Orientation Of Lamellar Crystals Research Articles

Additive Manufactured Poly(ε-caprolactone)-graphene Scaffolds: Lamellar Crystal Orientation, Mechanical Properties and Biological Performance.

Understanding the mechano–biological coupling mechanisms of biomaterials for tissue engineering is of major importance to assure proper scaffold performance in situ. Therefore, it is of paramount importance to establish correlations between biomaterials, their processing conditions, and their mechanical behaviour, as well as their biological performance. With this work, it was possible to infer a correlation between the addition of graphene nanoparticles (GPN) in a concentration of 0.25, 0.5, and 0.75% (w/w) (GPN0.25, GPN0.5, and GPN0.75, respectively) in three-dimensional poly(ε-caprolactone) (PCL)-based scaffolds, the extrusion-based processing parameters, and the lamellar crystal orientation through small-angle X-ray scattering experiments of extruded samples of PCL and PCL/GPN. Results revealed a significant impact on the scaffold’s mechanical properties to a maximum of 0.5% of GPN content, with a significant improvement in the compressive modulus of 59 MPa to 93 MPa. In vitro cell culture experiments showed the scaffold’s ability to support the adhesion and proliferation of L929 fibroblasts (fold increase of 28, 22, 23, and 13 at day 13 (in relation to day 1) for PCL, GPN0.25, GPN0.5, and GPN0.75, respectively) and bone marrow mesenchymal stem/stromal cells (seven-fold increase for all sample groups at day 21 in relation to day 1). Moreover, the cells maintained high viability, regular morphology, and migration capacity in all the different experimental groups, assuring the potential of PCL/GPN scaffolds for tissue engineering (TE) applications.

Electron microscopy for polymer structures

Abstract This paper reviews recent advances and perspectives of electron microscopy and its application to polymer hierarchical structures. Of the various kinds of hierarchical polymer structures, we placed particular emphasis on polymer nanocomposites and polymer crystals based mainly on our recent results. In those nanocomposites, the chemical bonding between the nanometer-size fillers and rubber matrix, a key contributor to the mechanical properties of the material, has been investigated by combining scanning transmission electron microscopy (STEM) with electron energy-loss spectroscopy (EELS). The position-dependent EELS spectrum with high spatial resolution of STEM successfully provided revealed the presence/absence of the chemical bonds across the interface. The mechanical properties and fracture mechanism of nanocomposites have been studied by combining structural observations made using transmission electron microscopy (TEM) with simulations. They have been further investigated using in situ TEM with a newly designed stretching holder, in which morphological changes, including cavity formation, were visualized and analyzed in terms of local strain distribution. The fracture processes of nanocomposite have been observed at nanometer resolution. The fundamental reinforcement mechanisms have been elucidated from morphological studies of nanocomposites under tensile deformation and during the fracture process. Moreover, nano-diffraction imaging, a position-resolved electron diffraction imaging with STEM, has been applied to a polymer crystal to evaluate the orientation of lamellar crystals at nanometer resolution. All these recent successes with radiation-sensitive polymer materials stemmed from developments made in electron optics and super-sensitive cameras used for advanced electron microscopy.

Mechanism of Elastic Properties of Biodegradable Poly[(R)-3-Hydroxybutyrate-co-4-hydroxybutyrate] Films Revealed by Synchrotron Radiation.

Reversible elastic films of biobased and biodegradable poly[(R)-3-hydroxybutyrate-co-4-hydroxybutyrate] [P(3HB-co-4HB)] were prepared by uniaxial drawing procedures. Mechanical properties and highly ordered film structures were investigated by tensile testing and both static-state and in situ wide-angle X-ray diffraction and small-angle X-ray scattering with synchrotron radiation during stretching and relaxing. Despite the crystalline nature of the polymers, the elongation at break of these films was greater than 1500%. Reversible elasticity was achieved after the first 10 times of uniaxial stretching. X-ray measurement results indicated that on stretching, β-form molecular chains with a planar zigzag conformation were introduced from molecular chains with random coils in the amorphous regions between α-form lamellar crystals. Notably, the orientation of the α-form lamellar crystals increased after relaxation of the molecular chains with a planar zigzag conformation (β-form) between the lamellar crystals (α-form). Reversible elastic properties were regenerated by a planar zigzag conformation between the lamellar crystals, the extension of molecular chains in lamellar crystals by the rotation of molecular conformation, and changes in the degree of orientation of the lamellar crystals.

Transition from chain- to crystal-network in extension induced crystallization of isotactic polypropylene

With a combination of extensional rheology and in-situ small-angle X-ray scattering measurements, the protocol of two-step extension is proposed to investigate the early stage of flow-induced crystallization (FIC) in supercooled isotactic polypropylene melt at 138 °C. After both step strains, the crystallization half-time presents a nonmonotonic dependence on the interval time between two extensional operations, based on which three different stages of structural evolution are defined. In stage I, both nucleation and chain relaxation occur, which enhances the crystallization rate but reduces the final lamellar crystal orientation. In this stage, each part of the melt is considered to have approximately the same dynamics to respond homogeneously to the second extension and thus the system is still dominated by a chain-network. When entering into stage II, the sparse large-scaled crystal is formed to construct a heterogeneous crystal-network superimposed on the chain-network, which decelerates the second extension induced crystallization by causing stress concentration on the crystal-network at low faction. In stage III, the crystal-network dominates the sample deformation due to the formation of abundant lamellar crystal, which recreates the approximately same dynamics for each part of sample and brings about an enhancement of crystallization rate again. The transition from chain- to crystal-network revealed in this work demonstrates a dynamical coupling of chain relaxation, crystal nucleation, and growth in FIC of polymers.

基板上の高分子薄膜で生成するラメラの選択的配向性

基板上の高分子薄膜で生成するラメラの選択的配向性

Orientation of lamellar crystals and its correlation with switching behavior in ferroelectric P(VDF-TrFE) ultra-thin films

The ferroelectric properties of poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) thin films mainly depend on the crystal orientation and crystallinity. We demonstrate here the thermal history and thickness dependent crystal orientation in P(VDF-TrFE) ultrathin films, and its correlation with local polarization reversal. Upon annealing in the paraelectric phase after spin-cast, the lamellar crystals grow preferentially along the crystallographic a axis, with the c axis (chain axis) parallel to the substrate. In contrast, when crystallized in the paraelectric phase from melt, the lamellar crystals take a flat-on orientation with the crystallographic c-axis normal to the substrate. In addition, local measurement by piezoresponse force microscopy indicates that the flat-on crystals do not display any polarization switching, whereas the edge-on crystals exhibit proper switching upon application of a vertical electric field. Importantly, the coercive field measured from the piezoresponse hysteresis loops does not change with the film thickness in the edge-on oriented lamellar crystals.

Confinement Induced Preferential Orientation of Crystals and Enhancement of Properties in Ferroelectric Polymer Nanowires.

The physical properties of polymers strongly depend on the molecular or supermolecular order and orientation. Here we demonstrate the preferential orientation of lamellar crystals and the enhancement of ferro/piezoelectric properties in individual poly(vinylidene fluoride-co-trifluoroethylene) (P(VDF-TrFE)) nanowires fabricated from anodic alumina oxide (AAO) templates. The crystallographic a axis of P(VDF-TrFE) was found to be aligned along the long axis of nanowires due to geometrical confinement and grapho-expitaxial crystals growth. The alignment of lamellar crystals in P(VDF-TrFE) nanowires and enhancement of crystallization translated into improved ferro/piezoelectric properties such as lower coercive field and higher piezoelectric coefficient, testified by piezoresponse force microscopy images and piezoresponse hysteresis loops.

Study on Lamellar Crystal Orientation and Instable Growth in Polymer Ultrathin Films

The lamellar crystal orientation and crystal instable growth in polymer ultrathin films were studied. The orientation of lamellar crystal transforms from edge-on to flat-on with decreasing film thickness due to the enhancement of restriction effect of polymer-substrate on the edge-on lamellar crystals. Lamellar crystals in ultrathin films have a instable growth and become a dendrite at the growth front because of the diffusion-controlled growth.

Microbeam X-ray diffraction of non-banded polymer spherulites of it-polystyrene and it-poly(butene-1)

The orientation of lamellar crystals in non-banded spherulites of it-polystyrene and it-poly(butene-1) was investigated by microbeam X-ray diffraction. The two-dimensional intensity map of diffraction enables us to examine the local orientation of lamellar crystallites in the non-banded spherulites. The obtained results indicated the re-orientation of crystallites in non-banded spherulites and confirmed our previous observation on the anisotropic birefringence of a group of crystal stacks by polarizing optical microscopy.

Crystal Growth Transition from Flat-On to Edge-On Induced by Solvent Evaporation in Ultrathin Films of Polystyrene-b-Poly(ethylene oxide)

The transition of lamellar crystal orientation from flat-on to edge-on in ultrathin films of polystyrene-b-poly(ethylene oxide) (PS-b-PEO) via solvent vapor (toluene) treatment was investigated. When the as-prepared film was treated in saturated solvent vapor, breakout crystals could form quickly, and then they transformed from square single crystals (flat-on lamellae) to dendrites and finally to nanowire crystals (edge-on lamellae). Initially, heterogeneous nucleation at the polymer/substrate interface dominated the structure evolution, leading to flat-on lamellar crystals orientation. And the transition from faceted habits to dendrites indicated a transition of underlying mechanism from nucleation-controlled to diffusion-limited growth. As the solvent molecules gradually diffused into the polymer/substrate interface, it will subsequently weaken the polymer-substrate interaction. In this case, lower free energy lamellae of an edge-on orientation could be obtained by the gain of free volume of the soluble PS block.

Thickness-Dependent Molecular Chain and Lamellar Crystal Orientation in Ultrathin Poly(di-n-hexylsilane) Films

The molecular chain and lamellar crystal orientation in ultrathin films (thickness < 100 nm) of poly-(di-n-hexylsilane) (PDHS) on silicon wafer substrates have been investigated by using transmission electronic microscopy, wide-angle X-ray diffraction, atomic force microscopy, and UV absorption spectroscopy. PDHS showed a film thickness-dependent molecular chain and lamellar crystal orientation. Lamellar crystals grew preferentially in flat-on orientation in the monolayer ultrathin films of PDHS, i.e., the silicon backbones were oriented along the surface-normal direction. By contrast, the orientation of lamellar crystals was preferentially edge-on in ultrathin films thicker than ca. 13 nm, i.e., the silicon backbones were oriented parallel to the substrate surface. We interpret the different orientations of molecular chain and lamellar crystal as due to the reduction of the entropy of the polymer chain near the substrate surface and the particularity of the crystallographic (001) plane of flat-on lamellae, respectively. A remarkable influence of the orientations of the silicon backbone on the UV absorption of these PDHS ultrathin films was observed due to the one-dimensional nature of sigma-electrons delocalized along the silicon backbone. With the silicon backbones perpendicular or parallel to the surface of the substrate, the UV absorbance increased or decreased with an increase of the angle between the incident UV beam direction and direction normal to the thin film, respectively.

Ultrathin Films of Poly(ethylene oxides) on Oxidized Silicon. 1. Spectroscopic Characterization of Film Structure and Crystallization Kinetics

We have characterized the structure, molecular orientation, and crystallization kinetics of isothermally crystallized thin (film thickness d < 500 nm) and ultrathin films (d < 100 nm) of poly(ethylene oxides) on oxidized silicon substrates by a combination of microscopic and spectroscopic methods. In situ hot stage atomic force microscopy (AFM) reveals a preferred flat-on orientation of lamellar crystals in films thinner than ca. 300 nm. The mean orientation of the polymer molecules, as measured by transmission and grazing angle reflection FT-IR spectroscopy, fully agrees with the preferred orientation of the PEO helices parallel to the surface-normal direction, as inferred from the AFM data. In addition to a strong film thickness dependence of this preferred chain orientation, the FT-IR data indicate that the degree of crystallinity decreases steadily when the film thickness becomes smaller than ∼200 nm. The local environment of pyrene end-labels in derivatized PEO was characterized by steady-state fluor...

Influence of temperature and humidity on ionic conduction of mixed metal hydroxides

Results of conductivity (ac) measurements are reported for synthetic mixed metal hydroxides as a function of temperature, ranging from 323 to 353°K, and relative humidity, ranging from 50 to 80%. The polycrystalline compacts of layered mixed hydroxides show protonic conductivities arising from the interlayer region. The conductivity is strongly dependent on the temperature, relative humidity and composition of mixed metal hydroxides. The conductivities are anisotropic with the direction of forming pressure owing to the preferential orientation of lamellar crystals, particularly evident with powders characterized by high crystal size.

Etching and the morphology of branched and cross-linked polyethylene

A permanganic acid etching technique was used to characterize the genuine morphologies of bulk crystallized samples of linear, branched, and crosslinked polyethylene. Four different kinds of artifacts as large as 15–20 μm were detected. It was also noted that artifact density could be drastically reduced by proper selection of etching conditions. The morphologies of artifacts were independent of specimen crystallinity and the orientation of lamellar crystals; however, their rate of production was susceptible to the crystallinity of samples. The formation of a fresh surface involved the preferential etching of edge-on and secondary lamellae. As the growth features of cross-linked polyethylene were less than 6 μm and were mainly sheaf-like structures, growth features could be readily distinguished from the artifacts by transmission electron microscopy. Another type of artifact (globules morphology) produced at angstrom scale was found to depend on the crystallinity and the surface structure of lamellar crystals. It is demonstrated that the fine lamellar details of branched and cross-linked polyethylene can be observed using transmission electron microscopy. Increasing the number of cross-links in polyethylene, modifies the size and nature of supermolecular structures, the periodic twisting of lamellar crystals, and the nature of the nucleation process. The study of a blend of branched and cross-linked polyethylene showed that the structurally dissimilar molecules co-crystallized at low crystallization temperatures, while there is a tendency for molecular fractionation at higher crystallization temperatures.