Phase separation polymer blend thin films

Many previous studies have considered the influence of confinement on the equilibrium phase separation temperature of thin films [50,51], and of spatial dimensionality on the kinetics of spinodal decomposition [52]. The latter study involved a combination of these factors, whereby an examination was made of the effect of finite film thickness on the kinetics of spinodal decomposition over a film thickness range where transition occurred from three-dimensional bulk-like to near two-dimensional kinetics. In particular, the 1000 A thin polymer blend films of PS and polybutadiene on a silicon substrate were found to be sufficiently thick to exhibit phase-separation kinetics similar to those of bulk blends. The thinner film (L w 200 A) exhibited a different phase-separation kinetics, however [47]. 

In the last decade, there has been great interest in the imdeistanding of the phase-separation process in the production of the thin films of immiscible polymer blends. This curiosity is as a direct consequence of the particular surface structures found in polymer blends that will define their final surface properties. Due to the extensive literature concerning both theoretical [9,17-21], and experimental [22] studies this part will be limited in order to briefly present the main concepts on polymer blends thin films. 

Dalnoki-Veress K, Forrest JA, Stevens JR, Dutcher JR. Phase separation morphology of thin films of polystyrene/polyisoprene blends. J Polym Sci B 1996 34 3017-24. 

Scheme 1 Different possible donor-acceptor active layer morphologies in OPV devices. The left drawing depicts a macrophase separated polymer blend morphology with undefined domains. In the middle and at the right, schematic active layer morphologies of disordered and vertically aligned microphase separated block copolymer thin films are shown. Dark and light grey domains correspond to the donor and the acceptor phase, respectively...

Ogawa, H., Kanaya, T., Nishida, K., Matsuba, G. Phase separation and dewetting in polystyrene/poly(vinyl methyl ether) blend thin films in a wide thickness range. Polymer 49(1), 254-262 (2008)... 

LEDs were fabricated with TA-PPP as the emissive layer. Single-layer devices of ITO/PEDOT/TA-PPP/Ca/Al were fabricated. PEDOT, poly(3,4-ethylenedioxythiophene), was used to enhance hole injection from the anode. Charge injections of the single layer LEDs were clearly hole dominant The barrier for electron injection, around 1.0 eV, is too high. Electron dominant materials such as DO-PF and 2-(4-t-butylphenyl)-5-biphenyloxadiazole (t-PBD) were used to enhance electron injection. The thin film of a TA-PPP and PF blend (95 5 weight ratio) was phase separated. Atomic force microscopy (AFM) showed PF spheres, close to 1 pm in diameter, dispersed in the TA-PPP matrix (Figure 6). This type of phase separation is common in blends of stiff and soft polymers. The PL emission of die blend film was characteristic of TA-PPP. However, once thermally treated, the spectrum shifted bathochromically much like PF. The EL spectrum from LEDs based on the blend thin film contained much emission from PF in the 500-700 nm regime. The device efficiency was about 0.43 cd/A. TA-PPP/PF double layer LEDs were also fobricated. But the efficiency was not improved because when PF was spin coated onto TA-PPP, the PF solution washed out most of the TA-PPP layer. 

The properties of the external surface can significantly alter phase separation in a thin polymer film. In extensive experimental investigations of the phase separation of polymer blends directed by patterned substrates [1,56,58,60,80-88] it was observed that the domain size evolved in a power law relation with time. The composition wave was normal to, and propagates inward from, the functionalized substrate. Likewise, processing parameters such as pattern size in the substrate were seen to affect refinement of the morphology. 

Phase Separation in Thin Polymer-Blend Films. 120... 

The bilayer morphology of thin asymmetric films of may be unstable. A regularly corrugated surface structure of the films was ascribed to spinodal transition into a laterally phase separated structure, where the surface morphology depended on the polymer incompatibility and the interfacial interactions [347, 348]. Recently, the phase separation and dewetting of thin films of a weakly incompatible blend of deuterated PS and poly(p-methylstyrene) have been monitored by SFM [349, 350]. Starting from a bilayer structure, after 454 h at T= 154 °C the film came to the final dewetting state where mesoscopic drops of... 

A mean field theory has recently been developed to describe polymer blend confined in a thin film (Sect. 3.2.1). This theory includes both surface fields exerted by two external interfaces bounding thin film. A clear picture of this situation is obtained within a Cahn plot, topologically equivalent to the profile s phase portrait d( >/dz vs < >. It predicts two equilibrium morphologies for blends with separated coexisting phases a bilayer structure for antisymmetric surfaces (each attracting different blend component, Fig. 32) and two-dimensional domains for symmetric surfaces (Fig. 31), both observed [94,114,115,117] experimentally. Four finite size effects are predicted by the theory and observed in pioneer experiments [92,121,130,172,220] (see Sect. 3.2.2) focused on (i) surface segregation (ii) the shape of an intrinsic bilayer profile (iii) coexistence conditions (iv) interfacial width. The size effects (i)-(iii) are closely related, while (i) and (ii) are expected to occur for film thickness D smaller than 6-10 times the value of the intrinsic (mean field) interfacial width w. This cross-over D/w ratio is an approximate evaluation, as the exact value depends strongly on the... 

Kim J-S, Ho PKH, Murphy CE, Friend RH (2004) Phase separation in polyfiuorene-based conjugated polymer blends lateral and vertical analysis of blend spin-cast thin films. Macromolecules 37 2861... 

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