Wan Shik Ha

Structural changes and their effect on mechanical properties of silk fibroin/chitosan blends

Silk fibroin/chitosan blend films were prepared by the solvent casting method. Miscibility between silk fibroin and chitosan was examined by dynamic mechanical thermal analysis. Structural changes of silk fibroin by the addition of chitosan were investigated by IR spectroscopy. The conformational transition of silk fibroin from random coil form to β-sheet structure induced by blending with chitosan resulted in the increase of crystallinity and density of the blend films. The blend film containing 30 wt % chitosan exhibited a maximum increase in crystallinity and density. It was found that the tensile strength and initial tensile modulus of blend films were greatly enhanced with increasing the chitosan content and showed a maximum value at the composition of 30 wt % chitosan.

Structure and properties of microfibrillar poly(vinyl alcohol) fibres prepared by saponification under shearing force of poly(vinyl pivalate)

Abstract Novel, high-strength, microfibrillar poly(vinyl alcohol) fibres (PVA fibrils) having a number-average degree of polymerization ( P n ) of 4500-20 000, a degree of saponification ( DS ) of 85.0–99.9% and a syndiotactic dyad (S-dyad) content of 57–64% could be prepared successfully by saponifying under shearing force poly(vinyl pivalate) having a P n of 11 300-33 000. The saponifying agent used was a mixture of potassium hydroxide, water and methanol. The PVA fibrils had dimensions of 1–50 μm in diameter, 0.5–200 mm in length, irregular cross-sections and needle-point-like ends. The high-strength PVA fibrils prepared by the fibrillation method had an orientation index of over 0.88, a crystallinity of over 48%, a crystal melting temperature of higher than 260°C and an accumulated ultrafine microfibril structure. The PVA fibrils having a P n of over 10 000 exhibited higher than 12 g den −1 of maximum tensile strength and their resistance to hot water was excellent. Moreover, these properties were enhanced by increases in the P n , S-dyad content and DS of the PVA fibrils.

In situ fibrillation of poly(vinyl alcohol) during saponification of poly(vinyl ester) (1). Chemorheological and morphological investigations of in situ fibrillation

Abstract The syndiotacticity-rich (syndiotactic diad content of 57–65%) high molecular weight (number-average degree of polymerization of 4500–20000) poly(vinyl alcohol) (PVA) microfibrillar fiber was directly produced via the saponification reaction of poly(vinyl pivalate) (PVPi) by controlling structural factors such as molecular weight and stereoregularity of PVA chains, and appetence between PVA and solvent without existing spinning procedures. By examining and observing the changes of flow birefringence, shear viscosity of reaction solution, degree of saponification, and shape of reaction product during the saponification reaction, the following novel formation mechanism of high strength microfibrillar PVA fiber is proposed. As PVPi converts into syndiotactic PVA by means of saponification using saponifying agents containing water, an oriented gel structure appears first by the interaction between water and hydroxyl groups of syndiotactic PVA formed during the saponification reaction, and then the microfibrillar structure is formed by the collapse of water-PVA interbridges and the resultant chain packing between syndiotactic PVA molecules. The larger the syndiotacticities of both PVPi and PVA, the higher the molecular orientation and the smaller the hydroxyl groups in P(VPi-VA) copolymer needed for in situ fibrillation. The well oriented microfibrillar PVA fiber prepared by such a manner had irregular cross-sections, needle-point-like ends, accumulated ultrafine microfibril structure.

Relationships between antithrombogenicity and surface free energy of regenerated silk fibroin films

Silk fibroin (SF) was dissolved in calcium chloride/ethanol/water mixture (1/2/8 in mole ratio) at 70°C for 4 h. The dissolved silk fibroin was regenerated by casting the dialyzed solution into the films. The films were treated with 50% aqueous solution of methanol for different times, and their antithrombogenicity was evaluated byin vitro andin vivo tests.In vivo blood tests were made by a method of peripheral vein indwelling suture. It was found that the silk fibroin had a good anti-thrombogenicity and an absorbability even though the polymer showed foreign body reaction. Finally, the blood compatibilty of silk fibroin films which were subjected to structural change by the methanol treatment, was examined in connection with their interfacial surface energy, and a correlation between these properties was found to be present.

Preparation of high molecular weight polyvinylalcohol by low temperature azoinitiator

Abstract Vinylacetate was bulk-polymerized at 30, 40, and 50 °C using a low temperature initiator, 2,2′-azobis(2,4-dimethylvaleronitrile) (ADMVN), and effects of polymerization temperature and initiator concentration were investigated in terms of polymerization behavior and molecular structures of polyvinylacetate (PVAc) and the corresponding polyvinylalcohol (PVA) obtained by saponifying with sodium hydroxide. Low polymerization temperature and low conversion by adopting ADMVN proved to be successful in obtaining PVA of high molecular weight. At conversion of below 20%, PVAc having a number-average degree of polymerization (Pn) of 6800-10,100 was obtained, whose degree of branching for acetyl group was calculated to be 0.60-0.70,0.75-0.90, and 1.05-1.45 at 30,40, and 50 °C, respectively. Saponifying this PVAc yielded PVA having a Pn of 3100–6100.

Preparation of poly(ethylene terephthalate-co-isophthalate) by ester interchange reaction in the PET/PEI blend system

Optimum conditions for the synthesis of PEI of considerable molecular weight have been established. Poly(ethylene terephthalate-co-isophthalate) (PETI) has been prepared through the ester interchange reaction of a blend of poly(ethylene terephthalate) (PET) and poly(ethylene isophthalate) (PEI). NMR analysis has indicated that the PETI changes from a block-type copolymer to a random type copolymer as the ester interchange reaction proceeds. If the reaction is limited to 20 min, the resulting PETI is crystallizable. The effects of catalysts that have been used during the synthesis of PEI on the characteristics of PETI are also discussed.

Syndiotacticity-rich ultrahigh molecular-weight poly(vinyl alcohol) film. I. Determination of optimum polymer concentration by zone-drawing method in film preparation

A new method using a simple zone-drawing technique has been suggested for determining the optimum initial concentration of a polymer solution that has suitable macromolecular entanglements. This method was developed to replace the incorrect inherent viscosity-measuring method for syndiotacticity-rich (syndiotactic diad content of 63.4%) ultrahigh molecular-weight (number-average degree of polymerization of 12,300) (UHMW) poly(vinyl alcohol) (PVA) solution. Syndiotacticity-rich UHMW PVA films were prepared from dimethyl sulfoxide (DMSO) solutions with different initial concentrations: of 0.1, 0.2, 0.3, 0.4, and 0.5 g/dL. In order to investigate the drawing behavior of the syndiotacticity-rich UHMW PVA films with different solution concentrations, the films were drawn under various zone-drawing conditions. Through a series of experiments, it was discovered that the initial concentration of PVA solution in DMSO caused significant changes in the draw ratio of the syndiotacticity-rich UHMW PVA film. That is, the one-step and maximum zone draw ratios of the film at an initial concentration of 0.3 g/dL exhibited its maximum values and gradually decreased at higher or lower concentrations. Thus, it was disclosed that the initial concentration of 0.3 g/dL is the optimum polymer concentration to produce a maximum draw ratio in this work. Based on the above results, it may be concluded that the optimum concentration of the initial PVA solution can be determined directly by measuring the zone draw ratio. The draw ratio, birefringence, crystallinity, degree of crystal orientation, tensile strength, and tensile modulus of the maximum drawn PVA film were 32.9, 0.0449, 0.61, 0.991, 1.91, and 46.2 GPa, respectively.

Preparation and characterization of syndiotacticity-rich ultra-high molecular weight poly(vinyl alcohol)/imogolite blend film

To enhance the physical properties of syndiotacticity-rich (syndiotactic diad content 63·4%) ultra-high molecular weight (UHMW) (number-average degree of polymerization 12300) poly(vinyl alcohol) (PVA) film, it was solution blended with rigid-rod imogolite in dimethyl sulphoxide. In addition, the blend film prepared was stretched using a high-temperature zone drawing technique for effective orientation of the film. Through a series of experiments, it was found that imogolite caused significant changes in the structure and properties of syndiotacticity-rich UHMW PVA film, i.e. imogolite acted as an important agent which increased crystal orientation of syndiotacticity-rich UHMW PVA, resulting in enhanced tensile strength of the film. However, imogolite played a hindering role in raising the amorphous orientation of syndiotacticity-rich UHMW PVA. The maximum tensile modulus of 19·8GPa and maximum tensile strength of 1·8GPa could be obtained at the maximum draw ratio of 7·45 for PVA/imogolite blend film. In the case of PVA homo film, the highest tensile modulus and strength were 25·2GPa and 1·4GPa, respectively.

Preparation of high molecular weight atactic poly(vinyl alcohol) by photo-induced bulk polymerization of vinyl acetate

Vinyl acetate was polymerized in ultraviolet-ray initiated bulk system at low temperatures using 2,2′-azobis(2,4-dimethylvaleronitrile) (ADMVN) or 2,2′-azobis(isobutyronitrile) (AIBN) as the photoinitiator, respectively. High molecular weight (HMW) poly(vinyl alcohol) (PVA) having number-average degree of polymerization (Pn) of 3,900–7,800 and syndiotactic diad (S-diad) content of 52.5–54.0% could be prepared by complete saponification of synthesized linear poly(vinyl acetate) (PVAc) havingPn of 5,900–9,400 obtained at conversion of below 30%.Pn of PVA using ADMVN was larger than that of PVA using AIBN. On the other hand, conversion of the former was smaller than that of the latter, and it was found that the initiation rate of the ADMVN was lower than that of AIBN. This could be explained by a fact that the rate of photolysis of AIBN is faster than that of ADMVN due to the higher quantum yield or dissociation rate constant of AIBN than that of ADMVN. ThePn, syndiotacticity, and whiteness of PVA from PVAc polymerized at lower temperatures were superior to those of PVA from PVAc polymerized at higher temperatures.

Kinetics of copolycondensation of bis(4‐hydroxybutyl) terephthalate and bis(2‐hydroxyethyl) terephthalate by ester‐interchange reaction

The kinetics of polycondensation and copolycondensation reactions were investigated using bis(4-hydroxybutyl) terephthalate (BHBT) and bis (2-hydroxyethyl) terephthalate (BHET) as monomers. BHBT was prepared by ester interchange reaction of dimethyl terephthalate and 1,4-butanediol. BHBT and BHET were polymerized at 270°C in the presence of titanium tetrabutoxide (TBT) as a catalyst. Applying second-order kinetics for polycondensation, the rate constants of polycondensation of BHBT and BHET, k11 and k22, were calculated as 3.872 min−1 and 2.238 min−1, respectively. BHBT and BHET were also copolymerized at 270°C using TBT. The rate constants of crossreactions in the copolycondensation of BHBT and BHET, k12 and k21, were obtained by using the results obtained from a proton nuclear magnetic resonance (1H-NMR) spectroscopy and a high-performance liquid chromatography (HPLC). It was found that the rate constants during the copolycondensation of BHBT and BHET at 270°C decreased in the order k21 > k11> k22 > k12 and the monomer reactivity ratio of BHBT was four or five times larger than that of BHET. In calculating the crossreactions, the method by the 1H-NMR spectroscopy gave more accurate results than that by the HPLC.