PIV measurements of flow in drying polymer solutions during solvent casting

An experimental method based on confocal microscopy and particle image velocimetry (PIV) is used to characterize the flow in a polymer solution during solvent casting. The flow inside a 200-μm-thick film of a poly(vinyl alcohol) (PVA) solution is visualized near a vertical wall of a mold using confocal microscopy of seed particles during solvent evaporation at 25, 35, and 45°C, and the corresponding velocity vector fields are determined from projections of the confocal images. Flow toward the vertical wall is observed inside the film as well as a slower Marangoni-type counter flow at the film surface during the initial phase of solvent evaporation, resulting from a polymer concentration gradient along the film due to a local variation in evaporation rate. Total volume of the polymer solution in the observation volume as well as solvent evaporation rate are determined as a function of time, both revealing close correlation to average horizontal velocity data from PIV. The PIV measurements show significant differences in the flow velocity fields at different temperatures. The PIV measurements correlate with the solvent evaporation rates as well as the final polymer thicknesses on the vertical wall of the mold. Surface tension and viscosity measurements are taken for different concentrations of PVA solution.     

Rheological evidence of modifications of polypropylene by β-irradiation

Electron beam irradiation can be used to induce chemical changes in polymers. The resulting reactions lead to both degradation and crosslinking of polymer chains, depending on reaction conditions. In neat polypropylene, degradation dominates and results in a decrease of molecular weight and worsening of mechanical properties. Enhanced crosslinking can be achieved by utilising a polyfunctional monomer. Triallylisocyanurate (TAIC) serves this purpose and can be used to crosslink polypropylene effectively. The corresponding changes of the rheological properties can be observed using oscillatory and creep experiments when changing the amount of TAIC added as well as the absorbed radiation dose. Depending on these parameters, we attribute the rheological properties to chain degradation or formation of a network and gelation. The phase angle vs the dynamic modulus plot is a useful analytical tool for characterisation of the resultant structures. Some samples showed behaviour that leads us to assume broadening of the molecular weight distribution and long chain branching. Resulting from the parameter dependencies, a topological state diagram is proposed to map parameter values to corresponding polymer structures.     

Rheological studies of the gelation of aqueous solutions of acrylamide-acrylic acid copolymers in the presence of chromium chloride

The gelation kinetics of aqueous solutions of acrylamideacrylic acid copolymers in the presence of chromium ions was studied with different rheometers. The gel time is found to vary with the principle of the apparatus because of a low shear dependence of the elongation of the macromolecules. The variations of the gel time with the experimental conditions of the gel preparation were investigated and we conclude that the gelation is governed by the oligornerization kinetics of the chromium ions which are instantaneously, complexated by the carboxylate groups of the polymer. The variation laws of the gel time and of the elastic modulus with chromium and polymer concentrations, temperature and ionic strength are in qualitative agreement with the structure of the temporary network of the semi-dilute solutions of these copolymers.     

Gelation behavior of polysaccharide-based interpenetrating polymer network (IPN) hydrogels

We report the preparation and rheological characterization of interpenetrating polymer network (IPN) hydrogels made from alginate and hydrophobically modified ethyl hydroxyl ethyl cellulose (HMEHEC). To our knowledge, there have been no studies of the gelation behavior of IPNs. We found that the rheology of these systems can be easily tuned, with the elastic modulus of the IPN strongly dependent on the relative ratio of HMEHEC to alginate. The sol–gel transition of these systems was found to satisfy the Winter–Chambon criterion for gelation at various crosslinker densities. From the power law relationship of the dynamic moduli (G ^′ ~G ^″ ~ω ⁿ), the exponent n appears to be dependent on both the crosslinker density and relative amount of two polymers. The value of n was found to be ~0.5 for all samples for stoichiometric amounts of crosslinker. The effect of molecular weight of HMEHEC on the gel point and viscoelastic exponent has also been reported. Alginate seems to dominate the kinetics of the process but the effect of high molecular weight HMEHEC on the gel point, especially at lower proportion was also evident.     

Effect of chemical structure on the crosslinking behavior of bismaleimides: Rheological study

The effect of molecular structure on the cure characteristics of bismaleimides (BMIs) was investigated by rheological measurements. BMI resins of different chemical structures were used, prepared from diglycidyl ether of bisphenol A and N-(3-carboxy phenyl) maleimide or N-(4-carboxy phenyl) maleimide. Temperature dependence of the rheological data was correlated to the Arrhenius equation, from which the activation energy of crosslinking was calculated.Upon heating in dynamic curing, viscosity passed through a minimum then sharply increased due to increasing amount of the crosslinking reaction. The temperature at minimum viscosity increased as the chain length became longer. After passing the minimum point, viscosity increased much faster for meta BMIs (3BE1, 3BE2) than para BMIs, however, the difference in the chain length did not show any appreciable difference in the viscosity increase rate (dη^∗/dT). Nonetheless, the final viscosity was lower for longer chains, because their crosslinking density would be lower.In isothermal curing, the final crosslinking density was expected to increase with temperature, which was shown via the increase in the final viscosity. The gelation time decreased with temperature, and the activation energy of the crosslinking reaction was obtained. Using the reaction kinetics parameters obtained from dynamic scanning calorimeter and Arrhenius type equation, the viscosity change during the isothermal curing was simulated and compared with the measurements.     

Rheological behavior of suspensions of modified and unmodified cellulose nanocrystals in dimethyl sulfoxide

The rheological behavior of cellulose nanocrystal (CNC) and modified CNC (mCNC) suspensions in dimethyl sulfoxide (DMSO) was investigated. The efficiency of the surface modification of CNCs by grafting an organic acid chloride to produce hydrophobic CNCs has been verified by X-ray photoelectron spectroscopy (XPS). The thermal degradation temperature of the mCNCs was found to be 165 versus 275 °C for CNCs. The CNC suspensions in DMSO at 70 °C underwent gelation at very low concentration (1 wt%) after 1 day. The network formation was temperature sensitive and did not occur at room temperature. For gels containing 3 wt% CNCs, the complex viscosity at 70 °C increased by almost four decades after 1 day. For the mCNCs in DMSO, a weak gel was formed from the first day and temperature did not affect the gelation. Finally, the effect of adding 10 wt% of polylactide (PLA) to the solvent on the rheological properties of CNC and mCNC suspensions was investigated. The properties of suspensions containing 1.9 wt% CNCs and mCNCs increased during the first and second days, and PLA did not prevent gel formation. However, the reduced viscosity and storage modulus of the CNC and mCNC gels with PLA were lower than those of samples without PLA.     

Viscoelastic properties of phosphoric and oxalic acid-based chitosan hydrogels

In a previous work, we have shown that chitosan true physical gelation occurs in some organic and inorganic acids (Hamdine et al. 2004). Two systems presenting similar gelation mechanisms were characterized furthermore in order to investigate the sol–gel transition: the chitosan–phosphoric acid and the chitosan–oxalic acid systems. By performing rheological measurements in the framework of linear viscoelasticity, we have investigated the effect of time, temperature, and polymer concentration on the gelation evolution. For both acid-based systems, gelation occurred above a critical polymer concentration around 5% w/v (g/100 ml) of chitosan. Isothermal time sweep experiments showed that the gelation occurs in three stages: (i) incubation; (ii) rapid increase of G′; and (iii) a last stage where G′ slowly reached its equilibrium value due to slow molecular diffusion. At the gel point, G′ and G′′ scaled with ω ⁿ , with n=0.55 for both acid-based systems and a fractal dimension d _f of 1.9. Cooling–heating cycles revealed that the gels showed thermoreversibility after one sequence, but became permanent during subsequent cycles.This revised version was published online in October 2005 with corrections to the author's name.     

Thermoreversible gelation of hydroxypropylcellulose aqueous solutions

The thermoreversible gelation of hydroxypropylcellulose (HPC)/water solutions is investigated by means of rheological and optical techniques. The linear viscoelasticity of solutions at different concentrations is measured during heating ramps from room temperature. The experimental results suggest that the solution microstructure changes according to two sequential steps, i.e., precipitation and gelation. Temperature, temperature rate of change, and concentration are found to be relevant parameters controlling the phase transition.     

Physical gelation under shear for gelatin gels

Physical gelation is the process of crosslinking which reversibly transforms a solution of polymers into a gel. The crosslinks of the network have a physical origin (hydrogen bonding, Van der Waals forces... ) and therefore are sensitive to variations of temperature, pH, ionic content, etc. (non-permanent crosslinks). Physical and chemical gelation have been extensively studied in quiescent conditions, where rheology experiments have been performed to follow the network formation without disturbing the process. In this study we consider gelation of a well known physical, thermoreversible, gel (gelatin gel), which proceeds under flowing conditions. The gelling solution is submitted to a shearing, with imposed, permanent shear stresses or imposed, permanent, shear rates. Under flow, a competition arises between the formation of clusters by physical crosslinking and their disruption by the shear forces. This investigation defines the flowing conditions which either allow or impede gel formation. In particular, a critical shear rate , related to the gelation temperature and gelatin concentration, is identified which separates the two regimes. A microscopic model is proposed, based on the analysis of flow curves and dynamic measurements, which describes the structure of the gelling solution: microgel particles grow to a maximum size which depends on the flow. When the volume fraction of particles is high enough, percolation between particles occurs suddenly and a yield stress fluid is formed (particulate gel). The differences between gels made in quiescent conditions and gels made under flow are underlined.     

Viscoelastic behaviour of partly hydrolysed polyacrylamide/chromium(III) gels

This paper reports an experimental investigation of the crosslinking process of high molecular weight partly hydrolysed polyacrylamides (HPAAm) in aqueous brine solution by trivalent chromium ions (Cr(III)). Crosslinking took place in the presence of a retardant agent (sodium citrate). First, sol-gel phase diagrams (in the polymer and concentration space) were established using the tilting tube method. Then, for a fixed composition, the gelation process was monitored systematically using dynamic viscoelastic measurements, varying the main parameters (pH, time, temperature and retardant concentration). Network formation proceeds rather slowly and an equilibrium state was not reached within 12 h. The gel was formed only at pHs between 5 and 9 and thus two gel points (i.e. at two pHs) were determined with the Winter-Chambon method. This is in agreement with the chemistry of aqueous chromium and of acrylic acid groups along the polymer backbone. Kinetics of network formation depends strongly on retardant concentration. Temperature plays an important role: network formation proceeds much faster at high temperature, in agreement with chemical kinetics.     

Time- and temperature-dependent crosslinking behaviour of a silicone resin

The crosslinking behaviour of a silicone resin which is interesting from a technical point of view was investigated by means of rheology. In order to accelerate the crosslinking process, zinc acetylacetonate and aluminium acetylacetonate were applied as latent catalysts. The effect of the type of catalyst, its concentration, and the temperature on crosslinking was determined by isothermal dynamic-mechanical measurements. A radial gradient in crosslinking causes the gel point to be reached earlier at the outer edge of the sample in the rheometer. This radial gradient is averaged when measuring G′ and G″. Therefore, since the physically well-defined state of critical gelation (gel point) could not be obtained from the data, the time at which a distinct crosslinking state is reached was determined by the crossover of the moduli G′ and G″. For this distinguished point, the denotation gelation index GI is introduced. The gelation indices measured at different temperatures follow an Arrhenius-type relationship. Activation energies between 89 and 126 kJ/mol were determined. They were found to be dependent on the type of catalyst used but independent of its concentration. The activation energies of the crosslinking processes enable the calculation of the gelation index at temperatures not measured directly.     

Effects of anionic and cationic surfactants on the rheological properties and kinetics of bovine serum albumin hydrogel

The effects of the anionic surfactant, sodium dodecyl sulfate (SDS), and of the cationic surfactant cetyltrimethylammonium bromide (CTAB) on the gelation kinetics of bovine serum albumin (BSA) hydrogel were investigated by rheological measurements using surfactant concentrations of 0–0.05 M, and BSA concentrations of 5, 7, and 10 wt%. It was found while an increase in CTAB concentration accelerated the rate of gelation of BSA solution under temperature jump and temperature ramp conditions, BSA solutions containing SDS exhibited a heat-dependent protective effect against thermal denaturation and gelation. Under temperature ramp conditions, inhibition of BSA gelation by SDS was diminished by increasing SDS concentration, while under temperature jump conditions, inhibition of BSA gelation increased with SDS concentration. That is, gel temperature (T_gel) under temperature ramp decreased with increasing CTAB and with SDS concentration, but under temperature jump the gel time (t_gel) decreased with increasing CTAB concentration but increased with SDS concentration. Furthermore, BSA/CTAB solutions were found to gel more rapidly than BSA/SDS solutions, which was in line with the lower activation energy of BSA/CTAB gel. In support of experiments, molecular dynamics (MD) simulations and dynamic light scattering (DLS) revealed the faster rate of BSA denaturation in the presence of CTAB was responsible for the increased gelation rate of BSA/CTAB solutions, whereas BSA was found to be protected by SDS against thermal denaturation leading to the slower gelation rate of BSA/SDS solutions.     

The effect of NaCl addition on the rheological behavior of cetyltrimethylammonium <Emphasis Type="Italic">p</Emphasis>-toluenesulfonate (CTAT) aqueous solutions and their mixtures with hydrophobically modified polyacrylamide aqueous solutions

In this paper, the influence of NaCl addition, up to very large concentrations, on the rheological properties of cetyltrimethylammonium p-toluenesulfonate (CTAT) solutions and their mixtures with two hydrophobically modified polyacrylamides (HMPAM) has been studied under simple shear. The CTAT concentrations employed were above the critical rod concentration. As salt is added to CTAT aqueous solutions, the zero-shear viscosity first increases, goes through a maximum, and at very high ionic strengths increases once more. The overlap concentration of worm-like micelles decreases as the concentration of NaCl increases. The results are explained by the salt addition-induced growth of worm-like micelles and salting out effects at the highest contents of NaCl. The influence of ionic environment on the rheological properties of CTAT with two HMPAM solutions with different contents of hydrophobic moieties was also studied under simple shear. When NaCl is added to HMPAM/CTAT solutions, the same trends observed in CTAT/NaCl solutions were repeated but the viscosity increases were largely magnified. The large viscosity enhancements with salt increments in HMPAM/CTAT solutions were explained by the formation of an interpenetrated network of hydrophobically modified polymer chains and worm-like micelles with hydrophobic sequences embedded within its structure.     

Rheological properties of suspensions of TiO2 particles in polymer solutions. 1. Shear viscosity

This paper presents results on the rheological behaviour of suspensions of two kinds of TiO₂ particles in two different polymer solutions. The particles differ in their hydrophilic or hydrophobic properties. The dispersing media are a solution of high molecular weight polyisobutylene in decalin and a solution of a low molecular weight polybutene in decalin. The concentrations of polymer are adjusted in order to get the same zero shear viscosity. The shear viscosity measurements display an apparent yield stress in some cases. The existence and the values of the yield stress depend on the volume fraction of solid particles and on the type of particles. The evolution of the intrinsic viscosity and of the maximum packing fraction vs the shear rate is interpreted in terms of evolution of the size and of the shape of aggregates of particles under shear. The effect of temperature on the development of the yield stress is also discussed. The results are completed by microscopic observations.     

Effect of polymer bridging on rheological properties of dispersions of charged silica particles in the presence of low-molecular-weight physically adsorbed poly(ethylene oxide)

 The effect of a low-molecular-weight physically adsorbed poly(ethylene oxide) on the rheological behavior of aqueous dispersions of silica particles (as a model system) has been investigated. Particular attention is given to the evolution of the rheological behavior with increasing polymer concentration in the system at different volume fractions of the particles. Experiments were performed in the absence of salt and just the pH of the dispersion was adjusted to 9.5, a condition at which the system is electrostatically stable and electrostatic repulsive forces are long range in nature. It was observed that the shear viscosity and the linear viscoelastic functions of the dispersion at 55 vol% increase initially through the addition of polymer, reach a maximum, and then decrease to a minimum with further addition of polymer to the system. At higher polymer concentrations, there may be an increase in the viscosity of the dispersion owing to an increase in the concentration of free polymer chains in the medium causing depletion flocculation in the system. The increase in the rheological behavior of the dispersion at low polymer coverage is attributed to polymer bridging flocculation caused by a low-molecular-weight poly(ethylene oxide) in the system. Comparison of the data given here with the results of earlier studies on the viscosity behavior of the system in the presence of salt (0.01 M) indicates that the range of the electrostatic repulsion has a significant role in the rheological behavior of the system. Received: 7 February 2001 Accepted: 18 October 2001     

Influence of surfactant addition on the rheological properties of aqueous Welan matrices

In the present work the effects produced by the presence of two different surfactants (Abil B 8842 and Triton N 101) on the rheological properties of aqueous welan matrices are studied, both in steady and in oscillatory shear conditions. Welan is an acidic microbial polysaccharide having high thermal, pH, and salt stability. At sufficiently low concentrations it forms aqueous weak gel matrices which can be profitably used to regulate the rheological properties of disperse systems and improve their stability. Different systems are examined, having the same polysaccharide concentration (0.25 wt%) and different surfactant concentrations (up to 40 wt%, far beyond the range of practical interest for emulsion preparation). All the systems exhibit marked shear-thinning properties which can be described quite satisfactorily by the Cross equation. The concentration dependence of the zero-shear-rate viscosity as well as the mechanical spectra confirm that, in the concentration range considered, the aqueous welan systems are typically weakly structured fluids. The influence of both surfactants is examined in detail by comparing the behavior of the different classes of systems. Both surfactants reduce the polymer contribution at low shear, whereas an opposite action is exerted at high concentration and shear. These contrasting effects are ascribed to the different structural features of the polymer matrix under low stresses and high shear conditions, respectively. Received: 6 February 2000 Accepted: 1 November 2000     

On the time-dependency of the flow-induced dynamic moduli of a liquid crystalline hydroxypropylcellulose solution

Some unusual rheological features of a liquid crystalline solution of hydroxypropylcellulose (HPC) in water have been investigated. Measurements have been performed by using a variety of different apparatuses with cone and plate geometries. Particular attention has been devoted to the experimental procedures, including the use of different sealing techniques, which are necessary to avoid solvent evaporation during the very long transients. Shear fracture effects, and their dependence on the type of sealing agents have also been studied. In steady shear, the HPC solution shows some rheological features which are common to other lyotropic systems, such as a three-region viscosity curve, and a double sign change in the first normal stress difference vs shear rate curve. The structural changes which take place after cessation of shear flow have been investigated by following the evolution of the dynamic moduli as a function of the time elapsed after the shear flow is stopped. It was found that the rate of the previously applied shear strongly affects both the kinetics and the asymptotic, long time values of the dynamic properties. Possible explanations for such behavior in terms of microstructure evolution are presented and discussed.     

Rheology and gelation kinetics of PVC plastisols

Oscillatory rheological experiments at different temperatures and over a wide range of frequencies have been used to investigate the gelation process and, more particularly, the sol–gel transition of various poly(vinyl chloride) (PVC) plastisols. The sol–gel transition process was found to be universal with respect to the temperature and solid volume fraction according to the similarity of the fractal structure in PVC plastisols. The variation of the gel time (t _gel) with temperature for any composition of PVC plastisols was predicted from the Dickinson’s model (E. Dickinson, J Chem Soc Faraday Trans, 93:111–114, 1997). Dynamic viscoelastic properties of PVC plastisols have also been studied as a function of temperature that allowed us to follow the gelation process of various plastisols. Thus, the influence of the type and concentration of PVC resins in gelation process was investigated. The variation of the complex shear modulus at a constant frequency was depicted by a master curve regarding the dependence of the moduli on PVC concentrations.     

Critical concentrations of polymers in solutions according to measurements of the viscosity and specific surface area of aerogels resulting after sublimation of the solvent

Conclusion Measurement of the viscosities of polystyrene and cyclolinear polyphenylsiloxane depending on their concentrations has revealed critical concentrations of these polymers to exist corresponding to the formation of a fluctuating continuous supermolecular network in the solutions. It has been shown on the example of polystyrene solutions that changing from a good to a poor solvent has little effect on the value of the critical concentration. Calculation of the critical concentration with the use of the parameters employed in the free volume theory after the Pezzin method shows good agreement with the critical concentrations determined from the shape of the viscosity vs. concentration dependences of polystyrene solutions.A method is described for producing polymer aerogels by sublimation of the frozen solvent from polymer solutions. It is shown that to obtain aerogels with very high specific surface areas from crystallized solutions is necessary to fulfil at least two conditions: 1. to freeze them quickly; 2. to sublimate the solvent in vacuo at a sufficiently low temperature. Special attention is drawn to the fact that the effectivity of sublimation drying should be estimated by the specific surface area of the preparations obtained as a result of drying.Mesurement of the specific surface areas of aerogels obtained under appropriate conditions from polymer solutions of different concentrations shows that with a poor solvent the aerogels have specific surface areas one decimal order lower. Hence it is concluded that solutions of polymers in poor solvents should yield stronger and less permeable polymeric systems. In all the cases studied the dependence of the specific surface area of the aerogel on the concentration of the polymer in the solution has a distinct maximum which corresponds to the critical concentration determined viscometrically. This is evidence of the correspondence between the structure of the aerogels and the structure of the initial solutions, at least, at concentrations in the region of their critical and above-critical values. Sublimation of the solvent from frozen polymer solutions results in contraction of the samples, which is the most considerable at polymer concentrations below critical. This is also connected with the relatively low specific surface areas of aerogels obtained from solutions of low concentrations.     

Process rheokinetics and microstructure of recycled EVA/LDPE-modified bitumen

Knowledge of the kinetics of the manufacturing process of polymer–bitumen blends is of great interest because it provides information on the behaviour of the binder at different stages of the mixing operation, which is useful for the establishment of the optimum processing conditions, involving temperature and operation time. The purpose of this research was to study the evolution of the rheological properties and microstructure of a polymer-modified bitumen during its processing. A 60/70-penetration grade bitumen and recycled EVA/LDPE were mixed under different processing conditions. Measurements of the evolution of viscosity with time, at different temperatures and agitation speeds, were carried out with an experimental device known as ‘rheomixer’, that is, a helical ribbon impeller inside a mixing vessel coupled with the transducer and motor of a conventional rheometer. Under the experimental conditions selected (within the laminar region, Re<10), temperature is the most important processing variable. Hence, low agitation speeds and a processing temperature of around 180°C should be chosen for bitumen modification with the polymer used.