Grafted Systems

The dose and dose rate of irradiation are important factors in any radiation grafting system. In the direct method the total dose determines the number of grafting sites, while the dose rate determines the number of grafting sites, and the dose rate determines the length of the grafted branches. The length of the branches is also con-... 

Partial replacement of ethanol by methanol has nearly no effect. In the case of propanol an increase in grafting is visible. This can be attributed to the mixing of higher carbon alcohols, e.g., butanol and isobutanol, with the active solvent methanol, which increases the miscibility of the monomer in these grafting systems and, consequently, increases the penetration of monomer to the active sites on the cellulose chains. 

Similar studies were undertaken as a comparison in the styrenic systems, although in these systems we also were able to compare the graft systems to diblock systems of similar composition. DSC also shows a depression of the high temperature Tg for all of the styrenes for the IK graft systems. The 5K systems showed a smaller depression but still a slight lowering of the Tg (Table VIII). Clearly, the t-butylstyrene system is most influenced, as anticipated. 

ESCA analysis showed a similar trend of incomplete surface coverage for the IK systems. Also, no domains were visible in any of the IK styrenic graft systems by TEM. There is an expected trend with respect to solubility parameter, p(t-butyl styrene) (6 s 8.1) has a solubility parameter much closer to that of polydimethylsiloxane (S a 7.3) than does p(p-methyl styrene) which is closer than p(styrene) (S s 9.1). 

For the PDMS-grafted systems a somewhat different method was used since PDMS is not soluble in DMF. An anionic polymerisation method was again used to produce "living" PDMS chains, but in this case these were reacted with acetic acid to give hydroxyl-terminated chains. The silica particle dispersions in ethanol were stable, and remained stable on adding n-heptane to give a 1 1 (by volume) solvent mixture in which PDMS is still soluble. 

It is now proposed that acid acts in an analogous manner in the styrene-methanol grafting system. No data for the scavenging role of styrene in this reaction is yet available although the data from preliminary results are consistent with this concept (10). It is thus proposed that acid affects the stability and reactivity of the intermediate charge-transfer complex formed in... 

Antiserum can be raised againsf lymphocyfes or fhymo-cyfes by fhe repealed injection of human cells into an appropriate recipient, usually a horse. The use of such antiserum or the immune globulin fraction derived from it has been used to produce immunosuppression. Although antilymphocytic serum can suppress cellular and often humoral immunity against a variety of tissue graft systems, the responses are variable, particularly from one batch of serum to another. 

The dose rate affects both the yield and chain length of the grafted material. Air has a detrimental effect on grafting because it inhibits the reaction, which is consistent with other radiation-induced free radical reactions. Increasing the temperature of the grafting system increases the yield. This is very likely because raising the temperature increases the diffusion rate of the monomer into the substrate.241... 

If the reaction takes place in the presence of monomer, grafting occurs in the usual manner. The fact that some homopolymerization also occurs in the redox-catalyzed grafting system can be explained by a chain-transfer mechanism (Reaction 10). The growing polymer radicals can abstract hydrogen atoms from the monomer, forming monomer radicals and thereby initiating homopolymerization. 

Aqueous solution of acrylamide has been chosen as the grafting system for polyamidic filaments that had been preheated under gamma-rays, in air (35,48). If the filaments were irradiated under nitrogen, grafting occurs on the surface, but if the high energy treatment is carried on in air, the side chains are formed also inside the fiber (61). 

They also studied the melting characteristics of the powders with surface-grafted MMA polymer by DSC [38]. Grafting of Kevlar 49, or poly(p-phenylene terephthalamide), fiber with GMA and AAm polymers without exposing to air was studied by Yamada et al. [39]. According to their grafting system, Kevlar yarn was suspended in an upper plasma-irradiation vessel and irradiated for 1 min at an Ar gas pressure of 20 Pa, an rf frequency of 13.56 MHz, and an rf power of... 

Homopolymerisation is the most undesired phenomenon associated with surface grafting and has remained the main drawback of the radiation-induced grafting systems. During the last two and a half-decades, different homopolymerisation inhibitors [192, 193, 194] have been employed in various grafting systems and their efficiencies evaluated. Some of the latest developments in improving the grafting efficiency and reducing associated homopolymerisation are presented herewith. 

Metallic ions such as Fe+3, Fe+2 and Cu+2 are known to have an inhibitory effect on free-radical polymerisation and have been used for several years in the radiation grafting systems to interdict homopolymer formation. The mechanism of their action is meticulously discussed in the literature [195,... 

For example, these workers (103) found that polybutadiene can be successfully metalated by n-BuLi-TMEDA, and the product subsequently used to graft styrene monomer to form poly(butadiene-g-styrene). This result showed that the grafting efficiency was 100%. Catalyst efficiency was found to be 75-95%. Thus, the n-BuLi-TMEDA metalating and grafting system is particularly effective for polydiene backbones. In fact, Falk and Schlatt (104) showed that poly(butadiene-g-styrene) has excellent tensile strength. 

As the consequence of the use of large amount of ceric salt in copolymerization reaction severe deviation from the true value of grafting percentage may result if the initial products of graft copolymerization are oven-dried before removing homopolymers from the grafting system. 

Addition of 1.33% of soda lignin to the grafting systems inhibited the grafting reactions completely (zero grafting yields) (Figures 1, 3, 4 and 5). 

In terms of paper conservation, this work is of value since it is an attempt to determine (i) if UV and EB grafting times for cellulose copolymerization can be shortened significantly by the inclusion of appropriate additives and (ii) if such a process is capable of incorporating into cellulose properties required for preservation, particularly wet and dry strength and flexibility. These studies also should indicate the feasibility of extrapolating data from the present solvent grafting systems to processes based on solvent-free, rapid-cure UV and electron beam (EB) which are the ideal polymerization conditions for preservation applications. 

Many authors elucidated functionalization of polymers containing reactive oxirane moieties. Epoxidized NR, BR, IR and/or the respective model hydrocarbons, poly (butadiene-co-isoprene, various epoxy resins, poly (2,3-epoxypro-pyl methacrylate) and its copolymers or grafted systems were mostly exploited. Stabilizers based on epoxidized unsaturated rubbers are of the top interest. The mechanism of the functionalization process was studied in details by means of 3,4-epoxy-4-methylheptane and 1,2-epoxy-3-ethyl-2-methylpentane as model compounds [289]. The ring opening of the asymmetric oxirane is regiospecific. Aliphatic primary amines attack the least substituted carbon atom and can be involved in crosslink formation. Aromatic primary and secondary amines are less reactive than aliphatic ones because of their lower basicity the attack on the least substituted carbon atom is however preferred too. 

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