摘要:

Thin fibers of calf thymus Na-DNA were electrospun from aqueous solutions with concentrations from 0.3% to 1.5%. In electrospinning, a high voltage is used to create an electrically charged jet of liquid solution, which dries to leave a polymer fiber. The electrospun DNA fibers have diameters around 50 to 80 nm. The diameter of the electrospun fibers is an order of magnitude or more smaller than that of previously reported fibers. The DNA fibers were observed by optical microscopy, scanning electron microscopy, and transmission electron microscopy. Bead-like structures were observed on many of the fibers. During electrospinning a process called splaying causes the jet to split longitudinally into two smaller jets, which split again, repeatedly, until the very small diameter fibers are formed. The small-diameter fibers are transparent in ordinary 100 kV electron microscopes. Fibers can be spun from samples of DNA as small as 1 mg.

ISSN:0022-2348    

DOI:10.1080/00222349708220422

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The block copolyetheresters with hard segments of poly(alkylene-p, p'-bibenzoate) and soft segments of poly(tetramethylene ether) have been investigated by two x-ray diffraction techniques and differential scanning calorimetry (DSC). The results from the counter diffractometry technique indicate the effect of composition on the diffraction peaks and relative crystallinity of the block copolyetheresters. The results from the photographic technique not only show the presence of the diffraction peaks qualitatively, but also indicate the layer spacings relating to the smectic phase for certain block copolyetheresters. The successive DSC heating curve after cooling from the isotropic (I) state to a temperature below the I-S (smectic) transition but well above the S-K (crystalline) transition indicates the nature of the S-I transition for some block copolyetheresters. The S-I transition peak overlaps with the K-S transition peak, or is even replaced by a K-I transition peak. Thus it is difficult to distinguish between the K-S, S-I transitions and a K-I transition in second-run DSC heating curves for the block copolyetheresters.

ISSN:0022-2348    

DOI:10.1080/00222349708220423

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

The effect of cryogenic treatment on dilute polystyrene solutions of different thermal history was studied by viscometry, size-exclusion chromatography, and static laser light-scattering measurements. The lowering of apparent molar mass during freezing of the solvent has its origin in a more compact conformation of the polymer chain resulting from the formation of intrachain segmental cohesion. Prior hypotheses about cryogenic degradation or chain scission in the polymer solution are incorrect.

ISSN:0022-2348    

DOI:10.1080/00222349708220424

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Single-chain single crystals of gutta-percha have been observed by transmission electron microscopy of atomizer-sprayed particles deposited from a very dilute solution in chloroform onto a carbon film which had a filter paper wetted with ethanol in contact with its under side. Selected-area electron diffraction patterns of crystals having no definite crystal profiles showed that the crystals were of the low-melting crystalline form and that the chain segments in the single crystals were standing up from the substrate during crystallization. In cases of single crystals showing sharply defined crystal profiles, electron diffraction patterns showed that they were neither of the low-melting form nor the high-melting form. The structure of this new crystalline modification needs further studies.

ISSN:0022-2348    

DOI:10.1080/00222349708220425

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Selectively deuterated poly(ethylene-2,6d-naphthalene dicarboxylate) (PEN) was synthesized, obtaining a material in which either the ethylene groups (PEN-Ed1or the naphthalene ring (PEN-Nd6) was deuterated. By means of deuteron nuclear magnetic resonance (2H-NMR), the molecular mobility of the deuterated groups was investigated as a function of temperature. The spectra were measured by the solid echo method, applying different waiting times twThe longitudinal relaxation curves were obtained by determination of the maximum intensity of the echo as a function of twIt was found that in the amorphous regions of PEN, the molecular mobility of the ethylene groups is higher than in the amorphous regions of PET. In PEN crystals in the β-modification form, trans-gauche jumps of the ethylene groups with almost equal occupation of the trans and the gauche conformation take place already at 90°C. This is not the case in either PEN crystals in the a-modification form or in PET crystals. The rotational motion of the naphthalene ring, which according to dynamic mechanical analysis at 20 Hz starts at 60°C, could not be observed in the NMR spectra. This is explained by the high frequency applied in the NMR experiment.

ISSN:0022-2348    

DOI:10.1080/00222349708220426

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

A new energy dispersive x-ray diffraction method was used to observe isothermal phase transitions of a polyethylene oxide sample contained in a variable temperature cell. A series of scattering measurements were performed to follow the time evolution of the system at each temperature. The collected spectra were suitably processed and the transition coordinatexwas obtained as a function of time. A detailed treatment of the errors was accomplished to evaluate the reliability of the method. The experimental points (dx/dt)max—Twere plotted both for the amorphous-crystalline (below the melting temperatureTm) and for the crystalline-amorphous (overTm) transitions. They reproduce the usual behavior forT < Tmand show a characteristic unexpected feature forT>Tmwhich is partially justified.

ISSN:0022-2348    

DOI:10.1080/00222349708220427

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Blends of poly(etheretherketone) (PEEK) and poly(etherimide) (PEI) were prepared directly during the plasticization step of an injection-molding process. The phase behavior and mechanical properties of the blends at both 23° and 125°C were studied. On the basis of differential scanning calorimetry (DSC) and dynamic-mechanical analysis (DMTA), all the blends exhibited a single amorphous phase. Both the as-molded and annealed blends showed at 23°C an overall synergistic behavior in ductility and in impact strength. The different crystalline levels influenced the properties but when there was no crystallinity effect, the intrinsic compatibility of PEEK and PEI was also clear and appeared as the probable main reason for the observed mechanical behavior. At 125°C the modulus of elasticity and tensile strength of annealed blends and the modulus of elasticity of as-molded blends were below the additive values. However, in the as-molded blends, the tensile strength and, mainly, the ductility, were synergistic.

ISSN:0022-2348    

DOI:10.1080/00222349708220428

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Morphology was studied in ternary polymer blends containing a polycarbonate (PC) matrix, a thermotropic HBA/HNA copolyester (LCP), and a poly(ethyleneterephthalate) (PET) or a poly-ϵ-caprolactone (PCL) as the minor components. LCP is immiscible in the other three polymers, while PET and PC exhibit a partial mutual miscibility, and PC/PCL is a miscible blend in the studied composition range, as evidenced byTgmeasurements. The melt rheology of the binary and ternary blends, studied using a capillary rheometer over a wide range of shear rates, shows unexpected dependency on the blend composition, which may partially stem from some degree of possible intermolecular reactions during melt blending, but is mainly due to morphological effects. The capillary extrudate morphology of the miscible blends (90PC/10PCL and 90PC/10PET) is that of a single-phase system. The morphology of the 85PC/15LCP and 60PC/40PET blends consists of a dispersed fibrillar minor phase oriented along the flow direction. However, no LCP fibrils, but rather particles, were observed in the [90/10]/15 [PC/PET or PCL]/LCP filaments. An intriguing morphology was observed in the [60/40]/15 [PC/PET]/LCP extrudates. As in the PC/LCP filaments, LCP particles, either spherical, elongated, or fibrillar, depending on the filaments' preparation conditions, were also observed in the ternary blends. However, they were engulfed by a PET-rich PET/PC layer. The preferred localization, engulfing the LCP particles rather than separately dispersed, of the PET-rich minor phase is expected by consideration of thermodynamic (interfacial tensions and “spreading coefficients”) and kinetic (viscosity ratios) parameters.

ISSN:0022-2348    

DOI:10.1080/00222349708220429

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Lamellar α-form single crystals of poly(butylene terephthalate) (PBT) have been prepared by our confined thin film melt polymerization method at polymerization temperatures of 190° to 210°C. These lamellar PBT single crystals have a thickness of ca. 100 Å, with (010) twinned crystals frequently observed. Six different zonal electron diffraction patterns were obtained, from which the lattice parameters are determined asa= 4.94 Å,b= 5.98 Å,c= 11.56 Å, α= 99.8°, β = 116.5°, and γ = 111.15° with a volume of 261.86 Å3, corresponding to a calculated density of 1.397 g/cm3for this “perfect” PBT single crystal. Chain packing and molecular conformation in the triclinic unit cell (space groupP1) were simulated by the Cerius2modeling software.

ISSN:0022-2348    

DOI:10.1080/00222349708220430

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor

摘要:

Commercial poly(ethylene terephthalate) yarn was heat set at various temperatures between 100° and 255°C, while held in the taut and slack states, respectively. The lattice parameters and crystal sizes of the as-drawn and heat-set fibers were measured with the help of wide-angle x-ray scattering while the long period was estimated from the small-angle x-ray scattering patterns. Combining the results from these two studies, information was obtained on the type of coupling between the crystalline and amorphous phases. From intensity scans and visual inspection of meridional layer lines of small-angle patterns, the tilt of the lamellar plane normals with respect of the fiber axis was estimated. The formation and reorganization of crystallites on heat setting and their temperature dependence have also been considered. The phase separation was shown to be much more distinct in the samples which were slack annealed at the higher temperatures. The implications of this study for structure-property relationships in polyester fibers are briefly discussed.

ISSN:0022-2348    

DOI:10.1080/00222349708220431

出版商:Taylor & Francis Group    

年代:1997

数据来源: Taylor