Full Citation: Fluorescence Analysis of Thermoresponsive Polymers. A.G. Ryder and C. Morris, Reviews in Fluorescence 2015 ,
Annual Volumes, Vol. 8, pp. 97-126, (2015). ISBN: 978-3-319-24607-9 (Hardcover), 978-3-319-24609-3 (ebook) Springer.
Page 18 of 31
Fluorescence intensity and position of emission band maxima as a function of polymer concentration at 20C.
Reproduced with permission from ref [124]. Copyright © 2005, Springer Berlin / Heidelberg.
These fluorescence results support the results from DLS that suggest for the telechelic PNIPAm
in solution, the degree of aggregation within the rosettes increases as the concentration increases, and
that the steric crowding results in expulsion/release of polymer bound water in order to accommodate
the increased steric pressure.
5.5
PHASE TRANSITIONS AT THE LCST.
The phase transitions that occur at the LCST are the defining feature of thermoresponsive
polymers. While the phase transition is often easily observed (with high concentration solutions or
when the polymer has been fabricated into a film), there are cases where the use of sensitive
fluorescence methods are warranted. This is particularly the case where the polymer concentration
is low, or where a non-contact method is required. Both the simple freely diffusing probe and the
more complex covalently bound probe approaches have been successfully employed. Winnik
utilized pyrene labeled PNIPAm to study the temperature induced phase transitions in aqueous
solutions [105,125]. One study showed that the complex pyrene photophysics when covalently
labeled with PNIPAm was dependent on the degree of labeling and involved emission from monomers
and excimers [105]. At ambient temperature in water, the presence of ground-state pyrene dimers
and higher aggregates was observed for the pyrene labeled PNIPAm. These aggregates can form
between pyrene attached to the same chain or between pyrene probes located on different chains.
Heating solutions of the labeled PNIPAm above the LCST results in disruption of the pyrene
aggregates and the quantum yield of the monomeric species increases relative to that of the excimers.
This dissociation is complete in the case of the sparsely labeled polymer(PNIPAm/Py/200) but only
partial in solutions of the more highly labeled polymer (PNIPAm/Py/20). When trace amounts of the
labeled PNIPAm were added to solutions of unlabeled PNIPAm, changes in pyrene fluorescence could
be used to ascertain the interaction between chains. They found that below the LCST, there was no
indication of interactions between labeled and unlabeled polymers, whereas above the LCST, the
labeled polymers were incorporated into the PNIPAm-rich phase. These fluorescence studies also
showed that in the low concentration limit (<1 ppm) for highly labeled polymers there was evidence
for the formation of single-polymer chain micelles. More recently, pyrene was employed to study
long-range polymer chain dynamics for PNIPAm using a variety of models including the Fluorescence
Blob Model (FBM) [126,127]. Chee et al. have also investigated in detail the interactions between
PNIPAm and pyrene using time resolved fluorescence spectroscopy and concluded that above the
LCST PNIPAm is capable of solubilizing hydrophobic guests such as pyrene but that below the LCST
much of this capability is lost [128].
A more facile method for assessing the effects of the phase changes in aqueous solutions of
Annual Volumes, Vol. 8, pp. 97-126, (2015). ISBN: 978-3-319-24607-9 (Hardcover), 978-3-319-24609-3 (ebook) Springer.
Page 18 of 31
Fluorescence intensity and position of emission band maxima as a function of polymer concentration at 20C.
Reproduced with permission from ref [124]. Copyright © 2005, Springer Berlin / Heidelberg.
These fluorescence results support the results from DLS that suggest for the telechelic PNIPAm
in solution, the degree of aggregation within the rosettes increases as the concentration increases, and
that the steric crowding results in expulsion/release of polymer bound water in order to accommodate
the increased steric pressure.
5.5
PHASE TRANSITIONS AT THE LCST.
The phase transitions that occur at the LCST are the defining feature of thermoresponsive
polymers. While the phase transition is often easily observed (with high concentration solutions or
when the polymer has been fabricated into a film), there are cases where the use of sensitive
fluorescence methods are warranted. This is particularly the case where the polymer concentration
is low, or where a non-contact method is required. Both the simple freely diffusing probe and the
more complex covalently bound probe approaches have been successfully employed. Winnik
utilized pyrene labeled PNIPAm to study the temperature induced phase transitions in aqueous
solutions [105,125]. One study showed that the complex pyrene photophysics when covalently
labeled with PNIPAm was dependent on the degree of labeling and involved emission from monomers
and excimers [105]. At ambient temperature in water, the presence of ground-state pyrene dimers
and higher aggregates was observed for the pyrene labeled PNIPAm. These aggregates can form
between pyrene attached to the same chain or between pyrene probes located on different chains.
Heating solutions of the labeled PNIPAm above the LCST results in disruption of the pyrene
aggregates and the quantum yield of the monomeric species increases relative to that of the excimers.
This dissociation is complete in the case of the sparsely labeled polymer(PNIPAm/Py/200) but only
partial in solutions of the more highly labeled polymer (PNIPAm/Py/20). When trace amounts of the
labeled PNIPAm were added to solutions of unlabeled PNIPAm, changes in pyrene fluorescence could
be used to ascertain the interaction between chains. They found that below the LCST, there was no
indication of interactions between labeled and unlabeled polymers, whereas above the LCST, the
labeled polymers were incorporated into the PNIPAm-rich phase. These fluorescence studies also
showed that in the low concentration limit (<1 ppm) for highly labeled polymers there was evidence
for the formation of single-polymer chain micelles. More recently, pyrene was employed to study
long-range polymer chain dynamics for PNIPAm using a variety of models including the Fluorescence
Blob Model (FBM) [126,127]. Chee et al. have also investigated in detail the interactions between
PNIPAm and pyrene using time resolved fluorescence spectroscopy and concluded that above the
LCST PNIPAm is capable of solubilizing hydrophobic guests such as pyrene but that below the LCST
much of this capability is lost [128].
A more facile method for assessing the effects of the phase changes in aqueous solutions of