PHYS01200704032 Debes Ray - Homi Bhabha National Institute

Chapter 4: Optimization of the Block Copolymer-mediated Synthesis of Gold Nanoparticles
SANS studies were performed to understand the mechanism of varying concentration
of block copolymer leading to their different behaviour. Figure 4.12 shows the SANS data of
pure 0.5, 1 and 2 wt% P85 block copolymer solutions, where the scattering cross-section
increases with the block copolymer concentration. The fitted parameters of the micelles at
different concentrations are given in Table 4.1. It is found that irrespective of the block
copolymer concentration, micelles have the same PPO core radius (R c = 3.63 nm) surrounded
by Gaussian PEO chain (R g = 1.2 nm) and aggregation number (N = 53). Only the number
density of micelles (n) for different systems varies in proportion to the block copolymer
concentration. The increase in the number density of the micelles means there will be larger
probability to the nucleation of the gold nanoparticles and hence the enhancement in the yield
observed at higher block copolymer concentration.
The above SANS results have been further confirmed by the DLS data of the intensity
autocorrelation function [g I ()] as shown in Figure 4.13. The functionality of the
autocorrelation function depends on the diffusion coefficient of the particles and the data
obtained suggest that it is dominated by only one type of the particles (i.e. micelles). The
analysis shows that irrespective of the block copolymer concentration, micelles have a
diffusion coefficient of 30 10 -6 cm 2 /sec, which corresponds to the hydrodynamic radius of
about 9 nm. This hydrodynamic size of the micelles comprises PPO core plus PEO shell along
with hydration attached to the shell. It has also been found out that there is not any significant
change in the DLS data of these block copolymer samples with nanoparticles, which is
consistent with SANS results of a very small fraction of nanoparticles as compared to that of
the free block copolymer micelles. The linear dependence of count rate at the detector with
variation in block copolymer concentration suggests increase in the number density of the
micelles consistent to that observed by SANS (inset of Figure 4.13).
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