The discussed simple model of a linear chain in a solvent becomes more
complex in the case of macromolecules with branched architecture including networks, dendrimers, star-like and brush-like. They usually have an ‘inhomogeneous’
structure in the molecular scale containing solvophilic centres distributed between
solvophobic parts. As a result, various types of intermolecular interactions (sensitive to various factors) should be taken into consideration to properly describe a
given system. These interactions are also a factor that determines the type of a
stimulus to which the polymer is sensitive. The case of polymer–water systems is
extremely difficult, as both static and dynamic properties are strongly dependent on
water behaviour, well-known as anomalous. Techniques based on vibrational
spectroscopy are invaluable tools in this field, as it is discussed in the next section.
8.2 Water and Polymer–Water Systems as Seen
by Vibrational Spectroscopy
In most aqueous polymer systems, water constitutes the major component (in hydrogels, it is usually over 90% of the weight fraction), which substantially contributes to the final properties of polymer solutions, dispersions and gels. The
presence of a high fraction of water is responsible for high biocompatibility and
biodegradability of most of these systems, their ability to dissolve additional
components like salts, non-ionic or even apolar molecules. Water also facilitates
fast transport of both mass and energy through polymer solutions or gels.
What is crucial from the perspective of SRPS are water properties and water–
polymer interactions which are decisive for the stimuli-responsiveness itself.
Regardless of actual triggers and the nature of mixing/demixing transitions of these
systems, the observed macroscopic action is most commonly based on highly
cooperative polymer–solvent interactions. To be more precise, stimuliresponsiveness lies in the delicate balance between hydrophilic and hydrophobic
interactions in the solution which, when perturbed, provokes micro- and macroscopic phase transitions. Breaking of hydrogen bonds between hydrophilic groups
of a polymer and water molecules leads to their phase separation in temperatureresponsive systems. The decrease in the hydration of ionic polymer groups after
their neutralisation (protonation of anionic or deprotonation of cationic groups)
provokes demixing in pH-responsive systems. Other methods for triggering
demixing transition, like addition of a co-solvent or a specific molecule, are based
on very similar mechanisms associated with weakening of water–polymer and
strengthening of polymer–polymer interactions. It is also important to stress that the
presence of a dominating fraction of water in the system is responsible for transportation of a triggering signal through a stimuli-responsive material, and thus, it
limits the rate of transition. Water has a thermal conductivity (0.591 W/m •K) much
higher than other popular solvents (alcohols, oils around 0.1 W/m •K) and many
times higher than most polymers, which plays a key role in homogenous transition
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M. Kozanecki et al.
complex in the case of macromolecules with branched architecture including networks, dendrimers, star-like and brush-like. They usually have an ‘inhomogeneous’
structure in the molecular scale containing solvophilic centres distributed between
solvophobic parts. As a result, various types of intermolecular interactions (sensitive to various factors) should be taken into consideration to properly describe a
given system. These interactions are also a factor that determines the type of a
stimulus to which the polymer is sensitive. The case of polymer–water systems is
extremely difficult, as both static and dynamic properties are strongly dependent on
water behaviour, well-known as anomalous. Techniques based on vibrational
spectroscopy are invaluable tools in this field, as it is discussed in the next section.
8.2 Water and Polymer–Water Systems as Seen
by Vibrational Spectroscopy
In most aqueous polymer systems, water constitutes the major component (in hydrogels, it is usually over 90% of the weight fraction), which substantially contributes to the final properties of polymer solutions, dispersions and gels. The
presence of a high fraction of water is responsible for high biocompatibility and
biodegradability of most of these systems, their ability to dissolve additional
components like salts, non-ionic or even apolar molecules. Water also facilitates
fast transport of both mass and energy through polymer solutions or gels.
What is crucial from the perspective of SRPS are water properties and water–
polymer interactions which are decisive for the stimuli-responsiveness itself.
Regardless of actual triggers and the nature of mixing/demixing transitions of these
systems, the observed macroscopic action is most commonly based on highly
cooperative polymer–solvent interactions. To be more precise, stimuliresponsiveness lies in the delicate balance between hydrophilic and hydrophobic
interactions in the solution which, when perturbed, provokes micro- and macroscopic phase transitions. Breaking of hydrogen bonds between hydrophilic groups
of a polymer and water molecules leads to their phase separation in temperatureresponsive systems. The decrease in the hydration of ionic polymer groups after
their neutralisation (protonation of anionic or deprotonation of cationic groups)
provokes demixing in pH-responsive systems. Other methods for triggering
demixing transition, like addition of a co-solvent or a specific molecule, are based
on very similar mechanisms associated with weakening of water–polymer and
strengthening of polymer–polymer interactions. It is also important to stress that the
presence of a dominating fraction of water in the system is responsible for transportation of a triggering signal through a stimuli-responsive material, and thus, it
limits the rate of transition. Water has a thermal conductivity (0.591 W/m •K) much
higher than other popular solvents (alcohols, oils around 0.1 W/m •K) and many
times higher than most polymers, which plays a key role in homogenous transition
228
M. Kozanecki et al.