Effective Chain Length Research Articles

The main transition enthalpy of the gel-to-liquid crystal phases for a series of asymmetric chain length d-erythro (2S, 3R) sphingomyelins

A series of d-erythro (2S, 3R) sphingomyelins (SMs) whose acyl chain was 16, 18, 20, 22, and 24 carbons long was synthesized by the acylation of d-erythro-sphingosyl-phosphorylcholine of 18 carbons long. All the SMs were found to exist as asymmetric chain-length lipids. DSC heating scans of the respective SM dispersions showed two transitions that were reversible and reproducible upon reheating. However, the enthalpy change (ΔHM) of the main gel-to-liquid crystal phase transition exhibited a slight decrease with an increase in the effective chain length (N), which contrasted with the generally accepted phenomenon for lipid-water systems. The behavior of ΔHMvs. N curve was discussed from the viewpoint of the effect of a mismatched chain-length segment in the acyl chain on the chain–chain van der Waals interaction energy that accounts for 2/3 of ΔHM, on the basis of a partial interdigitation for the chain packing arrangement of asymmetric chain length lipids.

Composition-based effective chain length for prediction of protein folding rates

Folding rate prediction is a useful way to find the key factors affecting folding kinetics of proteins. Structural information is more or less required in the present prediction methods, which limits the application of these methods to various proteins. In this work, an "effective length" is defined solely based on the composition of a protein, namely, the number of specific types of amino acids in a protein. A physical theory based on a minimalist model is employed to describe the relation between the folding rates and the effective length of proteins. Based on the resultant relationship between folding rates and effective length, the optimal sets of amino acids are found through the enumeration over all possible combinations of amino acids. This optimal set achieves a high correlation (with the coefficient of 0.84) between the folding rates and the optimal effective length. The features of these amino acids are consistent with our model and landscape theory. Further comparisons between our effective length and other factors are carried out. The effective length is physically consistent with structure-based prediction methods and has the best predictability for folding rates. These results all suggest that both entropy and energetics contribute importantly to folding kinetics. The ability to accurately and efficiently predict folding rates from composition enables the analysis of the kinetics for various kinds of proteins. The underlying physics in our method may be helpful to stimulate further understanding on the effects of various amino acids in folding dynamics.

Comparative Study on the Effect of NaBr on the Interaction Between Alkyltrimethylammonium Bromide and β-Cyclodextrin

The effect of NaBr on the interaction between alkyltrimethylammonium bromide (CnTAB (n = 14 for TTAB and 16 for CTAB)) and ß-CD was studied by surface tension method. The first decrease in the γcmc of TTAB/ß-CD in the presence of NaBr mainly due to the elongated effective chain length of the hydrophobic part of the TTAB/ß-CD complexes; while the increase in the γcmc of both of the TTAB/ß-CD and CTAB/ß-CD in the presence of NaBr could be attributed to the close pack of CnTAB at the air/water interface.

Structure and optical characteristics of poly(p-phenylenevinylene) prepared by vapor deposition polymerization

Films based on poly(p-phenylenevinylene) are prepared by pyrolitic polymerization of α,α′-dichloro-p-xylene. During monomer precipitation, the temperature on a substrate is 25, 50, or −196°C. Subsequent annealing of the precursor at 250°C yields the final product: the copolymer of p-phenylenevinylene and p-xylylene with an approximate composition of 4: 1. The surface morphology, structure, and optical characteristics of the polymer are studied. The mean-square surface roughness of the precursor is 5 nm. Thermal treatment increase the samples’ roughness up to 10 nm. When the precursor is transformed into poly(p-phenylenevinylene), the roughness coefficient decreases from 0.85 ± 0.05 to 0.74 ± 0.05 owing to the formation of a rougher surface. Characterization of the optical characteristics of the synthesized poly(p-phenylenevinylene) shows that the maximum effective conjugation chain length achieves 12 units in the copolymer prepared when the temperature on the substrate is −196°C. As the temperature on the substrate increases, the conjugation length decreases to 8 units upon precipitation. Luminescence analysis reveals the effective excitation-energy transfer from short chain fragments of poly(p-phenylenevinylene) to long chain fragments. Electron parameters of the material are estimated: i.e., the band gap, the Huang-Rhys factor, the Stokes shift, and the oscillation energy of molecules.

Self-Diffusivity Estimation by Molecular Dynamics

Reference fluid simulations are combined with a mean field correction for attractive effects to achieve a correlation for diffusivity of unentangled species over a wide range of density, temperature, and molecular weight. Reference fluids are composed of hard united atom descriptions including branching, rings, bond angles, and chain length. The molecular model is consistent with the one studied previously by thermodynamic perturbation theory. Molecular simulations of methane through triacontane provide the basis for a diffusivity correlation of reference n-alkanes from zero density to the glass transition. The low density limit is shown to exhibit Rouse scaling, contradicting typical hard sphere models. Experimental data for n-alkanes ranging from methane to hexadecane are used to correlate the correction for attractive forces and for the softness of the potential function. The average deviation is 8.6% for 657 data points. Reference simulations of nonalkanes show that the reference simulations can be interpolated smoothly with the n-alkane correlation by treating the effective chain length as an adjustable parameter. Applying the corresponding alkane mean field correction for attractive forces results in poor predictions for nonalkanes. The attractive energy must be treated as a single adjustable parameter to achieve reasonable accuracy. Existing correlations require two parameters to achieve comparable accuracy, and the physical meaning of the two parameters is difficult to interpret. A correlation between diffusivity and entropy suggested by Rosenfeld is shown to be problematic for chain molecules. Overall, diffusivity data are treated for 35 compounds with 1801 data points with 14% deviation by the correlation developed here.

Poly(thiophene) Nanoparticles Prepared by Fe3+-Catalyzed Oxidative Polymerization: A Size-Dependent Effect on Photoluminescence Property

Poly(thiophene) (PTh) nanoparticles with various sizes were directly prepared by Fe3+-catalyzed oxidative polymerization of thiophene with varying hydrogen peroxide (H2O2) concentrations. In the polymerization, the FeCl3/H2O2 (catalyst/oxidant) combination system was used as an initiator couple. Under the optimized reaction condition, percentage monomer conversions were >90%. With increasing H2O2 molar concentration from 2.35 to 5.88 M, the average sizes of the PTh nanoparticles decreased from 51 to 12 nm, and their photoemission wavelengths shifted from red to blue color at the maximum excitation wavelength (λUVmax = 400 nm). However, the molecular weights of all PTh nanoparticles were ∼3500 g/mol. As a result, we could tune the emitting colors that resulted from variations of the effective conjugation chain length by manipulating the size of PTh nanoparticles.

QSPR for HLB Values of Nonionic Surfactants Using Two Simple Descriptors

By using the quantitative structure-property relationship (QSPR) method, the relationship between the hydrophilic-lipophilic balance (HLB) value and the chemical structure of surfactants were studied for nonionic surfactants of the ethoxylated alcohols and ethoxylated alkylphenols, for which the number of EO groups is less than 20. The obtained best QSPR model contains only two simple descriptors. One is zero order of Kier-Hall index (KH0); the other is the number of oxygen atoms in the hydrophilic group (N o). The correlation coefficient (R 2) is as high as 0.984. The resulted QSPR model shows a significantly higher accuracy with respect to the effective chain length method.

Particle-induced network formation in linear PDMS filled with silica

We study the formation of permanent elastomers from linear PDMS chains by solution blending with up to 25wt% fumed silica. The physical networks are characterized by time-domain multiple-quantum NMR. Based upon dynamic parameters measured for the linear precursor polymer, we develop a reliable strategy for component separation in this complex heterogeneous system, providing information on the amount of monomers involved in network-like material, in elastically inactive yet entangled linear chains, and isotropically mobile chain ends, as well as on the effective network chain length as measured via the average residual dipolar coupling constant. The use of untreated silica leads to permanent networks, for which the NMR results correlate well with macroscopic determinations of the relaxed Young modulus and the degree of swelling. Surface-modified silicas do not lead to percolated network structures, but still lead to the formation of 20–40% network-like material, with effective network chain lengths that depend on the surface functionalization and thus on the nanoparticle dispersion. Characteristic changes in the mobile chain end fraction with temperature, in particular its decrease with increasing degree of filling are interpreted as a consequence of altered contour-length fluctuations. An aging experiment conducted on a sample prepared by melt blending reveals the microscopic changes in the network structure occurring over many months.

Barotropic and thermotropic bilayer phase behavior of positional isomers of unsaturated mixed-chain phosphatidylcholines

The bilayer phase transitions of six kinds of mixed-chain phosphatidylcholines (PCs) with an unsaturated acyl chain in the sn-1 or sn-2 position, 1-oleoyl-2-stearoyl- (OSPC), 1-stearoyl-2-oleoyl- (SOPC), 1-oleoyl-2-palmitoyl- (OPPC), 1-palmitoyl-2-oleoyl- (POPC), 1-oleoyl-2-myristoyl- (OMPC) and 1-myristoyl-2-oleoyl- sn-glycero-3-phosphocholine (MOPC), were observed by means of differential scanning calorimetry (DSC) and high-pressure light transmittance measurements. Bilayer membranes of SOPC, POPC and MOPC with an unsaturated acyl chain in the sn-2 position exhibited only one phase transition, which was identified as the main transition between the lamellar gel (L β) and liquid crystalline (L α) phases. On the other hand, the bilayer membranes of OSPC, OPPC and OMPC with an unsaturated acyl chain in the sn-1 position exhibited not only the main transition but also a transition from the lamellar crystal (L c) to the L β (or L α) phase. The stability of their gel phases was markedly affected by pressure and chain length of the saturated acyl chain in the sn-2 position. Considering the effective chain lengths of unsaturated mixed-chain PCs, the difference in the effective chain length between the sn-1 and sn-2 acyl chains was proven to be closely related to the temperature difference of the main transition. That is, a mismatch of the effective chain length promotes a temperature difference of the main transition between the positional isomers. Anomalously small volume changes of the L c/L α transition for the OPPC and OMPC bilayers were found despite their large enthalpy changes. This behavior is attributable to the existence of a cis double bond and to significant inequivalence between the sn-1 and sn-2 acyl chains, which brings about a small volume change for chain melting due to loose chain packing, corresponding to a large partial molar volume, even in the L c phase. Further, the bilayer behavior of unsaturated mixed-chain PCs containing an unsaturated acyl chain in the sn-1 or sn-2 position was well explained by the chemical-potential diagram of a lipid in each phase.

Synthesis and characterization of poly(3-octylthiophene)/γ-Fe 2O 3 nanocomposite—A promising combination of superparamagnetic–thermoelectric–conducting properties

We report on the synthesis of poly(3-octylthiophene)/γ-Fe 2O 3 nanocomposite by realizing the transfer of maghemite nanoparticles from aqueous solution to organic media. Monomer was polymerized in non-aqueous chloroform in the presence of the dispersed iron-oxide particles. The iron-oxide content of the composite was determined by atom absorption spectroscopy (ICP-AAS) and energy dispersive X-ray microanalysis technique (EDX). The molecular structure was characterized by UV–vis spectroscopy and photoacoustic Fourier transform infrared spectroscopy (PAS-FTIR). X-ray diffraction (XRD) was used to characterize the supramolecular structure. Morphology change could be followed by scanning electron microscopy (SEM) which shows that the presence of maghemite particles during the synthesis results in a more compact structure consisting of spherical particles. The effective chain-length did not alter in comparison with the net polymer, moreover the structural change can be well correlated with the conductivity and capacity increase, determined from AC impedance spectroscopy measurements. The successful synthesis of the new composite possessing conducting, superparamagnetic and thermoelectric properties, can lead to possible applications.

A kinetic theory description of the viscosity of dense fluids consisting of chain molecules

An expression for the viscosity of a dense fluid is presented that includes the effect of molecular shape. The molecules of the fluid are approximated by chains of equal-sized, tangentially jointed, rigid spheres. It is assumed that the collision dynamics in such a fluid can be approximated by instantaneous collisions between two rigid spheres belonging to different chains. The approach is thus analogous to that of Enskog for a fluid consisting of rigid spheres. The description is developed in terms of two molecular parameters, the diameter sigma of the spherical segment and the chain length (number of segments) m. It is demonstrated that an analysis of viscosity data of a particular pure fluid alone cannot be used to obtain independently effective values of both sigma and m. Nevertheless, the chain lengths of n-alkanes are determined by assuming that the diameter of each rigid sphere making up the chain can be represented by the diameter of a methane molecule. The effective chain lengths of n-alkanes are found to increase linearly with the number C of carbon atoms present. The dependence can be approximated by a simple relationship m=1+(C-1)3. The same relationship was reported within the context of a statistical associating fluid theory equation of state treatment of the fluid, indicating that both the equilibrium thermodynamic properties and viscosity yield the same value for the chain lengths of n-alkanes.

Micro‐Raman spectroscopy of pigments contained in different calcium carbonate polymorphs from freshwater cultured pearls

AbstractThe metastable calcium carbonate polymorph vaterite has recently been found to occur commonly in freshwater cultured pearls from Japan and China. Organic pigment molecules in these vaterite regions of freshwater cultured pearls were analysed by resonance and near‐resonance micro‐Raman spectroscopy, in single spectra and in mapping modes and are compared with their aragonitic counterparts. Four different excitation wavelengths (487.9, 514.5, 532.2 and 632.8 nm) were used, resulting in differential increase of resolution for the pigments. Although vaterite areas were always lightly coloured (light yellow, light brown or white), ten different pigments were identified and a single colour is formed by a combination of three and up to nine polyenes of different lengths. The molecules were identified as unmethylated polyenes with effective chain lengths between 8 and 12 conjugated double bonds and FWHMs of 10 ± 2 cm−1. Additionally, and for the first time in pearls, methylated polyenes were detected together with unmethylated pigments in aragonite. Polyenes with chain lengths shorter than 8 or longer than 12 were not found in vaterite, whereas both shorter and longer pigment chains were observed in aragonite from the same pearls. Pigments have lower concentrations in vaterite and are distributed more homogeneously, while they have higher concentrations in aragonite and are arranged along the annual growth lines of the pearls. Copyright © 2008 John Wiley & Sons, Ltd.

Synthesis and characterization of the swelling and mechanical properties of amphiphilic ionizable model co-networks containing hydrophobic blocks.

Amphiphilic ionizable model co-networks based on near-monodisperse, linear ABA and BAB triblock and statistical copolymers of 2-(dimethylamino)ethyl methacrylate (DMAEMA, hydrophilic ionizable) and n-butyl methacrylate (BuMA, hydrophobic non-ionic) were synthesized using group-transfer polymerization in tetrahydrofuran (THF) with the use of the hydrophobic ethylene glycol dimethacrylate (EGDMA) as cross-linker. Seven model co-networks were prepared in which the architecture and copolymer composition were varied systematically. One randomly cross-linked copolymer co-network was also prepared. The co-networks were characterized in terms of their degree of swelling in water as a function of pH and in THF. An increase in the aqueous degree of swelling was observed below pH 6 because of the ionization of the DMAEMA residues. The aqueous degrees of swelling at low pH decreased with the co-network composition in hydrophobic BuMA units. The maximum aqueous degrees of swelling of the copolymer co-networks were architecture-dependent, with the co-networks comprising the statistical copolymer chains swelling about 4 times more than their triblock copolymer counterparts. This was attributed to microphase separation in the triblock copolymer co-networks, which reduced the effective chain length between cross-links due to the collapse of the hydrophobic blocks. The mechanical properties of the water-swollen co-networks at pH 3 and 9 were investigated by determining the co-network uniaxial compression modulus. This modulus was higher at pH 9 than 3, and increased linearly with the BuMA content.

Stochastic chain simulation of wall slip in entangled polymer melts

The stochastic chain model described by Xu et al. [J. Rheology 50, 477–494 (2006)] provides fundamental insight into the mechanism of apparent wall slip in entangled polymer melts. Apparent slip is shown to be a consequence of a rapid decrease in the entanglement density for chains in the region of the wall. There is good agreement between model predictions and polydimethylsiloxane experiments of Durliat et al. [Europhys. Lett. 38, 383–388 (1997)] in which a surface layer containing a known density of tethered chains contacts a bulk melt. The model cannot provide quantitative information about apparent slip when chains are simply adsorbed at the wall, since the surface density and effective chain length are required inputs to the calculation, but it does agree in broad terms with experiments of Mhetar and Archer [Macromolecules 31, 8607–8616 (1998)] on 1,4-polybutadiene.

Evaluation of New Stationary Phases for the Separation of Fatty Acid Methyl Esters

In the analysis of fatty acids, one of the most commonly used tools is a GC separation of the fatty acid methyl esters (FAME). Many researchers perform this separation using a non-polar phase such the ubiquitous 5% phenyl / 95% methyl capillary columns found in most every chromatography laboratory. Numerous laboratories have also turned recently to polar phases such as 70% cyanopropyl columns, as this type of chemistry provides increased selectivity for unsaturated compounds, and thus improved separation of cis/trans and ω3/ω6 FAME isomers. Here, a series of columns nominally having 60, 70, 80, and 90% bis-cyanopropyl content have been tested for the separation of FAME isomers. Trends in retention and the influence of increasing phase polarity on effective and fractional chain lengths are highlighted to provide the FAME chromatographer with insight into which of these novel stationary phases might be best suited to their particular application. In addition, the elution temperatures (Te) of the FAME and linear alkane standards are presented, as this information will be of value to comprehensive two-dimensional multidimensional GC (GC × GC) users who wish to use these columns in the primary dimension separation.

Calculation of hydrophile–lipophile balance for polyethoxylated surfactants by group contribution method

Based on Davies' group contribution method, hydrophile–lipophile balance (HLB) values were calculated by effective chain length (ECL) instead of actual chain length of straight alkyl chain, polyoxyethylene (EO) chain, and polyoxypropylene (PO) chain. Linear equations were adopted for the effective chain length of straight alkyl chain and PO chain; a Gamma probability density function (PDF) was used to describe the contribution of each EO group to HLB and the effective chain length of EO chain could be obtained by integrating Gamma PDF. The HLB values of 224 nonionic surfactants were calculated and the average absolute error is less than 1.5, which is much better than the results obtained by Davies.

Studies on urethane–allophanate networks based on hydroxyl-terminated polybutadiene: Modeling of network parameters and correlation to mechanical properties

Hydroxyl-terminated polybutadiene (HTPB)-based allophanate–urethane networks were prepared by reacting HTPB with di-isocyanates, such as toluene–di-isocyanate (TDI), isophorone–di-isocyanate (IPDI), and 4,4′-di(socyanatocyclohexyl)methane (H12MDI) at stoichiometric ratios (r-values) ranging from 1.0 to 1.5. The networks were characterized for mechanical and swell properties. The network parameters, such as “X,” which is the fraction of urethane groups involved in the allophanate formation, and effective chain length (Lx) were calculated from experimental crosslink density values determined from swell data, using α-model equations developed by Marsh. Excellent linear correlations were obtained between mechanical properties and the calculated network parameters. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 101: 2986–2994, 2006

Effect of initial monomer concentration on the equilibrium swelling and elasticity of hydrogels

The linear swelling ratio α and the effective network chain length N of a series of poly( N, N-dimethylacrylamide) (PDMAAm) hydrogels were investigated as a function of the gel preparation concentration ν 2 0 . PDMAAm hydrogels were prepared at a fixed cross-linker ratio but at various initial monomer concentrations. It was found that α is not a monotonic function of ν 2 0 . As ν 2 0 is increased, α first decreases up to about ν 2 0 = 0.10 and remains constant in a narrow range of ν 2 0 , but then it increases continuously. The ν 2 0 -dependence of α is due to the variation of the network chain length N depending on the gel preparation concentration. In the range of ν 2 0 below 0.1, N follows the scaling relationship N ≈ ν 2 0 - 2 , while at higher concentrations, N varies only slightly with ν 2 0 . The increase of α with N obeys the relation α ≈ ν 2 0 N 1 / 5 , as predicted by the Flory–Rehner theory.

Growth inhibition and apoptosis induction of human melanoma cells by ??-hydroxy fatty acids

We examined the anti-tumor activity and structure-activity requirements of omega-hydroxy fatty acids (omega-HFAs) on the human melanoma cell line G361. The omega-hydroxystearic acid (omega-HSA) had strong growth-inhibiting and cytotoxic activity. Although omega-hydroxypalmitic acid (omega-HPA) also had growth-inhibiting and cytotoxic activity, these effects were relatively low. The effects of both these acids were dose and time dependent. Further, DNA laddering, which is an index of apoptosis, was also observed in G361 cells on treatment with these compounds. On the other hand, the omega-HFAs tested in this study, omega-hydroxymyristic acid and omega-hydroxyeicosanoic acid, had no growth-inhibiting or cytotoxic activity. Treatment for 12 h with 100 microM of omega-HPA and omega-HSA resulted in the expression of caspase-3 activity, and then increased upon 24 h, suggesting that the cell death induced by omega-HPA and omega-HSA was apoptosis. Fatty acids and dicarboxylic acids, which are analogs of omega-HFAs, had no cytotoxicity. However, fatty alcohols and diols, which have a 16- to 18-carbon chain length had weak cytotoxicity. From these results, the most effective carbon chain length is 18. Furthermore, the hydroxyl group at one end of the carbon chain and the carboxyl group at the other end seem to be required for the cytotoxic effect. At least one end of the carbon chain must have a hydroxyl group. The carbon chain length of omega-HFAs appears to be closely related to the cytotoxicity. This study revealed the potent cytotoxic actions of omega-HFAs on the human melanoma cell line G361.

Artificial multiple criticality and phase equilibria: an investigation of the PC-SAFT approach

The perturbed-chain statistical associating fluid theory (PC-SAFT) is studied for a wide range of temperature, T, pressure, p, and (effective) chain length, m, to establish the generic phase diagram of polymers according to this theory. In addition to the expected gas-liquid coexistence, two additional phase separations are found, termed "gas-gas" equilibrium (at very low densities) and "liquid-liquid" equilibrium (at densities where the system is expected to be solid already). These phase separations imply that in one-component polymer systems three critical points occur, as well as equilibria of three fluid phases at triple points. However, Monte Carlo simulations of the corresponding system yield no trace of the gas-gas and liquid-liquid equilibria, and we conclude that the latter are just artefacts of the PC-SAFT approach. Using PC-SAFT to correlate data for polybutadiene melts, we suggest that discrepancies in modelling the polymer density at ambient temperature and high pressure can be related to the presumably artificial liquid-liquid phase separation at lower temperatures. Thus, particular care is needed in engineering applications of the PC-SAFT theory that aims at predicting properties of macromolecular materials.