Structural changes in glassy polycarbonate induced by cyclic stresses (original) (raw)
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Journal of Polymer Science Part B: Polymer Physics, 1992
Positron annihilation lifetime spectroscopy (PALS) is used to probe structural changes in glassy polycarbonate in terms of changes in the hole volume and the number density of holes during fatigue (cyclic stress) aging. The ortho-positronium (0-Ps) pickoff annihilation lifetime T~, as well as the intensity 13, were measured as a function of cyclic stresses and various previous thermophysical aging histories. It is found that T~, the longest of the three lifetime components resolved in the PALS of glassy polycarbonate, increases when a cyclic stress is applied. These results indicate that there is a structural change during fatigue aging. The "holes" where 0-Ps can localize become larger upon fatigue aging. These results also suggest that a significant distinction exists between structural changes induced by thermophysical aging and fatigue aging. The 0-Ps annihilation intensity, which is a relative measure of the hole density in a material, showed a continuous decrease upon fatigue aging, indicating the possibility of hole coalescence, which could be a precursor of crazing. The interaction between thermophysical aging and fatigue aging corresponds very well with the enthalpy relaxation behavior as reported previously, uiz., a well-aged sample is much more sensitive to cyclic stress. More importantly, it is hypothesized that fatigue failure initiation is probably closely related to hole size and density fluctuation. Keywords: polycarbonate, stress-induced structural changes in positron annihilation spectroscopy of glassy polycarbonate after cyclic stress glassy polymer, structural changes in stress and aging positron annihilation study of
Effect of cyclic stress on enthalpy relaxation in polycarbonate
Journal of Polymer Science Part B: Polymer Physics, 1992
Possible effects of cyclic stress on physical aging in polycarbonate were investigated using differential scanning calorimetry (DSC ) measurements. When the enthalpy overshoot by DSC of specimens of different previous thermophysical aging histories is measured as a function of the cyclic stress amplitudes, two characteristic regimes are observed. By correlating with optical microscopic observations, these regimes are identified as the incubation and crazing stages (denoted regimes I and 11, respectively). The enthalpy relaxation behavior in Regime I is similar to thermophysical aging, indicating that the glassy structure as a whole is initially shifted to one where molecular mobility is retarded by relatively low amplitude cyclic stress. A strong interaction is also seen between the enthalpy overshoot and previous physical aging. That is, the more the material is previously aged, and the shorter the incubation period, the longer the crazing region is. As a result, brittle failure occurs over a wider load range compared with less aged specimens.
Physical review b, 2004
Dielectric permittivity measurements and free volume determination by means of positron annihilation lifetime spectroscopy (PALS) were used to monitor physical aging of polycarbonate (PC) far below the glass transition temperature sTgd. The effect of film thickness and cooling rate from the melt was investigated to give new insight about 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. A strong indication for a fully diffusive mechanism of physical aging is given, at least at the beginning of the aging process, where free volume holes disappear at a boundary which can either be the external surface of the sample or an internal surface defined by the presence of some low-density regions, which are created during the cooling process and have a concentration dependent on the cooling rate. In particular, the amount of internal surface is proportional to the cooling rate from the melt. Evidence for the existence of the low-density regions was provided by the absorption of ethylene glycol (EG), a molecule with a strong tendency to aggregate, in PC samples cooled down at different rates. The amount of absorbed EG was found to be proportional to the cooling rate. In addition, through dielectric spectroscopy in combination with thermogravimetric analysis, it was found that EG tends to locate in regions with large open space rather than in the bulk of PC. The physical origin for the formation of such regions might be related to the evolution of spatial heterogeneities during cooling from the supercooled state and its dependence on the cooling rate.
Fatigue Life Predictions for Glassy Polymers: A Constitutive Approach
Macromolecules, 2008
Long-term failure under constant or cyclic load is governed by the same process as short-term failure at constant rate of deformation. Failure proves to originate from the polymer's intrinsic deformation behavior, more particularly the true strain softening after yield, which inherently leads to the initiation of localized deformation zones. In a previous study we developed, and validated, a 3D constitutive model that is capable to predict the occurrence of these plastic instabilities, yielding quantitative predictions of the lifetime of polycarbonate under constant load. 1 Here we demonstrate that the same approach is also applicable to predict the life span of polycarbonate under cyclic loading conditions, over a large range of molecular weights and thermal histories, with a single parameter set only. The model incorporates the influence of physical aging, accelerated by the applied cyclic stress. For low cycle fatigue, at large stress amplitudes, where failure is thermally dominated, it is shown that the current constitutive model has to be extended to a multirelaxation time expression to properly describe the (evolution of the) energy dissipation.
Macromolecules, 2003
For the first time, load and depth sensing indentation (DSI) was used in order to monitor physical aging of bisphenol A polycarbonate for 30 months at room temperature and for 1 month at an elevated temperature. The DSI experiments were combined with differential scanning calorimetry, gel permeation chromatography, and infrared spectroscopy. The endothermic peak of polycarbonate shifted toward higher temperatures upon aging at an elevated temperature and did not change its location upon aging at room temperature. The elastic modulus and hardness of polycarbonate increased in a stepwise fashion during aging at room temperature. The molecular weight distribution broadened slightly, and the trans-trans conformational population increased during annealing. No simple correlation between changes in the mechanical properties and the shift of the endothermic peak during annealing was found. These changes seem to be caused by phenomena of different nature; namely, the changes in the mechanical properties appeared to have a reasonable correlation with free volume relaxation of the polymer, whereas the changes in the endothermic peak may be associated with internal energy changes. The similarities and differences between our results and the results of others are discussed.
The Journal of chemical …, 2005
In this study, polycarbonate (PC) and polystyrene (PS) are subjected to plastic deformation by means of cold rolling and the resulting variation of the free volume and its subsequent time evolution after rolling is investigated by means of positron annihilation lifetime spectroscopy (PALS). The value of the long lifetime component that is attributed to the decay of ortho-positronium (tau o-Ps) and its intensity (I o-Ps) are used to characterize, respectively, the size and the concentration of the free-volume holes. In addition to the PALS experiments, the effect of plastic deformation on the dynamic tensile modulus is investigated. The PALS results show that both for well-aged PC and PS an increase of to-Ps and a decrease of Io-Ps occur upon plastic deformation. During the subsequent aging, to-Ps tends to return to the value assumed before plastic deformation, while Io-Ps remains constant with time. These results corroborate the idea of an amorphous-amorphous transition, rather than that of a “mechanical rejuvenation” as proposed in the past to explain the ability of plastic deformation to reinitiate physical aging. Finally, a linear relation between the size of the free-volume holes and the dynamic tensile modulus is found, which suggests that the stiffness of amorphous glassy polymers is fully determined by their nanoscopic structure.
The effect of annealing on the local structure of glassy polycarbonate
Colloid & Polymer Science, 1985
Annealing of polycarbonate glasses at temperatures below Tg leads to an increase in yield stress and a drop in the impact strength. Although such behaviour may be related to the corresponding reduction in free volume upon annealing, variations in the wide-angle X-ray scattering curves indicate some modification to the local structure. The area of an intrachain peak at s-0.7/~l-i is monitored with respect to annealing temperature and time. It is proposed that the variations may be described by an increasing level of interlocking or (nesting) between neighbouring chain segments, a process which is a natural consequence of the molecular shape of polycarbonate.