Extended-chain 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.

Fabricating High-Thermal-Conductivity, High-Strength, and High-Toughness Polylactic Acid-Based Blend Composites via Constructing Multioriented Microstructures.

The massive accumulation of plastic waste has caused a serious negative impact on the human living environment. Replacing traditional petroleum-based polymers with biobased and biodegradable poly(l-lactic acid) (PLLA) is considered an effective way to solve this problem. However, it is still a great challenge to manufacture PLLA-based composites with high thermal conductivity and excellent mechanical properties via tailoring the microstructures of the blend composites. In the present work, a melt extrusion-stretching method is utilized to fabricate biodegradable PLLA/poly(butylene adipate-co-butylene terephthalate)/carbon nanofiber (PLLA/PBAT/CNF) blend composites. It is found that the incorporation of the extensional flow field induces the formation of multioriented microstructures in the composites, including the oriented PLLA molecular chains, elongated PBAT dispersed phase, and oriented CNFs, which synergistically improve the thermal conductivity and mechanical properties of the blend composites. At a CNF content of 10 wt %, the in-plane thermal conductivity, tensile strength, and elongation at break of the blend composite reach 1.53 Wm-1 K-1, 66.8 MPa, and 56.5%, respectively, which increased by 31.9, 73.5, and 874.1% compared with those of the conventionally hot-compressed sample (1.16 Wm-1 K-1, 38.5 MPa, and 5.8%, respectively). The main mechanism for the improved thermal conductivity is that the multioriented structure promotes the formation of a CNF thermal conductive network in the composites. The strengthening mechanism is attributed to the orientation of both PLLA molecular chains and CNFs in the stretching direction, restricting the movement of PLLA molecular segments around CNFs, and the toughening mechanism is due to the transformation of PLLA molecular chains from low-energy gt conformers to high-energy gg conformers induced by extensional flow field. More interestingly, after the extrusion-stretched samples are annealed, the oriented PLLA molecular chains form oriented crystal structures such as extended-chain lamellae, common "Shish-kebabs," and hybrid Shish-kebabs, which further enhance the thermal conductivity and heat resistance of the samples. This work reveals the effects of the orientation of the matrix molecular chains and crystallites on the thermal conductivity and mechanical properties of composites and provides a new way to prepare high-performance PLLA-based composites with high thermal conductivity, excellent mechanical properties, and high heat resistance.

Constructing Bone-Mimicking High-Performance Structured Poly(lactic acid) by an Elongational Flow Field and Facile Annealing Process.

Poly(lactic acid) (PLA) as one of the most promising biodegradable polymers is being tremendously restricted in large-scale applications by the notorious toughness, ductility, and heat distortion resistance. Manufacturing PLA with excellent toughness, considerable ductility, balanced strength, and great heat distortion resistance simultaneously is still a great challenge. Natural structural materials usually possess excellent strength and toughness by elaborately constructed sophisticated hierarchical architectures, however, completely reproducing natural structural materials' architecture have evidenced to be difficult. Inspired by the hierarchical construction of the compact bone, an innovational method with an intensive and continuous elongational flow field and facile annealing process was developed to create bone-mimicking structured PLA at an industrial scale. The bone-mimicking structured PLA with unique and novel hierarchical architectures of interlocked 3D network lamellae and large extended-chain lamellae connecting the regular lamellae was constructed by in situ formed oriented thermoplastic poly(ether)urethane nanofibers (TNFs) acting as "collagen fibers", orderly staggered PLA lamellae behaving as "hydroxyapatite (HA) nanocrystals", and the tenacious interface functioning as a "soft protein" adhesive layer. Attributed to the unique structure, it possesses super toughness (90.3 KJ/m2), high stiffness (2.15 GPa), balanced strength (52.6 MPa), and notable heat distortion resistance (holding at 163 °C for 1 h) simultaneously. These excellent performances of the structured PLA provide it with immense potential applications in both structural and bio-engineering materials fields such as artificial bones and tissue scaffolds.

Crystallization of Poly(ethylene oxide) with a Well-Defined Point Defect in the Middle of the Polymer Chain

Poly(ethylene oxide) (PEO) is a polymer of great interest due to its prevalence in biomedical, pharmaceutical, and ion conductive systems. In this study, the crystallization behaviors of a PEO with 22 monomer units (PEO22) and a PEO having the same degree of polymerization but with an additional 1,4-disubstituted 1,2,3-triazole ring in central position of the chain (PEO11-TR-PEO11) are investigated. PEO11-TR-PEO11 shows one type of lamella crystal after cooling to T = 0 °C, but structural changes during heating below their final melting are detected by WAXS, DSC, POM, and solid-state NMR spectroscopy. The lamella thickness increases, but simultaneously the helix–helix distance decreases and an additional Bragg reflection appears at 2θ = 22.1°. A model is proposed which explains these structural changes by incorporation of the TR ring into the crystals which are additionally stabilized by attractive C–H···π interactions of the TR rings. Additionally, two different types of extended chain lamella crystals a...

PTH-028 How About Resurrecting The Research of Mechanically Actuated Robotic Capsule Endoscopy by Designing Novel Pressure Crystallised Self-Powering Composite Structures?

Introduction Mechanically actuated robotic capsule endoscopes for active locomotion have demonstrated considerable clinical potential for conducting biopsies or therapeutic intervention in human gastrointestinal (GI) tract.1 Nevertheless, the research began to slow down owing to on-board power limitations and mechanical complexity in miniaturising electronic elements incorporated in the capsule body.2 Ideally, such swallowable capsules should be self-powered in vivo by converting the numerous GI-providing energy, such as the energy from the peristalsis near gastroesophageal junction and the high shear intestinal fluid flow.3 Methods Graphene nanoplatelet (GNP), consisting of stacked layers of graphene to enable the delivery of a broad range of therapeutics, was pre-mixed and then melt blended with poly (vinylidene fluoride) (PVDF). The as fabricated binary composites were crystallised at pressure with a piston-cylinder apparatus. Results Controllable rapid growth of unique self-powering 3 D hybrid nanoarchitectures, assembled from 1 D PVDF micro/nanowires on GNPs, was achieved by the pressure crystallisation of GNP/PVDF composites (Figure 1 a-c). By controlling the crystallisation conditions, the PVDF nanowires, with piezoelectric crystalline beta phase, were obtained respectively with folded- and extended-chain lamellae as their substructures. The GNPs may play the role as a catalyst in the composite system, due to their size effect, which catalysed the self-assembly of the PVDF molecules into nanowires and then resulted in the radial nanowire clusters. Although the original surfaces of GNP/PVDF composites were high hydrophobic, they were successfully converted into hydrophilic surfaces through the pressure treatment followed by appropriate etching process (Figure 1 d and e). Conclusion The pressure crystallised GNP/PVDF composites may permit niche applications in the fabrication of a new-generation of self-powering robotic capsule endoscopes to meet the environmental conditions of each section of GI tract for energy scavenging. This study was funded by the National Natural Science Foundation of China (51373139). References 1 Munoz F, Alici G, Li WH. Adv Drug Deliver Rev 2014;71:77–85. 2 Slawinski PR, Obstein KL, Valdastri P. World J Gastroentero 2015;21:10528–41. 3 Wang ZL, Song J. Science 2006;312:242–6. Disclosure of Interest None Declared

Extended Chain Lamella Formation Characteristics of Main-Chain Smectic Liquid Crystalline Copolyesters Comprising Different Length Units

A series of liquid crystal (LC) PB-8/12 copolyesters have been synthesized from 4,4′-biphenol with sebacic acid and tetradecanedioic acid, and their LC structures and morphologies have been examined. The copolyesters formed smectic I (SmI) LCs similarly as to the corresponding PB-8 and PB-12 homopolyesters; however, dissimilarity of the comonomer lengths decreased the smectic layer order while sustaining the hexagonal order in the lateral packing of the chains. The SmI LCs consisted of 100-nm-thick lamellae stacked along the polymer chain direction. The lamella thicknesses are more than two times greater than the thicknesses of the homopolyesters and comparable to or greater than the chain contour lengths, indicating the formation of extended chain lamellae.

Pressure-controlled growth of piezoelectric low-dimensional structures in ternary fullerene C60/carbon nanotube/poly (vinylidene fluoride) based hybrid composites

Hybrid composites, consisting of polymers and nanostructured carbon allotropes, can exhibit different properties than their constituent components. In this work, the synergistic action of fullerene C60 and carbon nanotube (CNT) on poly (vinylidene fluoride) (PVDF) was evaluated, in terms of the dispersion of the carbonaceous fillers in the polymer matrix during the composite preparation, and the control of crystalline morphology of the polymer at high pressure. The synergistic effect of the zero-dimensional and one-dimensional carbon materials resulted in a well dispersed ternary C60/CNT/PVDF based composite, which was simply fabricated by a physical and mechanical route. Furthermore, the pressure-controlled growth of piezoelectric low-dimensional crystalline structures of PVDF, including hollow nanowires and extended-chain lamellae, was achieved by the simultaneous introduction of C60 and CNT. Especially, PVDF nanowires with folded- and extended-chain lamellae as their substructures were obtained respectively by controlling the crystallization conditions of the ternary composite at high pressure. Under specific conditions, composite samples, which crystalline structures were totally with extended-chain β- or γ-form lamellae, further self-reinforced with extended-chain β-form nanowires, were successfully crystallized. The present study provides a facile and effective approach for the fabrication and the multi-level control of polar crystalline structures of a polymer based hybrid composite, with an overall good dispersion of zero- and one-dimensional nanostructured carbonaceous materials.

Crystallization and Melting of Poly(ethylene oxide) in Blends and Diblock Copolymers with Poly(methyl acrylate)

Blends of poly(ethylene oxide) (PEO) and poly(methyl acrylate) (PMA) as well as the respective diblock copolymers PEO-b-PMA form a homogeneous melt and undergo crystallization of PEO upon cooling. Although an identical PEO (Mn = 5000 g/mol) is used in blends and diblock copolymers, crystallization and melting behavior at comparable PMA contents differs strongly as revealed by temperature-resolved small-angle X-ray scattering (TR-SAXS) and differential scanning calorimetry (DSC) measurements. After isothermal crystallization, PEO lamellae in the blends thicken during heating from once-folded to extended chain crystals prior to melting as revealed by TR-SAXS. Contrarily, in PEO-b-PMA a thickening to extended chain lamellae is impossible when the PMA block exceeds an Mn of about 3000 g/mol. This behavior is caused by a balance between the tendency of the crystallizable block to form extended chain crystals, the tendency of the noncrystallizable chains to adopt a maximum in conformational entropy, and the spa...

Fracture and Deformation of Extended-Chain Crystals in Poly(ethylene Terephthalate)/Bisphenol-A Polycarbonate Blends Crystallized at High Pressure

With a combination of three-dimensional crystal ordering and long-chain molecular orientational ordering, the extended-chain crystals of polymers are ideal system for the studies on low-dimensional physics, and have potential applications as functional components. In this study, poly (ethylene terephthalate) / bisphenol-A polycarbonate (PET/BAPC) blend samples were prepared with solid phase forming technique under high pressure, and extended-chain single crystals with large c-axis thickness were formed within a relative short time. The fracture and deformation behaviors of the grownup crystals were investigated using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Morphological observation showed that most of the extended-chain crystals were endowed with perfect appearance after the sample fracture. However, there were still a few crystals fractured apart, along or perpendicularly to their c-axis direction, or with cohesive fracture surfaces. Fractured spherulites with extended-chain lamellae as substructures were also disclosed in extended-chain crystals with AFM. Furthermore, the parallel lamellae deformed and changed their striation direction during the crystal growth, as was observed on a kind of extended-chain crystals grown in a PET oligomer and BAPC blend (PETO/BAPC). This study may be instructive to the improvement of the separation technique of such single crystals, so as to hasten their real applications.

Morphology of High-Pressure Crystallized Poly (Ethylene Terephthalate)/Bisphenol-A Polycarbonate Blends

The high-pressure crystallized poly (ethylene terephthalate) / bisphenol-A polycarbonate (PET/BAPC) blend samples were investigated using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Besides the extended-chain single crystals with high quality and large c-axis thickness, the results showed that some novel crystal morphologies were formed in the multi-phase system within a relative short time. Spherulitic structures with different characteristics were observed, in and out the cultured polyester extended-chain crystals, respectively, and their substructures were found to be extended-chain lamellae. Furthermore, bud-like nano-crystals with zero- or one-dimensional structures, some of which aggregated and then evolved into three-dimensional nano-dendrites, were grown during the cooling process of the polyester blends at high pressure. The disclosed entities during this study may promote the understanding of formation process and mechanism for both spherulite and extended-chain crystals of polyesters, so as to instruct the processing of such alloyed engineering plastics with high performance.

Macromolecular effect on crystal pattern formation in ultra-thin films: Molecular segregation in a binary blend of PEO fractions

In this work we report our investigation on the crystal patterns of a 50/50 blend of two polyethylene oxide (PEO) fractions with different molecular weights ( M ¯ w = 5040 and 35000 g / m o l ) in ultra-thin films. Using AFM with a hot stage the samples on the surface of silicon wafer were isothermally crystallized at 20.0 ≤ T c ≤ 60.0 °C. The crystal patterns are different from those of the two pure fractions. Co-crystallization, partial segregation and full segregation have been observed. Especially, within 47.0 ≤ T c ≤ 54.0 °C dual thickness crystals formed. The thickness of the middle part of the crystal corresponds to the lamella thickness of 35k-PEO fraction with multiple folds, while the thickness of the edge part is nearly equal to an extended-chain lamella thickness of 5k-PEO fraction. We suppose that the appearance of the dual thickness crystals is due to molecular partial segregation. Utilizing in-situ AFM, the growth of the crystal with dual thicknesses as a function of time was monitored at T c = 54.0 °C. Two growth processes were observed and a pattern formation mechanism was suggested on the basis of specified molecular motion and chain-fold crystallization of polymers.

TIME-RESOLVED SAXS/WAXS STUDY OF PHASE BEHAVIOR AND CRYSTALLIZATION IN POLYMER BLENDS

Real-time SAXS and WAXS patterns have been simultaneously obtained during isothermal melt-crystallization of blends of low-molecular-weight poly(ethylene oxide) (PEO) and poly(methyl methacrylate) (PMMA). The analysis of results shows that the originally homogeneous, single-phase polymer blend separates into two phases. The PMMA molecules diffuse from the blend and form completely segregated regions while PEO starts to crystallize. The first and dominating effect at the beginning of crystallization is the formation of unstable lamellae of nonintegrally folded chains (NIF). The real-time crystallinity and density of the PEO crystalline phase in absolute units were obtained from the time-resolved SAXS/WAXS results. The structure development proceeds in two steps. A very fast evolution of PEO crystals from the melt starts to crystallize in disordered NIF lamellae with thick amorphous interlayers and with a lower density of crystalline phase. The steep growth of crystallinity and crystalline density mean quick thickening of crystalline part of lamellae and improvement of their crystalline structure. In the second step, the structure of the crystalline phase gradually improves and crystallinity grows very slowly. The recrystallization of NIF lamellae into extended chain lamellae (EC) and lamellae with once folded chains (1F) proceeds during both stages of crystallization.

Morphology and thermal properties of polyethylenes made by metallocene and Ziegler-Natta catalysts

Morphology and thermal behavior of high density polyethylene and ethylene-octene copolymers prepared with classical Ziegler-Natta (zHDPE) and metallocene catalysts (mHDPE and mLLDPE) are comparatively discussed. Wide angle X-ray diffraction (WAXD) data at room temperature showed that mHDPE crystals are more perfect with smaller unit cell dimensions and have higher crystalline density and larger crystallite sizes when compared to zHDPEs. In the case of copolymers, the unit cell dimensions did not change significatively when the number of branches was increased. Average interplanar chain distances in the amorphous phase increased with increasing octene content in the copolymer, showing a marked influence on the packing of the amorphous phase and confirming that the branches are concentrated in this phase. The existence of second phase in the ethylene-octene copolymers prepared with metallocenes was identified by calorimetry. The existence of this phase is time-dependent and was interpreted as the result of hexyl branches ordering in the amorphous phase. A morphological model based on Vile three-phase model, WAXD results and calculations involving empirical equations for fold and extended chain lamellae crystals was proposed to explain the time-dependent thermal behavior of these copolymers.

A study of the morphologies and growth kinetics of three monodisperse n-alkanes: C122H246, C162H326 and C246H494

An optical and electron microscopic study has been made of three monodisperse n-alkanes crystallising as extended-chain lamellae. All showed similar morphologies at the same supercooling and the same progressive changes of texture with crystallization temperature. This is, with increasing supercooling, from a morphology composed essentially of individual lamellae arranged in parallel stacks and radiating from common nuclei through coarse, somewhat branched microstructures whose parallel stacks diverge, to finer, more-branched pseudo-spherulitic textures. Quenched, once-folded, C246 shows a random spherulitic texture with a dominant/subsidiary microstructure as for polymeric systems. The kinetics of extended-chain growth increase linearly with supercooling, as predicted by Hoffman, except for a local dip at the onset of lamellar branching. This new phenomenon occurs within one quantized state of lamellar thickness in contrast to ‘self-poisoning’ when once-folded forms give way to extended-chain crystallization. When dominant lamellae are no longer parallel but diverge, their mutual splaying angles increase with supercooling for all the three n-alkanes studied. The values are less, for the same supercooling, for the longer homologues with their thicker lamellae. This behaviour is as expected of transient ciliation due to the excess of molecular length over that of the secondary nucleus and thereby reinforces previous evidence demonstrating the responsibility of ciliation for spherulitic development in polymers.

Extended chain lamellae self-assembled in liquid crystal phase of soluble polydiacetylenes through chain segregation

This review article highlights our efforts in the past years toward a fundamental understanding of the supramolecular structures of a soluble but rigid polydiacetylene. In our earlier works we presented evidence to indicate that this polymer formed a lyotropic phase in solutions. TEM, AFM and SEM observations on solidified thin films showed an extended chain lamellar structure which is surmised to be self-assembled in the lyotropic phase through chain segregation. Our results revealed that the supramolecular structures formed are closely associated with the structures formed in the lyotropic state. Due to three attributes of rigid macromolecules at the molecular level-high molecular weight, broad molecular weight distribution, and worm-like chain conformation, the supramolecular structures observed are also greatly different from those of conventional nematics or smectics.

Topographical Structure of Solid Films of Poly[5,7-dodecadiyne-1,12-diolbis(4-butoxycarbonyl methylurethane)] Self-Assembled from Its Lyotropic Liquid Crystal Phase

The topographical structures of film samples of a soluble polydiacetylenepoly[5,7-dodecadiyne-1,12-diolbis(4-butoxycarbonylmethylurethane)]solidified from its lyotropic liquid crystal phase were characterized by means of scanning electron microscopy and atomic force microscopy. The observations reveal a structural feature of extended-chain lamellae of the film samples and furthermore a three-dimensional chain segregation with respect to molecular length in a layer thinner than 5 nm on the free surface of the samples.

The crystallization kinetics of monodisperse C98H198 from the melt

The crystallization kinetics of the monodisperse n-alkane C98H198, growing as extended-chain lamellae from the melt, have been measured as part of a wider programme on these novel model systems for polymeric materials. Crystal growth rates decrease linearly, by a factor of 30, over the temperature range of 113.5–114.9°C, an interval which encompasses a significant change in habit from individual through parallel-stacked lamellae to a type of coarse spherulite. The data differ considerably in character from those reported previously for extended chain crystallization of n-C198H398 but are comparable to earlier studies of purified n-C94H190 and are sensibly linear with supercooling as proposed in Hoffman's theory of extended-chain crystallization.

Structure and Phase Transitions of Poly(diethylsiloxane)

Phase transitions and morphology of poly(diethylsiloxane) have been studied with help of differential scanning calorimetry and transmission electron microscopy on carbon−platinum replicas of freeze-fractured samples. The TEM experiments showed distinct lamellae in the case of the β and γ crystalline polymorphs, whose thicknesses correlated well with the calculated chain lengths, demonstrating an extended chain conformation for both polymorphs. A low heating rate dependence of the β → μ transition temperature in DSC and a linear relationship between isotropization temperature and reciprocal molecular weight indicated the existence of extended chain lamellae for the mesophase as well. The α crystalline polymorph displayed a greater softness than β- and γ-PDES, and no lamellar structures could be observed. DSC experiments on bimodal mixtures of PDES samples with strongly different molecular weights demonstrated molecular weight dependent phase separation in the case of β-PDES and the occurrence of mixed lame...

On the γ-Phase of Isotactic Polypropylene

The crystalline morphology of a relatively high molecular weight isotactic polypropylene (i-PP) (Mw = 18000 g/mol) with well-defined regio- and stereoirregularities is studied by means of atomic force microscopy (AFM), wide-angle X-ray scattering (WAXS), small-angle X-ray scattering (SAXS), and light microscopy. This i-PP crystallizes under certain crystallization conditions nearly exclusively in the γ-modification. The growth of the γ-phase is often influenced by the initial growth of the α-phase, but also the neat γ-phase is observable. The formation of the γ-phase can be related to stereo- and regioirregularities in the polymer chain caused by the polymerization mechanism using metallocene catalysts. WAXS measurements show that at large supercoolings the formation of α-phase i-PP is preferred, whereas at low supercoolings this sample crystallizes nearly exclusively in the γ-modification. Light microscopy and AFM reveal for the γ-phase i-PP morphologies different from the well-known spherulites formed by the α- or β-modification. However, it can be shown that the morphology development of γ-phase i-PP can be widely influenced by very small amounts of α-phase i-PP. A lamella thickness of about 4 nm and a long period of 12 nm of γ-phase i-PP isothermally crystallized at 100 °C are obtained from SAXS measurements. At very low supercoolings, supermolecular structures of the neat γ-modification are formed in competition with mixed (α and γ) morphologies. In thin films the neat γ-modification shows network structures, which are formed by triangular entities. Etched bulk samples reveal columns with a rectangular cross-section which are densely packed. On the molecular scale, the surface topography of the flat-on extended chain lamellae with periodic distances of rows of methyl groups of 3.6 Å is measured. This value is in excellent agreement with the distance of the methyl group rows on the crystallographic (010) plane according to the nonparallel chain packing model by Brückner and Meille.

Folded Chain Lamellar Crystal Self-Decoration of Extended Chain Crystals of a Random Terpolymer Main Chain Liquid Crystal Polymer

Abstract Surface melting of extended chain crystals of a flexible segment containing liquid crystal polymer followed by quenching is shown to result in the epitaxial crystallization thereon of folded chain lamellae, i.e. self decoration. It is proposed that the initial extended chain lamellae form by chain folding in the mesomorphic state followed by rapid chain extension by the “chain sliding mechanism”, the same mechanism that has been in the literature for the proposed similar crystallization of polyethylene under anabaric conditions.