A systematic experimental study of moiré patterns in the electron image of solution-grown polyethylene crystals

A systematic study of solution-grown polyethylene crystals in the electron microscope using dark-field imaging in several diffracted beams has warranted a detailed analysis of the moiré patterns recorded. Direct proof for the existence of tilt moiré patterns has been obtained for the first time, while parallel and tilt patterns have been positively distinguished. Consideration of the formation conditions and properties of moiré patterns in dark field has been extended to include interference between an electron beam diffracted in both participating crystals with one diffracted in only one of them. Examples of moiré patterns depending thus on double diffraction have been identified.     

Method for the Manual Analysis of Moiré Structures in STM images

A method is presented to manually determine the lattice parameters of commensurate hexagonal moiré structures resolved by STM. It solves the problem that lattice parameters of moiré structures usually cannot be determined by inspection of an STM image, so that computer-based analyses are required. The lattice vector of a commensurate moiré structure is a sum of integer multiples both of the two basis vectors of the substrate and of the adsorbed layer. The method extracts the two factors with respect to the adsorbed layer from an analysis of the Fourier transform of an STM image. These two factors are related to the two factors with respect to the substrate layer. Using the cell augmentation method, six possible moiré structures are identified by algebra. When the orientation and lattice constant of the substrate are roughly known, this information is usually sufficient to determine a unique moiré structure and its lattice parameters.     

Collapse of pyramidal polyethylene crystals

An assessment of the possible collapse mechanisms for solution-grown pyramidal polymer crystals is made, taking into account electron microscope observations and the mechanics of the loading situation. That comprehensive collapse always occurs is shown not to be as self-evident as is normally assumed, even though the more exacting criteria for its establishment are indeed usually met. Bending of lamellar crystals is found to account for many observations, although an alternative means of tilting molecular chains, the “differential shear tilt model” is also considered.     

Crystallization of isotactic polystyrene from solution

Difficulties previously encountered in the growth of chain-folded single crystals of isotactic polystyrene suitable for study by electron microscopy and electron diffraction have been overcome using very poor solvents (including atactic polystyrene of low molecular weight). The hexagonal lamellar crystals produced are relatively stable under electron bombardment and, as a consequence, dark-field moiré patterns produced by double diffraction from overlapping layers are easy to study. These patterns show no evidence of differences in lattice spacing between fold and nonfold planes such as have been reported in single crystals of several other polymers. Such differences were attributed to congestion at fold surfaces and their absence in polystyrene, for which the surface energy of fold surfaces is small, supports this interpretation. A comparison of crystallization kinetics of polystyrene crystals grown from good and from poor solvents reveals differences in growth rates of three or more orders of magnitude at comparable supercoolings. This disparity cannot be accounted for by acceptable adjustments of thermodynamic parameters in current theories of crystallization with chain folding. The role of molecular conformation in solution appears to exert an unexpectedly large influence on crystallization rate.     

Observations on Solution Crystallization of Poly(vinyl alcohol) in the Presence of Single‐Wall Carbon Nanotubes

PVA/SWNT dispersions yield aloe plant‐like crystals, where the leaves are single crystals templated by PVA coated SWNT. Longer growth times (≈18 months) lead to hexagonal rod‐like crystals. HR‐TEM images show evidence that PVA molecules are aligned parallel to the SWNT axis. WAXD, electron diffraction, and HR‐TEM observations of these aloe plant and hexagonal crystals suggests evidence for possible PVA‐SWNT epitaxy. Wide‐angle and electron diffraction data of these crystals also show that the structure seems to mimic the 2D hexagonal crystal packing of SWNT. PVA lattice images and moiré fringes were also observed in the leaf‐like crystals.

     

Polymer decoration study in chain folding behavior of solution-grown poly(ethylene oxide) crystals

The polymer decoration method based on the vaporization and condensation-crystallization of polyethylene (PE) upon the fold surface of polymer crystals has been widely used to study the chain folding behavior of the crystals. When this method was utilized to study solution-grown high molecular weight poly (ethylene oxide) (PEO) lamellar crystals, the highly anisotropic, low molecular weight fragment PE decorated become oriented parallel to the fold direction and form rods, which can be observed by transmission electron microscopy (TEM) and electron diffraction (ED). The growth sectors were clearly observed. From the ED patterns the {200} planes of the orthorhombic low molecular weight PE rod crystals can be observed, and the c-axis of these crystals is aligned parallel to the {120} growth planes of the PEO crystals. The decoration results indicate that the major fold orientation of high molecular weight PEO single crystals grown from dilute solution is along the {120} planes. © 1995 John Wiley & Sons, Inc.     

Solution crystallization of nylon 12

Electron microscopy and x-ray diffraction data have been obtained on nylon 12 crystallized from 1-hexanol, 1,6-hexanediol, and hexylene glycol. Ribbonlike lamellar crystals of the γ form are obtained by crystallization from all the solutions and elongated flat crystals of the α form by crystallization from the 1-hexanol and hexylene glycol solutions. The direction of the hydrogen bond in these crystals is almost parallel to that of maximum crystal elongation. α- and γ-form crystals both grow from 1-hexanol and hexylene glycol at appropriate crystallization temperatures. γ-form crystals alone are obtained from 1,6-hexanediol solution at every crystallization temperature. The long periods measured by small-angle x-ray diffraction for the solution-grown crystals are in the range 7.6–10.6 nm. The melting behavior of the solution-grown crystals is examined and discussed. The melting temperatures of the γ form may be lower than that of the α form. An equilibrium melting temperature of 208.4°C for γ-form crystals is obtained by using a relation between thickness of lamellar crystals and their melting temperatures observed by differential scanning calorimeter measurements. Solvents affect the growth of the two crystalline forms in solution crystallization.     

Densely packed overlayer of iron phthalocyanine molecules grown on single-layer graphene

The FePc molecules form a series of order superstructures on single-layer graphene grown on Ru(0001) with increasing molecular coverage.     

Morphology and fold length of single crystals in linear aliphatic polyesters

The marked effects of chemical structure on the morphology of single crystals in linear aliphatic polyesters have been investigated. Drawn fibers and solution-grown crystals of poly(ethylene sebacate), poly(hexamethylene sebacate), and poly(decamethylene 1,16-hexadecanedicarboxylate) have been studied by small-angle and wide-angle x-ray diffraction. Electron microscopy, and broadline NMR provided supplementary information. The crystal disorder due to irregularity in the cross section of the molecule due to ester groups along the chain direction increases with increase of the methylene sequence length in the chemical repeat unit. On the basis of the x-ray data and electron microscopy, it is proposed that the nature of the fold surface of single crystals of a given polymer, especially a choice of (001) or (hkl) fold surface (flat lamellae or hollow pyramidal crystals), is determined by energetic competition effects between fold geometry and unit cell symmetry. A smooth increase of the long period with increasing crystallization and annealing temperature seems to be general behavior even when a pronounced stepwise increase might be anticipated.     

Spot Moiré Fringes: Determination of Domain Boundaries and Structural Parameters in Ordered Nanoporous Structures

Spot moiré fringes are generated by the superposition between a nanoporous structure and a digital three‐way grating. The spot moiré fringes are useful for the characterization of the domain boundaries and structural parameters in ordered nanoporous materials. The pitches and the orientations of the nanopore arrays in three directions can be simultaneously determined in a large view field.     

Solution crystallization of nylon 6

Electron microscopy and x-ray diffraction data have been obtained on nylon 6 which has been crystallized from solutions in 1,6-hexanediol and 1,2,6-hexanetriol. Lamellar single crystals and spherulites of the γ form are obtained by crystallization from 1,2,6-hexanetriol. The morphology of the single crystals is different from that obtained from glycerine solutions. The spherulites of the γ form are composed of larger lamellae. Sheaflike crystals of the α form are obtained from both solvents. α-form and γ-form crystals both grow from 1,2,6-hexanetriol at appropriate crystallization temperatures. α-form crystals alone are obtained from 1,6-hexanediol solution at every crystallization temperature. The long periods measured by small-angle x-ray diffraction for the solution-grown crystals are in the range 56 to 66 Å. The melting behavior of the solution-grown crystals is examined and discussed. Effects of solvent on growth of the two crystalline forms from solution are investigated.     

Morphology of polypropylene crystals. III. Lamellar crystals of thermally decomposed polypropylene

Morphology and crystal structure of solution-grown and melt-grown crystals of thermally decomposed polypropylene have been studied by electron microscopy and x-ray diffraction. By crystallization from dilute α-chloronaphthalene or carbitol solutions well-defined lamellar crystals about 100–150 A. in thickness are obtained from fractions (number-average molecular weights 1600–2100) of thermally decomposed crystalline polypropylene. The structure is monoclinic as crystallized from very dilute α-chloronaphthalene or carbitol solutions (0.02–0.005 wt.-%). However crystals of the triclinic as well as of the monoclinic forms are precipitated from carbitol solutions of higher concentrations (0.05-1 wt.-%). On the other hand, the separated triclinic form has been obtained from the melt. Crystals of both modifications have similar morphology. In addition, the chain molecules cannot be expected to fold, within the thickness of lamellae in crystals of either modification prepared from the low molecular weight fractions used in this study.     

Bending contribution to line-broadening in polymer diffraction

It is proposed that bending of solution-grown polymer crystals causes the observed diffraction line-broadening in many cases. Independent estimates and observations of the severity of bending which can be sustained are used to calculate corresponding apparent crystal sizes, where the apparent crystal size is that obtained when it is assumed that broadening is caused by the small crystal size effect only. Bending broadening is found to be of sufficient magnitude to produce the observed broadening reported in the literature. Some of the steps taken so far in the development of line-broadening analysis for polymer crystals show similarities with the evolution of line-broadening analysis for graphitic carbons and a brief review of the latter is included.     

Identifying hexagonal boron nitride monolayers by transmission electron microscopy

Multislice simulations in the transmission electron microscope (TEM) were used to examine changes in annular-dark-field scanning TEM (ADF-STEM) images, conventional bright-field TEM (BF-CTEM) images, and selected-area electron diffraction (SAED) patterns as atomically thin hexagonal boron nitride (h-BN) samples are tilted up to 500 mrad off of the [0001] zone axis. For monolayer h-BN the contrast of ADF-STEM images and SAED patterns does not change with tilt in this range, while the contrast of BF-CTEM images does change; h-BN multilayer contrast varies strongly with tilt for ADF-STEM imaging, BF-CTEM imaging, and SAED. These results indicate that tilt series analysis in ADF-STEM image mode or SAED mode should permit identification of h-BN monolayers from raw TEM data as well as from quantitative post-processing.     

Long-range structural order, moiré patterns, and iridescence in latex-stabilized foams

We describe the facile production of highly stable foams stabilized solely by micrometer-sized, sterically stabilized polystyrene (PS) latex particles. Such foams can survive for more than one year in the wet state and remain intact after drying. In contrast, foams stabilized with either sodium dodecyl sulfate or poly(N-vinylpyrrolidone) were destroyed after removal of the aqueous phase. Scanning electron microscopy studies reveal hexagonally close-packed arrays of PS particles within the dried foam, which suggests high colloid stability for the PS particles prior to their adsorption at the air-water interface. Localized moiré patterns are observed by optical microscopy due to the formation of well-defined latex bilayers with exquisite long-range order. Moreover, the dried foams are highly iridescent in bright transmitted light, which may offer potential applications in security inks and coatings.     

Morphology of polypropylene crystallized from the melt

Polypropylene was oxidized with nitric acid in a manner contrived to remove its amorphous component. Concurrent with the loss in weight there was an increase in crystallinity, a decrease in molecular weight, a drop in melting point, and a change in visible structure. The crystalline component, which survived from polymer which had orginally crystallized isothermally, consisted of separated spherulites. These spherulites could be broken apart into lamellae which gave electron diffraction patterns like those now associated with a variety of solution-grown single crystals. As opposed to the high molecular weight of the unoxidized parents, the lamellae which survived the acid treatment consisted of short chains whose length depended on the temperature at which the parents had crystallized. If, as diffraction studies indicate, these short molecules are lined up in rank and file, then the length of the molecules and the height of the lamellae should correspond. The heights of the lamellae could not be measured precisely enough to establish an exact correlation, but the variations in average chain length calculated from viscosity data at least approximated the heights of the lamellae which were observed visually.     

Measurement of flexural modulus of polymeric foam with improved accuracy using moiré method

The flexural modulus of polymeric foams determined from three-point bending tests is usually inaccurate due to the local deformation undergone by the material during testing. The machine used in the test gives deflection values larger than the actual deflection of the foam specimen due to the deformation of the material at the loading point. This leads to errors in the computation of the modulus value. In this work, the deflection values of a beam made of polymeric foam in a three-point bending test were determined using the moiré method. The change in the moiré pattern at the neutral axis of the foam during loading was recorded and converted into deflection values. The deflection data were used to generate the stress–strain curve from which the flexural modulus of the foam material was determined. The proposed method was verified using aluminum beams, where a high correlation between the deflection data from the machine readings and the moiré method was obtained. The flexural modulus of the foam determined using the moiré method was found to be within 3% of the value published in the material data sheet.     

BUNDLE: A program for building the transmembrane domains of G-protein-coupled receptors

The only information available at present about the structural features of G-protein-coupled receptors (GPCRs) comes from low resolution electron density maps of rhodopsin obtained from electron microscopy studies on 2D crystals. Despite their low resolution, maps can be used to extract information about transmembrane helix relative positions and their tilt. This information, together with a reliable algorithm to assess the residues involved in each of the membrane spanning regions, can be used to construct a 3D model of the transmembrane domains of rhodopsin at atomic resolution. In the present work, we describe an automated procedure applicable to generate such a model and, in general, to construct a 3D model of any given GPCR with the only assumption that it adopts the same helix arrangement as in rhodopsin. The present approach avoids uncertainties associated with other procedures available for constructing models of GPCRs based on a template, since sequence identity among GPCRs of different families in most of the cases is not significant. The steps involved in the construction of the model are: (i) locate the centers of the helices according to the low-resolution electron density map; (ii) compute the tilt of each helix based on the elliptical shape observed by each helix in the map; (iii) define a local coordinate system for each of the helices; (iv) bring them together in an antiparallel orientation; (v) rotate each helix through the helical axis in such a way that its hydrophobic moment points in the same direction of the bisector formed between three consecutive helices in the bundle; (vi) rotate each helix through an axis perpendicular to the helical one to assign a proper tilt; and (vii) translate each helix to its center deduced from the projection map.     

Utilization of Moiré interferometry to study strain transfer across polymer interfaces within multilayer thermoplastic elastomers

In this paper, a moiré interferometry technique was used to study the viscoelastic strain distributions within and across the interfaces of multilayer thermoplastic elastomers, which were fabricated as models for functionally modulus‐graded materials. Two types of interfaces, those formed from miscible and immiscible pairs of the same thickness, were fabricated within multilayer thermoplastic samples. The analysis of the moiré fringe patterns indicated that there was a large normal strain, ε_y, concentration at the polymer interfaces formed from the immiscible or partially miscible pairs. However, this large normal strain was not observed at the interfaces formed from the miscible pairs. These results suggest that the magnitude of strain concentrations at polymer interfaces within functionally modulus‐graded materials could be significantly reduced by extensive chain interdiffusion, which could be promoted by careful selection of miscible polymer pairs. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 2196–2206, 2005     

Refined NMR analysis of the phase structure of solution-crystallized linear polyethylene

Nuclear magnetic resonance (NMR) spectroscopy reveals that solution-grown polyethylene samples have a unique phase structure independent of molecular weight. The lamellar crystallites are composed of about 85% crystalline material with the noncrystalline overlayer as large as 15%. The molecular motion in the overlayer is comparatively hindered and the liquid-like component, which is generally recognized in melt-grown crystals, cannot be produced appreciably, even at 60°C. Such hindered molecular mobility can be understood in terms of a rather restricted conformation of the molecular chains in the noncrystalline overlayer, arising from the special mode of crystallization from dilute solution.