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Articles 91 - 120 of 159
Full-Text Articles in Polymer Science
Effects Of Photointensity Gradient On Directional Crystal Growth In Blends Of Crystalline Polymer And Photoreactive Monomer Undergoing Photopolymerization-Induced Phase Transformation, Pankaj Rathi, Soo Jeoung Park, Thein Kyu
Effects Of Photointensity Gradient On Directional Crystal Growth In Blends Of Crystalline Polymer And Photoreactive Monomer Undergoing Photopolymerization-Induced Phase Transformation, Pankaj Rathi, Soo Jeoung Park, Thein Kyu
College of Polymer Science and Polymer Engineering
Effects of light intensity gradient on development of intricate hierarchical morphology of semicrystalline polyethylene oxide (PEO) and photoreactive diacrylate (DA) blends undergoing photopolymerization-induced crystallization have been demonstrated experimentally and theoretically. The melting temperature of PEO was found to decline upon addition of DA monomer. A solid-liquid phase diagram has been established by self-consistently solving the combined phase field free energy of crystal solidification and Flory-Huggins (FH) free energy of liquid-liquid demixing. Dynamic calculations were performed using time-dependent Ginzburg-Landau (model C) equations by incorporating the combined phase field and FH free energy densities coupled with the photopolymerization kinetics. The spatiotemporal development …
Application Of The Thermal Flash Technique For Low Thermal Diffusivity Micro/Nanofibers, Michael T. Demko, Zhenting Dai, Han Yan, William P. King, Miko Cakmak, Alexis R. Abramson
Application Of The Thermal Flash Technique For Low Thermal Diffusivity Micro/Nanofibers, Michael T. Demko, Zhenting Dai, Han Yan, William P. King, Miko Cakmak, Alexis R. Abramson
College of Polymer Science and Polymer Engineering
The thermal flash method was developed to characterize the thermal diffusivity of micro/nanofibers without concern for thermal contact resistance, which is commonly a barrier to accurate thermal measurement of these materials. Within a scanning electron microscope, a micromanipulator supplies instantaneous heating to the micro/nanofiber, and the resulting transient thermal response is detected at a microfabricated silicon sensor. These data are used to determine thermal diffusivity. Glass fibers of diameter 15 mu m had a measured diffusivity of 1.21x10(-7) m(2)/s; polyimide fibers of diameters 570 and 271 nm exhibited diffusivities of 5.97x10(-8) and 6.28x10(-8) m(2)/s, respectively, which compare favorably with bulk …
Theory And Computation Of Photopolymerization-Induced Phase Transition And Morphology Development In Blends Of Crystalline Polymer And Photoreactive Monomer, Pankaj Rathi, Thein Kyu
Theory And Computation Of Photopolymerization-Induced Phase Transition And Morphology Development In Blends Of Crystalline Polymer And Photoreactive Monomer, Pankaj Rathi, Thein Kyu
College of Polymer Science and Polymer Engineering
A hypothetical phase diagram of a crystalline polymer/photoreactive monomer mixture has been calculated on the basis of phase field (PF) free energy of crystal solidification in conjunction with Flory-Huggins (FH) free energy of liquid-liquid demixing to guide the morphology development during photopolymerization of poly(ethylene oxide)/triacrylate blend. The self-consistent solution of the combined PF-FH theory exhibits a crystalline-amorphous phase diagram showing the coexistence of solid+liquid gap bound by the liquidus and solidus lines, followed by an upper critical solution temperature at a lower temperature. When photopolymerization was triggered in the isotropic region, i.e., slightly above the crystal melting transition temperatures, the …
Shear Banding Or Not In Entangled Dna Solutions Depending On The Level Of Entanglement, Pouyan E. Boukany, Shi-Qing Wang
Shear Banding Or Not In Entangled Dna Solutions Depending On The Level Of Entanglement, Pouyan E. Boukany, Shi-Qing Wang
College of Polymer Science and Polymer Engineering
Entangled DNA solutions are ideal as a model system to examine nonlinear shear flow behavior. Even when the number of entanglements per chain, Z, is higher than 100, the solution is still soft enough with an elastic plateau modulus under 100 Pa and is thus amenable to experimental study by commercial rotational rheometry without ambiguity and uncertainty. We have investigated nonlinear flow behavior of three entangled DNA solutions with Z=24, 60, and 156, respectively, using a combination of particle-tracking velocimetric (PTV) and conventional rheometric measurements. We explore questions such as (a) whether shear banding also occurs in moderately entangled solutions, …
Understanding Rubber Friction In The Presence Of Water Using Sum-Frequency Generation Spectroscopy, Kumar Nanjundiah, Ping Yuan Hsu, Ali Dhinojwala
Understanding Rubber Friction In The Presence Of Water Using Sum-Frequency Generation Spectroscopy, Kumar Nanjundiah, Ping Yuan Hsu, Ali Dhinojwala
College of Polymer Science and Polymer Engineering
Infrared-visible sum-frequency-generation spectroscopy (SFG) was used to study the molecular structure of water between a poly(dimethylsiloxane) (PDMS) and a sapphire substrate. The observation of SFG peaks associated with the dangling surface hydroxyl groups (3690 cm(-1)) and water bands (3000-3400 cm(-1)) indicates that the contact spot between the PDMS lens and the sapphire substrate is heterogeneous. Within the contact spot there are regions where the methyl groups of the PDMS chains are in direct contact with the surface hydroxyl groups on the sapphire substrate. In the other regions, a thin water layer is trapped between the two surfaces with spectral features …
Photopolymerization-Induced Crystallization And Phase Separation In Poly(Ethylene Oxide)/Triacrylate Blends, Soo Jeoung Park, Thein Kyu
Photopolymerization-Induced Crystallization And Phase Separation In Poly(Ethylene Oxide)/Triacrylate Blends, Soo Jeoung Park, Thein Kyu
College of Polymer Science and Polymer Engineering
The present article describes experimental and theoretical investigations of miscibility and crystallization behavior of blends of poly(ethylene oxide) (PEO) and triacrylate monomer (TA) using differential scanning calorimetry and optical microscopy. The PEO/TA blends manifested a single T(g) varying systematically with composition suggestive of a miscible character in their amorphous states. Moreover, there occurs melting point depression of PEO crystals with increasing TA. A phase diagram was subsequently established that exhibited a solid+liquid coexistence region bound by the liquidus and solidus lines, followed by an upper critical solution temperature (UCST) at a lower temperature. The emerging phase morphology was investigated to …
From Elastic Deformation To Terminal Flow Of A Monodisperse Entangled Melt In Uniaxial Extension, Yangyang Wang, Shi-Qing Wang
From Elastic Deformation To Terminal Flow Of A Monodisperse Entangled Melt In Uniaxial Extension, Yangyang Wang, Shi-Qing Wang
College of Polymer Science and Polymer Engineering
Using a well-entangled monodisperse styrene-butadiene random-copolymer (SBR) melt as a model system, we illustrate generic features of uniaxial extension behavior that may be shared by all well-entangled thermoplastic and elastomeric materials. Depending on the imposed extensional rate, the same sample may behave like a viscous liquid or an elastic "solid." Analogous to the recently revealed shear inhomogeneity, the SBR melt inevitably undergoes cohesive failure in the form of sample breakage whenever the Weissenberg number is much greater than unity, making it challenging to reach steady state. In the elastic deformation regime where the external deformation rate is faster than Rouse …
Partial Crystallinity In Alkyl Side Chain Polymers Dictates Surface Freezing, Shishir Prasad, Zhang Jiang, Sunil K. Sinha, Ali Dhinojwala
Partial Crystallinity In Alkyl Side Chain Polymers Dictates Surface Freezing, Shishir Prasad, Zhang Jiang, Sunil K. Sinha, Ali Dhinojwala
College of Polymer Science and Polymer Engineering
We have studied the structure of a novel crystalline surface monolayer on top of a disordered melt of the same material [poly(n-alkyl acrylate)s] using grazing incidence x-ray diffraction. The grazing incidence x-ray diffraction, surface tension, and bulk latent heat results show that side chains crystallize except the nine methylene units of the alkyl side chains closest to the polymer backbone. The partial crystallinity along with a thicker surface layer, due to the additional length of the linker group, explains why the difference between the surface order-to-disorder transition temperature and bulk melting temperature increases with a decrease in the length of …
Steady State Measurements In Stress Plateau Region Of Entangled Polymer Solutions: Controlled-Rate And Controlled-Stress Modes, Sham Ravindranath, Shi-Qing Wang
Steady State Measurements In Stress Plateau Region Of Entangled Polymer Solutions: Controlled-Rate And Controlled-Stress Modes, Sham Ravindranath, Shi-Qing Wang
College of Polymer Science and Polymer Engineering
Despite decades of efforts, reliable measurements of nonlinear flow behavior of well-entangled polymers in continuous shear have been challenging to obtain. The present work attempts to accomplish three important tasks: (A) overcome this challenge by adopting a strategy of decoupling theological measurements from the outer meniscus region in a cone-partitioned plate (C/PP) setup; (B) determine whether well-entangled solutions indeed undergo a flow transformation under creep that can be taken to phenomenologically define an entanglement-disentanglement transition (EDT); (C) provide the velocity profiles of such solutions undergoing either controlled-stress or controlled-rate shear by carrying out in situ particle-tracking velocimetric (PTV) measurements. Upon …
Universal Scaling Characteristics Of Stress Overshoot In Startup Shear Of Entangled Polymer Solutions, Sham Ravindranath, Shi-Qing Wang
Universal Scaling Characteristics Of Stress Overshoot In Startup Shear Of Entangled Polymer Solutions, Sham Ravindranath, Shi-Qing Wang
College of Polymer Science and Polymer Engineering
We have carried out conventional theological measurements to explore the well-known stress overshoot behavior in startup shear of eight entangled polybutadiene solutions. In the elastic deformation regime with (gamma)over dot tau(R)> 1, we have identified universal scaling features associated with the stress maximum sigma(max) for samples of different levels of entanglements per chain ranging from 27, 40 to 64. Specifically, at the moment t(max) of the peak stress sigma(max) where the applied strain is gamma(max) = (gamma)over dott(max): (a) sigma(max) varies linearly with gamma(max), (b) sigma(max) scales with t(max) as sigma(max)similar to(t(max))-1/2. The combination of (a) and (b) yields a …
Sticky Gecko Feet: The Role Of Temperature And Humidity, Peter H. Niewiarowski, Stephanie Lopez, Liehui Ge, Emily Hagan, Ali Dhinojwala
Sticky Gecko Feet: The Role Of Temperature And Humidity, Peter H. Niewiarowski, Stephanie Lopez, Liehui Ge, Emily Hagan, Ali Dhinojwala
College of Polymer Science and Polymer Engineering
Gecko adhesion is expected to be temperature insensitive over the range of temperatures typically experienced by geckos. Previous work is limited and equivocal on whether this expectation holds. We tested the temperature dependence of adhesion in Tokay and Day geckos and found that clinging ability at 12 degrees C was nearly double the clinging ability at 32 degrees C. However, rather than confirming a simple temperature effect, our data reveal a complex interaction between temperature and humidity that can drive differences in adhesion by as much as two-fold. Our findings have important implications for inferences about the mechanisms underlying the …
Large Amplitude Oscillatory Shear Behavior Of Entangled Polymer Solutions: Particle Tracking Velocimetric Investigation, Sham Ravindranath, Shi-Qing Wang
Large Amplitude Oscillatory Shear Behavior Of Entangled Polymer Solutions: Particle Tracking Velocimetric Investigation, Sham Ravindranath, Shi-Qing Wang
College of Polymer Science and Polymer Engineering
Large amplitude oscillatory shear (LAOS) has been employed widely to probe the nonlinear behavior of complex fluids. Typically, the system undergoing LAOS has been assumed to experience homogeneous shear in each cycle so that material functions can be introduced to analyze the nonlinear dependence of these functions on the amplitude and frequency. Using particle-tracking velocimetric technique, we have carried a more systematic investigation on four different entangled polybutadiene (PBD) solutions with the number of entanglements per chain Z=13, 27, 64 and 119, extending well beyond the initial observations of shear banding in entangled PBD solutions under LAOS [Tapadia , Phys. …
Electrical, Structural, And Chemical Properties Of Semiconducting Metal Oxide Nanofiber Yarns, A. F. Lotus, E. T. Bender, E. A. Evans, R. D. Ramsier, Darrell Hyson Reneker
Electrical, Structural, And Chemical Properties Of Semiconducting Metal Oxide Nanofiber Yarns, A. F. Lotus, E. T. Bender, E. A. Evans, R. D. Ramsier, Darrell Hyson Reneker
College of Polymer Science and Polymer Engineering
The electrical, structural, and chemical properties of twisted yarns of metal-oxide nanofibers, fabricated using a modified electrospinning technique, are investigated in this report. In particular, synthesized zinc oxide and nickel oxide yarns having diameters in the range of 4-40 mu m and lengths up to 10 cm were characterized, whose constituent nanofibers had average diameters of 60-100 nm. These yarns have one macroscopic dimension for handling while retaining some of the properties of nanofibers. (C) 2008 American Institute of Physics.
Elastic Breakup In Uniaxial Extension Of Entangled Polymer Melts, Yangyang Wang, Pouyan Boukany, Shi-Qing Wang, Xiaorong Wang
Elastic Breakup In Uniaxial Extension Of Entangled Polymer Melts, Yangyang Wang, Pouyan Boukany, Shi-Qing Wang, Xiaorong Wang
College of Polymer Science and Polymer Engineering
Five entangled melts, with the number of entanglements per chain ranging from 25 to 160, have been studied to illustrate how cohesive strength can be overcome in either continuous or interrupted extension (i.e., during or after uniaxial stretching). The internal elastic stress due to chain deformation from imposed strain appears to be the cause of the observed yielding behavior that reveals scaling laws. The visual signature of the elastic breakup is the occurrence of nonuniform extension. The yield phenomena may be understood at a force level.
New Theoretical Considerations In Polymer Rheology: Elastic Breakdown Of Chain Entanglement Network, Shi-Qing Wang, Sham Ravindranath, Yangyang Wang, Pouyan Boukany
New Theoretical Considerations In Polymer Rheology: Elastic Breakdown Of Chain Entanglement Network, Shi-Qing Wang, Sham Ravindranath, Yangyang Wang, Pouyan Boukany
College of Polymer Science and Polymer Engineering
Recent experimental evidence has motivated us to present a set of new theoretical considerations and to provide a rationale for interpreting the intriguing flow phenomena observed in entangled polymer solutions and melts [P. Tapadia and S. Q. Wang, Phys. Rev. Lett. 96, 016001 (2006); 96, 196001 (2006); S. Q. Wang , ibid. 97, 187801 (2006)]. Three forces have been recognized to play important roles in controlling the response of a strained entanglement network. During flow, an intermolecular locking force f(iml) arises and causes conformational deformation in each load-bearing strand between entanglements. The chain deformation builds up a retractive force f(retract) …
Photopolymerization-Induced Directional Crystal Growth In Reactive Mixtures, Soo Jeoung Park, Pankaj Rathi, Thein Kyu
Photopolymerization-Induced Directional Crystal Growth In Reactive Mixtures, Soo Jeoung Park, Pankaj Rathi, Thein Kyu
College of Polymer Science and Polymer Engineering
Photopolymerization-induced crystallization has been demonstrated in blends of polyethylene oxide-diacrylate at temperatures above the depressed melting temperature of the crystalline component. Upon exposure to ultraviolet irradiation, the melting transition curve moves upward and eventually surpasses the reaction temperature, thereby inducing phase separation as well as crystallization. The present paper demonstrates the occurrence of directionally solidified interface morphologies of polymer crystals subjected to a photointensity gradient. The epitaxially grown seaweed or degenerate structures were observed at the circumference (low-intensity region) while the dense branched spherulites developed at the core high-intensity region.
Discrepancy In Determination Of Chi Parameters By Melting Point Depression Versus Small-Angle Neutron Scattering In Blends Of Deuterated Polycarbonate And Isotactic Poly(Methyl Methacrylate), Thein Kyu, Rushikesh A. Matkar, Dong Soo Lim, Chechian Ko
Discrepancy In Determination Of Chi Parameters By Melting Point Depression Versus Small-Angle Neutron Scattering In Blends Of Deuterated Polycarbonate And Isotactic Poly(Methyl Methacrylate), Thein Kyu, Rushikesh A. Matkar, Dong Soo Lim, Chechian Ko
College of Polymer Science and Polymer Engineering
The discrepancy in the chi interaction parameters of deuterated polycarbonate/isotactic poly(methyl methacrylate) blends as determined by the melting point depression approach and the small-angle scattering technique is reported. We have modified the Flory diluent theory by removing the inherent assumption of complete rejection of the solvent from the crystal solid by taking into consideration the crystal-amorphous, amorphous-crystal, and crystal-crystal interactions. The discrepancy in chi values obtained by the two methods is discussed.
A Correlation Between Velocity Profile And Molecular Weight Distribution In Sheared Entangled Polymer Solutions, Pouyan E. Boukany, Shi-Qing Wang
A Correlation Between Velocity Profile And Molecular Weight Distribution In Sheared Entangled Polymer Solutions, Pouyan E. Boukany, Shi-Qing Wang
College of Polymer Science and Polymer Engineering
In this work we attempt to answer several questions concerning the flow characteristics of entangled polymer solutions in a sliding plate shearing cell. We explore (a) how the molecular weight distribution affects the velocity profile in simple shear, (b) whether the observed shear banding is consistent with a nonmonotonic constitutive model, (c) whether the flow response and velocity profiles are different in simple shear depending on the different modes of shear. Our results provide a comparison with recent reports on a polydisperse polymer sample [Tapadia and Wang, Phys. Rev. Lett. 96, 016001 (2006); Tapadia, et al., Phys. Rev. Lett. 96, …
How Does The Coupling Of Secondary And Tertiary Interactions Control The Folding Of Helical Macromolecules?, Vikas Varshney, Gustavo A. Carri
How Does The Coupling Of Secondary And Tertiary Interactions Control The Folding Of Helical Macromolecules?, Vikas Varshney, Gustavo A. Carri
College of Polymer Science and Polymer Engineering
The authors study how the simultaneous presence of short-range secondary and long-range tertiary interactions controls the folding and collapse behavior of a helical macromolecule. The secondary interactions stabilize the helical conformation of the chain, while the tertiary interactions govern its overall three-dimensional shape. The authors have carried out Monte Carlo simulations to study the effect of chain length on the folding and collapse behavior of the chain. They have calculated state diagrams for four chain lengths and found that the physics is very rich with a plethora of stable conformational states. In addition to the helix-coil and coil-globule transitions, their …
Nonquiescent Relaxation In Entangled Polymer Liquids After Step Shear, Shi-Qing Wang, Sham Ravindranath, Pouyan Boukany, Michael Olechnowicz, Roderic P. Quirk, Jimmy Mays
Nonquiescent Relaxation In Entangled Polymer Liquids After Step Shear, Shi-Qing Wang, Sham Ravindranath, Pouyan Boukany, Michael Olechnowicz, Roderic P. Quirk, Jimmy Mays
College of Polymer Science and Polymer Engineering
Large step shear experiments revealed through particle tracking velocimetry that entangled polymeric liquids display either internal macroscopic movements upon shear cessation or rupturelike behavior during shear. Visible inhomogeneous motions were detected in five samples with the number of entanglements per chain ranging from 20 to 130 at amplitudes of step strain as low as 135%.
Interfacial Stick-Slip Transition In Simple Shear Of Entangled Melts, Pouyan E. Boukany, Prashant Tapadia, Shi-Qing Wang
Interfacial Stick-Slip Transition In Simple Shear Of Entangled Melts, Pouyan E. Boukany, Prashant Tapadia, Shi-Qing Wang
College of Polymer Science and Polymer Engineering
This article describes a systematic investigation of a discontinuous interfacial stick-slip transition (SST) in simple shear of monodisperse entangled 1,4-polybutadiene (PBD) and polyisoprene (PIP) melts with different molecular weights and architecture, using a specially designed controlled-force shear rheometer. The magnitude of the transition is found to be determined by the level of chain entanglement. Specifically, the dependence of extrapolation length b on molecular weight as b similar to M-w(3.4) and of the melt viscosity as b similar to eta is consistent with the observations based on capillary rheometric studies [X. Yang et al., Rheol. Acta 37, 415-423 (1998)]. The interfacial …
Porous Fiber Formation In Polymer-Solvent System Undergoing Solvent Evaporation, Pratyush Dayal, Thein Kyu
Porous Fiber Formation In Polymer-Solvent System Undergoing Solvent Evaporation, Pratyush Dayal, Thein Kyu
College of Polymer Science and Polymer Engineering
Temporal evolution of the fiber morphology during dry spinning has been investigated in the framework of Cahn-Hilliard equation [J. Chem. Phys. 28, 258 (1958)] pertaining to the concentration order parameter or volume fraction given by the Flory-Huggins free energy of mixing [P. J. Flory, Principles of Polymer Chemistry (Cornell University Press, Ithaca, NY, 1953), p. 672] in conjunction with the solvent evaporation rate. To guide the solvent evaporation induced phase separation, equilibrium phase diagram of the starting polymer solution was established on the basis of the Flory-Huggins free energy of mixing. The quasi-steady-state approximation has been adopted to account for …
Breakup Of Spiral And Concentric Ringed Spherulites In Polymer Crystallization, Haijun Xu, Hao-Wen Chiu, Yoshifumi Okabe, Thein Kyu
Breakup Of Spiral And Concentric Ringed Spherulites In Polymer Crystallization, Haijun Xu, Hao-Wen Chiu, Yoshifumi Okabe, Thein Kyu
College of Polymer Science and Polymer Engineering
Recently, the breakup of spiral and concentric ringed spherulites has been observed experimentally in isothermal crystallization of poly(vinylidene fluoride) in its blends with ethylene vinyl acetate copolymer. To elucidate the phenomenon of spiral breakup in polymer spherulites, a theoretical simulation has been undertaken in the framework of the phase field model of solidification based on time-dependent Ginzburg-Landau equations (TDGL model C) pertaining to the conserved concentration and nonconserved phase field order parameters. The concentration order parameter was described in terms of the Landau-type potential, while the crystal order parameter was expressed in terms of an asymmetric double-well potential to account …
Crystal-Liquid Crystal Binary Phase Diagrams, P Dayal, Ra Matkar, Thein Kyu
Crystal-Liquid Crystal Binary Phase Diagrams, P Dayal, Ra Matkar, Thein Kyu
College of Polymer Science and Polymer Engineering
We propose a new theoretical scheme for the binary phase diagrams of crystal-liquid crystal mixtures by a combination of a phase field model of solidification, the Flory-Huggins theory for liquid-liquid mixing and Maier-Saupe-McMillan (FH-MSM) model for nematic and smectic liquid crystal orderings. The phase field theory describes the crystal phase transition of anisotropic organic crystal and/or side chain liquid crystalline polymer crystals while the FH-MSM model explains isotropic, nematic and smectic-A phase transitions. Self-consistent calculations reveal several possible phase diagram topologies of the binary crystal-liquid crystal mixtures. The calculated phase diagrams were found to accord well to the reported experimental …
Coupling Between Lysozyme And Trehalose Dynamics: Microscopic Insights From Molecular-Dynamics Simulations, Taner E. Dirama, Joseph E. Curtis, Gustavo A. Carri, Alexei P. Sokolov
Coupling Between Lysozyme And Trehalose Dynamics: Microscopic Insights From Molecular-Dynamics Simulations, Taner E. Dirama, Joseph E. Curtis, Gustavo A. Carri, Alexei P. Sokolov
College of Polymer Science and Polymer Engineering
We have carried out molecular-dynamics simulations on fully flexible all-atom models of the protein lysozyme immersed in trehalose, an effective biopreservative, with the purpose of exploring the nature and extent of the dynamical coupling between them. Our study shows a strong coupling over a wide range of temperatures. We found that the onset of anharmonic behavior was dictated by changes in the dynamics and relaxation processes in the trehalose glass. The physical origin of protein-trehalose coupling was traced to the hydrogen bonds formed at the interface between the protein and the solvent. Moreover, protein-solvent hydrogen bonding was found to control …
Coupling Between Helix-Coil And Coil-Globule Transitions In Helical Polymers, Vikas Varshney, Gustavo A. Carri
Coupling Between Helix-Coil And Coil-Globule Transitions In Helical Polymers, Vikas Varshney, Gustavo A. Carri
College of Polymer Science and Polymer Engineering
We explore the coupling between the helix-coil and coil-globule transitions of a helical polymer using Monte Carlo simulations. A very rich state diagram is found. Each state is characterized by a specific configuration of the chain which could be a helix, a random coil, an amorphous globule, or one of various other globular states which carry residual helical strands. We study the boundaries between states and provide further insight into the physics of the system with a detailed analysis of the order parameter and other properties.
Confinement-Induced Ordering Of Alkanes Between An Elastomer And A Solid Surface, Kumar Nanjundiah, Ali Dhinojwala
Confinement-Induced Ordering Of Alkanes Between An Elastomer And A Solid Surface, Kumar Nanjundiah, Ali Dhinojwala
College of Polymer Science and Polymer Engineering
We have studied the molecular structure of liquid alkanes confined between a flexible elastomeric poly(dimethyl siloxane) lens and a rigid sapphire substrate using surface-sensitive infrared-visible sum frequency generation spectroscopy. The reduction in the gauche defects suggests ordering of liquid alkanes under confinement. The cooling of confined liquid below the freezing temperature leads to crystallization with alkane molecules lying on the substrate with the symmetry axis parallel to the surface normal. This structure is very different from the bulk alkane crystals next to sapphire or air interfaces.
Effect Of Thermal Transport On Spatiotemporal Emergence Of Lamellar Branching Morphology During Polymer Spherulitic Growth, Haijun Xu, Wirunya Keawwattana, Thein Kyu
Effect Of Thermal Transport On Spatiotemporal Emergence Of Lamellar Branching Morphology During Polymer Spherulitic Growth, Haijun Xu, Wirunya Keawwattana, Thein Kyu
College of Polymer Science and Polymer Engineering
Spatiotemporal emergence of lamellar branching morphology of polymer spherulite has been investigated theoretically in the framework of a phase field model by coupling a crystal solidification potential pertaining to a nonconserved crystal order parameter with a temperature field generated by latent heat of crystallization. A local free-energy density having an asymmetric double well has been utilized to account for a first-order phase transition such as crystallization. To account for the polymorphous nature of polymer crystallization, the phase field order parameter of crystal at the solidification potential of the double-well local free-energy density is modified to be supercooling dependent. The heat …
Rupture Of A Two-Dimensional Alkane Crystal, Shishir Prasad, Ali Dhinojwala
Rupture Of A Two-Dimensional Alkane Crystal, Shishir Prasad, Ali Dhinojwala
College of Polymer Science and Polymer Engineering
We have studied the breaking of a two-dimensional alkane crystal above the disordered melt using an oscillating bubble rheometer. Surface tension changes abruptly during the expansion and contraction cycle. We postulate that this is due to rupture of the 2D crystal at grain boundaries. The magnitude of the abrupt change in surface tension decreases with a decrease in the rate of change of bubble surface area with a power law exponent of 0.8. The interfacial area formed after rupture decreases with a decrease in rate. These results provide new insights in understanding defect-mediated rupture in confined geometry.
Branching In Electrospinning Of Nanofibers, A. L. Yarin, W. Kataphinan, Darrell Hyson Reneker
Branching In Electrospinning Of Nanofibers, A. L. Yarin, W. Kataphinan, Darrell Hyson Reneker
College of Polymer Science and Polymer Engineering
Electrospinning of polymer nanofibers often begins with a single, straight, elongating, and electrified fluid jet that emanates from a droplet tip when the electric field at the surface is high enough. After some distance an electrically driven bending instability of the elongating jet occurs. For a polymer solution suitable for electrospinning, capillary instability does not cause the jet to become a spray of droplets. Under some conditions, a sequence of secondary jet branches emanates from the primary jet. This paper describes an experiment in which many closely spaced branches along the jet were observed during the electrospinning of a polycaprolactone …