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46 Kumar et al.
4. Notes
1. In synthesizing the poly(VI-co-NIPAM) copolymers, it is important to optimize
the concentration of VI comonomer. Very high concentrations of VI affect
drastically the precipitation behavior of the copolymer. Poly(VI-co-NIPAM)
copolymers incorporated with about 30 mol% of VI can be precipitated from the
solution by heating, but above that concentration, precipitation of the copolymer
becomes difficult (27). High concentrations of VI are useful in providing high
metal-binding capacity for the copolymer. However, this can lead to poor precipitation
behavior, which makes it difficult to recover the synthesized copolymer
and thus gives low yields. VI concentrations in the range of 15–25 mol% are
considered to be optimal, which provide efficient precipitation properties for the
copolymers and also give sufficient binding capacity for the metal ions.
2. Separating the precipitated poly(VI-NIPAM) copolymer from the liquid phase
will mainly depend upon the type of precipitate aggregates formed. In such cases,
making the copolymers with about 6–12% (w/v, total commoner concentrations
in the starting reaction mixture) ensures easily aggregated precipitate formation.
The precipitate can simply be collected by decanting the liquid, which also allows
the precipitate to resolubilize fast by adding excess water. Too low concentrations
of comonomers (20% w/v) of
comonomers in the reaction mixture will produce gel type copolymers, which
can be rather difficult to handle for subsequent precipitation and solubilization.
3. Washing and recovery of the synthesized poly(VI-co-NIPAM) copolymer is
carried out by precipitating the copolymer by heating in the presence of 0.4–0.6
M NaCl. Keeping temperatures as high as possible up to 60°C and incubating
for about 5–10 min at this temperature will ensure better aggregation of the
copolymer. The pH of the copolymer solution is an important factor for the
better precipitation of the copolymer and should be kept in the range of 7–8. The
incorporation of relatively hydrophilic imidazole moieties hinder the hydrophobic
interactions of the native poly(NIPAM) and results in substantial increase in
the precipitation temperature (27). The effect is more pronounced at lower pH
values (