VTechWorks Repository :: Browsing by Author "Ducker, William A."

Browsing by Author "Ducker, William A."

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Adsorption in Confined Aqueous Films
Gaddam, Prudhvidhar Reddy (Virginia Tech, 2019-07-24)
This thesis describes direct measurements of equilibrium adsorption of ions in thin (< 100 nm) aqueous films. Adsorption in thin films is important because it is through adsorption that the stability of colloidal suspensions is frequently tuned. The vast majority of measurements of adsorption to date have been to a single interfaces, whereas the subject of this thesis is adsorption in a thin film between two interfaces. There are two isolated interfaces when particles in a suspension are far apart, but during the collision, a thin film forms between the particles, and the properties of the thin film determines the stability of the colloid. Thus, adsorption in the thin film determines the stability of the colloidal dispersion. There is a distinct gap in the scientific literature concerning adsorption in thin films mainly because there is no technique for measuring such adsorption. To fill this gap in knowledge, I first developed of a technique to directly measure adsorption in thin films, and then applied this technique to explore the behavior of co-ions near charged interfaces as a function of bulk solution composition and the thickness of the film. The adsorption behavior of fluorescein, a di-anion, to negatively charged silica interfaces was studied in dilute electrolytes. The focus was on the effect of the electrostatic screening length, or Debye-length. The separation was measured using interference microscopy and the adsorption of fluorescein was measured using fluorescence microscopy. The Debye-length was altered by variation of the background salt (NaCl) concentration in dilute (<1 M) solution. The surface excess of adsorption for fluorescein was shown to depend on both the Debye-length and the separation distance between two interfaces. Increasing the Debye-length from 4 nm to 21 nm increased the plateau surface excess at large separations, and decreasing the separation lead to a monotonically decreasing surface excess. The surface excess varied over a range that scaled with the Debye-length. The results were compared to solution of the Poisson-Boltzmann model and good agreement was found between the model and the experiment. The effect of background salt concentration on fluorescein adsorption was also studied in concentrated electrolytes (2.5 – 10 M) for various monovalent salts (LiCl, NaCl and CsCl). The results showed that the fitted electrostatic screening length showed an opposite trend to predictions from Poisson-Boltzmann, with the screening-length increasing with increasing salt concentration. That is, the Debye-length prediction was quantitatively incorrect and predicts the incorrect trend. For example, in 10 M LiCl where the Debye-length is 0.1 nm, and therefore colloidal chemists would traditionally predict that double-layer forces are negligible, my results show that the actual decay length is about 10 nm, which is about the same as in 10-3 M LiCl solution. The rate of increase of screening-length as a function of concentration was also an ion specific effect. In addition, the results show that there is an inversion of the surface charge in concentrated salt solution. The original device on which all the above measurements were made had two limitations: (1) the maximum film thickness was 50 nm and (2) the film was asymmetric, which hampered calculation of the surface excess and increased the number of degrees of freedom in modeling of the adsorption. In the last part of my thesis, I describe development of a symmetric sample which (1) enables measurement of films up to 1 µm, (2) simplifies modeling of the optics by eliminating optical interference of the fluorescence excitation, and reduces the number of parameters when comparing to models.
Adsorption of Novel Block Copolymers for Steric Stabilization and Flocculation of Colloidal Particles in Aqueous Environments
Krsmanovic, Jody Lynn (Virginia Tech, 2003-01-15)
The adsorption of several homopolymer polypeptides on Al2O3 and SiO2 particles and surfaces was investigated to identify possible anchor and tail blocks for brush-forming block copolypeptides. Poly-L-(glutamic acid) (GLU) and poly-L-(aspartic acid) (ASP) were found to adsorb on positively charged and nearly neutral Al2O3, while the GLU did not adsorb on negatively charged SiO2. Poly-L-proline (PRO) adsorbed only slightly on the alumina, but showed high affinity adsorption on silica. These results are useful in designing a brush forming block copolymer with the GLU acting as the anchor block and the PRO as the tail block. An important finding in this work is that these unstructured polypeptides, or proteins that only have primary and secondary structure, have adsorption behavior that is similar to that of synthetic polymers. The complexation between a random copolymer of two amino acids, glutamic acid and tyrosine, and poly(ethylene oxide) (PEO) was studied using an in-situ adsorption experiment. It was shown that the adsorption of the random copolymer greatly increased the adsorption of PEO. It was found that the conformation of the copolymer on the surface was controlled by the ionic strength, and the conformation of the adsorbed PEO was controlled by the PEO molecular weight. Both of these factors affected the molar complexation ratio between the PEO and the tyrosine repeat units. The adsorption of two novel triblock copolymers, with PEO tails and anionic hydrophobic center blocks, was studied on alumina and silica surfaces. On silica the adsorption was due to the PEO tails, resulting in low adsorbed amounts. The adsorption was much greater on alumina, indicating either brush formation on the surface or the adsorption of micelles, which are present in solution. The effect of adsorbed polymer on the steric stabilization of alumina particles was studied using sedimentation and electrophoretic mobility experiments. These results do not show conclusively that the triblock copolymer adsorption led to particle stabilization. It is possible that better colloid stabilization of the alumina may be realized by changing the triblock composition to get greater extension and higher packing of the PEO tails.
AFM-Assisted Nanofabrication using Self-Assembled Monolayers
Jang, Chang-Hyun (Virginia Tech, 2003-12-19)
This study describes the covalent and the electrostatic attachment of molecules, nano-particles, and proteins to patterned self-assembled monolayers. A scanning probe nanografting technique was employed to produce patterns of various sizes, down to 10 nm. Thus, we are able to demonstrate a degree of surface patterning which is an order of magnitude smaller than that used in the semiconductor industry. One efficient strategy for creating chemically specific nanostructures is to use the extraordinary catalytic properties of enzymes. However, as the dimension of a catalyst patch is reduced down to nanometer scale, it is difficult to detect the very low concentration of product. This study resolves the problem by developing a new strategy: the surface trapping of a product generated by a nanometer-scale patch of surface-bound enzyme. An array of proteins finds use when the array contains a number of different proteins. Toward this end, a new and convenient method for immobilizing enzymes is developed, which will allow the preparation of thin films containing several different catalytically-active enzymes on the nanoscale. The disadvantage of scanning probe nanografting technique is that the AFM tip loses resolution through wear during the patterning procedure. This study examines the possibility of developing a new AFM lithographic method to avoid wear: the use of enzymes covalently attached to a tip as a site-specific catalyst.
The Asymmetric Synthesis Of The C1'-C10' Portion of Pamamycin-621A
Bi, Feng Jr. (Virginia Tech, 1998-08-04)
This thesis describes the synthesis of the C1'-C10' portion 72 of the pamamycin-621A using a cuprate conjugate addition to join enone fragment 52 and organostannane fragment 64a. Fragments 52 and 64a were both synthesized from (S)-methylketene dimer 51.
An atomic force microscope tip as a light source
Lulevich, V.; Honig, Christopher D. F.; Ducker, William A. (AIP Publishing, 2005-12-01)
We present a simple method for causing the end of a silicon nitride atomic force microscope (AFM) tip to emit light, and we use this emitted light to perform scanning near-field optical microscopy. Illumination of a silicon nitride AFM tip by blue (488 nm) or green (532 nm) laser light causes the sharp part of the tip to emit orange light. Orange light is emitted when the tip is immersed in either air or water; and while under illumination, emission continues for a period of many hours without photobleaching. By careful alignment of the incident beam, we can arrange the scattered light to decay as a function of the tip-substrate separation with a decay length of 100-200 nm. The exponential decay of the intensity means that the emitted light is dominated by contributions from parts of the tip that are near the sample, and therefore the emitted orange light can be used to capture high-resolution near-field optical images in air or water. (c) 2005 American Institute of Physics.
ATR-FTIR Measurements of Cationic Surfactant Exchange Rates at the Solid-Liquid Interface
Clark, Spencer C. (Virginia Tech, 2003-05-01)
In many experiments, surfactant adsorption and desorption at solid-liquid interfaces is found to be quite slow, considering that surfactants are small molecules. Attenuated total reflectance Fourier transform infrared spectroscopy was used to study the adsorption, desorption, and exchange of tetradecyltrimethylammonium bromide (C₁₄TABr) at the silicon oxide surface. The exchange of surfactant was monitored using protonated and perdeuterated C₁₄TABr. The data show that exchange of C₁₄TABr between the surface and the bulk solution is very fast, complete exchange occurs in less than 10 seconds. A simple exchange model suggests that the disassociation rate constant of a single monomer is no less than 1 s^-1, which is ~ 10⁴ times slower than monomer exchange in bulk solutions. The actual exchange rate may be greater than observed in the present work due to transport phenomena. The rates of exchange are similar at concentrations above and below the critical micellar concentration. Adsorption is similarly rapid, but under some circumstances there is a small residue of surfactant that is slow to desorb. Desorption experiments utilizing KBr solutions of high and low ionic strength show that two thirds of each adsorbed micelle is held by hydrophobic association, and the other third is electrostatically bound. Adsorption, desorption, and exchange experiments at temperatures of 11°C above and 8°C below the Krafft temperature (14.4°C) show similar kinetics.
Connecting Thermodynamics and Kinetics of Ligand Controlled Colloidal Pd Nanoparticle Synthesis
Li, Wenhui (Virginia Tech, 2019-04-24)
Colloidal nanoparticles are widely used for industrial and scientific purposes in many fields, including catalysis, biosensing, drug delivery, and electrochemistry. It has been reported that most of the functional properties and performance of the nanoparticles are highly dependent on the particle size and morphology. Therefore, controlled synthesis of nanomaterials with desired size and structure is greatly beneficial to the application. This dissertation presents a systematic study on the effect of ligands on the colloidal Pd nanoparticle synthesis mechanism, kinetics, and final particle size. Specifically, the research is focused on investigating how the ligand bindings to different metal species, i.e., metal precursor and nanoparticle surface, affect the nucleation and growth pathways and rates and connecting the binding thermodynamics to the kinetics quantitatively. The first part of the work (Chapters 4 and 5) is establishing isothermal titration calorimetry (ITC) methodology for obtaining the thermodynamic values (Gibbs free energy, equilibrium constant, enthalpy and entropy) of the ligand-metal precursor binding reactions, and the simultaneous metal precursor trimer dissociation. In brief, the binding products and reactions were characterized by nuclear magnetic resonance (NMR), and an ITC model was developed to fit the unique ITC heat curve and extract the thermodynamic properties of the reactions above. Furthermore, in Chapter 6, the thermodynamic properties, especially the entropy trend changing with the ligand chain length was investigated on different metal precursors based on the established ITC methodology, showing that the entropic penalty plays a significant role in the binding equilibrium. The second part of the dissertation (Chapter 7 and 8) presents the kinetic and mechanistic study on size-tuning of the colloidal Pd nanoparticles only by changing different coordinating solvents as ligands together with the trioctylphosphine ligand. In-situ small angle X-ray scattering was applied to characterize the time evolutions of size, size distribution, and particle concentration using synthesis reactor connected to a capillary flow cell. From the real-time kinetic measurements, the nucleation and growth rates were calculated and correlated with the thermodynamics, i.e., Gibbs free energies of solvent-ligand-metal precursor reactivity and ligand-nanoparticle surface binding which were modified by the coordination of different solvents. Higher reactivity leads to faster nucleation and high nanoparticle concentration, and stronger solvent/ligand-particle coordination energy results in higher ligand capping density and slower growth. The interplay of both effects reduces the final particle size. Furthermore, because of the significance of the ligand-metal interactions, the synthesis temperature and ligand to metal precursor ratio were systematically to modify the relative binding between the ligand and precursor, and the ligand and nanoparticle, and determine the effect on the nucleation and growth rates. The results show that the relative rates of nucleation and growth is critical to the final size. A methodology for using the in-situ measurements to predict the final size by developing a kinetic model based is discussed.
Controlling Colloidal Stability using Highly Charged Nanoparticles
Herman, David J. (Virginia Tech, 2015-02-27)
This dissertation focused on the potential use of highly charged nanoparticles to stabilize dispersions of weakly charged microparticles. The experimental components of the project centered on a model colloidal system containing silica microparticles at the isoelectric point where the suspensions are unstable and prone to flocculation. The stability of the silica suspensions was studied in the presence of highly charged nanoparticles. Initial experiments used polystyrene latex with either sulfate or amidine surface groups. Effective zeta potentials were measured with nanoparticle concentrations ranging from 0.001% to 0.5% vol. Adsorption levels were determined through direct SEM imaging of the silica microparticles, showing that the nanoparticles directly adsorbed to the microparticles (amidine more than sulfate), producing relatively large effective zeta potentials. However, stability experiments showed that the latex nanoparticles did not stabilize the silica but merely provided a reduction in overall flocculation rate. It was concluded that the zeta potential was an insufficient predictor of stability as there was still sufficient patchiness on the surface to allow for the silica surfaces to aggregate. Experiments using zirconia and alumina nanoparticles did achieve effective stabilization; both types stabilized the silica suspensions for longer than the observation period of approximately 15 hours. Stability was observed at concentrations of 10^-4% to 1.0% (zirconia) and 10^-2% vol. (alumina). These particles adsorbed directly to the microparticles (confirmed via SEM) and produced increasing effective zeta potentials with increasing nanoparticle concentrations. The adsorption resulted in significant electrostatic repulsion that was determined to be effectively irreversible using colloidal probe AFM. The improved stabilizing ability was attributed to the increased van der Waals attraction between the oxide nanoparticles (compared to polystyrene). Finally, an unexpected result of the CP-AFM force measurements showed that the repulsive forces between a nanoparticle-coated particle and plate lacked the normal dependence on the radius of the probe as predicted by the Derjaguin approximation. The forces observed in nanoparticle suspensions were virtually identical for 5 µm and 30 µm probes. Based on calculations of the shear rate in the gap, it was theorized that this phenomenon may have resulted from the shearing of adsorbed particles from the surfaces, which leads to similar interaction geometries for the two probe sizes.
Controlling Microbial Colonization and Biofilm Formation Using Topographical Cues
Kargar, Mehdi (Virginia Tech, 2015-01-13)
This dissertation introduces assembly of spherical particles as a novel topography-based anti-biofouling coating. It also provides new insights on the effects of surface topography, especially local curvature, on cell–surface and cell–cell interactions during the evolution of biofilms. I investigated the adhesion, colonization, and biofilm formation of the opportunistic human pathogen Pseudomonas aeruginosa on a solid coated in close-packed spheres of polystyrene, using flat polystyrene sheets as a control. The results show that, whereas flat sheets are covered in large clusters after one day, a close-packed layer of 630–1550 nm monodisperse spheres prevents cluster formation. Moreover, the film of spheres reduces the density of P. aeruginosa adhered to the solid by 80%. Our data show that when P. aeruginosa adheres to the spheres, the distribution is not random. For 630 nm and larger particles, P. aeruginosa tends to position its body in the confined spaces between particles. After two days, 3D biofilm structures cover much of the flat polystyrene, whereas 3D biofilms rarely occur on a solid with a colloidal crystal coating of 1550 nm spheres. On 450 nm colloidal crystals, the bacterial growth was intermediate between the flat and 1550 nm spheres. The initial preference for P. aeruginosa to adhere to confined spaces is maintained on the second day, even when the cells form clusters: the cells remain in the confined spaces to form non-touching clusters. When the cells do touch, the contact is usually the pole, not the sides of the bacteria. The observations are rationalized based on the potential gains and costs associated with cell-sphere and cell-cell contacts. I concluded that the anti-biofilm property of the colloidal crystals is correlated with the ability to arrange the individual cells. I showed that a colloidal crystal coating delays P. aeruginosa cluster formation on a medical-grade stainless-steel needle. This suggests that a colloidal crystal approach to biofilm inhibition might be applicable to other materials and geometries. The results presented in appendix 1 suggest that colloidal crystals can also delay adhesion of Methicillin resistant staphylococcus aureus (MRSA) while it supports selective adhesion of this bacterium to the confined spaces.
The Correlated Dynamics of Micron-Scale Cantilevers in a Viscous Fluid
Robbins, Brian A. (Virginia Tech, 2014-12-08)
A number of microcantilever systems of fundamental importance are explored using theoretical and numerical methods to quantify and provide physical insights into the dynamics of experimentally accessible systems that include a variety of configurations and viscous fluids. It is first shown that the correlated dynamics of both a laterally and vertically offset cantilever pair can be accurately predicted by numerical simulations. This is verified by comparing the correlated dynamics yielded by numerical simulations with experimental measurement. It is also demonstrated that in order to obtain these accurate predictions, geometric details of the cantilever must be included in the numerical simulation to directly reflect the experimental cantilever. A microrheology technique that utilizes the fluctuation-dissipation theorem is proposed. It is shown that by including the frequency dependence of the fluid damping, improvements in accuracy of the predictions of the rheological properties of the surrounding fluid are observed over current techniques. The amplitude spectrum of a 2-D cantilever in a power-law fluid is studied. The resulting amplitude spectrum yielded a curve similar to an overdamped system. It is observed that the amplitude and noise spectrum yield the same qualitative response for a 2-D cantilever in a shear thinning, power-law fluid. The correlated dynamics of a tethered vertically offset cantilever pair is investigated. It is shown that for a range of stiffness ratios, which is the ratio of the spring constant of the tethering relative to the cantilever spring constant, the change in the correlated dynamics of a Hookean spring tethered cantilever pair can be seen in the presence of fluid coupling. The dynamics of a spring-mass tethered, vertically offset cantilever pair is qualitatively studied by simplifying the model to an array of springs and masses. The resulting study found that the correlated dynamics of the displacement of mass of the tethered object yielded newly observed features and characteristics. It is shown that the curve shape of the cross-correlation of the displacement of the mass of the tethered object is similar to that of the auto-correlation of the displacement of the mass representing a step forced cantilever. The cross-correlation of the displacement of the mass of the tethered object, however, is found to be significantly more dependent on the stiffness ratio than the auto-correlation of the displacement of the mass representing a cantilever for t > 0. At t = 0, it is observed that the mass of the tethered object yields the same finite value for the cross-correlation for all studied values of the stiffness ratio. This characteristic is a result of the symmetry of the studied spring-mass system.
A correlation force spectrometer for single molecule measurements under tensile load
Radiom, Milad; Honig, Christopher D. F.; Walz, John Y.; Paul, Mark R.; Ducker, William A. (American Institute of Physics, 2013-01-07)
The dynamical-mechanical properties of a small region of fluid can be measured using two closely spaced thermally stimulated micrometer-scale cantilevers. We call this technique correlation force spectroscopy (CFS). We describe an instrument that is designed for characterizing the extensional properties of polymer molecules that straddle the gap between the two cantilevers and use it to measure the stiffness and damping (molecular friction) of a dextran molecule. The device is based on a commercial atomic force microscope, into which we have incorporated a second antiparallel cantilever. The deflection of each cantilever is measured in the frequency range dc-1 MHz and is used to generate the cross-correlation at equilibrium. The main advantage of cross-correlation measurements is the reduction in thermal noise, which sets a fundamental noise limit to force resolution. We show that the thermal noise in our cross-correlation measurements is less than one third of the value for single-cantilever force microscopy. The dynamics of the cantilever pair is modeled using the deterministic motion of a harmonic oscillator initially displaced from equilibrium, which yields the equilibrium auto and cross-correlations in cantilever displacement via the fluctuation-dissipation theorem. Fitted parameters from the model (stiffness and damping) are used to characterize the fluid at equilibrium, including any straddling molecules. (C) 2013 American Institute of Physics. [http://dx.doi.org/10.1063/1.4772646]
Correlation Force Spectroscopy for Single Molecule Measurements
Radiom, Milad (Virginia Tech, 2014-07-24)
This thesis addresses development of a new force spectroscopy tool, correlation force spectroscopy (CFS), for the measurement of the mechanical properties of very small volumes of material (molecular to µm³) at kHz-MHz time-scales. CFS is based on atomic force microscopy (AFM) and the principles of CFS resemble those of dual-trap optical tweezers. CFS consists of two closely-spaced micro-cantilevers that undergo thermal fluctuations. Measurement of the correlation in thermal fluctuations of the two cantilevers can be used to determine the mechanical properties of the soft matter, e.g. a polymeric molecule, that connects the gap between the two cantilevers. Modeling of the correlations yields the effective stiffness and damping of the molecule. The resolution in stiffness is limited by the stiffness of the cantilever and the frequency by the natural frequency of the cantilevers, but, importantly, the damping resolution is not limited by the damping of the cantilever, which has enabled high-resolution measurements of the internal friction of a polymer. The concept of CFS was originally presented by Roukes' group in Caltech [Arlett et al., Lecture Notes in Physics, 2007]; I developed the first practical versions of CFS for experimentation, and have used it in two applications (1) microrheology of Newtonian fluids and (2) single molecule force spectroscopy. To understand the correlation in thermal fluctuations of two cantilevers I initially validated the theoretical approach for analyzing correlation in terms of deterministic model using the fluctuation-dissipation theorem [Paul and Cross, PRL, 2004]. I have shown that the main advantages of such correlation measurements are a large improvement in the ability to resolve stiffness and damping. Use of CFS as a rheometer was validated by comparison between experimental data and finite element modeling of the deterministic vibrations of the cantilevers using the known viscosity and density of fluids. Work in this thesis shows that the data can also be accurately fitted using a simple harmonic oscillator model, which can be used for rapid rheometric measurements, after calibration. The mechanical properties of biomolecules such as dextran and single stranded DNA (ssDNA) are also described. CFS measurements of single molecule properties of ssDNA reveal the internal friction of the molecule in solution.
Correlations between the thermal vibrations of two cantilevers: Validation of deterministic analysis via the fluctuation-dissipation theorem
Honig, Christopher D. F.; Radiom, Milad; Robbins, Brian A.; Walz, John Y.; Paul, Mark R.; Ducker, William A. (AIP Publishing, 2012-02-01)
We validate a theoretical approach for analyzing correlations in the fluctuations of two cantilevers in terms of a deterministic model, using the fluctuation-dissipation theorem [M. R. Paul and M. C. Cross, Phys. Rev. Lett. 92, 235501 (2004)]. The validation has been made possible through measurement of the correlations between the thermally stimulated vibrations of two closely spaced micrometer-scale cantilevers in fluid. Validation of the theory enables development of a method for characterizing fluids, which we call correlation force spectrometry. (C) 2012 American Institute of Physics. [doi:10.1063/1.3681141]
Crystallization and Melting Behavior of Linear Polyethylene and Ethylene/Styrene Copolymers and Chain Length Dependence of Spherulitic Growth Rate for Poly(Ethylene Oxide) Fractions
Huang, Zhenyu (Virginia Tech, 2004-09-24)
The crystallization and melting behavior of linear polyethylene and of a series of random ethylene/styrene copolymers was investigated using a combination of classical and temperature modulated differential scanning calorimetry. In the case of linear polyethylene and low styrene content copolymers, the temporal evolutions of the melting temperature, degree of crystallinity, and excess heat capacity were studied during crystallization. The following correlations were established: 1) the evolution of the melting temperature with time parallels that of the degree of crystallinity, 2) the excess heat capacity increases linearly with the degree of crystallinity during primary crystallization, reaches a maximum during the mixed stage and decays during secondary crystallization, 3) the rates of shift of the melting temperature and decay of the excess heat capacity lead to apparent activation energies that are very similar to these reported for the crystal ac relaxation by other techniques. Strong correlations in the time domain between the secondary crystallization and the evolution of the excess heat capacity suggest that the reversible crystallization/melting phenomenon is associated with molecular events in the melt-crystal fold interfacial region. In the case of higher styrene content copolymers, the multiple melting behavior at high temperature is investigated through studies of the overall crystallization kinetics, heating rate effects and partial melting. Low melting crystals can be classified into two categories according to their melting behavior, superheating and reorganization characteristics. Low styrene content copolymers still exhibit some chain folded lamellar structure. The shift of the low melting temperature with time in this case is tentatively explained in terms of reorganization effects. Decreasing the crystallization temperature or increasing the styrene content leads to low melting crystals more akin to fringed-micelles. These crystals exhibit a lower tendency to reorganize during heating. The shift of their melting temperature with time is attributed to a decrease in the conformational entropy of the amorphous fraction as a result of constraints imposed by primary and secondary crystals. To further understand the mechanism of formation of low melting crystals, quasi-isothermal crystallization experiments were carried out using temperature modulation. The evolution of the excess heat capacity was correlated with that of the melting behavior. On the basis of these results, it is speculated that the generation of excess heat capacity at high temperature results from reversible segmental exchange on the fold surface. On the other hand, the temporal evolution of the excess heat capacity at low temperature for high styrene content copolymers is attributed to the reversible segment attachment and detachment on the lateral surface of primary crystals. The existence of different mechanisms for the generation of excess heat capacity in different temperature ranges is consistent with the observation of two temperature regimes for the degree of reversibility inferred from quasi-isothermal melting experiments. In a second project, the chain length and temperature dependences of spherulitic growth rates were studied for a series of narrow fractions of poly(ethylene oxide) with molecular weight ranging from 11 to 917 kg/mol. The crystal growth rate data spanning crystallization temperatures in regimes I and II was analyzed using the formalism of the Lauritzen-Hoffman (LH) theory. Our results are found to be in conflict with predictions from LH theory. The Kg ratio increases with molecular weight instead of remaining constant. The chain length dependence of the exponential prefactor, G0, does not follow the power law predicted by Hoffman and Miller (HM). On this basis, the simple reptation argument proposed in the HM treatment and the nucleation regime concept advanced by the LH model are questioned. We proposed that the observed I/II regime transition in growth rate data may be related to a transition in the friction coefficient, as postulated by the Brochard-de Gennnes slippage model. This mechanism is also consistent with recent calculations published by Toda in which both the rates of surface nucleation and substrate completion processes exhibit a strong temperature dependence.
Decreasing the Energy of Evaporation Using Interfacial Water: Is This Useful for Solar Evaporation Efficiency?
Ducker, William A. (American Chemical Society, 2023-04)
Evaporation of water using solar power is an economical and environmentally friendly method for purification of aqueous solutions. It has been suggested that intermediate states can be used to lower the enthalpy of evaporation of water and therefore to increase the efficiency of evaporation that uses absorption of sunlight. However, the relevant quantity is the enthalpy of evaporation from bulk water to bulk vapor, which is fixed for a given temperature and pressure. The formation of an intermediate state does not alter the enthalpy of the overall process.
Design and Characterization of Central Functionalized Asymmetric tri-Block Copolymer Modified Surfaces
Wang, Jianli (Virginia Tech, 2001-10-30)
Well-defined central functionalized asymmetric tri-block copolymers (CFABC) were designed as a new type of polymer brush surface modifier, with a short central functionalized block that can form chemical bonds with a suitable substrate surface. A combination of sequential living anionic polymerization and polymer modification reactions were used for the synthesis of two CFABCs: PS-b-poly(4-hydroxystyrene)-b-PMMA and PS-b-poly(4-urethanopropyl triethoxysilylstyrene)-b-PMMA. GPC and NMR characterization indicated that the block copolymers possessed controlled molecular weights and narrow molecular weight distributions. CFABC polymer brushes were successfully prepared by chemically grafting PS-b-poly(4-urethanopropyl triethoxysilylstyrene)-b-PMMA onto silicon wafer surfaces. AFM, XPS and ellipsometry were used to confirm the CFABC polymer brush structures and thickness. The surface properties of CFABC polymer brush modified silicon wafer substrates subjected to different environmental parameters were studied. Reversibly switchable surface energies were observed when the polymer brush modified surfaces were exposed to solvents with different polarities. The phenomenon was attributed to the chain configuration auto-adjustment in the polymer brush systems. The same mechanism was also used to explain the enhanced adhesion capability between the modified surfaces and different polymer materials (PS and PMMA). Phase behaviors of polymer thin films on unmodified and CFABC polymer brush modified silicon wafer surfaces were also studied. For thin films of polymer blends, PS blend PS-co-PMMA, the effects of film thickness, chemical composition and temperature on the phase separation mechanism were investigated. The phase behavior in thin films of triblock copolymers with or without central functionalities were compared to reveal the role of the central functionalized groups in controlling film structures. Finally, the presence of CFABC polymer brush at the interface between PS-b-PMMA diblock copolymer thin film and silicon wafer substrate was found to decrease the characteristic lamellar thickness in the thin film. A mechanism of tilted chain configurations in the thin film due to the interactions with the CFABC polymer brushes was proposed.
The Design of Biodegradable Polyester Nanocarriers for Image-guided Therapeutic Delivery
Jo, Ami (Virginia Tech, 2018-09-12)
Multiple hurdles, such as drug solubility, stability, and physical barriers in the body, hinder bioavailability of many promising therapeutics. Polymeric nanocarriers can encapsulate the therapeutics to protect non-target areas from side effects but also protect the drug from premature degradation for increased circulation and bioavailability. To capitalize on these advantages, the polymer nanoparticle must be properly engineered for increased control in size distribution, therapeutic encapsulation, colloidal stability, and release kinetics. However, each application requires a specific set of characteristics and properties. Being able to tailor these by manipulation of different design parameters is essential to optimize nanoparticles for the application of interest. This study of nanoparticle fabrication and characterization takes us a step closer to building effective delivery systems tailored for specific treatments. Poly(ethylene oxide)-b-poly(D,L-lactic acid) (PEO-b-PDLLA) based nanoparticles were produced to range from 100-200 nm in size. They were fluorescently labeled with a hydrophobic dye 6-13 bis(triisopropylsilylethynyl) pentacene (TIPS pentacene) at an optimal loading of 0.5 wt% with respect to the core. Surfaces were successfully coated with streptavidin to be readily functionalized with various biotinylated compounds such as PD-L1 antibodies or A488 fluorophore. Using the same PEO-b-PDLLA, iron oxide and a conjugated polymer poly(2- methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV) were co-encapsulated to form fluorescently labeled magnetic particles. Using poly(lactic-co-glycolic acid), CRISPR-Cas9 plasmids were encapsulated at 1.6 wt% and most of the payload released within the first 24 hours. The incorporated plasmids were intact enough to have mammalian macrophages successfully express the bacterial protein Cas9. Using similar PLGA based particles, the surface was functionalized with streptavidin and bound to the surface of bacteria as an active carrier for increased penetration of solid tumors averaging ~23 particles per bacterium. PEO-b-PLGA based particles were used in conjunction with a hydrophobic salt former to encapsulate a peptide designed to reduce platelet binding to cancer cells and mitigate extravasation. The peptide encapsulated was increased from < 2 wt% without salt former to 8.5 wt% with the used of hexadecyl phosphonic acid. Although the applications across these projects can be broad, the fundamentals and important design parameters considered contribute to the overarching field of effective carriers for drug delivery.
The Design of Stable, Well-Defined Polymer-Magnetite Nanoparticle Systems for Biomedical Applications
Miles, William Clayton (Virginia Tech, 2009-08-10)
The composition and stability of polymer-magnetite complexes is essential for their use as a treatment for retinal detachment, for drug targeting and delivery, and for use as a MRI contrast agent. This work outlines a general methodology to design well-defined, stable polymer-magnetite complexes. Colloidal modeling was developed and validated to describe polymer brush extension from the magnetite core. This allowed for the observation of deviations from expected behavior as well as the precise control of polymer-particle complex size. Application of the modified Derjaguin-Verwey-Landau-Overbeek (DLVO) theory allowed the determination of the polymer loading and molecular weight necessary to sterically stabilize primary magnetite particles. Anchoring of polyethers to the magnetite nanoparticle surface was examined using three different types of anchor groups: carboxylic acid, ammonium, and zwitterionic phosphonate. As assessed by dynamic light scattering (DLS), the zwitterionic phosphonate group provided far more robust anchoring than either the carboxylic acid or ammonium anchor groups, which was attributed to an extremely strong interaction between the phosphonate anchor and the magnetite surface. Coverage of the magnetite surface by the anchor group was found to be a critical design variable for the stability of the zwitterionic phosphonate groups, and the use of a tri-zwitterionic phosphonate anchor provided stability in phosphate buffered saline (PBS) for a large range of polymer loadings. Incorporation of an amphiphlic poly(propylene oxide)-b-poly(ethyelene oxide) (PPO-b-PEO) diblock copolymer attached to the magnetite surface was examined through colloidal modeling and DLS. The relaxivity of the complexes was related to aggregation behavior observed through DLS. This indicated the presence of a hydrophobic interaction between the PPO layers of neighboring complexes. When this interaction was large enough, the complexes exhibited an increased relaxivity and cellular uptake. Thus, we have developed a methodology that allows for design of polymer-magnetite complexes with controlled sizes (within 8% of predicted values). Application of this methodology incorporated with modified DLVO theory aids in the design of colloidally stable complexes with minimum polymer loading. Finally, determination of an anchor group stable in the presence of phosphate salts at all magnetite loadings allows for the design of materials with minimum polymer loadings in biological systems.
Development and Characterization of Advanced Polymer Electrolyte for Energy Storage and Conversion Devices
Wang, Ying (Virginia Tech, 2017-01-09)
Among the myraid energy storage technologies, polymer electrolytes have been widely employed in diverse applications such as fuel cell membranes, battery separators, mechanical actuators, reverse-osmosis membranes and solar cells. The polymer electrolytes used for these applications usually require a combination of properties, including anisotropic orientation, tunable modulus, high ionic conductivity, light weight, high thermal stability and low cost. These critical properties have motivated researchers to find next-generation polymer electrolytes, for example ion gels. This dissertation aims to develop and characterize a new class of ion gel electrolytes based on ionic liquids and a rigid-rod polyelectrolyte. The rigid-rod polyelectrolyte poly (2,2'-disulfonyl-4,4'-benzidine terephthalamide) (PBDT) is a water-miscible system and forms a liquid crystal phase above a critical concentration. The diverse properties and broad applications of this rigid-rod polyelectrolyte may originate from the double helical conformation of PBDT molecular chains. We primarily develop an ionic liquid-based polymer gel electrolyte that possesses the following exceptional combination of properties: transport anisotropy up to 3.5×, high ionic conductivity (up to 8 mS cm⁻¹), widely tunable modulus (0.03 – 3 GPa) and high thermal stability (up to 300°C). This unique platform that combines ionic liquid and polyelectrolyte is essential to develop more advanced materials for broader applications. After we obtain the ion gels, we then mainly focus on modifying and then applying them in Li-metal batteries. As a next generation of Li batteries, the Li-metal battery offers higher energy capacity compared to the current Li-ion battery, thus satisfying our requirements in developing longer-lasting batteries for portable devices and even electric vehicles. However, Li dendrite growth on the Li metal anode has limited the pratical application of Li-metal batteries. This unexpected Li dendrite growth can be suppressed by developing polymer separators with high modulus (~ Gpa), while maintaining enough ionic conductivity (~ 1 mS/cm). Here, we describe an advanced solid-state electrolyte based on a sulfonated aramid rigid-rod polymer, an ionic liquid (IL), and a lithium salt, showing promise to make a breakthrough. This unique fabrication platform can be a milestone in discovering next-generation electrolyte materials.
Development and Mechanism of Action of Antimicrobial Coatings
Behzadinasab, Saeed (Virginia Tech, 2023-07-14)

Browsing by Author "Ducker, William A."

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