Highly Water-Soluble Thermally Responsive Poly(thiophene)-Based Brushes

Polymer Chemistry for Haptics, Soft Robotics, and Human-Machine Interfaces

Progress in the field of soft devices-i.e., haptics, robotics, and human-machine interfaces (HRHMIs)-has its basis in the science of polymeric materials and chemical synthesis. However, in examining the relevant literature, we find that most developments have been enabled by off-the-shelf materials used either alone or as components of physical blends and composites. In this Progress Report, we take the position that a greater awareness of the capabilities of synthetic chemistry will accelerate the capabilities of HRHMIs. Conversely, an awareness of the applications sought by engineers working in this area may spark the development of new molecular designs and synthetic methodologies by chemists. We highlight several applications of active, stimuli-responsive polymers, which have demonstrated or shown potential use in HRHMIs. These materials share the fact that they are products of state-of-the-art synthetic techniques. The Progress Report is thus organized by the chemistry by which the materials were synthesized, including controlled radical polymerization, metal-mediated cross-coupling polymerization, ring-opening polymerization, various strategies for crosslinking, and hybrid approaches. These methods can afford polymers with multiple properties (i.e. conductivity, stimuli-responsiveness, self-healing and degradable abilities, biocompatibility, adhesiveness, and mechanical robustness) that are of great interest to scientists and engineers concerned with soft devices for human interaction.

Shape-Changing Bottlebrush Polymers

Bottlebrush polymers (BBPs), composed of relatively short polymeric side chains densely grafted on a polymer backbone, exhibit many unique characteristics and hold promise for a variety of applications. This Perspective focuses on environmentally induced shape-changing behavior of BBPs at interface and in solution, particularly worm/star-globule shape transitions. While BBPs with a single type of homopolymer or random copolymer side chains have been shown to undergo pronounced worm-to-globule shape changes in response to external stimuli, the collapsed brushes are unstable and prone to aggregation. By introducing a second, solvophilic polymer into the side chains, either as a distinct type of side chain or as the outer block of block copolymer side chains, the collapsed brushes not only are stabilized but also create unimolecular micellar nanostructures, which can be used for, e.g., encapsulation and delivery of substances. The current challenges in the design, synthesis, and characterization of stimuli-responsive shape-changing BBPs are discussed.

Printable Off-On Thermoswitchable Fluorescent Materials for Programmable Thermally Controlled Full-Color Displays and Multiple Encryption

Thermoswitchable fluorescent materials (TFMs) have received special attention due to their unique fluorescent colorimetric responses to temperature. Conventional TFMs generally display unicolor with switching from one color to another, showing unprintable and unsatisfied performances. These limitations greatly hinder their development and expansion toward advanced applications. Herein, the superior integration of full-color, off-on switching mode, printability, and high performance to TFMs is achieved successfully. The success is due to a thermally induced synchronous "dual/multichannel" stimulus-response mode regulated by a self-crystalline phase-change material; that is, synergistic changes of the molecular existence states and subsequent colors/spectra of the fluorescent modifier and fluorophores, accompanied by corresponding high-efficiency on-off switching of Förster resonance energy transfer. These TFMs are simple to prepare and show good performance, such as high fluorescence emission contrast (>100), great reversibility (>200 cycles), and easy-to-adjust response temperature. Particularly, these R/G/B TFMs can be prepared as tricolor fluorescent inks, and thus full-color emissions on flexible substrate can be easily obtained by printing. Finally, their great potential in switchable dynamic interior decoration, programmatic temperature-control information display, and senior information encryption are illustrated. This successful exploration offers a new perspective for designing and optimizing various other switchable materials with higher comprehensive performances.

Development of A Thermo-Responsive Conjugated Polymer with Photobleaching-Resistance Property and Tunable Photosensitizing Performance

A thermo-responsive conjugated polymer, PFBT-gPA is synthesized by grafting the poly(N-isopropylacrylamide) (PNIPAAm) to the side chains of a conjugated polyfluorene derivative through atom transfer radical polymerization (ATRP). PFBT-gPA undergoes a reversible phase transition in water below and above the lower critical solution temperature (LCST) and the process is studied by differential scanning calorimetry (DSC) analysis and UV/vis absorption spectra. PFBT-gPA shows a good photostability under UV light irradiation especially above the LCST. Moreover, the photosensitizing performance of PFBT-gPA could be tuned simply by changing temperature. The unique properties of PFBT-gPA promise its potential applications in sensing and photodynamic therapy.

Bioinspired design of underwater superoleophobic Poly(N-isopropylacrylamide)/ polyacrylonitrile/TiO2 nanofibrous membranes for highly efficient oil/water separation and photocatalysis

Inspired by fish scales, this study prepares a thermo-responsive underwater oleophobic PNIPAM/PAN/TiO₂ nanofibrous membranes by traditional electrospinning technique using poly-N-isopropylacrylamide (PNIPAM) and polyacrylonitrile (PAN). Thermal properties, mechanical properties, surface chemical composition, wettability, photocatalysis, and oil/water separation of PNIPAM/PAN/TiO₂ membrane are explored compared to pure PNIPAM membrane. Result reveals that PAN/TiO₂ compounds make PNIPAM membrane with a smaller fiber diameter of 141 nm and high tensile stress of 7.4 MPa, and also decompose 98% of rhodamine B after UV light radiation. This bioinspired design structure endows the membrane with superhydrophilicity with a low water contact angle, and underwater superoleophobicity with a high oil contact angle of 157° (petroleum ether) and 151° (dichloromethane). This membrane can efficiency separate oil/water mixture with a high separation efficiency. Moreover, the resultant PNIPAM/PAN/TiO₂ membrane has the bionic fish scale structure, and has wettability respond at lower critical solution temperature making the water flux decreased from 10013 ± 367 L m^-2·h^-1 to 7713 ± 324 L m^-2·h^-1, and thus has a potential to be used in purification of reclaimed water and separation of oil from water.

Synthesis, thermoresponsivity and multi-tunable hierarchical self-assembly of multi-responsive (AB)mC miktobrush-coil terpolymers

Stimuli-responsive miktobrush-coil terpolymers can exhibit unique physical properties and hierarchical self-assembly behaviors dependent on composition, concentration and external stimuli.

Light-Induced Conformational Change of Uracil-Anchored Polythiophene-Regulating Thermo-Responsiveness

Tuning the electronic structure of a π-conjugated polymer from the responsive side chains is generally done to get desired optoelectronic properties, and it would be very fruitful when light is used as an exciting tool that can also affect the backbone chain conformation. For this purpose, polythiophene- g-poly-[ N-(6-methyluracilyl)- N, N-dimethylamino chloride]ethyl methacrylate (PTDU) is synthesized. On exposure to diffuse sunlight, the uracil moieties of the grafted chains cause the absorption maximum of PTDU solution to show gradual blue shift of 87 nm and a gradual blue shift of 46 nm in the emission maximum, quenching its fluorescence with time. These effects occur specifically at the absorption range of polythiophene (PT) chromophore on direct exposure of light of different wavelengths, and the optimum wavelength is found to be 420 nm. Impedance study suggests a decrease in charge transfer resistance upon exposure because of conformational change of PTDU. Theoretical study indicates that on exposure to visible light, uracil moieties move toward the backbone to facilitate photoinduced electron transfer between the PT and the uracil, attributing to the variation in optoelectronic properties. Morphological and light-scattering studies exhibit a decrease in particle size because of coiling of the PT backbone and squeezing of the grafted chain on light exposure. The transparent orange-colored PTDU solution becomes hazy with a hike in emission intensity on addition of sodium halides and becomes reversibly transparent or hazy on heating or cooling. The screening of cationic centers of PTDU by varying halide anion concentration tunes the phase transition temperature. Thus, the light-induced variation in the backbone conformation is responsible for tuning the optoelectronic properties and regulates the thermos-responsiveness of the PTDU solution in the presence of halide ions.

Optoelectronic Properties of Self-Assembled Nanostructures of Polymer Functionalized Polythiophene and Graphene

In this Feature Article, we discuss the variation of optoelectronic properties with the aggregation style of polythiophene (PT) graft copolymers and polymer-modified graphene systems. Grafting of flexible polymers on a PT chain exhibits several self-organized patterns under various conditions, causing different optical and electronic properties, arising from the different conformational states of the conjugated chain. Graphene, a zero band gap material, is functionalized with polymers both covalently and noncovalently to create a finite band gap importing new optoelectronic properties. The polymer-triggered self-assembled nanostructures of PT and graphene-based materials bring unique optical/electronic properties suitable for sensing toxic ions, nitroaromatics, and surfactants, for drug delivery, and also for fabricating molecular logic gates, electronic rectifiers, photocurrent devices, etc.

Facile synthesis of cylindrical molecular brushes via Lewis pair-mediated polymerization

A general strategy for creating molecular brushes via Lewis pair-mediated polymerization is described.

Light-triggered unfolding of single linear molecular bottlebrushes from compact globular to wormlike nano-objects in water

The photocleavage of o-nitrobenzyl moieties drives shape transitions from globular to wormlike in stimuli-responsive homografted and binary heterografted molecular bottlebrushes.

Tailoring Conformation-Induced Chromism of Polythiophene Copolymers for Nucleic Acid Assay at Resource Limited Settings

Here we report on the design and synthesis of cationic water-soluble thiophene copolymers as reporters for colorimetric detection of microRNA (miRNA) in human plasma. Poly(3-alkoxythiophene) (PT) polyelectrolytes with controlled ratios of pendant groups such as triethylamine/1-methyl imidazole were synthesized for optimizing interaction with target miRNA sequence (Tseq). Incorporation of specific peptide nucleic acid (PNA) sequences with the cationic polythiophenes yielded distinguishable responses upon formation of fluorescent PT-PNA-Tseq triplex and weakly fluorescent PT-Tseq duplex, thereby enabling selective detection of target miRNA. Unlike homopolymers of PT (hPT), experimental results indicate the possibility of utilizing copolymers of PT (cPT) with appropriate ratios of pendant groups for miRNA assay in complex matrices such as plasma. As an illustration, colorimetric responses were obtained for lung cancer associated miRNA sequence (mir21) in human plasma, with a detection limit of 10 nM, illustrating the feasibility of proposed methodology for clinical applications without involving sophisticated instrumentation. The described methodology therefore possesses high potential for low-cost nucleic acid assays in resource-limited settings.

Controlling molecular ordering in solution-state conjugated polymers

Rationally encoding molecular interactions that can control the assembly structure and functional expression in a solution of conjugated polymers hold great potential for enabling optimal organic optoelectronic and sensory materials. In this work, we show that thermally-controlled and surfactant-guided assembly of water-soluble conjugated polymers in aqueous solution is a simple and effective strategy to generate optoelectronic materials with the desired molecular ordering. We have studied a conjugated polymer consisting of a hydrophobic thiophene backbone and hydrophilic, thermo-responsive ethylene oxide side groups, which shows a step-wise, multi-dimensional assembly in water. By incorporating the polymer into phase-segregated domains of an amphiphilic surfactant in solution, we demonstrate that both chain conformation and degree of molecular ordering of the conjugated polymer can be tuned in hexagonal, micellar and lamellar phases of the surfactant solution. The controlled molecular ordering in conjugated polymer assembly is demonstrated as a key factor determining the electronic interaction and optical function.

Fluorescent Molecular Rotor-in-Paraffin Waxes for Thermometry and Biometric Identification

Novel thermoresponsive sensor systems consisting of a molecular rotor (MR) and paraffin wax (PW) were developed for various thermometric and biometric identification applications. Polydiphenylacetylenes (PDPAs) coupled with long alkyl chains were used as MRs, and PWs of hydrocarbons having 16-20 carbons were utilized as phase-change materials. The PDPAs were successfully dissolved in the molten PWs and did not act as an impurity that prevents phase transition of the PWs. These PDPA-in-PW hybrids had almost the same enthalpies and phase-transition temperatures as the corresponding pure PWs. The hybrids exhibited highly reversible fluorescence (FL) changes at the critical temperatures during phase transition of the PWs. These hybrids were impregnated into common filter paper in the molten state by absorption or were encapsulated into urea resin to enhance their mechanical integrity and cyclic stability during repeated use. The wax papers could be utilized in highly advanced applications including FL image writing/erasing, an array-type thermo-indicator, and fingerprint/palmprint identification. The present findings should facilitate the development of novel fluorescent sensor systems for biometric identification and are potentially applicable for biological and biomedical thermometry.

Amphiphilic and Thermoresponsive Conjugated Block Copolymer with Its Solvent Dependent Optical and Photoluminescence Properties: Toward Sensing Applications

Herein, we present a new class of amphiphilic, thermoresponsive rod-coil conjugated block copolymer having regioregular poly(3-hexyl thiophene) and poly(N-isopropylacrylamide). Optical and luminescence properties of theses polymers significantly depend on the self-assembled nanostructures formed in different solvent and are easily tailored by chnaging the solvent composition or external stimuli like heat. Unique optical and electronic properties of this block copolymer are believed to make it promising for applications like sensor, fluorescence thermometer, optoelectronic, and bioelectronics devices.

Multi-responsive behavior of highly water-soluble poly(3-hexylthiophene)-block-poly(phenyl isocyanide) block copolymers

Highly water-soluble block copolymers containing conjugated poly(3-hexylthiophene) and hydrophilic poly(phenyl isocyanide) segments were found to be responsive to multiple stimuli, such as temperature, pH and solvents.

Synthesis of multifunctional poly(1-pyrenemethyl methacrylate)-b-poly(N-isopropylacrylamide)-b-poly(N-methylolacrylamide)s and their electrospun nanofibers for metal ion sensory applications

PPy-b-PNIPAAm-b-PNMA multifunctional triblock copolymers based electrospun nanofibers detected temperature variation or metal-ions with a high sensitivity.

Water soluble polythiophenes: preparation and applications

Different synthetic procedures for water soluble polythiophenes and their applications in sensing, detection of biomolecules and optoelectronic devices are discussed.

Molecular Mechanochemistry: Engineering and Implications of Inherently Strained Architectures

Mechanical activation of chemical bonds is usually achieved by applying external forces. However, nearly all molecules exhibit inherent strain of their chemical bonds and angles as a result of constraints imposed by covalent bonding and interactions with the surrounding environment. Particularly strong deformation of bonds and angles is observed in hyperbranched macromolecules caused by steric repulsion of densely grafted polymer branches. In addition to the tension amplification, macromolecular architecture allows for accurate control of strain distribution, which enables focusing of the internal mechanical tension to specific chemical bonds and angles. As such, chemically identical bonds in self-strained macromolecules become physically distinct because the difference in bond tension leads to the corresponding difference in the electronic structure and chemical reactivity of individual bonds within the same macromolecule. In this review, we outline different approaches to the design of strained macromolecules along with physical principles of tension management, including generation, amplification, and focusing of mechanical tension at specific chemical bonds.

Shifting Electronic Structure by Inherent Tension in Molecular Bottlebrushes with Polythiophene Backbones

Bottlebrush macromolecules can be regarded as molecular tensile machines, where tension is self-generated along the backbone due to steric repulsion between densely grafted side chains. This intrinsic tension is amplified upon adsorption of bottlebrush molecules onto a substrate and increases with grafting density, side chain length, and strength of adhesion to the substrate. To investigate the effects of tension on the electronic structure of polythiophene (PT), bottlebrush macromolecules were prepared by grafting poly(n-butyl acrylate) (PBA) side chains from PT macroinitiators by atom transfer radical polymerization (ATRP). The fluorescence spectra of submonolayers of PT bottlebrushes were measured on a Langmuir-Blodgett (LB) trough with the backbone tension adjusted by controlling the side-chain length, surface pressure, and chemical composition of a substrate. The wavelength of maximum emission has initially red-shifted, followed by a blue-shift as the backbone tension increases from 0 to 2.5 nN, which agrees with DFT calculations. The red-shift is ascribed to an increase in the conjugation length due to the extension of the PT backbone at lower force regime (0-1.0 nN), while the blue-shift is attributed to deformations of bond lengths and angles in the backbone at higher force regime (1.0-2.5 nN).

Synthesis, morphology, and sensory applications of multifunctional rod-coil-coil triblock copolymers and their electrospun nanofibers

We report the synthesis, morphology, and applications of conjugated rod-coil-coil triblock copolymers, polyfluorene-block-poly(N-isopropylacrylamide)-block-poly(N-methylolacrylamide) (PF-b-PNIPAAm-b-PNMA), prepared by atom transfer radical polymerization first and followed by click coupling reaction. The blocks of PF, PNIPAAm, and PNMA were designed for fluorescent probing, hydrophilic thermo-responsive and chemically cross-linking, respectively. In the following, the electrospun (ES) nanofibers of PF-b-PNIPAAm-b-PNMA were prepared in pure water using a single-capillary spinneret. The SAXS and TEM results suggested the lamellar structure of the PF-b-PNIPAAm-b-PNMA along the fiber axis. These obtained nanofibers showed outstanding wettability and dimension stability in the aqueous solution, and resulted in a reversible on/off transition on photoluminescence as the temperatures varied. Furthermore, the high surface/volume ratio of the ES nanofibers efficiently enhanced the temperature-sensitivity and responsive speed compared to those of the drop-cast film. The results indicated that the ES nanofibers of the conjugated rod-coil block copolymers would have potential applications for multifunctional sensory devices.

Responsive nematic gels from the self-assembly of aqueous nanofibres

Aqueous nanofibres constructed by the self-assembly of small amphiphilic molecules can become entangled to form hydrogels that have a variety of applications including tissue engineering, and controlled drug delivery. The hydrogels are formed through the random physical cross-linkings of flexible nanofibres. Here we report that self-assembled nanofibres with a nematic substructure are aligned into a nematic liquid crystal and are spontaneously fixed in the aligned state to give rise to anisotropic gels. The liquid-crystal gels respond to temperature by transforming into a fluid solution upon cooling. Thus, the nanofibre solution can be mixed with cells at room temperature and then can be transformed into gels to encapsulate the cells in a three-dimensional environment upon being heated to physiological temperatures. We found that the cells grow within the three-dimensional networks without compromising the cell viability, and that subsequent cooling triggers the encapsulated cells to be released through a sol-gel transition.