Urea induced changes in self-assembly and aggregate microstructures of amphiphilic star block copolymers with widely different hydrophobicity

The presence of excess of denaturants like urea unfolds proteins and disfavours micellization of surfactants; the mode of interaction of urea has always been a matter of debate. Inspired by this need, the effect of urea on the monolayer and micellar characteristics of three polyethylene oxide/propylene oxide (PEO/PPO) starblock copolymers Tetronics® viz. T1301, T1304 and T1307 (with almost same molecular weight of PPO but varying% PEO as 10, 40 and 70%, respectively) in aqueous urea solutions was investigated through scattering/spectral technique and Langmuir film balance.The three copolymers showed markedly different behaviour. In the absence of urea, the most hydrophobic T1301 forms vesicular structures (few hundred nm), T1304 with moderate hydrophobicity forms spherical micelles (∼10-20 nm) with low polydispersity while very hydrophilic T1307 exists as a highly polydispersed system with unimers (∼3-5 nm), loose aggregates (∼10-20 nm) and clusters (with few hundred nm size). However, the addition of urea (0-8 M) to these copolymer solutions leads to the disruption of any loose aggregates formed which is probably due to its binding with the copolymer chains that enhances the solubility of Tetronics® and overall polydispersity of the copolymer solutions. In support to our hypothesis, the binding of urea was validated using fluorescence measurements/Langmuir film balance and it was established that urea follows the direct mechanism (as proposed in earlier reports) and the intercalation of urea between the PEO chains leads to the extended conformations as observed by Langmuir film balance. Although, urea had a common denaturing effect on all the three copolymers, the hydrating effect of urea was most evident for copolymers with more hydrophilic character.

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