Bulletin of the American Physical Society
APS March Meeting 2022
Volume 67, Number 3
Monday–Friday, March 14–18, 2022; Chicago
Session W42: Polymer Rheology and MechanicsInvited Live Streamed
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Sponsoring Units: DPOLY Chair: Michelle Calabrese, University of Minnesota Room: McCormick Place W-375A |
Thursday, March 17, 2022 3:00PM - 3:36PM |
W42.00001: Probing Dynamics of Photo-Gelation in Polymeric Systems Using Rheology Invited Speaker: Saad Khan Light-mediated crosslinked polymers and gels are being considered for a wide range of applications including 3D printing, coatings, photolithography and drug development. Understanding the mechanism of gelation, when and if the system forms a critical gel, 'dark' curing and competition between polymerization and crosslinking are crucial to the design of photo-cured polymer materials. In this presentation, we elucidate such characteristics using three systems of interest, acrylate-based free-radical crosslinking polymers, photo-active ion-containing biopolymers and photo-polymerizing coordinated ionic liquids. The critical gel point, as envisaged by the Winter-Chambon criteria is clearly demarcated in the acrylate system, but not so much in the ion-containing biopolymer. An alternative approach based on reduction in linear strain limit as the sample approaches gel point is proposed in this regard. We find that all these systems exhibit 'dark' curing, i.e., changes in elastic (G'') and viscous (G'') moduli after cessation of UV radiation. The traditional acrylate system exhibits such behavior only past the gel point with trapped free radicals furthering network formation and modulus increase only when exposed to additional deformation. In contrast, the ion-containing polymer system shows dark curing at all states as the diffusion of the ions which leads to gelation, occurs with and without radiation. Finally, the coordinated ionic liquid system containing vinyl monomers and salts simultaneously undergoes both polymerization and network formation. The ratio of monomer to salt dictates its rheological behavior - so one could potentially obtain systems with high G' that could either be a gel or a viscoelastic fluid. These examples illustrate that while UV crosslinkable systems have some commonality (e.g., modulus enhancement, network formation, dark cure), each is unique with its own mechanism, making them such exciting systems to probe. |
Thursday, March 17, 2022 3:36PM - 4:12PM |
W42.00002: New insights from transient nonlinear recovery rheology Invited Speaker: Simon A Rogers The transient nonlinear rheology of polymeric materials is of interest for a range of energy and environmental applications including rubber bushings and tires and inks for additive manufacturing. Typical observations include that of the Payne effect, where an overshoot is observed in the dynamic loss modulus during oscillatory shear straining of increasing magnitude. We show this overshoot to be due to an increase in energy dissipated through unrecoverable processes via an iterative recovery rheological protocol. Studying filled polymers and di-block bottlebrush block co-polymers, we show that the dynamic loss modulus can be experimentally decomposed into two components relating to recoverable and unrecoverable processes that may be associated with conformational changes and center of mass motions, respectively. Our experimental observations inspire the construction of a toy model in which the rate of conformation change influences the center of mass motion. In the linear viscoelastic regime, our model resembles the retarded Maxwell or Jeffreys models, but the nonlinear behavior predicts a frequency dependence to the Payne effect. The model also provides insight into the applicability of the Cox-Merz rule that relates the response to small dynamic perturbations to the highly out-of-equilibrium steady-state flow behavior. The model predicts that Cox-Merz is followed unchanged in the terminal regime, where unrecoverable processes account for almost all the energy dissipation, but requires rescaling at higher frequencies, where recoverable and unrecoverable processes dissipate similar amounts of energy. We verify the applicability of the modified rule by comparing it against published data. |
Thursday, March 17, 2022 4:12PM - 4:48PM |
W42.00003: Dynamic microenvironments created by human mesenchymal stem cells Invited Speaker: Kelly M Schultz During wound healing, human mesenchymal stem cells (hMSCs) orchestrate the healing process by regulating inflammation and coordinating tissue regeneration. Cell-laden hydrogels are designed to deliver additional hMSCs to wounds to enhance or restart healing. These scaffolds are designed to mimic native tissues, including physical and chemical cues. We characterize the feedback between microenvironmental cues presented to cells and the microenvironments cells engineer in response. We encapsulate hMSCs in a well-defined hydrogel that recapitulates aspects of the native extracellular matrix. Our hydrogel consists of a 4-arm poly(ethylene glycol) end-functionalized with norbornene which is cross-linked with a matrix metalloproteinase (MMP) degradable peptide that is cleaved by hMSC secretions. We use multiple particle tracking microrheology (MPT) and bi-disperse MPT to characterize spatio-temporal cell-mediated degradation. In MPT, particles are embedded in the material and their Brownian motion is measured and related to rheological properties. Bi-disperse MPT embeds two different particle sizes into the material to simultaneously measure lengthscale-dependent rheology. Using MPT, we determine hMSCs create a microenvironment where the cross-link density decreases as distance from the cell increases, which enables spreading and attachment prior to motility. The cell simultaneous secretes scaffold degrading MMPs and tissue inhibitors of metalloproteinases (TIMPs), which inhibit MMP activity and scaffold degradation. We reverse this degradation profile by inhibiting TIMPs, which increases hMSC motility. Using bi-disperse MPT, we simultaneously measure cell-mediated degradation and reversible remodeling. This work highlights the ability for a cell to selectively remodel their microenvironment during motility. These measurements can inform design of implantable biomaterials that instruct cellular processes for cell delivery to wounds. |
Thursday, March 17, 2022 4:48PM - 5:24PM |
W42.00004: Dynamics of semi-dilute polymer-particle solutions Invited Speaker: Jeremy C Palmer The dynamics of polymer-particle solutions are of great interest for applications in materials processing, drug delivery and oil recovery. When the particles are significantly larger than the polymers, the dynamics of these systems are well-described theoretically. Deviations from predicted behavior arise, however, when the size of the particles and length scales in the polymer solutions are comparable. We explore the polymer-particle dynamics in this limit using multiparticle collision dynamics (MPCD), an advanced simulation algorithm for modeling solvent-mediated hydrodynamic interactions in mesoscale systems. The MPCD simulations provide new insight into the coupling between linear polymer chains and suspended particles on short- and long- time scales. Findings from our recent studies examining the influence of chain stiffness and morphology (e.g., rings versus linear chains) on the dynamics of polymer-particle solutions will also be discussed. |
Thursday, March 17, 2022 5:24PM - 6:00PM |
W42.00005: TBD Invited Speaker: Nicole Demarquette
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