摘要:

Experimental data are presented on thermal field-flow fractionation (TFFF) of anionic polystyrene (PS) samples in the rangeM= (4–12)·106. They show extensive degradation of macromolecular chains at relatively low rate gradients (G< 30 s−1). The possibility of the influence of relaxation effects on the shape of fractograms and the elution volumes of the samples was taken into account. Mean concentrations in accumulative zones were evaluated. It was shown that, in all cases when degradation was observed, the accumulative zones are the layers of entangled macromolecules. The use of the scaling approach made it possible to simulate layer extension toward the channel center under the influence of the rate gradient. It was shown that, during stretching, the layer is destroyed into blobs, the size of which is determined by experimental conditions. An expression for the critical gradient leading to layer degradation was derived. Quantitative evaluations of fragment sizes and critical gradients obtained from the model are in good agreement with experimental data. The model developed for specific experimental conditions confirms the proposed general mechanism of the so-called shear degradation of macromolecules. The physical picture of degradation in the TFFF channel was considered.

ISSN:0022-2348    

DOI:10.1081/MB-100102473

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Dynamic mechanical and thermal behavior of ordered off-stoichiometric polyurethane (PU) systems, before and after the gel point, based on the mesogenic diol 6,6′;-[ethylenebis(1,4-phenyleneoxy)]dihexan-1-ol (D),2(4)-methyl-1,3-phenylene diisocyanate (DI), and poly(oxypropylene)triol (T) were studied. Polymer samples were prepared at various initial molar ratios of the reactive groups,r= [OH]T/[NCO]DI/[OH]D, ranging from 1/10/9 to 5/10/9 (the ratio [NCO]DI/[OH]D= 10/9 was constant); the total mole ratio of hydroxy (OH) and isocyanate (NCO) groups, rOH = [OH]/[NCO] = ([OH]D)/[NCO]DI,changed from 1 to 1.4. Dynamic mechanical measurements during the curing reaction showed that the power law parameters that characterize the critical gel state (gel strengthSand relaxation exponentn) are dependent on the initial composition (the ratiorOH). The gel-point critical ratio of reactive groupsrcOH, found during curing in the ordered state of the diol (at low curing temperature), has revealed that the critical gel (CG) structure is determined by a contribution of strong physical interactions as well as chemical junctions and does not correspond to pure chemical gelation (CG structure formed at low temperature exhibits flow at elevated temperatures in the isotropic state). This fact suggests that formation of the mesophase enhances the connectivity of the molecular structure at the gel point. Dynamic mechanical behavior of fully cured chemical networks (rOH<rOHc) and un-cross-linked (rOH>rOHc) samples (and a CG sample) has also been investigated. Decreasing the rOHratio (increasing concentration of chemical cross-links in the systems) inhibits conformational rearrangements required for ordering; at the same time, the intensity of the slow relaxation process in the rubbery region decreases.

ISSN:0022-2348    

DOI:10.1081/MB-100102474

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The issue of isotropic-anisotropic equilibria contributions to structure formation of phase inversion (PI) membranes of semiflexible polymers such as cellulose derivatives is addressed. Two types of ternary phase diagrams pertinent to the PI of semiflexible polymer dopes are suggested, and modes of phase separation that control structure formation are described. Alternative interpretations for certain earlier structural observations in cellulose acetate (CA) membranes are proposed. Discussions of the lyotropic polymer gelation modes and the effect of glass formation on the progress of PI structural transformations are included.

ISSN:0022-2348    

DOI:10.1081/MB-100102475

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Bulk high-density polyethylene (HDPE) films crystallized from isotropic melts show a spherulite structure; the tensile stress-strain (S-S) curves of such melt-pressed (M-P) films show a characteristic shape caused by necking. On the other hand, extruded-blown (E-B) HDPE films show an anisotropic structure in which lamellae are stacked normal to the extrusion direction. The S-S curves of E-B films showed various shapes for various elongation directions relative to the lamellar orientation direction. When drawn perpendicular to the extrusion direction (parallel to the lamellar orientation direction), the S-S curve showed typical necking behavior resulting from unfolding from one part of the lamellae. When drawn diagonal to the extrusion direction (diagonal to the lamellar orientation direction), the yield stress was smaller due to slippage between the lamellae. When drawn parallel to the extrusion direction (perpendicular to the lamellar orientation direction), the S-S curve was smooth, but it was shown that micronecking occurred repeatedly in various parts of the specimen. The characteristic shape of the S-S curve of the M-P films was reconstructed as the sum of all S-S curves of the E-B films in various directions. From a structural viewpoint, the deformation behavior of the bulk film, which has a spherulite structure, can be simulated at least qualitatively by the deformation behavior of anisotropic E-B films in various directions.

ISSN:0022-2348    

DOI:10.1081/MB-100102476

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Melting behavior in associating poly(4-vinylphenol)–N,N-dimethyloctadecylamine mixtures (PVPh-NDOA), which form comb-shaped aggregates, was investigated with various end functionality of the NDOA. Melting temperatures of the comb-shaped aggregates were depressed with slightly decreased end functionality of the surfactant. However, it was found that the repeating unit of PVPh and the dimethylamino group of NDOA formed 1:1 stoichiometric aggregates regardless of the decrement of the end functionality of the NDOA. In addition, it was shown that the excess surfactants, which did not have polar groups on the alkyl chain ends, act only as an impurity for the side-chain crystals of the PVPh-NDOA aggregates.

ISSN:0022-2348    

DOI:10.1081/MB-100102477

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The rigid polymer poly(p-phenylene benzobisthiazole) (PBZT) was crystallized from dilute solution. Electron microscopy showed that, on quenching, flat fibrils several nanometers thick were produced. Subsequent heat treatment in a solvent changed the morphology from fibrillar into “segmented ribbon” structure. Isothermal crystallization at a temperature of about 30°C below the dissolution temperature, in general, resulted in aggregation of rod crystals. The polymer chains were oriented normal to the rod crystals. The width of the rod crystal increased with average molecular length, but saturated to a value much smaller than the average molecular length. In the shorter molecular length range, the rod crystals clustered in a fanned-out manner, while with a medium molecular length (ca. 70–120 nm), all rods crystals in a cluster aligned parallel to each other and were of the same length. With longer molecular length (more than ca. 180 nm), the rod growth slowed because of small diffusion constants of molecular chains to the growing face. Based on observation of the morphology and the crystallization process, an isothermal crystallization mechanism is proposed. Because of the rigidity and wide length distribution of polymer chains, the chain ends were inevitably included within the crystals, resulting in crystal defects such as axial shifts, lattice curvatures, and edge dislocations, all of which were observed directly by lattice imaging.

ISSN:0022-2348    

DOI:10.1081/MB-100102478

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Due to the presence of two alternating bond angles in the backbone of the polymeric chains, polyisobutylene (PIB) possesses four different rotational isomeric states. Accordingly, conformations of PIB could be represented by a four-state rotational isomeric scheme with only one adjustable statistical weight parameter. Rotational isomeric states (RIS) were determined at +25°, −25°, +120°, −120°. Conformations were divided into a “+class” and “−class,” with bond conformations tended to be followed by those from the same class and with changes from one class to the other rare. Since the configurational and thermodynamic properties of PIB depend to a great deal on the conformational characteristics of the polymer, a modified rotational isomeric state approximation was used to generate the initial configurations of the polymeric chains without allowing for any segment-segment overlap. Attempts were made to test the configurational properties of these systems against those determined experimentally to ensure that these configurations do represent realistically the polymeric system. Furthermore, these configurations were used to perform subsequent molecular dynamics runs to elucidate the effect of the molecular weight of the polymer and the temperature on some of its important thermodynamic properties, such as self-diffusion coefficient, thermal pressure coefficient, heat capacity, and dielectric constant.

ISSN:0022-2348    

DOI:10.1081/MB-100102479

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

In several acrylonitrile-butadiene-styrene (ABS) copolymers, some amounts of polybutadiene (PB) laTeX grafted with styrene-acrylonitrile (SAN) copolymer were replaced by acrylonitrile-butadiene rubber (NBR) copolymer, and the variations of morphology, mechanical properties, and rheological properties were examined. The impact strength of ABS, with a bimodal distribution of rubber size, was improved by the presence of the NBR, which distributes coarsely in the SAN matrix. Yield behavior in the rheological response due to the presence of rubber particles in the SAN matrix was enhanced by the coarser NBR particles, especially at high temperature.

ISSN:0022-2348    

DOI:10.1081/MB-100102480

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Permanganic etching was performed on high-speed spun (HSS) and regular fibers of poly(ethylene terephthalate) (PET), and their surface morphologies were investigated via the two-stage carbon replica method using a transmission electron microscope (TEM). The HSS PET fibers, with disordered amorphous regions, showed peculiar surface morphology; many small warts corresponding to the pits of etched disordered amorphous regions were observed. Such unevenness, however, was hardly observed on the surface of the permanganic-etched regular PET fibers, with well-oriented amorphous regions, or on the surface of alkali-etched HSS PET fibers. The permanganic etchant removed the disordered amorphous regions more preferentially compared with the alkali etchant.

ISSN:0022-2348    

DOI:10.1081/MB-100102481

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The effect of the temperature scale on the nonisothermal crystallization of a polypropylene (PP) was studied with the Nakamura equation and a Tobin-type equation for nonisothermal crystallization. The calibration on cooling of differential scanning calorimetry (DSC) data was performed, and the scanning rate dependence of the sample’s thermal resistance was evaluated and taken into account to correct the nonisothermal crystallization data obtained from the DSC measurements. The description of the crystallization was performed with only one varying parameter, which changed regularly with the scanning rate, making possible predictions of real nonisothermal crystallization behavior at intermediate or high scanning rates.

ISSN:0022-2348    

DOI:10.1081/MB-100102482

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor