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1. Insights into the thermomechanical and interfacial behaviors of polymer‐clay nanocomposites via coarse‐grained molecular dynamics simulations

   https://doi.org/10.1002/pc.28357  

   Nie, Wenjian; Liao, Yangchao; Ghazanfari, Sarah; Wang, Yang; Wang, Xingyu; Huang, Ying; Xia, Wenjie (March 2024, Polymer Composites)

   Abstract Polymer‐clay nanocomposites (PCNs) are commonly applied as multi‐functional structural materials with exceptional thermomechanical properties, while maintaining the characteristics of lightweight and optical clarity. In this study, building upon previously developed coarse‐grained (CG) models for nanoclay and poly (methyl methacrylate) (PMMA), we employ molecular dynamics (MD) simulations to systematically investigate the thermomechanical properties of PCNs when arranged in stacked configurations. Incorporating stacked clay nanofillers into a polymer matrix, we systematically conduct shear and tensile simulations to investigate the influences of variations in weight percentage, system temperature, and nanoclay size on the thermomechanical properties of PCNs at a fundamental level. The weight percentage of nanoclay in nanocomposites proves to have a significant influence on both the shear and Young's modulus (e.g., the addition of 10 Wt% nanoclay leads to an increase of 32.6% in the Young's modulus), with each exhibiting greater mechanical strength in the in‐plane direction compared to the out‐of‐plane direction, and the disparity between these two directions further widens with an increase in the weight percentage of nanoclay. Furthermore, the increase in the size of nanoclay contributes to an overall modulus enhancement in the composite while the growth reaches a saturation point after a certain threshold of about 10 nm. Our simulation results indicate that the overall dynamics of PMMA are suppressed due to the strong interactions between nanoclay and PMMA, where the confinement effect on local segmental dynamics of PMMA decays from the nanoclay‐polymer interface to the polymer matrix. Our findings provide valuable molecular‐level insights into microstructural and dynamical features of PCNs under deformation, emphasizing the pivotal role of clay‐polymer interface in influencing the thermomechanical properties of the composite materials. HighlightsCG modeling is performed to explore the thermomechanical behavior of PCN.Effects of nanoclay weight percentage and size on modulus are studied.Interface leads to nanoconfinement effect onT_gand molecular stiffness.Correlations between molecular stiffness and modulus are identified.Simulations show spatial variation of dynamical heterogeneity. 

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2. Impact of salt and fillers on the rheological properties of polymer composites

   https://doi.org/10.1002/pc.29409  

   Teymoory, Parya; Driscoll, Stephen_Burke; Zhao, Jingzhou; Shen, Caiwei (December 2024, Polymer Composites)

   Abstract Polymer composites with salts or conductive fillers are promising for various solid‐state energy storage applications, where processability is often determined by their rheological properties. This study investigates the effect of lithium salts and conductive fillers on the rheological behavior of polylactic acid (PLA)‐based composites. We specifically examine how these additives influence complex viscosity and the interactions between the salt, fillers, and polymer. Our findings reveal that adding salt to the polymer reduces its viscosity, whereas adding conductive fillers imparts a shear‐thinning property, which is advantageous for thermal processing methods like thermal drawing, injection molding, or 3D printing. The combination of salt and conductive fillers results in multifunctional electrode‐electrolyte composites with enhanced shear‐thinning behavior and improved storage modulus. Characterizations through x‐ray diffraction, electrical measurements, and transmission electron microscopy link the electrical properties and morphology with rheological behavior. The formation of a robust filler network in these composites ensures stable viscoelastic behavior across a range of temperatures and frequencies, indicating their suitability for efficient manufacturing of polymer‐based solid‐state electrode‐electrolyte composites via thermal processing. HighlightsShear‐thinning behavior enhanced by conductive fillers.Viscosity increased with CB and CNT fillers, forming robust networks.Salt reduced viscosity but filler networks dominated flow behavior.Filler combinations led to stable viscoelastic properties across temperatures.Polymer electrolyte–electrode composites improved processability and storage modulus. 

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3. Effects of Dye Addition on the Rheological Properties of Aqueous Polymer Solutions

   https://doi.org/10.1021/acs.langmuir.4c01533  

   Upoma, Marufa Akter; He, Ziwen; Tran, Huy; Sivells, Tiara; Cyran, Jenée D; Pack, Min Young (September 2024, Langmuir)

   In many commercial applications, polymer–dye interactions are frequently encountered from food to wastewater treatment, and while shear rheology has been well characterized, the extensional properties are not well known. The extensional viscosity ηE and relaxation time λE are the extensional rheological parameters that provide valuable insights into how aqueous polymers respond during deformation, and this study investigated the effect of dyes on the extensional rheology of three different aqueous polymer solutions (e.g., anionic, cationic, and neutral) paired with two different dye salts (e.g., anionic and cationic) using drop pinch-off experiments. We have found that the influence of dyes on the pinch-off dynamics is complex but generally leads to a decrease in, for example, the apparent extensional relaxation time. We have utilized the dripping-onto-substrate method to probe the uniaxial deformation of widely used polymers such as xanthan gum (XG), poly(diallyldimethylammonium chloride) (PDADMAC), and poly(ethylene oxide) (PEO) as the anionic, cationic, and neutral polymers, respectively, paired with either fluorescein (Fl) or methylene blue (MB) as the anionic and cationic dyes, respectively. Polymer–dye pairs with opposite charges (e.g., XG–MB and PDADMAC–Fl) displayed a pronounced decrease in pinch-off times, but even PEO, which is a neutral polymer, resulted in decreased pinch-off times, which was restored by the addition of NaCl. The pinch-off times for the Boger fluid (mixture of poly(ethylene glycol) and PEO), however, were surprisingly uninfluenced by dyes. These results showed that not only did the small addition of dyes strongly decrease the polymer relaxation times, but the relative importance of the dye salts on the polymer pinch-off dynamics was also different from that of pure salts such as NaCl. 

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4. Dynamics and extensional rheology of polymer–surfactant association complexes

   https://doi.org/10.1039/D1SM00335F  

   Martínez Narváez, Carina D.; Mazur, Thomas; Sharma, Vivek (June 2021, Soft Matter)

   null (Ed.)

   Understanding and characterizing the influence of polymers and surfactants on rheology, application, and processing is critical for designing complex fluid formulations for enhanced oil recovery, pharmaceuticals, cosmetics, foods, inks, agricultural sprays, and coatings. It is well-established that the addition of anionic surfactant like sodium dodecyl sulfate (SDS) to an aqueous solution of an oppositely-charged or uncharged polymer like poly(ethylene oxide) (PEO) can result in the formation of the polymer–surfactant association complexes (P 0 S − ACs) and a non-monotonic concentration-dependent variation in zero shear viscosity. However, the extensional rheology response of polymer–surfactant mixtures remains relatively poorly understood, partially due to characterization challenges that arise for low viscosity, low elasticity fluids, even though the response to strong extensional flows impacts drop formation and many processing operations. In this article, we use the recently developed dripping-onto-substrate (DoS) rheometry protocols to characterize the pinching dynamics and extensional rheology response of aqueous P 0 S − solutions formulated with PEO (P 0 ) and SDS (S − ), respectively. We find the PEO–SDS mixtures display a significantly weaker concentration-dependent variation in the extensional relaxation time, filament lifespan, and extensional viscosity values than anticipated by the measured shear viscosity. 

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5. Evaporation-controlled dripping-onto-substrate (DoS) extensional rheology of viscoelastic polymer solutions

   https://doi.org/10.1038/s41598-022-08448-x  

   Robertson, Benjamin P.; Calabrese, Michelle A. (March 2022, Scientific Reports)

   Abstract Extensional flow properties of polymer solutions in volatile solvents govern many industrially-relevant coating processes, but existing instrumentation lacks the environment necessary to control evaporation. To mitigate evaporation during dripping-onto-substrate (DoS) extensional rheology measurements, we developed a chamber to enclose the sample in an environment saturated with solvent vapor. We validated the evaporation-controlled DoS device by measuring a model high molecular weight polyethylene oxide (PEO) in various organic solvents both inside and outside of the chamber. Evaporation substantially increased the extensional relaxation time$$\lambda _{E}$$ ${\lambda }_E$for PEO in volatile solvents like dichloromethane and chloroform. PEO/chloroform solutions displayed an over 20-fold increase in$$\lambda _{E}$$ ${\lambda }_E$due to the formation of an evaporation-induced surface film; evaporation studies confirmed surface features and skin formation reminiscent of buckling instabilities commonly observed in drying polymer solutions. Finally, the relaxation times of semi-dilute PEO/chloroform solutions were measured with environmental control, where$$\lambda _{E}$$ ${\lambda }_E$scaled with concentration by the exponent$$m=0.62$$ $m=0.62$. These measurements validate the evaporation-controlled DoS environment, and confirm that chloroform is a good solvent for PEO, with a Flory exponent of$$\nu =0.54$$ $\nu =0.54$. Our results are the first to control evaporation during DoS extensional rheology, and provide guidelines establishing when environmental control is necessary to obtain accurate rheological parameters. 

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