Temperature of maximum loss

It has been asserted (14) that above their melting points, the structural relaxation times in polymer fluids would be much less than 10" sec. This proved to be true for molten PE. However, for polyethylene oxide (PEO) a temperature of maximum loss was observed at approximately 60°C at a frequency of 6.06 GHz (J5). The melting point of PEO is near 60°C. The temperature of maximum loss for bisphenol-A polycarbonate (16) was 280°C at a frequency of 5.43 GHz. The melting point of bisphenol-A polycarbonate is 240°C. Thus any general correlation between Tm and structural relaxation in fluids seems unwarranted. 

Activation Energies. We can relate measured activation energies to the size of the cooperatively rearranging regions at the temperature of maximum loss using the following relation for relaxation rates (17),... 

H - surface magnetic field strength He - azimuthal magnetic field strength Jc = critical current density Pn = loss per unit volume of normal substrate Psc = loss per unit volume of superconducting substrate Tsc = critical temperature of substrate = temperature of maximum loss... 

Dynamic storage modulus ( ) is the most important property to assess the load-bearing capability of a composite material. The ratio of the loss modulus (E") to be storage modulus (F) is known as a mechanical loss factor (tan S), which quantifies the measure of balance between the elastic phase and the viscous phase in a polymeric structure. This can relate to impact properties of a material. Generally, the tan S peak (at low frequency) is at a temperature 10-20 °C above the Tg as measured by dilatometer or differential thermal analysis (DTA). The temperature of maximum loss modulus E" is very close to Tg. 

Fig. 37 Dependence of the temperature of maximum losses (1) and activation energy (2) on the volume fraction of the second network [192]...

In the absence of any inorganic additive, the resin loses about 80% of its weight during the initial decomposition stage, this corresponding to the loss of styrene and HBr, Q3) followed by char oxidation (13-7% loss) at a DTG max (temperature of maximum rate of weight loss) of ca. 528°C. 

Table I presents the results of "isothermal" simultaneous thermoanalytical (STA) runs, at 573 K and 773 K, for all three products. Similar data, at a fixed heating rate is shown in Table II. One of the crucial parameters is the temperature of maximum weight loss rate, corresponding to the time when dehydrochlorination of PVC starts becoming important. This temperature is close to 573 K in all cases. In fact, at a relatively fast heating rate, almost no decomposition occurs at temperatures under 563 K. If the materials are heated at 573 K for a prolonged period, complete dehydrochlorination takes place, but no further stages of PVC decomposition occur. None of the three materials investigated decomposes completely until a temperature of ca. 773 K is attained. Even then only a certain fraction of the entire mass of the samples is volatilised, due to the presence of inorganic fillers in their composition.

The degradation onset and the temperatures of maximum degradation were determined from the derivative plot of weight loss versus temperature (as illustrated in figure 8) and are displayed in table 4. A composite plot for the degradation of the 4-vinylpyridine containing polymers is displayed in figure 9. 

However, mechanical measurements show that especially at low intensities, the photopolymerization process continues for a considerable time at a rate which cannot be detected by DSC. At equal doses the temperatures of maximum mechanical loss, T(tan S ax), were observed to be the same. 

DMT A measurements were made with a Polymer Labs instrument. Samples were clamped in the single cantilever mode in a frame of 22 mm using 6 mm clamps with 0.5 mm faces. The sample length between the clamps was 8 mm. Measurements were performed at a frequency of 1 Hz, a strain amplitude of 0.063 mm and a heating rate of 5 K.min . Clamping was checked by monitoring the strain amplitude on an oscilloscope. The measurements were carried out in air. Values of the temperature of maximum mechanical loss, T (tan 5max). were reproducible to 2 K. 

Symbol of Sample Type of lignin P Content, X Temperature of Maximum Rate of Mass Loss, °C Range of Temp. of Max. Rate of Mass Loss, °C 100°C Percentage of Mass Loss at Different TemDeratures 200°C 300°C 400°C 500°C 600°C... 

In all the above three polymers only a single process is apparently observed in the time window for PCS (10-6 to 100 s). The shape of the relaxation function is independent of temperature. The temperature dependence of (r) follows the characteristic parameters observed for mechanical or dielectric studies of the primary (a) glass-rubber relaxation. Relaxation data obtained by many techniques is collected together in the classic monograph of McCrum, Read and Williams41. The data is presented in the form of transition maps where the frequencies of maximum loss are plotted logarithmically... 

In addition to knowing the temperature shift factors, it is also necessary to know the actual value of ( t ) at some temperature. Dielectric relaxation studies often have the advantage that a frequency of maximum loss can be determined for both the primary and secondary process at the same temperature because e" can be measured over at least 10 decades. For PEMA there is not enough dielectric relaxation strength associated with the a process and the fi process has a maximum too near in frequency to accurately resolve both processes. Only a very broad peak is observed near Tg. Studies of the frequency dependence of the shear modulus in the rubbery state could be carried out, but there... 

Temperature of onset of degradation, as indicated by IR spectroscopy. Temperature of maximum rate of weight loss from TGA. 

The dynamic mechanical properties of four 2,6-T-2P samples containing from 19 to 43 wt% hard segments are summarized in Figure 10. A low temperature s relaxation is apparent at about — 70°C for all compositions examined. The transition temperatures of these loss maxima and the associated activation energies are given in Table III. A second process, the c relaxation, can be noted as a shoulder on the high temperature side of the 8-loss maximum. The conclusion of this relaxation is marked by a change in slope of the loss modulus vs. temperature plots... 

Criteria of Stability. Table I identifies the pulps used in this study. Table II shows that for a heating rate of 10°C/min the temperature at which 10% decomposition (10% DT) and 50% decomposition (50% DT) occurred, together with the temperature of maximum rate of volatilization (MRT). The rate of loss at this temperature is also given (MRV). Samples are ranked in terms of decreasing temperatures and decreasing rates of decomposition since it has been shown that higher rates of losses are usually associated with more thermally stable materials. 

Tmr corresponds to the temperature of maximum rate of weight loss under air atmosphere at a heating rate of 5°C/min, and D to the average bundle diameter obtained by SEM micrographs. Polyetherimide (PEI) is the compatibilizing agent. 

Fig. 3.13 The temperature of maximum dielectric loss at 1 Hz for crystalline long carbon-chain compounds, Tmax, as a function of number, n, of CH2 units in the chain. After Hoffman, Williams and Passaglia (1966). Reproduced by permission of John Wiley Sons, Inc.

Td.max Temperature of maximum rate of decomposition (weight loss), in degrees Kelvin. Td,o Temperature of initial decomposition (weight loss), in degrees Kelvin. 

Usually the primary (P) glass-rubber relaxation cannot be resolved from the secondary relaxation at hypersonic frequencies. However, this is not always the case. The Brillouin frequencies Awd) and tan 8 for polypropylene glycol (PPG) (13) are plotted versus temperature in Figure 9. Two tempratures of maximum loss are observed. The higher temperature loss at 100 °G and a frequency of 4.40 GHz correlates very well with the primary glass-rubber relaxation line determined by dielectric relaxation at gigahertz frequencies (13), The lower temperature loss at 50°G and a frequency of 5.43 GHz correlates with an extension of the secondary transition line. The transition map is shown in Figure 10. 

Ti and Tj are temperatures where 1 and S wt % of weight loss, lespectivdy, is observed, taken as referoice temperature at which no weight loss related to evqxnation process is occurring Tp is the peak temperature of maximum rate decomposition and is the cmset decomposition temperature at the crossover of tangents drawn on bodi sides of the decomposition trace. 

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