polymers over the whole range of temperatures investigated. This indicates that
CBR and SBR remain in the solution during the initial cooling period of the
gelation reactions so that only the cryogelation mechanism is responsible for
formation of the pores in these networks. However, the swelling capacity of PIB
gel strongly depends on the temperature (Fig. 24a); the gel continuously deswells as
the temperature is decreased and benzene becomes a poor solvent at low temperatures. Thus, PIB chains are not only expelled from the solution due to cryoconcentration but they also undergo a cooling-induced phase separation to form
agglomerates of various sizes. Accordingly, both cryogelation and cooling-induced
phase separation mechanisms govern the process of formation of the pores in PIB
networks. The formation of millimeter-sized large pores also supports the idea that
two mechanisms are operative in the formation of the porous structure in PIB
networks.
The total pore volume V p of the cryogels formed at C R ¼ 5 % (w/v) was
7 Æ 0.5 mL/g, independent of the type of rubber and the crosslinker concentration.
The value of V p further increased with decreasing the rubber concentration C R to
2.5 % (w/v) [55]. Typical stress–strain data of the cryogels derived from PIB, CBR,
and SBR are shown in Fig. 24b. None of the rubber cryogels broke, even at a strain
of 99.9 %. Sorption tests show that the cryogels are effective sorbents for various
pollutants, e.g., crude oil, gasoline, diesel, and olive oil [54, 55, 69]. The sorption
rate of all cryogels is very rapid due to their interconnected pore structure and they
absorb the pollutants in less than 10–20 min to attain thermodynamic equilibrium.
Moreover, PIB gels exhibit the fastest uptake rate for the pollutants, probably due to
their large pores formed by phase separation. In Fig. 25a, the maximum sorption
capacities of CBR, SBR, and PIB cryogels are compared for various pollutants.
SBR or CBR cryogels sorb 35–38 g crude oil or 23–26 g olive oil per gram, as
compared to 23 g/g and 9 g/g, respectively, using the PIB gel. We should mention
that the widely used oil sorbents based on polypropylene have sorption capacities
for both crude oil and olive oil of about 15 g/g [54]. Thus, SBR and CBR gels have
sorption capacities about twice that of the commercial oil sorbents. Another point
shown in Fig. 25a is that the sorption capacity of the cryogels is highest for crude
oil, i.e., for the pollutant with highest viscosity. This implies that, although
increased viscosity of the pollutant decreases the rate of sorption, favorable hydrophobic interactions between the crude oil and the hydrophobic polymer dominate
the sorption process so that more oil is retained within the rubber gels.
The reusability of rubber cryogels and their continuous sorption capacities were
also demonstrated by conducting sorption–squeezing cycles [54, 55]. The cycles
were repeated 20 times to obtain the recycling efficiency and continuous extraction
capacity of the cryogels. The results are shown in Fig. 26, where the uptake capacity
of the gels in each cycle for various pollutants is plotted against the number of
sorption–squeezing cycles. The amount of pollutant sorbed in each cycle is almost
150
O. Okay and V.I. Lozinsky
CBR and SBR remain in the solution during the initial cooling period of the
gelation reactions so that only the cryogelation mechanism is responsible for
formation of the pores in these networks. However, the swelling capacity of PIB
gel strongly depends on the temperature (Fig. 24a); the gel continuously deswells as
the temperature is decreased and benzene becomes a poor solvent at low temperatures. Thus, PIB chains are not only expelled from the solution due to cryoconcentration but they also undergo a cooling-induced phase separation to form
agglomerates of various sizes. Accordingly, both cryogelation and cooling-induced
phase separation mechanisms govern the process of formation of the pores in PIB
networks. The formation of millimeter-sized large pores also supports the idea that
two mechanisms are operative in the formation of the porous structure in PIB
networks.
The total pore volume V p of the cryogels formed at C R ¼ 5 % (w/v) was
7 Æ 0.5 mL/g, independent of the type of rubber and the crosslinker concentration.
The value of V p further increased with decreasing the rubber concentration C R to
2.5 % (w/v) [55]. Typical stress–strain data of the cryogels derived from PIB, CBR,
and SBR are shown in Fig. 24b. None of the rubber cryogels broke, even at a strain
of 99.9 %. Sorption tests show that the cryogels are effective sorbents for various
pollutants, e.g., crude oil, gasoline, diesel, and olive oil [54, 55, 69]. The sorption
rate of all cryogels is very rapid due to their interconnected pore structure and they
absorb the pollutants in less than 10–20 min to attain thermodynamic equilibrium.
Moreover, PIB gels exhibit the fastest uptake rate for the pollutants, probably due to
their large pores formed by phase separation. In Fig. 25a, the maximum sorption
capacities of CBR, SBR, and PIB cryogels are compared for various pollutants.
SBR or CBR cryogels sorb 35–38 g crude oil or 23–26 g olive oil per gram, as
compared to 23 g/g and 9 g/g, respectively, using the PIB gel. We should mention
that the widely used oil sorbents based on polypropylene have sorption capacities
for both crude oil and olive oil of about 15 g/g [54]. Thus, SBR and CBR gels have
sorption capacities about twice that of the commercial oil sorbents. Another point
shown in Fig. 25a is that the sorption capacity of the cryogels is highest for crude
oil, i.e., for the pollutant with highest viscosity. This implies that, although
increased viscosity of the pollutant decreases the rate of sorption, favorable hydrophobic interactions between the crude oil and the hydrophobic polymer dominate
the sorption process so that more oil is retained within the rubber gels.
The reusability of rubber cryogels and their continuous sorption capacities were
also demonstrated by conducting sorption–squeezing cycles [54, 55]. The cycles
were repeated 20 times to obtain the recycling efficiency and continuous extraction
capacity of the cryogels. The results are shown in Fig. 26, where the uptake capacity
of the gels in each cycle for various pollutants is plotted against the number of
sorption–squeezing cycles. The amount of pollutant sorbed in each cycle is almost
150
O. Okay and V.I. Lozinsky