proposed a very useful procedure to study the hydration degree of particular
polymer groups by IR spectroscopy. This procedure is based on the analysis of
differential spectra and was originally proposed to monitor VPT in
thermo-responsive systems by IR spectroscopy. However, the method is universal
and may be applied to study VPT triggered by other stimuli as well as other types of
the phase transition. Firstly, a series of time-resolved vibrational spectra should be
collected. Next, the differential spectra between the spectra at various temperatures
(T) and the spectrum collected at the reference temperature (T 0 ) should be obtained
DA (T–T0) = A T –A T0 , where A relates to the signal intensity (absorbance for
absorption IR spectroscopy). After that, the difference between the intensity of
positive and negative peaks of a selected vibrational mode in each difference
spectrum should be calculated [146, 157]:
DDA TÀT 0 m 1 À m 2
ð
Þ¼DA TÀT 0 m 1
ð Þ À DA TÀT 0 m 2
ð Þ
ð8:5Þ
where m 1 and m 2 relate to the wavenumber at the positive and negative peaks in
difference spectrum, respectively. It was assumed that m 1 < m 2 , so positive values of
DDA ðTÀT 0 Þ m 1 À m 2
ð
Þcorrespond to the redshift of a given band, while the negative
ones are characteristic of the blue shift. The procedure proposed by Maeda et al.
allows investigator to monitor easily the shift of all bands in the spectrum and also
to cancel the baseline drift, which often accompanies thermal experiments. The
DDA ðTÀT 0 Þ m 1 À m 2
ð
Þparameter can be very useful to study hydration of particular
groups in the system and to monitor VPT as it is shown in Fig. 8.9.
In the case of polymers with relatively simple chemical structure and only one
type of hydrophilic centres (PVME or poly(N-isopropylacrylamide)—PNIPAM),
the analysis of hydration degree is relatively easy. More complex systems with
several hydrophilic centres require a more complex spectroscopic analysis and
should be often supported by computer simulations. As an example of such a
system, poly(2-(2-methoxyethoxy) ethyl methacrylate PMEO 2 MA with four different oxygen atoms may be pointed out. As it was shown by Maeda et al. [146] for
the linear polymer and by Olejniczak et al. [161] for PMEO 2 MA hydrogels, the
energy of water–oxygen interactions is different and decreases in series: carbonyl
group > ether bridge in the middle of side chain > terminal methoxy
group. Moreover, it was shown that both Raman and IR spectroscopy may be used
to estimate the percentage of hydrophilic centres accessible for water. The analysis
is based on the shift towards lower wavenumbers of the mode characteristic for
C=O stretching (from ca. 1728 to 1709 cm
−1 for PMEO 2 MA) induced by the
formation of C=OÁÁÁ water complex. Thus, the fraction of the hydrated (H-bonded)
carbonyl groups (f H-bond ) may be calculated according to the general formula:
f HÀbond ¼
1
1 þ
e 1709 A 1728
e 1728 A 1709
ð8:6Þ
240
M. Kozanecki et al.
polymer groups by IR spectroscopy. This procedure is based on the analysis of
differential spectra and was originally proposed to monitor VPT in
thermo-responsive systems by IR spectroscopy. However, the method is universal
and may be applied to study VPT triggered by other stimuli as well as other types of
the phase transition. Firstly, a series of time-resolved vibrational spectra should be
collected. Next, the differential spectra between the spectra at various temperatures
(T) and the spectrum collected at the reference temperature (T 0 ) should be obtained
DA (T–T0) = A T –A T0 , where A relates to the signal intensity (absorbance for
absorption IR spectroscopy). After that, the difference between the intensity of
positive and negative peaks of a selected vibrational mode in each difference
spectrum should be calculated [146, 157]:
DDA TÀT 0 m 1 À m 2
ð
Þ¼DA TÀT 0 m 1
ð Þ À DA TÀT 0 m 2
ð Þ
ð8:5Þ
where m 1 and m 2 relate to the wavenumber at the positive and negative peaks in
difference spectrum, respectively. It was assumed that m 1 < m 2 , so positive values of
DDA ðTÀT 0 Þ m 1 À m 2
ð
Þcorrespond to the redshift of a given band, while the negative
ones are characteristic of the blue shift. The procedure proposed by Maeda et al.
allows investigator to monitor easily the shift of all bands in the spectrum and also
to cancel the baseline drift, which often accompanies thermal experiments. The
DDA ðTÀT 0 Þ m 1 À m 2
ð
Þparameter can be very useful to study hydration of particular
groups in the system and to monitor VPT as it is shown in Fig. 8.9.
In the case of polymers with relatively simple chemical structure and only one
type of hydrophilic centres (PVME or poly(N-isopropylacrylamide)—PNIPAM),
the analysis of hydration degree is relatively easy. More complex systems with
several hydrophilic centres require a more complex spectroscopic analysis and
should be often supported by computer simulations. As an example of such a
system, poly(2-(2-methoxyethoxy) ethyl methacrylate PMEO 2 MA with four different oxygen atoms may be pointed out. As it was shown by Maeda et al. [146] for
the linear polymer and by Olejniczak et al. [161] for PMEO 2 MA hydrogels, the
energy of water–oxygen interactions is different and decreases in series: carbonyl
group > ether bridge in the middle of side chain > terminal methoxy
group. Moreover, it was shown that both Raman and IR spectroscopy may be used
to estimate the percentage of hydrophilic centres accessible for water. The analysis
is based on the shift towards lower wavenumbers of the mode characteristic for
C=O stretching (from ca. 1728 to 1709 cm
−1 for PMEO 2 MA) induced by the
formation of C=OÁÁÁ water complex. Thus, the fraction of the hydrated (H-bonded)
carbonyl groups (f H-bond ) may be calculated according to the general formula:
f HÀbond ¼
1
1 þ
e 1709 A 1728
e 1728 A 1709
ð8:6Þ
240
M. Kozanecki et al.