摘要:

The onset of sharkskin extrudate was investigated in terms of molecular structure, molecular weight, temperature, and die geometry. The recoverable shearSRor the ratio of wall shear stress over the storage modulus was determined and found to be independent of molecular weight, temperature, and die geometry for linear low-density polyethylenes (LLDPEs), but it depended critically on these factors and the molecular weight distribution for high-density polyethylenes (HDPEs). The plot ofSRversus λs/ϵs, or the sharkskin wavelength roughness amplitude ratio, constituted a sharkskin marginal stability curve common for all molecular weights, temperatures, and die geometry investigated; it is unique for each polymer type or molecular structure. Sharkskin instability occurred without hysteresis effect.

ISSN:0022-2348    

DOI:10.1081/MB-100102483

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

Maleic acid-alt-styrene (MAaS) copolymer with number-average molecular weight [Mbar]n= 2500 was used as a compatibilizer in blends of poly(ethylene oxide) (PEO) and poly(styrene) (PS). PEO with weight-average molecular weight [Mbar]w= 105(PEO100) and two PS samples with [Mbar]w= 9 × 104and 4 × 105, respectively (PS90and PS400, respectively) were used. A depression of the melting temperatureTmof PEO in blends containing MAaS relative to pure PEO and PEO/PS blends was observed. The melting enthalpy ΔHmfor the PEO/PS blends containing MAaS was lower than those of pure PEO and PEO/PS blends without compatibilizer. The crystallization kinetics of PEO and the blends were studied by differential scanning calorimetry (DSC) at different crystallization temperaturesTc. Flory-Huggins interaction parameters χ12for the blends were estimated. Their values are in good agreement with those obtained for similar systems and suggest that the free energy of mixing ΔGmixshould be negative. Polarized optical microscopy shows differences in the macroscopic homogeneity of the blends containing compatibilizer that could be attributed to a compatibilization process.

ISSN:0022-2348    

DOI:10.1081/MB-100102484

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The elastoplastic properties of injection-molded starch and starch-cement composites were investigated by load-displacement analysis from depth-sensing experiments. The creep behavior under the indenter was studied. Hardness data from the depth-sensing and imaging methods were shown to be in good agreement. The Young modulus values derived from the compliance method were studied; results are discussed in the light of the various materials used. Finally, the influence of annealing temperature on the microhardness of starch and of the starch-cement composites, processed under a water vapor atmosphere, was investigated. Results reveal a hardening of the materials on thermal treatment. This hardness increase is associated with an equilibrium water content decrease detected within the samples.

ISSN:0022-2348    

DOI:10.1081/MB-100102485

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

The dielectric relaxation behavior of a series of ethylene-vinylacetate (EVA) copolymers was investigated by measuring the complex dielectric permittivity in a broad frequency and temperature range. Crystallinity of EVA copolymers was estimated by differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS). The shape of the higher temperature relaxation, appearing above the glass transition temperatureTgdepends on the VA content. It was found that this relaxation was asymmetric for VA concentrations higher than 40 wt% and changed to a symmetric shape at lower VA values. Concurrently, as the VA content decreased, a major broadening of the relaxation over a wide frequency range was observed. It is found that the dielectric relaxation was preserved on going through the melting range of the semicrystalline samples, although it exhibited changes of its characteristic parameters that are typical for segmental relaxation appearing atTg. This finding allows one to associate this relaxation to the segmental motions atTgin the amorphous phase and not to the existence of interfacial regions.

ISSN:0022-2348    

DOI:10.1081/MB-100102486

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor

摘要:

In this manuscript, we present a new technique for using a differential scanning calorimeter (DSC) to study isothermal bulk crystallization and the kinetics of the process at “high” crystallization temperatures or “low” degrees of undercooling. It is based on measurements of the enthalpy of fusion from the subsequent melting endotherms after isothermal crystallization for various time intervals. Syndiotactic polypropylene (s–PP) was used as the model system. The benefits of the new technique, based on the results we present here, are threefold: (1) it allows an accurate determination of the induction time; (2) it offers a means to extend the use of DSC to obtain kinetics parameters for isothermal crystallization at temperatures at which the traditional technique is not applicable; and (3) it gives an insight into certain mechanistic aspects of the isothermal crystallization process as it occurs at different crystallization time intervals.

ISSN:0022-2348    

DOI:10.1081/MB-100102487

出版商:Taylor & Francis Group    

年代:2000

数据来源: Taylor