摘要:

Phonon dispersions for helical s(2/l)2 syndiotactic polystyrene (sPS) are obtained and compared with the dispersions for its planar form. Optically active frequencies corresponding to the zone center and zone boundary are identified and discussed. In general the dispersion in the helical form is less as compared to the planar form. Some of the distinguishing features such as crossing over, repulsion of modes, and profile of the acoustic modes are discussed and possible explanations are given.

ISSN:0022-2348    

DOI:10.1080/00222349508219484

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The irradiation of polymers can bring about significant changes in the polymer structure and ultimately in their performance. Poly-(phenylene sulfide) (PPS) is an engineering polymer which is mostly used in applications where it is subjected to high-energy radiation. This paper reports the results of gamma irradiation of PPS. The technique of differential scanning calorimetry was used to monitor the changes in the melting and crystallization parameters of irradiated PPS. The heat of fusion was found to decrease upon irradiation, indicating a lower degree of crystallinity; however, a marginal increase was observed in the melting point. The nonisothermal crystallization of irradiated PPS indicated reduced crystallizability. This contention is also supported by the isothermal crystallization studies in which the crystallization rate for irradiated PPS was found to be decreased. The retardation in the crystallization of irradiated PPS has been attributed to the possible changes in the structure of the polymer.

ISSN:0022-2348    

DOI:10.1080/00222349508219485

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Previous work has shown that under mechanical deformation, three grades of polyethylene which differ mainly in terms of branch content exhibit two yield points. The first yield point occurs at low applied strains and marks the onset of temporary plastic deformation; the second yield point occurs at higher strains and marks the onset of permanent plastic deformation, and is associated with the development of a neck in tension. The present work describes structural measurements carried out on these three polyethylene samples before and subsequent to different levels of tensile deformation. X-ray diffraction, transmission electron microscopy, and optical microscopy were carried out on the isotropic materials and after the first and second yield points under tensile deformation. The results show that the first yield point marks the onset of a recoverable reorientation process of the lamellae within the spherulites, with little or no destruction of the lamellae themselves. This reorientation is explained in terms of the relative lamellar orientation to the draw direction due to the initial spherulitic structure. The reorientation leads to the lamellae oriented predominantly at approximately 45° to the draw direction. The second yield point is associated with the destruction of the lamellae lying at 45° to the draw direction bycshear.

ISSN:0022-2348    

DOI:10.1080/00222349508219486

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

A theory is proposed indicating that the induction time characterizes the nucleation process in polymer crystallization. A microscopic method has been developed to monitor early stages of the spherulite growth. Using the method, based on photometric measurement of the depolarized light intensity during isothermal crystallization from the melt, nonzero nucleation induction time values were found for polypropylene (PP). From comparison of the theory with the experimental results obtained from polypropylene crystallization, it follows that the formation of PP crystal nuclei on admixtures is affected both by the formation of the first crystalline layer and by the size of the critical nucleus.

ISSN:0022-2348    

DOI:10.1080/00222349508219487

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Early stages of polymer crystallization are pronouncedly influenced by the presence of a foreign phase, the surface of which may increase the formation of crystal nuclei. However, measurement of the initial stages of the crystallization with the aid of conventional methods is extremely difficult. The possibility of using the Avrami equation in its classic form for that purpose is discussed and found inappropriate. Characterization of the initial crystallization stages is corroborated using the crystallization induction time, which represents the most probable value of the nucleation times of individual spherulites. The induction time of polypropylene without a nucleation agent was found to be positive.

ISSN:0022-2348    

DOI:10.1080/00222349508219488

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

X-ray diffraction profile analysis was applied to the study of polymers by Buchanan and Miller for the first time in 1966 [l]. They used the double-peak Fourier analysis described by Warren and Averbach [2]. In the case of polymers, two peaks of different orders of diffraction are not so easy to record, Single-peak analysis has been developed by many people [3-7] but very few results have been reported for the deformed structure of polymers. In this work the fine structure of a series of deformed i-polypropylene (i-pp) films was studied by a single-peak profile analysis based on the data obtained from a special arrangement of samples in the diffractometer. The crystallinity and degree of orientation during deformation were also studied.

ISSN:0022-2348    

DOI:10.1080/00222349508219489

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The effects of thermal treatments on the phase organization in some polymorphic forms of syndiotactic polystyrene have been investigated. In particular the thermal stability of the delta form was studied using x-ray diffraction and infrared spectroscopy as analytical techniques. The obtained results show that this form, characterized by chains in helix conformation, is affected by non-equilibrium phenomena. Moreover, the transition from the delta to the gamma form occurs through an intermediate form which maintains the helix chain conformation but partially loses the crystalline order.

ISSN:0022-2348    

DOI:10.1080/00222349508219490

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The molecular orientation, thermal behavior, and crystal lattice structure in extruded strands of a thermotropic liquid crystalline polymer (LCP) were studied with wide-angle x-ray diffraction (WAXD) and differential scanning calorimetry (DSC). The purpose of this work is to elucidate the effects of draw-down ratio and annealing treatment on the structure development in the LCP strands. The crystal orientation function markedly increased with increasing draw-down ratio, but the increase of orientation function saturated at higher draw-down ratio. Annealing treatment below 250°C slightly increased the degree of orientation, whereas the molecular orientation was relaxed by annealing at 270°C. In addition, the thermal properties and crystal lattice structure were sensitive to the annealing treatment. The change of DSC curves with annealing temperature suggested that the initial crystalline texture of as-extruded samples was reorganized into a more ordered structure by the annealing treatment. Draw-down ratio had some effects on the thermal properties. The molecular orientation facilitated the crystallization during annealing.

ISSN:0022-2348    

DOI:10.1080/00222349508219491

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

The structure and properties (in both the molten and solid state) of polypropylene and the copolymers of propylene may be modified by gamma irradiation, which is frequently used for sterilization in diverse medical applications. Copolymers of propylene [poly-(ethylene-co-propylene) and poly-(ethylene-b-propylene)] nucleated with 1,3,2,4-di-para-methylbenzylidene (MDBS) were irradiated with γ rays. The radiation dose was 5 Mrad. The influence of radiation on the structure and properties was measured and is discussed. The molecular weight decreased to about 50% of the value of the nonirradiated samples [Mn(5 Mrad)/Mn(0 Mrad) = 0.51] while a slightly lower decrease of the molecular weight was observed for the samples nucleated with MDBS. For the irradiated polymers, in all cases, a decrease of the DSC determined melting temperature of the propylene and ethylene blocks was noted (ΔTm= 1°C to 5°C); on the other hand, for the ethylene blocks a certain increase in the crystallization temperature (1°C to 5°C) as a result of the gamma irradiation was ascertained. A significant decrease in the melt viscosity was noted (about 9 times, for the lower shear rates, and about 1.5 times, for the higher shear rates), as well as a decrease in the impact resistance due to the gamma radiation.

ISSN:0022-2348    

DOI:10.1080/00222349508219492

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor

摘要:

Suspension-polymerized polyvinyl chloride (PVC) is a powder consisting of particles 100 ∼ 150 μm in diameter, calledgrains. The grains have a unique hierarchical structure consisting of a skin and internal tight agglomerates of about 10 μm diameter. The agglomerate consists of primary particles of about 1 μm diameter, which in turn consist of domains of about 0.1 μm. The domain is composed of microdomains of about 10 nm. In this work, mechanical mixing of PVC with a miscible polymer, nitrile rubber containing 30% acrylonitrile (NBR-30), was conducted in an internal mixer and the significant events in the mixing period of 150 sec were examined with scanning and transmission electron microcopes. The process of mixing was very different from that of two amorphous polymers in that the skins of the PVC grains were first peeled off and then the agglomerates were broken down to the subsequent smaller particles, eventually becoming microdomains dispersed in the rubber. The NBR was found to be an effective medium for the breakdown of the particulate structure of PVC. The high storage modulus of NBR-30 and the strong adhesion due to the molecular mixing at the interface of the two polymers effectively transmitted the stress from the machine to the PVC particles. Also, the viscous heating resulting from the high loss modulus of NBR-30 brought about a temperature rise which weakened the interparticulate bonds and melted some microcrystallites of PVC, thereby facilitaing the disintegration of the PVC particles.

ISSN:0022-2348    

DOI:10.1080/00222349508219493

出版商:Taylor & Francis Group    

年代:1995

数据来源: Taylor