Kinetics, glass transition temperature

Migliaresi C, Fambri L, Kolarik J (1994) Polymerization kinetics, glass transition temperature and creep of acrylic bone cements. Biomaterials 15 875—881... 

The kinetic theories are dealt with in section 5.3.2. According to these theories there is no true thermodynamic glass transition. The kinetic theories predict that the glass transition is a purely kinetic phenomenon and that it appears when the response time for the system to reach equilibrium is of the same size as the time-scale of the experiment. The theory predicts that a lowering of the cooling rate will lead to a decrease in the kinetic glass transition temperature. 

A simple algorithm [17] makes it possible to find the probability of any fragment of macromolecules of Gordonian polymers. Comparison of these probabilities with the data obtained by NMR spectroscopy provides the possibility to evaluate the adequacy of a chosen kinetic model of a synthesis process of a particular polymer specimen. The above-mentioned probabilities are also involved in the expressions for the glass transition temperature and some structure-additive properties of branched polymers [18,19]. 

The so-called glass transition temperature, Tg, must be considered below this temperature the liquid configuration is frozen in a structure corresponding to equilibrium at Tg. Around Tg a rather abrupt change is observed of several properties as a function of temperature (viscosity, diffusion, molar volume). Above 7 , for instance, viscosity shows a strong temperature dependence below Tg only a rather weak temperature dependence is observed, approximately similar to that of crystal. Notice that 7 is not a thermodynamically defined temperature its value is determined by kinetic considerations it also depends on the quenching rate. 

A sample of the polymer to be studied and an inert reference material are heated and cooled in an inert environment (nitrogen) according to a defined schedule of temperatures (scanning or isothermal). The heat-flow measurements allow the determination of the temperature profile of the polymer, including melting, crystallization and glass transition temperatures, heat (enthalpy) of fusion and crystallization. DSC can also evaluate thermal stability, heat capacity, specific heat, crosslinking and reaction kinetics. 

The glass transition temperature is thus closely related to kinetic parameters and to the duration of the experiment conducted on the material. Thus, the glass transition temperature is an increasing function of the quenching rate. In practice a variation of about 10-20 K for T may be observed for the same glass (Menetrier, Hojjaji, Estournes and Levasseur, 1991). Note also that for a well defined compound which may be obtained in the form of a glass T and Tq are linked by an empirical... 

Fig. 51. Glass transition temperature of solvent-modified, semi-porous and macroporous epoxies prepared via kinetically controlled CIPS with 1 wt% catalyst...

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