Symmetry-Dictated Mesophase Formation and Phase Diagram of

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Symmetry-Dictated Mesophase Formation and Phase Diagram of Perfluorinated Polyhedral Oligomeric Silsesquioxanes Yu Shao,†,‡ Xian Xu,† Guang-Zhong Yin,‡ Shuai-Yuan Han,‡ Di Han,§ Qiang Fu,*,§ Shuguang Yang,*,† and Wen-Bin Zhang*,‡

Macromolecules Downloaded from pubs.acs.org by EAST CAROLINA UNIV on 03/12/19. For personal use only.

†

Center for Advanced Low-Dimension Materials, State Key Laboratory for Modification of Chemical Fibers and Polymer Materials, College of Material Science and Engineering, Donghua University, Shanghai 201620, P. R. China ‡ Beijing National Laboratory for Molecular Sciences, 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 § College of Polymer Science & Engineering, State Key Laboratory of Polymer Materials Engineering, Sichuan University, Chengdu 610065, P. R. China S Supporting Information *

ABSTRACT: We report the symmetry-dictated mesophase formation and phase diagram of a series of T8-polyhedral oligomeric silsesquioxane (POSS) derivatives bearing perfluoroalkyl chains and hydroxyl alkyl groups. The phase structures and phase behaviors of these molecular Janus particles were studied by DSC, POM, FT-IR, and WAXD experiments. It was found that introducing one hydroxyl alkyl group leads to a decreased crystal melting point, and incorporating two hydroxyl alkyl groups at different POSS positions causes the formation of lamellar liquid crystal mesophases with feature sizes and transition temperatures that depend on the symmetry of these regioisomers. Interestingly, installing only one substituent with two hydroxyl groups leads to monotropic phase transitions, where the mesophase appears only upon cooling from the isotropic melt within the narrow supercooling window. Phase diagrams were systematically constructed for these compounds and understood based on the fine influence of symmetry on their possible molecular packing scheme and their distinct hydrogen bonding patterns, as evidenced by their infrared spectra at different temperatures. These results are not only fundamental to understanding the effects of regioisomerism on giant molecule self-assembly but also important for the development of hybrid materials with tailored nanostructures.

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between small molecules and block copolymers.3,4 These molecules’ self-assemblies exhibit remarkable sensitivity to their primary structures, which provides a versatile platform for the fundamental study of structure−property relationships.4 Giant molecules are typically constructed based on molecular nanoparticles that include, but are not limited to, polyhedral oligomeric silsesquioxanes (POSS), [60]fullerene, polyoxometalate (POM), and globular proteins.4−7 Being particle-like and shape-persistent, these molecules possess precise surface functional groups to direct their self-assembly. This “molecular Lego” approach is powerful in preparing monodisperse giant molecules with high molecular weight and controlled sequences and topology.8−10 Among these molecules, POSS compounds stand out due to their high molecular symmetry and multiple vertices that can be easily functionalized. Based on the T8-POSS scaffold, many Janus derivatives have been prepared, such as

INTRODUCTION Symmetry is a critical molecular parameter that dictates the molecular packing and resulting properties.1 While high symmetry is aesthetically appealing, symmetry breaking is often considered the cause of diversity and plays an important role in determining a material’s property.2 This is best exemplified in regio- or stereoisomers with distinct properties and bioactivities as typically observed in small molecules. Intriguingly, isomerism has been a relatively understudied topic in macromolecular science. Although the significance of controlling regio- and/or stereochemistry in synthetic macromolecules is clearly demonstrated in the dramatically different physical properties of isotactic, syndiotactic, and atactic polymers, these isoforms actually result from the collective behavior of multiple stereocenters instead of the change of one single chiral center/regio-configuration as often observed in small molecules or biomacromolecules. It has thus become a challenge to achieve a comparable level of precision and sensitivity in synthetic macromolecules. In recent years, giant molecules have emerged as a special class of precise macromolecules bridging the gap © XXXX American Chemical Society

Received: January 19, 2019 Revised: February 26, 2019

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DOI: 10.1021/acs.macromol.9b00138 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Scheme 1. Chemical Structure and Symmetry of Perfluorinated POSS Compounds

monoadducts,11 regioisomeric diadducts and triadducts,12,13 and even the tetra-adduct regioisomers.14,15 We have previously achieved the successful separation of different regioisomers in moderate yields which facilitates the study of their structure− property relationships.12,13 It has been shown that the regioconfiguration does exert a profound influence on the selfassembly of regioisomeric giant surfactants and shape amphiphiles, causing systematic changes in phase structure and phase stability.16−19 It remains to be demonstrated if such effects are general to other giant molecules, such as the molecular Janus particles.2,4,20 Fluorinated compounds possess salient features far different from their alkyl analogues, including good chemical and thermal stabilities, low surface free energy, low friction coefficient, biocompatibility, and nonstick behavior.21−27 Functionalizing POSS with perfluoroalkyl chains imparts unique properties to the resulting FPOSS compounds.28 For example, Mabry and coworkers29−32 have shown that superoleophobic surfaces can be obtained by blending polymers with FPOSS. Similar studies have also been reported on surface modification to obtain superomniphobic surfaces.28,33−38 In addition, fluorous interactions have been used to build up materials with complex hierarchical structures. For example, one of the topological isomers of PS-b-PEO block copolymer tethered with FPOSS at different locations forms a concentric lamellae structure due to the intrinsic truncated-wedge-like shape of the building blocks.39 The rigid-rod-like conformations of perfluorous alkyl chains are also important components in liquid crystalline small molecules40−43 and polymers.44−48 In this article, we report the synthesis and characterization of a series of molecular Janus particles based on FPOSS with the general formula of T8Fm(OH)n. These particles are derived from Oh-symmetric T8F8 with continuous symmetry breaking, thereby constituting a family of Janus FPOSS compounds with distinct symmetry. Upon progressively lowering the symmetry

and the introduction of hydroxyl groups (Scheme 1), liquid crystalline mesophases with varying stability in their phase diagram have been discovered. Monotropic phase behavior also emerged in one of the samples.

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RESULTS AND DISCUSSION Leveraging Symmetry and Interaction in Molecular Design. Symmetry is an intrinsic molecular attribute that guides molecular packing and interactions which, in turn, dictates the properties of materials in various states. Fully perfluoroalkylsubstituted POSS compounds possess superior omniphobic properties, however, they are less amenable to chemical modifications. We have previously shown that Janus T8FPOSS cages with varying numbers of perfluorinated and hydrophilic side chains could be easily prepared via sequential thiol−ene “click” reactions.13 In this work, specifically, six compounds were designed with continuous symmetry breaking from Oh to D3d and further to Cnv (Scheme 1). The synthesis and characterization of T8F8,33 T8F7(OH),49 and T8F6(OH)2 diadducts13 have been reported in the literature. It is noted that with changing symmetry the number of hydroxyl groups in the molecule also varies. The incorporation of hydroxyl groups is expected to promote the self-assembly of these compounds by both intermolecular hydrogen bonding and nanophase separation from fluorous chains. To illustrate how the number of hydroxyl groups affects assembly, a monoadduct with two hydroxyl groups attached at one vertex, T8F7(OH)2, was also designed. This monoadduct exhibits identical symmetry to T8F7(OH) but possesses the same number of hydroxyl groups as diadduct T8F6(OH)2. The synthesis followed the reported procedure for preparing similar compounds.13 Full characterization details can be found in the Supporting Information (Figures S1−S4). Interestingly, these compounds can also be regarded as patchy particles with different patchy patterns.50 Compared with colloidal patchy particles, these molecular B

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Macromolecules Table 1. Summary of Characterization Results for FPOSS Derivatives samples T8F8 T8F7(OH) T8F7(OH)2 p-T8F6(OH)2 m-T8F6(OH)2 o-T8F6(OH)2

Tda (°C)

Tmb (°C)

323 346 342 343 351

112 90 86 87 77 71

Tisob (°C)

dCrc (nm)

81d 98 90 73

3.57 3.63 3.45 3.57 3.51 3.57

dLAMc (nm)

3.07e 3.05 3.11 3.25

mesophase

symmetry

LAM LAM LAM LAM

Oh C3v C3v D3h C2v C2v

a Decomposition temperature (Td) at 5% weight loss was measured using TGA with a heating rate of 10 °C/min. bMelting temperatures (Tm) and isotropic temperatures (Tiso) were obtained from DSC upon heating at a heating rate of 10 °C/min. cThe d-spacing values of the crystalline state (dCr) and mesophase (dLAM) were calculated from WAXD using the equation d = 2π/q1. dThis value is obtained from the cooling curve due to monotropic behavior. eDue to the peak overlap in the WAXD pattern, this value was deduced from the second-order peak using the equation dLAM = 2∗2π/q2.

Figure 1. DSC curves of T8F8 (A), T8F7(OH) (B), para-T8F6(OH)2 (C), meta-T8F6(OH)2, (D) ortho-T8F6(OH)2 (E), and T8F7(OH)2 (F). The first cooling curve and the second heating curve are shown. The heating and cooling rates are both 10 °C/min.

thermal stability for all samples with 5% weight loss temperature (Td) typically above 320 °C (Table 1 and Figure S5), which is consistent with a previous report.36 Then, the phase structures and phase behaviors of these derivatives were investigated using combined techniques of differential scanning calorimetry (DSC) (Figure 1), polarized optical microscopy (POM) (Figures 2−5), and temperature-dependent wide-angle X-ray diffraction (TD-WAXD) (Figures 2−5). The results are summarized in Table 1.

patchy particles are much smaller (∼3.5 nm in the longest dimension) and may serve as model patchy nanoparticles. It would be intriguing to study the phase structures and phase behaviors of these Janus POSS compounds, to shed light on structure−property relationship, and broaden the scope and potential applications of FPOSS-based materials. Mesophase Formation in Janus FPOSS Compounds. We first performed thermal analysis on this series of FPOSS derivatives. Thermogravimetric analyses (TGA) show excellent C

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Figure 2. Phase diagram (A), POM images (B), and the corresponding WAXD patterns (C) of para-T8F6(OH)2 at 30, 90, and 120 °C, respectively. The scale bar is 50 μm.

Figure 3. Phase diagram (A), POM images (B), and the corresponding WAXD patterns (C) of meta-T8F6(OH)2 at 30, 80, and 120 °C, respectively. The scale bar is 50 μm.

Figure 4. Phase diagram (A), POM images (B), and the corresponding WAXD patterns (C) of ortho-T8F6(OH)2 at 30, 72, and 90 °C, respectively. The scale bar is 50 μm.

From the DSC thermograms shown in Figure 1, it is clear that T8F8 and T8F7(OH) have only one phase transition upon heating and cooling, which corresponds to the melting and crystallization processes, respectively (Figure 1A,B). There appears to be supercooling in both samples. Interestingly, all three [2:6] regioisomeric diadducts of T8F6(OH)2 show

mesophase formation upon heating or during cooling (Figure 1C−E). Considering the apparent supercooling and relatively large heat of fusion, the first transition at a lower temperature can be recognized as crystal melting/crystallization. The second transition at a higher temperature does not involve much supercooling and only exhibits a relatively small heat of fusion, D

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Figure 5. Phase diagram (A), POM images (B), and the corresponding WAXD patterns (C) of T8F7(OH)2 at 30, 78, and 100 °C, respectively. The scale bar is 50 μm.

them for 2D WAXD experiments. The results verify the LAM morphology and show that the lamellar normal direction is perpendicular to the shear direction (Figure S9), which is consistent with a previous report.53 Monotropic Phase Behavior of T8F7(OH)2. Monotropic phase behavior is an interesting phenomenon that emerges from metastable states.55−57 For T8F7(OH)2, we observed a typical monotropic phase behavior: the mesophase only develops upon cooling but not upon heating. This characterization is presented in Figure 5. Apparently, the mesophase region is very narrow. Rapid crystallization of fluorous chains may quickly take over the mesophase formation. Hence, we could not capture the liquid crystalline textures under POM on cooling. Instead, only the birefringence from crystals was observed. We speculated that mesophase formation may further lower the kinetic barrier to crystallization. In other words, mesophase formation may “catalyze” the transition from isotropic melt to crystalline state, making it difficult to stay in mesophase. However, we did capture the intermediate state in WAXD by quickly quenching the isotropic melt to 78 °C. Both crystalline state and mesophase coexist, as reflected by the two individual sets of lamellar diffraction peaks (Figure 5C, middle). It is clear that the metastable phase in this instance is also a LAM mesophase, and its isotropic temperature (Tiso) is lower than the melting temperature (Tm). As a result, it is not possible to observe mesophase upon heating. Because of much smaller supercooling, the LAM mesophase only has to overcome a small barrier for formation upon cooling and its growth rates are much faster than the crystal phase at higher temperatures, resulting in monotropic phase behavior. However, since the transition from LAM mesophase to crystalline state is also fast, there is only a small window for mesophase. At this stage, the entire phase diagram can be constructed for all samples (Figure 6). Phase Diagram of FPOSS Derivatives. From the phase diagram shown in Figure 6, we can see that the parent compound T8F8 with the highest symmetry has the highest melting temperature (Tm) of 112 °C. This is followed by T8F7(OH) at 90 °C and T8F7(OH)2 at 86 °C. Because the hydroxyl alkyl substituents (−CH2CH2OH) or (−CH2CHOHCH2OH) are much shorter than the perfluorinated alkyl chains (−CH2CH2(CF2)7CF3), it is speculated that their introduction (even as few as one) would hamper the proper crystal close packing and lead to depressed melting points. With two hydroxylalkyl substituents, the T8F6(OH)2 diadducts

which suggests a transition from mesophase to the isotropic state. The transitions were fully reversible even after many cycles of heating and cooling. Interestingly, mesophase formation was also observed in T8F7(OH)2, but only upon cooling within the narrow window of supercooling (Figure 1F). This is typical monotropic phase behavior,51 which will be discussed later in detail. We then examined the structural changes in different states using POM and WAXD. The phase behaviors of T8F8 and T8F7(OH) are relatively simple, showing only one transition from crystalline state to isotropic phase. This is supported by the observation of Maltese crosses for crystalline spherulites in the POM images (Figure S6) and the corresponding WAXD patterns (Figures S7 and S8). The characterization results for para-, meta-, and orthoT8F6(OH)2 are shown in Figures 2−4. For para-T8F6(OH)2, POM images were recorded at 120, 90, and 30 °C upon cooling and equilibrating at that specific temperature, which corresponds to the isotropic state, mesophase, and crystalline state, respectively (Figure 2B). At the isotropic state (Iso), the POM image is completely dark, as expected. Correspondingly, the WAXD pattern shows only amorphous halos (Figure 2C). At 90 °C, fan-shaped textures were observed, which are characteristic of mesophase formation. WAXD data indicate a lamellae (LAM) structure, as reflected by the q ratio of 1:2:3. This state was thus assigned LAM mesophase. While fan-shaped textures are typically observed in hexagonal columnar phases (Colh),52 it has been reported that fluorine-containing liquid crystals can exhibit this type of texture even though they are not Colh.41,48,53 In this state, there are no diffraction peaks characteristic of perfluorinated chain crystals (q ∼ 12). Upon further cooling to 30 °C, many small grains appear in the POM image that disrupt the fan-shaped textures. In the WAXD profile, there are a set of diffraction peaks with q ratios of 1:2:3:4:5:6 and characteristic diffraction peaks for the fluorous crystals at q ∼ 12. These results indicate that the fluorous side chains crystallize, leading to the much ordered structure. Thus, this state is assigned the crystalline state (Cr). Notably, d-spacing in the crystalline state is much larger than that in the LAM state (Table 1) because fluorous chains adopt an extended-chain conformation in the crystalline state54 but become considerably more flexible and coiled when melted. The other two regioisomers of T8F6(OH)2 show very similar characterization results (Figures 3 and 4). To further confirm the LAM morphology, we performed shear experiment on these samples in their mesophase states to align E

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this case, hydrogen bonding between hydroxyl groups is the key intermolecular interactions. We then examined the differences in hydrogen bonding behaviors in these samples using temperature-dependent infrared spectroscopy (Figure 7). It should be noted that in this system hydrogen bonding mainly occurs between hydroxyls but not between hydroxyls and fluorides. The fluorine atoms in the fluoroalkyls are not effective hydrogen bond acceptors compared to oxygen atoms.58 It is well-known that hydrogen bonding causes O−H stretching peaks in the IR spectra to shift to a lower wavenumber. Figure 7A shows the FT-IR spectra of T8Fm(OH)n compounds in the crystalline state at 30 °C. All spectra show strong absorption bands at ∼3500 cm−1, which is characteristic for intermolecular hydrogen bonding, with T8F7(OH)2 and para-T8F6(OH)2 being the strongest. The possible influence from residual moisture on the spectra is excluded because the characteristic absorption band of H2O at ∼1630 cm−1 could not be detected during heating and cooling (Figure S10).59 In addition, the absorption band of orthoT8F6(OH)2 in the FT-IR spectrum shows a distinct tailing toward the lower wavenumber region (down to ∼3100 cm−1). The hydrogen bonding in this region may likely arise from intramolecular interactions or from short oligomers like dimers and trimers rather than extended oligomers. This is quite reasonable considering the spatial proximity of the two hydroxyl groups in the ortho-isomer. Upon increase of the temperature, the hydrogen bonds would become weaker and more dynamic.60,61 The scenario is similar for the para- and meta-isomers (Figure 7D,E), as evidenced by the weakening absorption at ∼3500 cm−1 with increasing temperature. By contrast, for the ortho-isomer, not only does the absorption band at ∼3500 cm−1 decrease, but there is also a

Figure 6. Phase diagram of T8F8, T8F7(OH), T8F7(OH)2, paraT8F6(OH)2, meta-T8F6(OH)2, and ortho-T8F6(OH)2. The brown shaded region in T8F7(OH)2 indicates a metastable LAM mesophase accessible only upon cooling.

exhibit LAM mesophases. While the substituents further discourage FPOSS crystallization, the presence of two hydroxyl groups enhances intermolecular interactions and promotes mesophase formation. Phase stability is dependent on molecular symmetry. Both Tm and Tiso follow the order of para- > meta- > ortho-. The meta-isomer has the broadest range of liquid crystalline phase. Interestingly, the Tiso of the para-isomer (D3h) is only lower than the Tm of T8F8 (Oh) and the Tiso of the metaisomer (C2v) is comparable to the Tms of the monoadducts with Cnv symmetry, T8F7(OH) and T8F7(OH)2, showcasing a strong correlation between symmetry and phase stability. However, the ortho-isomer exhibits considerably lower Tm and Tiso despite its similar symmetry to the meta-isomer. These results suggest that molecular symmetry may result in different intermolecular interaction patterns that subsequently change phase stability. In

Figure 7. (A) FT-IR spectra of FPOSS compounds in the crystalline state (30 °C). (B) FT-IR spectra of T8F6(OH)2 regioisomers in the LAM mesophase. (C) FT-IR spectra of FPOSS compounds in the isotropic state (110 °C). Temperature-dependent FT-IR spectra of (D) para-T8F6(OH)2 at 30 °C (Cr), 90 °C (LAM), and 110 °C (Iso), (E) meta-T8F6(OH)2 at 30 °C (Cr), 80 °C (LAM), and 110 °C (Iso), and (F) ortho-T8F6(OH)2 at 30 °C (Cr), 72 °C (LAM), and 110 °C (Iso). F

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Figure 8. (A) Molecular model of T8F8 simulated by Materials Studio. (B) Cartoon illustration of molecular packing in the crystalline states for the three diadduct regioisomers. (C) Cartoon illustration of the POSS packing within the LAM mesophase and the postulated hydrogen bonding patterns for para- (left), meta- (middle), and ortho-isomers (right).

resonances for the hydroxyl protons. The long perfluoroalkyl chains may shield the shorter hydroxyl groups, making them hard to detect. It is also likely that the resonances of the hydroxyl protons are located between 3 and 4 ppm and overlap with other stronger signals. In this case, solid-state NMR techniques cannot provide much more information beyond that by the FT-IR spectra. Molecular Packing and Hydrogen-Bonding Patterns. To rationalize the observed phase stability and phase behaviors, we proposed the possible molecular packing schemes for these samples. FPOSS compounds are highly crystalline. The crystal structure of a similar FPOSS compound has been reported previously.30 It was found that the fluorous chains adopt extended conformations and segregate onto two sides of POSS. This rod-like motif further packs into the crystal where POSS units are in close vicinity and the fluorous chains pack together, resembling a “phase-separated” lamellar structure when viewed along the a- or b-axis. On the basis of the crystal structure, we built the molecular model for T8F8 in Materials Studio where the fluorous chains are extended and segregated to two sides similar to that in the reported single crystal (Figure 8A). The lateral distance between the terminal fluorides at two ends was measured to be ∼3.5 nm. This is in close agreement with the measured dCr (Table 1) and suggests that crystallization of the fluorous chains dominates the structure formation at lower temperatures (Figure 8B), which also explains the similar dimension of dCr for all these samples. The different Tms for the

considerable shift toward the higher wavenumber region (Figure 7F). Upon comparison of the IR spectra of the three diadduct regioisomers in the mesophase, the ortho-isomer still has the most absorption tailing at the lower wavenumber region, suggesting the highest fraction of hydrogen bonds from intramolecular interactions or short oligomers (dimers and trimers) similar to that in the crystalline state (Figure 7B). In the isotropic state (110 °C), hydrogen bonds are almost absent in T8F7(OH). There are mainly intermolecular hydrogen bonds in the para-isomer, as evidenced by the relatively narrow and small absorption band at ∼3500 cm−1. The spectra for T8F7(OH)2, meta-T8F6(OH)2, and ortho-T8F6(OH)2 are almost identical, which indicates that their interactions are very similar and contain minor contributions from transient short oligomers (Figure 7C). On the basis of these facts, we inferred that molecular symmetry dictates their hydrogen-bonding patterns and results in different phase stabilities, as best demonstrated in the diadduct regioisomers. To corroborate the findings in FT-IR spectrometry, we have also used solid-state NMR techniques to study these samples. Our initial attempts show that it did not have enough resolution to differentiate the three diadduct regioisomers, and no signal could be detected for hydroxyl protons in their solid-state NMR spectra (Figure S11). The temperature-dependent experiments were also performed for the meta-isomer. With increasing temperature, the solid-state NMR spectra become better resolved (Figure S12). Nonetheless, there is still no clear G

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intramolecular interaction or the formation of short oligomers, like dimers, the mesophase stability is quite limited. The low Tiso leads to a very narrow window for the mesophase formation. Only the meta-isomer has the proper geometry for intermolecular POSS−POSS interactions to promote mesophase formation while discouraging crystallization, giving the broadest temperature range for the mesophase. Similarly, for T8F7(OH)2, the presence of two hydroxyl groups enhances POSS−POSS interactions to promote mesophase formation. However, phase stability is rather limited due to the spatial constraints of the hydroxyl groups that prohibit extensive interaction. Because crystallization of the seven fluorous side chains dominates the process, metastable mesophase can only form upon cooling, leading to a typical monotropic phase behavior and a notably small window for the pure LAM mesophase.

three diadduct regioisomers could then be understood. The para-isomer has the symmetry most compatible with the geometry of the crystal and is the most stable, while the orthoisomer is not favored in the lamellar crystal packing and possesses the lowest Tm. After crystal melting, if the interaction between POSS motifs is strong enough to hold them together, a mesophase will form with “phase-separated” fluorous layers and POSS layers (Figure 8C). As exemplified in the diadduct regioisomers T8F6(OH)2, the key factors affecting phase stability are hydrogen bonding and the interface between the POSS core and the fluorous side chains. The flexible side chains lead to much smaller d-spacing of the mesophases. Depending on the molecular symmetry, the POSS motifs might have to adopt slightly different packing to maximize the intermolecular hydrogen bonding and minimize the interface. While the para-isomer with 180° separated hydroxyl groups allows for an almost linear head-to-tail configuration with fully segregated fluorous side chains, the meta-isomer with the two hydroxyls separated by ∼109° 28′ on the POSS scaffold might have to twist a little to form a zigzag-like configuration, leading to slight expansion in the d-spacing. This also results in partial interdigitation of fluorous chains across the POSS domain, which decreases the phase stability. Along this line, the ortho-isomer must offset even further to accommodate intermolecular hydrogen bonding, and so the d-spacing is increasingly larger. Meanwhile, it creates more interface between POSS and fluorous chains. The intramolecular hydrogen bonding and hydrogen bonding between short oligomers (such as dimers) are also in active competition with the formation of extensive intermolecular hydrogen-bonding network. Both factors dramatically destabilize the mesophase formation. Further disruption of the stable interactions between POSS motifs eventually leads to isotropic states. It is worth mentioning that the crystal phase obtained during multiple cycles of heating and cooling may not be the most ideal ordered structure, but rather the relatively small metastable crystals trapped by the crystallization of fluorous chains. Indeed, highly ordered crystal forms can be developed from solution, which shows many more diffraction peaks in WAXD (see, for example, Figure S13). Bulk crystallization prevents the formation of such single crystals, and thus, they are not further investigated. Our previous studies on double-chain giant surfactants (DPOSS-2PSn) showed the opposite order of phase stability with the ortho-isomer being the most stable one in most cases (ortho- > meta- > para-),16,18,19 which may seem contradictory to the current result at first glance. However, a second thought would clarify that the underlying principles are similar. In double-chain giant surfactants, phase separation occurs between the POSS head with two tethered chains; in the current example, the phase separation occurs between POSS and the remaining six tethered fluorous chains. The geometric requirements are just the opposite in these two cases, and so is the order of phase stability. Now, we understand that the crystal structure formation is driven by the crystallization of perfluoroalkyl chains and the LAM mesophase formation is driven by the phase separation between POSS and perfluoroalkyl chains and stabilized by the hydrogen bonds. The interplay between phase separation and hydrogen bonding determines the temperature range of the mesophases. Although the Tiso of the para-isomer is high due to strong POSS−POSS interactions, it also has a high Tm because of high symmetry, which limits the mesophase temperature range. The ortho-isomer has a low Tm, but since it favors

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CONCLUSIONS We designed a series of Janus FPOSS compounds with continuous symmetry breaking from the Oh-symmetric T8F8 precursor and reported a complete phase diagram for these compounds. At lower temperatures, the crystallization of fluorous chains dictates. Upon increasing temperature, the diadduct regioisomers T8F6(OH)2 develop LAM mesophases due to enhanced POSS−POSS interactions. The phase stability varies tremendously depending on the molecular symmetry and follows the order of para- > meta- > ortho-. This prediction was rationalized by the distinct hydrogen-bonding patterns for these regioisomers as supported by FT-IR spectra. Interestingly, the monoadduct with two hydroxyl groups, T8F7(OH)2, exhibits unique monotropic phase behavior, forming a transient, metastable LAM mesophase only upon cooling. This can be explained by the kinetically more accessible LAM mesophase over crystallization at higher temperatures. This study adds to our understanding of structure−property relationships, and the results are important for symmetry-guided structure design with tuned interactions for advanced hybrid materials.

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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/acs.macromol.9b00138.

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1 H NMR, 13C NMR, 29Si NMR, and MALDI-TOF MS spectra of T8F7(OH)2 in CHCl3 (Figures S1−S4); TGA curves (Figure S5); POM images (Figure S6); WAXD patterns (Figures S7−S9 and S13); temperature-dependent FT-IR spectra (Figure S10); 1H solid-state MAS NMR spectra (Figures S11 and S12) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yu Shao: 0000-0003-3546-2574 Di Han: 0000-0001-5794-6145 Qiang Fu: 0000-0002-5191-3315 Shuguang Yang: 0000-0003-2257-5457 Wen-Bin Zhang: 0000-0002-8746-0792 H

DOI: 10.1021/acs.macromol.9b00138 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Notes

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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 and 91427304) and the National Key R&D Program of China (2018YFB0703702). We thank the Beamline 16B1 at the Shanghai Synchrotron Radiation Facility and Beamline 1W2A at the Beijing Synchrotron Radiation Facility for the assistance with the WAXD experiments. We also thank Dr. Hui Fu of Peking University and Mr. Meng Zhou, Mr. Qin Li, and Dr. Haijun Yang of Tsinghua University for assistance with the solidstate MAS NMR experiments.

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DOI: 10.1021/acs.macromol.9b00138 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.9b00138 Macromolecules XXXX, XXX, XXX−XXX