*3.1. Materials*

Dibutyltin-dilaurate catalyst (Sn\_DL)—Tib kat 218 (produced by TIB Chemicals) and Hempel´s Nexus II 27400 was kindly provided by Hempel. Linear, trimethoxysilaneterminated polysiloxanes ((MeO)3Si-PDMS-Si(OMe)3)—Silmer TMS Di-10, Silmer TMS Di-50, and Silmer TMS Di-400), were purchased from Siltech. The molecular weights of (MeO)3Si-PDMS-Si(OMe)3 were determined by SEC and 1H NMR (Tables 1 and A1). The names of the polysiloxane cross-linkers are shortened to Di-10, Di-50, and Di-400, respectively, throughout the manuscript. Silanol-terminated polydimethylsiloxane (HO-PDMS-OH; C2T) was obtained from Wacker Chemie. The molecular weight of C2T was determined by SEC (Table 1). Ethoxy-terminated octakis(dimethylsiloxy)-T8-silsesquioxane ((QMOEt)8) was synthetized via ethanolysis of octakis(dimethylsiloxy)-T8-silsesquioxane ((QMH)8) in the presence of Karstedt´s catalyst [30]. Hostaphan RN 190/190 μm (polyester plastic carrier) was supplied by Mitsubishi Polyester Film. Syringe filter (diameter: 25 mm, pore size: 0.45, PTFE membrane) was purchased from Fisher Scientific.


**Table 1.** Summary of compounds used in the elastomer formulations.

\* Number average molecular weight (Mn) determined by SEC; \*\* determined from molecular structure.

#### *3.2. Elastomer Film Preparation*

Example of the preparation procedure for elastomer E\_C2T+Di-10\_r3: Silanol-terminated polydimethylsiloxane—C2T and trimethoxysilane-terminated polysiloxane—Di-10 were mixed in a mixing container using a FlackTech speed mixer at 2700 rpm for 3 min. The stoichiometric ratio between cross-linker and polymer functional groups (*r* = 6[(MeO)3Si-PDMS-Si(OMe)3]/2[HO-PDSM-OH]) was *r* = 3. Subsequently, 0.5 wt% of Sn\_DL was

added before mixing in a FlackTech speed mixer at 2700 rpm for another 3 min. After a homogeneous mixture was obtained, the sample was coated into a thin film by a coating knife with a nominal coating height of 200 μm onto a polyester carrier. For the scratch resistance measurements, the mixture was coated onto a tie-coat—Hempel´s Nexus II 27400, which was previously coated onto a polyester carrier using a coating knife with a nominal coating height of 200 μm. The films were subsequently cured in a climate chamber at 25 ◦C and 80% relative humidity.

The elastomer compositions for the long-term stability study can be found in Table A2. The elastomer compositions for tensile, electrical breakdown strength, and scratch resistance measurements can be found in Table A3.

The abbreviation, molecular weight, and chemical structure of the compounds used in the elastomer formulations are summarized in Table 1.

#### *3.3. Evaluation of the Elastomer Film Stability*

The silicone elastomer films, for which composition details can be found in Table A2, were used to study the effect of elastomer composition on network structure and stability. The following methods were used to characterize film stability: **Mechanical stability** (Figure 14a) was evaluated by tracking changes in Young´s modulus over time. The Young´s modulus was determined from the tangent of the linear region of the stress– strain curves at low strains (up to approximately 20% strain). Standard deviations were calculated from five tensile measurements for each sample composition. The tensile measurements were performed on an ARES-G2 rheometer using SER geometry according to a previously reported procedure [10]. **Chemical composition of the extract and wt% of extractable PDMS/cross-linker** (Figure 14b) was determined using Proton Nuclear Magnetic Resonance (1H NMR). A specimen 25 mm in length and 6 mm in width was cut from the silicone elastomer film (~100 μm), weighed, and extracted in 1 mL of chloroform-*d* for 24 h. After the extraction, a known amount of naphthalene was added, and the solution was transported into a NMR tube. 1H NMR characterization was performed on a 7 Tesla Spectrospin-Bruker AC 300MHz spectrometer at room temperature. 1H NMR spectra were analyzed using MestReNova. Detailed calculation of the wt% of extractable PDMS can be found in Figures A2 and A3. **Molecular weight of extractable PDMS** (Figure 14c) was obtained using Size-Exclusion Chromatography (SEC). Two specimens, 25 mm in length and 6 mm in width, were cut from the silicone elastomer film (~100 μm), weighed, and extracted in 1.2 mL of toluene for 24 h. The extract was filtered through a syringe filter. SEC was performed on a TOSOH EcoSEC HLC-8320GPC system equipped with an EcoSEC RI detector. This system was fitted with two SDV LINEAR S 5 μm 8 × 300 mm columns in series, protected by a GUARD column (SDV 5 μm 8 × 50 mm), all supplied by PSS. Samples were run in toluene at 35 ◦C. Molar mass characteristics were calculated using PSS WinGPC Unity, Build 9350 software, and linear PDMS standards acquired from PSS.

#### *3.4. Evaluation of the Silicone Elastomer Films Performance*

The silicone elastomer film compositions can be found in Table A3. The following properties were tested in order to evaluate elastomer performance: **Sol fraction** was determined using both 1H NMR (sol fraction consisting purely from the extractable PDMS species) and sample weight lost (sol (%) = (mi − md)/mi × 100, where mi is the original sample weight and md is the sample weight after the extraction and drying). For more details on the extraction, see the procedure in Section 3.3. The 24-h extraction time was found to be sufficient, as no additional weight loss was observed after a second extraction of the same specimen. **Tensile properties** were performed on an ARES-G2 rheometer using SER geometry according to a previously described procedure [10]. **Scratch resistance** was measured using a Motorized Clemen Scratch Tester equipped with a Ø 1mm ball tool. Film destruction was observed visually. Two different parameters were evaluated—"single" scratch resistance and "multiple" scratch resistance. The "single" scratch resistance was determined as the maximum load at which the coating continues to resist penetration.

The "multiple" scratch resistance was determined as the maximum load at which the coating remained unpenetrated after three consecutive scratches at the same place. Both "single" and "multiple" scratch procedures were repeated three times for each coating composition. If the coating was penetrated during one or more of the three repetitions, the load was lowered and the whole procedure was repeated. **Electrical breakdown strength** was measured using an instrument built in-house following the international standards (IEC 60243-1 (1998) and IEC 60243-2 (2001)) [31,32]. A silicone elastomer film approximately 100 μm thick was placed on a plastic frame and positioned inside the breakdown instrument between two semi-spherical stainless-steel electrodes. A voltage ramp of 100 V/s was then applied until the elastomer short-circuited. The electrical breakdown strength was calculated as the voltage at breakdown divided by sample thickness. The standard deviation was calculated from 10 breakdown measurements for each elastomer film. **Scanning electron microscope (SEM)**—Inspect S, FEI company—was used to evaluate the changes in the elastomer E\_C2T+(QMOEt)8\_15 before and after stretch. Images were taken in a low vacuum mode. The sample preparation can be found in Figure A5.

**Figure 14.** Methods used to evaluate the stability and network structure of elastomer films: (**a**) ARES-G2 rheometer equipped with SER geometry was used to track changes in Young´s modulus over time;(**b**) 1H NMR was used to analyze the chemical composition of the extract and to determine the wt% of extractable PDMS/cross-linker; (**c**) SEC was used to measure the molecular weight of extractable PDMS.

The exact sample thickness for the tensile, scratch resistance, and electrical breakdown strength measurements was determined using an optical microscope according to a previously described procedure [10].
