Dynamic mechanical loss curves

Figure 7.4 Dynamic mechanical loss curves of polyesters prepared from PET modified with 3 mol% of CHDM (relative to PCT)...

From the behaviour of the BXT copolymers, the authors concluded that j3 blocks of seven to nine units are required for a BPA-PC-like j3 transition. Such a conclusion can be questioned since the tan 8 curves for B3T and B5T copolymers show a clear shoulder in the - 100 °C region, suggesting that in these copolymers the motions involved in the ft transition of BPA-PC can occur within the B blocks. The larger tan 8 peak around - 25 to 0 °C reflects the motions encountered in the alternate BT copolymer. However, it is worth noting that a deeper analysis of the whole set of results would require consideration of the dynamic mechanical loss compliance, /", as done in Sects. 6, 7 and 8. 

Viscoelastic measurements of ionomers have been used to indirectly characterize the microstructure and to establish property structure relationships. Forming an ionomer results in three important changes in the viscoelastic properties of a polymer. First, T usually increases with increasing ionization. This is a conseqi nce of the reduced mobility of the polymer backbone as a result of the formation of physical, ionic crosslinks. Second, an extended rubber plateau is observed in the modulus above T, again as a result of the ionic network. Third, a high temperaturi mechanical loss is observed above T, which is due to motion in the ion-rich phase. The dynamic mechan cal curves for SPS ionomers shown in Fig. 9 clearly demonstrate these three characteristics. 

Dynamic mechanical analysis curve Graphical representation of the data collected by a dynamic mechanical analyser, where the dynamic loss modulus, dynamic storage modulus and tan 5 are plotted as a function of temperature (scanning mode) or time (isothermal mode). 

Figure 11.7 shows the dynamic mechanical spectrum reported by Celli and Scandola [42] for PLLA after heating the sample at 200°C in order to erase the thermal history. The solid line refers to a sample quenched in a water-ice mixture after extrusion, while the broken line depicts an immediate rerun on the same sample, after cooling from 160°C. Below room temperature, no relaxation process is apparent in either curves, that is, the dynamic mechanical loss tangent is as low as 10 over the range —150-20°C [42]. The absence of any loss phenomena below Tg capable of mechanical energy dissipation is likely the reason for the observed brittleness of glassy PLLA and induces failure of lower molecular... 

Crystalline Dynamic Mechanical Loss Peaks 3x 10- at 1 kHz (curve flat to 1 GHz) 16a... 

Relaxations of a-PVDF have been investigated by various methods including dielectric, dynamic mechanical, nmr, dilatometric, and piezoelectric and reviewed (3). Significant relaxation ranges are seen in the loss-modulus curve of the dynamic mechanical spectmm for a-PVDF at about 100°C (a ), 50°C (a ), —38° C (P), and —70° C (y). PVDF relaxation temperatures are rather complex because the behavior of PVDF varies with thermal or mechanical history and with the testing methodology (131). 

In the case of dynamic mechanical relaxation the Zimm model leads to a specific frequency ( ) dependence of the storage [G ( )] and loss [G"(cd)] part of the intrinsic shear modulus [G ( )] [1]. The smallest relaxation rate l/xz [see Eq. (80)], which determines the position of the log G (oi) and log G"(o>) curves on the logarithmic -scale relates to 2Z(Q), if R3/xz is compared with Q(Q)/Q3. The experimental results from dilute PDMS and PS solutions under -conditions [113,114] fit perfectly to the theoretically predicted line shape of the components of the modulus. In addition l/xz is in complete agreement with the theoretical prediction based on the pre-averaged Oseen tensor. 

Figures 3-5 that the dielectric relaxation again reveals only a single a relaxation for the mixtures. These are, however, noticeably broader than the a relaxation of the pure polymers. The temperatures of the loss maxima, when plotted (Figure 7) as a function of wu the weight fraction of PPO in the mixtures, do not display the smooth monotonic increase in T0 vs. Wi that was shown by both the Vibron and the DSC results. Instead, there is a pronounced increase in Tg above = 0.5 to give a sigmoid curve for this relation. Some reservations should be attached to this observation inasmuch as data for only three polyblend compositions are available nevertheless a qualitatively similar phenomenon is observed in the analysis of the intensity of the y peak (below). Further, if only the stronger maxima in the dynamical mechanical data are considered— i.e.y if the secondary peaks and shoulders which led to the identification of two phases are omitted—then a similar sigmoid curve is found. The significance of this observation is discussed later.

The upper and lower curves for the dynamical mechanical data (110 Hz) correspond to the maxima in the resolvable loss curves. Dielectric data at 100 Hz. 

The tan 8 loss curves obtained at 1 Hz for the PET blends with the DMT and TPDE additives [13] are shown in Fig. 23. In contrast to what happens in the dynamic mechanical experiments, the additives lead to only a small shift of the curves relative to the case of pure PET and to the same peak amplitude as for pure PET. Furthermore, the activation energies derived for the p peak obtained from dielectric measurements are the same as the ones for pure PET (Table 1) and the activation entropies are in the same range (Table 2). 

The dynamic viscoelasticity and the thermal behaviour of films of Thermoelastic 125 cast from solutions in four solvents - toluene (T), carbon tetrachloride (C), ethyl acetate (E), and methyl ethyl ketone (M) — have been studied by Miyamato133 The mechanical loss tangent (tan 8) and the storage modulus E dependences exhibit two transitions at —70 °C and 100 °C which have been attributed to onset of motion of polybutadiene and polystyrene segments, respectively. The heights of the polybutadiene peaks on tan 6 curves decrease in the order C > T > E > M, while for polystyrene the order is reversed C < T < E < M. These phenomena have been related to the magnitude of phase separation of the polystyrene and polybutadiene blocks. 

In the present case, all of our dynamic mechanical data could be reduced successfully into master curves using conventional shifting procedures. As an example, Figure 7 shows storage and loss-modulus master curves and demonstrates the good superposition obtained. In all cases, the shifting was not carried out empirically in order to obtain the best possible superposition instead the appropriate shift factors were calculated from the WLF equation (26) ... 

Dielectric Measurements. The dielectric loss (c") curves at different frequencies for samples containing 100, 80, 40, and 0% PVC, respectively, are shown in Figures 4, 5, 6, and 7. Figure 8 is a composite of the dielectric loss data at 1 kHz for each sample. The general characteristics of a and p relaxation peaks of the component polymers and their mixtures parallel the results of dynamic mechanical measurements. For each... 

Another possibility of determining the gel point with the help of rheological methods is dynamical mechanical spectroscopy. Analysis of change of dynamic mechanical properties of reactive systems shows that the gel point time may be reached when tan S or loss modulus G" pass a miximum [3,4,13], Some authors proposed to correlate the gel point with the intersection point of the curves of storage and loss moduli, i.e., with the moment at which tan 5 = 1 [14-16], However, theoretical calculations have shown that the intersection point of storage modulus and loss modulus meets the gelation conditions only for a certain law of relaxation behavior of the material and the coincidence erf the moment of equality G = G" with the gel point is a particular case [17]. The variation of the viscosity... 

Figure 5. Dynamic mechanical spectrum (torsion pendulum) of a cured film of Epon 828/PACM-20. Both the shear modulus G ( Z ) and its approximation O (0) are plotted on the upper curve the lower three curves are loss modulus G" (CT), logarithmic decrement A ( ), and damping coefficient a (+j-...

Using a computerized data reduction scheme that incorporates a generalized WLF equation, dynamic mechanical data for two different polymers were correlated on master curves. The data then were related to the vibration damping behavior of each material over a broad range of frequencies and temperatures. The master curves are represented on a novel reduced temperature nomograph which presents the storage modulus and loss tangent plots simultaneously as functions of frequency and temperature. ... 

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