Lamella, lamellar, lamellae

Fartasch, M., Bassuskas, I.D., and Diepgen, T.L., Structural relationship between epidermal lipid lamellae, lamellar bodies and desmosomes in humans epidermis an unltrastructural study, Br. J. Dermatol.,... 

In copolymer ordered structures, the segregated microphases can be spheres, cylinders or lamellae. The lamellae tend to form a regularly repeating lamellar sequence, the cylinders arrange themselves in a bidimensional hexagonal lattice, while the spheres give rise to cubic lattices. 

Zoning and internal structure—note differences in zonal birefringence and refractive index (on broken crystals) twinning or crystal intergrowths number of sets of lamellae lamellar width variations. 

Keywords chain fold crystal growth, lamella, lamellar thickness distribution, nucleation, secondary crystallization, etching, lamellae detection, small angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), optical microscopy, spherulite, morphology, electron microscopy. 

Crystallisation of polymers such as PCL, which crystallise to give spherulitic structures, starts from a nucleus which subdivides at the growth surface to generate a series of very thin (typically 10 nm thick) crystalline lamellae. The lamellae continue to grow and sub-divide to establish the spherically-symmetric structures (spherulites) which consist of a series of crystalline fibrils, bundles of lamellar crystals, extending from the nucleus in all directions, with a constant... 

Smectic phases are characterized by a layered structure, in which a two-dimensional fluid order prevails. In Fig. 3.4a, a schematic picture of the skeleton structure of a smectic phase is shown. The two-dimensional fluid layers are stacked upon each other with the periodicity distance d, causing a one-dimensional positional order along the direction of the layer normal k. In the case of the lyotropic lamellar L phase one smectic layer is usually referred to as a lamella. The lamella can be separated into two parts, as shown in Fig. 3.4b. The first part is a surfactant bilayer, in which the molecules are on the average oriented perpendicular to the layer plane. For conventional lyotropic mixtures polar solvents are used, which cause the hydrophobic chains to point towards the middle of the bilayers. This arrangement can be inverted by using apolar solvents, i.e. alkyls. If the surfactant molecules are interdigitated to some degree, the term partial bilayer is used. The second part of the lamella is a layer of solvent molecules, in which the molecules are believed to solely possess a fluid-like order. The solvent layers separate the surfactant bilayers from each other and should thus inhibit the transfer of information from one surfactant layer to the next. Consequently, the lamellar L phase is the only fluid. 

Figure 3.37 Optical micrographs of single compact polyethylene axiahte grown from a 1% solution in xylene at ca. 86°C. View (a) is nearly normal to the truncated lonzenge lamellar surfaces, (b) is 90° away, showing the edges of lamellae/lamellar bundles, and (c) is along the axis of the symmetry axis or spine. The width of each optical micrograph is 60 pm. From Bassett et al. [54] with permission of John Wiley Sons, Inc.

The effect of different types of comonomers on varies. VDC—MA copolymers mote closely obey Flory s melting-point depression theory than do copolymers with VC or AN. Studies have shown that, for the copolymers of VDC with MA, Flory s theory needs modification to include both lamella thickness and surface free energy (69). The VDC—VC and VDC—AN copolymers typically have severe composition drift, therefore most of the comonomer units do not belong to crystallizing chains. Hence, they neither enter the crystal as defects nor cause lamellar thickness to decrease, so the depression of the melting temperature is less than expected. 

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