ISSN:0022-2348    

DOI:10.1080/00222348708248069

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

The crystal and amorphous orientation functions of polypropylene (PP) and high-density polyethylene (HDPE) sheets, stretched by the roller-drawing technique, were measured by means of infrared dichroism and visible-ultraviolet dichroism. The molecular chains in the crystalline and amorphous regions lined up parallel to the draw direction in the roller-drawing process. At higher draw ratio (λ > 7–13), the crystal orientation function approached a constant value, whereas the amorphous orientation function increased monotonically with draw ratio. Film tilting measurements revealed that the deviation from symmetric uniaxial orientation was small for the roller-drawn HDPE and PP sheets. In the case of HDPE, the fraction of—GTTG—,—4TG—, and—GG—sequences decreased, and that of the trans-rich sequences increased in the roller-drawing process. These changes were interpreted in terms of the Peterlin model. The orientation function calculated from visible-ultraviolet dichroism decreased greatly with decreasing length of solute (dye) molecules. The reasons for this phenomenon are discussed.

ISSN:0022-2348    

DOI:10.1080/00222348708248070

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

The crack layer theory has been well established as a good description of the energy dependence of fatigue crack propagation in thermoplastics associated with multiple crazes or large deformation preceding the crack tip. Since PMMA usually displays crack propagation preceded by a single craze, the applicability of the theory in such extreme cases needs to be tested. Fatigue crack propagation data obtained in a previous interferometric investigation in PMMA have been utilized here. Two different approaches are attempted for estimation of the energy dissipated in craze formation and growth. Using both approaches, the crack layer formalism provides a reasonably good description of the crack propagation behavior in PMMA.

ISSN:0022-2348    

DOI:10.1080/00222348708248071

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

Microstructures and their orientation behavior in quenched films of a thermotropic polyester, poly(chloro-1,4-phenylene-trans-1,4-cyclohexanedicarboxylate), were studied by electron microscopy, polarized-light microscopy, and x-ray diffraction. Oriented films were obtained by shearing the anisotropic melt of the polymer and cooling to 0°C. Fibrous structures shaped like ribbons or prisms were observed in fractured lateral surfaces of quenched films. Prismlike fibrils are composed of ribbonlike fibrils, the thickness of which is less than 0.1 μm. Streaks suggesting the end of ribbonlike fibrils were observed on the surface of fibrils. The fibrils occur almost parallel to the direction of orientation of films. In the case of a poorly oriented film, the direction of orientation of the fibrils alters in waves. The period of the waves becomes longer with increasing degree of orientation. Bandlike structures 50–100 nm in width were observed on the surfaces of the films. They are arranged almost perpendicular to the direction of orientation of the films. The features of the microstructures observed in the wavelike fractured lateral surfaces of the thermotropic polyester film are schematically represented.

ISSN:0022-2348    

DOI:10.1080/00222348708248072

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

The nonisothermal and isothermal crystallization behavior of the thermoplastic copolyester based on 1–4 cyclohexane dimethanol, ethylene glycol, and terephthalic acid was investigated by differential scanning calorimetry. The copolyester is normally an amorphous polymer with the cyclic moiety in the main chain suppressing crystallization. However, the amorphous pellets crystallize during the heating mode. The crystallization behavior from the glassy solid state was found to be influenced by the melt temperature and the dwell time at the melt temperature. The onset of crystallization shifts progressively to lower temperatures, and the peak becomes narrower with increasing melt temperature and dwell time. The isothermal crystallization time at a given temperature of crystallization was also found to decrease significantly with increasing melt temperature. Thus, the crystallizing ability of the polymer and the rate of crystallization were found to be influenced by the melt temperature and the dwell time. The results are interpreted in terms of the temperature–dependent equilibrium ratio between the two possible chain conformations corresponding to the “Trans” and “gauche” forms of the cyclohexane moiety in the chain.

ISSN:0022-2348    

DOI:10.1080/00222348708248073

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor

摘要:

Computed-aided molecular modeling methods are presented to estimate the melt transition temperatures, Tm′s, of polymers in solution and in the crystalline state. The molecular geometries and energetics are computed using fixed valence geometry molecular mechanics with the charge densities computed for monomers using the CNDO/2 semiempirical molecular orbital method. A calibration-based melt transition model can be used on a congeneric series of polymers where at least one known Tmis available to scale the computation. This model is applied to a set of collagen triple-helix tripeptide and hexapeptide analogs, and to four classes of crystalline synthetic polymers. The estimated Tm′s are in good agreement with the reported experimental values. Moreover, the trends to increase or decrease Tmas a function of change in chemical structure of the polymer are correctly predicted by the model. A first-principle phase transition formalism to predict the thermodynamic properties of polymer crystal melting is also presented. This model is based upon Ising-mean-field theory. Application of this formalism to polymethylene and polytetrafluoroethylene yields accurate predictions of Tmand the entropy of melting. However, the formalism is quite computationally intense and not meant for routine applications.

ISSN:0022-2348    

DOI:10.1080/00222348708248074

出版商:Taylor & Francis Group    

年代:1987

数据来源: Taylor