AbstractThe degree to which a polymer film develops plastic flow depends largely on the total strain ϵ and the elongation time th. The magnitude and the time dependence of the elastic component ϵe of the total deformation are controlling factors, respectively, in the solubility and diffusion processes. The plastic deformation ϵpl seems not to contribute to the transport properties. The nonlinearity in solubility due to ϵ may be conveniently handled. In this study, the solubility of ethyl acetate vapor in poly(vinylidene fluoride) was determined as a function of pressure and total elongation at 30°C. These results suggest that the strain magnitude and time de pendence of the component deformations play important roles in transport behavior.

AbstractAt a fixed vapor pressure p of the penetrant and constant temperature of the experiment, the sorption S = c/p or concentration c of the ethylacetate vapor in the uniaxially strained low density polyethylene (LDPE) increases most rapidly at low strains. If, however, on the basis of strain relaxation one separates the total strain ϵ into an elastic ϵe, and a plastic ϵpl, deformation, one obtains an almost linear increase of the concentration c or sorption S of the sorbate with elastic strain ϵe. The separation of ϵ = ϵe + ϵpl depends very much on the time th the sample is kept elongated and the vapor pressure p of the sorbate. The elastic component decreases and the complementary plastic fraction increases with th and p. An almost stationary state is reached after th of about 1/2 h. The calculation of the diffusion coefficient Ds1 from the first sorption immediately after the stretching is affected by this slow adjustment in the interval 0 ≤ th ≤ ½h and shows a pseudo maximum at a strain of ϵ∼ = 10 percent. The first desorption experiment and all the later sorptions and desorptions yield the same DD = DS < DS1 that is the correctly calculated diffusion coefficient D. The coefficient D decreases with the strain ϵ or ϵe in contrast with the expected increase of Da of the amorphous component. Such an increase of Da is expected as a consequence of the fractional free volume (FFV) increase caused by the elongation. According to the FFV concept, a decrease of the measured apparent diffusion coefficient D = ψDa requires that with increasing ϵ, the tortuosity factor ψ decreases faster than the increase in Da.

AbstractA new apparatus has been developed for optical measurement of sorption/desorption in transparent polymer films at a given strain or stress. The technique utilizes a chosen infrared absorption frequency of the diffusing vapor in a spectral region where the film has negligible absorption. From the time dependence of the IR absorption at this frequency the sorption/desorption behavior of the film may be determined at any strain or stress. The simultaneous measurement of mechanical relaxation as a function of the amount of sorbed vapor is also possible. The results presented here show the applicability of the apparatus for determining the transport and mechanical properties of a low‐density polyethylene film in ethyl acetate vapor at 30°C.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTLow-frequency Raman study of drawn polyethyleneR. G. Snyder, J. R. Scherer, and A. PeterlinCite this: Macromolecules 1981, 14, 1, 77–82Publication Date (Print):January 1, 1981Publication History Published online1 May 2002Published inissue 1 January 1981https://pubs.acs.org/doi/10.1021/ma50002a014https://doi.org/10.1021/ma50002a014research-articleACS PublicationsRequest reuse permissionsArticle Views193Altmetric-Citations21LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InRedditEmail Other access optionsGet e-Alertsclose Get e-Alerts

Abstract The minimum requirement of the free enthalpy increase for the formation of any type of critical size nucleus determines the probability for the nucleation from supercooled melt or solution and for the subsequent crystal growth. The minimum demand decreases as the inverse square of the supercooling. This yields a rapid increase of the nucleation and growth rate with decreasing temperature. On the other hand, the viscous resistance of the melt or solution to the chain transfer from the liquid to the crystalline phase increases with the temperature approaching that of the glass transition. Both effects together yield a maximum of the nucleation and growth rates at a finite supercooling with a subsequent drastic drop at a higher supercooling. The extension and alignment of the polymer chains in the liquid by applied mechanical forces lowers the entropy and to some extent also the enthalpy of the liquid state, thus raising the equilibrium melting temperature of the system and increasing the effective supercooling. As a consequence the rate of nucleation and crystal growth is drastically increased. Moreover, the shape of the primary nuclei becomes substantially linear with orientation in the main strain direction. Since these linear elements carry most of the applied load or stress, the rest of the liquid can relax to such an extent that by epitaxial overgrowth the macromolecules are able to be deposited in more or less conventional lamellae perpendicular to the stress. This yields the shish-kebab structure in the stirred or sonicated solutions and the cylindritic structure in hard elastomers solidified from the extruded melt.

The diffusion coefficient of butane in linear polyethylene at room temperature as a function of the vapor pressure of butane was measured by the spin-echo method with a pulsed magnetic field gradient. For the Special morphology of randomly oriented stacks of parallel lamellas the detour factor is 1/3. As long as the blocking factor and migration through the lamellas can be neglected, the local diffusion coefficient Da of the small molecules through the amorphous layers in the direction parallel to the lamellas is three times the apparent diffusion coefficient D derived from the decay of the amplitude of the spin echo under the assumption of an infinitely extended homogeneous medium. The diffusion coefficient and the spin–spin relaxation time both increase exponentially with increasing pressure, i.e., butane concentration in the polymer, while the spin-lattice relaxation time is pressure independent and seems to be determined by interaction with the amorphous polyethylene matrix.

Summary The plastic deformation of fibrous material occurs primarily by a sliding motion of fibrils. To a first approximation, the displacement of their centers of mass can be well described by an affine transformation corresponding to the deformation of the bulk sample. Such a sliding motion of fibrils does not affect the morphology of the microfibrils. But by chain unfolding, it smoothes the surface inhomogeneities of the fibrils caused by microfibril ends which act as point defects of the microfibrillar lattice. This makes possible a more perfect lateral contact between adjacent fibrils and results in a steadily increasing resistance to plastic deformation. The sliding motion of fibrils produces a shear stress on skewed fibrils and this causes a slight shear displacement of microfibrils. But in spite of its smallness, this shear displacement enormously extends the interfibrillar tie molecules by chain unfolding and thus increases their fraction in the amorphous layers.

AbstractThe ductile behavior of crystalline polymers between liquid nitrogen and room temperatures is primarily due to enhancement of crazing by the presence of gases (N2, A2, O2, CO2) of high thermodynamic activity, i.e., close to their condensation point. Without the presence of such a gas, at temperatures sufficiently above its condensation point so that its activity has dropped to a few percent, the crystalline material is rather brittle although some very limited crazing can be detected. As expected, the ductility and the amount of crazing are very nearly the same in helium as under vacuum. The crazes in smectic PP are extremely long, often traversing the entire width (0.5 cm) of the sample, and generally very similar to crazes in glassy amorphous polymers. They are thin and become thicker at higher temperatures. In monoclinic PP with well developed spherulitic structure, crazing occurs along spherulite diameters that are perpendicular to the stress direction but not along the boundaries between spherulites. In general, the length of a craze equals the spherulite diameter. At low temperatures all crazes originate on the outer surface of the sample which is in direct contact with the gas.The action of the gas is due to two mechanisms. First, the adsorbed gas reduces the surface energy of the polymer, facilitating the creation of a new surface in the holes and voids of the craze. Second, the gas becomes highly absorbed at the tip of an incidental flaw or of an existing craze, since these are regions of high dilatant stress. The locally absorbed gas acts as a plasticizer easing the flow involved in the nucleation and growth of the craze, i.e., in the formation of fibrillar material of the craze.

The failure of an axially strained polymer solid having a fibrous structure is caused by formation, coalescence, and growth of microcracks up to critical size crack, which then propagates catastrophically through the cross-section of the sample. The primary candidates for microcrack formation are the ends of microfibrils where the material connection by tie molecules to the rest of the sample is almost completely interrupted. The opening of microcracks and sliding motion of fibrillar elements ruptures locally the most strained taut tie molecules and, thus, produces radicals detectable by ESR. But, chain rupture is the consequence and not the cause of displacement of the strong fibrillar elements. It also does not substantially affect the load carrying properties of the sample which mainly depend on the lateral autoadhesion of microfibrils and fibrils and on their quasi-viscous resistance to axial displacement. Hence, one has to reject the completely inadequate models trying to base the observed load-elongation curve of such samples on the load carrying properties of those tie molecules which are eventually ruptured upon straining. Some examples of these models are treated explicitly.

ESR detects radicals formed in a strained polymer sample as a consequence of chain rupture and, hence, for a while was considered the best method for investigation of molecular effects during tensile straining. It turned out that a sufficiently high number of radicals for ESR detection is only obtained in highly oriented material with well developed fibrous structure.

AbstractDuring extrusion the main deformation and orientation of macromolecules is achieved by the flow component with longitudinal gradient. The orientation increases drastically if some solidification occurs during flow, yielding row‐nucleated cylindrites and even fully oriented hard elastomers. In all cases the basic elements are stacks of very thin (∼100Å) folded‐chain lamellae connected by very few tie molecules. The plastic deformation of the solid transforms the original lamellar material into the extremely well oriented fibrous structure with high anisotropy of physical properties. The basic element are the highly aligned, very long and thin microfibrils bundled into fibrils. The axial strength of microfibrils is caused by the great many taut tie molecules connecting as almost crystalline bridges the crystalline blocks across the interposed amorphous layers. In plastic deformation of fibrous material the fibrils are sheared and longitudinally displaced. The latter mode is responsible for almost all the observed elongation. It smooths the structural defects on the surface of fibrils caused by the ends of microfibrils and thus produces a better lateral fit of fibrils resulting in rapidly increasing resistance to plastic deformation. The former mode extends the interfibrillar tie molecules and hence drastically increases their fraction per amorphous layer.

AbstractStress crazing is studied in three forms of crystalline, isotactic polypropylene (PP): (1) smectic/nonspherulitic, (2) monoclinic/nonspherulitic, and (3) monoclinic/spherulitic PP. Optical and scanning electron microscopy as well as stress—strain measurements are used to characterize crazing behavior in these three forms as a function of temperature (−210 to 60°C) and of the gaseous environment (vacuum, He, N2, Ar, O2, and CO2). Forms 1 and 2 are found to craze much like an amorphous, glassy polymer in the temperature range between −210 and −20°C, irrespective of environment. The plastic crazing strain is large close to the glass‐transition range (ca. −20°C) of amorphous PP and in the neighborhood of the condensation temperature of the environmental gas. Near condensation, the gas acts as a crazing agent inasmuch as the stress necessary to promote crazing is lower in its presence than in vacuum. A gas is the more efficient as a crazing agent, the greater is its thermodynamic activity.Spherulitic PP (form 3) crazes in an entirely different manner from an amorphous, glassy polymer, showing that the presence of spherulites influences crazing behavior much more profoundly than the mere presence of a smectic or monoclinic crystal lattice. Below room temperature, crazes are generally restricted in length to a single spherulite, emanating from the center and going along radii perpendicular, within about 15°, to the direction of stress. They never go along spherulite boundaries. Gases near their condensation temperature act as crazing agents much as in nonspherulitic PP. Above room temperature the crazes are no longer related to the spherulite structure, being extremely long and perfectly perpendicular to the stress direction. Apparently the crystals are softened enough by thermally activated segmental motion to permit easy propagation of the craze. The morphology of the fracture surfaces and its dependence on temperature and environment is described and discussed. Concerning the action of gases as crazing agents it is argued that the gas is strongly absorbed at the craze tip, where stress concentration increases both the equilibrium gas solubility and the diffusion constant. Hence, a plasticized zone is formed having a decreased yield stress for plastic flow. This is considered to be the main mechanism by which the gas acts as a crazing agent. In addition, reduction of the surface energy of the polymer by the adsorbed gas eases the hole formation involved in crazing.