摘要:

In our previous study on thermodynamic equilibrium at infinite dilution, chain segments at the periphery of a polymer coil were shown to be more extended than the average expansion of the coil. This is because such segments are in contact with pure solvent, whereas inside the coil there is a concentration of the segments. In a subsequent work examining the low concentration range, this excess extension of the segments was shown to diminish as the concentration was increased. Having more and more neighboring coils, the chain segments at the periphery of the coil have less and less excess extension, because the concentration difference between the inside and the outside of the coil decreases. In the present work the range below the theta temperature is examined; for polymers of finite molecular weights there are soluble ranges including partial and critical miscibility. When the limits of miscibility are approached either by lowering the temperature at a constant concentration or by increasing concentration at a constant temperature, the excess extension of the chain segments at the periphery of the coil disappears. The calculation makes use of a modified Maron theory. The required input data are intrinsic viscosity and polymer-solvent interaction parameter as a function of polymer concentration. These parameters must be known as functions of temperature and molecular weight. The example used in this work is the polystyrene-cyclohexane system.

ISSN:0022-2348    

DOI:10.1080/00222349308215476

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Structural properties of random propylene ethylene copolymers with low ethylene contents between 0 % and 11 mol % were investigated using mainly X-ray techniques. One of the essential issues of this paper is that a cocrystallization of the comonomer units does not take place. This is based on the following observations: The long period and the thickness of the crystalline lamellae decrease with increasing ethylene content. The electron density difference between crystalline and amorphous phase, however, as determined with the invariant, does not show any changes. The heat of fusion, normalized to the degree of crystallinity, does not decrease. This leads to the conclusion that the observed changes are not due to an incorporation of the ethylene units as defects, but to a reduced extension of the crystalline regions, now confined by the ethylene units which interrupt the helical chain conformation. The γ-modification, detectable by means of wide-angle X-ray diffractometry from an ethylene content of 5 mol % on, partially converted into the α-modification on annealing.

ISSN:0022-2348    

DOI:10.1080/00222349308215477

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The effects of different plasticizers on the structural features of PVC were investigated. Particular emphasis was on the fine structure as denoted by small-angle X-ray scattering (SAXS). The influence of plasticizer content on the mechanical response was followed by dynamic mechanical spectroscopy. The particular plasticizers investigated principally were diethylhexyl succinate and dibutyl phthalate, although other phthalate plasticizers were investigated including dibutyl succinate and dibutyl sebacate. It was noted that structural features at the 100– to 200–å level were systematically influenced by the type and amount of plasticizer and in particular by its solubility parameter. To account for the observed behavior, a model was proposed which speculates that the absorption of plasticizer is nonuniform within the system and in particular within the local regions where crystallites exist. The level of swelling of plasticizer into the amorphous regions at some distance from the crystallites is different (and greater) than that in disordered regions near the crystallites. This degree of difference in swelling, however, is dependent upon the solubility parameter of the plasticizer. This model, which is in general support of an earlier model, also helps to account for the large difference in temperature-dependent dynamic mechanical behavior introduced by the different plasticizer species.

ISSN:0022-2348    

DOI:10.1080/00222349308215478

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

In the presence of solvent, the catalytic rate in a polymeric metal complex catalyst (2)-substrate (1)-solvent (0) system is related to the solubility parameters (δi) of the three components. According to thermodynamic and kinetic theories, an approximate relationship between the reaction rate and solubility parameters has been deduced as follows:

ISSN:0022-2348    

DOI:10.1080/00222349308215479

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

The phospholipids exhibit a variety of phase transitions such as gel ⇌ liquid crystalline and pretransition which involves the rotation of the whole hydrocarbon-type chain. These transitions are accompanied by specific heat anomaly. In some situations the order of these transitions is believed to be fractional in nature, and in some other cases the transition has a fine structure which results from the presence of transitions due to the multiple components in the system. In the present communication, Zimm and Bragg theory has been suitably modified to interpret the specific heat measurements in the phosphatidylcholines containing sulfur atoms in thioester and thioether linkages. The thermodynamic parameters associated with the transition are those reported by Hajdu and Sturtevant. The sharpness of transition (△H|MXTc) and the variation of nucleation parameter [sgrave] are consistent with the structural differences in the phospholipids studied. The asymmetry at the tail ends of specific heat is a result of the presence of multiple components and asymmetry associated with the head group interaction.

ISSN:0022-2348    

DOI:10.1080/00222349308215480

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

Simultaneous interpenetrating polymer networks (SINs) were prepared from nadic methyl anhydride-cured epoxy resin and bismaleimide (BMI) coreacted with one of three different allylesters under different gelling conditions for both the epoxide resin and BMI-allylester comonomer. Dynamic mechanical and tensile properties were measured for these SINs. The rubbery modulus increased monotonically with increasing gel time ratio of the epoxide resin to that of the comonomer. On the other hand, the glass transition temperature (Tg) and tensile strength ([sgrave]b) exhibited maxima when the two gelations were most nearly simultaneous. This behavior was observed in all series of the SINs. irrespective of the kinds of the allylesters. The behavior of the rubbery modulus can be explained by the hypothesis that the BMI-allylester copolymer networks become more continuous as the comonomer reacts faster than the epoxide resin does. In addition, the maxima inTgand[sgrave]bcan be explained in terms of the greater extent of molecular mixing attained in a truely simultaneous gelation.

ISSN:0022-2348    

DOI:10.1080/00222349308215481

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

A numeric method for calculating the molecular weight (MW) and molecular weight distribution (MWD) from the terminal zone of viscoelasticity of narrow distribution linear polymers had been developed by one of the authors in previous work. In this work a modification of our previous method, by considering the shift factor, for calculating the polymer molecular weight and molecular weight distribution from the terminal zone of the storage modulusG'(ω) of a broad molecular weight distribution linear polymer is presented. Two narrow distribution polystyrene and one broad distribution polystyrene samples were used as a demonstration to calculate the molecular weight distribution. Good agreement between the results calculated from our method and those obtained from GPC measurements was found.

ISSN:0022-2348    

DOI:10.1080/00222349308215482

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor

摘要:

As-cast poly(ethylene terephthalate) (PET) film was drawn at 90 °C in water to different draw ratios between 2 to 5, resulting in flow-drawn samples which had no detectable birefringence or crystallinity. However, the shrinkage characteristics and crystallization kinetics of these samples were indicative of the presence of global orientation. The molecular networks in these samples were characterized within the framework of the statistical theory of rubber elasticity. For this, load-elongation studies on the flow-drawn samples were first made, and the network draw ratio was then estimated by matching these curves with that for the as-cast film through horizontal shifting. The network characteristics of the flow-drawn samples of draw ratios 2 to 5 were found to be essentially similar, and it was noteworthy that the network had fewer entanglements compared to those in axially oriented crystalline PET films or in as-spun PET fibers. This observation is consistent with the models of flow-drawn samples. A new method for shifting the load-elongation curves for estimation of network draw ratio has been proposed and is shown to give consistent results.

ISSN:0022-2348    

DOI:10.1080/00222349308215483

出版商:Taylor & Francis Group    

年代:1993

数据来源: Taylor