Addition of Block or Graft Copolymers

Table 1, Improvement of ultimate mechanical properties of LDPE/PS blends by addition of block or graft copolymers (from ref, 31)...

Recently, much attention has been paid to the possibility of controlling morphology development in immiscible polymer blends by the addition of interfacially active nanoparticles. In fact, it was found that some nanoparticles could refine and stabilize the morphology, whereas the addition of others leads to an increase in the dispersed particle size [162]. Generally, the optimization of this approach seems to be much more difficult compared to the incorporation of block or graft copolymers. One probable reason for this is that the polymer-filler interactions at the interface are weaker than in the case of copolymers anchored in the respective polymer phases. A critical comprehensive review on this topic was produced by Fenouillot et al. [163]. 

Surface active agents are important components of foam formulations. They decrease the surface tension of the system and facilitate the dispersion of water in the hydrophobic resin. In addition they can aid nucleation, stabilise the foam and control cell structure. A wide range of such agents, both ionic and non-ionic, has been used at various times but the success of the one-shot process has been due in no small measure to the development of the water-soluble polyether siloxanes. These are either block or graft copolymers of a polydimethylsiloxane with a polyalkylene oxide (the latter usually an ethylene oxide-propylene oxide copolymer). Since these materials are susceptible to hydrolysis they should be used within a few days of mixing with water. 

New Orleans, La., August 1999, p.752-3 POLYSTYRENE/POLYPROPYLENE POLYMER BLEND COMPATIBILISATION WITHOUT ADDITION OF PREMADE BLOCK OR GRAFT COPOLYMERS OR FUNCTIONALISATION Furgiuele N Khait K Torkelson J M (ACS,Div.of Polymer Chemistry)... 

It should be noted that the majority of the methods of synthesis of block and graft copolymers yield, in addition to the copolymer, fractions of the associated homopolymers to a greater or lesser extent. Therefore in order to determine the composition and the structure of the copolymer, it is first of all necessary to separate it from any homopolymer which may be present. This separation is usually based upon solubility differences between the different components (203). Although the turbidimetric titration may be very useful for analytical purposes (157), nevertheless it gives samples too small for further examination preparative methods, which are based on fractional precipitation, selective extraction or selective precipitation should be preferred. It is beyond the scope of this review to discuss these separation methods the reader is referred to existing literature concerning this subject (51). 

We will now consider successively the different reactions (4) and (5) of this reaction scheme in order to examine what is their relative importance for obtaining block or graft copolymers. Initially it was admitted that the graft copolymers were produced only by reaction (4b), considering that in the competition between monomer and substrate for primary initiator radicals the addition of vinyl monomers [reaction (2)] is much easier than the abstraction of an atom of another molecule [reaction (4 a)]. This assumption is based on the higher value of the activation energy for a chain transfer reaction than for a monomer addition, at least in the case of saturated molecules (88). 

The effects of improved wettability, entropic repulsion, and sterical hindrance undoubtedly play a role in stabilizing dispersed solid particles by block or graft copolymers. However, since the dispersions of titanium dioxide in toluene stabilized by carboxylated styrene-butadiene block copolymers are so much more stable than dispersions stabilized by carboxylated homopolymers under otherwise identical conditions, we must assume that an additional factor comes into play when block copolymers are used. The model in Figure 1 is an attempt to explain this additional... 

Block or graft copolymers in a selective solvent can form structures due to their amphiphilic nature. Above the critical micelle concentration (CMC), the free energy of the system is lower if the block copolymers associate into micelles rather than remain dispersed as single chains. Often the micelles are spherical, with a compact core of insoluble polymer chains surrounded by a corona of soluble chains (blocks) [56]. Addition of a solvent compatible with the insoluble blocks (chains) and immiscible with the continuous phase leads to the formation of swollen micelles or polymeric micro emulsion. The presence of insoluble polymer can be responsible for anomalous micelles. 

Block and graft copolymerizations by free-radical mechanism are usually conducted in a mixture of the parent polymer, the monomer(s) to be grown on the parent polymer, and fresh initiator. However, the product obtained in this case is likely to be a mixture. Thus, in addition to the desired block or graft copolymer, it may contain homopolymer of fresh monomer and parent homopolymer molecules that did not take part in the copolymerization. 

One of the most studied approaches to compatibilize a blend is the addition of a third component, such as a block or graft copolymer. Copolymers that contain segments chemically identical to the blend components are frequently used because they enhance the miscibility between the copolymer segments and the corresponding blend component (Fig. 27.5). 

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