A Molecular Rheology Dynamics Study on 3D Printing of Liquid Crystal Elastomers

This work presents a rheological study of a biocompatible and biodegradable liquid crystal elastomer (LCE) ink for three dimensional (3D) printing. These materials have shown that their structural variations have an effect on morphology, mechanical properties, alignment, and their impact on cell response. Within the last decade LCEs are extensively studied as potential printing materials for soft robotics applications, due to the actuation properties that are produced when liquid crystal (LC) moieties are induced through external stimuli. This report utilizes experiments and coarse-grained molecular dynamics to study the macroscopic rheology of LCEs in nonlinear shear flow. Results from the shear flow simulations are in line with the outcomes of these experimental investigations. This work believes the insights from these results can be used to design and print new material with desirable properties necessary for targeted applications.

Influence of solute association on the phase behavior of 12-hydroxystearic acid/n-alkane solutions

The phase behavior of 12-hydroxystearic acid (12-HSA) in even-numbered alkanes ranging from octane (C₈) to hexatriacontane (C₃₆) was measured by visual observation of liquid + solid to liquid and liquid-liquid to liquid cloud points and liquid + solid to liquid + liquid transitions. In general solid phases were stabilized to low concentration and higher temperature with increasing alkane length. Liquid-liquid immiscibility was observed in larger alkanes starting with octadecane. The liquidus lines of shorter alkanes (octane to hexadecane) showing only liquid to liquid + solid transitions were fit with an attenuated associated solution model based on the Flory-Huggins lattice model assuming that 12-HSA forms a carboxylic acid dimer over all concentrations investigated. The fit results show that 12-HSA forms associated structures with degrees of association ranging from 3.7-4.5 dimers in the neat 12-HSA. At low concentrations, the 12-HSA is dissociated into dimers, however the free energy cost of dissociation stabilizes the solid phase giving a sharp knee at low concentrations. The role of 12-HSA association in its phase behavior and gelation behavior are discussed. More broadly, the importance of solute association in small molecule organogelators and its potential as a molecular design parameter similar to other component thermodynamic parameters, such as melting temperature and heat of fusion, is discussed.

Osmotic Contribution of Synthesized Betaine by Choline Dehydrogenase Using In Vivo and In Vitro Models of Post-traumatic Syringomyelia

Introduction

Syringomyelia (SM) is a debilitating spinal cord disorder in which a cyst, or syrinx, forms in the spinal cord parenchyma due to congenital and acquired causes. Over time syrinxes expand and elongate, which leads to compressing the neural tissues and a mild to severe range of symptoms. In prior omics studies, significant upregulation of betaine and its synthesis enzyme choline dehydrogenase (CHDH) were reported during syrinx formation/expansion in SM injured spinal cords, but the role of betaine regulation in SM etiology remains unclear. Considering betaine's known osmoprotectant role in biological systems, along with antioxidant and methyl donor activities, this study aimed to better understand osmotic contributions of synthesized betaine by CHDH in response to SM injuries in the spinal cord.

Methods

A post-traumatic SM (PTSM) rat model and in vitro cellular models using rat astrocytes and HepG2 liver cells were utilized to investigate the role of betaine synthesis by CHDH. Additionally, the osmotic contributions of betaine were evaluated using a combination of experimental as well as simulation approaches.

Results

In the PTSM injured spinal cord CHDH expression was observed in cells surrounding syrinxes. We next found that rat astrocytes and HepG2 cells were capable of synthesizing betaine via CHDH under osmotic stress in vitro to maintain osmoregulation. Finally, our experimental and simulation approaches showed that betaine was capable of directly increasing meaningful osmotic pressure.

Conclusions

The findings from this study demonstrate new evidence that CHDH activity in the spinal cord provides locally synthesized betaine for osmoregulation in SM pathophysiology.

Supplementary Information

The online version of this article contains supplementary material available 10.1007/s12195-022-00749-5.

Creep and Recovery Behavior of Vitrimers with Fast Bond Exchange Rate

Vitrimers encompass the desirable mechanical properties of thermosets with the recyclability of thermoplastics. This ability arises from the rearrangement of the vitrimer covalent network upon heating via a bond shuffling mechanism while its cross-link density remains preserved. This unique feature makes vitrimers interesting candidates for the design of materials that combine dimensional stability at high temperatures and solvent resistance with the ability to be reshaped and processed. Despite these advantages, vitrimer exhibits significant creep at operating conditions where thermosets show little or no creep. As the mechanical properties of vitrimers not only depend on their chemical composition but also on the dynamics of the polymer chains, molecular dynamics (MD) simulations can provide detailed molecular mechanisms of the system of interest under macroscopic stress-induced deformations. In this regard, the recently developed MD/Monte Carlo simulation methodology capable of capturing the bond exchange mechanics in vitrimers is used to study the creep and recovery response of a coarse-grained model thermoset and vitrimer with a fast bond exchange rate. The time-stress superposition principle is then successfully applied to the creep response. The resulting universal curves enable us to predict the long-time creep behavior of both systems extending the timescale from 4 to over 10 orders of magnitude.

Extending the timescale of molecular simulations by using time-temperature superposition: rheology of ionic liquids

Molecular dynamics simulations are used to determine the temperature dependence of the dynamic and rheological properties of a model imidazolium-based ionic liquid (IL). The simulation results for the volumetric properties of the IL are in good agreement with the experimental results. The temperature dependence of the diffusion coefficient of anions and cations follows the Vogel-Fulcher-Tammann equation over the range of the temperatures studied. The shear viscosity of the IL shows a Newtonian plateau at low shear rates and shear-thinning behavior at high shear rates. The dynamic modulus values indicate that the IL behaves like a viscous liquid at high temperatures and low frequencies, while its viscoelastic response becomes similar to that of an elastic solid at low temperatures and high frequencies. Using the time-temperature superposition (TTS) principle, the dynamic moduli, shear viscosity, and mean squared displacement of cations and anions in the diffusive regime can be collapsed onto master curves by applying a single set of shift factors. Due to the large mismatch in the timescale investigated by the atomistically detailed simulations and experiments, the glass transition temperature predicted in simulations shifts to higher values. When this timescale mismatch is accounted for by using appropriate shift factors, the master curves of the dynamic moduli obtained in simulations closely match those obtained in experiments. This result demonstrates the exciting ability of TTS to overcome the large timescale disparity between simulations and experiments which will enable the use of molecular simulations for quantitatively predicting the rheological property values at frequencies of practical interest.

Molecular Dynamics Simulations of Aqueous Nonionic Surfactants on a Carbonate Surface

The interactions and structure of secondary alcohol ethoxylates with 15 and 40 ethoxylate units in water near a calcite surface are studied. It is found that water binds preferentially to the calcite surface. Prediction of the free-energy landscape for surfactant molecules shows that single-surfactant molecules do not adsorb because they cannot get close enough to the surface because of the water layer for attractive ethoxylate-calcite or dispersion interactions to be significant. Micelles can adsorb onto the surface even with the intervening water layer because of the integrative effect of the attractive interactions of all the surfactant molecules. Adsorption is found to increase because of the closer proximity of the micelles to the surface due to a weakened water layer at higher temperatures. The free-energy well and barrier values are used to estimate surface to bulk partition coefficients for different surfactants and temperatures, and qualitative agreement is found with experimental observations. The combined effect of surfactant-water and surfactant-solid interactions is found to be responsible for an increased adsorption for nonionic surfactants as the system approaches the cloud point.

On the universality of the flow properties of soft-particle glasses

We identify the minimal interparticle interactions necessary for a particle dynamics simulation to predict the structure and flow behaviour of soft particle glasses (SPGs). Generally, two kinds of forces between the particles must be accounted for in simulations of SPGs: viscous or frictional drag forces and elastic contact forces. Far field drag forces are required to dissipate energy in the simulations and capture the effect of the rheology of the suspending fluid. Elastic forces are found to be dominant compared to near-field drag or other forms of friction forces and are the most important component to compute the rheology. The shear stress, the first and second normal stress differences for different interparticle force laws collapse onto universal master curves of the Herschel-Bulkley form by non-dimensionalizing the stress with the yield stress and the shear rate with the viscosity of the suspending fluid divided by the low-frequency shear modulus. The Herschel-Bulkley exponents are close to 0.5 with a slight dependence on the repulsive pairwise elastic forces.

Swelling of Random Copolymer Networks in Pure and Mixed Solvents: Multi-Component Flory-Rehner Theory

A generalized extension of Flory-Rehner (FR) theory is derived to describe equilibrium swelling of polymer networks, including copolymers with two or more chemically distinct repeat units, in either pure or mixed solvents. The model is derived by equating the chemical potential of each solvent in the liquid and gel phases at equilibrium, while assuming the deformation of the network chains is affine. Simplifications of the model are derived for specific cases involving homopolymer networks, copolymer networks, pure solvents, and binary solvent mixtures. With reasonable assumptions, the number of polymer-solvent interaction parameters that must be determined by experiments can be reduced to two effective parameters (θ₁ and θ₂), which describe the net interactions between water/copolymer (θ₁) and ethanol/copolymer (θ₂), respectively. Experimental measurements of the swelling of random copolymer networks of n-butyl acrylate and 2-hydroxyethyl acrylate in water, ethanol, and a 100 g/L ethanol/water mixture are utilized to validate the model. For a random copolymer network, θ₁ and θ₂ can be obtained by fitting the three-component FR model to equilibrium swelling data obtained in the pure solvents. Predicted solvent volume fractions for swelling in water-ethanol mixtures obtained by inserting fitted values of θ₁ and θ₂ into the four-component FR model are in reasonable agreement with experimental measurements.

Cosolvents as Liquid Surfactants for Boron Nitride Nanosheet (BNNS) Dispersions

Despite a range of promising applications, liquid-phase exfoliation of boron nitride nanosheets (BNNSs) is limited, both by low yield in common solvents as well as the disadvantages of using dissolved surfactants. One recently reported approach is the use of cosolvent systems to increase the as-obtained concentration of BNNS; the role of these solvents in aiding exfoliation and/or aiding colloidal stability of BNNSs is difficult to distinguish. In this paper, we have investigated the use of a t-butanol/water cosolvent to disperse BNNSs. We utilize solvent-exchange experiments to demonstrate that the t-butanol is in fact essential to colloidal stability; we then utilized molecular dynamics simulations to explore the mechanism of t-butanol/BNNS interactions. Taken together, the experimental and simulation results show that the key to the success of t-butanol (as compared to the other alcohols of higher or lower molecular weight) lies in its ability to act as a "liquid dispersant" which allows it to favorably interact with both water and BNNSs. Additionally, we show that the stable dispersions of BNNS in water/t-butanol systems may be freeze-dried to yield nonaggregated, redispersible BNNS powders, which would be useful in an array of industrial processes.

Liquid phase exfoliation and crumpling of inorganic nanosheets

Experiment and simulation demonstrate the polymer-assisted dispersion of inorganic 2D layered nanomaterials such as boron nitride nanosheets (BNNSs), MoS₂ nanosheets, and WS₂ nanosheets; spray drying can be used to alter such nanosheets into a crumpled morphology.

Effect of carbon nanotube functionalization on mechanical and thermal properties of cross-linked epoxy-carbon nanotube nanocomposites: role of strengthening the interfacial interactions

We have used amido-amine functionalized carbon nanotubes (CNTs) that form covalent bonds with cross-linked epoxy matrices to elucidate the role of the matrix-filler interphase in the enhancement of mechanical and thermal properties in these nanocomposites. For the base case of nanocomposites of cross-linked epoxy and pristine single-walled CNTs, our previous work (Khare, K. S.; Khare, R. J. Phys. Chem. B 2013, 117, 7444-7454) has shown that weak matrix-filler interactions cause the interphase region in the nanocomposite to be more compressible. Furthermore, because of the weak matrix-filler interactions, the nanocomposite containing dispersed pristine CNTs has a glass transition temperature (Tg) that is ∼66 K lower than the neat polymer. In this work, we demonstrate that in spite of the presence of stiff CNTs in the nanocomposite, the Young's modulus of the nanocomposite containing dispersed pristine CNTs is virtually unchanged compared to the neat cross-linked epoxy. This observation suggests that the compressibility of the matrix-filler interphase interferes with the ability of the CNTs to reinforce the matrix. Furthermore, when the compressibility of the interphase is reduced by the use of amido-amine functionalized CNTs, the mechanical reinforcement due to the filler is more effective, resulting in a ∼50% increase in the Young's modulus compared to the neat cross-linked epoxy. Correspondingly, the functionalization of the CNTs also led to a recovery in the Tg making it effectively the same as the neat polymer and also resulted in a ∼12% increase in the thermal conductivity of the nanocomposite containing functionalized CNTs compared to that containing pristine CNTs. These results demonstrate that the functionalization of the CNTs facilitates the transfer of both mechanical load and thermal energy across the matrix-filler interface.