Standoff Screws Used In Composite Joists

Type of Document Master's Thesis
Author Alander, Chad C. M.S.
Author's Email Address calander@vt.edu
URN etd-4798-112921
Title Standoff Screws Used In Composite Joists
Degree Master of Science
Department Civil Engineering
Advisory Committee
Advisor Name Title
W. Samuel Easterling Committee Chair
Thomas E. Cousins Committee Member
Thomas M. Murray Committee Member
Keywords

* shear connectors
* composite joists
* standoff screws

Date of Defense 1998-05-05
Availability unrestricted
Abstract

The purpose of this study is to evaluate the performance

of the 5/16 in. diameter Elco Grade 8 standoff screw as

a mechanical shear connector in composite joists.

Standoff screws are being investigated as an alternative

to welded shear studs in short span composite joists.

The data and results obtained from 106 pushout tests

performed on the Elco Grade 8 standoff screw are

presented. The test parameters include: standoff screw

height, quantity of standoff screws per deck rib, standoff

screw position, slab depth, base angle thickness, deck

type, and amount of transverse reinforcement.

The test results from this study are compared to those

obtained in previous research performed by Hankins

(1994), and the applicability of Hankins' predictive

equation for the shear strength of the Elco Grade 8

standoff screw is investigated. The influences of various

test parameters on the shear strength of the standoff

screw are evaluated and the effects of grouping the

standoff screws in the deck ribs are examined. The

performance of the standoff screw in solid slab

applications is also ivestigated. Predictive equations for

the shear strength of the Elco Grade 8 standoff screw,

based on screw-related failure modes, concrete rib

failures, and longitudinal splitting of solid concrete slabs,

are presented.

free literature

FACTORS AFFECTING MOBILITY OF COPPER IN SOIL-WATER MATRICES

Type of Document Dissertation
Author Al-Taher, Hossam
Author's Email Address haltaher@vt.edu
URN etd-02062001-154844
Title FACTORS AFFECTING MOBILITY OF COPPER IN SOIL-WATER MATRICES
Degree PhD
Department Civil Engineering
Advisory Committee
Advisor Name Title
Dr. A. Dietrich Committee Co-Chair
Dr. J.T. Novak Committee Co-Chair
Dr. D. Gallagher Committee Member
Dr. J.M. Hughes Committee Member
Dr. W.L. Daniels Committee Member
Keywords

* copper
* mobility
* soil
* horizon

Date of Defense 2001-02-02
Availability unrestricted
Abstract

Copper is applied to many crops as a fungicide/bactericide, including plasticulture tomato growing operations. Field tests have shown that copper is sometimes found in ground water near these fields. Therefore, a laboratory study was undertaken to determine if this copper can result from plasticulture application and to determine the mechanisms that account for the movement through soil to the subsurface. From the factors that may affect this mobility process; TOC, dryness and its temperature cycle and water content of the soil were selected for study. These factors were investigated in both batch and continuous flow (column) processes. Copper mobility through soil columns was associated with TOC mobility, and soil drying had a major effect on both copper and TOC mobility. The concentration of copper eluted from columns containing dried soils was up to 20 times higher from those containing wet soils. The extent of dryness was found to affect mobility. First-flush-pattern for both copper and TOC from the columns was observed in all columns studies.

In this research the copper mobility through the A- and B-horizon of Bojac sandy loam from the Eastern Shore of Virginia was studied. This study included both batch and column processes. The effects of pH, TOC and humic substances were investigated. The mobility of copper was found to be higher through A-horizon soil. The pH was found to have a considerable effect on the mobility of copper and TOC. The highest mobility of copper was achieved at pH 6.24 and its mobility through both soil horizons was associated with the mobility of TOC. A study of the humic substances indicated that fulvic acids had 4 times higher adsorption capacity for copper than humic acids. The association between fulvic acids and copper, coupled with the mobility of TOC and fulvic acids in the soil, accounted for transport of copper through soil columns.

free literature

Two-Dimensional Finite Element Analysis of Porous Geomaterials at Multikilobar Stress Levels

Type of Document Dissertation
Author Akers, Stephen Andrew
Author's Email Address akerss@wes.army.mil
URN etd-12132001-095648
Title Two-Dimensional Finite Element Analysis of Porous Geomaterials at Multikilobar Stress Levels
Degree PhD
Department Civil Engineering
Advisory Committee
Advisor Name Title
Thangavelu Kuppusamy Committee Chair
George M. Filz Committee Member
J. Michael Duncan Committee Member
Marte S. Gutierrez Committee Member
Thomas L. Brandon Committee Member
Keywords

* Geomaterials
* Finite Element
* Effective Stress
* Porous

Date of Defense 2001-12-07
Availability unrestricted
Abstract

A technique was developed for analyzing and developing mechanical properties for porous geomaterials subjected to the high pressures encountered in penetration and blast-type loadings. A finite element (FE) code was developed to verify laboratory test results or to predict unavailable laboratory test data for porous media loaded to multikilobar stress levels. This FE program eliminates a deficiency in the process of analyzing and developing mechanical properties for porous geomaterials by furnishing an advanced analysis tool to the engineer providing properties to material modelers or ground shock calculators. The FE code simulates quasi-static, axisymmetric, laboratory mechanical property tests, i.e., the laboratory tests are analyzed as boundary value problems. The code calculates strains, total and effective stresses, and pore fluid pressures for fully- and partially-saturated porous media. The time dependent flow of the pore fluid is also calculated. An elastic-plastic strain-hardening cap model calculates the time-independent skeletal responses of the porous solids. This enables the code to model nonlinear irreversible stress-strain behavior and shear-induced volume changes. Fluid and solid compressibilities were incorporated into the code, and partially-saturated materials were simulated with a "homogenized" compressible pore fluid. Solutions for several verification problems are given as proof that the program works correctly, and numerical simulations of limestone behavior under drained and undrained boundary conditions are also presented.

free literature

MODELING LIGHT DUTY VEHICLE EMISSIONS BASED ON INSTANTANEOUS SPEED AND ACCELERATION LEVELS

Type of Document Dissertation
Author Ahn, Kyoungho
Author's Email Address kahn@vt.edu
URN etd-07102002-170729
Title MODELING LIGHT DUTY VEHICLE EMISSIONS BASED ON INSTANTANEOUS SPEED AND ACCELERATION LEVELS
Degree PhD
Department Civil Engineering
Advisory Committee
Advisor Name Title
Antonio Trani Committee Co-Chair
Hesham Rakha Committee Co-Chair
Dusan Teodorovic Committee Member
Francois Dion Committee Member
Pushkin Kachroo Committee Member
Keywords

* Mobile Source Emission Model
* High Emitter
* Engine Cold Start
* Microscopic
* Fuel Consumption

Date of Defense 2002-05-29
Availability unrestricted
Abstract

This dissertation develops a framework for modeling vehicle emissions microscopically. In addition, the framework is utilized to develop the VT-Micro model using a number of data sources. Key input variables to the VT-Micro model include instantaneous vehicle speed and acceleration levels. Estimating accurate mobile source emissions is becoming more and more critical as a result of increasing environmental problems in large metropolitan urban areas. Current emission inventory models, such as MOBILE and EMPAC, are designed for developing large scale inventories, but are unable to estimate emissions from specific corridors and intersections. Alternatively, microscopic emission models are capable of assessing the impact of transportation scenarios and performing project-level analyses.

The VT-Micro model was developed using data collected at the Oak Ridge National Laboratory (ORNL) that included fuel consumption and emission rate measurements (CO, HC, and NOx) for five light-duty vehicles (LDVs) and three light-duty trucks (LDTs) as a function of the vehicle??s instantaneous speed and acceleration levels. The hybrid regression models predict hot stabilized vehicle fuel consumption and emission rates for LDVs and LDTs. The model is found to be highly accurate compared to the ORNL data with coefficients of determination ranging from 0.92 to 0.99. The study compares fuel consumption and emission results from MOBILE5a, VT-Micro, and CMEM models. The dissertation presents that the proposed VT-Micro model appears to be good enough in terms of absolute light-duty hot stabilized normal vehicle tailpipe emissions. Specifically, the emission estimates were found to be within the 95 percent confidence limits of field data and within the same level of magnitude as the MOBILE5a model estimates. Furthermore, the proposed VT-Micro model was found to reflect differences in drive cycles in a fashion that was consistent with field observations. Specifically, the model accurately captures the increase in emissions for aggressive acceleration drive cycles in comparison with other drive cycles.

The dissertation also presents a framework for developing microscopic emission models. The framework develops emission models by aggregating data using vehicle and operational variables. Specifically, statistical techniques for aggregating vehicles into homogenous categories are utilized as part of the framework. In addition, the framework accounts for temporal lags between vehicle operational variables and vehicle emissions. Finally, the framework is utilized to develop the VT-Micro model version 2.0 utilizing second-by-second chassis dynamometer emission data for a total of 60 light duty vehicles and trucks.

Also, the dissertation introduces a procedure for estimating second-by-second high emitter emissions. This research initially investigates high emitter emission cut-points to verify clear definitions of high emitter vehicles (HEVs) and derives multiplicative factors for newly developed EPA driving cycles. Same model structure with the VT-Micro model is utilized to estimate instantaneous emissions for a total of 36 light duty vehicles and trucks.

Finally, the dissertation develops a microscopic framework for estimating instantaneous vehicle start emissions for LDVs and LDTs. The framework assumes a linear decay in instantaneous start emissions over a 200-second time horizon. The initial vehicle start emission rate is computed based on MOBILE6??s soak time function assuming a 200-second decay time interval. The validity of the model was demonstrated using independent trips that involved cold start and hot start impacts with vehicle emissions estimated to within 10 percent of the field data.

The ultimate expansion of this model is its implementation within a microscopic traffic simulation environment in order to evaluate the environmental impacts of alternative ITS and non-ITS strategies. Also, the model can be applied to estimate vehicle emissions using instantaneous GPS speed measurements. Currently, the VT-Micro model has been implemented in the INTEGRATION software for the environmental assessment of operational-level transportation projects.

free literature

Microscopic Fuel Consumption and Emission Modeling

Type of Document Master's Thesis
Author Ahn, Kyoungho
Author's Email Address kahn@vt.edu
URN etd-122898-094232
Title Microscopic Fuel Consumption and Emission Modeling
Degree Master of Science
Department Civil Engineering
Advisory Committee
Advisor Name Title
Michael W. Van Aerde Committee Chair
Antonio A. Trani Committee Co-Chair
Hesham Rakha Committee Member
Wei H. Lin Committee Member
Keywords

* ITS evaluation
* Microscopic modeling
* Transportation
* Fuel consumption and emission modeling

Date of Defense 1998-11-10
Availability unrestricted
Abstract

Mathematical models to predict vehicle fuel consumption and emission metrics are presented in this thesis. Vehicle fuel consumption and emissions are complex functions to be approximated in practice due to numerous variables affecting their outcome. Vehicle energy and emissions are particularly sensitive to changes in vehicle state variables such as speed and acceleration, ambient conditions such as temperature, and driver control inputs such as acceleration pedal position and gear shift speeds, among others.

Recent empirical studies have produced large amounts of data concerning vehicle fuel consumption and emissions rates and offer a wealth of information to transportation planners. Unfortunately, unless simple relationships are found between fuel consumption and vehicle emission metrics, their application in microscopic traffic and macroscopic planning models becomes prohibitive computationally. This thesis describes the development of microscopic energy and emission models using nonlinear multiple regression and neural network techniques to approximate vehicle fuel consumption and emissions field data. The energy and emission models described in this thesis utilized data collected by the Oak Ridge National Laboratory. The data include microscopic fuel consumption and emission measurements (CO, HC, and NOx) for eight light duty vehicles as a function of vehicle speed and acceleration. The thesis describes modeling processes and the tradeoffs between model accuracy and computational efficiency. Model verification results are included for two vehicle driving cycles. The models presented estimate vehicle fuel consumption within 2.5% of their actual measured values. Vehicle emissions errors fall in the range of 3-33% with correlation coefficients ranging between 0.94 and 0.99.

Future transportation planning studies could also make use of the modeling approaches presented in the thesis. The models developed in this study have been incorporated into a microscopic traffic simulation tool called INTEGRATION to further demonstrate their application and relevance to traffic engineering studies. Two sample Intelligent Transportation Systems (ITS) application results are included. In the case studies, it was found that vehicle fuel consumption and emissions are more sensitive to the level of vehicle acceleration than to the vehicle speed. Also, the study shows signalization techniques can reduce fuel consumption and emissions significantly, while incident management techniques do not affect the energy and emissions rates notably.

free literature

A Mechanistic Analysis Based Decision Support System for Scheduling Optimal Pipeline Replacement

Type of Document Dissertation
Author Agbenowosi, Newland Komla
URN etd-11302000-223612
Title A Mechanistic Analysis Based Decision Support System for Scheduling Optimal Pipeline Replacement
Degree PhD
Department Civil Engineering
Advisory Committee
Advisor Name Title
G. V. Loganathan Committee Chair
David F. Kibler Committee Member
James Campbell Committee Member
Richard Greene Committee Member
Tamim Younos Committee Member
Keywords

* Water Distribution System
* Decision Support System
* Optimal Scheduling
* Rehabilitation

Date of Defense 2000-09-08
Availability unrestricted
Abstract

Failure of pipes in water distribution systems is a common occurrence especially in large cities. The failure of a pipe results in: loss of water; property damage; interruption of service; decreased system performance; and the financial cost of restoring the failed pipe. The cost of replacement and rehabilitation in the United States is estimated at 23 plus billion dollars. It is virtually impossible to replace all vulnerable pipes at the same time. As a result, there is a need for methods that can help in progressive system rehabilitation and replacement subject to budgetary constraints. If delaying is considered a good strategy due to the time value of money then, the timing of preventive maintenance becomes a crucial element for system maintenance and operation. The central under pinning element in the decision process for scheduling preventive maintenance is the deteriorating nature of a pipe under a given surrounding. By planning to replace pipes before they fail, proper planning can be put in place for securing of finances and labor force needed to rehabilitate the pipes. With this approach, service interruptions are minimized as the loss of service time is limited to the time used in replacing the pipe.

In this research, a mechanistic model for assessing the stage of deterioration of an underground pipe is developed. The developed model consists of three sub-models namely, the Pipe Load Model (PLM), the Pipe Deterioration Model (PDM), and the Pipe Break Model (PBM). The PLM simulates the loads and stresses exerted on a buried water main. These loads include the earth load, traffic load, internal pressure, expansive soil loads, thermal, and frost loads. The PDM simulates the deterioration of the pipe due to corrosion resulting from the physical characteristics of the pipe environment. The pipe deterioration effect is modeled in two stages. First, the thinning of the pipe wall is modeled using a corrosion model. Second, the localized pit growth is used to determine the residual strength of the pipe based on the fracture toughness and the initial design strength of the pipe.

The PBM assesses the vulnerability of a pipe at any time in terms of a critical safety factor. The safety factor is defined as the ratio of residual strength to applied stress. For a conservative estimate the multiplier effect due to thermal and frost loads are considered. For a chosen analysis period, say 50 years, the pipes with safety factors less than the critical safety factor are selected and ordered by their rank. Aided by the prioritized list of failure prone pipes, utilities can organize a replacement schedule that minimizes cost over time.

Additionally a physically based regression model for determining the optimal replacement time of pipe is also presented. A methodology for assessing the consequences of accelerated and delayed replacement is also provided. The methodologies developed in this dissertation will enable utilities to formulate future budgetary needs compatible with the intended level of service. An application of the model and results are included in the dissertation.

free literature

Experimental Evaluation and Analytical Modeling of Shear Bond in Composite Slabs

Type of Document Dissertation
Author Abdullah, Redzuan
Author's Email Address rabdulla@vt.edu, due@gawab.com
URN etd-08012004-185803
Title Experimental Evaluation and Analytical Modeling of Shear Bond in Composite Slabs
Degree PhD
Department Civil Engineering
Advisory Committee
Advisor Name Title
W. Samuel Easterling Committee Chair
Carin L. Roberts-Wollmann Committee Member
Finley A. Charney Committee Member
Thomas E. Cousins Committee Member
Thomas M. Murray Committee Member
Keywords

* Composite slabs
* Small scale test
* Elemental test
* Partial shear connection
* Shear bond
* Steel-concrete composite

Date of Defense 2004-06-25
Availability unrestricted
Abstract

The strength and behavior of composite slabs are governed by the shear interaction between the concrete and the steel deck. The interaction property depends on several factors and it is not possible to express the relationship from a purely analytical basis. As such, analysis and design methods available today use the interaction property derived from full scale performance tests. In numerical modeling, the interaction property is obtained from a variety of elemental push off tests which, for the most part, do not represent actual slab bending.

This research comprises experimental, analytical and numerical investigations of composite slabs. The central objective of the experimental work is to develop a new small scale test method for evaluating the performance and behavior of composite slabs and also for determining the shear interaction property for use in numerical analysis. The characteristics of the new test specimen are simple, easy and economical to conduct, as well as comparable in performance and behavior with the more common full slab test.

The analytical study was conducted to determine whether data from small scale tests can be used in the present analytical methods to predict the strength of the actual slabs, to use the same test data for input in the numerical analysis, and to improve the present Partial Shear Connection (PSC) design procedure. A model that relates the shear bond stress to slab slenderness, which can be used to estimate the shear interaction property for slabs with any slenderness, was developed.

Finally, a finite element study was conducted to develop a simple modeling method that is suitable for analyzing composite slabs with variable slenderness. Parametric analyses to determine the effect of slenderness on the performance and behavior of composite slabs, and on the accuracy of the present design methods were also conducted.

The results of this investigation demonstrate that the small scale test is feasible as a replacement for the full scale test. Data from the small scale test can be used not only in the analytical methods but also in the numerical analysis, thus eliminating the need for separate push off type tests.

free literature

Study and Evaluation of Traffic Responsive Control on a Large Arterial Network

Type of Document Master's Thesis
Author Abdelaziz, Sherif Lotfy Abdel Motaleb
Author's Email Address saziz@vt.edu
URN etd-08012008-162548
Title Study and Evaluation of Traffic Responsive Control on a Large Arterial Network
Degree Master of Science
Department Civil Engineering
Advisory Committee
Advisor Name Title
Montasir M. Abbas Committee Chair
Antoine G. Hobeika Committee Member
Hesham A. Rakha Committee Member
Keywords

* Traffic Scenarios
* Patterns
* Traffic responsive
* Pedestrians
* Signal Control

Date of Defense 2008-08-04
Availability unrestricted
Abstract

Traffic responsive mode of operation with its two mechanisms, threshold-based and pattern matching, is considered one of the effective and efficient signal control modes. This operation mode is underutilized due to its cumbersome configuration procedure. The research presented in this thesis aims to give some guidelines regarding traffic responsive and issues that might improve the system performance.

Four different issues related to traffic responsive are considered: The first issue is the generation of different traffic scenarios that drive the design of the system. This point is not limited to traffic responsive only but it is more general for different traffic engineering applications that need different traffic scenarios. The second issue is presenting an approach to implement traffic responsive control mode of operation in a large arterial network in Northern Virginia. Pattern matching mechanism is used for this application. Compared to time-of-day control mode, traffic responsive control saves up to 26.94% of the average delay and 21.13% of average number of stops for Reston Parkway network.

The third issue is an attempt to improve the current threshold mechanism by relaxing the threshold constraints and using variable thresholds for different levels of plan selection parameters. The last issue is a study for the pedestrian effect on the performance of networks operating by traffic responsive control. The effects of pedestrian calls and pedestrian phases on traffic responsive control are compared and the results shows that pedestrian calls are better for low pedestrian volumes while pedestrian phases are better for high pedestrian volumes.

free literature

Phase Behavior and Phase Separation Kinetics in Polymer Solutions under High Pressure

Type of Document Dissertation
Author Zhang, Wei
Author's Email Address wezhang2@vt.edu
URN etd-04182005-115303
Title Phase Behavior and Phase Separation Kinetics in Polymer Solutions under High Pressure
Degree PhD
Department Chemical Engineering
Advisory Committee
Advisor Name Title
Dr. Erdogan Kiran Committee Chair
Dr. Garth L. Wilkes Committee Member
Dr. John C. Hassler Committee Member
Dr. Larry T. Taylor Committee Member
Dr. S. Ted Oyama Committee Member
Keywords

* Phase Behavior
* High Pressure
* Polymer Solution
* Phase Separation Kinetics
* Crystallization
* Light Scattering
* Supercritical Fluids

Date of Defense 2005-04-05
Availability unrestricted
Abstract

The phase behavior and phase separation kinetics in polymer solutions in binary mixtures of supercritical carbon dioxide (CO2) and organic solvents were studied for two systems. Solutions of polyethylene (PE) in CO2 + n-pentane were selected as one model system to study both the solid-fluid (S-F) and liquid-liquid (L-L) phase transitions as well as the interplay of these two types of phase separations on the final morphological and thermal properties of PE crystals. Solutions of polysulfone (PSF) in CO2 + tetrahydrofuran (THF) were selected as another model system because of the technological importance of this membrane forming polymer and because of the broad interest in developing new solvent/non-solvent systems for forming microporous materials. These phase boundaries were determined using a high-pressure view-cell and optical techniques over a temperature range of 90-165 oC and pressures up to 55 MPa for PE/n-pentane/CO2 system, and over a temperature range of 25 to 155 oC and pressures up to 70 MPa for PSF/THF/CO2 system.

For PE solutions, it has been found that the addition of CO2 to the PE/n-pentane system shifts the L-L phase boundary to significantly higher pressures, but moves the S-F phase boundary only slightly to higher temperatures. The S-F phase boundary which represents the crystallization/melting process in the polymer solution was about 10 oC lower than the crystallization/melting temperatures of the neat polyethylene samples determined by differential scanning calorimetry (DSC). It was further found that the S-F phase boundary in n-pentane displays a unique sensitivity to the pressure-temperature conditions and moves to lower temperatures in the pressure range from 38 to 42 MPa. This effect even though not as augmented remains also for the S-F boundary in the solutions in CO2 + n-pentane mixtures.

The miscibility of PSF in THF + CO2 was investigated at CO2 levels up to 14 wt %. This system shows lower critical solution temperature (LCST)-type phase behavior at low CO2 content, which is shifted to upper critical solution temperature (UCST)-type at higher CO2 levels along with an increase in the miscibility pressures. In contrast to the PE system, this system was found to display multiple miscibility windows. A ‘U’-shaped phase boundary in 92 % THF + 8 % CO2 mixture was observed to transfer to a “W”-shaped phase boundary at 10 wt % CO2, which was further separated into a double ‘U’-shaped phase boundary at 13 wt % CO2. The specific volume of the polysulfone solutions were found to display a variation parallel to this changing pattern in the phase boundaries, with reduced miscibility being accompanied with an increase in the specific volume.

The phase separation kinetics in these two polymer solutions were investigated using time- and angle-resolved light scattering techniques. With the PE solutions, the focus was on the kinetics of S-F phase separation (crystallization) and miscibility and (melting) in n-pentane. Experiments were conducted with relatively dilute solutions at concentrations up to 2.3 wt %. The results show that the crystallization which was induced by cooling at constant pressure is dominated by a nucleation and growth process. In the majority of the experiments the particle growth process was observed to last for about 1 minute with a slight dependence on the crystallization pressure.

The phase separation kinetics in PSF solutions were conducted only in a solvent mixture containing 90 wt % THF and 10 wt % CO2. Polymer concentrations were varied up to 3.3 wt %. This system was also observed to undergo phase separation by only nucleation and growth mechanism under these conditions upon reducing the pressure at constant temperature. Several experiments were conducted using a multiple rapid pressure drop technique to identify the depth of the metastable region.

PE crystals that were produced by crossing the S-F boundary by different paths were collected and characterized by field emission scanning electron microscopy (FESEM) and DSC. Crystallization was carried out either by cooling at constant pressure, or by cooling without pressure adjustment, or by first crossing the L-L boundary via pressure reduction at a constant temperature followed by cooling. For crystal recovery, the system was depressurized to ambient conditions irrespective of the path. It was found that all of the crystals formed from these solutions show multiple melting peaks in their first DSC heating scans, which however collapse into one crystallization peak in the cooling scans and one melting peak in the second heating scans. The temperatures corresponding to the multiple melting peaks were lower than the single melting temperature of the original PE sample and the melting temperature observed in the second heating scans for all samples. The multiple melting peaks were attributed to the presence of different lamellar thickness that are formed in the crystallization, final depressurization and sample collection stages. Depending upon the crystallization path some differences were noted. The crystals formed by first going through L-L phase separation displayed predominately double melting peaks in the first DSC scan. It was observed that the overall crystallinity is increased by more than 10 % to about 75 % compared to the crystallinity of the original PE sample, which is about 63 %.

FESEM characterization showed that the prevailing morphology is composed of plate-like lamellae that show different level of agglomeration depending on the crystallization conditions. The overall structures of the particles were ellipsoid for crystals formed from dilute solutions. For crystals formed from the 1% PE solution, crystal sizes ranged from 4 mm ´ 10 mm for crystals formed at 14 MPa to 30 mm ´ 45 mm at 45 MPa. The crystals formed from 5 wt % solutions in n-pentane at pressures in the range of 38-54 MPa showed different morphologies with features of shish-kebab like structures which were however absent in crystals formed from n-pentane + CO2 solutions. The crystals that were formed from first crossing the L-L phase boundary followed by cooling showed two distinct particle size ranges that were attributed to crystals formed from the polymer-rich and polymer-lean phases that evolve when the L-L phase boundary is crossed.

free literature

Development and Evaluation of a Comprehensive Tropospheric Chemistry Model for Regional and Global Applications

Type of Document Dissertation
Author Zaveri, Rahul A.
Author's Email Address zaveri@vtaix.cc.vt.edu
URN etd-7197-18361
Title Development and Evaluation of a Comprehensive Tropospheric Chemistry Model for Regional and Global Applications
Degree Doctor of Philosophy
Department Chemical Engineering
Advisory Committee
Advisor Name Title
Leonard K. Peters Committee Chair
John C. Little none
Rick D. Saylor none
Wayne L. Neu none
William L. Conger none
Keywords

* Air quality model
* monoterpenes
* dimethylsulfide
* aerosol chemistry
* aqueous chemistry

Date of Defense 1997-06-27
Availability unrestricted
Abstract

Accurate simulations of the global radiative impact of anthropogenic

emissions must employ a tropospheric chemistry model that predicts

realistic distributions of aerosols of all types. The need for a such

a comprehensive yet computationally efficient tropospheric chemistry

model is addressed in this research via systematic development of the

various sub-models/mechanisms representing the gas-, aerosol-, and

cloud-phase chemistries.

The gas-phase model encompasses three tropospheric chemical regimes -

background and urban, continental rural, and remote marine.

The background and urban gas-phase mechanism is based on the paradigm

of the Carbon Bond approach, modified for global-scale applications.

The rural gas-phase chemistry includes highly condensed isoprene and

a-pinene reactions. The isoprene photooxidation scheme is adapted for

the present model from an available mechanism in the literature, while

an a-pinene photooxidation mechanism, capable of predicting secondary

organic aerosol formation, is developed for the first time from the

available kinetic and product formation data. The remote marine gas-

phase chemistry includes a highly condensed dimethylsulfide (DMS)

photooxidation mechanism, based on a comprehensive scheme available

in the literature. The proposed DMS mechanism can successfully explain

the observed latitudinal variation in the ratios of methanesulfonic

acid to non-sea-salt sulfate concentrations.

A highly efficient dynamic aerosol growth model is developed for

condensing inorganic gases. Algorithms are presented for calculating

equilibrium surface concentrations over dry and wet multicomponent

aerosols containing sulfate, nitrate, chloride, ammonium, and sodium.

This alternative model is capable of predictions as accurate for

completely dissolved aerosols, and more accurate for completely dry

aerosols than some of the similar models available in the literature.

For cloud processes, gas to liquid mass-transfer limitations to

aqueous-phase reactions within cloud droplets are examined for all

absorbing species by using the two-film model coupled with a

comprehensive gas and aqueous-phase reaction mechanisms. Results

indicate appreciable limitations only for the OH, HO2, and NO3

radicals. Subsequently, an accurate highly condensed aqueous-phase

mechanism is derived for global-scale applications.

free literature

Ozone Decomposition and Acetone Oxidation on Manganese Oxide Catalysts

Type of Document Master's Thesis
Author Xi, Yan
Author's Email Address yanxi@vt.edu
URN etd-05222005-141806
Title Ozone Decomposition and Acetone Oxidation on Manganese Oxide Catalysts
Degree Master of Science
Department Chemical Engineering
Advisory Committee
Advisor Name Title
S. Ted Oyama Committee Chair
Brian E. Hanson Committee Member
David F. Cox Committee Member
Keywords

* Manganese oxide catalysts
* VOCs
* ozone
* acetone
* foam
* X-ray absorption spectroscopy

Date of Defense 2005-05-11
Availability unrestricted
Abstract

This thesis describes the preparation and characterization of manganese oxide catalysts and their application in the oxidation of acetone, a typical volatile organic compound (VOC), and ozone decomposition. This topic is of great value because of environmental concerns of the elimination of the harmful VOCs and ozone. Manganese oxide was chosen because it is a well-known complete oxidation catalyst for VOCs and also an active catalyst for ozone decomposition. Two cases of studies were carried out in this work.

The first study involved the oxidation of acetone using ozone on silica- and alumina-supported manganese oxide catalysts deposited on aluminum oxide foam substrates. The characteristics of the catalysts were determined through various techniques, including x-ray diffraction (XRD), x-ray absorption spectroscopy (XAS), Brunauer-Emmett-Teller (BET) surface area analysis, temperature-programmed reduction (TPR), and oxygen chemisorption. The use of these techniques allowed better understanding of the nature of the catalysts. Activity tests were carried out in the acetone oxidation reaction and it was found that the usage of ozone substantially reduced the oxidation temperature. Steady-state in situ Raman spectroscopy was also carried out to better understand the mechanism of the acetone oxidation reaction using ozone.

The second study involved an investigation of structural and electronic properties of manganese centers of the MnOx/SiO2 and MnOx/Al2O3 catalysts during the ozone decomposition reaction using in situ x-ray absorption spectroscopy (XAS). The number of surface active sites was again determined through TPR and oxygen chemisorption measurements. The performance of the catalysts with different loadings and supports were also compared.

free literature

Novel, High Activity Hydroprocessing Catalysts: Iron Group Phosphides

Type of Document Dissertation
Author Wang, Xianqin
Author's Email Address xiwang2@vt.edu
URN etd-03272002-001809
Title Novel, High Activity Hydroprocessing Catalysts: Iron Group Phosphides
Degree PhD
Department Chemical Engineering
Advisory Committee
Advisor Name Title
S. Ted Oyama Committee Chair
Brian E. Hanson Committee Member
David F. Cox Committee Member
Paul A. Deck Committee Member
Ravi F. Saraf Committee Member
Keywords

* Iron group phosphide
* Phosphorus effect
* Hydrodesulfurization
* Hydrodenitrogenation
* Hydrodenitrogenation mechanism
* EXAFS
* Structure-sensitivity

Date of Defense 2002-03-21
Availability unrestricted
Abstract

A series of iron, cobalt and nickel transition metal phosphides was synthesized by means of temperature-programmed reduction (TPR) of the corresponding phosphates. The same materials, Fe2P, CoP and Ni2P, were also prepared on a silica (SiO2) support. The phase purity of these catalysts was established by x-ray diffraction (XRD), and the surface properties were determined by N2 BET specific surface area (Sg) measurements and CO chemisorption. The activities of the silica-supported catalysts were tested in a three-phase trickle bed reactor for the simultaneous hydrodenitrogenation (HDN) of quinoline and hydrodesulfurization (HDS) of dibenzothiophene using a model liquid feed at realistic conditions (30 atm, 370 oC). The reactivity studies showed that the nickel phosphide (Ni2P/SiO2) was the most active of the catalysts. Compared with a commercial Ni-Mo-S/g-Al2O3 catalyst at the same conditions, Ni2P/silica had a substantially higher HDS activity (100 % vs. 76 %) and HDN activity (82 % vs. 38 %).

Because of their good hydrotreating activity, an extensive study of the preparation of silica supported nickel phosphides, Ni2P/SiO2, was carried out. The parameters investigated were the phosphorus content and the weight loading of the active phase. The most active composition was found to have a starting synthesis Ni/P ratio close to 1/2, and the best loading of this sample on silica was observed to be 18 wt.%.

Extended x-ray absorption fine structure (EXAFS) and x-ray absorption near edge spectroscopy (XANES) measurements were employed to determine the structures of the supported samples. The main phase before and after reaction was found to be Ni2P, but some sulfur was found to be retained after reaction.

A comprehensive scrutiny of the HDN reaction mechanism was also made over the Ni2P/SiO2 sample (Ni/P = 1/2) by comparing the HDN activity of a series of piperidine derivatives of different structure. It was found that piperidine adsorption involved an a-H activation and nitrogen removal proceeded mainly by means of a b-H activation though an elimination (E2) mechanism. The relative elimination rates depended on the type and number of b-hydrogen atoms. Elimination of b-H atoms attached to tertiary carbon atoms occurred faster than those attached to secondary carbon atoms. Also, the greater the number of the b-H atoms, the higher the elimination rates. The nature of the adsorbed intermediates was probed by Fourier transform infrared spectroscopy (FTIR) and temperature-programmed desorption (TPD) of the probe molecule, ethylamine. This measurement allowed the determination of the likely steps in the hydrodenitrogenation reaction.

free literature

Purification and Characterization of Proteoglycan from Bovine Aortic Endothelial Cells Conditioned Media, and Its Interaction with Basic Fibroblast Gr

Type of Document Master's Thesis
Author Wang, Ningling III
Author's Email Address nwang@vt.edu
URN etd-82197-95920
Title Purification and Characterization of Proteoglycan from Bovine Aortic Endothelial Cells Conditioned Media, and Its Interaction with Basic Fibroblast Growth Factor (bFGF)
Degree Master of Science
Department Chemical Engineering
Advisory Committee
Advisor Name Title
Kimberly E. Forsten Committee Chair
R. Michael Akers Committee Member
William H. Velander Committee Member
Keywords

* bovine aortic endothelial cells
* basic fibroblast growth factor (bFGF)
* proteoglycan

Date of Defense 1997-08-27
Availability unrestricted
Abstract

Cultured bovine aortic endothelial (BAE) cells were found to synthesize and secrete heparan sulfate proteoglycans (HSPG), which bound basic fibrobalst growth factor (bFGF). bFGF is a known mitogen for vascular smooth muscle cells, and is indicated to have a role in some proliferative vascular disorders. In the present study, we have purified proteoglycans from BAE cells conditioned media (BAE PG), and further separated the PG into two fractions, PG-I and PG-II, by ion exchange chromatography on a Q-Sepharose column using a linear salt gradient (0.15 M to 1.2 M). PG-I and PG-II elute at 0.85M salt and 0.1M salt respectively. BAE PG is primarily composed of heparan sulfate, which is accessible to the digestion of Heparinase I/III and nitrous acid treatment; and a small amount of chondroitin sulfate, which can be digested by Chondroitinase ABC. Gel filtration chromatography (Sepharose CL-2B and CL-4B columns) showed that BAE PG consisted of two different sized peaks, and had an average molecular weight of approximately 5 x 10 5 Da. SDS-PAGE with silver staining indicated that BAE PG had two core proteins with estimated sizes of 300kDa and 320kDa, which corresponded to the core protein of PG-I and PG-II respectively. Western blotting with anti-perlecan primary antibody recognized the core proteins of BAE PG. Size exclusion chromatography (Sepharose CL-6B column) following b-elimination showed that BAE PG had GAG chains with an estimated size less than 2 x 10 5 Da.

A protocol to investigate the cell free binding of bFGF with purified BAE PG was established using the BioRad Bio-Dot apparatus - the cationic filtration assay (CAFAS). Using a simple monovalent binding model, we obtained values for the equilibrium dissociation constant, KD, of (1.6 ? 0.8) x 10 -10 M; the dissociation rate constant, kr, of 0.01 min -1; the association rate constant, kf, of 6.2 x 10 7 M -1 min -1 and the total binding sites of the proteoglycan, RT, of 0.1~0.2 (# of site)/(molecule of PG). The comparison of experimental data with model predictions indicates that when the number of binding sites provided by the PG is similar or greater than that of bFGF, the monovalent binding model is valid. When the number of binding sites is less than that of bFGF, one possibility is that the binding might not be the described simple monovalent reaction, and bFGF might bind to the PG as dimers or oligomers. In addition, a model is proposed for BAE PG, in which 5 ~ 10 BAE PG molecules form a high affinity binding site for bFGF. Experimentally we find that exogenous heparan sulfate competes with BAE PG for binding with bFGF, while chondroitin sulfate seems to facilitate the binding. This result may be a useful consideration when we want to design possible pharmaceutical compounds.

free literature

Polymer Surface Modification With Plasma Reaction For Materials Integration

Type of Document Master's Thesis
Author Thurmond, Brian Victor
URN etd-01182002-134625
Title Polymer Surface Modification With Plasma Reaction For Materials Integration
Degree Master of Science
Department Chemical Engineering
Advisory Committee
Advisor Name Title
Dr. Ravi Saraf Committee Chair
Dr. David Cox Committee Member
Dr. David Dillard Committee Member
Keywords

* plasma
* polystyrene
* photoluminescence

Date of Defense 2002-01-04
Availability unrestricted
Abstract

Surface modification of polystyrene thin films was achieved using a plasma process with reactive gases to form functional groups. Advancing contact angles were measured after modification. Polystyrene surfaces were observed to reach a minimum average wetting contact angle of 7 degrees. The time required to achieve this contact angle decreased significantly by increasing the power of the discharge or by lowering the discharge source closer to the polymer substrate. Characterization studies of power, height, and corona exposure time versus contact angle led to the formation of surface energy gradients across the substrate.

Photoluminescent tagging agents were used to quantify the degree of carboxyl modification achieved with water plasma and amine modification achieved with ammonia plasma. AMCA (7-amine-4-methyl coumarin hydrazide) was used to show that surface modification reaches a maximum functionalization before degradation of the polymer substrate occurs with water vapor. A parallel study with OPA (O-phthaldialdehyde) yielded similar results when ammonia was ionized over the surface.

Additionally, stable surfaces were created by chemical reaction of zinc acetate with the freshly modified polymer. Zinc sulfide particles were formed within the polymer surface by reaction with hydrogen sulfide gas. Flourescence spectroscopy was used to verify the formation of zinc sulfide.

free literature

Computer Simulation and Optimization of the NOx Abatement System at the Radford Facility and Army Ammunition Plant

Type of Document MS
Author Sweeney, Andrew Jeffrey
Author's Email Address sweeneya@usit.net
URN etd-03042000-11000048
Title Computer Simulation and Optimization of the NOx Abatement System at the Radford Facility and Army Ammunition Plant
Degree Master of Science
Department Chemical Engineering
Advisory Committee
Advisor Name Title
Dr. Y.A. Liu Committee Chair
Dr. Joseph T. Sullivan Committee Member
Dr. William L. Conger Committee Member
Keywords

* nitric oxide
* nitric acid
* environmental engineering
* nitric acid production
* retrofit design
* nitrogen oxides
* chemical engineering
* catalyst
* scrubbing
* nitrogen dioxide

Date of Defense 1999-03-21
Availability unrestricted
Abstract

This thesis discusses findings gained through work with the NOx abatement system at Radford Facility and Army Ammunition Plant (RFAAP). Removal of harmful substances from flue-gas emissions has garnered increased priority in the chemical industry in preceding decades, as governmental restrictions on these substances become more stringent and as national awareness concerning environmental quality and resource utilization continues to grow. These reasons make the study of NOx abatement an important and challenging endeavor. This work concerns itself specifically with reduction of NOx in flue-gas emissions from stationary sources. First we present an overview of current technology and approaches to controlling NOx for stationary sources. Next, we focus in on one particular approach to control of NOx within the context of a case study of the technology used at the Radford Facility and Army Ammunition Plant. RFAAP employs a scrubber/absorber tower followed in series by a selective catalytic reduction (SCR) reaction vessel in their NOx abatement system. We use as the method of study computer simulations within ASPEN Plus, a process simulation software package for chemical plants. We develop three different models with which to characterize NOx abatement at RFAAP, a conversion model, an equilibrium model and a kinetic model. The conversion-reaction model approximates the absorption and SCR reactions with constant percentage extent-of-reaction values. Though useful for initial investigation and mass balance information, we find the conversion model's insensitivity to process changes to be unacceptable for in-depth study of the case of NOx absorption and SCR. The equilibrium-reaction model works on the assumption that all the reactions reach chemical equilibrium. For the conditions studied here, we find the equilibrium model accurately simulates NOx absorption but fails in the case of SCR. Therefore, we introduce a kinetic-reaction model to handle the SCR. The SCR reactions prove to be highly rate-dependant and the kinetic approach performs well. The final evolution of the ASPEN Plus simulation uses an equilibrium model for the absorption operation and a kinetic model for the SCR. We explore retrofit options using this combined model and propose process improvements. We end this work with observations of the entire project in the form of conclusions and recommendations for improving the operation of the NOx abatement system through process-parameter optimization and equipment-retrofit schemes.

By leading the reader through the process by which we arrived at a successful and highly informative computer model for NOx absorption and SCR, we hope to educate the reader on the subtleties of NOx abatement by absorption and SCR. We attempt to break down the numerous complex processes to present a less daunting prospect to the engineer challenged with the application of current NOx removal technology. In addition, we introduce the reader to the power and usefulness of computer modeling in instances of such complexity. The model teaches us about the details of the process and helps us develop concrete information for its optimization. Ideally, the reader could use a similar approach in tackling related operations and not confine the usefulness of this thesis to NOx absorption and SCR.

The audiences that we think would benefit from exposure to this thesis are the following: · Environmental engineers with a NOx problem; · Process engineers interested in optimization tools; · Design engineers exploring flue-gas treatment options; · Combustion engineer desiring to learn about SCR; · Chemists and mathematicians intrigued by the complexities of NOx absorption chemistry.

free literature

Interface Driven Dynamics at Nanoscales:Polymer thin films and Electrical Double Layer

Type of Document Dissertation
Author singh, gaurav
Author's Email Address gsingh@vt.edu
URN etd-12252006-024053
Title Interface Driven Dynamics at Nanoscales:Polymer thin films and Electrical Double Layer
Degree PhD
Department Chemical Engineering
Advisory Committee
Advisor Name Title
David F Cox Committee Chair
Donald G Baird Committee Member
Nammalwar Sriranganthan Committee Member
Ravi F Saraf Committee Member
Richey M Davis Committee Member
Keywords

* differential interferometry
* polyelectrolyte dynamics
* combinatorial electrochemistry

Date of Defense 2006-12-04
Availability unrestricted
Abstract

The electrical double layer (EDL) is formed due to the accumulation of charge at the interface of a metal surface in contact with an electrolyte. The total charge in the EDL compensates the charge on the metal surface. As EDL is the layer that ?connects? the electrode to the ?bulk?, all electrode mediated transport and redox reaction depends on the structure and dynamics of the ions in the EDL. Thus the ion dynamics in the EDL are critical to a wide range of physical and biological phenomena such as electrochemical reaction, flow in channels of nanofluidic devices, wetting of fluids; to biology, for example, folding and function of proteins, conformation change of DNA and ionic flow through cell membranes.

EDL polarization is the ion accumulation or depletion in the EDL due to the potential of the metal surface. The conventional method of measuring the EDL polarization is by monitoring the current flowing through the electrochemical system. Thus, the electrical characteristics of the EDL are inferred indirectly from the total current that is implicitly related to effects such as the impedance of the bulk solution. We have developed a sensitive optical interferometric technique to directly measure the polarization of the metal-electrolyte interface. The key advantage of our method is high sensitivity, and the measurement is specific only to the changes at the metal-electrolyte interface. The ion accumulation in the EDL of a simple salt like NaCl is studied as a function of the frequency and the amplitude of the applied potential on the metal electrode. The amplitude of modulation of the ions is linearly proportional to the amplitude of the applied AC potential. The linearity is observed up to high amplitude (up to 2V) and salt concentration as high as 0.5M. Furthermore, the local segmental dynamics of polyelectrolytes such as polystyrene sulfonate have been measured.

Next we extend this novel technique to study electrochemical redox reactions. The oxidation of the widely used redox ion [Fe(CN)6]4- is followed by measuring the response to an AC potential (amplitude ~100mV) as a function of a superimposed saw-tooth potential ramp, at a time period 106 fold slower and amplitude 5-10 fold larger than the AC potential. The sensitivity of the optical method is significantly better than the measurement of the AC current. For a redox process on the electrode, the change in the optical signal is over two orders of magnitude larger than the electrical signal. Using the optical technique, we can separate the kinetic events in redox processes: transport of charged species to the electrode surface and charge transfer across the electrode-electrolyte interface. Because we measure the local electrochemical process, the method can be used to probe redox reaction at multiple spots on the same electrode (i.e., combinatorial electrochemistry).

free literature

Evaluation of Phenomena that Determine the Performance of Immunoaffinity, Peptide-Based and Ion Exchange Affinity Sorbents

Type of Document Dissertation
Author Sines, Brian James
URN etd-12012000-111836
Title Evaluation of Phenomena that Determine the Performance of Immunoaffinity, Peptide-Based and Ion Exchange Affinity Sorbents
Degree PhD
Department Chemical Engineering
Advisory Committee
Advisor Name Title
William H. Velander Committee Chair
John J. Tyson Committee Member
Joseph T. Sullivan Committee Member
Kimberly E. Forsten Committee Member
Richey M. Davis Committee Member
William L. Conger Committee Member
Keywords

* affinity chromatography
* protein purification

Date of Defense 2000-11-03
Availability unrestricted
Abstract

Insufficient supply and pathogen safety concerns regarding plasma-derived therapeutic proteins, such as fibrinogen and immunoglobulins, have been the impetus for the development of genetic engineering techniques and separations methods for the economical and safe production of these proteins. This study is concerned with the isolation of these important therapeutics from complex media. Immunoaffinity chromatography has been an important method in the isolation of these products, typically being implemented as the final cleanup step yielding an extremely pure, homogenous final product. However, the use of immunoaffinity chromatography in large-scale purification processes have been precluded due to high capital costs and the inherent lability of immunosorbents. Peptide-based affinity sorbents are being developed in order to surmount the inherent limitations posed by monoclonal antibodies that are used as ligands in immunosorbents.

The objective of this research is to quantitatively assess the impact of affinity ligand orientation, local density and transport phenomena on peptide-based affnity sorbent performance. The peptides under study herein can form high-affinity complexes with their protein targets, thus these ligands are one of the newest technologies arising from combinatorial chemistry with applications to the difficult problem of purifying high-molecular weight proteins from complex mixtures. Two types of structural motifs which are common to small peptide affinity ligands derived from combinatorial chemistry are studied here: a linear peptide which is comprised of the affinity recognition sequence in its entirety and a chain structure which displays multiple branches of the recognition sequence emanating from a central lysinic core structure. Two recognition sequences are studied here which bind plasma proteins. One peptide recognition sequence forms a high affinity complex with fibrinogen. Another peptide recognition sequence binds the Fc region of immunoglobulins. Immunglobulins are plasma proteins which range in molecular weight from 155 to 900-kDa and are valuable for therapeutic uses for imparting passive immunity.

This study seeks to identify factors analogous to those manifested in immunosorbent performance that may also be important in the optimal design of peptide-based affinity sorbents. In general, previous research with the design of immunosorbents have found that immunosorbent performance, i.e., target-binding efficiency or activity, is substantially dependent upon several factors which include effects associated with ligand orientation, and local density as related to steric incumbrance of target binding sites, and transport phenomena as related to under utilization of intra matrix volume. In summary, this study asks the questions: (1) What factors regarding ligand orientation, local ligand density, and intraparticle transport phenomena, are important in the optimal design of peptide affinity sorbents?; and (2) Are these effects analogous to those manifested in immunosorbent performance?

This study seeks to investigate the use of techniques used to mitigate the effects associated with these negative factors upon immunosorbent performance in order to elucidate the nature of these same effects upon peptide-based affinity sorbents. For example, oriented ligand immobilization can be facilitated through selective coupling chemistries and the premasking of ligand binding domains prior to immobilization. In addition, the manipulation of local ligand density using novel spatially controlled matrix activation and ligand immobilization methods can be assessed and implemented for the optimization of the performance and design of peptide-based affinity sorbents. This study has found that enhanced transport phenomena into the matrix interior volume can be achieved by using low solids content cellulose matrices having a low extent of crosslinking. This study demonstrates the effective use of these large-particle diameter, low-solids content cellulose hydrogel matrices in immunoaffinity, peptide-based affinity and ion exchange chromatography in the separation of high-molecular weight therapeutic proteins.

free literature

Investigation of the Influence of Selected Variables on the Solid State Structure-Property Behavior of Segmented Copolymers

Type of Document Dissertation
Author Sheth, Jignesh Pramod
URN etd-10282004-165218
Title Investigation of the Influence of Selected Variables on the Solid State Structure-Property Behavior of Segmented Copolymers
Degree PhD
Department Chemical Engineering
Advisory Committee
Advisor Name Title
Garth L. WIlkes Committee Chair
Erdogan Kiran Committee Member
Richey M. Davis Committee Member
Thomas C. Ward Committee Member
Timothy E. Long Committee Member
Keywords

* segmented copolymer
* microphase separation
* poly(ether-block-amide)
* polyurethane
* hydrogen bonding
* structure-property behavior
* atomic force microscopy

Date of Defense 2004-10-22
Availability unrestricted
Abstract

Segmented copolymers are a commercially important class of materials that are utilized in a wide variety of applications. In these systems a relatively large number of variables such as backbone chemistry, segment molecular weight, and the overall molecular weight of the copolymer can be independently controlled to engineer materials with targeted properties. Such versatility also means that a large number of variables can influence the morphology and therefore, properties and performance of segmented copolymers. In this dissertation, the influence of selected variables on the solid state structure-property behavior of segmented poly(ether-block-amide), polyurethane, polyurethaneurea, and polyurea copolymers is explored. The specific variables which have been utilized singly or in conjunction with others are hard segment crystallizability, crystallization conditions, hard segment content, soft segment type and molecular weight, nature of hydrogen bonding, extent of inter-segmental hydrogen bonding, segment symmetry, and chain architecture.

In poly(ether-block-amide)s, it was found that the morphology of both the crystalline and the amorphous phase depend upon the polyamide content of the sample and, as expected, the crystallization conditions.

A comparison of polydimethylsiloxane based segmented polyurethanes with their polyurea counterparts demonstrated that for a constant hard segment content the soft segment molecular weight particularly governs the extent of microphase separation in these materials. The nature of hydrogen bonding, monodentate or bidentate, also strongly influences their mechanical response. Remarkably, the polyurea sample with a polydimethylsiloxane molecular weight of 7000 g/mol and a hard segment content of 25 wt % exhibited a remarkable service temperature window (for rubber-like behavior) of ca. 230 ?C (from -55?C to 175?C) whereas it was ca. 200 ?C wide (from -55?C to 145?C) for the equivalent polyurethane sample. The extremely high chemical incompatibility between the polydimethylsiloxane of sufficiently high molecular weight and urethane or urea segment is expected to generate a relatively sharp interface between the soft matrix and the dispersed hard domains. Therefore, a polyether co-soft segment was incorporated in a controlled manner along the chain backbone, which resulted in inter-segmental hydrogen bonding between the ether and the urea segments. The consequent segmental mixing gave rise to a gradient interphase, which led to a significant improvement in the tensile strength, and elongation at break in selected polydimethylsiloxane segmented polyurea copolymers.

The importance of the hydrogen bonding network in model polyurethaneurea copolymers was also explored by utilizing LiCl as molecular probe. It has been demonstrated that hydrogen bonding plays an important role, over and above microphase separation, in promoting the long-range connectivity of the hard segments and the percolation of the hard phase through the soft matrix. The incorporation of hard segment branching in these polyurethaneurea also reduced the ability of the hard segments to pack effectively and establish long-range connectivity. The disruption of the percolated hard phase resulted in a systematic softening of the copolymers.

The role of chain architecture in governing the structure/property/processing of segmented was also investigated by comparing highly branched segmented polyurethaneureas with their linear analogs. These copolymers were based on poly(propylene oxide) or poly(tetramethylene oxide) as the soft segments The highly branched copolymers utilized in this dissertation were able to develop a microphase morphology similar to their linear analogs. Particularly noteworthy, and surprising, was the observation of weak second order interference shoulder in the respective small angle X-ray scattering profiles of the highly branched samples based on poly(propylene oxide) of MW 8200 and 12200, indicating the presence of at least some level of long-range order of the hard domains in these samples. Tapping-mode atomic force microscopy phase images of these two samples clearly confirmed the small angle X-ray scattering results. In addition to the strain induced crystallization of the poly(tetramethylene oxide) MW 2000 g/mol based linear polyurethaneureas, the highly branched analog of this sample also exhibited similar behavior at ambient temperature and uniaxial deformation of ca. 400 % strain. Wide angle X-ray scattering confirmed the above observation. The reduced ability of the branched polymers to entangle resulted in slightly poorer mechanical properties, such as tensile strength, elongation at break, and stress relaxation as compared to their linear analogs. However, primarily due to their reduced entanglement density, the branched polyurethaneureas had significantly lower ambient temperature solution viscosity as compared to their linear polyurethaneurea analogs. Therefore, these highly branched polyurethaneureas can be more easily processed than the latter materials.

Finally, it was demonstrated that non-chain extended segmented polyurethane and polyurea copolymers in which the hard segment is based on only a single diisocyanate molecule may well exhibit properties, such as the breadth of the service window, the average plateau modulus, stiffness, tensile strength, and elongation at break that are similar to chain extended segmented copolymers that possess distinctly higher hard segment content. A careful control of the hard segment symmetry and the nature of the hydrogen bonding is necessary to achieve such improved performance in the non-chain extended systems. Therefore, the results of this study provide new direction for the production of thermoplastic segmented copolymers with useful structural properties.

free literature

Nonlinear Optically Active Ionically Self-Assembled Monolayer Thin Films of Organic Polymers Intercalated with an Inorganic Hectorite, Laponite RD

Type of Document Master's Thesis
Author Shah, Smital S
URN etd-02192003-120507
Title Nonlinear Optically Active Ionically Self-Assembled Monolayer Thin Films of Organic Polymers Intercalated with an Inorganic Hectorite, Laponite RD
Degree Master of Science
Department Chemical Engineering
Advisory Committee
Advisor Name Title
Dr. Richey M. Davis Committee Chair
Dr. Kevin E. Van Cott Committee Member
Dr. Randy Heflin Committee Member
Keywords

* ionically self-assembled monolayers
* laponite RD
* hectorite
* thin films
* nonlinear optics
* second harmonic generation

Date of Defense 2002-08-30
Availability unrestricted
Abstract

Detailed studies are presented of thin films containing a polycation, a nonlinear optically (NLO) active chromophore, and a synthetic hectorite that self-assemble into the noncentrosymmetric structure required for second order nonlinear optical responses. UV/Vis spectroscopy and ellipsometry were used as probes to monitor film growth for upto 25 deposition cycles. Exceptionally homogeneous films were obtained with regular film growth for up to the 25 cycles deposited.

ISAM films self-assemble from polyelectrolyte solutions due to coulombic interactions between a charged substrate and the charged polymer in solution. Charges accumulating at the surface restrict further growth due to charge overcompensation at the surface. The entire process occurs relatively quickly as compared to other competing assembly techniques such as Langmuir-Blodgett assembly and covalent self-assembly.

Previous studies indicated that second harmonic signal diminishes after the deposition of the first few bilayers. This is potentially due to adjacent layer interpenetration, which becomes prominent moving further away from the hard substrate interface. Laponite RD, a synthetic hectorite was introduced in the films in an effort to minimize interpenetration of adjacent bilayers and hence maintain chromophore orientation in every bilayer of the ISAM film. The film was deposited in quadlayers that have the following sequence:

Polycation—Laponite—Polycation—Chromophore.

This study is unique in its approach as it investigates the possible implications of film interpenetration on the NLO-activity of ISAM films that can be easily fabricated. It also shows the effects of different interfaces on the NLO-activities of the films.

We have investigated the effect of changing the polycation from poly(allylamine hydrochloride) (PAH) and poly(diallyldimethylammonium chloride) (PDDA) and the solution pH to see how these variables affect the growth and NLO properties of ISAM films. At lower pH values (specify relevant range of values), for both polycations, intrachain and interchain repulsion is strong due to little electrostatic screening. This results in polycation deposition in relatively thin, train-like layers. At higher pH levels (specify relevant range of values here) the electrostatic screening is greater due to a higher effective ionic strength. At these conditions, intrachain and interchain repulsion is reduced and so the polymers adsorb to form thicker layers with more loops and tails than for the case at lower pH. This also results in a higher density of the chromophore in the film.

Extremely smooth surfaces of Laponite RD in film were obtained as confirmed by AFM imaging on glass. Regular quadlayer growth monitored by UV/Vis spectroscopy and ellipsometry was observed for up to 25 quadlayers. Second harmonic generation (SHG) signal was not conclusively affected by the presence of laponite as the decrease of signal was seen after the first few layers in the laponite containing films. This decrease was however was not as sharp in the films containing laponite as in the films that did not contain laponite. It was also noted that the SHG was not so much affected by the number of layers deposited but more so by the distance of the chromophore layer from the hard glass interface. This study thus brings to light the very important effect of the glass interface on the NLO-activity of these films.

free literature

RESEARCH AND DEVELOPMENT OF SIMULATION AND OPTIMIZATION TECHNOLOGY FOR COMMERCIAL NYLON-6 MANUFACTURING PROCESSES

Type of Document Dissertation
Author Seavey, Kevin Christopher
Author's Email Address kseavey@vt.edu
URN etd-04172003-134414
Title RESEARCH AND DEVELOPMENT OF SIMULATION AND OPTIMIZATION TECHNOLOGY FOR COMMERCIAL NYLON-6 MANUFACTURING PROCESSES
Degree PhD
Department Chemical Engineering
Advisory Committee
Advisor Name Title
Y. A. Liu Committee Chair
Chau-Chyun Chen Committee Member
D. G. Baird Committee Member
R. M. Davis Committee Member
S. Adjerid Committee Member
Tim Bremner Committee Member
Keywords

* vacuum devolatization
* nylon-6
* modeling
* finishing reactor
* polycondensation

Date of Defense 2003-04-14
Availability unrestricted
Abstract

This dissertation concerns the development of simulation and optimization technology for

industrial, hydrolytic nylon-6 polymerizations. The significance of this work is that it is a

comprehensive and fundamental analysis of nearly all of the pertinent aspects of

simulation. It steps through all of the major steps for developing process models,

including simulation of the reaction kinetics, phase equilibrium, physical properties, and

mass-transfer- limited devolatization. Using this work, we can build accurate models for

all major processing equipment involved in nylon-6 production.

Contributions in this dissertation are of two types. Type one concerns the formalization

of existing knowledge of nylon-6 polymerization mixtures, mainly for documentation and

teaching purposes. Type two, on the other hand, concerns original research contributions.

Formalizations of existing knowledge include reaction kinetics and physical properties.

Original research contributions include models for phase equilibrium, diffusivities of

water and caprolactam, and devolatization in vacuum- finishing reactors.

We have designed all of the models herein to be fundamental, yet accessible to the

practicing engineer. All of the analysis was done using commercial software packages

offered by Aspen Technology, Cambridge, MA. We chose these packages for two

reasons: (1) These packages enable one to quickly build fundamental steady-state and

dynamic models of polymer trains; and (2) These packages are the only ones

commercially available for simulating polymer trains.

free literature