摘要:

The crystallization, melting, and glass-rubber relaxation characteristics of a series of random poly(phenylene sulfide) (PPS) copolymers based on sodium hydrosulfide (NaSH) and 1,4- and 1,3-dichlorobenzene monomers have been investigated; sample compositions included the PPS homopolymer, and copolymers incorporating 5%, 8%, and 10%meta-phenylene linkages. The introduction of increasing levels ofmeta-phenylene units led to a modest decrease in amorphous glass transition temperature (from 86°C [0%] to 74°C [10%]) and a strong decrease in equilibrium melting temperature as determined by Hoffman-Weeks extrapolation(from 331°C[0%] to 273°C [10%]). Wide-angle x-ray scattering (WAXS) studies indicated that the meta units were excluded as defects from the bulk crystal structure, with no variation in unit cell parameters. Measurements of glass transition temperature for the crystallized polymers revealed a significant positive offset inTgwith the presence of crystallinity owing to the constraints imposed by the crystallites on the amorphous phase. The magnitude of this offset was observed to decrease with increasing meta-phenylene incorporation, suggesting that the meta defects act to loosen the constraints imposed at the crystal lamellar surface. Determination of the rigid amorphous phase fraction atTg, however, revealed no dependence on copolymer composition.

ISSN:0022-2348    

DOI:10.1080/00222349608212379

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The relaxation characteristics of a series of random PPS copolymers based on sodium hydrosulfide (NaSH) and 1,4- and 1,3-dichlorobenzene monomers have been investigated via dielectric and dynamic mechanical methods. Comparison of dielectric and dynamic mechanical glass-rubber (α) relaxation temperatures measured for the crystallized copolymers with their amorphous analogues indicated that the presence ofmeta-phenylene defects led to a progressive reduction in the degree of constraint imposed by the crystallites on the relaxing amorphous chains. Estimation of the mobile amorphous phase fraction in the crystallized samples based on dielectric relaxation intensity revealed complete mobilization of the rigid amorphous phase aboveTgfor those copolymers containing 8% and 10%meta-phenylene units. Examination of the subglass (β) relaxation in the amorphous samples showed that the introduction of meta segments in the polymer chain backbone produced a positive offset in relaxation temperatures and a corresponding increase in apparent activation energy. These results indicated that the noncooperative phenylene motions of the β-relaxation occurred more readily in the PPS homopolymer.

ISSN:0022-2348    

DOI:10.1080/00222349608212380

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

A new application of energy dispersive x-ray diffraction to the kinetics of phase transitions is reported. The aim of this work is to provide a new investigation tool to follow the evolution of polymeric systems from the initial to the final phase. The theoretical treatment is developed both for constant density transitions and for the variable density ones. The experimental route is illustrated by an example.

ISSN:0022-2348    

DOI:10.1080/00222349608212381

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Rotational shear was imposed on two samples of polyethylacrylate rubber with differing types and amounts of gel in order to observe the stress-growth and subsequent quasi-steady state. The deformation rate was low, 0.02 s−1, to avoid slippage at the sample-instrument interface. The stress-growth behavior was represented by a strain-time correspondence of the form given by η+= (σε)αt, where the symbols are the stressing viscosity, η+, shear stress, σ, shear strain, ρ the corresponding extension ratio, α, and time,t. Eventually the growth of strain deviates from that predicted from the linear behavior at a point before the quasi-steady state is reached. The magnitude of shear strain at this point is estimated to be 0.36 to 2.6, depending upon the sample and temperature. Upon continued shearing, this strain memory gradually changes to a level of 0.66 to 1.35. These values were estimated from stress relaxation measurements after cessation of the quasi-steady state. The sample containing about 10% macrogel displayed a higher stress level and a larger magnitude of strain memory; this may be explained on the basis of an extensive entangling of long branches. The other sample, containing 70% microgel, gave a lower stress level and a smaller strain memory. The microgel is a crosslinked particle and less effective in entangling with its neighbors.

ISSN:0022-2348    

DOI:10.1080/00222349608212382

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The transport behavior of aromatic hydrocarbons in crosslinked nitrile rubber (NBR) membranes has been studied in the temperature interval of 28°–70°C. NBR was cured by different vulcanizing techniques, viz., efficient, conventional, peroxide, and mixed. The peroxide system showed the lowest solvent uptake; and the conventional system, the highest. The effect of penetrant size on the sorp-tion behavior of NBR was investigated. In all the systems the mechanism of diffusion showed an anomalous behavior. From the sorption data, the interaction parameter, molecular weight between crosslinks, Arrhenius activation energies, and thermodynamic quantities such as enthalpy and entropy changes for sorption processes were calculated. The first-order kinetic equation was used to study the sorption kinetics. Finally, the experimental sorption data were compared with theoretical predictions.

ISSN:0022-2348    

DOI:10.1080/00222349608212383

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

Poly(ethylene terephthalate) (PET) films were obtained by cold crystallization. Samples with different crystallinities were produced by annealing amorphous samples at different temperatures and for different times. The structural organization was analyzed by x-ray, density, transport properties, and Fourier-transform infrared measurements. The presence of trans conformer is associated with the appearance in the infrared spectrum of a band at about 973 cm−1. A correlation between the intensity of this band and the structural order of different semicrystalline PET films was proposed. Assuming that the fraction of the amorphous component (αa) can be determined from the sorption measurements, at low dichlorometh-ane activity, the fraction of the ordered phase (1-αa) could be correlated with the intensity of the infrared band at 973 cm−1. The result was consistently found to be higher than the fraction of crystalline phase derived from x rays and density measurements. This led to the suggestion that in these samples, besides the crystalline phase, there is also a fraction of a mesomorphic phase, impermeable to dichloromethane at low activity.

ISSN:0022-2348    

DOI:10.1080/00222349608212384

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor

摘要:

The morphology of the delta and gamma forms of syndiotactic polystyrene (sPS) was investigated using scanning electron microscopy. The crystalline forms, delta and gamma, characterized by the chains in helical conformation, were obtained by solvent-induced crystallization from the amorphous glassy polymer. Dichloromethane and acetone were used to obtain the delta and the gamma form, respectively. The last form was obtained by thermal treatments of the delta form, too. The surface of the solvent-crystallized samples shows a texture characterized by oriented elements and cavities, whereas the internal part is microspherulitic in the case of the delta form and is more complex in the gamma form. At variance, the gamma form obtained by annealing the delta form shows a much more regular microspherulitic texture.

ISSN:0022-2348    

DOI:10.1080/00222349608212385

出版商:Taylor & Francis Group    

年代:1996

数据来源: Taylor