Microdomains conformations

While thin polymer films may be very smooth and homogeneous, the chain conformation may be largely distorted due to the influence of the interfaces. Since the size of the polymer molecules is comparable to the film thickness those effects may play a significant role with ultra-thin polymer films. Several recent theoretical treatments are available [136-144,127,128] based on Monte Carlo [137-141,127, 128], molecular dynamics [142], variable density [143], cooperative motion [144], and bond fluctuation [136] model calculations. The distortion of the chain conformation near the interface, the segment orientation distribution, end distribution etc. are calculated as a function of film thickness and distance from the surface. In the limit of two-dimensional systems chains segregate and specific power laws are predicted [136, 137]. In 2D-blends of polymers a particular microdomain morphology may be expected [139]. Experiments on polymers in this area are presently, however, not available on a molecular level. Indications of order on an... 

Hasegawa H., Hashimoto T., Kawai H., Lodge T.P., Amis E.J., Glinka C.J., and Han C. SANS and SAXS studies on molecular conformation of a block copolymer in microdomain space. 2. Contrast matching technique. Macromolecules, 18, 67, 1985. 

FRET-FLIM has been applied to numerous biological problems, centering on protein-protein interactions, protein conformation, posttranslational modifications, and activation state of enzymes, with lipid microdomains. Each of these applications takes advantage... 

Unlike the bulk morphology, block copolymer thin films are often characterized by thickness-dependent highly oriented domains, as a result of surface and interfacial energy minimization [115,116]. For example, in the simplest composition-symmetric (ID lamellae) coil-coil thin films, the overall trend when t>Lo is for the lamellae to be oriented parallel to the plane of the film [115]. Under symmetric boundary conditions, frustration cannot be avoided if t is not commensurate with L0 in a confined film and the lamellar period deviates from the bulk value by compressing the chain conformation [117]. Under asymmetric boundary conditions, an incomplete top layer composed of islands and holes of height Lo forms as in the incommensurate case [118]. However, it has also been observed that microdomains can reorient such that they are perpendicular to the surface [ 119], or they can take mixed orientations to relieve the constraint [66]. 

In the case of t < Lo, it has been suggested that perpendicular lamellae are favored in the boundary-symmetric confined film because they avoid the entropic penalty associated with the compressed chain conformations in parallel-oriented microdomains [109]. In boundary-asymmetric substrate-supported films, various kinds of morphologies, including hybrid morphologies that combine surface-parallel and surface-perpendicular components, are predicted, as well as observed, depending on the film thickness difference and surface/interface energies [14,41,120]. 

The surface of bulk block copolymer samples has been studied using TEM by Turturro et al. (1995). They report that non-equilibrium structures with lamellar and cylindrical microdomains oriented normal to the free surface can result from solvent casting, with a high evaporation rate. However, slower evaporation of solvent from their PS-PB diblocks resulted in the equilibrium conformation with domains parallel to the free surface. Perpendicular orientation of PS-PB lamellae at the free surface was observed earlier by Henkee et al. (1988) who studied thin films prepared by solvent casting. They observed that a reduction of this orientation occurs in favour of the parallel one on annealing the sample. 

These results imply that homopolymer PS is not always miscible with the PS blocks of the copolymer, i.e. confinement of PS to an interface in a block copolymer can lead to immiscibility with homopolymer PS (Hashimoto et al. 1990). This has been interpreted in terms of the enthalpic and entropic contributions to the free energy (Hasegawa and Hashimoto 1996). For a < 1 uniform solubilization increases the translational entropy of the homopolymer, but chain stretching in the homopolymer and in the PS chain of the diblock leads to a decrease in conformational entropy. At the same time, the lateral swelling of microdomains leads... 

When the block length becomes comparable with Ny distinctions between the behaviors of two copolymers practically disappear. At L > 200, one observes that T Ly with y = 4/3. In this case, the characteristic scale of the microdomain structure behaves as r Ls with <5 = 1/2. This dependence is caused by the fact that flexible chains in the melt have a Gaussian conformation, and the average size of any chain section of n units is proportional to ft1/2 [75]. Hence, for sufficiently large Vs, the spatial scale of microinhomogeneities in the system is determined only by the block size. However, the behavior of the random-block copolymer at L < 102 is more complicated. In particular, r has a minimum at L 10. 

Even though some plasma membranes, such as nerve myelin membranes, contain a high concentration of lipids that form gel phase bilayers, the presence of cholesterol keeps these membranes in a fluid phase. However, interaction with the rigid cholesterol ring affects hydrocarbon chains of lipids in the liquid crystal phase (L ) and leads to formation of a new phase, the liquid ordered (Lq) phase (27). The phase is well characterized by a variety of physical methods and does not exist in pure lipids or their mixtures. In the liquid ordered phase, the long axis rotation and lateral diffusion rates are similar to the La phase, but the acyl chains are predominantly in an all-trans conformation and, hence, the order parameters are similar to the Lp phase (see Table 1). Recently, the cholesterol-rich Lq phase has been strongly associated with microdomains in live cells—the so-called lipid rafts. ... 

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