Pressure-Sensitive Supramolecular Adhesives Based on Lipoic Acid and Biofriendly Dynamic Cyclodextrin and Polyrotaxane Cross-Linkers

Slide-ring materials are polymer networks with mobile cross-links that exhibit impressive stress dissipation and fracture resistance owing to the pulley effect. On account of their remarkable ability to dissipate the energy of deformation, these materials have found their way into advanced materials such as abrasion-resistant coatings and elastic battery electrode binders. In this work, we explore the role of mobile cross-links on the properties of a biofriendly pressure-sensitive adhesive made using composites of cyclodextrin-based macromolecules and poly(lipoic acid). We modify cyclodextrin-based hosts and polyrotaxanes with pendant groups of lipoic acid (a commonly ingested antioxidant) to incorporate them as cross-links in poly(lipoic acid) networks obtained by simple heating in open air. By systematically varying the adhesive formulations while probing their mechanical and adhesive properties, we uncover trends in structure-property relationships that enable one to tune network properties and access biofriendly, high-tack adhesives.

Synthesis and Characterization of Cyclodextrin-Based Polyhemiaminal Composites with Enhanced Thermal Stability

Polyhemiaminal (PHA) polymers are a new class of thermosetting polymers that have recently gained attention owing to their high mechanical strength and excellent recycling behavior. However, low thermal stability is a common issue in PHA polymers due to the thermally labile crosslinked knots. Herein, crosslinked PHA polymer composites were synthesized by reacting formaldehyde with a precursor solution of 4,4ʹ-oxydianiline (ODA) and cyclodextrins (CDs) (α-, β-, and γ-). The material obtained under optimal conditions (ODA:CD molar ratio of 1:0.5, 37% aqueous solution of formaldehyde (formalin)) exhibited good film formability and high thermal stability with two characteristic decomposition phenomena and a high char yield. The early decomposition of CDs and char formation led to high thermal stability. Time-resolved NMR analysis was conducted to study hemiaminal bond formation via a condensation reaction between ODA and formaldehyde. Furthermore, PHA matrix formation was confirmed by the dissolution of the deposited CD layer in a solution of N-methyl-2-pyrrolidinone containing 8–9 wt.% LiBr at 80 °C and FTIR analysis. Based on the elemental analysis results, PHA network formation was confirmed by considering a single unit of the PHA network with CD composition, including the solvent and water.

Post-synthesis modification of slide-ring gels for thermal and mechanical reconfiguration

Ring-sliding behavior in polyrotaxanes imbues gels, elastomers, and glasses with remarkable stress-dissipation and actuation properties. Since these properties can be modulated and tuned by structural parameters, many efforts have been devoted to developing synthetic protocols that define the structures and properties of slide-ring materials. We introduce post-synthetic modifications of slide-ring gels derived from unmodified α-cyclodextrin and poly(ethylene glycol) polyrotaxanes that enable (i) actuation and control of the thermo-responsive lower critical solution temperature (LCST) behavior of ring-modified slide-ring hydrogels, and (ii) chemically bonding separate gels into hybrid or shape-reconfigured macro-structures with a slide-ring adhesive solution. The mechanical properties of the post-modified gels have been characterized by shear rheology and uniaxial tensile tests, while the corresponding xerogels were characterized by wide-angle X-ray scattering. These demonstrations show that post-synthetic modification offers a practical solution for re-configuring the properties and shapes of slide-ring gels.

Emerging Nanoassembly of Polyrotaxanes Comprising Acetylated α-Cyclodextrins and High-Molecular-Weight Axle Polymer

Acetylated α-cyclodextrin (α-CD)/poly(ethylene glycol) (PEG)-based polyrotaxanes (Ac-PRXs) with varying degrees of acetylation (DA) and molecular weight of axle PEG were synthesized and their solubility in aqueous solutions was investigated. Ac-PRXs with low DA (less than 35%) were dissolved in aqueous solutions without considering the molecular weight of axle PEG, whereas Ac-PRXs with high DA (more than 40%) and low molecular weight of axle PEG (less than 35000) were precipitated into the solutions. Interestingly, Ac-PRXs with high DA and high molecular weight of axle PEG (100000) exhibited a colloidal dispersion in aqueous solutions. It is considered that the threaded acetylated α-CDs formed hydrophobic microenvironments via hydrophobic interactions and the noncovered segments of axle PEGs provided colloidal stability. Furthermore, the potential application of Ac-PRX_100k as a drug carrier was examined and it was established that Ac-PRX_100k can encapsulate a hydrophobic drug. Accordingly, acetylation of PRXs is a viable approach to promote solubility in aqueous solutions and prepare self-assembled nanoparticles.

Synthesis of graft polyrotaxane by simultaneous capping of backbone and grafting from rings of pseudo-polyrotaxane

Graft polyrotaxanes, with poly(ε-caprolactone) (PCL) graft chains on the ring components were synthesized by the simultaneous ring-opening polymerization of ε-caprolactone from both ends of the backbone polymer, an end-functionalized polyethylene glycol (PEG) and the formation of inclusion complexes with α-cyclodextrin (α-CD). PEG with multiple functional groups at each end was prepared by the condensation of PEG-amine and D-gluconic acid; the PEG derivative formed an inclusion complex with α-CD. The polymerization of multiple hydroxy groups at the backbone ends resulted in a star-shaped end group, which served as a bulky capping group to prevent dethreading. In contrast, PEG with only one hydroxy group at each end did not produce polyrotaxanes, indicating that single PCL chains were too thin to confine α-CDs to the complex. In addition, the grafting polymerization proceeded properly only when robust hydrogen bonds formed between α-CDs were dissociated using a basic catalyst. Since the dissociation also induced dethreading, kinetic control of the polymerization and dissociation were crucial for producing graft polyrotaxanes. Consequently, this three-step reaction yielded graft polyrotaxanes in a good yield, demonstrating a significant simplification of the synthesis of graft polyrotaxanes.

Preparation of a "sliding graft copolymer", an organic solvent-soluble polyrotaxane containing mobile side chains, and its application for a crosslinked elastomeric supramolecular film

A novel "sliding graft copolymer" (SGC), in which many linear poly-ε-caprolactone (PCL) side chains are bound to cyclodextrin rings of a polyrotaxane, was prepared by ring-opening polymerization of ε-caprolactone initiated by hydroxyl groups of the polyrotaxane. An amorphous, flexible, and sufficiently tough elastomer film was prepared by crosslinking the obtained SGC-a supramolecule possessing a number of mobile side chains-with hexamethylene diisocyanate (HMDI).

Recent advances in the preparation of cyclodextrin-based polyrotaxanes and their applications to soft materials

The present review article deals with recent novel studies on the preparation and application of polyrotaxanes comprised of cyclodextrins (CDs) and various linear polymers, especially poly(ethylene glycol) (PEG). First, a brief introduction of the historical background of the pioneering work on the preparation of an inclusion complex and polyrotaxane is provided. Subsequently, the authors have focused on the recently developed solvent systems for the polyrotaxane. These new solvents are interesting from two fundamental viewpoints: (1) from the perspective of the clarification of the hydrogen-bonding-based dissolution mechanism of polyrotaxanes; and (2) from the practical viewpoint of the preparation of modified polyrotaxanes or slide-ring gels containing ionic liquids. A wide variety of polyrotaxane derivatives, whose cyclodextrin moiety was modified to carry various functional groups, and their intriguing characteristics are introduced in this article. Finally, many instances of the application of the PEG-CD polyrotaxane to soft materials, such as gels, molecular tubes and multivalent ligand systems, are summarized.

Liquid–liquid equilibria of polyrotaxane and poly(vinyl alcohol)

Liquid-liquid equillibria (LLE) of the tertiary system of hydroxypropylated polyrotaxane (HPPR)-poly(vinyl alcohol) (PVA)-solvent have been investigated by focusing on the internal structures of HPPR-PVA blend gels. The phase diagrams of the HPPR-PVA aqueous systems displayed two liquid phases at a high concentration and molecular weight of PVA. This result was consistent with the prediction of the Flory-Huggins lattice model. On the contrary, the HPPR-PVA-DMSO system exhibited only a single phase. The HPPR-PVA blend gels crosslinked in dimethylsulfoxide (DMSO) were highly transparent over a wide concentration range, while the gels prepared in water were opaque at high polymer concentrations. Spherical domains were observed in the opaque gels by laser scanning confocal microscopy, and the sizes of the domains were significantly dependent on the amount of cross-linking reagent utilized. These results indicated that the transparency of the HPPR-PVA blend gels was strongly affected by the competition between the liquid-liquid two-phase separation and the crosslinking HPPR and PVA polymers during the preparation of the blend gels.

Thermoreversible sol–gel transition of an aqueous solution of polyrotaxane composed of highly methylated α-cyclodextrin and polyethylene glycol

Novel thermo-gelling aqueous solution systems are developed with a polyrotaxane, consisting of methylated alpha-cyclodextrin and polyethylene glycol, based on the slide-ring properties of the macrocycles along the linear chain.