water) formed around 50-unit PNIPAM chain was investigated with MD simulations [209]. At 300 K, it was found ca. 19% smaller than that of bulk water.
Moreover, D for water molecules surrounding the PNIPAM oligomers doped with
amino acid-based co-monomers was found as characteristically dependent on the
distance from the solute molecule [219]. The D values were small in the range of
short distances and asymptotically increased far apart the polymer. Water domains
sufficiently far from macromolecule (>2 nm) revealed D values close to those found
for bulk water at the same temperature. Such behaviours were related to the water–
polymer interaction, including, in particular, the water bridges between distant
oligomer units. Moreover, all investigated doped oligomers revealed the same
influence on the solvent. MD simulations performed by Tavagnacco et al. [214]
showed a correlation between polymer segmental dynamics and a diffusion motion
of bound water occurring with the same activation energy. Below the transition
temperature, the ratio of bonded water diffusion coefficient to bulk water diffusion
coefficient ranged from 0.2 to 0.3 and it increased up to 0.4 for temperature above
the coil-to-globule transition. In case of PVA, PVME and PNIPAM, the mobility of
water molecules was highly reduced around the polymer chains for both translational and rotational motions [220]. There are two reasons for this reduction: (i) the
existence of hydrogen bonds between water and polymers around the hydrophilic
groups and (ii) structuration of water around the hydrophobic groups. The mean
square displacement for water, used to calculate D, was approximately linear only
in time of 0.5–5 ps due to statistics in the selected region. For PVA and PNIPAM,
the water motions were suppressed more significantly in the hydrophilic region than
in the hydrophobic region with diffusion coefficients approximately 10–40% of
those in pure water. For the PVME hydrogel in the hydrophilic region, the motions
were hindered because of tight hydrogen bonds between water and polymers and
stabilization of water–water hydrogen bonds. In the hydrophobic region, the
motions were hindered because of the structuration of water. For the PVME,
hydrogels with the water content higher than 75 wt%, D values in the bulk region
were almost the same as for pure water, while for the systems with a lower amount
of water D decreased in the bulk region, influenced by the presence of polymers.
For the high PVME concentration (> 54 wt%), a water sub-diffusive behaviour was
observed for time below * 0.1 ns (the mean square displacement slope in the log–
log scale was lower than unity) and a major decrease of water diffusion coefficients
(connected with the slope in the normal scale) [213]. It is important to notice that
water does not crystallise any more in the PVME–water system with the polymer
content higher than 61% [154].
Studies of PDMAEMA revealed that water dynamics in the vicinity of the
carbonyl group was more restricted than that of other functional groups (it is in
agreement with the strongest energy of this interaction, as it was mentioned above)
with the increasing temperature. The extended residence time for water molecules
surrounding the polymer chain resulted from both, a relatively slow decrease of
relaxation time of water–C=O group H-bonds, and the extended relaxation time of
water–water H-bonds in the vicinity of the carbonyl group [217].
254
M. Kozanecki et al.
Moreover, D for water molecules surrounding the PNIPAM oligomers doped with
amino acid-based co-monomers was found as characteristically dependent on the
distance from the solute molecule [219]. The D values were small in the range of
short distances and asymptotically increased far apart the polymer. Water domains
sufficiently far from macromolecule (>2 nm) revealed D values close to those found
for bulk water at the same temperature. Such behaviours were related to the water–
polymer interaction, including, in particular, the water bridges between distant
oligomer units. Moreover, all investigated doped oligomers revealed the same
influence on the solvent. MD simulations performed by Tavagnacco et al. [214]
showed a correlation between polymer segmental dynamics and a diffusion motion
of bound water occurring with the same activation energy. Below the transition
temperature, the ratio of bonded water diffusion coefficient to bulk water diffusion
coefficient ranged from 0.2 to 0.3 and it increased up to 0.4 for temperature above
the coil-to-globule transition. In case of PVA, PVME and PNIPAM, the mobility of
water molecules was highly reduced around the polymer chains for both translational and rotational motions [220]. There are two reasons for this reduction: (i) the
existence of hydrogen bonds between water and polymers around the hydrophilic
groups and (ii) structuration of water around the hydrophobic groups. The mean
square displacement for water, used to calculate D, was approximately linear only
in time of 0.5–5 ps due to statistics in the selected region. For PVA and PNIPAM,
the water motions were suppressed more significantly in the hydrophilic region than
in the hydrophobic region with diffusion coefficients approximately 10–40% of
those in pure water. For the PVME hydrogel in the hydrophilic region, the motions
were hindered because of tight hydrogen bonds between water and polymers and
stabilization of water–water hydrogen bonds. In the hydrophobic region, the
motions were hindered because of the structuration of water. For the PVME,
hydrogels with the water content higher than 75 wt%, D values in the bulk region
were almost the same as for pure water, while for the systems with a lower amount
of water D decreased in the bulk region, influenced by the presence of polymers.
For the high PVME concentration (> 54 wt%), a water sub-diffusive behaviour was
observed for time below * 0.1 ns (the mean square displacement slope in the log–
log scale was lower than unity) and a major decrease of water diffusion coefficients
(connected with the slope in the normal scale) [213]. It is important to notice that
water does not crystallise any more in the PVME–water system with the polymer
content higher than 61% [154].
Studies of PDMAEMA revealed that water dynamics in the vicinity of the
carbonyl group was more restricted than that of other functional groups (it is in
agreement with the strongest energy of this interaction, as it was mentioned above)
with the increasing temperature. The extended residence time for water molecules
surrounding the polymer chain resulted from both, a relatively slow decrease of
relaxation time of water–C=O group H-bonds, and the extended relaxation time of
water–water H-bonds in the vicinity of the carbonyl group [217].
254
M. Kozanecki et al.