G. S. Y. Yeh

The macroscopic yield behaviour of polymers

A yield criterion, not previously compared with the actual macroscopic behaviour of polymers, is herein compared with the pressure-modified octahedral shear stress criterion earlier suggested by others. This new relation, which is a version of the von Mises criterion, accommodates differences in tensile and compressive yield strengths and accounts for any dependence of yielding on the hydrostatic component of the applied stress state.With the use of thin-wall tubes accounting for the majority of experimental points, the yield behaviour of polycarbonate and polyvinylchloride was investigated. Besides these findings, results previously reported by others have also been utilized in this paper. Since these various studies employed quite different polymers, the excellent overall correlation of experiment with prediction should merit the serious attention of persons interested in the macroscopic yield behaviour of polymers.Comparisons between this new criterion and the modified octahedral shear stress are also made in regard to the effect of pressure on subsequent yield behaviour. Although not fully verified, it is suggested that the predictions which result using this new criterion, seem a little more reasonable.

A structural model for the amorphous state of polymers: Folded-chain fringed micellar grain model

Abstract A more realistic, two-phase structural model is proposed for the amorphous solid state of polymers. The model is based on structural evidence reported to date, in particular the evidence from our own electron diffraction and microscopy studies of crystallizable and non-crystallizable amorphous polymers, including near-molten rubbers. This model is different from either Kargins chain-packet or Hose-manns paracrystalline model. The two major elements of the model are the grain, which consists of an ordered domain and a distinct grain boundary, and the intergrain region, which consists of molecules that are in a more truly random conformation. In the ordered domain (∼20∼40 A) the molecular chain segments are aligned parallel to one another with a nearly constant spacing. A natural consequence of the present model is the concentration of excess free volume in the intergrain region. The amount of excess free volume has been estimated for typical polymers in their glassy amorphous state and found to ...

Order in amorphous polystyrenes as revealed by electron diffraction and diffraction microscopy

Abstract Ordered regions ranging in size between 15 and 45 A have been established in several atactic polystyrenes of various molecular weights (4800; 51,000; and 1,800,000) as well as in an amorphous isotactic polystyrene by means of electron diffraction and high-resolution diffraction microscopy. The order is due to long-chain polymer molecules which tend to align more-or-less parallel to one another with a constant average spacing. The orderly chain packing can be disturbed upon cross-linking by electron irradiation, the disturbance being manifested in the irreversible changes in the d spacing and broadening of one of the diffuse rings which is inter-molecular in origin. From the results obtained, the high-resolution diffraction microscopy technique appears to have the potential for resolving the question regarding the presence of order in unoriented amorphous polymers. The technique can also differentiate between a broadened diffuse ring due to small crystallites and another due to paracrystals contai...

Annealing effects of polymers and their underlying molecular mechanisms

Abstract This paper indicates that changes in chain mobility, heat capacity, WAXS crystallinity, SAXS long period, SAXS peak intensity, specific volume and morphology as a function of increasing temperature, occur in three fairly distinct annealing ranges (I, II and III) that are more or less the same for all crystallized polymers with a lamellar morphology. It is shown that none of the proposed molecular models to date, including the well-known fold surface premelting model, can satisfactorily account for all the experimental data. However, a new molecular interpretation, based primarily on electron microscopy and SAXS studies of changes such as lateral ‘melting’ from edges of microparacrystallites (mPC) within the lamellae seen at the annealing temperatures can account for the data. With our new molecular interpretation, the effect of temperature increase is established to result in a slight breakup of the laterally aligned mPC within the lamellae at low annealing temperatures in range I, and selective lateral ‘melting’ of the exposed mPC and recrystallization at higher annealing temperatures in ranges II and III, with the recrystallization being very limited in range III. Annealing effects seen in cold- or hot-drawn polymers with a fibrillar morphology can also be readily accounted for by this very general molecular mechanism occurring in the same annealing temperature ranges.

Similarity between craze morphology and shear-band morphology in polystyrene

The formation of shear bands and crazes in thin films as well as in bulk samples of polystyrene were examined in the electron microscope using a variety of replication techniques. The morphologies of shear bands and crazes are quite similar both depending initially upon the relative shear displacement of 400 to 1000 Å domains. As deformation continues and orientation increases, fibrils varying from 50 to 700 Å are formed within the deformation zone, lateral constraint of the normal Poisson contraction causing voids to form in the crazes but not in the shear bands. Shear-band width was found not to be a unique function of either temperature or strain-rate and both craze and shear-band morphologies were found not to be strong functions of molecular weight. Regardless of molecular weight, fibrils formed within the deformation zone were always on the order of a few hundred Angstroms in diameter. However, for thin films of molecular weight less than 20 000 insufficient numbers of tie molecules between fundamental structural units or domains made it difficult for these fibres to span the craze width.

Morphology of strain-induced crystallization of natural rubber. Part II. X-Ray studies on cross-linked vulcanizates

Abstract The results of wide- and small-angle X-ray studies on bulk vulcanizates of natural rubber are described. At room temperature no discrete X-ray peaks occur at small angles, even for highly strained (700%) specimens for which a significant degree of crystallinity is present at room temperature. Below the isotropic melting point of the network, discrete small-angle X-ray scattering develops for both unstrained and strained specimens. The Bragg periodicity corresponds closely to the longitudinal lamellar spacing from electron micrographs of thin films under corresponding conditions. The X-ray long period exhibits significant thermal reversibility. Upon heating, following initial crystallization at −25°C, the long period continuously increases from about 140 A at −25°C to about 250 A at 17°C. After heating, progressive cooling from room temperature results in continuous decreases in long period and at least 80% of the initial increase on heating is recovered. These results in conjunction with electron...

Morphology of polyethylene microfibrils and “shish kebabs”

Abstract Gold decoration was applied to elucidate the morphology of annealed and unannealed oriented structures produced by shearing of polyethylene single crystals of known thicknesses, and the morphology of drawn and undrawn “shish kebabs” obtained by stirrer-induced crystallization. Distinct Au periodicities were observed perpendicular and parallel to the unannealed oriented structure, revealing distinctly the presence of microfibrils and an alternation of crystalline and noncrystalline regions within a given microfibril of which the periodicity is, however, found to be much larger than the corresponding original crystal thickness. Annealing resulted in fibrils being completely replaced by lamellae standing edgewise with the Au particles decorating directly on top of the edge but concentrated mainly on the folds leaving the crystalline region almost entirely void of gold. Deformation studies revealed an unusual ductility associated with the shish kebab structure, with the kebab splitting first, then di...

Mechanism of yielding and cold flow in glassy polymers

Abstract 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 Tg does a transition to cooperative long-chain motion occur. Equivalent values for shear activation volume ...

Radiation-induced crosslinking. II, Effect on the crystalline and amorphous densities of polyethylene

Small-angle x-ray scattering (SAXS) was used to determine the structural changes in polyethylene induced by radiation. The changes in densities of the crystalline and amorphous phases, ρc and ρa, were calculated after direct determination of the mean square density fluctuation . ρa increases with increasing radiation dose for both linear and branched polyethylene. This accounts for the serious discrepancy between crystallinities determined from wide-angle x-ray scattering and density measurements. This study confirms our previous proposal that crosslinks occur primarily in the noncrystalline phase, most likely at the defects in the lateral grain boundary regions.

Cold extrusion and cold drawing of polymeric rod: The influence on subsequent tensile and compressive mechanical properties

Abstract Nylon 6-6, polycarbonate and high density polyethylene were extruded and drawn through a 40% reduction die in one pass at room temperature. Certain mechanical properties of the extrudate were measured by subsequent tension and compression tests. For both forming processes and for all three polymers, the tensile true stress-true strain curve is raised compared with the behavior of the original material while the comparable compressive curve is lowered. It was also found that the onset of tensile instability was extended to larger tensile strains when these polymers were first cold worked by extruding or drawing. The elastic modulus was increased when PC was extruded or drawn while this property exhibited a decrease, due to cold working, for both other polymers. All three materials exhibited pronounced directional effects when subjected to compressive loading both parallel to and perpendicular to the longitudinal axis of the extrudate. In a qualitative manner this seems to correlate with greater chain alignment brought about by the forced flow through the die throat. Exploratory temperature measurements indicate that a maximum rise of 20 deg F resulted for this severe reduction of area; this would not cause the glass transition temperature of the PC and Nylon to be exceeded.