Highly Stretchable and Rapid Self-Recoverable Cryogels Based on Butyl Rubber as Reusable Sorbent

Abstract

Cryogels based on hydrophobic polymers combining good mechanical properties with fast responsivity are attractive materials for many applications, such as oil spill removal from water and passive sampler for organic pollutants. We present, here, cryogels based on butyl rubber (BR) with a high stretchability, rapid self-recoverability, and excellent reusability for organic solvents. BR cryogels were prepared at subzero temperatures in cyclohexane and benzene at various BR concentrations in the presence of sulfur monochloride (S₂Cl₂) as a crosslinker. Although the properties of BR cryogels are independent of the amount of the crosslinker above a critical value, the type of the solvent, the cryogelation temperature, as well as the rubber content significantly affect their properties. It was found that benzene produces larger pore volumes as compared to cyclohexane due to the phase separation of BR from benzene at low temperatures, producing additional pores. Increasing cryogelation temperature from -18 to -2 °C leads to the formation of more ordered and aligned pores in the cryogels. Increasing BR content decreases the amount of unfrozen microphase of the frozen reaction solution, leading to a decrease in the total porosity of the cryogels and the average diameter of pores. Cryogels formed at -2 °C and at 5% (w/v) BR in cyclohexane sustain up to around 1400% stretch ratios. Cryogels swollen in toluene can completely be squeezed under strain during which toluene is released from their pores, whereas addition of toluene to the squeezed cryogels leads to recovery of their original shapes.

Keywords: butyl rubber; cryogels; macroporous rubber gels; mechanical properties; organogels.

Scheme 1

Cartoon showing cryogelation of a polymer solution containing a chemical crosslinker.

Figure 1

Weight and volume swelling ratios m_rel, V_rel, q_w, and q_v (a,b), swollen state porosities P (c), and pore volumes V_p (d) of BR-B (triangles) and BR-C cryogels (circles) shown as a function of BR% (w/v). T_cry = −18 °C. S₂Cl₂ = 10% (v/w). (n = 5, M ± SD).

Figure 2

(a) Fraction f of the unfrozen domains in cyclohexane solution of BR at −18 °C plotted against BR% (w/v). (n = 5, M ± SD). (b) DSC thermograms of BR solutions in cyclohexane. BR concentrations (in % (w/v)) are indicated. (c) True BR concentration (BR_true%) in the unfrozen domains at −18 °C (circles) and its relative value (BR_true,rel) (triangles) as a function of BR% (w/v). The arrows indicate the applicable axes.

Figure 3

SEM images of BR-B (a) and BR-C cryogels (b) prepared at 5 (1), 15 (2), and 20% (w/v) BR (3). S₂Cl₂ = 10% (v/w). T_cry = −18 °C. Scaling bars are 100 µm.

Figure 4

Average pore diameter D of BR-B (triangles) and BR-C (circles) cryogels plotted against the BR% (w/v). T_cry = −18 °C. (n = 5, M ± SD).

Figure 5

SEM images of BR-C cryogels prepared T_cry = −18 (a) and −2 °C (b). BR = 5% (w/v). S₂Cl₂ = 10% (v/w). Scaling bars are 100 µm.

Figure 6

Tensile stress–strain curves of BR-C cryogels prepared at various BR% (w/v) and at T_cry = −2 °C (a) and −18 °C (b).

Figure 7

Young’s modulus E and fracture stress σ_f of BR-C cryogels formed at −2 °C (circles) and −18 °C (triangles) plotted against BR% (w/v). n = 5, M ± SD. Error bars are smaller than the symbols where the bars are not shown.

Figure 8

Photographs BR-C cryogels formed at −18 (a) and −2 °C (b) during the compression tests. BR = 15% (w/v). The images a1–a3 show almost complete compression of a cryogel specimen formed at −18 °C under strain and immediate recovery of its initial shape after unloading. The images b1–b3 show rupture of the cryogel specimen formed at −2 °C upon compression.

Figure 9

Sorption capacity of BR-C cryogels prepared at various BR% (w/v) indicated. T_cry = −18 °C (a) and −2 °C (b). The lines are best linear fits to the data. BR% (w/v) = 5 (●), 10 (▲), 15 (■), and 20 (▼).