Chemistry Reference
In-Depth Information
Figure 3.9 Supramolecular bridges between two surfaces.
two separate surfaces, and covalent polymer bridging has received extensive theoreti-
cal (Fleer and Scheutjens 1986) and experimental (Jeppesen et al. 2001) attention.
When the potential bridges are linear SPs that can adjust their size and shape in
response to the steric constraints imposed by the surfaces (van der Gucht 2002,
2003), the bridging structure and the resulting material properties reflect the chem-
istry of small-molecule self-assembly. Even sterically induced entropic penalties as
small as kT [Boltzmann thermal energy (Boltzmann constant
temperature)] per
chain are large enough to shift the chemical potential that drives polymerization
and reduce the average SP molecular weight by
40%. In situations where mechan-
ical properties scale, for example, as MW
3.5
(molecular weight raised to the power of
3.5), order of magnitude changes in properties might result from the fairly modest
steric energetics.
The theory of SP bridging in this context has begun to receive attention in recent
years. Van der Gucht and cohorts (2003) have reported intersurface forces and scaling
relationships that differ from covalent analogs. Chen and Dormidontova (2006) have
examined the attractive forces of end-adsorbed SPs and their ability to bind to
complementary surfaces. These studies show that the molecular weights of supra-
molecular structures on surfaces vary substantially from those in solution as the con-
centration of surface absorption sites are increased. The chain orientation relative to
the surface is further affected by the concentration of polymer in solution. The
dynamic nature of the SP chains allows for surface remodeling in response to
changing environment. For example, an increase in solution concentration leads
to more dense, rather than taller, surfaces.
The first experimental investigations of SP bridging have also been reported only
recently. One series of studies makes use of the DNA-based SPs described in Section
3.2 (Fogleman et al. 2002; Xu et al. 2004). The DNA-based SP monomers comprise
