摘要:

Solutions of poly-p-phenylenebisbenzthiazole (PBZT) in phosphorous acid were examined in a rheogoniometer for shear response and relaxation as a function of concentration and temperature based on knowledge of the phase diagram of the system. It was found that the various regions of the phase diagram (i.e., the isotropic, the nematic, and the biphasic regions) and the various kinds of crystal solvates were reflected by their rheological behavior. This shows that rheological properties can be used to map, at least, the broad features of phase diagrams; conversely, the knowledge of the phase diagram is a prerequisite for the appreciation of rheological behavior. At the same time, the stress involved in the rheological tests themselves can modify, to a lesser extent, the positions of the phase dividing lines compared to those in the static system, which at appropriate points in the phase space can lead to stress-induced phase transitions. The principal rheological effect, here used diagnostically, is in itself novel; it is a spontaneous stress buildup in the stationary state of the rheometer after the stresses generated by the preceding shearing have decayed following stoppage of the rotor. The effect itself will be followed up in a separate study.

ISSN:0022-2348    

DOI:10.1080/00222349808220482

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The morphology of poly(ethylene naphthalene-2,6-dicarboxylate) (PEN) with a wide range of crystallinities has been determined by transmission electron microscopy (TEM). PEN thin films undergoing cold crystallization during annealing at temperatures aboveTghave been investigated after selective chemical staining and cutting in an ultramicrotome. Crystallization appears in amorphous areas distributed in a noncrystallizable matrix by formation of lamellae inside these amorphous areas. For low annealing temperatures (Tc∼ 145°C), the emerging morphology of PEN is seen prior to the occurrence of crystallinity in the x-ray diffraction patten. With increasing time and temperature of crystallization, the number and thickness of lamellae increase, in accordance with increasing crystallinity and crystalline thickness, as determined by x-ray investigations. Dendritic superstructures are also observed with a rough starting internal structure that becomes finer with increasing crystallization time.

ISSN:0022-2348    

DOI:10.1080/00222349808220483

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Two structural changes in poly(vinylidene fluoride) are known to be caused by application of an electric field: a phase transition from form II to form I via polar II and a poling-induced structural change in form I. In the phase, transition from form II to polar II, the molecular orientation must change. This is caused by an inversion motion near the domain boundary of the antiphase domain structure as the polar domain grows in the direction of theb-axis. In the phase transition from polar II to form I, the conformational change fromTGTGto the planar zigzag is caused by the molecular motion fromGTGtoTTTvia the incommensurate form. Two models have been proposed for the reorientation of the molecule in form I crystals induced by poling: the 60° step rotation model and the 180° reversal model. In the present study, the doubly oriented samples in which theb-axis (i.e., the direction of the dipole moment of the molecule) is parallel to the film plane and thec-axis is parallel to the machine direction were poled. No structural change could be observed due to the poling. This supports the 180° reversal model.

ISSN:0022-2348    

DOI:10.1080/00222349808220484

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The structure, phase composition, and temperature behavior of twotrans-1,4-polybutadienes (TPBs) were studied by means of x-ray scattering and differential scanning calorimetry (DSC) techniques. The two samples examined were (1) PB synthesized using an immobilized titanium-magnesium catalyst and (2) a random copolymer based on PB prepared with a homogeneous vanadium-containing catalyst used as a reference material. It was found that the nascent structure of the first PB involves three phases: crystalline, mesomorphic (low-temperature form), and amorphous. In the vicinity of 65°C, a first-order phase transition occurs. The system becomes biphasic and contains the high-temperature form of the mesophase, as well as the amorphous phase, component. Above 165°C, the polymer melts to form a single-phase isotropic melt with a structure typical of liquids. The lateral dimension of crystallites reversibly changes at the crystal-mesophase transition. It is suggested that during annealing of the mesophase formed by cooling of the isotropic melt, the chains acquire an extended conformation. Loss of regularity of the structure of macromolecules of TPB causes a reduction of phase transition temperatures, an increase of the imperfection of the crystalline phase, and a contraction of the temperature range of existence of the mesophase.

ISSN:0022-2348    

DOI:10.1080/00222349808220485

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The thermotropic polyester prepared from phenyl-terephthalic acid and hydro-quinone is highly crystalline, despite the probable random 2-and 3-disposition of the phenyl substituents. The x-ray pattern of melt-spun fibers contains 18 Bragg reflections that are indexed by a monoclinic unit cell with dimensionsa= 28.0 Å.b= 4.89 Å,c= 12.48 Å (fiber axis), and γ = 114.8°, containing monomer units of four chains. In theac-plane, the chains are arranged in pairs with the phenyl side chains interdigitated; successive pairs of chains are staggered by aboutc/2. We have used molecular mechanics modeling to simulate arrays of chains with random 2-and 3-disposition of the side chains on the terephthalic acid units and have compared the results with those for a similar structure in which all the substituents were at the 2-position. The refined model for random substitution is distorted, but the average separations of the monomer units are within experimental error of the observed unit cell dimensions, and their standard deviations are very similar to those derived from the line-broadening data. The potential energy of the random substitution model is only about 1 kcal/mol of monomer higher than that for the model with all 2-substitution, indicating the random substitution is not a problem for the formation of an ordered structure.

ISSN:0022-2348    

DOI:10.1080/00222349808220486

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

A brief introduction to the construction, operation, and performance of gas-filled proportional detectors with LC delay line readout is given. Measurements illustrating some of the fundamental physical principles involved and a method for obtaining accurate response curves for linear detectors are described. The reaction-diffusion properties of the detection process are illustrated by Turing patterns that can be observed during homogeneous irradiation.

ISSN:0022-2348    

DOI:10.1080/00222349808220487

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Time-resolved small-angle x-ray scattering (SAXS) has been used to probe the crystallization dynamics of poly(vinylidenefluoride) (PVF2) in its miscible blend with 30 wt% poly(l,4-butylene adipate) (PBA), as well as that of the pure PVF2homopolymer. The samples were quenched from 190°C (in the single-phase region for the blend) to 130°C to crystallize the PVF2. The results show that the isothermal crystallization rate of PVF2in the blend is slightly faster than that of the pure PVF2homopolymer. Based on the time variation of the invariant, the faster crystallization rate of the blend could be due to a faster growth of the PVF2lamellar stacks in the blend. The blend has a lower glass transition temperature than that of the homopolymer, hence the PVF2component in the blend has a higher mobility, which may result in a faster growth of the PVF2lamellar stacks. During the primary crystallization process, the pure PVF2homopolymer shows a decrease in the average thickness of the PVF2crystalline lamellae, suggesting a progressive insertion of thinner PVF2crystalline lamellae, but an approximately constant long period implies an increase in the average thickness of the interlamellar amorphous region. This increase suggests that the thinner PVF2crystalline lamellae insertion could occur in the free space leftbetweenthe existing lamellar stacks, rather thanwithinthe existing lamellar stacks. On the other hand, the blend shows a constant average thickness of the PVF2crystalline lamellae and a constant long period, suggesting that the insertion (if it exits) of PVF2crystalline lamellae should also be in the free space left between the existing lamellar stacks.

ISSN:0022-2348    

DOI:10.1080/00222349808220488

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

Poly(ethylene 2,6-naphthalate) (PEN) can be formed into the glassy state by rapid quenching, or it can form semicrystalline structure by either annealing the amorphous precursor at elevated temperature or slow cooling from the melt. During isothermal crystallization, relatively high crystallinity develops, with a time dependence described by the Avrami equation with the exponentn= 2.5. The activation energy for isothermal crystallization was determined to be 60 kcal/mol. The modified Avrami equation proposed by Velisaris was found to describe the primary and secondary crystallization reasonably well. For nonisothermal studies, PEN was crystallized by cooling at rates ranging from 1°C/min to 5°C/min from the melt. The models proposed by Ozawa and Nakamura were used to describe the nonisothermal crystallization kinetics of PEN. The exponentnand rate constantkof the nonisothermal crystallization determined by Ozawa analysis showed good agreement with those of isothermal crystallization. The Nakamura model predicted the development of relative crystallinity reasonably well. The crystal growth habit during isothermal crystallization was studied using an optical microscope under crossed polars. The dominant morphology was found to be spherulitic, indicating regime II growth. When PEN crystallized at high temperatures (245°C–265°C), elliptical-shaped hedritelike crystals were grown. These structures are formed under regime I kinetics. Crystals grown by continuous cooling showed highly distorted, banded spherulitic structure.

ISSN:0022-2348    

DOI:10.1080/00222349808220489

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

The microstructure of branched polyethylene/polypropylene blends, to 30% of polypropylene, has been investigated with the transmission electron microscope (TEM) following permanganic etching. The gross structure is one of polypropylene dispersed within a polyethylene-rich matrix, reflecting a degree of liquid/liquid phase separation in the melt. For concentrations of polypropylene above approximately 20%, heterogeneous nuclei cause polypropylene to crystallize before, and independent of, the polyethylene. At lower concentration, such nuclei become too dispersed to affect the majority of the polypropylene, almost all of which then solidifies after the polyethylene. Polyethylene tends to nucleate around polypropylene spheres, but the nature of the interface differs according to which polymer crystallizes first. If it is polyethylene, the two regions do not interpenetrate, but they do when polypropylene crystallizes first because polypropylene lamellae then grow out from polypropylene droplets into the polyethylene-rich matrix, strengthening the mutual interface. When lamellae cross into the matrix, their isothermal growth continues, but more slowly. They are thicker, further apart on average, and show little or no cross-hatching. These are all consequences of growth occurring at much reduced supercooling because of the lowered equilibrium melting temperature for polypropylene in the miscible blend. The presence or absence of similar changes may be used in other systems to indicate whether or not there is a local change of composition affecting polymeric growth from the melt.

ISSN:0022-2348    

DOI:10.1080/00222349808220490

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor

摘要:

In addition to homopoly(butylene terephthalate) (homo-PBT), the microhardness technique was applied to its multiblock copolymers for examination of the stress-induced polymorphic transition. Drawn and annealed at 170°C for 6 h with fixed ends in a vacuum, bristles of poly(ether ester) (PEE) having PBT as hard segments and poly(ethylene oxide) (PEO) with molecular weight 1000 as soft segments (PBT/PEO = 57/43 wt%) were characterized with respect to their microhardnessHat various stages of tensile deformation. A sharpHdecrease (by 20%) in a narrow deformation interval (2—4%) due to the stress-induced α-β polymorphic transition is observed followed by an increase and decrease ofH. The transition is registered at much higher relative tensile deformations (between 25% and 30%) compared with the case of homo-PBT. This is explained by the presence of a very soft amorphous phase of PEO, which deforms first. The same phase, distinguished by low viscosity in which the PBT crystallites are “floating,” is the reason for the very lowHvalues measured (between 25 and 35 MPa, depending on the amount of α-and β-modifications). The stress-induced polymorphic transition in copolymers of PBT with PEO is demonstrated for the first time, also supporting the recently proposed concept of the microhardness depression effect.

ISSN:0022-2348    

DOI:10.1080/00222349808220491

出版商:Taylor & Francis Group    

年代:1998

数据来源: Taylor