of 10–13 molecules per monomer unit in the swollen state. Some light was shed on
these, rather unexpected, behaviours by MD simulations, which showed that no
more than two hydration shells differ in their properties from bulk water [213]. It
was also found that the amount of not structured water molecules (one-bonded or
non-bonded to other water molecules) increased with polymer concentration at the
expense of double water–water hydrogen-bonded molecules. At the sufficiently
high polymer concentration, a significant amount of water was hydrogen bonded to
two polymer units [149].
Even for simple polymers, the knowledge about the strength of different types of
interactions (water–water, water–polymer, polymer–polymer) is important.
Saramak et al. [149] used ab initio QM to calculate the interaction energy as a
function of a distance of water molecules from the PVME trimer. Water–O–CH 3
group and water–water interactions were found to have similar strength with the
energy minimum at *3 Å. Other interactions were significantly weaker. In case of
polymers with more than one hydrophilic centre (e.g. PMEO 2 MA) and many
co-polymers, it is important to know the thermal stability of hydrogen bonds
between water and a particular polar group, as a crucial issue influencing the
kinetics of the coil-to-globule or volume phase transitions. Maeda et al. [146]
showed for PMEO 2 MA that the strongest interactions are formed between water
and a carbonyl group, while the energy of water–ether oxygen interactions
decreases with increasing the distance from the main chain. They also showed that
the H-bond between the ether oxygen and water influences vibrations relating to the
stretching of adjacent alkyl groups. They can thus be experimentally monitored by
both the shift of C–O stretching modes as well as by the shift of C–H stretching
modes. However, in the second case two mechanisms for observed changes can be
proposed: (i) a direct interaction between water and the C–H groups (including
formation of the C–HÁÁÁO–H systems) and (ii) hyperconjugation between the ether
oxygen and the adjacent alkyl groups leading to the shortening of C–H bond. This
fact is important in the light of the results discussed in the previous section, where it
was shown that the C–H stretching vibrations are very useful to study the hydration
degree in polymer gels and solutions.
The dehydration during the coil-to-globule transition was also investigated using
computer methods. It was found that for polymers exhibiting LCST hydration
decreased with increasing the temperature primarily due to the fact that the polymer
hydrophobic groups tend to hide from the solvent. For PNIPAM, the number of
water molecules residing in the first coordination shell is ca. 6% lower at 310 K
than at 300 K [209]. A little bit higher value (ca. 14%) was given by Tavagnacco
et al. [214], who performed MD simulations of oligomer containing 30 monomer
units. However, in this case the range of studied temperatures was significantly
broader (from 243 to 323 K). Below the coil-to-globule transition temperature,
PNIPAM was surrounded by a network of hydrogen-bonded water. The change of
hydrogen bond pattern between water molecules as well as between water and
amide groups was observed above the transition temperature. Moreover, the hydration shell was only partially lost during the coil-to-globule transition and even
above the LCST polymers still remained largely hydrated. It is fully compatible
252
M. Kozanecki et al.
these, rather unexpected, behaviours by MD simulations, which showed that no
more than two hydration shells differ in their properties from bulk water [213]. It
was also found that the amount of not structured water molecules (one-bonded or
non-bonded to other water molecules) increased with polymer concentration at the
expense of double water–water hydrogen-bonded molecules. At the sufficiently
high polymer concentration, a significant amount of water was hydrogen bonded to
two polymer units [149].
Even for simple polymers, the knowledge about the strength of different types of
interactions (water–water, water–polymer, polymer–polymer) is important.
Saramak et al. [149] used ab initio QM to calculate the interaction energy as a
function of a distance of water molecules from the PVME trimer. Water–O–CH 3
group and water–water interactions were found to have similar strength with the
energy minimum at *3 Å. Other interactions were significantly weaker. In case of
polymers with more than one hydrophilic centre (e.g. PMEO 2 MA) and many
co-polymers, it is important to know the thermal stability of hydrogen bonds
between water and a particular polar group, as a crucial issue influencing the
kinetics of the coil-to-globule or volume phase transitions. Maeda et al. [146]
showed for PMEO 2 MA that the strongest interactions are formed between water
and a carbonyl group, while the energy of water–ether oxygen interactions
decreases with increasing the distance from the main chain. They also showed that
the H-bond between the ether oxygen and water influences vibrations relating to the
stretching of adjacent alkyl groups. They can thus be experimentally monitored by
both the shift of C–O stretching modes as well as by the shift of C–H stretching
modes. However, in the second case two mechanisms for observed changes can be
proposed: (i) a direct interaction between water and the C–H groups (including
formation of the C–HÁÁÁO–H systems) and (ii) hyperconjugation between the ether
oxygen and the adjacent alkyl groups leading to the shortening of C–H bond. This
fact is important in the light of the results discussed in the previous section, where it
was shown that the C–H stretching vibrations are very useful to study the hydration
degree in polymer gels and solutions.
The dehydration during the coil-to-globule transition was also investigated using
computer methods. It was found that for polymers exhibiting LCST hydration
decreased with increasing the temperature primarily due to the fact that the polymer
hydrophobic groups tend to hide from the solvent. For PNIPAM, the number of
water molecules residing in the first coordination shell is ca. 6% lower at 310 K
than at 300 K [209]. A little bit higher value (ca. 14%) was given by Tavagnacco
et al. [214], who performed MD simulations of oligomer containing 30 monomer
units. However, in this case the range of studied temperatures was significantly
broader (from 243 to 323 K). Below the coil-to-globule transition temperature,
PNIPAM was surrounded by a network of hydrogen-bonded water. The change of
hydrogen bond pattern between water molecules as well as between water and
amide groups was observed above the transition temperature. Moreover, the hydration shell was only partially lost during the coil-to-globule transition and even
above the LCST polymers still remained largely hydrated. It is fully compatible
252
M. Kozanecki et al.