Interplay between Hydrogen Bonding and Macromolecular Architecture Leading to Unusual Phase Behavior in Thermosensitive Microgels

Acrylamide precipitation polymerization in a continuous flow reactor: an in situ FTIR study reveals kinetics

In this work, we present a combination of a continuous flow reactor with in situ monitoring of the monomer conversion in a precipitation polymerization. The flow reactor is equipped with a preheating area for the synthesis of thermoresponsive microgels, based on N-isopropylacrylamide (NIPAM). The reaction progress is monitored with in situ FTIR spectroscopy. The monomer conversion at defined residence times is determined from absorbance spectra of the reaction solutions by linear combination with reference spectra of the stock solution and the purified microgel. The reconstruction of the spectra appears to be in good agreement with experimental data in the range of 1710 to 1530 cm− 1, in which prominent absorption bands are used as probes for the monomer and the polymer. With increasing residence time, we observed a decrease in intensity of the ν(C=C) vibration, originating from the monomer, while the ν(C=O) vibration is shifted to higher frequencies by polymerization. Differences between the determined inline conversion kinetics and offline growth kinetics, determined by photon correlation spectroscopy (PCS), are discussed in terms of diffusion and point to a crucial role of mixing in precipitation polymerizations.

Core-shell microgels as thermoresponsive carriers for catalytic palladium nanoparticles

Responsive core-shell microgels are promising systems for a stabilization of Pd nanoparticles and control of their catalytic activity. Here, poly-N-n-propylacrylamide (PNNPAM) was copolymerized with methacrylic acid to yield microgel core particles, which were subsequently coated with an additional, acid-free poly-N-isopropylmethacrylamide (PNIPMAM) shell. Both core and core-shell systems were used as pH- and temperature-responsive carrier systems for the incorporation of palladium nanoparticles. The embedded nanoparticles were found to have a uniform size distribution with diameters at around 20 nm. Their catalytic activity was investigated by following the kinetics of the reduction of p-nitrophenol to p-aminophenol using UV-vis spectroscopy. For the PNNPAM microgel core, the temperature dependence of the rate constant followed the Arrhenius equation, which is an unusual behaviour for thermoresponsive carrier systems but common for passive systems such as polyelectrolyte brushes. In contrast, the catalytic activity of nanoparticles embedded in microgel core-shell systems decreased drastically at the volume phase transition temperature (44 °C) of the PNIPMAM shell. Accordingly, a promising architecture of passive nanoparticle-carrying core and thermoresponsive shell was realized successfully.

Synthesis of smart dual-responsive microgels: correlation between applied surfactants and obtained particle morphology

Thermo- and pH-responsive copolymer microgels were obtained by surfactant-assisted precipitation polymerization of N-isopropylacrylamide (NIPAM) and acrylic acid (AAc). The surfactants used were sodium dodecylsulfate (SDS), dodecyltrimethylammonium bromide (DTAB) and the nonionic n-octyl-β-d-glucopyranoside (C8G1). We investigate the influence of the surfactants on the acrylic acid incorporation rate, the particle size, particle morphology, and the swelling behaviour at pH 4 and pH 7, at which AAc is neutral or charged, respectively. It is shown that each surfactant has a specific influence, which is connected to its role in the polymerization mechanism and its charge. A combined FTIR and PCS study reveals that the particles undergo a temperature-induced change in microstructure, even if the particle hydrodynamic radius does not change significantly.

Swelling behaviour of core–shell microgels in H2O, analysed by temperature-dependent FTIR spectroscopy

The structural basis for linear thermoresponses of smart core–shell microgels is elucidated by FTIR spectroscopy, being sensitive to core processes.

Effect of Network Density in Surface-Anchored Poly(N-isopropylacrylamide) Hydrogels on Adsorption of Fibrinogen

We describe a simple approach to generate surface-attached biocompatible hydrogels with tunable cross-link density and employ them to study the effect of gel structure on protein adsorption. Using free-radical polymerization, we synthesize a series of random copolymers comprising N-isopropylacrylamide (NIPAm) and the photoactive curing agent 4-methacryloyl-oxy-benzophenone (MABP) of mole fractions ranging from 2.5 to 10%. We deposit a thin film of the precursor copolymer (∼150 nm) on a silicon or glass substrate, which is precoated with monolayers of benzophenone-silane, then cross-link it through UV irradiation at 365 nm (dose ≈ 6-10 J/cm²) to generate surface-attached networks. A systematic investigation of the network properties such as gel fraction, cross-link density, and swelling ratio reveals that gels with higher MABP content (≥5%) produce densely cross-linked hydrophobic networks with low or no swelling in an aqueous medium. We study the adsorption of fibrinogen (Fg) on such hydrogel substrates and establish that the amount of adsorbed Fg depends on the degree of cross-linking and the swelling capacity of the networks. Specifically, although Fg adsorbs heavily on denser networks, loosely bound gels that swell in aqueous medium repel proteins. We attribute the latter behavior to entropic shielding and size-exclusion factors.

Poly[tri(ethylene glycol) ethyl ether methacrylate]-coated surfaces for controlled fibroblasts culturing

Well-defined thermosensitive poly[tri(ethylene glycol) monoethyl ether methacrylate] (P(TEGMA-EE)) brushes were synthesized on a solid substrate by the surface-initiated atom transfer radical polymerization of TEGMA-EE. The polymerization reaction was initiated by 2-bromo-2-methylpropionate groups immobilized on the surface of the wafers. The changes in the surface composition, morphology, philicity, and thickness that occurred at each step of wafer functionalization confirmed that all surface modification procedures were successful. Both the successful modification of the surface and bonding of the P(TEGMA-EE) layer were confirmed by X-ray photoelectron spectroscopy (XPS) measurements. The thickness of the obtained P(TEGMA-EE) layers increased with increasing polymerization time. The increase of environmental temperature above the cloud point temperature of P(TEGMA-EE) caused the changes of surface philicity. A simultaneous decrease in the polymer layer thickness confirmed the thermosensitive properties of these P(TEGMA-EE) layers. The thermosensitive polymer surfaces obtained were evaluated for the growth and harvesting of human fibroblasts (basic skin cells). At 37 °C, seeded cells adhered to and spread well onto the P(TEGMA-EE)-coated surfaces. A confluent cell sheet was formed within 24 h of cell culture. Lowering the temperature to an optimal value of 17.5 °C (below the cloud point temperature of the polymer, TCP, in cell culture medium) led to the separation of the fibroblast sheet from the polymer layer. These promising results indicate that the surfaces produced may successfully be used as substrate for engineering of skin tissue, especially for delivering cell sheets in the treatment of burns and slow-healing wounds.

Concerted interaction between pnicogen and halogen bonds in XCl-FH2P-NH3 (X=F, OH, CN, NC, and FCC)

We analyze the interplay between pnicogen-bonding and halogen-bonding interactions in the XCl-FH(2)P-NH(3) (X=F, OH, CN, NC, and FCC) complex at the MP2/aug-cc-pVTZ level. Synergetic effects are observed when pnicogen and halogen bonds coexist in the same complex. These effects are studied in terms of geometric and energetic features of the complexes. Natural bond orbital theory and Bader's theory of "atoms in molecules" are used to characterize the interactions and analyze their enhancement with varying electron density at critical points and orbital interactions. The physical nature of the interactions and the mechanism of the synergetic effects are studied using symmetry-adapted perturbation theory. By taking advantage of all the aforementioned computational methods, the present study examines how both interactions mutually influence each other.