To investigate bigger systems, less molecularly detailed methods (using the
coarse-graining procedures) are useful. As an example, studies performed for the
model PVME–water system on the 3D lattice containing 50
3 beads with the use of
DLL algorithm can be referred [149]. PVME chains were polymerized up to
DP % 190, which for 250 chains in the calculation box allowed to achieve the
maximal water–polymer molar content of 3:1 (this value corresponds approximately to 500,000 atoms constituting polymer chains in the system). The diffusive
behaviour of water close to the chains (bound water) and more far away (bulk
water) was analysed as a function of the polymer content. The chains influence
strongly the water mobility only in the first coordination shell. However, only the
athermal casus was considered (such a system corresponds to h conditions). These
studies were continued to check the influence of both polymer chain length and
concentration on the mobility of solvent molecules in the PVME–water system by
Kozanecki et al. [148]. The obtained results for self-diffusion coefficients showed a
good agreement with a free volume theory. In diluted solutions, diffusion of the
solvent near the chains was affected by the polymer chain length. In dense solutions, the effect of polymer concentration was dominating. This behaviour was well
correlated with the spatial polymer homogeneity in the system.
The last part of this chapter is devoted to conformational changes induced in
water–polymer systems by the temperature. The conformational changes of
PNIPAM hydrogel as a function of the temperature was first studied by Walter et al.
[215] using MD simulations. The authors reported that these changes are caused by
a conformational transition of the single polymer chains in the hydrogel backbone.
The studies were performed for chains containing 15, 30, 50 and 75 monomer units
starting from a stretched initial conformation. It was found that sufficiently long
chains (with more than 15 monomer units) collapse and remain collapsed in the
equilibrium. Moreover, longer chains need more time for the conformation transition (from 3 to 7.5 ns for 30 and 75 monomers, respectively). Different force
fields for describing the hydrogel backbone and water molecules were compared
showing surprisingly that not all combinations are able to reveal the conformational
transition. Only two combinations (GROMOS-96 53A6 + TIP4P and
OPLS-AA + SPC/E) yielded the conformational change upon the temperature
variation. It was also shown that the results concerning h-temperature for
OPLS-AA + SPC/E (320 K) were in fair agreement with the experimental results
(305 K) [221].
Tucker and Stevens [222] investigated the polymer length dependence (over the
range of 3–30 mers) of the transition temperature for a single syndiotactic PNIPAM
oligomers and confirmed higher LCST for shorter chains [223]. The interactions
were simulated in a longer than 100-ns MD simulation. The differences in the
vibrational spectra in PNIPAM–water system at various temperatures (during
heating and cooling cycles) were investigated with MD simulation [224] to
understand the coil-to-globule and globule-to-coil transitions and identify the
changes in the relative interactions between various polymer groups and water.
Oligomer–water interactions were found to be dominant below the LCST, whereas
oligomer–oligomer interactions predominated above the LCST. Moreover,
8 Vibrational Spectroscopy in Analysis of Stimuli-Responsive …
255
coarse-graining procedures) are useful. As an example, studies performed for the
model PVME–water system on the 3D lattice containing 50
3 beads with the use of
DLL algorithm can be referred [149]. PVME chains were polymerized up to
DP % 190, which for 250 chains in the calculation box allowed to achieve the
maximal water–polymer molar content of 3:1 (this value corresponds approximately to 500,000 atoms constituting polymer chains in the system). The diffusive
behaviour of water close to the chains (bound water) and more far away (bulk
water) was analysed as a function of the polymer content. The chains influence
strongly the water mobility only in the first coordination shell. However, only the
athermal casus was considered (such a system corresponds to h conditions). These
studies were continued to check the influence of both polymer chain length and
concentration on the mobility of solvent molecules in the PVME–water system by
Kozanecki et al. [148]. The obtained results for self-diffusion coefficients showed a
good agreement with a free volume theory. In diluted solutions, diffusion of the
solvent near the chains was affected by the polymer chain length. In dense solutions, the effect of polymer concentration was dominating. This behaviour was well
correlated with the spatial polymer homogeneity in the system.
The last part of this chapter is devoted to conformational changes induced in
water–polymer systems by the temperature. The conformational changes of
PNIPAM hydrogel as a function of the temperature was first studied by Walter et al.
[215] using MD simulations. The authors reported that these changes are caused by
a conformational transition of the single polymer chains in the hydrogel backbone.
The studies were performed for chains containing 15, 30, 50 and 75 monomer units
starting from a stretched initial conformation. It was found that sufficiently long
chains (with more than 15 monomer units) collapse and remain collapsed in the
equilibrium. Moreover, longer chains need more time for the conformation transition (from 3 to 7.5 ns for 30 and 75 monomers, respectively). Different force
fields for describing the hydrogel backbone and water molecules were compared
showing surprisingly that not all combinations are able to reveal the conformational
transition. Only two combinations (GROMOS-96 53A6 + TIP4P and
OPLS-AA + SPC/E) yielded the conformational change upon the temperature
variation. It was also shown that the results concerning h-temperature for
OPLS-AA + SPC/E (320 K) were in fair agreement with the experimental results
(305 K) [221].
Tucker and Stevens [222] investigated the polymer length dependence (over the
range of 3–30 mers) of the transition temperature for a single syndiotactic PNIPAM
oligomers and confirmed higher LCST for shorter chains [223]. The interactions
were simulated in a longer than 100-ns MD simulation. The differences in the
vibrational spectra in PNIPAM–water system at various temperatures (during
heating and cooling cycles) were investigated with MD simulation [224] to
understand the coil-to-globule and globule-to-coil transitions and identify the
changes in the relative interactions between various polymer groups and water.
Oligomer–water interactions were found to be dominant below the LCST, whereas
oligomer–oligomer interactions predominated above the LCST. Moreover,
8 Vibrational Spectroscopy in Analysis of Stimuli-Responsive …
255