Unentangled Regime Research Articles

Single Polymer Dynamics in A Random Flow

SummaryThe dynamics and conformations of a single fluorescently stained T4 DNA molecule are studied in a random flow of elastic turbulence created by the same unlabeled molecules. The explored polymer concentration covers the dilute and semi‐dilute unentangled regimes, according to the measurement of the longest relaxation time by extension relaxation of single polymer chains. The criterion of the coil‐stretch transition was found to be close to the theoretically predicted value. Using measured polymer stretching statistics and its known elastic properties, the elastic stress in elastic turbulence is obtained over a broad range of Weissenberg number and polymer concentration. The existing theory of elastic turbulence is disproved by the measurements of the elastic stress and the degree of polymer stretching. The role of increasing shear rate on polymer extension and angular statistics in a random flow is also studied and compared with theory and numerical simulations.

Near Superhydrophobic Fibrous Scaffold for Endothelialization: Fabrication, Characterization and Cellular Activities

In this work, we have applied an electrospinning method to control wettability and further hydrophobic modification of a hydrophobic polymer mat of poly(vinylidene fluoride-co-hexafluoropropylene). A correlation between the processing parameters, rheological properties of polymer solutions, and electrospinning ability was made using the polymer's critical entanglement concentration, the boundary between the semidilute unentangled regime and the semidilute entangled regime. The wetting behavior, structural and thermal characteristics of electrospun (ES) mats were evaluated and compared with solvent cast sample using advancing and receding contact angle analyses, differential scanning calorimetry, and small-angle X-ray scattering. To demonstrate the feasibility, the best optimized ES samples were examined for their potential and ability to support bone marrow derived endothelial cell seeding efficiency, adhesion and proliferation. Our studies show that, while different processing techniques can effectively modulate physical and morphological changes such as porosity and hydrophobicity, the cellular adhesion and proliferation are highly time-dependent and controlled by chemical factors. As such, these results suggest that it is the interplay of both physical and chemical factors that determine the endothelialization of porous near superhydrophobic scaffolds. The developed electrospun samples demonstrate their feasibility for endothelialization.

Viscosity of ring polymer melts.

We have measured the linear rheology of critically purified ring polyisoprenes, polystyrenes and polyethyleneoxides of different molar masses. The ratio of the zero-shear viscosities of linear polymer melts η0,linear to their ring counterparts η0,ring at isofrictional conditions is discussed as function of the number of entanglements Z. In the unentangled regime η0,linear/η0,ring is virtually constant, consistent with the earlier data, atomistic simulations, and the theoretical expectation η0,linear/η0,ring=2. In the entanglement regime, the Z-dependence of rings viscosity is much weaker than that of linear polymers, in qualitative agreement with predictions from scaling theory and simulations. The power-law extracted from the available experimental data in the rather limited range 1<Z<20, η0,linear/η0,ring ~ Z1.2±0.3, is weaker than the scaling prediction (η0,linear/η0,ring ~ Z1.6±0.3 ) and the simulations (η0,linear/η0,ring ~ Z2.0±0.3 ). Nevertheless, the present collection of state-of-the- art experimental data unambiguously demonstrates that rings exhibit a universal trend clearly departing from that of their linear counterparts, and hence it represents a major step toward resolving a 30-year old problem.

Flow behavior and linear viscoelasticity of cellulose 1-allyl-3-methylimidazolium formate solutions

The rheological properties of α-cellulose 1-allyl-3-methylimidazolium formate solutions were investigated using shear viscosity and dynamic rheological measurements in a large range of concentrations (0.1–10wt%) at 25°C. In steady shear measurement, the overlap concentration (c*) and the entanglement concentration (ce) were determined to be 0.5 and 2.0wt% respectively, and the exponents of the specific viscosity (ηsp) versus the concentration (c) were determined as 1.0, 2.0 and 4.7 for dilute, semidilute unentangled and entangled regimes respectively, which were in accordance with the scaling prediction for neutral polymer in θ solvent. The slopes of the relaxation time (τ) against the concentration for semidilute unentangled and entangled regimes were observed as 1.0 and 2.5 respectively. In dilute and semidilute unentangled regimes, failure of the Cox-Merz rule with steady shear viscosity larger than complex viscosity was observed; while the deviation from the Cox-Merz rule disappeared in semidilute entangled regime.

Influence of hydrophobe fraction content on the rheological properties of hydrosoluble associative polymers obtained by micellar polymerization

AbstractThe synthesis, characterization and rheological properties in aqueous solutions of water-soluble associative polymers (AP’s) are reported. Polymer chains consisting of water-soluble polyacrylamides, hydrophobically modified with low amounts of N,N-dihexylacrylamide (1, 2, 3 and 4 mol%) were prepared via free radical micellar polymerization. The properties of these polymers, with respect to the concentration of hydrophobic groups, using steady-flow and oscillatory experiments were compared. An increase of relaxation time (TR) and modulus plateau (G0) was observed in all samples studied. Two different regimes can be clearly distinguished: a first unentangled regime where the viscosity increase rate strongly depends on hydrophobic content and a second entangled regime where the viscosity follows a scaling behavior of the polymer concentration with an exponent close to 4.

Behavior of Hydrophobic Polyelectrolyte Solution in Mixed Aqueous/Organic Solvents Revealed by Neutron Scattering and Viscosimetry

We investigate in this paper the influence of the improvement of the solvent quality on the structure and the viscous properties of solutions of an hydrophobic polyelectrolyte, poly(styrene-co-sodium styrenesulfonate): PSS. The solvent used is a mixture of water and an organic solvent, THF, which is also slightly polar. We use small angle neutron scattering in the semidilute regime and viscosimetry as a function of concentration in dilute and semidilute unentangled regime. The structure, namely the scattering from all chains, is characterized by a maximum ("polyelectrolyte peak"). Its position and amplitude depends, at a given sulfonation rate of PSS, on the solvent quality through the added amount of organic solvent (THF). These evolutions with the THF amount are more pronounced when the sulfonation rate f is low (more hydrophobic polyelectrolyte) and the amount of added THF is high. Adding THF to hydrophobic PSS (f = 0.50 or f = 0.38), diminishes also the "shoulder" visible in the log I - log q plot and associated with the pearl size. It is therefore proposed that when THF is added to aqueous polyelectrolyte solutions, the pearls are dissolved and the chain conformation evolves from the pearl-necklace shape already reported in pure water toward the string-like conformation in pure water for fully sulfonated PSS. An addition of THF also reduces the important low q upturn found with hydrophobic polyelectrolyte solutions: the large aggregates are dissolved by THF. The upturn can become for PSSNa f = 0.38, after adding enough THF (50%), even smaller than that for the charged hydrophilic case PSSNa f = 0.82, in water. This can mean that in the quasi-fully charged PSS at f = 0.82 there are still hydrophobic effects in water, which is disagreeing with our recent reports, or that the electrostatics contribution to the upturn is reduced due to a lower dielectric permittivity. Concerning the hydrophilic polyelectrolyte, poly(sodium-2-acrylamido-2-methylpropanesulfonate)-co-(acrylamide): AMAMPS, no evolution in structure occurs until 25% THF. The viscosimetry variation with THF fraction is in good agreement with the scattering one up to 25%: though little dependent on THF for AMAMPS, and for hydrophilic PSSNa, it increases for hydrophobic PSSNa in agreement with the chain expansion signaled by scattering. At 50% THF concentration, the hydrophilic polyelectrolyte shows new surprising behaviors: the scattering of PSSNa is no longer characteristic of polyelectrolytes, and AMAMPS solutions display an unexpected viscosity decrease.

Role of Molecular Entanglements in Starch Fiber Formation by Electrospinning

We have demonstrated a method of fabricating pure starch fibers with an average diameter in the order of micrometers. In the present study, correlation between the rheological properties of starch dispersions and the electrospinnability was attempted via the extrapolation of the critical entanglement concentration, which is the boundary between the semidilute unentangled regime and the semidilute entangled regime. Dispersions of high amylose starch containing nominally 80% amylose (Gelose 80) required 1.2-2.7 times the entanglement concentration for effective electrospinning. Besides starch concentration, molecular conformation, and shear viscosity were also of importance in determining the electrospinnability. The rheological properties and electrospinnability of different starches were studied. Hylon VII and Hylon V starches, containing nominally 70 and 50% amylose, respectively, required concentrations of 1.9 and 3.7 times their entanglement concentrations for electrospinning. Only poor fibers were obtained from mung bean starch, which contains about 35% amylose, while starches with even lower amylose contents could not be electrospun.

Rheological properties of cellulose/ionic liquid/dimethylsulfoxide (DMSO) solutions

Rheological properties of cellulose dissolved in two ionic liquids (ILs), 1-allyl-3-methylimidazolium chloride (AmimCl) and 1-butyl-3-methylimidazolium chloride (BmimCl), with co-solvent dimethylsulfoxide (DMSO), are studied in the concentration range of cellulose from 0.070 to 6.0 wt%. The viscosities of ILs are exponentially decreased by adding DMSO in the concentration range of 0–100 wt%. The co-solvent DMSO decreases the monomer friction coefficient in cellulose solutions and has no significant change for the entanglement state of cellulose, thus results in the reduced solution viscosity, shortened relaxation time and unchanged moduli of the cross-over point. For cellulose solutions, dilute regime, semidilute unentangled regime and semidilute entangled regime were determined by steady shear experiments. In semidilute entangled regime, the specific viscosities ηsp, relaxation time τ, and plateau modulus GN, exhibit concentration dependences as ηsp ∼ C4.4, τ ∼ C2.2, andGN ∼ C1.9, respectively, in AmimCl-DMSO (80/20 w/w); and ηsp ∼ C4.3, τ ∼ C2.0, and GN ∼ C2.1, respectively, in BmimCl–DMSO (80/20 w/w). Therefore, the rheological properties of cellulose/IL/DMSO solutions are approximately of IL-independence in this study. The dependence of ηsp upon cellulose concentration shows that the IL–DMSO mixture is more like a θ solvent for cellulose, and the thermodynamic properties of IL–DMSO mixtures are similar with those of ILs for cellulose at 25 °C. The conformation of cellulose in ILs would not be changed with the addition of DMSO not only in the dilute regime but also in the entanglement regime.

Synthesis and solution rheology of adenine-containing polyelectrolytes for electrospinning

Conventional free radical copolymerization of 9-vinylbenzyladenine (VBA) and 2-(dimethylamino)ethyl methacrylate (DMAEMA) with subsequent protonation afforded the synthesis of adenine-containing polyelectrolytes. All adenine-containing polyelectrolytes exhibited classical polyelectrolyte solution rheological behavior with scaling factors near 0.6 and 1.6 in the semidilute unentangled and semidilute entangled regimes, respectively. However, the adenine-containing polyelectrolytes deviated from polyelectrolyte behavior in the concentrated regime with increasing scaling factors as adenine-incorporation increased due to intermolecular association. The electrospinning behavior exhibited a strong dependence on adenine incorporation. Higher adenine-incorporation decreased the normalized concentration for fiber formation from 4.5Ce for PDMAEMA•HCl to 2.9Ce for 35 mol% VBA. The required zero-shear viscosities for electrospinning were 312 cP for PDMAEMA•HCl and 116 cP for the 35 mol% VBA copolymer. Increasing the adenine concentration also increased the fiber diameters presumably due to adenine–adenine interactions. These adenine-decorated electrospun mats exhibit potential in a variety of applications including filtration, purification, and tissue scaffolding.

Weak Temperature Dependence of Structure in Hydrophobic Polyelectrolyte Aqueous Solution (PSSNa): Correlation between Scattering and Viscosity

We study in this paper, either in the semidilute regime by small angle scattering or in dilute and semidilute unentangled regimes by viscosimetric measurements, the influence of temperature on the structure of aqueous solutions of an hydrophobic polyelectrolyte, poly(styrene-co-sodium styrene sulfonate). There is a strong consistency between the two methods: the very weak dependence over temperature at high chemical charge is increased for lower charge fractions, i.e., when hydrophobicity is increased. The solvent quality is thus modified, whereas, for comparison, the hydrophilic polyelectrolyte poly(acrylamide-co-sodium-2-acrylamido-2-methylpropane sulfonate), shows a structure totally independent of temperature, for all the charge fractions.

Multiple Dynamic Processes Contribute to the Complex Steady Shear Behavior of Cross-Linked Supramolecular Networks of Semidilute Entangled Polymer Solutions

Molecular theories of shear thickening and shear thinning in associative polymer networks are typically united in that they involve a single kinetic parameter that describes the network -- a relaxation time that is related to the lifetime of the associative bonds. Here we report the steady-shear behavior of two structurally identical metallo-supramolecular polymer networks, for which single-relaxation parameter models break down in dramatic fashion. The networks are formed by the addition of reversible cross-linkers to semidilute entangled solutions of PVP in DMSO, and they differ only in the lifetime of the reversible cross-links. Shear thickening is observed for cross-linkers that have a slower dissociation rate (17 s(-1)), while shear thinning is observed for samples that have a faster dissociation rate (ca. 1400 s(-1)). The difference in the steady shear behavior of the unentangled vs. entangled regime reveals an unexpected, additional competing relaxation, ascribed to topological disentanglement in the semidilute entangled regime that contributes to the rheological properties.

Rheology and viscosity scaling of the polyelectrolyte xanthan gum

AbstractThe viscosity as a function of concentration for xanthan gum in both salt‐free solution and in 50 mM NaCl is measured and compared with a scaling theory for polyelectrolytes. In general, the zero shear rate viscosity and the degree of shear thinning increase with polymer concentration. In addition, shear thinning was observed in the dilute regime in both solvents. In salt‐free solution, four concentration regimes of viscosity scaling and three associated critical concentrations were observed (c* ≈ 70 ppm, ce ≈ 400 ppm, and cD ≈ 2000 ppm). In salt solution, only three concentration regimes and two critical concentrations were observed (c* ≈ 200 ppm and ce ≈ 800 ppm). In the presence of salt, the polymer chain structure collapses and occupies much less space resulting in higher values of the critical concentrations. The observed viscosity‐concentration scaling is in very good agreement with theory in the semidilute unentangled and semidilute entangled regimes in both salt‐free and 50 mM NaCl solution. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Polymerized Ionic Liquids: Solution Properties and Electrospinning

In this study, the solution properties and electrospinning of a polymerized ionic liquid was explored. Polymerized ionic liquids are synthesized from polymerizing ionic liquid monomers, where ionic liquids are of great interest due to their unique physiochemical properties. Compared to other polyelectrolyte solutions, this polymerized ionic liquid solution exhibits similar viscosity scaling relationships in the semidilute unentangled and semidilute entangled regimes. However, the electrospraying-electrospinning transition occurs at similar polymer solution concentrations compared to neutral polymers, where electrospinning produced beaded fibers and defect-free fibers at ∼1.25 and ∼2 times the entanglement concentration, respectively. Due to high solution conductivities, electrospinning produces fibers approximately an order of magnitude smaller than neutral polymers at equivalent normalized solution concentrations. In addition, a high ionic conductivity of the solid-state fiber mat was observed under dry conditions and even higher conductivities were observed for polyelectrolyte fiber mats produced from electrospinning polyelectrolyte−ionic liquid solutions, where both anion and cation are mobile species.

Influence of methylcellulose on dynamic rheology of dilute wheat gliadin solution in 50% (v/v) aqueous propanol

Influences of methylcellulose on dynamic rheological behaviors of 50 g L −1 (dilute) wheat gliadins in 50% (v/v) aqueous propanol were investigated as a function of methylcellulose concentration C ranging from 5 to 20 g L −1. The mixture solutions exhibit shear rate thinning that becomes pronounced with increasing C. Specific viscosities of the first and the second Newtonian plateau viscosities, as well as the steady-flow viscosity as a function of C, reveal that methylcellulose is in the semi-dilute unentangled regime at temperatures from 5 to 65 °C. The intermolecular interaction between methylcellulose and gliadins provides high flow resistance, improved shear thinning and steady-flow activation energy of the mixture solution in comparison with pure gliadins solution.

Counterion condensation of differently flexible polyelectrolytes in aqueous solutions in the dilute and semidilute regime

The low-frequency limit of the electrical conductivity (dc conductivity) of differently flexible polyions in aqueous solutions has been measured over an extended polyion concentration range, covering both the dilute and semidilute (entangled and unentangled) regime, up to the concentrated regime. The data have been analyzed taking into account the different flexibility of the polymer chains according to the scaling theory of polyion solutions, in the case of flexible polyions, and according to the Manning model, in the case of rigid polyions. In both cases, the fraction f of free counterions, released into the aqueous phase from the ionizable polyion groups, has been evaluated and its dependence on the polyion concentration determined. Our results show that the counterion condensation follows at least three different regimes in dependence on the polyion concentration. The fraction f of free counterions remains constant only in the semidilute regime (a region that we have named the Manning regime), while there is a marked dependence on the polyion concentration both in the dilute and in the concentrated regime. These results are briefly discussed in the light of the scaling theory of polyelectrolyte aqueous solutions.

Dielectric properties of differently flexible polyions: a scaling approach

The dielectric relaxations associated with counterion polarization along some typical polyion lengths have been measured in an extended frequency range (from 10 kHz to 2 GHz) for four different polyelectrolyte solutions, differing for the polyion molecular weight and the backbone stiffness. Here, we deal with the so-called intermediate dielectric relaxation, falling between the polarization process concerning the whole polyion chain and the polarization process associated with the field-induced re-orientation of the water molecule dipoles. These observed intermediate relaxations have been characterized by means of two parameters, i.e., the dielectric strength Deltaepsilon and the relaxation frequency nu0, and their dependence on the polyion concentration has been described according to the scaling model of a polyelectrolyte solution. These dependencies follow the expected exponents of the scaling laws, both for the dilute and semidilute (unentangled and entangled) regimes. The different concentration regimes evidenced from dielectric relaxation measurements are in very good agreement with the ones determined by means of the zero-shear viscosity measurements. Our results confirm that the intermediate dielectric relaxation in polyelectrolyte solutions should be attributed to counterion fluctuations along some segments (e.g. the subunits of the Mandel model) of the polyion chain, independently of its overall stiffness. This counterion polarization effect is rather confined to the local structure of the polyion chain instead of the whole chain and it is largely independent of the polyion conformation.

Influence of Chain Charge and Complexation on the Overlap and Entanglements Formation in Poly(acrylic acid) Salt-Containing Aqueous Solutions

Dilute-semidilute regime crossover in aqueous solutions of partly neutralized poly(acrylic acid) and of its complex with tetradecyltrimethylammonium bromide was studied by light scattering and viscometry methods. The chain charge growth causes the decrease of overlap concentration (c*) and the increase of the entanglements formation concentration (ce), hence, the semidilute unentangled regime of solution expands. Complexation of the polyelectrolyte with an oppositely charged surfactant leads to c* increase and to ce decrease. It is shown that in semidilute entangled solutions the surfactant acts as an effective structuring agent because of the binding of polyelectrolyte chains via surfactant micelles.

Local Dynamics and Primitive Path Analysis for a Model Polymer Melt near a Surface

By applying local primitive path and Rouse modes analysis we study the chains conformations, local dynamics and viscosity of a model polymer melt in a polymer-wall interface. We establish that the presence of a repulsive wall leads to acceleration of the dynamics both for unentangled and weakly entangled melts and to a depletion in the entanglement density in the wall vicinity. When the surface bears some grafted chains, we show that the melt chains are accelerated in the unentangled regime and slowed down in the entangled regime. By analyzing the primitive paths we attribute the observed slowdown to an increase in the entanglement density in the interfacial layer. The presence of a relatively small density of grafting sites thus leads to improved mechanical properties (reinforcement) and decreases locally the entanglement length even if the surface is repulsive.

Electrospinning of polyacrylonitrile nanofibers

AbstractPolyacrylonitrile (PAN) was electrospun in dimethylformamide as a function of electric field, solution flow rate, and polymer concentration (C). The fiber diameter increased with C and ranged from 30 nm to 3.0 μm. The fiber diameter increased with the flow rate and decreased when the electric field was increased by a change in the working distance; however, it did not change significantly when the electric field was varied by a change in the voltage at a given working distance. The fibers below about 350 nm diameter contained beads, whereas above this diameter, bead‐free fibers were obtained. For PAN with a molecular weight of 100,000 g/mol, the fiber diameter scaled as C1.2 and C7.5 at low (5.1–16.1 wt %) and high (17.5–22.1 wt %) C values, respectively. Both concentrations were in the semidilute entangled regime, where the specific viscosity scaled as C4.4, consistent with De Gennes's scaling concepts. In the semidilute unentangled regime (0.5–3.1 wt %), where the viscosity scaled as C1.3, microscopic or nanoscopic particles rather than fibers were obtained. Concentration‐ dependent electrospinning studies were also carried out for higher molecular weight PAN (250,000 and 700,00 g/mol). The results of these studies are also presented and discussed. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 1023–1029, 2006

Charge density effects in salt‐free polyelectrolyte solution rheology

AbstractSolution rheology of 2‐vinyl pyridine andN‐methyl‐2‐vinyl pyridinium chloride random copolymers in ethylene glycol was studied over wide ranges of concentration and effective charge. The fraction of quaternized monomers α and the fraction of monomers bearing an effective chargefof these copolymers were measured using counterion titration and dielectric spectroscopy, respectively. Ethylene glycol is a good solvent for neutral poly(2‐vinyl pyridine), with very few ionic impurities. The viscosity η and relaxation time τ of dilute and semidilute unentangled solutions exhibit the scaling with concentration and effective charge expected by the Dobrynin model. Reduced viscosity data are independent of concentration in dilute solution, giving an intrinsic viscosity that depends on effective charge, and the experimental data obey the Fuoss law in the semidilute unentangled regime. Scaling concentration with the overlap concentration (c/c*) reduces these data to common curves, andc*∼f−12/7as predicted by the Dobrynin model, wherefis the fraction of monomers bearing an effective charge. While the overlap concentration depends strongly on effective charge until counterion condensation occurs, the entanglement concentrationceis surprisingly insensitive to effective charge, indicating that entanglement effects are not understood using the Dobrynin model. The terminal modulusG= η/τ depends only on the number density of chainsG=ckT/Nforc* &lt;c&lt;ce, andG∼c3/2forc&gt;ceindependent of the effective charge. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2001–2013, 2006