The big challenge still ahead for the time-resolved vibrational spectroscopy
methods is to fully determine kinetics and identify molecular origins of all the
stages of various types of stimuli-responsive phase transitions revealed by SRPSs.
This task is difficult for several reasons. First of all, there is a problem of delivering
an instantaneous trigger of a phase transition in the whole studied volume of a
sample. In the case of thermo-responsive polymers, such a trigger could be a
high-energy near-infrared pulse being in resonance with a water absorption band.
The vibration excitation of water would be transferred into heat within tens of
picosecond in the irradiated volume of a sample (an excellent review of the
so-called temperature-jump method can be found in [245]). In the case of
pH-responsive systems, a fast decrease in the pH of a system may be provided, e.g.
by addition of the so-called photoacids into a sample. A unique feature of these
compounds is a large change in their acidity constant pKa upon electronic excitation to S 1 state. In the aqueous solution or gel, it may lead to the dissociation of
photoacid into a photobase and a proton. The proton stays in a solution as long as
the photoacid is in its S1 state. An example of similar ‘pH-jump method’ can be
found in [246].
Another problem is that most likely particular stage of stimuli-responsive phase
transition spans from picoseconds (e.g. proton transfer [54]) to milliseconds (collapsing of a polymer chain into a globule [247] or even to minutes (water diffusion
[248])—see Fig. 8.17. In a typical ultrafast time-resolved spectroscopy system, a
probe pulse is delayed with respect to a pump due to a different length of paths they
travel (provided by motorized delay stages). Such a method provides a time window of a few nanoseconds. Longer delays (which provide an observation time
window of many microseconds) may be achieved by using the system of two
electronically synchronized lasers, which is a very expensive and technically difficult solution.
Different computational methods are very useful to predict the structure and
properties of stimuli-responsive materials in different ranges of time and size.
Furthermore, they can be a good support for experimental techniques, as the results
delivered by simulations correspond well to experimental ones. However, up to
now molecular simulations have been limited to a few hundred thousands of atoms
and times of a few hundreds of nanoseconds. To be able to fully compare simulations with experiments of both time and length scales simulated in ‘all-atom’
approach would require, at least, 2–3 orders of magnitude more. Currently, the
spatial and temporal resolutions easily accessible for computer simulations and
experiments are practically disjunctive in the field of SRPS. The local phenomena
occurring at short times could be precisely modelled, but require very sophisticated
and expensive equipment to be studied experimentally. Contrarily, slow macroscopic processes may be monitored by various simple techniques, while their
investigations by computer methods have been still suppressed by hardware limitations. Therefore, further progress in the field of both computational and experimental methods is needed. In the case of simulations, it is directly connected with
the development of faster hardware and new algorithms involving parallel computing on clusters, support of GPU-based calculations, etc., enabling the
8 Vibrational Spectroscopy in Analysis of Stimuli-Responsive …
259
methods is to fully determine kinetics and identify molecular origins of all the
stages of various types of stimuli-responsive phase transitions revealed by SRPSs.
This task is difficult for several reasons. First of all, there is a problem of delivering
an instantaneous trigger of a phase transition in the whole studied volume of a
sample. In the case of thermo-responsive polymers, such a trigger could be a
high-energy near-infrared pulse being in resonance with a water absorption band.
The vibration excitation of water would be transferred into heat within tens of
picosecond in the irradiated volume of a sample (an excellent review of the
so-called temperature-jump method can be found in [245]). In the case of
pH-responsive systems, a fast decrease in the pH of a system may be provided, e.g.
by addition of the so-called photoacids into a sample. A unique feature of these
compounds is a large change in their acidity constant pKa upon electronic excitation to S 1 state. In the aqueous solution or gel, it may lead to the dissociation of
photoacid into a photobase and a proton. The proton stays in a solution as long as
the photoacid is in its S1 state. An example of similar ‘pH-jump method’ can be
found in [246].
Another problem is that most likely particular stage of stimuli-responsive phase
transition spans from picoseconds (e.g. proton transfer [54]) to milliseconds (collapsing of a polymer chain into a globule [247] or even to minutes (water diffusion
[248])—see Fig. 8.17. In a typical ultrafast time-resolved spectroscopy system, a
probe pulse is delayed with respect to a pump due to a different length of paths they
travel (provided by motorized delay stages). Such a method provides a time window of a few nanoseconds. Longer delays (which provide an observation time
window of many microseconds) may be achieved by using the system of two
electronically synchronized lasers, which is a very expensive and technically difficult solution.
Different computational methods are very useful to predict the structure and
properties of stimuli-responsive materials in different ranges of time and size.
Furthermore, they can be a good support for experimental techniques, as the results
delivered by simulations correspond well to experimental ones. However, up to
now molecular simulations have been limited to a few hundred thousands of atoms
and times of a few hundreds of nanoseconds. To be able to fully compare simulations with experiments of both time and length scales simulated in ‘all-atom’
approach would require, at least, 2–3 orders of magnitude more. Currently, the
spatial and temporal resolutions easily accessible for computer simulations and
experiments are practically disjunctive in the field of SRPS. The local phenomena
occurring at short times could be precisely modelled, but require very sophisticated
and expensive equipment to be studied experimentally. Contrarily, slow macroscopic processes may be monitored by various simple techniques, while their
investigations by computer methods have been still suppressed by hardware limitations. Therefore, further progress in the field of both computational and experimental methods is needed. In the case of simulations, it is directly connected with
the development of faster hardware and new algorithms involving parallel computing on clusters, support of GPU-based calculations, etc., enabling the
8 Vibrational Spectroscopy in Analysis of Stimuli-Responsive …
259