Mechanistic Investigation Of Heterogeneous Catalysis By Transient Infrared Methods,
2014
The University of Akron
Mechanistic Investigation Of Heterogeneous Catalysis By Transient Infrared Methods, Steven Chuang, Felipe Guzman
Steven S.C. Chuang
This paper provides an overview of the use of various transient infrared methods to determine the role of infrared observable species in the mechanisms of the NO–CO reaction, heterogeneous ethylene hydroformylation, and photocatalytic oxidation of ethanol. The transient infrared methods with a judicious choice of ways in changing the concentration of reactants and their isotope counterparts produce responses, allowing (i) identification of the spectators, (ii) determination of active adsorbed species, and (iii) verification of kinetic models and their parameters. The method has also been recently extended to monitor infrared absorbance of photogenerated electrons during photocatalysis, correlating variation in the concentration …
The Effect Of O2 On The No--Co Reaction Over Ag--Pd/Al2o3: An In Situ Infrared Study,
2014
The University of Akron
The Effect Of O2 On The No--Co Reaction Over Ag--Pd/Al2o3: An In Situ Infrared Study, Steven Chuang
Steven S.C. Chuang
The effect of O2 on the NO–CO reaction over Pd/Al2O3 and Ag–Pd/Al2O3 has been studied by in situ infrared (IR) spectroscopy coupled with the O2 pulse technique at 373–573 K. Pulsing O2 into the NO–CO flow at 373 K causes the oxidation of Pd+–NO to gaseous NO2 over Pd/Al2O3 and the emergence of Pd0–NO on Ag–Pd/Al2O3. The formation of Pd0–NO during the O2 pulse over Ag–Pd/Al2O3 catalyst suggests that (i) O2 disrupts the Ag overlayers on the Pd surface, allowing the Pd surface to be exposed to NO; and (ii) Ag species is able to keep the Pd surface in …
How Super Is Supercontraction? Persistent Versus Cyclic Responses To Humidity In Spider Dragline Silk,
2014
The University Of Akron
How Super Is Supercontraction? Persistent Versus Cyclic Responses To Humidity In Spider Dragline Silk, Todd Blackledge, Cecilia Bountry, Shing-Chung Wong, Avinash Baji, Ali Dhinojwala, Vasav Sahni, Ingi Agnarsson
Ali Dhinojwala
Spider dragline silk has enormous potential for the development of biomimetic fibers that combine strength and elasticity in low density polymers. These applications necessitate understanding how silk reacts to different environmental conditions. For instance, spider dragline silk ;supercontracts' in high humidity. During supercontraction, unrestrained dragline silk contracts up to 50% of its original length and restrained fibers generate substantial stress. Here we characterize the response of dragline silk to changes in humidity before, during and after supercontraction. Our findings demonstrate that dragline silk exhibits two qualitatively different responses to humidity. First, silk undergoes a previously unknown cyclic relaxation-contraction response to …
Rotational Diffusion Of Nonlinear Optical Chromophores In Rubbery And Glassy Polymers: Ï¡-Relaxation Dynamics Probed By Second Harmonic Generation And Dielectric Relaxation,
2014
University of Akron
Rotational Diffusion Of Nonlinear Optical Chromophores In Rubbery And Glassy Polymers: Ï¡-Relaxation Dynamics Probed By Second Harmonic Generation And Dielectric Relaxation, Ali Dhinojwala
Ali Dhinojwala
No abstract provided.
In Dynamics In Small Confining Systems Ii,
2014
University of Akron
In Dynamics In Small Confining Systems Ii, Ali Dhinojwala
Ali Dhinojwala
No abstract provided.
Dependent Droplet Impact Dynamics On Flat And Textured Surfaces,
2014
University of Akron
Dependent Droplet Impact Dynamics On Flat And Textured Surfaces, Ali Dhinojwala
Ali Dhinojwala
Dropletimpactdynamics determines the performance of surfaces used in many applications such as anti-icing, condensation, boiling, and heat transfer. We study impactdynamics of water droplets on surfaces with chemistry/texture ranging from hydrophilic to superhydrophobic and across a temperature range spanning below freezing to near boiling conditions. Dropletretraction shows very strong temperature dependence especially on hydrophilicsurfaces; it is seen that lower substrate temperatures lead to lesser retraction. Physics-based analyses show that the increased viscosity associated with lower temperatures combined with an increased work of adhesion can explain the decreased retraction. The present findings serve as a starting point to guide further studies …
Small Molecule Probe Diffusion In Thin Polymer Films Near The Glass Transition: A Novel Approach Using Fluorescence Nonradiative Energy Transfer,
2014
University of Akron
Small Molecule Probe Diffusion In Thin Polymer Films Near The Glass Transition: A Novel Approach Using Fluorescence Nonradiative Energy Transfer, Ali Dhinojwala
Ali Dhinojwala
A novel experimental approach involving fluorescence nonradiative energy transfer (NRET) is employed to study the Fickian diffusion of small molecules in rubbery polymer films near the glass transition. A theoretical formalism has been developed which directly relates the small molecule translational diffusion coefficient, , to changes in the energy transfer efficiency, E. Values of as low as 5 × 10-16 cm2/s have been measured. In this approach, two thin polymer films are sandwiched together, one labeled with either NRET donor or acceptor chromophores and the second doped with the complementary chromophore. Upon annealing for a time t, dopant chromophore diffusion …
Cobweb-Weaving Spiders Produce Different Attachment Discs For Locomotion And Prey Capture,
2014
The University Of Akron
Cobweb-Weaving Spiders Produce Different Attachment Discs For Locomotion And Prey Capture, Vasav Sahni, Jared Harris, Todd Blackledge, Ali Dhinojwala
Ali Dhinojwala
Spiders' cobwebs ensnare both walking and flying prey. While the scaffolding silk can entangle flying insects, gumfoot silk threads pull walking prey off the ground and into the web. Therefore, scaffolding silk needs to withstand the impact of the prey, whereas gumfoot silk needs to easily detach from the substrate when contacted by prey. Here we show that spiders accomplish these divergent demands by creating attachment discs of two distinct architectures using the same pyriform silk. A 'staple-pin' architecture firmly attaches the scaffolding silk to the substrate and a previously unknown 'dendritic' architecture weakly attaches the gumfoot silk to the …
In Dynamics In Small Confining Systems Iii,
2014
University of Akron
In Dynamics In Small Confining Systems Iii, Ali Dhinojwala
Ali Dhinojwala
No abstract provided.
Adsorption,
2014
University of Akron
New Approaches To Measure Interfacial Rheology Of Confined Fluids,
2014
University of Akron
New Approaches To Measure Interfacial Rheology Of Confined Fluids, Ali Dhinojwala
Ali Dhinojwala
Several instrumental developments to examine the rheology of ultrathin confined liquids are described. These include capacitance sensing of the film thickness, concurrent measurement of normal and shear forces by piezoelectric methods, application of periodic forces in the normal direction to determine dynamic oscillatory shear moduli, and quantification of apparatus misalignment. The method to separate apparatus from sample contributions is stressed.
Contact Of Oil With Solid Surfaces In Aqueous Media Probed Using Sum Frequency Generation Spectroscopy,
2014
The University Of Akron
Contact Of Oil With Solid Surfaces In Aqueous Media Probed Using Sum Frequency Generation Spectroscopy, Ping Hsu, Ali Dhinojwala
Ali Dhinojwala
We have studied the interface between hexadecane droplets and sapphire substrates in water using infrared-visible sum frequency generation spectroscopy (SFG). At high pH and above the isoelectric point of the sapphire substrate, the hexadecane drop is repelled due to electrostatic forces. The SFG measurements are consistent with the observation that a thick layer of water is present between the oil and the sapphire substrate. Below the isoelectric point of the sapphire substrate, the hexadecane drops stick to the sapphire surface. Surprisingly, the SFG results show the presence of a thin layer of water between hexadecane drop and the sapphire substrate. …
Translation-Rotation Paradox For Diffusion In Glass-Forming Polymers: The Role Of The Temperature Dependence Of The Relaxation Time Distribution,
2014
University of Akron
Translation-Rotation Paradox For Diffusion In Glass-Forming Polymers: The Role Of The Temperature Dependence Of The Relaxation Time Distribution, Ali Dhinojwala
Ali Dhinojwala
Comparisons are made of the translational and rotational diffusion of small-molecule probes in a polymer near its glass transition temperature, Tg. In the rubbery state, 1.1Tg>T>Tg, translational diffusion is much less temperature dependent than rotational reorientation; in a ``quenched'' glass, translation and rotation have similar temperature dependencies. This is explained to be a consequence of the fact that in the rubbery state near Tg the breadth of the polymer relaxation distribution is strongly temperature dependent, while in the quenched glass it is temperature invariant.
Probing Polymer/Polymer Interfaces,
2014
The University Of Akron
Probing Polymer/Polymer Interfaces, Gary Harp, Keshav Gautam, Ali Dhinojwala
Ali Dhinojwala
Infrared-visible sum frequency generation spectroscopy (SFG) has been used to study the interface between poly(vinyl-N-octadecylcarbamate-co-vinyl acetate) (Comb) and deuterated or hydrogenated polystyrene (dPS or hPS) films. Strong methyl symmetric and Fermi resonance bands associated with the alkyl side chains of the Comb polymer are observed in the SFG spectra. In addition, for Comb/hPS spectra, symmetric and asymmetric vibration modes of phenyl groups are observed. The presence of asymmetric modes indicates the phenyl rings are tilted with respect to the interface normal.
Novel Experimental Protocol For Studying Temporal Decay Of Second-Order Nonlinear Optical Polymer Properties: Quantitative Analysis Of Coupling To Polymer Dynamics And Conditions For Temporal Stability,
2014
University of Akron
Novel Experimental Protocol For Studying Temporal Decay Of Second-Order Nonlinear Optical Polymer Properties: Quantitative Analysis Of Coupling To Polymer Dynamics And Conditions For Temporal Stability, Ali Dhinojwala
Ali Dhinojwala
A new experimental approach has been developed which allows characterization of rotational dynamics of NLO chromophores in amorphous polymers over 12 decades in time, from 10-6 to 106 sec. Measurements over such a broad distribution in relaxation times are important not only in determining how the chromophore orientational dynamics are coupled to polymer dynamics but also in making accurate predictions of the temporal stability of technological interest. In particular, accurate characterization over a broad range of time scales reveals that NLO polymers meet technological requirements more easily than is predicted using a less complete characterization.
Interfacial Properties Of Free-Standing Poly(3-Hexyl Thiophene) Films,
2014
The University Of Akron
Interfacial Properties Of Free-Standing Poly(3-Hexyl Thiophene) Films, Yeneneh Yimer, Ali Dhinojwala, Mesfin Tsige
Ali Dhinojwala
Using full atomistic classical molecular dynamics simulations, the interfacial properties of free-standing poly(3-hexylthiophene) (P3HT) films have been investigated. The orientations of different parts of the P3HT chain and the surface tensions of the films were calculated in a temperature range of 540 K-600 K. At the liquid/vacuum interface, the P3HT chain shows ordering by exposing hexyl groups at the interface, while the chain backbone lays flat with the thiophene ring preferentially tilt toward the surface. At the interface, the terminal methyl groups of hexyl side chains are in excess compared to the methylene groups or thiophene rings. The surface tension …
Optorheological Studies Of Sheared Confined Fluids With Mesoscopic Thickness,
2014
University of Akron
Optorheological Studies Of Sheared Confined Fluids With Mesoscopic Thickness, Ali Dhinojwala
Ali Dhinojwala
Fluids of mesoscopic thickness can be sheared and their molecular orientation probed concurrently with the new instrument described in this paper. The fluid is confined between parallel optically flat windows whose spacing is controlled, using piezoelectric inchworms, from submicrometer thickness to 500 μm, with no essential lower limit apart from surface roughness. Capacitance sensors or optical interferometry is used to monitor spacing between the windows with submicrometer accuracy. Piezoelectric bimorphs are used to apply periodic shear displacements with amplitude 0.l−10 μm and frequency 0.1−700 Hz. Shear-induced molecular alignment during sinusoidal shear cycles is determined, with up to 5 μs time …
Direct Probe Of Interfacial Structure During Mechanical Contact Between Two Polymer Films Using Infrared Visible Sum Frequency Generation Spectroscopy,
2014
The University Of Akron
Direct Probe Of Interfacial Structure During Mechanical Contact Between Two Polymer Films Using Infrared Visible Sum Frequency Generation Spectroscopy, Gary Harp, Ali Dhinojwala
Ali Dhinojwala
Infrared-visible sum frequency generation spectroscopy (SFG) has been used to study the interface between poly(vinyl-N-octadecylcarbamate-co-vinyl acetate) (PVNODC) and polystyrene (PS) films during mechanical contact. The films were contacted using a deformable semispherical poly(dimethylsiloxane) (PDMS) lens that can be easily adapted to incorporate friction and adhesion force measurements. A strong methyl symmetric peak and Fermi resonance band associated with the alkyl side chains of PVNODC are observed in the SFG spectrum. This suggests that the interface structure during mechanical contact is more ordered than the structure observed for PS/PVNODC bilayer films annealed above the melting and glass transition temperatures of PVNODC …
Rotational Dynamics In Polystyrene Above And Below The Glass Transition Temperature: Second Harmonic Generation And Dielectric Relaxation Measurements,
2014
University of Akron
Rotational Dynamics In Polystyrene Above And Below The Glass Transition Temperature: Second Harmonic Generation And Dielectric Relaxation Measurements, Ali Dhinojwala
Ali Dhinojwala
No abstract provided.
Sum Frequency Generation Vibrational Spectroscopy Of Silicone Surfaces And Interfaces,
2014
The University Of Akron
Sum Frequency Generation Vibrational Spectroscopy Of Silicone Surfaces And Interfaces, Dongchan Ahn, Ali Dhinojwala
Ali Dhinojwala
While several general reviews of the applications of sum frequency generation vibrational spectroscopy (SFG) appear in the literature, none have focused specifically on the application of SFG to silicones. The unique and somewhat dichotomous surface properties of silicones, and their ever-increasing use in surface and interface-dependent applications such as lubricants, adhesives, micro-fluidic materials, sensors and matrices or scaffolds for nano-composites, calls for increased fundamental understanding that has motivated the use of SFG analysis. This chapter focuses on the combination of this uniquely surface sensitive tool to study applications using PDMS and other silicone-based materials. Because the interpretation of SFG spectra …
