Effects of the Chiral Interface and Orientation-Dependent Segmental

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Effects of the Chiral Interface and Orientation-Dependent Segmental Interactions on Twisting of Self-Assembled Block Copolymers Tao Wen and Rong-Ming Ho* Department of Chemical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan S Supporting Information *

ABSTRACT: The chirality effect on the self-assembly of block copolymers (BCPs) composed of chiral entities, denoted as chiral BCPs (BCPs*), gives rise to the formation of the helical phase (H*) due to intermolecular chiral interactions. The corresponding twisting of self-assembled H* might be initiated from the microphase-separated chiral interface and/or attributed to chiral orientation-dependent segmental interactions. To examine the origins of the twisting mechanisms, a series of polylactide-containing BCPs* with different sequences and molecular weights of chiral block, poly(L-lactide) (PLLA), and achiral block, poly(D,L-lactide) (PLA), were designed for self-assembling. Accordingly, the impact of sequence and length of the blocks on self-assembled BCPs* can be scrutinized. For the ones with the PLLA as a middle block, polystyrene-b-poly(L-lactide)-bpoly(D,L-lactide), the formation of H* can be achieved even with a short PLLA segment. By contrast, the self-assembled morphology of polystyreneb-poly(D,L-lactide)-b-poly(L-lactide) with the PLA as a middle block is dependent upon the length of the PLA block. On the basis of the self-assembled results, the chirality effect on the self-assembly of BCP* is originated from both the chiral interface and the chiral orientation-dependent segmental interactions. The present work on the study of the formation mechanisms of the H* thus provides insights into the intermolecular chiral interactions from the self-assembly of BCP*.

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The formation of helical superstructures or phases from the self-assembly of chiral systems is attributed to the twisting of the self-assembled textures resulting from the packing of chiral molecules or macromolecules due to intermolecular chiral interaction. The origins of the twisting have been intensively investigated in the past decades.19 A simple elastic model was first proposed by Helfrich to describe the twisting mechanism at which self-assembled helices are formed by the spontaneous torsion of bilayer edges and the bending of the ribbon.20 Subsequently, an elastic theory was developed by Helfrich and Prost to describe the intrinsic bending of the bilayer ribbon.21 By drawing an explicit analogy between titled chiral lipid bilayers (TCLBs) and cholesteric liquid crystal, two-dimensional elastic constants for the bilayer ribbon were expressed by Ou-Yang and Liu in terms of three-dimensional Frank constants for liquid crystal,22,23 giving the tilting of the director of liquid crystal from molecules packing on a curved interface determined by a chiral curvature modulus. On the basis of the elastic model proposed by Helfrich, a microscopic modeling of chiral aggregates for the packing of chiral amphiphilic molecules was then developed by Nandi and Bagchi,24 at which each molecule is regarded as a single chiral carbon and the molecules with same chirality pack at an angle with respect to their

lock copolymers (BCPs) consisting of components with different chemical structures can self-assemble into various ordered phases such as sphere (S), hexagonally packed cylinders (HC), double gyroid (DG), and lamellae (L).1−3 By introducing chirality effect on BCP self-assembly, a helical (H*) phase besides conventional nanostructured phases can be found in the self-assembly of polystyrene-b-poly(L-lactide) (PS− PLLA).4−8 The H* phase possesses interdigitated PLLA helices hexagonally packed in a PS matrix with the space group of P622.6 As a universal self-assembling behavior for the chirality effect on BCP self-assembly, the H* phase can also be found in the self-assembly of poly(4-vinylpyridine)-b-poly(L-lactide).9 Accordingly, the BCP composed of chiral entities such as the one with chiral polylactide described above is denoted as a chiral block copolymer, BCP*.4−8 Moreover, chirality transfer at different length scales can be achieved by self-assembling the BCP*, suggesting the behaviors of homochiral evolution from the molecular chirality of chiral polylactide to the hierarchical chirality of the H* phase in the self-assembly of polylactidecontaining BCPs*.10 Homochiral evolution in self-assembled chiral materials is critical to the chemical and biological processes in nature, such as communication, replication, and enzyme catalysis, and is the vital factor in chemistry and materials for functions and complexity.11−14 The corresponding behaviors have been extensively studied experimentally and theoretically in a broad range of chiral systems such as liquid crystals,15 biomolecules,16,17 and filaments.18 © XXXX American Chemical Society

Received: February 22, 2017 Accepted: March 21, 2017

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DOI: 10.1021/acsmacrolett.7b00138 ACS Macro Lett. 2017, 6, 370−374

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ACS Macro Letters

Figure 1. TEM micrographs of self-assembled S0.63−LA0.18−A0.19 (a) and S0.63−A0.17−LA0.20 (b).

axis as shown in Figure S3. The PS microdomain appears dark, whereas the PLLA microdomain is bright because of the mass− thickness contrast from RuO4 staining for the PS block. A hexagonally packed structure can be observed at the tilting angle of 0° (Figure S3a). With the increase of tilting angle for projection, the projection image gradually changes to crescentlike projection as shown in Figure S3b−S3f, which is the unique projection character of the H* phase under TEM observations.6 On the basis of the projection results, the forming H* phase is thus identified. In the following studies, the identification of the H* phase is based on the method described above, and only one specific projection will be used as a representative for the projection of the self-assembly phase. Figure S4a shows the 1D SAXS profile of self-assembled S0.63−LA0.18−A0.19 at which the subscripts denote the respective volume fraction of each block; the reflections at the relative q values of 1:√4:√7:√13 can be found, suggesting hexagonally packed structure. Similarly, the self-assembled S0.63−A0.17− LA0.20 gives hexagonally packed structure with the relative q values of 1:√3:√7 in the 1D SAXS profile (Figure S4b). Figure 1a shows the corresponding TEM micrograph of selfassembled S0.63−LA0.18−A0.19 at which crescent-like projection can be observed from the titling experiments (Figure S5); by contrast, self-assembled S0.63−A0.17−LA0.20 exhibits cylinder projection only (Figure 1b and Figure S6). Those results suggest that, with equivalent volume fraction of polylactide, the formation of the H* phase is indeed dependent upon the location of chiral block in the BCPs*; the one with PLLA middle block gives H* phase, but the one with PLA middle block gives HC phase. According to our previous studies,6 owing to the presence of a specific geometrical property (i.e., helical steric hindrance), helical assembly with chiral curvature can be formed at the microphase-separated interface of PS− PLLA. It is similar to the helical packing of chiral amphiphilic molecules.24 Moreover, the incompatibility between chemically connected PS and PLLA blocks gives rise to the stretching of polymer chains that can further stabilize the helical conformation of the PLLA chain and to enhance the helical steric hindrance (i.e., to amplify the effect of chiral interface on twisting of self-assembled BCP). As a result, the presence of helical steric hindrance at the microphase-separated interface in S0.63−LA0.18−A0.19 leads to the formation of the H* phase. By contrast, the absence of such chiral effect on the self-assmebled S0.63−A0.17−LA0.20 results in the formation of an HC phase. As evidenced above, the sequence of chiral and achiral block in the BCP* is critical to the self-assembled phase due to the

neighbors, resulting in twisting for the formation of helical texture. Recently, an “orientational self-consistent field” (oSCF) theory was developed by Grason and co-workers to describe the mean-field relationship between chain configurations, composition profiles, and mean orientation of self-assembled BCP, and the theory was further developed to comprehend the formation of helical morphologies in self-assembled BCP*.25−27 The theoretical study proposed that twisting of self-assembled BCP* is attributed to domain deformation/segment polarization resulting from segment orientation with intermolecular chiral interactions.27 Herein, we aim to systematically examine the origins of the twisting and to achieve an in-depth understanding of the forming mechanisms for the H* phase in the self-assembly of BCPs*. In particular, we are interested in clarifying the effects of the helical steric hindrance from the microphase-separated chiral interface and the chiral orientation-dependent segmental interactions on twisting of BCP* self-assembly. For systematic comparison, a series of polystyrene-b-poly(L-lactide)-b-poly(D,L-lactide) (PS−PLLA−PLA, denoted as S−LA−A) and polystyrene-b-poly(D,L-lactide)-b-poly(L-lactide) (PS−PLA− PLLA, denoted as S−A−LA) with equivalent polylactide composition but different block sequences and lengths of chiral (PLLA) and achiral (PLA) blocks were synthesized for self-assembly to examine the accessibility of forming a helical phase. Those block copolymers were synthesized by sequential polymerization similar to our previously reported works.5,28 The synthetic routes and procedures of the designed BCPs* were described in detail in the Supporting Information (see Tables S1 and S2 for the corresponding characterization). For all the polylactide-containing BCPs* synthesized, the total volume fraction of polylactide is in the composition range to give the formation of H* or HC phases.6 For an H* phase, the one-dimensional small-angle X-ray scattering (1D SAXS) profile exhibits a reflective mode of hexagonally packed structure in reciprocal space; this profile is similar to the one from an HC phase.6,7,25 The identification of the H* phase thus relies on real-space observations through transmission electron microscopy (TEM) projection. It is noted that two-dimensional (2D) TEM imaging of the H* phase is complicated, and the projection image is dependent upon sample thickness from microsectioning and projection direction to the helical axis.6 Following the methodology developed in our laboratory,6 a series of TEM micrographs for a 70 nm microsection of H*-forming PS−PLLA were acquired at different tilting angles with the tilting axis normal to the helical 371

DOI: 10.1021/acsmacrolett.7b00138 ACS Macro Lett. 2017, 6, 370−374

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Figure 2. TEM micrographs of self-assembled S0.66−LA0.11−A0.23 (a) and S0.66−LA0.06−A0.28 (b).

Figure 3. TEM micrographs of self-assembled S0.63−A0.14−LA0.23 (a) and S0.63−A0.08−LA0.29 (b).

chain parameters of PLLA in crystal, which is much larger than the persistence length.32 Accordingly, although intermolecular chiral interaction might be reduced with the length of the chiral block, the helical steric hindrance at the chiral interface in S0.66−LA0.06−A0.28 is still sufficient to induce the twisting of selfassembled BCP for the formation of the H* phase. In contrast to S−LA−A BCPs*, the achiral block in the middle of S−A−LA BCPs* results in the absence of the chiral interface from self-assembly. As a result, no H* phase but an HC phase can be found in self-assembled S0.63−A0.17−LA0.20 as shown in Figure 1. To further examine the effect of achiral middle block on the self-assembly of BCP*, S−A−LA BCPs* with different lengths of achiral block were synthesized for selfassembly (see Table S2 for detailed characterization and Figure S10 for the 1D SAXS profiles). Figure 3a shows the TEM micrograph of self-assembled S0.63−A0.14−LA0.23 at which cylinder projection can be observed, suggesting the formation of the HC phase as further confirmed by tilting experiment (Figure S11). The morphological results are in line with the speculation at which the required chiral interface for the formation of the H* phase is absent due to the achiral middle block. Unexpectedly, with further reducing the length of the PLA segment, crescent-like projection can be observed in selfassembled S0.63−A0.08−LA0.29 (Figure 3b), and the formation of the H* phase can be clearly identified from the tilting experiment (Figure S12). These results indicate that the H* phase can be formed even without the presence of a chiral interface at which the length of the middle achiral block becomes the critical factor to the twisting of self-assembled BCP*. Owing to the linear molecular architecture of the BCP

helical steric hindrance initiated from the microphase-separated interface (referred to as a chiral interface). It is intuitive to query how effective the helical steric hindrance from the chiral interface can be to give the formation of the H* phase. To examine the limitation of the chirality effect at the microphaseseparated interface, the length of the chiral block in the S−LA− A BCPs* was reduced. Figure 2 shows the TEM micrographs of self-assembled S−LA−A BCPs* with shorter PLLA length, S0.66−LA0.11−A0.23 and S0.66−LA0.06−A0.28 (see Table S1 for detailed characterization and Figure S7 for the 1D SAXS profiles). As shown in Figure 2a, crescent-like projection can be found in self-assembled S0.66−LA0.11−A0.23, and the formation of H* can be confirmed by tilt experiment (Figure S8). Most surprisingly, the H* phase can be still identified in selfassembled S0.66−LA0.06−A0.28 (Figure 2b and Figure S9) at which the molecular weight of the PLLA segment is only 2800 g/mol which is in the range of oligomeric molecular weight. The formation of the H* phase from the S−LA−A BCPs* with such a low-molecular-weight chiral block clearly demonstrates the significant effect of helical steric hindrance from the chiral interface on twisting of the self-assembled BCP*, further evidencing the significance of the chiral interface in the formation of H* phase. The persistence length of polylactides is determined by the ratio of D- and L-lactide and increases from 0.95 nm for racemic PLA (D-/L-lactide ∼1) to an estimated 1.18 nm for chiral PLLA (D-/L-lactide = 0).29,30 The larger persistence length of PLLA is attributed to the formation of helical conformation as compared to the random coil of the racemic PLA chain.31 Note that the chain length of the PLLA with 2800 g/mol is estimated to be 10.8 nm on the basis of the 372

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intrinsic chirality at the microphase-separated interface is significant to induce the formation of helical curvature in the H* phase at which the twisting can occur even with the short PLLA block as demonstrated. As the achiral segment locating in the middle of the BCPs* (i.e., S−A−LA), PLA block gives an achiral interface to the self-assembly of BCPs*. However, the stretching and intrinsic chirality of the PLLA chain give the anisotropic character similar to chiral cholesteric liquid crystal during self-assembly that creates a specific force field from chiral orientation-dependent segmental interactions between PLLA chains. Once such a force field is able to pass into the achiral microdomain and reach the interface of PS and PLA, it is possible to give the twisting of microphase-separated microdomains. Thus, the final morphology of self-assembled S−A−LA is dependent upon the length of the achiral block. With a long PLA block, the chiral force field is blocked by the achiral layer (Figure 4b). The absence of the chiral interface and chiral segmental interactions gives rise to the formation of the HC phase. As the achiral block is shortened, the chiral force field can pass to the interface of PS and PLA (Figure 4c). As a result, the H* phase can be formed even without the existence of a chiral interface. In conclusion, as demonstrated in the self-assembly of polylactide-containing BCPs* containing chiral and achiral blocks with different block sequences and lengths, the formation of the H* phase in BCPs* is attributed to the chiral interface and chiral orientation-dependent segmental interactions. The direct connection of a chiral block to PS block in S−LA−A gives a chiral interface from self-assembly, and thus a short PLLA block is able to induce the H* phase due to the significant effect of helical steric hindrance on BCP selfassembly. By contrast, the self-assembled morphology of S−A− LA is determined by the length of the PLA block due to the absence of a chiral interface. A long achiral block in S−A−LA could block the transfer of force field from chiral orientationdependent segmental interactions to the interface, resulting in the formation of the HC phase. As the achiral block is shortened, such a chiral force field is able to pass into the achiral microdomain and reach the interface of PS and PLA, giving the formation of H* phase.

and the stretching of polymer chains due to the repulsive force from the constituted blocks, the segments of the chiral block will interact with each other to give preferred orientation in space due to chain stretching and chirality. Similar to liquid crystal molecules, those segment−segment interactions comprising intermolecular chiral interaction give rise to the formation of anisotropically ordered phases with orientational distortions that can be described by a generalized elastic theory.26 Once such interactions (specific force field resulting from chirality) can pass into the achiral microdomain and reach the interface of PS and PLA, helical curvature at the microphase-separated interface can be formed, as evidenced by the formation of the H* phase in self-assembled S0.63−A0.08− LA0.29. There will be a threshold for transferring the force field to form the suggested helical curvature from twisting. As a result, with the increase of the achiral chain length, the chiral orientation-dependent segmental interactions resulting from chiral effect will be blocked by the self-assembled microdomain of the achiral middle block, resulting in the formation of the HC phase as shown in Figure 3a. Figure 4 schematically illustrates the mechanisms of the twisting in self-assembled BCPs* with different sequences and

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00138. Materials, sample preparation, TEM and XPS experiments, GPC and NMR measurements, SAXS profile, and additional figures and tables (PDF)

Figure 4. Schematic illustration of the origins of twisting in selfassembled S−LA−A and S−A−LA BCPs*.

lengths of chiral PLLA and achiral PLA in polylactidecontaining BCPs*. The presence of intrinsic chiral centers in PLLA block results in one-handed helical conformation. As the PLLA block directly connects with PS block, the helical conformation of the PLLA chain can give rise to the chiral interface from the self-assembly of the BCP*, and the chain packing results in helical steric hindrance at the microphaseseparated interface. As a result, the location of the chiral block is critical to the final morphology of self-assembled BCP*. For S−LA−A BCPs*, the intrinsic chirality of the middle block gives rise to a chiral interface during self-assembly (Figure 4a). On the basis of energetic consideration, the helical steric hindrance gives the twisting of self-assembled BCPs to create the helical curvature at the microphase-separated interface. The

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Tao Wen: 0000-0002-8376-0608 Rong-Ming Ho: 0000-0002-2429-7617 Notes

The authors declare no competing financial interest. 373

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ACKNOWLEDGMENTS Authors gratefully acknowledge the financial support of the Ministry of Science and Technology, Taiwan (MOST 1052119-M-007-011 and MOST 105-2811-E-007-014).

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DOI: 10.1021/acsmacrolett.7b00138 ACS Macro Lett. 2017, 6, 370−374