94
6 Ultrasound as Mechanical Force
Fig. 6.5 Schematic diagram of the flow cell employed for measuring real-time ultrasonic activation rates of the spiropyran mechanophore. Copyright 2016 by the American Chemical Society.
Reproduced with permission
the rate of polymer chain scission (Thomas 1959). With the advent of mechanophores,
the effect of structure on chain breakage can be re-assessed in detail, while the effects
of MW and polymerization degree can be decoupled. In fact, Moore and co-workers
have investigated the mechanochemical ring opening of spiropyran, which was used
as a model mechanophore and embedded into a variety of acrylate polymers with
different repeating units and side chain compositions, as well as its conversion into the
merocyanine form (Fig. 6.3, top). Kinetic information was obtained using real-time
changes in UV–Vis absorption spectra (May et al. 2016). Continuous ultrasound
irradiation (10.7 W/cm
2 at 3–5 °C), in methyl ethyl ketone (in which all of the
polymers had good solubility), was implemented in a flow set-up at a constant flow
rate of 4.5 mL/min (Fig. 6.5).
As shown by plots of kinetic constants versus degree of polymerization, for which
all data collapse onto a single linear relationship, it is polymerization degree that was
found to be the crucial descriptor of mechanochemical activity, regardless of MW
and side chain constitution. Conversely, plots of rate constant against MW gave rise
to differing linear regressions depending on the pendant group (Fig. 6.6).
Cavitation is a prototypical example of mechanochemistry in solution as it exerts
its mechanical action through solvodynamic shear forces. While mass and degree
of polymerization are the principle characteristics at play, polymer architectures can
also be decorated with other structural motifs, such as cross-linking and hydrogen
bonding, which ultimately determine folding properties, like in proteins and their
analogues. In fact, sonication has been applied to dilute samples and the changes in
6 Ultrasound as Mechanical Force
Fig. 6.5 Schematic diagram of the flow cell employed for measuring real-time ultrasonic activation rates of the spiropyran mechanophore. Copyright 2016 by the American Chemical Society.
Reproduced with permission
the rate of polymer chain scission (Thomas 1959). With the advent of mechanophores,
the effect of structure on chain breakage can be re-assessed in detail, while the effects
of MW and polymerization degree can be decoupled. In fact, Moore and co-workers
have investigated the mechanochemical ring opening of spiropyran, which was used
as a model mechanophore and embedded into a variety of acrylate polymers with
different repeating units and side chain compositions, as well as its conversion into the
merocyanine form (Fig. 6.3, top). Kinetic information was obtained using real-time
changes in UV–Vis absorption spectra (May et al. 2016). Continuous ultrasound
irradiation (10.7 W/cm
2 at 3–5 °C), in methyl ethyl ketone (in which all of the
polymers had good solubility), was implemented in a flow set-up at a constant flow
rate of 4.5 mL/min (Fig. 6.5).
As shown by plots of kinetic constants versus degree of polymerization, for which
all data collapse onto a single linear relationship, it is polymerization degree that was
found to be the crucial descriptor of mechanochemical activity, regardless of MW
and side chain constitution. Conversely, plots of rate constant against MW gave rise
to differing linear regressions depending on the pendant group (Fig. 6.6).
Cavitation is a prototypical example of mechanochemistry in solution as it exerts
its mechanical action through solvodynamic shear forces. While mass and degree
of polymerization are the principle characteristics at play, polymer architectures can
also be decorated with other structural motifs, such as cross-linking and hydrogen
bonding, which ultimately determine folding properties, like in proteins and their
analogues. In fact, sonication has been applied to dilute samples and the changes in