Highly stretchable or extremely soft silicone elastomers? One reaction to make them all - from easily available materials!

: An easy curing reaction to prepare silicone elastomers is reported, in which a platinum-1 catalyzed reaction of telechelic/multi-hydrosilane (Si-H) functional polydimethylsiloxane (PDMS) in the presence of oxygen and water leads to slow crosslinking. This curing chemistry allows versa-3 tile tailoring of elastomer properties, which exceed their intrinsic limitations. Both highly stretcha-4 ble silicone elastomers and extremely soft silicone elastomers are prepared by creating highly en-5 tangled elastomers and bottle-brush elastomers from commercial precursor polymers, respectively. 6 The highly stretchable elastomers can be uniaxially stretched to a maximum strain of 2800% and 7 their areas can be biaxially extended 180-fold. The extremely soft silicone elastomers exhibit shear 8 moduli of 1.2-7.4 kPa, depending on composition, values that are comparable to hydrogels and hu-9 man soft tissues. The reported curing chemistry can be used to prepare a range of silicone elasto-10 mers with carefully tailored mechanical properties. 11


INTRODUCTION 13
Highly stretchable, soft silicone elastomers are of great interest for the fabrication of stretchable 14 electronics, soft actuators, medical devices, and microfluidics. [1][2][3][4][5] High stretchability provides long-15 term device stability in various distortion scenarios and permits exceptional deformations. Signifi-16 cant effort has been devoted to preparing silicone elastomers with a combined softness and elastic-17 ity resembling that of human soft tissue for use in soft robotics. 6,7 18 resulting silicone elastomers is proportional to 0.5 based on the Kuhn model, where is the aver-25 age molar mass of network strands. 9 The ultimate extensibility is usually less than 900%, and it is 26 difficult to further increase this value by using longer precursor polymers, since they would bring 27 difficulties to the fabrication process due to their high viscosity. 10 The elastic modulus of the silicone 28 elastomers is determined by the crosslinking density-namely, the molar density of mechanically 29 active strands = / , where is the density. However, the lowest achievable elastic modulus for 30 ideal elastomer networks is around 0.6 MPa due to the fact that entanglements in the cross-linked 31 networks act as topological crosslinks once the molecular weight of network strands (M) exceeds 32 the entanglement molecular weight. 11 33 Several strategies have been explored for overcoming limitations on the ultimate extensibility and 34 softness of silicone elastomers. For example, elastomers prepared from long precursor polymers in 35 solution and subjected to solvent evaporation after curing, supramolecular elastomers with mova-36 ble cross-links, and concatenated ring elastomers have all been developed to improve ultimate ex-37 tensibility. [12][13][14][15][16][17] Approaches such as adding external sol molecules, sparse crosslinking, and cross-38 linking bottle-brush PDMS have also been used to prepare soft silicone elastomers. 18-23 While the 39 above strategies improve elastomer extensibility or softness to some extent, they either increase 40 process complexity or lead to mechanical instabilities in the resulting elastomers. What is more, 41 none of them are versatile enough to enable the preparation of silicone elastomers that are both 42 highly stretchable and very soft. There is thus a lack of available chemistries capable of efficiently 43 preparing silicone elastomers with superior stretchability and softness. 44 Silicone elastomers have been prepared from Si-H functionalized polymers at 250°C in air, where 45 the crosslinking mechanism was found to originate from oxidative crosslinking of Si-H groups in the 46 presence of oxygen. 24 In this study, silicone elastomers are prepared from PDMS with 47 telechelic/multiple Si-H groups and a platinum catalyst in air at the much lower temperature of 48 100℃ (see Figure 1b). In contrast to classical curing chemistries where the network strands are 49 directly related to the length of the precursor polymers, the curing chemistry presented here, when 50 combined with the hydrosilylation reaction, allows network strands to be tailored from normal lin-51 ear precursors during the curing process. Specifically, we report highly stretchable silicone elasto-52 mers with ultra-long network strands and extremely soft silicone elastomers with bottle-brush 53 strands, both of which are easily prepared from commercially available linear precursors. Both 54 novel silicone elastomers are based on sequential crosslinking mechanisms in one-pot reactions, 55 where the fast hydrosilylation reaction is followed by a slow crosslinking of residual Si-H functional 56 groups. This allows independent control of network strand size and structure, as well as of cross- In our previous work with platinum-catalyzed hydrosilylation reactions, we observed that elasto-67 mers were formed when only telechelic Si-H-functional PDMS and a platinum catalyst were present. 68 This discovery of a potentially simple method for preparing elastomers prompted us to perform 69 further studies in order to elucidate the mechanism of this formation. 70 Hydrosilanes readily undergo hydrolysis and alcoholysis reactions with water and alcohols, respec-71 tively, under basic or strongly acidic conditions or in the presence of radicals, metals, or transition 72 metal complexes. 25,26 In the presence of a platinum catalyst and water, telechelic Si-H functional 73 PDMS can thus be hydrolyzed into Si-OH, which may further undergo condensation to form ex-74 tended chains. However, the hydrolysis and condensation reactions cannot account for the for- wet N2 (a sample containing water with ~4 molar equivalent of hydrosilanes under dried nitrogen), 84 dry air (a dried sample under dried air), and wet air (a sample containing water with ~4 molar 85 equivalent of hydrosilanes under dried air). DMS-H11 was converted into a solid elastomer only 86 under wet air conditions. As measured by 1 H NMR, the conversion efficiency of Si-H groups for the 87 three liquid products under dry N2, wet N2, and dry air conditions were found to be 6.6%, 29.5%, 88 and 52.9%, respectively ( Figure 2a). These findings indicate that both water and oxygen participate 89 in the reaction. A peak at 2.27 ppm in the 1 H nuclear magnetic resonance (NMR) spectrum of the 90 sample from the wet N2 atmosphere is assigned to a Si-OH structure (Figure 2a), suggesting a hy-91 drolysis process of Si-H during the reaction. 27 A further condensation process of Si-OH results in 92 chain extension, as evidenced by the increased molecular weight of the sample ( Figure S1, ESI). An-93 other new peak that appears at 3.47 ppm (Figure 2a) on the 1 H spectrum of the sample from dry air 94 atmosphere is contributed to a silyl ether (Si-O-CH2-Si) structure which has been reported to origi-95 nate from oxidation reactions of Si-H with oxygen and methyl groups at much higher temperatures 96 in the absence of a platinum catalyst. 24 The presence of this silyl ether suggests that branched chains 97 are formed during the oxidation of Si-H. The integration of the 1 H spectrum shows that the amount 98 of hydrogen on silyl ether only accounts for 1.4% of the Si-H loss (Table S3,

108
Direct investigations of the chemical composition of solid elastomers cured under a normal air at-109 mosphere and a precursor polymer DMS-H11 were performed using 29 Si solid-state NMR. Figure 2b  110 shows that a peak at -7 ppm on the spectrum of the precursor polymer is assigned to Si-H functional 111 groups. 28 This peak vanishes on the spectra of the elastomers, suggesting an efficient conversion of 112 Si-H after curing. New peaks located at 7 ppm and -64 ppm are observed on spectra of the elasto-113 mers and are assigned to (CH3)3SiO(or (CH3)2CH2SiO) and CH3SiO3, respectively. 28-30 Since the hy- groups. Integration of the spectra shows that Si atoms associated with these newly formed struc-117 tures account for 1.5-2.5% of Si atoms in the elastomers (Table S4, ESI), suggesting that these oxi-118 dized structures play a major role in crosslinking. 119 It should be noted that the same radicals mentioned above were also used to explain crosslinking 132 during the oxidization of Si-H functional PDMS at high temperature (250℃) in the absence of a cat-133 alyst, in which a silyl ether (Si-O-CH2-Si) structure was the main oxidized structure produced. 24 In 134 our work, however, the oxidation process is faster and occurs at a much lower temperature due to 135 the use of a platinum catalyst; the main oxidized structures produced are (CH3)2CH2SiO and CH3SiO3.  In order to compare the reaction kinetics of the Si-H functional PDMS system with those of the hy-153 drosilylation reaction between Si-H and vinyl groups, platinum-catalyzed reactions of mono-Si-H 154 functional PDMS and mono-Si-H functional PDMS with mono-vinyl functional PDMS, respectively, 155 were conducted at 100℃. The total concentration of functional groups was the same for both reac-156 tions. Figure 3a shows that the reaction of mono-Si-H functional PDMS takes 6 h to complete 100%, 157 compared to 2 min for the hydrosilylation reaction. In addition, the reaction of mono-Si-H functional 158 PDMS products a fraction of products that are more than 10-time molecular weight of precursor 159 polymers ( Figure 3a). This is consistent with the branching nature of Si-H oxidation. In comparison, 160 the hydrosilylation reaction of mono-Si-H functional PDMS with mono-vinyl functional PDMS exclu-161 sively produces chains with double initial molecular weight ( Figure 3a).

168
Curing chemistries for highly stretchable/extremely soft silicone elastomers. 169 Preparing silicone elastomers with long strands is one way to efficiently obtain high stretchability. 170 However, doing so using conventional curing chemistry requires exceptionally long precursor poly-171 mers whose high viscosity brings difficulties to fabrication process. 9,40 Here, highly stretchable sili-172 cone elastomers are prepared from a platinum-catalyzed reaction between telechelic Si-H func-173 tional PDMS and telechelic vinyl functional PDMS using a small excess of Si-H groups (Figure 1c). In 174 a one-pot reaction, both the hydrosilylation reaction between Si-H and vinyl groups and the water 175 and oxygen-mediated crosslinking of Si-H (see above) take place. Due to the significant kinetic ad-176 vantage of the hydrosilylation reaction over Si-H crosslinking (Figure 3a), the hydrosilylation reac-177 tion is expected to proceed to high conversion before any significant crosslinking occurs. Assuming 178 the two reactions happen strictly in sequence, the hydrosilylation reaction results in extended 179 chains which are subsequently cross-linked into elastomers through the reaction of excess Si-H with 180 oxygen and water. In this case, the average molar mass of the network strands can be expressed where brush (Equation S3, ESI) and brush (Equation S4, ESI) are the molecular weight and number 203 of excess hydrides per chain, respectively, after full side chain grafting. 204 Representative curing reactions for preparing highly stretchable, extremely soft elastomers were 205 investigated by tracing the evolution of the storage and loss moduli during the two curing reactions 206 ( Figure 3b). Gel points are reached within 5 min at 100℃, suggesting fast curing processes. 207 208

Properties of highly stretchable silicone elastomers 209
A number of stretchable silicone elastomers were prepared using different hydrosilane-to-vinyl-210 functional polymer ratios as well as polymers of different molecular weights, as shown in Table 2. 211 Figure 4a shows that, when using the same precursor polymers DMS-H21 and DMS-V22, the tensile 212 strain increases from 1040% to 2400% when R decreases from 1.15 to 1.05. Tensile strain can be 213 further increased to 2800% by using longer starting polymers DMS-H25 and DMS-V25. Longer ex-214 tended chains improve tensile strain by enabling larger slippage lengths upon deformation (Table  215 2). The Kuhn model is widely used to estimate the ultimate extensibility of elastomers as max = 216 /ℎ, where is the strand length in a fully stretched state, and ℎ is the strand length in a random 217 coil state. 9 max is thus proportional to 0.5 based on the relations of ∝ and ℎ ∝ 0.5 . A linear 218 relation between max and theo 0.5 complies with the Kuhn model ( Figure S3), suggesting that equa-219 tion 1 is a valid description of the molar mass of the network strands, which supports the proposed 220 curing route. Silicone elastomers with tailored stretchability can thus be realized by designing 221 strand lengths based on equation 1. Elastomers' linear viscoelastic responses are shown in Figure  222 4b. The storage modulus of the conventional elastomer reaches a plateau at low frequencies, while 223 the storage moduli of the highly stretchable elastomers continue to decrease as the frequency ap-224 proaches zero. This unusual behavior is explained by stress relaxation from entanglements of highly 225 extended strands upon deformation. Stretchable elastomers are often biaxially stretched in practi-226 cal use. 42,43 Figure 4c shows the area of stretchable silicone elastomer, Ela_DMS-H21_DMS-227 V22_R1.05, is biaxially extended 180-fold from an initial state-20 times greater extension than that 228 of the conventional silicone elastomer Ref_DMS-V41. This significantly enhanced stretchability 229 demonstrates the very high mechanical integrity of the elastomers studied here. 230 231   Table 3 shows that the molecular weights of bottle-brush network strands ( c ) (Equation S6, 248 ESI) are larger than the average molecular weight between Si-H groups on the bottle-brush chains 249 ( c_SiH ). Specially, c >10 c_SiH for the elastomers with MCR-V21 side chains. The large differences 250 between c and c_SiH can be explained by the preferentially intramolecular reactions of the multi-251 functional bottle-brush chains, which result in a large fraction of elastically inactive loops and dan-252 gling. 45 Figure 5b shows the compressibility of the bottle-brush elastomer Ela_HMS-064_MCR-253 V21_R1.05 compared to that of the conventional elastomer Ref_DMS-V41. The bottle-brush elasto-254 mer is compressed to a strain of 88% under a pressure of 0.16 MPa, while the conventional elasto-255 mer only shows a compression strain of 19% under the same pressure. Importantly, despite the 256 large compression strain imposed on the bottle-brush elastomer, it recovers to its initial state al-257 most instantaneously upon pressure being released, displaying superior elasticity compared to nor-258 mal soft elastomers, which often recover only partially.    Silicone elastomers with bespoke properties can be prepared via classical hydrosilylation chemistry 270 combined with a platinum-catalyzed reaction of telechelic/multi-hydride functional PDMS, without 271 using any additional cross-linker. The mechanism of the curing reaction is consistent with platinum-272 mediated crosslinking of hydrosilanes in the presence of trace water and oxygen, and thus may be 273 considered a side-reaction in conventional formulations. Compared with classical curing chemis-274 try-i.e., hydrosilylation reaction-Si-H crosslinking in the presence of moisture and oxygen pro-275 ceeds much more slowly, thereby providing formulations with an inherent delayed crosslinking op-276 portunity and allowing the preparation of highly diverse networks using simple one-pot reactions.  Table S1, in which all the chemicals 440 were purchased from Gelest, except that Catalyst 511was purchased from Hanse Chemie. As a precursor 441 polymer for condition controlling experiments, DMS-H11 (10 mL) was diluted in hexane (20 mL, ≥95%, 442

Materials Information of chemicals used in this studies is shown in
Sigma-Aldrich) and dried with silica gel (5 g, particle size of 63-200 μm, high-purity grade, Sigma-Al-443 drich) in a sealed flask at a room temperature for 2 days. The upper layer was transferred into a dried flask 444 through a syringe mounted with a filter. Subsequently, the hexane was thoroughly distilled under a vac-445 uum pressure condition at room temperature for 6 h. Red pigment (PGRED01, 50% in silicone oil) was 446 purchased from Gelest. 447 Table S1 were used to prepare silicone elastomers, 449 respectively. A representative procedure is as follows: DMS-H11 (10 g, 1.00× 10 −2 mol) was mixed with 450 catalyst SIP 6830.3 (2 mg, 3.08× 10 −7 mol) using a speed mixer (DAC150FVZ, Hauschild Co.) at 3000 451 rpm for 2 min. The mixture was poured into a mold and placed in an oven at 100°C for 24 h. 452

Silicone elastomers prepared from hydrosilane (Si-H) containing PDMS. All the telechelic Si-H func-448 tional PDMS and multi-Si-H functional PDMS in
Highly stretchable silicone elastomers and extremely soft silicone elastomers Vinyl functional PDMS 453 and catalyst SIP 6830.3 (2 mg, 3.08× 10 −7 mol) were well mixed using a speed mixer. Subsequently, Si-454 H functional PDMS was added into the mixture and well-mixed. The final mixture was poured on a mold 455 and placed in an oven at 100°C for 24 h. The prepared elastomers listed in Table S2 are named according  456 to the precursor polymer used. 457 Conventional silicone elastomers Part A and part B were prepared before the curing reaction. For a 458 reference sample of Ref_DMS-V25, part A was prepared by mixing DMS-V25 (5 g, 3.3 × 10 −4 mol) 459 with HMS-301 (0.69 g, 3.6 × 10 −4 mol). Part B was prepared by mixing DMS-V25 (5 g, 3.3× 10 −4 mol) 460 with catalyst 511 (2 mg, 1.0 × 10 −7 mol). For a reference sample of Ref_DMS-V41, part A was prepared 461 by mixing DMS-V41 (10 g, 1.5 × 10 −4 mol) with HMS-301 (0.14 g, 7.3 × 10 −5 mol). Part B was pre-462 pared by mixing DMS-V41 (4.8 g, 7.7× 10 −5 mol) with catalyst 511 (3 mg, 1.5 × 10 −7 mol). Parts A 463 and B were then mixed together using a speed mixer at 3000 rpm for 30 s. The final mixture was poured 464 on the surface of a polyethylene terephthalate (PET) substrate and evenly distributed by applying an au-465 tomatic applicator. The PET substrate together with the mixture was placed in an oven at 100°C for 5 h. 466   as same as protocol (a) but replacing the dry N2 with dry air. (d) Wet air was created as same as protocol 477 (b) but replacing the dry N2 with dry air. Subsequently, the four flasks were heated at 100 ℃ for 48 h. 478 The resulting liquid products were analyzed by 1 H nuclear magnetic resonance (NMR) and size-exclusion 479 chromatography (SEC). 480 Kinetics study Two sets of reactions (a) MCR-H21 (2 g) mixed with catalyst SIP6830.3 (2 mg), and (b) 481 MCR-H21 (1 g) mixed with MCR-V21 (1 g) and SIP6830.3 (2 mg) were run in an oven at 100℃. Samples 482 Compression test Two pieces of cylinder elastomers (8 mm in didameter) were stacked with a thickness 506 of around 2 mm. Rheometer ARES G2 was used to compress the elastomers by applying two round plates 507 (8 mm in diameter). Applied forces and gaps between plates were recorded during the compression. 508 SEC measurement SEC was performed on a Tosoh EcoSEC HLC8320GPC instrument equipped with 509 RI and UV detectors and SDV Linear S columns from Polymer Standards Service (PSS). Samples were 510 run in toluene at 35°C at a rate of 1 mL min -1 . Molecular weights and ĐM were calculated using WinGPC 511 Unity 7.4.0 software and standard linear PDMS were acquired from PSS. 512 NMR measurement 1 H NMR spectra of samples were performed on a Bruker 300 MHz spectrometer 513 on 50 mg mL -1 solutions in CDCl3. 29 Si solid-state NMR MAS spectra of investigated elastomers were 514 acquired on a Bruker Avance III HD spectrometer operating at a magnetic field of 14.05 T ( L ( 29 Si) = 515 119.2 MHz) and equipped with a 4 mm CP/MAS broadband probe. The spectra were acquired with a 516 spinning frequency of 6 kHz, a /2 pulse of 4.75 ms, an acquisition time of 35 ms and 10 seconds of 517 interscan delay. This was determined to be sufficient for full relaxation of the two observed signals for 518 elastomers. High-power 1 H SPINAL64 decoupling ( RF = 100 kHz) was employed during acquisition. 519 The prepared elastomers were cut into smaller pieces and packed in 4 mm o.d. zirconia rotors. Chemical 520 shifts are reported relative to TMS (0.0 ppm). High-resolution 29 Si NMR spectrum of a precursor PDMS 521 (liquid state) was acquired on a Bruker Avance II spectrometer operating at a magnetic field of 9.4 T ( L 522 ( 29 Si) = 79.495 MHz) and equipped with a 5mm BBFO probe. The NMR samples were prepared as-523 received (i.e. no deuterated solvent was added) and chemical shifts are referenced using the lock-field 524 determined for a secondary CDCl3 sample. A Pi/6 pulse was used for excitation with an interscan delay 525 of 15 seconds. Inverse-gated 1H decoupling was applied during acquisition. Data were analyzed using 526  Theoretical average molecular weight of extended chains 528 For a platinum-catalyzed reaction system of telechelic Si-H functional PDMS with telechelic vinyl func-529 tional PDMS using a small excess of Si-H groups, both the hydrosilylation reaction between Si-H and 530 vinyl groups and the crosslinking of Si-H take place. Assuming the two reactions happen strictly in se-531 quence, the hydrosilylation reaction results in extended chains, which are cross-linked into elastomers by 532 subsequent crosslinking of excess Si-H. According to mass balance equation: 533 Where DMS−H is the molecular weight of telechelic Si-H functional PDMS, DMS−v is the molecular 534 weight of telechelic vinyl functional PDMS, extended is the average molar mass of the extended network 535 strands and R is the molar ratio of the Si-H to vinyl functional groups. extended is expressed as: 536

Theoretical molecular weight between Si-H groups on intermediate bottlebrush polymers 537
Assuming crosslinking of Si-H takes place strictly after the full grafting of side chains, molecular weight 538 Where is the density of silicone elastomer. is the gas constant. is the absolute temperature. ∅ is the 545 gel fraction of elastomers. is the shear modulus of elastomers. 546  Figure S1. SEC curves of starting polymer DMS-H11 and its liquid products after heating at 100℃ for 557 48 h under dry N2, wet N2 and dry air atmospheres, respectively. 558  Figure S4. Uniaxial stress-strain curves of extremely soft silicone elastomers and a conventional silicone 568 elastomer. 569