Classifying deep eutectic solvents for polymer solvation via intramolecular dimer formation

Abstract

Deep eutectic solvents (DESs) have emerged as versatile and inexpensive solubilizing media with widely varying physicochemical properties. Establishing the characteristics of a novel solvent milieu for polymer dissolution is an important exercise. The assessment of two DESs obtained by mixing choline chloride (ChCl) salt with H-bond donors (HBDs) glycerol (Gly) and urea in a 1 : 2 molar ratio (named ChCl:Gly and ChCl:urea, respectively) as solvents for polydimethylsiloxane (PDMS) solvation is carried out via the investigation of intramolecular dimerization by pyrene (Py) end-tagged PDMS of MW 3100 (Py-PDMS-Py) as a fluorescent probe in the temperature range of 293.15–363.15 K. The outcomes are compared with those in liquid PDMS of average MW 2000 (PDMS2000; a similar solvent to that of the polymer) and in glycerol (the HBD constituent of the DES ChCl:Gly). While the intramolecular dimerization by Py-PDMS-Py occurs exclusively in the excited state in liquid PDMS2000, wavelength-dependent fluorescence excitation spectra along with excited-state intensity decay kinetics reveal the presence of ground-state interactions between the pyrenyl moieties in ChCl:Gly, ChCl:urea and glycerol. This leads to the proposition that PDMS prefers to stay in predominantly the coiled form in DESs and glycerol, as opposed to that in PDMS2000 where the PDMS–PDMS contact is maximized leading to the absence of ground-state heterogeneity. Thus, while the liquid PDMS2000 is characterized as a ‘good’ solvent, the DESs ChCl:Gly and ChCl:urea along with glycerol may be designated as ‘bad’ solvents for PDMS dissolution.