Viscoelastic measurements temperature

The specimen was prepared by the following method. After mixing HAF carbon black (50 phr) with natural rubber (NR) in a laboratory mixer, carbon gel was extracted from unvulcanized mixture as an insoluble material for toluene for 48 h at room temperamre and dried in a vacuum oven for 24 h at 70°C. We made the specimen as a thin sheet of the carbon gel (including carbon black) by pressing the extracted carbon gel at 90°C. The cured specimen was given by adding sulfur (1.5 phr) to the unvulcanized mixture and vulcanized for 30 min at 145°C. The dynamic viscoelastic measurement was performed with Rheometer under the condition of 0.1% strain and 15 Hz over temperatures. 

The dynamic viscoelasticity of particulate gels of silicone gel and lightly doped poly-p-phenylene (PPP) particles has been studied under ac excitation [55]. The influence of the dielectric constant of the PPP particles has been investigated in detail. It is well known that the dielectric constant varies with the frequency of the applied field, the content of doping, or the measured temperature. In Fig. 11 is displayed the relationship between an increase in shear modulus induced by ac excitation of 0.4kV/mm and the dielectric constant of PPP particles, which was varied by changing the frequency of the applied field. AG increases with s2 and then reaches a constant value. Although the composite gel of PPP particles has dc conductivity, the viscoelastic behavior of the gel in an electric field is qualitatively explained by the model in Sect. 4.2.1, in which the effect of dc conductivity is neglected. 

All the viscoelastic measurements were carried out in the Rheometrics Dynamic Spectrometer RDS-770 at a frequency of 1Hz, a strain of 0.1%, and a temperature range of -140° to 140°C incremented every 2 degrees. The Texas Instrument Terminal Silent 700 was tapped to provide a hookup to an IBM 308X main frame computer located some miles away. The output of the Rheometrics unit was converted to a data file to be used in conjuc-tion with SAS (1). All statistical manipulations, software developments, and the necessary graphics that are reported here were carried out with the aid of SAS. 

Under viscoelastic measurements poly(cycloalkyl methacrylates) show a loss maximum (designated y), located in the very low temperature range (T <-60 °C), as illustrated in Fig. 6 in the case of poly(cyclohexyl methacrylate). Such a series of polymers has been extensively studied by Heijboer in his Ph.D. thesis [5], by performing viscoelastic studies at 1 Hz (sometimes 180 kHz) as a function of temperature and exploring quite a large number of cycloalkyls, either substituted or not. In cyclopentyl, cyclohexyl, cyclohep-tyl derivatives, the y transition was shown to occur at ca. - 185 °C (180 Hz), - 80 °C (1 Hz), - 180 °C (1 Hz), respectively. The associated activation energies, a> are 13, 47, 26kJmol 1 for the cyclopentyl, cyclohexyl, cycloheptyl derivatives, respectively. 

The linear viscoelastic measurements of the end-tethered nanocomposites reveal several features unique to these materials. Time-temperature superposi-... 

Thermal analysis is capable of providing accurate information on the phase transition temperatures, degradation temperatures, heat capacity, and enthalpy of transition of polymers using comparatively simple DTA, DSC, and TG instruments. The measurement time is short compared with other techniques, such as viscoelastic measurement and nuclear magnetic resonance spectroscopy. Moreover, any kind of material, e.g., powders, flakes, films, fibers, and liquids, may be used. The required amount of sample is small, normally in the range of several milligrams. 

Figure 19 shows dynamic shear modules (G ) and loss tangent (tan 8) as a functions of temperature for carboxymethylated wood binding various metal ions [49]. The content of carboxymethyl groups in treated wood is about 1.07 mmol/g for each specimen. Dynamic viscoelastic measurements were carried out under vacuum. There are three dispersions in the range below 1(X)°C the P dispersion near 50°C for the carboxymethylcellulose main chain motion in the modified wood, the 7 diversion near 0°C for local mode of wood components related to water, and the 8 dispersion near - 60°C for the side chain... 

An empirical equation was used to predict the peak loss factor temperature of dynamic viscoelastic measurements from DSC T data. The frequency for DSC T was given an arbttuary value of 10 Hz. 

Empirical Relationship - Empirical relationships correlating glass transition temperature of an amorphous viscoelastic material with measurement temperature and frequency, such as the William Landel Ferry equation (17) and the form of Arrhenius equation as discussed, assume an affine relationship between stress and strain, at least for small deformations. These relationships cover finite but small strains but do not include zero strain, as is the case for the static methods such as differential scanning calorimetry. However, an infinitely small strain can be assumed in order to extend these relationships to cover the glass transition temperature determined by the static methods (DSC, DTA, dilatometry). Such a correlation which uses a form of the Arrhenius equation was suggested by W. Sichina of DuPont (18). 

The E value determined by dynamic viscoelasticity measurements is 2 x 1011 dyne/ cm3 at room temperature. It decreases abruptly in the temperature range 140-150°C, but the net decrease of E within this temperature range is relatively small. 

In the present paper, after a rapid presentation of the reptation model in its simplest version, in order to pinpoint the underlying hypothesis, we discuss the interest of complementary self diffusion and viscoelastic measurements, and present the currently available methods for measuring diffusion in entangled polymer systems. Then, results obtained on polydimethylsiloxane (PDMS), a model liquid polymer well above its glass temperature at room temperature will be described, and the consequences on the limits of the entangled regime as seen from diffusion measurements, compared to what is observed in rheometry, will be discussed. 

Viscoelastic measurements in oscillatory shear were performed using a Rheometrics System Four rheometer in the frequency range 10 3 rad/sec < co < 102 rad/sec. 8 mm diameter parallel plates were used for temperatures below 80°C and 25 mm diameter parallel plates were used for T>80°C, with plate separations of 1.1 0.2 mm. Comparison of... 

Another important result deals with the temperature dependence of the correlation times of the elementary motions, which agrees fairly well with the prediction of the William, Landel, Ferry equation, using the phenomenological coefficients obtained from low frequency viscoelastic measurements. Tlf s means that the elementary motions which are observed by FAD and... 

The particle size was determined using dynamic light scattering (also termed photon correlation spectroscopy PCS), using a Malvern PCS instrument. The equilibrium sediment and cream volumes were recorded using measuring cylinders at room temperature, and viscoelastic measurements were made using a Bohlin VOR rheometer. 

Photophysical and photochemical processes in polymer solids are extremely important in that they relate directly to the functions of photoresists and other molecular functional devices. These processes are influenced significantly by the molecular structure of the polymer matrix and its motion. As already discussed in Section 2.1.3, the reactivity of functional groups in polymer solids changes markedly at the glass transition temperature (Tg) of the matrix. Their reactivity is also affected by the / transition temperature, Tp, which corresponds to the relaxation of local motion modes of the main chain and by Ty, the temperature corresponding to the onset of side chain rotation. These transition temperatures can be detected also by other experimental techniques, such as dynamic viscoelasticity measurements, dielectric dispersion, and NMR spectroscopy. The values obtained depend on the frequency of the measurement. Since photochemical and photophysical parameters are measures of the motion of a polymer chain, they provide means to estimate experimentally the values of Tp and Tr. In homogeneous solids, reactions are related to the free volume distribution. This important theoretical parameter can be discussed on the basis of photophysical processes. 

In the first experiments over an extended frequency range, the biaxial viscoelastic as well as uniaxial viscoelastic properties of wet cortical human and bovine femoral bone were measured using both dynamic and stress relaxation techniques over eight decades of frequency (time) [Lakes et al, 1979]. The results of these experiments showed that bone was both nonlinear and thermorheologically complex, that is, time-temperature superposition could not be used to extend the range of viscoelastic measurements. A nonlinear constitutive equation was developed based on these measurements [Lakes and Katz, 1979a]. 

Viscoelastic measurements can be performed at a series of absolute temperatures T. The temperatures intervals need to be small when near the glass... 

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