AbstractFibers of isotactic polypropylene drawn at different temperatures show aging phenomena after drawing. In particular a modulus stiffening is observed at room temperature where the modulus increases with the aging time. In the same time scale the birefringence is constant, while drastic effects on the transport properties are observed. The aging phenomenon can be explained in terms of two mechanisms: a secondary crystallization and/or a densification of the amorphous component.

AbstractQuenched films of isotactic polypropylene were drawn at 110°C up to draw ratio λ = 18. The axial elastic modulus was measured as function of λ up to the highest achieved λ. The sorption and diffusion of CH2Cl2 at 25°C in the undrawn and drawn samples were studied. Exclusively transparent samples were used for the measurement of the density and transport properties. This reduces the maximum usable draw ratio to 15. The drawing process is inhomogeneous with neck propagation. In the neck the draw ratio increases by about 6. As a consequence of the increasing fraction of taut tie molecules the axial elastic modulus increases faster than the draw ratio. The transport parameters D, S, and λ indicate that the original lamellar morphology is completely transformed into the microfibrillar structure.

AbstractLinear polyethylene both as drawn, or drawn and subsequently annealed with free ends, changes its length, density, crystallinity, elastic modulus, sorption, and diffusivity as the sample stands completely unrestrained at room temperature. Most of these changes occur during the first few hours. But they are important on a molecular scale since they suggest strongly that drawn, and drawn and annealed samples are far from equilibrium. As a consequence of the tendency of each mobile tie molecule in the amorphous conformation to retract and to crystallize, the specimen approaches but does not reach complete equilibrium. The transient seems to be caused by slow crystallization of tie molecules which creates crystalline bridges across the amorphous layers.

The additional attenuation of the spin echo of a pulsed magnetic field gradient NMR caused by the diffusion of the spins carrying molecules is calculated for a time dependent diffusion coefficient. The cases of a Constant D0, of an exponential decay with time, and an abrupt drop from D0 to zero at time t0 are considered. In all cases the time dependence of the experimentally observed attenuation is less steep than that of D. The time scale, however, Changes very little. The calculation was based on the consideration that the diffusion transports the precessing spins to a location with a different Larmor frequency so that a complete reversal after the 180° pulse is not possible any more. Hence, any deviation from a straight line in a plot of the excess attenuation versus δ2(1 − δ/3Δ) may by interpreted in terms of a time dependence of the diffusion coefficient.

AbstractThe values of drawing dependence of the density ρ, axial elastic modulus E, and maximum draw ratio λ of crosslinked low‐density polyethylene (CLPE) rather similar to those obtained with un‐crosslinked branched material of similarly low density. Very much the same applies to the equilibrium concentration of sorbed methylene chloride in the amorphous component and the zero‐concentration diffusion coefficient D0. The exponential concentration coefficient γD, however, even at the maximum draw ratio, shows no indication of the rapid increase so characteristic of the completed transformation from the lamellar to the fibrous structure. On the basis of this finding, one can understand the small deviations in the dependence of the mechanical properties between the crosslinked and uncrosslinked branched material. The segments between the crosslinks, much shorter than the free molecules, favor the formation of the interfibrillar tie molecules that limit the drawability of the sample. But since they cannot be extended to the same length as the free molecules, they contribute less to the total fraction of tie molecules per amorphous layer and hence yield a smaller axial elastic modulus.

AbstractAll the improvements of the independent‐rod model of longitudinal accordian‐type acoustic mode (LAM) oscillations have assumed that the oscillation energy is retained either on the isolated macromolecule oscillating in a vacuum or in a narrow cylinder containing the straight sections of the macromolecules in the crystal lattice and their straight continuations through the amorphous layers. According to such concepts, concentration of the oscillation energy in gauche defects or amorphous layers occurs, respectively, whenever the axial elastic modulus of the straight sections (crystal lattice) is very much larger than that of the kinked sections (amorphous layers). The effect is enhanced by low crystallinity. Actually such behavior has never been observed. To agree with experimental data the model has to be modified in such a manner that the oscillation amplitude in the amorphous layer steadily decreases with increasing distance from the boundary between the two phases. The necessary large damping of the LAM oscillation in the kinked sections results from true damping in the viscoelastic amorphous component and energy transfer to adjacent chains which turns out to be just as easy as energy conduction along the kinked chain. Such a transfer is equivalent to radiation of the oscillation energy in all directions in the kinked phase. As a consequence of damping, the coupling of chains in adjacent crystals becomes so small that it may be completely neglected. Such a model explains in a satisfactory manner the observed accordion Laser‐Raman spectra of the semicrystalline polymers and the infrared absorption of paraffins in the liquid state.

AbstractShrinkage of unconstrained low‐density polyethylene samples and the retractive stress of samples with ends fixed have been investigated as a function of the annealing time tA and temperature TA on material drawn at room temperature to draw ratios λ between 4 and 6. The shrinkage increases with tA and TA. The retractive stress on a sample annealed with ends fixed goes through a maximum as the sample is annealed and then drops to a limiting value which increases with TA as long as TA is at or below 80°C and rapidly decreases with higher TA. The drop from the maximum to the limiting retractive stress, slow at lower TA and rapid at higher TA, seems to be a consequence of rapid pulling of chain segments out of crystal block in which interfibrillar tie molecules are anchored. This process is facilitated by the high TA, which softens the crystal matrix. At constant end‐to‐end distance, the contour length of the tie molecules is irreversibly increased, and this causes a reduction in the contribution of the affected tie molecules to the overall retraction stress. Hence one finds a substantially higher retraction stress during first heating than during subsequent cooling and heating of the drawn sample.

AbstractOne can reproduce the observed accordion‐type laser‐Raman (ALR) scattering of highly drawn linear polyethylene if one assums that any gauche defect in the crystal lattice which interrupts the all‐trans conformation sequence of the molecular chain completely decouples the accordion‐type longitudinal oscillations of the two sections on both sides of the defect. Each oscillates independently of the rest. The length of the section, smaller than the full length of the straight chain between the crystal surfaces, determines the frequency of the ALR absorption. One such defect per five chain stems of the ideal crystal yields a straight‐length distribution which agrees sufficiently well with that derived from the ALR spectrum. Small‐angle x‐ray scattering very generally registers the resulting decrease of the electron density of the crystalline component without yielding more detailed information about the location and frequency of such gauche defects.

AbstractThe acoustic emission from a crazing polyvinyltoluene in a tensile and bending experiment is described. Acoustic emission appears as a series of bursts which most likely correspond to the initiation and growth of crazes. The emission intensity is characterised by acoustic activity (pulse rate) measured by the ring‐down technique. The average activity increases with strain. During repeated loading the acoustic activity shows a measurable intensity and significant rise only beyond the maximum strain of the former runs. This is equivalent to Kaiser's effect in metals. Acoustic emission during the creep experiment occurs in three characteristic periods. They are characterized as the relaxation, fatigue, and breakdown periods. Visual observations indicate that the relaxation period corresponds to the initiation, and the fatigue period to the growth of crazes. In the breakdown period a macroscopic crack develops and the sample fails.

In the creep experiment the brittle fracture of the unoriented semicrystalline polymers at very small and very high tensile load with the intermediate ductile region may be explained by the competition between crazing and shear band formation during the microcrack growth phase. The former type of microcrack growth leads to brittle fracture while the latter type yields necking which transforms the original lamellar structure into the final fibrous structure. The actual fate of the strained sample depends on the growth time of the craze, tg, and of the shear band formation time, ts. If tg ts, the material will deform plastically. The failure of the fibrous material seems to occur when the ratio between the average distance and diameter of the microcracks reaches a value about 3. The microcracks seem to form primarily at defects of the microfibrillar structure, i.e., at the ends of microfibrils where the axial connection of subsequent crystal blocks through the amorphous layers by a great many taut tie molecules is either completely interrupted or at least drastically reduced. The stress concentration resulting from the opening of these defects into microcracks may rupture also some of the adjacent microfibrils. Such nucleation and subsequent lateral growth of the microcrack ruptures the taut tie molecules in its path. The ruptured molecules yield free radicals which can be monitored by electron spin resonance.

An empirical relationship between the diffusion coefficients of gases in three polyethylenes and their diffusion coefficient in natural rubber is given. The diffusion coefficients are given within a relative error of 4.7%. This relationship is interpreted according to the fractional free volume theory. The ratios of the fractional free volumes in the polyethylenes to the fractional free volume in natural rubber are derived.

The specific concentration c a of methylene chloride, the zero‐concentration diffusion coefficient D 0 , and the concentration coefficient γ D of the diffusivity in drawn and annealed LDPE were measured. The influence of the drawing rate, of annealing with the ends of the sample free and fixed and the effects of time of standing at room temperature after annealing were investigated. The observed transport properties are in good agreement with the microfibrillar model of fibrous structure, its relaxation during annealing, and the slow crystallization of relaxed tie‐molecules upon standing at room temperature.

AbstractIn the last few years some new features of the so‐called type II diffusion have been established which confirm the first theoretical description of such a material transport into a semi‐infinite glassy medium which at a certain concentration of the sorbate is transformed into a gel. The boundary between the glass and the gel progresses at a constant velocity into the interior of the sample thus yielding a linear term in the weight gain. The gradual establishment of the concentration profile in front of this boundary yields at the beginning a square root term in the weight gain. A detailed analysis of the extensive measurements of Hopfenberg, et al. of the diffusion of n‐hexane into extremely small polystyrene spheres demonstrates that the weight gain always starts with a square root of time term. In sufficiently large spheres this contribution is soon completely overridden by the term linear in time. The spherical geometry substantially modifies the concentration profile and the weight gain. In particular the weight gain divided by the square root of time vs the square root of time shows a maximum as soon as the geometrical factors prevail over the effect of the constant velocity progression of the boundary between the glass and the gel.

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTChain Folding in Lamellar CrystalsA. PeterlinCite this: Macromolecules 1980, 13, 4, 777–782Publication Date (Print):July 1, 1980Publication History Published online1 May 2002Published inissue 1 July 1980https://pubs.acs.org/doi/10.1021/ma60076a001https://doi.org/10.1021/ma60076a001research-articleACS PublicationsRequest reuse permissionsArticle Views250Altmetric-Citations23LEARN 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

The load-elongation curve of a semicrystalline polymer with lamellar structure may have four distinct parts: the initial elastic part, the yield area, the necking area where the lamellar morphology is transformed into the fibrous structure, and the drawing area of the fibrous material up to the failure of the sample. The neck formation stops the elastic deformation of the lamellar structure and hence reduces the upper yield load to the load needed for the continuous necking. The drawing of the fibrous structure extends the interfibrillar tie molecules and shears the crystal blocks. The increase of the contribution of taut interfibrillar tie molecule to the axial elastic modulus of the drawn sample comes to an end as soon as the tie molecule gets so far unfolded that at least one end is completely pulled out of the crystal block it was originally anchored in. Swelling of the amorphous regions so much weakens these areas that they are not able to exert enough force on the lamellae for their complete transformation into microfibrils. But the easy separation of the lamellae perpendicular to the direction of the applied tensile force creates a great many channels which are responsible for the high gas permeability. The bending of the same lamellae yields the high rubber-like strain behavior in the draw direction. The fracture of the sample is caused by the growth and coalescence of microcracks up to a critical size crack. The process involves the rupture of most molecules which connect the opposite sides of any microcrack. This rupture increases the work per surface area of the microcrack but does not cause the failure of the sample.

AbstractAs a rule, the large increase of elastic modulus with increasing draw ratio obtainable in highly‐drawn or extruded semicrystalline polymers is not reflected in a similarly large increase of strength. This is closely connected with the wellknown fact that with increasing plastic deformation one obtains fibrous material with decreasing strain to break. The axial elastic modulus is mainly caused by the taut tie molecules which bridge the amorphous layers between consecutive crystal blocks and thus provide an efficient axial force transmission through the sample. The defects at the ends of microfibrils interrupt this transmission because they contain few if any taut tie molecules connecting the end of microfibril with adjacent fibrillar elements. As a consequence of the small number of such ends, they only marginally reduce the elastic modulus. But as the mechanically weakest areas of the fibrous material, they drastically depress the strength. They fail as soon as the strain concentration upon them reaches their strain to break. The growth and coalescence of resulting microcracks finally lead to bulk fracture as the growing crack reaches critical dimensions.

If one interprets the ALR absorption lines of crystalline polymers in terms of oscillating elastic rods of length equal to the thickness L of the crystalline lamellae, or the thickness D of the crystalline core of the lamellae, one obtains for the elastic modulus of these rods values which are substantially higher than the axial elastic modulus Ec of the crystal lattice. The effect is particularly conspicous with polymers which in the crystalline state exhibit a helical-chain conformation. The explanation of this effect seems to be the elastic coupling of longitudinal oscillations of chain stems of subsequent lamellae by the intervening amorphous layers. The apparent increase of the ALR oscillation frequency by such coupling is particularly large if the elastic modulus Ea of the amorphous layers is equal or even higher than that of the crystal lattice. The high value of Ea derived from such an analysis is a consequence of the very high ALR frequency (∼0.3 THz), which makes the amorphous layer react as a rigid glass.