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The method is an extension of the classic membrane inflation test (41, 42) to nanometric thin films deposited onto filter templates having micrometer size through channels. The filter templates had 10- and 20-μm-diameter channels in the current study. The 20-μm templates were used for temperatures below 110°C and for the 200-nm-thick film because of the high stiffnesses of the films. Detailed procedures for the creep compliance analysis are described elsewhere (41, 42). The inflated bubbles were monitored using an atomic force microscope, and the creep response was obtained by measuring the bubble radius as a function of time at constant pressure. The nanobubble inflation measurements were performed by following the O’Connell and McKenna (41) bubble inflation procedures. The creep experiments were performed for temperatures ranging from 63° to 135°C, following Struik’s protocol (fig. S1) (5).

A comment here is that the film at 63°C was very stiff, with the result that the heights of the inflated bubbles did not reach the membrane regime that requires a minimum bubble height of three times the film thickness to apply the membrane approximation. Prior bubble inflation studies (41, 42) report that the bending energy needs to be included in the analysis when the bubble heights are less than three times the film thickness. In Fig. 2A, the creep compliances at 63°C were calculated by membrane analysis; hence, the actual compliance would be lower than that calculated from the membrane analysis. However, the purpose of the experiments at 63°C is to show the aging response rather than the absolute creep compliance, which we could not obtain at this low temperature and high material stiffness. The results illustrate that aging occurs but the quantitative aspects are only approximate. Except for the 63°C data, the heights of all the bubbles in the current work were in the membrane regime. In addition, we also note that 63°C was the lowest temperature of the current experiments.

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