platelets. The relaxation time is shown to have an Arrhenius temperature dependence which does not extrapolate to the high-temperature behavior. This behavior is in accordance with the proposed so-called “fragile-strong” transition of supercooled bulk water, which implies a change in the temperature dependence of the relaxation time from a high-temperature non-Arrhenius to low-temperature Arrhenius behavior. However, this transition is experimentally elusive as it would occur in an inaccessible (due to the ease of crystallization) temperat...

Relaxation dynamics of a polymer in a 2D confinement

The molecular dynamics of oligomeric poly(propylene glycol) (PPG) liquids (MW=1000, 2000, and 4000 g/mol) confined in a two-dimensional layer-structured Na-vermiculite clay has been studied by broadband dielectric spectroscopy. The alpha-relaxation and the normal mode relaxation processes were studied for all samples in bulk and confinement. The most prominent experimental observation was that for the normal mode process: the relaxation rate in the clay is drastically shifted to lower frequencies compared to that of the bulk material. This slowing down is probably caused by the strongly reduced number of accessible chain conformations in two dimensions. Also the temperature dependence of the relaxation time for the normal mode process is strongly affected by the confinement. In contrast, for the alpha-relaxation of the confined polymers we observed only a slight increase of the relaxation rate at high temperatures compared to the corresponding bulk samples, and a decrease of its relaxation strength relative to the beta relaxation. Thus, the glass transition is unaffected by the 2D confinement, suggesting that the underlying phenomena responsible for the glass transition is the same as in bulk. Moreover, in the clay the intensity of the normal mode is stronger than that of the alpha-process, in contrast to the bulk samples where the opposite behavior is observed.

Slow Debye-type peak observed in the dielectric response of polyalcohols

Dielectric relaxation spectroscopy of glass forming liquids normally exhibits a relaxation scenario that seems to be surprisingly general. However, the relaxation dynamics is more complicated for hydrogen bonded liquids. For instance, the dielectric response of monoalcohols is dominated by a mysterious Debye-like process at lower frequencies than the structural alpha-relaxation that is normally dominating the spectra of glass formers. For polyalcohols this process has been thought to be absent or possibly obscured by a strong contribution from conductivity and polarization effects at low frequencies. We here show that the Debye-like process, although much less prominent, is also present in the response of polyalcohols. It can be observed in the derivative of the real part of the susceptibility or directly in the imaginary part if the conductivity contribution is reduced by covering the upper electrode with a thin Teflon layer. We report on results from broadband dielectric spectroscopy studies of several polyalcohols: glycerol, xylitol, and sorbitol. The findings are discussed in relation to other experimental observations of ultraslow (i.e., slower than the viscosity related alpha-relaxation) dynamics in glass formers.