parameters for the solvent and the polymer, respectively. The solubility of a
polymer in a solvent is favored when (δ 1 Àδ 2 )
2 is minimized, i.e., when the
solubility parameters of the two components are most closely matched. The value
of δ 1 for aliphatic and some aromatic hydrocarbons, as a model for crude oil, is
close to the δ 2 of rubbers such as PIB, CBR, and SBR (16.5–16.8, 18.0, and
18.1 MPa
1/2 , respectively) [51, 167, 168]. As a consequence, crude oil is a good
solvent for these rubbers and can be used for the production of cryogels for use as
oil sorbents.
As detailed in Sect. 2.1, the dilute solutions of rubbers such as PIB, SBR, and
CBR can be crosslinked using sulfur monochloride (S 2 Cl 2 ) as a crosslinking agent.
Experiments show that S 2 Cl 2 is an efficient crosslinking agent in organic solutions
such as benzene or cyclohexane, even at very low reaction temperatures down to
À22
C and at crosslinker ratios down to about 0.9 mol S 2 Cl 2 /mol internal vinyl
group on the rubber [52, 53, 55]. The reaction between S 2 Cl 2 and the unsaturated
groups of the rubbers PIB, CBR, and SBR in frozen organic solutions is due to the
effect of cryo-concentration below the freezing point of the solvent. For instance,
when a 5 % (w/v) PIB solution in benzene is frozen at À18
C, 14 % of the benzene
remains unfrozen in the apparently frozen system, making the PIB concentration in
the unfrozen regions about 36 %, which is high enough to conduct the crosslinking
reactions even at À18
C [52].
Figure 23 shows SEM images of CBR, SBR, and PIB cryogels formed in
benzene at T prep ¼ À18
C and at a rubber concentration C R of 5 % (w/v)
[55]. The crosslinker (S 2 Cl 2 ) concentration is 6 and 12 % in the upper and lower
panels, respectively. The cryogels derived from CBR and SBR exhibit an aligned
porous structure consisting of regular pores with sizes 10
1 –10
2
μm, separated by
pore walls of 10–20 μm in thickness. Decreasing the concentration of CBR or SBR
in the feed further increased the regularity of the porous structure [55]. As indicated
in Sect. 4.1, the regularity of the pore structure increases with decreasing freezing
rate of the reaction solution, or by conducting the cryogelation reactions under
isothermal conditions. The latter facilitates homogeneous nucleation of solvent
crystals so that the polymer network formed will exhibit monodisperse pores. In
the present case, gelation occurs non-isothermally during the initial cooling period
of the reaction solution from 20
C to À18
C, which takes place in about 5 min
[55]. Since decreasing the polymer concentration C R also decreases the rate of the
crosslinking reactions, the gel point is shifted towards longer reaction times as C R is
decreased, so that gelation occurs isothermally in the apparently frozen system,
leading to the formation of more regular pores. In contrast, gel networks formed
from PIB exhibit a broad size distribution of irregular pores from micrometer to
millimeter sizes. Thus, the use of PIB in the gel preparation destroys the regularity
of the porous structure in the organogels.
The variation in the morphologies of the cryogels depends on the type of rubber
and originates from the different solvating power of benzene for different types of
rubber. For instance, Fig. 24a shows the normalized weight swelling ratio m rel of
conventional gels based on PIB, CBR, and SBR in benzene plotted against the
swelling temperature [55]. Both CBR and SBR gels exhibit temperatureindependent swelling behavior and, thus, benzene is a good solvent for these
148
O. Okay and V.I. Lozinsky
polymer in a solvent is favored when (δ 1 Àδ 2 )
2 is minimized, i.e., when the
solubility parameters of the two components are most closely matched. The value
of δ 1 for aliphatic and some aromatic hydrocarbons, as a model for crude oil, is
close to the δ 2 of rubbers such as PIB, CBR, and SBR (16.5–16.8, 18.0, and
18.1 MPa
1/2 , respectively) [51, 167, 168]. As a consequence, crude oil is a good
solvent for these rubbers and can be used for the production of cryogels for use as
oil sorbents.
As detailed in Sect. 2.1, the dilute solutions of rubbers such as PIB, SBR, and
CBR can be crosslinked using sulfur monochloride (S 2 Cl 2 ) as a crosslinking agent.
Experiments show that S 2 Cl 2 is an efficient crosslinking agent in organic solutions
such as benzene or cyclohexane, even at very low reaction temperatures down to
À22
C and at crosslinker ratios down to about 0.9 mol S 2 Cl 2 /mol internal vinyl
group on the rubber [52, 53, 55]. The reaction between S 2 Cl 2 and the unsaturated
groups of the rubbers PIB, CBR, and SBR in frozen organic solutions is due to the
effect of cryo-concentration below the freezing point of the solvent. For instance,
when a 5 % (w/v) PIB solution in benzene is frozen at À18
C, 14 % of the benzene
remains unfrozen in the apparently frozen system, making the PIB concentration in
the unfrozen regions about 36 %, which is high enough to conduct the crosslinking
reactions even at À18
C [52].
Figure 23 shows SEM images of CBR, SBR, and PIB cryogels formed in
benzene at T prep ¼ À18
C and at a rubber concentration C R of 5 % (w/v)
[55]. The crosslinker (S 2 Cl 2 ) concentration is 6 and 12 % in the upper and lower
panels, respectively. The cryogels derived from CBR and SBR exhibit an aligned
porous structure consisting of regular pores with sizes 10
1 –10
2
μm, separated by
pore walls of 10–20 μm in thickness. Decreasing the concentration of CBR or SBR
in the feed further increased the regularity of the porous structure [55]. As indicated
in Sect. 4.1, the regularity of the pore structure increases with decreasing freezing
rate of the reaction solution, or by conducting the cryogelation reactions under
isothermal conditions. The latter facilitates homogeneous nucleation of solvent
crystals so that the polymer network formed will exhibit monodisperse pores. In
the present case, gelation occurs non-isothermally during the initial cooling period
of the reaction solution from 20
C to À18
C, which takes place in about 5 min
[55]. Since decreasing the polymer concentration C R also decreases the rate of the
crosslinking reactions, the gel point is shifted towards longer reaction times as C R is
decreased, so that gelation occurs isothermally in the apparently frozen system,
leading to the formation of more regular pores. In contrast, gel networks formed
from PIB exhibit a broad size distribution of irregular pores from micrometer to
millimeter sizes. Thus, the use of PIB in the gel preparation destroys the regularity
of the porous structure in the organogels.
The variation in the morphologies of the cryogels depends on the type of rubber
and originates from the different solvating power of benzene for different types of
rubber. For instance, Fig. 24a shows the normalized weight swelling ratio m rel of
conventional gels based on PIB, CBR, and SBR in benzene plotted against the
swelling temperature [55]. Both CBR and SBR gels exhibit temperatureindependent swelling behavior and, thus, benzene is a good solvent for these
148
O. Okay and V.I. Lozinsky