Temperature relaxation

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). 

The method (27) can best be explained with reference to Figure 2. After stretching to 10, the force f is measured as a function of time. The strain is kept constant throughout the entire experiment. At a certain time, the sample is quenched to a temperature well below the glass-transition temperature, Tg, and cross-linked. Then the temperature is raised to the relaxation temperature, and the equilibrium force is determined. A direct comparison of the equilibrium force to the non-equilibrium stress-relaxation force can then be made. The experimental set-up is shown in Figure 4. 

EVALUATION OF MELTING AND CRYSTALLINE RELAXATION TEMPERATURES OF FATTY ACID MONOLAYERS ON THE WATER SURFACE... 

Table 1. Melting temperature, Tm, and crystalline relaxation temperature, Tctc> of fatty acid monolayers on the water surface. 

Fracture toughness may correlate with the 0 relaxation temperature for the polymer. After irradiation, the 0 relaxation temperature increases with a corresponding broadening and decrease in intensity which can be seen in Figure 1. This result is consistent with the results of Hinkley et. al. (13) who observed the same phenomenon for polyether sulfone irradiated with electron beam irradiation above Tg. 

As mentioned above, spectroscopic data on a number of proteins suggest that there is a relaxation temperature at about 180 K. Unfortunately, the present data for myoglobin cluster in points far removed from this value. The data in Fig. 23 could be fitted by a single straight... 

Fukada and Sakurai (1971) measured the temperature dependence of df3 and df2 for a drawn and polarized PVDF film (Fig. 30). The relaxational behavior is somewhat masked by the rapid monotonic increase with increasing temperature on account of the decrease in the elastic modulus. Details of the curves, however, seem to indicate that d has a maximum at relaxational temperatures and, in accordance with this, d" has a maximum and a succeeding minimum at these temperatures. 

Such increases of y- and -relaxation temperatures with increasing draw ratio are thought to be due to the limited mobility of molecular chains in the noncrystalline region. As pointed out in the foregoing section, the amorphous molecular chains align fairly well parallel to the drawing direction. Therefore, the conformational versatility and mobility of such molecular chains are considered to be much restricted. 

When one considers the effect of variation of temperature during a test there are two distinct cases. First in long term tests such as creep it is important that the temperature control is very good ( 0.1°) naturally the precision of temperature control becomes more important close to relaxation temperature. Secondly in short term tests we must decide whether the experiment is conducted isothermally, or adiabatically. The moduli will of course be different in the two cases. 

Table 1. Polymethacrylates and polyacrylates measured - [CH2-CR,(COOR2)-] and their relaxation temperatures... 

The increase in the 8-relaxation temperature for 2,6-T-lP samples with increasing hard-segment content is caused by extreme broadening of the relaxation maximum attributable to increasing hard block crystallinity. These effects are similar to changes in dynamic mechanical and... 

Figure 5.2. A schematic representation of typieal alpha and beta relaxations observed from mechanical and dielectric properties. The observed relaxation temperatures increase with increasing frequency.

Activation energy Relaxation temperature (K) (kJ/mol) Molecular motion... 

The characteristics of the dynamic mechanical spectrum of SMAA show drastic changes compared with those of the aPS homopolymer even at very low molar fractions of the added comonomer. All the changes observed reflect the ionic interactions. The a relaxation temperature increases with increasing methacrylic acid content as a consequence of a stable network of chemical crosslinks due to anhydride bridge formation. The y relaxation could be related to local motion of methacrylic acid due to the breakdown of the weakest hydrogen bonds. The 3 relaxation could be attributed to local motion of the backbone chain induced by the breakdown of stronger hydrogen bonds than those invoked for the y relaxation. 

Wagner and Robeson [54] have shown that for (f> values lower than 20%, the a relaxation temperature of rubber phase, 7, is always lower than that for the pure rubber, Tar. The absolute value of the shift between Tapr and Tar decreases while the strength of the relaxation increases with increasing (f> at constant rubber weight fraction. It must be pointed out that in this research an increase in (f>, by inclusion of polystyrene sub-particles inside the rubber phase, led to an increase in the particle size of the rubber phase. 

---