Activation energies, glass transition polystyrene

Glass transitions involve mainly the onset or freezing of cooperative, large-amplitude motion and can be studied using thermal analysis. Temperature-modulated calorimetry, TMC, is a new technique that permits to measure the apparent, fiequency-dependent heat capacity. The method is described and a quasi-isodiermal measurement method is used to derive kinetic parameters of the glass transitions of poly(ethylene terephthalate) and polystyrene. A first-order kinetics expression can describe the approach to equilibrium and points to the limits caused by asymmetry and cooperativity of the kinetics. Activation energies vary from 75 to 350 kJ/mol, dependent on thermal pretreatment. The preexponential factor is, however, correlated with the activation energy. 

Fig. 20 Apparent activation energy of glass transition (a-relaxation) vs activation energy of p-relaxation dependence obtained at 10-2-1 Hz obtained by DSC or DMA techniques for polymers (1) polyisoprene (2) poly(dimethylsiloxane) (3) polyethylene (4) polypropylene (5) polyamide-6 (6) poly(ethylene terephthalate) (7) poly(vinyl acetate) (8) poly(vinyl chloride) (9) poly(vinyl alcohol) (10) poly(methyl methacrylate) (11) boron oxide glass (12) polyacrylonitrile (13) polycarbonate (14) poly(cyclohexyl methacrylate) (15) poly(a-methylstyrene) (16) polystyrene (17) polyarylate (18) polyimide I (19) polyimide II (20) polyimide III (21) poly(m-phenylene isoph-thalamide)...

APPJ could also be successfully used for the improvement of surface adhesion due to the activation of the surface. Dowling et al. [50] reported results of treatment of PP, polystyrene (PS), and polycarbonate (PC) substrates with air plasma. It was found that in the case of PP and PC polymers, the activation energy of the surface could be increased, whereas the surface energy of PS was decreased hy the plasma treatment. This unexpected result could he attrihuted to some decomposition of the polymer surface due to relatively low-glass transition tem-... 

In the introduction of this communication we pointed out the similarities between the pressure dependence, the activation energy, and the annealing effects of the (Tp< Tg) and the (T/ > Tg) transitions, which strongly suggests that these transitions are the manifestation of the same relaxation process, probably issued from the complexity of the kinetic mechanism responsible for the glass transition temperature itself. In the second part of this communication we present and reanalyze DSC studies of atactic Rheomolded polystyrene specimens treated with various thermal-mechanical histories. The effect of frequency, amplitude of vibration, and annealing time... 

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