摘要:

A new approach pertaining to the hole and lattice theories of polymeric systems is developed to interpret the universal inverse relationship between the glass transition temperature (Tg) and the difference in thermal expansion coefficients for liquid and glassy polymers (δ α). The success of this particular theoretical consideration rests upon the familiar hypothesis of the iso-free volume state at Tgand the entropy contribution to the hole-polymer segment interaction energy. Based on the experimental data available, we have established the free volume fraction at Tg, vg= 0.025, which is identical with the assessment of the empirical Williams-Landel-Ferry (WLF) equation for the temperature-dependence shift factor aT. However, appreciably higher values of vgare reported by other contemporary models. The present findings justify an alternative form of the WLF equation with the product Tg, δ α playing the role of δ α. The universality of this novel expression is demonstrated, by means of statistical analysis, to be remarkably more warrantable than its original version.

ISSN:0022-2348    

DOI:10.1080/00222348808260837

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

Permanganic etching is a technique of removing material selectively to reveal lamellae in crystalline polymers. It is generally found that populations of lamellae with different thicknesses are removed to different depths. In linear poly-ethylene we have previously shown that this discrimination is not an inherent property but a consequence of differential deformation during cutting to expose the surface prior to etching. The work is now extended to linear low-density polyethylene to test whether the changed basis of molecular segregation (with more branched molecules crystallizing later) alters the etching behavior. It is shown that thinner lamellae are etched at a faster rate which is only slightly reduced after removing damage due to cutting. This intrinsic differentiation between crystal thicknesses accompanies faster etching rates than for the linear material. It is inferred that this is due to faster chain scission at fold surfaces which are more penetrable for the linear low-density than for the linear polymer; however, chain scission remains slow compared to stem removal. Fold stems are probably removed as entities rather than incrementally, but it is evident that a fold surface region needs to be etched away before a stem can be removed.

ISSN:0022-2348    

DOI:10.1080/00222348808260838

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

Preoriented yarns (POY fiber) of polyethylene terephthalate spun at low speed shrink crystallize above Tgand show no orientation. It is shown that the kinetics of crystallization and the morphology (long period) are dependent on the molecular weight and on the spinning velocity. These differences observed in isotropic spun fibers are explained by the disentanglement of the chains, which depends on the mechanical history of the melt state. It is emphasized that these memory effects indicate that the time for the spun melt to recover its equilibrium properties is longer than the terminal time ιt. Similar conclusions were obtained for polyethylene melts subjected to temperature jumps.

ISSN:0022-2348    

DOI:10.1080/00222348808260839

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

The process of water vapor sorption into the polyelectrolyte complex of poly(acrylic acid)/poly(4-vinyl-N-ethylpyridinium bromide) has been investigated; the complex is classified as being of the weak polyelectrolyte/strong polyelectrolyte type. Films of stoichiometric (1/1 molar ratio) and nonstoichiometric (1/2 molar ratio) complex were employed in the experiments. The successive differential sorptions demonstrated that these complexes have an analogous morphological structure and undergo a very similar structural change during moisture sorption. The structural change is discussed by considering the network structure of the complexes. The integral sorption data indicated that the sorption rate for the 1/2-complex is greater than that for the 1/1-complex. For the present complexes, it has been found that the magnitude of the sorption rate can be interpreted in terms of the ionic character of the materials. The sorption rate was also evaluated from a theory available for non-Fickian processes that has been presented by Long and Richman.

ISSN:0022-2348    

DOI:10.1080/00222348808260840

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

ISSN:0022-2348    

DOI:10.1080/00222348808260841

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

Solvent crazing has been investigated for bisphenol-A poly-carbonate exposed to the liquids of n-alcohols, di-n-alkylphthalates, and adipic acid polyesters. The critical strain at which crazes appeared was determined with a Bergen elliptical strain device. In the case of the n-alcohol and di-n-alkylphthalate liquids with a small number of carbons in the alkyl chain, crazes spread rapidly to areas of lower strain with time and ceased within 30 min at room temperature. Such a lower limiting value of critical strain can be predicted by using polar-nonpolar solubility parameter plotting representations employing the molar volume term. On the other hand, craze initiation was more delayed for two higher molecular weight liquids, di-n-octylphthalate and adipic acid polyester. Sorption studies were also conducted on polycarbonate immersed in both liquids. The values of the activation energy and diffusion coefficient estimated from the experimental data on craze initiation were found to be nearly comparable with the ones from the sorption experiments. In conclusion, the induction time-that is, the time at which crazes appear for a given strain–is controlled by the diffusion of the crazing liquid into the polymer. Thus, in the case of these liquids, which are miscible with the polymer, the crazing mechanism can be explained in terms of facilitated craze formation of a plasticized polymer.

ISSN:0022-2348    

DOI:10.1080/00222348808260842

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

The temperature dependence of the elastic modulus E1of crystalline regions of poly(ethylene terephthalate) (PET) in the direction parallel to the chain axis has been investigated by x-ray diffraction. The axial chain contraction coefficient is constant from 25°C to 200°C, and correspondingly the value of E1(108 GPa) remain unchanged up to 215°C. This is in contrast with the temperature dependence of E1values of various polymers previously reported. It is considered for PET that the axial thermal molecular motions which cause the decrement of E1do not take place in the crystalline regions, and that the force constant of bond stretching and that of bond angle bending are constant over both the temperature and the stress range studied. It is reasonable to conclude that the mechanical structure of drawn PET is composed of the series model.

ISSN:0022-2348    

DOI:10.1080/00222348808260843

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

摘要:

The phase behavior of blends of polystyrene and poly(vinyl methyl ether) undergoing shearing flow was examined. Experiments conducted at a constant level of flow-induced strain found that such flow elevates the phase boundary and pro-motes miscibility. In the framework of the Cahn-Hilliard model for spinodal decomposition, an expression was developed which predicts the effect of an external flow field on the spinodal. The basis for this expression is the minimization of strain energy in a two-component polymer system that is in a thermodynamic state near its phase-separation point. The zero shear viscosities of the blends were measured and demonstrated to be an indication of one- or two-phase flow.

ISSN:0022-2348    

DOI:10.1080/00222348808260844

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

ISSN:0022-2348    

DOI:10.1080/00222348808260845

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor

ISSN:0022-2348    

DOI:10.1080/00222348808260846

出版商:Taylor & Francis Group    

年代:1988

数据来源: Taylor