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1. Increasing valence pushes DNA nanostar networks to the isostatic point

   https://doi.org/10.1073/pnas.1819683116  

   Conrad, Nathaniel ; Kennedy, Tynan ; Fygenson, Deborah K. ; Saleh, Omar A. ( March 2019 , Proceedings of the National Academy of Sciences)

   The classic picture of soft material mechanics is that of rubber elasticity, in which material modulus is related to the entropic elasticity of flexible polymeric linkers. The rubber model, however, largely ignores the role of valence (i.e., the number of network chains emanating from a junction). Recent work predicts that valence, and particularly the Maxwell isostatic point, plays a key role in determining the mechanics of semiflexible polymer networks. Here, we report a series of experiments confirming the prominent role of valence in determining the mechanics of a model system. The system is based on DNA nanostars (DNAns): multiarmed, self-assembled nanostructures that form thermoreversible equilibrium gels through base pair-controlled cross-linking. We measure the linear and nonlinear elastic properties of these gels as a function of DNAns arm number, f, and concentration [DNAns]. We find that, as f increases from three to six, the gel’s high-frequency plateau modulus strongly increases, and its dependence on [DNAns] transitions from nonlinear to linear. Additionally, higher-valence gels exhibit less strain hardening, indicating that they have less configurational freedom. Minimal strain hardening and linear dependence of shear modulus on concentration at high f are consistent with predictions for isostatic systems. Evident strain hardening and nonlinear concentration dependence of shear modulus suggest that the low-f networks are subisostatic and have a transient, potentially fractal percolated structure. Overall, our observations indicate that network elasticity is sensitive both to entropic elasticity of network chains and to junction valence, with an apparent isostatic point$5<f_c\le 6$in agreement with the Maxwell prediction.

    

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2. Optical-Tweezers-integrating-Differential-Dynamic-Microscopy maps the spatiotemporal propagation of nonlinear strains in polymer blends and composites

   https://doi.org/10.1038/s41467-022-32876-y  

   Peddireddy, Karthik R. ; Clairmont, Ryan ; Neill, Philip ; McGorty, Ryan ; Robertson-Anderson, Rae M. ( December 2022 , Nature Communications)

   Abstract How local stresses propagate through polymeric fluids, and, more generally, how macromolecular dynamics give rise to viscoelasticity are open questions vital to wide-ranging scientific and industrial fields. Here, to unambiguously connect polymer dynamics to force response, and map the deformation fields that arise in macromolecular materials, we present Optical-Tweezers-integrating-Differential -Dynamic-Microscopy (OpTiDMM) that simultaneously imposes local strains, measures resistive forces, and analyzes the motion of the surrounding polymers. Our measurements with blends of ring and linear polymers (DNA) and their composites with stiff polymers (microtubules) uncover an unexpected resonant response, in which strain alignment, superdiffusivity, and elasticity are maximized when the strain rate is comparable to the entanglement rate. Microtubules suppress this resonance, while substantially increasing elastic storage, due to varying degrees to which the polymers buildup, stretch and flow along the strain path, and configurationally relax induced stress. More broadly, the rich multi-scale coupling of mechanics and dynamics afforded by OpTiDDM, empowers its interdisciplinary use to elucidate non-trivial phenomena that sculpt stress propagation dynamics–critical to commercial applications and cell mechanics alike. 

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3. Modeling solvent evaporation during thin film formation in phase separating polymer mixtures

   https://doi.org/10.1039/c7sm02560b  

   Cummings, John ; Lowengrub, John S. ; Sumpter, Bobby G. ; Wise, Steven M. ; Kumar, Rajeev ( January 2018 , Soft Matter)

   Preparation of thin films by dissolving polymers in a common solvent followed by evaporation of the solvent has become a routine processing procedure. However, modeling of thin film formation in an evaporating solvent has been challenging due to a need to simulate processes at multiple length and time scales. In this work, we present a methodology based on the principles of linear non-equilibrium thermodynamics, which allows systematic study of various effects such as the changes in the solvent properties due to phase transformation from liquid to vapor and polymer thermodynamics resulting from such solvent transformations. The methodology allows for the derivation of evaporative flux and boundary conditions near each surface for simulations of systems close to the equilibrium. We apply it to study thin film microstructural evolution in phase segregating polymer blends dissolved in a common volatile solvent and deposited on a planar substrate. Effects of the evaporation rates, interactions of the polymers with the underlying substrate and concentration dependent mobilities on the kinetics of thin film formation are studied. 

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4. Spatiotemporal signatures of elastoinertial turbulence in viscoelastic planar jets

   https://doi.org/10.1103/PhysRevFluids.8.064610  

   Yamani, Sami ; Raj, Yashasvi ; Zaki, Tamer A. ; McKinley, Gareth H. ; Bischofberger, Irmgard ( June 2023 , Physical Review Fluids)

   The interplay between viscoelasticity and inertia in dilute polymer solutions at high deformation rates can result in inertioelastic instabilities. The nonlinear evolution of these instabilities generates a state of turbulence with significantly different spatiotemporal features compared to Newtonian turbulence, termed elastoinertial turbulence (EIT). We ex- plore EIT by studying the dynamics of a submerged planar jet of a dilute aqueous polymer solution injected into a quiescent tank of water using a combination of schlieren imaging and laser Doppler velocimetry (LDV). We show how fluid elasticity has a nonmonotonic effect on the jet stability depending on its magnitude, creating two distinct regimes in which elastic effects can either destabilize or stabilize the jet. In agreement with linear stability analyses of viscoelastic jets, an inertioelastic shear-layer instability emerges near the edge of the jet for small levels of elasticity, independent of bulk undulations in the fluid column. The growth of this disturbance mode destabilizes the flow, resulting in a turbulence transition at lower Reynolds numbers and closer to the nozzle compared to the conditions required for the transition to turbulence in a Newtonian jet. Increasing the fluid elasticity merges the shear-layer instability into a bulk instability of the jet column. In this regime, elastic tensile stresses generated in the shear layer act as an “elastic membrane” that partially stabilizes the flow, retarding the transition to turbulence to higher levels of inertia and greater distances from the nozzle. In the fully turbulent state far from the nozzle, planar viscoelastic jets exhibit unique spatiotemporal features associated with EIT. The time-averaged angle of jet spreading, an Eulerian measure of the degree of entrainment, and the centerline velocity of the jets both evolve self-similarly with distance from the nozzle. The autocovariance of the schlieren images in the fully turbulent region of the jets shows coherent structures that are elongated in the streamwise direction, consistent with the suppression of streamwise vortices by elastic stresses. These coherent structures give a higher spectral energy to small frequency modes in EIT characterized by LDV measurements of the velocity fluctuations at the jet centerline. Finally, our LDV measurements reveal a frequency spectrum characterized by a −3 power-law exponent, different from the well-known −5/3 power-law exponent characteristic of Newtonian turbulence. 

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5. Polymer blend-filled nanoparticle films via monomer-driven infiltration of polymer and photopolymerization

   https://doi.org/10.1039/C7ME00099E  

   Qiang, Yiwei ; Manohar, Neha ; Stebe, Kathleen J. ; Lee, Daeyeon ( January 2018 , Molecular Systems Design & Engineering)

   Incorporation of nanoparticles into polymer blend films can lead to a synergistic combination of properties and functionalities. Adding a large concentration of nanoparticles into a polymer blend matrix via conventional melting or solution blending techniques, however, is challenging due to the tendency of particles to aggregate. Herein, we report a straightforward approach to generate polymer blend/nanoparticle ternary composite films with extremely high loadings of nanoparticles based on monomer-driven infiltration of polymer and photopolymerization. The fabrication process consists of three steps: (1) preparing a bilayer with a nanoparticle (NP) layer atop a polymer layer, (2) annealing of the bilayer with a vapour mixture of a monomer and a photoinitiator, which undergoes capillary condensation and imparts mobility to the polymer layer and (3) exposing this film to UV light to induce photopolymerization of the monomer. The monomer used in this process is chemically different from the repeat unit of the polymer in the bilayer and is a good solvent for the polymer. The second step leads to the infiltration of the plasticized polymer, and the third step results in a blend of two polymers in the interstices of the nanoparticle layer. By varying the thickness ratio of the polymer and nanoparticle layers in the initial bilayers and changing the UV exposure duration, the volume fraction of the two polymers in the composite films can be adjusted. This versatile approach enables the design and engineering of a new class of nanocomposite films that contain a nanoscale-blend of two polymers in the interstices of a nanoparticle film, which could have combinations of unique mechanical and transport properties desirable for advanced applications such as membrane separations, conductive composite films and solar cells. Moreover, these polymer blend-filled nanoparticle films could serve as model systems to study the effect of confinement on the miscibility and morphology of polymer blends. 

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