Remarkably Rich Variety of Nanostructures and Order–Order

Jun 20, 2013 - Ling-Ying Shi, Yu Zhou, Xing-He Fan, and Zhihao Shen* ... amorphous PMPCS, and a HEX-to-LAM transition occurs in the BCP when the LC ...
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Remarkably Rich Variety of Nanostructures and Order−Order Transitions in a Rod−Coil Diblock Copolymer Ling-Ying Shi, Yu Zhou, Xing-He Fan, and Zhihao Shen* Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China S Supporting Information *

ABSTRACT: A remarkably rich variety of nanophase-separated structures and various order−order transitions are observed in a series of low-molecular weight (MW) rod−coil block copolymers (BCPs) with the rod blocks of different lengths (LRod’s). The rod−coil diblock copolymer studied herein is poly(dimethylsiloxane)-b-poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene} (PDMS-b-PMPCS), in which PMPCS is a rod-like polymer and exhibits an MW-dependent liquid crystalline (LC) phase behavior. When the polymerization degree of the PMPCS rod block (NRod) is less than 32 (LRod < 8 nm), the PMPCS block is always amorphous in the entire temperature range. And the corresponding PDMS-b-PMPCS BCPs with NRod from 11 to 29 and the volume fraction of the PMPCS rod ( f Rod) from 43% to 67% self-assemble into various equilibrium nanostructures after annealed at temperatures above the glass transition temperatures of the PMPCS blocks. When NRod = 11 and f Rod = 43%, the BCP forms a lamellar structure (LAM); when NRod = 15 and f Rod = 51%, the BCP forms a double gyriod structure (GYR) ; when NRod = 20 and f Rod = 57%, the BCP forms a GYR structure after annealed below 180 °C and transforms to the Fddd structure after annealed above 180 °C; when NRod = 29 and f Rod = 67%, the nanostructure of the BCP after annealed below 180 °C is hexagonally packed cylinders (HEX) and changes to a body centered cubic structure (BCC) after annealed above 180 °C. When NRod > 32 (LRod > 8 nm), the PMPCS rod block is amorphous at low temperatures and transforms to a stable columnar LC phase after annealed at high temperatures. Correspondingly, the PDMS-b-PMPCS BCP with NRod = 44 and f Rod = 75% forms a HEX structure after annealed at lower temperatures at which the PMPCS block is amorphous, and the nanostructure transforms to LAM after the sample is annealed at higher temperatures at which the PMPCS block enters into the LC phase. Therefore, only by a small change of the rod length in the low-MW PDMS-b-PMPCS rod−coil BCPs, various nanostructures including LAM, GYR, Fddd, HEX, and BCC are obtained. In addition, by increasing annealing temperatures, GYR-to-Fddd and HEX-to-BCC transitions are observed in the BCPs with the amorphous PMPCS, and a HEX-to-LAM transition occurs in the BCP when the LC PMPCS block undergoes an isotropic-to-LC phase transformation.

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INTRODUCTION

Compared with the self-assembly of coil−coil diblock copolymers, there are two additional factors that influence the phase behaviors of rod−coil BCPs containing liquid crystalline (LC) rod-like polymers, including geometric asymmetric factor between the rod and coil blocks and the aligning interaction or LC interaction between the rod-like blocks themselves.12−18 Consequently, the self-assembling behaviors of rod−coil BCPs are different from those of coil− coil ones in several aspects. First, rod−coil BCPs have relatively strong phase-separation abilities due to the additional LC interaction which also facilitates microphase separation besides the Flory−Huggins interaction. Thus, rod−coil small molecules, oligomers, and BCPs with low molecular weights (MWs) can nanophase separate into ordered morphologies.19,20 For

The strategies for facilitating the self-assembly of block copolymers (BCPs) and the various periodic nanostructures provided by the self-assembly of BCPs have great applications in nanotechnology.1−3 The crucial parameters which determine the final morphologies of BCPs include the volume fraction f of the individual block and χN, where χ is the Flory−Huggins interaction parameter and N the total degree of polymerization of the copolymer. Depending on these parameters, the coil− coil diblock copolymers can self-assemble into equilibrium lamellae (LAM), bicontinuous double gyroid (GYR), hexagonally packed cylinders (HEX), and body centered cubic arrays of spheres (BCC).4−7 In addition, some other nanostructures such as the metastable hexagonally perforated lamellae (HPL) and the stable Fddd phase which is an orthorhombic network morphology with a 3-fold symmetry can be observed in some weakly segregated coil−coil diblock copolymers.8−11 © XXXX American Chemical Society

Received: May 6, 2013 Revised: June 7, 2013

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the volume fraction of PMPCS is larger than 80%. However, the phase behaviors of BCPs with low-MW, amorphous PMPCS blocks have not been studied, and the difference of the nanophase-separated structures of BCPs with PMPCS blocks of relatively high MWs before and after the LC phase formation have also yet to be investigated. Furthermore, with the use of relatively low-MW BCPs, ordered nanostructures with smaller periodic sizes can be generated. In this work, we report the phase behaviors of a series of lowMW PDMS-b-PMPCS (DmMn, m and n are the degrees of polymerization of PDMS and PMPCS, respectively) with lowMW PMPCS in the amorphous state and another one with relatively high-MW, LC PMPCS. The nanophase-separated structures and phase transitions of these BCPs are studied by small-angle X-ray scattering (SAXS), one-dimensional wideangle X-ray scattering (1D WAXS), and transmission electron microscopy (TEM) experiments. Interestingly, LAM, GYR, Fddd, HEX, and BCC nanostructures as well as GYR-to-Fddd, HEX-to-BCC, and HEX-to-LAM transitions are observed. As far as we know, this is the first report on a rod−coil BCP system which contains such a wide variety of nanophases including GYR and Fddd structures and various OOTs by a small change in the NRod value or in the annealing temperature. In addition, the dimensions of the nanostructures of these BCPs are below or close to 20 nm, which allows the possibility of obtaining sub-10 nm templates for patterning.

example, Lee et al. have reported a series of rod−coil oligomers with different compositions self-assembling into various nanostructures.19 Second, due to the conformational or geometric asymmetry between the rod and coil blocks, the phase diagrams are always asymmetric with respect to composition. Because long rods prefer a flat intermaterial dividing surface (IMDS),13,15 lamellar phases are stable in a wide range of composition even in some rod−coil BCPs with high compositional asymmetry.16,21 For instance, Ober et al. have reported that poly(hexyl isocyanate)-b-polystyrene (PHIC-b-PS) rod−coil BCPs form different lamellar phases with the volume fraction of PS changing from relatively symmetric 0.58 to extremely asymmetric 0.02.21 In addition, sphere and double gyroid phases with curved interfaces are observed in some low-MW rod−coil molecules15 and in some rod−coil BCPs with low volume fractions of the rod-like block,22,23 but are rarely observed in rod−coil BCPs with long rods. Third, the rod-like blocks prefer to aggregate to form ordered LC phases. Thus, rod−coil BCPs can form interesting hierarchical structures.15 During the self-assembling process, the nanophase separation and the LC phase formation compete with and influence each other.15,24 Finally, because of the above characteristics, rod−coil BCPs exhibit some interesting phases which cannot form in coil−coil BCPs, such as smectic A and smectic C morphologies 16,22,25 as well as tetragonally perforated lamellae (TPL).26 Among the different ordered nanostructures of BCPs, many order−order transitions (OOTs) can be induced simply by the change in annealing temperatures.27,28 For common coil−coil BCP systems, temperature-induced OOTs are attributed to the decreases in Flory−Huggins interaction parameters accompanying with the increase in temperature. For liquid crystalline BCPs, the LC phase transitions of the LC blocks can trigger OOTs of the nanophase-separated structures of BCPs.29−31 For example, Sänger et al. have reported that a BCP containing an LC polymer forms a HEX structure when annealed at the temperature at which the LC block is in the nematic phase but forms a BCC structure when annealed at the temperature above the clearing temperature of the LC block.29,32 In our previous work on a rod−coil BCP containing a mesogen-jacketed liquid crystalline polymer (MJLCP), thermoreversible transitions between the low-temperature isotropic state and the hightemperature LC phase induce thermoreversible OOTs between BCC and HEX nanostructures.30 The LC phase behaviors of MJLCPs are always MWdependent.33−35 For example, poly{2,5-bis[(4-methoxyphenyl)oxycarbonyl]styrene} (PMPCS) with MW < 12 000 g/mol (N < 32) is always amorphous in the entire temperature range before decomposition owing to the small aspect ratio, and PMPCS with MW > 13 000 g/mol (N > 32) is amorphous at low temperatures and enters into the columnar LC phases after annealed at high temperatures.33 The nanophase separation behaviors of PMPCS-containing rod−coil BCPs with LC PMPCS have been studied in several BCP systems. For example, Li et al. have reported that the polystyrene-b-poly{2,5bis[(4-methoxyphenyl)oxycarbonyl]styrene} (PS-b-PMPCS) BCPs with LC PMPCS blocks of relatively high MWs form a bilayer smectic A phase and a TPL structure.26 In our previous work, poly(dimethylsiloxane)-b-poly{2,5-bis[(4methoxyphenyl)oxycarbonyl]styrene (PDMS-b-PMPCS) BCPs with high-MW LC PMPCS blocks as the major component self-assemble into a stable LAM phase with a PMPCS volume fraction of 79% and a HEX morphology when

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EXPERIMENTAL SECTION

Synthesis and Characterization of PDMS-b-PMPCS BCPs. The PDMS-b-PMPCS BCPs were synthesized and characterized by the same methods as previously reported.32 The MWs of the BCPs were determined with the combination of gel permeation chromatography (GPC), 1H NMR, and the absolute molecular weight of the macroinitiator.32 GPC experiments were conducted on a Waters 2410 instrument equipped with a Waters 2410 RI detector, with pure tetrahydrofuran (THF) as eluent (1.0 mL/min). The calibration curve was obtained with linear polystyrenes as standards. 1H NMR spectra were obtained with a Bruker 400 MHz spectrometer. Differential scanning calorimetry (DSC) measurements were carried out on a TA Q100 DSC calorimeter in a nitrogen atmosphere. Morphology Investigation. Small-Angle X-ray Scattering (SAXS) and One-Dimensional Wide-Angle X-ray Scattering (1D WAXS). The BCP samples for SAXS and 1D WAXS experiments were first dissolved in dry THF solvent, which was evaporated slowly. The dry samples were annealed at each desired temperature for 48 h for the nanophase-separated structures to reach equilibrium. And then the samples were quenched in liquid nitrogen from the annealing temperatures ranging from 125 to 200 °C. The SAXS and 1D WAXS experiments on the samples after annealing were carried out on a SAXSess instrument (Anton Paar) using Cu Kα radiation at a wavelength of 0.154 nm. The working voltage and current were 40 kV and 40 uA, respectively. The scattering profiles of both SAXS and WAXS were simultaneously recorded on an imaging plate (IP) with a pixel size of 42.3 × 42.3 μm2 which extended to the high-angle range (the q range covered by the IP was from 0.06 to 29 nm−1). The scattering peak positions were calibrated with silver behenate for the small-angle region and silicon powder for the wide-angle region, respectively. The scattering vector q is defined as q = 4π/λ[sin θ], where the scattering angle is 2θ, and the d-spacing (d) is given by 2π/ q. Transmission Electron Microscopy (TEM). The samples for TEM experiments were annealed with the same procedure for SAXS experiments. Then the samples were embedded in an epoxy resin and ultramicrotomed to be thin sections with thicknesses of about 80 nm. And the thin sections were collected on the carbon-coated 400-mesh copper grids. The bright-field images were obtained with a Tecnai T20 TEM instrument using an accelerating voltage of 120 kV. B

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MW in the literature.39 Thus, the Rg/LRod values are calculated from the Rg value of PDMS and LRod’s of PMPCS blocks (Table 1). As the length of the PMPCS rod increases, the ν value decreases, and the degree of geometric asymmetry between the rod and coil blocks increases. The glass transition temperatures (Tg’s) of PMPCS blocks are determined from DSC experiments. And the LC phase behaviors of the PMPCS blocks are determined from 1D WAXS experiments. For a PMPCS sample which can form an LC phase after annealed at high temperatures, the LC phase will permanently maintain after the sample is cooled to ambient temperature. Therefore, the 1D WAXS patterns at ambient conditions were studied after the samples were annealed at high temperatures. As shown in Figure 1, the 1D WAXS patterns of

RESULTS AND DISCUSSION Compositions and Geometric Parameters of the PDMS-b-PMPCS Rod−Coil BCPs and the Phase Behaviors of the PMPCS Blocks. A series of PDMS-b-PMPCS BCPs (Chart 1) with relatively low MWs of PMPCS are Chart 1. Chemical Structure of PDMS-b-PMPCS Block Copolymer

synthesized and characterized by the same methods as described in our previous work.32 The number-average MW, degree of polymerization, and polydispersity index (PDI) of the PDMS coiled block are 4 332 g/mol, 58, and 1.02, respectively.32 The absolute molecular weights and the degrees of polymerization of the PMPCS rod-like blocks (NRod’s) are from 4 400 to 17 800 g/mol and from 11 to 44, respectively, which are determined from the 1H NMR results of the BCPs. And the overall absolute MWs of the BCPs range from 8,900 to 22 300 g/mol determined from 1H NMR results, and the PDI values of all BCPs are lower than 1.10 determined from GPC results (Figure S1 in Supporting Information and Table 1). In these BCPs, the volume fractions of the PMPCS rod ( f Rod’s) are from 42% to 75% (Table 1) calculated from the absolute MWs and the densities of the PDMS and PMPCS blocks.32 Because of the steric requirement of the side chains, PMPCS takes a stiffened chain conformation and is a rod-like polymer, as suggested by the reported large persistent length of poly{2,5bis[(4-methoxybenzoyl)oxy]styrene} (which has almost the same structure and LC phase behavior as those of PMPCS) in THF.36 For PMPCS in the extended-chain conformation, the vinyl backbone is in the trans-conformation with a bond angle of 104°.33 Thus, the length of the PMPCS rod (LRod) has the relationship with NRod as LRod = 0.154 nm × 2NRod × sin52° ≈ 0.24NRod nm. The calculated lengths of PMPCS rods in these BCPs are listed in Table 1. The geometric asymmetry parameter, ν, is the ratio of the radius of gyration of the coiled block to the length of the rod block (Rg/LRod).15,16,37 For PDMS with an MW of 4 332 g/mol, Rg is 1.78 nm calculated from the reported relationship of ⟨R2⟩0 (=6⟨Rg2⟩)38 with the

Figure 1. 1D WAXS profiles of D58M11, D58M15, D58M20, and D58M29 after annealed at 200 °C for 48 h and those of D58M44 after annealed at 125, 150, and 200 °C for 48 h.

D58M11, D58M15, D58M20, and D58M29 after annealed at a high temperature of 200 °C still only display a broad scattering halo in the low-angle region, indicating that the PMPCS blocks in these four BCP samples are always amorphous. The 1D WAXS patterns of these four BCPs after annealed at different temperatures for SAXS experiments also only display amorphous halos as shown in Figure S2 in Supporting Information. The 1D WAXS pattern indicates that D58M44 is amorphous after annealed below 150 °C, and a sharp and intense peak at q* ≈ 4.04 nm−1 corresponding to a d-spacing of 1.56 nm appears after annealed above 150 °C, which indicates the formation of a columnar nematic (ΦN) LC phase33

Table 1. Molecular Weights, Polydispersity Indexes, and f Rod, LRod, and ν Values of the Diblock Copolymers as well as Tg Values and Phase Transition Temperatures of PMPCS Blocks sample

Mna (g/mol)

Mn(PMPCS)a (g/mol)

PDIb

f Roda (%)

LRod (nm)

ν (Rg/LRod)

Tg(PMPCS)c (°C)

TLC(PMPCS)d (°C)

D58M11 D58M15 D58M20 D58M29 D58M44

8900 10 600 12 600 16 200 22 300

4400 6100 8100 11 700 17 800

1.05 1.05 1.04 1.04 1.07

42 51 57 67 75

2.7 3.7 4.9 7.0 10.7

0.659 0.481 0.363 0.254 0.166

98 100 102 105 110

−e − − − 150

a

Determined from 1H NMR results of the macroinitiator and the block copolymers. And the values of the degrees of polymerization of PMPCS blocks noted in the sample names were also calculated from 1H NMR results. bDetermined from GPC results using linear polystyrene standards. c Glass transition temperatures of PMPCS blocks determined from DSC results. dLC phase formation temperatures of PMPCS blocks determined from 1D WAXS results. eNot applicable. C

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Figure 2. SAXS profiles of D58M11 samples after thermally annealed at each indicated temperature for 48 h (a), TEM micrographs of D58M11 samples after thermally annealed at 125 °C (b) and at 200 °C (c), the model of the LAM structure (d), and the possible arrangement of the BCP molecules in the LAM structure (e).

Figure 3. SAXS profiles of D58M15 samples after thermally annealed at indicated temperatures for 48 h (a), TEM micrographs of D58M15 samples after thermally annealed at 125 °C (b and c) and at 200 °C (d and e), the model of the GYR structure (f), and the possible molecular arrangement in a node of the GYR structure (g).

The primary reflection peak in the SAXS curve of the sample after annealed at 125 °C is at q* = 0.469 nm−1, corresponding to a d-spacing of 13.4 nm. And the q* and d values of the samples annealed at different temperatures are almost the same within the range of errors. In addition, results in the in situ SAXS experiments during heating are similar. Then TEM experiments on the microtome-sliced thin films of D58M11 samples after annealed at 125 and 200 °C were conducted to confirm the nanostructures. In the TEM micrographs, the dark part is the PDMS microdomain and the bright part is the PMPCS microdomain because PDMS has a higher electron density.40,41 TEM micrographs of the D58M11 samples after annealed at 125 and 200 °C as shown in parts b and c of Figure 2 confirm the lamellar morphologies. Therefore, the D58M11 BCP forms stable LAM structures after annealed at

(D58M44 curves indicated in Figure 1). As the temperature is increased, the intensity of the peak increases, indicating that the LC phase developed becomes more ordered. Therefore, the PMPCS blocks in D58M11, D58M15, D58M20, and D58M29 are always amorphous, while that in D58M44 is amorphous below 150 °C and forms an LC phase after annealed above 150 °C. LAM Structure of D58M11 ( f Rod = 43%, LRod = 2.7 nm). The nanophase separation behavior of the D58M11 BCP with LRod of 2.7 nm and f Rod of 43% was first investigated. The D58M11 samples were annealed at 125, 150, and 200 °C for 48 h in vacuum, respectively, and then characterized by SAXS experiments conducted under ambient conditions. As shown in Figure 2a, the SAXS profiles of D58M11 samples after annealed at different temperatures have two diffraction peaks with the scattering vector ratios of 1:2, characteristic of LAM structures. D

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Figure 4. SAXS profiles of D58M20 samples after thermally annealed at indicated temperatures for 48 h (a), TEM micrographs of D58M20 samples after thermally annealed at 125 °C (b and c) and at 200 °C (d and e), and the models of the GYR (f) and Fddd (g) structures.

temperatures ranging from 125 to 200 °C. From the volume fraction of the PMPCS block and the d-spacing of D58M11, the thickness of the PMPCS layer is 13.4 nm × 43% = 5.7 nm, which agrees well with the width of the brighter PMPCS layer (about 6 nm) in the TEM micrographs. The model of the LAM structure of D58M11 is shown in Figure 2d. In addition, because LRod is 2.7 nm, the PMPCS rods in the PMPCS lamellae should arrange in a double-layer manner. The possible molecular arrangement in the LAM structure is schematically shown in Figure 2e. The agreement of the thickness of the PMPCS lamellae with the 2LRod in return also indicates that the PMPCS block in D58M11 really has the extended-chain conformation. However, due to their small aspect ratio, the short PMPCS rods in the lamellae are amorphous, without orientational ordering.33 GYR Structure of D58M15 ( f Rod = 51%, LRod = 3.6 nm). When NRod changes to 15, LRod and f Rod increase to 3.6 nm and 51%, respectively. In the SAXS profile of the D58M15 sample after annealed at 125 °C (Figure 3a), the scattering vector ratio is 1:3.06, and the q* is at 0.460 nm−1, corresponding to a dspacing of 13.7 nm. And SAXS profiles of the D58M15 samples after annealed at higher temperatures up to 200 °C are almost the same. Because of the absence of the reflection peak at 2q*, the nanostructure cannot be assigned as a LAM structure. In order to further confirm the structure of D58M15, TEM experiments were conducted on microtome-sliced thin films. In TEM micrographs of the samples after annealed at 125 °C (parts b and c of Figure 3) and at 200 °C (parts d and e of Figure 3), the morphologies belong to those in the [111] projection of a GYR structure. The scattering vector ratio 1:3.06 in SAXS results equals to √6:√56, which can be found in the scattering result of a GYR phase.42 The primary reflection peak in the SAXS profile of a GYR structure is always indexed as (111).42 And the d-spacing obtained from SAXS results is consistent with the distance between the centers of the nearby PDMS domains in the [111] projection observed in the TEM micrographs. Thus, D58M15 forms a GYR structure after annealed at temperatures between 125 and 200 °C. The model of the GYR structure is shown in Figure 3f.

In the GYR structure, the PDMS blocks form the interweaving network which is composed of “tubes” (or “necks”) and “nodes,” with each node formed by the intersection of three tubes.6,43 The flexible PDMS chains can stretch to reach the center of the bulky nodes. The possible molecular arrangement in a node of the GYR structure is schematically shown in Figure 3g. The short PMPCS rods are always in the isotropic and amorphous state, and they can freely arrange in the matrix of the GYR structure. Although the composition of the D58M15 BCP is almost symmetric, the geometry and conformation between PMPCS and PDMS blocks are asymmetric with a geometric asymmetry parameter of 0.481. In addition, D58M15 with a relatively low overall MW is a relatively weakly segregated BCP. Therefore, it is reasonable for D58M15 to form a GYR structure. GYR-to-Fddd Phase Transition of D58M20 ( f Rod = 57%, LRod = 4.9 nm). For the D58M20 BCP, LRod increases to 4.9 nm and f Rod increases to 57%. In SAXS profiles of the D58M20 samples after annealed at 125 °C and at 150 °C as shown in Figure 4a, the diffraction peaks have scattering vector ratios of 1:1.16:1.80:2.51:3.50, which agrees with the scattering vector ratio of a GYR phase. The primary reflection is at q* = 0.372 nm−1, corresponding to a d-spacing of 16.9 nm. And the GYR phase is also confirmed by the observed morphology of the [111] projection of the GYR structure in the TEM results shown in parts b and c of Figure 4. In addition, the distance between the centers of the nearby PDMS domains in the [111] projection in the TEM micrograph is about 17 nm, consistent with the d-spacing obtained from SAXS results. Thus, the D58M20 BCP forms GYR structures after annealed at 125 and 150 °C. When D58M20 samples are annealed at 180 °C or even higher temperatures, the scattering vector ratio of the diffraction peaks in SAXS profiles changes to 1:1.22:1.72:2:2.65:3.65. And the q* value changes to 0.32 nm−1, corresponding to a d-spacing of 19.6 nm. The significant changes of the scattering vector ratio and the d value indicate the occurrence of an OOT in this BCP. And the scattering vector ratio of D58M20 annealed above 180 °C agrees with that of an Fddd structure as reported in the literature.9,10,44 Moreover, TEM micrographs of the sample E

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Figure 5. SAXS profiles of D58M29 samples after thermally annealed at indicated temperatures for 48 h (a), TEM micrographs of D58M29 samples after thermally annealed at 125 °C (b and c) and at 200 °C (d and e), and the models of the HEX (f) and BCC (g) structures.

annealed at 200 °C (parts d and e of Figure 4) clearly show that the dark PDMS ovals are connected with the trivalent junctions, which is consistent with the representative pattern of the [111] projection of an Fddd structure.10 The distance between the nearby PDMS ovals is about 21 nm, in agreement with the d-spacing from SAXS results within the range of errors. Therefore, the D58M20 BCP forms a GYR phase after annealed below 180 °C and transforms to an Fddd structure after annealed above 180 °C. The Fddd structure is a network structure with a 3-fold symmetry. It is suggested to be a stable phase for diblock copolymer melts in a narrow region overlapping with the weak segregation part of the gyroid region and is preferred over the GYR phase during weak segregations as predicted by the selfconsistent field theory (SCFT).45 And the Fddd structure is observed for the PS-b-PIP coil−coil BCP within a narrow range of composition.44 Although the GYR phase is difficult to be observed in high-MW rod−coil BCPs, there are some reports on this phase in low-MW rod−coil BCPs.15 However, this is the first report of the Fddd structure observed in rod−coil BCPs. The structural models of the GYR and Fddd structures are shown in parts f and g of Figure 4, respectively. And the possible molecular arrangements in these two structures of D58M20 should be similar to that in the GYR phase of the D58M15 BCP. The mechanism and process of the GYR-Fddd transition are in further research. HEX-to-BCC Phase Transition of D58M29 ( f Rod = 67%, LRod = 7.0 nm). D58M29 is the sample with a relatively long PMPCS rod block which still cannot form an LC phase after heated to high temperatures. In the SAXS profile of the D58M29 BCP after annealed at 125 °C (Figure 5a), the diffraction peaks have a scattering vector ratio of 1:√4:√7:√13. And the primary reflection is at q* = 0.332 nm−1, corresponding to a dspacing of 18.9 nm. In TEM micrographs shown in parts b and c of Figure 5, projections of hexagonally packed cylinders both along and perpendicular to the cylinder long axis are observed. Combining the SAXS and TEM results, D58M29 after annealed at 125 °C has the HEX nanostructure, the model of which is shown in Figure 5f. For the HEX structure, the distance between the PDMS cylinders (a) equals 2/√3d. Thus, a is 21.8

nm. The distance between the PDMS cylinders observed in TEM is about 22 nm, which agrees with the SAXS results. The radius of the PDMS cylinders (rCyl.) of D58M29 is about 6.6 nm, which can be calculated from the composition and the dspacing (rCyl. = (2f PDMS/√3π)1/2d). As a result, a − 2rCyl. is about 8.5 nm, which is a little larger than LRod. Therefore, the PMPCS rods may arrange in an interdigitated fashion in the matrix. However, the PMPCS rods have no orientational ordering, and the PMPCS matrix is still amorphous. When the annealing temperature is increased to 180 °C, a new peak at √5q* appears, and the scattering vector ratio changes to 1:√4:√5:√7:√13, which indicates that the nanostructure of D58M29 changes to BCC. The q* barely changes, which is consistent with the reported relationship of d110, BCC = d100, HEX in a BCP with a HEX−BCC transition.46 In the TEM micrographs, both rectangularly packed (Figure 5d) and hexagonally packed (Figure 5e) spheres are observed, which are the morphologies of [110] and [111] projections of the BCC structure, respectively. And the distance between the PDMS spheres in the [111] projection of the TEM micrograph is about 22 nm, which agrees with the calculated value ((2/ √3)d110 = 22.3 nm) from SAXS results. In the BCC structure, the radius of the PDMS spheres (rSph.) equals (3f PDMS/ 8π)1/3(√2d) and is calculated to be 9.2 nm. Therefore, the spheres in two (or more) layers overlap when viewed along the [110] direction in the TEM experiment.30 However, the PMPCS rods in the matrix are still amorphous. In addition, when the temperature is increased to 200 °C, the nanostructure is still BCC. In in situ SAXS experiments during heating, the profiles of the sample at different temperatures are similar to those of samples quenched from the corresponding annealing temperatures. Therefore, for D58M29, the HEX-to-BCC phase transition occurs as the annealing temperature is increased. Interestingly, the composition of the D58M29 BCP with f Rod = 67% is relatively symmetric. However, the HEX structure is observed at low temperatures and BCC at high temperatures. Because of the relatively small N value of the BCP, the χN value is small. As the annealing temperature is increased, the interaction parameter of the BCP decreases, which leads to the decrease in the χN value. Moreover, as the temperature F

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Figure 6. SAXS (a) and 1D WAXS (b) profiles of D58M44 samples after thermally annealed at indicated temperatures for 48 h, TEM micrographs of D58M44 samples after thermally annealed at 125 °C (c and d) and at 200 °C (e and f), the model of the HEX structure with the partially interdigitated packing of PMPCS rods in the amorphous PMPCS matrix (g), and the model of the LAM structure with the bilayered packing of PMPCS rods in the LC PMPCS lamella (h).

appearance of a sharp peak at q = 4.03 nm−1 in the 1D WAXS profile shown in Figure 6b, the SAXS scattering vector ratio changes to 1:√3:2:3:√13:5, indicating possible coexistence of the LAM and HEX structures. As the annealing temperature is further increased, the scattering vector ratio of the diffraction peaks in the SAXS profile of the sample changes to 1:2:3:4:5:6, which indicates that the nanostructure changes to a highly ordered LAM. And the q* changes to 0.270 nm −1 , corresponding to a d-spacing of 23.3 nm. TEM micrographs of the sample after annealed at 200 °C (parts e and f of Figure 6) also show lamellar morphologies. And the periodic size is about 25 nm, in agreement with the SAXS results within the range of errors. On the basis of the d-spacing of the LAM structure and the volume fraction of the PMPCS block, the thickness of the PMPCS layer is calculated to be about 17.5 nm (about 18 nm from the TEM result), which is much larger than LRod but a little less than 2LRod. Because the PMPCS rods in the ΦN phase are not completely parallel to the lamellar normal, they may arrange into a bilayer in the LAM structure. The scheme of the LAM structure, the bilayered packing model of the D58B44 BCP molecules, and the ΦN LC phase of PMPCS are shown in Figure 6h. Moreover, after annealed at temperatures above the LC phase formation temperature, the LC phase is stable even when the temperature is decreased. The LAM structure of the BCP after annealed at high temperatures is also stable and cannot transform back to the HEX structure even after annealed at lower temperatures for a relatively long time. From the above

increases, the compositional fluctuation and anisotropic fluctuation of the cylinders occur.28 Consequently, the HEXto-BCC transition occurs. HEX-to-LAM Phase Transition of D58M44 ( f Rod = 75%, LRod = 10.7 nm) Accompanied with the Isotropic-to-LC Phase Transformation of PMPCS. For comparison, the phase behavior of D58B44 is also studied. In this BCP, the PMPCS block is amorphous at low temperatures and forms a stable LC phase after annealed at high temperatures. When D58B44 is annealed at 125 °C which is above the Tg but below the LC formation temperature of the PMPCS block, the diffraction peaks in the SAXS profile (Figure 6a) have a scattering vector ratio of 1:√3:2:√7:3, which is characteristic of a HEX structure. And the primary reflection is at q* = 0.246 nm−1, corresponding to a d-spacing of 25.5 nm. The HEX structure is confirmed by the TEM results shown in parts c and d of Figure 6. The distance between the PDMS cylinders, a, in the TEM results is about 28 nm, while the a value calculated on the basis of the d-spacing from SAXS results is 29.4 nm. The TEM and SAXS results are consistent within the range of errors. In the HEX structure, the radius of the PDMS cylinders, rCyl., is calculated to be 7.8 nm. Thus, a − 2rCyl. is 13.8 nm, which is a little larger than the length of the PMPCS rod (LRod = 10.7 nm). Therefore, the PMPCS rods in the matrix of the HEX structure may be packed in a partially interdigitated manner as schematically shown in Figure 6g. When the sample is annealed at 150 °C at which the LC phase of the PMPCS block starts to develop as indicated by the G

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work,32 the nanostructure is LAM with PMPCS in the LC phase. And the morphology of D58M54 after annealed at lower temperatures is then confirmed in this work. As shown in part a of Figure S3 in Supporting Information, the diffraction peaks in the SAXS profile of D58M54 after annealed at 125 °C has a scattering vector ratio of 1:√3:3, and the TEM micrographs (parts c and d of Figure S3 in Supporting Information) show both HEX and modulated LAM morphologies. For the PDMSb-PMPCS BCPs with high-MW PMPCS blocks after annealed at 125 °C, which is just above the Tg values of the PMPCS blocks, for a relatively long time, the PMPCS blocks have already partially entered into LC phases as indicated by 1D WAXS patterns of D58M54 shown in part b of Figure S3 in Supporting Information. For the D58M54 sample after annealed above 150 °C, the scattering vector ratio of the diffraction peaks in the SAXS profile changes to 1:2:3:4, and the TEM micrographs (parts e and f of Figure S3 in Supporting Information) also confirm the LAM structure, consistent with the results reported previously.32 As the molecular weight of PMPCS is further increased, the composition becomes even more asymmetric, and the BCPs with f PMPCS values larger than 80% form stable HEX structures with LC PMPCS blocks as the matrix.32

results, the nanostructure of D58B44 is HEX when the PMPCS is amorphous and irreversibly transforms to LAM accompanying with PMPCS transforming to the LC phase. The unusual sequence of the phase transition from HEX to LAM is directly associated with the phase transformation of PMPCS from the amorphous state to the LC phase which leads to an increase in the interaction parameter, similar to the unique OOT during heating in another BCP system we recently reported,30 although in that case the transition is thermally reversible. Furthermore, the preferentially parallel packing of PMPCS rods in the LC phase is readily realized in the LAM structure compared with the HEX structure, which facilitates the HEXto-LAM transition. Apparent Phase Diagram of PDMS-b-PMPCS. Because of the large difference between the solubility parameters of PDMS and PMPCS, the Flory−Huggins interaction parameter is relatively large.32 Thus, the PDMS-b-PMPCS BCPs with relatively low overall MWs (therefore, low N values) can still segregate into ordered nanostructures. From the self-assembled nanostructures of the low-MW PDMS-b-PMPCS BCPs with amorphous PMPCS blocks, as the volume fraction of the rodlike PMPCS block changes from 43% to 67%, LAM, GYR, Fddd, HEX, and BCC structures as well as GYR-to-Fddd and HEX-to-BCC structural transitions are observed. For the BCP with an LC PMPCS block, the favorable rod−rod interaction helps bring the system to the strongly segregated regime.15 And the PDMS-b-PMPCS BCP with a PMPCS volume fraction of 75%, in which the PMPCS block enters into the LC phase after annealed above 150 °C, forms a HEX structure when the PMPCS block is in the amorphous state and transforms to a LAM structure following the isotropic-to-LC phase transition of the PMPCS block. Therefore, the nanophase-separated structures and order−order phase transitions of these PDMSb-PMPCS BCPs with the volume fraction of PMPCS block ranging from 43% to 75% can be summarized in a simplified apparent phase diagram (annealing temperature vs f Rod) as shown in Figure 7. However, this is not a true phase diagram because the morphologies at low annealing temperatures for the BCP with the LC PMPCS block are not equilibrium structures. In addition, for the D58M54 BCP with a PMPCS volume fraction of 79% which has been reported in our previous

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CONCLUSIONS

In summary, a series of well-defined PDMS-b-PMPCS rod−coil BCPs with amorphous or LC PMPCS rod-like blocks are investigated. For the PDMS-b-PMPCS BCPs with low-MW, amorphous PMPCS rods, as the polymerization degree of the PMPCS block is only increased from 11 to 29 and the volume fraction of PMPCS is increased from 42% to 67%, LAM, GYR, Fddd, HEX, and BCC nanostructures are observed. In addition, a GYR-to-Fddd phase transition in the BCP with NRod = 20 and f Rod = 57% and a HEX-to-BCC phase transition in the BCP with NRod = 29 and f Rod = 67% are induced by increasing the annealing temperature. These low-MW rod−coil BCPs have the phase separation abilities but are located in the weakly separated region of the phase diagram due to the relatively small N values. Thus, the nanostructures are determined by the composition and the annealing temperature, and the nanostructural transitions are induced by the variation in the composition or the annealing temperature. Moreover, although the conformations of the PMPCS and PDMS blocks are different, the PMPCS rods in the BCPs with short lengths and therefore small aspect ratios are always amorphous and have no orientational ordering. Thus, GYR, Fddd, HEX, and BCC structures with curved interfaces can stably exist as equilibrium phases in these BCPs. For the PDMS-b-PMPCS BCP with the LC PMPCS block (NRod = 44 and f Rod = 75%) which is amorphous at low temperatures and liquid crystalline after annealed at high temperatures, it forms a HEX structure when PMPCS is in the amorphous state and transforms to a LAM structure when PMPCS enters into the LC phase. And the nanostructures of PDMS-b-PMPCS BCPs with LC PMPCS blocks of even higher MWs are HEX structures when the compositions become much more asymmetric as reported in our previous work. In this series of rod−coil BCPs containing PMPCS rod blocks with low to relatively high MWs, interesting phase behaviors with various nanostructures and OOTs are observed. Therefore, this block copolymer is an excellent model rod−coil BCP system to form different well-defined nanostructures as well as to establish various phase transitions. In

Figure 7. Simplified apparent phase diagram, annealing temperature vs f Rod, of low-MW PDMS-b-PMPCS BCPs having different volume fractions of PMPCS rods. The drawing of the regions above 200 °C is rather arbitrary and hypothetical, without experimental proof. H

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addition, such a BCP system may be useful as templates for sub-10 nm patterning.

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ASSOCIATED CONTENT

S Supporting Information *

GPC curves of PDMS-b-PMPCS block copolymers, 1D WAXS patterns of the PDMS-b-PMPCS block copolymers with amorphous PMPCS at different temperatures, and SAXS and 1D WAXS profiles as well as TEM micrographs of the D58M54 BCP after annealed at different temperatures. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: (Z.S.) [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Natural Science Foundation of China (Grant 21174006) is gratefully acknowledged.

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