Naresh C. Osti

Effect of Metal Ion Intercalation on the Structure of MXene and Water Dynamics on its Internal Surfaces

MXenes are a recently discovered class of 2D materials with an excellent potential for energy storage applications. Because MXene surfaces are hydrophilic and attractive interaction forces between the layers are relatively weak, water molecules can spontaneously intercalate at ambient humidity and significantly influence the key properties of this 2D material. Using complementary X-ray and neutron scattering techniques, we demonstrate that intercalation with potassium cations significantly improves structural homogeneity and water stability in MXenes. In agreement with molecular dynamics simulations, intercalated potassium ions reduce the water self-diffusion coefficient by 2 orders of magnitude, suggesting greater stability of hydrated MXene against changing environmental conditions.

Luminescent phosphonium polyelectrolyte prepared from a diphosphine chromophore: synthesis, photophysics, and layer-by-layer assembly

A luminescent phosphonium-containing polyelectrolyte (P1-OHx) has been prepared by polymerization of a chromophore-containing diphosphine. The resultant cationic polyelectrolyte displays solvatochromism and undergoes facile layer-by-layer assembly with anionic poly(acrylic acid) to form smooth, well-defined films. Solution and film characterization by absorption and photoluminescence spectroscopy, and AFM imaging of films, are reported.

Bipyridyl-modified phosphonium polyelectrolytes: synthesis, photophysics, metal ion coordination and layer-by-layer assembly with anionic conjugated polymers

A novel class of chromophoric polyelectrolytes bearing phosphonium moieties in their backbones has been prepared to incorporate metal-binding 2,2′-bipyridyl (bipy) units. The effect of metal ion binding on the photophysical properties of the polyelectrolyte was probed. By binding metal ions to the polycation, the relative charge loading could be progressively increased as a strategy to control its supramolecular assembly with polyanions. The metallated and metal-free polyelectrolytes were utilized in layer-by-layer self-assembly with poly(acrylic acid) or anionic conjugated polymers including a poly(p-phenylene vinylene) and polythiophene derivative. The photophysical properties, nanomorphology, and substrate dependence of the resultant multilayer films were examined, and indicate that layer density, composition, and morphology can be tuned by metallation and ion pairing effects.

Solvent Polarity Governs Ion Interactions and Transport in a Solvated Room-Temperature Ionic Liquid

We explore the influence of the solvent dipole moment on cation-anion interactions and transport in 1-butyl-3-methyl-imidazolium bis(trifluoromethylsulfonyl), [BMIM+][Tf2N-]. Free energy profiles derived from atomistic molecular dynamics (MD) simulations show a correlation of the cation-anion separation and the equilibrium depth of the potential of mean force with the dipole moment of the solvent. Correlations of the ion diffusivity with the dipole moment and the concentration of the solvent were further demonstrated by classical MD simulations. Quasi-elastic neutron scattering experiments with deuterated solvents reveal a complex picture of nanophase separation into the ionic liquid-rich and solvent-rich phases. The experiment corroborates the trend of concentration- and dipole moment-dependent enhancement of ion mobility by the solvent, as suggested by the simulations. Despite the considerable structural complexity of ionic liquid-solvent mixtures, we can rationalize and generalize the trends governing ionic transport in these complex electrolytes.

Water dynamics in rigid ionomer networks

The dynamics of water within ionic polymer networks formed by sulfonated poly(phenylene) (SPP), as revealed by quasi-elastic neutron scattering (QENS), is presented. These polymers are distinguished from other ionic macromolecules by their rigidity and therefore in their network structure. QENS measurements as a function of temperature as the fraction of ionic groups and humidity were varied have shown that the polymer molecules are immobile while absorbed water molecules remain dynamic. The water molecules occupy multiple sites, either bound or loosely constrained, and bounce between the two. With increasing temperature and hydration levels, the system becomes more dynamic. Water molecules remain mobile even at subzero temperatures, illustrating the applicability of the SPP membrane for selective transport over a broad temperature range.

Association of a multifunctional ionic block copolymer in a selective solvent

The self-assembly of multiblock copolymers in solutions is controlled by a delicate balance between inherent phase segregation due to incompatibility of the blocks and the interaction of the individual blocks with the solvent. The current study elucidates the association of pentablock copolymers in a mixture of selective solvents which are good for the hydrophobic segments and poor for the hydrophilic blocks using small angle neutron scattering (SANS). The pentablock consists of a center block of randomly sulfonated polystyrene, designed for transport, tethered to poly-ethylene-r-propylene and end-capped by poly-t-butyl styrene, for mechanical stability. We find that the pentablock forms ellipsoidal core-shell micelles with the sulfonated polystyrene in the core and Gaussian decaying chains of swollen poly-ethylene-r-propylene and poly-t-butyl styrene tertiary in the corona. With increasing solution concentration, the size of the micelle, the thickness of the corona, and the aggregation number increase, while the solvent fraction in the core decreases. In dilute solution the micelle increases in size as the temperature is increased, however, temperature effects dissipate with increasing solution concentration.

Multimodality of Structural, Electrical, and Gravimetric Responses of Intercalated MXenes to Water

Understanding of structural, electrical, and gravimetric peculiarities of water vapor interaction with ion-intercalated MXenes led to design of a multimodal humidity sensor. Neutron scattering coupled to molecular dynamics and ab initio calculations showed that a small amount of hydration results in a significant increase in the spacing between MXene layers in the presence of K and Mg intercalants between the layers. Films of K- and Mg-intercalated MXenes exhibited relative humidity (RH) detection thresholds of ∼0.8% RH and showed monotonic RH response in the 0-85% RH range. We found that MXene gravimetric response to water is 10 times faster than their electrical response, suggesting that H2O-induced swelling/contraction of channels between MXene sheets results in trapping of H2O molecules that act as charge-depleting dopants. The results demonstrate the use of MXenes as humidity sensors and infer potential impact of water on structural and electrical performance of MXene-based devices.

Self-assembly of a semi-fluorinated diblock copolymer in a selective solvent

The self-assembly of a highly incompatible siloxane containing semi-fluorinated diblock copolymer, polytrifluoro propyl methylsiloxane-b-polystyrene (SiF-PS), in toluene, a selective solvent for polystyrene, was studied using Small Angle Neutron Scattering. Incompatibility is often enhanced by inserting fluorinated segments into one of the blocks and as a result not only the interchain interactions are changed but also the rigidity of the blocks. Herein the incorporation of siloxane into the backbone of a semi-fluorinated block maintains its flexibility and allows separation of the effects of direct interactions due to fluorine atoms from those of rigidity. Measurements were carried out in dilute solutions below 1 wt%, at volume fractions ϕSiF ranging from 0.0 to 0.5. The high incompatibility of the SiF block drives aggregation at low volume fractions of the SiF block, where spherical core–Gaussian shell aggregates are detected at ϕSiF = 0.16. In the symmetric SiF-PS complex fluid, elongated micelles were observed. The micelles exhibited unique temperature stability in comparison with the aggregates formed by diblock-copolymers in the lower segregation regime. As the temperature increases the micelles dissociate into free chains to form unimolecular micelles.

Origin of dielectric relaxor behavior in PVDF-based copolymer and terpolymer films

Relaxor ferroelectrics exhibit frequency-dispersion of their dielectric permittivity peak as a function of temperature, the origin of which has been widely debated. Microscopic understanding of such behavior for polymeric ferroelectrics has presented new challenges since unlike traditional ceramic ferroelectrics, dielectric relaxation in polymers is a consequence of short-range molecular dynamics that are difficult to measure directly. Here, through careful analysis of atomic-level H-atom dynamics as determined by Quasi-elastic Neutron Scattering (QENS), we show that short-range molecular dynamics within crystalline domains cannot explain the macroscopic frequency-dispersion of dielectric properties observed in prototypical polyvinylidene-fluoride (PVDF)-based relaxor ferroelectrics. Instead, from multiscale quantitative microstructural characterization, a clear correlation between the amount of crystalline-amorphous interfaces and dielectric relaxation is observed, which indicates that such interfaces pl...

Conjugated polymers with m-pyridine linkages: synthesis, photophysics, solution structure and film morphology

The synthesis of two π-conjugated polymers whose structures include m-pyridyl linkages that create regularly spaced bends and disrupt π-conjugation along the polymer backbones is described. The π-conjugated segments in the two polymers were comprised of p-(2,5-dialkoxyphenylene)vinylene, p-phenylenevinylene and 2,7-carbazole subunits. Additionally, each m-pyridylene unit is enshrouded within a steric cleft provided by 2,6-diaryl substitution of the pyridyl ring, thus minimizing inter-chain interactions of these sites. The concentration-dependence of photophysical characteristics along with small angle neutron scattering (SANS) was employed to resolve solution aggregation properties and atomic force microscopy was employed to assess the surface morphology of polymer films.