Mkf 115

In total, 80 Celluloid sheets were aged at 70 °C and 75% RH in a fan-assisted dynamic climate chamber (MKF 115, Binder), ensuring good aeration of the samples by hanging them on glass rods. After 10 days, 40 sheets achieved a moderate condition of being slightly yellowed. After 13 days, 40 sheets showed severe discoloration and physical alteration (Figure 3). In particular, bubbles, crazes and fractures were observed in the central area of the sheets, while no visual changes were visible along the borders.
After aging, both aged and unaged sheets were kept in the dark at room temperature inside a safety storage cabinet (Q90.195.120, asecos^®) with permanent filtered (active charcoal) air ventilation.
Four analysis points were considered along the diagonal of each sheet, from its center (A) to its corner (D) (Figure 4 and Figure 5). Micro-samples with a thickness of ca. 100–200 µm for each point were taken with a surgical scalpel at the surface and at the core (at a depth of ca. 500 µm). Sampling was performed under a stereo microscope, and depth reproducibility was ensured by documenting the sheets’ section before and after sampling with a Keyence VHX-1000 digital microscope under a 5–50x magnification.

Ion conductivity was measured using an Elins Z1500J (Chernogolovka, Russia) impedance meter (frequency range 1 kHz–1.5 MHz) on symmetric carbon/membrane/carbon cells with an active surface area S of 0.5 cm². The conductivity value σ (S∙cm^–1) was calculated from the resistance R found from the impedance hodographs from the cutoff on the axis of active resistances and the geometric dimensions of the membrane according to the Equation (5) The Binder MKF 115 constant climate chamber was used to set the required humidity and temperature during measurement.
$\sigma =\frac{l}{S\cdot R}$
where l is the membrane thickness in cm, S is membrane area in cm².
A typical impedance hodograph is shown in Figure S1. We considered the electrical equivalent circuit describing this system in Golubenko, D., Karavanova, Y., Yaroslavtsev’s article [29 (link)]. With an increase in the current frequency, the polarizing contribution of diffusion layers decreases, accompanied by a decrease in the real and imaginary parts of the complex resistance. At high frequencies, the imaginary part of the impedance decreases to zero, while the real part of the impedance is equivalent to the membrane’s ohmic resistance. We found the ohmic resistance directly by extrapolating the hodographs to the active resistance axis in this work.