Synthesis and Characterization of Stimuli-Responsive Polymer Brushes in Nanofluidic Channels

Theoretical investigation on the conformation of polymer brushes in mixtures of binary solvents

Polymer brushes are extensively used in various applications, such as antifouling coatings and biomedical sensors. Mixed solvents entail versatile regulations on the conformation of polymer brushes. The understanding of the conformation of polymer brushes in mixtures of two solvents, however, is far from mature. In this work, we develop a self-consistent field (SCF) theory and an Alexander-de Gennes (A-dG) theory to examine the chain conformation of polymer brushes in mixtures of two miscible solvents. We systematically investigate how the Flory-Huggins interaction parameters among the three components, the composition of the mixed solvent, the grafting density, and the chain length, influence the brush height and the density profiles of various species. Our calculations exhibit many non-trivial phenomena, such as the collapse of brushes in mixtures of two good solvents and the worsening of solvent quality when adding a good solvent to a poor solvent. The physical mechanisms of these intriguing phenomena are rationalized via the interplay among the chain conformation entropy, the mixing entropy of the two solvents, and the competition in the interactions among the three species. Quantitative comparison between the SCF and the A-dG theories demonstrates that the latter theory can qualitatively capture the variation trends of the brush height and the average concentrations of different species, while the former theory can provide more detailed descriptions on the density profiles of various species in the brush. Our results here not only exhibit the richness and complexity of polymer brushes in mixed solvents but also provide valuable principles for the rational design of stimuli-responsive brushes.

Tailoring Polymer Coatings and Grafting Structures for Photoswitchable Ionic Transport in Solid-State Nanochannels

Photoresponsive ion nanochannels have gained significant attention for their ability to regulate ionic transport in response to external stimuli. The potential of molecular and polymeric architectures in the nanochannels to further enhance and modulate these behaviors, however, remains underexplored. In this work, we explore the integration of spiropyran-based polymers into anodic aluminum oxide (AAO) nanochannels, resulting in tailored photoresponsive behaviors. Spiropyran undergoes reversible ring-opening isomerization upon UV irradiation, which leads to changes in the packing and polarity of polymer chains within the nanochannels. The polySp-coated and polySp-grafted AAO systems, fabricated via solution wetting and surface-initiated atom transfer radical polymerization (SI-ATRP), exhibit unique macroscopic and microscopic responses, including reversible color changes, wettability adjustments, and modulation of ion transport under UV and visible light. These findings demonstrate the potential of spiropyran-functionalized nanochannels for applications in optical information storage, photogated materials, and sensors. By manipulating molecular architecture and nanoconfinement, this work paves the way for the design of next-generation photoswitchable systems with enhanced multifunctionality.

Synthesis of Raspberry-like Nanoparticles via Surface Grafting of Positively Charged Polyelectrolyte Brushes: Colloidal Stability and Surface Properties

A method to synthesize stable, raspberry-like nanoparticles (NPs), using surface grafting of poly(glycidyl methacrylate) (PGMA) brushes on a polystyrene (PS) core with varying grafting densities, is reported. A two-step functionalization reaction of PGMA epoxide groups comprising an amination step first using ethylene diamine and then followed by a quaternization using glycidyltrimethylammonium chloride generates permanently and positively charged polyelectrolyte brushes, which result in both steric and electrostatic stabilization. The dispersion stability of the brush-bearing NPs is dramatically improved compared to that of the pristine PS core in salt solutions at ambient (25 °C) and elevated temperatures (60 °C). Additionally, the grafted polyelectrolyte chains undergo a reversible swelling in the presence of different ionic strength (IS) salts, which modulate the surface properties, including roughness, stiffness, and adhesion. An atomic force microscope under both dry and wet conditions was used to image conformational changes of the polyelectrolyte chains during the swelling and deswelling transitions as well as to probe the nanomechanical properties by analyzing the corresponding force-sample separation curves. The quaternized polyelectrolyte brushes undergo a conformational transition from a collapsed state to a swelled state in the osmotic brush (OB) regime triggered by the osmotic gradient of mobile ions to the interior of the polymer chain. At IS ∼ 1 M, the brushes contract and the globules reform (salted brush state) as evidenced by an increase in the surface roughness and a reduction in the adhesion of the brushes. Beyond IS ∼ 1 M, quartz crystal microbalance with dissipation monitoring measurements show that salt uptake continues to take place predominantly on the exterior surface of the brush since salt adsorption is not accompanied by a size increase as measured by dynamic light scattering. The study adds new insights into our understanding of the behavior of NPs bearing salt-responsive polyelectrolyte brushes with adaptive swelling thresholds that can ultimately modulate surface properties.