Poly blend with polystyrene, surface segregation

Figure 26.13 Surface segregation of cyclic poly(oxyethylene) (1.5kg/mol) from its blend with polystyrene by annealing in a high-humidity environment. Images of water drops are shown with then-contact angles above the as-spun-cast film (left) and after annealing (right).

Hyperbranched and comb polymers have also been used as surface active additive. Ariura et al. synthesized by combination of anionic and cationic polymerization a monodispersed hyperbranched polystyrene [73]. The authors proved by combination of DSIMS and neutron reflectivity the preferential surface enrichment of the branched protonated macromolecules when blended with its deuterated linear polystyrene counterparts with the same molar mass. Other systems involving the segregation of the branched macromolecules in binary blends were demonstrated such as in polyamide [74] or poly (methylmethacrylate) [75]. 

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... 

Poly(dimethylsiloxane) (PDMS) is a well-known hydrophobic polymer with higher repellency for water than PS crosslinked siUcone elastomers (WCA = 112° for a smooth film) are commonly used for fabricating microfluidic devices. But forming solid fibers comprised solely of linear PDMS is not possible, due to its low glass transition temperature. Instead of using linear homopolymer PDMS, Ma et al. [21] electrospun fibers of poly(styrene-b-dimethylsiloxane) block copolymers blended with 23.4 wt% homopolymer polystyrene (PS-PDMS/PS) from a solution in a mixed solvent of THF and DMF. The resultant fiber mat, with fiber diameters in the range of 150-400 nm, exhibited a WCA of 163° and a hysteresis of 15°. An illustration of water droplets beaded up on such a mat is provided in Fig. 3. A PS mat of similar fiber diameter and porosity exhibited a WCA of only 138°. The difference was attributed to the lower surface tension of the PDMS component, combined with its spontaneous segregation to the fiber surface. X-ray photoelec-... 

The characterization of surface structure for miscible blends is a more formidable task, requiring techniques that are sensitive to the composition of the blend within several nanometers of the surface. X-ray photoelectron spectroscopy (xps) provided the first direct and quantitative evaluation of surface composition and surface composition gradients for miscible polymer blends of poly(vinyl methyl ether) (PVME) and polystyrene (PS) (22,23). Since that time, the situation has changed dramatically with the advance of theory and the application of exciting new experimental techniques to this problem. In addition to xps and pendant drop tensiometry (22,23), forward recoil spectroscopy (28), neutron (29) and x-ray reflectivity (30), secondary ion mass spectroscopy (either dynamic or time-of-flight-static) (31,32), and attenuated total reflectance Fourier transform infrared spectroscopy (33-35), have been applied successfully to study surface segregation. The advent of these new tools has enabled a multitechnique experimental approach toward careful examination of the validity of current surface segregation theories (36-39). 

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). 

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