Modulation of Nanoscale Self-Assembled Structures of Highly Hydrophobic Block Copolymers by Urea, Alkylureas, and Thiourea: Vesicle to Micelle Transition.

Abstract

Self-assembly in poly(ethylene oxide, EO)-poly(propylene oxide, PO)-based highly hydrophobic block copolymer (BCPs) with 10%EO content (L31, L61, L81, L101, and L121) is investigated in the presence of urea, thiourea and alkyl ureas (methyl urea, dimethyl urea, and tetramethyl urea) using clouding, scattering, photophysical and computational simulation approach. This work explores BCPs with significantly lower %EO content, unreported in different areas, that highlight the novelty of our investigation. The results of our study conveyed an increase in the clouding phenomenon, the extent of which was dependent on the molecular characteristics of the BCPs. High-sensitivity differential scanning calorimetry was employed to investigate the thermodynamics of micellization. Fourier Transform Infrared spectral profile presented the involved interactions within the examined copolymeric systems. Two-dimensional nuclear Overhauser effect spectroscopy provided spatial correlation signals, confirming close molecular proximities and specific interactions in the micellar environment. Computational simulations were performed using the DFT/B3LYP method with the 3-21G basis set in Gaussian 5.0.9, and the optimized descriptors were evaluated. Interestingly, urea, thiourea, and alkyl urea, especially tetramethyl urea, significantly impacted the micellar structure, leading to demicellization in some BCPs. Here, the dimension expressed as hydrodynamic diameter (D_h) and micelle size changes in the observed system were monitored using dynamic light scattering analysis with the normalized intensity autocorrelation functions providing complementary information on size-dependent scattering dynamics. Furthermore, small-angle neutron scattering studies revealed that ureas enhance the solubility of the EO segments, inducing the structural transitions from multilamellar vesicles to unilamellar vesicles to spherical or Gaussian chains, attributed to the accumulation of water molecules in the micellar proximity. The effect of varying urea concentrations on the fluorescence behavior of Nile Blue A perchlorate (NBA) dye in 1%w/v L81 vesicles at 30 °C was investigated through absorbance, fluorescence emission, excitation, and lifetime decay measurements. Here, absorbance spectra exhibited a bathochromic shift with increasing urea concentration, indicating a change in micellar polarity. The fluorescence emission also showed bathochromic shift, suggesting the migration of NBA into a more hydrophobic environment within the micellar core as urea disrupted the micelle structure. Fluorescence emission lifetime decay analysis revealed complex decay at low urea concentrations, reflecting a heterogeneous dye environment, which simplified at higher urea concentrations due to micelle collapse or dye aggregation. These findings provide insight into the role of urea in altering micelle behavior and dye aggregation, with implications for micelle-based drug delivery systems.