the vicinity of water molecule reduces its probability to perform angular jumps and
slows down the rotational motion.
In the following years, the femtosecond pump–probe IR spectroscopy (fs-IR) has
been successfully applied to study liquid water and aqueous systems most intensly by
groups of Bakker, Nibbering [97, 98] and Tokmakoff [99]. Those researches resulted
in determination of the vibrational relaxation time 1 ! 0 of stretching OH
(T 1 = 0.26 ps) [100] and bending H–O–H mode (T 2 = 0.17 ps) [101] in neat H 2 O,
but also (with the use of two-colour experiments) the relaxation channel of OH
stretching excitation through H–O–H bending down to librational mode was unravelled [101–103]. It should be noted that extremely fast kinetics of these processes
result from the resonant energy transfer between H-bonded OH groups [92]—in
isotopically diluted water, this relaxation proceeds much more slowly; T 1 = 0.74 ps
for OH in HDO/D 2 O [104], while T 1 = 1.8 ps for OD in HDO/H 2 O [105] (all values
of vibrational relaxation times are given for the temperature of 298 K).
Nonlinear vibrational spectroscopy methods have been scarcely applied to aqueous polymer systems so far. Hunger, Mazur and co-workers applied fs-IR together
with dielectric spectroscopy and DSC to study water dynamics around polysaccharides (hyaluronan, dextran [106] and alginates [107]). Remarkably, they observed by
both fs-IR and DSC methods around 15 water molecules bound by hyaluronan
monomer unit. Moreover, their results suggest that the effect of studied polysaccharide on the dynamics of water is restricted solely to the first hydration shell.
Applications of surface-specific vibrational sum-frequency generation spectroscopies (VSFG) to studies of the water structure and dynamics at interfaces,
although of paramount significance, are not covered in this review due to the
breadth of the topic. Some VSFG results on aqueous pH-responsive polymer systems will be discussed in the next section.
8.3 pH-Sensitive Systems—Polyelectrolytes
The group of pH-responsive polymers is together with thermo-responsive polymers
(described in the next section) the most commonly synthesized and investigated
types of SRPS. They undergo some sort of a structural transition (polymer chain
collapse/extension, swelling/deswelling, micellization, gelation, etc.) in response to
the change in pH of their environment. The pH-responsiveness of these systems is
linked with ionizable, basic or acidic groups incorporated into a polymer main
chain. Henceforth, the pH-responsive polymers are classified as polyacids and
polybases. Some authors single out an additional category—natural pH-responsive
polymers like chitosan or some ionizable polypeptides (polyglutamic acid,
polyaspartic acid, etc.) [108]. The most often applied polyacids are poly (carboxylic
acids), poly (phosphoric acids), poly (sulphonic acids) and poly (aminoacids).
Popular polybases are polypyridines, polyimidiazoles and polymers containing
tertiary amino groups. It should be also mentioned that polyacids and polybases
may form a polymer complex stable at the given pH range [109].
8 Vibrational Spectroscopy in Analysis of Stimuli-Responsive …
235
slows down the rotational motion.
In the following years, the femtosecond pump–probe IR spectroscopy (fs-IR) has
been successfully applied to study liquid water and aqueous systems most intensly by
groups of Bakker, Nibbering [97, 98] and Tokmakoff [99]. Those researches resulted
in determination of the vibrational relaxation time 1 ! 0 of stretching OH
(T 1 = 0.26 ps) [100] and bending H–O–H mode (T 2 = 0.17 ps) [101] in neat H 2 O,
but also (with the use of two-colour experiments) the relaxation channel of OH
stretching excitation through H–O–H bending down to librational mode was unravelled [101–103]. It should be noted that extremely fast kinetics of these processes
result from the resonant energy transfer between H-bonded OH groups [92]—in
isotopically diluted water, this relaxation proceeds much more slowly; T 1 = 0.74 ps
for OH in HDO/D 2 O [104], while T 1 = 1.8 ps for OD in HDO/H 2 O [105] (all values
of vibrational relaxation times are given for the temperature of 298 K).
Nonlinear vibrational spectroscopy methods have been scarcely applied to aqueous polymer systems so far. Hunger, Mazur and co-workers applied fs-IR together
with dielectric spectroscopy and DSC to study water dynamics around polysaccharides (hyaluronan, dextran [106] and alginates [107]). Remarkably, they observed by
both fs-IR and DSC methods around 15 water molecules bound by hyaluronan
monomer unit. Moreover, their results suggest that the effect of studied polysaccharide on the dynamics of water is restricted solely to the first hydration shell.
Applications of surface-specific vibrational sum-frequency generation spectroscopies (VSFG) to studies of the water structure and dynamics at interfaces,
although of paramount significance, are not covered in this review due to the
breadth of the topic. Some VSFG results on aqueous pH-responsive polymer systems will be discussed in the next section.
8.3 pH-Sensitive Systems—Polyelectrolytes
The group of pH-responsive polymers is together with thermo-responsive polymers
(described in the next section) the most commonly synthesized and investigated
types of SRPS. They undergo some sort of a structural transition (polymer chain
collapse/extension, swelling/deswelling, micellization, gelation, etc.) in response to
the change in pH of their environment. The pH-responsiveness of these systems is
linked with ionizable, basic or acidic groups incorporated into a polymer main
chain. Henceforth, the pH-responsive polymers are classified as polyacids and
polybases. Some authors single out an additional category—natural pH-responsive
polymers like chitosan or some ionizable polypeptides (polyglutamic acid,
polyaspartic acid, etc.) [108]. The most often applied polyacids are poly (carboxylic
acids), poly (phosphoric acids), poly (sulphonic acids) and poly (aminoacids).
Popular polybases are polypyridines, polyimidiazoles and polymers containing
tertiary amino groups. It should be also mentioned that polyacids and polybases
may form a polymer complex stable at the given pH range [109].
8 Vibrational Spectroscopy in Analysis of Stimuli-Responsive …
235