Physical aging in PMMA/silica nanocomposites: Enthalpy and dielectric relaxation

Abstract

We have investigated the physical aging below the glass transition temperature, namely the slow evolution occurring in non-equilibrium glasses, of poly(methyl methacrylate)/silica (PMMA/silica) nanocomposites. To do so we have followed the time evolution of the enthalpy and that of the dielectric strength of PMMA β process during isothermal annealing. The results indicate that physical aging is generally accelerated in all nanocomposites in comparison to pure PMMA, despite the lack of effect of the nanoparticles on PMMA molecular dynamics. Furthermore, the shorter is the interparticle distance, and hence the higher is the area/ volume of silica in PMMA, the more pronounced is such acceleration. The acceleration of the physical aging together with the invariance of PMMA dynamics in the nanocomposites in comparison to pure PMMA poses serious questions on the idea that the molecular mobility is the only responsible parameter for the rapidity of physical aging. Thus, an interpretation based on the free volume holes diffusion towards the external surface, in this case represented by the polymer/silica interface, is provided.

Physical aging and glass transition of hairy nanoparticle assemblies

“…The constant T g of the PS matrix was attributed to a neutral interface with no attractive interactions between polymer and the NP. This is in contrast to strong interactions, which increase T g or repulsive interfaces which decrease T g . Herein, the phenyltrimethoxysilane functionalization of the silica terminates the hydrophilic hydroxyl‐rich silica surface with phenyl groups, and likely produces a neutral surface to PS.…”

“…Even in the glass state where segmental relaxation is localized; such factors impact the evolution of the molecular structure toward its ideal melt‐extrapolated state. For example, nanofillers have been shown to alter physical aging, as well as T g , ranging from no effect, to either an increase or decrease depending on the interactions between the polymer and NPs . These observations are generally consistent with the thin film perspective that interface interactions (repulsive or attractive) alter local mobility of segments relative to regions far from the interface, manifesting in a shift (increase or decrease) of the effective bulk glass transition temperature and physical aging rates .…”

Physical aging in polycarbonate nanocomposites containing grafted nanosilica particles: A comparison between enthalpy and yield stress evolution

“…Fillers are known to affect the physical aging behavior of the host polymer. Boucher et al showed that the addition of surface‐functionalized silica and gold particles accelerated physical aging of poly (methyl methacrylate) (PMMA) and poly(styrene) (PS), respectively . They showed that functionalization did not alter the molecular mechanism for physical aging, but increased the surface‐to‐volume ratio accelerated physical aging.…”

Free volume holes diffusion to describe physical aging in poly(mehtyl methacrylate)/silica nanocomposites

“…Available theories [8][9][10] and a number of observations 11, 12 mainly focus on the local nature of physical aging, but do not provide any explanation on the acceleration of physical aging recently encountered in thin films [13][14][15][16][17][18][19] and nanocomposites [20][21][22][23] when the typical size of the system (the thickness in thin films and the interparticle distance in nanocomposites) is of the order of (or lower) than several micrometers. In particular, Boucher et al 20,22 -monitoring the evolution of the dielectric strength of PMMA secondary relaxation 20 and the enthalpy recovery 22 occurring during physical aging of poly(methylmethacrylate) (PMMA) in nanocomposites with silica particles displaying a diameter of the order of several hundreds of nanometers-observed a) Electronic mail: swxcacad@sw.ehu.es.…”

“…[21][22][23] To do so, we have solved numerically the second equation of Fick with a free volume dependent diffusion coefficient. [21][22][23] To do so, we have solved numerically the second equation of Fick with a free volume dependent diffusion coefficient.…”