Interlamellar Slip Research Articles

Deformation of Poly-l-lactid acid (PLLA) under Uniaxial Tension and Plane-Strain Compression.

The ability of PLLA, either amorphous or semicrystalline, to plastic deformation to large strain was investigated in a wide temperature range (Td = 70–140 °C). Active deformation mechanisms have been identified and compared for two different deformation modes—uniaxial drawing and plane-strain compression. The initially amorphous PLLA was capable of significant deformation in both tension and plane-strain compression. In contrast, the samples of crystallized PLLA were found brittle in tensile, whereas they proved to be ductile and capable of high-strain deformation when deformed in plane-strain compression. The main deformation mechanism identified in amorphous PLLA was the orientation of chains due to plastic flow, followed by strain-induced crystallization occurring at the true strain above e = 0.5. The oriented chains in amorphous phase were then transformed into oriented mesophase and/or oriented crystals. An upper temperature limit for mesophase formation was found below Td = 90 °C. The amount of mesophase formed in this process did not exceed 5 wt.%. An additional mesophase fraction was generated at high strains from crystals damaged by severe deformation. After the formation of the crystalline phase, further deformation followed the mechanisms characteristic for the semicrystalline polymer. Interlamellar slip supported by crystallographic chain slip has been identified as the major deformation mechanism in semicrystalline PLLA. It was found that the contribution of crystallographic slip increased notably with the increase in the deformation temperature. The most probable active crystallographic slip systems were (010)[001], (100)[001] or (110)[001] slip systems operating along the chain direction. At high temperatures (Td = 115–140 °C), the α→β crystal transformation was additionally observed, leading to the formation of a small fraction of β crystals.

Onset critical strains as an effective parameter for compatibilizer efficiency in a polypropylene‐poly(ethylene terephthalate) blend

AbstractThe effectiveness of a compatibilizer is responsible for improved mechanical properties of immiscible blends. The enhanced interactions between the phases are assessed via critical plastic strain (onset of fine slip caused by local intra‐/interlamellar slip) and critical elastic strain (onset of fibrillation of crystalline skeleton). Polypropylene‐poly(ethylene terephthalate) (PP‐PET) blends were compatibilized with three maleic anhydride grafted compatibilizers with different backbones: PP, SEBS, and POE. Using this critical onset strain method, via free shrinkage experiments, allowed to identify the effect of PET on the matrix's deformation and also the different contributions of the compatibilizers to the deformation mechanisms. Results showed that PPgMAH promoted best PET's plastic deformation to the matrix, relative to the other compatibilizers. However, SEBSgMAH's elastomeric backbone provide better stress dissipation before onset of fibrillation compared to the binary blend and matrix. This method can be used as a way to assess the effectiveness of a compatibilizer in an immiscible blend.

Influence of nucleating agent self-assembly on structural evolution of isotactic polypropylene during uniaxial stretching

Dotlike, needlelike, and treelike morphologies of N,N′-dicyclohexyl-2,6-naphthalene dicarboxamide (NJS) are induced via self-assembly. Isotactic-polypropylene (iPP)/NJS composite with treelike NJS shows the highest β-iPP relative crystallinity (Kβ). During uniaxial stretching, a higher Kβ could increase the number of voids. However, the size of voids is similar regardless of the NJS morphology. The β-α phase transition takes place after void formation. During intralamellar and interlamellar slip, no obvious polymer chains orientation can be found for α-iPP. In the strain range of 0.1–0.6, the c-axis of the β-iPP crystal tends to orient perpendicular to the stretching direction. This is caused by lamellae twisting, a unique deformation mode of β-iPP lamellae. The lamellae twisting is proposed to be responsible for the intense voids formation of the composite with higher Kβ. At a strain larger than 0.6, the drastic increase of orientation is mainly caused by the rotation of the residual fragmented lamellae and the orientation of the newly formed crystals.

Water droplet erosion mechanism of nearly fully-lamellar gamma TiAl alloy

Water droplet erosion (WDE), is a known mechanical degradation and progressive damage for turbine blades in the power generation and aerospace industries. Hence, water droplet erosion study of TiAl as an important structural material in such industries would be very attractive. The current work is to investigate the erosion damage mechanism of nearly fully lamellar Ti45Al2Nb2Mn0.8TiB2 (45-2-2XD) alloy. This alloy was subjected to water droplet erosion using 460μm droplets and 350ms−1 impact speed. In order to track the erosion damage, WDE test was interrupted at different stages; incubation, maximum erosion rate and final steady state stages. The eroded specimens were characterized using optical microscope (OM), scanning electron microscope (SEM) and atomic force microscope (AFM). The qualitative and quantitative study reveals that the erosion damage of TiAl initiates by inhomogeneous and localized material flow followed by crack network generation on the surface. Such cracks mainly nucleate on the interlamellar slip bands and cause delamination of lamellae. Further droplet impacts result in the cracks coalescence and subsequent micro-pitting within the colonies. In the advanced stages, the erosion damage was governed by periodic water roughening and water polishing, which were observed on the bottom and the sidewalls of deep erosion craters.

The effect of comonomer content on structure and property relationship of propylene-1-octene copolymer during uniaxial stretching

The structure and property relationships of propylene-1-octene random copolymer having different octene comonomer concentrations were investigated. In specific, the crystal structure evolution in these copolymers during uniaxial stretching at 60 °C was characterized by in-situ synchrotron wide-angle X-ray diffraction (WAXD) and small-angle X-ray scattering (SAXS) techniques. With high octene content, the copolymer behaved like an elastomer with small elastic modulus and yield stress, low crystallinity and low crystal orientation under stretching. Furthermore, step-cycle tensile test showed that the copolymer with high octene content had a high recovery ratio. With low octene content, the copolymer behaved like a plastomer with large elastic modulus and yield stress, high crystallinity and high crystal orientation under stretching. From 2D SAXS results, it was found that in low octene content sample, lamellar fragmentation occurred resulting in a significant decrease in lamellar lateral size. In contrast, in high octene content sample, stress might be mainly concentrated on the amorphous matrix, leading to an inter-lamellar slip and a small decrease in lamellar lateral size. Schematic structural changes of propylene-1-octene copolymer under tensile deformation were illustrated to explain the different elasticity behavior in these copolymers.

Influence of Trace Amount of Well-Dispersed Carbon Nanotubes on Structural Development and Tensile Properties of Polypropylene

Functionalized multiwalled carbon nanotubes (F-MWCNTs) were individually dispersed in a commercial polypropylene (PP) matrix using our well-established α-zirconium phosphate (ZrP) nanoplatelet-assisted dispersion approach. The F-MWCNTs remained uniformly dispersed in PP after melt mixing and injection-molding and were found to remarkably enhance modulus and strength at only 0.1 wt % loading. The individual F-MWCNTs were mostly oriented and extended in the flow direction and were shown to be integrated within the crystalline structure of the matrix. The change in mechanical properties is attributed to both the modification in crystal structure due to MWCNT-induced nucleation and the direct reinforcement of crystalline lamellae and amorphous regions by the MWCNTs. We propose that the compatibilized F-MWCNTs exhibit sufficient interfacial interaction with the PP matrix to anchor lamellae stacks and resist interlamellar slip. The F-MWCNTs also reinforce amorphous domains between crystallites and behave as “su...

Plastic deformation behavior of β phase isotactic polypropylene in plane‐strain compression at elevated temperatures

AbstractIsotactic polypropylene (iPP) rich in β crystal modification was deformed by plane‐strain compression at T = 55–100 °C. The evolution of phase structure, morphology, and orientation were studied by DSC, X‐Ray, and SEM. The most important deformation mechanisms found were interlamellar slip operating in the amorphous layers, resulting in numerous fine deformation bands and the crystallographic slip systems, including the (110)[001]β chain slip and (110)[$1{\bar {1}}0$]β transverse slip. Shear within deformation bands leads to β→α solid state phase transformation in contrast to β→smectic transformation observed at room temperature. Newly formed α crystallites deform with an advancing strain by crystallographic slip mechanism, primarily the (010)[001]α chain slip. As a result of deformation and phase transformation within deformation bands β lamellae are locally destroyed and fragmented into smaller crystals. Deformation to high strains, above e = 1, brings further heavy fragmentation of lamellae, followed by fast rotation of crystallites with chain axis towards the direction of flow FD. This process, together with still active crystallographic slip, leads to the final texture with molecular axis of both crystalline β and α phase oriented along FD. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 92–108, 2008

Plastic Deformation of the γ Phase Isotactic Polypropylene in Plane−Strain Compression at Elevated Temperatures

Plastic deformation behavior of iPP homopolymer crystallized exclusively in the γ modification was studied. Samples of γ-iPP were obtained by isothermal crystallization under pressure of 200 MPa. Deformation experiments in plane−strain compression were performed in the temperature range of 55−100 °C. Samples of γ-iPP demonstrated higher modulus, higher yield stress, and flow stress, yet slightly lower ultimate strain comparing to α-iPP in the entire range of temperature studied. During plastic deformation numerous fine shear bands, initiated by the interlamellar shear of the amorphous layers, start to develop already at the yield point. Their propagation across the sample causes a limited destruction of γ lamellae oriented perpendicularly to the direction of the band. Destroyed fragments of crystallites partially reconstruct into either mesophase (smectic) domains or crystals of α phase, depending on the deformation temperature. Mesophase is produced upon deformation at room temperature, while at 55 °C and above the crystalline α phase is formed instead. With increasing strain shear bands multiply and tilt toward the flow direction. Fragmented lamellae undergo kinking and rotation, which results in formation of a chevron-like lamellar morphology. This leads also to the development of a weak crystalline texture. Both crystalline texture and lamellae orientation emerge due to the same deformation mechanism of interlamellar slip, produced by the shear within interlamellar amorphous layers. The activity of any crystallographic deformation mechanism within the crystalline component was not detected at any temperature. The interlamellar amorphous shear appears to be the primary deformation mechanism of γ-iPP. The other identified mechanisms, i.e., γ−smectic and γ−α transformations, play a supplementary role in the deformation sequence.

Dynamic mechanical properties of random ethylene/1-octene copolymers prepared by rapid cooling

The dynamic mechanical properties of a well-characterized series of homogeneous ethylene/1-octene copolymers with different random hexyl branch contents and prepared using different cooling conditions have been examined using dynamic mechanical analysis (DMA). It was confirmed that the relaxation behavior of copolymers varied continuously with the branch content: the magnitude of the β relaxation increased with branch content while the intensity of the α relaxation decreased with the branch content; both relaxation temperatures decreasing with increasing branch level in the copolymers. Copolymers prepared at different cooling conditions were further examined and strikingly continuous changes were found for the first time. The β relaxation was shown to correlate to the amorphous region, while the α 1 and α 2 relaxations can be clearly differentiated for some samples and are assumed to be associated with the interlamellar slip and intra-crystalline c-shear processes respectively. With increasing cooling rate, the relative intensity of α 1 relaxation to α 2 relaxation was found to decrease while the β relaxation did not change. The most informative data is determined from deconvolution of tan δ spectra. In higher crystallinity polymers the α 1 and α 2 relaxations are closely related in activation energy but have different temperature locations. For lower crystallinity systems, where the α 1 relaxation cannot be observed, the α 2 and β relaxations are closely linked, with activation energies approaching one another. These results show very clearly that, although the observed relaxation data can be separated through deconvolution into three separate peaks, the behaviors are closely linked. Presumably, this a clear reflection of the role of tie molecules in binding phases together and in influencing dynamic mechanical behavior. A clear change of behavior has also been observed in the β relaxation when a distinct amorphous phase exists outside of the spherulites, confirming the general belief that the crystalline phase influences the amorphous phase when it is confined within a spherulite. Again, this behavior is reflecting the role of tie molecules in binding together the nanocomposite structure of a spherulite.

Plastic deformation behavior of β-phase isotactic polypropylene in plane-strain compression at room temperature

Isotactic polypropylene (iPP) rich in β crystal modification (constituting 92% of crystalline phase) was deformed by the plane-strain compression with constant true strain rate, at room temperature. The evolution of phase structure, morphology and orientation was studied by DSC, X-ray and SEM. The deformation sequence and the active deformation mechanisms were found out. The most important mechanisms were interlamellar slip operating in the amorphous layers, resulting in numerous fine deformation bands due to localization of deformation and the crystallographic slip systems, including the (110)[001] chain slip and (110)[1 1 ¯ 0] transverse slip. Shear within deformation bands leads to β → smectic and β → α solid state phase transformations. At room temperature the β → smectic transformation appeared to be the primary transformation, yielding the oriented smectic phase with high concentration of 19 wt.% at the true strain of e = 1.49. The β → α yields only about 4 wt.% of new α-phase at the same strain. As a result of the deformation and phase transformation within numerous fine deformation bands β-lamellae are locally destroyed and fragmented into smaller crystals. Another deformation mechanism is the cooperative kinking of lamellae, leading to their reorientation and formation of a chevron-like lamellar arrangement. At high strains, above e = 1, an advanced crystallographic slip and high stretch of amorphous material due to interlamellar shear bring further heavy fragmentation of lamellar crystals, earlier fragmented partially by deformation bands. This fragmentation is followed by fast rotation of small unconstrained crystallites with chain axis towards the direction of flow, FD. This process leads to development of the final texture of the highly deformed β-iPP with molecular axis of both crystalline and smectic phases oriented along FD.

Plastic Deformation of the γ Phase in Isotactic Polypropylene in Plane-Strain Compression

Morphology and deformation behavior of samples of iPP homopolymer containing exclusively γ-modification with only minor traces of α-crystals, obtained by isothermal crystallization at high pressure of 200 MPa, were investigated. It was found that the growth of γ-lamellae is initiated on “seeds” consisting of a spine of single α-lamella and several shorter α-lamellae branching at the angle of 80°. Epitaxial growth of γ-lamellae on (010) faces of α-parent and daughter lamellae of the seed leads to the formation of immature spherulites, which eventually fill completely the sample volume. The plane-strain and uniaxial compression tests demonstrated higher modulus, higher yield stress and flow stress, yet slightly lower ultimate strain of γ-iPP as compared to α−iPP. During plastic deformation numerous fine shear bands, initiated by the interlamellar shear of the amorphous layers start to develop already at the yield point. Their propagation across the sample causes a limited destruction of γ-lamellae oriented perpendicularly to the direction of the band. Destroyed fragments of crystallites transform partially into a smectic phase. No γ−α phase transformation was detected. With increasing strain the shear bands multiply and tilt gradually toward the flow direction. Lamellae, already fragmented within shear bands, undergo kinking and rotation, resulting in the formation of a chevron-like lamellar morphology. Simultaneously, a relatively weak one-component crystalline texture is developed. This texture is described by the orientation of c crystallographic axis along the constrained direction, b axis 10−30° away from the loading direction toward the flow direction, and a axis 10−30° away from FD. Both crystalline texture and lamellae orientation are developed due to the activity of the same deformation mechanismthe interlamellar slip produced by the shear within interlamellar amorphous layers. Activity of any crystallographic deformation mechanism within crystalline component was not detected. The interlamellar amorphous shear appears the primary deformation mechanism of γ-iPP. The other identified mechanism, γ-smectic phase transformation, plays a minor, supplementary role in the deformation sequence.

Molecular deformation mechanisms of isotactic polypropylene in α‐ and β‐crystal forms by FTIR spectroscopy

AbstractThe orientation behavior of isotactic polypropylene (iPP) in α‐ and β‐crystal form was investigated by rheo‐optical Fourier transformed infrared (FTIR) spectroscopy. This method enabled quantification of the degree of orientation as a feature of structural changes during uniaxial elongation in not only the crystalline phase but also the amorphous one. Molecular orientation mechanisms can be successfully derived from experimental results. Generally, three mechanisms were detected for iPP: (1) interlamellar separation in the amorphous phase, (2) interlamellar slip and lamellar twisting at small elongations, and (3) intralamellar slip at high elongations. The third mechanism was favored by α‐PP, whereas β‐PP favored the second mechanism, which, in fact, was responsible for the different mechanical properties of both materials at the macroscopic level. On the other hand, crystallization conditions may have significantly affected the amorphous orientation. Nevertheless, for both iPP types the chains in the amorphous phase always oriented less than did those in the crystalline phase. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4478–4488, 2004

Analysis of elastic-plastic deformation in TiAl polycrystals

A computational model is described for analyzing stress variations within polycrystals of γ-TiAl, including the effect of anisotropic yielding and small-scale plastic flow. Interlamellar (soft mode) slip behavior is controlled by a separate collection of slip systems whose properties are derived from measurements on polysynthetically twinned (PST) specimens. When used to represent several hundred randomly oriented material grains, the model provides distributions and statistical data about the local stress, strain, and plastic deformation resulting from a prescribed macroscopic loading.

CRITICAL STRAINS DETERMINING THE YIELD BEHAVIOR OF s-PP1*

Video-controlled tensile deformation experiments giving true stressstrain curves were carried out on samples of s-PP. Dividing the total tensile deformation into elastic and plastic parts shows three critical strains (A, B, C) for which the differential compliance and the recovery properties change. It was found that all critical strains remain constant on varying the crystallite thickness or the testing temperature. Point A marks the end of the linearelastic range. The yield point, as given by the maximum on the engineering stress-strain curve for a necking sample, essentially corresponds to the second critical point, although the yield point is shifted relative to B to higher strains and varies with temperature. It is associated with the collective onset of inter- and intralamellar slip processes. At the third critical strain (C), the lamellae become destroyed, and fibrils are formed. The texture changes accompanying the drawing determined by a simultaneous measurement of wide-angle X-ray scattering (WAXS) patterns indicate a common activity of intra- and interlamellar slip processes setting in at A and a dominance of intralamellar block slips at moderate deformations above B. The Young's modulus and the yield stress show a large decrease with temperature, whereas there are only minor changes in the crystallinity. We understand it as resulting from a change of coupling, both between the crystalline lamellae and of the blocks within the lamellae. *Dedicated to Prof. Francisco J. Baltá Calleja on the occasion of his 65th birthday.

Melting behavior of the oriented β-phase polypropylene texture crystallized by the temperature slope method

Isotactic polypropylene consisting of uniaxially oriented P-phase lamellae was crystallized in a temperature gradient. The β → α transition was investigated by simultaneous measurements with differential scanning calorimetry (DSC) and X-ray diffraction using synchrotron radiation (SR). To compare the transition mechanism, the β-phase sample was deformed by rolling it along the direction of the crystallization. During rolling, the β-crystal is deformed by interlamellar and interchain slip, which induces c-axis-oriented molecules along the rolling direction. The melting behavior is changed by the rolling deformation. For the as-grown β-crystal, the DSC thermogram has three peaks: the β-melting endotherm at 150°C, an exotherm by recrystallization into the °-form, and the endotherm at 167°C caused by melting of the recrystallized α-form. After the rolling deformation, the β-endotherm is extinguished by the successive exotherm. Simultaneous X-ray measurements reveal that the β → α transition is shifted to a lower temperature and that the recrystallized α-form has a c-axis-orientation caused by the rolling deformation. In the process of the β→ α transition, higher-order lamellar structure is developed earlier than formation of the crystalline structure. In this study, the heating phenomena, such as the β α transition and thickening of the β- and α-lamellae, are consistently explained by a mechanism involving melting and subsequent recrystallization.

Structure and mechanical properties of nylon 6.12 prepared by temperature slope crystallization. II. Rolling deformation and microhardness of the oriented negative spherulite

Oriented negative spherulites of nylon 6.12 were crystallized by the temperature slope method. Rolling deformation of the negative texture was performed in three directions. The deformation mechanism of the negative spherulites was investigated by x-ray diffraction and microhardness measurements. Slip deformation between crystalline lamellae (interlamellar slip) was observed in the first stage of deformation. The results are compared with the rolling deformation of other polymer textures such as the positive spherulites of nylon 6.12 and β-phase isotactic polypropylene. In the rolling deformation of nylon 6.12, hydrogen-bonded (010) planes play an important role. For large deformations (λ > 1.5), preferential slip between (010) planes appears, resulting in a lamellar inclination of 60° and a decrease of the lamellar thickness. Microhardness measurements after rolling deformation of the oriented negative texture show good agreement with the structural analysis. The rapid decrease in the microhardness with increasing deformation can be explained by the lamellar thickness decrease brought about by the preferential slip between the (010) planes.

Morphology and texture of high-density polyethylene-polystyrene blends deformed by plane-strain compression

The blends of high-density polyethylene (HDPE) with atactic polystyrene (PS) were deformed plastically by plane-strain compression in a channel die. The samples were deformed up to the true strain 1.8 (compression ratio 6) in three temperature regimes: below, near, and above the glass transition temperature of polystyrene component. The morphology and the texture of crystalline component in the deformed blend samples were investigated by means of scanning electron microscopy and wide angle X-ray diffraction (pole figures technique). It was found that the deformation process in the blend of immiscible HDPE and PS does not differ markedly from the deformation of the one-component system from the point of view of the deformation mechanisms involved. The crystalline textures of the blend samples are qualitatively the same as in the plain HDPE deformed under similar conditions. The active deformation mechanisms are the same in deformation of both the plain HDPE and HDPE/PS blend. The mechanism identified are crystallographic slips: (100)[001], (100)[010], and (010[001] supported by the interlamellar slip. The presence of PS in blends modifies to some extent the deformation process and resulting orientation of the crystalline component of HDPE by modification of the stress distribution within HDPE matrix around PS inclusions. This influence is much stronger at low deformation temperatures, when PS is in a glassy state, than at temperatures above Tg of PS. © 1996 John Wiley & Sons, Inc.

Toughened blends of poly(butylene terephthalate) and BPA polycarbonate

The toughening mechanisms of blends of poly(butylene terephthalate) (PBT) and bisphenol-A polycarbonate (PC) toughened with core/shell impact modifier have been studied by transmission electron microscopy, notched impact testing and uniaxial tensile dilatometry. It was found that in both toughened PBT and toughened PBT/PC blends, shear deformation is the major toughening mechanism. Brittleness at low temperatures is caused by a reduction in the ability of the matrix to undergo shear deformation. In tensile dilatometry this effect is indicated by an increase in the extent of internal cavitation of the impact modifier particles. The low-temperature impact toughness of toughened PBT/PC blends is significantly greater than that of toughened PBT. Modification of PBT with partially miscible PC appears to have a beneficial effect on the ability of PBT to undergo shear deformation. This effect has been attributed to the PC residing in the amorphous interlamellar regions of the PBT spherulites, thus facilitating interlamellar slip.

Plastic Deformation of Oriented Lamellae. II. Hot Rolling of β-Phase Isotactic Polypropylene

Isotactic polypropylene was crystallized under temperature gradient and an oriented β-phase sample was obtained. The β-phase film was rolled in three orthogonal directions at 68, 96, 122, and 142°C. The deformation behavior was investigated by wide angle and small angle X-ray diffraction methods. Anisotropy in deformation was observed in each case, including the rotation of lamellae, interlamellar slip, longitudinal and transversal chain slip. At high temperatures, the deformation proceeded mainly by interlamellar slip whereas chain slip could hardly be observed. During the hot roll deformation, the original β-phase crystal was destroyed and then recrystallized into the c-axis-oriented α-phase crystal. The amount of the β→α phase transition increased with increasing rolling temperature and draw-ratio. The spacing of the newly appeared α-phase lamellae changed, depending only on the rolling temperature. The final c-axis-oriented texture was composed of α-phase crystals. From these results, it is considered that the deformation proceeds, not by incorporation of the original lamellar blocks, but through the melting or unfolding of the original β-phase lamellae and subsequent recrystallization into the c-axis-oriented α-phase lamellae.

Plastic deformation of oriented lamellae: 1. Cold rolling of β-phase isotactic polypropylene

Isotactic polypropylene was crystallized by the oriented growth method and the oriented β-phase obtained. This has unidirectional lamellar orientation with the lamellar long axis parallel to the growth direction, the lamellae being twisted along this direction. The sample plates were cold-rolled in three orthogonal directions, and the deformation behaviour of each case was investigated chiefly by wide-angle and small-angle X-ray diffraction methods. It was revealed that deformation takes place by a different mechanism in each case, including rotation of lamellae, interlamellar slip, chain-directional and transversal chain slip. These results are discussed in connection with the anisotropic structure of these samples due to the lamellar orientation. When the β-phase samples are rolled, α-phase crystals appear with c-axis orientation and the proportion increases with draw ratio. For crystallographic reasons it is concluded in this case that by stretching the c-axis orientation is brought about not through block formation of the original β-phase lamellae and incorporation of these blocks into microfibrils, but by melting or unfolding of the original β-phase lamellae and recrystallization to the c-axis-oriented new α-phase.