Rationally Designing the Supramolecular Interfaces of Nanoparticle Superlattices with Multivalent Polymers.

In supramolecular materials, multiple weak binding groups can act as a single collective unit when confined to a localized volume, thereby producing strong but dynamic bonds between material building blocks. This principle of multivalency provides a versatile means of controlling material assembly, as both the number and the type of supramolecular moieties become design handles to modulate the strength of intermolecular interactions. However, in materials with building blocks significantly larger than individual supramolecular moieties (e.g., polymer or nanoparticle scaffolds), the degree of multivalency is difficult to predict or control, as sufficiently large scaffolds inherently preclude separated supramolecular moieties from interacting. Because molecular models commonly used to examine supramolecular interactions are intrinsically unable to examine any trends or emergent behaviors that arise due to nanoscale scaffold geometry, our understanding of the thermodynamics of these massively multivalent systems remains limited. Here we address this challenge via the coassembly of polymer-grafted nanoparticles and multivalent polymers, systematically examining how multivalent scaffold size, shape, and spacing affect their collective thermodynamics. Investigating the interplay of polymer structure and supramolecular group stoichiometry reveals complicated but rationally describable trends that demonstrate how the supramolecular scaffold design can modulate the strength of multivalent interactions. This approach to self-assembled supramolecular materials thus allows for the manipulation of polymer-nanoparticle composites with controlled thermal stability, nanoparticle organization, and tailored meso- to microscopic structures. The sophisticated control of multivalent thermodynamics through precise modulation of the nanoscale scaffold geometry represents a significant advance in the ability to rationally design complex hierarchically structured materials via self-assembly.

Controlling the dynamics of elastomer networks with multivalent brush architectures.

Herein, we report a design strategy for developing mechanically enhanced and dynamic polymer networks by incorporating a polymer with multivalent brush architecture. Different ratios of two types of imidazole functionalized polymers, specifically poly(n-butyl acrylate) (PnBA) and poly(poly(n-butyl acrylate)) (PPnBA) were blended with Zn(II) ions, thereby forming a series of elastomers with consistent composition but varying network topologies. As the weight fraction of PPnBA increased, the melting temperature, plateau modulus, and relaxation time of the melt increased because of the increase in the crosslinking density and coordination efficiency. Remarkably, however, the activation energy of the flow, Ea, decreased with increasing amounts of PPnBA despite the observed increases in mechanical properties. This unique behavior is attributed to the multivalent nature of the brush polymer, which allows the PPnBA to generate a higher crosslinking density than networks of linear PnBA, even though the brush polymers contain a lower weight fraction of the imidazole crosslinks. This method of lowering Ea, while improving the mechanical properties of the elastomers has great potential in the development of various soft materials such as self-healing or 3D-printable elastomeric structures.

Fabrication of Self-Oscillating Micelles with a Built-In Oxidizing Agent.

Various biological behaviors are fueled by "respiration", which is an example of catabolism. So far, we have reported various self-oscillating soft materials exhibiting bioinspired dynamic movements. These autonomous polymer systems are driven by the Belousov-Zhabotinsky (BZ) reaction, which is analogous to the tricarboxylic acid (TCA) cycle that is an integral part of respiration. However, in the BZ reaction, the external addition of an oxidizing agent is necessary to initiate the oxidation process, which is realized by intracellular moieties such as ubiquinone in living systems. Herein, we realized self-oscillating micelles that are driven without the external addition of an oxidizing agent. This was achieved by embedding the oxidizing source into the structure of the self-oscillating AB diblock copolymers. This strategy introduces a new function equivalent to intracellular oxidizing moieties, and is useful for the design of completely autonomous bioinspired materials.

Self-oscillating AB diblock copolymer developed by post modification strategy.

We prepared AB diblock copolymer composed of hydrophilic poly(ethylene oxide) segment and self-oscillating polymer segment. In the latter segment, ruthenium tris(2,2'-bipyridine) (Ru(bpy)3), a catalyst of the Belousov-Zhabotinsky reaction, is introduced into the polymer architecture based on N-isopropylacrylamide (NIPAAm). The Ru(bpy)3 was introduced into the polymer segment by two methods; (i) direct random copolymerization (DP) of NIPAAm and Ru(bpy)3 vinyl monomer and (ii) post modification (PM) of Ru(bpy)3 with random copolymer of NIPAAm and N-3-aminopropylmethacrylamide. For both the diblock copolymers, a bistable temperature region (the temperature range; ΔTm), where the block copolymer self-assembles into micelle at reduced Ru(bpy)3(2+) state whereas it breaks-up into individual polymer chain at oxidized Ru(bpy)3(3+) state, monotonically extends as the composition of the Ru(bpy)3 increases. The ΔTm of the block copolymer prepared by PM is larger than that by DP. The difference in ΔTm is rationalized from the statistical analysis of the arrangement of the Ru(bpy)3 moiety along the self-oscillating segments. By using the PM method, the well-defined AB diblock copolymer having ΔTm (ca. 25 °C) large enough to cause stable self-oscillation can be prepared. The periodic structural transition of the diblock copolymer in a dilute solution ([Polymer] = 0.1 wt. %) is closely investigated in terms of the time-resolved dynamic light scattering technique at constant temperature in the bistable region. A macroscopic viscosity oscillation of a concentrated polymer solution (15 wt. %) coupled with the periodic microphase separation is also demonstrated.

Design and Synthesis of Quick Setting Nonswelling Hydrogels via Brush Polymers.

Brush polymers have emerged as components of novel materials that show huge potential in multiple disciplines and applications, including self-assembling photonic crystals, drug delivery vectors, biomimetic lubricants, and ultrasoft elastomers. However, an understanding of how this unique topology can affect the properties of highly solvated materials like hydrogels remain under investigated. Here, it is investigated how the high functionality and large overall size of brush polymers enhances the gelation kinetics of low polymer weight percent gels, enabling 100-fold faster gelation rates and 15-fold higher stiffness values than gels crosslinked by traditional star polymers of the same composition and polymer chain length. This work demonstrates that brush polymer topology provides a useful means to control gelation kinetics without the need to manipulate polymer composition or crosslinking chemistry. The unique architecture of brush polymers also results in restrained or even nonswelling behavior at different temperatures, regardless of the polymer concentration. Brush polymers therefore are an interesting tool for examining how high-functionality polymer building blocks can affect structure-property relationships and chemical kinetics in hydrogel materials, and also provide a useful rapidly-setting hydrogel platform with tunable properties and great potential for multiple material applications.

Reactivity Control of Polymer Functional Groups by Altering the Structure of Thermoresponsive Triblock Copolymers.

A thermoresponsive ABA triblock copolymer bearing an aldehyde group on the thermoresponsive A segments was synthesized. The polymer formed a micellar assembly due to the hydrophobic interactions of the thermoresponsive segment above the lower critical solution temperature (LCST). In contrast, the ABA polymer assembly decomposed upon lowering the temperature below the LCST. Using this structural change, the reactivity of the aldehyde group toward primary amines of albumin and poly(allylamine) was investigated. When the ABA polymer assembly and reactant were mixed above the LCST, Schiff base formation was suppressed because of the aldehyde group being protected by the hydrophobic thermoresponsive core. In contrast, Schiff base formation between the ABA triblock copolymer and the primary amine moiety on the molecules was confirmed below the LCST. The reactivity of the aldehyde functional group can therefore be controlled by altering the structure of the thermoresponsive ABA polymer.

Precisely Tunable Sol-Gel Transition Temperature by Blending Thermoresponsive ABC Triblock Terpolymers.

Here, we report a facile methodology to control the sol-gel transition temperature (Tgel) of a physically cross-linked hydrogel by blending two kinds of ABC triblock terpolymers. Well-defined triblock terpolymers including thermosensitive N-isopropylacrylamide (NIPAAm), ABC1, and ABC2, were prepared by sequential reversible addition-fragmentation chain transfer polymerization. The chemical structure as well as the molecular weight of the A and B blocks for both polymers are identical, whereas the C blocks are different. The C block of ABC1 (C1) is a statistical copolymer of NIPAAm with hydrophobic n-butyl acrylate (BA), while that of ABC2 (C2) is a PNIPAAm homopolymer. Independently prepared ABC triblock terpolymer solutions exhibit well-defined sol-gel transitions. The Tgel of ABC1 is lower than that of ABC2 since hydrophobic BA is copolymerized into block C1. Remarkably, the Tgel varies linearly within this temperature range by simply blending the two polymers, while the resultant gel strength (∼G') remains almost unchanged. Therefore, the Tgel can be precisely adjusted by the mixing ratio of the two polymers. This method for straightforward manipulation of Tgel has great potential for various soft material applications such as biomaterials for tissue engineering, drug delivery systems, and injectable gels.

Amoeba-like self-oscillating polymeric fluids with autonomous sol-gel transition.

In the field of polymer science, many kinds of polymeric material systems that show a sol-gel transition have been created. However, most systems are unidirectional stimuli-responsive systems that require physical signals such as a change in temperature. Here, we report on the design of a block copolymer solution that undergoes autonomous and periodic sol-gel transition under constant conditions without any on-off switching through external stimuli. The amplitude of this self-oscillation of the viscosity is about 2,000 mPa s. We also demonstrate an intermittent forward motion of a droplet of the polymer solution synchronized with the autonomous sol-gel transition. This polymer solution bears the potential to become the base for a type of slime-like soft robot that can transform its shape kaleidoscopically and move autonomously, which is associated with the living amoeba that moves forward by a repeated sol-gel transition.

Multiblock copolymers exhibiting spatio-temporal structure with autonomous viscosity oscillation.

Here we report an ABA triblock copolymer that can express microscopic autonomous formation and break-up of aggregates under constant condition to generate macroscopic viscoelastic self-oscillation of the solution. The ABA triblock copolymer is designed to have hydrophilic B segment and self-oscillating A segment at the both sides by RAFT copolymerization. In the A segment, a metal catalyst of chemical oscillatory reaction, i.e., the Belousov-Zhabotinsky (BZ) reaction, is introduced as a chemomechanical transducer to change the aggregation state of the polymer depending on the redox states. Time-resolved DLS measurements of the ABA triblock copolymer confirm the presence of a transitional network structure of micelle aggregations in the reduced state and a unimer structure in the oxidized state. This autonomous oscillation of a well-designed triblock copolymer enables dynamic biomimetic softmaterials with spatio-temporal structure.