Lamellar Orientation Research Articles

Toward High-Performance Poly(l-lactide) Fibers via Tailoring Crystallization with the Aid of Fibrillar Nucleating Agent

As a sustainable alternative to conventional petrochemical-based polymers, biobased and biodegradable poly(l-lactide) (PLLA) exhibits tremendous application potential in the textile industry due to its attractive elastic recovery, moisture regain, and flammability. However, the commercial adoption of PLLA textile fibers still faces some hurdles mainly associated with their poor heat resistance (i.e., high thermal shrinkage or low dimensional stability) because the low crystallization rate makes PLLA difficult to crystallize during melt spinning. Herein, we report a simple but robust strategy to address this hurdle via simultaneously manipulating crystallinity and lamellae orientation with the aid of a highly active nucleating agent (NA) that can be completely dissolved in PLLA melt and reorganize into fine fibrils upon cooling. By taking full advantage of strong elongational flow field involved in the melt spinning, the NA fibrils with high nucleation efficiency on PLLA crystallization tend to align along...

New Water Vapor Barrier Film Based on Lamellar Aliphatic-Monoamine-Bridged Polysilsesquioxane.

Siloxane-based hybrid lamellar materials with ordered nanostructure units paralleling to the substrate have been widely used for water vapor barrier. However, it is very difficult to control the orientation of the lamellar units at molecular level. In this Research Article, a new lamellar bridged polysilsesquioxane (BPSQ) film, whose voids between lamellae were filled by pendant alkyl chains in the organic bridge, was prepared via the stoichiometric reaction between 3-glycidoxypropyltrimethoxysilane and aliphatic monoamine at 60 °C without catalyst. Experimental evidence obtained from FT-IR, MS, NMR, and GIXRD techniques suggested that the as-prepared BPSQ films were constructed by lamellar units with disordered orientation. Nonetheless, they possessed satisfactory water vapor barrier performance for potassium dihydrogen phosphate (KDP) and deuterated potassium dihydrogen phosphate (DKDP) optical crystals, and the water vapor transmission rate through BPSQ film with thickness of 25 μm was as low as 20.3 g·m(-2)·d(-1). Those results proved that filling the voids between molecular lamellae with alkyl chains greatly weakened the effect of lamellar unit orientation on the vapor barrier property of BPSQ film.

Modeling orthotropic elasticity, localized plasticity and fracture in trabecular bone

This work develops a model for the mechanical response of trabecular bone including plasticity, damage and fracture. It features a resultant lamellar orientation that captures trabecular strut anisotropic elasticity, and introduces asymmetric J2 plasticity with isotropic hardening to capture evolving strut tensile and compressive dissipative properties. A continuum compatibility based damage and fracture criterion is also proposed to model fracture surface generation. We investigated fracture of a trabecular bone network under a compressive load, for which failure modes of both tension and compression were identified at the strut level. The predicted trabecular network response was found to fall within the range of experimental results reported in literature. We also investigated the response of idealized struts under compression, tension and bending using our model. Individual struts were found to exhibit micro-buckling under compression and micro-necking under tension. These instabilities are however masked by the multiplicity and complexity of strut orientations at the trabecular network level.

Crystallization of Polymer Chains Chemically Attached on a Surface: Lamellar Orientation from Flat-on to Edge-on.

Crystallization of polymer chains confined on a surface greatly influences surface properties. We have grafted comb-like copolymer, consisting of poly(2-hydroxyethyl methacrylate) (PHEMA) backbone and semicrystalline poly(ε-caprolactone) (PCL) side chains, on silicon surface and investigated the crystallization of such confined PCL chains upon solvent evaporation by using atomic force microscopy (AFM), grazing incidence wide-angle X-ray scattering (GI-WAXS), and polarized optical microscope (POM). Our studies reveal that the PCL chains align and form "flat-on" lamellae at a low PCL chain density. As the chain density increases, the comb-like polymer (PHEMA-g-PCL) chains undergo pancake-to-mushroom-to-brush transition, and the lamellae turn from "flat-on" to "edge-on" in orientation. Further increasing PCL chain density leads the "edge-on" lamellae to change from a quasi-two-dimensional (quasi-2D) to quasi-three-dimensional (quasi-3D). Because of the confinements of polymer chains, we can observe the evolution of spherulites at different stages during the mushroom-to-brush transition of PHEMA-g-PCL chains. The confinements also result in knobbly substructures in the edge-on lamellae.

Transient microstructure in long alkane segment polyamide: Deformation mechanism and its temperature dependence

Deformation-induced microstructure evolution of Polyamide 1012, with an emphasis on lamella development, polymorphism transition and molecular orientation, is investigated. When deformed above Tg (100 °C), a series of complex SAXS patterns are continuously identified at intermediate strain including four-point pattern, figure-8 pattern and X-shaped pattern, accompanied with two-bar pattern on the meridian, which corresponds to the transient microstructure. Such particular structure is resulted from the approach of tilted lamellae along the drawing direction, the insertion and the orientation of new lamellae. To investigate the temperature dependence, the microstructure developments at 30 °C (below Tg), 60 °C (close to Tg) and 100 °C (above Tg) are compared. Considering 38% enhancement of the fracture stress with elevated temperature (from 60 °C to 100 °C), the temperature dependence of particular SAXS patterns further suggests that the elongation above Tg favors higher level of stretching-induced crystallization and better aligned microfibrillar structure, which competes with thermal-induced increased chain mobility. Based on the comprehensive results, the correlation between microstructure and mechanical response has been successfully established, which is featured by the synchronous occurrence of transient structure with slight strain hardening.

Comparison of microstructures and mechanical properties of as-cast and directionally solidified Ti-47Al-1W-0.5Si alloy

The as-cast Ti-47Al-1W-0.5Si alloys were prepared by induction skull melting (ISM) technology under argon atmosphere. Some bars were cut from as-cast ingot longitudinally and then were directionally solidified by Bridgman solidification technique. The microstructures and mechanical properties of the as-cast and directionally solidified Ti-47Al-1W-0.5Si alloy ingots with different lamellar orientations were investigated. After directional solidification, the grain boundaries were eliminated and the lamellar orientation was nearly parallel to the growth direction. The interlamellar spacing of directionally solidified samples decreased from average 1500 nm to less than 1200 nm and the interlamellar spacing was more homogeneous. In addition, the shape of Ti5Si3 transformed from short rod to strip along the growth direction, and the B2 phase disappears after directional solidification. At room temperature, the compressive yield strength of Ti-47Al-1W-0.5Si alloy increases from 573 in as-cast condition to 651 Mpa after directional solidification. The fracture mode transforms from inter-granular fracture to inter-lamellar fracture after directional solidification.

Toughening of syndiotactic polypropylene with chalk

ABSTRACTWe hypothesized that polymer crystal anisotropy is advantageous for toughening of polymer composites involving easy slip network of oriented crystalline layers around filler particles. To this end, composites of syndiotactic polypropylene (sPP) with high concentration of submicrometer calcium carbonate particles were prepared and examined because usual sPP crystals exhibit high packing anisotropy. The specific orientation of sPP lamellae around chalk grains was found, which is supposed to facilitate the plastic deformation of polymer matrices. The compression molded bars of the composite exhibited markedly higher Izod impact strength than those of neat sPP. Toughening was even enhanced in the injection molded composite, for which 4.5‐fold increase in the impact strength was achieved. Injection‐induced orientation of the disordered form I sPP crystals was enhanced in the composite. The injection molded tensile specimens exhibited also a good drawability. Debonding at chalk–sPP interface occurred both during the impact and tensile tests facilitating the plastic deformation of sPP matrix. Chalk did not have any significant influence on the thermal properties of the composites but it affected the rheological behavior, increasing the loss and storage moduli, and the viscosity. Highly filled sPP composite exhibited solid‐like behavior in a molten state with the storage modulus exceeding the loss modulus in the entire frequency range. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016, 133, 43651.

Stereocomplex- and homo-crystallization of blends from 2-armed poly(l-lactide) and poly(d-lactide) with identical and opposite chain directional architectures and of 2-armed stereo diblock poly(lactide)

Ethylene glycol (EG) and succinic anhydride (SA)-based 2-armed PLLA and PDLA correspondingly with tail-to-tail (T–T) and head-to-head (H–H) architectures, together with SA-based linear 2-armed stereo diblock copolymer, were synthesized and the SC crystallization and homo-crystallization behavior of blends of 2-armed PLLA and PDLA with various combinations of molecular architectures, together with that of neat polymers, were investigated. The crystallizability was higher for SC crystalline samples than for homo-crystalline samples during solvent evaporation and slow cooling from the melt. The hydrogen bonds of hydroxyl terminals and/or low molecular weight of EG-based polymer rather than specific chain directional architecture or its combination disturbed and delayed the crystallization, lowering the crystallization temperature (Tc), crystalline], the reciprocal of experimental crystallization half time [1/tc(1/2) (exp)], spherulite growth rate (G), enthalpy of crystallization (ΔHc) for 1st cooling, and total enthalpy of cold crystallization and melting [ΔH(tot)] for 2nd heating of both SC crystalline and homo-crystalline samples, the melting temperature (Tm) for 1st and 2nd heating, nucleus density and the degree of lamella orientation of spherulites of SC crystalline samples. The opposite chain directional architectures of polymers in blends should have increased ΔH(tot) for 1st heating, ΔHc for 1st cooling, ΔH(tot) for 2nd heating, and G of SC crystalline samples. The effects of hydrogen bonds of hydroxyl terminal groups, Mn (or Mn per one arm or block), and chain directional combination were determined by the crystallization temperature range and the presence or absence of a solvent. The Tm values of SC crystalline samples were determined by Mn (or Mn per one arm or block), whereas those of homo-crystalline samples did not show clear dependence on Mn (or Mn per one arm or block) values per one arm, hydroxyl terminal groups (hydrogen bonding), or chain direction. The intermolecular SC crystallization in the enantiomeric polymer blends was favored compared with intramolecular SC crystallization in the stereo diblock copolymer.

Extending the Anomalous Dilation in CO2 to Thick Polymer Blend Films: A Neutron Reflectivity Study

We investigated the effect of density fluctuation of supercritical carbon dioxide (scCO2) on anomalous swelling of multilayer polymer thin films on the ridge in the pressure–temperature phase diagram of CO2. In order to measure the swelling ratio along the film depth, we alternately deposited hydrogenated poly(methyl methacrylate) (PMMA) and deuterated polystyrene (dPS) thin films and performed the neutron reflectivity measurements as a function of CO2 pressure at 36 °C. The results showed that, in contrast to previous studies, CO2 was to penetrate deeply throughout the multilayer thin film where the magnitude of swelling along the density fluctuation ridge of CO2 was independent of film thickness. Block copolymer thin films of dPS-b-PMMA with a parallel lamellar orientation also showed similar swelling behavior in scCO2. However, it is well-known that single-layer polymer thin films exhibit anomalous swelling behavior only near the film surface. This difference is probably due to the fact that the multilayer thin films have the CO2-philic PMMA layer sandwiched between dPS layers, which can function as a CO2 reservoir, thereby transferring the CO2 molecules from the PMMA layers to the adjacent dPS layers. Furthermore, we found that the interaction between polymers and substrates was not significant in scCO2 from diffusion dynamics results using neutron reflectivity, thereby facilitating anomalous dilation of polymers near the substrates without a pinning effect.

Intervertebral Disc Maintenance and Repair Potential in Mice with Different Repair Potential

Introduction Intervertebral disc (IVD) degeneration is a major cause of back pain, affecting quality of life. Current treatments are limited to salvage surgical operations. Biological treatments to relieve symptoms or restore disc are not available as we know little about the biology of disc degeneration and its potential to repair/regeneration. While most people will develop disc degeneration with aging, there are individuals who are protected even at old age, suggesting the presence of protective genes. Progenitor cells within the IVD are thought to play important roles in disc homeostasis. A hypothesis is that genetic factors can confer a protection against disc degeneration via better maintenance of endogenous progenitor cells. There exist strains of “healer” mice (LG/J, MRL/MpJ) with superior repair potential of cartilaginous tissues. We propose to study the maintenance of the IVD tissues in healer mice and assess potential relationship to progenitor cells, extracellular matrix and inflammatory response changes, to degeneration and repair/regeneration processes. Materials and Methods “Healer” (MRL/MpJ) and “poor healer” (C57/BL6C) mice were used in this study. Histological comparison of tail disc was assessed from 8 to 24 weeks of age. Different NP cell populations and ECM markers were assessed using immunohistochemistry with specific cell markers. Tail-looping at 8-week of age for a fixed period was used as an environmental perturbation that will induce degeneration. Unlooping the tail after certain period of looping can assess repair processes with appropriate controls. Results A comparison of MRL and C57 mice showed neither observable histological differences, nor signs of degenerative processes from 8-week to 24-week of age. Following tail looping for 4, 5, 6 and 8 weeks, there were significant distortion of the annulus fibrosus (AF) and NP at the compressed side, in terms of NP cell loss, AF tears and ruptures, and cell death in the AF. After the tails were unlooped for 4 weeks or 8 weeks, there were restorations of NP and AF structures in both strains of mice. However, superior repairing was seen in MRL mice at all time-points studied, in which the disc structure restored better via continuous expansion of NP region, cell repopulation and lamellae orientation recovered in the compressed AF sides with a clear NP-AF boundary. In C57 mice, the AF lamellae structure remained disorganized following unlooping. NP cell population analysis including Tie2, GD2, Sox9 and T showed different expression pattern in MRL and C57, from which showed that GD2 positive cells may be the functioning NP cell in disc maintenance. Besides, MRL maintained better Col I, Col II and aggrecan content during the repair processes. Interestingly, lower IL-1β was found in the “healer” mice, which suggested less inflammatory response may contribute to a better disc recovery after injury. Conclusion By comparing the genetically different “healer” and “poor healer” mice, we showed a population of novel marked NP progentiors that may play important roles in the maintaining and repair/regenerative of IVD. The better repairing features in the “healer” mice are associated with better functional cell population maintenance and less inflammatory response. Acknowledgment The project is supported by AOSPINE (SRN 2012_8), China's National Strategic Basic Research Program (“973”) Grant 2014CB942901 and Research Grants Council of Hong Kong (T12–708/12N)

Polarimetric Interferometry for Assessment of Corneal Stromal Lamellae Orientation.

To analyze corneal stromal lamellae orientation in normal subjects and patients with various corneal conditions using a polarimetric interferometer. Thirty-two healthy control subjects (59 eyes) and 16 patients (22 eyes) with corneal conditions or postkeratoplasty were included in the study. All patients were imaged using the interferometer and slit lamp photography. The interferometer produces an orthogonal cross-like image of stromal lamellae by illuminating the cornea in phase-light polarization angle. Corneal haze was graded on a scale from 0 to 4. Interferometer cross-like images were graded on a scale from 0 to 5 by a masked observer. Keratometry, corneal central pachymetry, and optical densitometry were obtained with Scheimpflug corneal tomography. The cross-like image was observed in 31 of 32 healthy control subjects (58 of 59 eyes) and in 13 of 16 patients (19 of 22 eyes). The image was not detectable in 3 patients, 1 with total corneal neovascularization and scarring, 1 with central leukoma, and 1 with failed deep lamellar keratoplasty. Corneal haze was the main factor obscuring the cross-like image (P < 0.05). Clarity of the images was influenced by older age, steeper keratometry, higher pachymetry, and optical density (P < 0.05). There was no correlation between the orientation of the keratometric axes and the orientation of the 2 arms of the cross-like image (P > 0.05). Corneal stromal lamellae orientation can be efficiently assessed and displayed as a cross-like image by the polarimetric interferometer. Stromal lamellae orientation imaging may have potential implications in corneal diagnostics and surgery.

Solvothermal Vapor Annealing of Lamellar Poly(styrene)-block-poly(d,l-lactide) Block Copolymer Thin Films for Directed Self-Assembly Application.

Solvothermal vapor annealing (STVA) was employed to induce microphase separation in a lamellar forming block copolymer (BCP) thin film containing a readily degradable block. Directed self-assembly of poly(styrene)-block-poly(d,l-lactide) (PS-b-PLA) BCP films using topographically patterned silicon nitride was demonstrated with alignment over macroscopic areas. Interestingly, we observed lamellar patterns aligned parallel as well as perpendicular (perpendicular microdomains to substrate in both cases) to the topography of the graphoepitaxial guiding patterns. PS-b-PLA BCP microphase separated with a high degree of order in an atmosphere of tetrahydrofuran (THF) at an elevated vapor pressure (at approximately 40-60 °C). Grazing incidence small-angle X-ray scattering (GISAXS) measurements of PS-b-PLA films reveal the through-film uniformity of perpendicular microdomains after STVA. Perpendicular lamellar orientation was observed on both hydrophilic and relatively hydrophobic surfaces with a domain spacing (L0) of ∼32.5 nm. The rapid removal of the PLA microdomains is demonstrated using a mild basic solution for the development of a well-defined PS mask template. GISAXS data reveal the through-film uniformity is retained following wet etching. The experimental results in this article demonstrate highly oriented PS-b-PLA microdomains after a short annealing period and facile PLA removal to form porous on-chip etch masks for nanolithography application.

Lamellar orientation control of Ti–47Al–0.5W–0.5Si by directional solidification using β seeding technique

In this paper, we report a β seeding technique for the lamellar orientation controlling in TiAl alloy, which is a novelty and effective method for aligning the lamellar orientation of Ti–47Al–0.5W–0.5Si with primary α phase. The shorter composition transition zone and simpler process procedure can improve the deficiency of α seeding technique. The proper temperature gradient and normal growth rate are necessary for aligning the lamellar orientation in TiAl alloy with primary α phase using β seeding technique.

Influence of thermal stabilization treatment on microstructure evolution of the mushy zone and subsequent directional solidification in Ti-43Al-3Si alloy

Thermal stabilization (TS) and directional solidification (DS) experiments were performed on Ti-43Al-3Si alloy in a Bridgman-type furnace. The effects of thermal stabilization time on the microstructure evolution of mushy zone and initial interface of directional solidification were investigated. The results show that the volume fraction of liquid in mushy zone and the length of mushy zone decrease as the thermal stabilization time increases. The concentrations of Al and Si exhibit a decreasing trend in the complete liquid zone with increasing TS time, while the solute concentrations of Al and Si in complete liquid zone are always higher than those of the original as-cast alloy. A long TS time promotes solute diffusion and provides a solute homogeneous initial interface for subsequent DS process. Isolated α grains and Ti5Si3 particles in mushy zone tend to coarsen with the increase of thermal stabilization time. Long time TS treatment also leads to growth of Ti5Si3 particles which may act as nucleation sites to terminate the continuous growth of dendrite and inheritance of the lamellar orientation. Directionally solidified samples with aligned lamellar structures can be obtained within 30min TS treatment in Ti-43Al-3Si alloy.

Orientation relationship of eutectoid FeAl and FeAl2.

Fe-Al alloys in the aluminium range of 55-65 at.% exhibit a lamellar microstructure of B2-ordered FeAl and triclinic FeAl2, which is caused by a eutectoid decomposition of the high-temperature Fe5Al8 phase, the so-called ∊ phase. The orientation relationship of FeAl and FeAl2 has previously been studied by Bastin et al. [J. Cryst. Growth (1978 ▸), 43, 745] and Hirata et al. [Philos. Mag. Lett. (2008 ▸), 88, 491]. Since both results are based on different crystallographic data regarding FeAl2, the data are re-evaluated with respect to a recent re-determination of the FeAl2 phase provided by Chumak et al. [Acta Cryst. (2010 ▸), C66, i87]. It is found that both orientation relationships match subsequent to a rotation operation of 180° about a 〈112〉 crystallographic axis of FeAl or by applying the inversion symmetry of the FeAl2 crystal structure as suggested by the Chumak data set. Experimental evidence for the validity of the previously determined orientation relationships was found in as-cast fully lamellar material (random texture) as well as directionally solidified material (∼〈110〉FeAl || solidification direction) by means of orientation imaging microscopy and global texture measurements. In addition, a preferential interface between FeAl and FeAl2 was identified by means of trace analyses using cross sectioning with a focused ion beam. On the basis of these habit planes the orientation relationship between the two phases can be described by ([Formula: see text]01)FeAl || (114)[Formula: see text] and [111]FeAl || [1[Formula: see text]0][Formula: see text]. There is no evidence for twinning within FeAl lamellae or alternating orientations of FeAl lamellae. Based on the determined orientation and interface data, an atomistic model of the structure relationship of Fe5Al8, FeAl and FeAl2 in the vicinity of the eutectoid decomposition is derived. This model is analysed with respect to the strain which has to be accommodated at the interface of FeAl and FeAl2.

Effect of growth rate on microstructures and microhardness in directionally solidified Ti–47Al–1.0W–0.5Si alloy

Abstract

Characterization of structural knot distributions in UHMWPE fibers

Microbeam wide-angle X-ray diffraction (WAXD) experiments were carried out at different structural knot positions of SIOC and M4 fibers of ultra-high molecular weight polyethylene (UHMWPE). The optical microscope images revealed that SIOC fiber had bamboo-like structural knots, and M4 fiber had chaotic distribution of structural knots. WAXD patterns showed the monoclinic unit cell in the whole M4 fiber, but different lamellar orientations in the bamboo joint of SIOC fiber. In addition, small-angle X-ray scattering (SAXS) patterns confirmed that the SIOC fiber contained uniform distribution of shish structures, and differential scanning calorimetry (DSC) measurements showed that its less branched and short chains benefited the orthorhombic-hexagonal phase transformation.

Intraretinal variability and specialization of cones in Japanese anchovy (Engraulis japonicus, Engraulidae)

The retina of anchovies is characterized by an unusual arrangement and ultrastructure of cones. In the retina of Japanese anchovies, Engraulis japonicus, three types of cones are distributed into rows. The nasal, central, temporal, and ventro-temporal regions of the retina were occupied exclusively by the long and short cones. Triple cones, made up of two lateral components and one smaller central component, were found only in the dorsal and ventro-nasal retinal regions. In the outer segments of all short and long cones from the ventro-temporal region, the lamellae were oriented along the cell axis and were perpendicular to the lamellae in the long cones, providing a morphological basis for the detection of polarization. This lamellar orientation is unique to all vertebrates. The cones were examined with respect to regional differentiation in their size and spectral properties via light microscopy, transmission electron microscopy, and microspectrophotometry. Various dimensions of cones were measured in preparations of isolated cells. The cones from the ventro-temporal region had different dimensions than cones of the same type located in other retinal regions. Triple cones from the dorsal region were significantly larger than triple cones from the ventro-nasal region. The spectral absorbance of the lateral components of triple cones in the ventro-nasal retina was identical to the absorbance of all long and short cones from the ventro-temporal region. These are shifted to shorter wavelengths relative to the absorbance of the lateral components of the triple cones located in the dorsal retina. Thus, the retina of the Japanese anchovy shows some features of regional specialization common in other fishes that improves spatial resolution for the upwards and forwards visual axis and provides spectral tuning in downwelling light environment. That results from the differentiation of cone types by size and by different spectral sensitivity of various retinal areas.

Anisotropic deformation characteristics of an ultrafine- and nanolamellar pearlitic steel

Micromechanical experiments with 3 × 3 × 6 μm3 sized micro pillars were used to examine orientation dependencies of the mechanical properties in a severely plastically deformed high strength steel and compared with the undeformed state. For the synthesis, an initially ultrafine-lamellar (UFL) fully pearlitic steel was subjected to high pressure torsion (HPT) transforming the steel into a nanolamellar (NL) composite. Both microstructural states were then tested in-situ inside a scanning electron microscope. Within the individual micro pillars, fabricated by focused ion beam milling, the ferrite and cementite lamellae were aligned parallel, normal or inclined to the loading direction. The main findings are: First, the strength and strain hardening capacity is more than doubled comparing the UFL with the NL composite. Second, an anisotropic mechanical response exists in terms of i) strain hardening capacity and ii) stress level at the onset of plateau formation. Third, deformation and localization mechanisms at large compressive strains vary with the lamellae orientation, however they are independent of the lamellae thickness.

Phase Behavior and Its Effects on Crystallization in a Poly(trimethylene terephthalate)/Phenoxy Resin Blend.

Phase behavior and its effects on crystallization in an extruded poly(trimethylene terephthalate) (PTT)/phenoxy resin blend were studied with time-resolved light scattering (TRLS), optical microscopy (OM), differential scanning calorimetry (DSC), and small-angle X-ray scattering (SAXS). During annealing in the molten state, a two-phase structure with unique periodicity and phase connectivity was developed by liquid–liquid phase separation. After the formation of the phase-separated structure, the blend was homogenized by the interchange reactions between the two polymers. The crystallization behavior of PTT predominantly depended on the phase morphology developed during annealing. The pre-existing phase structures disturbed the lamellar orientation, resulting in a poorly ordered spherulitic superstructure.