PHYS01200704032 Debes Ray - Homi Bhabha National Institute

Chapter 4: Optimization of the Block Copolymer-mediated Synthesis of Gold Nanoparticles
gold nanoparticles or form their own micelles. In the case of nanoparticles, block copolymers
get coated on the particles, and will have a very different scattering pattern than that for their
micelles. Therefore, the scattering from block copolymer-coated gold nanoparticles is
expected to be significantly different than that from the micelles. The fact that the scattering
curve does not change with varying salt concentration, even when the yield of the
nanoparticles is maximum, indicates only a very small fraction of gold nanoparticles has been
formed in these systems. In other words, the most of the block copolymers seem not part of
the coating on the nanoparticles and form their own micelles, which could be because of very
low number density of gold nanoparticles compared to the micelles. It is thus not possible to
separate the small scattering contribution of block copolymer coating from that of the
micelles.
The scattering for block copolymer micelles are fitted as core-shell particles with
different scattering length densities for core and shell. The structure of these micelles is
described using a model consisting of non-interacting Gaussian PEO chains attached to the
surface of the PPO core. The form factor (intraparticle structure factor) of the micelles
comprises four terms: the self-correlation of the core, the self-correlation of the chains, the
cross term between core and chains, and the cross term between different chains. It is given
by [181]
F Q N b F Q N b F Q N N b F Q N b b F Q
2 2 2 2 2
m
( )
s s s( ) 2
s c c( ) 2
s(2 s
1) c cc
( ) 4
s s c sc( )
where b s and b c are excess scattering length of the core and chain, respectively and N s is the
aggregation number of the micelles. The subscript s (= core) and c (= chain) are used here.
They can be calculated as b s = V s ( s – solv ) and b c = V c ( c – solv ), respectively, where V s and
V c are the total volumes of a block in the core and in the corona. s and c are the
corresponding scattering length densities and solv is the scattering length density of the
surrounding solvent. The structure factor is taken to be unity, as valid in the case of dilute
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