Light-responsive polymer surfaces via postpolymerization modification of grafted polymer-brush structures

Spiropyran-Based Soft Substrate with SPR, Anti-Reflection and Anti-NRET for Enhanced Visualization/Fluorescence Dual Response to Metal Ions

The photoluminescence of modified spiropyran on solid surfaces is poor, and the fluorescence intensity of its MC form is weak, which affects its application in the field of sensing. In this work, a PMMA layer containing Au nanoparticles and a spiropyran monomolecular layer are coated on the surface of a PDMS substrate with inverted micro-pyramids successively by means of interface assembly and soft lithography, and the overall structure is similar to insect compound eyes. The anti-reflection effect of the bioinspired structure, the SPR (surface plasmon resonance) effect of the Au nanoparticles and the anti-NRET (non-radiation energy transfer) effect of the PMMA isolation layer raise the fluorescence enhancement factor of the composite substrate vs. the surface MC form of spiropyran to 5.06. In the process of metal ion detection, the composite substrate can achieve both colorimetric and fluorescence response, and the detection limit for Zn²⁺ can reach 0.281 μM. However, at the same time, the lack of the ability to recognize specific metal ions is expected to be further improved by the modification of spiropyran.

Arylazopyrazole-Modified Thiolactone Acrylate Copolymer Brushes for Tuneable and Photoresponsive Wettability of Glass Surfaces

Photoswitches have long been employed in coatings for surfaces and substrates to harness light as a versatile stimulus to induce responsive behavior. We previously demonstrated the viability of arylazopyrazole (AAP) as a photoswitch in self-assembled monolayers (SAMs) on silicon and glass surfaces for photoresponsive wetting applications. We now aim to transfer the excellent photophysical properties of AAPs to polymer brush coatings. Compared to SAMs, polymer brushes offer enhanced stability and an increase of the thickness and density of the functional organic layer. In this work, we present thiolactone acrylate copolymer brushes which can be post-modified with AAP amines as well as hydrophobic acrylates, making use of the unique chemistry of the thiolactones. This strategy enables photoresponsive wetting with a tuneable range of contact angle change on glass substrates. We show the successful synthesis of thiolactone hydroxyethyl acrylate copolymer brushes by means of surface-initiated atom-transfer radical polymerization with the option to either prepare homogeneous brushes or to prepare micrometer-sized brush patterns by microcontact printing. The polymer brushes were analyzed by atomic force microscopy, time-of-flight secondary ion spectrometry, and X-ray photoelectron spectroscopy. Photoresponsive behavior imparted to the brushes by means of post-modification with AAP is monitored by UV/vis spectroscopy, and wetting behavior of homogeneous brushes is measured by static and dynamic contact angle measurements. The brushes show an average change in static contact angle of around 13° between E and Z isomer of the AAP photoswitch for at least five cycles, while the range of contact angle change can be fine-tuned between 53.5°/66.5° (E/Z) and 81.5°/94.8° (E/Z) by post-modification with hydrophobic acrylates.

Immobilization of Polymers to Surfaces by Click Reaction for Photocatalysis with Recyclability

A surface-bound photocatalyst offers advantages of reusability and recyclability with ease. While it can be immobilized by spin coating or drop-casting, a more reliable and durable method involves the formation of a self-assembled monolayer (SAM) on a suitable surface using designer molecules. In this paper, we report devising a practical, durable, and recyclable photocatalytic surface using immobilized polytriazoles of diketopyrrolopyrrole (DPP). While the SAM formation techniques were utilized for superior results, conventional coatings of polymers on surfaces were performed for comparison. Different methods confirmed efficient immobilization and high grafting density for the SAM technique. Computational models suggested favorable energy parameters for active materials. Photocatalytic studies were performed using both immobilized polymers and polymers in solution for comparison. These findings are important for understanding various physicochemical characteristics of polytriazole-functionalized surfaces.

Influence of the Glass Transition Temperature and the Density of Crosslinking Groups on the Reversibility of Diels-Alder Polymer Networks

The interest in self-healing, recyclable, and adaptable polymers is growing. This work addresses the reversibility of crosslink formation based on Diels-Alder reaction in copolymer networks containing furfuryl and maleimide groups, which represent the "diene" and the "dienophile," respectively. The copolymers are synthesized by atom transfer radical polymerization (ATRP) and free radical polymerization. The diene bearing copolymers are crosslinked either with a small molecule containing two dienophiles or with a dienophile bearing copolymer. The influence of the crosslinking temperature on the Diels-Alder reaction is analyzed. Furthermore, the influence of the glass transition temperature and the influence of the density of crosslinking groups on the thermo-reversibility of crosslinking are investigated by temperature dependent infrared spectroscopy and differential scanning calorimetry. It is shown that the reversibility of crosslinking is strongly influenced by the glass transition temperature of the system.

Surface Modification of Parylene C Film via Buchwald-Hartwig Amination for Organic Solvent-Compatible and Flexible Microfluidic Channel Bonding

Surface modification offers an efficient and economical route to installing functional groups on a polymer surface. This work demonstrates that primary amine groups can be introduced onto a polymer surface via Buchwald-Hartwig amination, and the functionalized substrates can be chemically bonded to produce functional microfluidic devices. By activating the CCl bond in commercially used poly(chloro-p-xylylene) (parylene C) by Pd catalyst and substituting Cl with the amine source, the amine groups are successfully installed in a facile and recyclable manner. The substrates can be covalently bonded with each other via amine-isocyanate chemistry, providing much higher bonding strength compared to previous methods based on noncovalent adhesive coatings. As a result, transparent and flexible microfluidic channels can be fabricated that are compatible with organic solvents and high pressure. Retention of amine group reactivity in the channel suggests the potential of this methodology for the surface immobilization of functional molecules for microfluidic reactors and biosensors.

A Hierarchical Nanoparticle-in-Micropore Architecture for Enhanced Mechanosensitivity and Stretchability in Mechanochromic Electronic Skins

Biological tissues are multiresponsive and functional, and similar properties might be possible in synthetic systems by merging responsive polymers with hierarchical soft architectures. For example, mechanochromic polymers have applications in force-responsive colorimetric sensors and soft robotics, but their integration into sensitive, multifunctional devices remains challenging. Herein, a hierarchical nanoparticle-in-micropore (NP-MP) architecture in porous mechanochromic polymers, which enhances the mechanosensitivity and stretchability of mechanochromic electronic skins (e-skins), is reported. The hierarchical NP-MP structure results in stress-concentration-induced mechanochemical activation of mechanophores, significantly improving the mechanochromic sensitivity to both tensile strain and normal force (critical tensile strain: 50% and normal force: 1 N). Furthermore, the porous mechanochromic composites exhibit a reversible mechanochromism under a strain of 250%. This architecture enables a dual-mode mechanochromic e-skin for detecting static/dynamic forces via mechanochromism and triboelectricity. The hierarchical NP-MP architecture provides a general platform to develop mechanochromic composites with high sensitivity and stretchability.

Polymer on Top: Current Limits and Future Perspectives of Quantitatively Evaluating Surface Grafting

Well-defined polymer strands covalently tethered onto solid substrates determine the properties of the resulting functional interface. Herein, the current approaches to determine quantitative grafting densities are assessed. Based on a brief introduction into the key theories describing polymer brush regimes, a user's guide is provided to estimating maximum chain coverage and-importantly-examine the most frequently employed approaches for determining grafting densities, i.e., dry thickness measurements, gravimetric assessment, and swelling experiments. An estimation of the reliability of these determination methods is provided via carefully evaluating their assumptions and assessing the stability of the underpinning equations. A practical access guide for comparatively and quantitatively evaluating the reliability of a given approach is thus provided, enabling the field to critically judge experimentally determined grafting densities and to avoid the reporting of grafting densities that fall outside the physically realistic parameter space. The assessment is concluded with a perspective on the development of advanced approaches for determination of grafting density, in particular, on single-chain methodologies.

Phototriggered Growth and Detachment of Polymer Brushes with Wavelength Selectivity

Both phototriggered growth and removal of polymer chains from surfaces are efficient ways to finely tune interface properties. Combining these two capabilities in one system with independent control can significantly increase the feasibility of photoregulation on surface modification but has not been reported yet. Herein we describe a novel approach to control both the growth and the detachment of polymer brushes independently by light with different wavelengths. The approach is based on a nitrodopamine-based initiator (NO₂-BDAM) which contains a catechol structure for surface modification, alkyl bromide group for radical polymerization, and o-nitrophenyl ethyl moiety for photolysis. When dimanganese decacarbonyl (Mn₂(CO)₁₀) was applied together with NO₂-BDAM as an initiating system, visible light (460 nm) can be used to trigger the site-specific growth of polymer brushes. Resulting polymer brushes can be selectively removed by UV light (360 nm). This method is suitable for different monomers on various substrates, suggesting a facile and robust method to regulate surface properties.

Light- and pH-dually responsive dendrimer-star copolymer containing spiropyran groups: synthesis, self-assembly and controlled drug release

Dendrimer-star copolymer containing spiropyran groups could self-assemble into micelles and presented light- and pH-dually responsive properties.

The influence of surface potential on the optical switching of spiropyran self assembled monolayers

Surfaces whose macroscopic properties can be switched by light are potentially useful in a wide variety of applications. One such promising application is electrochemical sensors that can be gated by optically switching the electrode on or off. One way to make such a switchable electrode is by depositing a self-assembled monolayer (SAM) of bistable, optically switchable molecules onto an electrode surface. Quantitative application of any such sensor requires understanding how changes in interfacial field affect the composition of photostationary states, i.e. how does electrode potential affect the extent to which the electrode is on or off when irradiated, and the structure of the SAM. Here we address these questions for a SAM of a 6-nitro-substituted spiro[2H-1-benzopyran-2,2'-indoline] covalently attached through a dithiolane linker to an Au electrode immersed in a 0.1 M solution of Tetramethylammonium hexafluorophosphate in Acetonitrile using interface-specific vibrational spectroscopy. We find that in the absence of irradiation, when the SAM is dominated by the closed spiropyran form, variations in potential of 1 V have little effect on spiropyran relative stability. In contrast, under UV irradiation small changes in potential can have dramatic effects: changes in potential of 0.2 V can completely destabilize the open merocyanine form of the SAM relative to the spiropyran and dramatically change the chromophore orientation. Quantitatively accounting for these effects is necessary to employ this, or any other optically switchable bistable chromophore, in electrochemical applications.

Reversible Light-Switching of Enzymatic Activity on Orthogonally Functionalized Polymer Brushes

Copolymer brushes, composed of glycidyl methacrylate and a furan-protected maleimide-containing monomer, were grafted from radical initiators at the surface of irradiation-activated fluoropolymer foils. After postpolymerization modification with enzymatically active microperoxidase-11 and photochromic spiropyran moieties, the polymer brushes catalyzed the oxidation of 3,3'5,5'-tetramethylbenzidine. Exposure to either UV or visible-light allowed switching the turnover by more than 1 order of magnitude, as consequence of the reversible, light-induced spiropyran-merocyanine transition. The modified samples were integrated into an optofluidic device that allowed the reversible switching of enzymatic activity for several cycles under flow, validating the potential for application in smart lab-on-a-chip systems.

From pH- to Light-Response: Postpolymerization Modification of Polymer Brushes Grafted onto Microporous Polymeric Membranes

A microporous pH- and light-responsive membrane that enables remote control over its interfacial properties has been fabricated. pH-Responsiveness was imparted to a porous polypropylene film via grafting of poly(methacrylic acid) brushes from the substrate using argon-plasma-induced free-radical graft polymerization. Morphological changes as a function of grafting level were analyzed using atomic force microscopy. Conversion into a light-responsive membrane was performed via postpolymerization modification to covalently attach photochromic spiropyran moieties to the grafted polymer brushes. Reversible switches in wettability and permeability were determined upon changing from acidic to basic pH or upon alternating UV- and visible-light irradiation. Additionally, light-responsive membranes show a switch in color upon UV exposure.

Photo-responsive polymers: synthesis and applications

Photo-responsive polymers are able to change their structure, conformation and properties upon light irradiation.

Electron-Beam Lithographic Grafting of Functional Polymer Structures from Fluoropolymer Substrates

Well-defined submicrometer structures of poly(dimethylaminoethyl methacrylate) (PDMAEMA) were grafted from 100 μm thick films of poly(ethene-alt-tetrafluoroethene) after electron-beam lithographic exposure. To explore the possibilities and limits of the method under different exposure conditions, two different acceleration voltages (2.5 and 100 keV) were employed. First, the influence of electron energy and dose on the extent of grafting and on the structure's morphology was determined via atomic force microscopy. The surface grafting with PDMAEMA was confirmed by advanced surface analytical techniques such as time-of-flight secondary ion mass spectrometry and X-ray photoelectron spectroscopy. Additionally, the possibility of effective postpolymerization modification of grafted structures was demonstrated by quaternization of the grafted PDMAEMA to the polycationic QPDMAEMA form and by exploiting electrostatic interactions to bind charged organic dyes and functional proteins.

Multipurpose Polymeric Coating for Functionalizing Inert Polymer Surfaces

In this work, we report on the development of a highly functionalizable polymer coating prepared by the chemical coupling of trichlorosilane (TCS) to the vinyl groups of poly(vinylmethyl siloxane) (PVMS). The resultant PVMS-TCS copolymer can be coated as a functional organic primer layer on a variety of polymeric substrates, ranging from hydrophilic to hydrophobic. Several case studies demonstrating the remarkable and versatile properties of PVMS-TCS coatings are presented. In particular, PVMS-TCS is found to serve as a convenient precursor for the deposition of organosilanes and the subsequent growth of polymer brushes, even on hydrophobic surfaces, such as poly(ethylene terephthalate) and polypropylene. In this study, the physical and chemical characteristics of these versatile PVMS-TCS coatings are interrogated by an arsenal of experimental probes, including scanning electron microscopy, water contact-angle measurements, ellipsometry, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy, and near-edge X-ray absorption fine structure spectroscopy.

Poly(N,N-dimethylaminoethyl methacrylate) Brushes: pH-Dependent Switching Kinetics of a Surface-Grafted Thermoresponsive Polyelectrolyte

The temperature-dependent switching behavior of poly(N,N-dimethylaminoethyl methacrylate) brushes in alkaline, neutral, and acidic solutions is examined. A novel microscopic laser temperature-jump technique is employed in order to study characteristic thermodynamic and kinetic parameters. Static laser micromanipulation experiments allow one to determine the temperature-dependent variation of the swelling ratio. The data reveal a strong shift of the volume phase transition of the polymer brushes to higher temperatures when going from pH = 10 to pH = 4. Dynamic laser micromanipulation experiments offer a temporal resolution on a submillisecond time scale and provide a means to determine the intrinsic rate constants. Both the swelling and the deswelling rates strongly decrease in acidic solutions. Complementary experiments using in situ atomic force microscopy show an increased polymer layer thickness at these conditions. The data are discussed on the basis of pH-dependent structural changes of the polymer brushes including protonation of the amine groups and conformational rearrangements. Generally, repulsive electrostatic interactions and steric effects are assumed to hamper and slow down temperature-induced switching in acidic solutions. This imposes significant restrictions for smart polymer surfaces, sensors, and devices requiring fast response times.

Fabrication of Thiol-Ene "Clickable" Copolymer-Brush Nanostructures on Polymeric Substrates via Extreme Ultraviolet Interference Lithography

We demonstrate a new approach to grafting thiol-reactive nanopatterned copolymer-brush structures on polymeric substrates by means of extreme ultraviolet (EUV) interference lithography. The copolymer brushes were designed to contain maleimide functional groups as thiol-reactive centers. Fluoropolymer films were exposed to EUV radiation at the X-ray interference lithography beamline (XIL-II) at the Swiss Light Source, in order to create radical patterns on their surfaces. The radicals served as initiators for the copolymerization of thiol-ene "clickable" brushes, composed of a furan-protected maleimide monomer (FuMaMA) and different methacrylates, namely, methyl methacrylate (MMA), ethylene glycol methyl ether methacrylate (EGMA), or poly(ethylene glycol) methyl ether methacrylate (PEGMA). Copolymerization with ethylene-glycol-containing monomers provides antibiofouling properties to these surfaces. The number of reactive centers on the grafted brush structures can be tailored by varying the monomer ratios in the feed. Grafted copolymers were characterized by using attenuated total reflection infrared (ATR-IR) spectroscopy. The reactive maleimide methacrylate (MaMA) units were utilized to conjugate thiol-containing moieties using the nucleophilic Michael-addition reaction, which proceeds at room temperature without the need for any metal-based catalyst. Using this approach, a variety of functionalities was introduced to yield polyelectrolytes, as well as fluorescent and light-responsive polymer-brush structures. Functionalization of the brush structures was demonstrated via ATR-IR and UV-vis spectroscopy and fluorescence microscopy, and was also indicated by a color switch. Furthermore, grafted surfaces were generated via plasma activation, showing a strongly increased wettability for polyelectrolytes and a reversible switch in static water contact angle (CA) of up to 18° for P(EGMA-co-MaMA-SP) brushes, upon exposure to alternating visible and UV-light irradiation.