Glass transition temperature from heat capacity

The glass transition temperatures and heat capacity increments for the transitions have been determined for a number of mono- and oligo-saccharides using differential scanning calorimetry. Aqueous gels and micellar fibres prepared from TV-alkylaldonamide mixtures have been studied by electron microscopy. ... 

The increase in heat capacity at the glass transition temperature is 5.4 cal deg mole or half of this amount per chain unit, which is close to average increase in heat capacity per bead of 41 glasses analyzed by Wunderlich (1960). Above the glass transition temperature, the heat capacity of polyisobutylene is about 20% higher and increases 20% faster than the heat capacity of polypropylene. One would expect this from the increased number of optical vibrations. 

Figure 3.25 shows the changes of heat capacity with temperature for the polyepichlorohydrin (PECH)/poly(vinyl acetate) (PVAc) combination at different diffusion times. In the glass transition region, the heat capacity traces are different for the different diffusion times.However, it is difficult to draw out more detailed information from these traces. The dCp/dT curves, however, clearly showed that an interface is formed by thermal diffusion, (see Figure 3.26). This is shown by the increase in the dCp/dT signal between the two glass transitions. With increasing diffusion time, the concentration of the interface will change and its thickness will increase.

Figure 4.55 shows a plot of various heat capacity data of PTT as shown in Figure 4.54 [65]. The fully amorphous point was calculated from the heat capacity of the glass and the melt, both extrapolated to the glass transition temperature. The heat of fusion of the 100% crystalline sample agrees also with a discussion of the entropies expected from similar polymers. The data points with somewhat lower ACp are most likely due to a small amount of RAF [64], frozen at the crystal interface, as indicated by the thin line.

Isotactic polystyrene can also be obtained in an amorphous modification at low temperature by quenching to below the glass transition temperature from above the melting temperature. Only Abu-Isa and Dole (1965) and Karasz, Bair, and O Reilly (1965) measured the heat capacity of amorphous isotactic polystyrene below the melting transition. Both measurements show almost identical values to the atactic sample and also a similar glass transition temperature. At about 400° K the amorphous isotactic sample is not metastable any more, but crystallizes slowly to become semicrystalline. 

Material properties can be further classified into fundamental properties and derived properties. Fundamental properties are a direct consequence of the molecular structure, such as van der Waals volume, cohesive energy, and heat capacity. Derived properties are not readily identified with a certain aspect of molecular structure. Glass transition temperature, density, solubility, and bulk modulus would be considered derived properties. The way in which fundamental properties are obtained from a simulation is often readily apparent. The way in which derived properties are computed is often an empirically determined combination of fundamental properties. Such empirical methods can give more erratic results, reliable for one class of compounds but not for another. 

Measurements of heat capacity jumps at the glass-transition temperatures, Tg, in the matrix material and the composites, carried out from heat-capacity experiments, were intimately related to the extent of the mesophase thickness. Further accurate measurements of the overall longitudinal elastic modulus of the composites and the... 

Moreover, the mesophase-volume fractions Oj for the same inclusion-contents were determined from the experimental values of heat-capacity jumps ACp at the respective glass transition temperatures T f by applying Lipatov s theory. Fig. 7 presents the variation of the differences Ars oi the radii of the mesophase and the inclusion (rf), versus the inclusion volume content, uf, for three different diameters of inclusions varying between df = 150 pm and df = 400 pm. 

Indeed, the multi-layered model, applied to fiber reinforced composites, presented a basic inconsistency, as it appeared in previous publications17). This was its incompatibility with the assumption that the boundary layer, constituting the mesophase between inclusions and matrix, should extent to a thickness well defined by thermodynamic measurements, yielding jumps in the heat capacity values at the glass-transition temperature region of the composites. By leaving this layer in the first models to extent freely and tend, in an asymptotic manner, to its limiting value of Em, it was allowed to the mesophase layer to extend several times further, than the peel anticipated from thermodynamic measurements, fact which does not happen in its new versions. 

To investigate the internal plasticization of polystyrene (Tg = 105 C) by insertion of methyl acrylate (Tg = 4°C),the samples are run on a DSC.Therefore, approximately 15 mg of each of the well dried polymers are weighed into small aluminum pans and measured by two heat-cool runs in a DSC apparatus.The glass transition is found as a characteristic jump in the heat capacity in the system.The glass transition temperature is evaluated after the second heating from the DSC plot. 

The glass transition temperature can be measured in a variety of ways (DSC, dynamic mechanical analysis, thermal mechanical analysis), not all of which yield the same value [3,8,9,24,29], This results from the kinetic, rather than thermodynamic, nature of the transition [40,41], Tg depends on the heating rate of the experiment and the thermal history of the specimen [3,8,9], Also, any molecular parameter affecting chain mobility effects the T% [3,8], Table 16.2 provides a summary of molecular parameters that influence the T. From the point of view of DSC measurements, an increase in heat capacity occurs at Tg due to the onset of these additional molecular motions, which shows up as an endothermic response with a shift in the baseline [9,24]. 

While physicochemical and spectroscopic techniques elucidate valuable physical and structural information, thermal analysis techniques offer an additional approach to characterize NOM with respect to thermal stability, thermal transitions, and even interactions with solvents. Information such as thermal degradation temperature (or peak temperature), glass transition temperature, heat capacity, thermal expansion coefficient, and enthalpy can be readily obtained from thermal analysis these properties, when correlated with structural information, may serve to provide additional insights into NOM s environmental reactivity. 

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