Structure lamellar

Rgure 2.1 Model of lamellar structure, showing the arrangement of macromolecules (right)  

Similar microscopic images of lamellae can be achieved in different ways. Fig. 2.2 compares some useful techniques to reveal the lamellar structure of semicrystaiiine polymers (here, iPP). Electron microscopic imaging of any structural detail usually requires the application of contrast-enhancement steps. In Fig. 2.2(a), for 

In contrast to amorphous polymers, structural details of lamellae can be imaged using electron diffraction contrast in TEM. In this case, unstained uitrathin sections or solution-cast films are used. Electron diffraction patterns allow structural analysis at the level of the crystalline unit cell, and they can be used to detect the crystal orientations of a polymer. An example with a sheaf-like morphology of PE with low molecular weight is shown in Eig. 2.3 by different modes in TEM. 

A dark-field image is formed if only diffracted electrons are allowed to contribute to image formation (crystaiiine regions appear bright, Fig. 2.3(a)). The arrangement of parallel lamellae is reveaied after chemicai staining (Fig. 2.3(b)). The electron diffraction pattern shows the crystalline structure and orientation of the crystals (Fig. 2.3(c)). 

for irradiation above the melting range, the whole sample exists in the molten (amorphous) state. Cross-linking of macromolecules by irradiation also yields a stabilization of this phase after cooling down to room temperature (i.e., any crystallization is prevented, and no lamellar structures are visible in TEM). 

---

V. A. Drits and C. Tchoubar, X-Ray diffraction by disordered lamellar structures. Springer-Verlag, Berlin (1990). 

The period of the lamellar structures or the size of the cubic cell can be as large as 1000 A and much larger than the molecular size of the surfactant (25 A). Therefore mesoscopic models like a Landau-Ginzburg model are suitable for their study. In particular, one can address the question whether the bicontinuous microemulsion can undergo a transition to ordered bicontinuous phases. 

The length scale A is the wavelength of the lamellar structure. Minimization of Eq. (100) with respect to A gives... 

The single crystal of a polymer is a lamellar structure with a thin plateletlike form, and the chain runs perpendicular to the lamella. The crystal is thinner than the polymer chain length. The chain folds back and forth on the top and bottom surfaces. Since the fold costs extra energy, this folded chain crystal (FCC) should be metastable with respect to the thermodynamically more stable extended chain crystal (ECC) without folds. 

Similar lamellar structures are formed for l-alkyl-3-methylimida2olium cations with [PdCl4] when n > 12. As with the pyridinium systems, mesomorphic liquid crystal structures based on the smectic A structure are formed [24]. 

DOia/TigAl. These materials possess a lamellar structure consisting of layers of twin-related TtAl and layers of TigAl in single crystalline form they are called "polysynthetically twinned (PST) crystals" (for a recent review see Yamaguchi, et al. 1995). 

A new class of compounds able to form lamellar structures consisting of bilayers separated by water-layers are dialkylammonium-salts of 2-[((trifluormethyl)-phenyl)-amino]-benzoic acid (Flufenamic acid) as was found by Eckert and Fischer 80). 

With diblock copolymers, similar behavior is also observed. One component is enriched at the surface and depending on miscibility and composition a surface-induced ordered lamellar structure normal to the surface may be formed. Recent investigations include poly (urethanes) [111], poly(methoxy poly (ethyleneglycol) methacrylate)/PS [112] and PS/PMMA [113, 114]. In particular the last case has been extensively studied by various techniques including XPS, SIMS, NR and optical interferometry. PS is enriched at the surface depending on blockcopolymer composition and temperature. A well ordered lamellar structure normal to the surface is found under favourable conditions. Another example is shown in Fig. 6 where the enrichment of poly(paramethylstyrene), PMS(H), in a thin film of a di-... 

The first section involves a general description of the mechanics and geometry of indentation with regard to prevailing mechanisms. The experimental details of the hardness measurement are outlined. The tendency of polymers to creep under the indenter during hardness measurement is commented. Hardness predicitions of model polymer lattices are discussed. The deformation mechanism of lamellar structures are discussed in the light of current models of plastic deformation. Calculations... 

Microhardness of Lamellar Structures 6.1 Detoi mation Mechanism... 

Well-ordered lamellar structures. The lamellae are arranged in parallel, giving rise to long-range order. Examples are soaps, phosphoUpids and clays. 

Figure 5.4-47. Formation of lamellar structure through action of vorticity (adapted from Baldyga and Bourne, 1986).

The ionic polymerisation of styrene is as dangerous. Interlaminar compounds of sodium or potassium with graphite catalyse the polymerisation of styrene. This method can usually be controlled. Nevertheless, it gives rise to detonations. It was assumed that in these cases the lamellar structure of graphite is destroyed and the metallic particles dispersed. 

Dye structures of passive tracers placed in time-periodic chaotic flows evolve in an iterative fashion an entire structure is mapped into a new structure with persistent large-scale features, but finer and finer scale features are revealed at each period of the flow. After a few periods, strategically placed blobs of passive tracer reveal patterns that serve as templates for subsequent stretching and folding. Repeated action by the flow generates a lamellar structure consisting of stretched and folded striations, with thicknesses s(r), characterized by a probability density function, f(s,t), whose... 

Polar lipids form different kinds of aggregates in water, which in turn give rise to several phases, such as micellar and liquid crystalline phases. Among the latter, the lamellar phase (La) has received the far greatest attention from a pharmaceutical point of view. The lamellar phase is the origin of liposomes and helps in stabilizing oil-in-water (O/W) emulsions. The lamellar structure has also been utilized in creams. We have focused our interest on another type of liquid crystalline phase - the cubic phase... 

Certain azacrown-appended cholesterol derivatives can form unique vesicular or lamellar structures in the absence and the presence of metal salts in aqueous solution.187-190 These superstructures created from the azacrown-appended cholesterol derivatives are useful as a template for the transcription into the silica structure. Azacrown-appended cholesterol gelator creates the novel multilayered spherical structure in acetic acid, whereas this gelator results in the fluffy globular aggregates in the presence of Pd(N03)2.191... 

The structures of electroplated hard alloys have been less extensively studied than those of similar electrolessly deposited materials. Sallo and co-workers [118-120] have investigated the relationship between the structure and the magnetic properties of CoP and CoNiP electrodeposits. The structures and domain patterns were different for deposits with different ranges of coercivity. The lower-f/c materials formed lamellar structures with the easy axis of magnetization in the plane of the film. The high-Hc deposits, on the other hand, had a rod-like structure, and shape anisotropy may have contributed to the high coercivity. The platelets and rods are presumed to be isolated by a thin layer of a nonmagnetic material. 

Nylon crystallites consist of sheets of chains that are hydrogen-bonded to their neighbors. On a supermolecular scale, crystallites have a lamellar structure, that is they are many times longer and broader than they are thick. When nylon crystallizes from an isotropic molten state, it generally forms spherulites, which consist of ribbon-like lamellae radiating in all directions... 

Recent developments have allowed atomic force microscopic (AFM) studies to follow the course of spherulite development and the internal lamellar structures as the spherulite evolves [206-209]. The major steps in spherulite formation were followed by AFM for poly(bisphenol) A octane ether [210,211] and more recently, as seen in the example of Figure 12 for a propylene 1-hexene copolymer [212] with 20 mol% comonomer. Accommodation of significant content of 1-hexene in the lattice allows formation and propagation of sheaf-like lamellar structure in this copolymer. The onset of sheave formation is clearly discerned in the micrographs of Figure 12 after crystallization for 10 h. Branching and development of the sheave are shown at later times. The direct observation of sheave and spherulitic formation by AFM supports the major features that have been deduced from transmission electron and optical microscopy. The fibrous internal spherulite structure could be directly observed by AFM. 

If the intended evaluation can be carried out on isotropic material, and thus the observed anisotropy is rather an obstacle than an advantage, the fiber pattern can be isotropized (cf. Sect. 8.4.2). This may, in particular, be helpful if lamellar structures are analyzed. If the focus of the study is on the anisotropic structure, the multidimensional CDF (cf. Sect. 8.5.5) may be a suitable tool for analysis. Several studies have demonstrated the power of the CDF method for the study of structure evolution during straining [174,177,181-183],... 

---