Session G17: Padden Award Symposium

Elastic turbulence generates anomalous flow resistance in porous media

Polymer solutions are often injected in porous media for applications such as oil recovery and groundwater remediation. In many cases, the macroscopic flow resistance abruptly increases above a threshold flow rate in a porous medium, but not in bulk solution. The reason why has been a puzzle for over half a century. Here, by directly visualizing the flow in a transparent 3D porous medium, we demonstrate that this anomalous increase is due to the onset of an elastic instability in which the flow exhibits strong spatio-temporal fluctuations reminiscent of inertial turbulence, despite the vanishingly small Reynolds number. We quantitatively establish that the energy dissipated by unstable pore-scale fluctuations generates the anomalous increase in flow resistance through the entire medium. Moreover, we show that this finding applies in porous media with different mean grain sizes and generalize it to media that are more structurally heterogeneous akin to those that arise in many natural settings.  Thus, by linking the onset of unstable flow at the pore scale to transport at the macroscale, our work provides generally-applicable guidelines for predicting and controlling polymer solution flows in a variety of porous media.   

Direct visualization of bottlebrush polymers in the solid state with super-resolution microscopy

Although the behavior of single chains informs our understanding of polymer physics, a clear image of single chains that depicts their behavior within a bulk, solid state has eluded the polymers community for decades. We use super-resolution optical microscopy to capture clear and convincing images of individual chains within a bulk material using a model system of densely grafted chains called bottlebrush polymers. In this work, we directly visualize individual bottlebrush polymers by mixing dilute quantities of fluorescently-labeled bottlebrush chains with unlabeled linear polymers in films prepared by spin coating. We use these images to directly study chain conformation and orientation. We have found chains in this system are more rigid that previously reported in dilute solutions, and we explain these discrepancies by examining backbone conformation and its dependence on side chain length, grafting density, and molecular weight of the surrounding matrix. We have also found that chains orient in the direction parallel to centripetal force during spin coating. We investigate this phenomenon further by studying the dependence of chain orientation on location relative to center of the sample, solvent used for spin coating, and spin speed.   

Adhesive Contact of a Pressure Tunable, Patterned Elastomer

Applications like soft robotics and pick-and-place manufacturing benefit from using materials capable of achieving discrete levels of adhesion strength. Many studies have investigated switchable or singular tunable adhesion, but not the former. In this work, we develop a pressure tunable adhesive (PTA) with this capability by patterning an elastomer surface with rigid aspereties. The adhesive strength of the PTA is tuned by controlling the amount of pressure applied to the material. Flat punch indentation testing is used to characterize the adhesion strength of PTAs with varying pattern dimensions (asperity size and spacing). We show that the critical pull-off load , P_c, to separate the flat punch from the PTA increases with the amount of compressive load applied, P_m. Additionally, PTAs with small pattern dimensions have a high sensitivity to P_m. Increasing pattern dimensions reduces sensitivity to P_m and overall adhesion strength. A mechanics analysis is presented to describe the adhesive contact between a rigid body and the PTA. Additionally, we show that pressure tunable adhesion is possible for other thin film-bulk elastomer systems and demonstrate their capabilities for pick and place manufacturing.   

Ion correlations and transference numbers in polyelectrolyte solutions for Li-ion batteries

Nonaqueous polyelectrolytes have been presented in recent years as promising high cation transference number (t₊) alternatives to conventional Li-ion battery electrolytes: it is thought that by slowing the motion of the anions via attachment to a polymer chain, the cation will carry the majority of the electrolyte current. Experimental characterization of t₊ in these polyelectrolytes via the Nernst-Einstein (ideal solution) approximation corroborates this intuition, yielding t₊ values approaching unity. Here, we present the Onsager transport framework as a means of rigorously computing the transference number, without neglecting non-idealities, from molecular dynamics simulations. We demonstrate for a range of polyelectrolyte solutions that the Nernst-Einstein assumption does not yield a physically meaningful approximation for polyelectrolyte transport due the substantial anion-anion and cation-anion correlations in these systems, and that t₊ is generally lower in a polyelectrolyte than in a conventional battery electrolyte. This work has thus demonstrated critical flaws in the standard methods for experimentally characterizing transport in polyelectrolytes and has dispelled the notion that polyelectrolytes may yield promising transport behavior for next-generation batteries.   

Understanding chain motion at glass-forming polymer interfaces: dynamical gradients yield rubbery surfaces

Over 30 years of research has probed alterations in the dynamical response of polymers confined to nanoscale domains. These alterations include profound changes in rheological response that are central to these materials’ performance and poorly understood at a fundamental level. A central question is how alterations in Tg and segmental dynamics in these settings drive changes in polymer chain motion and rheology. Because the Rouse theory that underpins polymer dynamics assumes homogenous segmental dynamics, addressing this question demands development of a new fundamental understanding of chain motion in massively heterogeneous environments. Beyond its fundamental implications, this problem has received broad interest because of its implications for the rheological response of nanostructured and nano dimensioned polymers ranging from thin films to polymeric nanocomposites.

Here we report on simulations and theory, supported by experiment, providing the first predictive understanding of polymer chain relaxation in a Tg gradient at the surface of a film. Our simulation results indicate that the gradient in segmental relaxation timescales near a polymer interface drives emergence of rubbery-like surface behavior, even in unentangled polymers, with a lifetime that grows upon cooling. As a consequence, time temperature superposition (TTS) breaks down at surface of polymer films for both entangled and unentangled polymers. We establish a new theory of Rouse motion near polymer interfaces involving large Tg gradients, with this theory predicting that the Tg gradient produces a diffuse transient tethering effect that in turn leads to the emergence of surface rubbery-like behavior on cooling. These results provide a new basis for understanding and analyzing rheological response data in polymer films in the glass-formation range and likely has major implications for the rheological response of nanocomposites as well.   

Circumventing physical limits of liquid crystalline elastomer actuation

Liquid crystalline elastomers (LCEs) are functional materials widely studied for their ability to produce large deformations. This deformation is a result of order disruption between mesogenic moieties incorporated in the polymer network. Often, reports detail that LCEs undergo an order-disorder transition analogous to low molar mass liquid crystalline systems.  However, in practice, LCEs deviate from this behavior. Prior investigation has associated the 2^nd order nature of the phase transition of LCEs to the retention of order in the crosslinks of the polymer network.

These physical limitations inspire and motivate our research, which has pursued theory-driven experimental study to enhance the stimuli-response.  In the preparation of LCE, we demonstrate the considerable influence of the orientational genesis (e.g., ordering of mesogens) during crosslinking in the LCEs. Theory suggests that the alignment method, which is associated with the phase conditions, will dramatically affect the stimuli-response. We examine this in developing a reaction chemistry amenable to both surface and stretch alignment, to prepare compositionally identical LCEs. As predicted by theory, the thermotropic response of LCEs is shifted by as much as 75°C between the two LCEs.  From this basis, we then detail that the thermotropic response of LCE can be further enhanced by reducing the strength of intermolecular interactions between mesogenic components in these LCEs.   

Impact of placement and composition of hydrogen bonding groups along polymer chains on blend morphology using coarse-grained molecular dynamics simulations

In this talk, we will present our work involving molecular dynamics (MD) simulations and a recently developed coarse-grained (CG) model for polymers with directional interactions [Macromolecules (2019) 52 (7) 2725-2735] to predict polymer blends’ morphology for varying composition (i.e., fraction of monomers with H-bonding acceptor/donor groups along polymer chains) and placement of H-bonding acceptor/donor groups along polymer chains. We first validate our CGMD simulation approach by reproducing previously published theoretical phase diagrams for end-associating polymers at varying H-bonding strength vs. polymer segregation strength. We use the validated CGMD approach to elucidate how blends’ morphology varies with random and regular placement of multiple H-bonding groups along the polymer chains. For these varying placements of multiple H-bonding groups, we characterize the blend morphology (e.g., two-phase, lamellar, bicontinuous microemulsion, disordered and disordered microphase) and domain sizes as a function of varying H-bonding attraction vs. polymer segregation strength. Through the results from this study, we establish design rules for incorporating H-bonding functional groups along polymer chains to achieve precisely tuned blend morphology.    

Dynamic Associations via Stickers, Micelles, or Particles Facilitate Macromolecular Engineering of Aqueous Formulations

Hydrophobic stickers, charged micelles, and charged particles provide hydrophilic flexible polymers with temporary, dynamic junctions with association-disassociation time that changes with polymer concentration and deformation. Macromolecular engineering of formulations typically relies on the characterization of response to shear flow, with velocity gradients perpendicular to flow direction, emulating processing flows through channels and drag flows near moving solid surfaces. However, characterizing extensional rheology response has remained a longstanding challenge, even though streamwise velocity gradients, associated with extensional flows, often arise during processing especially during dispensing and liquid transfer by dripping, jetting, or spraying. In this contribution, we examine the contrasting influence of dynamic associations on macromolecular dynamics and formulation rheology in response to shear flows using torsional rheometry and extensional flows with dripping-onto-substrate (DoS) rheometry. We find that dynamic associations help to tweak the rate-dependent rheological response, to facilitate easier dispensing and coating, better sagging, leveling, and storage while reducing misting and stringiness.      

Sub-3-Nanometer Domain Spacings of Ultrahigh-χ Multiblock Copolymers with Pendant Ionic Groups: Morphologies, Phase Diagram, and Applications

We investigated the temperature-dependent phase behavior and interaction parameter of polyethylene-based multiblock copolymers with pendant ionic groups. These step-growth polymers contain short polyester blocks with a single SO₃^-Li⁺ group strictly alternating with polyethylene blocks of x-carbons (PESxLi, x = 12, 18). At room temperature, these polymers exhibit layered morphologies with semicrystalline polyethylene blocks. Upon heating above the melting point, PES18Li shows two order-to-order transitions involving  gyroid and hexagonal morphologies. For PES12Li, an order-to-disorder transition accompanies the melting of the polyethylene blocks. Notably, a Flory-Huggins interaction parameter was determined from the disordered PES12Li using mean-field theory: χ (T) = 77.4/T + 2.95 (T in K) and χ (25°C) ~ 3.21. This ultrahigh-χ indicates that the polar ionic and non-polar polyethylene blocks are highly incompatible, and affords well-ordered morphologies with sub-3 nm domain spacings. The experimental phase diagram of these polymers contradicts the self-consistent field theory of neutral multiblock copolymers. Finally, the potential applications of these multiblock copolymers as ion-conducting membranes and templating nanoscale materials will be discussed.   

Mechanisms of Self-Diffusion of Linear Associative Polymers Studied by Brownian Dynamics Simulation

Anomalous diffusion of associative polymers has been attributed to a coexistence of multiple diffusive modes, though their role in networks with sticker densities below the mean-field limit is unclear. Here, Brownian dynamics simulations are performed to study self-diffusion of linear associative polymers with various sticker densities, revealing a rich interplay between segmental motion, walking diffusion, and hopping diffusion on different length scales. This interplay creates several self-diffusive regimes, including two apparent superdiffusive regimes before terminal Fickian diffusion. The two superdiffusive regimes have distinct origins: while one occurs from a transition from walking to hopping, the second occurs from walking alone due to changes in the chain pervaded volume over time. Each regime is highly sensitive to the sticker density and binding kinetics due to their effects on the walking and hopping modes. Notably, increasing a chain's sticker density promotes loops and enhances the hopping likelihood, resulting in a non-monotonic effect of sticker density on the overall chain diffusivity. Scaling arguments are developed to predict characteristic walking and hopping diffusivities, with good agreement with simulation.   

Non-exponential stress relaxations in arrested gels

Non-exponential stress relaxation processes occur ubiquitously in arrested soft materials such as glasses, colloidal suspensions, biological networks, and highly associated polymer networks. Despite this ubiquity, the microscopic origins of such relaxation processes remain unclear. Here, we directly study this phenomenon through a combination of rheology and x-ray photon correlation spectroscopy on a model arrested associative gel. By investigating the microscopic relaxation processes in the gel in quiescent and in driven conditions, we find that these slow relaxation processes are governed by an interplay of microscopic fluctuations in the elastic stresses accrued during arrest, and of the elastic avalanches generated by small mechanical perturbations onto the system. We thus show that non-exponential stress relaxations in strongly arrested materials are a signature of non-linear relaxation processes governed by internal stress heterogeneities, and a manifestation of the fractal potential energy landscapes underlying such materials.   

Supramolecular Bonds, Phase Equilibrium, and Self-Assembly in Telechelic Polymer Blends

We study a binary blend of telechelic homopolymers that can form reversible AB-type bonds at chain ends. Previous studies of reversibly-bonding polymer systems have been limited by the computational demand of accounting for an infinite number of possible reaction products in a spatially inhomogeneous self-assembled structure. We demonstrate that newly developed theoretical models and numerical methods enable the simultaneous computation of phase equilibrium, reaction equilibrium and self-assembly via self-consistent field theory. Phase diagrams are computed at a variety of physically relevant conditions and are compared with non-reactive analogues. We briefly survey future work that extends these methods to network-forming polymers.