G. D’Angelo

Relaxation dynamics of H-bonded liquids confined in porous silica gels by Rayleigh wing spectroscopy

Reorientational dynamics of neat liquid propylene glycol molecules is compared with that of confined molecules in 25 and 75 A pores of a sol-gel porous glass by means of depolarized light-scattering spectroscopy. The experimental spectra, performed as a function of temperature, show unambiguously the slowing down of the collective reorientational processes triggered by the confinement effects. Such results confirm the main role played by the strong dipolar interactions on the dynamics of hydrogen-bonded liquids diffusing within nanoscopic spherical pores.

Multi-component modeling of quasielastic neutron scattering from phospholipid membranes

We investigated molecular motions in the 0.3-350 ps time range of D2O-hydrated bilayers of 1-palmitoyl-oleoyl-sn-glycero-phosphocholine and 1,2-dimyristoyl-sn-glycero-phosphocholine in the liquid phase by quasielastic neutron scattering. Model analysis of sets of spectra covering scale lengths from 4.8 to 30 Å revealed the presence of three types of motion taking place on well-separated time scales: (i) slow diffusion of the whole phospholipid molecules in a confined cylindrical region; (ii) conformational motion of the phospholipid chains; and (iii) fast uniaxial rotation of the hydrogen atoms around their carbon atoms. Based on theoretical models for the hydrogen dynamics in phospholipids, the spatial extent of these motions was analysed in detail and the results were compared with existing literature data. The complex dynamics of protons was described in terms of elemental dynamical processes involving different parts of the phospholipid chain on whose motions the hydrogen atoms ride.

Locally heterogeneous dynamics in miscible blends of poly(methyl methacrylate) and poly(vinylidene fluoride)

Comparative measurements of specific heat capacities (temperature interval between 2 and 500 K), and of low frequency mechanical spectroscopy (temperature interval between 120 and 400 K) in poly(methyl methacrylate) (PMMA) and poly(vinylidene fluoride) (PVDF) amorphous blends show the existence of single calorimetric and mechanical glass transition temperatures, as a clear indication of the existence of homogeneous single-state structures. Below T g , the experimental data reveal distinct local relaxation processes within the backbone of the individual components, while the heat capacities below 15 K can be explained in terms of a two-phase model (i.e., a simple linear overlap of the contributions from wholly amorphous PMMA and PVDF, weighted by their proportions). These findings are associated with locally heterogeneous relaxation and vibrational motions, and are regarded as experimental evidence for the existence of a nanoscopic length scale where the dynamics of a blend exhibits a heterogeneous regime.

Lipid Diffusion in Alcoholic Environment

We have studied the effects of a high concentration of butanol and octanol on the phase behavior and on the lateral mobility of 1,2-palmitoyl-sn-glycero-3-phosphocholine (DPPC) by means of differential scanning calorimetry and pulsed-gradient stimulated-echo (PGSTE) NMR spectroscopy. A lowering of the lipid transition from the gel to the liquid-crystalline state for the membrane-alcohol systems has been observed. NMR measurements reveal three distinct diffusions in the DPPC-alcohol systems, characterized by a high, intermediate, and slow diffusivity, ascribed to the water, the alcohol, and the lipid, respectively. The lipid diffusion process is promoted in the liquid phase while it is hindered in the interdigitated phase due to the presence of alcohols. Furthermore, in the interdigitated phase, lipid lateral diffusion coefficients show a slight temperature dependence. To the best of our knowledge, this is the first time that lateral diffusion coefficients on alcohol with so a long chain, and at low temperatures, are reported. By the Arrhenius plots of the temperature dependence of the diffusion coefficients, we have evaluated the apparent activation energy in both the liquid and in the interdigitated phase. The presence of alcohol increases this value in both phases. An explanation in terms of a free volume model that takes into account also for energy factors is proposed.

Low-temperature excess specific heat and fragility in polymers: Crystallinity dependence

A study of the mechanical characteristics and the low-temperature specific heat has been performed on a class of polymers whose crystallinity degree varies in a wide range, from a wholly amorphous to a highly crystalline structure. Typical features of the amorphous phase have been analyzed in order to shed further light on their correlations: the deviation from the exponentiality and the “fragility” obtained by modeling the mechanical αa-relaxation and the excess specific heat over the predictions of the Debye theory. It has been found that increasing crystallinity enhances the nonexponentiality of the αa-relaxation, and decreases both the fragility and the excess specific heat. This behavior is intrinsic to semicrystalline polymers. Consequently, the recently suggested correlation between the degree of fragility and the additional low-energy vibrations causing the low-temperature excess specific heat in glasses fails in semicrystalline polymers.

Why many polymers are so fragile: A new perspective

Many polymers exhibit much steeper temperature dependence of their structural relaxation time (higher fragility) than liquids of small molecules, and the mechanism of this unusually high fragility in polymers remains a puzzle. To reveal additional hints for understanding the underlying mechanism, we analyzed correlation of many properties of polymers to their fragility on example of model polymer polystyrene with various molecular weights (MWs). We demonstrate that these correlations work for short chains (oligomers), but fail progressively with increase in MW. Our surprising discovery is that the steepness of the temperature dependence (fragility) of the viscosity that is determined by chain relaxation follows the correlations at all molecular weights. These results suggest that the molecular level relaxation still follows the behavior usual for small molecules even in polymers, and its fragility (chain fragility) falls in the range usual for molecular liquids. It is the segmental relaxation that has this unusually high fragility. We speculate that many polymers cannot reach an ergodic state on the time scale of segmental dynamics due to chain connectivity and rigidity. This leads to sharper decrease in accessible configurational entropy upon cooling and results in steeper temperature dependence of segmental relaxation. The proposed scenario provides a new important insight into the specifics of polymer dynamics: the role of ergodicity time and length scale. At the end, we suggest that a similar scenario can be applicable also to other molecular systems with slow intra-molecular degrees of freedom and to chemically complex systems where the time scale of chemical fluctuations can be longer than the time scale of structural relaxation.

Raman spectroscopic and low-temperature calorimetric investigation of the low-energy vibrational dynamics of hen egg-white lysozyme

The low-frequency vibrational dynamics of chicken hen egg-white lysozyme were investigated using Raman spectroscopy and low-temperature calorimetry. An amorphous-like behaviour of low-frequency dynamics was observed in this protein. By using inelastic light scattering data and resorting to a fitting procedure, the low-temperature specific heat trend was theoretically reproduced, confirming that, as in disordered systems, the same low-energy excitations give rise to the observed anomalies in low-frequency vibrational and low-temperature thermal properties. A further study of polarised and depolarised Raman spectra has allowed us to infer information about the symmetry of these modes. The frequency dependence of the light–vibrational coupling constant has also been analysed.

Prepeak and First Sharp Diffraction Peak in the Structure Factor of (Cs2O)0.14(B2O3)0.86 Glass: Influence of Temperature

Neutron diffraction measurements on (Cs(2)O)(0.14)(B(2)O(3))(0.86) glass were performed at varying temperature over an extended range from room temperature to 800 K. It was found that, in the same Q range where the first sharp diffraction peak (FSDP) is observed in the static structure factor of almost all glass-forming systems, cesium borate glass shows two peaks. The intensities of these peaks increase with temperature, and their positions shift to lower Q values, in agreement with the peculiarities of the FSDP of network glasses. A description of this anomalous temperature dependence in terms of thermal relaxations of strained bonding arrangements of boron oxide units lying on the boundaries of cages present in the boron skeleton matrix is suggested. By comparing the diffraction patterns of a (Cs(2)O)(0.14)(B(2)O(3))(0.86) sample before and after a high-temperature thermal treatment with the spectra of cesium crystals, a correspondence between the medium-range structure in the glass and the related crystalline phases has been inferred.

Physical study of dynamics in fully hydrated phospholipid bilayers

We studied the thermotropic phase behavior of a phospholipid membrane bilayer of dimyristoylphosphatidylcholine (DMPC) under excess water conditions. We also investigated the effect induced by the antimicrobial peptide, gramicidin D (GrD), when inserted into the membrane bilayer. Several techniques were used to collect information on different properties and various spatial and temporal regimes, including differential scanning calorimetry, fixed energy window neutron scattering, pulsed-field gradient nuclear magnetic resonance and molecular dynamics simulations. We obtained data on the main phase transition (gel–liquid crystalline) temperature of the bilayer system and on the long-range diffusion coefficient for both phospholipids and hydration water. Moreover, we demonstrated the effect of transition on microscopic molecular mobility, perpendicular and parallel to the membrane plane. The influence of gramicidin on these properties is also discussed.

Local and cooperative molecular mobilities in thermoplastic polymers

Comparative measurements of calorimetry and low-frequency mechanical spectroscopy (temperature interval between 120 and 400 K) in a thermoplastic interpenetrating polymer network based on semicrystalline poly(urethane) and a styrene–acrylic acid block copolymer show the existence of distinct calorimetric and mechanical transitions associated with the two components, as a clear indication of a multiple-phase heterogeneous structure. In contrast with the apparent thermodynamic incompatibility of the components, significant deviations in the magnitudes of local and cooperative transitions from a simple dilution effect are revealed. These findings are associated with a limited miscibility due to weak interactions (hydrogen bondings) between the functional groups of the two polymeric components.