Mechanical relaxations in heat‐aged polycarbonate. Part I: Comparison between two molecular weights

Abstract

As part of a wider research program related to polycarbonate embrittlement, the effects of heat-aging on mechanical relaxation behavior have been studied by examining the relationship between secondary transitions and stress relaxation behavior. In this Part I, the differences in response between two molecular weight polycarbonates (PC) are compared for injection molded samples. Dynamic mechanical spectra showed that the presence of an intermediate p transition (-80°C) is strongly dependent on molecular weight and heat-aging. However, the p1 (35°C) and the y (-100°C) peaks are generally insensitive to either effect. The study also attempted to interpret the similarities and differences in relaxation response using free volume and conformational change arguments, which have been subject to much scrutiny. Using the KWW stretched exponential to characterize stress relaxation, it appeared that bulk free volume recovery concepts could explain differences in stress relaxation response but not the corresponding losses in toughness. Hence, it is proposed that changes in relaxation response are most likely due to an interplay of relatively large scale molecular volume and molecular conformation processes that affect intermolecular cooperativity. These high-activation processes are related to the broad p region I P1 < T < T,).

Mechanical and thermal properties of polycarbonate, part 1: Influence of free quenching

“…The existence of the β 1 mode for the quenched samples should be evidenced in the future by complementary studies at lower frequencies, for example, or by a dielectric study. Lee‐Sullivan has already observed β 1 and β modes at the same time during a DMA study of PC; this tends to prove that the β 1 mode exists independently of the thermal treatment 32. The existence of this relaxation mode at about 35°C should be at the origin of the behavior of PC samples after a second quenching; the difference between the first and the second quenching is probably linked to the relaxation time of the molecular segments which are moving at 40°C.…”

“…Lee‐Sullivan et al as well as other authors have observed this peak and they have concluded that it was insensitive to thermal history, elusive and unaffected by thermal ageing 32, 38, 39. The origin of this relaxation mode remains unclear; it is simply described to be due to segmental motions of intermediate length, i.e., between long chain motions occurring at the α glass transition and short chain motions responsible for the γ relaxation 32…”

“…Nevertheless, a maximum of rupture properties is observed for a second quench at 40°C. This behavior could be linked to the existence of a molecular relaxation around 35°C 32. A DMA study of pure PC samples submitted to the same thermal treatments has revealed the existence of such a molecular relaxation.…”

High Frequency Rheometry of Viscoelastic Coatings with the Quartz Crystal Microbalance

“…A strength of our technique is that it is very sensitive to the low phase angles that are typical of glassy polymers. As a result, physical and chemical aging processes, such as shifts in the β relaxation 40 that affect the fracture behavior of glassy polymers, 41 can be readily investigated. The phase angles for each cross-linker ratio (sample 1 from Table 1 in each case) are plotted as a function of curing time in Figure 11.…”

Mechanical and thermal properties of polycarbonate. II. Influence of titanium dioxide content and quenching on pigmented polycarbonate

“…As for the case of neat PC, this behavior could be linked to the existence of a molecular relaxation located around 358C. 11 We will recall here that DMA experiments presented in part I have demonstrated the existence of this relaxation in neat PC specimens submitted to annealing treatments.…”