Diblock copolymers emulsification

There is a vast body of diblock copolymer studies since block choice can be such that they resemble amphiphilic surfactants. For the sake of brevity, we will skip them. Instead, we present an interesting case of triblock copolymers of poly(ethylene oxide), PEO, and poly(propylene oxide), PPO, commonly known by one of its trade names, Pluronics [117]. They have been used as non-ionic surfactants for a variety of applications such as in emulsification and dispersion stabilization. In aqueous solutions, these copolymers form micelles, and there exists a well-defined critical micelle concentration that is experimentally accessible. Several groups have investigated colloidal suspensions of these polymers [118-122], The surface properties of the adsorbed monolayers of the copolymers have been reported with respect to their structures and static properties [123-126]. 

One possible way of reducing interfacial tension and improving phase adhesion between PP-based blend phases is to use a selected copolymeric additive that has similar components to the blend, as a compatibilizer in the blend system. Well-chosen diblock copolymers, widely used as compatibilizing agents in PP-based blends, usually enhance interfacial interaction between phases of blends (15, 16), reduce the particle dimensions of the dispersed phase (16, 17), and stabilize phase dispersion against coalescence (16-18) through an emulsification effect, thus improving the mechanical properties (15-19). 

The recent successful synthesis in our laboratory of styrene-propylene diblock copolymer (iPS-fc-iPP), with crystalline isotactic structure in each block (20, 21), makes it possible to attempt toughening of the iPS-iPP blend by the method of block copolymer emulsification of the diblock copolymer. This... 

Core-sheath nanofibers of PEG-PLA and PEG can be prepared by electrospinning a water-in-oil emulsion in which the aqueous phase consists of a PEO solution in water and the oily phase is a chloroform solution of an amphiphilic PEG-PLLA diblock copolymer. The fibers obtained are composed of a PEO core and a PEG-PLA sheath with a sharp boundary in between. By adjusting the emulsion composition and the emulsification parameters the overall fiber size and the relative diameters of the core and the sheath can be changed. As shown in Fig. 5.15, a mechanism is proposed to explain the process of transformation fi-om the emulsion to the core-sheath fibers, i.e., the stretching and evaporation induced de-emulsification. In principle, this process can be applied to other systems to... 

Gailard and coworkers [215, 266] demonstrated the surface activity of block copolymers by studying the interfacial tension reduction in demixed polymer solutions. Addition of a FS-b-FBD diblock copolymer to the PS/PBD/styrene ternary system showed first a characteristic decrease in interfacial tension followed by a leveling off, which is similar to the evolution of interfacial tensions for oil-water systems in the presence of surfactants. The early investigations were more of case studies that demonstrated the phenomenon without giving the fundamental detail required to help the understanding of the emulsification process and the factors that govern it. 

HDPE + nylon 6 was compatibilized by adding polyethylene ionomer, which gave finer domain size due to emulsification and lower interfacial tension [70]. LLDPE + polyvinyl chloride was compatibilized by adding up to 5% of isoprene-4-vinyl pyridine diblock copolymer, acting as an emulsifier to reduce phase diameter and interfacial tension. Polystyrene-acrylic acid diblock copolymer was somewhat less effective [49]. 

The effect of AB diblock size relative to the homopolymers on the compati-bilization of A/B homopolymer blends was examined using numerical self-consistent field theory (in two dimensions) by Israels et al. (1995). They found that the interfacial tension between homopolymers can only be reduced to zero if the blocks in the diblock are longer than the corresponding homopolymer. Short diblocks were observed to form multilamellar structures in the blend, whereas a microemulsion was formed when relatively long copolymers were added to the homopolymer mixture. These observations were compared to experiments on blends of PS/PMMA and symmetric PS-PMMA diblocks reported in the same paper. AFM was used to measure the contact angle of dewetted PS droplets on PMMA, and the reduction in the interfacial tension caused by addition of PS-PMMA diblocks was thereby determined. The experiments revealed that the interfacial tension was reduced to a very small value by addition of long diblocks, due to emulsification of the homopolymer by the diblock, in agreement with the theoretical expectation (Israels et al. 1995). 

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