Confined microdomains

Some SR compartments (jSR) lie just beneath special microdomains of PM, and are joined to this PL by electron-dense processes (observed with electron microscopy). Na+ pumps with high-ouabain-affinity a. subunits (a2/a3 subunits), NCX and SOCs, appear to be confined to these PL microdomains. These PL microdomains, the subjacent jSR, and the intervening tiny cytosolic volume, form functionally specialized units we call PLasmERosomes. Through the operation of these units (which are apparently present in many types of cells), modulation of Na+ pump activity may have a profound influence on Ca2+ signalling in smooth muscles and many other types of cells. 

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]. 

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... 

The intrinsic 3D interfacial curvature in compositionally asymmetric block copolymers provides extra degrees of freedom for the phase behavior in hexagonally structured microdomains. It is now well established that confinement of a cylinderforming block copolymer to a thickness other than the characteristic structure dimension in bulk, together with surface fields, can cause the microstructure to deviate from that of the corresponding bulk material. Surface structures in Fig. 1 are examples of simulated [57-59] and experimentally observed morphologies [40, 49, 60-62] that are formed in thin films of bulk cylinder-forming block copolymers. 

Another interesting featiu e of these block copolymers is the fact that the double bonds present in the PB block can be used for crosslinking the PB part [64,65]. This can lead to the creation of nanoobjects, i.e., through crosslinking of the PB of aggregates in aqueous solution. Alternatively, crosslinking of the PB microdomains in the bulk or thin film state can create a network, producing variable confinement of the crystaUizable PEO chains and thus affect their crystallization characteristics. 

A similar ordering is found within each of the perovskite layers in the reduced n=3 Ruddlesden-Popper phase SrjMn Og, derived from the fuUy oxidised Sr Mn O,. Under normal preparation methods the ordering is confined to the perovskite layers and does not extend to three dimensions in the macroscopic crystal, although electron microscopy suggests that microdomains of such three-dimensionally ordered stmctures do exist. These structures are similar to those of Mn-containing brownmiUerite-related compounds (Section 2.5.1). 

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