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This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 3665–3667 3665
Cite this: Chem. Commun., 2012, 48, 3665–3667
Helical phase from blending of chiral block copolymer and homopolymer
Hsiao-Fang Wang,aHsin-Wei Wang
aand Rong-Ming Ho*
ab
Received 30th December 2011, Accepted 15th February 2012
DOI: 10.1039/c2cc18163k
The phase behavior of the binary blends of polystyrene-b-
poly(L-lactide) chiral block copolymer (BCP*) and polystyrene
homopolymer (HS) is found to be strongly dependent on the
molecular weight (Mn) of the HS. A helical phase is formed in
the blends with low-Mn HS due to an enhancement of helical
steric hindrance.
Self-assembly is the spontaneous organization of components
into patterns or structures by cooperating secondary interactions
(i.e., non-covalent bonding forces).1,2 Nature uses the self-
assembly of molecules and supramolecules for structuring
substances so as to form various biological architectures.3 Of
them, helical morphology is perhaps the most fascinating
morphology in nature. The chirality of compounds has been
identified as one of the main origins of the formation of helical
textures.4–7 Block copolymers (BCPs) are able to self-assemble into
periodic nanostructures in bulk because of the incompatibility
and the chemical connection between constituent blocks.8–11
Hierarchical superstructures with a helical sense can be obtained
by self-assembling amphiphilic BCPs containing a charged chiral
block in solution, suggesting that the effect of chirality might play
an important role in the formation of helical nanostructures.12
Recently, block copolymers composed of chiral blocks (denoted
chiral block copolymers (BCPs*)), such as poly(styrene)-b-
poly(L-lactide)s (PS-PLLA)s, have been designed for self-
assembly.13–15 Twisted textures under transmission electron
microscopy (TEM) projection can be observed in the bulk
samples of the PS-PLLA, whereas no such projection image in
racemic BCPs (poly(styrene)-b-poly(D,L-lactide) (PS-PLA)) can
be found, suggesting the chirality effect on BCP self-assembly.13
Consequently, a helical phase possessing hexagonally packed
PLLA helical nanostructures in a PS matrix is identified as a
new phase with the space group of P622.15
A hypothetical mechanism for the formation of the helical
phase is proposed.15 Owing to the effect of chirality (the helical
steric hindrance) on molecular packing and microdomain stacking,
each microphase-separated domain will twist and shift toward the
others during morphological evolution from self-assembly, yielding
the helical curvature at the interface, and giving a helical
nanostructure to develop. Namely, the formation of helical
microdomains in the PS-PLLA is initiated from the microphase-
separated interface, and then amplified by the incompatible
PS block. Blending homopolymer into BCP can obviously
enrich the phase behavior of the BCP. For the blends of BCP
with a selective homopolymer, i.e., compatible with one of the
constituent blocks in the BCP, the phase behavior for the binary
blends depends strongly on the compatibility resulting from the
affinity of the homopolymer with the constituent blocks and the
corresponding molecular weight (Mn) of the homopolymer to
that of the selected block in the BCP.16–18 In this study, we aim
to investigate the feasibility of forming helical phase by blending
PS homopolymers (HS) with different Mns into the PS-PLLA
for self-assembly so as to examine the suggested mechanism for
the formation of the helical microdomains initiating from the
microphase-separation interface. The chemical structures of
PS-PLLA and HS are illustrated in Scheme 1.
On the basis of the phase behavior of the PS-PLLAs,15 a
PS-PLLA with a lamellar phase (symmetric composition) was
used for the preparation of the binary blends so as to bring the
expected compositions for the formation of the helical phase
from blending. The binary blends were prepared by solution
casting followed by rapid cooling from the microphase-separated
melt at 175 1C at the cooling rate of 150 1C min�1 to 25 1C.
Consequently, amorphous microphase-separated phase can be
formed without the effect of the PLLA crystallization on the self-
assembled morphology. The PS-PLLA with PLLA volume
fraction (fvPLLA) of 0.48, exhibiting lamellar phase, was used for
blending. The fvPLLA in the blends was 0.33 for all of the blends
examined in this study. Fig. 1(a) shows the TEM micrograph for
the blends of PS-PLLAwith high-Mn HS (Mn= 39000 gmol�1).
RuO4-stained PS microdomains appear as dark regions, whereas
polylactide microdomains appear as bright regions, suggesting the
preservation of lamellar phase (namely, there is no occurrence of
phase transformation). The corresponding 1D SAXS profile
Scheme 1 Chemical structures of PS-PLLABCP* and PS homopolymer.
aDepartment of Chemical Engineering, National Tsing Hua University,Hsinchu, 30013, Taiwan, R.O.C. E-mail: [emailprotected];Fax: +886-3-5715408; Tel: +886-3-5738349
b Frontier Research Center on Fundamental and Applied Sciencesof Matters, National Tsing Hua University, Hsinchu, 30013,Taiwan, R.O.C.
ChemComm Dynamic Article Links
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3666 Chem. Commun., 2012, 48, 3665–3667 This journal is c The Royal Society of Chemistry 2012
(Fig. 2(a)) shows reflections at q* ratios of 1 : 2 : 3 : 4, further
demonstrating the preservation of lamellar phase. Moreover,
no macrophase separation is observed under TEM, and signifi-
cant shifting of reflection peaks to lower q in the SAXS result is
found, suggesting that the introduction of the HS should be
localized in between the PS blocks (as illustrated in Fig. 3(b))
so as to enlarge the size of the PS microdomains from 32.5 to
49.0 nm, as determined from the primary peaks of the SAXS
profiles. Fig. 1(b) shows the TEM micrograph for the blends of
the PS-PLLA with intermediate-Mn HS (Mn = 13400 g mol�1).
Hexagonally packed cylinders can be identified under TEM
observation. Also, the hexagonally packed cylindrical phase
can be clearly identified by the corresponding 1D SAXS profile
(Fig. 2(b)) in which the reflections occur at q* ratios of
1 :ffiffiffi4p
:ffiffiffi7p
:ffiffiffi9p
:ffiffiffiffiffi17p
, inferring a phase transformation from
lamellae to cylinders. We speculate that the phase transforma-
tion from lamellae to cylinders is driven by a solubilization of
introducing HS, as illustrated in Fig. 3(c). The introduction of the
HS gives rise to the swelling of the PS microdomains both laterally
and longitudinally, resulting in an expansion of the average
distance between chemical junctions and a change in microdomain
size. Subsequently, the increase in the enthalpy penalty due to the
incompatibility between the PLLA and PS polymer chains will be
balanced by a reduction of interfacial area so as to result in the
phase transformation from lamellae to cylinders.
Most interestingly, while the Mn of introduced HS is much
lower than that of the PS block in the PS-PLLA, for instance
Mn = 3000 g mol�1, complicated projections under TEM can
be obtained (Fig. 1(c)). Instead of cylindrical projection,
twisted morphology can be found in the blends. The reflection
peaks in the corresponding 1D SAXS profile occur at q* ratios
of 1 :ffiffiffi4p
:ffiffiffi7p
:ffiffiffiffiffi13p
(Fig. 2(c)), suggesting a hexagonally
packed character. Notably, in contrast to the blends with
intermediate-Mn HS, a helical phase instead of a cylindrical
phase is found in the blends using low-Mn HS for blending.
We speculate that the low-Mn HS can be homogeneously
distributed into the PS microdomains of the PS-PLLA so as to
give a higher tendency to allocate the HS near the microphase-
separated interface than that in the blends with higher Mn. The
phase behavior of the allocation of the HS near the interface is
Fig. 1 TEM micrographs for the blends of PS-PLLA with high-Mn
HS (Mn= 39000 gmol�1) (a), intermediate-Mn HS (Mn=13400 gmol�1)
(b), and low-Mn HS (Mn = 3000 g mol�1) (c).
Fig. 2 1D SAXS profiles for the blends of PS-PLLA with high-Mn
HS (Mn= 39000 gmol�1) (a), intermediate-Mn HS (Mn=13400 gmol�1)
(b), and low-Mn HS (Mn = 3000 g mol�1) (c).
Fig. 3 Illustration of (a) the intrinsic lamellae of PS-PLLA, and the
molecular dispositions for the blends of PS-PLLA with (b) high-Mn
HS, (c) intermediate-Mn HS, and (d) low-Mn HS. The sizes of the PS
microdomain and the PLLA microdomain are determined based on
the primary peaks of the SAXS profiles. Fig. 3(c) and (d) show the
calculation results of the radian between two chemical junctions in the
PLLA microdomain (rPLLA) as indicated by the red arc line. rPLLA is
approximately 6.731 in helical phase, and 5.501 in cylindrical phase.
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This journal is c The Royal Society of Chemistry 2012 Chem. Commun., 2012, 48, 3665–3667 3667
similar to the BCP composites with inorganic nanoparticles in
which the nanoparticles with small size tend to aggregate near
the microphase-separated interface on the basis of theoretical
predictions.19 Subsequently, the occurrence of the local segre-
gation of the HS near the microphase-separated interface gives
rise to significant helical steric hindrance for the formation of
helical curvature through a twisting and shifting mechanism,
and eventually helps develop a helical phase. We speculate that
the behavior of the enhancement in the helical steric hindrance
is similar to the morphological change by blending with a
small concentration of compatible polymer diluents in which
the effect of helical steric hindrance will be significantly
enhanced by increasing the diluents fraction.20,21 As a result,
as illustrated in Fig. 3, the radian between two chemical
junctions in the PLLA microdomains with helical phase
should be larger than that in the blends with cylindrical phase.
To further examine the suggested hypothesis, systematic
calculation is conducted on the basis of the SAXS results.
By assuming that the number of PLLA chains comprising a
microdomain is equal to the ratio of the microdomain volume
(Vdomain) to the molecular volume of a PLLA chain (vPLLA),
the radian between two chemical junctions in the PLLA
microdomain can thus be determined.
Vdomain
vPLLA¼ number of chains ¼ Sdomain
sð1Þ
Sdomain is the microdomain surface area. s is the area at the
surface of microdomain occupied by a single PLLA chain,
which can be replaced by s = 2vPLLA/RPLLA because the
surface-to-volume ratio of microdomain for both cylinder and
helix is 2/RPLLA,22 where RPLLA is the radius of the PLLA
microdomain and can be determined by the following equation,
RPLLA ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi3p
f vPLLA=2pq
D. D is the interplanar spacing,
determined by the primary peak of the SAXS profile. The
calculated results are summarized in Table 1. Also, vPLLA can
be replaced by the equation vPLLA = MPLLA/(rB � NAv),
where MPLLA is the molar mass of the PLLA chain, NAv is
Avogadro’s number, and rB is the density of the PLLA chain.
For instance, in our blending system, MPLLA is 39 600 g mol�1
and rB is 1.248 g cm�3. Accordingly, the calculated result of
vPLLA is 52.88 nm3 and s is 4.68 nm2 for the cylindrical phase
and 5.36 nm2 for the helical phase. Furthermore, the number
of PLLA chains within the microdomain (N) can be described
as N ¼ ðp� R2PLLA �
ffiffiffispÞ=vPLLA.16 As a result, the radian
between two chemical junctions in the PLLA microdomain
(rPLLA = 3601/N) in the helical phase (approximately 6.731) is
higher than the radian in the cylindrical phase (approximately
5.501), implying that the low-Mn HS indeed tends to be
allocated near the microphase-separated interface so as to
enlarge the radian between two chemical junctions in the
PLLA microdomain.
In summary, a helical phase can be formed by blending the
PS-PLLA and the low-Mn HS, and examined by TEM and
SAXS. The formation of helical phase is attributed to
the enhancement of helical steric hindrance, resulting from
the occurrence of the allocation of the low-Mn HS near the
microphase-separated interface. Consequently, the mechanism
of forming helical phase by blending BCP* and homopolymer
may provide further understanding for the formation of the
helical phase due to the chirality effect on BCP self-assembly.
We thank National Science Council of Taiwan (NSC 99-2120-
M-007-003) for financial support.
Notes and references
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Table 1 Characterization of blends of PS-PLLA with HS
MorphologyD/nm
RPLLA/nm
DPSa/
nmDPLLA
b/nm
Blends with high-Mn HS Lamellae 73.1 — 49.0 24.1Blends with intermediate-Mn HS
Cylinder 74.8 22.6 — —
Blends with low-Mn HS Helix 65.4 19.7 — —
a DPS is the size of PS microdomain. b DPLLA is the size of PLLA
microdomain.
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