interactions between oligomer–oligomer and between the oligomer and water were
less perturbed during the cooling cycle. Results suggested that a hysteresis, if it
occurs, is a consequence of ultrafast heating–cooling kinetics, which results in the
insufficient relaxation for the solvated oligomer. The coil-to-globule transition was
also confirmed for PMEO 2 MA by MD simulations [212]. The analysis of the pair
interaction between PNIPAM polymer chains, in a concentration regime close to
the chain overlap concentration, by comparing atactic and isotactic-rich PNIPAM
solutions [225] provided information on the features of interchain junctions as a
function of tacticity. MD simulations carried out above the LCST showed the
PNIPAM transition to the insoluble state and do not reveal much influence of
stereochemistry on the structure of the polymer ensemble. The results for the
temperature below LCST gave the estimate of the swelling ratio of
non-stereocontrolled PNIPAM microgels as equal to 12, which is a typical value—
see section Thermo-responsive systems—“non-ionisable” polymers.
Additives can influence the coil-to-globule transition; e.g., a comparison of pure
PNIPAM with PNIPAM doped with amino acid-based co-monomers showed that
the presence of co-monomers in the chain prevents against the chain collapse upon
heating above 315 K [219]. MD simulations were successfully used to model
PNIPAM oligomers with chain lengths of 8, 18 and 30 units. The results compared
with the PNIPAM–alkyl copolymer surfactant revealed that in case of pure
PNIPAM oligomers, the backbone folded onto itself above the LCST to shield the
hydrophobic groups from water. The surfactant, however, did not show conformational change with the temperature [226]. The effect of salts addition (NaCl,
NaBr, NaI, KCl) on lowering the LCST temperature for a single 50-unit PNIPAM
chain was simulated by Du et al. [227]. The authors found that cations had strong
affinity with the amide oxygen in the polymer. However, the affinity can be
weakened by a stronger cation–anion interaction. Coarse-grained MD simulations
were used to study thermo-responsive and pH-responsive behaviours for
PAA-PDMAEMA co-polymers in water [228]. In these studies, water was treated
as a continuous medium, and for polymer short-range, long range Coulombic
interactions and hydrogen bonding were taken into account as a bead-spring model.
It was found that a maximum pH value exists for which PAA-PDMAEMA solutions exhibit pronounced thermo-responsive behaviours. The effect of the polymer
molecular mass on the aggregation behaviour had been more profound than that of
the polymer composition.
PNIPAM was also investigated in a form of brush grafted onto the silica surface
[229]. It was showed that above the LCST, the PNIPAM chains (50-unit oligomers)
aggregate on the silica support forming a hydrophobic surface. At temperatures
below the LCST, the PNIPAM chains were stretched and broadcast the surface
properties to the hydrophilic one. A more sophisticated grafting was investigated by
Lin et al. [230], i.e. the effect of grafting PNIPAM on the edge of nanoscale silicate
platelets. In this case, the DPD method was used to study the effects of the
polymer-grafted architecture and grafting density on the micellar behaviour.
The use of a finite-difference simulation (FDM) to calculate the electric field
intensity and its distributions in smart thermo-stimulated PNIPAM/silver
256
M. Kozanecki et al.
less perturbed during the cooling cycle. Results suggested that a hysteresis, if it
occurs, is a consequence of ultrafast heating–cooling kinetics, which results in the
insufficient relaxation for the solvated oligomer. The coil-to-globule transition was
also confirmed for PMEO 2 MA by MD simulations [212]. The analysis of the pair
interaction between PNIPAM polymer chains, in a concentration regime close to
the chain overlap concentration, by comparing atactic and isotactic-rich PNIPAM
solutions [225] provided information on the features of interchain junctions as a
function of tacticity. MD simulations carried out above the LCST showed the
PNIPAM transition to the insoluble state and do not reveal much influence of
stereochemistry on the structure of the polymer ensemble. The results for the
temperature below LCST gave the estimate of the swelling ratio of
non-stereocontrolled PNIPAM microgels as equal to 12, which is a typical value—
see section Thermo-responsive systems—“non-ionisable” polymers.
Additives can influence the coil-to-globule transition; e.g., a comparison of pure
PNIPAM with PNIPAM doped with amino acid-based co-monomers showed that
the presence of co-monomers in the chain prevents against the chain collapse upon
heating above 315 K [219]. MD simulations were successfully used to model
PNIPAM oligomers with chain lengths of 8, 18 and 30 units. The results compared
with the PNIPAM–alkyl copolymer surfactant revealed that in case of pure
PNIPAM oligomers, the backbone folded onto itself above the LCST to shield the
hydrophobic groups from water. The surfactant, however, did not show conformational change with the temperature [226]. The effect of salts addition (NaCl,
NaBr, NaI, KCl) on lowering the LCST temperature for a single 50-unit PNIPAM
chain was simulated by Du et al. [227]. The authors found that cations had strong
affinity with the amide oxygen in the polymer. However, the affinity can be
weakened by a stronger cation–anion interaction. Coarse-grained MD simulations
were used to study thermo-responsive and pH-responsive behaviours for
PAA-PDMAEMA co-polymers in water [228]. In these studies, water was treated
as a continuous medium, and for polymer short-range, long range Coulombic
interactions and hydrogen bonding were taken into account as a bead-spring model.
It was found that a maximum pH value exists for which PAA-PDMAEMA solutions exhibit pronounced thermo-responsive behaviours. The effect of the polymer
molecular mass on the aggregation behaviour had been more profound than that of
the polymer composition.
PNIPAM was also investigated in a form of brush grafted onto the silica surface
[229]. It was showed that above the LCST, the PNIPAM chains (50-unit oligomers)
aggregate on the silica support forming a hydrophobic surface. At temperatures
below the LCST, the PNIPAM chains were stretched and broadcast the surface
properties to the hydrophilic one. A more sophisticated grafting was investigated by
Lin et al. [230], i.e. the effect of grafting PNIPAM on the edge of nanoscale silicate
platelets. In this case, the DPD method was used to study the effects of the
polymer-grafted architecture and grafting density on the micellar behaviour.
The use of a finite-difference simulation (FDM) to calculate the electric field
intensity and its distributions in smart thermo-stimulated PNIPAM/silver
256
M. Kozanecki et al.