One reaction to make highly stretchable or extremely soft silicone elastomers from easily available materials

An easy curing pathway to silicone elastomers is presented, in which telechelic/multi-hydrosilane functional polydimethylsiloxane in presence of oxygen and water leads to crosslinking. This curing chemistry allows versatility preparing both highly stretchable and extremely soft silicone elastomers.
Published in Materials
One reaction to make highly stretchable or extremely soft silicone elastomers from easily available materials
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Highly stretchable, soft silicone elastomers are of great interest for the fabrication of stretchable, soft devices. Low limit of softness and ultimate extensibility of the mechanically stable silicone elastomers are inherently limited to around 0.6 MPa and 900%, respectively. Several strategies have been explored for overcoming the limitations. However, there is a lack of available chemistries capable of efficiently preparing silicone elastomers with superior stretchability and softness.

In our previous work with platinum-catalyzed hydrosilylation reactions, we observed that elastomers were formed when only telechelic hydrosilane (Si-H) functional polydimethylsiloxane (PDMS) and a platinum catalyst were present. The mechanism of the curing reaction is consistent with platinum-mediated crosslinking of hydrosilanes in the presence of trace water and oxygen, and thus may be considered a side-reaction in conventional formulations. Compared with classical curing chemistry—i.e., hydrosilylation reaction—Si-H crosslinking in the presence of moisture and oxygen proceeds much more slowly, thereby providing formulations with an inherent delayed crosslinking opportunity and allowing the preparation of highly diverse networks using simple one-pot reactions.

Highly stretchable silicone elastomers and extremely soft silicone elastomers were developed by combining this curing chemistry with hydrosilylation reactions: the fast hydrosilylation reactions controlled the size and structures of network strands, after which elastomers were created through the much slower crosslinking of Si-H functional groups. Specifically, highly stretchable silicone elastomers were prepared by creating highly entangled (long-chain) silicone elastomers from the reaction between telechelic Si-H functional PDMS and telechelic vinyl functional PDMS. Tensile strains could be tailored from 1500% to 2800% by varying precursor length and the molar ratio of Si-H-to-vinyl groups (Figure 1). We demonstrated a 180-fold extension in area by biaxial stretch for one such highly stretchable silicone elastomer. Extremely soft silicone elastomers were made by creating bottle-brush silicone elastomers from the reaction between multi-Si-H functional PDMS and mono-vinyl functional PDMS. The shear moduli of the prepared bottle-brush elastomers could be adjusted from 1.2 kPa to 7.4 kPa by changing the molar ratio of reactive groups and the side chain lengths (Figure 2).

Both highly stretchable silicone elastomers and extremely soft silicone elastomers can be easily prepared via one-pot reactions using commercial precursors. In addition to enabling the preparation of highly stretchable or extremely soft elastomers, the general methodology based on slow crosslinking presented here enables the easy development of silicone elastomers with a wide range of functionalities.

Figure 1 Properties of highly stretchable silicone elastomers and a conventional silicone elastomer. (a) Uniaxial stress-strain curves. (b) The highly stretchable silicone elastomer is marked with a red 1 cm2 square; the area of the same film is manually extended 180-fold.

Figure 2 Properties of extremely soft silicone elastomers and a conventional silicone elastomer (a) Frequency dependence of storage and loss moduli (G´ and G´´) of extremely soft silicone elastomers and a conventional silicone elastomer measured at room temperature. (b) Extremely soft specimens are compressed to a strain of 88%, and two stacked conventional specimens were compressed to a strain of only 19% under the same pressure.

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