Blended polymers surface segregation

In polymer blends, or mixtures, the primary question is whether one of the components segregates preferentially to the surface. One of the reasons this is of interest is because most commercial polymers contain more than one component and a surface segregation of one of the components from a miscible mixture during, for example, extrusion of the material, could affect the surface finish of the product. Because polymer blends are generally dense liquids, from the previous discussion it is clear that packing effects are expected to dominate the surface properties. 

The quantitative study of polymer surfaces and interfaces is about ten years old and so is a relatively recent subject. It is therefore not surprising that there are many possibilities for future research. Many of the phenomena that occur at polymer interfaces are related to self-assembly. Even the simplest example, that of surface segregation of one component of a polymer blend to an interface is self-assembly. This is because where we initially had a homogeneous film, we now have one with two layers, a bulk layer and a surface segregated layer. We have shown how even such a well-studied phenomenon as surface segregation can have a future because with it one can generate a surface which could provide an excellent precursor to an interface. 

A conventional understanding of the surface segregation from polymer blends is that the surface should be enriched in the component with lower bare surface free energy fs, regardless of the value of bulk composition This is however true only when (-dfs/d(j))s does not change its sign when surface concentration is varied (see Fig. 14b). For such blends, surface enrichment in the same... 

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... 

Finally, it has been found that the deuterium staining of individual molecules, commonly used in condensed matter studies, in case of polymers can lead to serious consequences in bulk and surface thermodynamics. This was shown in this work by the phase separation of isotopic blends (Sect. 2.2.2), isotope swapping effect in blend miscibility (Sect. 2.2.3) and surface segregation (Sect. 3.1.2.5) as well as by the specific scaling law (Eq. 61) which governs the polymer brush conformation (Sect. 4.2.1). 

Chen, J. X., and Gardella, J. A. (1998). Quantitative ATR FT-IR analysis of surface segregation of polymer blends of polystyrene/poly(dimethylsiloxane)-co-polystyrene. Appl. Spectrosc. 52, 361-366. 

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. 

Within this context, this chapter discusses the possibilities to produce surface patterns by surface segregation of an additive towards the interface in polymer blends. Surface segregation is the result of the preferential migration of one blend component to the interface thereby inducing selective enrichment at the nearsurface level. As will be discussed below in detail, this effect is directed by the surface thermodynamics that favors the presence at the interface of the component of a mixture lowering the surface tension. As a consequence, this phenomenon is the cause of having large differences between the surface and the bulk composition. 

In this chapter, we aim to provide an overview of the possibilities of using surface segregation not only to functionaUze and but also to nanostructure polymer surfaces. For this purpose, we first discuss the factors that may favor or reduce the presence of a particular additive at the interface. As depicted here, the molecular structure, the functional groups contained within the polymer, and other factors such as hydrophilicity of the environment or the temperature will play a key role on the migration of an additive towards the surface. More interestingly, the use of block copolymers within the blends permits the formation, by self-assembly, of different structures at the nanometer scale, thus providing an interesting way to fabricate different nanometer scale structures at the interface. 

It has to be mentioned at this point that this chapter focuses on the segregation and eventually interfacial self-assembly of nonmiscible blends. These blends are susceptible to form microstructured and nanostructured domains at the polymer interface. Readers interested in surface segregation in homopolymer miscible blends should refer to the following references [14—22]. 

Fig. 5.1 Representation of the surface segregation phenomena in a polymer blend of a surface active component in a matrix...

Other groups have developed alternative models in order to explain the surface segregation in multicomponent systems, briefly discussed in the next part. However, it is outside the scope of this chapter to thoroughly describe the theoretical models developed in this topic but rather provide a simple overview of the main aspects involved in the surface segregation of polymer blend. Those readers interested in the theoretical approaches reported to understand the surface segregation phenomena are referred to the following references [18, 38 1]. 

From this preliminary definition, we can conclude that, by taking advantage of this spontaneous phenomenon and considering the aspects that rule the migration of a particular component in a polymer blend we will be able to vary the surface behavior. In the next paragraphs of this chapter we illustrate how this phenomenon has been employed both to modify the chemical composition of the polymer surface and to produce nanostructured interfaces. Moreover, the adaptive/responsive behavior of the polymer blend surfaces as a function among others of the environment, temperature or pH will be also described. But first of aU we have to review the various parameters that require a consideration in order to obtain functional surfaces by segregation. 

Factors Involved in the Surface Segregation of Polymer Blends... 

As a consequence it is obvious that polymer dispersity will have an influence on surface segregation. Smaller chains in the samples will migrate at the interfaces [62]. Tanaka et al. used scanning force microscopy in order to investigate the surface molecular motion of PS films. It was revealed that the surface was in a glass/rubber transition state at 293 K due to the surface segregation of the lower molecular weight chains of a polydisperse blend (compared with 373 K in the bulk) [63]. 

They demonstrated the surface eiuichment of the star in isotopic blend with a linear polystyrene counterpart by dynamic secondary ion mass spectroscopy. However, in this study the star polymer was the deuterated component and had a lower molar mass than the linear matrix. Therefore, one could question if these two factors aren t themselves driving the surface segregation. 

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