Browsing Scholarly Works, Chemical Engineering by Issue Date
Now showing 1 - 20 of 151
Results Per Page
- A systematic study of the effect of casting solvent on the bulk properties of linear SBS block copolymersWilkes, Garth L.; Bagrodia, Shiriam; Ophir, Zohar H.; Emerson, John A. (American Institute of Physics, 1978)Using a well_characterized series of styrene_butadiene triblock copolymers, a systematic study was carried out to determine the influence of casting solvent, used for film preparation, on the final mechanical properties. The results clearly indicate that in the range of styrene contents of 30–40 mole%, the modulus behavior varies greatly as the solubility parameter _ of the casting solvent is varied. Specifically, the modulus increases as _ increases. At lower or higher styrene content, the properties are influenced but to a much lesser degree. Interpretation of the measured mechanical behavior is given in light of morphological data obtained by rheo_optical methods.
- Sub-Tg annealing studies of rubber-modified and unmodified epoxy systemsOphir, Zohar H.; Emerson, John A.; Wilkes, Garth L. (American Institute of Physics, 1978)Epoxy network systems based on DGEBA (bisphenol_A_diglycidyl ether) and NMA (nadic methyl anhydride) systems modified with the low molecular weight CTBN (carboxyl_terminated butudiene_acrylonitrile copolymer)rubbers were prepared and studied. It was found that below the glass transition of the epoxy matrix these materials displayed time_dependent changes in their mechanical properties; specifically, the strain to break as well as the rate of stress relaxation were observed to decrease in a near_linear behavior with the logarithm of time at sub_T g annealing. Calorimetric methods clearly showed a simultaneous decrease in enthalpy with time that behaved in a similar fashion as the time_dependent mechanical properties. All the calorimetric and mechanical data are qualitatively related. The importance of this phenomena is considered in view of the widespread use of epoxys. Similar behavior is expected for other network glasses thermally quenched into a nonequilibrium state.
- Comparison Of The Rheological Properties Of Concentrated-Solutions Of A Rodlike And A Flexible Chain PolyamideBaird, Donald G.; Ballman, R. L. (AIP Publishing, 1979)The steady state shear rheological properties of solutions of a rodlike polyamide, poly_p_phenyleneterephthalamide (PPT), in 100% sulfuric acid have been compared with those of solutions of a flexible chain polyamide, nylon 6,6 in the same solvent. For solutions of similar concentration (c) and molecular weight (M), it was found that the primary normal stress difference (N1) and the viscosity (_), compared at the same shear rate (__), were an order of magnitude greater for solutions of PPT. It was believed that this behavior could be accounted for through the formation of an enhanced entanglement network in the PPT solutions. Plots of the zero shear viscosity(_0) versus cM_w, where M_w is the weight average molecular weight, for both systems revealed that "bends" occurred in the data corresponding to a critical entanglement molecular weight (Mc) of 1180 for PPT (this corresponds to 30 main chain atoms (z)) and to 5260 (z=330) for nylon 6,6. More significantly, _0 was found to be proportional to (cM_u)6.8 for solutions of PPT and to (cM_w)3.4 for nylon 6,6 solutions. _ versus __ curves were similar in shape for both systems and could be reduced to the same master curve with the only difference being that the relaxation times or shifting factors were considerably greater for the PPT solutions. This suggested that the process of destroying entanglements may be similar for both polymers. The overlap parameter c[_], where [_] is the intrinsic viscosity, provided a much better correlation of _0 data from the two sets of solutions than did the segment contact parameter cM_w. This suggested that the structural variable controlling the onset of entanglements may be a parameter such as the radius of gyration. Because of the inability of rodlike molecules to coil around each other, further insight into the nature of entanglements is obtained.
- Rheological Properties Of Liquid-Crystalline Solutions Of Poly-P-Phenyleneterephthalamide in Sulfuric-AcidBaird, Donald G. (AIP Publishing, 1980)The steady state shear and linear viscoelasticproperties of both isotropic and liquid crystalline solutions of the rigid chain polymer poly_p_phenyleneterephthalamide in sulfuric acid have been determined. Both the primary normal stress coefficient and the linear viscoelasticproperties show the same characteristic concentration behavior as does the viscosity. The unique rheological properties of the anisotropic phase can be attributed to the formation of a suspension of highly ordered regions in a matrix of isotropic fluid. The rheological properties were compared against the Bird_Carreau model and Hand's anisotropic fluid theory. Remarkably good agreement was found between the Bird_Carreau model and data especially at high shear rates. This suggested that at high shear rates very little difference may exist between anisotropic and isotropic phases. Hand's model was more applicable to solutions which were only slightly anisotropic.
- Role Of Solvent Nature on Rheological Properties Of Nylon 6,6 SolutionsBaird, Donald G. (AIP Publishing, 1981)The rheological properties of concentrated solutions of nylon 6,6 of various molecular weights in various solvents have been determined in steady shear flow. Four solvents including 90% formic acid,m_cresol, 97% H2SO4, and 100% H2SO4 were selected based on their effect on the ionic nature of nylon 6,6 in dilute solutions. The magnitude of the rheological properties of concentrated solutions depended on the solvent when compared at the same shear rate (__) and segment contact parameter (cM_w, where c is the concentration and M_w is the weight average molecular weight). However, as observed by others, the critical value of cM_w for the onset of entanglements was independent of the solvent. The contact parameter was effective in reducing values of _0 versus cM_w to a single curve for three of the solvents but values of _0 for formic acidsolutions were consistently two orders of magnitude lower than for the other solutions. Values of the equilibrium compliance, J0e, were highest for the formic acidsolutions. However values of the reduced compliance (JeR) for all four solutions were around 0.4 which is in reasonable agreement with the Rouse theory. The onset of non_Newtonian viscosity depended on the solvent but the shape of the flow curves was similar for all polymer/solvent systems. It is concluded that the solventviscosity may contribute more to the rheological properties of concentrated solutions than the solvent's influence on the ionic nature of polymer chains.
- Rheological Properties Of Copolyester Liquid-Crystalline Melts .1. Capillary RheometryJerman, R. E.; Baird, Donald G. (AIP Publishing, 1981)Rheological properties of two copolyesters which exhibit liquid crystalline behavior in the melt state were determined using an Instron capillary rheometer. The two polymer melts with nematic liquid crystalline order consisted of copolymers of polyethylene terephthalate (PET) and 60 and 80 mole % of p_hydroxybenzoic acid (PHB). Data was also obtained on PET which was used as a control. Measurements included the temperature and shear rate dependence of viscosity, entrance pressure losses (_Pent), and die swell (Dj/D). The viscosity of the liquid crystalline melts are as much as two orders of magnitude less than those of PET at temperatures at which they are ordinarily processed. The extrudate actually contracts at the lower end of processing temperatures but does increase to values greater than one with increasing temperature for the liquid crystalline polymers. Although the die swell data indicate that there is negligible elastic recovery at the capillary exit, values of the ratio of _Pent to wall shear stress (_w) are considerably higher for liquid crystalline melts than for PET. Some explanation of these data is presented but more rheological measurements are needed before our understanding of these unique systems is complete.
- Initial Results of a Flow Birefringence Study of the Hole Pressure for Polymer MeltsPike, Daniel; Baird, Donald G. (AIP Publishing, 1984)The initial results of this study on a polymer melt shows that, for the present experimental conditions, stress field asymmetry has a negligible effect on the validity of the HP theory and that the theory may be extended to yield a method [i.e., Eq. (2)] by which values of N1 may be calculated directly and accurately from slit die PH and σω data. Additional work is needed, however, to determine the range of conditions for which this is true. Current research efforts are being directed toward determining the effects of slot dimensions, rounding of slot corners, and melt rheological properties on stress asymmetry and the corresponding effect on the magnitude of PH and the predictions of the HP theory.
- Rheological Properties Of Liquid-Crystalline Copolyester Melts .2. Comparison Of Capillary And Rotary Rheometer ResultsGotsis, A. D.; Baird, Donald G. (AIP Publishing, 1985)In an earlier paper capillary rheometer results were presented for two liquid crystalline copolyesters consisting of 60 mole % and 80 mole % p_hydroxybenzoic acid (pHBA) and polyethyleneterephthalate (PET). In this paper obtained by means of cone_and_plate (CP) and parallel plate (PP) attachments of a Rheometrics Mechanical Spectrometer are compared with the capillary data. In particular, it is observed that over the range of conditions studied, viscosity data obtained in the CP and PP agree with those obtained by means of the capillary rheometer. Furthermore, there is no dependence of the viscosity on geometric or dimensional factors as is observed for low molecular weight liquid crystals under some conditions. Although the viscosity is still shear rate dependent for shear rates as low as 5_10_2_sec_1,
- Studies on The Transient Shear-Flow Behavior Of Liquid-Crystalline PolymersViola, G. G.; Baird, Donald G. (AIP Publishing, 1986-06-01)An understanding of the flow behavior of liquid crystalline polymers (LCP's) is of immense practical value because of the potential to form high modulus materials from these polymers. These fluids exhibit a high degree of structure even in the quiescent state, as evidenced by their ability to transmit polarized light. In an effort to understand how the structure changes during flow, we have carried out a study on the transient shear flow properties of two thermotropic copolyesters of 60_ and 80_mole % para_hydroxybenzoic acid (PHB) and polyethyleneterephthalate (PET) and a lyotropic system of poly_p_phenyleneterephthalamide (PPT) in 100% sulfuric acid. In one of the first theories concerned with the flow behavior of liquid crystalline fluids, which was proposed by Ericksen, the transient flow properties are all predicted to be due to changes in orientation of a director which describes the orientation of packets of rod_like molecules. Stress growth, interrupted stress growth, and stress relaxation experiments are carried out on the three LCP's and at first sight are in qualitative agreement with the predictions of Ericksen's theory. However, wide angle X_ray scattering analysis of quenched samples subjected to shear flow along with annealing experiments on oriented samples indicate that these materials do not orient readily in shear flow. Furthermore, orientation generated during extensional flow relaxes at a rate much faster than is indicated by the interrupted stress growth experiments. It is concluded that the stress growth response of LCP's is due to a disruption of a domain structure which exists within the fluid rather than to orientation changes of the domains of rod_like molecules.
- Comparison Of Flow Birefringence Data with A Numerical-Simulation Of The Hole PressureBaird, Donald G.; Read, M. D.; Reddy, Junuthula N. (AIP Publishing, 1988-08-01)The penalty_Galerkin finite_element method is used to simulate the flow of a polystyrene melt over a rectangular slot placed perpendicular to the flow direction. The White_Metzner constitutive equation is used with a Carreau model viscosity function and a shear rate_dependent relaxation time defined so that the primary normal stress difference is exactly reproduced by the model in simple shear flow. Values of the stress field predicted by the simulation are compared with those obtained experimentally by means of flowbirefringence. As observed by others, the limiting elasticity value as determined by the Weissenberg number (We) for convergence of the algorithm decreased with increased refinement of the mesh. However, good agreement is still found between predicted values of stress using the coarse mesh and those measured by means of flowbirefringence. This work suggests that there may be an optimum mesh for a given flow and constitutive equation which will still give physically realistic results. The Weissenberg number for the melt used in the experimental study asymptotically approached a value of about 1.5 with increasing shear stress, suggesting that it may not be necessary to reach excessively high values of We for simulations involving some polymer melts.
- Methanol decomposition on single-crystal Cu₂OCox, David F.; Schulz, Kirk H. (American Institute of Physics, 1990-05-01)CH₃OD decomposition has been studied over the stoichiometric, nonpolar (111) and polar, copper_terminated (100) surfaces of Cu₂O. Thermal desorption studies for monolayer coverages of methanol at 90 K show similar conversions, but the product distributions vary significantly, indicating a structure sensitive reaction. The (111) surface shows a maximum selectivity for complete oxidative dehydrogenation to CO, while the (100) surface shows a maximum selectivity for partial dehydrogenation to CH₂O. Thermal desorptionspectroscopy (TDS) and x-ray photoelectron spectroscopy (XPS) data are consistent with a decomposition pathway which proceeds through a methoxy intermediate for both surfaces. Possible origins for the structure sensitivity are discussed.
- Transient Flow Of Thermotropic Liquid-Crystalline Polymers in Step Strain ExperimentsDone, D.; Baird, Donald G. (AIP Publishing, 1990-07-01)The transient response of two thermotropic liquid crystalline polymers (a copolyester of 60 mol_% p_hydroxybenzoic acid, PHB, and 40 mol_% poly(ethylene terephthalate), PET, and a copolyester of PHB and 2_hydroxy_6_naphthoic acid, following step strains up to 20 strain units was measured. The relaxation curves typically show an initially rapid decay followed by a long relaxation tail. The lower the temperature, the more remarkable is the length of the long relaxation tail. This behavior makes the LCP uniquely different from most flexible chain polymers such as PET and polystyrene, which show a relaxation modulus which decreases continuously having no discontinuity in the slope. This behavior is probably due to unmelted solid phase which exists in the melt up to quite high temperatures. When the LCP's are heated to temperatures well above their melting points as determined by DSC, then the long relaxation tails are eliminated. Furthermore, on cooling the sample rapidly down from a temperature well above the melting point the relaxation modulus resembles that at the higher temperature as a result of the supercooling effect. The relaxation modulus was also determined for samples subjected to lubricated squeezing flow. Whereas for polystyrene the relaxation modulus determined in lubricated squeezing flow was equal to that determined in step shear strain experiments, this was not the case for the LCP's. It is not known whether the behavior reported here is common to nematic LCP's or to the multiphase structure (crystallites, nematic phase, isotropic phase) which might be present.
- Photoemission and low-energy-electron-diffraction study of clean and oxygen-dosed Cu₂O (111) and (100) surfacesSchulz, Kirk H.; Cox, David F. (American Physical Society, 1991-01-01)The geometric and electronic structure of clean and oxygen-dosed Cu₂O single-crystal surfaces was studied with x-ray and ultraviolet photoelectron (UPS) spectroscopies and low-energy electron diffraction. The nonpolar (111) surface can be prepared in a nearly stoichiometric (1×1) form by ion bombardment and annealing in vacuum. Oxygen adsorbs molecularly on the stoichiometric (111) surface at 300 K, but adsorbs dissociatively on a defective (111) surface prepared by ion bombardment. For the polar Cu₂O(100) face it was possible to prepare a reconstructed, Cu-terminated surface with a (3√2×√2)R45° periodicity by ion bombardment and annealing in vacuum. Preparation of an unreconstructed, (1×1), O-terminated (100) surface was possible by large (10⁹-L) oxygen exposures. UPS investigations of the O-terminated (100) surface suggest a mixture of incorporated (i.e., lattice) oxygen and adsorbed atomic oxygen (i.e., adatoms) in the terminating layer. The annealing behavior of the Cu₂O(100) surface was history dependent. Early in the sample history, bulk lattice oxygen diffused to the surface at temperatures above 800 K giving domains of (square-root 2×√2)R45° periodicity associated with half a terminating layer of oxygen atoms. After repeated ion bombardment and annealing cycles, heating above 800 K gave only a Cu-terminated surface, apparently because of a depletion of bulk lattice oxygen.
- An Evaluation Of The Doi-Ohta Theory For an Immiscible Polymer BlendGuenther, G. K.; Baird, Donald G. (AIP Publishing, 1996-01-01)The theory developed by Doi and Ohta was evaluated for its ability to predict the rheology of an immiscible polymer blend. The theory describes the additional stresses arising as a consequence of interfacial tension in two phase systems in which the constituents consist of Newtonian fluids and have equal viscosities. The blend considered in this paper consisted of an immiscible mixture of poly(ethylene terephthalate) (PET) and nylon 6,6 at a composition ratio of 25/75 w/w PET/nylon 6,6. The rheological properties of this blend were found to be stable for the time frame required for the rheological experiments used in this work (e.g., < 5 min). The Doi-Ohta theory was found to be capable of qualitatively predicting the extra stresses arising as a result of the interfacial tension as observed in the steady state viscosity and steady state first normal stress difference. The transient shear stress and first normal stress difference at the start up of steady shear flow were qualitatively predicted by the Doi-Ohta theory while the recovery of the initial overshoot observed experimentally was not. The overshoot observed experimentally during step-up experiments and the undershoot observed during step-down experiments were not predicted by the theory in which it was predicted that the stresses change monotonically with a stepwise change of the shear rate to the final steady state value. While the shear thinning behavior observed for this blend was not predicted by the theory, the scaling relation for the transient stresses predicted by the theory was found to hold for the blend using stepwise changes of shear rate at a constant step ratio. (C) 1996 Society of Rheology.
- Polyelectrolyte DNA conjugation and genetic transformation of an animal(United States Patent and Trademark Office, 1996-06-04)The present invention provides a method of obtaining an organism which has been characterized as having cells containing exogenous genetic material which includes any sequence of DNA that can be distinguished as exogenous by known molecular biological analysis by insertion of genetic material into an animal's genetic makeup. The insertion of the genetic material is done by inserting DNA that has been complexed with molecules that allow the DNA to be inserted into the chromosomes when injected into the cytoplasm, perivitelline space, or placed in surrounding culture media to be taken up and incorporated into the genome. When the DNA is complexed into the polyelectrolyte molecules by electrostatic attraction, the electric charge of DNA of the complex is partially to substantially neutralized. The present method does not require the genetic material to be introduced into the embryo at a particular stage in development.
- Influence Of Molecular Weight Distribution on The Melt Extrusion Of High Density Polyethylene (HDPE): Effects Of Melt Relaxation Behavior on Morphology And Orientation in HDPE Extruded Tubular FilmsYu, T. H.; Wilkes, Garth L. (AIP Publishing, 1996-11-01)The influence of molecular weight distribution and extrusion processing variables on the morphological features and orientation of high density polyethylene (HDPE) uniaxially extruded tubular films was investigated. In order to gain a better understanding of the orientation-crystallization behavior occurring during extrusion processing, the melt flow properties of the two HDPE resins with identical <(M)over bar (n)> (14 600) values but different molecular weight distributions (<(M)over bar (w)>/<(M)over bar (n)> = 10.3, 15.1), utilized in our previous study, were characterized by dynamic rheological experiments over the temperature range from 150 degrees C to 230 degrees C within the angular frequency range from 0.1 to 100 rad/s. The experimental data were shifted to produce master flow curves. The how activation energy calculated from the shifting process was found to be 25.9 kJ/mol for resin 1 and 29.1 kJ/mol for resin 2. The characteristic relaxation time at 190 degrees C obtained by use of a Carreau-Yasuda analysis for resin 2 having the broader molecular weight distribution was found to be 6.5 times greater than that of resin 1. This observation further supports our previous conjecture that the prominence of the fibril nuclei in resin 2 is due to its longer melt relaxation time behavior. The extrusion processing variables of melt temperature at the die exit, quench height (which is the distance from the exit of the die to the cooling ring), flow rate of the air through the cooling ring, film line speed, and die gap were varied to control the melt relaxation time of HDPE resins and the processing time frame for cooling. The results show that a longer melt relaxation time and a shorter cooling processing time can enhance the formation of fibril nuclei. The importance of melt relaxation behavior in influencing the final morphological structure in HDPE extruded films and their associated properties is clearly made evident in this paper. (C) 1996 Society of Rheology.
- Shear And Extensional Rheology Of Sparsely Branched Metallocene-Catalyzed PolyethylenesBin Wadud, S. E.; Baird, Donald G. (AIP Publishing, 2000-09-01)The purpose of this study was to identify any rheological effects that are consistent with the presence of sparse levels of long chain branching (LCB) in three commercial metallocene-catalyzed polyethylenes (MCPE) all of the same melt flow index of 1.0. Two Dow INSITE MCPEs with apparently varying levels of LCB of approximately 0.17 and 0.57/10 000 carbon atoms and one Exxon EXXPOL MCPE with no LCB were studied. The breadth of distribution as determined by M (w) / M (n) of the three samples was 2.11 for the Exxon and one of the Dow samples, and 2.42 for the other Dow sample that had the highest degree of LCB. The MCPE with the highest branching seemed to have a slightly higher molecular weight tail in the distribution. Both the Dow samples had significantly higher how activation energies than the Exxon sample, consistent with the presence of LCB, but this method could not distinguish between the two branched polymers. The differences in M (w) could also not account for the appreciably higher zero-shear viscosities of the branched samples relative to the linear sample. Despite the differences in M (w) and LCB content in the two Dow samples, they exhibited almost identical shear how curves at temperatures between 120 and 170 degrees C. They also exhibited very similar shear stress growth behavior. Under constant extension rate deformation, the two samples with LCB showed a significant degree of strain hardening relative to the linear sample. Comparison between the two Dow samples revealed that the sample with the higher degree of LCB showed a greater degree of strain-hardening behavior. The extensional behavior is consistent with the LCB content determined by means of solution light scattering. (C) 2000 The Society of Rheology. [S0148-6055(00)00905-6].
- Spontaneous planarization of nanoscale phase separated thin filmSaraf, Ravi F.; Niu, S.; Stumb, Eric (AIP Publishing, 2002-06-01)Structure of complex fluid at mesoscales is influenced by interfacial effects. We describe the dynamic response in such films to sudden change in interfacial tension. In a self-assembled block copolymer film, the monolayer of 15 nm diam cylindrical discrete phases close to the surface commence to sink at an average rate of 0.16 nm/day in response to the interfacial tension change. Surprisingly, this spontaneous planarization occurs, even though the cylinders are covalently stitched to the matrix. A simple model explains the observed behavior. The observation may lead to approaches to tailor the structure of mesoscale thin films of complex fluids for long-range order that are desirable for nanoscale device fabrication. (C) 2002 American Institute of Physics.
- Ratio of dynamic moduli and estimation of relaxation timesHuang, J. H.; Baird, Donald G. (AIP Publishing, 2002-07-01)In this paper, the theoretical interrelation between the ratio of dynamic moduli and the number and distribution of relaxation times required to fit the generalized Maxwell model to the data was investigated. Theorems were derived for making interval estimation of the relaxation times of the generalized Maxwell model from the ratio of linear combinations of the dynamic moduli at different frequencies. According to these theorems, given dynamic moduli (G' and G") at two frequencies a and b or three frequencies a, b, and (ab)(1/2), one must select at least one relaxation time in the relevant interval (tau(A), tau(B)) for the model to fit the data precisely. As a result, from a set of dynamic data G' and G", one can determine a series of (tau(A), tau(B)) from which the minimum number (N-min) and distribution (interval estimation of tau(i)) of relaxation times of the model can be estimated. The approach was applied to polystyrene data reported in the literature. The results were discussed and compared with those of relevant work, especially the nonlinear regression model-fitting procedure. (C) 2002 The Society of Rheology.