Lamellar spacing

In conclusion, it can be suggested that the lamellar spacing of miktoarm-star copolymers is controlled by two parameters (i) the molecular weight of the corresponding AB diblock, which also controls the segregation strength, and (ii) the functionality of the central core. 

The effect of constraints introduced by confining diblock copolymers between two solid surfaces was examined by Lambooy et al. (1994) and Russell et al. (1995). They studied a symmetric PS-PMMA diblock sandwiched between a silicon substrate, and silicon oxide evaporated onto the top (homopolymer PMMA) surface. Neutron reflectivity showed that lamellae formed parallel to the solid interfaces with PMMA at both surfaces. The period of the confined multilayers deviated from the bulk period in a cyclic manner as a function of the confined film thickness, as illustrated in Fig. 2.60. First-order transitions were observed at t d0 = (n + j)d0, where t is the film thickness and d0 is the bulk lamellar period, between expanded states with n layers and states with (n + 1) layers where d was contracted. Finally, the deviation from the bulk lamellar spacing was found to decrease with increasing film thickness (Lambooy et al. 1994 Russell et al. 1995). These experimental results are complemented by the phenomenologi-... 

Fig. 5.20 Calculated lamellar spacings for copolymers with (/,g)-folded PEO and PBO blocks (loo) divided by measured lamellar spacings (d) and plotted against lm (Mai et al. 1997). Crystallization conditions are indicated by ( ) cooled at 10°Cmin T (o) quenched (a) annealed ( ) self-seeded. The symbol (v) denotes results from Yang et al. (1995). The dashed lines indicate the integral values of la)/d expected for the conformations with...

Fig. 6.34 Dimensionless lamellar spacing A - d aNin for an /— 0.45 diblock at %N - 12 as four different homopolymers are added (with degrees of polymerization /ftV) (Matsen 19956). Dimensionless widths of the A- and B-rich lamellae are shown in (b) and (c) respectively.

The lamellar spacing of a monoglyceride gel phase as a function of water content is plotted in Figure 14. The gel phase of the neutral monoglyceride has a lipid bilayer thickness of 49.5 A, and it swells to a unit layer thickness of 64 A (20). If an ionic amphiphilic substance (e.g. a soap) is solubilized in the lipid bilayer, it is possible to obtain a gel phase with high water content. As with the gel phases with infinite swelling that were discussed above, there is, however, a minimum water layer thickness which in this monoglyceride gel is about 40 A. 

Figure 3.3 Idealized models of mesostmctured M41S materials (a) MCM-41 (2D hexagonal, space group pbnun ). (b) MCM-48 (cubic, space group Ia- id), and (c) MCM-50 (lamellar, space group pi).

Interactions between water and the polar head groups of lipid molecules are necessary for the formation of lamellar phases however, it appears that there is no free water associated with the 13 nm trilaminar units. This is supported by the observation that this periodicity does not increase with increasing stratum corneum water content.22 There is likely water hydrogen bonded to the polar regions of the lamellae. In contrast, the minor short periodicity swells from 5.8 to 6.6 nm as the water content of stratum corneum increases from 12 to 50%.23 This suggests that the lipid lamellae are simple individual bilayers and free water molecules can exist between adjacent bilayers, thus causing the increase in the lamellar spacing. 

Fig. 35 Temperature dependence of lamellar spacing (upper half) and angle of chain tiltwith respect to the lamellar normal (lower half) for extended-chain crystals of end-deuterated alkane C216H385D49 grown from toluene solution at 70 °C. SAXS spectra were recorded during heating from 60 °C to the melting point. Data for heating at 1 °C/min (solid diamonds) and 6 °C/min (open squares) are shown for comparison (from [84] by permission of American Chemical Society)...

Figure 7.8. Layer arrangement of the tin(II) phosphate. Note that the lone-pair of electrons points into the inter-lamellar space (Vaidhyanathan and Natarajan [26]).

Figure 7.26. A tin(II) oxalate showing the saw-tooth arrangement between two adjacent layers. The amine and water molecules occupy the inter-lamellar space (Natarajan...

There may also be more specific effects of certain liquids, such as the softening of the binders in bonded films suggested by Hopkins and Campbell . Any adsorption of a liquid on the surface basal planes, or any intercalation between lamellae would interfere with the ideal charge distributions and inter-lamellar spacing found with uncontaminated molybdenum disulphide. These effects would in fact be similar to the adverse effects of water vapour and other vapours on the friction of burnished films. 

Although a lamellar crystal structure is favourable for solid lubrication, the inter-lamellar spacing and the nature of the inter-lamellar bonding are of major importance in determining the resistance to inter-lamellar shear, and therefore the sliding friction, of lamellar compounds. 

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