Strain-induced low-frequency relaxation in colloidal DGEBA/SiO2 suspensions

Diglycidyl ether of bisphenol A (DGEBA) is widely exploited as an epoxy resin in adhesives and coatings. In this paper, it is used as an oligomer matrix for silica-filled nanocomposites. Rheological measurements show that the pure matrix obeys power-law relaxation dynamics in the vicinity of the dynamic glass transition of this low-molecular-weight glass former. In the filled systems, a low-frequency relaxation appears additionally to the structural α-process of the matrix. Considering the nanocomposites as Newtonian hard-sphere suspensions at low angular frequencies (or high temperatures), the modified terminal regime behavior of the matrix can be linked to strain-induced perturbations of the isotropic filler distributions. While in the low-frequency regime hydrodynamic stresses relax instantaneously, the Brownian stress relaxation is viscoelastic and can be evidenced by dynamic rheological measurements. At higher angular frequencies, the α-process of the matrix superimposes on the Brownian stress relaxation. In particular, we were able to depict the low-frequency anomaly for concentrated, semi-dilute, and even for dilute suspensions.     

Comparative rheological studies of polyamide-6 and its low loaded nanocomposite based on layered silicates

We study some rheological properties for polyamide-6 (PA-6) and a low concentrated clay nanocomposite melt based on polyamide-6 and montmorillonite. Simple shear experiments, carried out for both the neat system and nanocomposite at two different temperatures, include start up shear flows, stress relaxation after cessation of steady flow and oscillatory shear. The dynamic data for the neat PA-6 matrix differ markedly from that of the nanocomposite system, even if it has very low nanofiller concentration. Thermal stability of the PA-6 matrix imposed many restrictions on rheological studies of our systems. Therefore an experimental window was established via rheological and thermal characterization of the materials, wherein the polymer matrix was confirmed to be thermally stable. The relaxation spectra for both polymer systems were determined from linear dynamic experiments using the Pade-Laplace procedure. A rough estimation of nanocomposite volume fraction at percolation allowed us to attribute the occurrence of extra (relative to the neat polymer) Maxwell modes observed for the nanocomposite to the formation of a particulate network above the percolation threshold.     

Effect of poly(ethylene oxide) on the rheological behavior of silica suspensions

The steady-shear viscosity, dynamic viscoelasticity, and sedimentation behavior were measured for silica suspensions dispersed in aqueous solutions of poly(ethylene oxide) (PEO). For suspensions prepared with polymer solutions in which the transient network is developed by entanglements, the viscosity at a given shear rate decreases, shows a minimum, and then increases with increasing particle concentration. Because the suspensions are sterically stabilized under the conditions where the particle surfaces are fully covered with by a thick layer of adsorbed polymer, the viscosity decrease can be attributed to the reduction of network density in solution. But under the low coverage conditions, the particles are flocculated by bridging and this leads to a viscosity increase with shear-thinning profiles. The polymer chains with high molecular weights form flexible bridges between particles. The stress-dependent curve of storage modulus measured by a stress amplitude sweep shows an increase prior to a drastic drop due to structural breakdown. The increase in elastic responses may arise from the restoring forces of extended bridges with high deformability. The effect of PEO on the rheological behavior of silica suspensions can be explained by a combination of concentration reduction of polymer in solution and flocculation by bridging.     

A thermodynamic approach to the rheology of highly interactive filler-polymer mixtures. Part II. Comparison with polystyrene/nanosilica mixtures

The dynamic properties as a function of frequency and strain amplitude, steady-state viscosity as a function of shear rate, and transient shear stresses at startup and cessation of shear flow of polystyrene (PS)/fumed silica mixtures of various concentrations were investigated. An abrupt change in the viscoelastic properties was noticed at a concentration above 1% by volume. Observations by means of scanning electron microscopy (SEM) indicate the presence of a three-dimensional network through the bridging of filler particles by the adsorbed polymer. The viscoelastic behavior is simulated utilizing a theory proposed in Part I (Havet and Isayev 2001) based on a double network created by the entangled polymer matrix and the adsorbed polymer with filler concentration taken into account through the bridging density of polymer-filler interactions and a hydrodynamic reinforcement. The steps taken for determining the model parameters required to carry out the simulation are described. The major features of the rheological behavior of highly interactive polymer-filler mixtures are captured qualitatively and in some cases, quantitatively predicted.     

Small deformation viscoelastic response of gum and highly filled elastomers

Small deformation viscoelastic response has been investigated in a series of five elastomeric binders, both with and without nonreinforcing filler. The filled systems were found to be both nonlinear viscoelastic and thermorheologically complex. These behaviors suggest the existence of a secondary relaxation process. The origin of this secondary process was modeled as an interphase of polymer weakly adsorbed on the filler surface. Decomposition of timetemperature shift factors for filled vs unfilled properties showed that the mechanical response of this interphase followed Arrhenius behavior. Measured activation energies ranged from 24 to 76kJ/mole, depending on the cohesiveenergy density of the elastomeric binder. Finally, these activation energies were related to the strain amplitude dependent nonlinear factors for the polymeric systems which contained no polar groups in their backbone, suggesting that in these systems both the nonlinear and thermorheologically complex nature of the filled materials' viscoelastic response originate from relaxations within this interphase.     

Stress relaxation,creep, and internal stresses in high density polyethylene filled with calcium carbonate

The effect of filling high density polyethylene (HDPE) with calcium carbonate (up to 50% by weight) on the stress relaxation and the creep in uniaxial extension at room temperature was investigated. The addition of CaCO₃ was found to have a strong influence on the flow behaviour of HDPE. In particular, it was observed that the internal stress level, calculated from relaxation data, increased markedly with the filler content. The reduction in creep rate of the filled samples suggested that the CaCO₃-particles induce a change in the structure of the HDPE-interphase close to the filler surface. This was supported by dynamic mechanical measurements performed at low temperatures on swollen HDPE-CaCO₃ samples.     

A study of viscoelasticity and extrudate distortions of wood polymer composites

Natural fiber composites exhibit a characteristic surface tearing and extrudate distortions upon exiting from extrusion dies. This type of defect is characterized by highly rough, cracked, and distorted extrudate surface. In this study, the extrudate distortions and viscoelastic nature of metallocene-catalyzed polyethylene (mPE)/wood flour composites have been investigated. As the wood flour loading increases the region of linear viscoelasticity shortens. The first normal stress difference decreases, while the storage modulus increases. It was observed that increasing the wood flour loading up to 50 wt% aggravated the surface tearing; however, 60 wt% wood flour in mPE completely eliminated the surface defect. It was also found that increasing the shear rate improved the surface appearance of the filled compounds. This is due to the increased wall slip velocity of the composites at high shear rates and wood filler loadings. Increasing the diameter of the die at the same aspect ratio generally provides more severe surface tearing. This paper was presented at the 3rd Annual Rheology Conference, AERC 2006, April 27–29, 2006, Crete, Greece.     

Generalized Maxwell model for the viscoelastic behavior of a soda-lime-silica glass under low frequency shear loading

The linear viscoelastic behavior of a soda-lime-silica glass under low frequency shear loading is investigated in the glass transition range. Using the time-temperature superposition technique, the master curves of the shear dynamic relaxation moduli are obtained at a reference temperature of 566°C. A method to determine the viscoelastic constants from dynamic relaxation moduli is proposed. However, some viscoelastic constants cannot be directly measured from the experimental curves and others cannot be precisely obtained due to non-linearity effects at very low frequencies. The generalized Maxwell model is investigated from the experimental dynamic moduli without fixing the viscoelastic constants. A set of parameters is shown to be in good agreement with the experimental dynamic relaxation moduli, but does not give the correct values of the viscoelastic constants of the investigated glass. The soda-lime-silica glass exhibits a non-linear viscoelastic behavior at very low stress level which is usually observed for organic glasses. This non-linear behavior is questioned.     

Der Einfluß der elastischen Eigenschaften des Trägermediums auf das Fließverhalten von Kugel- und Fasersuspensionen

The influence of the elastic properties of the suspending medium on the flow of viscoelastic glass bead and glass fibre suspensions through flat orifices was investigated. The results are discussed by contrasting the flow behaviour of the viscoelastic suspensions with that of corresponding suspensions in a Newtonian suspending medium. For the suspensions in a Newtonian oil linear relationships were always found between the pressure loss and the effective velocity gradient in orifice flow. Thus it can be concluded that in this case the influence of the filler on the flow behaviour is independent of the imposed strain. Increasing the filler content or using more anisotropic particles led to higher viscosities and thus to larger pressure losses. It is well known that viscoelastic polyisobutene solutions show strain rate dependent flow behaviour. Due to the increasing influence of elasticity with increasing strain rate, the apparent flow curves can be divided into characteristic regions with different slopes. The addition of filler to such solutions altered the shape of the flow curves and it was found that the onset of “flow hardening” occurred at lower imposed strains. In addition, characteristic changes in the hardening behaviour and flow stability were observed; these were most pronounced for the fibre suspensions, even at low concentrations. For the fibre suspensions, these phenomena could be related to the influence of the fibres on the undisturbed flow field near the orifice, leading, in general, to higher strain velocities between the fibres. On the other hand, enhanced extensional strains are induced at the ends of the fibres and, at the same time, shear flow occurs along the fibres. These two effects counteract each other with respect to the hardening behaviour of the polymer. Similar effects may also occur in the glass bead suspensions, although they would certainly be less pronounced.     

Dynamic and transient rheological properties of glass filled polymer melts

The dynamic and transient rheological properties of a low density polyethylene melt and a plasticized polyvinylchloride melt filled with glass beads were measured at 200 °C and 180 °C respectively in a modified Weissenberg Rheogoniometer R-17. Its main modification consisted of the use of a piezoelectric transducer instead of the conventional torsion bar, and of the interfacing of a microcomputer Apple II plus to the Rheogoniometer for data acquisition and analysis. The glass beads were pretreated with silane and titanate coupling agents to observe the effect of the chemical modification of the polymer filler interface on these properties. It was observed that both the dynamic viscosity and the storage modulus increased with the weight fraction, but this last parameter did appreciably affect the stress growth and stress relaxation curves of the polymeric matrices at low shear rates. The effect of coupling agents on these properties was varied.