Grafting to/from

In the manufacture of highly resident flexible foams and thermoset RIM elastomers, graft or polymer polyols are used. Graft polyols are dispersions of free-radical-polymerized mixtures of acrylonitrile and styrene partially grafted to a polyol. Polymer polyols are available from BASF, Dow, and Union Carbide. In situ polyaddition reaction of isocyanates with amines in a polyol substrate produces PHD (polyhamstoff dispersion) polyols, which are marketed by Bayer (21). In addition, blending of polyether polyols with diethanolamine, followed by reaction with TDI, also affords a urethane/urea dispersion. The polymer or PHD-type polyols increase the load bearing properties and stiffness of flexible foams. Interreactive dispersion polyols are also used in RIM appHcations where elastomers of high modulus, low thermal coefficient of expansion, and improved paintabiUty are needed. 

Grafting reactions alter the physical and mechanical properties of the polymer used as a substrate. Grafting differs from normal chemical modification (e.g., functionalization of polymers) in the possibility of tailoring material properties to a specific end use. For example, cellulose derivatization improves various properties of the original cellulose, but these derivatives cannot compete with many of the petrochemically derived synthetic polymers. Thus, in order to provide a better market position for cellulose derivatives, there is little doubt that further chemical modification is required. Accordingly, grafting of vinyl monomers onto cellulose or cellulose derivatives may improve the intrinsic properties of these polymers. 

The growth in popularity of radiation as the initiating system for grafting arises from the improvement in availability and cost of ionizing radiation. This is due to the introduction of more powerful nuclear reactors. Apart from its inexpensiveness, radiation is a very convenient method for graft initiation because it allows a considerable degree of control to be exercised over such structural... 

The effect of acid concentration on the rate of grafting has been studied. It is clear that increasing the acid concentration from 0.5% to 1.0% increases not only the total monomer conversion percent from 60.1% to 61.5%, but also the grafting efficiency from 58.0% to 59.6%. This... 

The effect of the reaction temperature on the grafting process is discussed. Upon increasing the reaction temperature from 15°C to 40°C the grafting decreases from 30.7% to 29.1% and from 30.0% to 28.5% for cotton linters and viscose wood pulp, respectively. This can be... 

Chen et al. utUized a direct chemical reaction with a given solution (wet treatment) to modify the surface of the silicone rubber. The presence of a layer of PEO on a biomaterial surface is accompanied by reductions in protein adsorption, and cell and bacterial adhesion. In order to obtain a PEO layer on top of the silicone rabber surface, the surface was firstly modihed by incorporating an Si-H bond using (MeHSiO) , and followed by PEO grafting to the surface using a platinum-catalyzed hydrosilylation reaction. These PEO-modified surfaces were demonstrated by fibrinogen adsorption both from buffer and plasma, as well as albumin adsorption from buffer. Reductions in protein adsorption of as much as 90% were noted on these surfaces. 

The antioxidative activities of polymeric antioxidants prepared from Verona oil and the conventional phenolic antioxidant 3-(3,5-di-terf-butyl-4-hydroxyphenyl)propionic acid (DTBH), chemically grafted to polystyrene and polyurethanes, is similar and in some cases even better than that of the corresponding low-MW phenolic antioxidants [81]. 

Radiation Treatment NVP, 2-hydroxyethylmethacrylate (HEMA), and acrylamide (AAm) have been grafted to the surface of ethylene-propylene-diene monomer (EPDM) rubber vulcanizates using the radiation method (from a Co 7 source) to alter surface properties such as wettability and therefore biocompatibility [197]. Poncin-Epaillard et al. [198] have reported the modification of isotactic PP surface by EB and grafting of AA onto the activated polymer. Radiation-induced grafting of acrylamide onto PE is very important... 

An important polymer modification reaction is the grafting to or from a polymer backbone by some chemical method to produce a branched structure Q). The characterization of the products of these reactions is often somewhat less well defined than block copolymers (2) due to the complexity of the mixture of products formed. It is therefore useful to prepare and characterize more well defined branched systems as models for the less well defined copolymers. The macromonomer method (3 ) allows for the preparation of more well defined copolymers than previously available. 

The majority of analyses on neural crest cell migration, proliferation, and differentiation have been carried out in avian and amphibian embryos because of the relative ease of experimental manipulation (see Le Douarin, 1982). These experiments demonstrated that pigment cells arise from the neural crest. When pieces of neural folds were grafted to the... 

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