Aggregation and Rheology of an Azobenzene-Functionalized Hydrophobically Modified Ethoxylated Urethane in Aqueous Solution

Understanding associative polymer self-assembly with shrinking gate fluorescence correlation spectroscopy

The self-assembly of polymers is integral to their role in liquid formulations. In this study, we combine a dye whose lifetime is sensitive to the nanoviscosity of its local environment with shrinking gate fluorescence correlation spectroscopy (sgFCS) to study the self-assembly of a model telechelic polymer, hydrophobically modified ethoxylated urethane (HEUR). Fluorescence lifetime measurements show a monotonic increase in average lifetime with increasing HEUR concentration driven by a small fraction of dye (<1%) with long lifetimes strongly bound to HEUR. Despite this small fraction, sgFCS isolates the diffusional dynamics of the bound fraction with no a priori assumptions as to the distribution of lifetimes. Sensitivity is greatly enhanced compared to standard FCS, revealing micellar aggregates forming between 0.2 and 1 wt% followed by formation of a percolated network. This sgFCS approach, which we apply for the first time to polymers in this work, is readily extendable to any dye that changes lifetime on binding.

Wavelength-Gated Adaptation of Hydrogel Properties via Photo-Dynamic Multivalency in Associative Star Polymers

Responsive materials, such as switchable hydrogels, have been largely engineered for maximum changes between two states. In contrast, adaptive systems target distinct functional plateaus between these maxima. Here, we demonstrate how the photostationary state (PSS) of an E/Z-arylazopyrazole photoswitch can be tuned by the incident wavelength across a wide color spectrum, and how this behavior can be exploited to engineer the photo-dynamic mechanical properties of hydrogels based on multivalent photoswitchable interactions. We show that these hydrogels adapt to the wavelength-dependent PSS and the number of arylazopyrazole units by programmable relationships. Hence, our material design enables the facile adjustment of the mechanical properties without laborious synthetic efforts. The concept goes beyond the classical switching from state A to B, and demonstrates pathways for a truly wavelength-gated adaptation of hydrogel properties potentially useful to engineer cell fate or in soft robotics.

Linear Alternating Associative Polymer with Ultrahigh Molecular Weight: Facile Preparation by Self-Assembly Assisted Dimerization of Anthracene and Rheology in Aqueous Solution

Alternating associative polymers (AAPs) containing more than two species of alternating hydrophobic and hydrophilic units can form unique physical network and perform interesting rheological behavior in aqueous solution. In this work, an AAP was prepared through self-assembly assisted dimerization of an anthracene-functionalized telechelic associative polymer (AnTAP) in aqueous solution by light irradiation. It is demonstrated that AnTAP can in situ chain extend to AAP with well-defined linear structure and ultrahigh molecular weight through dimerization reaction of anthracene moieties in the core of micelle under light irradiation. Meanwhile, the solution changes from viscoelastic liquid to a free-standing gel, because a physical network that cannot relax in a finite time window has developed along with the dimerization process. The results are therefore of interest not only for understanding the network structure and rheological properties of AAP solution, but also for preparing AAPs with ultrahigh molecular weight by self-assembly assisted photodimerization reactions.

Controlled Self-Assembly of Multiple-Responsive Superamphiphilc Polymers Based on Host-Guest Inclusions of a Modified PEG with β-Cyclodextrin

Superamphiphilic polymers (SAPs) constructed by host-guest inclusion can self-assemble into various nanostructures in solution, which can find applications in many fields such as nanodevices, drug delivery, and template synthesis. Herein, we report the controlled self-assembly of multiple-responsive SAP based on a selective host-guest inclusion of β-cyclodextrin (β-CD) with a modified poly(ethylene glycol) (PEG) (FcC₁₁AzoPEG) consisting of a ferrocene (Fc) end group, a C₁₁ alkyl chain, an azobenzene (Azo) block, and a poly(ethylene glycol)methyl ether (PEG) chain. These SAPs can self-assemble into interesting nanostructures in water upon exposure to different stimuli because β-CD can be selectively included with different guests, such as Fc, Azo, and C₁₁ alkyl chain, under different stimuli. The inclusion complex of Fc with β-CD (Fc@β-CD SAP) can form nanowire micelles in aqueous solution. The nanowire micelles can be transformed into spindle micelles with the addition of oxidant because the majority of β-CDs dissociated from the complex Fc@β-CD SAP due to a conversion of Fc to Fc⁺ and will preferentially include with Azo group to form another dominant inclusion complex (Azo@β-CD SAP). After UV irradiation, the spindle micelles can be further transformed into spherical micelles because most of β-CDs are excluded from the complex Azo@β-CD SAP due to a trans- to cis-Azo conversion and then form a dominant inclusion complex with C₁₁ alkyl chains (C₁₁@β-CD SAP). This work not only demonstrates the selective host-guest inclusion of stimuli-responsive groups modified PEG with β-CD but also provides a useful approach for construction of diverse morphologies.

Synthesis of High Molecular Weight Poly(glycerol monomethacrylate) via RAFT Emulsion Polymerization of Isopropylideneglycerol Methacrylate

High molecular weight water-soluble polymers are widely used as flocculants or thickeners. However, synthesis of such polymers via solution polymerization invariably results in highly viscous fluids, which makes subsequent processing somewhat problematic. Alternatively, such polymers can be prepared as colloidal dispersions; in principle, this is advantageous because the particulate nature of the polymer chains ensures a much lower fluid viscosity. Herein we exemplify the latter approach by reporting the convenient one-pot synthesis of high molecular weight poly(glycerol monomethacrylate) (PGMA) via the reversible addition-fragmentation chain transfer (RAFT) aqueous emulsion polymerization of a water-immiscible protected monomer precursor, isopropylideneglycerol methacrylate (IPGMA) at 70 °C, using a water-soluble poly(glycerol monomethacrylate) (PGMA) chain transfer agent as a steric stabilizer. This formulation produces a low-viscosity aqueous dispersion of PGMA-PIPGMA diblock copolymer nanoparticles at 20% solids. Subsequent acid deprotection of the hydrophobic core-forming PIPGMA block leads to particle dissolution and affords a viscous aqueous solution comprising high molecular weight PGMA homopolymer chains with a relatively narrow molecular weight distribution. Moreover, it is shown that this latex precursor route offers an important advantage compared to the RAFT aqueous solution polymerization of glycerol monomethacrylate since it provides a significantly faster rate of polymerization (and hence higher monomer conversion) under comparable conditions.

Ferrocene-Functionalized Hydrophobically Modified Ethoxylated Urethane: Redox-Responsive Controlled Self-Assembly and Rheological Behavior in Aqueous Solution

In this work, we present a novel redox-responsive ferrocene-functionalized hydrophobically modified ethoxylated urethane (Fc-HEUR) model polymer. The effects of a redox-induced hydrophobicity change of ferrocenyl hydrophobes on the self-assembly and rheological properties of Fc-HEUR in aqueous solution were investigated. In view of the redox-induced change in the hydrophilic-lipophilic balance of polymers, the Fc-HEUR polymer in aqueous solution can reversibly self-assemble into spherical micelles and larger micellar aggregates of different nanoscales and also disassemble by redox reactions immediately. Moreover, we have demonstrated that a rearrangement of micellar junctions takes place through a bridge-loop or loop-bridge transition in the concentrated polymer solution followed by redox reactions, which induces a great change in the rheological properties of the polymer solution: a viscoelastic liquid for the reduction state Fc-HEUR and a viscous liquid for the oxidation state Fc⁺-HEUR, owing to their different relaxation behaviors. Particularly, the associative structures and rheological properties of the Fc-HEUR aqueous solution can be reversibly controlled by redox reactions. This work will be useful not only for understanding of the thickening mechanism of stimuli-responsive HEURs but also for the development of reversible self-assembly and controlled rheological fluids, which may have some special application in drug delivery systems, catalyst supports, sensors, and microfluidic devices.