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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 hom*opolymer

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

hom*opolymer (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 hom*opolymer into BCP can obviously

enrich the phase behavior of the BCP. For the blends of BCP

with a selective hom*opolymer, 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 hom*opolymer with the constituent blocks and the

corresponding molecular weight (Mn) of the hom*opolymer 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 hom*opolymers (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 hom*opolymer.

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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(PDF) Helical phase from blending of chiral block copolymer and hom*opolymer - DOKUMEN.TIPS (3)

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 hom*ogeneously

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 hom*opolymer

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

1 J. M. Lehn, Science, 1985, 227, 849–856.2 G. M. Whitesides and B. Grzybowski, Science, 2002, 295,2418–2421.

3 G. A. Petsko and D. Ringe, Protein Structure and Function,London, New Science Press, London, 2004.

4 H.-S. Kitzerow and C. Bahr, Chirality in Liquid Crystals, SpringerPress, New York, 2001.

5 J. J. L. M. Cornelissen, A. E. Rowan, R. J. M. Nolte andN. A. J. M. Sommerdijk, Chem. Rev., 2001, 101, 4039–4070.

6 M. M. Green, R. J. M. Nolte and E. W. Meijer, Materials-Chirality, vol. 24 of Topics in Stereochemistry, ed. S. E. Denmarkand J. Siegel, Wiley, Hoboken, NJ, 2003.

7 R.-M. Ho, Y.-W. Chiang, S.-C. Lin and C.-K. Chen, Prog. Polym.Sci., 2011, 36, 376–453.

8 E. L. Thomas, D. M. Anderson, C. S. Henkee and D. Hoffman,Nature, 1988, 334, 598–601.

9 F. S. Bates and G. H. Fredrickson, Annu. Rev. Phys. Chem., 1990,41, 525–557.

10 J. T. Chen, E. L. Thomas, C. K. Ober and G.-P. Mao, Science,1996, 273, 343–346.

11 F. S. Bates and G. H. Fredrickson, Phys. Today, 1999, 52,32–38.

12 J. J. L. M. Cornelissen, M. Fischer, N. A. J. M. Sommerdijk andR. J. M. Nolte, Science, 1998, 280, 1427–1430.

13 R.-M. Ho, Y.-W. Chiang, C.-C. Tsai, C.-C. Lin, B.-T. Ko andB.-H. Huang, J. Am. Chem. Soc., 2004, 126, 2704–2705.

14 R.-M. Ho, C.-K. Chen and Y.-W. Chiang, Adv. Mater., 2006, 18,2355–2358.

15 R.-M. Ho, Y.-W. Chiang, C.-K. Chen, H.-W. Wang, H. Hasegawa,S. Akasaka, E. L. Thomas, C. Burger and B. S. Hsiao, J. Am.Chem. Soc., 2009, 131, 18533–18542.

16 H. Tanaka, H. Hasegawa and T. Hashimoto, Macromolecules,1990, 23, 4378–4386.

17 H. Tanaka, H. Hasegawa and T. Hashimoto, Macromolecules,1991, 24, 240–251.

18 H. Tanaka, H. Hasegawa and T. Hashimoto, Macromolecules,1994, 27, 6532–6540.

19 R. B. Thompson, V. V. Ginzburg, M. W. Matsen and A. C. Balazs,Science, 2001, 292, 2469–2472.

20 H. D. Keith, F. J. Padden, Jr. and T. P. Russell, Macromolecules,1989, 22, 666–675.

21 C. C. Chao, C. K. Chen, Y. W. Chiang and R. M. Ho,Macromolecules, 2008, 41, 3949–3956.

22 O. Tcherkasskaya, S. Ni andM. A. Winnik,Macromolecules, 1996,29, 4241–4246.

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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