Self-organization of stack-up block copolymers into polymeric supramolecules
Nanoscale Res Lett
Self-organization of stack-up block copolymers into polymeric supramolecules
Yong J. Yuan 0
Ka-Wai Choi 0
Herbert Wong 0
0 Y. J. Yuan (&) K.-W. Choi H. Wong Industrial Research Ltd., Crown Research Institutes , 69 Gracefield Road, 31-310 Lower Hutt , New Zealand
Polyethylene oxide -b- polypropylene oxide -b- polyethylene oxide (EO106PO70EO106) block copolymer self-organizes into polymeric supramolecules, characterized by NMR as phase transition from the isotropic stack-up block structure to the ordered cubic polymeric supramolecular structure. Its dependence on both temperature and copolymer concentration is clearly shown by the changes in line shape and chemical shift of the PO70 block b, c resonances.
Self-assembly; NMR; Block co-polymer
-
Self-assembly of polymeric supramolecules is a
powerful tool for producing functional materials that
combine several properties [
1
]. Potential applications
include: information storage, magnetic fluid, medical
diagnosis, catalysis, ceramics, sensors, separations and
reactions involving large molecules, chromatographic
media, proton conducting materials, controlled release
of agrochemicals, hosts for supramolecular assembly,
and pigments/solubilising agent in paints and cosmetics
[
2
].
Commercially available non-ionic Pluronics or
Synperonics triblock copolymers [
3
] (polyethylene oxide–
polypropylene oxide–polyethylene oxide, EOm
POnEOm) are superior polymeric templates, which
produce material of a wide pore diameter and wall
thickness [
4, 5
]. The concept of stacking triblock
copolymers [4] was proposed to produce very
longrange linear nanostructures, due to extension more or
less indefinitely in both directions. The synthesized
conical molecules, which are shaped like a badminton
shuttlecock, were reported to stack together in a
directed manner [
6
]. The specific shapes open up the
huge potential for directionalities of alignment, causing
by hydrogen-bonding and/or weak van der Waals
interactions.
Pluronics F127 is the subject of interest for this
study. It has the formula of EO106PO70EO106. As
illustrated in Scheme 1, this triblock compound
consists of a hydrophobic PO70 block sandwiched by two
hydrophilic EO106 blocks. For simplicity, there are two
different modes of interaction for self-assembled block
copolymer, namely hydrophobic PO70 and hydrophilic
EO106 packing segments. In both cases, the packing of
large molecules, i.e., EO106PO70EO106, means that only
a fraction of molecules will be in direct contact due to
hydrogen-bonding, polar or van der Waals forces.
Because of the unique amphiphilic property, the
material self-assembles into stacking structures.
Hydrogen-bonding among the PO70 units are expected
to drive the triblock molecules to assemble into
linearrotating cylinder structures [
4
]. Its phase behavior is
temperature and concentration dependent, which
relies on the level of dehydration of EO106 and PO70
block. An additional self- assembly process pushes the
corona-surrounded domains into unusual anisotropic
interactions, which was suggested to be a cubic phase
[
7
].
NMR (nuclear magnetic resonance) for studying
liquid crystalline systems was discussed, [
8
] to elucidate
thermotropic and lyotropic phase transitions. The
studies of the 13C NMR of EO61PO41EO61 (F87) at
Scheme 1. Self-organization of stack-up EO106PO70EO106 into
polymeric supramolecules
low concentration less than 1% (w/w) have been
documented previously, [
9, 10
] even the self-assembly
behavior in water of a mixture of EO13PO30EO13 (L64)
and EO37PO58EO37 (P105), was explored [11] by 2H
NMR at 25 C. However, the experimental application
of these techniques and the interpretation of their
results are more complicated than in homogeneous
systems [
7, 12
]. To date, no complete NMR study of
F127 polymer has been published. This study is focused
on the 1H NMR analysis of F127 in D2O. All spectra
were recorded on samples dissolved in D2O contained
in a 5 mm o.d. NMR tube, on a Varian Unity 500 MHz
NMR Spectrometer equipped with a 5 mm inverse
probe. Excitation pulse width was approximately
81 (10 ls), data acquisition time 4.096 s, relaxation
delay time 6 s, pulse repeat time approximately 10 s.
The residual HDO peak was used as a secondary
reference as a function of temperature [13] to calibrate
the chemical shifts. Although not ideal, this should
remove the gross effects of temperature dependence of
the chemical shift.
As shown in Fig. 1, the chemical shifts of both PO70
and EO106 blocks appear to be
temperature-dependent. There is a fine structure (bCH2 or cCH) at 20 C,
and partial overlap with b‘CH2 units of EO106 block.
The spectra at 40 and 60 C are similar; the resonances
of 1H (aCH3, cCH and bCH2) of PO70 block decrease as
temperature increases. At temperatures above the
phase transition, [
7
] the signal is increased, due to an
increased relaxation rate of the interacting PO70
blocks, with the decrease of segmental mobility (...truncated)