The Py scale of solvent polarities

Abstract

This paper is dedicated to Professor Peter

Polarity Assessment of Thermoresponsive Poly(NIPAM-co-NtBA) Copolymer Films Using Fluorescence Methods

“…The changes observed in the spectra are due to dipole-induced dipole interactions based on vibronic coupling [22,23]. The emission spectra (Figure 2a) show that the shape and position of the vibronic bands in the emission spectra are not affected by the composition of the film and there is also no evidence of an excimer band at 470 nm which implies that emission occurs only from pyrene monomers in the poly(NIPAM-co-NtBA) films.…”

“…Using the py scale, we can compare polymer polarity ( Figure 2d) with liquid solvent data, and this indicates that these polymers have a polarity between that of the aprotic solvents chloroform (py = 1.25) and tetrahydrofuran (py = 1.35) [22]. However, the situation is not as simple as this, because polarity is a complex parameter.…”

“…According to the pyrene polarity scale, a lower intensity ratio is attributed to a less polar or more hydrophobic environment [22,25]. Thus, the higher NtBA content causes a reduction in the overall polarity of the film and increases its hydrophobicity.…”

“…However, the situation is not as simple as this, because polarity is a complex parameter. Pyrene being a nonpolar polyaromatic hydrocarbon means that the predominant mode of interaction with its local environment is dipolarity/polarizability [27], and as a consequence, the py scale is relatively insensitive to protic factors [22] which we know are significant for the NIPAM/NtBA copolymers [26]. Dong and Winnik [22] (particularly when the structure indicates the potential for hydrogen-bonding) one should also take into consideration all three (α, β, and π*) Kamlet-Taft parameters [26].…”

“…Pyrene being a nonpolar polyaromatic hydrocarbon means that the predominant mode of interaction with its local environment is dipolarity/polarizability [27], and as a consequence, the py scale is relatively insensitive to protic factors [22] which we know are significant for the NIPAM/NtBA copolymers [26]. Dong and Winnik [22] (particularly when the structure indicates the potential for hydrogen-bonding) one should also take into consideration all three (α, β, and π*) Kamlet-Taft parameters [26]. If one does this, then one can assess the gross polymer polarity of these films as being closer to the polarity of N-methylacetamide (C 3 H 7 NO on graph), even the py value for this solvent (py = 1.48 [22]) is significantly larger than those of the polymer film.…”