Chemistry Reference
In-Depth Information
cylinders and gyroid structures with increasing volume fraction and finally lamella at
near equal volume fractions for the two monomers (Leibler 1980). The introduction
of small molecule hydrogen bonding groups serves to control this volume fraction
through selective incorporation into particular domains, thereby influencing the
morphology. For example, Polushkin and colleagues studied blends of pentadecyl-
phenol and poly(styrene-b-4-vinylpyridine) (Polushkin et al. 2001; Albderda van
Ekenstein et al. 2003). In this case, two length scales were observed: a longer
length scale (20-90 nm) corresponding to microphase separation between poly-
styrene and poly(4-vinylpyridine) blocks and a smaller length scale (3.5 nm) corre-
sponding to the lamellar structure in the poly(4-vinylpyridine)-pentadecylphenol
comb domains. Mathers and colleagues (2007b) recently introduced phosphonium
ionic groups with hydrogen bonding functionality to complementarily functionalized
block copolymers. The block copolymers shifted from a cylindrical morpho-
logy to a lamellar morphology upon introduction of the hydrogen bonding phos-
phonium salt.
In block lamellar diblock copolymers, the molecular weight dependence of the
long period typically follows a dependence on the molecular weight to the 0.67
power in the case of strong phase separation (D
N
0.67
). However, in the case of
poly(styrene-b-4-vinylpyridine) with pentadecylphenol an exponent of 0.8 was
observed, suggesting weaker phase separation, which was attributed to the partial
miscibility of the pentadecylphenol with the styrenic lamella. High shear stability
of hydrogen bonded pentadecylphenol and poly(4-vinylpyridine) groups below
their ODT was demonstrated at strains of 100% and 0.5 Hz frequency. In the case
of poly(styrene-b-4-vinylpyridine) blends with pentadecylphenol, shearing was a
useful strategy for orienting the lamella parallel to the shear direction and the hydro-
gen bonded comb structure redeveloped upon cooling (Makinen et al. 2000).
Shearing of blends with cylindrical morphology resulted in the orientation of the hexa-
gonally packed cylinders along the shear direction. The soluble pentadecylphenol
was extracted from the oriented cylinders of poly(4-vinylpyridine) using methanol,
resulting in a nanoporous structure (Makki-Ontto et al. 2001).
Ten Brinke's group also studied the hydrogen bonding attachment of octyl gallate
(an octyl-functionalized trihydroxybenzene) to poly(isoprene-b-vinylpyridine) block
copolymers containing small 2- or 4-vinylpyridine blocks of 10-28 units (Bondzic
et al. 2004). For 4-vinylpyridine block copolymers, the morphologies consisted of
cylinders of 4-vinylpyridine and octyl gallate in a polyisoprene matrix, and cylinder
diameters increased with octyl gallate content. This was not surprising given that the
hard phase weight fraction ranged from 15.8 to 25.2 wt%. For the poly(2-vinylpyridine)
blocks, the attachment led to different morphologies, depending on the amount of
octyl gallate present. The morphology shifted to lamellar with increasing octyl
gallate content, and the morphologies exhibited temperature dependence such that
the lamellar morphology showed a decrease in lamellar spacing with temperature
and a transition to a cylindrical morphology at 150 8C (Fig. 4.16). Shifting of the mor-
phology and the changing sizes of the morphological features was attributed to dis-
sociation of hydrogen bonding guests, and the poly(2-vinylpyridine) blocks were
thought to relax from a stretched state upon the dissociation of the octyl gallate.
