down towards the polarized band observed in ice (at 3150 cm
−1 , see: spectrum of
ice, in Fig. 8.4) when the temperature is decreased, it has been linked to the
‘structured’ fraction of water [60, 61]. The ‘collective’ nature of this unique mode
has been recently confirmed with computational studies [62]. Other water modes of
the collective nature present in the Raman spectrum are two broad low-frequency
bands (marked with arrows in the inset to Fig. 8.4). They are both assigned to
inter-monomer damped harmonic vibrations of water molecules [63]. The band at
ca. 60 cm
−1 is interpreted as the inter-monomer bending of the whole H 2 O molecule engaging at least three molecules (O–O–O bending) and the mode centred at
ca. 175 cm
−1 is assigned to stretching of a hydrogen bond between two molecules
(O–O stretching). These two modes are located in the spectral range difficult to
access in IR, and thus, Raman spectra are an exclusive source of information.
The dynamic micro-structure of water is mostly manifested in intense stretch
vibrational band at 3100–3700 cm
−1 . The disturbance of bulk water structure
(decoupling of OH oscillators) is caused not only by the influence of temperature,
but also by the presence of ions, surfactants, polymer chains, etc., and it is reflected
in the shape of this broad band. It is worth noting here that the ratio of a low- and
high-frequency component of the OH stretching multi-mode band changes with the
temperature. This ratio may thus be used as a molecular thermometer capable of
measuring in situ temperature of a polymer–water system [60, 61, 64].
The analysis of the changes in water structure in polymer systems is often only
qualitative—the shapes of normalised spectra are compared [65, 66]. The attempts
of presenting such changes quantitatively usually comprise either determination of
the collective parameter I C (equal to difference between intensities of m s for parallel
and perpendicular polarizations normalized to the depolarization of the band), as
proposed by Green et al. [67] or by deconvolution of m s band into components and
subsequent calculation of a ratio of the integrated intensities of high I 3400 and low
I 3200 frequency components [68, 69]. The value of the I 3400 /I 3200 ratio in distilled
water at the room temperature was determined as ca. 0.87, and it is often taken as a
reference [69]. What has to be stressed is the fact that the value is strongly
dependent on the Raman excitation wavelength. Pastorczak et al. [70] showed that
the polarized component around 3200 cm
−1 is enhanced by the resonance with red
light. Weak absorption of water in the red part of the visible range comes from
vibrational overtones and combinational modes. It is supposed that strongly coupled OH oscillators, which contribute the most to the component at 3200 cm
−1 , are
much more anharmonic than OH vibrations of loosely bonded water, and therefore,
they are resonantly enhanced by excitation light from the red range [71] (Fig. 8.5).
As it was mentioned in many technically useful SRPS, such as solutions, dispersions and gels, water is a dominant component. Therefore, a spectroscopist
frequently faces a problem of a strong water signal (either in the OH stretching or
H–O–H bending region) masking polymer-related bands (it is especially frequent in
IR spectroscopy due to a much higher activity of water in this technique in comparison to Raman spectroscopy). In such a case, a useful solution is substituting
H 2 O with heavy water (D 2 O), whose bands are downshifted to 2504 cm
−1
(stretching) and 1209 cm
−1 (bending) [72]. Such an approach may be applied
230
M. Kozanecki et al.
−1 , see: spectrum of
ice, in Fig. 8.4) when the temperature is decreased, it has been linked to the
‘structured’ fraction of water [60, 61]. The ‘collective’ nature of this unique mode
has been recently confirmed with computational studies [62]. Other water modes of
the collective nature present in the Raman spectrum are two broad low-frequency
bands (marked with arrows in the inset to Fig. 8.4). They are both assigned to
inter-monomer damped harmonic vibrations of water molecules [63]. The band at
ca. 60 cm
−1 is interpreted as the inter-monomer bending of the whole H 2 O molecule engaging at least three molecules (O–O–O bending) and the mode centred at
ca. 175 cm
−1 is assigned to stretching of a hydrogen bond between two molecules
(O–O stretching). These two modes are located in the spectral range difficult to
access in IR, and thus, Raman spectra are an exclusive source of information.
The dynamic micro-structure of water is mostly manifested in intense stretch
vibrational band at 3100–3700 cm
−1 . The disturbance of bulk water structure
(decoupling of OH oscillators) is caused not only by the influence of temperature,
but also by the presence of ions, surfactants, polymer chains, etc., and it is reflected
in the shape of this broad band. It is worth noting here that the ratio of a low- and
high-frequency component of the OH stretching multi-mode band changes with the
temperature. This ratio may thus be used as a molecular thermometer capable of
measuring in situ temperature of a polymer–water system [60, 61, 64].
The analysis of the changes in water structure in polymer systems is often only
qualitative—the shapes of normalised spectra are compared [65, 66]. The attempts
of presenting such changes quantitatively usually comprise either determination of
the collective parameter I C (equal to difference between intensities of m s for parallel
and perpendicular polarizations normalized to the depolarization of the band), as
proposed by Green et al. [67] or by deconvolution of m s band into components and
subsequent calculation of a ratio of the integrated intensities of high I 3400 and low
I 3200 frequency components [68, 69]. The value of the I 3400 /I 3200 ratio in distilled
water at the room temperature was determined as ca. 0.87, and it is often taken as a
reference [69]. What has to be stressed is the fact that the value is strongly
dependent on the Raman excitation wavelength. Pastorczak et al. [70] showed that
the polarized component around 3200 cm
−1 is enhanced by the resonance with red
light. Weak absorption of water in the red part of the visible range comes from
vibrational overtones and combinational modes. It is supposed that strongly coupled OH oscillators, which contribute the most to the component at 3200 cm
−1 , are
much more anharmonic than OH vibrations of loosely bonded water, and therefore,
they are resonantly enhanced by excitation light from the red range [71] (Fig. 8.5).
As it was mentioned in many technically useful SRPS, such as solutions, dispersions and gels, water is a dominant component. Therefore, a spectroscopist
frequently faces a problem of a strong water signal (either in the OH stretching or
H–O–H bending region) masking polymer-related bands (it is especially frequent in
IR spectroscopy due to a much higher activity of water in this technique in comparison to Raman spectroscopy). In such a case, a useful solution is substituting
H 2 O with heavy water (D 2 O), whose bands are downshifted to 2504 cm
−1
(stretching) and 1209 cm
−1 (bending) [72]. Such an approach may be applied
230
M. Kozanecki et al.