Savvas Hatzikiriakos


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Dissertations completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest dissertations.

Stabilization of zirconium oxide suspensions by cellulose nanocrystal (CNC) (2023)

Colloidal gels of cellulose nanocrystals (CNCs) were prepared by increasing ionic strength. The ultimate objective was to investigate stabilization of zirconia suspensions by using a small amount of CNC by changing electrolyte concentration or pH. First, the rheological behavior of CNC hydrogels in the presence of a monovalent electrolyte (NaCl) as a function of CNC and salt concentration was explored using a variety of linear and nonlinear rheological tests. Two step-yielding behavior was observed for CNCs in the presence of high electrolyte concentration from amplitude sweep experiment. The first yielding corresponds to the maximum in the loss modulus at the crossover point between storage and loss modulus due to disconnection of link between CNC clusters. The second yield stress is due to the deformation of clusters to smaller flocs and individual nanorods (corresponding to the change in the slope of loss modulus at higher shear strains). Small angle light scattering measurements, confocal laser scanning microscopy and polarized microscopy images confirmed the gradual breakup of clusters to smaller ones and eventually to nearly individual fibers with an increase in the applied shear strain and rate.CNCs were used as a dispersant to stabilize water-based zirconia suspensions. The zirconia suspensions as functions of solid content, pH and CNC concentration were studied to produce stable highly concentrated suspensions. The achieved stability was found to be due to the adsorption of CNC nanofibers around the zirconia particles revealed by Scanning Electron Microscopy (SEM) images. Suspensions with the lowest viscosity and highest stability over 24 h achieved at pH 4 i.e., 30 wt.% zirconia particles stabilized with the addition of 1 wt.% CNC.Finally, the effect of CNC and NaCl concentration was studied on the stability, adsorption, zeta potential, size, and rheology of slurries. The results confirm that the adsorption capacity of CNC on the surface of zirconia particles increases as salt concentration increases, associated with rise in the viscoelastic properties and denser structure on the surface of adsorbent. It has been concluded that zirconium oxide suspensions of high concentration (>30 wt.%) can be effectively stabilized by using a small amount of CNC (
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Flow instabilities of PP filled with crosslinked EPDM rubber: using rheology to understand and control its occurrence (2021)

Thermoplastic vulcanizates (TPVs) are polymer blends that have good processability like their thermoplastic phase and good elasticity like their elastomeric phase. They consist of a high amount of dynamically cured rubber particles which make flow instability such as melt fracture intense and complicated. In this study a comprehensive rheological analysis is performed to gain a deep understanding of TPVs’ flow instabilities and identify the key parameters that control them.First, a thorough linear viscoelastic analysis is performed using several groups of TPVs which are systematically different in curing level, types of polymer components, and cured rubber content. All the TPVs show a non-terminal behavior reaching to an equilibrium modulus at low frequencies/high relaxation times. The equilibrium modulus, Gy, is an indication of the existence of yield stress and the linear modulus, G(t) can be modeled by a simple power-law model in the form of (?(?)−??)∝?−?, where t is the time. A yield stress is also observed in the extensional tests and the relationship between extensional and shear yield stress is investigated.Due to the presence of oil and high amount of rubber particles, TPVs show wall slip. The multimode integral Kaye-Bernstein-Kearsley-Zapas (KBKZ) constitutive model considering wall slip is applied to study and model the TPVs’ non-linear behavior. To incorporate slip into the model, a new way is used by applying a fraction of imposed nominal strain to match the experimental data. Moreover, it is assumed that the material does not slip in the linear unyielding region i.e., for shear stresses less than the yield stress. Applying these assumptions, the KBKZ captures the experimental data well.Finally, to study processing parameters that affect the melt fracture phenomenon, capillary experiments are performed. It is observed that TPVs slip massively in capillary flow. Surface fracture of the TPVs gets better with shear rate indicating that the origin of melt fracture is different with that of TPV’s pure component. Yield stress controls flow instability of the TPVs and to overcome and/or mitigate melt fracture, high shear rates are needed to cause flow and eliminate unyielded regions in the complex geometries used in real processing.

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Ionic diodes based on cellulose nanocrystals (CNC) and related rheological studies (2021)

In this thesis, cellulose nanocrystals (CNC) fibers were chemically modified to produce anionic (pCNC) and cationic (nCNC) polyelectrolyte hydrogels to fabricate ionic diodes. In the beginning, the rheological behaviour of cationic and anionic CNC was studied and compared to that of pristine CNC. It was demonstrated that in the sonicated state, anionic and cationic CNC form hydrogen bonding, which notably contributes to interparticle forces and gel strengths. These structures between individual rods defeat the purpose of flocculation and ultimately leading to a more stable suspension. Moreover, enhanced rheological properties were observed in the case of nCNC in comparison with the pCNC and this may be due to the extensive formation of hydrogen bonding. In addition, the surface-modified cellulose nanocrystals were used to fabricate ionic diodes. Rectification behaviour from two oppositely charged hydrogels doped with cellulose nanocrystals with positive and negative surface charges was observed. It was found that the current−voltage characteristics of the CNC−hydrogel diode are influenced by several parameters including gel thickness, hydrogel concentration, applied voltage, and scanning frequency. Pronounced rectification ratio and high current densities in forward bias occurred as a result of the high surface area followed by a high charge density.Analyzing the experimental data, we demonstrated that unidirectional current response originated from an anisotropic distribution of counterions at the interface between the two gels doped with oppositely charged CNCs. Moreover, the physical mechanism is described quantitatively by an electrochemical model. We investigated and validated the proposed electrochemical mechanism by the Yamamoto-Doi model using experimental data. We demonstrated that the diode works via a physical mechanism that involves the electrochemical generation of hydroxyl ions and protons at the electrodes to create current. Exponential currents (J) in the forward bias were observed while J = A√(-V) in the backward bias, which is in agreement with predictions of the electrochemical model proposed by Yamamoto-Doi ¹. The results of this thesis can be directly utilized to fabricate biodegradable diodes of good, stable rectification performance. Also, this work provides insight on how to control ionic movement in ionic devices.

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Investigation of polyelectrolyte gel based electronic devices (2020)

We present a molecular dynamics (MD) and experimental study of polyelectrolyte (PE) gel based electronic devices such as sensors and diodes. We first perform an MD study of two PE gels with different degrees of ionization coupled in a slab geometry. Our simulations show that a pressure gradient emerges between the two gels that results in the buildup of a Nernst-Donnan potential. The Nernst-Donnan potential at the interface is found to scale linearly with temperature with the coefficient of proportionality given by the fraction of concentrations of the uncondensed counterions. We show that the potential difference can also be expressed as a linear function of the lateral pressure, thus providing a molecular interpretation of the piezo-ionic effect. These findings provide further insight onto the behaviour of soft-sensors in the equilibrium regime with no salt ion/solvent fluxes.We also perform an MD study of a junction of two oppositely charged PE networks, and compare the ion densities and electrostatic field to a corresponding continuum Poisson-Boltzmann (PB) model. At low electrostatic coupling strength, the PB model reproduces the MD simulation results for density and electric field throughout the gel very well. At higher electrostatic coupling and higher degrees of ionization, the standard PB fails to predict the MD profiles at the diode interface due to counterion condensation, network collapse and field-induced gel deformation. In fact, MD simulations predict that the rectifying behavior of diodes operating in such regimes will be much reduced. We develop a modified PB model that accounts for these effects, show that it produces better agreement with the MD results, and can be used for improved modeling of Polyelectrolyte Gel Diodes (PGDs).Additionally, we perform a systematic stress-test of the predictions of the (Yamamoto and Doi 2014) theory of the PGDs under linear sweep and step bias. We have found that the predictions of the functional forms of current-voltage (I-V) curves of the Yamamoto theory hold well. They predict an exponential increase in the regime of the forward bias as well as the square-root dependency for the regime of the reverse voltage.

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Rheology of associating polymers: effects of ionic and hydrogen bonding interactions (2019)

Associating polymers, consisting of relatively small functional groups capable of forming reversible physical associations, represent an important class of materials since they can be used in stimuli-responsive systems. They have found numerous applications in self-healing systems, and they can be used as shape memory polymers and biomaterials. Their complex behavior depends on the delicate interplay between the chemistry of functional groups and polymer dynamics. Therefore, it is essential to understand the physics of these complicated networks to exploit their full potential for important/innovative applications.Two classes of associating polymers, namely ion-containing polymers (ionomers), and hydrogen bonding polymers, were studied. Using a rotational rheometer and the Sentmanat extensional fixture, a full rheological characterization of several commercial ionomers and a series of novel amine-containing polynorbornenes and polycyclooctenes was conducted. Emphasis has been placed on the distribution of the relaxation times to identify the characteristics times such as reptation, Rouse times and the characteristic lifetime of the ionic and hydrogen bonding associations. These characteristic relaxation times were related to the structure of these polymers using developed scaling theories.Complete removal of ions was performed to produce copolymers in order to study the relative effects of ionic associations. It was demonstrated that ionic interactions increase the linear viscoelastic moduli and the viscosity by up to an order of magnitude and cause significant strain hardening effects in uniaxial extension. Similarly, for the novel hydrogen bonding polymers, the reversible associations significantly increase the rheological properties and thus delay their relaxation, causing chains to strain-harden before failure. Additionally, it was discovered that the amine-containing polymers exhibit outstanding self-healing properties. Optimal, fast self-healing response was achieved by varying the molecular weight and structure of these materials.Finally, the capillary flow of several commercial ionomers was studied both experimentally and numerically. The excess pressure drop due to entry, the effect of pressure on viscosity, and the possible slip effects on the capillary data analysis were examined. Rheological data were successfully fitted to a viscoelastic model developed by Kaye, Bernstein, Kearsley, and Zapas, (known as the K-BKZ model) to perform relevant capillary flow simulations.

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Surface engineering of wood fiber/filler networks (2019)

The hydrophilic property of cellulose substrates and their sensitivity to moisture limits their use in certain applications. The aim of this study is to enhance the barrier properties of cellulosic and lignocellulosic paper by utilizing environmentaly benign techniques. Chemical Vapor Deposition (CVD), Plasma Enhanced Chemical Vapor Deposition (PECVD), and Atomic Layer Deposition (ALD) techniques were employed to deposit Dichlorodimethylsilane (DCDMS), tetrafluoromethane (CF₄), and aluminum oxide (Al₂O₃) on cellulosic and lignocellulosic papers, respectively. A wide range of fiber sizes from unrefined, 927 µm, to highly refined, 177 µm, were employed to make handsheets and the effect of the chemicals and the deposition techniques, mentioned above, on wettability and gas permeability of the handsheets was investigated. In this regard, the contact angles on handsheets prepared with unrefined fibers are significantly higher (140°-153°) than those of the refined ones (95°-120°) due to their higher surface roughness. However, on handsheets formed with refined fibers, although the treatments resulted in a hydrophobic surface, the water droplets absorb to the handsheets over time. It is also shown that at certain fiber size (561 µm) the water vapor transmission rate (WVTR) reaches its minimum value and further decrease of the fiber size does not significantly affect the WVTR. In terms of wettability, the cellulosic and lignocellulosic substrates coated by deposition of CF₄ resulted in the highest contact angles (120°-153°). However, regarding moisture barrier properties, the Al₂O₃ deposited substrates resulted in the lowest WVTRs (2.1 g·m-²·day-¹). Moreover, the impact of fabrication method was studied and the fiber drying mechanism during sheet formation was also elucidated. It was found that casting of a Micro Fiber (MF) suspension on hydrophobic substrates results in formation of optically translucent films with mechanical and barrier properties similar to micro and nano fibrillated cellulose films. The manufacturing of the latter is energy intensive and hence the new method has potential advantage. Finally, Janus (hydrophilic-hydrophobic) fillers were fabricated and the effect of filler’s dual functionality on barrier properties of handsheets loaded with Janus fillers was investigated. Silanization of handsheets substrates in conjunction with dual functionality of fillers results in formation of a superhydrophobic handsheet.

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Synthesis and rheological characterization of polyhydroxybutyrate with different topologies and microstructures (2018)

Series of monodispersed linear and star-shaped polyhydroxybutyrate (PHB)s were synthesized using controlled indium and zinc based complexes through immortal ring opening polymerization of β-butyrolactone (BBL) in the presence of benzyl alcohol, tris(hydroxymethyl)benzene, and dipentaerythritol chain transfer agents. The topologies of the prepared PHBs of various molecular weights were investigated using solution and melt rheological characterizations. The powerlaw relationship between the radius of gyration and hydrodynamic radii of the linear and star PHBs with the molecular weight confirmed that the molecules are self-similar. Reduced values of compactness factor relative to that of linear counterparts and exponential scalling of the zero-shear viscosity of the stars with span molecular weight confirmed the presence of branching on the PHB backbone. A series of racemic and enantiopure zinc complexes were synthesized and fully characterized for the polymerization of BBL to form high molecular weight syndiotactic PHBs (Pr up to 75%). Complex (±)-[(NNHOtBu)ZnOBn]₂ (9) showed unprecedented reactivity and control towards the polymerization of up to 20000 equivalents of BBL in the presence of 5000 equivalents of benzyl alcohol. Isothermal time sweep tests at temperatures above the melting point of the syndio-rich PHBs showed thermally stable behavior of these polymers at temperatures below 140 oC. The zero-shear viscosity of the syndio-riched PHBs was higher than their atactic counterparts and showed a power-law relationship with the molecular weight confirming the linear microstructure and the absence of cyclic or branched species in the melt. The extensional rheometry revealed high melt strength in a range of strain rates as a result of flow induced crystallization.Easy to make, indium-salan complexes were reported for the polymerization of as-received lactide. The solution state characterization of these polymers showed narrow molecular weight distributions with molecular weights closely matching the theoretical molecular weight as an indication of a robust catalytic system. These complexes are capable of polymerizing impure lactide isomers in the melt state under ambient atmospheric conditions to form high molecular weight symmetric star shaped multi-block PLAs with high melting points, up to 197 °C. This catalytic system can also be used for the formation of star-shaped PHB-PLA copolymers in inert atmosphere.

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Wall slip of polydisperse linear polymers: effects of molecular weight characteristics, and surface conditions (2018)

The classical no-slip boundary condition of fluid mechanics is not always a validassumption for the flow of complex fluids including polymer melts. Since the slip velocity ofpolymer melts complicates analysis of rheological data and it is needed for simulation of polymerprocesses and process optimization, a comprehensive predictive slip velocity model should bedeveloped. In this thesis, the slip behavior of monodisperse and polydisperse linear polymersincluding high-density polyethylenes (HDPEs), polybutadienes (PBDs), and polystyrenes (PSs) isstudied to fully understand the effect of molecular weight (MW) and molecular weight distribution(MWD). Concepts from double reptation mixing rule are used to develop an expression for slipvelocity of polydisperse polymers based on their MW and MWD. Very good agreement betweenexperimental data and predictions of proposed model is observed, validating the applicability ofthe model.Surface enrichment of short chains next to solid boundaries due to entropic effectscomplicates slip analysis specially in the case of bimodal polymers. To address surfacesegregation, the slip behavior of several HDPEs with broad range of molecular weight includingbimodals is studied. Moreover, the developed slip model coupled with a model of surfacemolecular weight fractionation is used to predict the slip velocity of the studied polymers. It isobserved that surface fractionation has a minor effect on slip of narrow to moderate MWDpolymers (particularly unimodal), but its role is significant for broad bimodal polymers.Moreover, the dynamic slip behavior of a polymer melt was investigated by performingdynamic shear experiments using the stress/strain controlled rotational rheometer equipped withparallel partitioned plate geometry. The multimode integral Kaye-Bernstein-Kearsley-Zapas(KBKZ) constitutive model is applied and it is found that a dynamic slip model with a sliprelaxation time is needed to adequately predict the experimental data at large shear deformations.Finally, the effects of surface topology and energy on slip velocity of high-density HDPEswas studied using treated and untreated smooth and patterned slit dies. It was found that the slipvelocity is decreased by roughness and is increased by silanization. These effects have beenincorporated into the slip velocity model.

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Crystallization of Polypropylene: experiments and modeling (2017)

In this study, the quiescent crystallization of several polypropylenes (PPs) with different molecular characteristics was first examined using Differential Scanning Calorimetry (DSC) and Polarized Optical Microscopy (POM). The Avrami/Nakamura equation was employed to fit and predict crystallization kinetics under isothermal and non-isothermal conditions. The Avrami/Nakamura model was found to predict the non-isothermal crystallization data of the various PPs very well over a range of cooling rates. POM was used in line with a rotational rheometer to further examine the behaviour under quiescent condition at different temperatures and/or cooling rates. The growth rate of crystals was impeded exponentially with increase of temperature. To study the effect of flow on crystallization behaviour of PPs, the Anton Paar MCR-502 rotational rheometer equipped with various fixtures including parallel-plate and POM to induce shear flow was used. Generally, an increase in strain and strain rate or decrease of temperature is found to decrease the thermodynamic barrier for crystal formation and thus enhancing crystallization kinetics at temperatures between the melting and crystallization points. Popular models based on suspension theory, which are often used to relate the degree of crystallinity to normalized rheological functions are validated experimentally. It is found that the constant(s) of various suspension models should be dependent on the flow parameters in order for the suspension models to describe the effect of shear on FIC, particularly at higher shear rates. Finally, a capillary rheometer was used to investigate flow-induced crystallization (FIC) of various resins at high shear rates relevant to polymer processing. It is found that the crystallization kinetics are enhanced with increasing molecular weight indicating the importance of high-end tail of MWD on FIC. Various dies with different physics were used to investigate the effect of flow on FIC. The Cogswell analysis was applied on the capillary data to obtain the apparent extensional strain rate and strain as well as the apparent extensional viscosity. FIC was found to depend strongly on the L/D ratio of the capillary die. Finally, temperature impacted the FIC behaviour extensively since it alters the activation energy needed for the formation of macroscopic structures.

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Extrudate Swell of High Density Polyethylene in Capillary and Slit Dies (2017)

The analysis of extrudate swell in polymer melts is of great importance in many polymer processing operations and has been the subject of interest both experimentally and numerically. The main objectives of this research work are to obtain systematic and reliable extrudate swell data of a high molecular weight HDPE, to identify a suitable constitutive model that can precisely represent extrudate swell phenomena and to predict extrudate swell accurately under various processing and operating conditions. A novel extrudate swell measuring system with an online data acquisition system is designed for the present work. This system allows one to measure extrudate swell profile under different conditions such as steady state or transient, gravity free, isothermal and non-isothermal conditions. Further, the set-up is suitable for both capillary and slit extrudates. A comprehensive analysis on the applicability and validity of various rheological (integral and differential/molecular) models in describing extrudate swell of a highly viscoelastic HDPE polymer over a broad range of shear rates (5 to 100s-¹) is carried out using FEM based ANSYS POLYFLOW®. The simulation results indicated that the integral constitutive equations of K-BKZ type can account for the significant memory effects of viscoelastic polymer melts such as HDPE. Overprediction of extrudate swell by the integral K-BKZ model invoked the importance of obtaining non-linear viscoelastic properties for a broader range of deformations/deformation rates. The newly available CPP fixture from AntonPaar is used to procure such non-linear viscoelastic data and thus to determine the accurate damping function. The simulation results of extrudate swell in capillary and slit dies are in good agreement with the experimental measurements using the newly determined damping function. In addition, non-isothermal extrudate swell of the HDPE polymer is studied using the pseudo-time integral K-BKZ Wagner (i.e., the non-isothermal form) model with the differential Nakamura equation for the crystallization kinetics. The model is implemented in ANSYS POLYFLOW®. Extrudate swell measurements are obtained by extruding the polymer melt at 200ºC through long capillary and slit dies to ambient air at 25ºC and 110ºC. The numerical results are found to be in very good agreement with the experimental observations.

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Role of fibrillation on Poisson's ratio of expanded polytetrafluoroethylene (PTFE) (2017)

The polytetrafluoroethylene (PTFE) paste extrusion was studied to elucidate the role of structure formation (fibrillation) on the Poisson’s ratio of final products such as stents and other implants. In particular two types of PTFE have been studied in capillary extrusion using dies having different reduction ratios (RR) and die entrance angles. The extrudates collected at different processing conditions, were tested in uniaxial extension to assess their mechanical properties. The tensile modulus, yield stress and ultimate tensile strength of the obtained extrudates were found to be increasing functions of reduction ratio, although the opposite effect was found for the ultimate elongational strain. Moreover, a PTFE paste was extruded using a capillary rheometer at various temperatures through cylindrical dies of different reduction ratios (1-D structure samples). Uniaxial tensile experiments were performed on the collected extrudates using the SER at different temperatures and Hencky strain rates. A nonlinear viscoelastic model (Matsuoka) was used to model the transient tensile results. An empirical model was also developed to predict the tensile ultimate strength as a function of processing conditions such as temperature and die reduction ratio, as well as, the testing operating conditions i.e., temperature and Hencky strain rate. PTFE flat profiles were extruded using slit dies, which promoted orientation of fibrils in two directions (2-D structure samples). Uniaxial tensile experiments were performed on the collected extrudates using the SER at different temperatures and Hencky strain rates to determine mechanical properties. Poisson’s ratio was determined using image analysis and the results were compared using data from the 1-D structure samples. Polarized Raman spectroscopy was used to gain additional information on the degree of fibril orientation at different locations along and across the width and length of the extrudates. Finally, a simple model was derived for the density change in tensile deformation by taking into the account the Poisson’s ratio and the strain recovery. Results of the Raman spectra and the strain recovery coefficient from density changes, were found to be in agreement with the fibril structure/morphology obtained from SEM micrographs.

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Superhydrophobic polymeric surfaces: fabrication, wettability, and antibbacterial activits (2017)

This PhD thesis studies fabrication of superhydrophobic polymeric surfaces, their wetting properties, and their antibacterial activities as potential application to medical sciences. A femtosecond laser technique was used to fabricate mico/nano- structures on the surface of PTFE and PU. The effect of laser parameters (fluence, scanning speed, and overlap) on the wettability of the resulted micro/nano-patterns was studied. Two techniques were used to laser-scan the surface, namely uniaxial and biaxial scan. Uniaxial scan creates channeled morphology with direction-dependent wettability. To produce uniform wettability independent of direction, biaxial scanning was examined, which creates well-defined pillars with very high contact angle (CA) and very low contact angle hysteresis (CAH).To facilitate and speed up the surface micro/nano-structuring, laser-ablation was coupled with thermal imprinting. The metallic femtosecond laser-ablated templates were employed to imprint micron/submicron periodic structures onto the surface of several polymers. The CA of imprinted polymers increased to above 160°, while their CAH varied significantly depending on the surface thermophysical and chemical properties.A unique technique was developed to create superomniphobic patterns on HDPE through hot embossing. The filefish skin dual scale superoleophobic patterns were used as a biological model to develop angled microfiber arrays on HDPE. The obtained bioinspired surface is highly capable of repelling both water and liquids with low surface tensions that meets the superomniphobic criteria.The effect of superhydrophobicity on protein adsorption and bacterial adhesion of laser-ablated PTFE substrates were investigated. Samples were incubated in Gram negative (E.coli) and Gram positive (S.aureus) bacteria cultures, BSA solution, IgG solution, and blood plasma for 4 hours. All superhydrophobic surfaces were found to be more resistant to protein /bacteria adhesion compared to the corresponding smooth samples. However, some of the most superhydrophobic PTFE surfaces were found to exhibit the highest adherence with protein/bacteria; while some other did not allow any adsorption/adherence of protein/bacteria respectively towards the end of the incubation. Besides the CA, CAH, average height of pillars, and spacing distance betweeniiithe pillars, this study showed that there are other roughness factors, which play crucial role in the durability of the superhydrophobic surfaces such as the distribution of pillar heights.

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Synthesis and rheology of poly(lactide)s and their lignin composites (2017)

Synthetic plastics were first introduced 180 years ago, but the materials we have produced are likely to persist on our earth for thousands of years. Global shifts in thinking have urged researchers to focus their attention on bio-derived and biodegradable polymers. One such polymer is poly(lactic acid) (PLA). Despite its environmental benefits, PLA has several material weaknesses which hinder it’s use as a replacement for commodity plastics. Highly active and selective indium catalysts for the ring-opening polymerization of lactide isomers have recently been developed by the Mehrkhodavandi group. By utilizing these catalysts, modification of tacticity and end-group functionality of PLAs are possible, permitting exploration into the effect of these modifications on chain interactions in PLA. The thermal and rheological behaviours of PLAs with different microstructures were compared. The molecular weight between entanglements was greatest for the syndiotactically enriched PLAs, giving rise to the lowest zero-shear viscosity. In addition, hetero- and isotactically enriched PLA had higher flow activation energies than syndiotactic variants, implying the inclusion of transient aggregate regions within these polymers due to enhanced L- and D-interactions. A series of aryl-capped PLAs were synthesized by living ring-opening polymerization with a chain transfer agent using a previously reported dinuclear indium catalyst, [(NNO)InCl]₂(μ-Cl)(μ-OEt) (A). Thermal, rheological and mechanical techniques were employed to understand the extent and strength of association caused by arylated chain ends. It is shown that the end-group has a greater effect on the properties of low molecular weight PLAs due to the larger number density of aryl end groups; significant interactions can be induced under oscillatory shear conditions in the low frequency flow regime (terminal zone).The lignocellulosic biorefinery industry has been expanding in recent years and now provides researchers access to a range of bio-based composite materials through blending and copolymerization. Lignin-graft-PLA copolymers were synthesized via different routes and the PLA products were analyzed. Polymers were found to have cyclic structures at low lignin loading and star-like structures at higher lignin loading. Rheological studies were undertaken to derive useful structure-property relationships and optimize material properties.

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Rheology of Pastes: Effects of Fibrillation, Thixotropy and Structure (2014)

Three different pastes (toothpaste, PTFE paste that is mixture of polytetrafluoroethylene of submicron size particles with a liquid lubricant, and chocolate) are investigated in this thesis as model paste systems to study their processing characteristics in capillary flow using various dies. The rheological behaviour of toothpaste and melt chocolate paste is identified as that of a yield-stress, thixotropic material with a time-dependent behaviour. The rheological data obtained from a parallel-disk were used to formulate a constitutive equation with a structural parameter which obeys a kinetic equation, typically used to model thixotropy. For semi-solid paste extrusion (PTFE paste and solid chocolate), a simple phenomenological mathematical model is developed. The model takes into account the elastic-plastic (strain hardening) and viscous nature of the material in its non-melt state. In addition, it takes into account the slip boundary condition at the paste/wall interface. To study scale-up possibilities, the rheology of non-melt processible polytetrafluoroethylene (PTFE) pastes is studied using three capillary rheometers having barrels of different diameter and equipped with capillary dies of various designs. The effects of process conditions on fibrillation and mechanical properties of polytetrafluoroethylene (PTFE) paste extrudates are also studied. To describe the effects of die design on the quality of the final product, a basic phenomenological mathematical model is developed. The model consists of a simple equation that explains fibril formation, due to the compression of PTFE resins, plus a kinetic equation, which is coupled with the “radial-flow” hypothesis to predict the structure and the tensile strength of extrudates. Model predictions for structural parameter compared with the tensile strength measurements, have shown a good qualitative agreement. For all paste systems, the pressure drop is measured as a function of apparent shear rate (flow rate), reduction ratio (cross sectional area of barrel to that of die), contraction angle, length-to-diameter ratio, and diameter of the barrel (scale-up). In all cases, model shown to have coefficient of determination (R2) above 0.84. Finally, extrusion pressure predictions based on the proposed models are compared with the experimental data obtained from macroscopic pressure drop measurements and are found to be consistent.

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Shear rheology of cellulose nanocrystal (CNC) aqueous suspensions (2014)

Scientific and commercial interests in renewable nanomaterials have been receiving increasing attention over the years. Cellulose nanocrystal (CNC) derived from entirely renewable resources promises wide applicability owing to its high strength, chirality, self-assembly and electromagnetic properties. In this thesis the rheology of CNC aqueous suspensions was studied and the rheological behaviour was correlated with their microstructure. It has been found that the CNC aqueous suspensions experience two microstructural transitions by increasing CNC concentration: a transition from isotropic to chiral nematic liquid crystal occurs above a first critical concentration, and by further increasing concentration, the suspensions go through another transition from chiral nematic liquid crystal to gel above a second critical concentration. The viscosity profile of anisotropic suspensions shows a three-region behaviour characteristic of liquid crystals, and after gel formation a single shear thinning is observed over the whole investigated range.CNC suspensions possessing a higher degree of sulfation have more tendency to form anisotropic chiral nematic structures, and form gels at relatively higher concentration compared to those with a lower degree of sulfation. Sonication up to 1000 J/g CNC, breaks all the aggregates in the system and significantly decreases the viscosity. Although the sonication-induced decrease in viscosity levels off through further sonication (>1000 J/g CNC), it still affects the viscosity of anisotropic suspensions at low shear rates by increasing the size of chiral nematic domains.The effects of adding NaCl to CNC aqueous suspensions have been evaluated in different concentration regimes: isotropic, anisotropic chiral nematic, and gel. For isotropic samples and gels, the viscosity decreases by the addition of NaCl up to 5 mM. For anisotropic samples, on the other hand, the viscosity at low shear rates increases by addition of NaCl up to 5 mM due to decrease in chiral nematic domain size. However, at high shear rates, where all the domains are broken, the viscosity decreases when adding NaCl. Further addition of NaCl (>5 mM) results in extensive aggregation in suspension, and thus the viscosity increases.

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Superhydrophobic Nanopatterned Interfaces: Optimization and Manufacturing (2014)

This work studies in detail the effect of femtosecond laser irradiation process parameters (fluence, scanning speed and scanning overlap) on the wettability of the resulted micro/nano-patterned morphologies on stainless steel. Depending on the laser parameters, four distinctly different nano-patterns were produced, namely nano-rippled, parabolic-pillared, elongated sinusoidal-pillared and triple roughness nanostructures. All of the produced structures were classified according to a newly defined parameter, the Laser Intensity Factor (LIF) that is a function of scanning speed and fluence of laser. By increasing LIF, the ablation rate and the periodicity of the asperities increase. In order to decrease the surface energy, all of the surfaces were coated with a fluorinated alkylsilane agent. Analysis of the wettability in terms of contact angle (CA) and contact angle hysteresis (CAH) revealed enhanced superhydrophobicity for most of these structures, particularly that possessing triple roughness pattern. This also exhibited a low CAH. The high permanent superhydrophobicity of this pattern is due to the special micro-nano structure of the surface that facilitates the Cassie-Baxter state. A new two-dimensional (2D) thermodynamic model is developed to predict the contact angle (CA) and contact angle hysteresis (CAH) of all types of surface geometries, particularly those with asperities having non-flattened tops. The model is evaluated by micro/nano sinusoidal and parabolic patterns fabricated by laser ablation. These microstructures are analyzed thermodynamically through the use of the Gibbs free energy to obtain the equilibrium CA and CAH. The effects of the geometrical details on maximizing the superhydrophobicity of the nano-patterned surface are also discussed in an attempt to design surfaces with desired and/or optimum wetting characteristics. The analysis of the various surfaces reveals the important geometrical parameters, which may lead to lotus effect (high CA>150° and low CAH150° and high CAH>>10°).

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The Flow Properties of Bitumen in the Presence of C02 (2014)

The present dissertation discusses the flow behaviour of bitumens in the presence of CO₂. Firstly, the viscoelastic behaviour of bitumen is studied and an appropriate constitutive equation is identified to describe its rheological behavior. The K-BKZ constitutive equation has been shown to represent accurately the rheological properties of bitumen. Analysis of experimental results revealed that either the Papanastasiou or the Marucci form of the damping function can be used in the K-BKZ constitutive equation. Moreover, the damping function was found to be independent of temperature (0°C-50°C). Secondly, the effects of temperature, pressure, dissolved carbon dioxide and shear rate on the rheological response of bitumen are investigated by using the reduced variable method at the temperature range of –10°C to 180°C and pressures up to 15 MPa. The double–log model is found to be the most accurate equation in describing the effect of temperature on the viscosity of bitumen over a wide range of temperature while the Barus model with the temperature–dependent parameter is found to be the most appropriate correlation to represent the effect of pressure. The Fujita–Kishimato equation, resulting from the free volume concept modelling, is employed to account for the effect of dissolved CO₂ on the viscosity of the bitumen–CO₂ mixture. The results show that the viscosity is influenced by the temperature and saturation pressure. Thirdly, the combined pressure-decay technique with rheometry is developed to measure the diffusivity of CO₂ in bitumen at the temperatures of 30˚C, 50˚C, 70˚C, 90˚C and 110˚C and saturation pressures of 2, 4 and 10 MPa. The impact of temperature on the diffusivity of CO₂-bitumen systems can be described by the Arrhenius equation. The diffusivity increases with pressure at gaseous CO₂ state. The increase is more dominant at lower temperatures while the diffusivity increase is 53% at 30˚C compared to 25% at 70˚C. It is shown that changing the state of CO₂ impacts the diffusivity values of in bitumen while the diffusivity is higher for the liquid CO₂ compared to supercritical CO₂. 

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Rheology and processing of biodegradable poly (E-Caprolactone) polyesters and their blends with polylactides (2013)

The solution rheological properties and melt viscoelastic behaviour of a number of commercial and newly synthesized PCLs with different molecular characteristics was investigated using both rotational and extensional rheometry. The variation of zero-shear viscosity and relaxation spectrum with molecular weight were found to be in agreement with theories for linear polymers. The classic Wagner model was found to represent the rheology of all PCL polymers quite well. In addition, the PCL processing instabilities were studied by capillary extrusion. Sharkskin and gross melt fracture was observed for the high molecular weight PCL at different shear rates. The onset of melt fracture occurred at 0.2 MPa at temperatures higher than 115°C. Moreover, addition of 0.5 wt% of a polylactide (PLA) into the PCL eliminated or delayed the onset of melt fracture to higher shear rates. The thermodynamics and rheological behavior of PCL/PLA blends has been studied in an attempt to find ways to improve the mechanical properties of PCL. The effects of shear and heating rates on the determination of the phase separation boundary of the PCL/PLA blend system were studied in detail. The lower critical solution temperature (LCST) phase boundary for this system is shifted based on the frequency, heating rate and concentration of the components. Additionally, higher molecular weight amorphous PLA leads to phase separation boundary shifted to lower temperatures. Differential scanning calorimetry (DSC) thermograms of PCL/PLA blends exhibit separate melting peaks, which are indicative of immiscible structure at all compositions in agreement with the phase diagram. Scanning electron microscopy (SEM) images have shown droplet morphology of PCL into PLA matrix up to 40wt% of PCL. Above this concentration the co-continuous morphology starts to appear, which becomes again droplet morphology for blends with PCL concentration above 60wt%. The viscoelastic studies in the phase separated region have shown the enhancement of the elastic modulus of blends at small frequencies, which is a signature behavior of immiscible systems due to the presence of interface and interfacial tension contribution to the stress. Emulsion models were found to be successful in the prediction of viscoelastic behavior at the compositions which corresponds to droplet morphology.

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Rheology and Processing of High-Density Polyethylenes (HDPEs): Effects of Molecular Characteristics (2012)

In this study, the linear viscoelastic properties of two series of Ziegler-Natta and metallocene HDPEs (ZN-HDPEs and m-HDPEs respectively) of broad molecular weight distribution (MWD) have been studied. Relationships between the zero-shear viscosity and molecular weight (Mw) and molecular weight distribution show that the breadth of the molecular weight distribution (MWD) for m-HDPEs plays a significant role. Other interesting correlations between the crossover modulus and steady state compliance with MWD of both these classes of polymers have also been derived. Finally, the steady-shear viscosities from capillary rheometry are compared with LVE data to check the applicability of the empirical Cox-Merz rule. It is shown that the original Cox-Merz rule is approximately applicable for HDPEs for narrow to moderate MWD and fails for those HDPEs having a wide MWD due to the occurrence of wall slip. The processing behavior of both series of HDPEs was investigated. Their melt fracture behaviour was studied primarily as a function of Mw and MWD, and operating conditions i.e. temperature and geometrical details and type of die (capillary, slit and annular). It is found that sharkskin and other melt fracture phenomena are very different for these two classes of polymers, although their rheological behaviors are nearly the same for many of these. It is also found that critical conditions for the onset of various melt fracture phenomena depend significantly on the type of die used for their study. The slip behaviour of these resins was also studied as a function of Mw and MWD. It is found that the slip velocity increases with decrease of Mw, which expected to decay to zero as the Mw approaches a value with characteristic molecular dimension similar to surface asperities. For HDPEs that exhibit stick-slip transition (narrow to moderate MWD), the slip velocity has been found to increase with increase of polydispersity. The opposite dependence is shown for HDPEs of wider molecular weight distribution that do not exhibit stick-slip transition. A criterion is also developed as to the occurrence or not of the stick-slip transition which is found to depend strongly on Mw and its distribution.

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Rheology and Processing of Poly (Lactides) and their Enantiomeric Copolymers and Blends (2012)

Poly(lactide) PLA, a biodegradable thermoplastic produced from corn and other renewable agricultural resources, has received a large share of the interest in biodegradable materials due to environmental concerns and desire to reduce dependence on finite petroleum reserves. In this study, nearly monodisperse controlled microstructure PLA samples synthesized using a novel chiral dinuclear indium catalyst; and studied thermorheologically. Specifically, the effects of molecular structural parameters (i.e. weight-average molecular weight (Mw) and different ratios of lactides) on solution and melt rheological properties under shear and extension were studied. The solution properties and linear viscoelasticity (LVE) of melts indicated linear structure behavior. The zero-shear viscosity and relaxation time of PLAs showed a power law scaling of 3.4 with Mw. The K-BKZ constitutive equation was used and proved that strain hardening occurs at low temperatures, which is due to the dynamics of molecular relaxation, when the longest relaxation time exceeded the characteristic time for deformation. In an attempt to reduce PLA brittleness, copolymers of L-lactide with its enantiomer D-lactide or racemic mixture DL-lactide were synthesized. The effects of Mw and block length ratio on the thermal, rheological and mechanical behavior of the diblock copolymers were investigated. For comparison, blends of PDLLA and PLLA homopolymers of equivalent Mw to the diblock copolymers were prepared. Despite different thermal behavior, the linear viscoelasticity of block copolymers and blends in disordered state are relatively similar. Improvement in elongation at break and tensile strength were observed as compared to their counterpart homopolymer blends. Furthermore, the wall slip and melt fracture behaviors of four commercial PLAs with Mw in the range of 10⁴ to 10⁵ g/mol were investigated. PLAs with Mw greater than a certain value slipped. The slip velocity increased with decrease of Mw. The onset of melt fracture for the high Mw PLAs occurred at about 0.2 to 0.3 MPa, depending on the geometrical characteristics of the dies and independent of temperature. Addition of 0.5 wt% of a poly(ε-caprolactone) (PCL) into the PLA that exhibits melt fracture was effective in eliminating and delaying its onset to higher shear rates.

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Rheology of low to medium consistencey pulp suspensions (2011)

Papermaking is a major industry to manufacture products vital to education, communication, and packaging. Most operations in this industry deal with the flow of different mass concentrations of pulp suspensions. Therefore, the flow properties (rheology) of pulp suspensions are of great importance for the optimal functionality of most unit operations in the industry. Yield stress is one of the most important rheological properties in designing process equipment, thus needs to be determined by a reliable technique. Two established and extensively used methods for determining yield stress were compared with a velocimetry technique. The yield stresses were determined for commercial pulp suspensions at fibre mass concentrations of 0.5 to 5 wt. %. The results were compared and models were proposed to predict the yield stress as a function of fibre mass concentration. The yield stress values obtained by the velocimetry technique were found to be the most reliable.Conventional rheometry and local velocimetry techniques were further used to study the flow behaviour of pulp suspensions beyond the yield stress. Pulp suspensions were found to be shear-thinning up to a certain high shear rate. The Herschel–Bulkley constitutive equation was used to fit the local steady-state velocity profiles and to predict the steady-state flow curves obtained by conventional rheometry. Conventional rheometry was found to fail at low shear rates due to the presence of wall slip. Consistency between the various sets of data was found for all suspensions studied.Finally, the same approach was used to study thixotropy and transient flow behaviour of concentrated pulp suspension of 6 wt.%. Pulp was found to exhibit a plateau in the flow curve where a slight increase in the shear stress generated a jump in the corresponding shear rate, implying the occurrence of shear banding. The velocity profiles were found to be discontinuous in the vicinity of the yielding radius where a Herschel-Bulkley model failed to predict the flow. Shear history and the time of rest prior to the measurement were found to play a significant role on the rheology and the local velocity profiles of pulp suspension.

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Microscopic ice friction (2010)

Microscopic ice friction was studied systematically across all to ice friction relevant friction regimes using several metallic interfaces. A rheometer with a newly designed fixture for friction measurements was used in these studies. The investigations focus on the influence of material properties, such as surface wettability, roughness, surface structure, surface nanopatterning, and thermal conductivity. Using a femtosecond laser process certain dual scale roughness structures were created to mimic the lotus leaf on the surface of inherently hydrophilic metal alloys. After laser irradiation the samples show initially superhydrophilic behavior with complete wetting of the structured surface. However, over time these surfaces become hydrophobic to superhydrophobic. The change in wetting behavior correlates with the amount of carbon found on the structured surface. The explanation for the time dependency of the surface wettability lies in the combined effect of surface morphology and surface chemistry.With regard to ice friction this controlled lotus-like roughness significantly increases the coefficient of friction at low sliding speeds and temperatures well below the ice melting point. However, at temperatures close to the melting point and relatively higher speeds, roughness and hydrophobicity significantly decrease ice friction. This decrease in friction is mainly due to the suppression of capillary bridges. The influence of surface structure on ice friction was also investigated isolated from the effect of surface roughness. It is shown that grooves oriented in the sliding direction also significantly decrease friction in the low velocity range compared to scratches and grooves randomly distributed over a surface.The isolated effect of thermal conductivity on ice friction is investigated by thermally insulating the slider and the friction fixture with fiberglass. A decrease of the friction coefficient in the boundary friction regime and an earlier onset of the mixed friction regime in terms of sliding velocity are reported. Furthermore, the dependence of the ice friction coefficient on sliding velocity is compared for different sliding materials. It was concluded that the influence of thermal conductivity decreases with increasing sliding velocity.

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Microscopic ice friction of polymeric substrates (2010)

Interest in snow and ice friction comes from the need to understand and control phenomena of practical importance such as glacier and avalanche movement, traction of automobile tires, snow and ice sports. The need to minimize friction on ice and snow in competitive winter sports is the main motivation behind the present work.A novel tribometer was designed and utilized in conjunction with a conventional rheometer for measuring and understanding the mechanisms of ice friction over polymeric surfaces. Experiments were performed to measure friction between ultra-high molecular weight polyethylene, polytetrafluoroethylene and poly(methyl methacrylate), and ice as a function of sliding velocity, temperature, surface roughness and hydrophobicity. Various techniques were utilized to modify the properties and characteristics of the polymeric surfaces. Light microscopy and scanning electron microscopy as well as surface profilometry were utilized to perform surface analysis and characterize the surface. A goniometer set-up was used for the measurement of the water contact angle measurements and X-ray photoelectron spectroscopy for conducting the elemental analysis.Overall it was found that the magnitude of the sliding velocity and temperature play important roles in ice friction. The more hydrophobic polymers exhibit a lower coefficient of friction. Liquid fluorinated additives as well as a plasma enhanced chemical vapour deposition in a fluorinated gas can improve the hydrophobicity of a polymer and decrease its coefficient of friction over ice. These two concepts can directly be applied in snow winter sports and more specifically in ski and snowboard bases production and preparation where greater speeds, shorter times and therefore less friction are in high demand.

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Thermorheology and processing of polyethylene blends : macromolecular structure effects (2008)

Rheological and processing behavior of a number of linear low-density polyethylene(LLDPE)/low-density polyethylene (LDPE) blends was studied with emphasis on the effects of long chain branching. First, a linear low-density polyethylene (LL3001.32) was blended with four LDPE's having distinctly different molecular weights. At high LDPE weight fractions, DSC melting thermograms have shown three different polymer phases; two for the pure components and a third melting peak of co-crystals. Different rheological techniques were used to check the thermo rheological behavior of all blends in the melt state and the effect of long chain branching. It was found that all blends are miscible in the melt state at small LDPE concentrations. The elongational behavior of the blends was studied using a uniaxial extensional rheometer, SER. The blends exhibit strain hardening behavior at high rates of deformation even at LDPE concentrations as low as 1%, which suggests the strong effect of branching added by the LDPE component. On the other hand, shear rheology was found to be insensitive to detect addition of small levels of LDPE up to lwt%.The second set of blends prepared and studied consisted of two Ziegler-Natta LLDPE's (LL3001.32 and Dowlex2045G) and two metallocene LLDPE's(AffinityPL1840 and Exact 3128) blended with a single LDPE. In DSC melting thermograms, it was observed that blends with metallocence LLDPE's exhibit a single melting peak at all compositions; whereas the Ziegler-Natta blends exhibit three melting peaks at certain compositions. It was found also that the metallocene LLDPE's are miscible with the LDPE at all concentrations. On the other hand, the Ziegler-Natta LLDPE's were found to be miscible with LDPE only at small LDPE concentrations.The processing behavior of all blends with emphasis on the effects of long chain branches was also studied in capillary extrusion. The critical shear stresses for the onset of sharkskin and gross melt fracture are slightly delayed with the addition of LDPE into LLDPE. Furthermore, the amplitude of the oscillations in the stick-slip flow regime, known as oscillating melt fracture, were found to scale with the weight fraction of LDPE. Amounts as low as 1 wt% LDPE have a significant effect on the amplitude of pressure oscillations. These effects are clearly due to the presence of LCB. It is suggested that the magnitude of oscillations in the oscillating melt fracture flow regime can be used as a method capable to detect low levels of LCB.Finally, the sharkskin and stick-slip polymer extrusion instabilities of a linear low-density polyethylene were studied as a function of the type of die geometry. The critical wall shear stress for the onset of flow instabilities, the pressure and flow rate oscillations, and the effects of geometry and operating conditions on the instabilities are presented for a LLDPE. It was found that sharkskin and stick-slip instabilities were present in the capillary and slit extrusion. However, stick-slip and sharkskin in annular extrusion are absent at high ratios of the inside to outside diameter of the annular die. This observation also explains the absence of these instabilities in polymer processing operations such as film blowing. These phenomena are explained in terms of the surface to volume ratio of the extrudates.

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Master's Student Supervision

Theses completed in 2010 or later are listed below. Please note that there is a 6-12 month delay to add the latest theses.

Thixotropy and slip of model kaolinite hybrid and oil tailings suspensions (2022)

In this thesis, two clay colloidal systems are studied and rheologically characterized. The first is a suspension of kaolinite particles with and without the addition of cationic and anionic surfactant, namely Cetyl Trimethyl Ammonium Bromide (CTAB), and sodium dodecyl sulfate (SDS). The second system is Mature Fine Tailings (MFT), a by-product of oil sands bitumen extraction. The former system is studied as a model system to understand the more complex behavior of oil tailings.The steady-shear behavior of the kaolinite suspensions with surfactant is explored above and below the critical micelle concentration (CMC). SDS was shown to weaken the network structure at concentrations below the CMC. At concentrations greater than the CMC, SDS causes an increase in the yield stress due to network formation of micellar structuresThe yield stress was found to increase monotonically with CTAB concentration, which can be explained by the improved network strength. Both kaolinite and kaolinite-CTAB systems were found to exhibit thixotropic and anti-thixotropic behavior at short and longtime scales respectively.The rheological characteristics of mature fine tailings (MFTs) were examined in steady shear flow. MFT samples exhibited thixotropic behavior as well as apparent slip, which was suppressed by using sandpaper of grit 80 (200 μm) at the wall of the parallel-plate geometry. The real yield stress of MFTs with shear-thinning behavior was retrieved after fitting to a Herschel–Bulkley equation. The presence of bitumen remaining in the MFTs (up to 2 wt. %) suppressed the apparent slip to a substantial extent due to the immobilization of the particles at the interface.Finally, the effect of temperature on the rheology of both suspension systems was studied in detail over the range of 0–50 °C, showing a minimum viscosity and yield stress at about 20 °C, demonstrating a non-monotonic increase with the increase in temperature. This was found to be due to the increase of kaolinite particle charge with increase of temperature.

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Flow-induced crystallization of polylactides in capillary flow (2021)

Polylactide (PLA) is a biodegradable thermoplastic polyester produced from renewable biomass sources. The low crystallinity and the low associated crystallization kinetics have a negative impact on its mechanical and barrier properties. In this Master thesis, the effect of flow on the PLA crystallization is studied. The flow-induced crystallization (FIC) of two PLA with different microstructures (different L-lactic acid content) is studied using simple shear, uniaxial extension and capillary flow experiments. In simple shear and capillary flow, increase in shear rate and decrease in temperature was found to enhance the crystallization kinetics particularly for Weissenberg numbers (based on the reptation relaxation time, Wi) greater than 1. On the other hand, in uniaxial extensional flow, once a critical Hencky strain is achieved, crystallization starts independently of strain rate and temperature. The amount of mechanical work per unit volume imposed/dissipated onto the polymers during flow to initialize crystallization was also calculated in capillary flow. The critical mechanical work for the onset of flow-induced crystallization was found to be independent of temperature and degree of molecular chain stretch (Wi) as Wi becomes greater than 1. Lastly, the PLA sample with higher content of PLLA showed slightly higher zero-shear viscosity and a smaller thermodynamic barrier for the onset of crystallization.

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Ionic interactions in EMAA ionomer blends: a rheological and mechanical property investigation (2021)

Polyethylene-co-methacrylic acid (EMAA) ionomers are functionalized polymers with up to 15% carboxylic acid groups neutralized by counterions such as sodium, zinc, or lithium. Due to the polarity difference between the polar cations and the nonpolar hydrocarbon chain, ionic groups aggregate into clusters which act as crosslinks leading to improved mechanical strength, viscosity, and self-healing properties. The analysis done in this study shows that binary mixtures of two different cations further enhance the rheological and mechanical properties such as complex viscosity, Young’s modulus, and yield strength. The enhancement is beyond what is calculated through the linear combination of the corresponding properties of the individual components forming the blends. The morphological change supporting the synergism is suggested to be an increased cluster strength and phase segregation for highly neutralized EMAA ionomers. However, this synergism is not observed for large stresses and deformations outside the linear region due to the deformation of clusters as a stress-release mechanism. Investigation of interaction in binary ionic binary blends of EMAA ionomers is measured using linear, nonlinear, and extensional rheology in the melt state, and using dynamic mechanical tensile testing in the solid state. A universal parameter, the “Molecular weight-normalized Neutralization” (NMW), is developed and found to be correlated to the presence of rheological and mechanical property enhancement for all binary blends. NMW is composed of the inverse of molecular weight and of neutralization. High NMW obtained through high degrees of combined cation neutralization and low combined molecular weight ionomers are correlated with higher degrees of enhancement. More significant synergism has been found for blends containing zinc counterions due to the high valency of their ionic charge compared to sodium or lithium counterions.

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Rheological characterization of CNC-CTAB network below and above the critical micelle concentration (CMC) (2021)

The network of Cellulose Nanocrystal (CNC) suspension is explored below and above the critical micelle concentration (CMC), in the presence of cetyltrimethylammonium bromide (CTAB) with a positively charged head using rheological characterization. CNC-CTAB gel showed shear thinning behaviour, complex relationship between strain amplitudes and CTAB concentration, diminishing thixotropic behaviour as a function of CTAB and single and two yielding stress maxima as a function of CTAB, resulting from different microstructure below and above CMC of CTAB. Comparing the flow curves of CNC-CTAB suspension/gel revealed the role played by CTAB content, CNC concentration and sonication energy in strengthening of the network. We analysed and obtained yield stress from steady shear, creep testing and oscillatory experiments and compared them.

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Rheology and capillary flow of sodium and zinc ionomers (2020)

Ionomers are an important class of polymers that contain a small number of ionic functionalities capable of forming reversible ionic associations. They are used in many applications such as self-healing materials, semi-permeable membranes, and in coating and food packaging. It is important to understand the interplay between the dynamics of these ionic functionalities and their role on the rheological and mechanical properties in order to explore their potential for new commercial applications.Using a parallel-plate rheometer equipped with a partitioned plate, and the Sentmanat extensional rheometer (SER) fixture, a full rheological characterization of several sodium and zinc poly(ethylene-co-methacrylic acid) and their corresponding parent copolymers has been carried out. Particular emphasis has been placed on the distribution of the relaxation times to identify the characteristic times, such as the reptation, Rouse, and lifetime of associations that are associated with entanglements, segmental dynamics and ionic and hydrogen bonding associations respectively. It was found that ionic interactions increase the linear viscoelastic moduli and the viscosity by up to one order of magnitude and cause significant strain hardening effects in the uniaxial extension of ionomers.The time-strain separability that can be used to formulate a K-BKZ constitutive equation (Wagner damping function), was found to hold for all ionomers and their corresponding copolymers. Their damping function was found to have different values indicating that their relaxation depends on the number of ionic associations. The relaxation of copolymers is faster mainly due to the absence of ionic interactions.Finally, the capillary flow properties of these ionomers were studied to assess their processability in terms of instabilities such as wall slip and melt fracture. It was found that the no-slip boundary condition is a valid assumption for these polymers due to the strong ionic associations and strong interactions with the capillary wall. All ionomers were found to exhibit gross melt fracture and no sharkskin, a characteristic of strain-hardening polymers. The critical shear stress for the onset of gross melt fracture was found to depend on the lifetime of associations, independent of temperature, molecular weight and type of ion.

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Rheology and dye adsorption of surfactant-cellulose nanocrystal complexes (2019)

The adaptability of Cellulose Nanocrystal (CNC) with reference to two of its major applications, namely viscosity modification in complex fluids and dye adsorption, is investigated. In the context of viscosity enhancement, the interaction of CNCs with two ionic surfactants, namely sodium dodecyl sulfate (SDS) and cetyltrimethylammonium bromide (CTAB), has been explored in this study. The effect of CTAB and SDS on surface charge of CNCs is analyzed and coupled with the rheological properties in order to investigate the mechanisms in CNC/surfactant and CNC/CNC interactions. It was found that both surfactants result in gelation of chiral nematic CNC suspensions by setting the stage for establishment of crosslinks between individual CNCs. In the case of CTAB, a gradual increase in the viscoelastic moduli was observed with the concentration of surfactant implying the induction of dominant attractive forces between CNCs by surfactant molecules. However, the presence of SDS results in the emergence of both attractive and repulsive forces whose dominance controls the viscoelastic properties. In the context of dye adsorption, a CNC based adsorbent was first synthesized by modifying the pristine CNC with various amounts of CTAB and consequently was used to study the adsorption behavior of Congo red (CR) in aqueous medium. The interaction of CTAB with CNC, and potential alterations on the chemical and physical structure of CNC was studied. The synthesized adsorbent, a modified cellulose nanocrystal (MCNC), was characterized using Fourier transform infrared (FTIR), X-ray diffraction (XRD), scanning electron microscopy (SEM) and zeta potential analysis. The amount of surfactant used for modification exhibited an optimum at which the adsorption capacity was shown a weak maximum. Furthermore, it was found that the amount of surfactant affects the CR-MCNC interactions and determines the mechanisms of adsorption. Characterization of the adsorbent, before and after adsorption, coupled with kinetics and isotherm studies implied electrostatic attraction, hydrogen bonding, and hydrophobic attraction as the main mechanisms of adsorption. Thermodynamic studies on the system suggested that the adsorption process is spontaneous and exothermic. Finally, stability analysis in aqueous environment revealed that the adsorbent is highly stable and retains its original adsorption capacity after successive dialysis cycles.

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Molecular Dynamics simulation of Polymer Crystallization process (2015)

Large scale molecular dynamics simulations were carried out to study the kinetics of polymer melt crystallization. A coarse-grained model CG-PVA developed by Meyer and Muller-Plathe is applied. A new algorithm for analyzing crystallization is proposed. It is based on the alignment of individual chains which speeds up previous similar calculation by a factor of ten. Moreover, it is found to be more suitable for investigating chain crystallinity in polydisperse systems. Different thermodynamic protocols of polymer crystallization were studied: deep quench, shallow quench and cooling with various rates, as well as polymer pre-stretching and consequent cooling and quenching. Cooling with the slowest rate was shown to generate the highest terminal crystallinity values. Resulting curves were fitted using the Avrami equation that showed good agreement at the early stages of crystallization. As a result shorter chains were found to exhibit higher terminal crystallinity value than the longer ones. Pre-stretching and subsequent quenching was found to have a minor effect on thefinal crystallinity, whereas pre-stretching followed by an intermediate rate cooling was found to increase the terminal crystallinity. The effect of polydispersity was modeled via two bidisperse melts comprising of different proportion of short and long chains. Due to the presence of two relaxation times in the melt, initial stages of bidisperse polymers crystallization were found to be dominated by the short chains, whereas the final stages were dominated by the long ones. Further investigation concluded that the behavior of bidisperse melts is governed by the proportion of short and long chains in the melt. When a critical fraction of the long chains was reached, they appeared to act as baby nuclei for the short chains to attach themselves onto resulting in bundle-like fringed micelle structures. Otherwise, they acted as "molecular traps" hindering crystallization of the short chains. When a critical fraction of the short chains was reached, they were found to assist crystallization of the long chains at the initial stages of crystallization but impede crystallization dynamics at the final stages.

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Flow-induced crystallization of HDPE: the effects of shear, uniaxial extension and temperature (2011)

The effects of shear, uniaxial extension and temperature on the flow-induced crystallization of two different types of two high-density polyethylenes (a metallocene and a Ziegler-Natta HDPE) are examined using rheometry. Shear and uniaxial extension experiments were performed at temperatures below and well above the peak melting point of the polyethylene’s in order to characterize their flow-induced crystallization behavior at rates relevant to processing. Generally, strain and strain rate found to enhance crystallization in both shear and elongation. In particular, extensional flow was found to be a much stronger stimulus for polymer crystallization compared to shear. At temperatures well above the melting peak point (up to 25°C), polymer crystallized under elongational flow, while there was no sign of crystallization under simple shear. A modified Kolmogorov crystallization model (Kolmogorov AN (1937) On the statistics of the crystallization process on metals. Bull Akad Sci. USSR, Class Sci, Math Nat. 1:355–359) proposed by Tanner (Tanner RI (2009) Stretching, shearing and solidification, Chem Eng Sci, 64:4576-4579) was used to describe the crystallization kinetics under both shear and elongational flow. The model was found to predict the FIC behaviour under low deformation rates and various temperatures well; however the predictions for the higher rates were not satisfactory.

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