Chemistry Reference
In-Depth Information
toward a truly biomimetic macromolecule. The purpose of this chapter is to provide a
brief overview of supramolecular assemblies formed by amphiphilic macromolecules
using a few selected examples.
2.2. AMPHIPHILIC BLOCK COPOLYMERS
A polymer is considered to be a copolymer when more than one type of repeat unit is
present within the chain. There are a variety of copolymers, depending on the relative
placement of the different types of repeat units. These are broadly classified as
random, block, graft, and alternating copolymers (see Fig. 2.1 for structural details;
Cheremisinoff 1997; Ravve 2000; Odian 2004). Among these structures, block copo-
lymers have attracted particular attention, because of their versatility to form well-
defined supramolecular assemblies. When a block copolymer contains two blocks
(hydrophobic and hydrophilic), it is called an amphiphilic diblock copolymer.
The immiscibility of the hydrophilic and lipophilic blocks in the polymers provides
the ability to form a variety of assemblies, the structures and morphologies of which
can be controlled by tuning the overall molecular weight and molar ratios of the
different blocks (Alexandridis et al. 2000).
These polymers are capable of forming self-assembled structures such as micelles,
vesicles, and bilayers (Figs. 2.2 and 2.3; van Hest et al. 1995; Allen et al. 1999;
Alexandridis et al. 2000; Discher and Eisenberg 2002; Choucair and Eisenberg
2003; Lazzari and Lopez-Quintela 2003; Chen and Jiang 2005; Ruzette and
Leibler 2005; Nakashima and Bahadur 2006). From a thermodynamic viewpoint,
formation of these amphiphilic assemblies is often thought to be an entropy-driven
process (Tanford 1980; De Maeyer et al. 1998). The entropy gain arises from 1)
the hydrophobic effect: the ordered water around the hydrophobic segments prior
to the formation of assembly are expelled into the bulk aqueous phase, resulting in
a large gain in entropy of water molecules; and 2) the entropy gain for the hydro-
phobic segments: when the hydrophobic segments are at the core of a micelle, for
Figure 2.1 Representation of different types of copolymers.
