Departure from the Vogel behaviour in the glass transition—thermally stimulated recovery, creep and dynamic mechanical analysis studies

Abstract

Elsevier Alves, N.; Mano, J.; Gómez Ribelles, JL.; Gómez-Tejedor, JA. (2004). Departure from the Vogel behaviour in the glass transition -thermally stimulated recovery, creep and dynamic mechanical analysis studies. Polymer. 45(3): 1007-1017. doi:10.1016/j.polymer.2003.04.002. N.M. Alves et al. / Polymer 45 (2004 AbstractIn this work the study of the dynamics of the segmental motions close to T g of a poly(methyl methacrylate), PMMA, network was analysed by distinct mechanical spectroscopy techniques. Three techniques were employed: dynamic mechanical analysis (DMA), creep and thermally stimulated recovery (TSR). The timetemperature superposition principle was applied to the DMA and creep results, and master curves were successfully constructed. A change from a Vogel to an Arrhenius behaviour was observed in these results.Above T g it was found a distinct temperature dependence for the retardation times calculated from creep and the relaxation times calculated from DMA. This unexpected behaviour was attributed to the merging of the and the relaxations that occurs in PMMA systems. The activation energies (E a ) were also calculated from DMA, creep and TSR experiments. Above T g the E a values obtained agreed very well for all the techniques. In addition, the fragility exhibited by this material was investigated by the mechanical spectroscopy techniques referred above and by differential scanning calorimetry (DSC). The obtained values of the fragility index m indicated that the PMMA network is a kinetically fragile system. The thermodynamic manifestation of the fragility was also analysed.

Dielectric and thermal studies of the segmental dynamics of poly(methyl methacrylate)/silica nanocomposites prepared by the sol–gel method

“…By following the evolution of the b relaxation toward the high-frequencies region, a change of the slope can be detected at about 358 K. This change reflects the beginning of the merging of the b with the a relaxations, which is a typical behavior for PMMA and acrylates in general. 17,[46][47][48]63 The observation of this change verifies the partially co-operative character of the b relaxation, which is strongly affected by the forthcoming segmental dynamics. Another point that should be considered is the second change of the slope, observed at lower temperatures (at about 303 K) mainly for the nanocomposites.…”

“…An interesting point in the Arrhenius plot is the region of the merging of a and b relaxations in a complex ab, as mentioned earlier. 17,[46][47][48] This complex process is faster than the b relaxation (smaller slope) and slower than the a relaxation (bigger slope). Also, the ab relaxation, even if it is attributed to the rubber state of the polymer above T g and it is originated from segmental molecular motions, is better described by an Arrhenius equation [eq.…”

“…46 This observation is very important for the study of the PMMA, because it reveals that the b relaxation has a partially co-operative character; therefore, several studies focused on this temperature region. 22,[46][47][48][49][50] This kind of secondary relaxation with a co-operative character is known as Johari-Goldstein relaxation 51 (JG b-relaxation here) and has been observed in many different systems under various conditions. 52 The b relaxation (Figure 4) is the dominant relaxation in the spectrum, because the molecular group that creates it has a very strong dipole moment; therefore, the material relaxes mainly through this relaxation.…”

“…(5)]. 47 Such behavior is typical for secondary relaxations and not for segmental processes. The equations used for describing the dependence of each relaxation from the temperature are the Arrhenius equation:…”

Temperature and time effects on the structural properties of a non-aqueous ethyl cellulose topical drug delivery system

“…Empirical rheology also indicates a rubbery state for the data in Fig. 3a, instead of a glassy consistency, as it pertains to oscillatory values of the complete dynamic-modulus function from the master curve of viscoelasticity (Alves, Mano, Gomez Ribelles, & Gomez Tejedor, 2004). However, the alternative framework of the reaction rate theory that translates to the following equation (Kasapis, 2008):…”