Preparation of Amino-Functionalized Poly(N-isopropylacrylamide)-Based Microgel Particles

Responsive cationic microgels are a promising building block in several diagnostic and therapeutic applications, like transfection and RNA or enzyme packaging. Although the direct synthesis of cationic poly(N-isopropylacrylamide) (PNIPAm) microgel particles has a long history, these procedures typically resulted in low yield, low incorporation of the cationic comonomer, increased polydispersity, and pure size control. In this study, we investigated the possibility of the post-polymerization modification of P(NIPAm-co-acrylic acid) microgels to prepare primary amine functionalized microgels. To achieve this goal, we used 1-ethyl-3-(3-(dimethylamino)propyl)carbodiimide hydrochloride (EDC) mediated coupling of a diamine to the carboxyl groups. We found that by controlling the EDC excess in the reaction mixture, the amine functionalization of the carboxyl functionalized microgel could be varied and as much as 6-7 mol% amine content could be incorporated into the microgels. Importantly, the reaction was conducted at room temperature in an aqueous medium and it was found to be time efficient, making it a practical and convenient approach for synthesizing primary amine functionalized PNIPAm microgel particles.

Copolymerization Kinetics of Dopamine Methacrylamide during PNIPAM Microgel Synthesis for Increased Adhesive Properties

Combining stimuli-responsive properties of gels with adhesive properties of mussels is highly interesting for a large field of applications as, e.g., in life science. Therefore, the present paper focuses on the copolymerization of poly(N-isopropylacrylamide) (PNIPAM) microgels with dopamine methacrylamide (DMA). A detailed understanding of reaction kinetics is crucial to figure out an optimized synthesis strategy for tailoring microgels with adhesive properties. The present study addresses the influence of relevant synthesis parameters as the injection time of DMA during the microgel synthesis and the overall reaction time of the microgel. Reaction kinetics were studied by mass spectrometry of time samples taken during the microgel synthesis. This allowed us to determine the monomer consumption of NIPAM, the cross-linker N,N'-methylenebisacrylamide (BIS), and DMA. A second-order reaction kinetics was found for DMA incorporation. The amount of DMA incorporated in the resulting microgel was successfully determined by a combination of UV-vis and NMR spectroscopy to level off limitations of both methods. The dependence of the hydrodynamic radius on temperature was determined by DLS measurements for the microgels. While an early injection of DMA stops the PNIPAM polymerization due to scavenging, it greatly enhances the reaction speed of DMA. The faster reaction of DMA and the incomplete NIPAM and BIS conversion also compensate for shorter reaction times with respect to the incorporated amount of DMA. On the contrary, a later injection of DMA leads to a full NIPAM monomer and BIS cross-linker consumption. An overall reaction time of 60 min ensures the DMA incorporation. Longer reaction times lead to clumping. First adhesion tests show an increased adhesion of P(NIPAM-co-DMA) microgels compared to pure PNIPAM microgels, when mechanical stress is applied.

Electrochemically Initiated Synthesis of Polyacrylamide Microgels and Core-shell Particles

Herein, we developed a simple procedure for synthesizing micrometer-sized microgel particles as a suspension in an aqueous solution and thin films deposited as shells on different inorganic cores. A sufficiently high constant potential was applied to the working electrode to commence the initiator decomposition that resulted in gelation. Under hydrodynamic conditions, this initiation allowed preparing different morphology microgels at room temperature. Importantly, neither heating nor UV-light illumination was needed to initiate the polymerization. Moreover, thin films of the cross-linked gel were anchored on different core substrates, including silica and magnetic nanoparticles. Scanning electron microscopy and transmission electron microscopy imaging confirmed the microgel particles' and films' irregular shape and porous structure. Energy-dispersive X-ray spectroscopy indicated that the core coating with the microgel film was successful. Dynamic light scattering measured the micrometer size of gel particles with different combinations of acrylic monomers. Thermogravimetric analysis and the first-derivative thermogravimetric analysis revealed that the microgels' thermal stability of different compositions was different. Fourier-transform infrared and 13C NMR spectroscopy showed successful copolymerization of the main, functional, and cross-linking monomers.

Ultralow Crosslinked Microgel Brings Ultrahigh Catalytic Efficiency

Microgel nanoreactors maintain the stability of metallic nanoparticles and regulate their catalytic activity. However, limited by the synthetic method, the recycling ability and long-lasting stability of microgel nanoreactors are challenged. Herein, a brand-new nanoparticle carrier, ultralow crosslinked poly(N-isopropylacrylamide-b-methacrylic acid) (P(NIPAm-b-MAA)) microgel, is synthesized based on the reversible addition-fragmentation chain transfer polymerization method and the self-crosslinking mechanism of PNIPAm. This carrier enables the easy preparation, low cost, long-lasting stability, and high catalytic efficiency of nanoreactors. As far as it is known, the catalytic reduction rates of several dye models used in this work are the highest ones in similar systems. In addition, the presence of the MAA block leads to the agglomeration and dispersion of the microgels under different pH conditions, thus realizing rapid recycling of the nanoreactors. This novel carrier has great potential for a wide range of applications in catalysis.

Ultralow Self-Cross-Linked Poly(N-isopropylacrylamide) Microgels Prepared by Solvent Exchange

We found that the poly(N-isopropylacrylamide) (PNIPAm) synthesized by free-radical polymerization in organic phase could also form stable microgels in water through solvent exchange without chemical cross-linkers. Dynamic light scattering and transmission electron microscopy showed the larger swelling ratio and higher deformability of these microgels. Nuclear magnetic resonance and infrared spectroscopy indicated that the self-cross-linking structures in these microgels were attributed to the hydrogen atom abstraction both from the isopropyl tert-carbon atoms and the vinyl tert-carbon atoms in PNIPAm chains and the organic solvents were important assistants in the hydrogen abstraction behavior. Our discovery revealed that the self-cross-linking of PNIPAm chains is a common phenomenon within their free-radical polymerization process, whether in aqueous phase or in organic phase. Besides, the addition of second monomers will not affect the cross-linkage of the PNIPAm portion, which may be of great significance for the synthesis of various functional ultralow cross-linking PNIPAm microgels.

Synthesis of Poly(N-vinylcaprolactam)-Based Microgels by Precipitation Polymerization: Pseudo-Bulk Model for Particle Growth and Size Distribution

Particle size distribution and in particular the mean particle size are key properties of microgels, which are determined by synthesis conditions. To describe particle growth and particle size distribution over the progress of synthesis of poly(N-vinylcaprolactam)-based microgels, a pseudo-bulk model for precipitation copolymerization with cross-linking is formulated. The model is fitted and compared to experimental data from reaction calorimetry and dynamic light scattering, showing good agreement with polymerization progress, final particle size, and narrow particle size distribution. Predictions of particle growth and reaction progress for different experimental setups are compared to the corresponding experimental data, demonstrating the predictive capability and limitations of the model. The comparison to reaction calorimetry measurements shows the strength in the prediction of the overall polymerization progress. The results for the prediction of the particle radii reveal significant deviations and highlight the demand for further investigation, including additional data.

Direct Monitoring of Microgel Formation during Precipitation Polymerization of N-Isopropylacrylamide Using in Situ SANS

Poly(N-isopropylacrylamide) microgels have found various uses in fundamental polymer and colloid science as well as in different applications. They are conveniently prepared by precipitation polymerization. In this reaction, radical polymerization and colloidal stabilization interact with each other to produce well-defined thermosensitive particles of narrow size distribution. However, the underlying mechanism of precipitation polymerization has not been fully understood. In particular, the crucial early stages of microgel formation have been poorly investigated so far. In this contribution, we have used small-angle neutron scattering in conjunction with a stopped-flow device to monitor the particle growth during precipitation polymerization in situ. The average particle volume growth is found to follow pseudo-first order kinetics, indicating that the polymerization rate is determined by the availability of the unreacted monomer, as the initiator concentration does not change considerably during the reaction. This is confirmed by calorimetric investigation of the polymerization process. Peroxide initiator-induced self-crosslinking of N-isopropylacrylamide and the use of the bifunctional crosslinker N,N'-methylenebisacrylamide are shown to decrease the particle number density in the batch. The results of the in situ small-angle neutron scattering measurements indicate that the particles form at an early stage in the reaction and their number density remains approximately the same thereafter. The overall reaction rate is found to be sensitive to monomer and initiator concentration in accordance with a radical solution polymerization mechanism, supporting the results from our earlier studies.

Synthesis and structure of deuterated ultra-low cross-linked poly(N-isopropylacrylamide) microgels

Partial deuteration of the N-isopropylacrylamide monomer reveals new insights into the self-cross-linking of polymer chains in ultra-low cross-linked microgels.

From Batch to Continuous Precipitation Polymerization of Thermoresponsive Microgels

Microgels are commonly synthesized in batch experiments, yielding quantities sufficient to perform characterization experiments for physical property studies. With increasing attention on the application potential of microgels, little attention is yet paid to the questions (a) whether they can be produced continuously on a larger scale, (b) whether synthesis routes can be easily transferred from batch to continuous synthesis, and (c) whether their properties can be precisely controlled as a function of synthesis parameters under continuous flow reaction conditions. We present a new continuous synthesis process of two typical but different microgel systems. Their size, size distribution, and temperature-responsive behavior are compared in depth to those of microgels synthesized using batch processes, and the influence of premixing and surfactant is also investigated. For the surfactant-free poly( N-vinylcaprolactam) and poly( N-isopropylacrylamide) systems, microgels are systematically smaller, while the actual size is depending on the premixing of the reaction solutions. However, by the use of a surfactant, the size difference between batch and continuous preparation diminishes, resulting in equal-sized microgels. Temperature-induced swelling-deswelling of microgels synthesized under continuous flow conditions was similar to that of their analogues synthesized using the batch polymerization process. Additionally, investigation of the internal microgel structure using static light scattering showed no significant changes between microgels prepared under batch and continuous conditions. The work encourages synthesis concepts of sequential chemical conditions in continuous flow reactors to prepare precisely tuned new microgel systems.

Stimuli-Responsive Microgels and Microgel-Based Systems: Advances in the Exploitation of Microgel Colloidal Properties and Their Interfacial Activity

In this paper, recent developments in the chemical design of functional microgels are summarized. A wide range of available synthetic methods allows the incorporation of various reactive groups, charges, or biological markers inside the microgel network, thus controlling the deformation and swelling degree of the resulting smart microgels. These microgels can respond to various stimuli, such as temperature, pH, light, electric field, etc. and can show unique deformation behavior at the interface. Due to their switchability and interfacial properties, these smart microgels are being extensively explored for various applications, such as antifouling coatings, cell encapsulation, catalysis, controlled drug delivery, and tissue engineering.

Dynamically Cross-Linked Self-Assembled Thermoresponsive Microgels with Homogeneous Internal Structures

The internal morphology of temperature-responsive degradable poly(N-isopropylacrylamide) (PNIPAM) microgels formed via an aqueous self-assembly process based on hydrazide and aldehyde-functionalized PNIPAM oligomers is investigated. A combination of surface force measurements, small angle neutron scattering (SANS), and ultrasmall angle neutron scattering (USANS) was used to demonstrate that the self-assembled microgels have a homogeneously cross-linked internal structure. This result is surprising given the sequential addition process used to fabricate the microgels, which was expected to result in a densely cross-linked shell-diffuse core structure. The homogeneous internal structure identified is also significantly different than conventional microgels prepared via precipitation polymerization, which typically exhibit a diffuse shell-dense core structure. The homogeneous structure is hypothesized to result from the dynamic nature of the hydrazone cross-linking chemistry used to couple with the assembly conditions chosen that promote polymer interdiffusion. The lack of an internal cross-linking gradient within these degradable and monodisperse microgels is expected to facilitate more consistent drug release over time, improved optical properties, and other potential application benefits.

Functional Microgels and Microgel Systems

Microgels are macromolecular networks swollen by the solvent in which they are dissolved. They are unique systems that are distinctly different from common colloids, such as, e.g., rigid nanoparticles, flexible macromolecules, micelles, or vesicles. The size of the microgel networks is in the range of several micrometers down to nanometers (then sometimes called "nanogels"). In a collapsed state, they might resemble hard colloids but they can still contain significant amounts of solvent. When swollen, they are soft and have a fuzzy surface with dangling chains. The presence of cross-links provides structural integrity, in contrast to linear and (hyper)branched polymers. Obviously, the cross-linker content will allow control of whether microgels behave more "colloidal" or "macromolecular". The combination of being soft and porous while still having a stable structure through the cross-linked network allows for designing microgels that have the same total chemical composition, but different properties due to a different architecture. Microgels based, e.g., on two monomers but have either statistical spatial distribution, or a core-shell or hollow-two-shell morphology will display very different properties. Microgels provide the possibility to introduce chemical functionality at different positions. Combining architectural diversity and compartmentalization of reactive groups enables thus short-range coexistence of otherwise instable combinations of chemical reactivity. The open microgel structure is beneficial for uptake-release purposes of active substances. In addition, the openness allows site-selective integration of active functionalities like reactive groups, charges, or markers by postmodification processes. The unique ability of microgels to retain their colloidal stability and swelling degree both in water and in many organic solvents allows use of different chemistries for the modification of microgel structure. The capability of microgels to adjust both their shape and volume in response to external stimuli (e.g., temperature, ionic strength and composition, pH, electrochemical stimulus, pressure, light) provides the opportunity to reversibly tune their physicochemical properties. From a physics point of view, microgels are particularly intriguing and challenging, since their intraparticle properties are intimately linked to their interparticle behavior. Microgels, which reveal interface activity without necessarily being amphiphilic, develop even more complex behavior when located at fluid or solid interfaces: the sensitivity of microgels to various stimuli allows, e.g., the modulation of emulsion stability, adhesion, sensing, and filtration. Hence, we envision an ever-increasing relevance of microgels in these fields including biomedicine and process technology. In sum, microgels unite properties of very different classes of materials. Microgels can be based on very different (bio)macromolecules such as, e.g., polysaccharides, peptides, or DNA, as well as on synthetic polymers. This Account focuses on synthetic microgels (mainly based on acrylamides); however, the general, fundamental features of microgels are independent of the chemical nature of the building moieties. Microgels allow combining features of chemical functionality, structural integrity, macromolecular architecture, adaptivity, permeability, and deformability in a unique way to include the "best" of the colloidal, polymeric, and surfactant worlds. This will open the door for novel applications in very different fields such as, e.g., in sensors, catalysis, and separation technology.