Chemistry Reference
In-Depth Information
Hawker et al. 2001; Hawker and Wooley 2005). Recent developments in living
radical polymerization allow the preparation of structurally well-defined block copo-
lymers with low polydispersity. These polymerization methods include atom transfer
free radical polymerization (Coessens et al. 2001), nitroxide-mediated polymerization
(Hawker et al. 2001), and reversible addition fragmentation chain transfer polymer-
ization (Chiefari et al. 1998). In addition to their ease of use, these approaches
are generally more tolerant of various functionalities than anionic polymerization.
However, direct polymerization of functional monomers is still problematic
because of changes in the polymerization parameters upon monomer modification.
As an alternative, functionalities can be incorporated into well-defined polymer back-
bones after polymerization by coupling a side chain modifier with tethered reactive
sites (Shenhar et al. 2004; Carroll et al. 2005; Malkoch et al. 2005). The modification
step requires a clean (i.e., free from side products) and quantitative reaction so that
each site has the desired chemical structures. Otherwise it affords poor reproducibility
of performance between different batches.
6.3. SELF-ASSEMBLY OF POLYMER-PARTICLE
NANOCOMPOSITES
In this section, we focus on the strategies of controlling nanoparticle assemblies
through functionalized polymer scaffolds, starting from interparticle spacing in
bulk aggregates to 3-D morphologically controlled hierarchical nanostructures.
6.3.1. Control of Interparticle Spacing
The overall optical and electronic properties of nanoparticle assembly are affected by
neighboring particles in a strongly distance-dependent fashion (Hovel et al. 1993;
Schmitt et al. 1997; Sandrock and Foss 1999; Taton et al. 2000; Ung et al. 2001).
Although the control of interparticle spacing on the assembly can be achieved by
manipulating the nanoparticle size and shapes, the use of separate entities as
spacers provides a more modular method to regulate the interparticle distances.
Structurally, regular dendrimers can provide useful “spacers” because of their glob-
ular geometry, and they allow systematic control the interparticle spacing through the
choice of dendrimer generations.
Rotello's research group first demonstrated the direct control of interparticle
spacing through self-assembly of gold nanoparticles with poly(amido-amine)
(PAMAM) dendrimers (Fig. 6.2; Frankamp, Boal, et al. 2002). The assembled nano-
composite was obtained by the formation of salt bridges between carboxylic acid-
functionalized gold nanoparticles and the peripheral amine group of PAMAM.
Small angle X-ray scattering (SAXS) revealed a monotonic increase of interparticle
distance from 4.1 to 6.1 nm with an increase of dendrimer generations from G0 to
G6. The same approach was applied to dendrimer-mediated assembly of gold nano-
particles with a larger diameter (6.0 nm; Srivastava, Frankamp, et al. 2005). The
interparticle spacing calculated from the SAXS spectra in the same PAMAM
