Poli(etilen glikol) fraksiyonlarının gama ışınları ile çapraz bağlanma mekanizması

Abstract

Ill SUMMARY In this study, gamma ray induced chain scission and crosslinking mechanism of PEG with low and narrow molecular weight distribution have been investigated with special consideration of the end group effect. PEG samples with narrow mole cular weight distributignjyere prepared from commercial PEG's having molecular weight distributions between 1.20-1.43 by fractional crystallization from their solutions of n-butanol and n-heptan. Methyl terminated PEG (PEGCH3 2000), having 2000 molecular weight has been used to examine the end group effect on crosslinking mechanism. PEGCH3 2000 samples crystallized at 60, 70 and 80 % were irradiated with gamma rays. Molecular weights of irradiated polymers were determined by Size Exclusion Chromatography and G(X), G(S) and G(S)/G(X) values were calculated by following the change in molecular weights with dose. The G(S)/G(X) value increased from 1.54±0.03 to 1.74±0.04 with the increase of amorphous part in the polymer. The G(S)/G(X) values were determined to be lower than 4 and this proved that crosslinking is predominating effect for this polymer. However the crosslinking tendency decreased with the decrease of amorphous part. The G(S)/G(X) values were compared with the values of poly(ethylene glycol) (PEGOH) with hydroxyl end groups having the same crystallinity. It has been found that hydroxyl terminated PEG has higher tendency to crosslinking than methyl terminated PEG with increasing amount of amorphous part. This showed that during the gamma irradiation of PEG the end groups were effective on crosslinking and chain scission. It has been proposed that the higher tendency of crosslinking of hydroxyl terminated PEG was due to higher probability of the intermolecular connections through the ends of the chains by the formation of hydro gen bonds via hydroxyl groups. The dimer peak in the irradiated PEGCH3's SE chromatograms has been observed at an elution volume 1 % lower than that of the molecular weight of a linear dimer. No shifts have been observed between the peak points of chromatograms ofIV irradiated and nonirradiated samples. It has been proved that crosslinks formed by end to end connection in the regions that were very close to chain end.The molecular weight of dimer molecule was observed to be 5 % smaller than linear dimer molecular weight in the SE chromatogram of hydroxyl terminated PEG. This showed that crosslinks in hydroxyl terminated PEG were mainly located at two monomer units before chain end. The difference chromatograms shoved that chain scission in both methyl terminated PEG and hydroxyl terminated PEG was not random. The new peak appea ring in difference chromatograms showed that chain scission takes place somewhere near to the middle of the chain. It has also been proved that scissioned and crosslinked units were involved in further radiation induced reactions.This has been concluded after a stoichiometric comparison of G(-monomer) and G(S) and G(X) yields. Polymer mixtures were used in order to find the contribution of amorphous region and chain end fringes on crosslinking with gamma radiation. Polymer mixtures were prepared by mixing 50 %-50 % of PEGOH 2010 with PEGOH 600 and PEGOH 3 100 with PEGOH 1050. 80-85 % of crystalline regions were formed by the compe- nents that have higher molecular weight and amorphous regions were formed by the component that have lower molecular weights. By analyzing the difference chroma tograms of irradiated mixtures it has been seen that crosslinking predominantly takes place in the amorphous region. It was not possible to determine the existance of chains formed by end to end connection of chains in the amorphous region with frin ges on lamella surface.