Drying dissipative structures of thermo-sensitive gel spheres of poly(<Emphasis Type="Italic">N</Emphasis>-isopropylacrylamide) with low degree of cross-linking

Drying dissipative patterns of deionized suspensions (colloidal crystal state at high concentrations) of the thermo-sensitive gels of poly(N-isopropylacrylamide) with low degree of cross-linking of 0.5% (318 nm and 116 nm in the hydrodynamic diameter at 25 °C and 45 °C) were observed at 20 °C and 45 °C on a cover glass, a watch glass and a Petri glass dish. The broad rings were observed and their size decreased as micro-gel concentration decreased. Formation of the monodispersed agglomerated particles and their ordered arrays were observed. This work clarified the formation of the drying microscopic structures of (a) flickering ordered spoke-lines, (b) ordered rings, (c) net structure, and finally (d) lattice-like ordered structures of the agglomerated particles. The net and lattice structures formed more favorably at higher temperatures and/or higher degree of cross-linking of the gels. Importance of the convectional flow of the agglomerated particles during the drying processes is supported for the ordered array formation. The role of the electrical double layers around the agglomerated particles and the interaction of the particles with the substrate surfaces during dryness are also important for the ordering. The microscopic drying patterns of gel spheres were quite different from those of linear-type polymers and also from typical colloidal spheres, though the macroscopic patterns such as broad ring formation at the edges of the dried film were similar to each other.     

Colloidal crystallization of thermo-sensitive gel spheres of poly (<Emphasis Type="Italic">N</Emphasis>-isopropyl acrylamide)

Morphology, phase diagram, and reflection spectroscopy of the colloidal crystals of thermo-sensitive gel spheres, poly (N-isopropylacrylamide) (224 nm in the hydrodynamic diameter at 25 °C) were studied. Giant colloidal single crystals formed at very low gel concentrations. Critical concentration of melting of gel spheres (0.8 wt.% without ion-exchange resins) decreased sharply to 0.01 wt.% as the gel suspension was deionized exhaustively with coexistence of the mixtures of cation- and anion-exchange resins and increased substantially as concentration of sodium chloride increased. These studies demonstrated that the colloidal crystallization takes place by the extended electrical double layers formed around the gel spheres in addition of the excluded-volume effect of the gels. Most of the researchers including the authors have believed that the crystallization of the gel spheres takes place by the excluded-volume effect, in other words, by the hard-sphere model, exclusively. However, the present work clarified that the colloidal interfaces, which are inevitable for the formation of the electrical double layers, are formed firmly between the water phase and gel spheres, though the gel spheres contain a lot of water molecules in the inner the sphere region.     

Temperature-responsive self-assembly of charged and uncharged hydroxyethylcellulose-<Emphasis Type=Italic>graft</Emphasis>-poly(<Emphasis Type=Italic>N</Emphasis>-isopropylacrylamide) copolymer in aqueous solution

Temperature-induced interchain association and contraction of species in aqueous solutions of charged (MHEC(−)-g-PNIPAAM) and uncharged (MHEC-g-PNIPAAM) modified hydroxyethylcellulose-graft-poly(N-isopropylacrylamide) copolymer have been studied with the aid of turbidimetry and dynamic light scattering (DLS). It was shown that by attaching PNIPAAM chains to the backbone of a hydrophilic cellulose derivative, a strongly temperature-responsive copolymer could be prepared. The results show an intriguing interplay between interchain association and contraction of the multichain species. The transition zone for compression is narrow, and the compaction effect is promoted by a low polymer concentration and charges on the polymer moieties. The findings from DLS revealed two populations of species, namely molecularly dispersed molecules or small clusters and interchain complexes, which exhibit temperature-induced collapse. The magnitude of the cluster contraction can be modulated by changing the polymer concentration and charge density of the copolymer.     

Colloidal crystallization of thermosensitive gel spheres of poly(<Emphasis Type="Italic">N</Emphasis>-isopropylacrylamide) with low degree of cross-linking

Morphology, phase diagram, and reflection spectroscopy of the colloidal crystals of thermosensitive gel spheres, poly(N-isopropylacrylamide) ((200–0.5), 318 and 116 nm in the hydrodynamic diameter at 25°C and 45°C, and 0.5% in the degree of cross-linking) were studied. Giant colloidal single crystals formed at very low gel concentrations. Densities of the gel spheres were 0.030 and 0.61 at 25°C and 45°C, respectively. Critical concentration of melting of gel spheres (0.8 wt.% without ion-exchange resins) decreased sharply to 0.015 wt.% at 25°C as the gel suspension was deionized exhaustively with coexistence of the mixtures of cation and anion exchange resins. These results demonstrate that the colloidal crystallization takes place by the extended electrical double layers formed around the gel spheres in addition of the excluded volume effect of the gels. Extent of the contribution of the electrical double layers on the crystallization increased sharply when the degree of cross-linking increased, the gel spheres shrank, and/or the density of the gel spheres increased.     

Syntheses proceeding from <Emphasis Type=Italic>N</Emphasis>-(oxiran-2-ylmethyl)-<Emphasis Type=Italic>N</Emphasis>-ethylaniline

The preparation of N-(oxiran-2-ylmethyl)-N-ethylaniline was developed. The compound was used in the synthesis of new N-derivatives of aromatic amines containing vicinal aminoalcohol moieties.     

Colloidal crystallization of thermo-sensitive gel spheres of poly (<Emphasis Type="Italic">N</Emphasis>-isopropyl acrylamide). Influence of gel size

Influence of the gel size on the morphology, phase diagram, and reflection spectroscopy of the colloidal crystals of thermo-sensitive gel spheres, poly (N-isopropylacrylamide) (pNIPAm), was discussed by adding the data of two gel samples of pNIPAm(400–5) and pNIPAm(600–5) of 412 nm (at 25 °C) and 220 nm (at 45 °C) and of 517 nm (at 20 °C) and 294 nm (at 45 °C), respectively. Colloidal single crystals formed, but not so large compared with the giant crystals of small pNIPAm gels reported previously. The suspensions even with ion-exchange resins were turbid and hard to observe the single crystals clearly with the naked eyes as gel size increased. The critical concentration of melting decreased sharply as the suspensions were deionized with coexistence of the mixtures of cation- and anion-exchange resins. The critical concentration increased as the gel size increased and/or dispersion temperature increased. Density of the gel spheres increased as their size increased. These results demonstrated that the colloidal crystallization takes place by the extended electrical double layers formed around the gel spheres in addition of the excluded volume effect of the gels. Contribution of the electrical double layers on the crystallization increased sharply as temperature increased and gel concentration decreased, respectively. The contribution also increased slightly as sphere size increased, when comparison was made at the same gel concentration in wt.%. The present work clarified that the colloidal interfaces, which are inevitable for the formation of the electrical double layers, are formed between the water phase and gel spheres, though the gel spheres contain a lot of water molecules at the inner sphere region.     

Morphological characteristics of modified freeze-dried poly(<Emphasis Type=Italic>N</Emphasis>-isopropylacrylamide) microspheres studied by optical microscopy,SEM, and DLS

Influence of the initiator and additional hydrophobic copolymer on the morphology of thermosensitive poly(N-isopropylacrylamide) (pNIPAM) microspheres, and their presumed application for the stabilization of biologically active molecules were evaluated in this study. Three different types of pNIPAM were synthesized, applying various components: PN1 is a polymer with terminal anionic groups resulting from potassium persulfate initiator; PN2 was synthesized with a 2,2′-azobis(2-methylpropionamidine) dihydrochloride initiator introducing cationic amidine terminal groups; in the PN3 polymer, anionic terminals were implemented, however, increased hydrophobicity was maintained using N-tert-butyl functional groups. Turbidity measurements of the obtained dispersions confirmed specific thermosensitivity of synthesized microspheres in the range of 32–33°C. The polymerization course was proved by infrared spectroscopy and ¹H NMR assessments, whereas the size of the synthesized microspheres, expressed as planar area, was evaluated by dynamic light scattering (DLS), scanning electron microscopy (SEM) and optical microscopy (OM). The respective surface patterns of the freeze-dried microspheres were evaluated by SEM. Planar area of the synthesized macromolecules was in the range between 0.41–3.22 μm, depending on the substrates composition and the method applied for the measurements. The assessments performed in the dry stage gave higher values of the diameter and planar area of the observed microspheres. The measured diameter and planar area increased in the following order for the PN3 microspheres: DLS, OM, SEM. In the case of PN1 and PN2, the observed diameters were positioned as: DLS, SEM, OM. These differences were assigned both to varied intramolecular hydrophobic-hydrophilic interactions of the polymer chains and to the environment, i.e. low pressure in the SEM conditions and aqueous solvent in the DLS measurements. The observed gaps in the freeze-dried PN2 polymer resulted in an attempt to evaluate the application of this polymer for mechanical stabilization of certain macromolecules or nanocrystals in the size range between 10 nm and 20 nm.     

Formation of pyrrolinium derivatives in the bromination reaction of <Emphasis Type=Italic>N</Emphasis>,<Emphasis Type=Italic>N</Emphasis>-dipropargyl ammonium salts

Bromination of ammonium salts containing two propargyl groups afforded bromine-substituted derivatives of pyrrolinium salts. The reactions of piperidinium and morpholinium analogs resulted in spirocyclic ammonium salts.     

Regioselective <Emphasis Type=Italic>N</Emphasis>-alkylation of (2-chloroquinolin-3-yl) methanol with <Emphasis Type=Italic>N</Emphasis>-heterocyclic compounds using the Mitsunobu reagent

The Mitsunobu reaction is a well-established fundamental reaction and has been widely applied in organic synthesis. In this paper, under Mitsunobu conditions dehydration proceeds between (2-chloroquinolin-3-yl)methanol and nitrogen heterocyclic compounds such as quinazolinone, pyrimidone, 2-oxoquinoline in dry THF in the presence of triethylamine, triphenylphosphane and diethyl azodicarboxylate to give the corresponding products. As part of our recent research, we attempted to couple two N-heterocyclic compounds under Mitsunobu reaction conditions to provide efficient building blocks for natural product synthesis.     

Synthesis and properties of symmetrical <Emphasis Type=Italic>N</Emphasis>,<Emphasis Type=Italic>N</Emphasis>'-Bis(R-adamantan-1-yl)ureas as target-oriented soluble epoxide hydrolase (sEH) inhibitors

Self-condensation of substituted adamantan-1-yl isocyanates in THF in the presence of DBU afforded 91–96% of symmetrical ureas as target-oriented soluble epoxide hydrolase (sEH) inhibitors. The described reaction avoids the use of the corresponding amine as counterpart. The obtained ureas are characterized by reduced melting points and improved solubility in water.     

Synthesis of cross-linked poly (<Emphasis Type="Italic">N</Emphasis>-isopropylacrylamide) microparticles in supercritical carbon dioxide

Herein, we report a new method that has been developed to prepare thermoresponsive polymers. The white, dry, fine powders were obtained directly from cross-linking polymerization of N-isopropylacrylamide in supercritical carbon dioxide (scCO₂) with N,N-methylenebisacrylamide as a cross-linker. The effects of the reaction pressure and time as well as the initial concentrations of the initiator, cross-linker, and monomer on the yield and morphology of the resulting polymer were investigated systematically. The polymer yield was increased with the concentrations of the cross-linker, monomer, and reaction time. Under the condition of using higher cross-linker concentrations, the cross-linked poly (N-isopropylacrylamide) microparticles with diameters of 50 nm were generated in scCO₂ in high-yield and short reaction times. These results suggest that the synthetic method using scCO₂ can be used to prepare biomedical materials such as the controlled drug-release system.     

Synthesis of 3-acetyl-6-(<Emphasis Type=Italic>o</Emphasis>-methyl benzoyl)-<Emphasis Type=Italic>N</Emphasis>-ethylcarbazole

3-Acetyl-6-(o-methyl benzoyl)-N-ethylcarbazole was prepared through successive o-methyl benzoylation and acetylation of N-ethylcarbazole in one pot. The overall yield was 85.6% and the structure was confirmed by ¹H-NMR and ¹³C-NMR. A preliminary investigation had also been carried out on the mechanism of the o-methyl benzoylation of N-ethylcarbazole.     

Glucose sensitive poly (<Emphasis Type="Italic">N</Emphasis>-isopropylacrylamide) microgel based etalons

Thermoresponsive microgels have been shown to be an excellent platform for designing sensor materials. Recently, poly (N-isopropylacrylamide)-co-acrylic acid (pNIPAm-co-AAc) microgel based etalon materials have been described as direct sensing materials that can be designed to have a single, unique color. These color tunable materials show immense promise for sensing due to their spectral sensitivity and bright visual color. Here, we describe a proof-of-concept for etalon sensing of glucose. We found that aminophenylboronic acid (APBA)-functionalized pNIPAm-co-AAc microgels in an etalon respond to 3 mg/mL glucose concentrations by red shifting their reflectance peaks by 110 nm up to 150 nm. Additionally, APBA-functionalized pNIPAm-co-AAc microgels have a depressed volume phase transition temperature at 18–20 °C, which shifts to 24–26 °C after glucose binding. We also demonstrate that these materials show a marked visual color change, which is a first step towards developing direct read-out sensor devices.