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

The study addresses the challenge of producing silicone elastomers that combine very high stretchability with extreme softness for applications in stretchable electronics, soft actuators, medical devices, and microfluidics. Conventional silicone elastomers are formed via hydrosilylation of telechelic vinyl PDMS with multi-hydrosilane cross-linkers, which constrains network strand length and mechanical properties. Ultimate extensibility typically remains below ~900% due to processing limits with very long precursors and entanglement effects, while the lowest elastic modulus for ideal networks is limited (~0.6 MPa) because entanglements act as topological crosslinks once strand molecular weight exceeds the entanglement molecular weight. The purpose is to overcome these intrinsic limitations using an alternative curing chemistry that decouples strand formation from crosslinking, enabling exceptionally long strands for stretchability and bottle-brush architectures for softness, all using accessible one-pot processes and commercial precursors.

Prior work highlights: (1) Classical hydrosilylation cures yield networks whose strand length mirrors precursor polymer length, limiting extensibility with λmax scaling as M^0.5 and practical strains <900%. (2) Entanglements set a lower bound on modulus (~0.6 MPa) in ideal networks once strands exceed entanglement molecular weight. (3) Strategies to increase extensibility include very long precursors (processing issues due to viscosity) and supramolecular approaches; (4) Extremely soft, entanglement-diluted bottle-brush elastomers (shear moduli ~1–100 kPa) have been reported, but typically require complex multistep syntheses to make bottle-brush polymers before crosslinking. (5) Oxidation or alcoholysis of Si–H groups and radical pathways are known in PDMS chemistry, and catalyst-free autoxidation crosslinking at high temperatures has been observed; however, using Pt catalysts and trace O2/H2O at moderate temperature to deliberately effect slow Si–H crosslinking offers a route to timed curing that has not been exploited for tuning network architecture in one-pot processes.

Core concept: Employ a platinum-catalyzed reaction of telechelic or multi-Si–H functional PDMS with dissolved oxygen and trace water to achieve slow Si–H crosslinking. Combine this with the much faster hydrosilylation of Si–H and vinyl groups in one-pot formulations. Kinetic hierarchy allows hydrosilylation to proceed to high conversion first, creating either (i) extended chains (for high stretchability) or (ii) bottle-brush chains (for extreme softness), which are then crosslinked via slow Si–H reactions mediated by O2/H2O.

Two formulation routes:

• Highly stretchable elastomers: Telechelic Si–H PDMS + telechelic vinyl PDMS at small excess Si–H (R = [Si–H]/[vinyl] slightly >1). Fast hydrosilylation first extends chains; subsequent slow Si–H crosslinking forms the network. The average strand molar mass increases sharply as R → 1 (derived in the text).
• Extremely soft elastomers (bottle-brush networks): Multi-Si–H PDMS + mono-vinyl PDMS with excess Si–H. Fast hydrosilylation grafts side chains, forming bottle-brush polymers; slow Si–H crosslinking later connects the brushes into a network. Side chain length (mono-vinyl PDMS Mn) and R control brush diameter and network softness.

Mechanistic and kinetic studies:

• Mechanism: 1H and 29Si NMR on telechelic Si–H PDMS (DMS-H11) heated at 100 °C for 48 h under controlled atmospheres (dry N2, wet N2, dry air) with Pt catalyst. Observed Si–H loss strongly depends on oxygen/water presence; identification of oxidized/branched silicon environments consistent with O2/H2O-mediated crosslinking via Pt.
• Kinetics: At 100 °C, Pt-catalyzed Si–H oxidation/crosslinking of mono-Si–H PDMS completes in ~6 h and produces high-MW branched products (>10× precursor Mn), whereas hydrosilylation between mono-Si–H and mono-vinyl PDMS completes in ~2 min and yields only doubled chain length, confirming strong kinetic separation.

Processing and characterization:

• Curing typically at 100 °C for 24 h (one-pot mixes), with Karstedt’s catalyst (Pt-divinyl tetramethyldisiloxane complex) primarily; Speier’s catalyst and a Rh catalyst also tested. Cure acceleration at higher temperature (e.g., 150 °C) and higher Pt concentration; e.g., DMS-H25 (Mn 14 kDa) cured to solid in 0.5 h at 150 °C with 30 ppm Pt.
• Rheology: Time sweeps at 100 °C, 1 Hz, 1% strain to determine gelation; frequency sweeps for linear viscoelasticity (LVE) at room temperature; master curves via TTS for stretchable elastomers.
• Mechanical testing: Uniaxial tensile (ASTM D-638 Type V), biaxial stretching demonstrations, compression tests on soft elastomers.
• Swelling to assess gel fraction and network structure. SEC and NMR (1H, 29Si; solution and solid-state) to follow reactions and structures.
• Materials from Gelest and others; detailed formulations provided (Tables 4 and 5) with R values and component masses/moles.

• Mechanism and atmosphere dependence: At 100 °C for 48 h with Pt, telechelic Si–H PDMS showed Si–H loss of 52.7% (dry air), 32.8% (wet N2), and 3.3% (dry N2), evidencing a requirement for O2/H2O to drive crosslinking. 1H/29Si NMR identified oxidized/branched motifs (e.g., SiOCH2Si and CH3SiO3), consistent with a Pt-mediated oxidation/hydrolysis mechanism at moderate temperatures.
• Kinetic separation: Hydrosilylation (Si–H + vinyl) completed in ~2 min at 100 °C, while Si–H oxidation/crosslinking required ~6 h under the same conditions. Si–H oxidation produced high-MW branched species (>10× initial Mn), whereas hydrosilylation yielded chains of exactly double Mn.
• Baseline Si–H-cured elastomers (no added vinyl): From telechelic or multi-Si–H PDMS, tensile strains of 70–360%, Young’s moduli 0.29–0.60 MPa, and tensile strengths 0.24–0.52 MPa, comparable to conventional hydrosilylation-cured elastomers.
• Highly stretchable elastomers (extended-strand networks): By tuning R (Si–H/vinyl) and precursor Mn, ultimate tensile strain increased dramatically. With DMS-H21/DMS-V22: λmax rose from 1142% (R=1.15) to 1602% (R=1.10) to 2493% (R=1.05). Using longer precursors (DMS-H25/DMS-V25) at R=1.05 achieved λmax ≈ 2864% (≈2800%). Biaxial stretching demonstrated a 180-fold area expansion, far exceeding a conventional elastomer (9-fold). LVE showed decreasing storage modulus at low frequency due to entanglement relaxation of long strands. Swelling indicated gel fractions of 67–83% and very high swelling ratio (up to 128) for the most stretchable sample, consistent with networks dominated by entanglements over chemical crosslinks.
• Extremely soft bottle-brush elastomers: Shear modulus G decreased from 7.4 kPa to 1.2 kPa by reducing R and/or increasing side-chain length (e.g., switching mono-vinyl PDMS from Mn 6 kDa to 23 kDa). LVE displayed near frequency-independent G′ at low frequencies (rubber-like). Network analysis showed Me » M_SiH (e.g., Me ≈ 10× M_SiH for MCR-V21 side chains), attributed to intramolecular reactions creating loops/dangling ends; gel fractions 52–83% vs 97% in conventional elastomers. Under 0.16 MPa, bottle-brush elastomers compressed to 88% strain with nearly instantaneous recovery; conventional elastomers compressed only to 19% under the same load.
• Practicality: One-pot reactions using commercial precursors and standard Pt catalysts enable facile preparation; gel points within ~5 min at 100 °C indicate fast network formation once initiated.

The study demonstrates that deliberately leveraging the slow, Pt-catalyzed crosslinking of Si–H groups in the presence of trace oxygen and water provides a built-in delayed curing step. Because hydrosilylation is much faster, network strand architecture can be established first (via chain extension or side-chain grafting) and only then fixed by slow Si–H crosslinking. This decoupling enables architectures that surpass the intrinsic limits of classical hydrosilylation-cured silicone networks. The extended-strand networks produce unprecedented stretchability (up to ~2800% tensile strain and 180× biaxial area expansion) by dramatically increasing strand length without relying on ultra-long, high-viscosity precursors. The bottle-brush route yields ultra-low shear moduli down to ~1.2 kPa—comparable to hydrogels and soft tissue—using simple, solvent-free, one-pot chemistry, unlike conventional multistep bottle-brush syntheses. The rheological and swelling data support the proposed network structures and curing sequence. Overall, the results indicate broad tunability of mechanical properties by adjusting R and component molecular weights, opening pathways to soft robotics, wearable electronics, and biomedical devices that require extreme deformability or very low modulus.

A simple one-pot curing strategy combining fast hydrosilylation with slow, Pt-catalyzed O2/H2O-mediated Si–H crosslinking enables silicone elastomers with properties beyond the limits of classical formulations. Using commercial telechelic and multi-Si–H and vinyl PDMS precursors, the authors prepared: (i) highly stretchable elastomers with tailored ultimate strains from ~1500% to ~2800%, and (ii) extremely soft bottle-brush elastomers with tunable shear moduli from ~1.2 to ~7.4 kPa. Mechanistic NMR studies and kinetic comparisons confirm the role of oxygen and water in slow crosslinking and the strong kinetic advantage of hydrosilylation. The approach is practical, scalable, and versatile, providing an accessible route to diverse network architectures and property sets. Future work could explore long-term durability, environmental stability, alternative catalysts or curing conditions, integration with functional fillers/electronics, and extension to other silicone architectures and chemistries.

• The slow Si–H crosslinking relies on the presence of oxygen and water and Pt catalysts; curing rates and network formation may be sensitive to environmental conditions (humidity, oxygen availability) and temperature, requiring process control.
• Networks formed via bottle-brush routes show lower gel fractions (52–83%) and large fractions of loops/dangling ends, which may affect mechanical strength and long-term stability compared to highly crosslinked conventional networks.
• Achieving maximal properties typically required elevated temperatures (100–150 °C) and hours-long cures; although feasible, this may limit certain substrates or processing windows.
• The study focuses on mechanical and rheological performance; comprehensive assessments of fatigue, aging, solvent resistance, and long-term cyclic durability were not reported in the provided text.