Oscillatory sweep experiment

In the oscillatory sweep experiment, the strain amplitude is kept constant in the linear viscoelastic region (one usually takes a point far from y, but not too low, i.e. in the midpoint of the linear viscoelastic region) and measurements are carried out as a function of frequency. This is schematically represented in Fig. 3.50 for a viscoelastic liquid system. 

An alternative rheological technique for assessing flocculation involves oscillatory measurements which, as noted above, can include two sets of experiments, namely strain and oscillatory sweep measurements. 

In oscillatory measurements, two sets of experiments are carried out, namely strain sweep measurements and oscillatory sweep measurement. 

In oscillatory measurements one carries out two sets of experiments, strain sweep and oscillatory sweep, which are detailed below. 

We used the Batchelor theory in combination with the MM-Model to calculate the interfacial stress waves for a number of varying emulsion parameters and frequencies as a function of strain amplitude yo resembUng a strain sweep experiment. Initially monodisperse emulsions were considered, characterized by a single droplet radius R, the interfacial tension F and the viscosities of the eonstituents tjd and r]m. The two functions f and fz depend on the viscosity ratio A and the capillary number Ca (Eq. 11). The modelled stress oscillatory signals were Fourier transformed and the relative intensities of the third and fifth harmonic were extracted from the spectra to obtain their ratio, I5/3 = h/i/h/i as a function of strain amplitude yo. 

In this section, we present the effect of thermal history on the oscillatory shear rheometry of block copolymers. We will show that the occurrence of a minimum in G in the isochronal dynamic temperature sweep experiment does not necessarily signify OOT for highly asymmetric block copolymers instead, it sometimes reflects imperfect bcc spheres, as determined by SAXS and TEM, due to an insufficient annealing of a specimen. Here, we will show that a minimum in G, observed for an unannealed specimen in the isochronal dynamic temperature sweep experiment, may disappear completely when the specimen is annealed for a sufficiently long time at an elevated temperature below the TmoT of highly symmetric SI diblock copolymer. 

An Advanced Rheometric Expansion System (ARES, TA Instruments) was used in oscillatory shear mode with parallel plate geometry. Strain amplitude was fixed at 2% and dynamic frequency sweep experiments with angular frequency ( ) from 0.1 to 100 s were performed at 280°C. PET and all blends were tested under nitrogen atmosphere, while pure LCP, which was found not to degrade, was tested under air. The complex viscosity ( 7 ), dynamic storage (GO and loss (G") moduli were obtained. All rheological measurements are an average of four runs. 

Oscillatory frequency sweep and strain amplimde sweep experiments were first performed in range of 0.01 -100 rad/s and 0.1% - 50%, respectively in order to establish the extent of the linear viscoelastic regime. Based on these data, all subsequent experiments were... 

The evolution of the dynamic viscosity rp (co, x) or of the dynamic shear complex modulus G (co.x) as a function of conversion, x, can be followed by dynamic mechanical measurements using oscillatory shear deformation between two parallel plates at constant angular frequency, co = 2irf (f = frequency in Hz). In addition, the frequency sweep at certain time intervals during a slow reaction (x constant) allows determination of the frequency dependence of elastic quantities at the particular conversion. During such experiments, storage G (co), and loss G"(co) shear moduli and their ratio, the loss factor tan8(co), are obtained ... 

Formulating appropriate rate laws for CO adsorption, OH adsorption and the reaction between these two surface species, a set of four coupled ordinary differential equations is obtained, whereby the dependent variables are the average coverages of CO and OH, the concentration of CO in the reaction plane and the electrode potential. In accordance with the experiments, the model describes the S-shaped I/U curve and thus also bistability under potentiostatic control. However, neither oscillatory behavior is found for realistic parameter values (see the discussion above) nor can the nearly current-independent, fluctuating potential be reproduced, which is observed for slow galvanodynamic sweeps (c.f. Fig. 30b). As we shall discuss in Section 4.2.2, this feature might again be the result of a spatial instability. 

Rheology. The rheological properties of the blends and their components were determined on a Rheometrics Mechanical Spectrometer (RMS 800). Three kinds of dynamic oscillatory measurements (i.e. temperature, time, and frequency sweeps) were carried out. All experiments were done by using a parallel plate attachment with a radius of 12.5 mm and a gap setting from 1.2 to 1.8 mm. There was no significant dependence of the experimental results on the gap setting. 

In oscillatory measurements one carries out two sets of experiments (i) Strain sweep measurements. In this case, the oscillation is fixed (say at 1 Hz) and the viscoelastic parameters are measured as a function of strain amplitude. This allows one to obtain the linear viscoelastic region. In this region all moduli are independent of the appUed strain amplitude and become only a function of time or frequency. This is illustrated in Fig. 3.50, which shows a schematic representation of the variation of G, G and G" with strain amplitude (at a fixed frequency). It can be seen from Fig. 3.49 that G, G and G" remain virtually constant up to a critical strain value, y . This region is the linear viscoelastic region. Above y, G and G start to fall, whereas G" starts to increase. This is the nonlinear region. The value of y may be identified with the minimum strain above which the "structure of the suspension starts to break down (for example breakdown of floes into smaller units and/or breakdown of a structuring agent). 

---