A. Peterlin

Molecular model of drawing polyethylene and polypropylene

Morphological studies of plastic deformation of single crystals, thin layers, and bulk samples together with mechanical, X-ray and infra-red data revealed the existence of three stages in cold drawing of crystalline polymer: the plastic deformation of the original spherulitic structure, the discontinuous transformation of the spherulitic into fibre structure by micronecking, and the plastic deformation of the fibre structure. The initial material, which has low strength and high ductility, consist of stacks of parallel lamellae with few interlamella links. It deforms plastically by stack rotation, sliding of lamellae, phase change and twinning of crystal lattice, chain slip and tilt until the predeformed lamellae reach the position of maximum compliance for fracture by micronecking. The micronecks transform every single lamella into microfibrils of between one and three hundred angstroms in width, consisting of folded chain blocks broken off the lamella primarily by chain slip in the boundary layers between adjacent mosaic blocks. The chains bridging the crack are partially unfolded during the micronecking process. They connect in axial direction the blocks in the microfibril as intrafibrillar tie molecules. The number of microfibrils per cm of crack length increases with molecular weight. The draw ratio of the microfibrils and the axial separation in the microfibril of the originally adjacent crystal blocks increase with the average distance between microfibrils and, hence, decrease with increasing molecular weight. The concentration of micronecks in every stack of lamellae in a thin destruction zone produces a bundle of microfibrils of rather uniform draw ratio. Such a fibril measuring a few thousand angstroms in width includes the interlamella ties of the original sample as interfibrillar tie molecules connecting adjacent microfibrils. The concentration of micronecks also provides the conditions for a nearly adiabatic heating of the generated fibril by the transformation work in the destruction zone. The local temperature rise imparts so much mobility to the chains in the crystal blocks that during subsequent cooling to ambient temperature, the long period becomes adjusted to this temperature. The more or less random distribution of destruction zones in the neck makes the transformation from spherulitic to fibre structure appear to be a gradual process in spite of the discontinuous transformation in the micronecks. The plastic deformation of the new fibre structure can proceed only by longitudinal sliding of microfibrils past each other, a process limited by interfibrillar tie molecules. Hence, high molecular weight samples with many interlamella links exhibit a smaller draw ratio than lower molecular weight material. The three stages are to some extent intermixed in the neck. In the initial neck characterised by a low draw ratio and rather gentle constriction, the transformation into the fibre structure is not complete, so that some of the remains of the original microspherulitic structure are still present in the necked portion. They are destroyed during subsequent drawing which completes the transformation and also deforms the fibre structure. The sharply constricted mature neck, however, yields a high draw ratio which is composed of the draw ratio of microfibrils and of subsequent sliding motion of the microfibrils. The technically important natural draw ratio is the maximum draw ratio obtained with the sample under the conditions of the experiment. It seems to be higher than the draw ratio of the microfibrils.

Dependence of diffusive transport on morphology of crystalline polymers

Abstract The sorption and diffusion of low molecular weight penetrants proceeds almost exclusively through the amorphous component of the semicrystalline polymer solid. The diffusive transport properties and geometrical distribution of the amorphous component are substantially modified by mechanical and thermal treatment. Deformation of spherulitic material first loosens the structure and then transforms it into a densely packed fibrous structure with a great many taut tie molecules in the amorphous component. Annealing lets the crystals grow in thickness, removes crystal defects, sharpens the boundaries between crystalline and amorphous component, and relaxes the taut tie molecules. The resulting changes of transport properties cannot be described in a satisfactory manner by crystallinity and orientation but requite a detailed consideration of morphology. The elastic tensile deformation enhances sorption and diffusion by reducing the density of amorphous component. The high anisotropy of diffusion and th...

Polymer Deformation. VI. Twinning and Phase Transformation of Polyethylene Single Crystals as a Function of Stretching Direction

The deformation occurring during the drawing of polyethylene single crystals consisting only of {110} fold domains was studied by means of electron diffraction and bright‐field and dark‐field electron microscopy. Samples of several types of polyethylene crystals were used; all crystals studied being drawn 25% on a Mylar substrate. This paper is particularly concerned with the relationship between the draw direction and the crystal axes as it affects the type of deformation. Four different types of deformation, involving different combinations of twinning and phase changes, can be distinguished as follows: (1) When the draw direction is near the b axis, a phase transformation from orthorhombic to monoclinic occurs such that (210)Mo corresponds to (200)Or. (2) When the draw direction lies at an angle between b and 〈110〉, i.e., is nearly perpendicular to a growth face, the phase transformations described here under 1 and 3 both occur. (3) When the draw direction is nearly parallel to 〈110〉, a phase transfor...

Plastic deformation of polymers with fibrous structure

SummaryThe 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.ZusammenfassungDie plastische Verformung von Fasermaterial kommt vorwiegend durch Gleitverschiebung der Fibrillen zustande. Die Verschiebung ihrer Schwerpunkte kann in erster Näherung durch eine der Probenverformung entsprechende affine Transformation beschrieben werden. Eine derartige Gleitverschiebung der Fibrillen beeinflußt nicht die allgemeine Morphologie der Mikrofibrillen. Durch die dabei auftretende Kettenentfaltung an den Mikrofibrillenden, die als Punktfehler des Mikrofibrillengitters wirken, werden jedoch die Oberflächenrauhigkeiten der Fibrillen weitgehend ausgeglättet. Als Folge davon verbessert sich der Seitenkontakt zwischen Nachbarfibrillen, was zu einer progressiven Widerstandserhöhung der plastischen Verformung führt. Die Gleitverschiebung der Fibrillen erzeugt an asymmetrischen Fibrillen auch eine Scherspannung, die eine kleine Scherverschiebung der Mikrofibrillen verursacht. Diese an und für sich kleine Verformung dehnt jedoch durch Kettenentfaltung ganz erheblich die interfibrillaren Verbindungsmoleküle aus und erhöht auf diese Weise ihren Anteil in den amorphen Schichten zwischen den Kristallblöcken.

Evaluation of Small‐Angle X‐Ray Scattering of Polymers

The influence of the particle factor and lattice factor on the positions of the small‐angle reflections of polymer crystalline samples is investigated. Since a single‐crystal cake or a polymer solid crystallized from melt supposedly consists of stacks of nearly parallel plate‐like lamellae, one may describe the scattering behavior of polymer crystals based on a periodic step function with fluctuating periods (linear paracrystal). In this model the particle factor as well as the lattice factor may shift the scattering maxima out of the positions given by the Bragg equation. Good agreement with the observation that the first order leads to a larger spacing than the second one may be obtained by introduction of an unsymmetrical distribution function for the lamellar thickness.

Infrared studies of drawn polyethylene part I. Changes in orientation and conformation of highly drawn linear polyethylene

Abstract Infrared spectroscopy is applied to discuss the orientation, the crystallinity, and the conformation of chain segments in the amorphous regions in drawn high-density polyethylene. The orientation of the crystals as well as the crystallinity are derived from the dichroism and the absorbance, respectively, of the band at 1894 cm-1. The orientation and some aspects about the conformation of the chain segments in the amorphous regions can be obtained from the bands in the 1400-1300 cm-1 region (gauche) and at 1078 cm-1 (gauche and trans). The dichroic studies show a high degree of orientation increasing with draw ratio λ for the chain segments in the crystals, but a low orientation reaching saturation at λ between 5 and 10 for those in the amorphous regions. The experiments indicate a change in crystallinity during the drawing process which depends on the thermal treatment of the undrawn sample. In the amorphous regions the number of CH2 groups in gauche conformations decreases up to λ between 10 and...

Surface Replicas of Drawn Polyethylene. III. The Morphology of Drawing with Neck Formation

During the first stage of plastic deformation before the neck formation the crystal lamellae are rotated into the position of maximum compliance to applied stress that causes the stress‐strain curve to drop to the plateau characteristic for deformation with neck propagation. The plastic‐flow pattern in the neck results in an asymmetry of strain momentum imposed on tie molecules which during annealing lets the lamellae on the sample surface rotate about an axis parallel to the surface and perpendicular to the draw direction. The temperature at which such a rotation starts increases with the draw ratio. Concurrently the c axis moves away from the draw direction. The rotation of lamellae in the surface layer is nearly prevented by keeping the sample at constant length during annealing and in the interior of the sample by the surrounding lamellae even if the sample is free to shrink. The difference in long period which depends on the temperature of drawing in bulk samples and on the thickness of original crys...

Plastic Deformation of Polypropylene. III. Small‐Angle X‐Ray Scattering in the Neck Region

The small‐angle x‐ray scattering (SAXS) pattern of polypropylene (PP) drawn at various temperatures (Td = 20°, 100°, and 135°C) was investigated in a range of very closely spaced draw ratio (λ) values through the neck region. In the zone before the neck starts (λ = 1.05–1.1) the uniform diffraction ring due to microspherulitic material with a long period L0 changes into a four‐point diagram superposed onto an elliptical halo. At the beginning of the neck (λ = 1.3–2.5) two meridional maxima arising from a new fibrous structure with a limiting long period LT appear. Irrespective of whether L0>LT or L0<LT the transition between L0 and LT is discontinuous. The four‐point pattern reminiscent of the spherulitic structure remains observable up to the end of the neck. In the same manner as in the case of PE, stacks of lamellae exhibiting the most favorable orientation for chain tilt and slip start being plastically deformed and finally break into folded‐chain blocks which are incorporated into microfibrils. The f...

RAMAN SCATTERING FROM LONGITUDINAL ACOUSTICAL VIBRATIONS OF SINGLE CRYSTALS OF POLYETHYLENE

Raman scattering from the longitudinal‐acoustical vibration of single crystals of polyethylene has been observed using an argon laser and an iodine filter. The frequencies observed are in the range 10–40 cm−1 and are inversely proportional to the thickness of the single crystals which varied between 90 and 250 A.

Drawing and annealing of fibrous material

The more‐than‐linear increase of elastic modulus with draw ratio, the gradual disappearance of meridional SAXS maximum, the drastic drop of elastic modulus after annealing and its recovery upon standing at room temperature if the sample was annealed with fixed ends so that it did not shrink, and the shape stability of such polyethylene samples and of superdrawn material (polyethylene, polypropylene, polyoxymethylene) during new annealing can be easily explained by the microfibrillar model of fibrous structure which was developed some years ago on the basis of electron microscopy and x‐ray and ir investigation of plastically deformed linear polyethylene and isotatic polypropylene.