Rapid induction of thermoreversible hydrogel formation based on poly(propylene glycol)-grafted dextran inclusion complexes

Poly(propylene glycol) (PPG)-grafted dextran was synthesized by conjugating amino-terminated PPG with hydroxyl groups of dextran, and its inclusion complexation property was investigated. The average number of grafted PPG per dextran was changeable in the range of 1.1 to 38.5. The formation of inclusion complexes between the PPG grafts and β-cyclodextrins (β-CDs) and their crystalline structures were characterized by ¹³C CP/MAS NMR and X-ray spectroscopies. These hydrogel systems showed a thermally reversible sol–gel transition based on supramolecular assembly and dissociation between host and guest moieties. The results of rheological measurements and sol–gel transition temperature of the hydrogels suggested that the aggregated channel-type crystalline domain was critical to control the transition temperature in terms of initial feed molar ratio of PPG and β-CD and the graft number of PPG constituents. These thermoreversible hydrogel systems showed rapid gelation properties, which may be useful for biomedical application, especially injectable drug delivery systems.

Sol–gel phase transition of ICs between PPG-grafted dextrans and β-CDs. (conc. of IC was fixed at 20 wt.-%).     

Surfacial modification of polymer powders as drug carriers

Preparation of warfarin methacrylate(WaMA) was carried out by esterification of warfarin as thromboembolism drug with methacryloyl chloride. Radical copolymerization of WaMA with 1-vinyl-2-pyrrolidone(VPr) was carried out in 1,4-dioxane at 60°C using 2,2′-azoisobutyronitrile as initiator. The surfacial modification of the copolymer powders was achieved using the freeze-drying by dissolving the copolymer in benzene and micellation by benzene-in-water and water-in-benzene systems. The hydrolysis of the copolymer including the drug was investigated under mild condition from a view point of released rate of the drug.     

An analysis of deformation process on poly‐p‐phenylenebenzobisoxazole fiber and a structural study of the new high‐modulus type PBO HM+ fiber

This study is concerned with fiber structure of new high‐modulus type PBO fiber. Crystal modulus and molecular orientation change with stress was surveyed. Standard‐modulus type PBO (AS) fiber has hysteresis effect to applied stress while high‐modulus type PBO (HM) fiber shows reversible change. In order to raise actual PBO fiber modulus higher, nonaqueous coagulation process was adopted with conventional heat treatment. The fiber (HM+) so made gives 360 GPa in the Young's modulus and an absence of small‐angle X‐ray scattering pattern that is characteristic for aqueous‐coagulated PBO fiber with heat treatment (Zylon™ HM). The crystal structure form and crystal size for the HM+ fiber are the same as those of the HM fiber. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 1605–1611, 2000     

Synthesis of α‐Cyclodextrin‐Conjugated Poly(ε‐lysine)s and Their Inclusion Complexation Behavior

Novel functional polymers utilizing specific host/guest interactions were designed by introducing α‐CD host molecules into poly(ε‐lysine) chains as side groups. An interesting phase separation was observed as a result of the inclusion complexation between the polymeric host and 3‐(trimethylsilyl)propionic acid as a model guest in aqueous media. This water‐soluble polymeric host would be useful for various applications, particularly drug delivery, due to its biodegradability, low toxicity, and unique functionality represented as a complexation‐induced phase separation.     

An investigation into the relationship between internal stress distribution and a change of poly‐p‐phenylenebenzobisoxazole (PBO) fiber structure

This study concerns stress distribution induced by external force in individual poly‐p‐phenylenebenzobisoxazole (PBO) molecules in fiber. In reality, there are no fibers having an ideal structure (i.e., composed of infinitely long complete crystal elongated parallel to the fiber axis without defects that disconnect stress transfer in the crystal structure). Normally, real fiber structure has some structural incompletion, such as molecular ends, molecular misorientation, and density fluctuation (inhomogeneity) along the fiber axis. They play the role of heterogeneous stress distribution and reduction of fiber modulus in the fiber under tensile deformation. To carry out such analysis, meridional X‐ray diffraction peaks of the PBO fiber under stress were measured and discussed. Distribution of the diffraction peak profile (half‐height width of the diffraction profile) was especially considered. Change of the molecular orientation induced by external stress to the fiber was also estimated by measuring distribution of equatorial spots along the Debye ring. It was found that the distribution of the meridional diffraction spots became wider in the meridian, while the peak profile along the azimuthal direction became narrower as external stress was added for all three fibers. The degrees of response against stress came in this order: AS (180 GPa) > HM (280 GPa) > HM+ (360 GPa). Hosemann's analysis was adopted to analyze real crystallite size and disorder parameter (g) of crystallites. It indicated that the crystalline size does not vary but the ordering of periodicity in the crystal lattice starts to loosen as applied stress to the fiber is increased. The stress seems to affect only local micro regions in the crystal structure. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2901–2911, 2000     

Morphological study on poly-p-phenylenebenzobisoxazole (PBO) fiber

Morphological survey on new PBO fiber (Zylon®) was conducted by X-ray and transmission electron microscopic studies. Crystal size, orientation of the crystal, fibrils, microvoids, and fine structure were discussed. It was found that the molecule in the fiber showed high orientation (more than 0.99 in Hermann's orientation function for heat-treated fiber) and relatively small crystal sizes in the longitudinal (160 Å) and the transverse (110 Å) directions. Crystal modulus estimated by extrapolation to perfect orientation on the plot of the fiber modulus as a function of fiber orientation (Northolt's method) shows discrepancy from the crystal modulus directly obtained by X-ray scattering. This discrepancy means that the Northolt's model is insufficient to describe the Young's modulus of PBO fiber. Microvoids elongated to the fiber direction were examined by small-angle X-ray scattering and transmission electron microscopic methods. The diameter of the microvoids was 20 Å to 30 Å and the fiber had a very thin microvoids-free layer (0.2 μm). Preferential orientation of the a-axis of crystal in the fiber was also confirmed. Summarizing these results, a structure model of the PBO fiber was proposed. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36 : 39–48, 1998     

Photochromism of 3-butyl-1-methyl-2-phenylazoimidazolium in room temperature ionic liquids

We studied the photochromism of a newly synthesized ionic liquid, [2PA-Bmim]Tf₂N ([2PA-Bmim]⁺: 3-butyl-1-methyl-2-phenylazoimidazolium, Tf₂N^?: bis(trifluoromethanesulfonyl)-amide) which is characterized by a phenylazo group substituted on the imidazolium ring. The melting point of [2PA-Bmim]Tf₂N is 329 K. The absorption spectrum of [2PA-Bmim]⁺ dissolved in conventional organic solvents or in ionic liquids changes drastically upon UV-light irradiation, which is attributed to the photoisomerization of the phenylazo group from E- to Z-forms during irradiation and the backward thermal isomerization from Z- to E-forms in the dark. The E–Z photoisomerization quantum yield, Φ_iso, was determined by 355 nm laser photolysis. The Φ_iso value slightly depends on solvent viscosity, from 0.12 in 3-butyl-1-methylimidazolium PF₆^? (η = 241 cP) to 0.19 in toluene (η < 1 cP). On the other hand, no solvent dependence was observed for Arrhenius parameters of the backward Z–E thermal isomerization. We discuss the isomerization mechanism and the reason why the E–Z photoisomerization yield depends on solvent viscosity.     

Spectral decomposition of the mean curvature vector field of surfaces in a Sasakian manifold R2n+1(−3)

We study surfaces in a Sasakian manifold R²ⁿ⁺⁺¹(?3) whose mean curvature vector fields admit a finite spectral decomposition with respect to certain elliptic linear differential operators.