Influence of physical aging on the thermal and mechanical behaviors of glassy polycarbonate
Polycarbonate (PC) is an amorphous plastic with very high impact strength, good ductility and fair stiffness. It is very difficult to break and the material is therefore considered fracture-proof. Due to physical aging phenomenon, the mechanical characteristics of PC products can be altered. The focus of this thesis is to study the effects of aging on the yielding and the fracture behaviors of various unmodified polycarbonates: high and low molecular-weight grades, injection molded bars, and extruded sheets. The effects of time and temperature on physical aging of PC have been firstly investigated by differential scanning calorimetry (DSC). Aging of PC is a progressive process involving several mechanisms. During annealing, rejuvenation and aging take place simultaneously. At the early stage of annealing, rejuvenation is more important and can be easily detected by DSC analysis. At a later annealing stage, aging becomes predominant and outweighs the rejuvenation process. The second part of this thesis is to investigate the effects of physical aging on the kinetics of yielding in PC. In terms of molecular movement, the yielding process is a thermally activated process involving inter- and intra-molecular motions. The time-temperature dependence of yielding behavior can be separated into two regions ([alpha] and [bêta]). Aging does not affect localized molecular motions of the [bêta] process during yielding. Heat aging also causes a decrease of the activation entropy ([delta]S) in PC, and this decrease is more important when the molecular weight is reduced. Increasing the aging time (t[subscript ag] ) and aging temperature (T[subscript ag] ) results in a continuous reduction of [delta]S . The rate of aging decreases with decreasing T[subscript ag] , and below about 30ÀC no aging takes place in yielding behavior. The kinetics of yielding and aging processes in PC are different. An increase in the strain rate does not have the same effect on the yield stress ([sigma][subscript y] ) as an increase in the aging time by a same factor. The third part of this thesis deals with the effects of physical aging and time-temperature on fracture behavior of PC. Measurements of the strain energy density showed a continuous reduction when t[subscript ag] and T[subscript ag] increase. These aging effects are confirmed by the change in fracture toughness, as measured by three-point bending tests. The variations of fictive temperature, measured by DSC, also show the same trends when t[subscript ag] changes. These results contradict the effects of aging on fracture toughness observed by the essential work of fracture (EWF) approach which shows anomalous regions of increasing fracture toughness with aging. The brittle-ductile transition in fracture behavior is analyzed by an activation energy approach. Aging increases the brittle-ductile transition temperature, and the effect is more pronounced for the lower molecular-weight sample.
- Génie – Thèses