Chain-folded Lamellae Research Articles

Plastic Deformation of High Density Polyethylene with Extended-Chain Crystal Morphology.

Samples of polyethylene with extended-chain crystal morphology, obtained by crystallization under high pressure, were subjected to uniaxial compression to various strains. Accompanying structural changes were analyzed using scanning electron microscopy. At the true strain of e = 0.2−0.3 the microbuckling instability was observed in longitudinally loaded lamellae, resulting in the formation of angular kinks. This induced a rapid reorientation of the lamellae, facilitating their further deformation by crystallographic slip. Microbuckling instability was found to occur earlier than in samples with folded-chain crystal morphology (e = 0.3−0.4) due to a smaller ratio of the amorphous to crystalline layer thickness. SEM observations demonstrated that the microbuckling instability begins with small undulation in long lamellae. Sharp angular lamellar kinks develop from the initial undulation through intense plastic deformation by crystallographic slip along the chain direction. The same slip system was found to operate throughout the kink, including the tip region as well as both limbs. In contrast to thin folded-chain lamellae that often undergo fragmentation during deformation, the thick extended-chain lamellae deform stably by chain slip and retain their continuity up to high strains, e > 1.6. This stability of deformation is related to the large thickness of extended-chain lamellae.

Bilayer-folded lamellar mesophase induced by random polymer sequence

Randomness is perceived in two different extremes, in macroscopic homogeneity and local heterogeneity, but apparently far away from order. Here we show that a periodic order spontaneously arises from a binary random copolymer when self-assembly occurs in an ensemble containing > 1015 possible chain sequences. A Bernoullian distribution of hydrophilic and hydrophobic side chains grafted onto a linear backbone was constructed by random copolymerization. When the polymer chains associate in water, a sequence matching problem occurs because of the drastic heterogeneity in sequence: this is believed to generate local curvature mismatches which deviate from the ensemble-averaged interfacial curvature. Periodic folding of the self-assembled bilayer stabilizes the curvature instability as recurring hinges. Reminiscent of chain-folded lamellae found in polymer crystallization, this new liquid crystalline mesophase, characterized as bilayer-folded lamellae, manifests itself as an anisotropically alignable birefringent hydrogel with structural hierarchy across multiple length scales.

Folded Chain Lamellae of Dynamic Helical Poly(phenylacetylene) in the Hexagonal Columnar Phase

Folded Chain Lamellae of Dynamic Helical Poly(phenylacetylene) in the Hexagonal Columnar Phase

Fibril microstructural changes of polyacrylonitrile fibers during the post-spinning process

Ultrathin sectioning and solvent etching were used to assess microstructural changes in polyacrylonitrile (PAN) fibrils following a dry-jet wet spinning process. Morphologic assessment revealed the coexistence of fibrils and lamellae within the PAN fibers, with a frill-like structure found on the fibrils. The size of the lamella-like layers was consistent with a crystal size of 13.2 nm as characterized by X-ray diffraction and microfibrils were composed of alternate dark lamella-like crystal layers and surrounded by bright amorphous chains. Further, fibril arrangement followed a weave-like pattern. During the post-spinning process, break-rearrangement of chain-folded lamellae had an important role in fibril structure evolution. The high stream stretching and heat treatment induced the breaking of lamellae and the formation of new lamellae through the molecular chains rotating and slipping, resulting in a decrease in voids and the coalescence of microfibrils. Consequently, the fibrils were arranged in a more orderly manner and interlinked more tightly, a process conducive for the preparation of high quality PAN fibers.

Rigid amorphous fraction and multiple melting behavior in poly(butylene terephthalate) and isotactic polystyrene

The multiple melting behavior of many semicrystalline polymers is generally attributed to fusion of original crystals reorganized/recrystallized upon heating at conventional scanning rates. In the present study, the triple and double melting behavior of poly(butylene terephthalate) (PBT) and isotactic polystyrene (iPS), which is observed after isothermal crystallization at low and high T c s, respectively, is put into relation with the presence and absence of rigid amorphous/crystal interphase around the original primary crystals. Such constrained amorphous interphase has been recognized as decisive for the definition of the mechanical, thermal, and barrier properties of semicrystalline polymers. The method for the approximate determination of the limit temperature for the presence of this rigid amorphous fraction (RAF), already applied to poly[(R)-3-hydroxybutyrate] (PHB) and poly(ethylene terephthalate) (PET), is here utilized for PBT and iPS. The complex melting behavior displayed by all these polymers is explained by assuming that two distinct morphologies of primary crystals develop during crystallization at temperatures lower than the RAF limit temperature, in a percentage that is a function of the crystallization temperature. Simultaneously with the RAF devitrification, these distinct morphologies differently reorganize/recrystallize upon heating at low scanning rate, originating the triple melting behavior. On the contrary, after crystallization at high T c s, a single reorganization route is followed by all the primary crystalline structures, characterized by the same morphology made of tightly chain-folded lamellae and absence of vitrified RAF at the amorphous/crystal interphase.

Plastic Deformation Mechanisms of Semicrystalline and Amorphous Polymers.

We use large-scale molecular dynamics simulations to investigate plastic deformation of semicrystalline polymers with randomly nucleated crystallites. The strain-softening regime is dominated by deformation of crystallites via reorientation of chain-folded lamellae toward the tensile axis, fragmentation of largest crystalline domains, and a partial loss of crystallinity. The strain-hardening regime coincides with unfolding of chains and recrystallization as a result of strain-induced chain alignment. These observed deformation mechanisms are consistent with experimental findings. We compare the tensile behavior of semicrystalline polymers with their amorphous counterparts at temperatures above and below the glass transition temperature.

X-Ray Diffraction of the Supramolecular Structure of Polypropylene Yarns

We show that for orientational drawing of polypropylene multifilament yarns within the draw ratio range 4.2-5.6 at elevated temperature, we observe practically complete transition of the folded-chain lamellar crystallites to fibrillar crystallite formations of stretched chains, resulting in a significant increase in the relative breaking load for the yarns. We established that increasing the preliminary spinneret draw ratio, which considerably reduces the possibility of formation of a*-oriented foldedchain lamellae, lets us obtain stronger yarns.

Enhancement of physical, mechanical, and gas barrier properties in noncovalently functionalized graphene oxide/poly(vinylidene fluoride) composites

Sulfonated poly(ether-ether-ketone) functionalized graphene oxide (SPG)/poly(vinylidene fluoride) (PVDF) composites have been prepared by a solvent evaporation technique. The –SO3H group of SPG interact with the >CF2 dipole of PVDF to integrate graphene uniformly into a PVDF matrix. The cross-sectional field emission scanning electron microscopy micrograph shows folded-chain lamella of spherulitic PVDF crystallites spread outwards from the growth center. The spherulitic crystallites decrease with the incorporation of SPG into the PVDF matrix and the composite containing 3wt.% SPG (SPG3) does not show spherulite, but fiber-like crystallites. Fourier transform infrared spectroscopy, wide angle X-ray scattering and differential scanning calorimetry (DSC) results suggest that piezoelectric β-polymorph formation begins with the incorporation of SPG into the PVDF matrix, and SPG3 shows the formation of the fully β-polymorph. DSC cooling curves show that SPG acts as an effective nucleating agent for the crystallization of PVDF. Study of the mechanical properties shows that there is a simultaneous enhancement of the stress and the strain at break indicating the enhanced toughness of the PVDF composites compared to pure PVDF. Young’s modulus increases 160% and the oxygen permeability coefficient decreases by 91% in the SPG3 composite film compared to pure PVDF.

Lamellae break induced formation of shish-kebab during hot stretching of ultra-high molecular weight polyethylene precursor fibers investigated by in situ small angle X-ray scattering

The extracted low-oriented ultra-high molecular weight polyethylene precursor fibers were stretched at the temperature of 80 °C. In situ small-angle X-ray scattering (SAXS) measurement using synchrotron radiation was applied to study the microstructure evolutions of fibers including the deformation and break of lamellae, the diminishing of microvoids, and the formation of shish-kebab. It was found that hot stretching could induce different kinds of SAXS patterns: two-point pattern indicating the stacking lamellae, X-shaped pattern indicating the tilted state of lamellae, the changing of streak pattern indicating the evolution of microvoids and shish structures respectively. During later stage of stretching, the shish structures along the fiber axis became remarkable to develop the final shish-kebab structures accompanied by the break-reorganization of original lamellae. The results also reveal the close relationship between macroscopic deformation and microstructural evolution that the macroscopic external stress loaded on internal microstructures can effectively promote the transformation from chain-folded lamellae to chain-extended crystals.

Uniaxial Tensile Deformation of Poly(ε-caprolactone) Studied with SAXS and WAXS Techniques Using Synchrotron Radiation

The structural evolution of poly(ε-caprolactone) (PCL) during uniaxial tensile deformation at 25 °C was examined using small- and wide-angle X-ray scatterings (SAXS and WAXS) techniques with simultaneous stress and strain (S–S) curves. A high-energy X-ray beam at the recently upgraded Pohang synchrotron radiation source revealed the complete lamellar deformation behavior of PCL. Slope-based division of the S–S curves indicated three distinct regions of elastic (region I), yielding (region II) and plastic deformations (region III). In region I, which showed elastic deformation, the WAXS patterns were isotropic, whereas the SAXS patterns became oblate due to elongation of the amorphous chains along the draw direction. In region II, which showed yielding deformation, the WAXS patterns showed a slight orientation, whereas the SAXS patterns exhibited a change from oblate to four-point and to six-point patterns due to the simultaneous fragmentation and melting of the chain-folded lamellae (leading to the four-point pattern) and the subsequent formation of chain-extended lamellae (adding another two maxima along the meridian). In region III, the WAXS patterns revealed the development of the orientation of PCL crystals, whereas SAXS patterns exhibited a two-point pattern. The newly formed chain-extended lamellae in regions II and III might produce network junctions that can transfer an applied force to the PCL crystals for increased orientation. The six-point pattern in region II for PCL was not observed or reported in the past during the uniaxial tensile deformation experiment. This might be due to fast acquisition of the X-ray patterns during mechanical drawing using synchrotron radiation.

THICKENING PROCESS OF LAMELLAR CRYSTALS OF A LOW MOLECULAR WEIGHT POLY(ETHYLENE OXIDE)

The thickening process of lamellar crystals from two folded-chain(2-FC) lamellae through one folded-chain(1-FC) lamellae finally to extended chain(EC) ones in bulk samples of a poly(ethylene oxide)(PEO) with a molecular weight of 5000 was studied.The samples with 2-FC crystals were prepared by quenching them from their melt to liquid nitrogen.During reheating them or treating them at selected annealing temperatures,we tracked the lamella-thickening process using small-angle X-ray scattering(SAXS).After analyzing the SAXS data and the corresponding one-dimensional correlation functions,we found that below 52℃ 2-FC crystals primarily thickened into 1-FC crystals;but above 52℃ 2-FC crystals mainly thickened into EC crystals.In the temperature region between 58℃ and melting point of the EC crystals,the newly formed 1-FC crystals will further thicken into EC crystals.Our observations using polarized light microscopy(PLM) and scanning electron microscopy(SEM) show no changes of crystalline superstructures,such as spherulites,formed by these crystals.Therefore,we can conclude that the thickening process,we observed in this crystalline PEO samples,do not cause any destruction of large crystalline superstructures,so it is a process only containing melting of thinner lamella crystals and then recrystallization of the molecules into thicker lamella crystals.More importantly,this process is strictly restricted within some local regions of large crystalline superstructures.

Spherulitic Growth of Poly (Ether Ether Ketone) Crystallized at High Pressure

High-pressure crystallized poly (ether ether ketone) (PEEK) samples were prepared with a piston-cylinder apparatus by varying temperature, pressure, crystallization time, and molecular weight, and were investigated using wide-angle X-ray diffraction (WAXD), differential scanning calorimetry (DSC), and scanning electron microscopy (SEM). The applied etching experiments showed that the chemical resistance of the polymer was significantly improved through the high-pressure treatment. The results also revealed that PEEK spherulites with folded-chain lamellae as substructures and with different characteristics were formed at high pressure. Crystalline elliptical micro-spheres consisting of flake-like lamellae with rugged surfaces were observed on the etched samples with SEM, which may diversify niche applications in functional fillers, carriers, adsorbents, etc. Morphological observations suggested that such micro-spheres might possibly have evolved from a novel dendritic crystal. Furthermore, a granular substructure of the lamellae was observed in these single-phase polymer systems with rigid molecular backbones, which further confirmed the model developed by Strobl for polymer crystallization.

Direct Formation of Nanohybrid Shish-Kebab in the Injection Molded Bar of Polyethylene/Multiwalled Carbon Nanotubes Composite

The formation of a nanohybrid shish-kebab (NHSK) superstructure, in which fibrillous carbon nanotubes (CNTs) act as shish while polymer lamellae act as kebab, is a novel way to bond the polymer and CNTs together and was first observed in the solution crystallization of polyethylene in the presence of CNTs. In this work, a direct formation of nanohybrid shish-kebab in the injection molded bar of high-density polyethylene (HDPE)/multiwalled carbon nanotubes (MWCNTs) composite, has been achieved, via a so-called dynamic packing injection molding technology (DPIM), in which oscillatory shear field was imposed on the gradually cooled melt during the packing solidification stage. Interestingly, whatever the long axis of CNTs is perpendicular to or parallel to the shear flow direction, the lamellae of PE is always perpendicular to long axis of CNTs. The three-step mechanism, including (1) the disentanglement and orientation of both PE and CNTs, (2) PE folded-chain lamellae directly nucleated on CNTs surface and/or first formation of PE extended-chain shish directly on CNTs surface then followed by nucleation of folded-chain lamellae, and (3) the crystallization of PE kebabs on the PE decorated CNTs fibrils, was proposed to understand the formation of NHSK under effect of shear. Importantly, the NHSK structure can bring significant mechanical reinforcement in the HDPE/MWCNTs composite. For the oriented composites containing 5% MWCNTs, its tensile strength is increased by 150% and 270%, compared to the oriented pure HDPE and the isotropic composites containing 5% CNTs, respectively; meanwhile, its Young’s modulus is enhanced by 130% and 180%, compared to the oriented pure HDPE and the isotropic composites containing 5% CNTs, respectively. This work is the first to enlarge the theoretical value and application potential of NHSK structure in the crystallizable polymer/CNTs composite.

Precursor of shish–kebab in isotactic polystyrene under shear flow

We performed polarized optical microscope (POM), depolarized light scattering (DPLS) and small- and wide-angle X-ray scattering measurements on the structure formation process or the crystallization process of isotactic polystyrene (iPS) under shear flow below and above the nominal melting temperature Tm. It was found that an anisotropic oriented structure termed here as a string-like object was formed in μm scale even above the nominal melting temperature and stable for more than 24h, but melted at around 270°C far above the nominal melting temperature. The string-like object acts as a nucleation agent for the folded chain lamella crystal (or the kebab), and was assigned to a precursor of the shish–kebab. We also examined the shear rate dependence of the structure formation to find a critical shear rate for the formation of the string-like object, suggesting the relaxation of the chains plays an important role in the formation of the structure. Based on the results we have discussed the inner structure of the string-like object.

Lamellar orientation on the surface of a polymer determined by ToF-SIMS and AFM

The fold and lateral surfaces of chain-folded lamellae of poly(bisphenol-A-co-etheroctane), containing both aliphatic CH 2 and aromatic segments, were investigated by time-of-flight secondary ion mass spectrometry (ToF-SIMS). At low and high crystallization temperatures, the surfaces of the polymer films were shown to consist mainly of edge-on and flat-on lamellae, respectively. Surfaces with a mixture of edge-on and flat-on lamellae were produced at intermediate temperatures. The edge-on and flat-on lamellae were identified by using ions that recognize the flexible and rigid segments of the polymer. Ion images produced using selected ions that are related to the edge-on or flat-on orientation can be used to identify the location of these lamellae on the polymer surface. Our results indicate that ToF-SIMS can be used to detect different lamellar orientations at the surfaces of semi-crystalline polymers.

Substrate-Controlled Reorganization of Solution-Grown Polyethylene Single Crystals through Partial Melting

By combining transmission electron microscopy (TEM) and atomic force microscopy (AFM), the annealing behavior of polyethylene (PE) single crystals exhibited when annealed on highly oriented pyrolytic graphite (HOPG) was investigated. Morphologically, fibrils were formed and ordered in the form of a leaf venation in an annealed single crystal. From TEM observations, PE chains were found to lay down on the HOPG substrate surface, performing the epitaxial fitting with it; (110)PE //(0001)HOPG and 〈001〉PE//〈112̄0〉HOPG. It was also confirmed that in veinlike fibrils forming the venation texture, molecular chains were oriented perpendicular to their length axis. The fibrillar crystals were chain-folded lamellae and were grown edge-on on the HOPG. Molecular orientation was crystallographically adjusted to the substrate through partial melting at a temperature below the melting point, performing lattice matching between polymer crystals and the substrate.

The role of interlamellar chain entanglement in deformation-induced structure changes during uniaxial stretching of isotactic polypropylene

In-situ small-angle X-ray scattering (SAXS), and wide-angle X-ray diffraction (WAXD) were carried out to investigate the deformation-induced structure changes of isotactic polypropylene (iPP) films during uniaxial stretching at varying temperatures (room temperature, 60°C and 160°C). From the WAXD data, mass fractions of amorphous, mesomorphic and crystal phases were estimated. Results indicate that at room temperature, the dominant structure change is the transformation of folded-chain crystal lamellae (monoclinic α-form) to oriented mesomorphic phase; while at high temperatures (>60°C); the dominant change is the transformation of amorphous phase to oriented folded-chain crystal lamellae. This behavior may be explained by the relative strength between the interlamellar entangled network of amorphous chains, which probably directly influence the tie chain distribution, and the surrounding crystal lamellae. It appears that during stretching at low temperatures, the interlamellar entanglement network is strong and can cause lamellar fragmentation, resulting in the formation of oriented mesomorphic phase. In contrast, during stretching at high temperatures, the chain disentanglement process dominates, resulting in the relaxation of restrained tie chains and the formation of more folded-chain lamellae.

Structural properties of crystallizable polymer melts: Intrachain and interchain correlation functions

We present results from constant pressure molecular-dynamics simulations for a bead-spring model of a crystallizable polymer melt. Our model has two main features, a chemically realistic intrachain rigidity and a purely repulsive interaction between nonbonded monomers. By means of intrachain and interchain structure factors we explore polymer conformation and melt structure above and below the temperature T{crys}{hom} of homogeneous crystallization. Here, we do not only determine average spatial correlations, but also site-specific correlations which depend on the position of the monomers along the polymer backbone. In the liquid phase above T{crys}{hom} we find that this site dependence can be well-accounted for by known theoretical approximations, the Koyama distribution for the intrachain structure and the polymer reference interaction site model (PRISM) for the interchain structure. This is no longer true in the semicrystalline phase. Below T{crys}{hom} short chains fully extend upon crystallization, whereas sufficiently long chains form chain-folded lamellae which coexist with amorphous regions. The structural features of these polymer crystals lead to violations of premises of the Koyama approximation or PRISM theory so that both theoretical approaches cannot be applied simultaneously. Furthermore, we find a violation of the Hansen-Verlet freezing criterion; our polymer melt crystallizes more easily than a simple liquid. This hints at the importance of the coupling between conformation (backbone rigidity) and density (packing constraints) for polymer crystallization.

Thermodynamics, Morphology, and Structure of the Poly(vinylidene fluoride)−Ethyl Acetoacetate System

The poly(vinylidene fluoride) (PVF2)−ethyl acetoacetate (EAA) system produces polymer−solvent compounds. The temperature−concentration phase diagram and the Tamman's diagram indicate that the polymer−solvent complexes are of two different types which are stable in different temperature regions; one is a congruently melting compound while the other is of the singular type. Also, a metatectic-like behavior is observed due to the peculiar melting behavior of PVF2. The morphology of the compounds resembles a “crumpled cloth” network, with the probable presence of chain-folded lamellae. SAXS study indicates presence of chain-folded lamellae of lamellar thickness 8 nm in the PVF2−EAA systems. FTIR and WAXS spectra indicate formation of α polymorphic structure of PVF2, and they are consistent with the occurrence of compounds. Indeed, two new X-ray diffraction peaks are observed for the system at 2θ = 22.2° and 24.0° near the polymer−solvent complex stoichiometric composition (WPVF2 = 0.33). The FTIR spectrum also shows a new peak at 592 cm-1 for the complex formation at the stoichiometric composition. Molecular modeling using the MMX program supports the polymer−solvent complexation in this system.

Shear-induced crystallization in isotactic polypropylene containing ultra-high molecular weight polyethylene oriented precursor domains

Melt blends of short ultra-high molecular weight polyethylene (UHMWPE) fibers and isotactic polypropylene (iPP) were subjected to shear at 145 °C, above the melting point of polyethylene (PE). Structural evolution and final morphology were examined by in situ synchrotron X-ray scattering/diffraction as well as ex situ microbeam X-ray diffraction and high resolution scanning electron microscopy, respectively. Results indicate that the presence of oriented UHMWPE molten domains significantly facilitated the crystallization of iPP and enhanced the initial ‘shish-kebab’ structure leading to the final cylindritic morphology. It is argued that shear flow aligns the fibrillar UHMWPE domains, where the interfacial frictions between PE and iPP effectively retards the relaxation of iPP chains, allowing the aligned iPP chains to create a shish-like structure. Nucleation on the iPP shish initiates the folded chain lamellae (kebabs), which grow perpendicularly to the iPP/PE interface.