摘要:

Normal modes and their dispersion are obtained for syndiotactic polystyrene (sPS) in the planar zigzag and s(2/1)2 helical conformations. Comparison is made with the spectra of isotactic polystyrene (ips) to identify the conformation-sensitive modes; for example, the 1221 cm−1mode is identified as a skeletal mode sensitive to chain structure and it also shows maximum dispersion. Bunching of dispersion curves and their intersection are among the characteristic features which are discussed. A check on the dispersion curves below 400 cm−1is made from the spectra of polystyrene oligomers.

ISSN:0022-2348    

DOI:10.1080/00222349408248083

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The structural relaxation of an atactic polystyrene under sub-Tgannealing at 50°C and at 70°C was investigated by the radial distribution function (RDF) derived from its wide-angle x-ray scattering (WAXS). By recording the changes in RDF (i.e., ΔH(r), whereris the radial distance from an arbitrary reference atom) after it had been annealed for a certain period of days, and taking the integration of ΔAH(r)2dr, we found that the atom density within structural domains in a size below 15 Å was changed dramatically, but that the tendency leveled off as annealing proceeded. However, the atom density outside the domains is barely changed by sub-Tgannealing. The size of the domain is similar to the statistical segment length reported in the literature. The behavior of the domains—that the segmental relaxation inside the domains in the initial sub-Tgannealing is unconstrained by their neighbors outside the domains—is also similar to the behavior of the statistical segments. The domains are believed to be composed of a statistical segment across the center, which has about 6 styrene repeat units, and 6 equal-distanced parallel segments tangent to the edge of the domain. On the other hand, as the annealing temperature is closer to the glass transition temperature, the frozen unstable chain segments reach equilibrium sooner and with less disturbance in their conformation.

ISSN:0022-2348    

DOI:10.1080/00222349408248084

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Five networks based on a diglycidylether of bisphenol A-dimethacrylate prepolymer of molar mass 1100 g· mol−1crosslinked with 35%, 40%, 45%, 50%, and 55% styrene mass fractions were characterized by Fourier transform infrared spectroscopy, cross-polarization/magic-angle spinning, nuclear magnetic resonance spectroscopy, styrene extraction, and rubber elasticity measurements. Various structural models were built up from the available analytical data, and their crosslink density was compared to the experimental data derived from rubber elastic moduli. Globally, network models based on a purely statistical approach but taking into account the incomplete cure conversion make it possible to predict the trends of modulus variations. The kinetic data on copolymerization show, however, that the material is diphasic, one phase being composed of the vinylester network and the other of practically pure polystyrene. We tried to take this heterogeneity into account in modulus predictions using a simple mixture rule, but the method failed for high styrene mass fractions (s≫ 0.45), for which the experimental modulus value is about 50% higher than the predicted one. The results suggest the existence of a morphological transition for a critical styrene weight fraction between 40% and 45%.

ISSN:0022-2348    

DOI:10.1080/00222349408248085

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

A simple technique is developed for predicting the dynamic viscosity from the steady shear viscosity data of polyelectrolyte solutions. The Carreau-type viscosity equation is used to represent both the steady shear and dynamic viscosity data. A shift factor β, defined as the ratio of the dynamic time constant to the steady shear time constant, is introduced. This dimensionless parameter enables all the dynamic viscosity data to be shifted onto the steady shear viscosity curve. The parameter β correlates well with the zero shear viscosity and has the following form:

ISSN:0022-2348    

DOI:10.1080/00222349408248086

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The degree of crystal perfection in semicrystalline poly(ethy1ene terephthalate) (PET) fibers and films, produced under different conditions, was estimated in terms of the Ruland-Vonk crystal disorder parameter with the help of x-ray diffraction studies. Fibers and films which were produced by low-speed melt extrusion followed by drawing in water at low strain rates in a low-temperature environment had the lowest x-ray crystallinity and the most imperfect crystals. Further evidence of greater crystalline imperfection in these samples was provided by their x-ray intensity plots and melting endotherms. The low x-ray crystallinity and high defect density shown by these samples have been attributed to (a) the slow drawing at relatively low temperature, which does not provide the necessary environment for crystallization; and (b) the presence of very small nuclei which may be crystalline remnants that are retained at the low speeds of production and which can act as seed crystals around which crystallization occurs with incorporation of defects. On heat setting of these samples at elevated temperatures, crystal perfection improves significantly. On the other hand, samples produced by drawing at a relatively higher effective temperature and high velocity have higher crystallinity and more perfect crystals which crystallize further on heat setting without any significant enhancement of crystal perfection, except at very high temperatures.

ISSN:0022-2348    

DOI:10.1080/00222349408248087

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Two nominally equivalent grades of polypropylene designed for similar applications were extruded into sheets, with one surface chilled rapidly, giving a smooth texture; and the other being rougher, with a structure of spherulites separated by crevices formed by retraction of the last traces of molten polymer. Differences in the two grades gave rise to differences in spherulitic morphology, in terms of both spatial distribution of spherulites among die lines and the proportions of α- and β-spherulites. The sheets were subjected to three kinds of deformation: unrestricted monoaxial, restricted monoaxial (plane strain), and biaxial. The effects of surface variations persisted right through the drawing process. The morphology of the rough surface in particular was observed (a) to dominate the process of deformation, especially necking as it initiated at surface crevices between spherulites; and (b) to affect the optical quality of the final product, most significantly in the structure and number of haze rings. Legacies of β-spherulites in fully drawn films can be observed at all levels, from the unaided eye down to lamellar structures observed in the transmission electron microscope.

ISSN:0022-2348    

DOI:10.1080/00222349408248088

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

A numerical method is presented for the accurate determination of the viscoelastic spectra from dynamic response function data. First, the technique is validated by application of the method to dynamic data for a hypothetical polymer with an assumed relaxation spectrum. It is shown that the method produces a single retardation spectrum which when used in viscoelastic integral equations yields excellent agreement with the functions from which the spectrum was determined in the first place. Also, upon applying computed transient functions to the linear viscoelastic convolution integral, excellent agreement is obtained. Finally, the method is applied to dynamic mechanical data for a vinyl ester resin system. It is shown that a single retardation spectrum can be generated that upon integration in appropriate integral equations yields response functions in excellent agreement with experiment.

ISSN:0022-2348    

DOI:10.1080/00222349408248089

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

Solution blends of a modified polyacrylonitrile (PAN) with polyurethane (PU) ionomers were prepared inN,N-dimethylformamide. PU anionomer (PUAn20) blends showed phase separation of up to several micrometers, whereas almost homogeneous morphology was obtained with PU ca-tionomer blends. In PU cationomer blends, the tan δ peaks of the soft segments of PU and PAN moved toward lower and higher temperature, respectively. Notably, the tensile strength of PU cationomer blend at 30/70 (PU/PAN by weight) increased to over 2.5 times that of PAN; for this blend the concentration of PU cation approximately matched that of the sulfonate anion of the modified PAN. The synergistic effect was interpreted in terms of ionic interactions between the main chains of the polymers.

ISSN:0022-2348    

DOI:10.1080/00222349408248090

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor

摘要:

The effect of different fluid environments on the properties of poly(ether sulfone) has been studied. It has been found that, among the solvents studied,ortho-dichlorobenzene greatly affects the ductility of the polymer, producing a catastrophic embrittlement. This results even after short sorption times at 20°C. Among the possibilities of solvent-induced crystallization, antiplasticization, chemical attack, and surface cavitation, the last seems to be the most responsible for the observed embrittlement.

ISSN:0022-2348    

DOI:10.1080/00222349408248091

出版商:Taylor & Francis Group    

年代:1994

数据来源: Taylor