Fig 1 (a) SAXS profile, (b) TEM micrograph, and (c) schematic illustration of the LAM phase self-assembled by D65M20 at 130 °C. The blue and red colors represent PMPCS and PDMS blocks, respectively.
Published:1 May 2024,
Published Online:28 January 2024,
Received:9 November 2023,
Revised:7 December 2023,
Accepted:21 December 2023
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The rich phase behavior of block copolymers (BCPs) has drawn great attention in recent years. However, the double diamond (DD) phase is rarely obtained because of the competition between the minimization of interfacial energy and packing frustration. Here, a rod-coil BCP containing mesogen-jacketed liquid crystalline polymer is designed to acquire ordered bicontinuous network nanostructures. The reduction of internal energy originating from the orientational interaction among the rod blocks can compensate for the free energy penalty of packing frustration to stabilize the DD structure. The resulting BCP can also experience lamellae-to-DD and double gyroid-to-lamellae transitions by changing the annealing temperature. These results make the rod-coil BCP an excellent candidate for the self-assembly of ordered network structures, demonstrating great potential in nanopatterning and metamaterials.
Block copolymer;
Rod-coil;
Ordered bicontinuous phase;
Double diamond;
Double gyroid
The self-assembly of block copolymers (BCPs) has drawn great attention in recent decades because of its potential in nanopatterning, metamaterials, etc.[
Generally, the DD phase with 4-fold nodes and Pn¯3 m symmetry is regarded as an unstable or metastable morphology compared with BCC, HEX, LAM, and DG because of competition between the minimization of interfacial energy and packing frustration.[
The reduction in packing frustration can compensate for the increased interfacial energy originating from a non-CMC surface. Therefore, the degree of deviation from the CMC surface can measure the stability of these complex structures. The calculated interface of DG deviates less from the CMC surface than that of DD.[
Herein, we report ordered bicontinuous phases, DD and DG, formed by a rod-coil BCP composed of the flexible polydimethylsiloxane (PDMS) and the rigid mesogen-jacketed liquid crystalline polymer (MJLCP) poly{2,5-bis[(4-methoxyphenyl)-oxycarbonyl]styrene} (PMPCS). The interplay between the microphase separation of the two blocks and the orientational interaction of the rod blocks makes its phase behavior different from that of regular coil-coil BCPs and stabilizes the DD structure by reducing internal energy to make up for the packing frustration. The LAM-to-DD and DG-to-LAM phase transitions can also be observed by changing the annealing temperature, demonstrating dramatic changes in the phase behavior from the variation in the segregation strength. This work can promote the development of ordered bicontinuous structures in BCPs and show great potential in nanotemplates, nanoporous materials, and metamaterials.
The synthetic route of the macroinitiator 2-bromoisobutyrate-terminated PDMS was shown in Scheme S1 and Fig. S1 (in the electronic supplementary information, ESI). The absolute molecular weight (MW) of the macroinitiator was determined by matrix-assisted laser desorption/ionization time of flight mass spectrometry (MALDI-TOF-MS) (Fig. S2 in ESI). PDMS-b-PMPCS was prepared by atom-transfer radical polymerization (Scheme S1 in ESI). The molar mass dispersity (ĐM) of the macroinitiator and PDMS-b-PMPCS was determined by size exclusion chromatography (SEC) (Figs. S3 and S4 in ESI), and the DP of PMPCS was determined by the absolute MW of the macroinitiator and the 1H-NMR result (Fig. S5 in ESI). The volume fraction of PMPCS, frod, was calculated assuming the densities of PDMS and PMPCS are 0.97 and 1.28 g/cm3, respectively.
Bulk samples of PDMS-b-PMPCS block copolymers were prepared through solution casting from tetrahydrofuran (THF) solutions overnight and then annealed at 130 and 190 °C for 2 days. Then, liquid nitrogen was used to quench the samples in order to maintain the high-temperature structures. After the small-angle X-ray scattering (SAXS) and wide-angle X-ray scattering (WAXS) experiments, the samples were embedded in epoxy resins and microtomed with a thickness of approximately 100 nm by an ultramicrotome for transmission electron microscopy (TEM) observation.
1H-NMR spectra were conducted on a Bruker-400 (400 MHz) spectrometer. The GPC experiments were performed on a Waters 1515 instrument with an isocratic HPLC pump and a Waters 2414 instrument with a refractive index detector using THF as eluant (1 mL/min). The polystyrenes were used as standards to plot the calibration curve. MALDI-TOF-MS results were obtained on an AB Sciex 5800 instrument. The SAXS and WAXS experiments were carried out on Xeuss 2.0 (Xenocs SA, France) at Institute of Chemistry, Chinese Academy of Sciences (ICCAS), the synchrotron X-ray beamline 1W2A at Beijing Synchrotron Radiation Facility (BSRF), and the synchrotron X-ray beamline BL16B1 at Shanghai Synchrotron Radiation Facility (SSRF).[
The characterization results of PDMS-b-PMPCS are listed in
Sample | Mn,D a(kg/mol) | Mn,M b(kg/mol) | ĐM a(Mw/Mn) | frod’s | Nanostructure c(130 °C) | Nanostructure c(190 oC) |
---|---|---|---|---|---|---|
D65M20 | 4.9 | 8.1 | 1.11 | 0.56 | LAM | DD |
D65M34 | 4.9 | 13.7 | 1.17 | 0.68 | DG | DG + LAM |
a Determined by SEC in THF, Mn,D is the number-averaged MW of the PDMS block and ĐM is the molar mass dispersity of the BCP; b Determined by 1H-NMR, Mn,M is the number-averaged MW of the PMPCS block; c The nanostructures were characterized by SAXS and TEM.
Fig 1 (a) SAXS profile, (b) TEM micrograph, and (c) schematic illustration of the LAM phase self-assembled by D65M20 at 130 °C. The blue and red colors represent PMPCS and PDMS blocks, respectively.
As described in the literature, ordered network nanostructures such as DD, DG, and Fddd can be regulated through the self-assembly of BCPs. The scattering vector ratio of D65M20 with the annealing temperature of 190 °C is 21/2:31/2:41/2:61/2:81/2, which is consistent with that of the DD phase ( 1] projection (
Fig 2 SAXS profile of the DD phase self-assembled by D65M20 at 190 °C with the blue and red lines representing the theoretical diffraction peak positions of Fddd and DG phases, respectively (a); TEM micrographs (b, c) and schematic illustration (d) of the DD phase. The blue and red colors represent PMPCS and PDMS blocks, respectively.
Based on the above results, the LAM structure should be the most stable structure at low temperatures. At high temperatures, the LAM structure is likely to be unstable because the thermal motion of the coil block increases and large interfacial areas between the layers are required to relieve the free energy of deformation originating from the coil packing.[ m symmetry. To understand the reason for the phase transition, WAXS experiments were carried out (Fig. S6 in ESI). In rod-coil BCPs, the other two parameters, the geometrical asymmetry interaction (ν) and the orientational interaction (μ), can also influence the structures. The liquid crystalline (LC) phase behavior of PMPCS is MW-dependent, and PMPCS rods in D65M20 are in the amorphous state during the annealing temperature range, which means that the strength of the orientational interaction among PMPCS rods remains unchanged. However, the decrease of the Flory-Huggins interaction parameter and the corresponding χN value will weaken the segregation strength between the two blocks when the annealing temperature is increased, and then the structure of D65M20 will have an order-order transition (OOT). Similar to the situation in sPP-b-PS that the ordered packing of helical sPP segments can reduce internal energy to make up for the packing frustration in the 4-fold node caused by the entropic loss of the stretched polymer chains,[
To further investigate the role of the orientational interaction among PMPCS rods on the phase behavior, the MW of PMPCS is increased to allow the formation of an LC phase upon annealing. The peaks of D65M34 annealed at 130 °C exhibit a scattering vector ratio of 61/2:81/2:161/2:241/2:321/2:421/2, which coincides with that of the DG phase (
Fig 3 SAXS profile (a) and TEM micrographs (b, c) of the DG phase self-assembled by D65M34 at 130 °C and cross-sectional view of possible molecular arrangement of the 3-fold node in the DG structure (d). The blue and red colors represent PMPCS and PDMS blocks, respectively.
The peaks can be assigned as the (211), (220), (400), (422), (440), and (541) diffractions. The DG structure can be further confirmed by TEM results (
With the annealing temperature rising to 190 °C, the intensities of the abovementioned peaks decrease, and several new peaks with the scattering vector ratio of 1:2:3:4 appear (
Fig 4 SAXS profile (a) and TEM micrographs (b, c) of D65M34 after thermal annealing at 190 °C and WAXS profiles of D65M34 after thermal annealing at 130 and 190 °C (d).
As mentioned above, the microstructures of rod-coil BCPs are influenced by not only f and χN but also the geometrical asymmetry interaction and orientational interaction. The phase diagram is shown in Fig. S7 (in ESI). When the MW of PMPCS is lower than the critical MW for LC formation, the segregation strength between the two blocks will be decreased because of the decrease in the Flory-Huggins interaction parameter with increasing annealing temperature. When the MW of PMPCS exceeds the critical MW, the segregation strength between the two blocks will be enhanced mainly owing to the increase in the orientational interaction of PMPCS and geometrical asymmetry between the two blocks caused by LC formation. The enhanced segregation strength caused by the formation of LC phase can also be observed in the BCP containing side-chain liquid crystalline polymers, which promotes the formation of low-curvature structures (e.g., BCC, HEX, and LAM), instead of DD.[
In conclusion, the rich structures of PDMS-b-PMPCS are studied because the geometrical asymmetry and orientational interaction play an important role in the self-assembly of rod-coil BCPs. By changing the MW of the rod block and annealing temperature, the BCP can self-assemble into LAM, DD, and DG nanostructures. In addition, for the first time the DD structure and a LAM-to-DD transition are observed in the bulk sample of a BCP containing the rigid MJLCP. The orientational interaction of PMPCS can reduce internal energy to minimize packing frustration for stabilizing the DD structure. Moreover, the transformation of PMPCS from the amorphous to the LC phase also induces a phase transition from DG to LAM. Therefore, various nanostructures, especially bicontinuous network structures (such as DD and DG), can be easily obtained in the rod-coil BCPs containing MJLCP rod blocks. Meanwhile, it is also a promising candidate to be used as nanotemplates, nanoporous materials, and metamaterials.
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