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.
Rheology of associating polymers: effects of ionic and hydrogen bonding interactions (2019)
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.
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.
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.
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.
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.
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.
Superhydrophobic polymeric surfaces : fabrication, wettability, and antibbacterial activity (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.
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.
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.
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.
Super-hydrophobic 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°).
The Flow Properties of Bitumen in the Presence of C02 (2014)
Rheology and processing of biodegradable poly (E-Caprolactone) polyesters and their blends with polylactides (2013)
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.
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.
Rheology of low to medium consistencey pulp suspensions (2011)
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.
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.
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.