Dynamic Mechanical Thermal spectra

Similarly to the didectric analysis, the modification of the a-rdaxation spectrum during crystallization of a semirigid chain polymer can also be monitored by dynamic mechanical thermal analysis (DMTA) In this case, due to a more restricted frequency... 

Figure 1. Dynamic mechanical spectrum (left log E (Pa), right tan S vs temperature (°C)) of the thermal cure of PMDA-ODA polyamic acid film at a.) 5 °C/min. and b.) 0.5 °C/min.

The dynamic mechanical spectrum of a PAN fiber obtained by Minami [205] is shown in Figure 12.20. The real modulus at room temperature is approximately 4 x 10 MPa. The modulus begins to decrease near 75°C and drops to 2 x 10 MPa, which is a decrease of slightly over one order of magnitude, and corresponds to the onset of the glass transition. The Tg based on the midpoint of the decrease in real modulus is approximately 115°C. This is higher than the values obtained using thermal expansion coefficient (85°C) or DSC (100°C), and simply reflects the temperature shift characteristic of dynamic techniques mentioned earlier. 

The T of the H-H polyacrylates were measured by DSC, (Table 5), by the determination of the thermal mechanical spectrum and by dynamic mechanical measurements. In general, the T of the polyacrylates increased with increasing bulkiness of the substituent the H-H polymers have higher T than the H-T polymers the individual values ranged from 27° to about 40°C. higher for the H-H polymer than for the H-T polymers. Small differences have been noted between the measurements of the T by DSC as compared to TMS. ... 

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... 

The value of the coefficient will depend on the mechanism by which heat is transferred, on the fluid dynamics of both the heated and the cooled fluids, on the properties of the materials through which the heat must pass, and on the geometry of the fluid paths. In solids, heat is normally transferred by conduction some materials such as metals have a high thermal conductivity, whilst others such as ceramics have a low conductivity. Transparent solids like glass also transmit radiant energy particularly in the visible part of the spectrum. 

The ions are not only modified by the solvent, but at the same time they function as centers for the modification of the liquid structure. This fact is commonly expressed by considering such ions as "structure breakers", as they interact with the water molecules more strongly than do the water molecules between each other. IR-and NMR-spectra indicate changes which simulate the effects of a decrease in temperature of the pure wafer spectrum(12). What is termed "thermal vibrations" is actually a complicated dynamic pattern with a character all of its own that reflects the energetic influence by the environment including that of temperature as well as these of pressure, irradiation, fields, mechanical forces, drop size etc. 

Melt rheology offers a spectrum of information about the polymer (or blend), its thermal stability, shear sensitivity, the mechanism of degradation (e.g., chain scission vs. branching and crosslinking), processability, etc. The preferred method of testing is dynamic between cone-and-plate or parallel plates. ... 

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