Ricerc@Sapienza

The effect of UV irradiation on the cloud points (CP) of aqueous solutions of
a random 1:1 copolymer of oligo(ethylene glycol) methyl ether methacrylate
(OEGMA500) and a 2-(hydroxyimino)aldehyde (HIA) functionalized methacrylate
is presented. CPs are determined by visible spectroscopy and dynamic light
scattering (DLS). 1H and 13C NMR experiments are carried out in D2O and
DMSO-d6 on the polymer and on an HIA-functionalized model of the photoresponsive
repeat unit. UV-irradiated solutions exhibit an unprecedented
increase of the phase-separation temperature for an OEGMA photoresponsive
copolymer
(10–22 °C, depending on concentration and irradiation conditions).
Phase separation is reversible with little hysteresis. With both pristine
and irradiated polymer solutions, aggregate dimensions are <10 nm (DLS) at
room temperature. Aggregates of >100 nm form at the CP and gradually grow
as temperature increases, whereas the light-induced processes of the repeat
unit model in DMSO-d6 are well identified (e.g., oxime E/Z isomerization and
Norrish-Yang cyclization of the aldehyde moiety), it is not straightforward to
extrapolate such behavior to the polymeric solution in water. The remarkably
large phototriggered thermal effect in the present work motivates further
investigations on the solvent-dependent photochemistry of HIA as a promising
functional group for the synthesis of multi-stimuli responsive materials.