Simple linear ionic polysiloxane showing unexpected nanostructure and mechanical properties

Polysiloxanes are ubiquitous materials in industry and daily life derived from silicates, an abundant resource. They exhibit various properties, which depend on the main-chain network structure. Linear (1D backbone) polysiloxanes provide amorphous materials. They are recognized as fluid materials in the form of grease or oil with a low glass transition temperature. Herein we report that a simple linear polysiloxane, poly(3-aminopropylmethylsiloxane) hydrochloride, shows an elastic modulus comparable to that of stiff resins such as poly(tetrafluoroethylene). By introducing an ammonium salt at all the units of this polysiloxane, inter- and intramolecular ionic aggregates form, immensely enhancing the elastic modulus. This polysiloxane is highly hygroscopic, and its modulus can be altered reversibly 100 million times between moist and dry atmospheres. In addition, it works as a good adhesive for glass substrates with a shear strength of more than 1 MPa in the dry state. Despite its simple structure with a flexible backbone, this polymer unexpectedly self-assembles to form an ordered lamellar nanostructure in dry conditions. Consequently, this work reveals new functions and possibilities for polysiloxanes materials by densely introducing ionic groups.

S−3 IR measurements, the polysiloxane films were prepared by spin-coating of a methanol solution (polymer concentration: 5 wt %) onto the UV-O3 cleaned Si wafers (CZ-grown p-type (100) silicon crystal doped with boron in the order of 5−20 •cm). The spincast condition was set at 800 rpm for 120 s. The film samples were set into CaF2 chamber. The spectral resolution was set at 4 cm −1 .
Thermogravimetric analysis (TGA) was taken by a DTG-60 (Shimadzu). The sample was heated up to 500 °C under 50 mL min −1 of N2 flow, but held at 110 °C for 1 h to avoid adsorbed water. Heating rate was set at 5 °C min −1 .

Syntheses of hygroscopic polysiloxanes
PSx(NH2) was synthesized by sol-gel reaction of APDMOS under basic condition as described below. 1.84g of APDMOS (1.1×10 −2 mol), 6.86 g of triethylamine (6.8×10 −2 mol), and 4.58 g of PW (2.5×10 −1 mol) were placed into a glass bottle, then the mixture was stirred at 70 °C for 5h. After the solvents were evaporated from the mixture, residual viscous liquid was vacuum dried overnight at 40 °C. Final product was 1.5 g of clear viscous liquid.
2.00 g of APDMOS (1.2×10 −2 mol) and 3.54 g of HCl (3.6×10 −2 mol) were placed into a glass bottle. This mixture was stirred at r.t. for 2h, successively stirred in open system at 70 °C until all of the solvent evaporated. Residual viscous solid was re-dissolved in a small portion of methanol and vacuum-dried for overnight, then 1.89 g of PSx(NH3 + Cl − ) was obtained. 1 H NMR spectra and 29 Si NMR spectra of the two-types polysiloxanes were shown in Fig. S1 and S2, respectively. In 29 Si NMR spectra, only the chemical shifts of a D unit structure of silicone were observed. The NMR spectra show the formation of polysiloxane without branched structure. Molecular weight and its distribution were characterized by SEC measurements. The SEC chart of PSx(NH3 + Cl − ) was shown in Fig. S3. Broad peak and overlapping sharp peak were detected. The entire peak was obtained corresponding to Mn = 3.2×10 3 and Mw/Mn = 3.67. The sharp peak is assignable to a ring polymer formation.
X-ray diffraction (XRD) measurements were taken by the same equipment described in the main manuscript. S−5

S−7
Hygroscopic nature of the polysiloxane Humidity-controlled IR spectra of the polysiloxane films on humidification process were shown in Fig. S4. The absorbance of the OH stretching vibration ( (OH)) around 3410 cm −1 and the OH bending vibration ( (OH) at 1640 cm −1 and 2 (OH) around 3250 cm −1 ) 1,2 increased on humidification process. The water peaks were not observed for the Si wafers only at RH = 90%, indicating that the assigned peaks can be attributed to water taken into the film. The increase in absorbance of PSx(NH3 + Cl − ) was larger than that of PSx(NH2). This is because the increase depends on the amount of moisture absorbed. This is due to the fact that PSx(NH3 + Cl − ) absorbs more moisture than PSx(NH2), as shown in Fig. 1.

Reversibility of moduli of the polysiloxane film
For the moist PSx(NH3 + Cl − ) spincast films, force curve measurements were performed.
Before measurements, the spincast films were placed in the chamber with RH=60% overnight.
The moist film was measured in an open system. After that, the film was exposed to the environment of N2 atmosphere and force curve was recorded. Finally, we again measured the moist film exposed to RH=60% overnight. As shown in Fig. S5, the elastic modulus changed with the relative humidity. For the moist films, the elastic moduli could not be determined accurately because the humidity is not accurately controlled during measurements and the cantilever did not follow the retract process due to the plastic deformation of the film, but the repetitive humidity response of the modulus change could be qualitatively confirmed. HCl to PSx(NH2) and stirring at r.t. for 2h. The resulting PSx(NH3 + Cl − ) showed scatterings derived from lamellar structure (Fig. S6b). It means amine hydrochloride is necessary for the formation of the lamellar structure. The scatterings disappeared at RH=80%, suggesting the melting of the lamellar structure due to waster absorption (Fig. S6c). The scattering peaks appeared again upon dehumidification, and the formation and deformation of the lamellar structure occurred reversibly depending on the humidity.

Thermal stability
As shown in Fig. S9, a weight loss of 15% was observed at 100 °C, corresponding to the evaporation of water adsorbed on PSx(NH3 + Cl − ). No significant decrease was observed at higher temperature up to 260 °C. This means that PSx(NH3 + Cl − ) did not be pyrolyzed by heating up to o 260 °C.