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Publication Date (Web): January 12, 2018 ... as an additional factor in tuning the self-assembly of giant surfactants at sub-10 nm or even sub-5 nm le...
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Influence of Regio-Configuration on the Phase Diagrams of DoubleChain Giant Surfactants Xiao-Man Wang,† Yu Shao,†,‡ Peng-Fei Jin,† Wenbo Jiang,§ Wei Hu,∥ Shuguang Yang,‡ Weihua Li,*,§ Jinlin He,*,⊥ Peihong Ni,⊥ and Wen-Bin Zhang*,† †

Key Laboratory of Polymer Chemistry & Physics of Ministry of Education, Center for Soft Matter Science and Engineering, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R. China ‡ State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Materials Science and Engineering, Center for Advanced Low-dimension Materials, Donghua University, Shanghai 201620, P. R. China § State Key Laboratory of Molecular Engineering of Polymers, Department of Macromolecular Science, Fudan University, Shanghai 200433, P. R. China ∥ Key Laboratory of Particle Astrophysics, Institute of High Energy Physics, Chinese Academy of Sciences, Beijing 100049, P. R. China ⊥ College of Chemistry, Chemical Engineering and Materials Science, State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials, Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, Suzhou Key Laboratory of Macromolecular Design and Precision Synthesis, Soochow University, Suzhou 215123, P. R. China S Supporting Information *

ABSTRACT: It has been established that a minute difference of the primary chemical and topological structures influences the self-assembly behaviors of giant surfactants; however, the regio-configuration effect has been rarely reported. Herein, we report a systematic study on the self-assembly behaviors of a series of double-chain giant surfactant regio-isomers, which consist of a hydrophilic polyhedral oligomeric silsesquioxane (POSS) head and two identical hydrophobic polystyrene (PS) tails with various molecular weights tethered in para-, meta-, and ortho-configurations, respectively. Small-angle X-ray scattering and transmission electron microscopy characterizations have been combined to investigate the phase behaviors of each sample and construct the phase diagrams for the three isomers with respect to the molecular weights of PS tails. It is observed that the regio-configuration significantly impacts the self-assembly behaviors of the giant surfactant isomers, including order-todisorder (ODT) and order-to-order (OOT) transitions. In each isomer system, the order-to-disorder transition temperature (TODT) changes nonmonotonically with the length of PS tails, which is generally similar to the variation of TODT for block copolymers but also exhibits some peculiar features associated with the rigid conformation of headgroup. With equal length of PS tails, TODT of the three isomers is in the descending order of ortho > meta > para. There is a pronounced and systematic phase boundary shift to lower volume fraction of PS ( f PS) and to lower temperatures from para to meta to ortho. This is closely related to the compounds’ regio-configuration and rigid 3D conformation of the headgroup and may be understood through the lower effective f PS as two tails get further apart. These findings elucidate the sophisticated effect of the regio-configuration on selfassembly behaviors. It suggests regio-configuration as an additional factor in tuning the self-assembly of giant surfactants at sub10 nm or even sub-5 nm length scales, which is of significant technical importance.

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INTRODUCTION In recent years, there has been a growing demand on engineering ordered patterns with small feature sizes for microelectronic applications. Besides the conventional “topdown” lithographical methods,1,2 the “bottom-up” nanopatterning and nanofabrication technologies enabled by selfor directed- assembly have also been extensively explored, among which block copolymers are widely investigated.3−5 It is well-known that when the χN value exceeds a critical value, where χ is the Flory−Huggins interaction parameter and N is the overall degree of polymerization, self-assembly will take © XXXX American Chemical Society

place and the block copolymer will form ordered nanostructures.6−9 For many traditional random-coil copolymers, such as poly(styrene-b-methyl methacrylate) (PS-b-PMMA), the interaction between two blocks is weak, so a large N is needed to drive the self-assembly, leading to a large feature size of >20 nm.10−12 To obtain higher areal density, the sub-10 nm and even sub-5 nm domain sizes are highly desired, which calls for Received: November 8, 2017 Revised: December 28, 2017

A

DOI: 10.1021/acs.macromol.7b02383 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Chemical structures and molecular architectures of (A) p-DPOSS-2PSn, (B) m-DPOSS-2PSn, and (C) o-DPOSS-2PSn. The DPOSS is functionalized by multiple hydroxyl groups.

Table 1. Molecular Characterization of the Regioisomers samples

na

Mn,PSb (kg/mol)

Mn,NMRc (kg/mol)

Đd

f PSe

Tgf

Tdg

p-DPOSS-2PS9 p-DPOSS-2PS11 p-DPOSS-2PS16 p-DPOSS-2PS20 p-DPOSS-2PS25 p-DPOSS-2PS28 p-DPOSS-2PS35 p-DPOSS-2PS37 m-DPOSS-2PS9 m-DPOSS-2PS11 m-DPOSS-2PS16 m-DPOSS-2PS20 m-DPOSS-2PS25 m-DPOSS-2PS28 m-DPOSS-2PS35 m-DPOSS-2PS37 o-DPOSS-2PS9 o-DPOSS-2PS11 o-DPOSS-2PS16 o-DPOSS-2PS20 o-DPOSS-2PS25 o-DPOSS-2PS28 o-DPOSS-2PS35 o-DPOSS-2PS37

9 11 16 20 25 28 35 37 9 11 16 20 25 28 35 37 9 11 16 20 25 28 35 37

1.2 1.5 2.0 2.4 2.9 3.2 4.0 4.2 1.2 1.5 2.0 2.4 2.9 3.2 4.0 4.2 1.2 1.5 2.0 2.4 2.9 3.2 4.0 4.2

4.0 4.4 5.4 6.3 7.3 7.9 9.4 9.8 4.0 4.4 5.4 6.3 7.3 7.9 9.4 9.8 4.0 4.4 5.4 6.3 7.3 7.9 9.4 9.8

1.06 1.05 1.05 1.04 1.05 1.06 1.03 1.04 1.04 1.04 1.03 1.05 1.06 1.03 1.06 1.05 1.06 1.04 1.04 1.05 1.06 1.03 1.04 1.05

0.69 0.73 0.78 0.82 0.84 0.86 0.88 0.89 0.69 0.73 0.78 0.82 0.84 0.86 0.88 0.89 0.69 0.73 0.78 0.82 0.84 0.86 0.88 0.89

42.2 44.2 50.6 63.4 68.8 74.8 79.7 82.4 40.2 43.6 50.2 62.7 68.3 74.7 78.3 82.0 40.7 43.2 50.0 62.6 67.1 74.6 78.0 82.4

193.1

a

1

200.5 190.3 189.4

198.5 200.1 209.7

202.2 205.5

b

Degree of polymerization for the PS tails, which was calculated based on H NMR of PSn-N3. Number-average molecular weights of one PS tail including initiator and linker group, kg/mol, as measured by 1H NMR. cNumber-average molecular weights of DPOSS-2PSn, kg/mol, as measured by 1 H NMR. dPolydispersity index, as measured by SEC. eVolume fraction of PS for DPOSS-2PSn. fGlass transition temperature, as measured by DSC with a heating rate of 20 °C/min. gDecomposition onset temperature as measured by TGA with a heating rate of 10 °C/min.

the development of high-χ polymers.13 Silicon-containing block copolymers, such as poly(styrene-b-dimethylsiloxane),14−17 poly(dimethylsiloxane-b-methyl methacrylate),18 and poly(styrene-b-trimethylsilylstyrene-b-styrene),19 are one kind of the most widely studied high-χ materials.20−22 It is reported that poly(dimethylsiloxane-b-2-vinylpyridine) can form cylinders with the smallest line width of 6 nm,23 and poly(styrene-bethylene oxide) forms ordered arrays of cylindrical microdomains about 3 nm in diameter.24 Other high-χ block copolymers include sugar-based polymers25−28 and poly(4-

vinylpyridine)-containing copolymers,29 all of which generates ordered nanostructures with small feature sizes. Giant molecules have emerged as a unique class of precise macromolecules built upon shape- and volume-persistent molecular nanoparticles for achieving sub-10 nm or even sub5 nm nanopatterns.30 Unlike block copolymers, the clustering of high-density functional groups on molecular nanoparticles leads to considerable collective interactions and dramatically increases the incompatibility even at low molecular weights.31 They typically include giant surfactants,32,33 shape amphiB

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Figure 2. Phase diagrams of giant surfactant regioisomers: (A) p-DPOSS-2PSn, (B) m-DPOSS-2PSn, and (C) o-DPOSS-2PSn.

philes,34,35 and giant polyhedra.36 Giant surfactants are constructed by tethering polymer chains onto precisely structured three-dimensional (3D) building blocks such as polyhedral oligomeric silsesquioxanes (POSS), fullerenes, and polyoxometalates. They possess features reminiscent of both small-molecule surfactants and block copolymers.30 The selfassembly of giant surfactants has been extensively studied both theoretically and experimentally.37−41 Giant surfactant consisting of one hydroxyl-functionalized POSS head (DPOSS) and one PS tail can self-assemble into lamellae (LAM), double gyroid (DG), hexagonally packed cylinders (HEX), and bodycentered cubic-packed spheres (BCC) nanostructures in bulk.30 Distinct nanostructures can be obtained with the same tail by adjusting the functional groups on the POSS head.42 Zhang et al.43 further show that giant molecules with linear or branched tandem interconnected POSS cages in specific sequences exhibited a full phase diagram including the inversed phase structures. Furthermore, by increasing the number of PS tails, unconventional ordered phases such as the Frank−Kasper phases and a dodecagonal quasi-crystal phase can be formed in these cone-like DPOSS-3PSn and DPOSS-4PSn systems.44 The “bolaform-like” giant surfactants composed of a PS chain endcapped with two distinctly functionalized POSS (BPOSS-PSnDPOSS) have also been reported and found to form hierarchical nanostructures in bulk with BPOSS crystallized as a thin layer in the middle of the LAM phase.45 It has been recognized that self-assembly of giant molecules is remarkably sensitive to their primary chemical structures, thus providing a versatile platform for tailoring nanostructures at a dimension right between small molecules and block copolymers.46 The influences of minute structural differences, such as regioconfiguration, on the self-assembly have been an intriguing inquiry. It is reported that the phase behaviors of isomeric pairs, AC60-2PSn and AC60-PS2n, which possess the same chemical composition and molecular weight but with different numbers of PS chains, are distinct.47 Similarly, the giant surfactant isomers with different nanoparticle tethering sites, such as AC60-PS-PEO vs PS-AC60-PEO and FPOSS-PS-PEO vs PSFPOSS-PEO, self-assemble into distinct ordered nanostructures.48−50 Nevertheless, it remains elusive how the variation of tethering positions of tails on a single head could affect their self-assembly. Recently, our group has successfully synthesized the para-, meta-, and ortho-DPOSS-2PSn double-chain giant surfactant regioisomers from the corresponding Janus POSS precursors51 and demonstrated the remarkable difference of a series of low-molecular-weight regioisomers in their phase

structures and behaviors.52 In this paper, we report the complete phase diagram of these regioisomers and demonstrate systematically the striking influence of regio-configuration on their self-assembly behaviors (Figure 1).

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RESULTS AND DISCUSSION General Design, Synthesis, and Assembly. A series of giant surfactant regioisomers were synthesized following the protocol reported in our recent paper.52 To cover a broad range of f PS, we chose several PS chains with molecular weights ranging from 1.2 to 4.2 kDa (Figure S1 and Table 1). The lowest molecular weights of PS were limited by the synthetically accessible, narrowly polydispersed chain length by anionic polymerization. By fixing the DPOSS head and changing the length of the PS tails, we obtained eight sets of regioisomeric samples; the detailed characterizations are summarized in Table 1. All giant surfactants have very narrow polydispersity index (Đ < 1.07). The samples were first studied by TGA and DSC to characterize their thermal stability as reflected by their decomposition temperature (Td) and glass transition temperature (Tg), respectively (Figures S2 and S3, Table 1). Although it was reported that block copolymers containing the ester and triazole linkages show significant thermal decomposition at relatively low temperatures (e.g., 140 °C),53 our samples are very stable at least until ∼190 °C (Figure S2). This is probably due to the presence of POSS that could enhance the thermal stability of the sample.54,55 The experimental values of Tg were compared to that calculated by empirical formula based on the overall molecular weight of PS.56 They agree very well with each other except for the ones having molecular weights lower than 3.0 kDa, which is the lowest limit that the empirical equation could apply (Figure S4).56 The self-assembly process was conducted by thermal annealing at a temperature higher than the corresponding Tg. Prior to the small-angle X-ray scattering (SAXS) experiments for structure determination, the samples were heated to a sufficiently high temperature yet below Td for several minutes to eliminate the thermal history and then quenched to the annealing temperature (Tanneal) immediately. The Tanneal was chosen to be higher than the corresponding Tgs for favorable relaxation kinetics but lower than T d to avoid any decomposition. For consistency, the lowest Tanneal was selected to be 80 °C for DPOSS-2PS9 and DPOSS-2PS11 with lower MWs, 100 °C for DPOSS-2PS16, DPOSS-2PS20, and DPOSS2PS25, and 120 °C for DPOSS-2PS28, DPOSS-2PS35, and DPOSS-2PS37. At these temperatures, the kinetics of bulk C

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Figure 3. SAXS profiles (A−I) of bulk samples and TEM images (J−R) of microtomed samples of p-DPOSS-2PSn samples annealed at specific temperature: (A, J) n = 9, at 80 °C, LAM phase; (B, K) n = 11, at 80 °C, LAM phase; (C, L) n = 16, at 100 °C, DG phase; (D, M) n = 20, at 100 °C, HEX phase; (E, N) n = 25, at 100 °C, HEX phase; (F, O) n = 28, at 120 °C, HEX phase; (G, P) n = 28, at 140 °C, BCC phase; (H, Q) n = 35, at 120 °C, BCC phase; (I, R) n = 37, at 120 °C, BCC phase. The insets in the TEM images are the corresponding FFT patterns. The scale bars are 25 nm.

DPOSS-2PS9 has been reported previously, which maintains a LAM phase during the heating process until becoming completely disordered; the ordered morphology forms at 80 °C are shown in Figure 3A.52 When f PS increases to 0.73, the sample of p-DPOSS-2PS11 remains in the LAM phase (Figure S5 and Figure 3B) with a slightly larger domain size at the same annealing temperatures (Table S1). Further increasing f PS to 0.78 (p-DPOSS-2PS16) leads to a transition to the DG phase indicated by the characteristic q ratio of √6:√8:√20:√22 (Figure S6 and Figure 3C) in the SAXS profile. The samples with f PS of 0.82 (p-DPOSS-2PS20) and 0.84 (p-DPOSS-2PS25) both exhibit the HEX phase indicated by the typical q ratio of 1:√3:2 in the SAXS profiles (Figures S7 and S8, Figure 3D,E). Interestingly, for p-DPOSS-2PS28 (f PS = 0.86), the HEX phase is observed at lower temperatures (e.g., 120 °C) (Figure 3F) and transfers to the BCC phase when temperature rises to 140 °C (Figure 3G) as revealed by the transformation of q ratio from 1:√3:2 to 1:√2:√3 in the SAXS profiles shown in Figure S9. This order-to-order transition (OOT) is also confirmed by the bright-field TEM images of the sample

assembly in giant surfactants was sufficiently fast owing to the lack of chain entanglement. The ordered nanostructure was usually developed within half an hour. To probe the phase behavior, temperature-dependent SAXS experiments were performed. The heating rate was set at 5 °C/min, and the Tanneal was set at 10 °C intervals. At each specific temperature, the assembled nanostructures of these samples were characterized by the SAXS diffraction patterns and further confirmed by microtoming and imaging in real space. The phase diagrams were then constructed for each series of regioisomers (Figure 2) and are discussed in detail in the following sections. Phase Diagram of p-DPOSS-2PSn. The phase diagram of the para-isomers with respect to the volume fraction of PS tails (f PS) and the annealing temperature in Figure 2A shows a relatively complete spectrum of traditional phases observed in block copolymers including lamellae (LAM), double gyroid (DG), hexagonally packed cylinders (HEX), and body-centered cubic-packed spheres (BCC) (Figures S5−S11). Figure 3 shows the corresponding SAXS and TEM images of each sample at specific temperatures. The self-assembly of pD

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Macromolecules annealed at 120 °C (Figure 3O) and 140 °C (Figure 3P). Moreover, both samples of p-DPOSS-2PS35 ( f PS = 0.88) and pDPOSS-2PS37 (f PS = 0.89) form BCC phases evidenced by the typical q ratios of 1:√2:√3 in the SAXS profiles shown in Figures 3H and 3I, respectively, and the temperature-dependent SAXS profiles can be found in Figures S10 and S11. All these ordered structures were confirmed by the bright-field TEM images in real space (Figure 3J−R). Unlike the single-tailed giant surfactants DPOSS-PSn,30 all of these double-chain giant surfactants p-DPOSS-2PSn show order-to-disorder transitions (ODT) at sufficiently high temperatures, which is indicated by the change of the SAXS profiles from multiple peaks to a single broad peak (e.g., Figures S5−S11). The TODT varies nonmonotonically with increasing lengths of PS tails. The sample with the shortest tail length (p-DPOSS-2PS9) showing the LAM phase has the lowest TODT of ∼130 °C. TODT increases to ∼150 °C for the DG phase (p-DPOSS-2PS16). The first sample in the HEX region exhibits the highest TODT of ∼180 °C (p-DPOSS2PS20). After passing through the peak of the first sample in the HEX phase region, the TODT drops off with increasing polymer length (p-DPOSS-2PS25 and p-DPOSS-2PS28) and rises up again with increasing f PS within the BCC phase region (pDPOSS-2PS35 and p-DPOSS-2PS37). This nonmonotonous change will be discussed in detail later. Phase Diagram of m-DPOSS-2PSn. In general, the phase diagram of the meta-isomer shown in Figure 2B is similar to that of the para-isomer. Four traditional ordered phases were also observed in the common sequence of LAM, DG, HEX to BCC with increasing f PS. The temperature-dependent SAXS profiles are shown in Figures S12−S18, and the ordered morphologies for each sample are also confirmed by TEM characterizations as shown in Figure S19. Compared with the phase diagram of the para-isomer, the phase boundaries are slightly shifted toward lower f PS. Specifically, in contrast to the DG phase region located at 0.78 for p-DPOSS-2PS16, the DG phase becomes stable at a much lower f PS of 0.69 for mDPOSS-2PS9. Moreover, the sample of m-DPOSS-2PS11 ( f PS = 0.73) shows the DG phase at all investigated temperatures below TODT. Consequently, the phase boundary between DG and HEX also shifts from 0.78 < f PS < 0.82 in Figure 2A to 0.73 < f PS < 0.78 in Figure 2B. The shift for the phase region of BCC is also noticeable. For example, the sample of m-DPOSS2PS28 with long PS tails forms the BCC phase in a wider range of temperatures than that of p-DPOSS-2PS28. The variation trend of TODT with the length of PS tails is similar to that in the para-isomers, indicating that they resemble the common mechanisms. However, the TODT of meta-isomers is in general slightly higher than that in the corresponding para-isomers except for the one with f PS = 0.82, which shows the highest TODT in para-isomers. Phase Diagram of o-DPOSS-2PSn. When the tethering positions of two polymer tails are further put closer, remarkable changes appear in the phase diagram of o-DPOSS-2PSn (Figure 2C). The temperature-dependent SAXS profiles of o-DPOSS2PSn and the TEM images confirming their phase structures are shown in Figures S20−S26 and Figure S27, respectively. In contrast to the previous two kinds of isomers, the phase boundaries are shifted further toward lower f PS. More surprisingly, the LAM phase is no longer observed in any sample of o-DPOSS-2PSn, while the DG phase with curved interfaces becomes favorable even in the samples containing PS tails with very low MWs. Both o-DPOSS-2PS9 and o-DPOSS-

2PS11 show the DG-to-HEX transition with the former having a slightly higher TOOT but lower TODT. In addition, an exceptionally broad range of the HEX phase was observed with the ortho-isomers. Over the entire range, the TODT values of the ortho-isomers are consistently higher than the other two isomers. For some of the samples, remarkably higher TODT values (∼210 °C for both o-DPOSS-2PS16 and o-DPOSS2PS20) were observed as compared to their corresponding regioisomers (by up to 60 °C) (Figure S28). Nevertheless, the trend of TODT variation profile persists in the ortho-isomer system. Phase Sequences, Phase Boundary Shift, and Molecular Packing. As discussed above, all the phase diagrams show the common sequence of LAM → DG → HEX → BCC with increasing f PS (or increasing temperature), except for the absence of LAM in the phase diagram of ortho-isomers. The phase sequence is similar to what observed before for singletailed and multitailed giant surfactants as well as those of traditional block copolymers,7,30,44 which could be understood through the general principle of creating curved interfaces to accommondate unbalanced pressure posed by the two incompatiable domains with increasing f PS. The phase boundaries are observed to shift toward lower f PS or lower temperature. In general, as the regio-configuration changes from para- to meta- and to ortho-isomers, it favors the formation of ordered phases with higher spontaneous curvature. As a result of such progressive phase boundary shift, LAM phase is completely absent for the ortho-isomers in the regions explored. It has been demonstrated that for the DPOSS-2PS 9 regioisomers giant surfactants adopt a packing model in which the two tails aggregate onto one side and the heads are partially interdigitated to minimize the interfacial area while maximizing the hydrogen bonding at the same time as illustrated in Figure 4B.52 It is reasonable to assume that as

Figure 4. Molecular packing model: (A) the bridging conformation and (B) the head-to-head packing with partial interdigitation.

the tail length increases, this molecular packing scheme would still hold. Nevertheless, for the para-isomer, it poses much more entropy penalty to bend two tails toward one side due to the constraint from the tethering points, and thus, there will also be a significant portion of molecules in bridging conformation (Figure 4A). By contrast, this packing scheme is most favored by the ortho-isomer, giving rise to the least proportion of bridging conformation. The meta-isomer is somewhere in between. The presence of bridging conformation would reduce the DPOSS domain size and broaden the interface, leading to higher free energy and thus a lower TODT. This speculation was evidenced by the facts that the dimension E

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Macromolecules of DPOSS domain for the LAM phase in p-DPOSS-2PS9 is much smaller than that of m-DPOSS-2PS9.52 The molecular packing in response to the regio-configuration should also play a critical role on impacting the order-to-order phase transitions. As illustrated in previous work, the interfacial area per molecule (A0) is an experimentally derived parameter critical for evaluating the phase stability of the system since the interfacial energy (estimated as γA0 with γ being the interfacial tension) is often the dominating contribution in the free energy of the nanophase-separated systems.52 In block copolymer system, when the two blocks possess different Rg, there will be a pressure on the interface that increases with increasing length of one block. Within the same structural domain, the pressure is alleviated by increasing the interfacial area. With further increasing length, at some point, the domain will have to adopt a curved interface to release the pressure and reduce the loss of configurational entropy, leading to the order-to-order phase transitions. The scenario also holds for the self-assembly of giant surfactants. The detailed calculation of A0 can be found in the Supporting Information, and the results are summarized in Figures 5−7 for the three regioisomers, respectively.

Figure 6. Summary of interfacial area per molecule (A0) of m-DPOSS2PSn at different temperatures in units of nm2 and the stretching parameter (S) as shown in parentheses.

Figure 7. Summary of interfacial area per molecule (A0) of o-DPOSS2PSn at different temperatures in units of nm2 and the stretching parameter (S) as shown in parentheses.

Figure 5. Summary of interfacial area per molecule (A0) of p-DPOSS2PSn at different temperatures in units of nm2 and the stretching parameter (S) as shown in parentheses.

chains thus favor a more relaxed conformation. By examining A0 and S at the same time, it is found that the phase transitions are accompanied by the reduction of the interfacial energy at the expense of further stretching the chain, confirming again that the interfacial energy is the dominant term in the selfassembly of these giant surfactants with relatively short PS tails. Phase Stability with Temperature and Tail Lengths. A brief examination of the A0 values reveals the following trends. First of all, for any sample, A0 increases with increasing temperature, and whenever an OOT occurs during heating (often for samples near the phase boundary), there is also a drop in A0. The highest A0 value in all of three systems upon heating is ∼2.03 nm2, the reason for which will be elaborated in later discussions. Elevating the temperature progressively increases the role of the configurational entropy, driving the polymer tails to become more relaxed while weakening the head-to-head interactions to increase the interfacial area. A larger A0 would imply a larger distance between DPOSS heads, which potentially weakens the collective hydrogen bonding interactions between the heads and synergistically reduces the

The phase stability and transformation are also influenced by the PS conformation, which is often characterized by the stretching parameter (S) defined as the ratio between the characteristic PS domain size (LPS) and the ideal size of the tethered PS chain (R0): S = LPS/R0. The LPS is a length characterizing the average size of the PS domain (characteristic domain size), and R0 is the average end-to-end distance of the unperturbed PS chain.50 The detailed calculation can be found in the Supporting Information, and the results are summarized in Figures 5−7. The results show that the values of S for all phases at different temperatures are around 1.0, indicating that the conformational entropy contribution is limited, and A0 is the critical parameter dominating the overall free energy of the phase structure. Generally, the S value decreases with elevated temperature for each sample, which is consistent with more relaxed conformation at higher temperatures; it also decreases with increasing f PS within each specific phase region at the same temperatures, which indicates that as the polymer tails become longer, the entropy penalty for stretching is bigger and the PS F

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Macromolecules Table 2. Phase Structure Characterizations of o-DPOSS-2PSn in the HEX Phase Region samples

f PSa

T (°C)b

phase structure

q1 (nm−1)c

d (nm)d

A0 (nm2)e

Sf

o-DPOSS-2PS9 o-DPOSS-2PS11 o-DPOSS-2PS16 o-DPOSS-2PS20 o-DPOSS-2PS25

0.69 0.73 0.78 0.82 0.84

140 140 140 140 140

HEX HEX HEX HEX HEX

0.99 1.00 0.95 0.92 0.85

6.32 6.25 6.63 6.83 7.40

1.63 1.74 1.84 1.93 1.93

1.13 1.05 0.98 0.94 0.94

a

Volume fraction of PS for DPOSS-2PSn. bTemperature at which the phase was developed and the SAXS experiments were performed. cData obtained from SAXS profiles collected in situ at specific temperature T. dData calculated from q1. eAverage interfacial area per molecule. fStretching parameter.

stability of the ordered nanostructures. When the energy gains from the hydrogen-bonding interactions as well as the reduced interfacial area associated from the phase separation are no longer able to compensate for the loss of configurational entropy, ODT occurs and thus the sample becomes completely disordered. When the length of PS tails increases, generally the A0 value increases and the S value decreases. Specifically, Table 2 summarizes the characterization data for ortho-isomers oDPOSS-2PSn forming the HEX phase at 140 °C. A systematic increase of A0 with increasing f PS is observed, indicating increasing interfacial energy. Nevertheless, once a phase boundary is crossed, a drop in A0 can be found. As a result, at relatively low f PS, increasing the length of PS tails enlarges the contribution of interfacial energy, thus increasing TODT. After passing a critical volume fraction similar to f = 0.5 in AB diblock copolymer, at which the contribution of interfacial energy reaches maximum, the entropic contribution of PS tails becomes dominant over the interfacial energy. Then TODT decreases as f PS increases (Figure 2C). The variation trend is most obvious in the ortho-isomers, and only the downward trend is observed in para- and meta-isomers due to the limited phase region and the progressive phase boundary shifts. The changing trend of TODT exhibits some similarity but also notable difference to that of flexible diblock copolymers. Note that our experimental phase diagrams are plotted with respect to f PS and temperature, where f PS is varied by increasing the length of PS tails while the volume of DPOSS head is unchanged. In other words, the total molecular weight is increased synchronously as f PS, which is different from the ordinary phase diagram of AB diblock copolymer in the f ∼ χN plane with a constant total chain length N. In order to make a direct comparison, we replot the phase diagram of AB diblock copolymer in the f B ∼ (χNA)−1 plane,57 where f B = NB/(NA + NB) is increased by increasing NB while NA is kept as constant (Figure 8). As we focus on the phase behaviors of f B > 0.5 mimicking the majority PS tails, we replot the right portion of the phase diagram. According to the usual relationship χ ∼ 1/T, (χNA)−1 can be simply regarded to be proportional to T. Although the regioisomers and the simple AB diblock copolymer are highly different, they still share the general self-assembly mechanism that the self-assembly is dictated by the competition between the interfacial energy and the entropic energy. Obviously, the variation of TODT as f B exhibits the similar nonmonotonous trend. Moreover, the phase diagrams of the two different systems resemble the similar phase sequence. From this aspect, the phase behavior of AB diblock copolymer provides a qualitative guide for those of the regioisomers. The above arguments based on the phase behavior of diblock copolymers generally hold for the current regioisomer systems,

Figure 8. Phase diagram of AB diblock copolymer replotted in the f B− (χNA)−1 plane, where f B = NB/(NA + NB) is increased by increasing NB while NA is kept constant.

except for a peculiar feature of increasing TODT once entering the BCC phase region. Judging from the trend of change with further increasing the lengths of PS tails, there may be double peak variation of TODT in these phase diagrams, which requires some more sophisticated explanations. Compared with flexible AB diblock copolymers, the regioisomers possess some unique features. The first one is that the POSS head is rigid, and thus its configurational entropy is negligible. Moreover, its excluded volume becomes a nontrivial factor. Both factors should have considerable effect on the ODT. The second feature is that collective hydrogen-bonding interactions exist between the POSS heads, which increases the immiscibility between the heads and the tails and thus favors the separation of the heads from the PS tails. As a result, the decreasing trend of TODT can be delayed by the increased effective interaction parameter χN. While there have been several reports on the self-assembly of XPOSS-mPSn (where X represents different hydrophilic functional groups and m is the number of tails ranging from 1 to 4) giant surfactants,44 the current system is quite unique in that a systematic change of ODT has been observed. In fact, the composition of DPOSS-2PSn in these two systems is comparable to previous reports except that there are only 12, rather than 14, hydroxyl groups on the POSS heads in this system. The tiny difference of only two hydroxyls dramatically reduces the TODT. It can be understood through the collective hydrogen-bonding effects among the head groups. On the other hand, it suggests that self-assembly of giant surfactants is indeed very sensitive to primary chemical structures, a feature usually unique to small-molecule systems. The reduced collective hydrogen bonding can also account for the large change of χeff upon increasing temperature, which also contributes to the ODT. Role of the Rigid Head and Regio-Configuration. For the system with identical f PS and same temperature, the A0 value generally follows the order of para ≥ meta > ortho. It is not surprising in cases when they exhibit different phases G

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Macromolecules Table 3. Phase Structure Characterizations of DPOSS-2PS25 Regioisomers samples

f PSa

T (°C)b

phase structure

q1(nm−1)c

d (nm)d

A0 (nm2)e

Sf

p-DPOSS-2PS25 m-DPOSS-2PS25 o-DPOSS-2PS25

0.84 0.84 0.84

120 120 140

HEX HEX HEX

0.88 0.87 0.84

7.16 7.19 7.47

2.00 1.99 1.91

0.91 0.92 0.95

a

Volume fraction of PS for DPOSS-2PS25. bTemperature at which the phase was developed and the SAXS experiments were performed. cData obtained from SAXS profiles collected in situ at specific temperature T. dData calculated from q1. eAverage interfacial area per molecule. fStretching parameter.

trated. It demonstrates that the minute structural difference can also endow distinct consequences on the self-assembly behaviors and suggests that the regio-configuration may serve as an additional dimension for fine-tuning the ordered phases, which may also be helpful for the design and discovery of new unconventional phases in soft matter.

considering the phase boundary shift. The rule also holds when the samples show the same phases (Table 3). It indicates that the phases of the ortho-isomers are often the most stable one among three isomers, which is consistent with their higher TODT. It may be due to the presence of higher proportion of bridging conformations in the other two regioisomers with two tails whose tethering sites are further apart from each other. The bridging conformation is expected to benefit the formation of the disordered phase at low f PS. The difference gradually tapers off as the chain length increases and becomes quite trivial for sufficiently long polymer tails. It suggests that the tethering sites have a persistent influence on the polymer chain. Within the polymer’s Kuhn length, the relative positions of the tethering sites are well preserved, and if the polymer chain is much longer, the effect is not as significant. Since the DPOSS head is an incompressible and impenetrable molecular nanoparticle, its projection area at the interface may vary from a lower value in the interdigitated double-layer model to a higher value in the exactly opposing double-layer model. Considering the diameter of DPOSS head (D) is ∼1.4 nm, the upper limit of their projected interfacial area is D2, which is ∼2.0 nm2. This is in good agreement with the experimental values. Above that value, the interaction between two heads will be dramatically weakened. This scenario is slightly different from block copolymers where the two blocks can be either stretched or compressed to accommodate changes in the interfacial area. Notably, the rigid conformation is also responsible for regio-configuration to exert its impact on the final assembly.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b02383. Experimental procedures, calculation details, SAXS data, and additional characterization data (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W.-B.Z.). *E-mail: [email protected] (W.L.). *E-mail: [email protected] (J.H.). ORCID

Yu Shao: 0000-0003-3546-2574 Shuguang Yang: 0000-0003-2257-5457 Weihua Li: 0000-0002-5133-0267 Jinlin He: 0000-0003-3533-2905 Peihong Ni: 0000-0003-4572-3213 Wen-Bin Zhang: 0000-0002-8746-0792

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Author Contributions

CONCLUSIONS In summary, we have performed a systematic investigation on the effect of regio-configuration on the self-assembly of doublechain giant surfactants. The phase structures were characterized by temperature-dependent SAXS and confirmed by bright-field TEM in real space. Three phase diagrams were constructed for the para-, meta-, and ortho-isomers. With increasing f PS, a phase transition sequence of LAM−DG−HEX−BCC is observed, which is similar to that found in the block copolymers. However, the phase boundaries between ordered structures obviously shift toward lower f PS from the para-, to meta- and ortho-isomers. Surprisingly, a nonmonotonic variation of orderto-disorder phase transition temperature (TODT) with two maximal peaks is observed, which is in contrast to that in traditional flexible diblock copolymers exhibiting only one maximum. The different phase behaviors are rationalized through the preference over different packing schemes for distinct regioisomers. It is speculated that the rigid conformation of POSS head as well as the hydrogen-bonding interactions between them plays a very important role in impacting the phase behaviors. This is the first time that the effect of regio-configuration on the self-assembly behaviors of double-chain giant surfactants has been systematically illus-

X-M.W. and Y.S. contributed equally to the work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for the financial support from the National Natural Science Foundation of China (Grants 21674003, 91427304, 21774081, and 21774025), 1000 Plan (Youth), Natural Science Foundation of Jiangsu Province (BK20171212), and Beijing National Laboratory for Molecular Sciences (20140152). We thank the Beamline 16B1 for the assistance with the SAXS experiments at the Shanghai Synchrotron Radiation Facility and Beamline 1W2A for the assistance with the SAXS experiments at the Beijing Synchrotron Radiation Facility.

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REFERENCES

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