PHYS01200704032 Debes Ray - Homi Bhabha National Institute

Chapter 2: Block Copolymer-mediated Synthesis of Gold Nanoparticles
Figure 2.7. This transition is generally observed to be reversible and results in a sharp
modification of the macroscopic characteristic features of the aqueous medium. For example,
one can expect a sharp modification to the viscosity of the solution once the system is shifted
from unimers to micelles. This stimulant can be temperature, pH, ionic strength etc
[125,126]. For example, in PEO-PPO block copolymers micellization is driven by an increase
in temperature [127]. This arises from the fact that dehydration of the PPO block takes place
with increasing temperature and the initial double hydrophilic copolymer then transformed
into an amphiphilic one above the CMT. At further higher temperatures, dehydration of the
PEO blocks results in a completely hydrophobic system, which precipitates out from the
solution (cloud point). A typical example of pH-responsive micellization has been observed
for P2VP-PEO copolymers that exist as unimers at pH < 5 and form micelles at higher pH
values [127,128]. This behaviour is directly linked to the deprotonation of the P2VP blocks
with increase in pH as P2VP blocks are protonated and positively charged at low pH and thus
water soluble. Deprotonation results in hydrophobic P2VP blocks, which then aggregate into
micellar cores. The hydrophobization of the P2VP blocks results in micelles rather than in
precipitated particles because the P2VP blocks are linked to water-soluble PEO segments.
Although the deprotonation process is continuous, it is more significant around the pKa of the
P2VP blocks, which explains why micellization is observed around pH = 5 [128].
Figure 2.7. Schematic of a stimuli-sensitive micellization process.
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