Nondestructive testing of defects in adhesive joints

It is known that for alkali and alkaline earth metals, usually the co-ordination number is from 6-
10 [32]. For the formation of a pseudohelical structure, the ligand should have 6 donor atoms. In
our case, this structure is obtained using five -[-CH2-CH2-O-] groups along with a 6 th -CH2-
CH2-O-group that forms the turn of the pseudohelix. The metal ion is accommodated in
the cavity of the spiral structure, depending upon its size, i.e. either by size comparable to that
of 15-crown-5 or 18-crown-6. Hence here the maximum extraction is for K + , which shows the
PEO chains prefer to take a pseudohelical structure similar to that of 18-crown-6. The cavity size
of 18-crown-6 [1,2] is 2.6A 0 which suits to K + ion and the extraction order is
K + >Rb + >Cs + >Na + >Li + . The situation can be diagrammatically represented in Fig 4.
The slight decrease in the affinity of block copolymer to form a strong complex with Ba ++ can be
explained as follows. Ba ++ has a slightly greater ionic radius compared to the cavity size of 18crown-6.
Therefore the whole helical structure has to expand a little to accommodate this larger
cation, which leads to lower absorption compared to K + .
Conclusion
The work shows that NR/PEO block copolymers can be used to extract alkali and alkaline earth
metals from aqueous media. The order of extractability is as follows: K + >Ba ++ >Ca ++ >Na + >Mg ++ .
The order of cation preference is nearly identical to that of 18-crown-6 derivatives and also of
noncyclic polyethers. We thus believe that a helical arrangement of inwardly directed oxygen
atoms forms the basis of complexation between cation and block copolymers.
Further investigation of NR/PEO extraction mechanisms is continuing in our laboratory. A
number of analytical and other applications are being investigated and will be reported shortly.
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