摘要:

It has been found that methylene stretching bands in polyacrylonitrile (PAN) occur at higher frequencies and have a lower intensity than in other hydrocarbon compounds. This is a spectroscopic manifestation of nitrile group intermolecular repulsion in PAN, which leads to helical winding of the polymer chains. The distortion of the elementary unit geometry occurs on spiraiization, changing the methylene band parameters. On heating, the polymer spiral unwinds, which restores the normal geometry, band position, and intensity. The unwinding of a polymer chain represents a thermal transition, occurring in PAN at a temperature of about 180°C. This transition is a necessary prerequisite for initiation of cyclization.

ISSN:0022-2348    

DOI:10.1080/00222349908212434

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

A nonconventional energy-depressive X-ray diffraction method applied to phase transitions (EDXD-PT) was used to measure polymer crystallization rates. It enables one to follow the kinetics of phase transitions by a direct comparison of diffraction patterns collected during the process and suitably normalized with respect to X-ray absorption spectra taken with the same time scan. With this technique, the memory that a polymer retains of its thermal history is investigated by correlating the crystallization rates to the residence time in the melt.

ISSN:0022-2348    

DOI:10.1080/00222349908212435

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

This paper reports the small-angle X-ray scattering (SAXS) study and flow- rate measurements of semipermeable porous polysulfone (PS) and sulfonated polycarbonate-doped polysulfone (SPC-PS) membranes. Flow rates of sodium chloride, sodium sulfate, and polyethylene glycol solutions through these membranes have been measured. For all the solutions, the flux through the SPC-PS membrane was found to be close to 1.65 times that through the PS membrane. SAXS measurements show that the pore morphologies of two membranes are mass fractal in nature with different fractal dimensions. The difference in flow rates through these membranes is explained in terms of porosity and fractal dimension.

ISSN:0022-2348    

DOI:10.1080/00222349908212436

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The Zimm and Bragg theory for helix-to-coil transitions has been suitably amended and applied to explain the two-step transition in the DNA triplex. The experimental measurements reported elsewhere have been successfully interpre- ted. The order-order and order-disorder transitions associated with the melting of the DNA triple helix, with and without netropsin binding, were characterized by the nucleation parameter, enthalpy change, sharpness of transition, and heat capacity. The destabilization of the triplex and stabilization of the duplex on netropsin binding are reflected in these characteristic parameters.

ISSN:0022-2348    

DOI:10.1080/00222349908212437

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Polyacrylonitrile (PAN) fibers were reinforced with three different types of polyurea (PU), which were synthesized in the presence of PAN dissolved in dimethyl formamide (DMF) (referred to as an in situ blend). Solution viscosity increased with increasing PU content, with viscosity yield seen at high PU content and an extended Newtonian plateau at low PU content. The reinforcing effect was greater with linear PUs, among which PU prepared from toluene diisocyanate (TDI) and ethylene diamine (EDA) gave the greatest improvement.

ISSN:0022-2348    

DOI:10.1080/00222349908212438

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The physical properties of commercially obtained metallocene and Ziegler-Natta (ZN) polyethylenes (PEs) were correlated to their backbone structure (i.e., the composition, ethylene sequence distribution, and short-chain branch length) using dynamic mechanical analysis and thermal fractionation. The uniformity or heterogeneity of the 1-olefin branch distribution on the ethylene backbone profoundly controls the mean crystalline ethylene segment lengths, the degree of crystallinity, the beta transition temperature and intensity, the plateau modulus, and the presence or absence of the alpha transition.

ISSN:0022-2348    

DOI:10.1080/00222349908212439

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Oxidation reactions are one of the main reasons for the failure of polymeric materials. Antioxidants, compounds designed to protect against oxidation, must meet three main requirements for success: (1) an efficient antioxidative mechanism, (2) compatibility with the oxidizing polymer, and (3) permanence Within the oxidizing polymer. Common antioxidants are low molecular weight materials that can easily diffuse, leach, or evaporate from the polymer they are designed to protect. An increase in the molecular weight of the antioxidant not only decreases diffusion and volatility, but also decreases compatibility since most high molecular weight polymers will not mix. Selective sterically hindered phenolics, however, are concurrently antioxidants and “compatibilizers” through hydrogen bond formation. Carefully designed copolymers containing a small percentage of 2,6-diisopropyl-4-vinylphenol were found to mix intimately with two readily oxidizing polyethers and to protect them against oxidation.

ISSN:0022-2348    

DOI:10.1080/00222349908212440

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Local compositional variations in blends are examined using micro-Raman im- aging. Raman images are compared with morphologies detected by transmission electron microscopy (TEM). In isothermally crystallized blends, with morpholo- gies composed of clusters of crystals rich in deuterated linear polyethylene (DLPE) in a matrix of quenched material, the results from both techniques are comparable. These results show that compositional variations of about 30% can be detected by Raman imaging on a scale as small as 2 fim. In rapidly quenched blends exhibiting morphological phase structures on a scale of 1–2 f Am in the electron microscope, the Raman images were homogeneous to within the level of uncertainty in composition. However, the scale of the morphological phase structures examined is close to the limit of resolution of Raman imaging. It is shown that if there are compositional differences between the morphological phases, then these must be less than 3% in the samples studied.

ISSN:0022-2348    

DOI:10.1080/00222349908212441

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

Blends of cellulose (CELL) with polyethylene glycol (PEG) were prepared from a mixed solution in dimethyl sulfoxide (DMSO)-paraformaldehyde (PF) by coagulation in a nonsolvent. The state of miscibility of the blends obtained over the composition range of 95/5 to 30/70 was characterized by wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry (DSC). The WAXD and DSC results indicate that the crystallinity and phase transition temperature of PEG within blends decreased with increasing content of CELL. The analysis of phase transition temperature depression shows that CELL and PEG within the blends were miscible. The investigation with Fourier transform infrared (FTIR) spectroscopy demonstrated that there was a strong intermolecular hydrogen bond between these two components, which perhaps was the intrinsic reason why CELL and PEG exhibited a high level of miscibility.

ISSN:0022-2348    

DOI:10.1080/00222349908212442

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor

摘要:

The phase transition properties of blends of polyethylene glycol (PEG) with cellulose (CELL) prepared from solution inN,N-dimethylacetamide/lithium chloride (DMAC/LiCI) and those from solution in dimethylsulfoxide/paraform- aldehyde (DMSO/PF) were found to be completely different. The differences of the phase transition properties were probably related to the different miscibilities of these two polymers in the two solvent systems. In DMAC/LiCl, the miscibility of CELL and PEG was limited; the composite obtained exhibited a solid- liquid phase transition and had a small phase transition enthalpy. However, in DMSO/PF, these two polymers had a high level of miscibility; the composite obtained exhibited a solid-solid phase transition and had a large phase enthalpy. It is suggested that the differences of miscibility and the phase transition properties were caused by the different dissolving mechanisms of CELL and the different interactions in these two solutions.

ISSN:0022-2348    

DOI:10.1080/00222349908212443

出版商:Taylor & Francis Group    

年代:1999

数据来源: Taylor