Physical aging of poly(vinyl acetate). A thermally stimulated depolarization current investigation

The influence of physical aging on the dielectric α-relaxation of poly(vinyl acetate) (PVAc) is investigated by means of the thermally stimulated depolarization currents (TSDCs) technique. The obtained TSDC data are analyzed in terms of the Kohlrausch–Williams–Watts (KWW) equation for an isothermal decay of a stored charge. Assuming that at temperatures less than the glass transition temperature the shape of the KWW function depends neither on the temperature nor on the thermal history, the generalized Bucci and Fieschi equation allows us to obtain the dependence of the KWW time. The results obtained are interpreted in the Adam and Gibbs theory framework.This work has been supported by the Spanish Ministry of Education (MEC) (project PB97-0638), the Government of the Basque Country (projects GV-PI98/20 and EX-1999-11), and the University of the Basque Country (projects 206.215-G20/98). The authors thank Iberdrola S.A. for the partial financial support.Peer reviewe

The Adam–Gibbs equation and the out-of-equilibrium α relaxation of glass forming systems

The temperature dependence of the α-relaxation time out of equilibrium has been investigated by means of dielectric relaxation in a series of fragile glass formers including several polymers. The influence of physical aging on this behavior has also been studied. The experimental results have been quantitatively compared with the predictions of the Adam–Gibbs equation. It has been found that, whereas for small molecule glass formers the experimental values of the apparent activation energy agree quite well with the prediction of the Adam–Gibbs equation, for polymers the experimental activation energy values are systematically higher. Moreover, whereas for small molecule glass formers the experimental values of the apparent activation energy remains essentially unaffected by physical aging, for polymers a pronounced reduction of the experimental apparent activation energy is observed. These results are found to be consistent with the Adam–Gibbs equation if a significant temperature variation of the configurational entropy in the investigated temperature range would occur for nonannealed polymers, being the possible variation hardly noticeable for the small molecules. With this assumption, all the obtained results would support the validity of the Adam–Gibbs equation for describing the temperature dependence of the time scale of the α-relaxation also out of equilibrium, at least for fragile glass formers.Peer reviewe

Dynamical and structural aspects of the cold crystallization of poly(dimethylsiloxane) (PDMS)

13 pages.A detailed study of the dynamical and structural aspects of the cold crystallization behavior of highly flexible poly(dimethylsiloxane)(PDMS) is presented. In order to understand the complete behavior, before, under and after crystallization, a wide range of experimental techniques have been employed. A particular emphasis is made on the interplay between dynamical and structural effects and how the properties of the amorphous phase evolve during the crystallization. This is highlighted by combining mobility sensitive techniques, involving broadband dielectric spectroscopy (BDS) and differential scanning calorimetry, with neutron scattering: wide and small-angle neutron scattering (WANS/SANS) which are sensitive to the relative ordering of the atoms. In this way, we are able to compare the structure associated with crystal formation with the evolution and modification of the amorphous phase. The kinetics deduced from WANS points toward a classical nucleation and growth behavior closely following a Avrami-like growth with an exponent of about n = 3 which is expected for athermal nucleation from fixed centers followed by three-dimensional crystal growth. Furthermore, the amorphous phase (deduced from BDS) decays in parallel with the emergence of the crystalline phase (from WANS/SANS) without any shift in the characteristic relaxation time. However a careful comparison of the crystallization at short times indicates that the amorphous phase seems to be affected before any measurable crystallization is detected by WANS. Although this might be compatible with the existence of mesomorphic phase, it may also be attributed to more simple precursors as initial crystalline "baby-like" nuclei. In this picture, these crystalline nuclei may be formed homogeneously in the system which in turn causes a constraint on the surrounding chains connected to these crystalline nuclei. This is manifested as a distinct relaxation contribution that is drastically slower and heterogeneous than the conventional amorphous -relaxation of the melt. It would also explain a signal in SANS before any accompanying crystallization signal in WANS. Once the crystal starts developing, the fraction of the slower amorphous phase (constrained amorphous phase, CAP) grows and the conventional amorphous phase gradually disappears. At the very end the growing crystalline fronts start to overlap and some of the remaining CAP becomes even more constrained due to cross-link strongly manifested in both the dielectric response and the heat capacity.The authors acknowledge the University of the Basque Country, the Basque Country Government (A9/UPV 00206.215-13568/2001), and the Spanish Ministery of Education (MAT 2004-01617) for support. The support of the European Community within the SoftComp Network of Excellence (NoE) program (Project No. NMP3-CT-2004-502235) is also acknowledged. Allocation of beamtime and support for the measurements carried out at the D11 and D1B instruments by ILL/NMI3 and the Spanish Initiative for Neutron Scattering (SpINS) respectively are greatly appreciated.Peer reviewe