Nondestructive testing of defects in adhesive joints

Complexation of metal ions: General procedure
A known mass of block copolymer (0.3g) was suspended in 50ml 2M metal salt solutions (NaCl,
KCl, MgSO4, BaCl2, CaCl2) at its natural pH, for 168 hours in a closed condition. The polymer
was then taken out and washed the surface instantly but carefully with de-ionized water to
remove any surface salt present. The metal ion intake of the block copolymer was obtained from
the determination of the concentration of the metal salts solutions before and after complexation.
From the decrease in concentration of metal ion solution, the amount of metal ion complexed by
the polymer was calculated. The concentrations of the metal salt solutions were determined by
volumetric methods-Na+ and K+ by Mohr’s method [28], Mg++ and Ca++ by complexometric
titrations [29], Ba++ by precipitation titration [30].
Results and discussion
Following the procedure under experimental part, sheets of polyether-natural rubber block
copolymers were prepared. Polyether is hydrophilic in nature and natural rubber provides the
nonpolar hydrophobic part. Series of NR/PEO-1000 and NR/PEO-4000 were synthesized by
solution polymerization. The course of the reaction and the structure of the block copolymer are
shown in Scheme I.
Interaction with metal salts
The interaction of NR/PEO block copolymers with various metal salts is due to the PEO content
in the samples. The polyethylene oxide bearing -[-CH2-CH2-O-]n- units can exhibit crown ether or
podand property by forming spiral or pseudo cavity of adequate size to accommodate the cations.
The present study on the competitive extraction of different cations is originated from the above
background. Even though the PEO part contains no crown ether like macrocycle, the long chain
with -[-CH2-CH2-O-]n- units can adopt a ring like structure by taking a spiral shape. The salt
solutions considered are NaCl, KCl, MgSO4, BaCl2, and CaCl2.
The interaction of dried NR/PEO-1000 and NR/PEO-4000 with the above salt solutions were
studied by estimating the cation impregnated in the polymer sample. The next step is to find the
order of competitive extraction of these metal cations. The results have been analyzed by the
overall metal complexation values [Table I]. The order of competitive extraction by these two
block copolymers are the same as given below K + >Ba ++ >Ca ++ >Na + >Mg ++ . The experiment was
repeated with another concentration (0.2M) of the metal salt solutions. In that case too the
extraction order was the same. From the above result it is clear that as the ionic radii increases
(Na + =1.96A 0 , K + =2.66A 0 , Ba ++ =2.83A 0 , Ca ++ =2.12A 0 , Mg ++ =1.56A 0 ) [31] complexation ability
also increases, except for Ba ++ as shown in Fig.1.
The HTNR and PEO used in the present study are made up of appreciably large number of
respective monomer unit. The copolymerisation of these two using TDI makes the block
copolymer system almost infinite size with alternate HTNR-PEO components. In this copolymer
the NR part behaves as the soft segment and it is completely amorphous in nature. The PEO is the
hard segment and in solution, not much ordering is expected from this fragment also. But when
treated with metal salt solution the PEO fragments with -[-CH2-CH2-O-]- units would tend to
interact strongly with the metal ions, resulting in a high degree of ordering as shown in Fig 2. The
corresponding situation with HTNR/PEO block copolymer is shown in Fig 3.