Exploring Aqueous Solution Dynamics of an Amphiphilic Diblock Copolymer: Dielectric Relaxation and Time-Resolved...

We explore in this work, after synthesizing and appropriately characterizing an amphiphilic diblock copolymer, its interaction with water molecules and the subsequent aqueous solution dynamics by employing time-resolved fluorescence measurements (TRF) and megahertz-gigahertz dielectric relaxation (DR) experiments. The synthesized amphiphilic diblock copolymer is poly(2-(((tert-butoxycarbonyl)alanyl)oxy)ethyl methacrylate)-b-poly(polyethylene glycol monomethyl ether methacrylate) (P(Boc-l-Ala-HEMA)-b-PPEGMA). Dynamic light scattering measurements of aqueous solutions indicate formation of 14-20 nm particles from a balance between the chain lengths of the hydrophobic (P(Boc-l-Ala-HEMA) and hydrophilic (PPEGMA) segments. Field-emission scanning electron microscopy, on the other hand, suggests a spherical shape for the dried micelles. The critical micelle concentration of the P(Boc-l-Ala-HEMA)-b-PPEGMA block copolymer at different block lengths in aqueous media, determined via steady-state fluorescence measurements, is very low (∼4-8 mg/L), and the resultant micellar size has been found to be insensitive to the polymer concentration. Interfacial and bulk aqueous dynamics have been investigated by tracking the solution frictional resistance on rotational motion of dissolved hydrophobic and hydrophilic dipolar solute probes of comparable sizes. TRF anisotropy measurements reflect the biphasic temporal profile for the frictional resistance. Interestingly, the hydrophobic probe, because of its preferential location at the micellar interface, experiences greater frictional resistance than the hydrophilic counterpart, although the latter reports stronger polymer concentration dependence of the frictional retardation than the former. DR measurements at the highest of the polymer concentrations considered suggest presence of aqueous dynamics slower than that for neat bulk water, although evidence for such "slow" dynamics at lower concentrations has not been detected in the present DR measurements.