subjected to mechanical cycles exhibit hysteresis whose extent continuously
increases with the maximum strain indicating breakage of intermolecular bonds
of varying strength [108]. However, if the cyclic tests are repeated several times
with a wait time of 7 min between cycles, all the cycles overlap well for a
given maximum strain, which is an indication of reformation of the broken bonds
during the wait time.
Figure 12b shows maximum strain dependence of the hysteresis energy U hys ,
calculated as the area surrounded by the loading and unloading curves, of
successive tensile (filled circles) and compressive cycles (open circles). Because
uniaxial compression ratio equals to the reciprocal of the square root of the biaxial
extension ratio, U hys data could be plotted against a common abscissa, namely, the
uniaxial (λ max ) and biaxial maximum stretch ratios (λ biax,max ). Over the whole
range of maximum strain, the hysteresis energies U hys fall into a single curve
revealing that U hys is independent on the type of strain, and it only depends on the
value of the maximum strain. Calculation of the fraction f v of reversibly broken
bonds during the mechanical cycles reveals that more than half of the intermolecular
bonds, i.e., up to at least f v ¼ 0.56, can be broken reversibly at a stretch ratio of
around 20 (Fig. 12c) [108]. This is a sign of a high self-healing efficiency of
PDMAA hydrogels without any external stimuli. Indeed, a complete healing in
these hydrogels was achieved by holding their cut surfaces together at 24
C for
20 min (Fig. 12d) [108].
The mechanism of autonomic self-healing in surfactant-containing hydrogels was
recently investigated by scanning force microscopy (SFM) measurements [102]. For
this purpose, the surface of a hydrophobically modified PAAm hydrogel containing
2 mol% C17.3M was first cut with the SFM tip to create trenches of 20–40 nm in
depth and protrusions of around 10 mm in height. Figure 13A shows topographic
images (left panel) and cross sections of the gel surface (right panel) just after cutting
(a), a few seconds after cutting (b), and after a wait time of 75 min (c). Interestingly, a
terraced topography was observed for both protrusions and trenches with a step
height between 3.8 and 5.0 nm, or multiples thereof, as indicated by the horizontal
dotted lines in Fig. 13A. XRD measurements revealed that the smallest step height is
close to the d spacing of the hydrogel (3.9 nm) [102]. This indicates the existence of
layered hydrophobic nanodomains in the hydrophobically modified hydrogels as
observed in comb-like polymers with alkyl side chains [110] and fluorocarbon-based
hydrogels [83, 84]. Moreover, immediately after damaging, both the trenches and
protrusions on the gel surface transform into rounded shapes without affecting their
depth and height, respectively (a to b in Fig. 13A). In contrast to the fast reshaping
process in the damaged area, the healing process, that is, the size reduction of the
holes and islands on the hydrogel surface, and their disappearance require a relatively long time (b to c in Fig. 13A). Thus, healing of the gel surface occurs in two
steps, namely, a fast reshaping of the damaged area into circular forms without
healing followed by slow size reduction of this area and finally complete healing to
recover the virgin surface [102]. The first step was attributed to the strong attractive
interactions between the alkyl side chains of hydrophobic units locating in close
42
O. Okay
increases with the maximum strain indicating breakage of intermolecular bonds
of varying strength [108]. However, if the cyclic tests are repeated several times
with a wait time of 7 min between cycles, all the cycles overlap well for a
given maximum strain, which is an indication of reformation of the broken bonds
during the wait time.
Figure 12b shows maximum strain dependence of the hysteresis energy U hys ,
calculated as the area surrounded by the loading and unloading curves, of
successive tensile (filled circles) and compressive cycles (open circles). Because
uniaxial compression ratio equals to the reciprocal of the square root of the biaxial
extension ratio, U hys data could be plotted against a common abscissa, namely, the
uniaxial (λ max ) and biaxial maximum stretch ratios (λ biax,max ). Over the whole
range of maximum strain, the hysteresis energies U hys fall into a single curve
revealing that U hys is independent on the type of strain, and it only depends on the
value of the maximum strain. Calculation of the fraction f v of reversibly broken
bonds during the mechanical cycles reveals that more than half of the intermolecular
bonds, i.e., up to at least f v ¼ 0.56, can be broken reversibly at a stretch ratio of
around 20 (Fig. 12c) [108]. This is a sign of a high self-healing efficiency of
PDMAA hydrogels without any external stimuli. Indeed, a complete healing in
these hydrogels was achieved by holding their cut surfaces together at 24
C for
20 min (Fig. 12d) [108].
The mechanism of autonomic self-healing in surfactant-containing hydrogels was
recently investigated by scanning force microscopy (SFM) measurements [102]. For
this purpose, the surface of a hydrophobically modified PAAm hydrogel containing
2 mol% C17.3M was first cut with the SFM tip to create trenches of 20–40 nm in
depth and protrusions of around 10 mm in height. Figure 13A shows topographic
images (left panel) and cross sections of the gel surface (right panel) just after cutting
(a), a few seconds after cutting (b), and after a wait time of 75 min (c). Interestingly, a
terraced topography was observed for both protrusions and trenches with a step
height between 3.8 and 5.0 nm, or multiples thereof, as indicated by the horizontal
dotted lines in Fig. 13A. XRD measurements revealed that the smallest step height is
close to the d spacing of the hydrogel (3.9 nm) [102]. This indicates the existence of
layered hydrophobic nanodomains in the hydrophobically modified hydrogels as
observed in comb-like polymers with alkyl side chains [110] and fluorocarbon-based
hydrogels [83, 84]. Moreover, immediately after damaging, both the trenches and
protrusions on the gel surface transform into rounded shapes without affecting their
depth and height, respectively (a to b in Fig. 13A). In contrast to the fast reshaping
process in the damaged area, the healing process, that is, the size reduction of the
holes and islands on the hydrogel surface, and their disappearance require a relatively long time (b to c in Fig. 13A). Thus, healing of the gel surface occurs in two
steps, namely, a fast reshaping of the damaged area into circular forms without
healing followed by slow size reduction of this area and finally complete healing to
recover the virgin surface [102]. The first step was attributed to the strong attractive
interactions between the alkyl side chains of hydrophobic units locating in close
42
O. Okay