96
6 Ultrasound as Mechanical Force
cross-linkage enhances the stability of the chains and the polymer becomes essentially insensitive to solvodynamic shear when containing above 10 mol% cross-link
density.
As mentioned earlier, it is not easy to distinguish thermal effects from purely
mechanical ones during cavitation because thermal activation will inevitably occur
at the bubble interface and in the bulk solution, even though high polymers will not
be activated inside the microbubble. Sijbesma and associates addressed this issue by
generating active carbene catalyst after mechanophore scission (see Fig. 6.3, bottom), and by performing the reaction under methane (CH 4 ), instead of argon. The
change in gas largely suppresses the thermal effects of ultrasound and concomitantly enhances the activity of the in situ generated catalyst, as displayed by the performance of mechanochemically activated transesterification and alkene metathesis
(Groote et al. 2012). The suppression of thermal effects caused by using CH 4 as the
saturation gas apparently increases catalyst lifetime and hence, catalytic activity. The
role that CH 4 plays can be rationalized using well-known sonochemical arguments.
First, the use of a polyatomic, instead of a monoatomic, gas makes liquid cavitation
difficult (Mason 1991). Accordingly, the enhanced survival of the free catalyst arises
from its avoiding secondary sonochemical reactions, such as the formation of radical species or products derived from solvent pyrolysis. Moreover, methane is more
soluble in organic solvents, hydrocarbons in particular, than argon, and this greater
solubility leads to less violent bubble collapse, which also translates into slower
polymer scission (Price and Smith 1993).
6.5 Conclusion
The last decade has seen numerous studies that have focused on the action
of ultrasound on polymer chains that contain weak bonds or functional groups
(mechanophores), revealing the mechanical effects of cavitational collapse. This
relatively new ultrasound-induced chemistry can be tailored for multiple productive
purposes, which include the design of smart and stimuli-responsive materials and the
high performance of sonochemically activated latent catalysts, among others. Such
mechanical effects depend strongly on molecular structure (degree of polymerization
in particular) and can be advantageously decoupled from thermal effects via accurate control of working parameters, such as temperature, intensity and the nature of
dissolved gases.
References
Berkowski KL, Potisek SL, Hickenboth CR, Moore JS (2005) Ultrasound-induced site-specific
cleavage of azo-functionalized poly(ethyleneglycol). Macromolecules 38:8975–8978
Boldyrev VV (1995) Mechanochemistry and sonochemistry. Ultrason Sonochem 2:S143–S145
6 Ultrasound as Mechanical Force
cross-linkage enhances the stability of the chains and the polymer becomes essentially insensitive to solvodynamic shear when containing above 10 mol% cross-link
density.
As mentioned earlier, it is not easy to distinguish thermal effects from purely
mechanical ones during cavitation because thermal activation will inevitably occur
at the bubble interface and in the bulk solution, even though high polymers will not
be activated inside the microbubble. Sijbesma and associates addressed this issue by
generating active carbene catalyst after mechanophore scission (see Fig. 6.3, bottom), and by performing the reaction under methane (CH 4 ), instead of argon. The
change in gas largely suppresses the thermal effects of ultrasound and concomitantly enhances the activity of the in situ generated catalyst, as displayed by the performance of mechanochemically activated transesterification and alkene metathesis
(Groote et al. 2012). The suppression of thermal effects caused by using CH 4 as the
saturation gas apparently increases catalyst lifetime and hence, catalytic activity. The
role that CH 4 plays can be rationalized using well-known sonochemical arguments.
First, the use of a polyatomic, instead of a monoatomic, gas makes liquid cavitation
difficult (Mason 1991). Accordingly, the enhanced survival of the free catalyst arises
from its avoiding secondary sonochemical reactions, such as the formation of radical species or products derived from solvent pyrolysis. Moreover, methane is more
soluble in organic solvents, hydrocarbons in particular, than argon, and this greater
solubility leads to less violent bubble collapse, which also translates into slower
polymer scission (Price and Smith 1993).
6.5 Conclusion
The last decade has seen numerous studies that have focused on the action
of ultrasound on polymer chains that contain weak bonds or functional groups
(mechanophores), revealing the mechanical effects of cavitational collapse. This
relatively new ultrasound-induced chemistry can be tailored for multiple productive
purposes, which include the design of smart and stimuli-responsive materials and the
high performance of sonochemically activated latent catalysts, among others. Such
mechanical effects depend strongly on molecular structure (degree of polymerization
in particular) and can be advantageously decoupled from thermal effects via accurate control of working parameters, such as temperature, intensity and the nature of
dissolved gases.
References
Berkowski KL, Potisek SL, Hickenboth CR, Moore JS (2005) Ultrasound-induced site-specific
cleavage of azo-functionalized poly(ethyleneglycol). Macromolecules 38:8975–8978
Boldyrev VV (1995) Mechanochemistry and sonochemistry. Ultrason Sonochem 2:S143–S145