Thin films phase-separation process

Phase separation of polymer blends and block copolymers. Confining polymer blends and block copolymers between surfaces may influence the phase separation process, as a consequence of the preferential affinity of one of the components for the interface. Since the pioneer works of Reich and Cohen [26] and later by Nesterov et al. [27], Ball and Essery [28], and Jones [29] amongst others much work has been done to understand the mechanisms of phase separation in polymer thin films. The presence of substrate-film and/or film-air interfaces introduces an additional complexity compared to bulk phase separation processes [30-35]. Complex structures can be produced by slight differences on parameters... 

When polymers undergo phase separation in thin films, the kinetic and thermodynamic effects are expected to be pronounced. The phase separation process can be controlled to effect desired morphologies. Under suitable conditions a film deposition process can lead to pattern replication. Demixing of polymer blends can lead to structure formation. The phase separation process can be characterized by the binodal and spinodal curves. UCST is the upper critical solution temperature, which is the temperature above which the blend constituents are completely miscible in each other in all proportions. LUST behavior is not found as often in systems other than among polymers. LUST is the lower critical solution temperature. This is the... 

Phase separation can emerge spontaneously in thin films of polymer blend, and it is this thermodynamically driven phase-separation process that produces the patterning of thin polymer blend films. It can be induced by several parameters, each with critical values, including (i) the ratio between the components [10,22] (ii) the demixing temperature [12] and (iii) the chemical nature of the solvent. 

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. 

The formation of structured thin films (both phase morphology and surface topography) is typically the result of the phase-separation processes, which are, in turn, strongly influenced by the presence of an interface. Therefore, these processes may lead to surface-oriented phase separation [27] among others, or the formation of a wetting layer [61], or, in the case of a partial wetting, the presence of a surface field that can induce the breakup of a surface layer [62,63], In this section, we will detail the main factors involved in the phase-separation phenomena. 

Illustration of the phase-separation stages during the spin-coating process. After the initial spin-off stage where both polymer and solvent are removed (i), (ii), the film separates into two layers (iii) and the film thins owing to solvent evaporation only. The interface between the polymers destabilizes (iv) and the film phase-separates laterally (v), (vi). 

As has been outlined previously, the close interaction with the substrate can enhance the phase-separation process and favor the formation of surface morphologies compared to the bulk [33], In effect, the structures formed can be explained taking into account several aspects. For instance, several studies have explained the thin film morphologies observed in terms of chain conformations directly related to the film thickness [29], Other studies highlighted the role of the surface tension on the boundary shape. According to these reports, the polymer-air interface is deformed due to the influence of the surface tension [27,33,114-118]. 

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. 

To maximise separation efficiency requires low H and high N values. In general terms this requires that the process of repeated partitioning and equilibration of the migrating solute is accomplished rapidly. The mobile and stationary phases must be mutually well-dispersed. This is achieved by packing the column with fine, porous particles providing a large surface area between the phases (0.5-4 m2/g in GC, 200-800 m2/g in LC). Liquid stationary phases are either coated as a very thin film (0.05-1 pm) on the surface of a porous solid support (GC) or chemically bonded to the support surface as a mono-molecular layer (LC). 

Phase separation controlled by diffusion exchange often results in a skin which is composed of a micellar assembly of nodules, as will be discussed below. When extremely hydrophobic polymers (e.g., modifled-PPO) are cast from dioxane into water (pg = p = p ) a dense polymer layer is formed at the solution s interface that somewhat resembles the type of layer formed by Interfacial polymerization. There is almost no inward contraction of the interfacial skin, and the coagulation process is controlled by diffusion through the dense, interfacial thin film. These result in an anisotropic membrane with a very fine "coral" structure (Figures 9 and 10). 

Mori R, Takahashi M, Yoko T (2005) Photoelectrochemical and photocatalytic properties of multilayered Ti02 thin films with a spinodal phase separation structure prepared by a sol-gel process. J Mater Res 20 121-127... 

Most commercially available RO membranes fall into one of two categories asymmetric membranes containing one polymer, or thin-film composite membranes consisting of two or more polymer layers. Asymmetric RO membranes have a thin ( 100 nm) permselective skin layer supported on a more porous sublayer of the same polymer. The dense skin layer determines the fluxes and selectivities of these membranes whereas the porous sublayer serves only as a mechanical support for the skin layer and has little effect on the membrane separation properties. Asymmetric membranes are most commonly formed by a phase inversion (polymer precipitation) process (16). In this process, a polymer solution is precipitated into a polymer-rich solid phase that forms the membrane and a polymer-poor liquid phase that forms the membrane pores or void spaces. 

Another process which leads to HIPE instability is gravitational syneresis, or creaming, where the continuous phase drains from the thin films as a result of density differences between the phases. This produces a separated layer of bulk continuous phase and a more concentrated emulsion phase. The separated liquid can be located either above or below the emulsion, depending on whether the continuous phase is more or less dense, respectively, than the dispersed phase. This process has been studied by Princen [111] who suggests that it can be reduced by a number of parameters, including a high internal phase volume, small droplet sizes, a high interfacial tension and a small density difference between phases. 

---