Damping peak

Figure 5. Effect of electron radiation fluence on the Tg damping peak width (at half height). (Reproduced from reference 8.)...

Another class of co-reactants leads to entirely different behavior, however. By providing a redox pathway to phthalocyanine (7), hydroquinone promotes a very different network structure, and this difference shows up clearly in Figure 4. This resin still shows the cryogenic damping peak, the +50°C peak which has been attributed to crosslinked structures is very prominent, but the Tg is hardly visible. 

The Tg of the neat thermoset was undetected. The triazlne network showed a small damping peak between -60 to -80 C. This was attributed to a beta transition due to rotation of the blsphenol-A unit. Other workers had reported glass transition points for the cured material at 2U °C. The conversions of those reactions were lower than encountered here. These results may indicate mobility of branch points ending with unreacted cyanates. 

The higher the temperature above Tg, the more we are removed from the damping peak, so the lower is tan 5. 

Fig. 20. Logarithmic decrement (related to tan 8 and loss modulus) vs temperature for a fluorocarbon dibenzoxazole (148). After drying up to 200°C, the experiment was conducted at 200 — —180 —> 200°C AT/ At = =tl.5°C/min in ahelium atmosphere. The T gives a sharp damping peak, whereas the...

Woodward and Sauer (1958), have reviewed the many studies of polyvinyl chloride. The only correlation that can be seen between the mechanical and calorimetric data is related to the glass transition which occurs at 92, and 107° C as detected by mechanical damping peaks using 0.67 and 500 cps respectively. These temperatures are to be compared... 

From the specific heat data of Wilhoit and Dole (1953) on the polyamide, 6-6 Nylon, there is no hint of the strong damping peak found by Woodward, Sauer, Deeley and Kline (1957) at 77° C. 

Figure 10.7 Comparison of the damping peaks of tetraglycidyl methylene diamine and diamino diphenyl sulphone (TGMDA-DDS epoxy network) (O) and poly(bismaleimide) (BMI network) ( ).

Earlier investigations on the dynamic mechanical properties of PPMI over a wide range of temperatures indicated the existence of two distinct relaxations at around 250 and 400 K [15,16]. The former relaxation was assigned to adsorbed water molecules in the polyimide chain and the latter was due to local relaxation modes of the backbone. Recently Ahlbom reported the mechanical relaxation of various polymers at low temperatures [17]. In the film sample of PPMI, the small relaxation at 93 K. attributed to the motion of phenyl rings was observed, although the dominant damping peak at 198 K is not yet explained. 

The sequence of curves in Figures 5-11 shows the stages of partial phase separation that occur in an amorphous multiblock polymer with polydisperse hard blocks. By systematically varying the block molecular weights and copolymer composition, the breadth and shape of the damping peaks changed in a systematic and predictable way. Controlled partial phase separation would appear to be a powerful way to tailoring polymer properties for a particular application. 

The resilience of a polymer will be high (i.e. tan <5 is small) in temperature regions where no mechanical damping peaks are found. This applies in particular to rubbery networks (T Tg), which therefore possess a high resilience. Various rubbers behave quite differently at room temperature the rebound resilience is for natural rubber, butadiene-styrene rubber and butyl rubber high, medium and low, respectively. In practice this means that tyres for cars must have medium rebound resilience high rebound resilience causes bumping on the road, whereas low rebound resilience causes a tyre to become very hot. 

In many investigations dynamic-mechanical properties have been determined not so much to correlate mechanical properties as to study the influence of polymer structure on thermo-mechanical behaviour. For this purpose, complex moduli are determined as a function of temperature at a constant frequency. In every transition region (see Chap. 2) there is a certain fall of the moduli, in many cases accompanied by a definite peak of the loss tangent (Fig. 13.22). These phenomena are called dynamic transitions. The spectrum of these damping peaks is a characteristic fingerprint of a polymer. Fig. 13.23 shows this for a series of polymers. 

The temperature at which the damping peak occurs is not the same as that at which the discontinuous change in a thermodynamic quantity is found. The damping peak will always nearly coincide with the point of inflection of the modulus-temperature curve, whereas the conventional transition temperature is at the intersection of the two tangents... 

If absorption is measured not too close to the damping peak (tan 8) both longitudinal and shear absorption often increase linearly with frequency. This is demonstrated in Fig. 14.5 for polyethylene (Hartmann and Jarynski, 1972). This phenomenon is called hysteresis behaviour in acoustics (it does not refer to absorption as is common among polymer scientists (Hartmann, 1990). At any frequency shear absorption is much higher than longitudinal absorption. 

The Gi-values in Table 14.6 demonstrate that the backbone motions and the moieties attached directly to the backbone, contribute the most to the damping peak and that long side chains act as "diluants". 

Figure 4. Shift of damping peak with frequency for an epoxy polymer. (Reproduced with permission from Ref. 11. Copyright 1984 Elsevier Applied Science Co. Inc.)...

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