Understanding the Extraordinary Flexibility of Polydimethylsiloxane through Single-Molecule Mechanics

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Linear (un-cross-linked) polydimethylsiloxane (PDMS) exists as a viscous fluid at room temperature even though its molecular weight reaches a high level (105 Da). Accordingly, one can infer the extraordinary flexibility of PDMS. However, the mechanism of the extraordinary flexibility is still unclear at present. Herein the single-chain mechanics of PDMS is measured by single-molecule atomic force microscopy, which is consistent with the results from quantum mechanical calculations. Then, the effects of several factors of microscopic structures of PDMS on the macroscopic mechanical properties are qualitatively analyzed. With the extremely low elasticity of the individual chain, the large effective cross-sectional area, and the very weak intra/interchain noncovalent interactions and their synergistic effect, the relatively small modulus of PDMS is reasonable. We further propose a method to estimate the modulus of an extended chain crystal of a polymer from the single-chain force–extension curve. For the polymer crystals in which only van der Waals forces are involved in the intra/interchain noncovalent interactions, the estimated moduli are very close to the measured values, which indicates that this method should be valid for PDMS. It is found that the estimated modulus of PDMS is smaller than those of most polymers, which directly shows its extraordinary flexibility at the macroscopic level. This work may be helpful for bridging the gap between the single-molecule and macroscopic properties of polymer materials.