We investigated the kinetics of enthalpy recovery of several glass-forming polymers at temperatures significantly below the glass transition temperature (Tg) and for aging times up to one year. We find a double-step recovery at relatively low aging temperatures for the longest investigated aging times. The enthalpy recovered after the two-step decay approximately equals that expected by extrapolation from the melt. The two-step enthalpy recovery indicates the presence of two time scales for glass equilibration. The equilibration time of the first recovery step exhibits relatively weak temperature dependence, whereas that of the second step possesses pronounced temperature dependence, compatible with the Vogel-Fulcher-Tammann behavior. These results, while leaving open the question of the divergence of the relaxation time and that of a thermodynamic singularity at a finite temperature, reveal a complex scenario of glassy dynamics.

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Physical aging is a ubiquitous phenomenon in glassy materials and originates from the fact that they are generally out-of-equilibrium. Due to the technological and fundamental implications, this phenomenon has been deeply investigated in the last decades especially in glassy polymers. Here we provide a critical review of the latest hot debated themes in the field of physical aging in polymers and polymer nanocomposites. We first summarize the fundamental aspects of physical aging, highlighting its relationship with the polymer segmental mobility. A review of the methods employed to monitor physical aging is also provided, in particular those probing the time dependent evolution of thermodynamic variables (or related to) and those probing the (quasi)instantaneous polymer segmental mobility. We subsequently focus our attention on the two following debated topics in the field of physical aging of polymers: (i) the fate of the dynamics and thermodynamics of glassy polymers below the glass transition temperature (T g ), i.e. the temperature below which physical aging occurs; (ii) the modification of physical aging induced by the presence of inorganic nanofillers in polymer nanocomposites. With respect to the former point particular attention is devoted to recent findings concerning possible deviations from the behavior normally observed above T g of both dynamics and thermodynamics deep in the glassy state.Regarding the effect of the presence of nanofillers on the rate of physical aging, the role of the modification of the polymer segmental mobility and that of purely geometric factors are discussed with particular emphasis on the most recent advances in the topic. The modification of the rate of physical aging in other nanostructured systems, such as polymer thin films, is discussed with particular emphasis on the analogy in terms of a large amount of interface with polymer nanocomposites.Daniele Cangialosi is a Tenured Scientist at the Material Physics Centre (Joint Centre of the UPV/ EHU and CSIC). He obtained his PhD at the University of Palermo and later moved to the Netherlands, Technical University of Del, for a 3 year post-doctoral fellowship. Before obtaining his current position, he was a postdoc at the Donostia International Physics Center (DIPC) and at the Material Physics Centre in San Sebastián (Spain). His specialization is in dielectric relaxation spectroscopy and calorimetry techniques. The focus of his recent research activity is the problem of the glass transition in the bulk and under nanoscale connement.

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We performed a systematic study on the recoverable enthalpy in several glass-forming polymers. We found that after prolonged isothermal physical aging the enthalpy reaches a plateau with values substantially larger than than those corresponding to the enthalpy state extrapolated from the melt state. Enthalpy recovery experiments after up-jumps indicate that the enthalpy state corresponding to the plateau found after simple down-jump experiments is restored after long-term aging. This result is interpreted considering the plateau in the enthalpy as a thermodynamically stable state. We argue on the possible scenarios emerging from this conclusion. In particular, we discuss whether polymer glasses in the achieved thermodynamic state are stable over any time scale, or rather this corresponds to a relative minimum with further evolution at much larger time scales. Finally, the shift factor obtained from aging time–temperature superposition of enthalpy recovery data was found to considerably deviate from the Vogel–Fulcher–Tammann equation, normally adequate to describe the segmental mobility above the glass transition temperature (T g). The deviation of thermodynamics and dynamics from the behavior expected extrapolating the behavior from above T g has been analyzed within the Adam–Gibbs framework, which actually relates the relaxation time and a thermodynamic magnitude, namely the configurational entropy. It has been found that, at least semiquantitatively for most of the investigated polymers, the connection between dynamics and thermodynamics holds also below T g.

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The physical aging of polystyrene (PS) free-standing films has been investigated as a function of the films thickness, ranging from several micrometers to tens of nanometers. In this range of thicknesses, unchanged segmental dynamics in comparison to the bulk was previously reported. This study has been carried out through differential scanning calorimetry (DSC), by following the enthalpy recovery to monitor the physical aging process at different temperatures. The temperature marking the onset of nonequilibrium effects, that is the onset of the glass transtion temperature, T g on , was also assessed, at different cooling rates. An acceleration of the physical aging process, and consequently a depression of T g on , is found with decreasing the films thickness, already for thicknesses in the micrometer range. Moreover, the onset of nonequilibrium effects is shown to be cooling rate dependent, this being more pronounced when the PS films get thinner. The thickness effects on the typical signatures of the out-of-equilibrium dynamics of the films, namely their physical aging and T g on , can be well accounted for by assuming an equilibration mechanism based on volume holes diffusion toward the interfaces of the films. The temperature dependence of the diffusion coefficient obtained within this framework is found to crossover from Vogel−Fulcher− Tammann (VFT) to Arrhenius when decreasing the temperature. The implications of these results are discussed. ■ INTRODUCTIONAmorphous polymer glasses always experience a slow evolution of their thermodynamic state toward equilibrium by a loss of the excess in the thermodynamic properties (volume, enthalpy, entropy). 1 This phenomenon, referred to as physical aging, induces an alteration of the materials engineering properties depending on the thermodynamic properties (e.g., mechanical, dielectric, diffusive properties), 2−4 and thus may result in technological troubles. These aging phenomena occur in laboratory times at temperatures some tens of degrees below the glass transition range, separating the metastable equilibrium state of the supercooled liquid from the out-of-equilibrium glassy state. This temperature range usually extends over few degrees but is conveniently characterized by a temperature referred to as the glass transition temperature, T g .Physical aging is a general feature of glassy systems, and is of special relevance in polymers. 1 Of particular interest nowadays is the physical aging behavior of polymers with thicknesses ranging from the micrometer to the nanometer range because of the great variety of technological applications based on glassy polymer films (nanoimprinting, optical coatings, photovoltaics, memory storage devices, etc.). 5−7 Consequently, a better fundamental understanding of this phenomenon in polymer materials confined to the micro-or nanoscale is required.Despite the vast interest of the scientific community for nanostructured systems in the last few decades, their physical aging and glass transition behaviors are still a matter of debat...

In this work, we have studied the effect of silica particles on the physical aging of nanocomposites based on poly(methyl methacrylate) (PMMA). To do that, we have followed the enthalpy relaxation by means of differential scanning calorimetry (DSC). In agreement with previous results carried out by means of broadband dielectric spectroscopy (BDS), we observe an acceleration of the physical aging process of PMMA nanocomposites in comparison to bulk PMMA. The fitting of the enthalpy relaxation results to the well-known Tool-Narayanaswamy-Moynihan (TNM) model gives rise to equal structural parameters for all investigated samples, including bulk PMMA. This implies that the molecular mechanism for physical aging in PMMA is not affected by the presence of silica particles. The only parameter changing is the pre-exponential factor setting the time scale of physical aging. The values obtained are correlated to the area/volume ratio of silica particles in the polymer and thereby to the silica interparticle distance in the nanocomposites. This latter observation is an indication that the physical aging process is driven by the diffusion of free volume holes toward polymer interfaces, as already proposed in the past.