Thermoreversible Order–Order Transition of a Diblock Copolymer

Dec 10, 2012 - Xing-He Fan,. † ... an fPBPCS value of 90% could completely transform to the HEX ... cylinders (HEX), bicontinuous gyroids, and body-...
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Thermoreversible Order−Order Transition of a Diblock Copolymer Induced by the Unusual Coil−Rod Conformational Change of One Block Ling-Ying Shi,† I-Fan Hsieh,‡ Yu Zhou,† Xinfei Yu,‡ Hai-Jian Tian,† Yu Pan,† 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 ‡ Department of Polymer Science, College of Polymer Science and Polymer Engineering, The University of Akron, Akron, Ohio 44325-3909, United States S Supporting Information *

ABSTRACT: Unusual liquid crystalline (LC) behavior and interesting nanophase separation behavior in the selfassemblies of a series of diblock copolymers (BCPs), poly(dimethylsiloxane)-b-poly{2,5-bis[(4-butoxyphenyl)oxycarbonyl]styrene} (PDMS-b-PBPCS), were observed. The synthesized PDMS-b-PBPCS BCPs with PBPCS blocks of different molecular weights had volume fractions of PBPCS ( f PBPCS’s) from 77% to 90%. At the molecular scale, the BCPs with f PBPCS values of 77% and 83% were in amorphous isotropic state in the entire temperature range studied, and thus, they were in coil−coil BCPs. For the BCPs with f PBPCS values of 87% and 90%, the PBPCS blocks displayed a thermodynamically stable isotropic phase at lower temperatures and an LC phase at higher temperatures. Therefore, they exhibited a transition from the coil−coil to rod−coil BCPs with increasing temperatures. At the nanophase structure scale, all of the BCPs were coil−coil type and self-assembled into body-centered cubic (BCC) structures in the low temperature range. Moreover, some of the nanophase-separated structures of these BCPs could be thermoreversibly transformed by changing temperatures. For the BCPs with f PBPCS values between 77% and 83%, they remained BCC nanophase-separated structures during heating (up to 260 °C) as well as during subsequent cooling. For the BCPs with f PBPCS values of 87% and 90%, when the temperature was higher than the isotropic-to-LC transition temperatures of the PBPCS blocks, the new nanophase-separated hexagonally packed cylinder (HEX) structures started to form. However, only the nanophase-separated structure of the BCP with an f PBPCS value of 90% could completely transform to the HEX structure when the temperature reached 250 °C. As the temperature was reduced, the nanophase-separated structures of these two samples gradually reverted to the BCC structures below the LC-to-isotropic transition temperatures of the PBPCS blocks.

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BCP compared with that of the coil−coil one,7−11 including the strong phase separation ability, the asymmetry of the phase diagram, and the appearance of some new morphologies.7,9,12,13 Moreover, the rod blocks in rod−coil BCPs tend to form ordered mesophase structures at the length scale of 1−5 nm. By combining nanophase separation and mesophase ordering, rod−coil BCPs can form hierarchical structures at a broader length scale.8,9,12 For a rod−coil BCP with an LC block, the phase behavior of the LC block influences the nanophase separation,9,14−16 and the LC phase transition may be used to trigger the phase transition of the nanophase-separated structures of LC BCPs.17−19 The order−disorder transition temperature of an

INTRODUCTION The self-assembly of block copolymers (BCPs) can provide a variety of periodic nanostructures which have potential applications in nanotechnology.1−3 Depending on the volume fraction f of the individual block and χN, where χ is the Flory− Huggins interaction parameter and N is the total degree of polymerization of the copolymer, simple coil−coil BCPs can self-assemble into lamellae (LAM), hexagonally packed cylinders (HEX), bicontinuous gyroids, and body-centered cubic (BCC) arrays of spheres.4−6 The phase behaviors of coil− coil BCPs have been well understood through theoretical descriptions and experimental studies.5,6 When a rod-like polymer such as a helical polypeptide, a conjugated polymer, and a liquid crystalline (LC) polymer is introduced to form a rod−coil BCP, the additional geometrical asymmetry between the rod and coil blocks and the aligning interaction between the rod blocks result in the different phase behavior of the rod−coil © 2012 American Chemical Society

Received: September 28, 2012 Revised: November 30, 2012 Published: December 10, 2012 9719

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oxycarbonyl]styrene} (PDMS-b-PBPCS, DB) block copolymers with PBPCS blocks of different MWs through the atom transfer radical polymerization (ATRP) method. We have studied the bulk phase behaviors of these BCPs and the influences of the temperature and phase transitions of PBPCS blocks on the nanophase separation of the BCPs utilizing smallangle X-ray scattering (SAXS) and transmission electron microscopy (TEM). When BCP samples with the volume fractions of PBPCS (f PBPCS’s) between 77% and 90% are annealed at relatively low temperatures, all samples exhibit the coil−coil BCPs and self-assemble into BCC structures. The nanophase-separated structures of BCPs containing low-MW PBPCS blocks maintain the BCC structures during heating and cooling. For the BCPs containing PBPCS blocks of relatively high MWs, thermoreversible order−order transitions of their nanophase-separated structures occur upon the phase transitions of the PBPCS blocks between the isotropic and LC phases.

LC BCP almost coincides with the clearing point of the LC block.18 In another example of poly(isobutyl methacrylate)-bpoly[2,5-di(isopropyloxycarbonyl)styrene] (PiBMA-b-PiPCS) BCPs containing PiPCS as a mesogen-jacketed LC polymer (MJLCP), the periodic size of the nanophase-separated structure increases or the BCP undergoes an irreversible order−order transition upon the development of the LC phase of the MJLCP.19,20 An ABA triblock copolymer with an LC block B as the matrix forms a HEX structure when annealed at a temperature at which the LC block is in a nematic phase but exhibits a BCC structure when annealed at a temperature above the clearing temperature of the LC block.17 When the LC block is in the smectic or nematic phase, the nanophase-separated structure prefers to be the LAM or HEX structure rather than the spherical morphology which has a high curvature at interfaces when the composition is quite asymmetric.17,21−23 Poly{2,5-bis[(4-butoxyphenyl)oxycarbonyl]styrene} (PBPCS) is an MJLCP exhibiting an unusual phase behavior.20,24 PBPCS with a low molecular weight (MW) (∼3.36 × 104 g/mol) displays a thermodynamically stable isotropic phase at lower temperatures and a hexatic columnar nematic LC phase at higher temperatures. The isotropic-to-LC transition temperature (Tiso−LC) of PBPCS in the heating cycle depends on the MW, and the LC-to-isotropic transition temperature (TLC−iso) during cooling is always lower than the corresponding Tiso−LC measured during heating. Higher entropy gains which are originated from the free mobility of bulky side chains in the LC phase have been proposed to be the factor for stabilizing the LC phase of PBPCS at high temperatures.24 We thus design a series of BCPs containing a coil polymer, poly(dimethylsiloxane) (PDMS), and a PBPCS block with different MWs and predict that this series of BCPs should exhibit interesting phase behaviors as summarized in Figure 1. Namely,

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EXPERIMENTAL SECTION

Materials. 2-Bromo-2-methylpropionyl bromide (Acros, 98%), monohydroxy-terminated poly(dimethylsiloxane) (PDMS-OH, Aldrich, 4670 g/mol, 0.97 g/cm3), and N,N,N′,N″,N″-pentamethyldiethylenetriamine (PMDETA, TCI, 98%) were used as received. Triethylamine (98%) and tetrahydrofuran (THF, Beijing Chemical Reagents Co., A.R.) were used after distilled. Chlorobenzene was purified by washing with concentrated sulfuric acid to remove residual thiophenes, followed by washing with a 5% sodium carbonate solution and with water, and then dried with anhydrous calcium chloride before it was finally distilled. CuBr (Beijing Chemical Reagents Co., A.R.) was purified by washing with acetic acid, followed by washing with methanol, and then dried for use. Synthesis of the Macroinitiator 2-Bromoisobutyrate-Terminated PDMS (PDMS-Br). The macroinitiator 2-bromoisobutyrateterminated PDMS was synthesized as reported previously.21 Synthesis of PDMS-b-PBPCS through ATRP. In a typical experiment for polymerization, the monomer {2,5-bis[(4butoxyphenyl)oxycarbonyl]styrene} (BPCS) (0.557 g, 1.15 mmol), PDMS-Br (0.05 g, 11.5 μmol), CuBr (1.66 mg, 11.5 μmol), PMDETA (2.00 mg, 11.5 μmol), and chlorobenzene (3.05 mL) were charged into a polymerization tube. After being stirred and degassed by three freeze−pump−thaw cycles, the tube was sealed under vacuum and subsequently immerged into an oil bath thermostated at 110 °C for 8 h. It was then quenched in liquid nitrogen and taken out to ambient condition. The solution was passed through a neutral alumina column in order to remove copper salt. Finally, the copolymer was precipitated in a large volume of methanol and dried in vacuum overnight. Figure S1 in the Supporting Information is the representative 1H NMR spectrum of the PDMS-b-PBPCS block copolymers. The resonance signals of protons of the methyl group in PDMS (a in Figure S1), vinyl backbone (b and b′ in Figure S1), disubstituted phenyl (c in Figure S1), trisubstituted phenyl (d in Figure S1), oxyethyl group (e in Figure S1), ethyl group (f in Figure S1), and methyl group (g in Figure S1) in PBPCS appeared at δ = −0.05−0.20, 1.52−1.82, 6.38−6.96, 7.2−7.82, 3.40−3.89, 1.20−1.49, and 0.62−0.95 ppm, respectively. Characterization Techniques. The molecular weights of BCPs were determined with the combination of GPC, 1H NMR, and the absolute molecular weight of the macroinitiator.21 GPC experiments were conducted on a Waters 2410 instrument equipped with a Waters 2410 RI detector, with pure THF as eluent (1.0 mL/min). The calibration curve was obtained with linear polystyrenes as standards. 1 H 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. SAXS experiments were carried out on a Micromax002+ SAXS instrument using Cu Kα radiation at a wavelength of 0.154 nm. The working voltage and current were 45 kV and 0.88 mA, respectively. The

Figure 1. PDMS-b-PBPCS block copolymers with PBPCS blocks of different molecular weights exhibiting the characteristics of coil−coil and rod−coil BCPs in different temperature ranges.

the BCPs containing low-MW PBPCS blocks will be the coil− coil type in the whole temperature range up to decomposition, while BCPs containing high-MW PBPCS blocks will thermoreversibly alternate between the coil−coil and rod−coil types following the phase transition of PBPCS between the isotropic state and the LC phase. In this article, we thus aim to synthesize a series of poly(dimethylsiloxane)-b-poly{2,5-bis[(4-butoxyphenyl)9720

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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. One-dimensional wide-angle X-ray scattering (1D WAXS) experiments were performed on a SAXSess instrument (Anton Paar) with the working voltage and current of 40 kV and 40 mA, respectively. The scattering patterns were recorded on an imaging plate (IP) with a pixel size of 42.3 × 42.3 μm2 which extended to high-angle range (the q range covered by the IP was from 0.06 to 29 nm−1). The diffraction peak positions in the wideangle region were calibrated with silicon powder. TEM bright-field images were obtained with a JEOL (1200 EX II) TEM instrument using an accelerating voltage of 120 kV.

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RESULTS AND DISCUSSION Synthesis and Characterization of PDMS-b-PBPCS BCPs. A series of PDMS-b-PBPCS BCPs with different molecular weights of PBPCS were synthesized through ATRP initiated by the same PDMS macroinitiator prepared from commercially available PDMS with a hydroxyl end group as described previously21 (Scheme 1). The number-average

Figure 2. GPC profiles of the macroinitiator and the block copolymers synthesized.

were no other phase transitions except the glass transitions, while D58B82 and D58B104 in the heating cycle had endothermic phase transition peaks at 221 and 203 °C, respectively. In the DSC curves of D58B82 and D58B104 during the second cooling cycle, the corresponding exothermic peaks were observed at relatively lower temperatures. Furthermore, the phase behaviors of PBPCS blocks were also investigated using 1D WAXS experiments as shown in Figure 3. The 1D WAXS patterns of D58B40 (parts a and b of Figure 3) and D58B60 (parts c and d of Figure 3) during heating and cooling only display a broad scattering halo in the entire temperature range, indicating that the PBPCS blocks in these two samples were always amorphous. On the other hand, 1D WAXS patterns of the D58B82 (parts e and f of Figure 3) and D58B104 (parts g and h of Figure 3) show the amorphous halo at temperatures below the corresponding phase transition temperatures obtained from the DSC results, and yet a sharp and intense peak at q* ≈ 3.373 nm−1 corresponding to a d-spacing of 1.86 nm at temperatures higher than the phase transition temperatures appears. The appearance of the reflection peak with a d-spacing of 1.86 nm is indicative of the formation of the hexatic columnar nematic LC phase of the PBPCS block.24 As the temperature is increased, the intensities of the diffraction peaks are also increased. During cooling, the diffraction peaks disappear. These results indicate that the PBPCS blocks in D58B82 and D58B104 form the LC phases above the corresponding phase transition temperatures, consistent with the results of the PBPCS homopolymers reported in the literature.24 Therefore, the D58B40 and D58B60 BCPs are always the coil− coil type in the entire temperature range, while D58B82 and D58B104 are the coil−coil BCPs below the phase transition temperatures of the PBPCS blocks and are the rod−coil BCPs above such temperatures. The latter two block copolymers thus show the characteristics of the coil−coil BCPs or those of rod− coil BCPs at different temperature ranges. Nanophase-Separated Structures of PDMS-b-PBPCS BCPs after Thermal Annealing at Relatively Low Temperatures. First, all PDMS-b-PBPCS samples were annealed at temperatures higher than their Tg values of the PBPCS blocks but lower than the Tiso−LC values of the PBPCS blocks of D58B82 and D58B104 to ensure that they were the coil− coil BCPs under the annealing conditions. After being annealed at 160 °C for 48 h in vacuum, the samples were characterized by SAXS and TEM. The SAXS experiments on these samples after thermal annealing were conducted under ambient

Scheme 1. Synthesis of PDMS-b-PBPCS Block Copolymers

molecular weight, degree of polymerization, and polydispersity index (PDI) of PDMS-Br were 4332 g/mol, 58, and 1.02, respectively.21 The monomer of the PBPCS block, BPCS, was prepared and characterized by the same method described in the literature.24 Using PDMS-Br as initiator, CuBr as catalyst, PMDETA as ligand, and chlorobenzene as solvent, a variety of PDMS-b-PBPCS diblock copolymers were synthesized by changing the ratio of macroinitiator to monomer. All PDMSb-PBPCS BCPs were characterized by 1H NMR (Figure S1) and GPC (Figure 2 and Table 1). The molecular weights ranged from 11 300 to 30 200 g/mol from GPC results. The f PBPCS value was from 77% to 90% calculated from the 1H NMR results of the BCPs using densities of 0.97 g/cm3 for PDMS (provided by the supplier) and 1.25 g/cm3 for PBPCS measured by the method reported in the literature25 (Table 1). All results indicated that these block copolymers had welldefined chemical structures with symmetric and narrow molecular weight distributions. LC Phase Behaviors of PBPCS Blocks of PDMS-bPBPCS BCPs. The LC phase behaviors of PBPCS blocks in all these BCPs were characterized by DSC (Figure S2 in Supporting Information) and 1D WAXS experiments (Figure 3). Figure S2 presents DSC curves of all block copolymers during the second heating cycle and the second cooling cycle at the heating or cooling rate of 10 °C/min. In the DSC curves of D58B40 and D58B60 during heating and cooling processes, there 9721

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Table 1. Molecular Weights, Polydispersity Indexes, f PBPCS Values of the Macroinitiator and the Diblock Copolymers, and Tg Values and Phase Transition Characteristics of PBPCS Blocks sample PDMS-Br D58B40 D58B60 D58B82 D58B104

Mna (g/mol) 4300 11 300 20 100 24 300 30 200

Mnb (g/mol) 4500 24 800 33 500 44 100 54 800

PDIa 1.07 1.08 1.08 1.09 1.09

f PBPCSb (%)

Tg(PBPCS)c (°C)

Tiso−LCc (°C)

TLC−isoc (°C)

ΔHiso−LCc (J/g)

0 77 83 87 90

− 102 104 108 110

− − − 221 203

− − − 185 174

− − − 0.051 0.345

d

a Determined from GPC results using linear polystyrene standards. bDetermined from 1H NMR results of the block copolymers and the absolute molecular weight of the PDMS block. Correspondingly, the values of polymerization degree of PBPCS blocks labeled in the subscripts of the sample names were calculated from the molecular weights determined from 1H NMR results. cDetermined from DSC results. dNot applicable.

Figure 3. 1D WAXS profiles of D58B40 (a, b), D58B60 (c, d), D58B82 (e, f), and D58B104 (g, h) diblock copolymers at indicated temperatures during heating and cooling.

conditions, while the TEM experiments were performed on the microtome-sliced thin films (85 nm, without staining) of the corresponding samples after SAXS experiments. The darker part in the TEM micrograph is the PDMS domain because of the relatively higher electron density of PDMS.26 The SAXS profiles of all samples are shown in Figure 4, and the q* and calculated d-spacing values of the peaks are shown in Table 2. The profile of D58B40 has only a primary diffraction at q* = 0.433 nm−1 corresponding to a d-spacing of 14.5 nm. Although there are no clear higher-ordered diffraction peaks, the structure of D58B40 is BCC, confirmed by the tetragonally and hexagonally packed spheres in the TEM micrographs as shown in Figure 5a. The profiles of D58B60, D58B82, and D58B104 have the scattering vector ratio of 1:√2:√3, a characteristic of a cubic structure. The primary diffractions are at q* = 0.369, 0.333, and 0.302 nm−1, corresponding to dspacing values of 17.0, 18.9, and 20.8 nm, respectively. Therefore, the nanostructures of the latter three samples also possess BCC structures. The TEM micrographs of the indicated cross sections of the BCC structures of D58B60, D58B82, and D58B104 are shown in parts b, c, and d of Figure 5, respectively.

Figure 4. SAXS profiles of all diblock copolymers after thermal annealing in vacuum at 160 °C for 48 h.

For these BCC nanostructures, the d-spacing values obtained from the SAXS experiments are d110’s which are equivalent to the a/√2 (where a is the d100) values. Figures 5e−h show the model of the BCC structure and the different patterns of (100), (110), and (111) cross sections which have been observed in 9722

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entire temperature range as described previously. In-situ SAXS experiments in heating and cooling runs were performed to determine the influence of temperature on the nanophaseseparated structures. Both samples were first thermally annealed at 160 °C for 48 h to have BCC nanostructures and measured at room temperature (RT) as indicated. The data collection at each temperature took about 5 min after the sample was annealed at the corresponding temperature for 30 min. During the heating process from 160 to 260 °C, the SAXS profiles of D58B40 (Figure 6) and D58B60 (Figure 7) exhibit no

Table 2. Nanostructure Characteristics of the Diblock Copolymers after Thermal Annealing in Vacuum at 160 °C for 48 h Obtained from SAXS Results sample

q* (nm−1)

d (nm)

nanostructure

D58B40 D58B60 D58B82 D58B104

0.433 0.369 0.333 0.302

14.5 17.0 18.9 20.8

BCC BCC BCC BCC

Figure 6. SAXS profiles of the D58B40 diblock copolymer at the indicated temperatures during heating (a) and cooling (b). The sample before heating was thermally annealed in vacuum at 160 °C for 48 h and was measured by SAXS at RT.

Figure 5. TEM micrographs of the indicated cross sections of D58B40 (a), D58B60 (b), D58B82 (c), and D58B104 (d) along with the model of the BCC structure (e) and the models of the patterns which could be observed by TEM experiments of the corresponding cross sections of the BCC structure including (100) (f), (110) (g), and (111) (h) cross sections. Figure 7. SAXS profiles of the D58B60 diblock copolymer at the indicated temperatures during heating (a) and cooling (b). The sample before heating was thermally annealed in vacuum at 160 °C for 48 h and was measured by SAXS at RT.

TEM experiments. The observed nearest sphere-to-sphere distance of (100) cross section is equivalent to a/√2. For the (110) cross section, the rectangular pattern with the length equivalent to a/√2 and the width equivalent to a/2 can be observed. The sphere-to-sphere distance of the (111) cross section is equal to (√2/√3)a. The indicated sphere-to-sphere distances of the corresponding cross sections in the TEM micrographs are consistent with the SAXS results within the range of experimental error. For D58B40 and D58B60 with relatively lower f PBPCS values, they have lower total polymerization degrees and, therefore, smaller χN values. Although D58B82 and D58B104 have relatively higher total polymerization degrees and larger χN values, they have much higher f PBPCS values. Therefore, it is reasonable for all these samples to form BCC structures of different d-spacing’s after annealing at 160 °C for 48 h with all PBPCS blocks in the isotropic state. Influence of Temperature on the Phase Behaviors of D58B40 and D58B60. For D58B40 (f PBPCS = 77%) and D58B60 (f PBPCS = 83%), the PBPCS blocks are always isotropic in the

dramatic changes of those diffraction peaks except for a small shift of the q values toward the lower q direction. Although the profiles of D58B40 always have smeared higher-ordered diffraction peaks, there are no discontinuous changes in the plots of the reciprocal of the maximum scattering intensity of the first-order diffraction (Im−1) as a function of the reciprocal of the absolute temperature (1/T) (Figure S3), indicating that D58B40 has no order−order or order−disorder transition in the temperature range studied.27 For D58B60, the scattering vector ratio is always 1:√2:√3 at different temperatures, revealing that D58B60 also maintains the BCC structure during heating. The subtle shift of the q value to the lower q direction is attributed to a small increase in the d-spacing due to thermal expansion of the nanostructure. During the subsequent cooling 9723

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process from 260 °C to RT, the nanostructures of D58B40 and D58B60 are still BCC, and the d-spacing’s shrink back to the initial values prior to the previous heating. Both D58B40 and D58B60 are always the coil−coil BCPs during heating and cooling. A significant difference between the Hildebrand solubility parameters of PBPCS (δPBPCS = 21.6 J1/2 cm−3/2 as calculated in the Supporting Information) and PDMS (δPDMS = 14.9 J1/2 cm−3/2) indicates that PDMS-b-PBPCS has a large Flory−Huggins interaction parameter and is a strongly phase-segregated system.28 For strong nanophase-separated block copolymers, the influence of temperature during heating thus stabilizes the nanophase-separated structures. Variation of the Nanostructure of D58B82 during Heating and Cooling. In D58B82 (f PBPCS = 87%), the PBPCS block transforms from the isotropic state to the LC phase at 221 °C with the ΔHiso−LC of about 0.051 J/g in the heating cycle at a rate of 10 °C/min. During the heating process of in-situ SAXS experiments as shown in Figure 8,

Table 3. Morphological Characteristics of D58B82 at Different Temperatures Obtained from SAXS Results T (°C)

q* (nm−1)

d (nm)

RT 160 230 260

0.333 0.330 0.330 0.326

18.8 19.0 19.0 19.3

qnew* (nm−1)

0.241 0.239

dnew (nm)

nanostructure

26.1 26.3

BCC BCC BCC and HEX BCC and HEX

can be ascribed to the fact that the PBPCS matrix is only partially in the LC phase confirmed by the existence of the amorphous halo at the left side of the diffraction peak in the 1D WAXS profile of D58B82. Moreover, the nanostructures forming at high temperatures cannot be fixed by quenching due to the wide temperature range of the isotropic state of the PBPCS block between the LC phase and the glassy state. Thermoreversible BCC−HEX Transition of D58B104 Accompanying the LC Phase Transition of the PBPCS Block. For D58B104 ( f PBPCS = 90%), the PBPCS block transforms from the isotropic state to the LC phase at 203 °C with a ΔHiso‑LC of 0.345 J/g in the heating cycle at a rate of 10 °C/min. In the in-situ SAXS measurements on D58B104 in the BCC structure during heating as shown in Figure 9, when

Figure 8. SAXS profiles of the D58B82 diblock copolymer at the indicated temperatures during heating (a) and cooling (b). The sample before heating was thermally annealed in vacuum at 160 °C for 48 h and was measured by SAXS at RT.

when the temperature is increased to be higher than the Tiso−LC of the PBPCS block, a new peak with a lower q value appears, indicating the formation of a new structure. With a further increase in temperature, the intensity of the diffraction peak of the new structure increases, while the intensities of the diffraction peaks of the initial BCC structure decrease. However, the new structure does not completely replace the initial BCC structure, and the two structures coexist even at the highest temperature in our experimental condition. The higherordered diffraction peaks of the new structure and those of the BCC structure overlap. The peaks in the profile of the new structure have a scattering vector ratio of 1:√3:√4:√7:√13, which indicates that it is a HEX structure with the qnew* of 0.239 nm−1 corresponding to a d-spacing of 26.3 nm at 260 °C. The nanostructural characteristics of D58B82 at several temperatures are listed in Table 3. During cooling, the intensities of the diffraction peaks of the HEX structure gradually decrease, and those of the BCC structure increase. When the temperature is decreased to be lower than the TLC−iso, the nanostructure returns back to the BCC completely. Accompanying with the phase transition of the PBPCS block, the nanophase-separated structure of the D58B82 block copolymer shows a thermoreversible order−order transition between the HEX and the BCC structures. The coexistence of HEX and BCC nanostructures of the BCP at high temperatures

Figure 9. SAXS profiles of the D58B104 diblock copolymer at the indicated temperatures during heating (a) and cooling (b). The sample before heating was thermally annealed in vacuum at 160 °C for 48 h and was measured by SAXS at RT.

the temperature is higher than the Tiso−LC of the PBPCS block, a new nanostructure is formed as evidenced by the appearance of a new diffraction peak with a lower q value. As the temperature is further increased, the higher-ordered diffraction peaks of the new structure appear, and the intensities of diffraction peaks attributed to the new structure increase, while those of peaks generated by the initial BCC structure decrease. When the temperature is increased to 250 °C at which the amorphous halo near the diffraction peak in the 1D WAXS profile almost disappears (part g of Figure 2), the diffraction peaks of the BCC structure in the SAXS profile also disappear. The scattering vector ratio of peaks from the new structure is 1:√3:√4:√7, a characteristic of a HEX structure. The primary diffraction has a q* of 0.234 nm−1 corresponding to a d-spacing of 26.8 nm. Thus, the cylinder-to-cylinder distance is about 31.0 nm (2dnew/√3), and the radius of PDMS cylinders is about 4.9 nm (dnew(2ϕPDMS/√3π)1/2), where ϕPDMS is the volume fraction of the PDMS block. The simulated run length of the PBPCS rod (using the reported method for MJLCPs29) is 9724

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packed cylinders.17 As the PBPCS blocks transform from the coiled to the rod-like chain, the rearrangement of the polymer chains must take place during the transition process of the nanostructure. Moreover, during such a transition process, there is a temperature range in which the BCC and HEX structures coexist, which can be ascribed to the coexistence of the amorphous and LC phases of the PBPCS blocks as described previously. Upon the LC-to-isotropic transition of the PBPCS blocks in the cooling cycle, D58B82 and D58B104 change back to coil−coil BCPs. Correspondingly, the nanostructures are reverted to the BCC structures because the composition of D58B82 and D58B104 are very asymmetric and the BCC structures are the stable structures for them in this case. Therefore, the order−order transitions are thermoreversible. The chain conformations of PBPCS segments, the phases of the PBPCS matrix, and the nanophase-separated structures of D58B104 in different temperature ranges are schematically drawn in Figure 10. Finally, the nanophase-separated structures of all PDMS-b-PBPCS BCPs studied in this work at different temperature ranges are summarized in Figure 11.

about 23.5 nm, which is close to the value of the cylinder-tocylinder distance minus the diameter of the PDMS cylinders within the range of error. Therefore, the PBPCS rods are packed in an interdigitated manner in the PBPCS matrix. The nanostructural characteristics at RT before heating and at 160, 220, and 250 °C during heating are shown in Table 4. During Table 4. Morphological Characteristics of D58B104 from SAXS Results at Different Temperatures during Heating T (°C) RT 160 220 250

q* (nm−1) d (nm) 0.302 0.300 0.298

20.8 20.9 21.1

qnew* (nm−1)

0.236 0.234

dnew (nm)

nanostructure

26.6 26.8

BCC BCC BCC and HEX HEX

cooling, the nanostructure is thermoreversibly transformed back to the BCC structure from the HEX structure. The temperature at which the HEX structure disappears is slightly lower than that at which the HEX structure forms during heating. These observations are consistent with DSC and WAXS results. For the order−order transitions of BCPs between BCC and HEX structures reported in the literature, the HEX structures always form in the lower temperature range, while the BCC structures appear at higher temperatures.17,30 However, it is the opposite case in this work. The BCC structure is stable at lower temperatures, while the HEX structure is stable at higher temperatures, which should be directly related to the unusual phase transitions of the LC PBPCS blocks. We are now trying to provide our explanations in the next section. Mechanism of the Thermoreversible Order−Order Transition of PDMS-b-PBPCS Induced by LC Phase Transitions of PBPCS Blocks. Compared to D58B40 and D58B60, D58B82 and D58B104 have more asymmetric compositions. For D58B40 and D58B60 containing PBPCS blocks which are always amorphous, no phase transition of their nanophaseseparated structures occurs with varying temperatures, while D58B82 and D58B104 have the nanostructural phase transitions accompanying with the mesophase transition of PBPCS blocks. Therefore, the thermoreversible order−order transitions of D58B82 and D58B104 between the BCC and HEX structures are induced by the phase transitions of the PBPCS blocks between the isotropic and LC phases. On the one hand, the increase in the stiffness of the PBPCS segments results in the increase of the conformational asymmetry between the PBPCS and PDMS segments. Thus, the repulsive interaction between PBPCS and PDMS segments increases when PBPCS changes from coil to rod, which contributes to the larger Flory−Huggins interaction parameter of the BCP.9 On the other hand, during the isotropic-to-LC transition of the PBPCS blocks, the additional aligning interaction between the rod PBPCS blocks and the geometrical asymmetry between the rod PBPCS block and the coil PDMS block result in the instability of the BCC nanostructure due to the highly curvatured interfaces.22,23 The transition of the BCC to the HEX structure minimizes the elastic energy.17 The BCC−HEX transition should be processed by the coalescence of the spheres along the (111) direction because the distance between the spheres along this direction is minimal ((√3/2)a). Only the coalescence of spheres in the BCC structure along the (111) direction may form the hexagonally

Figure 10. Chain conformations of PBPCS segments, phases of the PBPCS matrix, and the nanophase-separated structures of D58B104 in different temperature ranges.

Figure 11. Nanophase-separated structures of PDMS-b-PBPCS BCPs with varying compositions in different temperature ranges. 9725

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CONCLUSIONS In summary, we have synthesized a series of well-defined PDMS-b-PBPCS BCPs with f PBPCS values ranging from 77% to 90% through the ATRP method. The unusual LC phase behaviors of the PBPCS blocks in the BCPs have been studied utilizing DSC and 1D WAXS experiments. These PDMS-bPBPCS BCPs combine characteristics of the coil−coil and rod− coil BCPs. Their nanophase separation behaviors have been characterized by SAXS and TEM experiments. The influences of temperature and the mesophase transition of PBPCS blocks on the nanophase separation of the BCPs have also been monitored via in-situ SAXS experiments during both heating and cooling. When the BCPs are annealed at relatively low temperatures, all of them are the coil−coil types and selfassemble into the BCC nanostructures. For the BCPs with the low-MW PBPCS blocks always isotropic in the entire temperature range (D58B40 and D58B60), the nanophaseseparated structures maintain the BCC structures during heating and cooling. On the other hand, for D58B82 and D58B104, the PBPCS blocks have higher molecular weights, and the BCPs thermoreversibly change between the coil−coil and rod−coil types when the PBPCS blocks alternate between the isotropic and LC phases. Correspondingly, the nanophaseseparated structures are thermoreversibly changed between the BCC and HEX structures. Moreover, during the transition process, there is a temperature range in which the BCC and HEX structures coexist. By comparing the phase behaviors of the BCPs with those of the PBPCS blocks of different molecular weights during heating and cooling, it can be concluded that the thermoreversible order−order transitions of the BCPs are induced by the phase transitions of the PBPCS blocks. Compared with the HEX−BCC transitions reported in the literature in which the HEX structure is observed at lower temperatures and the BCC structure emerges at higher temperatures, the order−order transitions of the PDMS-bPBPCS BCPs are reversed mainly because isotropic−LC transitions of the PBPCS blocks occur in the opposite direction. Furthermore, these block copolymers with thermoreversible order−order transitions may be possible candidates for potential applications in thermal responsive nanomaterials.

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ASSOCIATED CONTENT

S Supporting Information *

Text and table about the Hildebrand solubility parameter of PBPCS; 1H NMR spectrum and DSC curves of PDMS-bPBPCS block copolymers; the plot of the Im−1 as a function of 1/T for the D58B40 diblock copolymer. 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 [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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REFERENCES

(1) Bates, F. S.; Fredrickson, G. H. Phys. Today 1999, 52, 32. 9726

dx.doi.org/10.1021/ma302048y | Macromolecules 2012, 45, 9719−9726