A variety of model end-linked poly(dimethylsiloxane) (PDMS) networks with a turbid, phase separated micro-structure are compared and contrasted. A critical review of the various PDMS end-linking techniques is presented to understand the phenomenology of the structure development. The necessary and sufficient requirements for developing such textures appear to be (ⅰ) large concentrations of short PDMS chains, and (ⅱ) large amounts of the catalyst used in the end-linking or cross-linking reactions. Scattering and microscopic techniques demonstrate that the observed structures are likely high cross-link density clustered phases. The phase separation is not the result of side reactions or system-specific artifacts, and thus appears to be a general phenomenon in such network-forming systems. The formation of the phases does not appear to be directly linked with the speed of the reactions, which suggests that the phases are not the result of a precipitation, or a semi-stable colloidal structure, etc. Interesting phase patterns can be developed, including an interpenetrating spinodal-like structure and spheres trapped within the network mesh. These structures bear a close similarity to structures expected from classical thermodynamic phase separation mechanisms (spinodal decomposition and binodal nucleation and growth); furthermore, these structures persist even after attempts at dispersal/redissolution. These results suggest that the observed structures are the unique result of a reaction induced phase separation which is in turn mediated by and eventually trapped by the extensive cross linking. Hydrosilylation cross linking favors the interpenetrating spinodal texture, while condensation cross linking favors dispersed spheres. Bimodal formulations do not appear to affect the observed hydrosilylation cross-linked structure, whereas bimodal and/or hybrid reactions tend to favor smaller/polydisperse spheres for condensation cross linking.
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