X. X. Zhu

Lower critical solution temperatures of N-substituted acrylamide copolymers in aqueous solutions

Abstract Several series of copolymers were prepared from N-substituted acrylamides by free radical polymerization in solution. We have selected a group of monomers with varying degree of hydrophilicity including acrylamide, N-ethylacrylamide, N,N-dimethylacrylamide, N,N-diethylacrylamide, and N-tert-butylacrylamide. The chemical composition in the final copolymers was found to be very close to the original monomer composition in the feed prior to polymerization. In an effort to elucidate the effect of the chemical composition on the phase separations of the aqueous solutions of the copolymers, the lower critical solution temperatures (LCST) of the copolymers in water were determined by differential scanning calorimetry and optical turbidimetry. In principal, the LCST of the copolymers can be adjusted within the freezing and boiling points of the solutions. The copolymers exhibit systematic changes in their LCSTs as a function of their comonomer composition, for which an empirical equation was established.

Layer-by-layer multilayer films linked with reversible boronate ester bonds with glucose-sensitivity under physiological conditions

Layer-by-layer (LBL) multilayer films were fabricated from poly(vinylalcohol) (PVA) and poly[acrylamide-co-3-(acrylamido)phenylboronic acid] [P(AAm-AAPBA)] by the use of covalent phenylboronate ester bonding as the driving force. The film thickness increases with decreasing pH and increasing ionic strength of the assembly solutions. Phenylboronate ester bonds in the film were confirmed from the IR marker mode at 1730 cm−1. Because the phenylboronate ester bonding is reversible, the PVA/P(AAm-AAPBA) films disassemble gradually when immersed in aqueous solutions. The disassembly rate increases with increasing pH and decreasing ionic strength of the aqueous solutions. Furthermore, the disassembly of the film is accelerated by the addition of glucose, which competes with PVA for phenylboronic acid groups and weakens the interaction between the two polymers. Interestingly, the PVA/P(AAm-AAPBA) film presents glucose-sensitive behavior even under physiological conditions. The enhanced glucose-sensitive behavior at physiological pH may originate from the stabilization of phenylboronate ester by the adjacent amide group. These glucose-sensitive LBL films hold promise for self-regulated insulin release.

Recent advances in entropy-driven ring-opening polymerizations

Entropy-driven ring-opening polymerization (ED-ROP) of unstrained macrocyclic monomers and/or oligomers employs the ring-chain equilibria between macrocycles and their corresponding polymers and the associated increase of conformational freedom to achieve high molecular weight materials. The principles of building macrocyclic compounds, their use in ED-ROP, and the practical considerations of polymerizations are described, and recent progress in this area is discussed through selected examples. The various polymerization techniques used for ED-ROP are discussed, including anionic, radical, coordination/insertion, ring-opening metathesis, and enzymatic polymerization methods. Emphasis is placed on the potential of ED-ROP in the synthesis of biomaterials and the development of enzyme-catalyzed green systems.

N-Alkylacrylamide copolymers with (meth)acrylamide derivatives of cholic acid: synthesis and thermosensitivity

Abstract Copolymers of N -alkylacrylamides with acrylamide or methacrylamide derivatives of cholic acid have been prepared to obtain copolymers with desired thermosensitivity and enhanced hydrophilicity. The thermosensitivity of these copolymers depends on their chemical composition as studied by microcalorimetry, differential scanning calorimetry and UV–visible spectroscopy. The lower critical solution temperature and the enthalpy change of the phase separation process of the copolymers in water decreased with increasing content of the bile acid residues in the polymers.

Molecular pockets derived from cholic acid as chemosensors for metal ions.

Molecular pockets in the form of a tripod made of cholic acid were found to be able to solubilize pyrene in polar media as a result of the facial amphiphilicity of bile acids. The trimer containing 1,2,3-triazole groups can complex with heavy metal ions, as clearly shown by electron paramagnetic resonance spectroscopy. Both the metal cation and the pyrene molecule can be brought close together in the cavity formed by the cholic acid trimer, resulting in significantly improved efficiency for fluorescence quenching of pyrene. The decrease of fluorescence intensity can be used for the detection of heavy metal ions, and the detection limit is about 1 microM in water, suggesting the usefulness of such molecules as chemosensors for such metal ions. A different trimer without the coordinating triazole groups is shown to shield pyrene away from metal ions, causing a much reduced fluorescence quenching.

Salt-induced erosion of hydrogen-bonded layer-by-layer assembled films

Hydrogen-bonded layer-by-layer assembled films from poly(vinyl pyrrolidone) (PVPON) and poly(acrylic acid) (PAA) were fabricated. Fabry–Perot fringes were used as a simple but efficient method for studying the structure changes in the films during salt treatment. Results indicate that these films are very sensitive to salt when incubated in water. A brief treatment in salt solution of low concentration results in microphase separation in the films, the extent of which increases with increasing pH and salt concentration. The microphase separation in PVPON–PAA films is explained by the enhanced dissociation of PAA, which partially breaks the hydrogen bonds in the films. More importantly, the erosion of the PVPON–PAA films accelerates greatly in salt solution. Morphology changes during the erosion suggest a dewetting process of the films, which facilitates their dissolution. Finally, salt-triggered, rapid release of ibuprofen was achieved by the quick, salt-induced erosion of the hydrogen-bonded films.

Copolymers of N-alkylacrylamides and styrene as new thermosensitive materials

New thermosensitive copolymers were obtained by copolymerization of styrene with N-methylacrylamide, N-ethylacrylamide or N,N-dimethylacrylamide. The turbidity measurements showed that these polymers can undergo a thermally induced phase transition at various temperatures, depending on their chemical composition, molecular weight and concentration in water. Fluorescence measurements indicated the occurrence of aggregation through hydrophobic interactions even below the lower critical solution temperature. The enthalpy and cooperativity of phase transition determined by microcalorimetry were strongly dependent on the chemical structure of the N-alkylacrylamide comonomer.

Fluorescence study of inclusion complexes between star-shaped cholic acid derivatives and polycyclic aromatic fluorescent probes and the size effects of host and guest molecules

Star-shaped host molecules containing two, three, and four cholic acid moieties have been used to form inclusion complexes with polycyclic aromatic hydrocarbon probes (guests) varying in size from four (pyrene) to five (benzo(e)pyrene) and seven aromatic rings (coronene) and investigated by steady-state fluorescence measurements and fluorescence lifetime techniques. The results indicated that these hydrophobic guest probes prefer to locate in the hydrophobic cavities formed by the host molecules in an aqueous solution. Further studies showed that the stoichiometric ratios of the complexes depended on the relative size of both the host and the guest. The complexes of 1:1 ratio (guest:host) were formed between pyrene and the host molecules of different sizes, while the complexes of 1:2 ratio (guest:host) were found for coronene in all cases. For benzo(e)pyrene with an intermediate size, the complexes with 1:1 and 1:2 ratios (guest:host) were formed depending on the relative sizes of the host molecules. The stability of the inclusion complexes was observed to change with the solvent polarity, indicative of an adaptation of the hydrophobicity of the host pockets to the polarity of the solvent. The formation of the complexes was driven by the solvophobic interactions.

Nitrocellulose-stabilized silver nanoparticles as low conversion temperature precursors useful for inkjet printed electronics

Silver nanoparticles were synthesized from silver nitrate and methanol and stabilized by the use of nitrocellulose and 3-aminopropyl alcohol. These colloidal solutions were found to be very stable, with no evidence of silver aggregation over a period of 9 months, and displayed properties (viscosity, surface tension and size of the nanoparticles) compatible with inkjet printing technologies. The two main advantages of using nitrocellulose are its ability to stabilize silver nanoparticles and its low converison temperature (degradation starts at 135 °C, and the typical annealing temperature was 190 °C). For the annealing of the precursor films, two different heating methods and their effect on the kinetics of degradation and morphology of the final cured films have been investigated.

Hydrogel thin film with swelling-induced wrinkling patterns for high-throughput generation of multicellular spheroids.

Three-dimensional (3D) multicellular spheroids (MCSs) mimic the structure and function of real tissue much better than the conventional 2D cell monolayers, however, their application was severely hindered by difficulties in their generation. An ideal method for MCS fabrication should produce spheroids with narrow size distribution and allow for control over their size. The method should also be simple, cheap, and scalable. Here, we use patterned nonadhesive poly(2-hydroxyethyl methacrylate) hydrogel films to guide the self-assembly of cells. The films were fabricated directly in the wells of cell culture plates. They were patterned spontaneously by swelling in water, without the use of any template or specialized facilities. When cell suspension is added, the cells settle down by gravity to the bottom. Because of the presence of the wrinkling pattern composed of uniformed microcaves, the cells accumulate to the center of the microcaves and gradually self-assemble into MCSs. Using this method, monodisperse MCSs were generated. The size of the spheroids can be facilely controlled by the number of cells seeded. The method is compatible with the conventional monolayer cell culture method. Thousands of spheroids can be generated in a single well. We expect this method will pave the way for the application of MCSs in various biomedical areas.