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Highly Asymmetric Phase Behaviors of Polyhedral Oligomeric Silsesquioxanes Based Multi-Headed Giant Surfactants Mingjun Huang, Kan Yue, Jiahao Huang, Chang Liu, Zhe Zhou, Jing Wang, Kan Wu, Wenpeng Shan, An-Chang Shi, and Stephen Z. D. Cheng ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.7b08687 • Publication Date (Web): 19 Jan 2018 Downloaded from http://pubs.acs.org on January 21, 2018

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Highly Asymmetric Phase Behaviors of Polyhedral Oligomeric Silsesquioxanes Based Multi-Headed Giant Surfactants Mingjun Huang 2, Kan Yue 1,3*, Jiahao Huang 2, Chang Liu 2, Zhe Zhou 2, Jing Wang 1,3, Kan Wu 2

, Wenpeng Shan 2, An-Chang Shi 4 & Stephen Z. D. Cheng 1,2,3*

1

South China Advanced Institute for Soft Matter Science and Technology, South China

University of Technology, Guangzhou 510640, China 2

Department of Polymer Science, The University of Akron, Akron, OH 44325, USA

3

State Key Laboratory of Luminescent Materials and Devices, School of Materials Science and

Engineering, South China University of Technology, Guangzhou 510640, China 4

Department of Physics and Astronomy, McMaster University, Hamilton, Ontario, Canada L8S

4M1

ABSTRACT: This work reports the molecular design, synthesis, and systematic study on the bulk self-assembly behaviors of three series of polyhedral oligomeric silsesquioxanes (POSS)based multi-headed giant surfactants XDPOSS-PSn (X = 2, 3 and 4), which are composed of two, three or four hydrophilic hydroxyl-group functionalized DPOSS cages attached via one junction point to a hydrophobic polystyrene (PS) chain. These series of hybrid polymeric amphiphiles with precisely defined chemical structure and controllable molecular architecture are synthesized

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by the sequential usage of “click” reactions. By tuning molecular weights of the PS tail, we established full phase diagrams of XDPOSS-PSn as a function of the volume fractions of PS chains ( ). We found that the self-assembled structures were greatly influenced by the molecular architecture. Strikingly, our results showed that the lamellar morphology, which usually existed at relatively symmetric compositions in common diblock copolymers, became the thermodynamically stable phase in the 3DPOSS-PSn and 4DPOSS-PSn samples even at an asymmetric composition up to = 0.842, with the ratio between the thicknesses of PS and DPOSS lamellae up to 5.32. This unusual phenomenon induced by molecular architectural variation could be explained by the large cross section area of DPOSS cages at the nanophaseseparated domain interface and high elastic deformation energy of clustered DPOSS cages which have relatively rigid conformation. The unique bulk self-assembly behaviors in our POSS-based multi-headed giant surfactants provide insights in developing hybrid nanomaterials towards unconventional nanostructures.

KEYWORDS: POSS, asymmetric lamella, giant surfactants, phase diagram, macromolecular architecture.

Hierarchically ordered supramolecular structures generated by the self-assembly of block copolymers have attracted great interest due to their potential applications in advanced material designs and nanofabrication via the “bottom-up” approach.1-6 To better complement the traditional “top-down” lithographical techniques, extremely small feature sizes of self-assembled structures from block copolymers (e.g., sub-10 nm) are desirable.7-9 To this end, block copolymers with relatively low molecular weights (MW) combined with large Flory-Huggins interaction parameters (χ) are required.8,10 Many of the so-called “high-χ” block copolymers,


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based on silicon-11-15 or metal-containing polymers,16 polylactides,12,17,18 oligosaccharides,19-23 and other types of polymeric blocks,24-27 have been developed to ensure the thermodynamic driving force for nanophase separation at such minute length scales. Among the various ordered structures, the lamellar phase (LAM) is of particular significance because it is the simplest ordered morphology and it could be used as the template to fabricate nano-sized strips or lines. Because most of the widely studied diblock copolymer systems are composed of coil-coil polymer blocks that are symmetric in terms of molecular architecture, the resulting characteristic phase diagram is also symmetric with respect to the volume fraction. Therefore, the lamellar phase of diblock copolymers generally appears around the center of the phase diagram (50/50 in volume) within a quite narrow volume fraction region.2,3,28 As a result, it theoretically remains a challenge to tune the size ratio of each lamellae domain, especially to a highly asymmetric extent. When the volume fraction of one block is much larger than 0.5, other ordered phases with curved interfaces, such as double gyroids (DG), cylindrical and spherical phases, would become the stable phase.2,3,28 Some recent studies revealed a highly asymmetric phase by blending two diblock copolymers with specific intermolecular interactions.29-32 Achieving similar highly asymmetric lamellae phase in a single-component block copolymer system has rarely been demonstrated, except in certain well-defined oligopeptide amphiphiles.33,34 In the past few years, our group has focused on using three-dimensional (3D) shape persistent molecules35,36 (also known as molecular nanoparticles, or MNPs), such as polyhedral oligomeric silsesquioxanes (POSS)37-41 and fullerenes (C60) derivatives40,42,43 as the building blocks to prepare polymeric amphiphiles with structural precision and high χ values. When tethered with flexible polymeric chains such as polystyrene (PS), the resulting hybrid macromolecules, termed


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as giant surfactants, could be regarded as diblock copolymer analogues.36 As the simplest example of such giant surfactants, DPOSS-PSn, which is composed of a hydroxyl-functionalized POSS (DPOSS) tethered with a PS tail, could self-assemble into various well-ordered nanostructures with feature sizes below 10 nm, depending on the volume fraction of the PS tail ( ).39 More importantly, the self-assembly phase diagram of DPOSS-PSn was found to be asymmetric with respect to , deviating from that of conformationally symmetric diblock copolymers. For instance, the boundary between LAM phase and DG phases in DPOSS-PSn system appeared at around = 0.78, in contrast to 0.68 in diblock copolymers.39 This deviation could be related to the nanometer size and rigid conformation of the DPOSS cages that induce interface asymmetry.39,44 Based on this unusual behavior, we hypothesize that further amplifying the molecular architectural asymmetry of giant surfactants by tethering more than one DPOSS cage with a polymer chain could create more asymmetric interfaces and thus shift the phase boundaries of LAM phase towards even higher volume fraction of the polymer tail. This proposed architecture would therefore realize the self-assembly of a highly asymmetric LAM phase. It is worth mentioning that recently a dramatic molecular architectural effect has been observed in multitailed POSS-based giant surfactants.45 With increasing the number of PS tails, molecular geometrical variations not only affect phase boundaries in terms of the PS volume fraction, but also open a window to stabilize supramolecular Frank-Kasper and quasicrystalline phases in the spherical phase region.45 To validate this hypothesis, we designed and prepared three series of POSS-based multi-headed giant surfactants XDPOSS-PSn, namely, bi-headed giant surfactants 2DPOSS-PSn, tri-headed


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giant surfactants 3DPOSS-PSn, and tetra-headed giant surfactants 4DPOSS-PSn, where two, three or four hydrophilic hydroxyl-group functionalized DPOSS cages are attached to a hydrophobic PS chain through relatively short linkers (Figure 1), and studied their bulk selfassembly behaviors. Indeed, unconventional phase behaviors that are distinct from traditional block copolymers were observed, which are significantly influenced by the molecular geometry. Specifically, the range of the LAM phase is found to be largely dependent on the number of POSS heads. In the 3DPOSS-PSn and 4DPOSS-PSn samples, stable LAM morphology could even exist in samples with as high as 0.84, which has an asymmetric domain ratio, R, between the thicknesses of PS and DPOSS lamellar domains (R = dPS/dDPOSS) up to 5.32. The thickness of the POSS domains in the self-assembled LAM structures is generally less than 5 nm, due to the relatively fixed shape and volume of the POSS cages, while the thickness of the PS domain is between 4.6 nm and 23.0 nm. The LAM periods can be tuned from ca. 10 nm to 28.0 nm. Notably, the resulted highly asymmetric LAM phases with such small feature sizes are also highly ordered, as illustrated by the observation of up to sixth-order diffractions in small-angle X-ray scattering (SAXS) profiles and the related autocorrelation analysis showing sharp interfaces. Moreover, the hybrid nature of 3DPOSS-PSn and 4DPOSS-PSn might guarantee high etching contrast between the POSS domain and the PS domain, which could have potential applications in lithographic technologies. This study reveals the unique feature of our POSSbased giant surfactants and provides insights to access more unconventional self-assembled structures through rational molecular design. RESULTS AND DISCUSSION. Molecular design and synthesis. Our group has been developing efficient synthetic strategies to synthesize POSS-based hybrid giant surfactants.46 The reported sequential “click” approach can


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quantitatively link functionalized POSS cages through an azide group on the polymer chain.47,48 To synthesize bi-headed and tri-tailed giant surfactants, two “click” adaptor molecules47,48 bearing both alkyne and two or three bromo groups were designed to change the number of azide groups in a PS tail (see Scheme S1 and S2). All the synthetic details and characterization data such as 1H-NMR, FT-IR, GPC, and MALDI-TOF MS results confirming the successful synthesis of multi-headed giant surfactants are provided in the Supporting Information. For the synthesis of 4DPOSS-PSn samples, we prepared a dendron-like molecule containing one alkyne group and four vinyl-functionalized POSS cages, and applied it in the sequential click strategy (Scheme S3). PS tails with different molecular weights are used to prepare three sets of multi-headed giant surfactants samples with different . This molecular design systematically varies the molecular geometry of giant surfactants by increasing the number of POSS heads, providing an ideal prototype platform to investigate the molecular architecture effect on the self-assembly of POSS-based multi-headed giant surfactants. Note that the introduction of fourteen hydroxyl groups on each DPOSS cage could result in a very high apparent χ value between DPOSS cages with PS tails,44 which also provides the sufficient thermodynamic driving force for self-assembly of amphiphilic DPOSS-based giant surfactants.


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Figure 1. Chemical structures of giant surfactants XDPOSS-PSn studied in this work. The blue spheres in cartoons represent the hydrophilic DPOSS cages while the pale red coils represent the hydrophobic PS tails. Self-assembly in the Bulk State. Since the PS tails in all the samples are shorter than the critical entanglement length of PS (ne = 250),49 self-assembled ordered structures could be easily obtained by a simple thermal annealing process at 150 °C. Kinetics of the ordered structure formation is in general fast, which is in good agreement with our previous studies on other POSS-based giant surfactants with relatively short PS tails.39,44,45 It typically requires a few minutes to less than 2 hours for the system to reach the final steady states. We combined SAXS profiles and transmission electron microscopy (TEM) results to identify phase formation of the samples. Detailed sample preparation procedures are included in the SI. The resulting structural identification of all the giant surfactants and several


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key parameters are summarized in Table 1. As a reference, for the previously reported DPOSSPSn system, the LAM phase ends at ca. = 0.77.39 Comparing to DPOSS-PSn, the bi-headed giant surfactant 2DPOSS-PSn has two functionalized POSS cages linked to one end of a PS tail through a common junction point. A series of five 2DPOSS-PSn samples with different PS tail lengths are studied (with = 0.641 to 0.826). Figures 2a - 2e show a set of SAXS patterns of the thermally treated 2DPOSS-PSn samples recorded at room temperature. The appearance of multiple diffraction peaks clearly indicated the formation of ordered structures. Within the region between 0.641 and 0.802, the selfassembled structures were all LAM phases, as supported by the q/q* ratio of the diffraction peaks being 1:2:3:…. It was found that the LAM period (d = dDPOSS + dPS) increased from 10.2 nm for 2DPOSS-PS33 to 15.9 nm for 2DPOSS-PS80 as shown in Table 1. Since the molecular weights of the DPOSS cages remained unchanged for these samples, the increased LAM period should be largely attributed to increased PS tail length. The individual thickness, dDPOSS and dPS, of the domains could be calculated by dDPOSS = d1 and dPS = d1 . As shown in Table 1, we found that dDPOSS values were in between 3.18 – 3.74 nm, with a slightly decrease with increasing the PS molecular weights and overall LAM periods. The self-assembled LAM structures were also confirmed by bright-field (BF) TEM images of microtomed thin slice samples (Figures 2f - 2j). Note that all these TEM images were obtained without any staining procedure. The image contrast is solely due to the high electron density difference between the silicon containing DPOSS domains and the PS domains. The darker and narrow lines were assigned to the DPOSS domains, while the lighter wide domains were composed of the PS tails. As the samples were not treated by the staining process, the measured


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domain widths in the TEM images should be close to the real values. Indeed, they matched well with the calculations based on volume fraction and the autocorrelation analysis. For example, the dDPOSS in 2DPOSS-PS63 was determined as 3.25 nm from the image shown in Figure 2h, and the calculated value was 3.37 nm.


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Table 1. Summary of Self-Assembled Structures of XDPOSS-PSn in the Bulk

a

Sample

Morphology

a

d1b

dDPOSSc

dPSd

Rge

Sf

Ag

DPOSS-PS15

LAM

0.623

7.41

2.81

4.64

1.08

1.08

1.06

DPOSS-PS23

LAM

0.706

8.10

2.38

5.72

1.33

1.08

1.32

DPOSS-PS33

LAM

0.770

9.02

2.07

6.95

1.59

1.09

1.56

2DPOSS-PS33

LAM

0.641

10.2

3.74

6.36

1.59

1.00

1.71

2DPOSS-PS38

LAM

0.669

10.9

3.59

7.27

1.71

1.06

1.72

2DPOSS-PS63

LAM

0.762

14.2

3.37

10.81

2.20

1.22

1.92

2DPOSS-PS80

LAM

0.802

15.9

3.18

12.73

2.48

1.28

2.07

2DPOSS-PS96

HEX

0.826

14.0

7.08

9.10

2.72

0.84

---

3DPOSS-PS19

Inversed HEX

0.440

10.6

3.71

8.52

---

---

---

3DPOSS-PS42

LAM

0.602

10.8

4.31

6.50

1.80

0.90

2.12

3DPOSS-PS96

LAM

0.762

17.1

4.06

13.03

2.72

1.20

2.42

3DPOSS-PS135

LAM

0.816

20.1

3.70

16.42

3.23

1.27

2.70

3DPOSS-PS163

LAM

0.842

22.0

3.48

18.51

3.54

1.31

2.89

3DPOSS-PS192

Ill-defined PFL

0.862

21.1

---

---

---

---

---

4DPOSS-PS38

Inversed HEX

0.509

10.9

3.15

9.42

---

---

---

4DPOSS-PS65

LAM

0.626

16.7

6.24

10.41

2.24

1.17

2.04

4DPOSS-PS87

LAM

0.687

18.5

5.78

12.70

2.59

1.23

2.25

4DPOSS-PS125

LAM

0.756

21.7

5.32

16.42

3.10

1.32

2.50

4DPOSS-PS192

LAM

0.824

27.8

4.90

22.90

3.85

1.49

2.76

4DPOSS-PS221

Ill-defined PFL

0.843

27.6

---

---

---

---

---

The volume fraction of PS is given by = MPS/ρPS/(MPS/ρPS + MDPOSS/ρDPOSS), where the

density values are 1.05 g/cm3 for PS and 1.43 g/cm3 for the DPOSS cage. For convenience, the extra molecular weight of adapter linkers were treated as in PS domain; b These values are the


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corresponding d-spacing of first order peaks calculated by d1 = 2π/q1 ; c Domain size of DPOSS in LAM phase is calculated by: dDPOSS = d1* (1- ). In HEX phase the diameter of the DPOSS cylinder domain can be calculated according to dDPOSS = 2*[(2*(1- )*d12)/(√3π)]1/2; d Domain size of PS in LAM phase is calculated by: dPS = d1 . In HEX phase the size of the PS matrix domain can be calculated according to dPS = 2 d1/√3 - dDPOSS;

e

The radius of gyration of PS

chain is calculated by: = (/6)/ , where b = 0.68 nm and n is the degree of polymerization of PS;50 f The stretching ratio (S) of PS chains is characterized by S = L/2Rg; area between DPOSS and PS domains =

g

The interfacial

.

Importantly, a comparison between the 2DPOSS-PSn system with the DPOSS-PSn system revealed significant molecular architecture effects on the self-assembly behaviors. The presence of two clustered DPOSS and one PS tail connected at the interface not only resulted in a larger cross-section area per molecule (see Table 1) than that of the DPOSS-PS system, but also created higher conformational asymmetry. As a result, the higher limit for the LAM phase in the 2DPOSS-PSn system was observed at ca. 0.802, which was larger than that in DPOSS-PSn system ( = 0.770).39 It should also be noted that at = 0.802, the asymmetric ratio R, defined as R = dPS/dDPOSS, reached at least 4.00 in 2DPOSS-PS80. This observation supports our hypothesis that increasing the architectural asymmetry could further shift the phase boundaries towards being more asymmetric. Moreover, the sample of 2DPOSS-PS96 exhibited a hexagonally packed cylindrical (HEX) phase structure based on the SAXS pattern with a scattering vector q ratio of 1:√3:√4: (Figure 2e) and a hexagonal packing scheme of columns in the BF TEM image (Figure 2j). At even larger , the thermally treated samples did not exhibit highly ordered structures. For example, the SAXS pattern of 2DPOSS-PS115 showed relatively broad higher


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order scattering peaks (Figure S12a), which were close to an ill-defined HEX structure. Only partial and ill-defined features of columns and parallel layers were observed in the BF TEM image of microtomed 2DPOSS-PS115 samples (Figure S12b). Neither ordered DG nor body centered cubic (BCC) phase was observed before entering the disordered structure.

Figure 2. (a-e) SAXS patterns of five representative 2DPOSS-PSn samples collected at room temperature after thermal annealing at 150 °C and (f-j) their corresponding BF TEM images of microtomed thin slices. The scale bars are 30 nm. The asymmetric molecular geometry could be further amplified in the 3DPOSS-PSn and 4DPOSS-PSn systems, which exhibited similar phase behaviors as shown in Figures 3 and 4. With a short PS tail and three DPOSS heads, 3DPOSS-PS19 ( = 0.440) showed an inverse HEX phase (Figure 3a), which has not yet been observed in the single-headed DPOSS-PSn or biheaded 2DPOSS-PSn. From the BF-TEM image (Figure 3f), the PS domains were located at the lighter area embedded in the darker DPOSS matrix, which exhibited hexagonal shaped boundary and represented a honeycomb-like frame structure. The thickness of the DPOSS domain was


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measured around 3.5 nm, stabilized by strong hydrogen bonding interactions between the hydroxyl groups at the peripheries of DPOSS cages. 3DPOSS-PSn samples with longer tails (n from 42 to 163) fell into the LAM phase region. For example, as shown in Figure 3d, the SAXS pattern of 3DPOSS-PS135 exhibited five diffraction peaks with the scattering vector q ratio being 1:2:3:4:6, clearly indicating the formation of highly ordered LAM morphology. The domain period calculated from the primary scattering peak for 3DPOSS-PS135 was 20.1 nm, and the thickness of the DPOSS domains was calculated to be 3.70 nm. The missing fifth order peak in SAXS data could be attributed to extinction, due to that the thickness ratio of the DPOSS and the PS domains was ~ 1:4. As the PS tail lengths increased from n = 42 to n = 163, the LAM period changed from 10.8 nm to 22.0 nm and the asymmetric ratio R increased from 1.51 to 5.32.

Figure 3. (a-e) SAXS patterns of representative 3DPOSS-PSn samples collected at room temperature after thermal annealing at 150 °C and (f-j) their corresponding BF TEM images of microtomed thin slices. The scale bars are 30 nm.


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Again, the highly asymmetric domain size in these LAM structures could be directly observed in BF-TEM images (Figures 3g - 3j) of the microtomed 3DPOSS-PSn samples without staining. It was evident that the widths of the DPOSS domains (darker lines) remained almost identical, while the widths of the PS domains (lighter area) became much broader as the lengths of PS tails increase. Furthermore, as shown in Figure S13, we also conducted autocorrelation analysis to the SAXS profile of 3DPOSS-PS96 sample, which supported the excellent orders and sharp interfaces of the formed LAM structures. Notably, the average dDPOSS for 3DPOSS-PS96 measured from the TEM images (3.85 nm) roughly matched with the calculated values (4.06 nm) from SAXS data using volume fractions and from the autocorrelation analysis (4.11 nm). The 3DPOSS-PS192 sample failed to self-assemble into well-ordered structures even after prolonged thermal annealing procedure (12 hours at 150 °C), as revealed by the poor quality of the SAXS pattern (Figures S14a). The apparent period calculated from the first order peak was 21.1 nm, which was smaller than the 3DPOSS-PS163 with shorter PS tails. We deduced that 3DPOSS-PS192 should have already passed the LAM phase region and entered into HEX or other phase structures, which could hardly be identified due to poor quality SAXS profile. BF-TEM images of microtomed slices showed certain ill-defined features of a perforated lamella (PL) with column structures perforated into the lamella (Figure S14b). When the molecular weight of PS tails further increased, the resulting SAXS profile became more disordered (Figure S15), not showing any ordered HEX or BCC structure. This indicated that forming curved interface in this series of sample was strongly resisted by the molecular architecture. Briefly, the boundary of the LAM phase from this 3DPOSS-PSn system could be approximately determined in a region between 0.602 and 0.842. To our knowledge, LAM morphologies with such small feature sizes with high orders and asymmetry are unprecedented in common amphiphilic block polymers.


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The 4DPOSS-PSn system exhibited very similar phase behaviors as 3DPOSS-PSn samples shown in Figure 4. At the low region ( = 0.509), 4DPOSS-PS38 showed an inverse HEX structure. The LAM structures were found between = 0.626 to 0.824 while an ill-defined perforated lamellar (PFL) structure was observed in 4DPOSS-PS221 ( = 0.843), shown in Figure S16. At even higher , rather disordered structure was observed. The dDPOSS of LAM phase in 4DPOSS-PSn ranged from 16.8 nm to 27.7 nm, and highly ordered LAM phase up to fifth order peaks could still be observed in 4DPOSS-PS192.

Figure 4. (a-e) SAXS patterns of representative 4DPOSS-PSn samples collected at room temperature after thermal annealing at 150 °C and (f-j) their corresponding BF TEM images of microtomed thin slices. The scale bars are 30 nm. Based on these results, we could construct a phase diagram (Figure 5) to summarize the phase behaviors and transition sequences of these four series of giant surfactants. It is evident that the phase boundaries of LAM phases shifted to higher values with an increasing number of DPOSS heads in 2DPOSS-PSn and 3DPOSS-PSn, yet reaching kind of saturation in 4DPOSS-


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PSn. This is caused by the presence of multiple rigid DPOSS heads at the junction point that create larger cross-section areas of DPOSS cages and more conformational asymmetry at the domain interfaces between DPOSS and PS. Interestingly, in the 4DPOSS-PSn series, the illdefined PFL phase appeared at even smaller PS volume fractions than in the 3DPOSS-PSn series (Figure 5). We reason that this might be attributed to the relatively long linkers between each POSS cage with the PS chain in the 4DPOSS-PSn samples, due to the use of a dendron-like molecule to attach four DPOSS cages to a PS chain. In general, a longer linker leads to a higher degree of freedom for the clustered DPOSS cages to arrange themselves, which to certain extend, lowers the molecular conformational asymmetry at the junction point. In addition, the window for phase structures with curved interface (DG, HEX, and BCC) was largely squeezed in the 2DPOSS-PSn system and finally suppressed in the 3DPOSS-PSn and 4DPOSS-PSn systems. This is in great contrast to the reported phase diagram of the DPOSS-PSn system, which showed a highly ordered bicontinuous double gyroid phase adjacent to the LAM phase.39 We reason that in multi-headed giant surfactants, the clustering of multiple DPOSS cages through relatively short linkers might limit the freedom of molecular packing into ordered structures with intrinsic curvature. As a result, even at asymmetric compositions, ordered phases with intrinsic interfacial curvature are not stabilized. At very high values, instead of packing into ordered phases with large curvature, these multi-headed giant surfactants would rather stay in the relatively disordered phase to lower free energy penalty associated with the restrained packing of the clustered DPOSS cages. Taking 3DPOSS-PS192 as an example, if the HEX structure was formed, the domain size of DPOSS columns would be calculated as 9.51 nm based on its volume fraction, in strong contrast to 3.48 nm in the LAM of 3DPOSS-PS163. Considering the size range between 3 nm to 5 nm of compact DPOSS domain packing, it would be inevitable


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to introduce large volumes of void spaces in the center of cylinder structure, which is energetically disfavored.

Figure 5 Experimental phase boundary diagrams of multi-headed giant surfactants XDPOSS-PSn with different molecular geometry. Regions for different phases are shown in different colors as indicated by the legends. The black squares, red dots, blue triangles, and purple triangles represent experimentally accessed data points for DPOSS-PSn, 2DPOSS-PSn, 3DPOSS-PSn, and 4DPOSS-PSn, respectively. Giant surfactant packing model and PS tail conformation. An interesting question is how the giant surfactants arrange themselves in these self-assembled structures, particularly in the highly asymmetric LAM phases. A double-layered giant surfactant packing model could be constructed based on the measured widths of the DPOSS domains (Scheme 1). The DPOSS cages are holding together by the collective inter- and intra-molecular hydrogen bonding interactions to form their own domains; while the PS tails form separated domains. We have raised the temperature to their decomposition temperatures of the samples and no order-disorder transitions could be observed, indicating that the systems are in the strong


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phase separation regions (Figures S17 and S18). As shown in Table 1, dDPOSS values have large increase from DPOSS-PSn (2.80 nm – 2.07 nm from n = 15 to 33), to 2DPOSS-PSn (3.74 nm – 3.18 nm from n = 33 to 80), 3DPOSS-PSn (4.31 nm – 3.48 from n = 42 to 163), and 4DPOSSPSn (6.24 nm to 4.90 nm from n = 65 to 192). Moreover, it is clearly observed that dDPOSS values decrease with increasing PS tail lengths in each series of samples (Table 1). If we consider the DPOSS cages as 3D nano-objects with fixed volumes, the decreased dDPOSS value following the increasing PS chain length should be associated with an increased cross-sectional area per molecule, which is dominated by the conformation of DPOSS cages. This observation strongly suggests that although the DPOSS cages possess a relatively rigid molecular structure, they could still be slightly deformed in the self-assembled phases to adopt an ellipsoidal or rugby-like shape (Scheme 1). In this case, the DPOSS cages are more expanded along the direction perpendicular to the normal direction of the LAM interface to partially release the crowdedness of the PS tails and decrease the PS stretching. Scheme 1. The packing model of representative multi-headed giant surfactant 3DPOSS-PSn in double-layered LAM structure


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On the other hand, the conformation of the PS chains in the self-assembled LAM structures could be estimated by comparing the thickness of the PS layer L in the LAM structure (L = dPS/2) and the radius of gyration of the PS chains Rg.43 The stretching ratio S of the PS chains could thus be defined as S = L/2Rg. A value of S larger than unity would indicate that the PS chains are stretched perpendicular to the lamellae. The stretching ratio of different giant surfactants in LAM phase region are summarized in Figure 6. From the calculated S, it is obvious that a positive correlation exists between the S-value and the length of the PS tails. For example, the S value increases from 0.90 for 3DPOSS-PS44, to 1.20 for 3DPOSS-PS96 and 1.27 for 3DPOSS-PS135 and finally, to 1.31 for 3DPOSS-PS163. At the same time, this trend of increasing S values with n is accompanied with a decrease in the dDPOSS (from 4.31 nm in 3DPOSS-PS44 to 3.48 nm in 3DPOSS-PS163). On the other hand, at the same degree of polymerization of the PS tail, the S value decreases with increasing the number of DPOSS head due to an increased cross-section area. Our previous studies of different mono-headed giant surfactants indicate that experimentally observed morphology transitions would occur successively (LAM → HEX → BCC) when the estimated S values in specific morphology exceed 1.1, to make S values close to unity (the PS-stretching free phases).51 However, the hydrogen bonding interactions between relatively rigid DPOSS cages and the covalently clustering of multiple DPOSS cages in multiheaded giant surfactants prevent such phase transitions and stabilize the LAM phase structure under strong stretching condition (e.g., S = 1.49 in 4DPOSS-PS192).


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Figure 6 Stretching ratio of PS chains in XDPOSS-PSn giant surfactants with different molecular geometries. To understand the driving force for the architectural effect on the morphology, we estimate the free energy of the self-assembled LAM structures as the sum of the three terms: # = # $ % $ #

(1)

where FDPOSS is the term describing the contribution from the DPOSS cages, γA represents the interface energy between the DPOSS domains and the PS domains, and # is the free energy due to the stretching of the PS chain. It is assumed that DPOSS cages form an elastic layer due to the strong H-bonding interactions. Each PS chain is associated with an interfacial area per chain, A. For a given value of A, the deformation energy of the DPOSS cages could be written as *

# = &'( ) , , where - is the area per chain of an undeformed DPOSS layer. On the * +

other hand, the stretching energy of the PS chain is given by # =

.

. /0

'(. The area per chain is


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related to the PS thickness via the volume conservation, =

12

, where 3 is the volume per

segment of the PS chain and n is the number of segments. Taking all these contributions together, the free energy per chain of the lamellae could be estimated as, 4

*

8

∝ & )* , $ 56 $ 19. ) 56 +

12 *

,

(2)

The equilibrium area per chain or thickness L is determined by minimizing the free energy per chain, . 2

9.

:* *.+

8

. 1 2

$ 56 − 9.

*
*.+

8

$ 56 ? =

. A general solution of this equation can be obtained but the expression is quite complex. It

is instructive to examine the solutions of this equation at two extreme cases. The first case is when the interfacial energy dominates the elastic deformation energy, leading to

8

? ~ 56

. 2

9.

or

~/?. The corresponding PS domain thickness scales as A~/~/?. This is the well-known scaling relation for diblock copolymers. On the other hand, the solution of the interfacial area per chain scales as B ~ or ~/B if the elastic deformation energy of DPOSS cages dominates over the interfacial energy. In this case the thickness would scale as the PS chain length as A~/~?/B . We then fitted the scaling relations by power function: A = C2 (Table S1). Here DPOSS-PSn series shows a scaling law of A~-.E. This relation indicates that in DPOSS-PS samples, the interfacial energy between DPOSS cages and PS chains plays the major role in dictating the LAM structure formation, and the entropic effect of the PS chains is similar to that in traditional diblock copolymers.


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However, the multi-headed giant surfactants, 2DPOSS-PSn, 3DPOSS-PSn, and 4DPOSS-PSn all transit to another extreme case with scaling law close to A~-.FE , in which the elastic deformation energy of DPOSS cages dominates over the interfacial energy. Although the DPOSS cages possess a relatively rigid molecular structure, they could still be slightly deformed to adopt ellipsoidal or rugby-like shapes (Scheme 1). In these multi-headed giant surfactants, increasing the PS tail length would be accompanied with more expended conformation of DPOSS cages along the direction parallel to LAM interface and increased cross-sectional area A per molecule. Nevertheless, the deformation of relatively rigid DPOSS cages is energy disfavored. In this case the LAM structure would become thermodynamically stable due to minimization of the DPOSS elastic deformation energy even at very high values, at the penalty of increasing PS stretching energy. The stability of highly asymmetric phase lamellae, especially in the case of 3DPOSS-PSn and 4DPOSS-PSn systems, could be understood by the following arguments: (1) Due to the cage structure and relatively rigid conformation of the DPOSS cages, the interfacial cross-sectional area of each giant surfactant can possess a relatively large value even at very low volume fraction of DPOSS composition, which efficiently help reduce the stretching of the PS tails by reaching an equilibrium between the interfacial energy cost and the entropic penalty from chain stretching; (2) When > 0.84, no comparably well-ordered structures can be experimentally observed in the 3DPOSS-PSn and 4DPOSS-PSn systems, suggesting that the formation of HEX phase (or DG and BCC phase, if any) is at least kinetically slow, or even thermodynamically unfavorable due to significant packing constrains of DPOSS cages (the energy term contributed from DPOSS cages, FDPOSS significantly increases, see Eq. 1). As a result, the free energy of HEX phase might even be higher than that of the disordered phase. (3) The observed transition at


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high is from LAM phase to ill-defined PL structure with poor order. The PL structure can simultaneously compromise the DPOSS cage packing in LAM matrix and PS chain stretching in column formations. Besides, in the relatively disordered PL phase, the partial mixing of PS chain and DPOSS cages provides extra entropy gain to minimize the free energy of the whole system. Overall, the formation of highly asymmetric LAM phase comes from the balance of DPOSS cage heads packing, PS chain stretching and their interfacial tension. The clustering of multiple DPOSS cages with short linkers limited the freedom of molecular packing into ordered structures with curvature, while the strong segregation tendency between DPOSS and PS favors the phase separation process. CONCLUSION. In summary, the molecular design, synthesis, and bulk self-assembly behaviors of three series of POSS-based multi-headed giant surfactants, 2DPOSS-PSn, 3DPOSS-PSn, and 4DPOSS-PSn are reported. Increasing the number of POSS cages not only leads to a larger conformational asymmetry at the interfaces, but also introduces enhanced packing constrains of the DPOSS cages into curved phases. As a result, significant influence of molecular architecture on the selfassembly behaviors of these POSS-based giant surfactants have been observed. The full phase diagram between and molecular geometry is established. Especially, the LAM phase covers the range of from 0.602 to 0.842 in 3DPOSS-PSn and 0.626 to 0.824 in 4DPOSS-PSn, revealing a highly asymmetric LAM phase with an asymmetric ratio R up to 5.32. The stability of such highly asymmetric phase is explained by analyzing the free energy terms. In addition, the window for phase structures with curved interface (DG, HEX, and BCC) is largely squeezed in 2DPOSS-PSn and finally suppressed in 3DPOSS-PSn and 4DPOSS-PSn. These results demonstrate a class of giant surfactants with unusual phase behaviors not yet been achieved in


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traditional flexible block copolymers, thus providing insights to the development of selfassembling polymer materials. EXPERIMENTAL SECTION. Small-Angle X-ray Scattering (SAXS). SAXS data were collected using a Rigaku MicroMax 002+ instrument with a multiwire area detector and a microfocus sealed copper tube, at a working voltage of 45 kV, a current of 0.88 mA, with the wavelength of the X-ray being 0.154 nm. The scattering vector q was calibrated using silver behenate as the standard with the primary reflection peak located at q = 1.067 nm−1. A typical recording time for each sample was 10−20 min depending on scattering intensity. Data analysis was performed with the SAXSgui software. Transmission Electron Microscopy (TEM). Bright field TEM images of thin-sliced bulk samples were recorded on a JEOL-1230 TEM instrument at an accelerating voltage of 120 kV using a digital CCD camera. Data processing was completed with the accessory digital imaging software. Sample Preparation. Samples for SAXS experiments were prepared by loading dried samples into a customized aluminum sample holder (15 mm × 10 mm × 1 mm) with a hole (5 mm in diameter) in the center. The aluminum holders were then sealed with Kapton tapes, heated to a desired temperature under nitrogen atmosphere, and annealed at that temperature for a desired time. Formation of ordered nanostructures was monitored by SAXS measurements. For TEM experiments, thin slices of the annealed samples were obtained using a Leica EM UC7 Ultracryomicrotome equipped with a Diatome Cryo diamond knife at room temperature, and subsequently transferred to copper grids coated with amorphous carbon. Typical thickness of the slices was around 70–100 nm. Typically no staining was necessary unless otherwise stated.


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ASSOCIATED CONTENT Supporting Information. The supporting Information is available free of charge on the ACS Publications website: http://pubs.acs.org. Materials, experimental methods, synthesis details, additional figures and tables as described in the text (PDF). AUTHOR INFORMATION Corresponding Author * Corresponding author: [email protected] (K.Y.), [email protected] (S.Z.D.C.). Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21774038), the Guangdong Innovative and Entrepreneurial Research Team Program (No. 2016ZT06C322), the National Key R&D Program of China (No. 2017YFC11050003), and the NSF grant (DMR1408872). A.-C. S acknowledges the support by the Natural Science and Engineering Research Council (NSERC) of Canada. K.Y. thanks the Youth Thousand Talents Program of China and the Fundamental Research Funds for the Central Universities (No. 2017JQ001) for support. REFERENCES (1) Kim, H.-C.; Park, S.-M.; Hinsberg, W. D. Block Copolymer Based Nanostructures: Materials, Processes, and Applications to Electronics. Chem. Rev. 2010, 110, 146-177.


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Cheng, S. Z. D. Giant Molecular Shape Amphiphiles Based on Polystyrene–Hydrophilic [60]Fullerene Conjugates: Click Synthesis, Solution Self-Assembly, and Phase Behavior. J. Am. Chem. Soc. 2012, 134, 7780-7787. (44) Yue, K.; Liu, C.; Huang, M.; Huang, J.; Zhou, Z.; Wu, K.; Liu, H.; Lin, Z.; Shi, A.-C.; Zhang, W.-B.; Cheng, S. Z. D. Self-Assembled Structures of Giant Surfactants Exhibit a Remarkable Sensitivity on Chemical Compositions and Topologies for Tailoring Sub-10 nm Nanostructures. Macromolecules 2016, 50, 303–314. (45) Yue, K.; Huang, M.; Marson, R. L.; He, J.; Huang, J.; Zhou, Z.; Wang, J.; Liu, C.; Yan, X.; Wu, K.; Guo, Z.; Liu, H.; Zhang, W.; Ni, P.; Wesdemiotis, C.; Zhang, W. B.; Glotzer, S. C.; Cheng, S. Z. Geometry Induced Sequence of Nanoscale Frank-Kasper and Quasicrystal Mesophases in Giant Surfactants. Proc. Natl. Acad. Sci. U. S. A. 2016, 113, 14195-14200. (46) Li, Y.; Dong, X.-H.; Zou, Y.; Wang, Z.; Yue, K.; Huang, M.; Liu, H.; Feng, X.; Lin, Z.; Zhang, W.; Zhang, W.-B.; Cheng, S. Z. D. Polyhedral Oligomeric Silsesquioxane Meets “Click” Chemistry: Rational Design and Facile Preparation of Functional Hybrid Materials. Polymer 2017, 125, 303-329. (47) Yue, K.; Liu, C.; Guo, K.; Yu, X.; Huang, M.; Li, Y.; Wesdemiotis, C.; Cheng, S. Z. D.; Zhang, W.-B. Sequential “Click” Approach to Polyhedral Oligomeric Silsesquioxane-Based Shape Amphiphiles. Macromolecules 2012, 45, 8126-8134. (48) Yue, K.; Liu, C.; Guo, K.; Wu, K.; Dong, X. H.; Liu, H.; Huang, M. J.; Wesdemiotis, C.; Cheng, S. Z. D.; Zhang, W. B. Exploring Shape Amphiphiles Beyond Giant Surfactants: Molecular Design and Click Synthesis. Polym. Chem. 2013, 4, 1056-1067. (49) Fetters, L. J.; Lohse, D. J.; Milner, S. T.; Graessley, W. W. Packing Length Influence in Linear Polymer Melts on the Entanglement, Critical, and Reptation Molecular Weights. Macromolecules 1999, 32, 6847-6851. (50) Tanaka, K.; Yoon, J.-S.; Takahara, A.; Kajiyama, T. Ultrathinning-Induced Surface Phase Separation of Polystyrene/Poly(Vinyl Methyl Ether) Blend Film. Macromolecules 1995, 28, 934938. (51) Hsu, C.-H.; Dong, X.-H.; Lin, Z.; Ni, B.; Lu, P.; Jiang, Z.; Tian, D.; Shi, A.-C.; Thomas, E. L.; Cheng, S. Z. D. Tunable Affinity and Molecular Architecture Lead to Diverse SelfAssembled Supramolecular Structures in Thin Films. ACS Nano 2015, 10, 919-929.


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