Session X52: Polymer and Polyelectrolyte Rheology II: Large Deformations

Entanglements in strongly strained high molecular weight polymer melts

We study highly entangled, fully equilibrated polymer melts [1] subject to strong isochoric uniaxial deformation in the non-linear viscoelastic regime by extensive molecular dynamics simulations. It is shown that the over-all conformations of the original paths as well as the primitive paths (PPs) affinely follow the deformation, while the distribution of entanglements along the PPs does not [2]. The signature of chain retraction predicted by the tube model right after deformation is verified qualitatively but not quantitatively in all directions [3]. Upon further relaxation an inhomogeneous distribution of topological constraints in the melts due to long-lived clustering of entanglements and the delayed relaxation in chain conformation is observed.

[1] H.-P. Hsu and K. Kremer, J. Chem. Phys. 144, 154907 (2016)
[2] H.-P. Hsu and K. Kremer, ACS Macro Lett. 7, 107 (2018)
[3] H.-P. Hsu and K. Kremer. Phys. Rev. Lett. 121, 167801 (2018)   

Strain Hardening in Extensional Rheology of Polyolefin Multilayer Films

The interfacial strength between isotactic polypropylene (iPP) and polyethylene (PE) depends on commercial grades, i.e., detailed molecular structures of the polymers.¹ We utilized extensional rheology to understand molten iPP/PE interfaces. We produced iPP/PE multilayer films prepared from Ziegler-Natta catalyzed high-density PE (zHDPE) and isotactic PP (ziPP) as well as metallocene catalyzed high-density PE (mHDPE), linear low-density PE (mLLDPE), and iPP (miPP). Films with 2, 160, and 640 alternating layers of ziPP/zHDPE, miPP/mHDPE, and miPP/mLLDPE were fabricated via multilayer coextrusion. Extensional rheology of the 160-layer miPP/mLLDPE system revealed a strain hardening behavior, which became more pronounced when the number of layers increased to 640, effectively quadrupling the interfacial area in multilayer films. The miPP/mHDPE system showed strain hardening only when the number of layers reached to 640. Strain hardening was not observed in individual mHDPE, mLLDPE, and miPP films, nor in their bilayers. We quantitatively model strain hardening in the multilayer films by adding the stress contribution from the equilibrium melt interfacial tension to the measured extensional contribution from the components.
¹ Jordan et al., Macromolecules 2018, 51, 2506.   

Molecular origin of extensional strain hardening in phenyl-containing polymer melts

We recently reported unexpectedly extensional strong strain hardening (SH) in poly(4-vinylbiphenyl) (PVBP) when subjected to uniaxial flow. We postulate that this behavior is due to a molecular rearrangement mechanism (supported by WAXS measurements) that involves flow-induced π- π stacking of the phenyl groups, which results in an enhancement of friction drag between polymer chains [PRL 119, 247801 (2017)]. To further elucidate the molecular mechanism of SH, we measure the time evolution of the molecular alignment using an extensional rheometer housed in a custom-built oven designed to facilitate in-situ synchrotron X-ray experiments. Based on the anisotropic 2D WAXS profiles, we defined two alignment factors, one for the backbone (A_bb) and one for the π-π stacking (A_ππ). Perfect correlation between A_bb and A_ππ are observed during extensional flow start-up and flow cessation. Moreover, linear relations between both A_bb and A_ππ and transient stress (σ_E⁺) are observed at stress values below 1 MPa. The resemblance between this behavior and the stress-optical rule observed in many polymeric liquids led us to propose a stress-WAXS rule (SWR): A_bb=C_bbσ_E⁺ and A_ππ=C_ππσ_E⁺. Finally, we will discuss the validity of the SWR in PVBP and polystyrene.   

Macromolecular Dynamics, Extensional Rheology, Pinch-off Dynamics, and Printability of Aqueous Solutions of Flexible and Semi-Flexible Polymers

Liquid transfer and drop formation/deposition processes associated with printing, spraying and coating flows involve complex free-surface flows including the formation of
columnar necks that undergo spontaneous capillary-driven instability, thinning and pinch-off. For
simple Newtonian fluids, the interplay of capillary, inertial and viscous
stresses determines the self-similar thinning and pinch-off dynamics. In rheologically complex fluids, extra elastic stresses alter the pinch-off dynamics. Here we show that dripping-onto-substrate (DoS) rheometry protocols that involve visualization and analysis of capillary-driven thinning of a columnar neck can be used for measuring extensional viscosity and extensional relaxation time of polymeric fluids. We investigate the role of chemical structure by contrasting behavior of aqueous solutions of flexible polyethylene oxide (PEO) with solutions of semi-flexible hydroxyethyl cellulose (HEC) and show that both flexibility and extensibility of chains dramatically influence the extensional rheology response and the macromolecular relaxation dynamics. We elucidate how macromolecular stretching and orientation modify excluded volume and hydrodynamic interactions, affecting extensional viscosity response as well as polymer relaxation dynamics.
  

Brownian dynamics simulations of linear and non-linear semidilute polymer solutions in extensional flow

Understanding the relationship between microstructure and bulk properties is an overarching goal in polymer rheology. This is particularly true for non-linear polymer architectures, where topological interactions resulting from chain crossing restrictions lead to flow rates which are locally much higher than the applied flow. Polymers can thus be deformed significantly from their expected conformations, modifying the molecular functionality and the bulk stress. Coarse-grained molecular simulations which satisfy crossing constraints are crucial for developing quantitative models of these phenomena. Using our iterative conformational averaging method for Brownian dynamics simulations, we present an explanation for unexpected dynamics in solutions of non-linear polymers first observed in single molecule imaging experiments. Starting from a solution of linear chains, we introduce trace ring or branched polymers and observe the effect on polymer relaxation and transient stretching in planar extensional flow. We determine the importance of hydrodynamic interactions for polymer solutions ranging from dilute to the entanglement concentration and show that non-linear polymers can exhibit accelerated or hindered relaxation below the overlap concentration.   

Pinch-off Dynamics, Extensional Rheology and Printability of Polyelectrolyte Solutions

Biological macromolecules like proteins, DNA and polysaccharides, and many industrial polymers, are classified together as polyelectrolytes for in solution, the repeat units in their backbone are decorated with disassociated, charge-bearing ionic groups, surrounded by counter-ions. In diverse applications like inkjet printing, sprayable cosmetics and insecticides, paints and coatings that involve the formation of fluid columns or sheets that undergo progressive thinning and pinch-off into drops, the dominant flow within the necking filament is extensional in nature. The extensional rheology response of the charged macromolecular solutions are not as well understood as that of their uncharged counterparts. Here focus on the characterization of capillary thinning and pinch-off dynamics, extensional rheology and printability of two model systems: sodium (polystyrene sulfonate) and poly(acrylic acid) by using dripping-onto-substrate (DoS) rheometry technique. Due to an interplay of hydrodynamics-induced and charged-induced stretching, both the measured extensional relaxation times and the extensional viscosity values show salt- and polymer concentration-dependent behavior that is not expected or anticipated from the typical shear rheology response.   

Plant-Sourced Polysaccharides for Turbulent Drag Reduction

The cost of polymer additives has been a key impediment in the adoption of polymer drag reduction techniques in large-scale marine applications. In this study, we investigate the mucilage extracted from the bran of plant seeds such as flax, chia and psyllium, as potential sources of inexpensive, water-soluble, high molecular weight polyelectrolytes for drag reduction in turbulent boundary layer flows. To this end, skin friction measurements in dilute solutions of both synthetic as well as plant-based high polymers are performed using a custom-built Taylor-Couette apparatus, operating in the fully turbulent flow regime, at Reynolds numbers between 10⁴ and 10⁵. As a specific example, we characterize the drag reducing properties of the aqueous mucilage extracted from flax seeds (Linum usitatissimum), and compare its performance to that of a commonly-used synthetic flexible homopolymer, namely, polyethylene oxide (PEO). The molecular and viscoelastic properties of the principal polysaccharide constituent in flax mucilage is also studied using size exclusion chromatography and extensional rheology techniques (CaBER). Finally, we compare the shear-induced degradation of both polymers under prolonged turbulent flow conditions, and explore the possibility of mitigating chain-scission processes by the use of appropriate ionic and non-ionic surfactant additives to modify the chain flexibility and develop polymer-surfactant complexes . The dilute mucilage solutions are seen to exhibit comparable drag reduction and degradation behavior as aqueous PEO, but on a much cheaper cost-basis, and can potentially serve as an effective, eco-friendly, and economical alternative to synthetic polymers in real-life drag reduction applications   

Rheology and Dynamic Adsorption of Polymer-Surfactant Complexes

The rheological properties of polymer-surfactant mixtures play a significant role in applications ranging from enhanced oil recovery, pharmaceutical and biological fluids, cosmetics, food, and coating. Addition of an ionic surfactant to an aqueous solution of neutral polymer like polyethylene oxide is known to result in a shear rheological response with non-monotonic concentration dependent variation, attributed to association complexes formed by hydrophobic interactions between surfactant monomers and polymers chains, as well as charge effects. Furthermore, the formation of association complexes changes both dynamic and equilibrium surface tension. However, due to a lack of suitable techniques, extensional rheology response of polymer-surfactant mixtures has not been characterized in adequate detail, even though most processing flows, especially those involving drop formation or liquid transfer are influenced by extensional rheology and pinch-off dynamics. In this study, we examine how pinch-off dynamics and the extensional rheological response of polymer solutions are modified by the addition of ionic surfactants. We utilize dripping-onto-substrate rheometry protocols and show that shear and extensional rheology response display contrasting concentration-dependent variation.
  

A Model for Salt Effects on Semidilute Polyelectrolyte Solutions: Equilibrium and Dynamics

Polyelectrolyte (PE) solutions are endowed with both viscous and elastic properties that differ significantly from uncharged polymer solutions. Owing to their universal existence and functionalities in biology and industry, PE solution physics have been widely studied yet are often restricted to specific assumptions, such as the presence of a theta solvent, rod/flexible chain configurations or weakly/strongly charge conditions. Here, we will present a theoretical model based on mean-field theory, which provides new insights and scaling laws for the chain conformation, relaxation time and viscosity of semidilute PE solutions in good solvents. By considering the elastic, excluded volume, and electrostatic energies due to the PE charges and double layer, our theoretical predictions agree well with experiments and offer interpretations for previously unexplained viscoelastic behavior of PE solutions in either high polymer density regimes or conditions with added salt. New criteria for the overlap concentration and other special regimes with varying salt concentrations and polymer density will also be discussed.
  

Entanglement density and crossovers of polyelectrolyte solutions

We present steady shear and oscillatory rheology data for flexible sodium polystyrene sulfonate (NaPSS) and semiflexible sodium carboxymethyl cellulose as a function of polymer concentration, degree of polymerisation and added salt. Semidilute non-entangled dynamics are well described by the Rouse model, in agreement with Dobrynin's model for flexible polyelectrolytes. Addition of salt leads to a decrease in chain dimensions and therefore in solution viscosity, the effect being much more pronounced for NaPSS than NaCMC, which can be understood as resulting from the intrinsic rigidity of NaCMC preventing chain collapse at high salt.

The entanglement concentration of NaCMC is found to be only weakly dependent on added salt concentration. This contrasts with the overlap crossover which increases by several orders of magnitude from salt-free conditions to 0.1 M NaCl. Further, we find that the entanglement density remains constant upon addition of salt despite significant changes in polymer conformation. Our results for entangled polyelectrolyte in salt-free and salt solutions strongly contradict several predictions of the current scaling models for polymer entanglement.   

Coarse grained simulations of migration of polyelectrolytes in a combination of flow fields and electric fields

Separations can be accomplished by controlling the relative rate at which different molecules travel through a microfluidic device. In a flow, the rate of travel of a molecule is largely determined by its relative position in the flow profile. Thus, if one can control molecules relative positions within a flow profile, one can influence separation. It has been found that the electrophoretic mobility of certain molecules, such as double-stranded DNA. is dependent on their conformation, which is in turn dependent on the local shear rate. This opens the door to using flow rate as a means to modify polymer conformation and by extension polymer mobility. Though this potential method opens the door to a variety of separations, there remain a number of fundamental questions about the underlying mechanism. To address these, we use a combination of theoretical calculations and Brownian dynamics simulations to probe how the combination of velocity gradients and electric field gradients impact particle mobility and migration.   

Ultralow interfacial tension of polyelectrolyte coacervates using drop retraction method

The interfacial tension of coacervates can be controlled by a number of physicochemical conditions, such as salt type and concentration, temperature, and polymer molecular mass. An improved understanding of these relationships will help advance applications such as polymer coatings, encapsulation media and wet adhesives. However, interfacial tension measurements are challenging due to the ultra-low magnitudes observed in polyelectrolyte coacervates. This presentation describes the use of shape retraction analysis of a deformed drop of dilute phase generated in situ in the coacervate. Our measurements confirm that the interfacial tension follows the mean field prediction of 3/2 scaling as a function of the difference between salt concentration and the critical salt concentration at which interfacial tension vanishes. The additional effects of temperatures and molecular mass will be described.   

Bubble dynamics in non-Newtonian fluids

Cavitation Rheology (CR) has been investigated as a novel technique for the characterization of the elastic and fracture properties of soft materials. In addition to measuring mechanical properties of solids, the formation and pressurization of a cavity, as dictated by some initial defect size, is an information-rich phenomenon that can illuminate the interfacial properties of fluids undergoing hydrostatic deformation. In this presentation, we explore the use of CR as a probe for the fluid properties of materials that display both Newtonian and non-Newtonian behavior. By tracking the evolution of the pressure inside the bubble and carefully analyzing the geometry of the bubble wall, we gain insight into properties of the fluid such as viscosity, bulk modulus and Poisson’s ratio. For the case of a model shear thickening fluid, we observe pressurization rate dependence on the critical cavitation pressure and evidence of transient fracture at sufficiently high pressurization rates. Our observations have deep implications in advancing the study of high-rate deformation of non-Newtonian fluids and soft materials.