Controlled stress rheometers

Plate-plate stress rheometer test. The resin is placed between the two steel plates of a stress-controlled rheometer, maintaining a gap larger than 0.5 cm. The upper plate oscillates at a given frequency whereas the lower plate is heated. The variation of the storage and loss moduli as a function of the temperature is monitored. Softening temperature can be estimated from the temperature at cross-over between the two moduli [26]. 

The flow curves can be established for different concentrations and different molar masses of HA samples, and at different temperatures for a better insight into the molecular properties of polymers. Fig. (14) shows results of a series of rheological tests of HA polymers with different molar masses at different concentrations. Fig. (14, left panel) shows the flow curves for three different HA samples with the Mw values of 850 kDa, 600 kDa, and 400 kDa. Fig. (14, right panel) exhibits the flow curves for an HA sample at four different concentrations ranging from 0.11% to 2.16%. The flow curves are obtained by using an AR 2000 stress-controlled rheometer from TA Instruments (New Castle, DE, USA). A cone/plate geometry is used. The rotor was made of the acrylic material, 4 cm of diameter and 1° of cone angle. The measurements were performed at 20 °C. 

FIG. 9.8 Plot of shear stress vs. shear rate for a commercial LADD. The rheograms were recorded on a Carri-Med CSL 100 stress-controlled rheometer using a 4 cm acrylic parallel plate configuration with gap setting of 1000 xm. 

Rheological measurements were performed in shear using a stress controlled rheometer (Carri-Med CSL 100) operating in cone-plate geometry. Each sample is submitted successively to a first frequency sweep in range 10 3-40 Hz under 3% strain, to a creep and recovery test, and finally to a second frequency sweep identical to the first one. The dynamical strain amplitude (3%) and the value of the creep stress (chosen so as to keep the maximum strain below 10%) were set in order to remain within the linear viscoelasticity domain. Creep and creep recovery were recorded during 20 h and 80 h, respectively, times which allowed the steady state to be reached in all cases. A fresh sample was used for each solvent/temperature combination. 

Rheological experiments were carried out on a stress-controlled rheometer type CVO-120 (Bohlin, Germany) with cone-plate geometry, cone-angle a = 1 , cone radius R = 20 mm. 

The rheological measurements were performed with a Haake RS 150 shear-stress controlled rheometer and a cone-plate sensor pC60/2° Ti) at 25 0.1 °C controlled by a Haake DC 30/K20 thermostat. Three types of measurements were performed ... 

The increase in viscosity follows on from the normal decrease of viscosity with increasing shear rate seen with all suspensions. The increase then seen becomes more and more abrupt as the concentration is increased. Since the viscosity is double-valued with respect to shear stress, strange things can happen when stress-controlled rheometers are used to measure this phenomena. For instance, if a stress-sweep programme is used, the shear rate will increase, but eventually decrease. 

Dynamic viscoelastic (DVE) measurements were made with a stress-controlled rheometer, CSLIOO (Carri-MED, ITS Japan) using a cone-and-plate geometry. The storage and the loss shear moduli, G co) and G"(m), of the samples were measured at a strain of 0.3 over an angular frequency, (o, from 0.07 to 100 rad/s at T = 25 °C. 

However, the software of advanced stress controlled instruments allows for running an experiment at variable strain amplitudes. In this operation mode, several iterative cycles have to be measured before the actual measurement. In these iterations, the applied torque is adjusted to produce the desired strain amplitude [27]. In contrast to the classical way of amplitude adjustment, new operating modes of stress controlled rheometers (termed Direct Strain Oscillation or Continuous Oscillation) use a feedback control to compare the current strain signal y(t) at time t to the desired pure sinusoidal signal yd t) = y o sin(control loop then adjusts the torque accordingly in order to minimize the difference Yd t + At) — y (t -I- At)I for the next step at t -I- At. This deformation control enables a stress controlled rheometer to mimic a strain controlled experiment [27]. This holds true even beyond the linear regime where nonlinear contributions to the strain wave are compensated for and are then transferred into the stress wave, as the control loop tries to make the appropriate adjustments to the torque within minimum time. 

Merger D, Wilhelm M (2014) Intrinsic nonlineeuity from LAOStiain—experiments on various strain and stress-controlled rheometers a quantitative comparison. Rheol Acta 53(8) 621-634... 

The rheological behavior of the copolymers was measured with a DynAlyser 100 stress-control rheometer (EUieoLogica) equi] d with a cone and plate at 25 C. The radius of the cone is 40 mm, and the angle between the cone and plate is 4.0°. Steady shear and oscillatory flow measurements were conducted to obtain the steady shear viscosity and dynamic viscoelastic properties of polymer solutions. 

On the other hand, for stress-controlled rheometers, the shear stress is applied as t(0 = to(sin cot) and the resulting shear strain is measured as y(t) = yo(sin cot + S). For a purely elastic material, it follows from Hooke s law that the strain and stress waves are always in phase (8 = 0°). On the other hand, while a purely viscous response has the two waves out of phase by 90° (8 = 90°). Viscoelastic materials give rise to a phase-angle somewhere in between (Fig. 3). 

The steady shear viscosities of the filled and unfilled samples were measured using a stress-controlled rheometers, MCR 500 from Anton Parr with 25mm plate-plate fixture at low shear rate range. A capillary rheometer, Rheograph 2003 from Goettfert was used at high shear rate range. The capillary with 1 mm diameter and L/D = 30/1 was used. 

The results of Equation (3.56) are plotted in Figure 3.14. It can be seen that shear thinning will become apparent experimentally at (p > 0.3 and that at values of q> > 0.5 no zero shear viscosity will be accessible. This means that solid-like behaviour should be observed with shear melting of the structure once the yield stress has been exceeded with a stress controlled instrument, or a critical strain if the instrumentation is a controlled strain rheometer. The most recent data24,25 on model systems of nearly hard spheres gives values of maximum packing close to those used in Equation (3.56). 

Sample, stored in an appropriate clean container (sealed if solvent loss is an issue) Controlled-rate or controlled-stress rotational rheometer with Computer control and appropriate software for instrument control, data acquisition, and model fitting... 

Load a sample on a controlled-rate or controlled-stress rotational rheometer and equilibrate to the test temperature (see Strategic Planning). 

The four variables in dynamic oscillatory tests are strain amplitude (or stress amplitude in the case of controlled stress dynamic rheometers), frequency, temperature and time (Gunasekaran and Ak, 2002). Dynamic oscillatory tests can thus take the form of a strain (or stress) amplitude sweep (frequency and temperature held constant), a frequency sweep (strain or stress amplitude and temperature held constant), a temperature sweep (strain or stress amplitude and frequency held constant), or a time sweep (strain or stress amplitude, temperature and frequency held constant). A strain or stress amplitude sweep is normally carried out first to determine the limit of linear viscoelastic behavior. In processing data from both static and dynamic tests it is always necessary to check that measurements were made in the linear region. This is done by calculating viscoelastic properties from the experimental data and determining whether or not they are independent of the magnitude of applied stresses and strains. 

This instrument operates by applying an oscillatory, sinusoidal stress and records the strain (Figure 17.16). The solid line corresponds to the applied stress, controlled by the instrument, and the sample s response strain appears as the dotted line. The rheometer measures the variation in strain as a function of applied stress and reports... 

Both strain- and stress-controlled rotational rheometers are widely employed to study the flow properties of non-Newtonian fluids. Different measuring geometries can be used, but coaxial cylinder, cone-plate and plate-plate are the most common choices. Using rotational rheometers, two experimental modes are mostly used to study the behavior of semi-dilute pectin solutions steady shear measurements and dynamic measurements. In the former, samples are sheared at a constant direction of shear, whereas in the latter, an oscillatory shear is used. 

Flow curves were evaluated using a stress-controlled Reologica Stresstech rheometer equipped with a bob-and-cup geometry at 20°C. A power law model was used for curve fitting over a shear rate range of 0.1 tol59 sec. Data reported are the average of three replications. 

The melt index is an industry standard test used to assess the processability of a polymer melt. A schematic of a melt indexer is shown in Fig. 10. It is essentially a stress-controlled capillary rheometer using a weight-driven piston to force material through a round-hole capillary die. A melt index is obtained by measuring the amount of time required for a specific volume of material to be extruded from the die, with the results presented in units of g/10 min. 

Today s modern rheometers allow the precise measurement of a complex material s response to an applied force (stress) or deformation (strain). Historically, rheometers were categorized as stress controlled (applies a force and measures the resulting deformation) or strain controlled (applies a deformation and measures the resulting force) (Macosko, 1994). Advances in instrument hardware and control have resulted in versatile instruments which can perform both types of tests. Strain controlled instruments are more expensive, but they can accurately probe higher oscillation frequencies and do not require frequent inertial calibration (Kavanagh Ross-Murphy, 1998). 

Figure 6.5.2 shows a flocculated suspension with a yield stress as well. The yield stress is best measured with a stress-controlled rotational rheometer but may be confirmed by rate-controlled and capillary measurements. In Chapter 8 we discuss... 

To study suspensions, the first choice is a narrow gap, concentric cylinder rheometer. The outer cylinder should rotate to avoid inertia problems. If there are no settling, large particle, or sensitivity limitations, the cone and plate is a good second choice. For either geometry, stress-controlled instruments (see Figure 8.2.10) provide the lowest shear rate data and best measure of yield stress. Most of the stress-controlled instruments can also do sinusoidal oscillations that allow determination of Yc and structure breakdown and recovery measures (see C hapter 10). 

There are two basic designs of drag flow rheometers controlled strain with stress measurement and controlled stress with strain measurement. Below we Hrst discuss strain control and torque measurement (Section 8.2.2) followed by instrument alignment problems (Section 8.2.3) and normal stress measurement (Section 8.2.4). Then we treat special design issues for stress control. Both designs use the same type of environmental control system, as discussed in Section 8.2.6. 

LAOS measurements for two samples, a polyisoprene melt (abbreviated PI-84k, Mw = 84,000 g/mol, PDI = 1.04) and a 10 wt% solution of poly isobutylene (abbreviated PIB, Af = 1.1 xlO g/mol) in oligoisobutylene, were conducted on four different rheometers. The first two were separated motor transducer(SMT)-rheometers, namely the ARES-G2 (TA Instruments) and the ARES-LS (TA Instruments) with a IKFRTNl transducer. The DHR-3 (TA Instruments) and the MCR501 (Anton Paar) are in principle stress controlled instruments, but can be used for strain controlled experiments when using the deformation control feedback option (called continuous oscillation for DHR-3 and direct strain oscillation for MCR501). 

Both controlled-angular displacement (strain-controlled) and controlled-torque (stress-controlled) rotational rheometers are used, with the former giving superior performance at high frequencies and the latter better precision at low frequencies. There have also recently appeared on the market instruments said to be capable of operating in both modes. Controlled torque instruments can also be used to make creep and creep recovery tests, which are described in the next section. In order to obtain a linear viscoelastic characterization that includes the terminal zone, it is sometimes useful to combine data from oscillatory flow with those from a creep experiment, and this is also discussed in the following section. Rheometrical methods are described in some detail in several books [3,8,9]... 

Since it is the long-time behavior that is closely related to molecular structure, this is the information that is most interesting in the present context. For example, the zero-shear viscosity describes behavior in the limit of zero frequency and is very sensitive to molecular weight. However, for a material whose longest relaxation time is quite large, neither step-strain nor oscillatory shear experiments are useful to probe the behavior at very long times or very low frequencies. The main problem is that the stress is so small that it is not possible to measure it precisely. It is in this region that creep measurements are most useful. This is because it is possible to make a very precise measurement of a displacement, and it is also possible to apply a very small controlled stress. Controlled-torque (controlled-stress) rheometers are available from several manufacturers. 

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