Relaxation of Free Volume in Polycarbonate and Polystyrene Studied by Positron Annihilation Lifetime Spectroscopy

We present the time evolution of polycarbonate and polystyrene free volume probed by positron annihilation lifetime spectroscopy, which occurs after thermal rejuvenation above the glass transition temperature (T$\text{}_{g}$) and after plastic deformation via cold rolling. The ortho-positronium (τ$\text{}_{o-Ps}$) lifetime and its intensity (I$\text{}_{o-Ps}$) are used to characterise, respectively, size and concentration of the free volume holes. Our results suggest that the effect of plastic deformation on the polymer microstructure is far different from that of a thermally rejuvenated sample contrarily to common beliefs. In particular, plastic deformation results in an increase inτ$\text{}_{o-Ps}$ and a decrease in I$\text{}_{o-Ps}$, whereas rejuvenation above T$\text{}_{g}$ results in an increase in I$\text{}_{o-Ps}$ whileτ$\text{}_{o-Ps}$ remains unchanged. In addition, physical aging below T$\text{}_{g}$ after plastic deformation manifests as a reduction of τ$\text{}_{o-Ps}$, whereas a decrease in I$\text{}_{o-Ps}$ is observed after thermal rejuvenation. These results corroborate the idea of an amorphous-amorphous transition induced by plastic deformation

Diffusion mechanism for physical aging of polycarbonate far below the glass transition temperature studied by means of dielectric spectroscopy

Dielectric permittivity measurements were used to monitor physical aging of polycarbonate (PC) at about T-g - 100 K. The effect of film thickness and cooling rate from the melt was investigated to give new insight into the microscopic nature of the physical aging process. The results show that both the film thickness and the cooling rate play an important role in the kinetics of physical aging. indications of a diffusive mechanism for physical aging are given, where free volume holes can disappear either at the external surface of the sample or at an internal surface created during the cooling process. Evidence for the existence of the an internal surface is provided. (c) 2005 Elsevier B.V. All rights reserved.status: publishe

Positron annihilation lifetime spectroscopy to study the structural relaxation of PC far below the glass transition temperature

We report the dynamics of the physical aging of polycarbonate (PC) far below T-g by means of positron annihilation lifetime spectroscopy (PALS). This technique is able to detect the isothermal relaxation of the free volume after rejuvenation above T-g. The isothermal relaxation was done at different temperatures far below T-g and for different aging times. The free volume relaxation results were modeled according to a phenomenological model presented by Struik, so as to obtain the equilibrium relaxation times of the aging process at each aging temperature. The results suggest two possible scenarios for aging far below T-g: a) the relaxation process of PC is dominated by the beta process, although the fast modes of the a process remain present even at the lowest investigated temperature; b) an Arrhenius type process, resulting from cooperative dynamics of several beta processes, determines the aging far below T-g.status: publishe

Relaxation of Free Volume in polycarbonate and Polystyrene Studied by Positron Annihilation Lifetime pectroscopy

We present the time evolution of polycarbonate and polystyrene free volume probed by positron annihilation lifetime spectroscopy, which occurs after thermal rejuvenation above the glass transition temperature (T$\text{}_{g}$) and after plastic deformation via cold rolling. The ortho-positronium (τ$\text{}_{o-Ps}$) lifetime and its intensity (I$\text{}_{o-Ps}$) are used to characterise, respectively, size and concentration of the free volume holes. Our results suggest that the effect of plastic deformation on the polymer microstructure is far different from that of a thermally rejuvenated sample contrarily to common beliefs. In particular, plastic deformation results in an increase inτ$\text{}_{o-Ps}$ and a decrease in I$\text{}_{o-Ps}$, whereas rejuvenation above T$\text{}_{g}$ results in an increase in I$\text{}_{o-Ps}$ whileτ$\text{}_{o-Ps}$ remains unchanged. In addition, physical aging below T$\text{}_{g}$ after plastic deformation manifests as a reduction of τ$\text{}_{o-Ps}$, whereas a decrease in I$\text{}_{o-Ps}$ is observed after thermal rejuvenation. These results corroborate the idea of an amorphous-amorphous transition induced by plastic deformation

Dynamics of polycarbonate far below the glass transition temperature: A positron annihilation lifetime study

In this study, we have extended the knowledge of the dynamics of polycarbonate (PC) far below T-g by means of positron annihilation lifetime spectroscopy (PALS), which is able to detect the isothermal relaxation of the free volume after rejuvenation above T-g. The free volume relaxation patterns at different temperatures below T-g were modeled according to a phenomenological model presented by Struik to obtain the equilibrium relaxation times of the process at each aging temperature. The dynamics of supercooled liquids above the glass transition temperature (T-g) is usually described by the Williams-Landel-Ferry (WLF) or Vogel-Fulcher-Tammann (VFT) equation. Recently, deviations from this behavior were found below T-g. In contrast, an Arrhenius-type temperature dependence was found. Our results suggest that, far below T-g, two possible scenarios can hold: (a) the relaxation process of PC is dominated by the beta process, though the fast modes of the alpha process still manifest even at the lowest investigated temperature; (b) an Arrhenius-type process, deriving from the coupling of few beta events, drives aging far below T-g. Finally, the aging process was related to a diffusion mechanism and a coupling parameter was introduced as a measure of the efficiency of the beta process in producing appreciable changes in the density. The magnitude of the coupling parameter suggests a weak coupling between the beta relaxation and the physical aging process.status: publishe

Accumulation of charges in polycarbonate due to positron irradiation

Positron annihilation lifetime spectroscopy is a powerful technique for the study of free volume in polymers. The lifetime of ortho-positronium (o-Ps), a bound state of an electron and a positron, can be used to assess the cavity size, while the intensity can be used to characterize the number of cavities. In the Fast, results have been published, which suffer from artifacts, whereby the drop in o-Ps intensity is not related to a decrease in the number of cavities. One of the possible artifacts is the build-up of an internal charge during long-term exposure to positron irradiation, which affects the o-Ps intensity. In this study, we focus on the charging created in polycarbonate by positron radiation, which we investigated by the laser intensity modulation method and by charge-decay experiments, both isothermally and with thermal stimulated discharge. From these measurements we conclude that: (1) the o-Ps intensity decrease during prolonged positron irradiation is due to the accumulation of a space charge; (2) in reverse, changes in the o-Ps intensity may be used to probe electric fields in dielectrics. (C) 2003 Elsevier Science Ltd. All rights reserved.status: publishe