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1. 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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2. Mapping of composition‐rheology relationships in polymer composite‐type precursors

   https://doi.org/10.1002/pc.29220  

   Grover, Caitlin_A; Bernal, Cindy_Bonilla; Sargin, Irmak; Beckman, Scott_P; Gozen, B_Arda (November 2024, Polymer Composites)

   Abstract Considering their simplicity, processibility, and tunable rheological properties, polymer composite‐type precursors hold exceptional promise in the processing of polymers, ceramics, metals, and their composites. This large variety of precursors used in many different applications cover a large compositional space with dramatically varying rheological properties. Understanding how precursor composition influences their rheological properties is a key need towards streamlining the design and implementation of these precursors. With regard to this design advancement, this study elucidates the composition‐rheology relationships of graphene‐poly(ethylene) oxide (PEO) composite inks as a sample polymer composite‐type precursor. To this end, shear and extensional rheology of numerous compositions were studied across a wide compositional space, which varied graphene concentration, total solid concentration, and binder molecular weight. These studies showed that composition greatly affected various rheological parameters, such as the overall presence of yielding behavior. Specifically, this study illustrated the influence of (i) binder structure, (ii) total solid loading, and (iii) binder‐filler interactions on ink rheology. Extensional rheology was studied to examine how relaxation behaviors were dependent on composition and explicate how relaxation behaviors coincide with responses to shear forces. In tandem, our results illuminate significant composition‐rheology relationships in polymer composite‐type precursors. HighlightsRheology of polyethylene oxide‐graphene composite precursors were studied.Shear and extensional rheology, and their correlations were investigated.Composition‐binder molecular weight‐yielding relationships were elucidated.Extensional relaxation regimes were identified with respect to composition.Results can be used to determine compositional ranges for different processes. 

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3. Techniques to characterize dynamics in biomaterials microenvironments: XPCS and microrheology of alginate/PEO–PPO–PEO hydrogels

   https://doi.org/10.1039/d0sm01628d  

   Quah, Suan P.; Zhang, Yugang; Fluerasu, Andrei; Yu, Xiaoxi; Zheng, Bingqian; Yin, Xuechen; Liu, Weiping; Bhatia, Surita R. (February 2021, Soft Matter)

   null (Ed.)

   Many recent studies have highlighted the timescale for stress relaxation of biomaterials on the microscale as an important factor in regulating a number of cell-material interactions, including cell spreading, proliferation, and differentiation. Relevant timescales on the order of 0.1–100 s have been suggested by several studies. While such timescales are accessible through conventional mechanical rheology, several biomaterials have heterogeneous structures, and stress relaxation mechanisms of the bulk material may not correspond to that experienced in the cellular microenvironment. Here we employ X-ray photon correlation spectroscopy (XPCS) to explore the temperature-dependent dynamics, relaxation time, and microrheology of multicomponent hydrogels comprising of commercial poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide) (PEO–PPO–PEO) triblock copolymer F127 and alginate. Previous studies on this system have shown thermoreversible behavior in the bulk oscillatory shear rheology. At physiological temperatures, bulk rheology of these samples shows behavior characteristic of a soft solid, with G ′ > G ′′ and no crossover between G ′ and G ′′ over the measurable frequency range, indicating a relaxation time >125 s. By contrast, XPCS-based microrheology shows viscoelastic behavior at low frequencies, and XPCS-derived correlation functions show relaxation times ranging from 10–45 s on smaller length scales. Thus, we are able to use XPCS to effectively probe the viscoelasticity and relaxation behavior within the material microenvironments. 

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4. 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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5. Interfacial electrostatics of poly(vinylamine hydrochloride), poly(diallyldimethylammonium chloride), poly- l -lysine, and poly- l -arginine interacting with lipid bilayers

   https://doi.org/10.1039/C7CP07353D  

   McGeachy, A. C.; Dalchand, N.; Caudill, E. R.; Li, T.; Doğangün, M.; Olenick, L. L.; Chang, H.; Pedersen, J. A.; Geiger, F. M. (January 2018, Physical Chemistry Chemical Physics)

   Charge densities of cationic polymers adsorbed to lipid bilayers are estimated from second harmonic generation (SHG) spectroscopy and quartz crystal microbalance with dissipation monitoring (QCM-D) measurements. The systems surveyed included poly(vinylamine hydrochloride) (PVAm), poly(diallyldimethylammonium chloride) (PDADMAC), poly- l -lysine (PLL), and poly- l -arginine (PLR), as well as polyalcohol controls. Upon accounting for the number of positive charges associated with each polyelectrolyte, the binding constants and apparent free energies of adsorption as estimated from SHG data are comparable despite differences in molecular masses and molecular structure, with Δ G ads values of −61 ± 2, −58 ± 2, −57 ± 1, −52 ± 2, −52 ± 1 kJ mol −1 for PDADMAC 400 , PDADMAC 100 , PVAm, PLL, and PLR, respectively. Moreover, we find charge densities for polymer adlayers of approximately 0.3 C m −2 for poly(diallyldimethylammonium chloride) while those of poly(vinylamine) hydrochloride, poly- l -lysine, and poly- l -arginine are approximately 0.2 C m −2 . Time-dependent studies indicate that polycation adsorption to supported lipid bilayers is only partially reversible for most of the polymers explored. Poly(diallyldimethylammonium chloride) does not demonstrate reversible binding even over long timescales (>8 hours). 

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