Interfacial morphology cases

Fig. 37. Theoretical predictions for thin film morphology of a phase-separating polymer mixture. Case I yA where yA and yB are the surface tensions of components A and B, respectively, and yAB is the interfacial tension. Case II yA

It has been indicated how interfacial morphology plays a crucial role in the transport properties of hybrid membranes, and conditions as close as possible to the ideal case must be achieved to ensure advanced separation performances. For this reason, several fabrication techniques have been developed to reduce as much as possible the influence of a nonideal interfacial morphology on the transport characteristics of the hybrid membranes. Among the most frequently used procedures there are solution blending, the sol-gel procedure, surface modification, and interfacial polymerization. In some cases a combination of these techniques is required to reach the best dispersion of the filler within the matrix. 

The appearance and persistence of core-shell structures as well as the occurrence of phase separation are attributed to a small asymmetry in the X -parameters (xPS-pi = 0.06, xpi-pdms = 0.09 and xps-pdms = 0.20). Hence, a PDMS core surrounded by a PI shell embedded in a PS matrix results in a smaller inner diameter interfacial area, relative to that for the PS-PI case. In a blend of a PS-fo-PI-fc-PDMS triblock with PS and PDMS homopolymers, more PS homopolymer is expected to be found in the corona of the PS block than PDMS homopolymer in the corona of the PDMS block because the penalty for contact between the PI block and PDMS homopolymer is larger. In consequence, the distribution of homopolymers favours an expanded PS-PI interface, making the core-shell morphologies, gyroid and cylinder, more prevalent. 

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]. 

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