Since the polymer hydrogels may be potentially applied as carriers for biologically important molecules (drugs, fertilizers and others), interactions between those
molecules and other components (water and polymer chains) seem to be especially
interesting. Moreover, the presence of additives usually changes the system stability
and impacts the phase transition temperature. Until now, it was found that many
various ionic substances, for example simple inorganic salts such as halogens or
surfactants [158, 163, 164], influence the VPT temperature and dynamics, shifting
the VPT to lower or to higher temperatures. It is postulated that an explanation of
this phenomenon relates to structure-making and structure-breaking properties of
various salts ordered in Hofmeister series—see the previous section [164, 165].
However, also non-ionic substances (for example hydroquinone, phenol, glycerol
or urea [166, 167]) strongly impact VPT.
Based on spectroscopic results, Maeda postulated that additives do not interact
with the polymer skeleton directly, but disturb the water structure simultaneously
influencing the polymer hydration. The direct proof of this thesis was delivered by
Olejniczak et al. [133]. Studies performed on PMEO 2 MA gels loaded by solutions
of various non-steroidal anti-inflammatory drugs (namely ibuprofen and naproxen
sodium salts) showed a significant increase in the VPT temperature, linearly
dependent on drug concentration. The comparison of Raman spectrum recorded for
the drug solution with the differential spectrum obtained by subtraction of Raman
spectrum characteristics for gels filled with deionized water from the spectrum
collected for gel loaded with an appropriate drug solution was performed. As a
result, both spectra should be representative of the hydrated polymer network. The
positions of all bands characteristics for polymer were found unchanged in the drug
presence as any shifts of Raman lines were not observed [133]. Interesting work
about the hydration of ionizable, amphiphilic amino acids in aqueous solutions was
published by Ide et al. [156]. The relative intensity of water collective band (normalized to the signal for pure water) was measured for aqueous solutions of amino
acids differing as regards hydrophobicity. It was found that at neutral pH, the water
structure is not changed by the presence of amino acids independently of their
chemical structure. In acidic conditions, the intensity of collective band decreases
with the increase in hydrophobicity, which revealed that the degree of H-bonding
between water molecules increases in the neighbourhood of hydrophobic species,
while the opposite tendency is observed next to hydrophilic centres.
The limits of application of classical methods of vibrational spectroscopy to
study changes in the hydration degree during VPT or the coil-to-globule transition
results from a relatively low time resolution restricted by the acquisition time of a
single spectrum with the quality sufficiently high for further mathematical treatment
(commonly it is a timescale of single minute for Raman and tens of seconds for
FTIR spectroscopy). Thus, many investigations of VPT are performed in
quasi-dynamic conditions (a sample is heated to a given temperature, shortly stabilized to reach the uniform temperature distribution, a spectrum is collected, next
the temperature is changed, and the procedure is repeated). It could be expected that
such a procedure does not mimic properly the dynamics of VPT. The comparison of
the thermo-optical analysis and Raman results shows a high agreement—see
8 Vibrational Spectroscopy in Analysis of Stimuli-Responsive …
243
molecules and other components (water and polymer chains) seem to be especially
interesting. Moreover, the presence of additives usually changes the system stability
and impacts the phase transition temperature. Until now, it was found that many
various ionic substances, for example simple inorganic salts such as halogens or
surfactants [158, 163, 164], influence the VPT temperature and dynamics, shifting
the VPT to lower or to higher temperatures. It is postulated that an explanation of
this phenomenon relates to structure-making and structure-breaking properties of
various salts ordered in Hofmeister series—see the previous section [164, 165].
However, also non-ionic substances (for example hydroquinone, phenol, glycerol
or urea [166, 167]) strongly impact VPT.
Based on spectroscopic results, Maeda postulated that additives do not interact
with the polymer skeleton directly, but disturb the water structure simultaneously
influencing the polymer hydration. The direct proof of this thesis was delivered by
Olejniczak et al. [133]. Studies performed on PMEO 2 MA gels loaded by solutions
of various non-steroidal anti-inflammatory drugs (namely ibuprofen and naproxen
sodium salts) showed a significant increase in the VPT temperature, linearly
dependent on drug concentration. The comparison of Raman spectrum recorded for
the drug solution with the differential spectrum obtained by subtraction of Raman
spectrum characteristics for gels filled with deionized water from the spectrum
collected for gel loaded with an appropriate drug solution was performed. As a
result, both spectra should be representative of the hydrated polymer network. The
positions of all bands characteristics for polymer were found unchanged in the drug
presence as any shifts of Raman lines were not observed [133]. Interesting work
about the hydration of ionizable, amphiphilic amino acids in aqueous solutions was
published by Ide et al. [156]. The relative intensity of water collective band (normalized to the signal for pure water) was measured for aqueous solutions of amino
acids differing as regards hydrophobicity. It was found that at neutral pH, the water
structure is not changed by the presence of amino acids independently of their
chemical structure. In acidic conditions, the intensity of collective band decreases
with the increase in hydrophobicity, which revealed that the degree of H-bonding
between water molecules increases in the neighbourhood of hydrophobic species,
while the opposite tendency is observed next to hydrophilic centres.
The limits of application of classical methods of vibrational spectroscopy to
study changes in the hydration degree during VPT or the coil-to-globule transition
results from a relatively low time resolution restricted by the acquisition time of a
single spectrum with the quality sufficiently high for further mathematical treatment
(commonly it is a timescale of single minute for Raman and tens of seconds for
FTIR spectroscopy). Thus, many investigations of VPT are performed in
quasi-dynamic conditions (a sample is heated to a given temperature, shortly stabilized to reach the uniform temperature distribution, a spectrum is collected, next
the temperature is changed, and the procedure is repeated). It could be expected that
such a procedure does not mimic properly the dynamics of VPT. The comparison of
the thermo-optical analysis and Raman results shows a high agreement—see
8 Vibrational Spectroscopy in Analysis of Stimuli-Responsive …
243