Dynamic rheological behavior of DBS-induced poly(propylene glycol) physical gels

Physical gelation can be induced in various organic and silicone-based liquids, as well as in polymeric melts, upon addition of 1,3:2,4-dibenzylidene sorbitol (DBS). Such gels are stabilized by the formation of a percolated DBS network composed of highly interconnected nanofibrils. In this study, we explore several factors affecting the rheological properties of poly(propylene glycol) (PPG) gelled by DBS. To ascertain the effect of PPG molecular weight (M_PPG) on gel formation and rheology, we have investigated three series of DBS-induced PPG gels in which M_PPG varies from 425 g/mol to 4000 g/mol. Dynamic stress measurements reveal that the DBS concentration identifying the onset of PPG gelation decreases with increasing M_PPG. Since the solubility parameter (δ_s) of PPG decreases sharply as M_PPG increases over this M_PPG range, this observation suggests that DBS gelation is sensitive to δ_s of the matrix liquid, in agreement with previously reported data collected from DBS-gelled solvents. Moreover, the elastic modulus and yield stress are found to increase with increasing (i) DBS concentration for the three series of PPG/DBS gels examined here and (ii) recovery time after cessation of an introductory shear. Received: 22 February 2000 Accepted: 9 August 2000     

Rheological properties of poly(vinyl chloride)/plasticizer systems—relation between sol–gel transition and elongational viscosity

Poly(vinyl chloride) (PVC)/di-isononyl phthalate systems with PVC content of 45.5 (PVC8) and 70.4 wt% (PVC6) were prepared by a hot roller at 150 °C and press molded at 180 °C. The dynamic viscoelasticity and elongational viscosity of PVC8 and PVC6 were measured in the temperature range from 150 to 220 °C. We have found that the storage and loss shear moduli, G′ and G″, of PVC8 and PVC6 exhibited the power-law dependence on the angular frequency ω at 190 and 210 °C, respectively. Correspondingly, the tan δ values did not depend on ω. These temperatures indicate the critical gel temperature T _gel of each system. The critical relaxation exponent n obtained from these data was 0.75 irrespective of PVC content, which was in agreement with the n values reported previously for the low PVC concentration samples. These results suggest that the PVC gels of different plasticizer content have a similar fractal structure. Below T _gel, the gradual melting of the PVC crystallites takes place with elevating temperature, and above T _gel, a densely connected network throughout the whole system disappears. Correspondingly, the elongational viscosity behavior of PVC8 and PVC6 exhibited strong strain hardening below T _gel, although it did not show any strain hardening above T _gel. These changes in rheological behavior are attributed to the gradual melting of the PVC crystallites worked as the cross-linking domains in this physical gel, thereby inapplicability of the of time–temperature superposition for PVC/plasticizer systems.     

Rheological behaviour of poly(ethylene terephthalate) and poly(butylene terephthalate) blends

The presence of up to 4% Poly(butylene terephthalate) (PBT) in blends of Poly(ethylene terephthalate) (PET) with PBT results in an increase in viscosity and a decrease in activation energy, which has been related to the entanglement density. However, further increases in the amount of PBT up to 10% result in decreases in the viscosity, which may be due to partial phase segregation. Various blending rules have been applied to correlate the experimental results and predict the viscosities of the blends.     

Viscoelastic properties of semidilute poly(methyl methacrylate) solutions

Viscoelastic properties were examined for semidilute solutions of poly(methyl methacrylate) (PMMA) and polystyrene (PS) in chlorinated biphenyl. The number of entanglement per molecule, N, was evaluated from the plateau modulus, G _N . Two time constants, _s and ₁, respectively, characterizing the glass-to-rubber transition and terminal flow regions, were evaluated from the complex modulus and the relaxation modulus. A time constant _k supposedly characterizing the shrink of an extended chain, was evaluated from the relaxation modulus at finite strains. The ratios ₁/ _s and _k / _s were determined solely by N for each polymer species. The ratio ₁/ _s was proportional to N ^4.5 and N ^3.5 for PMMA and PS solutions, respectively. The ratio _k / _s was approximately proportional to N ^2.0 in accord with the prediction of the tube model theory, for either of the polymers. However, the values for PMMA were about four times as large as those for PS. The result is contrary to the expectation from the tube model theory that the viscoelasticity of a polymeric system, with given molecular weight and concentration, is determined if two material constants _s and G _N are known.     

Nonlinear stability of timber column strengthened with fiber reinforced polymer

By taking into account the effect of the bi-modulus for tension and compression of the fiber reinforced polymer (FRP) sheet in the reinforcement layer, a general mathematical model for the nonlinear bending of a slender timber beam strengthened with the FRP sheet is established under the hypothesis of the large deflection deformation of the beam. Nonlinear governing equations of the second order effect of the beam bending are derived. The nonlinear stability of a simply-supported slender timber column strengthened with the FRP sheet is then investigated. An expression of the critical load of the simply-supported FRP-strengthened timber beam is obtained. The existence of postbuckling solution of the timber column is proved theoretically, and an asymptotic analytical solution of the postbuckling state in the vicinity of the critical load is obtained using the perturbation method. Parameters are studied showing that the FRP reinforcement layer has great influence on the critical load of the timber column, and has little influence on the dimensionless postbuckling state.     

Rheological properties of poly(vinyl alcohol)/sodium borate aqueous solutions

Rheological properties were studied for aqueous solutions of poly(vinyl alcohol) containing sodium borate. The solution exhibited a marked shear thickening at a certain critical rate of shear, _c and the flow was unstable at higher rates of shear. The loss modulus exhibited a maximum and the angular frequency corresponding to the maximum, _MAX, was approximately equal to . These features were common to several measurements including various molecular weights and concentrations of polymer and various temperatures. Possible mechanism of shear thickening was discussed based on a temporary network model composed of crosslink points of a finite lifetime.     

Structure and orientation dynamics of sepiolite fibers–poly(ethylene oxide) aqueous suspensions under extensional and shear flow,probed by in situ SAXS

The structure and orientation dynamics of sepiolite clay fibers about 1,000 nm long and 10 nm thick, suspended in an aqueous poly(ehtylene oxide) matrix of 10⁵ g/mol molecular mass, have been studied under control extensional and shear flow. A new extensional flow cell developed at the “Laboratoire de Rhéologie” and the combined rheology and small angle X-ray scattering (Rheo-SAXS) setup available at the European Synchrotron Radiation Facility have allowed access to in situ and time-resolved fiber orientations and structure properties in the volume of suspensions under flow. In the volume fractions and shear rate domains for which the suspensions exhibit shear-thinning properties, two regimes of orientation separated by a critical strain rate have been identified under extensional flow.     

A study on the melt viscosity of linear and branched poly(butyleneterephthalate)

Linear and branched PBTP samples were synthesized and characterized in terms of the intrinsic viscosity, the melt-flow-index and, for some, the melt viscosity over a range of shear rates at 250 °C.An exponent of 3.2 in the equation relating to was found for linear samples. Both linear and branched samples exhibited Newtonian behaviour over a wide range of shear rates, but for any given melt-viscosity the branched samples became shear thinning at lower shear rates than the linear ones. Correlation between a branching index,, and melt-visocity ratio (0,b/0,l) was in agreement with a previous theoretical study.     

Dispersion of multi-walled carbon nanotubes in poly（p-phenylene） thin films and their electrical characteristics

Dispersion of multi-walled carbon nanotubes in poly（p-phenylene） composite exposed to toluene was experimentally investigated. 3 mg of multi-walled carbon nanotubes with nominal size of 20 nm was compounded with 30 mg of poly（p-phenylene） with the presence of terpineol as binding initiator. To investigate an optimal condition for homogenizing all constituents, ultrasonication with an output power of 750W was employed with compounding time of 3, 10, 20 and 30 min. With FTIR analyses, it could be confirmed that homogeneous composite of multi-walled carbon nanotubes and poly（p-phenylene） could be prepared. SEM analyses were also conducted to examine the dispersion of multi-walled carbon nanotubes in the polymer matrix. Then intrinsic electrical resistance of the composites after being exposed to toluene was also investigated. It was found that the composite film prepared with ultrasonication for 20 min could provide sufficiently sensitive response with respect to varied concentration of toluene.     

Linear and branched poly(butyleneterephthalate): Activation energy for melt flow

Poly(butyleneterephthalate) (PBTP) samples of different molecular weights, both linear and branched, were synthetized by mass polymerization and studied in the molten state with a melt-flow-index apparatus at different temperatures in the range 245–270 °C. In our experimental conditions ( 20 s^–1) the behaviour of PBTP samples was Newtonian, as reported previously. The flow activation energyE _a,0 was found to increase with degree of branching: typicallyE _a,0 was about 47 and 63–79 kJ/mol for linear and branched polymers respectively.Presented in part at the IX. International Congress on Rheology, Acapulco, Mexico, October 8–13, 1984     

Rheology and structure of precipitated silica and poly(dimethyl siloxane) system

The reinforcing capability of precipitated silica in poly(dimethyl siloxane: PDMS) was characterized by means of bound rubber formation, solvent swelling, yield stress, rheological and dynamic properties. Volume concentration of precipitated silica in PDMS was varied from 0 to 0.16. The homogeneity of the compounds after mixing was confirmed by studying a uniformity of dispersion of silica particles in PDMS via SEM morphology of vulcanizates. Bound rubber measurements of the compounds and solvent swelling studies of vulcanizates showed that the precipitated silica exhibited much stronger interaction with PDMS than that of typical carbon black with rubbers but less than that of fumed silica with PDMS. At high volume concentrations of silica (0.128 and 0.160), a yield behavior was evident from the storage modulus measurements. The network formation due to an interaction between the precipitated silica and PDMS was visualized via dynamic property measurements.     

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     

Effect of maleic anhydride content on the rheology and phase behavior of poly(styrene-co -maleic anhydride)/ poly(methyl methacrylate) blends

 We investigated the thermo- rheological behavior of high glass transition, high molecular weight and small dynamic asymmetry blends of poly(styrene-co-maleic anhydride) (SMA) and poly (methyl methacrylate) (PMMA) with varying amounts of maleic anhydride (MA) content, namely 8 wt%, 14 wt% and 32 wt%, in the SMA component. The phase separation (binodal) temperature of each blend was determined rheologically using a combination of dynamic frequency and temperature sweeps in parallel plate geometry; it was marked by a change in slope of the elastic modulus and the occurrence of a peak in tan δ in temperature sweeps. Failure of the time-temperature superposition principle and observation of two peaks in the Cole-Cole plots corroborated these findings. The blends displayed lower critical solution temperature (LCST) behavior with the critical temperatures exhibiting a non-monotonic dependence on the MA content. From rheological and thermal measurements it was concluded that SMA/PMMA blends containing 14% MA were more miscible than those containing 8% or 32% MA, a finding attributed to the compositional dependence of the interplay between SMA-SMA and SMA-PMMA interactions in the different samples. MA also influenced the dynamic asymmetry and pretransitional concentration fluctuations. The phase diagrams corresponding to each blend were modeled using a two-parameter temperature dependent interaction parameter, based on the concept of generalized Gibbs free energy of mixing. The fitted values of interaction parameter were in good agreement with values calculated explicitly using the Flory-Huggins theory. Received: 16 February 2001 Accepted: 11 July 2001     

Orientation distribution of fibers and rheological property in fiber suspensions flowing in a turbulent boundary layer

A model relating the translational and rotational transport of orientation distribution function （ODF） of fibers to the gradient of mean ODF and the dispersion coefficients is proposed to derive the mean equation for the ODE Then the ODF of fibers is predicted by numerically solving the mean equation for the ODF together with the equations of turbulent boundary layer flow. Finally the shear stress and first normal stress difference of fiber suspensions are obtained. The results, some of which agree with the available relevant experimental data, show that the most fibers tend to orient to the flow direction. The fiber aspect ratio and Reynolds number have significant and negligible effects on the orientation dis- tribution of fibers, respectively. The additional normal stress due to the presence of fibers is anisotropic. The shear stress of fiber suspension is larger than that of Newtonian solvent, and the first normal stress difference is much less than the shear stress. Both the additional shear stress and the first normal stress difference increase with increasing the fiber concentration and decreasing fiber aspect ratio.     

Ultrasonic investigation of interpenetrating networks of poly(methyl methacrylate) and polyurethane

Interpenetrating networks are the most recent development in polymeric blend materials. Due to the crosslinking of both the continuous and dispersed phases, a high degree of molecular mixing is achieved in these materials. Notwith-standing that poly(methyl methacrylate)-polyurethane (PMMA-PUR) interpene-trating and semi-interpenetrating networks have been extensively investigated by Meyer et al., ultrasonic relaxation technique has been applied here for the first time. These materials were found to be highly ultrasound absorbing.It is observed that ultrasonic absorption has a peak at a particular composition of PMMA-PUR interpenetrating network. The absorption coefficient increases with frequencyf. The absorption is of relaxational nature and is not due to the scattering of ultrasonic waves by the domains of the dispersed phase. At every composition of the interpenetrating network, the/f ² vs.f curve indicates the presence of a relaxation frequency below 2 MHz and that the absorption increases with the temperature at some compositions which indicates the presence of thermal relaxation. An attempt is made to relate the absorption with the relaxation of pendent groups of polyurethane in the continuous phase.     

Rheo-optical measurements of the first and third normal stresses of homopolymer poly(vinyl methyl ether) melt

Normal stresses play a key role in polymer processing, yet accurate measurements are still challenging. Simultaneous rheo-optical measurements are conducted on a poly(vinyl methyl ether) homopolymer melt over a wide range of temperatures and oscillatory shear frequencies, in an effort to measure the normal stresses, by using quantitative flow birefringence measurements. The stress optical rule holds well for this polymer as expected, with the value of the stress optic coefficient of (6.38±0.19)×10⁻¹¹ cm²/dyn at 30°C. The first and third normal stress difference coefficients, calculated using a single memory constitutive equation applied to the stress and birefringence data, are in excellent agreement. The ratio of the measured third and first normal stress difference coefficients, (1−β)=0.71±0.05, agrees well with the result of the Doi–Edwards model with independent alignment approximation (β=0.28). The measurement of normal stress difference coefficients with such small deviations proves the robust nature of the improved rheo-optical instrument and its ability to measure complete stress tensor.     

Rheological analysis of the structural properties effecting the percutaneous absorption and stability in pharmaceutical organogels

This study presents the organogels of glyceryl monostearate emulsifiers and coconut oil as an alternative for developing the traditional organogels. We investigated how the emulsifier type affects the semisolid consistency and the drug release. In these aspects we compared glyceryl monostearate organogels (GMSO) to commercially available references, while studying the effect of the individual constituents on the structural and functional properties.Rheology provided indirect information on the structure, relevant from an application point of view. We observed that glyceryl monostearate as an organogelator results in a network with elastic nature and moderate crosslink energy. The products had low viscosity and low yield value which means practically an easily spreadable pharmaceutical dosage form with soft consistency.Modelling the percutaneous absorption in vitro, we observed that the diffusion of the piroxicam incorporated was significantly affected by the thermodynamical potency of piroxicam, which was favoured by the emulsifier. The glyceryl monostearate enhanced the partition of the suspended drug, resulting in higher drug release.This paper was presented at the first Annual European Rheology Conference (AERC) held in Guimarães, Portugal, September 11-13, 2003.