Self-Assembly of Thienopyrrole-Fused Thiadiazoles Containing an Amide Linker: Control of Supramolecular Polymerization Mechanism and Chiroptical Properties by Trialkoxy Side Chains.

Three thienopyrrole-fused thiadiazole (TPT) fluorescent dyes featuring a common amide linker and different alkoxy substituents on peripheral trialkoxybenzene moieties were synthesized, and their self-assembly behavior in solution was investigated. The obtained results revealed a substantial steric effect of the alkoxy substituents on the supramolecular polymerization mechanism, which results from a combination of π-stacking and hydrogen (H)-bonding interactions. Detailed spectroscopic measurements revealed that with increasing steric demand of the substituents, the supramolecular polymerization processes in pure methylcyclohexane (MCH) or a mixture of MCH and toluene become temperature-sensitive and enthalpically favorable, resulting in a change from the isodesmic assembly mechanism to the cooperative mechanism. Theoretical calculations suggested that in TPTs with bulky substituents, steric hindrance causes the H-bonding array of the amide moieties to be aligned along the stacking axis of the π-systems; thus, the H-bonding interactions are strengthened compared to those in TPTs with less bulky substituents, compensating for the weakened π-stacking interactions. A chiral TPT derivative with (S) stereogenic centers was found to form homochiral helical supramolecular assemblies that generate discernible circularly polarized luminescence. Achiral TPTs also generate helical assemblies to which preferential helicity can be imparted through the external chiral bias of the solvents (R)- and (S)-limonene.

Self-Wrinkling in Polyacrylamide Hydrogel Bilayers.

Swelling of a gel film attached to a soft substrate can induce surface instability, which results in the formation of highly ordered patterns such as wrinkles and folds. This phenomenon has been exploited to fabricate functional devices and rationalize morphogenesis. However, obtaining centimeter-scale patterns without immersing the film in a solvent remains challenging. Here, we show that wrinkles with wavelengths of up to a few centimeters can be spontaneously created during the open-air fabrication of film-substrate bilayers of polyacrylamide (PAAm) hydrogels. When the film of an aqueous pregel solution of acrylamide prepared on the PAAm hydrogel substrate undergoes open-air gelation, hexagonally packed dimples initially emerge on the surface, which later evolve into randomly oriented wrinkles. The formation of such self-organized patterns can be attributed to the surface instability resulting from autonomous water transport in the bilayer system during open-air fabrication. The temporal evolution of the patterns can be ascribed to an increase in overstress in the hydrogel film due to continued water uptake. The wrinkle wavelength can be controlled in the centimeter-scale range by adjusting the film thickness of the aqueous pregel solution. Our self-wrinkling method provides a simple mechanism for the generation of swelling-induced centimeter-scale wrinkles without requiring an external solvent, which is unachievable with conventional approaches.

Thermoresponsive properties of polymer hydrogels induced by copolymerization of hydrophilic and hydrophobic monomers: comprehensive study of monomer sequence and water affinity.

In order to raise the possibility of the practical use of thermoresponsive hydrogels in various fields, it is imperative to achieve on-demand control of responsive behavior especially by using a simple synthetic method with common monomers. To this end, we synthesized various hydrophilic/hydrophobic copolymer hydrogels from common acrylamide derivatives and acrylate monomers via free radical copolymerization, and examined the correlation between the structure and the swelling properties of the obtained gels, specifically from the viewpoint of the monomer sequence in the network chains and the affinity to water molecules. The obtained gels with a hydrophobic acrylamide monomer were shown to exhibit a sharp volume change in water upon heating at suitable monomer compositions. In contrast, the gels consisting of a hydrophobic acrylate monomer only decreased the swelling degree with no significant thermoresponsive volume change. The formation of a local amphiphilic structure without a long hydrophobic sequence is critical for achieving sharp thermoresponsiveness. Moreover, the water affinity was drastically changed at a sharp volume transition with the copolymer gels of a hydrophobic acrylamide. This transition was most likely driven by an entropic factor because of the strong contribution of the hydration/dehydration of the network chains. The comparison of the temperature-responsive behavior of the gels with that of the corresponding linear copolymers demonstrated that the crosslinking structure made significant hydrophobic contribution to the responsive behavior.

Implicit rule on the elastic function of a swollen polyacrylamide hydrogel.

A full understanding of the elastic properties of hydrogels under swelling is required for their practical application in the chemical and biomedical engineering fields. This is because hydrogels are expected to retain water during mechanical use in moist atmospheres. In the present study, we investigated the relationship between the elastic modulus and the swelling ratio in a specific type of hydrogel (a polyacrylamide gel). The elasticity and swelling data revealed that these two parameters are proportionally related in hydrogels comprising adequate amounts of monomers and crosslinkers. We also demonstrated that this proportional relationship inherently conforms to the linear elastic behaviour predicted by the Flory-Rehner free energy function (the F-R model). The implicit rule is established by the extended F-R model with two scaling exponents. The extended model is capable of representing the irregular elasticity of swollen gels formed from low- or high-molecular-weight polymers.

Multiarm Star-Crosslinked Hydrogel: Polymer Network with Thermoresponsive Free-End Chains Densely Connected to Crosslinking Points.

Soft tissue in biological system is a hydrogel with elaborate structure exhibiting repeatable dynamic function. In order to approach such sophisticated system, precise construction of a designed network with multi-components is desired. This communication presents a novel hydrogel having highly dense stimuli-responsive free-end chains around crosslinking structure. A key molecule is a core-crosslinked star-shaped polymer with multiple thermoresponsive arms, which can be prepared by reversible addition-fragmentation chain transfer polymerization of divinyl crosslinker with poly(N-isopropylacrylamide) (PNIPAAm) macro-chain transfer agent and have a number of unreacted carbon-carbon double bonds in the core. These unreacted double bonds can be utilized as a crosslinker for poly(acrylamide) (PAAm) gel synthesis by free radical polymerization. The obtained gel contains homogeneously dispersed star PNIPAAms as crosslinking points and exhibits thermoresponsive swelling behavior in water depending on the star contents. In particular, the gel with low content of the star crosslinker shows localized responsive behavior with expansion and shrinkage of the star in one molecule. The mechanical properties of the star-crosslinked gel are significantly high compared to the conventional PAAm gels particularly in compressive strength (≈9 MPa). Moreover, the star-crosslinked gel has thermoresponsive mechanical toughening property.

Crosslinker-Based Regulation of Swelling Behavior of Poly(N-isopropylacrylamide) Gels in a Post-Polymerization Crosslinking System.

A fundamental understanding of the effect of a crosslinker on gel properties is important for the design of novel soft materials because a crosslinking is a key component of polymer gels. We focused on post-polymerization crosslinking (PPC) system utilizing activated ester chemistry, which is a powerful tool due to structural diversity of diamine crosslinkers and less susceptibility to solvent effect compared to conventional divinyl crosslinking system, to systematically evaluate the crosslinker effect on the gel properties. A variety of alkyldiamine crosslinkers was employed for the synthesis of poly(N-isopropylacrylamide) (PNIPAAm) gels and it was clarified that the length of alkyl chains of diamine crosslinkers strongly affected the gelation reaction and the swelling behavior. The longer crosslinker induced faster gelation and decreased the swelling degree and the response temperature in water, while the crosslinking density did not significantly change. In addition, we were able to modify the polymer chains in parallel with crosslinking by using a monoamine modifier along with a diamine crosslinker. This simultaneous chain modification during crosslinking (SMC) was demonstrated to be useful for the regulation of the crosslinking density and the swelling behavior of PNIPAAm gels.

Evaluation of the effects of cross-linking and swelling on the mechanical behaviors of hydrogels using the digital image correlation method.

Experimental evaluation and modeling are important steps in the investigation of the mechanical behaviors of hydrogels in the small- to large-strain range. In this study, the effects of cross-linking and swelling on the true stress-strain response of a specific type of hydrogel (polyacrylamide) were evaluated using a uniaxial tensile test. The development of true strain on the surface of the hydrogel was measured using the digital image correlation method. The specimens with higher cross-link density exhibited a higher initial elastic modulus and earlier orientation hardening. The initial elastic modulus was reduced by the swelling, whereas the orientation hardening occurred in an earlier strain range in the swollen hydrogel. The mechanical responses of the as-prepared and swollen hydrogels with different cross-linker contents were fitted using a non-Gaussian statistical model. The conventional model underestimated the decrease in the elasticity owing to the swelling effect and overestimated the increase in the stress in the large-strain range. The mechanical model was suitably modified to yield an accurate reproduction of the mechanical responses. The proposed model, which was characterized by five material parameters, was found to reproduce the characteristics of the mechanical responses of the as-prepared and swollen hydrogels with different cross-linker contents.

Design of monoalcohol - Copolymer system for high quality silver nanowires.

Research to improve the dimensional properties of silver nanowires (Ag NWs) for transparent conductive film (TCF) applications are being carried out intensively. However, the protocol for the designed synthesis of high-quality Ag NWs is yet to be developed due to the inadequacy of knowledge on the role of parameters. Here, we attempt to elucidate the role played by the parameters and propose a monoalcohol-copolymer based system for the designed synthesis of Ag NWs superior in quality to the one synthesized using conventional ethylene glycol (EG)-polyvinylpyrrolidone (PVP) system. The key findings of the study are as follows: (1) the solubility of Ag source and the partially formed AgCl particles in monoalcohols was found to play an important role not only in the reduction to metallic Ag but also on the uniaxial growth, (2) the adsorption of capping agents on Ag NWs was carried through O and N atoms in the base molecule and their binding energies indicated that the strength is the key parameter to obtain Ag NWs with high aspect ratio, (3) the properties of nanowire could be enhanced through copolymerization of VP and base molecules that have O- and N-based ligands, and (4) the influence of copolymerization on the physical and chemical properties of the surface active agent has been theoretically and experimentally verified. Consequently, we succeeded in the development of a new technique to synthesize high yield of Ag NWs with improved aspect ratio than EG-PVP system by using benzyl alcohol as reducing solvent and N-vinylpyrrolidone/N,N-diethylaminoethyl metacrylate copolymer as a capping agent. The optical transmittance and electrical resistivity of TCFs prepared using the Ag NWs with an average diameter of 43 nm, and an average length of 13 µm were 98.6% and R: 49.1 Ω/□, respectively.

Effect of Monomer Sequence along Network Chains on Thermoresponsive Properties of Polymer Gels.

The effect of monomer sequence along the network chain on the swelling behavior of polymer gels should be clarified for the advanced control of swelling properties of gel materials. To this end, we systematically investigated the swelling properties of poly(acrylamide derivative) gels with the same composition but different monomer sequence by utilizing two gel synthetic methods: copolymerization giving a random network and co-crosslinking giving a blocky network. Both of the copolymerization and the co-crosslinking gels were prepared from the combination of two of the three following monomers: hydrophilic N,N-dimethylacrylamide (DMAAm), hydrophobic N-n-butylacrylamide (NBAAm), and thermoresponsive N-isopropylacrylamide (NIPAAm) with various monomer compositions. The swelling measurement of the obtained gels showed totally different behaviors between the copolymerization and the co-crosslinking gels, even with the same monomer composition. The copolymerization gels had the average property from the two monomers, depending on monomer composition, because random monomer distribution changed the affinity of each network chain to water. On the other hand, the co-crosslinking gels behaved as if two components independently contributed to the swelling properties, probably due to the domain structure derived from two kinds of prepolymers.

Designer template initiator for sequence regulated polymerization: systems design for substrate-selective metal-catalyzed radical addition and living radical polymerization.

"Template initiator"' platforms (1) have been designed for expressing the sequence information in a template in radical polymerization. Thus, we demonstrated the structural adequacy of 1 consisting of two initiating sites placed ortho to each other in benzene: one for living cationic polymerization to introduce a template carrying substrate-recognition tags, and the other for metal-catalyzed living radical polymerization to achieve sequence regulation. For example, for two positional isomers with an amine template for an acid monomer, only the ortho initiator induced selective radical addition of MAA (recognizable) over MMA (non-recognizable). Another version was an oligo(vinyl ether) with a multiple amine template, which demonstrated template effects for MAA recognition over BzMA in copolymerization.

Template-assisted selective radical addition toward sequence-regulated polymerization: lariat capture of target monomer by template initiator.

Surprisingly high monomer selectivity was demonstrated in competitive radical addition with two kinds of methacrylates carrying sodium and ammonium cation. Crucial is size-specific recognition by a lariat crown ether embedded close to the reactive halide in a designer template initiator. Especially, a combination with an active ruthenium catalyst led to outstanding selectivity at low temperature. This template system will open the way to unprecedented sequence-regulated polymerization.

Selective radical addition with a designed heterobifunctional halide: a primary study toward sequence-controlled polymerization upon template effect.

A ruthenium(II)-catalyzed, highly selective, quantitative radical addition of an alkene, methacrylic acid (MAA), has been achieved by using a template halide (2) containing a built-in amine group as a recognition site for the carboxyl group of the substrate. The specific ionic binding of MAA by the amine template (1:1 molar ratio) led to preferential formation of the 1:1 MAA-2 adduct, whereas a similar halide without a template induced MAA oligomerization even in the presence of an externally added amine. A competitive radical addition of MAA versus its ester form [methyl methacrylate (MMA)] on the halide further demonstrated that the substrate selectivity [k'(MAA)/k'(MMA)] for 2 is enhanced more than 10 times by the intramolecular introduction of the template relative to the result for the nontemplate halide. These specificities are most likely triggered by the specific interaction (recognition) of the carboxyl group in MAA via the acid-selective template amine, which is implanted in the close vicinity of the radical addition site in 2. These results intimate possibility of control over the repeat-unit sequence in precision polymerization.