Solutions Of High Molecular Weight Polymers Research Articles

Scission of flexible polymers in contraction flow: Predicting the effects of multiple passages

When injected through a contraction, high molecular weight polymer solutions exhibit a sharp increase of apparent viscosity which originates from stretching polymer chains above a critical extension rate. This chain stretching can also induce polymer scission, which then decreases the extensional viscosity. In practice, the two phenomena are difficult to separate. Moreover, these phenomena are often observed in situation where flow instabilities appear. In order to disentangle the two effects we have measured the pressure-flux relation for polymer solutions passing through a hyperbolic contraction. The ratio of the pressure drop to that of the (Newtonian) solvent has a maximum due to the competition between polymer extension and scission. We find a geometry-dependent relation between the flow rates at which the maximum occurs for successive passages in a given contraction, which appears to be independent of molecular weight, concentration, solvent quality and viscosity, and can be used to predict the scission under successive passes.

Nucleation effects of high molecular weight polymer additives on low molecular weight gels

Polymeric species have been introduced to low molecular weight gelators to tailor their nucleation and rheological behavior. This work combines polymers and molecular gels (MGs) in a different manner by using polymers as the major component in a solution. Additionally, using polymers above their entanglement molecular weight is a step towards building polymer–MG composite materials. Specifically, a cholesterol-pyridine (CP) molecular gel was introduced to poly(ethylene oxide-co-epichlorohydrin) (EO-EPI) and poly(vinyl acetate) (PVAc), which have dissimilar chain conformations in anisole. Dynamic light scattering, scanning electron microscopy, and temperature-dependent small- and wide-angle X-ray studies were utilized to investigate the influence of the solution properties of high molecular weight EO-EPI and PVAc on the CP network structure. The collapsed chain conformation and aggregation of EO-EPI led to isolated, branched CP fiber networks, resulting in unexpectedly high dissociation temperatures. In contrast, PVAc gels displayed transient fiber networks, as evidenced by fiber wrapping and bundling. Cooperative interactions between PVAc and CP resulted in gels with dissociation temperatures higher than those of pure CP gels. These structural characteristics significantly influenced the gel mechanics. The collapsed chain conformation of EO-EPI led to weaker, more viscous gels, and the freely extended PVAc chain conformation led to interconnected, elastic gels independent of the molecular gel concentration. Polymer chain conformation was utilized to control the nucleation of a cholesterol-pyridine molecular gel. Collapsed chain conformations influence gel structure and dissociation behavior by acting as physical barriers that lead to confinement effects and permanent networks. An extended polymer chain conformation allowed for polymer–molecular-gel interactions and increased dissociation temperatures due to its highly ordered structure, resulting in transient networks. Additionally, the high molecular weight polymer solution behavior guided solution mechanics, where collapsed chains lead to viscous solutions and gels and extended chains led to elastic gel networks.

Fabrication of aligned Eu(TTA)3phen/PS fiber bundles from high molecular weight polymer solution by electrospinning

Super-long aligned luminescent Eu(TTA)3phen/PS composite fibers (TTA = thenoyltrifluoroacetone, phen = 1,10-phenanthroline, PS = polystyrene) with diameter in the range of 1–10 μm were prepared via an electrospinning method. The key to the success of alignment of these fibers was the usage of high molecular weight PS in the electrospinning solution and the low speed collecting drum. Luminescent properties of the composite fibers were systemically studied in comparison with that of the corresponding pure europium complex Eu(TTA)3phen. The results showed that the fluorescence lifetime for the 5 D 0 state in the composite fibers became shorter compared to that in the pure europium complex and decreases gradually with the concentration of Eu(DBM)3phen complex.

Quantitative characterization of high molecular weight polymer solutions in microfluidic hyperbolic contraction flow

Nonlinear flows of polyacrylamide (PAAm) ( $$M_{\text{w}} = 5.7 \times 10^{6}\,{\text{g}}/{\text{mol}}$$ ) aqueous solutions through a micro-fabricated, hyperbolic contraction geometry with high Hencky strain ( $$\varepsilon_{{\text{H}}} = 3.7$$ ) have been characterized by micro-particle image velocimetry ( $$\mu $$ -PIV). Various flow dynamics regimes in a range of Weissenberg number (Wi) and Reynolds number (Re) are presented in a Wi–Re diagram. The symmetric corner vortices are only observed in the flow of low concentration PAAm solution ( $$c/c^{*}=3.3$$ ). In a higher concentration ( $$c/c^{*}=8.3$$ ), PAAm solution exhibits chaotic-like flow patterns in the strong nonlinear flow regime ( $$Wi>350$$ ). Extensional deformation in nonlinear flows of Wi up to 860 has been analyzed. Furthermore, the local stretch experienced by the polymer chain in complex flow is systematically quantified and linked to the corresponding velocity vector fields, which are valuable for understanding the highly nonlinear flow phenomena.

Self-Assembly and Orientation of Hydrogen-Bonded Oligothiophene Polymorphs at Liquid–Membrane–Liquid Interfaces

One of the challenges in organic systems with semiconducting function is the achievement of molecular orientation over large scales. We report here on the use of self-assembly kinetics to control long-range orientation of a quarterthiophene derivative designed to combine intermolecular π-π stacking and hydrogen bonding among amide groups. Assembly of these molecules in the solution phase is prevented by the hydrogen-bond-accepting solvent tetrahydrofuran, whereas formation of H-aggregates is facilitated in toluene. Rapid evaporation of solvent in a solution of the quarterthiophene in a 2:1:1 mixture of 1,4-dioxane/tetrahydrofuran/toluene leads to self-assembly of kinetically trapped mats of bundled fibers. In great contrast, slow drying in a toluene atmosphere leads to the homogeneous nucleation and growth of ordered structures shaped as rhombohedra or hexagonal prisms depending on concentration. Furthermore, exceedingly slow delivery of toluene from a high molecular weight polymer solution into the system through a porous aluminum oxide membrane results in the growth of highly oriented hexagonal prisms perpendicular to the interface. The amide groups of the compound likely adsorb onto the polar aluminum oxide surface and direct the self-assembly pathway toward heterogeneous nucleation and growth to form hexagonal prisms. We propose that the oriented prismatic polymorph results from the synergy of surface interactions rooted in hydrogen bonding on the solid membrane and the slow kinetics of self-assembly. These observations demonstrate how self-assembly conditions can be used to guide the supramolecular energy landscape to generate vastly different structures. These fundamental principles allowed us to grow oriented prismatic assemblies on transparent indium-doped tin oxide electrodes, which are of interest in organic electronics.

Pair collisions of fluid-filled elastic capsules in shear flow: Effects of membrane properties and polymer additives

The dynamics and pair collisions of fluid-filled elastic capsules during Couette flow in Newtonian fluids and dilute solutions of high-molecular weight (drag-reducing) polymers are investigated via direct simulation. Capsule membranes are modeled using either a neo-Hookean constitutive model or a model introduced by Skalak et al. [“Strain energy function of red blood-cell membranes,” Biophys. J. 13, 245 (1973)], which includes an energy penalty for area changes. This model was developed to capture the elastic properties of red blood cells. Polymer molecules are modeled as bead-spring trimers with finitely extensible nonlinearly elastic springs; parameters were chosen to loosely approximate 4000 kDa poly(ethylene oxide). Simulations are performed with a novel Stokes flow formulation of the immersed boundary method for the capsules, combined with Brownian dynamics for the polymer molecules. The results for isolated capsules in shear indicate that at the very low concentrations considered here, polymers have a little effect on the capsule shape. In the case of pair collisions, the effect of polymer is strongly dependent on the elastic properties of the capsules’ membranes. For neo-Hookean capsules or for Skalak capsules with only a small penalty for area change, the net displacement in the gradient direction after collision is virtually unaffected by the polymer. For Skalak capsules with a large penalty for area change, polymers substantially decrease the net displacement when compared to the Newtonian case and the effect is enhanced upon increasing the polymer concentration. The differences between the polymer effects in the various cases are associated with the extensional flow generated in the region between the capsules as they leave the collision. The extension rate is highest when there is a strong resistance to a change in the membrane area and is substantially decreased in the presence of polymer.

Dynamics of polymers in elongational flow studied by the neutron spin-echo technique

The nanoscale fluctuation dynamics of semidilute high molecular weight polymer solutions of polyethylenoxide (PEO) in D 2O under non-equilibrium flow conditions were studied by the neutron spin-echo technique. The sample cell was in contraction flow geometry and provided a pressure driven flow with a high elongational component that stretched the polymers most efficiently. Neutron scattering experiments in dilute polymer solutions are challenging because of the low polymer concentration and corresponding small quasi-elastic signals. A relaxation process with relaxation times of about 10 ps was observed, which shows anisotropic dynamics with applied flow.

Complex Fluids Based on Methacrylated Hyaluronic Acid

We present the preparation and characterization of viscoelastic formulations of hyaluronic acid functionalized with polymerizable methacrylate groups. We explored three different processing strategies for controlling microstructure and interchain interactions: lightly cross-linked near-gels, emulsion-cross-linked microspheres, and an elastic microgel formed through centrifuging the microspheres. The component structure and rheological properties of these formulations were compared to those of high molecular weight hyaluronic acid solutions, which displayed classical behavior of high molecular weight polymer solutions reported by other investigators. We demonstrate that these processing strategies allow the tuning of solution properties from strongly viscoelastic behavior, observed in lightly cross-linked near-gels and concentrated microsphere solutions to elastic behavior in elastic microgels, behaving like pseudoplastic liquids having a well-defined yield stress above which viscous behavior was observed. In the centrifuged microspheres, the hyaluronic acid degree of methacrylation was inversely proportional to the gel elasticity, and a mechanism based on failure due to microsphere brittleness is proposed to explain this behavior. These results suggest that processing methacrylated hyaluronic acid can lead to a diversity of solution properties, providing methods for delivering this biologically active polymer in a broad range of applications.

Multiscale simulation of dilute DNA in a roll-knife coating flow

The dynamics of dilute solutions of DNA flowing in a scaled down roll-knife coating flow are investigated on multiple scales. The flow is generated between a rotating roll and a stationary glass knife, and extension of fluorescently stained DNA molecules is measured at the minimum gap at low roll speeds. The macroscopic flow is computed by solving the continuum equations of motion with the finite element method; the microscale behavior of DNA molecules is predicted by Brownian dynamics combined with successive fine-graining. The simulations predict that the DNA should stretch almost to full extension near the roll surface in the region of minimum gap; this does not agree with experimental measurements. The assumption that the flow is nearly homogeneous on the length scale of the polymer molecules, commonly used in processing flows as well as Brownian dynamics simulations of simple flows, fails near free surfaces, and is the likely cause of the discrepancy. Evidence from the literature suggests that similar nonlocal effects may be present in coating and ink-jet printing flows of high molecular weight polymer solutions.

Displacement flows of dilute polymer solutions in capillaries

The two-dimensional, free surface flow of a viscoelastic liquid being displaced in a capillary tube by gas injection is analyzed by both experiments and theory. The experiments consisted of flow visualization and measurement of mass displaced by a gas bubble in a capillary tube filled with a viscoelastic liquids. Various solutions of low molecular weight polyethylene glycol (PEG) and high molecular weight polyethylene oxide (PEO) in water were used in order to evaluate the effect of viscoelastic behavior on the flow. The rheological properties of the solutions were evaluated both in shear and predominantly extensional flows. The theoretical analysis consisted of solving the conservation equations, together with appropriate constitutive models, with the finite element method. Models of flows of high molecular weight polymer solution must rely on theories that can account for the different behavior of microstructured liquids in shear and extensional flow. Moreover, displacement flows involve a free surface, and the domain where the differential equations are posed is unknown a priori, being part of the solution. These two characteristics make the problem extremely complex. Here, the viscoelastic behavior of the liquid was described using three differential constitutive equations that approximate dilute and semi-dilute polymer solutions, namely Oldroyd-B, FENE-P and FENE-CR . Both the theoretical predictions and experimental measurements were obtained at values of the elasto-capillary number much higher than previously reported. At this limit, a new phenomenon is observed, the film thickness attached to the capillary wall reaches a plateau as the liquid becomes more elastic.

Iterated stretching and multiple beads-on-a-string phenomena in dilute solutions of highly extensible flexible polymers

The dynamics of elastocapillary thinning in high molecular weight polymer solutions are reexamined using high-speed digital video microscopy. At long times, the evolution of the viscoelastic thread deviates from self-similar exponential decay and the competition of elastic, capillary, and inertial forces leads to the formation of a periodic array of beads connected by axially uniform ligaments. This configuration is itself unstable and successive instabilities propagate from the necks connecting the beads and the ligaments. This iterated process results in the development of multiple generations of beads in agreement with the predictions of Chang, Demekin, and Kalaidin [“Iterated stretching of viscoelastic jets,” Phys. Fluids 11, 1717 (1999)] although experiments yield a different recursion relation between successive generations. At long times, finite molecular extensibility truncates the iterated instability and axial translation of the bead arrays along the interconnecting threads leads to a progressive coalescence before the rupture of the filament.

Gelation of polymer solutions under shear flow

Abstract Gelling a polymer solution under constant shear rate was recently found to be an attractive procedure for preparing size-controlled microgels. In the theoretical approach developed in this paper, the microgels formed during gelling are modelled as non-covalent ‘star’ polymers. This approach provides scaling laws to predict the effects of the shear rate applied during gelation on: (1) the gelling kinetics in three flow regimes; (2) the microgel size dependence in weak and strong flows; and (3) the rheological and elastic properties of the microgel solutions. The reported experiments were carried out by gelling semi-dilute high molecular weight polymer solutions by a crosslinker small enough to diffuse inside macromolecules and thus able to form both intra and intermolecular cross-links. All available experimental results are in agreement with our theoretical predictions, but further experiments are needed for a complete validation of the proposed theory.

Porous polymeric structures for tissue engineering prepared by a coagulation, compression moulding and salt leaching technique

A technique for the preparation of porous polymeric structures involving coagulation, compression moulding and particulate leaching has been developed. The technique combines the advantages of thermal processing methods and particulate leaching. A high molecular weight polymer solution in an organic solvent containing dispersed water-soluble salt particles is precipitated into an excess of non-solvent. The polymer–salt composite is then processed by thermal processing methods into devices of varying shapes and sizes, which can subsequently be extracted to give the desired porous structures. The porosities of the scaffolds could be varied between 70% and 95% by adjusting the polymer to salt ratio and the pore size could be controlled independently by varying the leachable particle size. This versatility provides for the optimisation of scaffolds used in medicine and in tissue engineering. Compared with commonly used porosifying methods such as sintering, compression moulding combined with salt leaching, and freeze-drying, this process allows excellent control over pore size and porosity and yields scaffolds with a much more homogeneous pore morphology. We have prepared porous structures from several relevant polymers in the biomedical field: poly( d,l-lactide), poly( ε-caprolactone) and 1000PEOT70PBT30, a segmented poly(ether ester) based on polyethylene oxide and polybutylene terephthalate.

Binary miscible blends of poly(methyl methacrylate)/poly(α-methyl styrene-co-acrylonitrile). III. Investigation of the phase behavior and morphology during shear flow

The phase behavior and morphology of poly(methyl methacrylate) (PMMA) and poly(α-methyl styrene-co-acrylonitrile) (PαMSAN) blends have been investigated under different shear rates using a simple shear apparatus and transmission electron microscopy (TEM), respectively. The lower critical solution temperature-type (LCST-type) phase diagram detected under quiescent condition was shifted a few degrees to higher temperatures with increasing shear flow, i.e., the shear could suppress the phase separation and enlarge the homogenous region. There was no sign detected for shear-induced demixing over a wide range of shear rates and temperatures for all the measuring compositions. The normalized shift in the cloud point vs. shear rate for this blend was also investigated and compared with previous studies for high molecular weight polymer blends, oligomer mixtures, and polymer solutions as well as simple liquid mixtures. It was found that the characteristic relaxation time for concentration fluctuations is the most sensitive parameter, which controls the behavior of the mixtures under shear flow i.e., the larger the characteristic time for concentration fluctuations are, the larger the change in the cloud points under shear will be. The morphology of the critical blend composition was investigated as a function of shear rates at two different quenching temperatures (different quenching depth). At 189°C (17°C above the quiescent cloud point), the morphology of the blend was strongly elongated and oriented parallel to the flow direction and, shear-induced mixing was detected at a critical shear rate value (10 sec−1), at which no morphology was detected. At 193°C (21°C above the quiescent cloud point), the blend morphology showed a slightly elongated co-continuous two-phase structure with a significant decrease in the periodic distance; however, no shear-induced mixing could be detected at this temperature at shear rates up to 40 sec−1.

Aspects of the use of polyoxyalkylene glycols in polymer quenchants

AbstractSolutions of high molecular‐weight polymers, particularly polyoxyalkylene glycols (PAGs), are used widely in the heat treatment of metals known as ‘quenching’. This is mainly because of the non‐flammable nature of the polymer solution, compared with safety hazards associated with traditional mineral‐oil based quenchants. However, the use of PAG‐based quenchants for some heat treatment processes, particularly for slow cooling speeds, has been limited. This paper describes how, through an understanding of the quenching mechanism, the quench speed of PAG‐based quenchants can be slowed by the use of additives or controlling the conditions of operation. The paper also gives an overview of the experimental measurement of polymer quenching and the factors involved in quenching.

Hydrodynamic slippage inferred from thin film drainage measurements in a solution of nonadsorbing polymer

Thin film drainage measurements are presented for submicron films of an “ideal elastic” or Boger fluid, which is a high molecular weight polymer solution in a high viscosity solvent. The measurements are made in a surface force apparatus, with the fluid being squeezed between two mica surfaces in a crossed cylinder geometry and the film thickness measured as a function of time to study its drainage behavior. No equilibrium surface forces are detected in this system, indicating that the polymer is nonadsorbing. The effect of fluid elasticity is predicted to make drainage more rapid in a Boger fluid than for the equivalent Newtonian fluid. Qualitatively this is what is observed for films less than 600 nm thick, but the drainage is even more rapid than predicted for the elastic fluid. To account for this, it is suggested that slippage is occurring at the fluid–solid interfaces, and the data is analyzed in terms of a simple slip model. The slip length required to fit the data is in the range 30–50 nm.

Computational studies of nonlinear elastic dumbbell models of Boger fluids in a cross-slot flow

The flow of a viscoelastic fluid in a cross-slot flow is studied numerically for two different versions of the elastic-dumbbell model, namely, the Chilcott–Rallison model (FENE-CR) and the same model but with conformation-dependent friction modeled via the Hinch–de Gennes scaling (FENE-CD). It is shown that the FENE-CR and FENE-CD models both predict a local reduction in the streamwise velocity downstream of the stagnation point. The reduction in the velocity is shown to result from the presence of highly stretched dumbbells near the streamlines emanating from the stagnation point, and is in reasonable agreement with existing experimental observations on high molecular weight polymer solutions. However, the FENE-CR model is shown to produce a birefringent strand whose length in the streamwise direction is significantly shorter than observed experimentally. Better agreement with experimental observations is found with the FENE-CD model, which exhibits a much slower relaxation from the extended state, thus producing a birefringent strand which extends substantially farther downstream than for the FENE-CR model. The effect of a conformation-dependent friction coefficient similar to the one proposed by Larson [R.G. Larson, The unravelling of a polymer chain in a strong extensional flow, Rheol. Acta 29 (1990) 371–384] is also considered.

Formation of a macroscopic meniscus in the case of high molecular weight polymer solutions

The formation of the macroscopic meniscus, observed when a liquid rises along a vertical plate, is studied for different kinds of fluids: PDMS, decalin/polybutene oil mixtures, high molecular weight polymer solutions in a low viscous simple fluid (decalin) and in decalin/polybutene oil mixtures. The measurement of the force due to the surface tension exerted on the plate gives information on the evolution of the contact angle as a function of time. It is possible to determine a characteristic time of the process depending on the viscosity and on the surface tension. It clearly appears that the behaviour of polymer solutions, both in decalin and in decalin/polybutene mixtures, is markedly different from those for the other fluids. The influence of evaporation is discussed but it seems that the dynamics of the rise of the polymer solutions can be interpreted as resulting from a preferential wetting of the solid by the solvent which drags the polymer in its rise along the solid wall.

Supermolecular structures and flow birefringence in polymer solutions

Experimental studies of supermolecular structures and localized flow birefringence in solutions of high-molecular weight polymer are described. Advantage is taken of poly(ethylene oxide) and polyisobutylene. Supermolecular structures are examined with the aid of optical microscopy using freeze-dried samples of the polymer solutions. Birefringence is investigated that arises in planar elongational flow in a cross-slot cell. Flow velocities at which the onset of the localized birefringence occurs are determined. Then these velocities are correlated with viscoelastic characteristics of the solutions. The presence of a liquid-crystalline fibrillar network in the polymer solutions exhibiting flow birefringence is ascertained. The fibrils are birefringent objects. The fibrils are birefringent objects. The localized birefringence phenomenon is explained in term of the orientation of the fibrils in elongational flow. It has been shown that the onset of localized birefringence occurs at a critical Weissenberg number, the value of which is close to unity.

Phase equilibrium and conformation of a polymer in a binary solvent

The phase equilibrium behavior of a dilute high molecular weight polymer (poly-ethylene oxide PEO, molecular weight 9 × 10 5, concentration 0.001 g/cc) solution in a two-component liquid mixture (isobutyric acid + water, IBAW) is studied experimentally. Far above the critical temperature T c of IBAW, we found that although PEO dissolved in pure isobutyric acid (IBA) and pure water, mixtures of IBA and water with 0.1% PEO became cloudy within a certain composition range. Far below T c , when the binary mixture phase separates in two coexisting phases, the presence of PEO which dissolved in both phases did not affect the coexistence curve of the binary mixture. Around T c , three phases in coexistence were obsrrved in a narrow composition range near the critical composition of IBAW. At high temperature (≈ 24°C above T c ) the size of PEO in IBAW was measured by light scattering method at different IBAW compositions. We observed expansion of PEO molecules in the mixed solvent, which can be explained using the single liquid approximation by Scott. Such an expansion caused the macromolecules to overlap and separate from the solvent.