Lamellar stacks

11 A schematic representation of the macro-conformations of polymer chains. The vertices indicate the limiting cases. A, amorphous B, chain-folded C, chain-extended. The area indicates intermediate structures D, fringed micelle. (Reproduced by permission of Academic Press.) 

If the stacking is regular, as is often the case, 4 can be determined by SAXS, as described in section 3.4.2. If the stacking is very irregular, elee-tron microscopy must be used to sample different regions. Electron microscopy shows that the lamellae are not always flat but may be corrugated, particularly in melt-crystallised material. Electron diffraction shows that the chain axes are then usually perpendicular to the overall plane of the lamella rather than being normal to the local thickness direction. Information about the thicknesses of the crystalline layers can be obtained from Raman spectroscopy. 

Another method that can give information about the periodicity and at the same time can help to answer questions (a) and (c) is NMR spectroscopy, which is now considered in some detail because it provides a link with the discussion of molecular motion later in the chapter. 

In section 2.7.5 the idea of spin diffusion is introduced. In any kind of diffusion experiment the time taken for the diffusing species to pass through a particular thickness of material depends on the thickness and on the diffusion coefficient. If either of these is known, a measurement of the diffusion time can be used to determine the other. This idea can be appHed to yield information about the average separation of the phases in 

A suitable pulse sequence can then set up a state in which the net magnetisation of the crystalline phase is zero and that of the non-crystal-line phase is in equilibrium with the applied magnetic field Application of a 90° pulse after a time t and analysis of the subsequent FID then allows the determination of the magnetisation M t) of the crystalline phase as a function of time as it relaxes back towards equilibrium by the transference of magnetisation to it by spin diffusion from the non-crystalline phase. 

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Low density Linear low density 0.91-0.94 30-55 5-10 10-100 Small lamellar stacks Poorly defined spherulites... 

Except for biopolymers, most polymer materials are polydisperse and heterogeneous. This is already the case for the length distribution of the chain molecules (molecular mass distribution). It is continued in the polydispersity of crystalline domains (crystal size distribution), and in the heterogeneity of structural entities made from such domains (lamellar stacks, microfibrils). Although this fact is known for long time, its implications on the interpretation and analysis of scattering data are, in general, not adequately considered. 

To demonstrate the effect of misorientation or even isotropization let us consider a structural entity52 which is a perfect lamellar stack. Figure 8.12 demonstrates... 

No infinitely extended layers, several components with different topology (e.g. primary and secondary lamellar stacks)... 

Ixiqu t) = Ix(L2) was zero up to ri(OM) and it starts increasing after ti(OM). This is very different to Ix(qvbt). This suggests that the lamellar stacking is not superimposed during the induction period. The onset time of lamellar stacking ronset(L) is estimated as,... 

Figure 1.61 Schematic illustration of chain folding leading to lamellar crystallites (inset) and lamellar stacking to form spherulites.

Figure 8. Lamellar stacking in UP viewed along the crystallographic a axis, the shortest axis in the C222, unit cell. The c axis is vertical. Molecules drawn with heavy lines lie in grooves formed by their nearest neighbors, which lie behind them and are drawn with light lines. Reproduced from Ref. 33 with permission from Gordon and Breach Science Publishers S. A.

The distribution of chains in a lamellar film formed by a binary blend of symmetric PS-PMMA diblocks has been probed using neutron reflectivity by Mayes et al. (1994). The microphase self-assembled parallel to the substrate, giving a lamellar stack, as for pure diblocks (see Section 2.5.1). By blending mixtures of unlabelled and selectively labelled long and sort diblocks, it was determined that short diblock copolymer chains are localized at the PS-PMMA interface, while the longer chains are selectively located at the domain centres. These results... 

Figure 5 Transmission electron micrographs of human skin treated with liquid-state PEG-8-L L-595 CS (70 30 5) vesicles, nonocclusively. Detailed micrographs of skin treated for 16 h. Lamellar stacks are present in the intercellular spaces (see double arrow) only along the cell boundaries. The islands of lamellar stacks are found in the intercellular regions. These stacks most probably originate from vesicle material Arrow, lamellar stack C, corneocyte. Bar represents 100 nm. 

Figure 4.35 Calculated structure for an HT 3-hexylthiophene tetramer obtained by using molecular mechanics modeling, where the globally minimized tetramers have been docked in an idealized manner to X-ray structural parameters, (a) Intermolecular p-stacking between the thiophene rings as inferred from a (90 °C) X-ray pattern of the film, (b) Lamellar stacking as inferred from X-ray scans of intensity versus 20 data. Reprinted from R.D. McCullough, S. Tristram-Nagle, S.P. Williams, R.D. Lowe and M. Jayaraman, ]. Am. Chem. Soc., 115,4910 (1993). Copyright (1993) American Chemical Society...

These observations reveal that the periphery of the developing spherulites is not as smooth as that observed under OM. The growth front of the spherulite looks like a hedgehog and the lamellar stack develops into a spherulite shape only near the end of the growth, as shown in Fig. 37. Furthermore, at the early stage, the spherulite is not round in shape. 

The required crystal structure consists of the lamellar stacking of hexagonal crystals in which the repeating unit (unit cell) has hexagonal symmetry and includes two adjacent lamellae. This is conveniently described as a 2H arrangement of the unit... 

Highly ordered lamellar gel microstructures are formed by certain surfactants and mixtures of a surfactant and long-chain fatty alcohols in water. Using small angle X-ray scattering (SAXS), an ordered lamellar stack lattice model was proposed for the gel formed by 10% w/w cetostearyl alcohol containing 0.5% cetri-mide surfactant. In contrast, the microstructure of a Brij 96 gel depends on the surfactants concentration. A hexagonal liquid-crystalline gel structure was... 

These results demonstrate that the embrittlement of the PE implants accompanies a microhardening of a surface layer and an increase in crystallinity. The two pieces of evidence are complementary and imply a reduction in the crack-blunting ability of the material, i.e. a diminution of the number of interlamellar tie molecules which connect adjacent lamellar stacks. In consequence the elastic properties of the material diminish and cause the material to microharden during wear. The increase in microhardness at the wear surface is partly because the amorphous component decreases in quantity and partly because its chemical nature changes as it undergoes simultaneous microhardening and loss of elasticity. 

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