ABSTRACT Anti‐icing agents are highly interesting for a variety of applications such as the cryopreservation of cells, but also as coatings for outdoor technical appliances, which reduce wear and increase safety through ice‐phobic effects. In this work, microgels based on the biocompatible monomer N‐vinylcaprolactam (VCL) and hydrophilic co‐monomer 1‐vinyl‐3‐methylimidazolium (VIM+) are applied as anti‐icing coatings. Furthermore, microgels additionally containing the monomer 3‐(acrylamido)phenylboronic acid (PBA‐AA) are synthesized. These microgels bind to surface hydroxy groups (e.g., on glass or plasma‐activated polymer films). Coating quality and enhanced durability are successfully demonstrated with different microscopy techniques. Tensile force measurements with a customized setup reveal an up to 90% lower ice adhesion force for polymer substrates coated with the synthesized microgels. The structure of the lubricating hydration layer and especially the presence of interfacial quasi‐liquid water with restricted mobility, eventually leading to a freezing point depression, are studied by differential scanning calorimetry (DSC) and nuclear magnetic resonance (NMR) spectroscopy.

Stimuli-responsive microgels with ionizable functional groups offer versatile applications, e.g., by the uptake of oppositely charged metal ions or guest molecules such as drugs, dyes, or proteins. Furthermore, the incorporation of carboxylic groups enhances mucoadhesive properties, crucial for various drug delivery applications. In this work, we successfully synthesized poly{N-vinylcaprolactam-2,2'-[(5-acrylamido-1-carboxypentyl)azanediyl]diacetic acid} [p(VCL/NTAaa)] microgels containing varying amounts of nitrilotriacetic acid (NTA) using precipitation polymerization. We performed fundamental characterization by infrared (IR) spectroscopy and dynamic and electrophoretic light scattering. Despite their potential multiresponsiveness, prior studies on NTA-functionalized microgels lack in-depth analysis of their stimuli-responsive behavior. This work addresses this gap by assessing the microgel responsiveness to temperature, ionic strength, and pH. Morphological investigations were performed via NMR relaxometry, nanoscale imaging (AFM and SEM), and reaction calorimetry. Finally, we explored the potential application of the microgels by conducting cytocompatibility experiments and demonstrating the immobilization of the model protein cytochrome c in the microgels.

Microgels have a wide range of applications, from catalysis to biomedical applications such as drug delivery in dispersion, or as surface-attached materials to improve the biocompatibility of implants or as anti-fouling coatings. The first aim of this work is to understand the morphology of the supramacromolecular, tannic acid (TA) crosslinked microgels using two-dimensional H-1 NMR spectroscopy, and relaxometry. The second aim is to study the volume phase transition (VPT) of TA-crosslinked microgels with different crosslinker densities in dispersion using NMR. In this context,H- 1 high-resolution NMR spectroscopy is used in combination with the two-state model and the Boltzmann sigmoidal function to determine thermodynamic quantities of the VPT, such as temperature (T-t), width of transition (Delta T-t), and change in entropy (Delta S). Third, surface-deposited microgels are characterized in terms of their VPT using a quartz crystal microbalance (QCM-D) to study the properties of these microgels for applications where they are used as coatings. Finally, the T-t of microgels supramolecular crosslinked with TA are compared with microgels covalently crosslinked with N,N'-methylenebisacrylamide (BIS) in dispersion, using NMR, and deposited on a surface, using QCM-D. For this purpose temperature-responsive poly(N-vinylcaprolactam) and poly(N-isopropylacrylamide) microgels crosslinked with the plant-derived polyphenol TA or BIS are used.

In this study, we present a new synthesis methodology based on photo-crosslinking-assisted continuous precipitation polymerization which allows controlling the distribution of crosslinks in microgels. In our approach we substituted conventional crosslinking agent by a comonomer carrying photo-crosslinkable 4-oxocyclopent-2-en-1-yl group. Microgel size, morphology, distribution of crosslinks and packing density of the polymer chains are studied as a function of retention time (Rt) in the flow reactor. Dynamic and static light scattering (DLS and SLS) as well as small angle X-ray scattering (SAXS) proved an excellent level of control over the distribution of crosslinks in microgels during the polymerization process. These results were confirmed by atomic force microscopy (AFM), indicating a difference in microgel stiffness and arrangement of the polymer network as resulting from increased Rt.

We demonstrate reversible hydrogel formation by hydrophobic interaction for poly(N-alkyl-N-vinylacetamide)s with 3 mol % of alkyl chains (side chain length = 10, 12, 14, 16, and 18). The polymers dissolve in ethanol and can be cast to thin films in a mold. When immersed in water, they swell and form biocompatible hydrogels. Our synthesis ensures stochastic distribution of the n-alkyl chains along the water-soluble polymer backbone, and even in the case of the relatively long n-alkyl substituents (C-18), side-chain crystallization is fully suppressed. Polymers with alkyl substituents of different lengths demonstrate strong differences in the strength of the hydrophobic association. Relatively short alkyl substituents yield hydrogels, which flow upon shearing but still segregate from excess water, i.e., they exhibit syneresis. Longer alkyl substituents yield hydrogels with mechanical properties approaching those of permanently cross-linked hydrogels. Hydrogels were characterized by strain- and stress-controlled rheological experiments. Their rheological properties follow a time-temperature stickiness (TTS) superposition principle, indicating that the side chains serve as sticky substituents that extend the terminal relaxation time according to their hydrophobicity.

Mechanochemical approaches are widely used for the efficient, solvent-free synthesis of organic molecules, however their applicability to the synthesis of functional polymers has remained underexplored. Herein, we demonstrate for the first time that mechanochemically triggered free-radical polymerization allows solvent- and initiator-free syntheses of structurally and morphologically well-defined complex functional macromolecular architectures, namely stimuli-responsive microgels. The developed mechanochemical polymerization approach is applicable to a variety of monomers and allows synthesizing microgels with tunable chemical structure, variable size, controlled number of crosslinks and reactive functional end-groups.

Angewandte Chemie International EditionVolume 62, Issue 34 e202383411 Graphical AbstractFree Access Graphical Abstract: Angew. Chem. Int. Ed. 34/2023 First published: 17 August 2023 https://doi.org/10.1002/anie.202383411AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onEmailFacebookTwitterLinkedInRedditWechat Volume62, Issue34August 21, 2023e202383411 RelatedInformation

Stimuli-responsive polymer networks like microgels and hydrogels possess a variety of properties that need to be analyzed for their rational design and successful application in the targeted field of interest. Nanoscale characterization of gels can be achieved by highly selective and sensitive techniques including high-resolution NMR spectroscopy, relaxometry, and diffusometry. This review is focusing on recent results using H-1 and C-13 1D and 2D NMR techniques, which give chemically site selective information about the polymer network revealing the interplay between structure and dynamics of polymer networks at nanoscale level. By that, NMR can allow to obtain information about i) internal structure, ii) stimuli-induced phase transition, iii) mesh size, and iv) aggregation of microgels and hydrogels. The rational design of responsive microgels and hydrogels with application in drug delivery, cell carrier systems, catalysis, actuators, and as antibacterial agent can be made, if crucial polymer network properties can be correlated with cross-linking density, type of cross-link interactions, electrical charges, hydrophobic nano-structuring, and dispersion concentration.

Hydrogels as scaffoldsin tissue engineering have gained increasingattention in recent years. Natural hydrogels, e.g., collagen or fibrin,are limited by their weak mechanical properties and fast degradation,whereas synthetic hydrogels face issues with biocompatibility andbiodegradation. Therefore, combining natural and synthetic polymersto design hydrogels with tunable mechanical stability and cell affinityfor biomedical applications is of interest. By using fibrin with itsexcellent cell compatibility and dextran with controllable mechanicalproperties, a novel bio-based hydrogel can be formed. Here, we synthesizedfibrin and dextran-methacrylate (MA)-based hydrogels with tailorablemechanical properties, controllable degradation, variable pore sizes,and ability to support cell proliferation. The hydrogels are formedthrough in situ gelation of fibrinogen and dextran-MA with thrombinand dithiothreitol. Swelling and nuclear magnetic resonance diffusometrymeasurements showed that the water uptake and mesh sizes of fabricatedhydrogels decrease with increasing dextran-MA concentrations. Cellviability tests confirm that these hydrogels exhibit no cytotoxiceffect.

The functionality of stimuli-responsive microgels can be tailor-made by manipulating their internal nanostructure induced by the chemical composition and morphology of the polymer network. Microgels with phase-separated domains on a nanoscopic length scale were synthesized by copolymerization of N-vinylcaprolactam (VCL) and amphiphilic-to-hydrophobic 1-vinyl-3-alkylimidazolium (VIM+CnH2n+1) bromides (Br-) of different alkyl chain lengths (n = 12, 14, 16) as comonomers. These quaternized imidazoles provide dual functionality for immobilization of payload by electrostatic and hydrophobic interactions. The morphologies and properties of synthesized poly(VCL-co-VIM+CnH2n+1Br-) microgels with 10 mol % comonomer were investigated systematically by H-1 and C-13 high-resolution NMR spectroscopy and relaxometry. Chemical side-selective information about the monomers' volume-phase transition temperatures, width of transition, and change in transition entropy was reported and correlated to the alkyl chain length of the VIM+CnH2n+1Br- comonomer. C-13 NMR spectroscopy reveals the existence of trans and gauche conformers of alkyl chains, which depends on the alkyl chain length and temperature. Morphologies and dynamic contrasts of alkyl chain domains and VCL moieties of poly(VCL-co-VIM+CnH2n+1Br-) microgels were investigated by H-1 transverse magnetization relaxation (T-2-relaxation). Finally, the microgels were successfully applied in the uptake of the hydrophobic dye Nile red, proving their ability to solubilize hydrophobic substances. In addition, the poly(VCL-co-VIM+CnH2n+1Br-) microgels were utilized in electrostatic interactions, as well as simultaneous addition of hydrophobic and negatively charged payload as a proof of concept for dual functionality. This investigation will allow for a better understanding of the internal nanophase structure of complex poly(N-vinylcaprolactam) (PVCL)-based microgels comprising pH-independent positive charges, as well as hydrophobic compartments, which have potential application as dual-functional delivery systems.

Soft colloidal macromolecular structures with programmable chemical functionalities, size, and shape are important building blocks for the fabrication of catalyst systems and adaptive biomaterials for tissue engineering. However, the development of the easy upscalable and template-free synthesis methods to obtain such colloids lack in understanding of molecular interactions that occur in the formation mechanisms of polymer colloids. Herein, a computer simulation-driven experimental synthesis approach based on the supramolecular self-assembly followed by polymerization of tailored pyrazole-modified monomers is developed. Simulations for a series of pyrazole-modified monomers with different numbers of pyrazole groups, different length and polarity of spacers between pyrazole groups and the polymerizable group are first performed. Based on simulations, monomers able to undergo π-π stacking and guide the formation of supramolecular bonds between polymer segments are synthesized and these are used in precipitation polymerization to synthesize anisotropic microgels. This study demonstrates that microgel morphologies can be tuned from spherical, raspberry-like to dumbbell-like by the increase of the pyrazole-modified monomer loading, which is concentrated at periphery of growing microgels. Combining experimental and simulation results, this work provides a quantitative and predictive approach for guiding microgel design that can be further extended to a diversity of colloidal systems and soft materials with superior properties.

Stimuli-responsive microgel copolymer networks with ionizable functional groups have important applications for encapsulation of drugs, peptides, enzymes, proteins, or cells. Rational design of such networks can be based on characterization of stimuli-induced volume phase transition and spatial distribution of neutral and charged monomer units in crosslinked polymer chains. In this work we successfully synthesized poly(N-vinylcaprolactam-co-1-vinyl-3-methylimidazolium) (poly(VCL-VIM+)) microgels carrying permanent positive charges and demonstrate that 1H high-resolution NMR spectroscopy in combination with transverse (T2) magnetization relaxometry allows investigating separately the behavior of each functional group in the microgel network. The information about comonomer transition temperatures, width of transition, and change in transition entropy were reported and correlated with the concentration of charged functional groups and resulting electrophoretic mobility. A two-state approach was used to describe the temperature-induced volume phase transition separately for neutral and charged polymer segments. The core-corona architecture specific to each functional group was detected revealing that the charged methylated vinylimidazolium groups (VIM+) are concentrated mainly in the corona of the microgel. These biocompatible PVCL-based microgels functionalized with permanent positive charges are shown to serve as an antibacterial system against Gram-negative E. coli strains, due to the positive charge of the incorporated VIM+ comonomer in the polymer network.

Realization of self-healing polymer materials cannot rely on the wealth of active repair tools found in living systems but must focus entirely on the structural composition of the material and the properties of its constituents. Current challenges of the search for such compositions include healing of large-scale defects as well as the need for a healing process that is generated by the scission itself. Herein, we describe ionomer-rubber blends from poly(ethylene-co-methacrylic acid) and peroxide cross-linked ethylene-propylene-diene monomer (EPDM) that combine three types of cross-links: covalent links of a network of EPDM, clusters of aggregated ionic groups, and crystalline domains of longer ethylene sequences in the ionomer. Above the melting point of the latter, the components mix homogeneously, indicated by the clarity of the samples and supported by small-angle X-ray scattering (SAXS) and NMR. At ambient conditions, the samples are hard like a thermoplastic material. Self-healing after mechanical damage is enabled by two types of structural memory related to a hierarchy of deformation- and defect-caused stresses and their relaxation paths. Because of the solid-like character of the materials, damage-caused stress is retained by the micro deformation and rupture of the aggregates on small scales and on large scale-by the macroscopic shape memory effect of the deformed covalent network. When the samples get annealed at an elevated temperature, the former enables mending of fracture-caused surfaces and the latter-shape recovery. Based on a careful evaluation of the structural relaxation effects on the blends and their constituents (differential scanning calorimetry, NMR, and wide-angle X-ray scattering/SAXS), we demonstrate the repair of defects in the range of millimeters to centimeters by the defect-caused stresses. It is intrinsic to our concept that it holds only to damages such as scratches, small cuts, and microcracks, whereby the object is not fully fragmented, and that it will require thermal activation.

The fabrication of functional hydrogels with tuned thermoresponsivity is a major challenge. To meet this challenge we copolymerizeN-isopropylacrylamide (NIPAm) withN-vinylformamide (NVF) in different ratios with the formamide group being subsequently selectively hydrolyzed to the corresponding amine (VAm). The copolymers are crosslinked with phenylcarbonate telechelic glycol. The influence of the NIPAm : VAm ratio on the thermoresponsitiviy is investigated in terms of absorbance, rheology, NMR spectroscopy, relaxometry, and diffusometry. Phase transition temperatures, change in the entropy of the polymer-water system, and width of the transition in the process of coil-to-globule and swollen-to-collapsed network transitions were evaluated by a two state model and Boltzmann sigmoidal function.

We here performed an in-depth investigation of the behavior of microgels (mu gels) and their associated physicochemical transformations under shear force. Thermo- and mechanoresponsive poly(N-vinylcaprolactam) (PVCL) mu gels (d similar to 400 nm) cross-linked with a force-responsive mechanofluorophore in different crosslinking degrees were synthesized and examined. Fluorescence spectroscopy (FS), confocal laser scanning microscopy (CLSM), dynamic light scattering (DLS), cryogenic transmission electron microscopy (cryoTEM), high-resolution magic-angle sample spinning (HRMAS) nuclear magnetic resonance (NMR), Fourier-transform infrared (FTIR), and X-ray photoelectron spectroscopy (XPS) are used to characterize the mu gels before, during, and after shearing with different shear rates and intensities. The obtained results suggest nonuniform structural features consisting of a softer outer "corona" and a harder particle "core" (cross-linker-rich). Upon shearing, the mu gels rapidly lose their corona and the cores agglomerate altering mu gel functionality. Surprisingly, mu gels degrade promptly, even when subjected to low shear forces, such as the extrusion through a needle. This has potential implications for all applications in which shear forces in solution are expected, including extrusion, injection, and filtration processes involving colloidal mu gel solutions as well as circulation within the bloodstream of living organisms.

Microgels that host selenium and mimic the structure of the enzyme glutathione peroxidase are of great interest for biotechnological and catalytic applications. For this purpose selenium-functionalized thermoresponsive poly(N-vinylcaprolactam) (PVCL) microgels with cleavable diselenide crosslinks have been investigated. Thermodynamic and morphological parameters characterizing the temperature-induced phase transitions of dual crosslinked PVCL microgels were obtained using dynamic light scattering (DLS), 1H high-resolution magic-angle sample-spinning (MAS) NMR spectroscopy, and transverse magnetization (T2) NMR relaxometry. Quantities obtained from Flory-Rehner theory, a two-state model and Boltzmann sigmoidal function were used to relate the phase transitions of the dual crosslinked microgels to the transition temperature, entropy, temperature width of the phase transition, Flory interaction parameters, average number of strands, polymer volume fraction of the collapsed microgels, core-corona fractions and chain dynamics. The morphology of the selenium modified microgels after the oxidation and reduction processes was investigated by 1H T2 NMR and further correlated with the crosslink density.

AbstractAnhand des Vorbilds von Glutathionperoxidase (GPx) wurden Mikrogele mit Selenfunktionalitäten versehen und somit katalytisch aktive, kolloidale Mikrogele gewonnen. Diselenid‐basierte Vernetzer (Se X‐Linker) wurden synthetisiert und gemeinsam mit dem konventionellen Vernetzer N,N′‐Methylenbis(acrylamid) (BIS) mittels Fällungspolymerisation in Mikrogele eingebracht. Die in den Mikrogelen verteilten Diselenidbrücken konnten selektiv mittels Oxidation durch H2O2 gespalten und zu Selensäure umgesetzt werden, ohne dass die Mikrogele an struktureller Integrität einbüßten. Auf diese Weise wurden katalytisch aktive Mikrogele mit variablem Selensäuregehalt synthetisiert. Beachtenswert ist, dass die so erhaltenen Mikrogele in einer Modellreaktion, der Oxidation von Acrolein zu Acrylsäure (AA) und Methacrylsäure (MA), im Vergleich zum reinen Se X‐Linker eine erhöhte katalytische Aktivität aufweisen.

Active colloidal catalysts inspired by glutathione peroxidase (GPx) were synthesized by integration of catalytically active selenium (Se) moieties into aqueous microgels. A diselenide crosslinker (Se X-linker) was successfully synthesized and incorporated into microgels through precipitation polymerization, along with the conventional crosslinker N,N'-methylenebis(acrylamide) (BIS). Diselenide bonds within the microgels were cleaved through oxidation by H2O2 and converted to seleninic acid whilst maintaining the intact microgel microstructure. Through this approach catalytically active microgels with variable amounts of seleninic acid were synthesized. Remarkably, the microgels exhibited higher catalytic activity and selectivity at low reaction temperatures than the molecular Se catalyst in a model oxidation reaction of acrolein to acrylic acid and methyl acrylate.

The self-diffusion of various nano-objects investigated by high-resolution nuclear magnetic resonance diffusometry proves to be an efficient method for the characterization of dynamics, aggregation kinetic, and matrix morphology. This study investigates how the two-state model and Boltzmann function approach can be used for the evaluation of the thermodynamic parameters of temperature-induced phase transition encoded in polymer diffusivity. The characteristics of the phase transition given by the transition temperature, change of entropy, and width of transition are obtained for poly(N-isopropylacrylamide) (PNIPAm) linear polymers with hydrophilic and hydrophobic end-group functionalization. The effect of end groups upon the polymer diffusivity is investigated as a function of molecular weight (M-n), from which fractal dimensions and hydrodynamic drag coefficients are obtained. The PNIPAm diffusivity is affected strongly by the end groups, and it is reflected in the hydrodynamic radius dependence upon molecular weight that obeys different power-law relations. In this study, the synthesis of --heterotelechelic PNIPAm of different molecular weights with a thiol end group and a hydrophilic NIPAm-like as well as a hydrophobic benzyl end group are described by reversible addition-fragmentation chain-transfer polymerization.

Understanding the diffusion of gold nanorods (AuNRs) and their composites in dispersion is important at fundamental level and in fields as diverse as material science, nanobiotechnology to drug delivery. The translational and rotational diffusion of AuNRs decorated with thermoresponsive poly( N-isopropylacrylamide) brushes having hydrophilic and hydrophobic end groups was investigated in the dilute regime by dynamic light scattering. The same series of functionalized AuNRs were studied in the isotropic concentrated dispersions by high-resolution NMR diffusometry. The dependence of translational and rotational diffusivity upon molecular weight and polymer end group were measured as a function of temperature in the region of the brush phase transition. The effective hydrodynamic radius of AuNR composites proved to be the most sensitive quantity to the temperature-induced phase transition of brushes, allowing the evaluation of the brush thickness in the swollen and collapsed states.