Tunable Supramolecular Hexagonal Columnar Structures of

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Tunable Supramolecular Hexagonal Columnar Structures of Hydrogen-Bonded Copolymers Containing Two Different Sized Dendritic Side Chains Yongchen Cai,† Meiqing Zheng,† Yalan Zhu,† Xiao-Fang Chen,*,†,‡ and Christopher Y. Li‡ †

Suzhou Key Laboratory of Macromolecular Design and Precision Synthesis, Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, State and Local Joint Engineering Laboratory for Novel Functional Polymeric Materials, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, P. R. China ‡ Department of Materials Science and Engineering, Drexel University, Philadelphia, Pennsylvania 19104, United States S Supporting Information *

ABSTRACT: Polymer structures with tunable symmetry and sizes are desired for applications such as lithography, filtration membranes, and separation. Here we report the self-assembled supramolecular hexagonal columnar (ΦH) structures with tunable lattice size varying from 5 to 7 nm by constructing hydrogen-bonded copolymers bearing poly(4vinylpyridine) (P4VP) and two dendritic molecular additives, 1-[4′(3″,4″,5″-tridecyloxybenzoyloxy)phenyleneoxycarbonyl]-3-[(4″hydroxyphenyl)oxycarbonyl]benzene (12CBP) and 4-hydroxyphenyl (3,4,5-tridecyloxy)benzoate (12CTB). Despite the distinct molecular size difference between 12CBP and 12CTB, the resulting ternary supramolecular copolymers, P4VP(12CBP)x(12CTB)y, possess a homogeneous ΦH phase at x ≥ 0.1 and y ≥ 0.2. Each column is constructed with P4VP as the backbone tethered with mixed side chains. The column diameter is between the size of the corresponding P4VP(12CBP)x+y and P4VP(12CTB)x+y and could be easily tuned by varying x and y. The enhancement of ΦH in supramolecular copolymers is attributed to the entropy effect of the mixed side chain and enthalpy effect from hydrogen bonding interaction of the P4VP backbone and two molecules (12CBP and 12CTB).

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tunable polymers recently.5−7 Meanwhile, the diameter of the cylindrical polymer chain could be changed gradually along the polymer backbone in gradient copolymers.8 However, highly ordered structures formed by self-organization of dendronized copolymers are seldom reported. A general concern for the requirement of cylindrical polymer chains to self-organize into long-range ordered columnar phases is that the surface should be smooth, and the diameter along the cylinder should be identical. Apparently, copolymerization tends to increase the heterogeneity within the polymer chain, which seems unfavorable to the columnar phase formation. So far, only a codendronized polymer prepared via alternative copolymerization of different dendrons shows hexagonal columnar (ΦH) structure.9 With the aid of supramolecular approach,10 cylindrical dendronized polymers can also be realized by attaching dendritic molecules onto the polymer chain through the “grafting onto” method via noncovalent bonds, such as ion, metal coordination, or hydrogen bonding interaction.11,12 When the molar ratio of dendritic molecules to polymer

olumnar phases are a class of long-range-ordered twodimensional (2D) mesophases (except nematic columnar phase) formed by self-organizing of cylindrical assemblies that are comprised of dendrons,1 dendronized2 or mesogenjacketed 3 polymers, or discotic mesogens, 4 etc. As a prototypical cylindrical building block, the dendronized polymer has been considered as a type of well-defined shapepersistent and functional nanoobject. Incorporation of dendrons to linear polymer backbones endows these polymers’ cylindrical shape with tunable sizes and functionality. 2D columnar phases are readily formed by parallel packing of such kind of cylindrical polymers. Of great interest is that columnar phases formed in this system show different structural order on the length scale from sub- to a few nanometers, with tunable electrical, optical, and mechanical properties. They, therefore, have gained growing interest during last several decades for their extensive applications in organic semiconductors, ion conductors, sensors, catalyst supports, separation membranes, etc. Many potential applications such as ion-conducting and organic photovoltaics dictate precisely tuning the size and heterogeneity of the ordered structures. Copolymerization of dendritic monomers with nondendritic monomers or with different generation dendritic monomers has been proven to be the efficient and accessible way to achieve multifunctional © XXXX American Chemical Society

Received: February 24, 2017 Accepted: April 5, 2017

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DOI: 10.1021/acsmacrolett.7b00145 ACS Macro Lett. 2017, 6, 479−484

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ACS Macro Letters

formed the bands related to those stretching modes of pyridine at 1595, 1415, and 993 cm−1 shifted to higher wavenumbers. Figure 1(a) presents the SAXS profile of P4VP-

repeating unit (x) is less than 1, the supramolecular system mimics the copolymerization of dendritic monomers with nondendritic monomers. It has been known that the supramolecular dendronized polymers have the tendency to adopt cylindrical shape and further self-organize into the ΦH phase when increasing the grafting density. The diameter of each column is predominantly controlled by the size of the dendritic molecules. If we introduced two different sized dendritic molecules into the supramolecular polymer system, the selfassembly behavior of such kind of “supramolecular copolymer” would be interesting. Hypothetically several possibilities would happen, such as macrophase and/or microphase separation, homogeneous phase with different mesophase structures, or homogeneous phase without ordered structures. If the homogeneous mesophase was obtained, the lattice parameter could be easily tuned by exploring the ternary phase diagram. It would further broaden our understanding of how various parameters, such as molecular size, overall grafting density, and relative grafting densities of two tethered molecules, affect the phase behavior and properties of supramolecular polymers. In this work, we investigated a ternary system containing poly(4-vinylpyridine) (P4VP) and two different dendron-like molecules (12CBP and 12CTB) (Scheme 1). Both 12CBP and

Figure 1. (a) SAXS pattern of P4VP(12CBP)0.5(12CTB)0.5 and corresponding SAXS of P4VP(12CBP)1.0, P4VP(12CTB)1.0, and 12CBP/12CTB (molar ratio = 1:1) blends. (b) POM picture of P4VP(12CBP)0.5(12CTB)0.5 at 80 °C. The texture was obtained by slowly cooling the sample from the isotropic state to LC state. (c) Lattice parameter a of ΦH as a function of x for P4VP(12CBP)x(12CTB)y with x + y = 1.

Scheme 1. Chemical Structure of Supramolecular Copolymer P4VP(12CBP)x(12CTB)y

(12CBP)x(12CTB)y with x = 0.5 and y = 0.5, as well as the SAXS profiles of P4VP(12CBP)1.0, P4VP(12CTB)1.0, and 12CBP/12CTB (1:1) blends for comparison. SAXS of 12CBP and 12CTB blends only shows diffused lamellar scattering peaks. When P4VP was blended into the system, the resulting P4VP(12CBP)0.5(12CTB)0.5 shows characteristic columnar phase diffraction. Three diffraction peaks at 1.19, 2.07, and 2.38 nm−1 with the q ratio of 1:√3:2 could be clearly identified. Those reflection peaks can be indexed as (10), (11), and (20) planes of the hexagonal lattice. The calculated lattice parameter a is 6.09 nm, which is close to the diameter of each column by assuming a close packing geometry. Meanwhile, besides the hexagonal diffraction pattern, no additional peaks were observed in the small-angle region, suggesting that phase separation did not take place in this case. Therefore, the hydrogen bonding interaction between P4VP and 12CBP/ 12CTB did induce long-range ordered 2D ΦH structure. The POM micrograph (Figure 1b) taken at 80 °C clearly shows fanlike textures, which confirms the columnar phase formation. The diameter of P4VP(12CBP)0.5(12CTB)0.5 is 6.09 nm, which is smaller than the diameter of P4VP(12CBP)1.0 (6.91 nm) and greater than that of P4VP(12CTB)1.0 (5.00 nm). We further varied the relative molar ratio of 12CBP and 12CTB and kept x + y = 1. The ΦH structure was observed in all the blends explored (Figure S4). The lattice parameter a is in the range of 5.0−6.9 nm and increases with increasing content of 12CBP. That means the size of the supramolecular ΦH structure can be precisely controlled by varying the relative content of 12CBP and 12CTB. Figure 1(c) shows the plot of a2 vs x for P4VP(12CBP)x(12CTB)y with x + y = 1. The nearly linear relationship was observed, and the blue dotted line in Figure 1(c) shows a linear fit of the experimental data with

12CTB can complex with P4VP via hydrogen bonding interaction between phenolic and pyridine groups, giving rise to supramolecular polymer complexes. P4VP(12CBP)x at x ≥ 0.3 shows ΦH structure with the lattice parameter a around 7− 8 nm,12b while the a is around 5−6 nm for P4VP(12CTB)x at x ≥ 0.4.12c The entirely different a of the self-assembled structure is due to the distinct size difference between 12CBP and 12CTB. Assuming all molecules adopt extended chain conformation, the length is around 3.75 nm for 12CBP and 2.60 nm for 12CTB, respectively. Surprisingly, such big size difference (1.15 nm) did not induce any phase separation or inhibit mesophase formation in P4VP(12CBP)x(12CTB)y, and a homogeneous supramolecular columnar phase was detected instead. P4VP(12CBP)x(12CTB)y samples with different x and y were prepared via solution blending method. Generally, P4VP, 12CBP, and 12CTB were blended with the calculated ratio in CHCl3 solution. After drying slowly, the obtained complexes were further dried under vacuum for 2 days and then annealed at certain temperatures (depending on the Tg and Ti of the individual complex) for 8 h. FT-IR spectra (Figure S1) show the existence of hydrogen-bonding interaction between the pyridine and phenolic group. Once hydrogen bonds were

a 2 = 26.1x + 20.9

(1)

The nearly linear dependence in eq 1 is of great interest. As we exclude the possible macro- or microphase separation, the only 480

DOI: 10.1021/acsmacrolett.7b00145 ACS Macro Lett. 2017, 6, 479−484

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ACS Macro Letters

would increase the system entropy due to the mixing of two small molecules. Smaller 12CTBs will provide the neighboring 12CBPs more free volume especially in the alkoxy region, further increasing the conformational entropy of the side chain which stabilizes the formation of the columnar phase. Assuming that there is no volume change upon mixing in the present ternary systems, the proposed model in Figure 1 leads to

possible arrangement of 12CBP and 12CTB should be two molecules randomly attaching to the P4VP backbone, as shown in Figure 2. The P4VP polymer chains are surrounded by

2 2 2 a 2 = (a12CBP − a12CTB )x + a12CTB

Plug in the value of

a212CBP

and

a212CTB,

(2)

and we have

a 2 = 22.7x + 25

(3)

Equation 3 is the theoretic dependence of the column diameter on the molar percentage of 12CBP and is plotted as the red line in Figure 1(c). The red and blue lines are nearly overlapped with each other, confirming our hypothesis that the size of the supramolecular columns can be precisely tuned by introducing two side groups with different sizes. Note that the experimental data slightly deviate from the theoretical values at x ∼ 0.3−0.4 and 0.7−0.9, and this can be attributed to the additional free volume associated with the mismatch of two side groups. 12CBP and 12CTB have the same length of soft tails but different sized rigid parts. At x ∼ 0.3−0.4, the experimental a2 is slightly larger than the calculated one, indicating the 12CBP with larger rigid part has more influence on the size of the column. At x ∼ 0.7−0.9, 12CBP already dominates the column, and the size increase is due to the conformational change of soft tails. As we mentioned above, the self-assembly behavior is x dependent in dendronized supramolecular polymers. When the degree of complexation is above a critical point, the selfassembled structure tends to change from lamellar to ΦH structure. It would be interesting to find the x and y induced phase transition boundary in this case by keeping decreasing x and y. Based on the structural information listed in Table 1, when x + y < 1, most of P4VP(12CBP)x(12CTB)y samples are still capable to form the ΦH phase. Those samples also exhibit typical fan-like textures under POM (Figure S3). Different SAXS profiles appear in P4VP(12CBP)0.1(12CTB)0.1 and P4VP(12CBP)0.1(12CTB)0.2 (Figure S5). Due to the rather low grafting density, those two polymers only exhibit two rather diffused scattering peaks which refer to the lamellar-like structure. As both x and y reach to 0.2, the corresponding

Figure 2. Schematic representation of the possible molecular stacking of P4VP(12CBP)x(12CTB)y, P4VP(12CBP)x, and P4VP (12CTB)y in each column and (12CBP)x(12CTB)y in one layer. Mixed 12CBP and 12CTB are wrapping around the P4VP chain via hydrogen-bonding interaction in P4VP(12CBP)x(12CTB)y. The increased entropy compensates the mismatch and finally forms a homogeneous hexagonal columnar phase.

different molecules which would induce “defects” from the size difference of molecules. The cylinder surface would not be smooth like a homopolymer, while the results clearly show that such kind of “defects” did not prevent the mesophase formation. It has been known that equimolar discotic mixtures could be alternatively packed together to form a columnar phase with enhanced stability via donor−acceptor interaction or complementary polytopic interaction (CPI).13 In our work, there is no such strong donor−acceptor interaction to stabilize the columnar structure. Different from those equimolar discotic mixtures or alternative dendronized copolymers, the columnar phase could be formed at various ratios in our system. The diameter of the column could be tuned depending on the relative molar ratio of two molecules. The formation of a columnar phase with randomly distributed 12CBP and 12CTB Table 1. Structural Characterization of P4VP(12CBP)x(12CTB)y P4VP(12CBP)x(12CTB)y x

y

Tia (°C)

0.1 0.1 0.2 0.2 0.3 0.3 0.5 0.5 0.5

0.1 0.2 0.2 0.4 0.2 0.5 0.2 0.4 0.5

125 110 135 99 125 101 90

q (nm−1)b 0.98, 1.01, 1.06, 1.16, 1.06, 1.21, 1.09, 1.18, 1.19,

1.85 1.76, 1.81, 2.02, 1.82, 2.10, 1.87, 2.02, 2.07,

1.99 2.11 2.34 2.15 2.41 2.18 2.35 2.38

phasec

a3d (nm)

a1e (nm)/phase

a2f (nm) /phase

Lam ΦH ΦH ΦH ΦH ΦH ΦH ΦH ΦH

6.41 7.18 6.84 6.25 6.84 5.99 6.65 6.16 6.09

6.75/Lam 8.34/ΦH 7.80/ΦH 7.72/ΦH 7.72/ΦH 6.97/ΦH 7.11/ΦH 6.97/ΦH 6.90/ΦH

9.52/Lam 5.14/Lam 5.62/ΦH 5.49/ΦH 5.41/ΦH 5.33/ΦH 5.37/ΦH 5.11/ΦH 5.00/ΦH

Isotropic temperature Ti was measured from DSC (Figure S2) with the heating rate of 10 °C/min. bScattering vector q was obtained from 1D SAXS (Figures S4 and S5). q at x = 0.1 and y = 0.2 was measured from 2D SAXS (Figure 3b) . cPhase notation: ΦH, Hexagonal columnar phase; Lam, lamellar phase. dLattice parameter of P4VP(12CBP)x(12CTB)y. eLattice parameter of P4VP(12CBP)x+y. fLattice parameter of P4VP(12CTB)x+y. Lattice parameters were calculated by using a = (2/√3d10 + √3d11 + 2d20) for hexagonal structure and a = (d100 + 2d200)/2 for lamellar structure. a

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ACS Macro Letters polymer forms ΦH structure (Figure S5). So the critical point related to phase transition should be found in these three supramolecular polymers. We carried out high-resolution synchrotron 2D SAXS to further identify their phase structures. Figure 3 shows 2D SAXS patterns of the mechanically sheared

The ternary phase diagrams in Figure 3(d) and 3(e) clearly show the mixing property of P4VP(12CBP)x(12CTB)y. The minimum mole content of molecule additive (phase boundary) for columnar structure formation was 23% for P4VP(12CBP)x and 28.6% for P4VP(12CTB)x, respectively. Right now it is 7.7% of 12CBP and 15.4% of 12CTB for P4VP(12CBP)x(12CTB)y (Figure 3(d)), indicating the ternary system does not depress but slightly enhances the formation of the columnar phase. Regarding the phase diagram with mass fraction, 12CBP and 12CTB keep majority content in supramolecular copolymers, because the molecular weight of 12CBP or 12CTB is about 10 times larger than the P4VP repeating unit. Although P4VP mass fraction is less than 0.3 in the self-organized columnar structures, P4VP(12CBP)x(12CTB)y still exhibits significant polymeric properties, indicating hydrogen bonding interaction plays an important role in the ternary system. Furthermore, self-organized columnar structures of these dendronized supramolecular copolymers are also “visible” under AFM. Thin films of P4VP(12CBP)x(12CTB)y were obtained by spinning coating chloroform solution onto a silica wafer and further annealed at 70 °C for 10 h. The thickness of polymer thin films was kept less than 100 nm. The AFM phase image of P4VP(12CBP)0.2(12CTB)0.2 (Figure 4a) shows parallel aligned stripe-like topographies. Additionally, GISAXS was carried out to probe the interior structures and their

Figure 3. 2D SAXS patterns of P4VP(12CBP)x(12CTB)y when (a) x = 0.1, y = 0.1; (b) x = 0.1, y = 0.2; (c) x = 0.2, y = 0.2. Ternary phase diagrams with (d) mole fraction and (e) mass fraction of P4VP, 12CBP, and 12CTB.

samples with X-ray perpendicular to the shear direction. P4VP(12CBP)0.1(12CTB)0.1 shows a pair of diffused arcs, indicating the existence of lamellar-like structure (Figure 3a). At x = 0.1 and y = 0.2, although the intensity of (11) and (20) was very weak, we can still identify three pairs of scattering arcs in the meridian, which belong to ΦH structure (Figure 3b). At x = 0.2 and y = 0.2, ΦH phase structure could be clearly identified (Figure 3c). So it could be confirmed that P4VP(12CBP)x(12CTB)y tends to form ΦH structure when x ≥ 0.1 and y ≥ 0.2. Meanwhile, the lattice dimension of ΦH structure is controlled by the x and y. According to the lattice parameters shown in Table 1, the a of ΦH is between the size of corresponding structures of P4VP(12CBP)x+y and P4VP(12CTB)x+y. When keeping x as constant, the a of ΦH decreases gradually by increasing y. In this way, the total grafting density associated with x + y increases, which makes P4VP adopt more stretched conformation. Meanwhile, incorporation of 12CTB will give more free volume to 12CBP, which also decrease a to some extent. Nevertheless, their isotropic temperatures (Ti) are also x and y dependent. It has been reported that Ti is 120−160 °C for P4VP(12CBP)x12b and 80−100 °C for P4VP(12CTB)x.12c There is around 40 °C difference between corresponding binary complexes. If phase separation existed, it should be detected easily by appearing in at least two phase transitions in DSC heating or cooling curves, while only one phase transition could be detected in DSC (Figure S2). It further proves the formation of a homogeneous ternary system in this case. The Ti decreases gradually with increasing content of 12CTB.

Figure 4. AFM phase images (a, c, e) and GISAXS patterns (b, d, f) of thermal annealed P4VP(12CBP)x(12CTB)y thin films. The film thickness is ∼73 nm for P4VP(12CBP)0.2(12CTB)0.2, ∼ 68 nm for P 4 V P ( 1 2 C B P ) 0 . 2 ( 1 2 CT B ) 0 . 4 , a nd ∼ 5 7 n m f o r P 4 V P (12CBP)0.3(12CTB)0.5 respectively. 482

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orientation in the films. Figure 4b shows a typical hexagonal lattice, which is oriented with its [10] direction parallel to the substrate. So both AFM and GISAXS prove the parallel orientation of ΦH structure in the thin film. Further increasing x and y, the surface topographies appear as more regular fingerprint-like patterns composed of parallel stripes (Figure 4c and 4e, corresponding height images are listed in Figure S6). Meanwhile, the diffraction spots in GISAXS became smaller but stronger (Figure 4d and 4f). The distance between the adjacent stripes is 7.0 nm for P4VP(12CBP)0.2(12CTB)0.2 and decreases to 6.2 nm for P4VP(12CBP)0.2(12CTB)0.4 and 6.0 nm for P4VP(12CBP)0.3(12CTB)0.5. The periodicity of patterns decreases by increasing the content of 12CTB. It is coincident with previous SAXS results. So the AFM topographies and GISAXS patterns of those ternary systems also supported the formation of a homogeneous columnar phase. On the other side, it is also indicated that the surface pattern of such kind of supramolecular copolymer thin films could be controlled by simply varying x and y. In summary, we have developed a simple but effective strategy to tune the size of the columnar phase by building supramolecular copolymers with mixed dendritic side chains. Despite the significant size difference between side chains, a stable hexagonal columnar self-assembled structure could be formed. We can precisely control the size of columnar structures in the range from 5 to 7 nm by varying the grafting densities and the molar ratio of mixed side chains. The hydrogen bonding interaction between the polymer backbone and side chains, together with the reduced steric repulsion and gained entropy by mixed side chains, decreased the energy of mixing. The ternary phase diagram showed that there is ample design space available to tune the supramolecular ordering size for applications such as ion transport and gas separation. We anticipate that a plethora of supramolecular systems would be built up simply by modification of small molecular additives following the proposed framework.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00145. Experimental details, FT-IR, DSC, POM, and SAXS (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (X.-F. Chen). ORCID

Xiao-Fang Chen: 0000-0002-2973-0432 Author Contributions

The manuscript was written through contributions of all authors. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (No. 21174003, No. 21474073) and A Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) for financial supports. 483

DOI: 10.1021/acsmacrolett.7b00145 ACS Macro Lett. 2017, 6, 479−484

Letter

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DOI: 10.1021/acsmacrolett.7b00145 ACS Macro Lett. 2017, 6, 479−484