摘要:

The mechanical anisotropy of low density polyethylene sheets possessing orthorhombic symmetry and parallel lamellae morphology has been studied by making measurements of the creep behavior in extension and shear at room temperature. The behavior is interpreted in terms of two predominant deformation mechanisms taking place within and between the lamellae, i.e., the c-shear process and interlamellar shear, respectively, the latter involving both simple and pure shear of the interlamellar regions. Some of the creep compliances have unusually high time dependence and it is shown that this is related with intralamellar processes.

ISSN:0022-2348    

DOI:10.1080/00222347408204555

出版商:Taylor & Francis Group    

年代:1974

数据来源: Taylor

摘要:

Changes in supermolecular organization (SMO) of oriented films of low-density polyethylene were investigated by small (SAXS) and wide (WAXS) angle x-ray scattering. The films were oriented at different angles α with respect to the initial orientation. The temperatures of the initial orientation (T1) and of reorientation (T2) were varied as follows: (1) T1> T2(T1= 85°C, T2= 20°C); (2) T1-T2= 85°C; (3) T1= T2= 20°C. In all cases, at relatively high α, reversible skewing of crystallites with initial SMO occurs in the beginning of deformation and, after the critical angle of skewing is attained, the crystallites are destroyed and a new SMO is formed. At T1= T2the initial and the new SMO are not distinguished by x-ray diffraction patterns and changes in the latter correspond to a simple rotation of the structure in combination with a relatively small skewing of the crystallites. The first variant of the relationship, T1> T2, yields the most extensive information. In this case the initial and the new SMO greatly differ and when they are both present, two pairs of reflections are observed in SAXS patterns. The critical skewing angle after which the initial SMO is destroyed is 72° ± 5° (in the second and third variants it is 40—50°). The new SMO appears before the formation of macroscopic neck. The neck is interpreted as a result of merging of many local regions with new SMO when their concentration in the sample is comparatively high (75% by weight) At small 01 (520') the sample breaks before the neck is formed. At 01 = 90′ two crystallite systems skewed in opposite directions are formed. The results are interpreted in terms of melting of crystallites in the initial structure and crystallization of a new structure (T, > T2). At T, = T, the processes of meltingrecrystallization occur after critical skewing angles are attained. A phenomenon of reversible broadening of equatorial WAXS reflections has been observed.

ISSN:0022-2348    

DOI:10.1080/00222347408204556

出版商:Taylor & Francis Group    

年代:1974

数据来源: Taylor

摘要:

Tensile and compressive yield stress as well as tensile and compressive flow stress are determined as functions of strain rate, temperature, and thermal history for atactic polystyrene, isotactic polystyrene, polycarbonate, and poly-(methyl methacrylate). An Eyring-type activated flow analysis is employed to calculate both activation energy and shear activation volume over the range of experimental conditions studied, and the results are interpreted in terms of a two-phase amorphous microstructure consisting of both ordered and disordered regions. Similar values for yielding activation energies and activation energies obtained from dynamic mechanical analysis suggest that at temperatures well below Tg, yielding is controlled by cooperative local segmental motion. With increasing temperature, increasing degrees of local segmental motion are involved. Only when the test temperature approaches Tgdoes a transition to cooperative long-chain motion occur. Equivalent values for shear activation volume in both tension and compression suggest that tensile and compressive yielding mechanisms are very similar and that both may be controlled by localized volume dilations.

ISSN:0022-2348    

DOI:10.1080/00222347408204557

出版商:Taylor & Francis Group    

年代:1974

数据来源: Taylor

摘要:

The glass-transition temperature, Tg, is the narrow temperature interval (usually approximated by a point) above which long-range mobility of macromolecular segments exists and below which it ceases. For such mobility to be realized two prerequisites are required: (1) a free volume sufficiently large to accomodate the moving segment must exist, or be formed, adjacently to it, and (2) the segment itself must possess sufficiently high thermal energy to overcome the energy barrier confining it to its equilibrium position. The high segmental mobility reflects a relatively low packing density, D, and is reflected in a relatively high coefficient of thermal expansion, α.

ISSN:0022-2348    

DOI:10.1080/00222347408204558

出版商:Taylor & Francis Group    

年代:1974

数据来源: Taylor

摘要:

A series of poly(ethylene terephthalate) fibers of various molecular weights was first drawn to a practical maximum draw ratio and then allowed to contract thermally under tension for 10, 20, and 38%. These contracted fibers exhibit a high degree of plasticity even when tested at—100°C and extension rates of 1300%/sec. An attempt is made to explain this behavior by means of a systematic study of morphological changes which occur during thermal contraction. The interpretation of the results of small-angle and wide-angle x-ray diffractions, infrared spectroscopy, and birefringence suggest the existence of two types of amorphous domains; those separating the adjacent crystallites in the microfibril and those separating the microfibrils. It is speculated that the molecules in these two domains respond differently to thermal effects and stress, and that the interfibrillar amorphous domain consists of highly extended molecules.

ISSN:0022-2348    

DOI:10.1080/00222347408204559

出版商:Taylor & Francis Group    

年代:1974

数据来源: Taylor