Malleable, Mechanically Strong, and Adaptive Elastomers Enabled by Interfacial Exchangeable Bonds Zhenghai Tang, Yingjun Liu, Baochun Guo,*, and Liqun Zhang*, Department of Polymer Materials and Engineering, South China University of Technology, Guangzhou, 510640, P. R. China. State Key Laboratory of Organic/Inorganic Composites, Beijing University of Chemical Technology, Beijing, 100029, P. R. China. Determination of the concentration of carboxyl groups on CD. The concentration of carboxyl groups on CD was measured by potentiometric titration method using a MetrOhm 888 Titrando autotitrator. The electrode was calibrated against standard buffer solutions before use. For titration measurement, excess NaOH (n 0 mmol) was firstly added into V 0 ml of CD aqueous dispersion (2 mg/ml), then the mixture solution was titrated against HCl solution (0.16 mmol/ml) and the change in ph was monitored. The end point was identified by monitoring the greatest slope in the titration curve at the equivalence point. The volume (V ml) of the standard HCl solution consumed was noted to calculate the carboxyl group concentration (mmol/g), according to the following equation: (n 0-0.16V) 10 3 /2V 0. The titration instrument was fully automated and controlled using the Tiamo software from Metrohm. The measurement was repeated for three times and average value was given. The uncertainty of the measured concentration was estimated to be ±0.5%. Calculation of cross-linking density. Cross-linking density was determined by equilibrium swelling experiment in toluene based on Flory-Rehner equation. Preweighed vulcanizates were swollen in toluene at room temperature for 72 h, and the solvent is replaced with fresh solvent for each 24 h. After swelling, the solvent was wiped off quickly from the sample surface using filter paper, and the samples were immediately weighed and then dried in a vacuum oven at 60 C until constant
weight. Three specimens were measured for each sample. The volume fraction of rubber in the swollen gel ( ) was calculated according to the following equation: / / / where is the weight of the sample before swelling; and are the weights of the swollen and unswollen sample, respectively; φ is the weight fraction of the insoluble components, such as CD; and are the densities of the rubber and solvent, respectively. The elastically active network chain density can be calculated by the well-known Flory-Rehner equation 1 : ln 1 2 where is the volume fraction of polymer in the swollen sample, χ is the Flory-Huggins polymer solvent interaction parameter (0.341 for ENR and toluene) 2, and is the molar volume of the solvent (106.5 cm 3 /mol for toluene). Figure S1. TEM image for CD.
Figure S2. UV spectrum for CD aqueous dispersion. Table S1. Formulations for CE and SE. Samples CE-50 CE-40 CE-35 CE-30 CE-25 SE a Oxirane/carboxyl group mole ratio 50 40 35 30 25 35 CD (phr) 2.49 3.11 3.55 4.15 4.97 Zn(Ac) 2 2H 2 O (phr) 0.29 0.36 0.41 0.48 0.58 0.41 DMI (phr) 0.32 0.39 0.45 0.53 0.63 0.45 a Sebacic acid cross-linked ENR, the mole ratio of oxirane/carboxyl groups is 35.
Figure S3. Evolutions of carboxyl groups (1820-1560 cm -1 ) and epoxy (960-760 cm -1 ) regions of FTIR spectra during the curing of SE at 180 C. Figure S4. Cross-linking kinetics of CE-35, SE and ENR/DMI mixture.
Figure S5. Photos of CE-35 at visible and UV light (254 nm). Figure S6. Temperature dependence of the (a) storage moduli and (b) tan δ for CE and SE samples.
Figure S7. Creep experiment for CE-35 with a nominal stress of 0.1 MPa at 100 C. Relaxation time τ is calculated from elongational creep experiments (Figure 2c) according to the following equations 3 : η,, τ, that is τ, where η is the viscosity (Pa s), σ is the stress (Pa), ε' is the strain rate (s 1 ) and E is the Young s modulus at 180 o C (Pa). The parameters used to calculate the relaxation time were listed in Table S2. σ is a constant value of 0.1 MPa for all the samples. The strain rate ε' was determined from the slope of the linear fit from creep experiments. E is calculated from the slope of the stress-strain curves (0.5-3% strain) that are measured using DMA Q800 by stretching rectangular samples (10 mm 4 mm 1 mm) at 180 C with a load rate of 0.5 N/min.
Table S2. Strain rate, Young s modulus at 180 o C, and relaxation time. Samples ε' (10-3 min -1 ) E (MPa) τ (min) CE-50 3.8 0.82 31.5 CE-40 2.0 1.01 52.9 CE-35 1.5 1.53 56.2 CE-30 1.2 1.78 63.1 CE-25 0.6 1.81 126.0 SE 2.7 1.59 23.7 Figure S8. Stress relaxation experiments for all the CE samples at 180 o C with a constant strain of 5%.
Figure S9. Stress relaxation experiments for SE at different temperatures with a constant strain of 5%. Figure S10. Stress relaxation experiments for ENR/DMI sample at room temperature with a constant strain of 5%.
Characteristic relaxation times (τ*) follow an Arrhenius law with the temperature τ(t)= τ 0 exp(ea/rt), where τ 0 is the characteristic relaxation time at infinite T, Ea is the activation energy for the transesterification reactions, R is the universal gas constant and T is the temperature at the experiment was conducted. Figure S11. Fitting of τ* to the Arrhenius equation for CE-35 and SE. Figure S12. Comparison on the relaxation curves between CE-35 and uncatalysed CE-35 at 180 o C with a constant strain of 5%.
XPS survey spectrum reveals that our synthesized CD are composed of carbon (83.0%), oxygen (16.7%), and phosphorus (0.3%). In the high resolution P 2p spectrum, the peaks at binding energies of 132.7 and 133.7 ev are ascribed to P-C and P-O bonds, respectively, which confirms the successful doping of phosphorus. 4 Since the content of doped phosphorus is very low, the characteristic peaks associated with phosphorus cannot be well observed from the FTIR spectrum of CD. Figure S13. High resolution P 2p XPS spectrum of CD. Table S3. Mechanical properties of CE and SE samples. The error in the Table is standard deviation (6 specimens for each sample). Samples Young s modulus (MPa) Tensile strength (MPa) Elongation at break (%) CE-50 3.7±0.6 8.7±0.8 510±20 CE-40 10.1±0.4 13.0±1.9 470±20 CE-35 18.5±1.2 17.9±0.4 450±20 CE-30 152±8.6 22.0±1.3 380±10 CE-25 415±9.2 38.9±1.0 13±1 SE 1.7±0.1 5.9±0.2 410±10
Figure S14. Comparison on the stress-strain curves between CE-35 and SE. Figure S15. Loading-unloading curves of SE under different strain (100% and 300%) at room temperature.
Figure S16. Typical stress-strain curves of CE-40 after multiple cycles of recycle. Figure S17. FTIR spectra of CE-35 before and after reprocessing.
Figure S18. Crosslinking density of all the CE samples before and after reprocessing. Figure S19. TGA curves for CE-50, CE-35 and CE-25.
Figure S20. Isothermal TGA of CE-35 under nitrogen atmosphere at 180 o C. Figure S21. Quantitative shape memory and shape reconfiguration cycles for CE-35. Movie S1. Recovery from a temporary linear shape to a permanent helical fusilli-like shape. Movie S2. Recovery from a temporary reverse spiral shape to a permanent positive spiral shape. Movie S3. Step recovery from a temporary spiral shape to a permanent linear shape.
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