Dynamic polymers with both physical interactions and dynamic covalent bonds exhibit superior perf... more Dynamic polymers with both physical interactions and dynamic covalent bonds exhibit superior performance while achieving such dry polymers in an efficient manner remains a challenge. Herein, we report a novel organic solvent quenched polymer synthesis using a natural molecule, i.e., thioctic acid (TA), which has both dynamic disulfide bond and carboxylic acid. The effects of the solvent types and concentrations along with reaction times on the proposed reaction have been thoroughly explored for polymer synthesis. Solid-state proton nuclear magnetic resonance (1 H NMR) and first-principles simulations are carried out to investigate the reaction mechanism. It shows that the chlorinated solvent can efficiently stabilize and mediate the depolymerization of poly(TA) which is more kinetically favorable upon lowering the temperature. Attributed to the numerous dynamic covalent disulfide bonds and noncovalent hydrogen bonding, the obtained poly(TA) shows high extensibility, self-healable and re-processable properties. It can also be employed as an efficient adhesive even on a Teflon surface and 3D printed using the fused deposition modeling (FDM) technique. This new polymer synthesis approach on using organic solvents as catalysts along with the unique reaction mechanism provides a new pathway for efficient polymer synthesis, especially for those multi-functional dynamic polymers.
Despite the wide use of polymer nanocomposites (PNCs) in various applications, our understanding ... more Despite the wide use of polymer nanocomposites (PNCs) in various applications, our understanding of the microscopic parameters controlling their macroscopic properties remains limited. In this study, we examine the dielectric strength of segmental dynamics, Δε IL (T) in the interfacial polymer layer surrounding the nanoparticles in PNCs. The presented analysis reveals a significant drop in Δε IL (T) and its anomalous temperature dependence in the polymer layer adsorbed to nanoparticles. The drop in Δε IL (T) was observed in all samples regardless of whether segmental relaxation time in the interfacial layer was slower or faster than in the bulk polymer, excluding interpretation of the "dead" layer. We ascribe the observed decrease in the dielectric strength to the restricted amplitude of segmental relaxation in the interfacial/adsorbed layer. Our results provide a new perspective on discussion of dynamics in the interfacial layer in PNCs and thin polymer films, demonstrating that not only segmental relaxation time but also its amplitude can be strongly affected by the interface.
Probing the properties of the interfacial layer between the polymer matrix and nanoparticles in p... more Probing the properties of the interfacial layer between the polymer matrix and nanoparticles in polymer nanocomposites (PNCs) remains a challenging task. Here, we apply three methodsa single Havriliak−Negami (HN) function fit, a two HN functions fit, and the heterogeneous model analysis (HMA)to analyze the dielectric spectra of model poly(vinyl acetate)/SiO 2 nanocomposites for the thickness and the average slowing down in dynamics of the interfacial layer. We find the HMA presents the most accurate analysis on both the thickness and dynamics of the interfacial layer, in comparison to the other two methods that have been actively applied in the field. In addition, the dielectric spectra at low temperatures reveal unexpectedly nonmonotonous changes in the secondary relaxation of the polymer with nanoparticle loadings. These results clearly demonstrate that dielectric spectroscopy is an easy and robust method to study a wide range of dynamic properties of the interfacial layer in PNCs.
A theoretical model for the chain modes' contribution to the depolarized light scattering (DLS) s... more A theoretical model for the chain modes' contribution to the depolarized light scattering (DLS) spectrum of polymers is developed. It is shown that the frequency dependence of the DLS susceptibility spectrum is similar to that of the shear loss modulus. Specifically, the contribution of chain modes to the DLS spectrum is composed of a series of Lorentzians (single-exponential processes) having the same amplitude. The relaxation time associated with each mode follows the Rouse prediction, but with a value equal to one-half the mechanical Rouse time. DLS spectra of poly(dimethylsiloxane) melts with different molecular weights have been analyzed in the framework of the model. Reasonable agreement on both qualitative and quantitative levels is observed. In particular, the relaxation time of the longest Rouse mode extracted from DLS spectra is consistent with that from viscosity measurements.
Lithium-ion batteries (LIBs) have attracted worldwide research interest due to their high energy ... more Lithium-ion batteries (LIBs) have attracted worldwide research interest due to their high energy density and long cycle life. Solid-state LIBs improve the safety of conventional liquid-based LIBs by replacing the flammable organic electrolytes with a solid electrolyte. Among the various types of solid electrolytes, hybrid solid electrolytes (HSEs) demonstrate great promise to achieve high ionic conductivity, reduced interfacial resistance between the electrolyte and electrodes, mechanical robustness, and excellent processability due to the combined advantages of both polymer and inorganic electrolyte. This article summarizes recent developments in HSEs for LIBs. Approaches for the preparation of hybrid electrolytes and current understanding of iontransport mechanisms are discussed. The main challenges including unsatisfactory ionic conductivity and perspectives of HSEs for LIBs are highlighted for future development. The present review provides insights into HSE development to allow a more efficient and target-oriented future endeavor on achieving high-performance solid-state LIBs.
Using dielectric spectroscopy, we report the observation of highly decoupled conductivity in newl... more Using dielectric spectroscopy, we report the observation of highly decoupled conductivity in newly synthetized protic ionic conductor lidocaine di-(dihydrogen phosphate).
Analysis of temperature dependence of structural relaxation time τ(T) in supercooled liquids reve... more Analysis of temperature dependence of structural relaxation time τ(T) in supercooled liquids revealed a qualitatively distinct feature-a sharp, cusp-like maximum in the second derivative of log τ α (T) at some T max. It suggests that the super-Arrhenius temperature dependence of τ α (T) in glass-forming liquids eventually crosses over to an Arrhenius behavior at T<T max , and there is no divergence of τ α (T) at non-zero T. T max can be above or below T g , depending on sensitivity of τ(T) to change in liquid's density quantified by the exponent γ in the scaling τ α (T) ~ exp(A/Tρ-γ). These results might turn the discussion of the glass transition to the new avenue-the origin of the limiting activation energy for structural relaxation at low T.
At a given temperature during a heating/cooling cycle, lignin is more dynamic upon cooling, which... more At a given temperature during a heating/cooling cycle, lignin is more dynamic upon cooling, which may guide efficient biomass processing.
Despite its simple chemical structure water remains one of the most puzzling liquids with many an... more Despite its simple chemical structure water remains one of the most puzzling liquids with many anomalies at low temperatures. Combining neutron scattering and dielectric relaxation spectroscopy we show that quantum fluctuations are not negligible in deeply supercooled water. Our dielectric measurements revealed the anomalously weak temperature dependence of structural relaxation in vapor deposited water close to the glass transition temperature T g ~136K. We demonstrate that this anomalous behavior can be explained well by quantum effects. These results have significant implications for our understanding of water dynamics.
Analysis of temperature dependence of structural relaxation time τ(T) in supercooled liquids reve... more Analysis of temperature dependence of structural relaxation time τ(T) in supercooled liquids revealed a qualitatively distinct feature-a sharp, cusp-like maximum in the second derivative of log τ α (T) at some T max. It suggests that the super-Arrhenius temperature dependence of τ α (T) in glass-forming liquids eventually crosses over to an Arrhenius behavior at T<T max , and there is no divergence of τ α (T) at non-zero T. T max can be above or below T g , depending on sensitivity of τ(T) to change in liquid's density quantified by the exponent γ in the scaling τ α (T) ~ exp(A/Tρ-γ). These results might turn the discussion of the glass transition to the new avenue-the origin of the limiting activation energy for structural relaxation at low T.
Detailed analysis of the static structure factor S(Q) in several glass forming liquids reveals th... more Detailed analysis of the static structure factor S(Q) in several glass forming liquids reveals that the temperature variations of the width of the main diffraction peak ΔQ(T) correlates with fragility of these liquids. This observation suggests a direct connection between rather subtle structural changes and sharp slowing down of structural relaxation in glass forming liquids. We show that this observation can be rationalized using the Adam-Gibbs approach, through a connection between temperature variations of structural correlation length, l c ~ 2π/ΔQ, and the size of cooperatively rearranging regions.
In order to design more effective solid polymer electrolytes, it is important to decouple ion con... more In order to design more effective solid polymer electrolytes, it is important to decouple ion conductivity from polymer segmental motion. To that end, novel polymers based on oxanorbornene dicarboximide monomers with varying lengths of oligomeric ethylene oxide side chains have been synthesized using ring opening metathesis polymerization. These unique polymers have a fairly rigid and bulky backbone and were used to investigate the decoupling of ion motion from polymer segmental dynamics. Ion conductivity was measured using broadband dielectric spectroscopy for varying levels of added lithium salt. The conductivity data demonstrate six to seven orders of separation in timescale of ion conductivity from polymer segmental motion for polymers with shorter ethylene oxide side chains. However, commensurate changes in the glass transition temperatures T g reduce the effect of decoupling in ion conductivity and lead to lower conductivity at ambient conditions. These results suggest that both, an increase in decoupling and a reduction in T g might be required for developing solid polymer electrolytes with high ion conductivity at room temperature.
Low-frequency Raman coupling coefficient C(ν) of 11 different glasses is evaluated. It is shown t... more Low-frequency Raman coupling coefficient C(ν) of 11 different glasses is evaluated. It is shown that the coupling coefficient demonstrates a universal linear frequency behavior C(ν) ∝ (ν/νBP + B) near the boson peak maximum, νBP. Frequency dependence of C(ν) allows to separate the glasses studied into two groups: the first group has a frequency independent contribution B˜0.5, while the second one has B˜0. It was found that C(ν) demonstrates a superlinear behavior at very low frequencies. This observation suggests vanishing of the coupling coefficient when frequency tends to zero. The results are discussed in terms of the vibration wavefunction that combines features of localized and extended modes.
In this Letter we report the relation between ionic conductivity and structural relaxation in sup... more In this Letter we report the relation between ionic conductivity and structural relaxation in supercooled protic ionic liquids (PILs) under high pressure. The results of high-pressure dielectric and volumetric measurements, combined with rheological and temperature-modulated differential scanning calorimetry experiments, have revealed a fundamental difference between the conducting properties under isothermal and isobaric conditions for three PILs with different charge transport mechanisms (Grotthuss vs vehicle). Our findings indicate a breakdown of the fractional Stokes-Einstein relation and Walden rule when the ionic transport is controlled by fast proton hopping. Consequently, we demonstrate that the studied PILs exhibit significantly higher conductivity than one would expect taking into account that they are in fact a mixture of ionic and neutral species. Thus, the examined herein samples represent a new class of ''superionic'' materials desired for many advanced applications.
We report that the pressure coefficient of the glass transition temperature, dT g /dp, which is c... more We report that the pressure coefficient of the glass transition temperature, dT g /dp, which is commonly used to determine the pressure sensitivity of the glass transition temperature, T g , can be predicted in the thermodynamic scaling regime. We show that the equation derived from the isochronal condition combined with the well-known scaling, TV γ =const, predicts successfully values of dT g /dp for variety of glass-forming systems, including van der Waals liquids, polymers, and ionic liquids.
Tip-enhanced Raman spectroscopy (TERS) has been established as one the most efficient analytical ... more Tip-enhanced Raman spectroscopy (TERS) has been established as one the most efficient analytical techniques for probing vibrational states with nanoscale resolution. While TERS may be a source of unique information about chemical structure and interactions, it has a limited use for materials with rough or sticky surfaces. Development of the TERS approach utilizing a non-contact scanning probe microscopy mode can significantly extend the number of applications. Here we demonstrate a proof of the concept and feasibility of a noncontact TERS approach and test it on various materials. Our experiments show that non-contact TERS can provide 10 nm spatial resolution and a Raman signal enhancement factor of 10 5 , making it very promising for chemical imaging of materials with high aspect ratio surface patterns and biomaterials.
Polymer-based membranes play a key role in several industrially important gas separation technolo... more Polymer-based membranes play a key role in several industrially important gas separation technologies, e.g., removing CO 2 from natural gas, with enormous economic and environmental impact. Here, we develop a novel hybrid membrane construct comprised entirely of nanoparticles grafted with polymers. These membranes are shown to have broadly tunable separation performance through variations in graft density and chain length. Computer simulations show that the optimal NP packing forces the grafted polymer layer to distort, yielding regions of measurably lower polymer density. Multiple experimental probes confirm that these materials have the predicted increase in "polymer free volume", which explains their improved separation performance. These polymer-grafted NP materials thus represent a new template for rationally designing membranes with desirable separation abilities coupled with improved aging characteristics in the glassy state and enhanced mechanical behavior.
Experimental Details Synthesis of Silica Nanoparticles (NPs) and preparation of polymer nanocompo... more Experimental Details Synthesis of Silica Nanoparticles (NPs) and preparation of polymer nanocomposites (PNCs) The poly(2-vinyl pyridine) (P2VP) with molecular weights (MW) of 1, 9, 59, 100, 400 kg/mol and PDI of 1.07 was purchased from Scientific Polymer Products and used as received. Silica nanoparticles (NP) with radius R NP ~ 12.5 nm were synthesized by a modified Stöber's method [1, 2]. As prepared, the NPs were characterized by dynamic light scattering. P2VP of different molecular weights were mixed with silica nanoparticles in ethanol yielding formation of PNCs containing 40wt% (SetI) and 50wt% (Set II) of silica nanoparticles. The detailed sample preparation can be found elsewhere [3]. For XPS and SFG studies, samples were prepared through formation of a film. For XPS measurements of Set I, micron thick film was prepared on the surface of 1/2" Round Protected Aluminum Mirror (purchased from ThorLabs). For that, composite was drop casted from the ethanol solvent and dried at ambient condition followed by the high temperature annealing (150 o C) in the vacuum oven for two days. Set II was prepared by spincoating a composite solution on the surface of aluminum foil resulting in hundreds nanometer thick film. Aluminum foil was chosen in order to reduce the charging effect. The obtained films of PNCs on aluminum substrate were measured using AFM in order to confirm a good dispersion of nanoparticles. As prepared films were annealed at 150 o C for two days and used for XPS measurements. The films for SFG measurements (Set II) were prepared by spin-coating a composite solution on the surface of quartz glass resulting in a nanometer thick film. SFG experiments found little signal from the bare cleaned substrate. This is to be expected since the effective surface area is much greater for the PNCs as compared to the quartz surface and correspondingly more signal is expected based on the larger number density of surface species. As such, the SFG measurements discussed in the main text predominantly describe the-OH groups on the particle interfaces. Experimental techniques and sample preparation Thermogravimetry analysys of PNCs The weight fraction of silica nanoparticles was estimated using thermogravimetric analysis (TGA) (TA instrument, Discovery Q50). Specifically, heating from 20 o C to 800 o C at a rate of 20 o C/min in furnace is performed under air. The weight fraction of silica nanoparticles in composites are listed in Table S1 below. Dynamic Light Scattering (DLS) The DLS experiment was performed on the custom built device. We used the vertically polarized light from HeNe laser (Newport R-31425, 633nm 35mW) focused with F = 200mm lens into a micrometers sized spot in cylindrical vile with the sample. The scattered light was measured at 90 degrees using the similar lens and filter (632.8nm MaxLine laser clean-up filter) from Semrock. This specific filter was used to suppress fluorescent and Raman scattering from a sample. The filtered light passed through the "Enhances +Fiber Optical Beam Splitter" from ALV and hit the two detectors "Single Photon Counting Module" (SPCM, from Exilitas) which was connected to ALV-7004/Fast correlator, working in Crosscorrelation mode. The obtained correlation function was fitted to the single exponential function, yielding the characteristic decay time which was used to calculate the diffusion following [4]. Then, the size of the particles was calculated using Stock-Einstein equation for the diffusion of spherical particles. The average size of nanoparticles was found to be 25±1.5nm Atomic Force Microscopy AFM measurements were performed using SmartSPM (AIST-NT, Novato, USA). We used super sharp silicon tips with typical curvature radius of 2 nm (NanoSensor, USA).
Dynamic polymers with both physical interactions and dynamic covalent bonds exhibit superior perf... more Dynamic polymers with both physical interactions and dynamic covalent bonds exhibit superior performance while achieving such dry polymers in an efficient manner remains a challenge. Herein, we report a novel organic solvent quenched polymer synthesis using a natural molecule, i.e., thioctic acid (TA), which has both dynamic disulfide bond and carboxylic acid. The effects of the solvent types and concentrations along with reaction times on the proposed reaction have been thoroughly explored for polymer synthesis. Solid-state proton nuclear magnetic resonance (1 H NMR) and first-principles simulations are carried out to investigate the reaction mechanism. It shows that the chlorinated solvent can efficiently stabilize and mediate the depolymerization of poly(TA) which is more kinetically favorable upon lowering the temperature. Attributed to the numerous dynamic covalent disulfide bonds and noncovalent hydrogen bonding, the obtained poly(TA) shows high extensibility, self-healable and re-processable properties. It can also be employed as an efficient adhesive even on a Teflon surface and 3D printed using the fused deposition modeling (FDM) technique. This new polymer synthesis approach on using organic solvents as catalysts along with the unique reaction mechanism provides a new pathway for efficient polymer synthesis, especially for those multi-functional dynamic polymers.
Despite the wide use of polymer nanocomposites (PNCs) in various applications, our understanding ... more Despite the wide use of polymer nanocomposites (PNCs) in various applications, our understanding of the microscopic parameters controlling their macroscopic properties remains limited. In this study, we examine the dielectric strength of segmental dynamics, Δε IL (T) in the interfacial polymer layer surrounding the nanoparticles in PNCs. The presented analysis reveals a significant drop in Δε IL (T) and its anomalous temperature dependence in the polymer layer adsorbed to nanoparticles. The drop in Δε IL (T) was observed in all samples regardless of whether segmental relaxation time in the interfacial layer was slower or faster than in the bulk polymer, excluding interpretation of the "dead" layer. We ascribe the observed decrease in the dielectric strength to the restricted amplitude of segmental relaxation in the interfacial/adsorbed layer. Our results provide a new perspective on discussion of dynamics in the interfacial layer in PNCs and thin polymer films, demonstrating that not only segmental relaxation time but also its amplitude can be strongly affected by the interface.
Probing the properties of the interfacial layer between the polymer matrix and nanoparticles in p... more Probing the properties of the interfacial layer between the polymer matrix and nanoparticles in polymer nanocomposites (PNCs) remains a challenging task. Here, we apply three methodsa single Havriliak−Negami (HN) function fit, a two HN functions fit, and the heterogeneous model analysis (HMA)to analyze the dielectric spectra of model poly(vinyl acetate)/SiO 2 nanocomposites for the thickness and the average slowing down in dynamics of the interfacial layer. We find the HMA presents the most accurate analysis on both the thickness and dynamics of the interfacial layer, in comparison to the other two methods that have been actively applied in the field. In addition, the dielectric spectra at low temperatures reveal unexpectedly nonmonotonous changes in the secondary relaxation of the polymer with nanoparticle loadings. These results clearly demonstrate that dielectric spectroscopy is an easy and robust method to study a wide range of dynamic properties of the interfacial layer in PNCs.
A theoretical model for the chain modes' contribution to the depolarized light scattering (DLS) s... more A theoretical model for the chain modes' contribution to the depolarized light scattering (DLS) spectrum of polymers is developed. It is shown that the frequency dependence of the DLS susceptibility spectrum is similar to that of the shear loss modulus. Specifically, the contribution of chain modes to the DLS spectrum is composed of a series of Lorentzians (single-exponential processes) having the same amplitude. The relaxation time associated with each mode follows the Rouse prediction, but with a value equal to one-half the mechanical Rouse time. DLS spectra of poly(dimethylsiloxane) melts with different molecular weights have been analyzed in the framework of the model. Reasonable agreement on both qualitative and quantitative levels is observed. In particular, the relaxation time of the longest Rouse mode extracted from DLS spectra is consistent with that from viscosity measurements.
Lithium-ion batteries (LIBs) have attracted worldwide research interest due to their high energy ... more Lithium-ion batteries (LIBs) have attracted worldwide research interest due to their high energy density and long cycle life. Solid-state LIBs improve the safety of conventional liquid-based LIBs by replacing the flammable organic electrolytes with a solid electrolyte. Among the various types of solid electrolytes, hybrid solid electrolytes (HSEs) demonstrate great promise to achieve high ionic conductivity, reduced interfacial resistance between the electrolyte and electrodes, mechanical robustness, and excellent processability due to the combined advantages of both polymer and inorganic electrolyte. This article summarizes recent developments in HSEs for LIBs. Approaches for the preparation of hybrid electrolytes and current understanding of iontransport mechanisms are discussed. The main challenges including unsatisfactory ionic conductivity and perspectives of HSEs for LIBs are highlighted for future development. The present review provides insights into HSE development to allow a more efficient and target-oriented future endeavor on achieving high-performance solid-state LIBs.
Using dielectric spectroscopy, we report the observation of highly decoupled conductivity in newl... more Using dielectric spectroscopy, we report the observation of highly decoupled conductivity in newly synthetized protic ionic conductor lidocaine di-(dihydrogen phosphate).
Analysis of temperature dependence of structural relaxation time τ(T) in supercooled liquids reve... more Analysis of temperature dependence of structural relaxation time τ(T) in supercooled liquids revealed a qualitatively distinct feature-a sharp, cusp-like maximum in the second derivative of log τ α (T) at some T max. It suggests that the super-Arrhenius temperature dependence of τ α (T) in glass-forming liquids eventually crosses over to an Arrhenius behavior at T<T max , and there is no divergence of τ α (T) at non-zero T. T max can be above or below T g , depending on sensitivity of τ(T) to change in liquid's density quantified by the exponent γ in the scaling τ α (T) ~ exp(A/Tρ-γ). These results might turn the discussion of the glass transition to the new avenue-the origin of the limiting activation energy for structural relaxation at low T.
At a given temperature during a heating/cooling cycle, lignin is more dynamic upon cooling, which... more At a given temperature during a heating/cooling cycle, lignin is more dynamic upon cooling, which may guide efficient biomass processing.
Despite its simple chemical structure water remains one of the most puzzling liquids with many an... more Despite its simple chemical structure water remains one of the most puzzling liquids with many anomalies at low temperatures. Combining neutron scattering and dielectric relaxation spectroscopy we show that quantum fluctuations are not negligible in deeply supercooled water. Our dielectric measurements revealed the anomalously weak temperature dependence of structural relaxation in vapor deposited water close to the glass transition temperature T g ~136K. We demonstrate that this anomalous behavior can be explained well by quantum effects. These results have significant implications for our understanding of water dynamics.
Analysis of temperature dependence of structural relaxation time τ(T) in supercooled liquids reve... more Analysis of temperature dependence of structural relaxation time τ(T) in supercooled liquids revealed a qualitatively distinct feature-a sharp, cusp-like maximum in the second derivative of log τ α (T) at some T max. It suggests that the super-Arrhenius temperature dependence of τ α (T) in glass-forming liquids eventually crosses over to an Arrhenius behavior at T<T max , and there is no divergence of τ α (T) at non-zero T. T max can be above or below T g , depending on sensitivity of τ(T) to change in liquid's density quantified by the exponent γ in the scaling τ α (T) ~ exp(A/Tρ-γ). These results might turn the discussion of the glass transition to the new avenue-the origin of the limiting activation energy for structural relaxation at low T.
Detailed analysis of the static structure factor S(Q) in several glass forming liquids reveals th... more Detailed analysis of the static structure factor S(Q) in several glass forming liquids reveals that the temperature variations of the width of the main diffraction peak ΔQ(T) correlates with fragility of these liquids. This observation suggests a direct connection between rather subtle structural changes and sharp slowing down of structural relaxation in glass forming liquids. We show that this observation can be rationalized using the Adam-Gibbs approach, through a connection between temperature variations of structural correlation length, l c ~ 2π/ΔQ, and the size of cooperatively rearranging regions.
In order to design more effective solid polymer electrolytes, it is important to decouple ion con... more In order to design more effective solid polymer electrolytes, it is important to decouple ion conductivity from polymer segmental motion. To that end, novel polymers based on oxanorbornene dicarboximide monomers with varying lengths of oligomeric ethylene oxide side chains have been synthesized using ring opening metathesis polymerization. These unique polymers have a fairly rigid and bulky backbone and were used to investigate the decoupling of ion motion from polymer segmental dynamics. Ion conductivity was measured using broadband dielectric spectroscopy for varying levels of added lithium salt. The conductivity data demonstrate six to seven orders of separation in timescale of ion conductivity from polymer segmental motion for polymers with shorter ethylene oxide side chains. However, commensurate changes in the glass transition temperatures T g reduce the effect of decoupling in ion conductivity and lead to lower conductivity at ambient conditions. These results suggest that both, an increase in decoupling and a reduction in T g might be required for developing solid polymer electrolytes with high ion conductivity at room temperature.
Low-frequency Raman coupling coefficient C(ν) of 11 different glasses is evaluated. It is shown t... more Low-frequency Raman coupling coefficient C(ν) of 11 different glasses is evaluated. It is shown that the coupling coefficient demonstrates a universal linear frequency behavior C(ν) ∝ (ν/νBP + B) near the boson peak maximum, νBP. Frequency dependence of C(ν) allows to separate the glasses studied into two groups: the first group has a frequency independent contribution B˜0.5, while the second one has B˜0. It was found that C(ν) demonstrates a superlinear behavior at very low frequencies. This observation suggests vanishing of the coupling coefficient when frequency tends to zero. The results are discussed in terms of the vibration wavefunction that combines features of localized and extended modes.
In this Letter we report the relation between ionic conductivity and structural relaxation in sup... more In this Letter we report the relation between ionic conductivity and structural relaxation in supercooled protic ionic liquids (PILs) under high pressure. The results of high-pressure dielectric and volumetric measurements, combined with rheological and temperature-modulated differential scanning calorimetry experiments, have revealed a fundamental difference between the conducting properties under isothermal and isobaric conditions for three PILs with different charge transport mechanisms (Grotthuss vs vehicle). Our findings indicate a breakdown of the fractional Stokes-Einstein relation and Walden rule when the ionic transport is controlled by fast proton hopping. Consequently, we demonstrate that the studied PILs exhibit significantly higher conductivity than one would expect taking into account that they are in fact a mixture of ionic and neutral species. Thus, the examined herein samples represent a new class of ''superionic'' materials desired for many advanced applications.
We report that the pressure coefficient of the glass transition temperature, dT g /dp, which is c... more We report that the pressure coefficient of the glass transition temperature, dT g /dp, which is commonly used to determine the pressure sensitivity of the glass transition temperature, T g , can be predicted in the thermodynamic scaling regime. We show that the equation derived from the isochronal condition combined with the well-known scaling, TV γ =const, predicts successfully values of dT g /dp for variety of glass-forming systems, including van der Waals liquids, polymers, and ionic liquids.
Tip-enhanced Raman spectroscopy (TERS) has been established as one the most efficient analytical ... more Tip-enhanced Raman spectroscopy (TERS) has been established as one the most efficient analytical techniques for probing vibrational states with nanoscale resolution. While TERS may be a source of unique information about chemical structure and interactions, it has a limited use for materials with rough or sticky surfaces. Development of the TERS approach utilizing a non-contact scanning probe microscopy mode can significantly extend the number of applications. Here we demonstrate a proof of the concept and feasibility of a noncontact TERS approach and test it on various materials. Our experiments show that non-contact TERS can provide 10 nm spatial resolution and a Raman signal enhancement factor of 10 5 , making it very promising for chemical imaging of materials with high aspect ratio surface patterns and biomaterials.
Polymer-based membranes play a key role in several industrially important gas separation technolo... more Polymer-based membranes play a key role in several industrially important gas separation technologies, e.g., removing CO 2 from natural gas, with enormous economic and environmental impact. Here, we develop a novel hybrid membrane construct comprised entirely of nanoparticles grafted with polymers. These membranes are shown to have broadly tunable separation performance through variations in graft density and chain length. Computer simulations show that the optimal NP packing forces the grafted polymer layer to distort, yielding regions of measurably lower polymer density. Multiple experimental probes confirm that these materials have the predicted increase in "polymer free volume", which explains their improved separation performance. These polymer-grafted NP materials thus represent a new template for rationally designing membranes with desirable separation abilities coupled with improved aging characteristics in the glassy state and enhanced mechanical behavior.
Experimental Details Synthesis of Silica Nanoparticles (NPs) and preparation of polymer nanocompo... more Experimental Details Synthesis of Silica Nanoparticles (NPs) and preparation of polymer nanocomposites (PNCs) The poly(2-vinyl pyridine) (P2VP) with molecular weights (MW) of 1, 9, 59, 100, 400 kg/mol and PDI of 1.07 was purchased from Scientific Polymer Products and used as received. Silica nanoparticles (NP) with radius R NP ~ 12.5 nm were synthesized by a modified Stöber's method [1, 2]. As prepared, the NPs were characterized by dynamic light scattering. P2VP of different molecular weights were mixed with silica nanoparticles in ethanol yielding formation of PNCs containing 40wt% (SetI) and 50wt% (Set II) of silica nanoparticles. The detailed sample preparation can be found elsewhere [3]. For XPS and SFG studies, samples were prepared through formation of a film. For XPS measurements of Set I, micron thick film was prepared on the surface of 1/2" Round Protected Aluminum Mirror (purchased from ThorLabs). For that, composite was drop casted from the ethanol solvent and dried at ambient condition followed by the high temperature annealing (150 o C) in the vacuum oven for two days. Set II was prepared by spincoating a composite solution on the surface of aluminum foil resulting in hundreds nanometer thick film. Aluminum foil was chosen in order to reduce the charging effect. The obtained films of PNCs on aluminum substrate were measured using AFM in order to confirm a good dispersion of nanoparticles. As prepared films were annealed at 150 o C for two days and used for XPS measurements. The films for SFG measurements (Set II) were prepared by spin-coating a composite solution on the surface of quartz glass resulting in a nanometer thick film. SFG experiments found little signal from the bare cleaned substrate. This is to be expected since the effective surface area is much greater for the PNCs as compared to the quartz surface and correspondingly more signal is expected based on the larger number density of surface species. As such, the SFG measurements discussed in the main text predominantly describe the-OH groups on the particle interfaces. Experimental techniques and sample preparation Thermogravimetry analysys of PNCs The weight fraction of silica nanoparticles was estimated using thermogravimetric analysis (TGA) (TA instrument, Discovery Q50). Specifically, heating from 20 o C to 800 o C at a rate of 20 o C/min in furnace is performed under air. The weight fraction of silica nanoparticles in composites are listed in Table S1 below. Dynamic Light Scattering (DLS) The DLS experiment was performed on the custom built device. We used the vertically polarized light from HeNe laser (Newport R-31425, 633nm 35mW) focused with F = 200mm lens into a micrometers sized spot in cylindrical vile with the sample. The scattered light was measured at 90 degrees using the similar lens and filter (632.8nm MaxLine laser clean-up filter) from Semrock. This specific filter was used to suppress fluorescent and Raman scattering from a sample. The filtered light passed through the "Enhances +Fiber Optical Beam Splitter" from ALV and hit the two detectors "Single Photon Counting Module" (SPCM, from Exilitas) which was connected to ALV-7004/Fast correlator, working in Crosscorrelation mode. The obtained correlation function was fitted to the single exponential function, yielding the characteristic decay time which was used to calculate the diffusion following [4]. Then, the size of the particles was calculated using Stock-Einstein equation for the diffusion of spherical particles. The average size of nanoparticles was found to be 25±1.5nm Atomic Force Microscopy AFM measurements were performed using SmartSPM (AIST-NT, Novato, USA). We used super sharp silicon tips with typical curvature radius of 2 nm (NanoSensor, USA).
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Papers by Alexei Sokolov