Critical Entanglement Concentration Research Articles

Synthesis of a hydrophobic association polymer with an inner salt structure for fracture fluid with ultra-high-salinity water

This study used a compound initiation free radical polymerization process to synthesize a hydrophobic association polymer (PDMA1) with good salt resistance for use in ultra-high salinity reservoirs. The critical association and entanglement concentrations of PDMA1 in deionized (DI) water were 1596 and 2799 mg/L, respectively. The properties of the polymer in simulated formation (SF) water were investigated using viscosity change and conductivity tests, and the apparent viscosity of the PDMA1 solution was investigated using apparent viscosity, viscoelasticity, and thixotropy experiments. To a certain extent, the apparent viscosity of the solution was found to be influenced by metal ions. Under total dissolved solids (TDS) concentrations of up to 239,500 ppm, when the polymer concentration exceeded the critical entanglement concentration, the apparent viscosity of the polymer remained close to that of DI water. Additionally, the viscosity of the PDMA1 solution was affected by different types of metal ions, and the polymer solution thickened owing to divalent cations. PDMA1 is resistant to salt owing to hydrophobic interaction, electrostatic shielding, and double-layer compression. The results of the thixotropy and viscoelasticity tests revealed the thixotropy loop area of the PDMA1 solution in SF water to be greater than that in DI water. Under these conditions, PDMA1 is mostly viscous at low shear frequencies and mostly elastic at high shear frequencies. The PDMA1 inner-salt hydrophobic association polymer can be utilized as a chemical agent for tertiary recovery and fracturing, processes in high-salinity reservoirs, particularly for aggravating fracturing fluid.

Rheological aspects in fabricating pullulan-whey protein isolate emulsion suitable for electrospraying: Application in improving β-carotene stability

Beta-carotene has been used as a food colorant and functional ingredient, but these applications are limited due to its poor stability. The aim of this work was to improve the stability of β-carotene by encapsulating it in electrosprayed particles. The electrohydrodynamic behavior of pullulan solution was studied and the influence of encapsulation and core-to-wall ratio on the stability of β-carotene at different temperatures and relative humidity were investigated. The specific viscosity of the polymer solution increased linearly with the concentration until 5% w/w concentration. The critical entanglement concentration of the pullulan was found to be 4.58% wt. Among the various concentrations of pullulan studied, 3% w/w pullulan solution yielded spherical, monodisperse nanoparticles. Electrosprayed particles with 1:50 core-to-wall ratio had highest encapsulation efficiency. The rate of β-carotene loss displayed first order reaction kinetic within 14 days, while displayed diffusion limited mode after 14 days storage. The retention of β-carotene from the electrosprayed particles accelerated markedly with decreasing temperatures and relative humidity. This study provides a strategy for the preparation of promising electrosprayed particles to improve the stability of β-carotene.

Electrospun Alginate Fibers: Mixing of Two Different Poly(ethylene oxide) Grades to Improve Fiber Functional Properties.

The aim of the present work was to investigate how the molecular weight (MW) of poly(ethylene oxide) (PEO), a synthetic polymer able to improve alginate (ALG) electrospinnability, could affect ALG-based fiber morphology and mechanical properties. Two PEO grades, having different MWs (high, h-PEO, and low, l-PEO) were blended with ALG: the concentrations of both PEOs in each mixture were defined so that each h-PEO/l-PEO combination would show the same viscosity at high shear rate. Seven ALG/h-PEO/l-PEO mixtures were prepared and characterized in terms of viscoelasticity and conductivity and, for each mixture, a complex parameter rH/rL was calculated to better identify which of the two PEO grades prevails over the other in terms of exceeding the critical entanglement concentration. Thereafter, each mixture was electrospun by varying the process parameters; the fiber morphology and mechanical properties were evaluated. Finally, viscoelastic measurements were performed to verify the formation of intermolecular hydrogen bonds between the two PEO grades and ALG. rH/rL has been proved to be the parameter that better explains the effect of the electrospinning conditions on fiber dimension. The addition of a small amount of h-PEO to l-PEO was responsible for a significant increase in fiber mechanical resistance, without affecting the nano-scale fiber size. Moreover, the mixing of h-PEO and l-PEO improved the interaction with ALG, resulting in an increase in chain entanglement degree that is functional in the electrospinning process.

Particle tracking microrheology of the power-law viscoelasticity of xanthan solutions

We use particle tracking microrheology to obtain the power-law dynamical behaviour of xanthan in several aqueous solution conditions and illustrate that the probe particle motion is subdiffusive even at low xanthan concentrations. Analysis of the concentration dependence of xanthan’s viscoelasticity is described in terms of linear and comb polyelectrolyte rheological models. There is a clear distinction between two power-law regimes i.e. unentangled and entangled of the modulus prefactor as a function of concentration. The critical entanglement concentration and the power-law exponents are investigated in terms of their dependence on the content and valency of the added salts. Above the entanglement concentration the deviation of the particles motion from Brownian diffusion depends strongly on concentration. Linear complex viscosity is compared to nonlinear steady shear rate viscosity in an illustration of predicting the nonlinear rheology from particle tracking measurements in food biopolymer solutions. Finally in renatured xanthan solutions the viscoelasticity enhancement by temperature treatment is demonstrated to be more effective in the entangled region.

Electrospinning of poly(lactic acid): Theoretical approach for the solvent selection to produce defect‐free nanofibers

ABSTRACTIn this study an integrated methodology was proposed for the selection of solvent systems to produce electrospinnable solutions that form defect‐free poly(lactic acid) (PLA) fibers with narrow diameter distributions. The solvent systems were chosen using a thermodynamic approach, combined with electrical and rheological property criteria. More specifically, the three step methodology includes (1) initial choice of solvent by solubility evaluation to meet thermodynamic criteria, (2) electrical properties, that is, conductivity and dielectric constant adjustment by using solvent mixtures to meet electrical property criteria, and (3) critical entanglement concentration (Ce) determination by viscosity measurements, supported by elastic and plastic moduli measurements, followed by concentration adjustment to meet rheological criteria. All three criteria need to be met to ensure defect‐free nanofiber morphology. The methodology was demonstrated using PLA solutions that were characterized in terms of thermodynamic properties, conductivity, surface tension, and viscosity measurements. These data were analyzed and related to the nanofiber morphology and diameter as determined from scanning electron microscopy (SEM). Measurements of the elastic (G′) and the plastic (G″) moduli of PLA solutions showed a sharp increase of G′ at the chain entanglement concentration. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys.2016,54, 1483–1498

Electrospun poly lactic acid (PLA) fibres: Effect of different solvent systems on fibre morphology and diameter

The selection of an appropriate non-hazardous solvent or solvent system is essential to determine the rheological properties and electrospinnability of the solution, the productivity, and the morphology of nanofibres. In this study, poly lactic acid (PLA) solutions were prepared in various pure solvents and binary-solvent systems to investigate the effect of different solution properties on nanofibre morphology and diameter. Viscosity, conductivity and surface tension of each solution were measured. Nanofibre morphology was observed by scanning electron microscopy (SEM). Of all the solvent systems used acetone/dimethylformamide gave the highest fibre productivity and finest defect-free nanofibres. Therefore this solvent system was studied in more detail, varying the solvent ratio. Also the polymer concentration in this solvent system was varied to investigate the effect on nanofibre morphology and solution properties. Morphological investigations were done in correlation with rheological measurements: beaded nanofibrous structures were collected from solutions with concentration around the critical chain entanglement concentration (Ce), while defect-free nanofibres were produced when the concentration was increased to about twice the entanglement concentration. Further investigation of the effect of the PLA concentration on the elastic (G′) and the plastic (G″) moduli showed a sudden increase of the elastic moduli (G′) at the critical chain entanglement concentration. The results showed that the solvent properties, boiling point, viscosity, conductivity and surface tension, have a significant effect on process productivity, morphology and diameter distribution of the PLA nanofibres.

Fabrication of styrene–acrylonitrile random copolymer nanofiber membranes from N,N-dimethyl formamide by electrospinning

Ultrafine styrene–acrylonitrile random copolymer (SAN) nanofiber-based membranes were produced from N,N-dimethyl formamide solution by electrospinning. The purpose of this study was to find the optimum values of the electrospinning parameters and the influence of major significant parameters on the electrospun fiber morphology and the average fiber diameter ( Davg) and its standard deviation using design of experiment. A backward elimination model for multiple regression analysis was employed to obtain quantitative interactions among selected electrospinning parameters and the final fiber diameter. The dependence of the Davg and morphology on the critical entanglement concentration was also studied. Morphology of the electrospun nanofiber mats were examined by scanning electron microscopy. Davg of electrospun SAN fibers increased considerably with increasing solution concentration. Fibers with diameters ranging from 40 to 650 nm were obtained. Analysis of variance was utilized to identify the statistically significant parameters ( p < 0.05) and error variance.

Self-sustained webs of polyvinylidene fluoride electrospun nano-fibers: Effects of polymer concentration and desalination by direct contact membrane distillation

The effects of the polymer polyvinylidene fluoride (PVDF) concentration on the characteristics and direct contact membrane distillation (DCMD) desalination performance of self-sustained electrospun nano-fibrous membranes (ENMs) have been studied. Different polymer concentrations ranging from 15 to 30wt% were considered in the solvent mixture N,N-dimethyl acetamide and acetone, while all other electrospinning parameters were maintained the same. Viscosity, electrical conductivity and surface tension of the polymer solutions were measured and the effects of the PVDF concentration on fiber diameter, thickness, water contact angle, inter-fiber space, void volume fraction, liquid entry pressure, mechanical and thermal properties of the ENMs were investigated. The minimum polymer concentration, critical chain entanglement concentration, required for electrospinning beaded fibers and the concentration needed for the formation of bead-free fibers were localized. Two groups of ENMs were identified based on the surface structure of the ENMs, their void volume fraction and inter-fiber space. Bead-free ENMs, prepared with PVDF concentration higher than 22.5wt%, exhibit higher DCMD permeate flux than the beaded ENMs. Beaded ENMs can be used in desalination by DCMD. Among the prepared ENMs, the optimized membrane exhibiting the highest DCMD performance was prepared with 25wt% PVDF concentration.

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.

Electrospinning of poly(methyl methacrylate) nanofibers in a pump-free process

Abstract The effects of processing parameters, including solution concentration, viscosity, nozzle diameter, voltage bias and the nozzle to collector distance, on the morphology and diameters of poly(methyl methacrylate) (PMMA) fibers have been systematically investigated, using a unique pump-free electrospinning method. For PMMA solution concentrations less than the critical entanglement concentration, ce, prolate spheroid-shaped droplets or beads with fibers were formed, whereas at concentrations above ce, good quality bead-free fibers were formed. Quantitative analysis revealed a linear dependence between the solution viscosity and fiber diameter. Larger fiber diameters were achieved by increasing the nozzle diameter and voltage bias. Increasing the bias voltage has the additional effect of broadening the diameter distribution, as a result of splaying and splitting. By contrast, when the strength of the electrical field was reduced by increasing the distance between the nozzle and collector, the overall fiber diameter was reduced.

Molecular Size Characterization and Kinetics Studies on Hydrolysis of Pullulan by Pullulanase in an Entangled Alginate Medium

The behavior of a hydrolytic enzyme (pullulanase) toward its substrate (pullulan) in the presence of a nonsubstrate (alginate), both below and above the critical entanglement concentration (C*), was studied. The hydrolysis kinetics were studied with the enzyme and alginate concentrations varied using two main methods: a colorimetric assay of the reducing extremities (RE), which allowed the number-average molar masses (Mn) of the oligosaccharides to be determined, and size exclusion chromatography with on-line, multiangle light scattering, viscometer, and differential refractive index detectors, which allowed the average molar masses, Mn and Mw, of the oligosaccharides during hydrolysis to be determined. Free pullulanase acts via an "endo" process. The presence of alginate slows the hydrolysis kinetics, particularly when the alginate concentration is greater than the C*. These results were confirmed by the evolution of the kinetic parameters (KM, Vmax) obtained via isothermal titration calorimetry (ITC). The amount of oligosaccharides produced is not dependent on the alginate concentration, and the endo enzyme behavior is not modified by the entanglement in the medium. These observations were also confirmed by ITC analysis in the presence of degraded alginate (without entanglement). Our results correlated with the substrate diffusion in entangled media. The pullulanase reaction in the presence of alginate is shown to be diffusion-dependent.

Characterization and properties of electrospun thermoplastic polyurethane blend fibers: Effect of solution rheological properties on fiber formation

Abstract

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.

Electrospinning of hyaluronic acid nanofibers from aqueous ammonium solutions.

For several reasons, the electrospinning of nanofibrous mats comprised purely of biopolymers, such as hyaluronic acid (HA) has been difficult to achieve. Most notably, due to its polyelectrolytic nature, very low polymer concentrations exhibit very high solution viscosities. Thus, it is challenging to obtain the critical chain entanglement concentration necessary for biopolymer electrospinning to ensue. While the successful electrospinning of HA fibers from a sodium hydroxide:dimethylformamide (NaOH:DMF) system has been reported, the diameter of these fibers was well above 100nm. Moreover, questions regarding the degradation of HA within the solvent system arose. These factors supported our ongoing research into determining an improved solvent system. In this study, the use of a less basic (pH 11) aqueous ammonium hydroxide (NH4OH) solvent system, NH4OH:DMF, allowed for the fabrication of HA mats having an average fiber diameter of 39±12nm. Importantly, while using this solvent system, no degradation effects were observed and the continuous electrospinning of pure HA fibers was possible.

Drag reduction by DNA-grafting for single microspheres in a dilute λ-DNA solution

The fluid resistance of single micrometre-sized blank and DNA-grafted polystyrene microspheres under shear flow is compared in purified water and dilute λ-DNA solutions by means of optical tweezers experiments with a high spatial (±4 nm) and temporal (±0.2 ms) resolution. The measurement results show that the drag experienced by a colloid in a dilute λ-DNA solution (molecular weight of 48,502 bp per molecule, radius of gyration of 0.5 μm) is significantly decreased if the microsphere bears a grafted DNA brush. This newly discovered drag reduction effect is studied for different parameters, comprising the molecular weight of the grafted DNA molecules (250 bp, 1000 bp and 4000 bp), the concentration of the λ-DNA solution (11, 17 and 23 μg ml−1, all being significantly smaller than the critical entanglement concentration c∗), the microsphere core diameter (2 μm, 3 μm and 6 μm) as well as the flow speed of the medium (10–50 μm s−1). The maximum extent of the drag reduction is found to amount to (60 ± 20)% compared to the λ-DNA-induced contribution on the drag acting on blank colloids. We propose a theoretical explanation of this effect based on the combination of the dynamic density functional theory of Rauscher and co-workers [Rauscher M. J. Phys.: Condens. Matter 2010;22:364109] and the stagnation length theory of polymer brushes, as it was established by Kim, Lobaskin et al. [Kim et al. Macromolecules 2008;42(10):3650–3655]. In particular, the solution of the Stokes equation (i.e., the Navier–Stokes equation for creeping flow) for the studied system yields a numerical prediction that is found to be in full accord with our experimental results within measurement uncertainty.

Formation and morphological stability of polybutadiene rubber fibers prepared through combination of electrospinning and in-situ photo-crosslinking

Herein we report that fibers of polybutadiene rubber (BR) with high morphological stability were made through combination of electrospinning and in-situ photo-crosslinking. This study revealed that (1) the formation of electrospun BR fibers was correlated with the overlap/entanglement of BR macromolecules in the spinning solutions; for the formation of uniform fibers without beads and/or beaded fibers, the concentration of spinning solution had to be higher than the “critical chain entanglement concentration (Ce)”; and (2) the in-situ photo-crosslinking was an effective method to considerably improve the morphological stability of electrospun BR fibers; upon prolonged storage under ambient condition or even upon immersion in tetrahydrofuran (a good solvent for BR), the crosslinked BR fibers well retained their morphology. It is envisioned that electrospun rubber fibers could be utilized for the development of innovative composites with substantially improved toughness and/or impact strength.

Electrospinning Fundamentals: Optimizing Solution and Apparatus Parameters

Electrospun nanofiber scaffolds have been shown to accelerate the maturation, improve the growth, and direct the migration of cells in vitro. Electrospinning is a process in which a charged polymer jet is collected on a grounded collector; a rapidly rotating collector results in aligned nanofibers while stationary collectors result in randomly oriented fiber mats. The polymer jet is formed when an applied electrostatic charge overcomes the surface tension of the solution. There is a minimum concentration for a given polymer, termed the critical entanglement concentration, below which a stable jet cannot be achieved and no nanofibers will form - although nanoparticles may be achieved (electrospray). A stable jet has two domains, a streaming segment and a whipping segment. While the whipping jet is usually invisible to the naked eye, the streaming segment is often visible under appropriate lighting conditions. Observing the length, thickness, consistency and movement of the stream is useful to predict the alignment and morphology of the nanofibers being formed. A short, non-uniform, inconsistent, and/or oscillating stream is indicative of a variety of problems, including poor fiber alignment, beading, splattering, and curlicue or wavy patterns. The stream can be optimized by adjusting the composition of the solution and the configuration of the electrospinning apparatus, thus optimizing the alignment and morphology of the fibers being produced. In this protocol, we present a procedure for setting up a basic electrospinning apparatus, empirically approximating the critical entanglement concentration of a polymer solution and optimizing the electrospinning process. In addition, we discuss some common problems and troubleshooting techniques.

Electrospinning Fundamentals: Optimizing Solution and Apparatus Parameters

Electrospun nanofiber scaffolds have been shown to accelerate the maturation, improve the growth, and direct the migration of cells in vitro. Electrospinning is a process in which a charged polymer jet is collected on a grounded collector; a rapidly rotating collector results in aligned nanofibers while stationary collectors result in randomly oriented fiber mats. The polymer jet is formed when an applied electrostatic charge overcomes the surface tension of the solution. There is a minimum concentration for a given polymer, termed the critical entanglement concentration, below which a stable jet cannot be achieved and no nanofibers will form - although nanoparticles may be achieved (electrospray). A stable jet has two domains, a streaming segment and a whipping segment. While the whipping jet is usually invisible to the naked eye, the streaming segment is often visible under appropriate lighting conditions. Observing the length, thickness, consistency and movement of the stream is useful to predict the alignment and morphology of the nanofibers being formed. A short, non-uniform, inconsistent, and/or oscillating stream is indicative of a variety of problems, including poor fiber alignment, beading, splattering, and curlicue or wavy patterns. The stream can be optimized by adjusting the composition of the solution and the configuration of the electrospinning apparatus, thus optimizing the alignment and morphology of the fibers being produced. In this protocol, we present a procedure for setting up a basic electrospinning apparatus, empirically approximating the critical entanglement concentration of a polymer solution and optimizing the electrospinning process. In addition, we discuss some common problems and troubleshooting techniques.

Solution properties of hydrophobically modified polyelectrolytes synthesized via solution and micellar copolymerization

AbstractHydrophobically modified polyelectrolytes (HMPEs) were synthesized using sodium 2‐acrylamido‐2‐methyl‐propanesulfonate and N‐n‐dodecylacrylamide as monomers with the same feeding ratio via micellar and solution copolymerization. The effects of hydrophobic association and electrostatic interaction on the solution properties of the HMPEs were studied. Compared with HMPE obtained via solution copolymerization (s‐PAD), the hydrophobic interaction of HMPE obtained via micellar copolymerization (m‐PAD) is more obvious due to the micro‐blocky distribution of hydrophobic groups. The viscosity properties of m‐PAD in deionized water or brine follow well the scaling theory of polyelectrolytes. However, for s‐PAD, the concentration where zero‐shear viscosity (η0) and solvent viscosity (ηs) follow η0 ≈ 2ηs is more likely to be critical entanglement concentration (ce) rather than critical overlap concentration (c*). It is suggested that modifying of the transition region from c* to ce is valid and reasonable for s‐PAD. It is believed that the different solution properties of s‐PAD and m‐PAD should be attributed to the distributions of hydrophobic groups in the chains. Copyright © 2010 Society of Chemical Industry

Hydrophobically modified poly(sodium 2‐acrylamido‐2‐methylpropanesulfonate): synthesis and viscosity of aqueous solution

AbstractBACKGROUND: Hydrophobically modified polyelectrolytes are widely used polymers due to their good water solubility, stretched configuration in water and strong hydrophobic association. The study reported here aimed at researching the double action of hydrophobic association and electrostatic effect of novel hydrophobically modified polyelectrolytes in solution.RESULTS: A series of novel hydrophobically modified polyelectrolytes were synthesized by micellar copolymerization with various feed ratios of sodium 2‐acrylamido‐2‐methylpropanesulfonate, N‐n‐dodecylamine and sodium dodecylsulfonate. Their structure was characterized using Fourier transform infrared spectroscopy, nuclear magnetic resonance and gel permeation chromatography, and the viscosities of their aqueous and salt solutions were studied.CONCLUSION: The results show that the addition of the hydrophobic comonomer results in a decrease in molecular weight (Mw). The smaller the initial number of hydrophobes in one micelle, the higher is Mw of the resulting copolymer. The viscosity of PAD‐1.73 polyelectrolyte is less sensitive to salt than those of the others. According to the zero shear viscosity and corresponding concentration, the critical cluster‐forming concentration, critical overlap concentration and critical entanglement concentration of these polymer solutions were determined. Moreover, in the dilute regime the viscosity decreases with increasing salinity, while in the semi‐dilute regime the viscosity decreases first and then increases. It is suggested that in dilute and semi‐dilute regimes, hydrophobic intramolecular association and intermolecular association dominate, respectively. Copyright © 2009 Society of Chemical Industry