Grafting preferential

Composition of the Grafts—Preferential Solvation. A systematic study of different parameters has revealed that there could be an important difference in composition between grafted and non-grafted SAN. Especially at low conversion, this difference in AN content could be much greater than 4%, with resultant incompatibility of the two types of SAN prepared in the same batch (6). 

Monomer compositional drifts may also occur due to preferential solution of the styrene in the mbber phase or solution of the acrylonitrile in the aqueous phase (72). In emulsion systems, mbber particle size may also influence graft stmcture so that the number of graft chains per unit of mbber particle surface area tends to remain constant (73). Factors affecting the distribution (eg, core-sheU vs "wart-like" morphologies) of the grafted copolymer on the mbber particle surface have been studied in emulsion systems (74). Effects due to preferential solvation of the initiator by the polybutadiene have been described (75,76). 

The efficiency of the copolymers, either block or graft, acting as the compatibilizer depends on the structure of the copolymers. One of the primary requirements to get maximum efficiency is that the copolymer should be located, preferentially at the blend interface (Figs. 2a, b, and c). There are three possible conformations, as shown in the figure. Many researchers [10-12] found that the actual conformation is neither fully extended nor flat (Fig. 2c). A portion of the copolymers penetrates into the corresponding homopolymer and the rest re-... 

Hence the presence of acid and DVB not only increase grafting yields, they also enhance competing homopolymerisation, however the former reaction is preferentially affected to the benefit of the overall process. 

The accumulation of apo(a) in the aorta wall and in saphenous vein bypass grafts in relation to Lp(a) levels was recently demonstrated (C14, R3). Subsequently, the preferential deposition of extracellular apo(a) in atherosclerotic lesions of aortic and coronary artery tissue, in conjunction with the intracellular localization of apo(a) in macrophage-derived foam cells, has been the focus of a number of studies (N6, P7, S34, S35, W17). These careful studies also demonstrated the avid binding of Lp(a) to extracellular matrix components and the colocalization of fibrin and apo(a) in atheromatous lesions (N8, W16). 

The composition of the grafted side chain copolymer has also been determined by Sakurada (113) and found to be different from the normal copolymer formed with acrylonitrile and butadiene. With styrene the grafted copolymers were found to be richer in acrylonitrile than the normal copolymer. Similar differences were found by Resting (114) with methyl methacrylate and styrene grafted to cotton and by Odian et al. (115) with grafting mixed monomers to Teflon and to polyethylene. It is believed that one monomer may be preferentially sorbed or diffused faster than the other, leading to a different monomer ratio at the actual site of grafting. 

In a similar way, Iwasaki et al. [145] modified the surface of polyamide 6 films or fibers after ozonization. These authors grafted in a second step hydrophilic monomers such as acrylamide, and also vinyl acetate or methyl methacrylate. Once more, oxidation mechanisms and preferential sites of attack of ozone have not been clearly described in the literature. 

Figure 3.8 Illustration of the pore-blocking effect caused by bulky grafting species reacting preferentially at the pore openings and thus giving rise to a nonhomogeneous distribution of the organic components within the channel system.

The chief reason for the interest in graft copolymers originates from the incompatibility between polymer chains of different chemical nature. Intramolecular phase separation results, because grafts and backbone repell each other, and these compounds exhibit a marked tendency to form mesomorphic phases like block copolymers and soaps do. When these species are mixed with a solvent that exhibits a preferential affinity for one of the components (grafts or backbone) the incompatibility may be enhanced. This intramolecular phase separation has led to a number of applications. If small amounts of a graft copolymer are included into a homopolymer of the same nature as the grafts (or the backbone), surface modifications can result as described below. 

In the completed study, exact amounts of the powdered materials were sprinkled onto the wood-resin surface. Analysis of the test specimens revealed that under the press times and temperatures used the phase transition of the polystyrene side chains on the graft polymer was not efficient. Further, in order for the graft polymer to be effective as a interfacial agent, it must locate preferentially at the blend interface (SI), The research team hopes to develop procedures in the future to allow the polystyrene and graft polymers to be dissolved in an organic solvent for application to the wood resin surface. This should allow the graft polymer to locate at the blend interface and improve bonding efficiency. 

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