Model Networks of End-Linked Polydimethylsiloxane Chains. X. Bimodal Networks Prepared in Two-Stage Reactions Designed to Give High Spatial Heterogeneity

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Abstract
End-linking mixtures of very short and relatively long polydimethylsiloxane chains with a multi-functional alkoxy silane yields model elastomeric networks having a bimodal chain length distributions, and earlier studies have shown that this compositional heterogeneity increases the ultimate properties of the networks. The present investigation focuses on possible additional effects of the spatial heterogeneity introduced by the prereaction of some of the very short chains into tightly crosslinked domains. Increasing the extent of prereaction did not give any significant increases in modulus or ultimate strength. Thus, it is indicated that the unusual ultimate properties of bimodal networks are due to limited chain extensibilities, rather than to reinforcing effects.

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