Mechanical and transport properties of highly drawn isotactic polypropylene

Quenched 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.     

Plastic deformation of polytetramethylene oxide. I. Influence of molecular weight distribution,crystallinity, and structure

Polytetramethylene oxide with a planar zig-zag structure similar to polyethylene can be obtained with narrow molecular weight distributions. The plastic deformation of samples differing in molecular weight, molecular weight distribution, crystallinity, and structure has been studied. The degree of crystallinity of the undeformed annealed samples, as studied by NMR and DSC, leads to a value of the enthalpy of melting ΔH_m = 167 J g⁻¹, supporting the lower of two previously reported values. The low natural draw ratios and low Young's modulus of the drawn samples, together with the effect of blending a small amount of high molecular weight material into a low molecular weight sample, highlight the role of the flexibility and of the high molecular weight tail of the distribution in the plastic deformation process. The stress required for the propagation of the neck and the tensile strength are found to be linear functions of, respectively, the natural and maximum draw ratio.     

Polymer fibers drawn to the limit

The orientational drawing of polymers is known to be terminated because of sample rupture. The limiting draw ratio λ_lim reached may be different (either large or small) depending on the polymer and the actual drawing conditions. The purpose of the present work is to identify the change of supermolecular structure of polymer fibers which results in the termination of orientational drawing. Small-and wide-angle x-ray diffraction were used to study the variation of geometrical parameters of this structure with increasing draw ratio λ. The geometrical parameters discussed are the dispersions (fluctuation) of long periods and of longitudinal sizes of crystalline as well as amorphous regions. In this study we used fibers of poly(vinyl alcohol), poly(ε-caprolactam), polyoxymethylene, and poly(4,4′-diphenyloxide) pyromellitimide. It is found that the long period dispersion of these polymers, drawn under different conditions, increases to approximately the same value for different samples drawn to the limit, this relative standard deviation δ_L of long periods being 0.30-0.40. It is also found that the crystallite size dispersion does not increase with increasing λ; the increase of λ_L is due to increasing dispersion of the amorphous region lengths. For poly(vinyl alcohol) fibers drawn to the limit under different conditions and which have different λ_lim, the relative standard deviation of the sizes of amorphous regions δ_A turned out to be about the same (ca. 0.60). The latter evidence gives grounds to suggest that the rupture of polymers under drawing is associated with reaching a high degree of amorphous region size dispersion. In those regions which are considerably below the average size there probably will appear local overstress and molecular ruptures because the relative deformation of these regions is much larger than that of the adjacent regions in the cross section of the sample.     

Plastic deformation of polypropylene VII. Long period as function of temperature and rate of drawing

Polypropylene fibers prepared by quenching in ice-water were drawn at 25, 80, 120, and 140°C to a draw ratio between 6 and 8 at draw rates 0.05, 0.5, 5, and 50 cm/min. The long period increases almost linearly with the draw rate for drawing at 25°C and decreases for drawing at higher temperatures. The effect in the latter cases is an annealing effect. As a consequence of the shorter exposure of the drawn fibers to the high temperature at higher draw rate, the long-period growth proceeds for a shorter time and hence results in a smaller increase of long period. At 25°C, however, the long-period growth is negligible. The increase of long period with draw rate is the consequence of higher adiabatic heating as calculated from the energy input during the plastic deformation which transforms the spherulitic into the fibrous structure. One concludes that the long period established during this transformation depends on the maximum temperature reached in the micronecking zone and not on the macroscopically observed temperature of the sample in the neck.     

Mechanical and transport properties of drawn crosslinked low-density polyethylene

The 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.     

Mechanical properties of ultra-oriented polyethylene

Semicrystalline polymers generally exhibit moduli well below their theoretical limit due to chain folding and to lack of crystal alignment. Modulus increases attainable through standard drawing procedures are limited by sample fracture before large draw ratios are reached. Using an Instron capillary rheometer which allowed a draw ratio of > 300, transparent polyethylene strands of unusually high c-axis orientation have been produced by a combination of pressure and shear. The virtually perfect crystalline orientation and evidence for extended chains confirm that a significant improvement in modulus can be realized by this technique. The dynamic tensile storage modulus was measured by Vibron over the temperature range ?160°C to +120°C. Room-temperature moduli were 7 × 10¹¹ dyne/cm², higher than any reported values for drawn polyethylene. Values also remained above 10¹¹ dyne/cm² even at 120°C. The moduli and morphological data have been related by a model consisting of an extended-chain component in paralled with a conventional drawn morphology. Experimental and calculated moduli are compared and related to available theory.     

Plastic deformation of polypropylene. IV. Wide-angle x-ray scattering in the neck region

Wide-angle x-ray scattering (WAXS) patterns of two polypropylene samples, a quenched sample drawn at 21°C and an annealed sample drawn at 100°C, were investigated in a range of values of draw ratio λ very closely spaced through the neck region. In both cases, a range of small λ where deformation occurred by spherulite deformation was followed by one of higher λ where microfibrils were formed. The contribution to the WAXS pattern of microfibrils could be clearly distinguished from that of deformed spherulites because of the better orientation parallel to the draw direction of the former as compared to the latter. Additionally, for a drawing temperature of 21°C, microfibrils crystallize in the “smectic” phase as compared to the monoclinic phase for the initial sample and deformed spherulites. At this temperature, plastic deformation proceeds through the spherulite deformation mechanism up to λ = 1.4 accompanied by an increase in chain orientation with increasing λ. For λ > 1.4 plastic deformation appears to occur exclusively through microfibril formation. For drawing at 100°C, spherulite deformation is accompanied by very little change in chain orientation up to λ = 2, where microfibril formation begins. For λ > 2 (T_d = 100°C) plastic deformation is accompanied by both microfibril formation and some spherulite deformation as reflected by changes in both orientation and crystallite size. At this temperature the lateral crystallite size in the microfibrils is related to the long period according to the “equilibrium crystallite shape” previously found for annealed polypropylene.     

Structural changes on hot-rolling of polyethylene. I. Crystallite orientation and lamellar deformation

The crystallite orientation and lamellar deformation produced by hot-rolling in polyethylene have been investigated. In the lightly rolled stage, the [110]^* axis of polyethylene crystals orientates in the plane perpendicular to the rolling direction and the c axis becomes aligned with the rolling direction. In the heavily rolled stage the a, b, and c axes coincide with the macroscopic directions in the sample. These orientational changes are interpreted in terms of a slip mechanism. Small-angle x-ray scattering (SAXS) investigations of hot-rolled polyethylene show the following. (1) There are two kinds of lamellar structures; one in which inclined lamellae give a four-point diagram in the SAXS photograph and another in which lamellar normals are oriented around the rolling direction even at the lightly rolled stage. The latter structure is attributed to the mechanism of unfolding and recrystallization. (2) The chain-fold length in the original structure remains unchanged in the lamellae up to a roll ratio of four although the apparent long period decreases. This is explained by inclination of polymer chains in the lamellae. Further rolling aligns the polymer chains with the rolling direction and the fold length decreases.     

The drawing behavior of polyethylene copolymers

The tensile drawing behavior of a range of selected polyethylene copolymers has been studied. Sheets were prepared by quenching molten polymer into cold water. Two-centimeter-gauge-length samples were then drawn in air at 75 or 115°C in an Instron tensile testing machine at a crosshead speed of 10 cm/min. It was found that even at the very low concentration of one side branch per 1000 carbon atoms there was a very marked effect on the strain hardening behavior and the maximum draw ratio that could be achieved. The reduction in draw ratio increased with increasing branch concentration, and long branches were more effective than short branches in limiting the draw ratios achieved. The similarity between these effects and the effects of increasing M?_w or radiation crosslinking is noteworthy. This suggests that even a very small concentration of branches can significantly reduces the moleculer motions required for the process of plastic deformation. The Young's modulus/draw ratio relationship follows a pattern virtually identical to that observed in the case of homopolymers.     

Solid-state extrusion of chain-extended polyethylene

Billets of chain-extended polyethylene were prepared from Alathon 7050 (M_w 59,000, M_n 19,000) in an Instron capillary rheometer by crystallization at a constant pressure of 460 MPa, at a series of teimperatures from 198 to 221°C corresponding to varying degrees of undercooling. This gives chain-extended morphologies with a range of crystallinites and lamellar thicknesses. The billets were then solid-state extruded at 100°C through a conical die with 20° entrance angle up to an extrusion draw ration 23.4. Thermal behavior was studied with differential scanning calorimetry. The orientation function measured by wide-angle x-ray diffraction showed higher orientation function measured by wide-angle x-ray diffraction showed higher orientation at equivalent draw ratio when the initial billets were crystallized at lower temperatures. Drawing efficiency, defined as the ratio of molecular draw ratio (from shrinkage) to extrusion draw ratio correspondingly increases, reaching a maximum of 0.71 in our solid-state extrusion. These studies show that highly chain-extended polyethylene, i.e., with few chain entanglements, draws poorly. Drawability was improved by increasing chain entanglements by lowering the crystallization temperature. Electron micrographs of fracture surface replicas of extrudates revealed the coexistence of undeformed, tilted, partially drawn lamellae and fibrillar structure consistent with the cahange of morphologies in Peterlin's model of plastic deformation.     

A structural model for high-modulus polyethylene derived from entanglement concepts

Starting from the concept that the entanglement network is a controlling factor in polymer deformation, a molecularly based model has been constructed for polyethylene, drawn or extruded to high extension ratios λ. It predicts the experimentally observed form of the increase of Young's modulus E with λ: E^?1 = B + Cλ^?2. The model structure consists of imperfect crystalline microfibrils 10-30 nm in diameter and length αλ², about 1 μm at λ = 30. The microfibrils terminate at clusters of entanglements, and are embedded in a matrix of low modulus. This structure is very similar to that derived from solution-grown shish-kebab material. Available melting-point data for highly extended material fit the structural model well.     

Piled-lamellae structure in polyethylene film and its deformation

The fine structure of polyethylene film has been investigated by using a high-resolution scanning electron microscope equipped with a field emission source. The original film surface of a-axis-oriented blown polyethylene film and the surface of a necked region formed by drawing the film in the machine direction were observed. High magnification electron micrographs indicate that the basic unit of internal texture of this film consists of piled-lamellae units, each pile containing three to ten lamellar crystal sheets. The piled-lamellae unit acts as one body and does not separate into single lamellae during deformation. Many tie fibrils are formed between adjacent piled-lamellae units, when the film is drawn in the machine direction. Although little attention has been given to this mechanism, it is important in deformation. This fact seems to be reflected in different shapes of the stress-strain curves of films drawn the machine direction and perpendicular to it.     

Ultradrawing of high-molecular-weight polyethylene cast from solution. II. Influence of initial polymer concentration

Ultradrawing of films of high-molecular-weight polyethylene (M?_w = 1.5 × 10⁶) produced by gelation crystallization from solution is discussed. The influence of the initial polymer volume fraction (?) on the maximum draw ratio (λ_max) of the dried films is examined in the temperature region from 90–130°C. The results can be described very well by the relation λ_max = λ ?^?1/2 where λ is the (temperature-dependent) maximum draw ratio of the melt-crystallized film. An attempt is made to discuss the marked influence of the initial polymer volume fraction on λ_max in terms of the deformation of a network with entanglements acting as semipermanent crosslinks.     

Drawing of single-crystal mats of linear polyethylene

Drawing of single-crystal mats of linear polyethylene has been investigated. Drawing is possible at temperatures higher than about 90°C. The drawing is accompanied by distinct necking, with a large decrease in the thickness of the mat and a very high maximum draw ratio, sometimes over 30. The maximum draw ratio is approximately proportional to the thickness of the lamellae. This behavior strongly suggests the unfolding of chains during drawing. A change of orientation of crystal axes occurs before necking without change of lamellar orientation. The a axis orients in the drawing direction; the b axis orients perpendicular to the direction of drawing; and the chain axis tilts away from the thickness direction of the mat. The structure of films drawn from mats is characterized by a distinct double orientation of crystals. This biaxial orientation in the drawn films has a high degree of correlation with the orientation of crystal axes observed before necking, and suggests that necking takes place in such a way that the chain tilts gradually about the b axis and ultimately unfolds. The postulate of formation of transitory two-dimensional crystals in necking seems useful in explaining the double orientation in the drawn film. The orientation behavior of crystal axes observed before necking is not always similar to that observed in the deformation of a single crystal. The difference is thought to be due to the effect of forces induced by drawing that act in the direction normal to the lamellae within a mat.     

Oriented and thermal crystallization of poly(ethylene terephthalate)

The oriented and thermal crystallization of amorphous poly(ethylene terephthalate) (PET) films was investigated in terms of the morphological aspects. When the amorphous PET films were stretched up to the desired draw ratios in a hot water bath at 62, 72, and 80 °C, the birefringence of the specimens increased with increasing draw ratio (λ). This tendency becomes most significant when the specimen was drawn in the bath at 62 °C. The storage modulus of the specimen drawn at 62 °C was higher than those of the specimens drawn at 72 and 80 °C. The exothermic peak in the DSC curves was observed clearly for the specimen drawn up to λ=4 in the hot water bath at 80 °C, while the peak did not appear for the specimen drawn up to λ=4 at 62 °C. Under an H_v polarization condition, light scattering patterns from the specimens drawn in the hot water bath showed four lobes at small azimuthal angles and four sharp streaks at large azimuthal angles. Such a profile was independent of the drawing temperatures from 62 to 80 °C. Based on the observed H_v patterns, a model was proposed by assuming the existence of a row-nucleated sheaf-like structure whose rows were preferentially oriented at a particular angle with respect to the stretching direction. The patterns calculated by using the above model were rather close to the patterns observed. This agreement implies that row-nucleated sheaf-like texture arises with lamellar overgrowth.

     

Single-crystal-like orientation of ultrahigh-molecular-weight polyethylene by uniaxial stretching

Dried gel film of ultrahigh-molecular-weight polyethylene (UHMW PE) can be drawn to 370 times its original length at 135°C. Single-crystal mats and dried gel film of UHMW PE develop double orientation despite uniaxial stretching. This is unique for preferentially oriented UHMW PE systems. To elucidate the origin of this single-crystal-like orientation, precursors with different aspect ratios were prepared and drawn uniaxially. The degree of double orientation was measured by infrared spectroscopy. The origin of single-crystal-like orientation seems to reside in the necking region. The stacked lamellar structure is transformed into a fibrillar structure in a two-dimensional fashion. This condition is easily provided when UHMW PE single-crystal mat or dried gel film is drawn uniaxially. A draw ratio of 40 and aspect ratio of 40 are the optimal conditions to obtain a doubly oriented structure from UHMW PE single-crystal mat or gel film at 135°C.     

Stretching temperature and direction dependency of uniaxial deformation mechanism in overstretched polyethylene

In order to elucidate microscopic deformation behavior at different locations in isotropic semicrystalline polymers, the structural evolution of a preoriented high‐density polyethylene sample during tensile deformation at different temperatures and along different directions with respect to the preorientation was investigated by means of combined in situ synchrotron small‐angle X‐ray scattering (SAXS) and wide‐angle X‐ray diffraction (WAXD) techniques. For samples stretched along preorientation, two situations were found: (1) at 30 °C, the sample broke after a moderate deformation, which is accomplished by the slippage of the microfibrils; (2) at 80 and 100 °C, fragmentation of original lamellae followed by recrystallization process was observed resulting in new lamellar crystals of different thickness depending on stretching temperature. For samples stretched perpendicular or 45° with respect to the preorientation, the samples always end up with a new oriented lamellar structure with the normal along the stretching direction via a stress‐induced fragmentation and recrystallization route. The thickness of the final achieved lamellae depends only on stretching temperature in this case. Compared to samples stretched along the preorientation direction, samples stretched perpendicular and 45° with respect to the preorientation direction showed at least several times of maxima achievable stress before macroscopic failure possibly due to the favorable occurrence and development of microdefects in those lamellar stacks with their normal parallel to the stretching direction. This result might have significant consequence in designing optimal procedure to produce high performance polyethylene products from solid state. © 2014 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2014 , 52, 716–726     

Development of oriented structure and properties on drawing of poly(vinylidene fluoride) by solid‐state coextrusion

Gel films of poly(vinylidene fluoride) (PVDF) consisting of α‐form crystals were drawn uniaxially by solid‐state coextrusion to extrusion draw ratios (EDR) up to 9 at an optimum extrusion temperature of 160 °C, about 10°C below the melting temperature (T_m). The development of an oriented structure and mechanical and electrical properties on coextrusion drawing were studied as a function of EDR. Wide‐angle X‐ray diffraction patterns showed that the α crystals in the original gel films were progressively transformed into oriented β‐form crystals with increasing EDR. At the highest EDR of 9 achieved, the drawn product consisted of a highly oriented fibrous morphology with only β crystals even for the draw near the T_m. The dynamic Young's modulus along the draw direction also increased with EDR up to 10.5 GPa at the maximum EDR of 9. The electrical properties of ferroelectricity and piezoelectricity were also markedly enhanced on solid‐state coextrusion. The D–E square hysteresis loop became significantly sharper with EDR, and a remanent polarization P_r of 100 mC/m² and electromechanical coupling factor along the thickness direction k_t of 0.27 were achieved at the maximum EDR of 9. The crystallinity value of 73–80% for the EDR 9 film, estimated from these electrical properties, compares well with that calculated by the ratio of the crystallite size along the chain axis to the meridional small‐angle X‐ray scattering (SAXS) long period, showing the average thickness of the lamellae within the drawn β film. These results, as well as the appearance of a strong SAXS maximum, suggest that the oriented structure and properties of the β‐PVDF are better explained in terms of a crystal/amorphous series arrangement along the draw axis. Further, the mechanical and electrical properties obtained in this work are the highest among those ever reported for a β‐PVDF, and the latter approaches those observed for the vinylidene fluoride and trifluoroethylene copolymers. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 1371–1380, 2001     

Quantitative characterization of deformation in drawn polypropylene films

The submicroscopic morphology of uniaxially deformed isotactic polypropylene films has been examined by small-angle light scattering (SALS), electron microscopy, optical microscopy, small-angle x-ray scattering (SAXS), wide-angle x-ray diffraction, birefringence, sonic modulus, and density methods. Several new interpretations and extensions of existing theories are developed and verified experimentally as follows. (1) The V_v SALS pattern is shown to be a new tool for the identification of the sign of the birefringence of spherulites too small to be seen in the optical microscope. The theoretical dependence of the V_v SALS pattern is developed and verified experimentally with patterns from isotactic polypropylene, polyethylene, Penton, nylon 6,6, poly(ethylene terephthalate), and nylon 6,10. (2) Intraspherulitic lamellar behavior during deformation can be identified from the SAXS pattern. This includes quantitative evaluation of the long spacing between lamellae and their average orientation. (3) The two-phase sonic modulus theory is valid over the wide range of deformations, crystallinities, processing temperatures, and molecular weights used in this study. The deformation of isotactic polypropylene films drawn at 110 and 135°C. has been characterized quantitatively in terms of an integrated picture of mass movement on all morphological levels: the molecular, the interlamellar, and the spherulitic. At both temperatures, the spherulites deform affinely with extension, whereas the deformation mechanisms within the spherulite depend on the location of the radii with respect to the applied load. During spherulite deformation, lamellar orientation and separation processes predominate, whereas at high extensions, fibrillation occurs and crystal cleavage processes predominate. The noncrystalline region orients throughout the draw region. At 135°C. non-orienting relaxation processes appear in the noncrystalline region which retard the rate of molecular orientation with extension.     

Relevance of cage recombination in the plastic deformation of polymers

For a polymer in which permanent rupture of individual molecules is the rate-limiting process for plastic deformation, the kinetics of chain-end diffusion and secondary radical reactions should be compared with the kinetics of caged radical recombination in the calculation of activation parameters for plastic deformation. If mechanisms of cage escape are slower than those for cage recombination, the activation parameters for plastic deformation will differ from those for the initial bond-breaking process. For the case of polyethylene deformed in the vicinity of 250°K, the critical thermally activated event appears to involve scission of the polymer molecule near the site of an abstracted hydrogen atom. For this system the dominant cage-escape mechanism is diffusion, which is faster than either hydrogen abstraction or unzipping to the monomer. However, at low stresses the rate of cage recombination is expected to be higher than the rate of cage escape, so that the activation parameters for deformation should be the sum of those for chain scission and diffusion. The contribution of diffusion (ca. 15 kcal/mole) to the activation energy for deformation (E^*, extrapolated to zero stress conditions) is relatively modest. However, the calculated molar activation volume for deformation V^* increases by almost an order of magnitude, i.e., from ca. 10 to ca. 76 cm³/mole when diffusion is required. Consideration of experimental values of E^* and V^* for high molecular weight polyethylene indicates that, in the regime examined, chain scission plus chain-end diffusion is required to effect plastic deformation.