摘要:

Crystallization and melting of a copolymer depend on both intrachain and interchain distributions of noncrystallizable comonomer. Hydrogenated polybutadiene (HPB) is a model ethylene-butene (EB) random copolymer with known intrachain distribution and no interchain distribution of branching. Differential scanning calorimetry (DSC) was used to study HPB having 21 to 106 ethyl branches per 1000 backbone carbon atoms. Crystallization during continuous cooling occurs in two stages: a rapid primary transformation over a temperature interval of about 5°C, followed by more gradual crystallization at lower temperatures. The crystalline fraction α is below the value from Flory's theory, but the two quantities converge for high branching. Isothermal crystallization of these homogeneous copolymers results in crystals having two distinct melting ranges. This unexpected result is accounted for by a model that partitions crystallizable ethylene sequences into two fractions on the basis of lengths established by crystallization temperature and kinetics. The fraction of the longest sequences creates a kinetically controlled distribution of crystal thicknesslsimilar to that in a homopolymer. Segments of intermediate length form stable, but thinner, crystals that melt at a lower temperature. One ethylene-butene copolymer prepared with a metallocene catalyst behaves like the homogeneous model copolymers.

ISSN:0022-2348    

DOI:10.1081/MB-100100367

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

A new type instrument for dynamic light scattering (DLS) measurements was designed and set up to avoid the influences of absorption effect and emission from scatterers. The instrument was equipped with two kinds of incident laser light sources: He-Ne laser (λ = 632.8 nm) and Nd:YAG laser (λ = 1064 nm). The reproducible autocorrelation function (ACF) of blue-colored polystyrene latex (BPSL), as well as an ordinary colorless polystyrene latex (PSL), was measured successfully in the near-infrared region, and the diameters of BPSL could be evaluated accurately. On the other hand, the relationship between absorption coefficients and diffusive coefficients of BPSL measured in the visible region could be presented empirically.

ISSN:0022-2348    

DOI:10.1081/MB-100100368

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Impedance spectroscopy (IS) was to analyze the dielectric behavior of cross-linked polyethylene (XLPE) and to produce an electric analog equivalent circuit. In the high-frequency range, a peak was observed, and the values of the resistance for this particular frequency were calculated for the XLPE samples with different thicknesses. Two ohmic regions with different slopes were observed, suggesting the existence of two regions in the crystalline phase with the same morphological characteristics, but different sizes (i.e., similar time constants). A reduced two-phase model was used to explain the observed behavior of samples with different thicknesses. It is suggested that thinner samples present a crystalline layer with lamellae perpendicular to the surface, whereas in thicker samples, a bulk spherulitic morphology is dominant.

ISSN:0022-2348    

DOI:10.1081/MB-100100369

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Polymers of 2-methyl-1,3-pentadiene are capable of existing in both isotactic, as well as syndiotactic, configuration. The isotactic 1,4-cispolymer also exists in two stable forms that have different melting points. A complete study of normal modes and their dispersion was carried out using Higgs modification of Wilson's dynamical GF matrix method and the Urey Bradley force field. The force constants have been best fitted to the Fourier transform infrared (FTIR) frequencies. Characteristic features of dispersion curves, like bunching, repulsion, and exchange of character between various pairs of modes, were observed and are discussed here. In addition, the frequency distribution function derived from dispersion curves was used to calculate the heat capacity in the temperature range 50–450 K.

ISSN:0022-2348    

DOI:10.1081/MB-100100370

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Positive deviations of the modulusEband yield strengthSybof polymer blends consisting of partially miscible polymers are analyzed by combining models for miscible and immiscible blends. The effect of the composition of binary blends on the composition of conjugate phases over the entire composition range is described by an empirical formula that requires two parameters extractable from experimental data. Positive deviations ofEbandSybare ascribed to two effects: (1) respective properties of one or both conjugated phases are higher than those of parent polymers; (2) molecular mobility in both conjugate phases is reduced due to associative interactions (heterocontacts) between the chains of components. These effects are then taken into account in the predictive scheme, which combines (1) an equivalent box model (EBM) and (2) data on phase continuity rendered by the equations proposed by the percolation theory. Three typical situations are considered in model calculations: (1) components do not show any miscibility and interaction; (2) components are partially miscible, but their interaction in conjugate phases is negligible; (3) conjugate phases consist of strongly interacting components. The scheme satisfactorily fits available experimental data if associative interactions are encompassed by means of an adjustable interaction parameter.

ISSN:0022-2348    

DOI:10.1081/MB-100100371

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The effect of a compatibilizer on the breakup and coalescence of droplets in flowing polymer blends was analyzed. The analysis was based on theories developed in the literature for low molecular weight emulsions containing a surfactant. Analysis of the stepwise and transient breakup mechanisms led to the conclusion that the effect of a compatibilizer on these mechanisms is qualitatively different. The compatibilizer effect depends on the rate of its migration on the interface and between the interface and bulk phase. The presence of a compatibilizer suppresses coalescence of droplets due to the decrease in the interfacial tension and mobility of the interface. The importance of the compatibilizer effect increases with decreasing ratio of the viscosities of the dispersed phase and matrix.

ISSN:0022-2348    

DOI:10.1081/MB-100100372

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

In this paper, the effect of the inclusion of ethylene/vinyl alcohol (EvOH) copolymer in nylon 6 is analyzed with particular attention to the transport properties. Water vapor and dichloromethane transport measurements were performed to provide useful information about polymer-polymer interaction and segmental mobility. Sorption and diffusion tests were carried out at 25°C at several activities to determine both sorption isotherm and diffusivities. Moreover, permeability tests to gases (CO2, O2) were carried out at 30°C. The obtained results indicate the existence of strong hydrogen-bonded interactions of the two polar groups (amidic of nylon 6 and hydroxyl of EvOH) within the amorphous phase. This interaction strongly influences the transport properties of the nylon 6 amorphous component.

ISSN:0022-2348    

DOI:10.1081/MB-100100373

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Samples of nanopolymeric particles, each containing a single chain or a few chains, were prepared by a freeze-drying method from dilute solutions of isotactic polystyrene (iPS) in benzene. Thermal analysis of the particle samples revealed that the cold crystallization temperature greatly decreases and the crystallization rate remarkably increases as the solution concentration decreases. The increase in crystallization rate can be attributed to fewer interchain entanglements within and between particles, thus proving experimentally for the first time that entanglements can be a large barrier to the crystallization of polymers. Annealing of the particle samples at 373.2 K slightly changed the crystallization behavior, indicating that the interdiffusion of chains between these particles is rather sluggish in the vicinity ofTgof bulk iPS. Crystallization is assumed to be accomplished before the chain interdiffusion, and nanocrystals form in situ within compact single- and few-chain globules. The average size of crystals is larger than that of single-chain crystals because the single-chain crystals may initiate other chains to crystallize on the growing crystal surface. Meanwhile, we also found that the nanocrystals have a lower melting temperature than the bulk polymer as a result of their small size.

ISSN:0022-2348    

DOI:10.1081/MB-100100374

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Isotactic polypropylene (iPP) samples, quenched and then annealed at 70°C, 80°C, 110°C, 140°C, and 150°C, were investigated by X-ray diffraction. The transition from the quenched form (smectic) to the well-crystallized one is analyzed to explain better the morphological-structural changes that occur by annealing. For all the samples, both the quenched and annealed ones, the α-monoclinic structure was assumed, with different degrees of order and crystallite dimensions. The same α-monoclinic structure was also assumed for the so-called amorphous phase of the polymer, described here in terms of highly disordered nanocrystals. The high degree of disorder is due, in our interpretation, mainly to the disordered fraction inside the nanocrystals, which is considered wholly amorphous. The changes induced by annealing are interpreted as a rearrangement of the α-monoclinic structure, which presents both I and II types of disorder. The paracrystalline parameter α*, in its more detailed formulation, is introduced to obtain a more reliable estimation of the crystallite dimensions. The analysis of the transition process evidences two annealing temperature ranges: 70°C–80°C, involving a rearrangement in the chain axis direction mainly, and 80°C–150°C, with a prevalent rearrangement first in the directions normal to the chain axis and then in the chain direction again. During the whole process, the crystallites (crystalline phase of the polymer) undergo a remarkable increase of their dimensions, while the nanocrystals (microcrystalline phase) undergo significant changes in their degree of order only. The method adopted allows a more quantitative interpretation of the transition and provides a more structural criterion to define the amorphous phase and the crystallinity in the polymer samples.

ISSN:0022-2348    

DOI:10.1081/MB-100100375

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor