Chemistry Reference
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of the two homopolymers (Wang et al. 2001). Moreover, the polydispersity of the
micelles changed with the solvent composition and narrowed for less selective
solvent mixtures.
Other potential applications for hydrogen bond containing polymers exist, such as
reversible attachment of guest molecules (Ilhan et al. 2001), reversible cross-linking
(Thibault et al. 2003), improved melt processing behavior (Yamauchi, Lizotte,
Hercules, et al. 2002), self-healing materials (Lange et al. 1999), shape-memory poly-
mers (Liu et al. 2006), recyclable thermosets (Lange et al. 1999), induction of liquid
crystallinity (Gulik-Krzywicki et al. 1993), induction of phase mixing (Kuo et al.
2003) and demixing (de Lucca Freitas et al. 1998), templated polymerization
(Khan et al. 1999), drug-selective chromatographic media (Kugimiya et al. 2001),
hydrogen bonded layer by layer assemblies (Wang et al. 1999), and reinforcement
of the orientation of nonlinear optic materials (Huggins et al. 1997).
4.3. HYDROGEN BOND CONTAINING BLOCK COPOLYMERS
Hydrogen bonding telechelic and block copolymers both differ from randomly func-
tionalized copolymers in that the placement of the hydrogen bonding groups is loca-
lized to specific region(s) in the polymer backbone. This typically leads to different
behavior compared to random functionalization because of the presence of both phase
separation and hydrogen bonding interactions. Thus, in the bulk state, many of these
systems consist of both separated domains or blocks of hydrogen bonding groups and
nonhydrogen bonded regions.
One particular advantage of introducing hydrogen bonding groups in a block
copolymer structure is the multiplicative and cooperative effects of the hydrogen
bonding interactions that are attributable to the multitude of adjacent hydrogen
bonding groups (Pan et al. 2000). Pan and colleagues observed association constants
of (4.5 to 7.9)
10
5
M
21
for poly(4-vinylpyridine-b-NLO) with poly(4-hydroxysty-
rene-b-styrene) (a relatively good hydrogen bonding donor) and 4.4
10
3
M
21
for
poly(2-hydroxyethyl methacrylate-b-methyl methacrylate) (a relatively poor hydro-
gen bonding donor) in toluene/CH
2
Cl
2
(99:1) mixtures. These values are much
higher than those expected for a single vinylpyridine-hydroxystyrene hydrogen
bonding interaction. Kriz et al. (2006) found that the cooperativity of the association
of poly(4-vinylpyridine) and poly(4-hydroxystyrene) increased with the degree of
polymerization and reached a maximum at a 1:1 functional group molar ratio.
In addition, only short hydrogen bonding blocks are necessary to achieve phase
separation, because of the influence of the associations on the tendency of block
copolymers to microphase separate. Although this tendency is often measured
through the Flory-Huggins interaction parameter x (Leibler 1980; Lee and Han
2002b), hydrogen bonding interactions must be accounted for through additions of
other parameters into thermodynamic treatments, because x refers to nonspecific
interactions and is repulsive whereas hydrogen bonding is attractive. Coleman and
Painter (1995, 2006) thoroughly discussed thermodynamic treatments of hydrogen
bonding effects in polymer blend miscibility. Considerations must be made for
