Block copolymer blends separation

Keywords Self-Assembly Blend Block copolymer Microphase separation ... 

The effect of compatibilization of grafted or block copolymers as controllers of interphase interaction has been well known (1,28). Marosi and Bertlan (29), for example, described the compatibUizing effect of polybutylene terephthalate-polytetramethylene oxide block copolymers on PA6/HDPE blends. The addition of a block copolymer prevents separation in the blends during processing and increases the impact strength of the material several times. 

Phase Separation in Block Copolymers and Block Copolymer Blends... 

At the same time, based on the microphase separation occurring in block copolymers blends, including the latter may result in phase separated films exhibiting nano-domains. The formation of nanostructured domains at the interface induced by surface segregation of block copolymers is an interesting strategy to fabricate surfaces with controlled functionality and patterned in one single step. 

Commercial membranes for CO2 removal are polymer based, and the materials of choice are cellulose acetate, polyimides, polyamides, polysulfone, polycarbonates, and polyeth-erimide [12]. The most tested and used material is cellulose acetate, although polyimide has also some potential in certain CO2 removal applications. The properties of polyimides and other polymers can be modified to enhance the performance of the membrane. For instance, polyimide membranes were initially used for hydrogen recovery, but they were then modified for CO2 removal [13]. Cellulose acetate membranes were initially developed for reverse osmosis [14], and now they are the most popular CO2 removal membrane. To overcome state-of-the-art membranes for CO2 separation, new polymers, copolymers, block copolymers, blends and nanocomposites (mixed matrix membranes) have been developed [15-22]. However, many of them have failed during application because of different reasons (expensive materials, weak mechanical and chemical stability, etc.). 

The strain compatibilization of LCST blends was found to be similar [4] to that reported by Lyngaae-Jmgensen [237] for the styrene-butadiene-styrene (SBS) block copolymer that separates with the upper critical solution temperature, UCST. Thus, low, steady state shearing shifted the spinodal temperature Ts(y)—Ts(0) < 29 °C. 

Yielding in Biock Copoiymers. While polymer blends show macrophase-separated morphologies, which often lead to a deterioration of mechanical properties because of the immiscibility of the components, in block copolymers microphase-separated structures at the typical size scale 10-100 nm... 

Studies of block copolymer blends where one of the blocks is common to each of the block copolymers include PS-b-Pl with PS-b-EP [701], PS-b-PI with PI-b-PEO [702], and PS-b-PI with PS-b-P2VP [703]. Macrophase separation is often observed, but conditions exist where microphase separation results with imique morphologies. 

Figure 5.27 Experimental phase diagram of AB/ABA block copolymer blends dissolved in a B-selective solvent according to sol-gel and cloud-point measurements. A phase-separated regime is found to exist at low AB and total (AB + ABA) polymer concentrations. At higher AB or total polymer concentrations, the system forms a physical gel network. A further increase in AB content ultimately disrupts the ABA network and induces liquid-like behaviour. (Reprinted with permission from Vega, D. A., Sebastian, J. M., Loo, Y.-L. and Register, R. A. J. Polym. Sci. B Polym. Phys. 39, 2183, 2001. Copyright (2001) Wiley-Interscience.)...

Chu and coworkers [85,86] have demonstrated a new application for ethylene oxide/butylene oxide block copolymers. Blending the triblock polymers BO6EO46BO6 and BO10EO27BO10 formed a medium useful for the separation of double-stranded DNA. While neither block copolymer worked alone, various combinations performed well as the gel media. This use of block copolymers of ethylene oxide and butylene oxide extended and improved upon the work done by Rill et al. [87-89] and Chu et al. [90-93] with corresponding ethylene oxide/propylene oxide block copolymers as the separation medium for capillary electrophoresis. 

On the basis of these observations, criticize or defend the following proposition The fact that the separate spots fuse into a single spot of intermediate Rf value proves that block copolymers form between the two species within the blend upon heating. 

Block copolymers are closer to blends of homopolymers in properties, but without the latter s tendency to undergo phase separation. As a matter of fact, diblock copolymers can be used as surfactants to bind immiscible homopolymer blends together and thus improve their mechanical properties. Block copolymers are generally prepared by sequential addition of monomers to living polymers, rather than by depending on the improbable rjr2 > 1 criterion in monomers. 

Block (Star) Arrangement. The known star polymers, like their linear counterparts, exhibit microphase separation. In general, they exhibit higher viscosities in the melt than their analogous linear materials. Their rheological behavior is reminiscent of network materials rather than linear block copolymers (58). Although they have been used as compatibiUzers in polymer blends, they are not as effective at property enhancements as linear diblocks... 

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