Wide Range Of Angular Frequencies Research Articles

Rheology of Dispersions of High-Aspect-Ratio Nanofibers Assembled from Elastin-Like Double-Hydrophobic Polypeptides.

Elastin-like polypeptides (ELPs) are promising candidates for fabricating tissue-engineering scaffolds that mimic the extracellular environment of elastic tissues. We have developed a “double-hydrophobic” block ELP, GPG, inspired by non-uniform distribution of two different hydrophobic domains in natural elastin. GPG has a block sequence of (VGGVG)5-(VPGXG)25-(VGGVG)5 that self-assembles to form nanofibers in water. Functional derivatives of GPG with appended amino acid motifs can also form nanofibers, a display of the block sequence’s robust self-assembling properties. However, how the block length affects fiber formation has never been clarified. This study focuses on the synthesis and characterization of a novel ELP, GPPG, in which the central sequence (VPGVG)25 is repeated twice by a short linker sequence. The self-assembly behavior and the resultant nanostructures of GPG and GPPG were when compared through circular dichroism spectroscopy, atomic force microscopy, and transmission electron microscopy. Dynamic rheology measurements revealed that the nanofiber dispersions of both GPG and GPPG at an extremely low concentration (0.034 wt%) exhibited solid-like behavior with storage modulus G′ > loss modulus G” over wide range of angular frequencies, which was most probably due to the high aspect ratio of the nanofibers that leads to the flocculation of nanofibers in the dispersion.

Development of self-assembled polygalacturonic acid-peptide composites and their interactions with mesenchymal stem cells for potential applications in tendon tissue engineering

We have developed a new biomimetic scaffold for potential applications in tendon tissue engineering (TE). The scaffold template was synthesised by conjugating polygalacturonic acid with the dipeptide leucyl-leucine to mimic the leucine rich proteoglycans found in the extracellular matrix (ECM) of tenocytes. To the template, type I collagen and an elastin derived peptide were incorporated in order to form the final PG-Leu-Leu-Col-El scaffold. Results indicated the formation of gelatinous, fibrous scaffolds. DSC analysis showed phase changes that included crystallisation and thermal melting due to re-organisation of the scaffold components. Young's modulus was determined to be 832 ± 2 MPa. Rheology studies showed that the scaffold maintained a constant G´ / G˝ ratio over a wide range of angular frequency. Cell studies with bone marrow derived mesenchymal stem cells (BMSC) indicated that the scaffolds promoted cell proliferation and formed three dimensional cell-scaffold matrices. This newly developed scaffold may open new opportunities for tissue engineering applications.

Synthesis of amino acid based covalently cross-linked polymeric gels using tetrakis(hydroxymethyl) phosphonium chloride as a cross-linker

This work reports the design and synthesis of side-chain amino acid containing cross-linked polymer gels with high mechanical strength. The Boc-leucine methacryloyloxyethyl ester (Boc-Leu-HEMA) has been polymerized by reversible addition-fragmentation chain transfer (RAFT) polymerization to obtain the corresponding polymer, P(Boc-Leu-HEMA). After Boc group deprotection under acidic condition and pH neutralization amino groups remain in the polymer side chain, which were reacted with tetrakis(hydroxymethyl) phosphonium chloride (THPC) to obtain chemically cross-linked gels. The mechanical properties of gels have been studied by rheological measurements, which reveal that mechanical strength increases as weight% of polymer in solution, cross-linking density and polarity of solvents increases, and decreases as molecular weight of polymers increases. A plateau of storage modulus (G′) over a wide range of angular frequency demonstrates that viscoelastic property of gels is independent of angular frequency. Field emission scanning electron microscopy (FE-SEM) studies of cross-sectioned gels validate rheological observation that as the G′ values of gel increases, porosity of gel decreases.

Rheological and fluorescent properties of riboflavin–poly(N-isopropylacrylamide) hybrid hydrogel with a potentiality of forming Ag nanoparticle

The riboflavin (R) and poly(N-isopropylacrylamide) (PNIPAAM) hybrid hydrogels have been prepared using the free radical polymerization of N-isopropylacrylamide in the presence of N,N′-methylene bisacrylamide as a cross linker for 1, 2 and 3 mM concentrations of R. The invariance of storage (G′) and loss (G′′) moduli over a wide range of angular frequency, where G′ > G′′ for R–PNIPAAM system characterize it to be behaving as a gel in the hybrid state. Both G′ and G′′ decrease with the increase of R concentration, but the decrease is four times higher in the former than in the latter. R–PNIPAAM gel has a higher critical strain value than PNIPAAM gels and it gradually increases with the increase in R concentration indicating that R is acting as a supramolecular cross-linker. The fluorescent intensity of R–PNIPAAM gels increases with the increase in R concentration and its variation with temperature at different pH shows an increase in the intensity value with temperature, showing the maximum at ∼30 °C because of the coil-to-globule transition of PNIPAAM chains, suggesting that the R–PNIPAAM gel can be used to be a probe for temperature detection. The sensitivity index of the fluorescent temperature sensing of R–PNIPAAM gel is moderate and it is highest at pH 7. R becomes gradually released from the R–PNIPAAM gel when dipped into water (pH 7) at 20 °C, initially at a slower rate, then at a higher rate, and finally it shows a leveling for the release of 19% of embedded R at 24 h of aging. Silver nanoparticles that are grown in the R–PNIPAAM gels by immersing the slashed gel pieces in AgNO3 solution stay at both the surface and the interior of the R–PNIPAAM fibres, maintaining the gel structure intact with a decrease in critical strain and causing a quenching of the fluorescence intensity of R–PNIPAAM gels. The average size and size distribution of AgNPs linearly increase with R concentration at a constant AgNO3 concentration.

Tryptophan containing covalently cross-linked polymeric gels with fluorescence and pH-induced reversible sol–gel transition properties

pH-reversible covalently cross-linked polymeric gels from a side-chain polymer with multi-amino groups and a difunctional aldehyde cross-linker have been prepared by employing dynamic covalent chemistry in the form of reversible imine (CN) bond formation. Two different series of dual pH- and thermo-responsive amino acid based copolymers with primary amine groups located at different positions and various chemical compositions are synthesized by reversible addition fragmentation chain transfer (RAFT) copolymerization of tert-butyl carbamate (Boc)-tryptophan methacryloyloxyethyl ester (Boc-Trp-HEMA) with 2-(2-methoxyethoxy)ethyl methacrylate (MEO2MA) and N,N-(dimethylamino)ethyl methacrylate (DMAEMA) respectively, followed by Boc-group expulsion. The pH-induced thermoresponsive properties of the copolymers have been observed with the high content of thermoresponsive MEO2MA/DMAEMA segments (∼80%) in the copolymer. The tryptophanyl moiety present in the copolymer chain provided florescence characteristics, which were studied by florescence spectroscopy. Copolymers with free amino groups were employed for cross-linking with a difunctionalized aldehyde cross-linker, 4-formylphenyl 4′-formylbenzoate (FPFB), to form dynamic covalent gels through the formation of CN bonds in the network. Rheological measurements have been conducted with various ratios of CHO/NH2 from 1 to 0.33 for homopolymer gels and copolymer gels with different amino acid contents (20–80%), which showed a plateau of storage modulus (G′) over a wide range of angular frequencies indicating the independent nature of G′ with respect to angular frequency, and the mechanical properties of dynamic gels are a function of cross-linking density. The pH-dependent reversible sol–gel transition has also been investigated.

Rheokinetics of Ring-Opening Metathesis Polymerization of Bio-Based Castor Oil Thermoset

Ring-opening metathesis polymerization (ROMP) of norbornenyl-functionalized castor oil has been evaluated using small-amplitude oscillatory shear flow experiments as a function of angular frequency, temperature, and curing time. At the onset temperature of the curing process, an abrupt increase in dynamic shear moduli, G′ and G″, and complex shear viscosity, η*, was observed during the dynamic temperature ramps (2 °C/min heating rate) of the sample over a wide range of angular frequencies. A dramatic increase in zero-shear viscosity, η0, was also observed at the gelation temperature, Tgel. The value of Tgel obtained from the abrupt increase in η0 was found to be in good agreement with the value evaluated from the crossover point of G′ and G″. The real time curing kinetics was investigated under isothermal conditions over a wide range of angular frequencies at different constant curing temperatures (40, 45, 50, and 55 °C). The isothermal gelation kinetics was found to be strongly curing temperature depende...

The morphology of the thermal Sunyaev-Zel’dovich sky

At high angular frequencies the thermal Sunyaev-Zel'dovich (tSZ) effect constitutes the dominant signal in the CMB sky. The tSZ effect is caused by large scale pressure fluctuations in the baryonic distribution in the Universe so its statistical properties provide estimates of corresponding properties of the projected 3D pressure fluctuations. It's power spectrum is a sensitive probe of the density fluctuations, and the bispectrum can be used to separate the bias associated with pressure. The bispectrum is often probed with a one-point real-space analogue, the skewness. In addition to the skewness the morphological properties, as probed by the well known Minkowski Functionals (MFs), also require the generalized one-point statistics, which at the lowest order are identical to the skewness parameters. The concept of generalized skewness parameters can be extended to define a set of three associated generalized skew-spectra. We use these skew-spectra to probe the morphology of the tSZ sky or the y-sky. We show how these power spectra can be recovered from the data in the presence of arbitrary mask and noise templates using the well known Pseudo-Cl (PCL) approach for arbitrary beam shape. We also employ an approach based on the halo model to compute the tSZ bispectrum. The bispectrum from each of these models is then used to construct the generalized skew-spectra. We consider the performance of an all-sky survey with Planck-type noise and compare the results against a noise-free ideal experiment using a range of smoothing angles. We find that the skew-spectra can be estimated with very high signal-to-noise ratio from future frequency cleaned tSZ maps that will be available from experiments such as Planck. This will allow their mode by mode estimation for a wide range of angular frequencies and will help us to differentiate them from various other sources of non-Gaussianity.

Rheological Evaluation of Petroleum Jelly as a Base Material in Ointment and Cream Formulations : Linear Viscoelastic Behavior

ABSTRACT − The objective of the present study is to systematically characterize a linear viscoelastic behavior of petro-leum jelly in small amplitude oscillatory shear flow fields correspondent to the rheological ground state. With this aim, usinga strain-controlled rheometer, the dynamic viscoelastic properties of commercially available petroleum jelly have been mea-sured at 37 o C (body temperature) over a wide range of angular frequencies at an extremely small strain amplitude of 0.1%. In this article, the linear viscoelastic behavior was reported in detail and then explained from a structural view-point ofpetroleum jelly and discussed in depth with respect to the consumer’s requirements. Main findings obtained from this studycan be summarized as follows : (1) The storage modulus is always greater than the loss modulus over an entire range ofangular frequencies studied, meaning that the linear viscoelastic behavior of petroleum jelly is dominated by an elastic naturerather than a viscous nature. (2) Petroleum jelly shows a desirable linear viscoelastic behavior with respect to the consumer’srequirements because it is undesirable for the product to flow down from the skin at an initial stage upon contact with thehuman skin. (3) A fractional derivative model shows an excellent applicability to describe a linear viscoelastic behavior ofpetroleum jelly. However, this model should be used with a special caution because there exists no physical meaning forthe model parameters. (4) A modified form of the Cox-Merz rule gives a good ability to predict the relationship betweensteady shear flow properties (nonlinear behavior) and dynamic viscoelastic properties (linear behavior) for petroleum jelly.Key words

Optical properties of [formula omitted] revealed by ab initio calculations

Detailed optical functions and optical properties of a α -monoclinic phase of Selenium ( α -Se ) are presented over a wide range of angular frequencies ω by first-principles calculations. An optical band gap of 2.05 eV and a mobility gap of 2.17 eV±0.05 are observed in the present calculations. The imaginary part of the dielectric function has been obtained from the frequency dependent dielectric matrix and the use of the Kramers–Kronig (KK) relation allows us to determine the real part of the dielectric function. Optical functions and optical properties revealed the occurrence of a long range order in Se. For the first time, infrared and absorption spectra of monoclinic-Se were studied using the Kramers–Kronig (KK) relation by ab initio method. Finally, detailed studies of other dielectric related optical constants such as reflectivity, transmission, reflection and absorption coefficients were also presented.

Synthesis of Crosslinkable Core-Shell Emulsion and Rheology of the Core-Shell/Amine Crosslinking Process

Starve feed and semi-continuous seed emulsion polymerization were used to control the morphology of core shell latex particles with a vinyl acetate (VAc)/vinyl ester of versatic acid 10(VeoVa10) copolymer core surrounded by a poly(glycidyl methacrylate) (PGMA) shell. Pure core and core-shell structures were confirmed by TEM. The results suggest that core-shell morphology of the two stage emulsion was favoured by higher concentration of emulsifier in the seed latex: the particle size distribution of core-shell latex was broader than that of the core latex, and the average particle size of core-shell latex was larger than that of the core latex. The core-shell structure was not produced using seed emulsion with emulsifier concentration at or below the critical micelle concentration. The core shell emulsion containing epoxy functional group with added ethylene diamine showed an abrupt increase in dynamic shear moduli, G' and G'' and complex viscosity η* (several orders of magnitude) at about 35oC, during temperature ramps, over a wide range of angular frequencies. The time ramps showed that the crosslinking reaction did not occur at 15oC for the core-shell emulsion/amine system. The time for gel formation decreased with increase in temperature.

Dispersion dependence of second-order refractive index and complex third-order optical susceptibility in oxide glasses

A method correlates Fresnel-based spectrophotometric measurements and Lorentz dispersion theory is presented to study the dispersion of nonlinear optical parameters in particularly oxide glasses in a very wide range of angular frequency. The second-order refractive index and third-order optical susceptibility of Cr-doped glasses are determined from linear refractive index. Furthermore, both real and imaginary components of the complex susceptibility are carried out. The study reveals the importance of determining the dispersion of nonlinear absorption (two-photon absorption coefficient) to find the maximum resonant and nonresonant susceptibilities of investigated glasses. The present method is applied on Cr-doped lithium aluminum silicate (LAS) glasses due to their semiconductor-like behavior and also to their application in laser industry.

Rheologically determined negative influence of increasing nucleating agent content on the crystallization of isotactic polypropylene

Dibenzylidene sorbitol (DBS) exists in the form of fibril and usually acts as an effective nucleating agent to facilitate crystallization of polyolefin during manufacturing. In this research, the isothermal crystallization of isotactic polypropylene (iPP) containing different amounts of DBS was followed by dynamic rheometry, and described upon a viewpoint of viscoelastic property evolution. Since the adopted temperatures within the entire sample preparation and characterization process were below the melting point of DBS, the DBS additives played a role of only solid nucleating agent, thus the possible effect of a changed miscibility between iPP and DBS on the viscoelastic properties as change of temperature could be ignored. Although saturation of nucleating iPP was observed at 0.1% DBS concentration, a negative influence with further increasing DBS content on the crystallization of iPP was determined for the first time via time sweep of G′ upon a single angular frequency and application of “inverse quenching” protocol within a wide range of angular frequency. Even more, a largely increased G′ and viscosity were observed in the melt as decreasing the temperature of iPP containing 0.1% DBS, suggesting a strong quasi-solid like behavior before iPP crystallization. A complement for well understanding the crystallization of iPP containing nucleating agent was discussed based on the formation of the DBS fibrils' network, the interfacial tension between crystalline/amorphous phase, and the consistency of crystallizing lamellae. Our study demonstrates clearly that the crystalline characteristic upon saturation of heterogeneous nucleating is difficult to be detected by traditional means, but could be followed reasonably by rheological measurements which is much sensitive to the microstructural changes.

진동전단유동장에서 점탄성 반고형 연고기제(바셀린)의 레올로지 거동

Using a Rheometries Dynamic Analyzer (RDA II), the dynamic viscoelastic properties of a semi-solid ointment base (vaseline) in large amplitude oscillatory shear flow fields were measured over a temperature range of and the linear viscoelastic behavior in small amplitude oscillatory shear flow fields was investigated over a wide range of angular frequencies. In this article, the nonlinear viscoelastic behavior was reported from the experimentally obtained data and the effect of temperature on this behavior was discussed in detail. In addition, the angular frequency and temperature dependencies of a linear viscoelastic behavior were explained. Finally, the applicability of a time-temperature superposition principle originally developed for polymeric materials was examined using a shift factor. Main results obtained from this study can be summarized as follows : (1) At very small strain amplitude region, vaseline shows a linear viscoelastic behavior independent of the imposed deformation magnitudes. Above a critical strain amplitude , however, vaseline exhibits a nonlinear viscoelastic behavior ; indicating that both the storage modulus and dynamic viscosity are sharply decreased with increasing deformation magnitude. (2) In large amplitude oscillatory shear flow fields, an elastic behavior (storage modulus) has a stronger strain amplitude dependence and begins to show a nonlinear behavior at a smaller strain amplitude region than does a viscous behavior (dynamic viscosity). (3) In small amplitude oscillatory shear flow fields, the storage modulus as well as the loss modulus are continuously increased as an increase in angular frequency and an elastic nature is always superior to a viscous behavior over a wide range of angular frequencies. (4) A time-temperature superposition principle can successfully be applicable to vaseline. This finding allows us to estimate the dynamic viscoelastic behavior of vaseline over an extraordinarily extended range (11 decades) of angular frequencies inaccessible from the experimentally measured range (4 decades).

Viscoelastic Behavior of Scarcely Crosslinked Poly(dimethyl siloxane) Gel

Linear viscoelastic behavior was investigated for a poly(dimethyl siloxane) (PDMS) gel formed through a bulk double-liquid crosslinking reaction of two types of vinyl-terminated PDMS prepolymers of the molecular weights Mpre ≈ 35×103. Time-temperature superposition worked, and master curves at 20 °C were constructed for the storage and loss moduli, G' and G, in a wide range of angular frequency ω (= 103 −10−5 s−1) by combining the data obtained from dynamic oscillatory tests and creep tests. With decreasing ω to 10−1 s−1, G decreased in proportion to ω0.6 and G' rapidly decreased to its equilibrium plateau at the modulus Ge = 2800 Pa. On a further decrease of ω well in the plateau regime of G', G decreased in proportion to ω0.3. Thus, the gel exhibited the fast and slow relaxation processes characterized with these types of power-law behavior of G. The molecular weight between the crosslinks evaluated from the Ge data (as well as the equilibrium swelling ratio in toluene), Mc ≈ 340 ×103, was about ten times larger than Mpre. The crosslinking reaction was made in the bulk state but still gave such a scarce gel network (with Mc ≈ 10 Mpre) possibly because a large amount of sol chains and dangling chains had diluted the trapped entanglements during the reaction. From the analysis of the G' and G data on the basis of the above Mc value and the intrinsic Rouse relaxation time, the fast relaxation process was assigned as the Rouse-like constraint release (CR) process of individual gel strands. The polydispersity of the strands was found to be essential for the power-law behavior (G∝ωn with n ≈ 0.6) to be observed in the plateau regime of G'. The slow relaxation process was related to fluctuation of the crosslinking points, which is equivalent to cooperative Rouse-CR motion of many gel strands connected at these points.

Thermal-induced simultaneous liquid–liquid phase separation and liquid–solid transition in aqueous polyurethane dispersions

Thermal-induced simultaneous phase separation and liquid–solid transition (gelation) in waterborne polyurethane dispersions has been detected morphologically and rheologically. The viscoelastic material functions, such as dynamic shear moduli, G′ and G″ complex shear viscosity, η* and loss tangent, tan δ were found to be very sensitive to the structure evolution during the gelation process and the subsequent formation of a fractal polymer gel. At the onset temperature of the gelation process, an abrupt increase in G′, G″ and η* (several orders of magnitude) was observed during the dynamic temperature ramps (2 °C/min heating-rate) over a wide range of angular frequency. The temperature dependencies of G′, G″ and tan δ were found to be frequency independent at the gel-point, T gel, providing a fingerprint for determining T gel of the dispersions. Furthermore, a dramatic increase in zero-shear viscosity, η 0 (v-shape) was observed at T= T gel and found to be in good agreement with the value obtained from the tan δ versus T data. As expected, the time–temperature–superposition principle was found to be only valid for temperatures lower than the T gel; the principle failed at T≥70 °C. The morphology of the dispersions at 70 °C for 2 h showed for 36, 38 and 40 wt% formation of a network structure having a unique periodicity and phase connectivity. A lower critical solution temperature (LCST) phase diagram was estimated based on the different morphologies of the dispersions. The coexistence of liquid–liquid and liquid–solid transitions at the same temperature range confirmed the complex behavior of the polyurethane dispersions, pointing to the need for a new theory that explicitly takes this special behavior into account.

Unified theory of effects of vortex pinning and flux creep upon the rf surface impedance of type-II superconductors.

We present a new theoretical approach that permits the rf surface impedance ${\mathit{Z}}_{\mathit{s}}$ of type-II superconductors to be calculated over a wide range of angular frequencies \ensuremath{\omega}, magnetic inductions B, and temperatures T. Our approach, based on a self-consistent treatment of vortex dynamics, includes the influence of both vortex pinning and flux creep upon ${\mathit{Z}}_{\mathit{s}}$ in a unified manner, by making use of continued-fraction-expansion results for the dynamic mobility of a particle undergoing Brownian motion in a periodic potential.

Acoustic wave propagation in a flowing liquid-vapour mixture

The two-fluid equations describing transient nonequilibrium liquid-vapour flow have been used to derive a general dispersion relation for acoustic waves. The analysis is valid in principle for both dispersed and separated flow regimes. In contrast to previous work, the predicted sound speeds and attenuations depend only on measurable properties of the flow. The model can apply over a wide range of angular frequencies (up to 1–10 6 Hz for steam and water), under conditions where the scattering of waves by individual bubbles and droplets is unimportant. Predictions are made for sound speeds and attenuations in both bubbly and annular flow at low Mach numbers. Agreement of the theory with available data is shown to be reasonable.