PHYS01200704032 Debes Ray - Homi Bhabha National Institute

Chapter 2: Block Copolymer-mediated Synthesis of Gold Nanoparticles
2.2.2. Amphiphilic Block Copolymers and Self-Assembly
Amphiphiles are molecules which have an affinity for two different types of environments.
This dual affiliation is built into the molecules by the covalent joining of blocks of different
chemical character and solution properties. Given the opportunity, the two (or more) different
blocks strive to minimize their contact, and can thus coerce the amphiphilic molecules to
attain a preferential orientation. As a result, amphiphilic molecules self-organize at interfaces
and in solution and, in doing so, modify the interfacial properties to a great extent.
Amphiphilic block copolymers self-assemble both in solution and in bulk and are capable of
generating a variety of micro domain morphologies due to the covalently bonded blocks with
different physical and chemical properties [117]. The two blocks are incompatible and
interact differently with their environment due to the chemical nature and also behave
distinctively in solution which results in micro phase separation not only in aqueous media
but also in organic solvents. This self-assembly process is driven by an unfavourable mixing
enthalpy and a small mixing entropy, while the covalent bond connecting the blocks prevents
macroscopic phase separation.
Block copolymers are known to self-assemble in block selective solvents, which
solubilize one but not the other block, forming micelles of various shapes [119,120]. In a
solvent, block copolymer phase behaviour is controlled by the interaction between the
segments of the polymers and the solvent molecules as well as the interaction between the
segments of the two blocks. A defining property of amphiphilic block copolymers is the
ability of individual block copolymer molecules (unimers) to self-assemble into micelles in
dilute solutions. The unimers form molecular solutions in water at block copolymer
concentrations below that of the critical micelle concentration (CMC). At concentrations of
the block copolymer above the CMC, unimer molecules aggregate and form micelles, a
process called „micellization‟ as depicted in Figure 2.4. The hydrophobic blocks of
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