Blend films segregation

The stracture formed by donor acceptor BHJ blends is dependent on the solvent used. Studies of polyfluorene PCBM blends show that choice of solvent controls the segregation of materials within the blend film so that a vertical multilayer structure forms spontaneously in some cases (Bjorstrom and Moons, 2007). Similarly, the choice of solvent in a polymer-polymer blend controls the vertical arrangement of materials and the device performance (Arias et al, 2002). Exploitation of these effects may open new avenues for materials-processing procedures in the organic photovoltaic field. 

Structural instability due to phase segregation in donor-acceptor blends over time is a function of materials and temperature. At moderate temperatures of 50-80 C, the problem is more severe in MDMO-PPV PCBM than P3HT PCBM blend films. This... 

As the use of aqueous base in the development step has been mandated, the wettability of resist films is very important and therefore water contact angles of polymer and resist films are measured often. The contact angle measurement can provide important information about surface segregation of blend films. 

Segregated polymer brushes were fabricated utilizing phase-separation phenomena in ultrathin polymer blend films (PS-COOH/PMMA) and the combination of grafting-to and grafting-from methods. 

For instance, surface segregation in water vapor of the blend films prepared with polystyrene-hZocfc-poly(L-glutamic acid) PS27-Zt-PGA2o improved the reorientation of the hydrophilic polypeptide block at the interface (Fig. 5.22).[129] Water contact angle measurements supported the surface enrichment of polypeptide segments as a... 

The surface chemistry of the substrate, even if the substrate is homogeneous, will affect the film phase segregation (see Fig. 6.2). In a two component blend film of two polymers—let us call them polymer A and polymer B— if the surface tension of polymer A and the substrate is less than the surface tension of polymer B and the substrate, polymer A will tend to sink towards the substrate and polymer B will tend to rise from the substrate, producing a vertically phase segregated film. Substrate interactions tend to dominate the atmospheric interactions, though a similar process can lead to vertical phase separation at the atmospheric interface. 

Solvent Effects The essence of choosing different solvent types, mixed solvents, and solvent additives is to better control the time and degree of phase segregation of the blending film. The spin casting is a short and dynamic process that, in most... 

Mielczarski, J. A. Mielczarski, E. GaUi, G. MoreUi, A. MartineUi, E. Chiel-lini, E., The Surface-Segregated Nanostructure of Fluorinated Copolymer-Poly(dimethylsiloxane) Blend Films. Langmuir 2010, 26, 2871-2876. 

Currently, AFM, neutron reflection (NR) spectroscopy and SIMS are used to examine phase separation in symmetrically segregating thin polymer blend films (ca. 1000 A). Phase separation in the film leads to undulations of the liquid-air interface, provided that the film is sufficiently thin to suppress any surface-directed spinodal decomposition waves. Flattened droplets are formed at a very... 

An excellent example of the capabilities of this technique was provided by Mokarian-Tabari et al. [167], who investigated the phase separation of immisdble polymers during spin coating from a common solvent. The FRES technique, combined with NRA, was used for a quantitative analysis of the polymer blend films prepared from a solution of polystyrene (d-PS) and poly(methylmethacrylate) (h-PMMA). The data obtained showed an evident stratification in the film, with a segregation of d-PS to the surface and an increase in PMMA content with distance from the surface (see Figure 23.37). 

Surface segregation of a variety of A-substituted HBPAs in a linear polystyrene matrix after aimealing of the blend films was also studied by means of X-ray photoelectron spectroscopy (XPS) [38]. When two polymers with a similar degree... 

As discussed in Section 2.23.1.2, one of the advantages of AFM over electron miaoscopy is that it enables 3D imaging of untreated samples in a physically relevant environment. Given the surface nature of the technique, initial AFM studies of phase separation processes in polymer mixtures have been primarily focused on thin or ultrathin films. " " The confinement in a film and component segregation to the interfaces have been shown to have a profound effect in both spinodal decomposition, and nucleation and growth processes of the phase separation. The spectrum of AFM applications has been extended to bulk phase separation of polymer mixtures using conventional and oblique microtoming of polymer blend films. 

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... 

F-BDAF Tg for various blend compositions, see Fig. 14. The microphase-separated morphology further manifests itself in the self-adhesion behavior of polyimide films derived from such mixtures. For mixture containing at least 25 wt% of the flexible component, peel tests of polyimide bilayer samples prepared by solution casting, bulk failure of the test specimens was observed. Since the flexible component contained fluorine, the samples could be examined by X-ray photoelectron spectroscopy to determine the surface composition. At only 10% loading, the flexible component comprised 100% of the top 75 A of the sample. The surface segregation of the flexible component is believed to be responsible for the adhesion improvements. 

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