8.4 Thermo-Responsive Systems—“Non-Ionisable”
Polymers
Thermo-responsive polymer systems are designed mainly with the use of
non-ionisable macromolecules. Therefore, a mechanism responsible for triggering
the coil-to-globule transition or VPT is different in comparison to pH-sensitive
systems and bases mainly on the disturbance of hydrogen bonds and/or dipole–
dipole and/or hydrophobic interactions . For gels, the most popular SRPS, a broad
review of dynamic and kinetic behaviours, was performed by Shibayama and
Tanaka [129] and lately by Koetting et al. [130]. The network formation strongly
reduces the number of freedom degrees in the polymer–solvent system. Thus, the
free transition between collapsed and expanded states becomes limited. As a result,
the volume phase transition (VPT) is observed (an interesting comparison between
coil-to-globule and volume phase transitions can be found in [131]. Similarly to
liquid–gas transition, it could be continuous or discontinuous and its kinetics
depends on the network elasticity, polymer–solvent friction and the presence of
additional substances [132, 133]. For practical applications, systems exhibiting
discontinuous VPT are more attractive.
For the first time, the volume phase transition was observed experimentally by Tanaka
for partially ionized acrylamide gel [134]. Interestingly, it was previously predicted by
Dusek and Patterson [135], who developed theoretical considerations for linear polymers
done by Pristin and Eizner, de Gennes and Lifshitz et al. [136–138] and assumed the
possibility of conformational transitions in gels analogously to the coil-to-globule
transition in the solution of linear polymers. Currently, the family of thermo-responsive
polymers is very broad and includes: poly (N-alkyl substituted acrylamides) [10], poly
(N-alkoxyalkylacrylamides) [11, 12], poly (N-vinylalkylamides) [12], polyvinylmethylether (PVME) [140, 141], poly (oligoether methacrylates and acrylates) [13, 142]
and their various co-polymers. Also, some natural polymers such as cellulose and its
derivatives are sensitive to temperature [143, 144].
As it was mentioned, one of the most studied issues related to thermo-responsive
polymers is the polymer chain hydration and its changes during the coil-to-globule
transition or VPT. The presence of polymer chains in water influences its
supramolecular structure leading to hydrogen bond redistribution. Especially, hydrophilic centres located along the polymer chain (or network) can strongly impact
the organization and dynamics of water molecules. However, also hydrophobic
parts can be stabilized by water molecules by so-called hydrophobic interactions
[76, 77, 95, 145–147].
As it was mentioned in the previous section, various states of water differ in their
thermal, diffusive and other properties [148–154]. They may be determined by various
techniques; however, they are mainly studied by vibrational spectroscopy—a fast,
cheap and non-invasive method giving a broad range of information, simultaneously,
about the state of both a polymer and water. Moreover, information on particular
components of the complex systems can be achieved and analysed separately.
The series of IR spectroscopic studies done for various polymers by Maeda and
co-workers deserves special attention [65, 74, 75, 110, 139, 146, 155–160]. They
8 Vibrational Spectroscopy in Analysis of Stimuli-Responsive …
239
Polymers
Thermo-responsive polymer systems are designed mainly with the use of
non-ionisable macromolecules. Therefore, a mechanism responsible for triggering
the coil-to-globule transition or VPT is different in comparison to pH-sensitive
systems and bases mainly on the disturbance of hydrogen bonds and/or dipole–
dipole and/or hydrophobic interactions . For gels, the most popular SRPS, a broad
review of dynamic and kinetic behaviours, was performed by Shibayama and
Tanaka [129] and lately by Koetting et al. [130]. The network formation strongly
reduces the number of freedom degrees in the polymer–solvent system. Thus, the
free transition between collapsed and expanded states becomes limited. As a result,
the volume phase transition (VPT) is observed (an interesting comparison between
coil-to-globule and volume phase transitions can be found in [131]. Similarly to
liquid–gas transition, it could be continuous or discontinuous and its kinetics
depends on the network elasticity, polymer–solvent friction and the presence of
additional substances [132, 133]. For practical applications, systems exhibiting
discontinuous VPT are more attractive.
For the first time, the volume phase transition was observed experimentally by Tanaka
for partially ionized acrylamide gel [134]. Interestingly, it was previously predicted by
Dusek and Patterson [135], who developed theoretical considerations for linear polymers
done by Pristin and Eizner, de Gennes and Lifshitz et al. [136–138] and assumed the
possibility of conformational transitions in gels analogously to the coil-to-globule
transition in the solution of linear polymers. Currently, the family of thermo-responsive
polymers is very broad and includes: poly (N-alkyl substituted acrylamides) [10], poly
(N-alkoxyalkylacrylamides) [11, 12], poly (N-vinylalkylamides) [12], polyvinylmethylether (PVME) [140, 141], poly (oligoether methacrylates and acrylates) [13, 142]
and their various co-polymers. Also, some natural polymers such as cellulose and its
derivatives are sensitive to temperature [143, 144].
As it was mentioned, one of the most studied issues related to thermo-responsive
polymers is the polymer chain hydration and its changes during the coil-to-globule
transition or VPT. The presence of polymer chains in water influences its
supramolecular structure leading to hydrogen bond redistribution. Especially, hydrophilic centres located along the polymer chain (or network) can strongly impact
the organization and dynamics of water molecules. However, also hydrophobic
parts can be stabilized by water molecules by so-called hydrophobic interactions
[76, 77, 95, 145–147].
As it was mentioned in the previous section, various states of water differ in their
thermal, diffusive and other properties [148–154]. They may be determined by various
techniques; however, they are mainly studied by vibrational spectroscopy—a fast,
cheap and non-invasive method giving a broad range of information, simultaneously,
about the state of both a polymer and water. Moreover, information on particular
components of the complex systems can be achieved and analysed separately.
The series of IR spectroscopic studies done for various polymers by Maeda and
co-workers deserves special attention [65, 74, 75, 110, 139, 146, 155–160]. They
8 Vibrational Spectroscopy in Analysis of Stimuli-Responsive …
239