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 9 of 31
when it is in solution or deployed as a particle or a thin film. In the first case Hydrogen-bonding, van
der Waals interactions, and conformational changes are all significant, particularly in solution. In the
second case where the polymer is fabricated into a higher density form, all these factors are again very
important, but one must also take into account solvent/water infiltration which can mediate the
chemical behavior of the polymer very significantly.
5.1
WATER SORPTION
Water uptake in a thin polymer film can lead to significant changes in the physicochemical
properties of the polymer [69,70]. In critical applications like medical devices, this may lead to such
problems as reduced adhesion and mechanical properties, pronounced physical and chemical aging,
and swelling and expansion, compromising the intended function of the polymer and also modulating
biocompatibility. The situation will be exacerbated when using thin films because the surface to mass
ratio is much larger which facilitates water uptake. One of the most important considerations with
thermoresponsive polymers is that they can be appreciably hygroscopic above and below the LCST.
For PNIPAm, thermal gravimetric measurements on bulk polymer, indicated that no water was
adsorbed at 40 °C (or more correctly, it was not possible to measure the low amount of adsorbed water)
[71]. However, when PNIPAm is fabricated as a thin film then one can observe significant water
absorption both below and above the LCST. For instance, a 10 m thick PNIPAm film in an
environment with a relative humidity of 90 %RH, was measured to have absorbed 26.5 % by weight
of water below the LCST (at 25 C). Above the LCST (at 37 C) the amount of adsorbed/absorbed
water was still a very significant 6.1 % by weight [61]. While the water absorption can be described
as a purely physical effect, it has consequences for the chemical properties of the polymer because it
affects the integral hydrogen bonding within the polymer. Thus for the relatively hydrophilic
polymers like PNIPAm in thin film form, there is a clear requirement to handle the polymers under
conditions of controlled humidity.
Fluorescent 3-hydroxyflavone (3-HF) derivatives have been used to monitor the infiltration of
water in PNIPAm thin films [61]. 3-HF emission is governed by an excited-state intramolecular
proton transfer (ESIPT) process and these probes demonstrate very strong solvatochromism and
electrochromism [72-74]. These 3-HF fluorophores thus exhibit dual band fluorescence emission
which is sensitive to environmental factors, and as a sensor/probe they have a clear advantage over
single band fluorophores [75-80]. One of these bands originates from the normal excited state (N*),
and the other is due to the ESIPT reaction product tautomer (T*). This emission, in terms of the
wavelengths of maximum emission and relative intensities of the two emission bands, is sensitive to
various environmental factors [72-74,81-83]. The most important emission parameter is the ratio of
the emission intensities from the N* and T* excited states, IN*/IT*, which is associated with the relative
populations of the N* and T* states and is a very sensitive indicator of solvent polarity [82]. The
behavior of these emission bands as well as the relationship of their intensities depends strongly on
Annual Volumes, Vol. 8, pp. 97-126, (2015). ISBN: 978-3-319-24607-9 (Hardcover), 978-3-319-24609-3 (ebook) Springer.
Page 9 of 31
when it is in solution or deployed as a particle or a thin film. In the first case Hydrogen-bonding, van
der Waals interactions, and conformational changes are all significant, particularly in solution. In the
second case where the polymer is fabricated into a higher density form, all these factors are again very
important, but one must also take into account solvent/water infiltration which can mediate the
chemical behavior of the polymer very significantly.
5.1
WATER SORPTION
Water uptake in a thin polymer film can lead to significant changes in the physicochemical
properties of the polymer [69,70]. In critical applications like medical devices, this may lead to such
problems as reduced adhesion and mechanical properties, pronounced physical and chemical aging,
and swelling and expansion, compromising the intended function of the polymer and also modulating
biocompatibility. The situation will be exacerbated when using thin films because the surface to mass
ratio is much larger which facilitates water uptake. One of the most important considerations with
thermoresponsive polymers is that they can be appreciably hygroscopic above and below the LCST.
For PNIPAm, thermal gravimetric measurements on bulk polymer, indicated that no water was
adsorbed at 40 °C (or more correctly, it was not possible to measure the low amount of adsorbed water)
[71]. However, when PNIPAm is fabricated as a thin film then one can observe significant water
absorption both below and above the LCST. For instance, a 10 m thick PNIPAm film in an
environment with a relative humidity of 90 %RH, was measured to have absorbed 26.5 % by weight
of water below the LCST (at 25 C). Above the LCST (at 37 C) the amount of adsorbed/absorbed
water was still a very significant 6.1 % by weight [61]. While the water absorption can be described
as a purely physical effect, it has consequences for the chemical properties of the polymer because it
affects the integral hydrogen bonding within the polymer. Thus for the relatively hydrophilic
polymers like PNIPAm in thin film form, there is a clear requirement to handle the polymers under
conditions of controlled humidity.
Fluorescent 3-hydroxyflavone (3-HF) derivatives have been used to monitor the infiltration of
water in PNIPAm thin films [61]. 3-HF emission is governed by an excited-state intramolecular
proton transfer (ESIPT) process and these probes demonstrate very strong solvatochromism and
electrochromism [72-74]. These 3-HF fluorophores thus exhibit dual band fluorescence emission
which is sensitive to environmental factors, and as a sensor/probe they have a clear advantage over
single band fluorophores [75-80]. One of these bands originates from the normal excited state (N*),
and the other is due to the ESIPT reaction product tautomer (T*). This emission, in terms of the
wavelengths of maximum emission and relative intensities of the two emission bands, is sensitive to
various environmental factors [72-74,81-83]. The most important emission parameter is the ratio of
the emission intensities from the N* and T* excited states, IN*/IT*, which is associated with the relative
populations of the N* and T* states and is a very sensitive indicator of solvent polarity [82]. The
behavior of these emission bands as well as the relationship of their intensities depends strongly on