Stimuli-responsive Network Research Articles

Polymer networks which locally rotate to accommodate stresses, torques, and deformation

Polymer network models construct the constitutive relationships of a broader polymer network from the behavior of a single polymer chain (e.g. viscoelastic response to applied forces, applied electromagnetic fields, etc.). Network models have been used for multiscale phenomena in a variety of contexts such as rubber elasticity, soft multifunctional materials, biological materials, and even the curing of polymers. For decades, a myriad of polymer network models have been developed with differing numbers of chains, arranged in different ways, and with differing symmetries. To complicate matters further, there are also competing assumptions for how macroscopic variables (e.g. deformation) are related to individual chains within the network model. In this work, we propose a simple, intuitive assumption for how the network locally rotates relative to applied loading (e.g. stresses, external fields) and show that this assumption unifies many of the disparate polymer network models–while also recovering one of the most successful models for rubber elasticity, the Arruda–Boyce 8-chain model. The new assumption is then shown to make more intuitive predictions (than prior models) for stimuli-responsive networks with orientational energies (e.g. electroactive polymers), which is significant for shape morphing and designing high degree of freedom actuators (e.g. for soft robotics). Lastly, we unveil some surprising consequences of the new model for the phases of multistable biopolymer and semi-crystalline networks.

Healable soft materials based on ionic liquids and block copolymer self-assembly

Ion gels, which are macromolecular networks filled with ionic liquids, are attracting significant interest owing to their unique physicochemical properties such as their nonflammability, negligibly low volatility, and high ionic conductivity. However, for greater applicability in electrochemical devices, these materials must have high mechanical strength while continuing to exhibit the desirable properties of ionic liquids. Therefore, functional ion gels that can self-heal in response to external stimuli or in an autonomous fashion are being investigated. In this review, we highlight the recent progress made by our group in the development of healable ion gels formed by the self-assembly of block copolymers in ionic liquids. First, photohealable ion gels that exploit the photoinduced morphological transitions of block copolymers in ionic liquids are described. Then, the molecular design of self-healing block copolymer-ion gels that show fast self-healing as well as high mechanical strength is discussed. Our recent development on healable soft materials that utilize block copolymer self-assembly in ionic liquids is reviewed. We begin by discussing photohealable ion gels that respond to specific wavelength of light irradiation by forming and collapsing stimuli-responsive networks. Then, we highlight on the self-healing ion gels based on synergetic combination of supramolecular interaction and block copolymer self-assembly, which realize high mechanical strength and fast self-healing ability in the absence of any external stimuli.

Controlling biocatalytic cascades with enzyme–DNA dynamic networks

Cellular transformations, such as gene expression or temporal protein activities, are controlled by complex stimuli-responsive network circuitries regulated by enzymes, metabolites or transcription factors. Inspired by nature, extensive research efforts are directed to mimic these processes by in vitro chemical systems. Here we report on the assembly of constitutional dynamic networks (CDNs), composed of nucleic acid–enzyme conjugates, that act as modules for triggered, network-driven, biocatalytic cascades and for the intercommunication of network-guided biocatalytic cascades. Two CDNs were assembled—one network includes a constituent module functionalized with glucose oxidase and horseradish peroxidase in spatially close positions, and the second CDN includes a constituent module modified at sterically intimate positions with nicotinamide adenine dinucleotide and alcohol dehydrogenase. Biocatalytic cascades proceed in the two networks and, on the triggered reconfiguration of the CDNs, controlled and switchable biocatalytic cascades in the CDNs are demonstrated. The two CDNs are coupled, and the triggered feedback-driven intercommunication of the networks is realized. Extensive research efforts in systems chemistry are directed to the development of in vitro systems that mimic complex natural networks. Now, stimuli-responsive nucleic acid-based networks conjugated to biocatalysts for the triggered and orthogonal control over biocatalytic cascades are reported.

Synthesis and Reactivity of Metallocarbene-Containing Polymers.

Metallopolymers are an emerging class of materials with potential utility as semiconductors, catalysts, optical device components, and stimuli responsive networks. While polymer frameworks have been decorated with an array of organometallic moieties, the incorporation of metallocarbenes has been largely overlooked. Here, we report ring-opening metathesis polymerization as a strategy for the synthesis of Fischer carbene-containing polymers. High degrees of polymerization were observed (>800 repeats), and the isolated materials exhibited exceptional solubility and thermal stability. The tungsten carbene subunits were readily incorporated into block copolymers and could be modified through subsequent transformations. Moreover, the metallocarbene polymers were found to release carbon monoxide upon exposure to light or oxygen, which is unusual for tungsten carbene complexes. These metallocarbene-containing polymers could represent new platforms for the development of functional materials.

β-Cyclodextrin based pH and thermo-responsive biopolymeric hydrogel as a dual drug carrier

Herein, a novel biocompatible and stimuli-responsive network gel has been developed by grafting and crosslinking poly(N-isopropyl acrylamide) and poly(methacrylic acid) on cyclic oligosaccharide β-cyclodextrin [β-CD-cl-(PNIPAm-co-PMAc)].

Switching "on" and "off" the adhesion in stimuli-responsive elastomers.

The present work aims at the preparation of dry adhesives with switchable bonding properties by using the reversible nature of the [4πs+4πs] cycloaddition of anthracenes. Photo-responsive hydrogenated carboxylated nitrile butadiene rubber with photo-responsive pendant anthracene groups is prepared by one-pot synthesis. The formation of 3D networks relies on the photodimerization of the anthracene moieties upon UV exposure (λ > 300 nm). Controlled cleavage of the crosslink sites is achieved by either deep UV exposure (λ = 254 nm) or thermal dissociation at 70 °C. The kinetics of the optical and thermal cleavage routes are compared in thin films using UV-vis spectroscopy and their influence on the reversibility of the network is detailed. Going from thin films to free standing samples the modulation of the network structure and thermo-mechanical properties over repeated crosslinking and cleavage cycles are characterized by low-field NMR spectroscopy and dynamic mechanical analysis. The applicability of the stimuli-responsive networks as adhesives with reversible bonding properties is demonstrated. The results evidence that the reversibility of the crosslinking reaction enables a controlled switching "on" and "off" of adhesion properties. The recovery of the adhesion force amounts to 75 and 80% for photo- and thermal dissociation, respectively. Spatial control of adhesion properties is evidenced by adhesion force mapping experiments of photo-patterned films.

Design and application of photo-reversible elastomer networks by using the [4πs+4πs] cycloaddition reaction of pendant anthracene groups

Abstract In this work, the photo-reversible [4πs+4πs] cycloaddition reaction of anthracene moieties is employed to impart wavelength dependent properties into carboxylated nitrile butadiene rubber networks. Crosslinking of the side chain functionalized rubber is achieved by UV induced dimerization of pendant anthracene groups at wavelengths above 300 nm whilst a subsequent deep UV exposure (λ = 254 nm) results in a well-defined cleavage of the crosslinks. Crosslink kinetics and photo-reversibility of the dimerization reaction are determined by spectroscopic measurements. The changes in network structure and chain dynamics over multiple crosslinking and cleavage cycles are studied in detail by low field NMR. In thin films, the wavelength dependent changes in solubility are employed for the preparation of reversible photoresists enabling the preparation of 2D microstructures and a switching between negative- and positive-type patterns. Along with thin films, the wavelength dependent modulation of thermo-mechanical properties is also demonstrated for macroscopic samples, which shows the versatility of these optically stimuli-responsive networks.

Stimuli–responsive networks grafted onto polypropylene for the sustained delivery of NSAIDs

Co-polymers of N-isopropyl acrylamide (NIPAAm) and N-(3-aminopropyl) methacrylamide hydrochloride (APMA) were grafted on polypropylene (PP) films by means of a γ-ray pre-irradiation method, with the aim of developing medical devices able to load non-steroidal anti-inflammatory drugs (NSAIDs) and to control their release under physiological conditions. The NIPAAm/APMA molar ratios in the grafts, estimated by Fourier transform infrared attenuated total reflection spectroscopy and X-ray photoelectron spectroscopy analysis, were 4.76 and 1.23 for PP-g-(1M NIPAAm-r-0.5M APMA) and PP-g-(1M NIPAAm-r-1M APMA), respectively. By varying the reaction time, different degrees of grafting were achieved, while the monomer ratio was kept constant. PP-g-(NIPAAm-r-APMA) films showed temperature-responsive swelling, smaller friction coefficients, hemolysis and thrombogenicity and higher cell compatibility, did not elicit secretion of cytokines, and took up remarkable amounts of diclofenac and ibuprofen and sustained delivery for several hours in phosphate buffer, pH 7.4. Coating with carboxymethyl dextran of diclofenac-loaded PP-g-(NIPAAm-r-APMA) films caused a minor discharge of the drug but did not alter the drug release rate.

Investigating the Molecular Origins of Responsiveness in Functional Silicone Elastomer Networks

Dielectric, calorimetric, and dynamic mechanical measurements were performed to delineate the types and dynamic rates of molecular scale motion in modified poly(vinylmethyl siloxane) (PVMS) stimuli-responsive networks, where pendent groups of the form -S-(CH2)n-OH were chemically attached to the vinyl moiety of PVMS. The glass transition temperature (Tg) for the unsubstituted PVMS network matches that previously reported for linear PVMS indicating that the flexibility of the polymer chains is unaffected by the network cross-linking. In contrast, Tg increases with the introduction of pendent groups of the type -S-(CH2)n-CH3 or -S-(CH2)n-OH, where n is 2, 6, or 11, as the different groups constrain the siloxane backbone to differing degrees. The macroscopic response time and amplitude, as previously measured bydynamic contactangle,arecorrelated withtheobservedglass transition temperatures. Onecon- clusion is that the flexibility of the network and the interactions between pendent groups affect responsiveness.

Stratification, stimuli-responsiveness, self-healing, and signaling in polymer networks

The design, development, and manufacturing of new materials continue to be an ongoing challenge for scientists and engineers. Because of similarities to biological systems having self-repairable properties, polymer networks are of particular importance and interest, and if designed properly, may provide an unprecedented opportunity for mimicking biological systems. This can be accomplished through the formation of nanostructured stimuli-responsive networks that individually or collectively respond to internal or external stimuli. This article outlines selected recent developments and future trends that will formulate foundation for the development of stratified heterogeneous stimuli-responsive polymer networks capable of reorganizing, self-healing, or signaling. The primary focus is on the physico-chemical attributes of multi-component polymer networks with localized glass transition temperatures capable of stratification and exhibiting stimuli-responsiveness or recognition attributes.

Bioinspired Vesicle Restraint and Mobilization Using a Biopolymer Scaffold

Biology employs vesicles to package molecules (e.g., neurotransmitters) for their targeted delivery in response to specific spatiotemporal stimuli. Biology is also capable of employing localized stimuli to exert an additional control on vesicle trafficking; intact vesicles can be restrained (or mobilized) by association with (or release from) a cytoskeletal scaffold. We mimic these capabilities by tethering vesicles to a biopolymer scaffold that can undergo (i) stimuli-responsive network formation (for vesicle restraint) and (ii) enzyme-catalyzed network cleavage (for vesicle mobilization). Specifically, we use the aminopolysaccharide chitosan as our scaffold and graft a small number of hydrophobic moieties onto its backbone. These grafted hydrophobes can insert into the bilayer to tether vesicles to the scaffold. Under acidic conditions, the vesicles are not restrained by the hydrophobically modified chitosan (hm-chitosan) because this scaffold is soluble. Increasing the pH to neutral or basic conditions allows chitosan to form interpolymer associations that yield a strong, insoluble restraining network. Enzymatic hydrolysis of this scaffold by chitosanase cleaves the network and mobilizes intact vesicles. Potentially, this approach will provide a controllable means to store and liberate vesicle-based reagents/therapeutics for microfluidic/medical applications.