6.2 Sonochemistry: Tribochemical Interpretations
89
Unlike solid–solid contact, during which even plasma states can be created, the
generation and transmission of large tensile forces in liquids is a more complex
process, one in which molecular control is difficult to attain. In fact, most scissions
in liquids stem from the radical species formed inside the bubbles, so long as the
parent reagents are volatile enough. However, shear forces may homolytically fragment large molecules (e.g. polymers) into smaller monomers, which can undergo
further reactions and/or recombination (Teo et al. 2012). The mechanical properties
of ultrasonic irradiation can be compared to those of an elongational flow field, in
that its strain rate depends on the time passed since the onset of bubble collapse,
and hence the radius of the imploding void, as well as on the distance to the bubble (Nguyen et al. 1997). Ultrasonic agitation provides high strain rates (up to 10
7
s
−1 ), compared to other flow fields, while the average masses required for chain scission or mechanophore activation (Sect. 6.3) are significantly lower, at ca. 10
4 –10
5
Da, and are synthetically attainable molecular weights. Polymers are therefore more
sensitive to the mechanical effects of sonication than small molecules, which may
undergo fragmentation induced by secondary radical species. The rest of the chapter
will give these issues the detailed attention they deserve as polymer mechanochemistry (Sohma 1989) is currently enjoying a significant resurgence, thanks, in part, to
ultrasound technology, and is becoming a solid foundation upon which a number of
innovations are being built (Cravotto et al. 2013; Li et al. 2015; Cintas et al. 2015).
6.3 The Mechanophore Concept: Ultrasonication
of Polymers
Although thermal and mechanical effects coexist during bubble collapse, bond rupture is not selective and thus limits the scope of sonication in site-selective reactions
in which numerous functional groups are present. Nevertheless, strained molecules
and weak bonds will undergo more facile fragmentation than others and this effect
may be amplified in molecules that have been adsorbed onto surfaces (i.e. heterogeneous sonochemistry), and when such groups are incorporated into polymer chains
that can act as tweezers and propagate the mechanical action further (Encina et al.
1980).
As early as the 1950s, Melville and Murray performed ultrasonication experiments on a styrene-containing poly(methacrylate) solution, which resulted in polymer degradation and repolymerization (Melville and Murray 1950). This clearly
provided indirect evidence of homolytic bond breaking to produce radical species
initiators. Cavitation-induced cleavage in polymers is not completely random and,
although it does depend somewhat on molecular weight (MW) and chain length (vide
infra), polymer chains tend to break near the middle of large chains. This tendency can
be harnessed by inserting small structural units between polymer chains, as demonstrated by Moore and associates when they placed an azo group near the centre of
poly(ethyleneglycol) as it undergoes specific cleavage under sonication (Berkowski
89
Unlike solid–solid contact, during which even plasma states can be created, the
generation and transmission of large tensile forces in liquids is a more complex
process, one in which molecular control is difficult to attain. In fact, most scissions
in liquids stem from the radical species formed inside the bubbles, so long as the
parent reagents are volatile enough. However, shear forces may homolytically fragment large molecules (e.g. polymers) into smaller monomers, which can undergo
further reactions and/or recombination (Teo et al. 2012). The mechanical properties
of ultrasonic irradiation can be compared to those of an elongational flow field, in
that its strain rate depends on the time passed since the onset of bubble collapse,
and hence the radius of the imploding void, as well as on the distance to the bubble (Nguyen et al. 1997). Ultrasonic agitation provides high strain rates (up to 10
7
s
−1 ), compared to other flow fields, while the average masses required for chain scission or mechanophore activation (Sect. 6.3) are significantly lower, at ca. 10
4 –10
5
Da, and are synthetically attainable molecular weights. Polymers are therefore more
sensitive to the mechanical effects of sonication than small molecules, which may
undergo fragmentation induced by secondary radical species. The rest of the chapter
will give these issues the detailed attention they deserve as polymer mechanochemistry (Sohma 1989) is currently enjoying a significant resurgence, thanks, in part, to
ultrasound technology, and is becoming a solid foundation upon which a number of
innovations are being built (Cravotto et al. 2013; Li et al. 2015; Cintas et al. 2015).
6.3 The Mechanophore Concept: Ultrasonication
of Polymers
Although thermal and mechanical effects coexist during bubble collapse, bond rupture is not selective and thus limits the scope of sonication in site-selective reactions
in which numerous functional groups are present. Nevertheless, strained molecules
and weak bonds will undergo more facile fragmentation than others and this effect
may be amplified in molecules that have been adsorbed onto surfaces (i.e. heterogeneous sonochemistry), and when such groups are incorporated into polymer chains
that can act as tweezers and propagate the mechanical action further (Encina et al.
1980).
As early as the 1950s, Melville and Murray performed ultrasonication experiments on a styrene-containing poly(methacrylate) solution, which resulted in polymer degradation and repolymerization (Melville and Murray 1950). This clearly
provided indirect evidence of homolytic bond breaking to produce radical species
initiators. Cavitation-induced cleavage in polymers is not completely random and,
although it does depend somewhat on molecular weight (MW) and chain length (vide
infra), polymer chains tend to break near the middle of large chains. This tendency can
be harnessed by inserting small structural units between polymer chains, as demonstrated by Moore and associates when they placed an azo group near the centre of
poly(ethyleneglycol) as it undergoes specific cleavage under sonication (Berkowski