Glass transition, definition

A plasticizer is a substance the addition of which to another material makes that material softer and more flexible. This broad definition encompasses the use of water to plasticize clay for the production of pottery, and oils to plasticize pitch for caulking boats. A more precise definition of plasticizers is that they are materials which, when added to a polymer, cause an increase in the flexibiUty and workabiUty, brought about by a decrease in the glass-transition temperature, T, of the polymer. The most widely plasticized polymer is poly(vinyl chloride) (PVC) due to its excellent plasticizer compatibility characteristics, and the development of plasticizers closely follows the development of this commodity polymer. However, plasticizers have also been used and remain in use with other polymer types. 

Since quench rates in simulations typically are artificially high, this leads to a special problem for comparison with experiment as well as to the question whether there is a more general way to determine the glass transition temperature from the structure of the system. The experimental definition of viscosity is certainly not apphcable to simulations. 

The definition of the extent of mesophase and the evaluation of its radius r, is again based on the thermodynamic principle, introduced by Lipatov 11), and on measurements of the heat-capacity jumps AC and ACf, of the matrix material (AC ) and the fiber-composites (ACP) with different fiber-volume contents. These jumps appear at the glass-transition temperatures Tgc of the composites and they are intimately related, as it has been explained with particulates, to the volume fraction of the mesophase. 

The glass transition temperature is generally measured- by experiments that correspond to a time scale of seconds or minutes. If the experiments are done more rapidly, so that the time scale is shortened, the apparent Tg value is raised. If the time scale is lengthened to hours or days, the apparent Tg value is lowered. Thus, as generally measured, Tg is not a hue constant but shifts with the time scale of the experiment or observation. Moreover, Tg is masked by experimental difficulties, compounded by multiple and often inaccurate definitions of Tg in the literature. The least... 

Fig. 1.13. Temperature as a function of the concentration water - glycerine mixture at which phase transformations occur (Fig. 14 from [1.10]). Definitions by Luyet AE, Forming of small crystals or molecular groups E, etectic point EB, forming of clusters R, irruptive recristallization G, glass transition.

It has been suggested that glass transition is an important physicochemical event that controls the phase transition process of starch (Biliaderis, 1998). According to Biliaderis (1998), the "fringe-micelle" model (Fig. 5.16) does not permit assignment of a definite Tg for most starches. This is because the change in heat capacity during phase... 

Usually an indication of molecular mixing can be theoretically calculated based on composite Tg s. When the glass transition temperature of the SIPN was between that of the neat components, molecular interactions were assumed. A definite increase in Tg resulted in these materials. No calculated Tg can be compared since the thermoset did not show a Tg. It was interesting to note that the beta transition of the triazine network was unaffected, but the Tg for the composite was much higher relative to the thermoplastic alone. System II showed a 20 C increase in Tg, while I showed a large increase, changing from 62 C to 125°C. 

Glass transition temperature or the fictive temperature may be investigated or diagrammed using different methods, resulting in different definitions. These... 

No thermodynamic signature at Tg is evident in specific heat data at equilibrium, but a peak is observed under nonequilibrium conditions and is often taken as the definition of the glass transition. Unfortunately, this nonequilibrium peak cannot be addressed within the LCT of glass formation. We strictly avoid a discussion of the specific heat, given the complications of interpreting these data for polymer materials and the omission of the important vibrational component in the LCT treatment. 

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