Gel-sol transition in gellan aqueous solutions

Gel-sol transition of sodium type gellan solutions with and without salts is studied by dynamic viscoelastic measurements and differential scanning calorimetry (DSC). Mechanical spectra show that gellan aqueous solutions behave as an entangled polymer solution in the concentration range around 2 wt.-% at temperatures >15°C and as a weak gel below this temperature. Concentrated solutions (> 3 wt.-%) show a true gel behavior below 30°C. The two step transition is observed for 2∼3 wt.-% gellan aqueous solutions in thermal scanning rheological (TSR) measurements; the transition at a higher temperature is attributed to a coil-helix transition whilst the transition at a lower temperature is attributed to sol-gel transition. The transition observed in dilute solutions of gellan is attributed to the coil-helix transition whilst the sol-gel transition occurs simultaneously with coil-helix transition in more concentrated solutions (>3 wt.-%). The sol-gel transition temperature shifts to higher temperatures with increasing concentration of the added salts. Junction zones formed in the presence of divalent cations are far more heat resistant than those with monovalent cations judging from both DSC and TSR, however, the possibility of the formation of junction zones by covalent bonds or by ionic bonds is excluded.     

Synthesis and Self‐assembly of a Helical Polymer Grafted from a Foldable Polyurethane Scaffold

Herein a polyurethane graft poly‐l ‐glutamate amphiphilic copolymer was synthesized from a polyurethane (PU)‐based macro‐initiator (containing pendant primary amine groups) through the ring opening polymerization of N‐carboxy anhydride of γ‐benzyl‐l ‐glutamate ( BLG‐NCA ). On average, twenty two l ‐glutamic acids were grafted from each amino group which was pendant on the polyurethane chain with 10 repeating units. The grafted polymer ( PU‐PP‐1 ) exhibits self‐assembly to produce a hydrogel in a wide pH window ranging from pH 5.0 to 8.0 with a critical gelation concentration (CGC) of 5.0 wt % (w/v) at pH 7.4. Furthermore, circular dichroism study revealed the transition of the α‐helix to a random coil upon increasing the pH. Due to the protonation of side chains at pH 4.0, PU‐PP‐1 adopted an α‐helical conformation whereas at pH >8.0 the side‐chain carboxylic acid groups of the PLGAs were ionized, leading to the formation of an extended random coil conformation as a result of charge repulsion. Conformational switching was also supported by FTIR spectroscopy.     

NMR observations of drawn polymers. V. Sorption into drawn and undrawn polyethylene

NMR measurements on undrawn polyethylene (PE) samples in contact with a solvent such as C₂Cl₄ indicate an increase in the mobility of the mobile chain segments as compared to dry samples. Highly drawn PE shows no such effect. This is because S_a, the sorption per unit mass of noncrystalline material present, decreases from 20.9 wt.-% (dry basis), found for undrawn quenched PE, to 0.63 wt.-% after drawing (S_a determined at 25°C. and 0.80 vapor activity). Drawing also reduces the segment mobility according to the NMR spectrum. It is shown that these effects are caused by considerable structural changes occurring in the noncrystalline regions of PE upon drawing. Annealing of drawn PE samples at successively higher temperatures leads to a gradual relaxation of the noncrystalline regions towards the state characteristic of undrawn PE. With increasing annealing temperature S_a as well as the mobility approach values found with undrawn PE.     

Another way to view the chain conformation broadening of the line-width distribution measured in dynamic light scattering

In dynamic laser light scattering (LLS), for a given polydisperse sample, a line-width distribution G(Γ) or the translational diffusion coefficient distribution G(D) can be obtained from the measured time correlation function. For rigid colloid particles, G(Γ) can be directly related to the hydrodynamic size distribution. However, for flexible polymer chains, G(Γ) depends not only on the chain length distribution, but also on the relaxation of the chain conformation; that is, even for a monodisperse polymer sample there still exists a chain conformation distribution. If the time scale of the chain conformation relaxation is comparable to that of the translational diffusion, such as in the case of a very long polymer chain, the conformation relaxation might lead to an additional broadening in G (Γ). This "conformation broadening" has been directly observed for the first time by comparing two G(Γ) s obtained from a poly(N-isopropyl-acrylamide) solution at～25℃ and～32℃ at which the solution is ther     

Folding kinetics of a naturally occurring helical peptide: implication of the folding speed limit of helical proteins

The folding mechanism and dynamics of a helical protein may strongly depend on how quickly its constituent alpha-helices can fold independently. Thus, our understanding of the protein folding problem may be greatly enhanced by a systematic survey of the folding rates of individual alpha-helical segments derived from their parent proteins. As a first step, we have studied the relaxation kinetics of the central helix (L9:41-74) of the ribosomal protein L9 from the bacterium Bacillus stearothermophilus , in response to a temperature-jump ( T-jump) using infrared spectroscopy. L9:41-74 has been shown to exhibit unusually high helicity in aqueous solution due to a series of side chain-side chain interactions, most of which are electrostatic in nature, while still remaining monomeric over a wide concentration range. Thus, this peptide represents an excellent model system not only for examining how the folding rate of naturally occurring helices differs from that of the widely studied alanine-based peptides, but also for estimating the folding speed limit of (small) helical proteins. Our results show that the T-jump induced relaxation rate of L9:41-74 is significantly slower than that of alanine-based peptides. For example, at 11 degrees C its relaxation time constant is about 2 micros, roughly seven times slower than that of SPE(5), an alanine-rich peptide of similar chain length. In addition, our results show that the folding rate of a truncated version of L9:41-74 is even slower. Taken together, these results suggest that individual alpha-helical segments in proteins may fold on a time scale that is significantly slower than the folding time of alanine-based peptides. Furthermore, we argue that the relaxation rate of L9:41-74 measured between 8 and 45 degrees C provides a realistic estimate of the ultimate folding rate of (small) helical proteins over this temperature range.     

Dynamic viscosity of dilute solutions of poly(L-glutamic acid)

The dynamic viscosity η′ of a dilute solution of poly(L-glutamic acid) (DP = 1370) in a mixed solvent made up of aqueous 0.2M NaCl and dioxane (2:1 by volume) is measured over the pH range 4.2–10 and in the frequency range 2–500 kHz. The frequency dependence of η′ in the helix region (low pH) is interpreted in terms of a model molecule consisting of n rigid helical segments connected by universal joints. The steady-flow viscosity, relaxation time, and high-frequency limiting viscosity at pH 4.75 (helical content 80%) are well explained by this model with n = 5. This value of n is consistent with that estimated from the nucleation parameter σ = 1.4 × 10^?3 obtained from the relation between reduced steady-flow viscosity and helical content. The high-frequency values of η′ in the coil region (high pH) are fitted by Peterlin's theory. The internal viscosity seems to arise in part from the polyelectrolytic character of the molecule. An additional relaxation at low frequencies in the coil region is ascribed to rotation of molecules elongated by the electrostatic interaction. The lower value of reduced steady-flow viscosity in the coil region in the mixed solvent compared with that in water is interpreted in terms of the lower degree of effective ionization and the selective solvation of water by the polypeptide. No anomaly is observed in the helix–coil transition region, indicating that the relaxation time for helix–coil equilibrium is less than 10^?6sec.     

Physical properties of xanthan,galactomannan and their mixtures in aqueous solutions

Some new information on the conformation of xanthan in aqueous solutions is given. A single helical chain conformation characterizes xanthan in the native state. When heated over the critical temperature for conformational change ( helixŕ coil), the xanthan is denatured and renatured when it is cooled down in a localy double helical structure. A galactomannan was also characterized and its persistence length was obtained (Lp ≈︁ 90A°). Then mixtures of the galactomannan and xanthan were investigated to propose a mechanism for the specific gelation. From the results of microcalorimetry and circular dichroism, it is concluded that a complex is formed between one disordered xanthan chain and one galactomannan chain and that an ordered conformation is stabilized at temperatures lower than 25°C when the galactomannan has a M/G ratio of ≈︁ 3. This temperature corresponds to the sol-gel transition. This is the first time that a structure of the crosslink points is demonstrated.     

Non-Newtonian flow behaviour of gellan gum aqueous solutions

Rheological properties of gellan gum solutions with and without salt have been monitored using oscillatory measurements and steady-shear viscosity measurements. The steady-shear viscosity measurements indicated that gellan gum solutions showed a wide Newtonian plateau when gellan gum molecules took a coil conformation, and that the shear-thinning behaviour became more conspicuous with conformational change of gellan gum molecules from coiled to helical, and the range of the Newtonian plateau became limited only to very low shear rates. When gellan gum solutions showed rheological behaviour as a dilute or concentrated polymer solution, these systems obeyed the Cox–Merz superposition of steady-state viscosity and dynamic viscosity. As gellan gum solutions formed a weak gel, the Cox–Merz rule was not valid; however, the deviation from this superposition was less significant than that of xanthan gum solutions which also show weak-gel behaviour. Received: 8 December 1998 Accepted in revised form: 5 March 1999     

Molecular dynamics simulations of polyelectrolyte-polyampholyte complexes. Effect of solvent quality and salt concentration

Complexation between polyelectrolyte and polyampholyte chains in poor solvent conditions for the polyelectrolyte backbone has been studied by molecular dynamics simulations. In a poor solvent a polyelectrolyte forms a necklace-like structure consisting of polymeric globules (beads) connected by strings of monomers. The simulation results can be explained by assuming the existence of two different mechanisms leading to the necklace formation. In the case of weak electrostatic interactions, the necklace formation is driven by optimization of short-range monomer-monomer attraction and electrostatic repulsion between charged monomers on the polymer backbone. In the case of strong electrostatic interactions, the necklace structure appears as a result of counterion condensation. While the short-range attractions between monomers are still important, the correlation-induced attraction between condensed counterions and charged monomers and electrostatic repulsion between uncompensated charges provide significant contribution to optimization of the necklace structure. Upon forming a complex with both random and diblock polyampholytes, a polyelectrolyte chain changes its necklace conformation by forming one huge bead. The collapse of the polyelectrolyte chain occurs due to the neutralization of the polyelectrolyte charge by polyampholytes. In the case of the random polyampholyte, the more positively charged sections of the chain mix with negatively charged polyelectrolyte forming the globular bead while more negatively charged chain sections form loops surrounding the collapsed core of the aggregate. In the case of diblock polyampholyte, the positively charged block, a part of the negatively charged block, and a polyelectrolyte chain form a core of the aggregate with a substantial section of the negatively charged block sticking out from the collapsed core of the aggregate. In both cases the core of the aggregate has a layered structure that is characterized by the variations in the excess of concentration of monomers belonging to polyampholyte and polyelectrolyte chains throughout the core radius. These structures appear as a result of optimization of the net electrostatic energy of the complex and short-range attractive interactions between monomers of the polyelectrolyte chain.     

Synthesis and characterization of multiblock copolyesters containing poly(dimethylsiloxane) in the soft segments

Synthesis and thermal properties of poly(aliphatic/aromatic-ester) copolymers containing additionally poly(dimethylsiloxane) (PDMS) chains in the soft segments are discussed. A two step method of transesterification and polycondensation from the melt was carried out in a presence of magnesium-titanate catalyst. An aliphatic dimer fatty acid was used as a component of the soft segments while poly(butylene terephthalate) (PBT) constituted the hard blocks. Effectiveness of the incorporation of PDMS into polymer chain was confirmed by the Soxhlet extraction and infrared spectroscopy of an excess of 1,4-butane diol destilled off from the polycondensation reaction. Multiblock copolymers showed microphase separation as determined by differential scanning calorimetry. Incorporation of a small amount of PDMS (up to 14.5 wt.-%) into polymer chain containg low concentration of hard segments of PBT lead to decrease in crystallinity of such copolymers. This may indicate that semicrystalline PBT are dissolved in the amorphous matrix of the soft segments.     

Halide-guided oligo(aryl-triazole-amide)s foldamers: receptors for multiple halide ions

We synthesized and characterized a series of oligo(phenyl-amide-triazole)s that can fold into a helical conformation guided by halide ions. Their binding models and affinities are highly dependent on the length of the foldamer, media and the inducing capability of halide ions. The short foldamer with one helical turn shows a 1:1 binding stoichiometry to all halides, while the longer foldamer with two or three helical turns in principle can form 1:2 complexes with chloride anions even bromide anions with an enhancement on binding affinities. A result of quantitative NOE calculations imply that the longer foldamer should increase its helical pitch so as to release the electrostatic repulsion between halide ions.     

Shear and extensional rheology of solutions of mixtures of poly(ethylene oxide) and anionic surfactants in ionic environments

Interactions between a high molecular weight poly(ethylene oxide) (PEO) and the anionic surfactant sodium dodecyl benzene sulfonate (SDBS) in aqueous solutions were investigated by shear and extensional rheometry. Results for mixtures between PEO and sodium dodecyl sulfate (SDS) are also presented for comparison purposes. Addition of anionic surfactants to PEO solutions above the critical aggregation concentration (CAC), at which micellar aggregates attach to the polymer chain, results in an increase in shear viscosity due to PEO coil expansion, and a strengthening of interchain interactions. In extensional flows, these interactions result in a decrease of the critical shear rate for the onset of the characteristic extension thickening of the PEO solutions that is due to transient entanglements of polymer molecules. The relaxation times associated with these transient entanglements are not directly proportional to the shear viscosity of the solutions, but rather vary more rapidly with surfactant concentration. In the presence of an electrolyte, coil contraction results in lower shear viscosities and a decrease in the extension thickening effects at surfactant concentrations just beyond the CAC. The relaxation times associated with transient entanglement reach a minimum at the same surfactant concentration as the shear viscosity, which indicates that coil contraction is responsible for the observed effects in both types of flow. However, the increase in extensional-flow entanglement relaxation times is much more abrupt than the decrease in shear viscosity. All these results point to a greater sensitivity of extensional flows on the molecular conformation of PEO/surfactant complexes.     

MOLECULAR DYNAMICS SIMULATION OF THE RELAXATION OF A FULLY EXTENDED POLYETHYLENE CHAIN

Molecular dynamics simulation of the relaxation at 300 K of a fully extended polyethylene chain of800 CH_2 units has been carried out by following the changes in morphology, van der Waals energy, radius ofgyration in the sense of mechanics and gyration radius in the sense of Flory, population of trans-conformation and orientation factor. The relaxation went through three stages: (1) relaxation from themorphology of a straight rod of 100 nm length to the molphology close to a random coil of gyration radius5.9 nm in 110 ps; (2) collapse of the morphology of a coil to a highly compact globule close to a sphere ofgyration radius 1.3 nm after 178 ps as the result of intersegmental van der Waals attractive interactions; (3)lateral ordering of the folded chain segments in the globule without appreciable changes in the chaindimension up to 1600 ps, the time limit of present simulation. Nearly complete relaxation of local segmentalorientation was performed much faster than the relaxation of globule chain orientation even for a single chainof low degree of polymerization and at a temperature some 155℃above its T_g. The lateral ordering of thechain segments during the period 178 to 680 ps of the simulation time was found to obey the Avramiequation with an Avrami index of 1 .44.     

The influence of chain length and ripening time on the self-assembly of alkylamines on mica

The influence of chain length and ripening time on the self-assembly of tetradecyl (C(14)), hexadecyl (C(16)), and octadecylamine (C(16)) on mica has been studied by atomic force microscopy. The overall process can be described in three stages characterized by different time scales. First, alkylamine molecules adsorb in a process controlled by water mediated interactions of the NH(2) head groups and mica. Second, surface diffusion and aggregation into islands takes place, driven by energy interactions between alkyl chains. The third stage consists of a progressive tilt of the self-assembled molecules towards the surface, driven by relaxation of the electrostatic repulsion between protonated amino groups due to water uptake from atmosphere.     

Conformational ordering of apolar, chiral m-phenylene ethynylene oligomers

A series of m-phenylene ethynylene oligomers containing nonpolar, (S)-3,7-dimethyl-1-octanoxy side chains have been synthesized and studied. In apolar alkane solvents, oligomers of sufficient length (n > 10) were found to adopt a helical conformation with a large twist sense bias. In contrast, in chloroform the oligomers adopt a random coil conformation. Surprisingly, the strong twist sense bias was determined to be highly time dependent and is partially attributed to intermolecular aggregation.     

Molecular dynamics simulation of swollen membrane of perfluorinated ionomer

Molecular dynamics simulations of the swollen membrane of perfluorinated ionomer, which is composed of poly(tetrafluoroethylene) backbones and perfluosulfonic pendant side chains, have been undertaken to analyze the static and dynamic properties of the water and the side chain in the membrane. The calculations were carried out for four different water contents, 5, 10, 20 and 40 wt %, at 358.15 K and 0.1 MPa. The results are summarized as follows: (1) The sulfonic acid is the unique site to which water molecules can bind, and the other sites in the pendant side chain have no bound water even at high water concentration. (2) Sulfonic acids aggregate in the short range within 4.6-7.7 A despite the electrostatic repulsion between them. In such aggregates, a water molecule bridges two sulfonic acids. (3) Pendant side chains prefer to orient perpendicular to the hydrophilic/hydrophobic interface, and long-range correlation of side chain orientations is observed at 20 and 40 wt % water uptake membranes. (4) In a low water uptake membrane, the dynamics of water is substantially restricted due to strong attractive interactions with acidic sites. In contrast, at high water content, even the water locating near the sulfonic acid is relatively mobile. The short residence time of the bound water reveals that such water can frequently exchange position with relatively free water, which locates in the center of water cluster, in highly swollen membranes.     

Helix-turn-helix motifs in unsolvated peptides

The conformations of unsolvated Ac-A14KG3A14K + 2H+ (Ac = acetyl, A = alanine, K = lysine, G = glycine) have been examined by ion mobility measurements and molecular dynamics simulations. This peptide was designed as a model helix-turn-helix motif. It was found to adopt three distinct geometries which were assigned to an extended helical conformation which is only stable at low temperatures (<230 K), a relatively high energy but metastable structure with exchanged lysines, and a coiled-coil. The coiled coil (which consists of an antiparallel arrangement of two helical alanine sections linked by a flexible glycine loop) is the dominant conformation. For temperatures >350 K, the experimental results indicate the helices uncouple and the loop randomizes. From equilibrium constants determined for this helix coupling right arrow over left arrow uncoupling transition, we found DeltaH degrees = -45 kJ mol-1 and DeltaS degrees = 114 J K-1 mol-1. -DeltaH degrees is essentially the enthalpy change for docking the two helices together while DeltaS degrees is essentially the entropy change for freeing up the glycine loop.     

Atomic force microscopy study of the adsorption and electrostatic self-organization of poly(amidoamine) dendrimers on mica

The adsorption behavior of poly(amidoamine) dendrimers to mica surfaces was investigated as a function of ionic strength and pH. The conformation and lateral distribution of the adsorbed dendrimers of generations G8 and G10 were obtained ex situ by tapping mode atomic force microscopy (AFM). The deposition kinetics of the dendrimers was found to follow a diffusion-limited process. Fractional surface coverage and pair correlation functions of the adsorbed dendrimers were obtained from the AFM images. The data are interpreted in terms of the random sequential adsorption (RSA) model, where electrostatic repulsion due to overlapping double layers is considered. Although the general trends typical for an RSA-determined process are well-reproduced, quantitative agreement is lacking at low ionic strengths.     

Thermomechanical study of low-density polyethylene, polyamide 6 and its blends

By using thermomechanical analysis (TMA) multiple relaxation transitions in the amorphous part of semi-crystalline polymers and their blends can be found. These result from differences in the interaction energies between segments of macromolecules, and as a result, in molecular mobility. TMA shows essential changes in the structure of low-density polyethylene (LDPE) resulting from the grafting of a maleic anhydride (LDPE-g-MAH) onto this semi-crystalline polymer. The grafting process did not suppress the ability of polyethylene to crystallize. Essential changes were found in molecular weight distribution and relaxation transitions of the ternary blends LDPE/PA6 (polyamide 6, PA6)/LDPE-g-MAH studied. For a concentration of PA6 up to 30 wt.%, a single relaxation transition is visible, which testifies that the components are miscible in the amorphous region. For blends with 40 and 50 wt.% of PA6, this structure was transformed and two relaxation transitions are visible. From the results obtained in this study it is concluded that the introduction 5 wt.% of grafted polyethylene is sufficient to produce a PA6/LDPE blend only for PA6 concentration up to 30 wt.% which is homogenous on “molecular” level. The transformation of the structure of the ternary polymer blend was explained by the interaction of the components during the melt mixing and changes in the structure of its amorphous regions.