Solution-Type Graft Copolymers

Two types of well defined branched polymers are acessible anionically star-shaped polymers and comb-like polymers87 88). Such macromolecules are used to investigate the effect of branching on the properties, 4n solution as well as in the the bulk. Starshaped macromolecules contain a known number of identical chains which are linked at one end to a central nodule. The size of the latter should be small with respect to the overall molecular dimensions. Comb-like polymers comprise a linear backbone of given length fitted with a known number of randomly distributed branches of well defined size. They are similar to graft copolymers, except that backbone and branches are of identical chemical nature and do not exhibit repulsions. 

Macromonomers provide an easy access to a large number of functional copolymers and controlled topologies, such as comb-like, star-like, bottle brush, and graft copolymers. These types exhibit exceptional solution or solid state properties compared to their linear homologues. 

Craft Copolymers with Low Backbone-Polymer Content. The procedure for preparing this kind of graft copolymer is based on the dissolution of the backbone polymer in the monomer, dispersion of this solution in water, and polymerization by means of an organic peroxide. It applies only to soluble backbone polymers, such as most EPR s. As the handling of a too-viscous vinyl chloride/backbone polymer solution is impractical, this procedure is normally used for preparing end products of the type VC/backbone polymer (95-5) or (90-10). 

A number of statistical thermodynamic theories for the domain formation in block and graft copolymers have been formulated on the basis of this idea. The pioneering work in this area was done by Meier (43). In his original work, however, he assumed that the boundary between the two phases is sharp. Leary and Williams (43,44) were the first to recognize that the interphase must be diffuse and has finite thickness. Kawai and co-workers (31) treated the problem from the point of view of micelle formation. As the solvent evaporates from a block copolymer solution, a critical micelle concentration is reached. At this point, the domains are formed and are assumed to undergo no further change with continued solvent evaporation. Minimum free energies for an AB-type block copolymer were computed this way. 

Several other groups have also studied the blood compatibility of silicones with various radiation grafted copolymer constituents. Chapiro, et al (25) have grafted N-vinyl pyrroli-done using Co-60 onto silicone in both "bulk and "solution type reactions. It is interesting to note that similar work using E-beam radiation has not been as successful (26). 

The fact that the decrease in molecular weight in the photodegradation of polymethylvinylketone does not continue at the same rate throughout the irradiation has been ascribed to the occurrence of a competing reaction that opposes the main chain scission process. This is assumed to be the formation of new polymer—polymer linkages by mutual recombination of macro-radicals resulting from the addition of CHj or CH3—CO to unsaturated chain ends formed in the Norrish type II reaction [55]. Evidence for the presence of such macro-radicals is found in the production of graft copolymers when solutions of polymethylvinylketone in various monomers (acrylonitrile, methyl methacrylate, vinyl acetate) are irradiated [57]. 

When block and graft copolymers are dispersed in solvents, the solutions have properties that depend on whether or not the copolymer is eventually fully solvated. If the solvent is a good solvent for both sequences—for example, chloroform in the case of natural rubber graft copolymerized with poly(methyl methacrylate) (Halasa et al., 1976)—then both segment types are expanded and films cast from dilute solutions will usually be intermediate in properties to the... 

Acrylonitrile is the starting material for the different acrylic compounds formed by various types of reactions. Fig.l shows the general scheme of the derivation of acrylonitrile and the polymerisation of the different monomers. The system within the dotted lines (Fig.l, right) is present in solutions for the modification techniques of polymers surfaces by radiation grafting. The use of this technique in technical application causes the need of a detailed analysis of the components of the system. The grafting by irradiation leads to a consumption of the monomer as well as to the formation of the so called homopolymers which are non-desired by-products of the graft copolymer. For the control of a grafting solution instrumental methods are needed which preferentially are based on electroanalytical techniques. 

It was found that, when the hydrophilic functional groups are linear or when the end-capped units of the hydrophilic groups are of hydroxy type, the hydrolytic stability decreases and chemical attack of silicone surfactants backbone occms easily in the aqueous solution. A way to avoid this is to synthesize grafted copolymers or to use PEO blocks having methoxy end groups [43]. 

The polymer-based miscible systems can be either intermolecular mixtures, for instance polymer solutions and blends, or intramolecular mixtures, such as block copolymers, star-shape multi-arm copolymers, grafted copolymers, random copolymers, and gradient copolymers with a composition gradient from one chain end to the other. Polymer-based miscible systems can phase separate into segregated phases with stable interfaces, or crystallize into crystalline ordered phases. In other words, there are two types of phase transitions, phase separation and crystallization. Under proper thermodynamic conditions, two phase transitions may occur simultaneously. The interplay of these two transitions will dictate the final morphology of the system. In the following, we will choose polymer solutions as typical examples to introduce the polymer-based miscible systems. 

---