Viscoelastic properties of polyamic acid solutions—precursors of polyimides

The rheology of polyamic acid (PAA) solutions, precursors of polyimides used in microelectronic device applications, has been investigated by dynamic (oscillatory) shear flow measurements. Frequency dependent storage and loss moduli and dynamic viscosity were measured in the frequency range 10^?1 to 10³ rad/s at 23°C. The storage modulus G′ (ω) and loss modulus G″ (ω) exhibited quadratic and linear dependence in frequency at low frequencies respectively, the viscoelastic fluid behavior commonly predicted for polymer solutions from many molecular theories. At high frequencies both dynamic moduli become proportional to ω^2/3. The results show that PAA solutions are very high loss viscoelastic fluids, judging from the loss tangent values which far exceed unity. It is suggested that dynamic viscoelastic properties could be used to monitor the degree of imidization since there is a gradual change from viscoelastic fluids to soft viscoelastic solids to hard viscoelastic solids as PAA is converted to polyimides. Onset of non-Newtonian flow as shown on the frequency dependent dynamic viscosity was in the range 30 to 200 rad/s. The viscoelastic constants, zero-shear rate viscosity η_o and steady-state compliance J_e⁰, where also determined from the dynamic data and compared to previous steady shear flow results.     

Thermorheological analysis of blend of high- and low-density polyethylenes

The effect of temperature on dynamic viscoelastic properties of high density polyethylene and low density polyethylene blends with different weight fractions was investigated in the molten state by means of small amplitude oscillatory shear rheometry. It was found that the blends at various compositions do follow well the time-temperature superposition principle and show thermorheologically simple behavior. This behavior is attributed to both similarity in glass-transition temperature of the constituents and phase stability in the blends at various temperatures. The latter was suggested via coincidence of G′-G′′ plots and δ-G^* plots at different temperatures. That was furthur supported using G′ vs. temperature curves which showed no breakdown in the linear relation. Horizontal shift factors, which reflect temperature dependence of relaxation times, obtained to draw G′ and G′′ master curves, followed an Arrhenius equation with temperature. Analysis of terminal relaxation times of components revealed that terminal dynamics of components is similar at limited particular temperatures but different at others. Moreover, depending to test temperature, dynamics of a given component in the blend may be faster or slower than in the pure state.     

Rheological characterization of concentrated cellulose solutions in 1‐allyl‐3‐methylimidazolium chloride

The rheological properties of high concentrated wood pulp cellulose 1‐allyl‐3‐methy‐limidazolium Chloride ([Amim]Cl) solutions were investigated by using steady shear and dynamic viscoelastic measurement in a large range of concentrations (10–25 wt %). The measurement reveals that cellulose may slightly degrade at 110°C in [Amim]Cl and the Cox–Merz rule is valid for 10 wt % cellulose solution. All of the cellulose solutions showed a shear thinning behavior over the shear rate at temperature from 80 to 120°C. The zero shear viscosity (η_o) was obtained by using the simplified Cross model to fit experimental data. The η_o values were used for detailed viscosity‐concentration and activation energy analysis. The exponent in the viscosity‐concentration power law was found to be 3.63 at 80°C, which is comparable with cellulose dissolved in other solvents, and to be 5.14 at 120°C. The activation energy of the cellulose solution dropped from 70.41 to 30.54 kJ/mol with an increase of concentration from 10 to 25 wt %. The effects of temperature and concentration on the storage modulus (G′), the loss modulus (G″) and the first normal stress difference (N₁) were also analyzed in this study. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Viscoelastic properties of a 60 mol% para-hydroxybenzoic acid/40 mol% poly(ethylene terephthalate) liquid crystalline copolyester. II: Effect of shear history

Temperature sweeps of dynamic viscoelastic properties have shown that phydroxybenzoic acid (PHB)-based liquid crystalline polyesters, specifically in this case those copolymerized with poly(ethylene terephthalate) (PET), can be subjected to considerable supercooling if initial heating curves are compared to subsequent cooling curves, indicating that this type of material can be in quite different states even at the same temperature, depending on thermal history. Utilizing this supercooling behavior, viscoelastic properties of a 60 mol% PHB/40 mol% PET material produced by Unitika were monitored before and, particularly, after large-scale shear deformation to determine how potential structure changes induced by the shear are reflected in viscoelastic properties immediately, and with time. According to dynamic viscoelastic temperature sweep data four quite different initial states were employed including conditions with, as well as largely free of, crystallites. However, in all cases, post-shear monitoring showed decreased G′ and G″ values with almost no evidence of return towards initial values within approximately 25 min. These results, in addition to furthering somewhat the fundamental understanding of the flow and relaxation properties of liquid crystalline polymers, may be useful in polymer processing, where large-scale shear deformations employed in forming processes appear to be capable of changing considerably the subsequent behavior of such materials.     

Rheometric and ultrasonic investigations of viscoelastic properties of fresh Portland cement pastes

The paper investigates the possibility of using a shear wave reflection technique to monitor the viscoelastic behavior (represented by storage shear modulus and viscosity) of Portland cement paste at very early age. Three cement pastes with water/cement ratios equal to 0.4, 0.5 and 0.6 cured under water at a constant temperature of 25 °C were studied. By measuring the wave reflection coefficients and the phase angles of reflected ultrasonic waves, the dynamic storage shear moduli and the viscosity of the cement paste can be calculated. The calculated results of the storage modulus were compared with the results obtained directly from the oscillatory rheometric measurement. In addition, the viscosity calculated from the wave reflection measurements was compared with results obtained directly from the step rheometric method and a qualitative agreement was found. The results show that as a non-destructive method, the ultrasonic wave reflection method provides useful information about both the elastic and viscous behavior of cement pastes at very early age.     

Rheological properties of cotton pulp cellulose dissolved in 1‐butyl‐3‐methylimidazolium chloride solutions

Rheological properties of cotton pulp dissolved in 1‐butyl‐3‐methylimidazolium chloride ([Bmim]Cl) solutions were characterized using an advanced rheometer. The complex viscosity, dynamic modulus, and shear viscosity at different temperature were studied. In the steady shear measurements, all the solutions show a shear‐thinning behavior at high shear rates. The complex viscosity as a function of frequency was fitted by extended Carreau–Yasuda model. In all cotton pulp/[Bmim]Cl solutions, the complex dynamic viscosity (η*) and steady shear viscosity (η_a) followed the Cox–Merz rule only at lower frequency. The effects of tested temperature on viscosity and viscoelastic behavior of the solutions were also investigated. The value of activation energy for the dissolution of cotton pulp in ionic liquids was 65.28 kJ/mol at the concentration of 10 wt% and was comparable with the ones for the dissolution of cellulose in NMMO. POLYM. ENG. SCI., 2011. © 2011 Society of Plastics Engineers     

Photocurrent response and semiconductor characteristics of Ce-Ce2O3-CeO2-modified TiO2 nanotube arrays

We reported Ce and its oxide-modified TiO₂ nanotube arrays (TNTs) and their semiconductor properties. The TNTs were prepared by anodic oxidation on pure Ti and investigated by electrochemical photocurrent response analysis. Then, the TNT electrodes were deposited of Ce by cathodic reduction of Ce(NO₃)₃ 6H₂O. After deposition, the TNT electrodes were fabricated by anodic oxidation at E = 1.0 V_(SCE) for various electricity as Ce-Ce₂O₃-CeO₂ modification. The Ce-deposited TNTs (band gap energy E_g = 2.92 eV) exhibited enhanced photocurrent responses under visible light region and indicated more negative flat band potential (E_fb) compared with the TNTs without deposition. After anodic oxidation, the mixed Ce and its oxide (Ce₂O₃-CeO₂)-modified TNT photoelectrodes exhibited higher photocurrent responses under both visible and UV light regions than the TNTs without deposition. The photocurrent responses and E_fb were found to be strongly dependent on the contents of Ce₂O₃ and CeO₂ deposited on TNTs. A new characteristic of E_g = 2.1 ± 0.1 eV was investigated in the Ce₂O₃- and CeO₂-modified photoelectrodes. X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) were also employed to characterize various modified TNTs photoelectrodes.     

超高分子量聚乙烯凝胶的流变行为

采用旋转流变仪研究了超高分子量聚乙烯(UHMWPE)凝胶的流变行为。通过动态应变扫描测定了UHMWPE凝胶的线性黏弹区;通过动态温度扫描、动态频率扫描和稳态速率扫描研究了温度、浓度、剪切速率对凝胶流变行为的影响。结果表明,浓度为2%~22%的UHMWPE凝胶的线性黏弹区对应的应变下限为2%,上限为40%,且温度对凝胶线性黏弹区的影响较大;浓度为6%的UHMWPE凝胶,在180℃时,弹性模量最大,凝胶内部的黏结性最强;UHMWPE凝胶熔体的黏度随扫描频率、剪切速率的升高而降低,呈现明显的剪切变稀行为,属于假塑性流体;剪切速率较高时,UHMWPE凝胶的黏度对温度的变化更敏感。     

Application of pH-Responsive Viscoelastic Surfactant as Recyclable Fluid at High Temperature

A pH-responsive amphiphilic surfactant stearic amide 3-(N,N-dimethylamino)propylamide (SAA) was synthesized and served as a thickener in aqueous solution to construct a switchable viscoelastic surfactant fluid (VES fluid). The structure of SAA was studied by ¹H NMR, and the viscoelastic behavior of VES fluid was studied in detail by rheological measurements. The viscosity of this VES fluid can be switched reversibly from low to high immediately by adjusting system pH value. Even at high shear rate (170 s⁻¹) and high temperature (90 °C), excellent viscoelastic behavior of this VES fluid can be observed, which is a key performance for fracturing applications. Meanwhile, the recycled VES fluid can still maintain good pH-responsive behavior even after more than three cycles. These unique performances of this VES fluid not only enhanced our understanding of the transformation of wormlike micelles at high temperature, but also enriched a large potential of VES fracturing fluid in the development of oil and gas reservoirs.     

Rheological characterization of HDPE/sisal fiber composites

The present paper summarizes an experimental study on the molten viscoelastic behavior of HDPE/sisal composites under steady and dynamic mode. Variations of the melt viscosity and die swell of the composites with an increase in shear rate, fiber loading, and coupling agent concentration have been investigated using capillary rheometer. The shear rate γ at the wall was calculated using Rabinowitsch correction applied to the apparent shear rate values. It was observed that the melt viscosity of the composites increased with the addition of fibers and maleic anhydride-grafted PE (MAPE). Die swell of HDPE also decreased with the addition of sisal fibers and MAPE. Further, the dynamic viscoelastic behavior of the composites was measured employing parallel plate rheometer. Time–temperature superposition was applied to generate various viscoelastic master curves. Temperature sweeps were also carried out to study the flow activation energy determined from Arrhenius equation. The fiber–matrix morphology of the extrudates was also examined using scanning electron microscopy. POLYM. ENG. SCI., 47:1634–1642, 2007. © 2007 Society of Plastics Engineers     

Rheological non-Newtonian behaviour of ethylene glycol-based Fe2O3 nanofluids

The rheological behaviour of ethylene glycol-based nanofluids containing hexagonal scalenohedral-shaped α-Fe₂O₃ (hematite) nanoparticles at 303.15 K and particle weight concentrations up to 25% has been carried out using a cone-plate Physica MCR rheometer. The tests performed show that the studied nanofluids present non-Newtonian shear-thinning behaviour. In addition, the viscosity at a given shear rate is time dependent, i.e. the fluid is thixotropic. Finally, using strain sweep and frequency sweep tests, the storage modulus G'', loss modulus G″ and damping factor were determined as a function of the frequency showing viscoelastic behaviour for all samples.     

A new method to disperse CdS quantum dot-sensitized TiO2 nanotube arrays into P3HT:PCBM layer for the improvement of efficiency of inverted polymer solar cells

We report that the efficiency of ITO/nc-TiO₂/P3HT:PCBM/MoO₃/Ag inverted polymer solar cells (PSCs) can be improved by dispersing CdS quantum dot (QD)-sensitized TiO₂ nanotube arrays (TNTs) in poly (3-hexylthiophene) and [6,6]-phenyl-C₆₁-butyric acid methyl ester (P3HT:PCBM) layer. The CdS QDs are deposited on the TNTs by a chemical bath deposition method. The experimental results show that the CdS QD-sensitized TNTs (CdS/TNTs) do not only increase the light absorption of the P3HT:PCBM layer but also reduce the charge recombination in the P3HT:PCBM layer. The dependence of device performances on cycles of CdS deposition on the TNTs was investigated. A high power conversion efficiency (PCE) of 3.52% was achieved for the inverted PSCs with 20 cyclic depositions of CdS on TNTs, which showed a 34% increase compared to the ITO/nc-TiO₂/P3HT:PCBM/MoO₃/Ag device without the CdS/TNTs. The improved efficiency is attributed to the improved light absorbance and the reduced charge recombination in the active layer.     

Synthesis, electrorheology and creep behavior of polyindole/polyethylene composites

In this study, polyindole (PIN) and polyindole/polyethylene (PIN/PE) conducting composites, having various amounts of PIN, were synthesized by chemical polymerization using FeCl₃ as an oxidizing agent and taking the ratio of salt:monomer as 3:1. The samples of PIN and PIN/PE composites were characterized by FTIR, UV–vis, TGA, SEM, Gouy scale magnetic susceptibility, conductivity (1.2 × 10⁻³ S cm⁻¹ > σ > 1.96 × 10⁻⁶ S cm⁻¹, at T = 25 °C) and density measurements. FTIR analysis suggested a 2,3-propagation mechanism for PIN formation. The ground milled samples were subjected to particle size analysis by dynamic light scattering (DLS) and a micron-sized particle distribution was obtained. A series of volume fractions ( = 10–25%) were prepared from the materials in silicone oil (SO) and their sedimentation stabilities determined. The most stable composite [PIN(89%)/PE(11%)] against gravitational sedimentation was subjected to flow-rate measurements under externally applied electric field strength (E) and an electrorheological (ER) activity was observed; threshold energies (E_t) were calculated. The effects of volume fraction, shear rate, external E, frequency and temperature onto ER activities of the suspensions were investigated. Enhancement in the electric field viscosities and shear thinning viscoelastic behaviors were observed for all the samples studied. Recoverable viscoelastic deformations were determined from the creep tests under external E.     

Estimating time and temperature dependent yield stress of cement paste using oscillatory rheology and genetic algorithms

A controlled shear stress–shear rate rheometer was used to determine the viscoelastic behavior of cement paste incorporating various superplasticizers and subjected to prolonged mixing at high temperature. At a low applied shear stress range, the oscillatory shear strain/stress curve of cement paste was characteristic of a linear elastic solid; while the higher stress range was characteristic of a viscous liquid exhibiting a linear strain increase with increasing applied shear stress. The transition from solid-like to liquid-like behavior occurred over a very narrow stress increment. This transition stress corresponded to the yield stress parameter estimated from conventional flow curves using the Bingham model. The yield stress from oscillatory shear stress tests was estimated using the intersection between the viscous part of the oscillatory shear strain/stress curve and the oscillatory shear stress axis. In this study, equations describing the variation of shear strain versus shear stress beyond the solid–fluid transition for cement pastes incorporating various superplasticizers at different ambient temperatures and mixing times were developed using genetic algorithms (GA). The yield stress of cement pastes was subsequently predicted using the developed equations by calculating the stress corresponding to zero strain. A sensitivity analysis was performed to evaluate the effects of the mixing time, ambient temperature, and superplasticizer dosage on the calculated yield stress. It is shown that the computed yield stress values compare well with corresponding experimental data measured using oscillatory rheology.     

Viscoelastic behavior of AN‐g‐casein copolymer concentrated solution of sodium thiocyanate

The viscoelastic behavior of sodium thiocyanate concentrated solution of graft copolymer of acrylonitrile onto casein (AN‐g‐casein) was investigated in detail by nitrogen pressure capillary rheometer. The experimental results show that the concentrated solution of AN‐g‐casein is a non‐Newtonian fluid of shear thinning. The end correction increases with the increase of shear rate. In the course of practical spinning, the real shear stress only attributes about 35% to the apparent shear stress, which confirms the essentiality of end correction for the capillary extrusion of the concentrated solution of the graft copolymer. By using the Dewitt model, the Couette correction ξ and recoverable shear strain S_R were analyzed and separated. Quantitative function relation of ξ and S_R versus shear rate and temperature were deduced. It is a common equation within the range of experiment. In practical spinning process, the real viscoelastic parameters can be easily predicted by only a piece of flowing curve based on the common equation. Real elastic model G under any shear rate in the range of experiment was calculated. It was found that G increases with an increase of shear rate, which is opposite to the conclusion reported by Hayahara. The main reason for the error may be that Hayahara used the same Couette correction at a higher shear rate. Further, the die swell in the spinning process can be estimated efficiently. The theoretical base for the stability of AN‐g‐casein spinning technology was provided. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 84: 1721–1728, 2002; DOI 10.1002/app.10554     

Viscoelastic properties of polyarylene ether nitriles/thermotropic liquid crystalline polymer blend

Polyarylene ether nitriles (PEN)/thermotropic liquid crystalline polymer (TLCP) blend was prepared via melt mixing. The immiscible phase morphologies, linear and nonlinear, as well as transient viscoelastic properties of the blend were studied using SEM, rheometer, and DMA. The linear dynamic viscoelastic behavior of the blend shows temperature dependence due to further evolution of the immiscible morphology and, as a result, the principle of time‐temperature superposition (TTS) is invalid. In the steady shear flow, the discrete TLCP phase is difficult to be broken up because of the high viscosity ratio of the blend systems, while is easy to be coarsened and followed by elongation, and finally, to form fibrous morphology at high TLCP content and high shear level. During this morphological evolution process, the transient stress response presents step increase and nonzero residual relaxation behavior, leading to increase of the dynamic viscoelastic responses after steady preshear. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Rheological properties of high concentrations of carboxymethyl cellulose solutions

The study was concerned with measurements of steady‐state parameters, the transient shear stress response, and the yield stress of carboxymethyl cellulose (CMC) solutions at high concentrations. Tests also included thixotropic and viscoelastic behavior and dynamic responses. The concentrations ranged by weight from 5 to 8% of CMC. The steady‐state shear flow showed that at higher shear rates the viscosities of CMC solutions tend to be less dependent on the concentration. The solutions showed rheopectic behavior for very small shear rates. No yield stress was detected. Measurements recorded the thixotropic behavior. At higher stress values, nonlinear viscoelastic effects were detected. Dynamic viscosities measured in a dynamic test were higher than were the shear viscosities at the same concentrations. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 79: 1787–1801, 2001     

Long-term creep of polyphenylene sulfide (PPS) subjected to complex thermal histories: The effects of nonisothermal physical aging

The long-term viscoelastic behavior of polymeric materials used below the glass transition temperature (T_g) is greatly affected by physical aging. In contrast to isothermal physical aging, long-term response under nonisothermal history has received far less attention. This paper reports experimental results and analytical methods of long-term creep behavior of polyphenylene sulfide (PPS) subjected to complex thermal histories in a temperature range below T_g. To characterize the effects of aging, creep tests were performed using a dynamic mechanical analyzer (DMA). Besides the long-term data, short-term creep tests in identical thermal conditions were also analyzed; these were utilized with effective time theory to predict long-term response under both isothermal and nonisothermal temperature histories. The long-term compliance after a series of temperature changes was predicted by the effective time theory using the KAHR-a_te model to obtain nonisothermal physical aging shift factors. Comparison of theoretical predictions with experimental data shows good agreement for various thermal histories.     

Rheological properties of liquid crystalline solutions of ethyl celluloses with different molecular weight in m-cresol

Steady-state shear rheological properties of liquid crystalline solutions of four ethyl celluloses (ECs) were determined at a low shear rate (1 s^?1) and at relatively high shear rates by using two rheometers (cone-plate and capillary types), and were compared with those of liquid crystalline hydroxypropyl cellulose (HPC). The effect of molecular weight (MW) on the viscoelastic behavior was also determined. The viscoelastic behavior was also determined. The viscometric behavior of EC solutions was similar to that of HPC solutions: (1) with respect to temperature, the shear viscosity (η) at shear rate of 1 s^?1 exhibited a minimum (η_min) and a maximum (η_max), and the concentration–temperature superposition for η could be applied; (2) the behavior of η at relatively high shear rates as a function of shear rate or polymer concentration was typical of lyotropic liquid crystals. The MW dependence of η_min was greater than that of η_max for EC solutions. The behavior of the elastic parameters such as Bagley correction factor (v), entrance pressure drop (ΔP_ent), and die swell (B) at relatively high shear rates for EC solutions was essentially similar to that for HPC solutions: (1) the shear rate or stress dependence of the elastic parameters was greatly dependent on whether the polymer solution was in a single phase or biphase; (2) with respect to concentration the elastic parameters showed a maximum and a minimum and the maximum or minimum point for each parameter was not always identical to each other. η for the isotropic or fully anisotropic solutions at a given concentration (C) increased, whereas η for the solutions in the vicinity of the biphasic region showed a minimum, with respect to MW. The slope of η at a given shear rate vs. CM _w depended on shear rate, and this slope for the isotropic solutions appeared to be greater than that for fully anisotropic solutions. ΔP_ent and v at a given concentration showed either a monotonical increase or a maximum or minimum with MW, and this behavior was not fully consistent with that of η. B for the isotropic solutions increased and B's for both biphasic and fully anisotropic solutions were almost constant, with MW.     

PSA performances and viscoelastic properties of SIS‐based PSA blends with H‐DCPD tackifiers

Hotmelt pressure sensitive adhesives (PSAs) usually contain styrenic block copolymers like styrene–isoprene–styrene (SIS), SBS, SEBS, tackifier, oil, and additives. These block copolymers individually reveal no tack. Therefore, a tackifier is a low molecular weight material with high glass transition temperature (T_g), and imparts the tacky property to PSA. The SIS block copolymer with different diblocks was blended with hydrogenated dicyclopentadiene (H‐DCPD tackifier), which has three kinds of T_g. PSA performance was evaluated by probe tack, peel strength, and shear adhesion failure temperature. PSA is a viscoelastic material, so that its performance is significantly related to the viscoelastic properties of PSAs. We tested the viscoelastic properties by dynamic mechanical analysis and the thermal properties by differential scanning calorimeter to investigate the relation between viscoelastic properties and PSA performance. © 2006 Wiley Periodicals, Inc. J Appl PolymSci 102: 2839–2846, 2006