Linear viscoelasticity in lipid bilayers of vesicles.

Keywords: encapsulation, microcapsule, protein, fibril, protein-polysaccharide complex, controlled release, interfacial rheology, lysozyme, ovalbumin This thesis describes the design of encapsulation systems using mesostructures from proteins and polysaccharides. The approach was to first investigate the physical properties of the encapsulating materials (protein fibrils and protein – polysaccharide complexes). Subsequently, microcapsules with tunable release rate and mechanical strength were developed. Firstly, the effect of steady shear and turbulent flow on the formation of protein fibrils from lysozyme was studied. We determined the conversion and size distribution of fibrils obtained by heating lysozyme solutions at pH 2. The formation of fibrils was quantified using flow-induced birefringence. The size distribution was fitted using decay of birefringence measurements and Transmission Electron Microscopy. The morphology of Lys fibrils and kinetics of their formation varied considerably depending on the flow applied. With increasing shear or stirring rate, more rod-like and shorter fibrils were obtained, and the conversion into fibrils was increased. Secondly, we have investigated the surface rheological properties of oil – water interfaces stabilized by fibrils from lysozyme (long and semi-flexible, and short and rigid ones), fibrils from ovalbumin (short and semi-flexible), lysozyme – pectin complexes, or ovalbumin – pectin complexes. We have compared these properties with those of interfaces stabilized by the native proteins. The surface dilatational and surface shear moduli were determined using an automated drop tensiometer, and a stress controlled rheometer with biconical disk geometry. Results show that interfaces stabilized by protein – pectin complexes have higher surface shear and dilatational moduli than interfaces stabilized by the native proteins only. At most of the experimental conditions, interfaces stabilized by protein fibrils have the highest surface rheological moduli. The difference between long semi-flexible lysozyme fibrils or short rigid lysozyme fibrils is not pronounced in interfacial dilation rheology but significant in interfacial shear rheology. The complex surface shear moduli of interfaces stabilized by long semi-flexible fibrils are about ten times higher than those of interfaces stabilized by short rigid fibrils, over a range of bulk concentrations. Interfaces stabilized by short and more flexible ovalbumin fibrils have a significantly higher surface shear modulus than those stabilized by the somewhat longer and more rigid short lysozyme fibrils. Finally, encapsulation systems are developed using layer-by-layer adsorption of food-grade polyelectrolytes on an emulsion droplet template. The first encapsulation system was built with alternating layers of ovalbumin fibrils and high methoxyl pectin. By varying the number of layers, the release of active ingredients can be controlled: increasing the number of layers of the shell from four to eight, decreases the release rate by a factor six. The other encapsulation systems were built with alternating layers of protein – pectin complexes and protein fibrils. Two types of proteins (ovalbumin and lysozyme) and three types of fibrils were used: short and semi-flexible from ovalbumin, short and rod-like, and long and semi-flexible from lysozyme. At low number of layers (less than five), microcapsules from ovalbumin complexes and fibrils were stronger than microcapsules prepared from lysozyme complexes and fibrils. Increasing the number of layers, the mechanical stability of microcapsules from lysozyme complexes and fibrils increased significantly, and capsules were stronger than those prepared from ovalbumin complexes and fibrils with the same number of layers. The contour length of the Lys fibrils did not have a significant effect on mechanical stability of the lysozyme complexes and fibrils capsules. These results show that mechanical properties of this type of capsule can be tuned by varying the flexibility of the protein fibrils.

The effect of pH on surface dilatational and shear properties of phospholipid monolayers

the determination of the surface shear properties of adsorbed protein layers

In this work, CuFe2O4 nanoparticles were synthesized by natural source of glucose and fructose (i.e., honey)—mediated sol–gel auto-combustion method. Grain size, cation distribution and crystal phase were further tuned through annealing at higher temperature 500, 700, 900 and 1100 °C. The structural investigation was performed using powder X-ray Diffraction, Raman Spectroscopy, Fourier Transform Infrared Spectroscopy, Scanning Electron Microscopy, Energy Dispersive X-ray Spectroscopy and X-ray Photoelectron Spectroscopy. X-ray diffraction study confirmed the phase transformation from cubic to tetragonal as a function of annealing temperature. Magnetic properties were investigated by using vibrating sample magnetometer under an applied magnetic field of 10 kOe at room temperature. The highest value of saturation magnetization (Ms) was 26 emu/g for ferrite nanoparticles annealed at 1100 °C, whereas the lowest value was 11 emu/g for annealed at 700 °C. The highest and lowest coercivity (Hc) was 1389 and 65 Oe for ferrite nanoparticles annealed at 900 and 1100 °C, respectively. Detailed study of modulus and impedance spectroscopy revealed the contribution of grain and grain boundary on electrical transport mechanism and relaxation process. Further, dependence of relaxation time, resistance and capacitance at grain and grain boundary on grain size, cation distribution and annealing temperature was noticed. The asymmetry of peak in imaginary part of modulus spectra indicates that the relaxation process is non-Debye type. At lower frequency, ac conductivity is frequency independent, whereas, at high frequency, it follows an apparent power law, σ(ω) α ωs. Dielectric parameters (real and imaginary part, dielectric loss) with variation of frequency (1 Hz to 10 MHz) are investigated and dependence with frequency and annealing temperature is observed.

Dynamics of the phosphate group in phospholipid bilayers. A 31P angular dependent nuclear spin relaxation time study

HypothesisEscin, a monodesmosidic triterpenoid saponin, was shown previously to form viscoelastic interfaces with a very high dilatational and surface shear storage modulus. This is expected to be due to the arrangement of Escin into 2D disordered soft viscoelastic solid interfacial structures, which results in turn in a distribution of relaxation times. ExperimentsThe responses to dilatational and surface shear deformations of Escin-stabilized air-water interfaces were studied, both in the linear viscoelastic (LVE) and non-linear (NLVE) regime. Step relaxation and amplitude sweeps were performed in dilatation experiments. For surface shear, amplitude sweeps and creep recovery experiments were performed. FindingsEscin stabilized-interfaces displayed a highly non-linear behavior in dilatation as seen in the Lissajous plots. In large oscillatory shear the Lissajous curves had a rhomboidal shape, indicating intracycle yielding and recovery, typical of glassy systems. The relaxation of the interface showed stretched exponential behavior, with stretched exponents typical of disordered solids with dynamic heterogeneity. The use of surface rheological measurements beyond the commonly measured LVE regime clearly has provided new insights into the behavior of these interfaces and their microstructure. These results highlight the need to reconsider other complex interfaces as disordered solids and not as 2D homogenous viscoelastic fluids.

The aging-time dependence of the segmental relaxation time of poly(vinyl acetate) (PVAc) in the glassy state is investigated in the bulk polymer and its nanocomposites with silica (SiO(2)). These systems present identical segmental dynamics, when this is probed in the equilibrium supercooled liquid by broadband dielectric spectroscopy. An acceleration of the physical aging process of PVAc with SiO(2) was detected by monitoring the enthalpy recovery through differential scanning calorimetry. The segmental relaxation time during physical aging, followed by means of BDS, has been shown to increase more rapidly the higher the SiO(2) concentration in PVAc is. Thermally stimulated depolarization current experiments show that this is the case over the whole probed glassy state. This means that nanocomposites displaying a relatively slow segmental mobility evolve toward equilibrium more rapidly than the bulk. Furthermore, despite the faster increase in the relaxation time with aging time, so-called self-retardation, the nanocomposites and their bulk counterpart reach the same values of equilibrium relaxation time. These findings not only confirm the assumption of identical equilibrium dynamics even in the aging regime for all nanocomposites and bulk polymers, proposed in previous works, but also highlight the fact that the physical aging rate is not determined solely by the polymer segmental dynamics, the amount of interface being an additional relevant parameter.

The thermal relaxation process for a spatially uniform two-dimensional plasma in a uniform d.c. magnetic field is simulated numerically. Thermal relaxation times are defined in terms of the time necessary for the numerically computed Boltzmann H function to decrease through a given part of the distance to its minimum value. Dependence of relaxation time on two parameters is studied: number of particles per Debye square n0 λ2D and ratio of gyrofrequency to plasma frequency Ω/ωp. When Ω2/ω2p becomes ≫[ln (L/2πλD)]−½, where L is the linear dimension of the system, it is found that the relaxation time varies to a good approximation as (n0 λ2D)½ and Ω/ωp.

The relaxation kinetic of the epoxy network diglycidyl ether of bisphenol A (BADGE n = 0) and m‐xylylenediamine (m‐XDA) was analyzed from DSC experimental data, using different theoretical models. Based on a Petrie model, which involved separate contributions of temperature and structure, three characteristic parameters were calculated: a preexponential factor A, an apparent activation energy EH, and a parameter C, which indicate the dependency of relaxation time on structure. This model allowed us to calculate the relaxation function at different ageing temperatures. Another method used to study a relaxation kinetic was the Kovacs–Hutchinson model, which takes into account the dependency of the relaxation time on temperature and structure. The last model used was a two‐parameter equation from Williams–Watts, where the relaxation time is independent of temperature. Using data of characteristic times a master curve for the relaxation function was obtained. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 1591–1595, 2005

Temperature dependence of the collective phonon relaxation time and acoustic damping in Ge and Si

A comparison of the data on nuclear spin relaxation and ultrasonic relaxation is made. It is suggested the correlation time, τc, used in explaining nuclear spin effects is related to the shear and volume relaxational processes occurring in a liquid. The results of comparing the nuclear and ultrasonic processes indicate the following. (1) The average shear relaxation time found for glycerol is close in value to τc (2) The value for τc for water is shown to be very close to the volume relaxation time calculated by Hall. (3) The variation in the nuclear relaxation times in going from water to glycerol are closely related to the variation in the volume relaxation times. (4) Setting τc, proportional to the average shear relaxation time offers an explanation as to why the pressure dependence of the nuclear relaxation time is not a simple function of the viscosity as predicted by present nuclear relaxation theory.

Study of the temperature dependence of dielectric relaxation time of cooperative processes in polyacyrlates and polymethacrylates

Temperature dependence of spin-phonon relaxation time in holmium

Spin-lattice relaxation in rare earth ethylsulphates. II

Aqueous suspension of nanoclay Laponite undergoes structural evolution as a function of time, which enhances its elasticity and relaxation time. In this work, we employ an effective time approach to investigate long-term relaxation dynamics by carrying out creep experiments. Typically, we observe that the monotonic evolution of elastic modulus shifts to lower aging times, while maxima in viscous moduli get progressively broader for experiments carried out on a later date after preparation (idle time) of the nanoclay suspension. Application of effective time theory produces a superposition of all the creep curves irrespective of their initial state. The resulting dependence of the relaxation time on aging time shows very strong hyper-aging dynamics at short idle times, which progressively weakens to demonstrate a linear dependence in the limit of very long idle times. Remarkably, this behavior of nanoclay suspensions is akin to that observed for polymeric glasses. Consideration of aging as a first-order process suggests that continued hyper-aging dynamics causes cessation of aging. The dependence of relaxation time on aging time, therefore, must attenuate eventually producing linear or weaker dependence on time in order to approach a progressively low-energy state in the limit of very long times as observed experimentally. We also develop a simple scaling model based on a concept of aging of an energy well, which qualitatively captures various experimental observations very well, leading to profound insight into the hyper-aging dynamics of nanoclay suspensions.