**4. Conclusions**

In this study, post-curing reactions occurring in condensation curing silicone elastomers were thoroughly investigated using rheology, SEC, and 1H NMR, findings from which were correlated to the elastomers' network structure. Depending on the stoichiometric ratio, two types of post-curing reaction were shown to occur: Reactions between unreacted and/or dangling polymer chains, and reactions between cross-linker molecules. The exact stoichiometry, *r,* required to initiate these post-curing reactions depends on many factors, such as the type and molecular weight of the cross-linker used, as well as the concentration of the catalyst. Thus, in order to provide a guideline on how to develop stable and reliable silicone elastomers via condensation curing, a formulation optimization study was conducted. For hereby studied elastomer formulations, a stoichiometric ratio between 3 and 5 was found to produce the fewest post-curing reactions. Lower *r* values

produced elastomers that become unstable over time due to the reaction between unreacted and/or dangling polymer chains. At higher *r* values, so-called cross-linker domains were formed via the reaction between redundant cross-linker molecules. In contrast to the condensation of unreacted and/or dangling polymer chains, this reaction occurred relatively quickly—within approximately 3 weeks—and therefore did not contribute to long-term elastomer instability.

The long-term stability study served as a tool for preparation of stable condensation curing silicone elastomers. The elastomer properties were then tailored by cross-linker domains density and concentration. The cross-linker domains density was altered by changing either cross-linker chain length (Di-10, Di-50, and Di-400) or cross-linker type ((MeO)3Si-PDMS-Si(OMe)3 and (QMOEt)8). The cross-linker domains concentration was altered by varying *r*. As expected, the Young´s modulus of the elastomers was increasing and elongation at break was decreasing with increasing *r.* The smallest change of these properties was observed when using the highest molecular weight cross-linker (Di-400), as the resulting cross-linker domains were less dense due to the long distance between cross-links. The presence of cross-linker domains was found to positively affect electrical breakdown strength, with the highest value obtained for E\_C2T+(QMOEt)8\_r15 (130 μm/V). Scratch resistance was found to be highly dependent on both the size and rigidity/elasticity of the cross-linker domains. Due to their high weight percentage in the elastomer network, cross-linker domains Di-400 and Di-50 were found to have negligible and negative effects, respectively, on scratch resistance. On the other hand, scratch resistance was significantly improved by the presence of both Di-10 and (QMOEt)8 cross-linker domains. Scratch resistance comparable to that of the reference coating (E\_Ref), which contained reinforcing fillers, was obtained for both E\_C2T+Di-10\_r15 and E\_C2T+(QMOEt)8\_15. While E\_C2T+Di-10\_r15 performed well in both "single" and "multiple" scratch resistance tests, "multiple" scratch resistance was significantly lower for E\_C2T+(QMOEt)8\_15, indicating the importance of not only cross-linker domains size, but also rigidity/elasticity.

**Author Contributions:** Conceptualization, A.J., A.L.S., and S.M.O.; formal analysis, A.J.; methodology, A.J.; investigation, A.J.; resources, S.M.O. and M.A.B.; data curation, A.J.; writing—original draft preparation, A.J.; writing—review and editing, A.J., A.L.S., S.M.O., M.A.B., and K.D.-J.; supervision, A.L.S. and S.M.O.; project administration, A.L.S., K.D.-J., and S.M.O.; funding acquisition, K.D.-J. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the financial support from the Hempel Foundation to CoaST. **DataAvailabilityStatement:** Thedatacanbeobtainedfromthecorrespondingauthor.

 **Acknowledgments:** Narayanan Rajagopalan is acknowledged for help with the SEM images.

**Conflicts of Interest:** The authors declare no conflict of interest.

**Sample Availability:** Samples of the compounds are not available from the authors.
