Segregation effects, polymer surfaces

The wall-PRISM theory has also been implemented for binary polymer blends. For blends of stiff and flexible chains the theory predicts that the stiffer chains are found preferentially in the immediate vicinity of the surface [60]. This prediction is in agreement with computer simulations for the same system [59,60]. For blends of linear and star polymers [101] the theory predicts that the linear polymers are in excess in the immediate vicinity of the surface, but the star polymers are in excess at other distances. Therefore, if one looks at the integral of the difference between the density profiles of the two components, the star polymers segregate to the surface in an integrated sense, from purely entropic effects. 

Surface segregation, which is also a common phenomenon in other materials, has received limited attention and most of the studies applied this concept for the control of the surface chemical composition. Moreover, surface segregation has been typically considered as a non-desirable effect. This is particularly true in the case of materials with precise bulk properties provided by the presence of different additives such as plasticizers or UV-absorbers. The segregation of these additives towards the interface modifies the bulk properties and can provoke large variatiOTis on their mechanical behavior. Several contributions have explored this phenomenon in order to reduce the amount of oligomers/polymeric additives that bloom to polymer surfaces [13]. However, in many other applications it is desirable to have surface properties that vary to a large extent from those found in the bulk. For example, to favor adhesion or increase the wettability, or the opposite, i.e., increase the hydrophobicity, to improve the biocompatibility of commercial polymers or to enhance the chemical resistance. 

Hariharan, A., Kumar, S.K., Russell, T.P. A lattice model for the surface segregation of polymer chains due to molecular weight effects. Macromolecules 23, 3584-3592 (1990)... 

Differences are observed for both surface and bulk conductivities of one CB in various polymers as for example when Narkis et al. [19] compare PP, PS, HIPS, PE, Noryl . There is even a subtle effect of composition on resistivity in the case of PP homopolymer versus P P copolymer because the latter contains a rubbery component (Table 18.4). The CB apparently segregates into the rubbery phase and a higher loading of CB is needed to reach levels of conductivity comparable to the homopolymer [19]. Effects oftitanate surface treatments on the conductivity of CB compounds are included in Chapter 5. 

Polymer blends may be expected to reflect the balance of forces that control their phase structure in the bulk. If two polymers are compatible in the bulk, they may or may not segregate in the surface depending on the balance of forces. Materials that are able to phase separate will segregate in the surface, and generally speaking the lower surface energy material will move towards the free air surface. This effect was illustrated in Chapter 9 in the case of the surface segregation of the ether soft block in the case of block polyurethane copolymers. 

Effects of surface segregation of polymer additives (intrinsic surface contamination) are discussed in the next subsection. 

The grand canonical ensemble describes a system of constant volume, but capable of exchanging both energy and particles with its environment. Simulations of open systems under these conditions are particularly useful in the study of adsorption equilibria, surface segregation effects, and nanoscopically confined fluids and polymers. Under these conditions, the temperature and the chemical potentials jti,- of the freely exchanged species are specified, while the system energy and composition are variable. This ensemble is also called the jx VT ensemble. In the case of a one-component system it is described by the equilibrium probability density... 

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... 

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