fiber is often directly controlled by the molecular weight of the diblock copolymer,
and therefore can be fine-tuned at will.
Using a similar strategy with an (A) m –(B) n –(C) p type triblock copolymer, but
with an additional feature of being able to transform (or degrade) the core block,
Liu and coworkers reported the preparation of nanotubes [38, 39]. An interesting example is based on a triblock copolymer, poly(n-butyl methacrylate)-blockpoly(2-cinnamoyloxyethyl methacrylate)-block-poly(tert-butyl methacrylate), labelled
B1270-C190-tB270, containing 1270, 190 and 270 repeat units within each of the
respective blocks, generated an equilibrium phase-separated morphology, in which
cylinders containing concentric layers of a PtBA core and PCEMA shell are present [39]. These cylinders are embedded in the majority PBA phase (Figure 15.8).
Photo-crosslinking of the PCEMA block enabled the fixation of the cylindrical
structures, wherein the cylindrical shell is crosslinked. Dissolution of the majority
PBA phase in a suitable medium disperses the nanocylinders. The core of these
cylinders, which is made up of PtBA is then hydrolyzed using a mild two-step
process (tert-butyl ester is first transformed to the trimethylsilyl ester and then hydrolysed) to generate a poly(acrylic acid) (PAA) core, which when dried collapses
on to the inner walls of the cylindrical shell to generate nanotubes. An interesting
feature of the PAA chains is that their conformation is very pH and ionic strength
dependent; a feature that has been utilized to prepare nanoporous membranes
having tunable permeability, as will be seen later.
The formation of cylindrical domains of one block in a matrix of another, in diblock copolymers, has also been exploited for the generation of nanochannels in
thin film membranes [40, 41]. Using diblock copolymers of PtBA-b-PCEMA, but
this time with a larger volume fraction (ca. 74 vol%) of the photo-crosslinkable
PCEMA block, Liu et al, prepared thin polymer films containing a densely packed
array of nanocylinders (diameter @ 22 nm) [40]. As expected, in these cases the
PCEMA block forms the continuous phase with nanocylinders of the hydrolyzable
PtBA blocks forming a densely packed array. Typically, thick films (or disks of ca.
1–4 mm thick) of these diblock copolymers were solvent cast and further thermally
annealed for extended periods to enable the formation of the equilibrium morphologies. Very thin sections, with thickness ranging from 50 nm to 2 mm, then
need be microtomed to generate membranes that contain the PtBA cylinders oriented perpendicular to the membrane surface. The existence of such regularly
packed cylindrical domains in these thin sections was confirmed by TEM studies.
Photo-crosslinking of the majority continuous PCEMA phase, followed by mild
selective hydrolysis of the tert-butyl groups in the PtBA block, generates hexagonally arranged nanochannels across an insoluble polymer film (Figure 15.9).
It must be recognized that these channels are not empty but have poly(acrylic
acid) (PAA) chains tethered to their inner walls. However, since the relative volume
occupied by these chains is significantly lower than the parent PtBA, these channels are rendered substantially porous. A very interesting feature of these porous
membranes is the ‘‘chemical valving effect’’, wherein the relative permeability of the
membranes was shown to vary by almost two orders of magnitude as a function of
pH, with the lowest permeability being witnessed at a pH of 3 [41]. The explana15.5 Nanostructures Based on Bulk Phase Separation 489
and therefore can be fine-tuned at will.
Using a similar strategy with an (A) m –(B) n –(C) p type triblock copolymer, but
with an additional feature of being able to transform (or degrade) the core block,
Liu and coworkers reported the preparation of nanotubes [38, 39]. An interesting example is based on a triblock copolymer, poly(n-butyl methacrylate)-blockpoly(2-cinnamoyloxyethyl methacrylate)-block-poly(tert-butyl methacrylate), labelled
B1270-C190-tB270, containing 1270, 190 and 270 repeat units within each of the
respective blocks, generated an equilibrium phase-separated morphology, in which
cylinders containing concentric layers of a PtBA core and PCEMA shell are present [39]. These cylinders are embedded in the majority PBA phase (Figure 15.8).
Photo-crosslinking of the PCEMA block enabled the fixation of the cylindrical
structures, wherein the cylindrical shell is crosslinked. Dissolution of the majority
PBA phase in a suitable medium disperses the nanocylinders. The core of these
cylinders, which is made up of PtBA is then hydrolyzed using a mild two-step
process (tert-butyl ester is first transformed to the trimethylsilyl ester and then hydrolysed) to generate a poly(acrylic acid) (PAA) core, which when dried collapses
on to the inner walls of the cylindrical shell to generate nanotubes. An interesting
feature of the PAA chains is that their conformation is very pH and ionic strength
dependent; a feature that has been utilized to prepare nanoporous membranes
having tunable permeability, as will be seen later.
The formation of cylindrical domains of one block in a matrix of another, in diblock copolymers, has also been exploited for the generation of nanochannels in
thin film membranes [40, 41]. Using diblock copolymers of PtBA-b-PCEMA, but
this time with a larger volume fraction (ca. 74 vol%) of the photo-crosslinkable
PCEMA block, Liu et al, prepared thin polymer films containing a densely packed
array of nanocylinders (diameter @ 22 nm) [40]. As expected, in these cases the
PCEMA block forms the continuous phase with nanocylinders of the hydrolyzable
PtBA blocks forming a densely packed array. Typically, thick films (or disks of ca.
1–4 mm thick) of these diblock copolymers were solvent cast and further thermally
annealed for extended periods to enable the formation of the equilibrium morphologies. Very thin sections, with thickness ranging from 50 nm to 2 mm, then
need be microtomed to generate membranes that contain the PtBA cylinders oriented perpendicular to the membrane surface. The existence of such regularly
packed cylindrical domains in these thin sections was confirmed by TEM studies.
Photo-crosslinking of the majority continuous PCEMA phase, followed by mild
selective hydrolysis of the tert-butyl groups in the PtBA block, generates hexagonally arranged nanochannels across an insoluble polymer film (Figure 15.9).
It must be recognized that these channels are not empty but have poly(acrylic
acid) (PAA) chains tethered to their inner walls. However, since the relative volume
occupied by these chains is significantly lower than the parent PtBA, these channels are rendered substantially porous. A very interesting feature of these porous
membranes is the ‘‘chemical valving effect’’, wherein the relative permeability of the
membranes was shown to vary by almost two orders of magnitude as a function of
pH, with the lowest permeability being witnessed at a pH of 3 [41]. The explana15.5 Nanostructures Based on Bulk Phase Separation 489