Reiner Zorn

Molecular dynamics of water in oriented DPPC multilayers studied by quasielastic neutron scattering and deuterium‐nuclear magnetic resonance relaxation

The dynamics of water between highly oriented multilayers of 1,2‐dipalmitoyl‐sn‐glycero‐3‐phosphocholine (DPPC) has been studied in two time domains at different hydration levels. Incoherent quasielastic neutron scattering (QENS) and deuterium‐nuclear magnetic resonance (NMR) longitudinal (T1) relaxation were employed to investigate both the high‐frequency motions of water (10−9–10−11 s time scale) and their anisotropy, while 2H‐NMR transverse (T2) relaxation was used for obtaining information on low frequency dynamical processes (microsecond time scale). Our results show that high frequency dynamics (picosecond‐time scale) at low hydration (three to four water molecules per lipid) can be understood solely as a uniaxial rotation of the water molecules tightly bound to DPPC head groups with a correlation time τrot≊62 ps at 55 °C and a rotational radius of 1±0.1 A, but with no detectable translational degrees of freedom. The 2H‐NMR T1 data (nanosecond‐time scale) can be explained satisfactorily on the basis...

Anisotropic Motion of Cholesterol in Oriented DPPC Bilayers Studied by Quasielastic Neutron Scattering: The Liquid-Ordered Phase

Quasielastic neutron scattering (QENS) at two energy resolutions (1 and 14 microeV) was employed to study high-frequency cholesterol motion in the liquid ordered phase (lo-phase) of oriented multilayers of dipalmitoylphosphatidylcholine at three temperatures: T = 20 degrees C, T = 36 degrees C, and T = 50 degrees C. We studied two orientations of the bilayer stack with respect to the incident neutron beam. This and the two energy resolutions for each orientation allowed us to determine the cholesterol dynamics parallel to the normal of the membrane stack and in the plane of the membrane separately at two different time scales in the GHz range. We find a surprisingly high, model-independent motional anisotropy of cholesterol within the bilayer. The data analysis using explicit models of molecular motion suggests a superposition of two motions of cholesterol: an out-of-plane diffusion of the molecule parallel to the bilayer normal combined with a locally confined motion within the bilayer plane. The rather high amplitude of the out-of-plane diffusion observed at higher temperatures (T >/= 36 degrees C) strongly suggests that cholesterol can move between the opposite leaflets of the bilayer while it remains predominantly confined within its host monolayer at lower temperatures (T = 20 degrees C). The locally confined in-plane cholesterol motion is dominated by discrete, large-angle rotational jumps of the steroid body rather than a quasicontinous rotational diffusion by small angle jumps. We observe a significant increase of the rotational jump rate between T = 20 degrees C and T = 36 degrees C, whereas a further temperature increase to T = 50 degrees C leaves this rate essentially unchanged.

Logarithmic moments of relaxation time distributions

In this paper a novel way to quantify “nonexponential” relaxations is described. So far, this has been done in two ways: (1) by fitting empirical functions with a small number of parameters, (2) by calculation of the underlying distribution function g(ln τ) of (exponential) relaxations using regularization methods. The method described here is intermediate, it does not assume a specific functional form but also does not aim at the complete distribution g(ln τ) but only its logarithmic moments 〈(ln τ)k〉. It is shown that these exist (in contrast to the linear moments) and can be calculated analytically for all currently used empirical descriptions of nonexponential relaxations. Therefore, the logarithmic moments represent a common basis for comparing literature data from authors which prefer different empirical formulas (e.g., those of Kohlrausch and Havriliak-Negami). The logarithmic moments are also shown to be related in a simple way to the (linear) moments of an underlying distribution of activation en...

Dynamics of polybutadienes with different microstructures. 2. Dielectric response and comparisons with rheological behavior

A series of 1,2-1,4-polybutadienes with varying 1,2 vinyl content was investigated using time-domain dielectric spectroscopy. The time range was 10 Ϫ5-300 s, which can be converted by Laplace transform to a frequency range of 10 Ϫ3-6000 Hz. The samples were the same as those used in a previous rheological study from these laboratories. Therefore, a direct comparison of dielectric and mechanical responses was possible. Within experimental uncertainty, the ␣ relaxation observed by both methods shows the same temperature dependence but there is an offset between the characteristic times of both methods, which increases with increasing vinyl content. This result can be qualitatively understood from the difference of the size of the dipolar groups, viz. cis and vinyl monomeric units, in the context of the DiMarzio-Bishop model. In addition, the question of time-temperature superposition was studied using the dielectric data. In the cases of a vinyl content у0.53 no deviations from time-temperature superposition were detected. Only for the sample with the lowest vinyl content 0.07 does the attempt to construct a master curve from the dielectric loss data fail. In this case a fit with a combination of a Havriliak-Negami and a Cole-Cole functions suggests that this deviation from time-temperature superposition is an intrinsic feature of the ␣ relaxation rather than an effect of its merging with the ␤ relaxation. The absence of indications for such a deviation in the rheological study can be explained by the smaller frequency range of the latter. This stresses the necessity of a large dynamic range in experiments aimed at the examination of the time-temperature superposition principle.

Neutron scattering experiments on the glass transition of polymers

Neutron scattering experiments have been performed to explore the dynamics of polymers near the glass transition. The experimental data show three distinct types of relaxations: 1.(1) Susceptibility spectra exhibit a contribution in addition to phonons around a temperature independent frequency.2.(2) Neutron-spin-echo (NSE) experiments show a slow relaxation of the stretched exponential type. Above 220 K, the characteristic time of this process strictly follows the Vogel-Fulcher dependence of viscosity data indicatings the direct connection between microscopic and microscopic relaxation.3.(3) Below 220 K, the temperature dependence of the relaxation observed by NSE changes to an Arrhenius form while the viscosity still follows the Vogel-Fulcher law. Analysis of the Q dependence from IN13 backscattering data reveals a change of mechanism at the crossover.

Comparative study of the segmental relaxation in polyisoprene by quasi-elastic neutron scattering and dielectric spectroscopy

Abstract We have used neutron spin echo and dielectric spectroscopy to investigate the relaxational properties of polyisoprene. NSE data reveal a relaxation similar to what was found previously in polybutadiene. The comparison to the dielectric relaxation and rheological data exhibits that all time scales shift with the same temperature dependent factor.

Quasielastic neutron scattering study of the methyl group dynamics in polyisoprene

In this paper the microscopic dynamics of methyl side groups in polyisoprene is studied by means of inelastic neutron scattering. By combining time-of-flight and backscattering technique a range of four decades can be obtained (0.2 ps–2 ns). The two experimental results were combined in the time domain by using an inverse Fourier transform. Multiple scattering effects were treated by a novel procedure acting on the time-dependent intermediate scattering function S(Q,t). In the description of the data incoherent and coherent scattering from “fixed” atoms was taken into account, i.e., atoms in the main chain that move too slow to be observed in the dynamical window of the experiment. In this way good agreement with the rotation rate distribution model of a threefold jump could be obtained. Seeming discrepancies of the elastic incoherent structure factor vanish after the corrections mentioned above. The distribution of activation energies can be expressed as a Gaussian with an average of 9.7 kJ/mol and a wid...

ABSENCE OF ANNEALING EFFECT IN THE VIBRATIONAL DENSITY OF STATES IN A GLASSFORMING POLYMER

We describe experiments devised to detect a possible effect of annealing on the vibrational density of states (DOS) in a glassforming polymer with high accuracy from inelastic incoherent neutron scattering. Such an effect could be expected from the argument that annealing changes the behavior of many measured quantities, especially the specific heat cp(T). For the experiment polybutadiene was chosen because the effect on cp(T) as well as the optimal conditions of annealing are well known for this material. Contrary to our expectations an annealing effect could be detected neither in the inelastic spectra taken on IN6 nor in the elastic scans performed on IN16 at ILL, Grenoble. From this we conclude that the observed annealing effect on cp(T) does not originate from a difference in the DOS but rather from other sources, e.g. a change of configurational entropy. This result is in agreement with a theory of DiMarzio which gives a good prediction of the step Δcp at the glass transition in the case of polybuta...

Plasticizer effect on the dynamics of polyvinylchloride studied by dielectric spectroscopy and quasielastic neutron scattering.

We have studied the influence of plasticization on the microscopic dynamics of a glass-forming polymer. For this purpose we studied polyvinylchloride (PVC) with and without the commercially used plasticizer dioctylphthalate (DOP). We used dielectric spectroscopy and inelastic neutron scattering employing the neutron spin echo (NSE) technique. For both kinds of spectra the alpha relaxation could be consistently described by a model involving a distribution of individual relaxations of the Kohlrausch type. In contrast to earlier studies it turned out that an asymmetric distribution is necessary to fit the data at the lower temperatures investigated here. The shape parameters of the distribution (width, skewness) for PVC and PVC/DOP turned out to coincide when the characteristic relaxation times were the same. This means that the plasticizer only induces a remapping of the temperature dependence of the alpha relaxation. Comparison of NSE spectra S(Q,t)S(Q) at different scattering vectors Q gave the result that the slowing down at the structure factor peak Q(max) is surprisingly small for PVC while it is in the normal range for PVC/DOP.

The fast relaxation process near the glass transition in amorphous polymers with different microstructure

Abstract Inelastic neutron scattering results on polymers with different microstructure and fragility (polybutadiene, polyisoprene, poly(ethylene-propylene) and polyisobutylene) are presented. The experimental results from polybutadiene are briefly reviewed and new data for the fast relaxation process which appears near the glass transition temperature in polyisoprene and poly(ethylene-propylene) are presented. The recent observation that the fast relaxation process is of inelastic nature in polyisobutylene, a weakly fragile glass former, is discussed, and also a correlation between the Boson peak position in the glass and the monomeric friction coefficient in the melt is pointed out. Finally the results in connection with the mode coupling theory and the soft potential model are discussed.