Thermotropic and Lyotropic Transitions of Concentrated Solutions of

Oct 6, 2017 - Rich phase structures and evolutions of the blends of smectic–nematic (S–N) liquid crystalline (LC) diblock copolymers and a nematic...
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Thermotropic and Lyotropic Transitions of Concentrated Solutions of Liquid Crystalline Block Copolymers in a Liquid Crystalline Solvent Wei Wei,† Donglei You,† and Huiming Xiong*,†,‡ †

Department of Polymer Science, School of Chemistry and Chemical Engineering and ‡Center for Soft Matter and Interdisciplinary Sciences, Shanghai Jiao Tong University, Shanghai 200240, P. R. China S Supporting Information *

ABSTRACT: Rich phase structures and evolutions of the blends of smectic−nematic (S−N) liquid crystalline (LC) diblock copolymers and a nematic solvent (5CB) are investigated in a concentrated regime. These types of fully LC systems are macroscopically homogeneous with ordered phase-separated structures on the nanometer scale. By varying the temperature as well as the content of 5CB, the phase diagram of the blends is constructed, where the order-to-order transition (OOT) between lamellar and cylindrical nanostructures can be induced either lyotropically or thermotropically. The temperature and concentration dual-responsive properties are found to be closely related to the selectivity and distribution of 5CB, in addition to its phase transformation at the nematic−isotropic transition. As inferred from the miscibility and phase behaviors of its blends with the respective smectic and nematic homopolymers, the partition of 5CB in the S−N diblock copolymers and its influence on the phase structures reveal the important role of the LC interactions on the self-assembly of the diblock copolymers.

1. INTRODUCTION The self-assembly behaviors and solution properties of block copolymers in solvents are of fundamental and practical importance in fabricating and engineering polymeric materials.1−17 The structure and morphology of block copolymer solution depend on molecular parameters including molecular weight and composition of the block copolymer and solution parameters such as selectivity of the solvent, concentration, and temperature.1,2 By tuning these parameters, a variety of morphologies have been observed in the block copolymer solutions. Among these, the nature of the solvent can play a key role on the equilibrium morphological structure of the diblock copolymers. In a selective solvent, micellar structures often form in the dilute solution.3−10 At higher concentrations, additional interactions among the polymer chains may lead to lyotropic phases in different orders and symmetries.11−17 Small LC molecules can be also used as solvents. Because of its anisotropic and responsive properties, blending of polymer and LC solvent has received considerable attention for several decades, for instance, polymer dispersed LC (PDLC) for LC display.18−20 Because of the free energy penalty from the competition between LC ordering and conformational entropy of polymer chains, macrophase separation usually occurs in polymer coil/LC systems. However, the compatibility between the amorphous polymer chains and LC solvents can be improved by utilizing LC polymers (LCP) instead.21−26 Particularly, LC-coil block copolymers in LC solvents have been reported.27−31 The abrupt change of structure and consequently the selectivity of LC solvent across the clearing point (Ti) can be utilized to tune the self-assembly processes. Finkelmann and co-workers are the first to investigate the © XXXX American Chemical Society

miscibility and self-assembly of LC-coil block copolymer in a LC solvent.27,28 They mapped the miscibility phase diagram and observed the forming of threadlike aggregates consisting of bilayered vesicles in the dilute nematic solution. Kornfield and co-workers constructed a homogeneous LC gel with a fast response by dissolving ABA type LC block copolymers in 5CB solvent, in which the LC-phobic end blocks form the physical cross-linking points and stabilize the networks.29 The order-todisorder transition (ODT) from spherical micelles to isotropic solution can be further triggered by the melting of 5CB, highlighting the switchable effect of LC solvents.30 To our best knowledge, most of the reported work focuses on the LC block copolymers in LC solvents in a dilute regime. In the dilute solution, the LC ordering of the solvent associated with its high elastic free energy may play a dominant role, which gives rise to the tendency of macrophase separation of the block copolymer and the LC solvent, especially when the fraction of the LC block is low in the block copolymer. In this work, we expand the concentration of block copolymer/LC blends to a full range, particularly in a concentrated regime, to potentially enrich the phase behavior and broaden the phase diagram window.21−24 Instead of using conventional LC-coil block copolymers,32−34 we choose fully LC diblock copolymer, an S−N double liquid crystalline block copolymer,35,36 which may represent a novel type of block copolymer integrating prototypic LC fields.35−39 Especially, the chemistry of the mesogen in the N block is very similar to that Received: August 3, 2017 Revised: October 2, 2017

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from the 1H NMR measurement. The volume fraction of the N block in the S−N diblock copolymer is around 0.51 at room temperature, as is estimated on the basis of the densities of the respective S and N homopolymers.36 The detailed chemical and thermal characterizations are described in Figures S1 and S2 as well as Table S1 in the Supporting Information. Our previous work demonstrates that the S− N block copolymers can form the Lo and Ho superstructures.35 In the Lo structure, the incompatible S and N blocks segregate into a lamellar structure; particularly, the smectic layers within the S domain take an interdigitated array with a half-layer offset between the nearest lamellae. At high temperature above the clearing point, the conventional lamellar nanophase-separated structure (Lam) forms. In the Ho structure, the smectic layers within the S cylinders similarly stack in an alternating half-a-layer shift in the adjacent rows of cylinders in the pseudohexagonal structure. 2.2. Measurement Techniques. Gel permeation chromatography (GPC) with a multiangle laser light scattering detector (Wyatt Dawn EOS) plus a differential refractometer detector (Waters Model 2414) has been used to determine the molecular weights of the polymers and their polydispersities. The NMR spectra were recorded on a Varian MERCURY plus-400 (400 MHz, 1H NMR) spectrometer with chemical shifts reported in ppm relative to the residual deuterated solvent. The thermal behaviors were investigated on a PerkinElmer differential scanning calorimeter DSC-8000 under the nitrogen atmosphere. A cooling process from isotropic melt was always carried out first. Temperature scans were performed at a rate of 10 °C/min. The measurements were repeated at least three times to obtain the averaged enthalpy value of the phase transitions, in which each cycle shows similar phase transition peaks. The LC texture was examined under the polarized optical microscope (Leica DMLP) coupled with a Linkam hot stage. Small-angle X-ray scattering (SAXS) experiments were carried out on the synchrotron X-ray beamline BL16B1 in Shanghai Synchrotron Radiation Facility (SSRF).41 The wavelength of the X-ray beam is 1.24 Å. The scattering vector q is defined as q = 4π sin θ/λ, where λ is the X-ray wavelength and 2θ is the scattering angle. The silver behenate was used to calibrate the q. A Linkam heating stage with a N2 protection environment and a MarCCD detector were used. 2D Wideangle X-ray diffraction (WAXD) measurements were conducted on the synchrotron X-ray beamline BL14B1 in SSRF. The neat S−N diblock copolymer was preannealed at 100 °C for 4 h in a vacuum before measurement. During the temperature-dependent X-ray diffraction experiment, the sample was always isothermally stabilized at the preset temperature for 10 min before starting the measurement.

of the chosen LC solvent (5CB), a simple nematic solvent, as shown in Figure 1. Moreover, the common nematic LC phase

Figure 1. (a) Schematic illustration of the system composed of 5CB (the upper part) and S−N (the lower part) in this study. (b) Chemical structures of the S−N diblock copolymer and 5CB solvent.

of 5CB and the N block in a certain overlapped temperature range provides great potential to tune the LC interactions and compatibility of LC blocks with LC solvents in a wide concentration window. Unlike LC-coil systems, the S block in the S−N diblock copolymer is also thermally sensitive upon isotropization. This type of fully LC featured blend is expected to be a unique platform toward a profound understanding of the thermodynamics governing self-assembly of block copolymers in complex fluids and is also potentially useful to create hierarchical nanomaterials with switchable functionalities.

2. EXPERIMENTAL SECTION 2.1. Materials. 5CB was obtained from Sigma-Aldrich. All the solvents were obtained from Shanghai Tansoole Company and used as received. The smectic (SP, Mn = 11.0 kg/mol) and nematic (NP, Mn = 10.8 kg/mol) LCP were synthesized via the anionic ring-opening polymerization, and the S−N diblock copolymer was synthesized via sequential anionic ring-opening polymerization as reported previously.35−37,40 The S−N diblock copolymer is narrow-disperse (PDI = 1.04) as shown in Figure S1. The molecular weight of the S block is 9.7 kg/mol (26 repeat units on average), and the molecular weight of the N block is 11.6 kg/mol (36 repeat units on average), as estimated

Figure 2. (a) DSC thermograms of the NP/5CB blends during heating and cooling at a scanning rate of 10 °C/min. From the top down, the heating curves correspond to those of the blends at different weight concentrations of NP: the neat 5CB, 10%, 24%, 39%, 55%, 71%, 85%, and the NP homopolymer; the cooling curves are in a reverse order. The LC transitions of NP and 5CB are indicated in the plot. (b) DSC phase boundary diagram of the NP/5CB blends in terms of temperature versus composition. B

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mesogens in NP and 5CB. Interestingly, the heat of fusions of N−I transitions of the miscible NP/5CB blends (ΔHN−I) derived from the DSC results are found to be larger than that of the neat NP if the composition is taken into account, as illustrated in Figure S2. This suggests that the N−I transition of the miscible NP/5CB blend may encompass an additional contribution from melting of an ordered phase of 5CB apart from the melting of the nematic phase of NP. This implies a cooperative LC interaction between the nematic fields from NP and 5CB. Although the clearing points of the blends are generally higher than that of the neat 5CB, the anchoring of 5CB on the side-chain NP polymer may still be able to induce synergistic orientational coupling, thus the ordering of 5CB. This solvation around the NP polymer chain might extend a few 5CB molecules along the LC director. When the 5CB content is beyond the threshold value (∼90 wt %), macrophase separation may take place and result in the crystallization of 5CB to form a separated phase. Its subsequent melting can be observed in DSC measurement and utilized to identify the emergence of macrophase separation.45,46 The broad exothermic peak around −20 °C of NP/5CB (10 wt % NP) in the heating curve is ascribed to the cold crystallization of 5CB, and the endothermic peaks at the temperature lower than the N−I transition temperature (∼35 °C) are due to the crystal melting of 5CB.44 Because of the high mobility of 5CB and its heterogeneity, the N−I transition shifts to a lower temperature and becomes broader. Therefore, the ΔHN−I of the miscible blend should compose of melting of both NP and the induced ordered state of 5CB. 3.2. Binary Blends of SP/5CB. In comparison with NP, SP possesses a positional order in addition to an orientational order. Figure 3 depicts the thermal behaviors of the SP/5CB

Atomic force microscopy (AFM) measurements were conducted using a Multimode NanoscopeIIIa instrument in a tapping mode. A sample was prepared by casting its chloroform solution with a concentration of 1 mg/mL on a silicon wafer, followed by slow evaporation and annealing under vacuum at 40 °C overnight. 2.3. Sample Preparation. The polymers and 5CB in different compositions were dissolved in a common solvent CH2Cl2 at a concentration of 10 wt %. The resulting mixture was left to evaporate at room temperature overnight. The sample was then annealed in a vacuum at 50 °C for 8 h. The volume fraction of polymer (ΦP) was calculated by using the density of each component at room temperature and assuming the additivity of volumes. Regarding the possible evaporation of 5CB during the thermal treatments or experiments, we specifically checked the compositions of the samples after the thermal annealing at 50 °C in a vacuum for 8 h and the S−N/ 5CB (83 wt % S−N) after the temperature-dependent SAXS experiment in which the highest temperature reached 150 °C by using NMR. We confirmed that the mass loss of 5CB is negligible within the accuracy of the NMR measurement (∼2%), which is also consistent with the observations of reproducible shape of curves and heat of fusion in the repeated DSC scanning and the reversibility of the structural formation during the temperature-dependent SAXS experiments. Particularly, we found in the literature that the vapor pressure of pure 5CB is only ∼1 Pa at 100 °C or ∼60 Pa at 150 °C.42 When 5CB was mixed with the high viscous block copolymer S−N, it could be more difficult for 5CB to diffuse out of the melt and evaporate. The negligible mass loss of 5CB in our experiment is thus understandable. In the content, only the data of the blends up to 160 °C will be discussed.

3. RESULTS AND DISCUSSION 3.1. Binary Blends of NP/5CB. To gain a better understanding of the self-assembly of S−N/5CB blends, we first investigate the miscibilities and the phase behaviors of binary blends of NP/5CB and SP/5CB in different compositions. By using DSC measurements, we explore the calorimetric behavior of the NP/5CB blends in a full range of compositions, as shown in Figure 2a. The NP homopolymer exhibits a phase sequence of g-24°C-N-121°C-I,36 while that of 5CB is K-23°CN-35°C-I.43 The multiple exothermic peaks of 5CB during the heating process are ascribed to the cold crystallization of 5CB, and the endothermic peaks other than the N−I transition at 35 °C are due to the melting of the formed crystals.44 It is found that the blends exhibit a single LC transition at Ti and a glass transition (Tg) at a lower temperature as the NP concentration is higher than 10 wt %. The LC transition is a transition from nematic to isotropic (N−I), as confirmed by POM (Figure S3) in the Supporting Information. Crystallization and LC transition of the pristine 5CB are efficiently suppressed due to the mixing with NP. With increase of the 5CB content, both the Tg and the Ti of the blends decrease. The LC transition becomes broader, and the peak temperature gradually shifts toward the Ti of the neat 5CB at 35 °C. When the NP concentration decreases to 10 wt %, 5CB in the blend starts to crystallize. Based on above experiments, the DSC phase boundary diagram of the NP/5CB blends can be established as shown in Figure 2b. The details about thermal transitions of the NP/5CB blends are summarized in Table S1. The calorimetric study indicates that NP and 5CB are miscible in a broad concentration range. The introduction of NP into 5CB efficiently suppresses the macrophase separation and crystallization of 5CB on the one hand; on the other hand, the LC phase transition temperature and glass transition temperature of the blends decrease due to the plasticizing effect of 5CB. This is understandable considering potentially the same LC phase state and the similar chemical structures of the

Figure 3. DSC thermograms of the SP/5CB blends at scanning rate of 10 °C/min. From the top down, the traces are arranged in order of the weight concentration of SP: the neat 5CB, 25.6%, 43%, 60%, 67%, 70%, 73%, 75%, 85%, and the neat SP, respectively. The transitions of 5CB solvent are indicated in the figure. The miscible and immiscible regions are indicated by different colors.

blends in different compositions. The neat SP exhibits two LC phase transitions: a SmB−SmA at 75 °C and a SmA−I at 140 °C.37 We found that when the content of 5CB in the blend is below 33 wt %, only the SmA phase can be detected, which has been confirmed by the POM (Figure S4) and SAXS (Figure S5) experiments in the Supporting Information. With more 5CB added into the blend, crystallization of 5CB and subsequent melting emerge as shown in Figure 3, which implies a potential macrophase separation or biphasic behavior C

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Macromolecules in the blend.45,46 The detailed information about thermal properties of the SP/5CB blends is summarized in Table S2. The onset concentration of the macro-phase separation (67 wt % for SP, equivalent to the mole ratio of the mesogens of SP to 5CB ∼ 1:0.74) should correspond to the limit of miscibility or represent as a saturation point. Above that, macrophase separation between 5CB and SP occurs. In contrast to the NP/ 5CB blends, the limit of miscibility of the SP/5CB blends is much lower. For a smectic phase, one-dimensional ordered layer structure of SP may constrain the growth of the nematic order of 5CB normal to the smectic layers unlike NP. The SAXS measurements (Figure 4) show that the thickness of

Figure 5. DSC thermograms of the S−N/5CB blends during heating and cooling at a scanning rate of 10 °C/min. From the top down, the traces are arranged in order of weight concentration of S−N diblock copolymers : the neat 5CB, 24%, 32%, 40%, 47%, 54%, 61%, 71%, 83%, 96%, and the neat S−N, respectively.

gradually approaches the Ti of the neat 5CB. The transformation process at the lower temperature resembles the behavior of the NP/5CB blends, in which the N−I transition shifts to the lower temperature due to the solvation of NP by 5CB. The Ti of the S−N/5CB blends is found to be related to the isotropization of the smectic phase, as proved by the temperature-dependent WAXD (Figure S6) of the S−N/5CB blends (e.g., 47 wt % of S−N) in the Supporting Information. 3.2.2. Ordered Nanostructures of S−N/5CB Blends. The phase structure of the neat S−N diblock copolymer is first investigated by the means of temperature-dependent SAXS experiments. The SAXS profiles of the S−N diblock copolymer illustrated in Figure 6a suggest the formation of the Lo structure below Ti at 141 °C, as reported previously.35 In the Lo structure, the smectic and nematic orders of the LC blocks coexist. The reflections at peak ratios q/q* of 1:2:3 at low angle region (q* = 0.538 nm−1) originate from the nanophaseseparated lamellar structure.35,36,47 The reflection near q ∼ 1.5 nm−1 corresponds to the characteristic scattering of the smectic layers, which is consistent with that of the SP homopolymer. The reflections at q ∼ 1.6 and 1.95 nm−1 result from the alternating arrangement of the smectic layers in the neighboring S domains. When the temperature rises above Ti, the Lo structure transforms into a regular lamellae structure accompanying the melting of LC order. This OOT is also thermoreversible during the cooling process, as revealed in Figure S9. The temperature-dependent SAXS profiles of the S−N/5CB blends with the polymer concentration higher than 61 wt % are shown in Figure 6 and Figure S10 (for 71 wt % S−N in 5CB). As indicated by the SAXS results in Figure 6, the S−N/5CB blends in the concentration range of 61−100 wt % form the Lo structure at low temperature, in which the satellite scatterings arising from the superstructure are around q ∼ 1.4 nm−1. When the temperature is above the isotropization point, OOT from the Lo to the Lam occurs. As the content of 5CB increasing, the TOOT of the system decreases in line with the decrease of Ti of the blends. The ODTs of the neat and 96 wt % S−N samples were not accessible until 160 °C, suggesting a strong phase segregation. In contrast, for 83, 71, and 61 wt % S−N diblock copolymers in 5CB, they show the ODTs in a decreasing order from 150 to 100 °C. The decease of TODT with the increase of its content indicates the role of 5CB to mediate the segregation

Figure 4. Periodicity and the reduced fraction of the thickness of smectic layers in the SP/5CB blends compared with that of SP as a function of the weight concentration of SP.

smectic layers (dSm) in the miscible region of the SP/5CB blends decreases as the content of 5CB increases. In contrast, the dSm keeps nearly identical in the biphasic window. This suggests that a certain amount of 5CB could have been partitioned into the smectic domain and distributed among the mesogens of SP, thus making the polymer backbone stretched and the smectic layers thinner. The maximum shrinkage of the layer thickness ∼6% has been observed with the introduction of 5CB, as shown in Figure 4. 3.3. Binary Blends of S−N/5CB. 3.3.1. Calorimetric Study of S−N/5CB. The much better miscibility of 5CB with NP than with SP indicates that 5CB can be used as a selective solvent. In the case of the S−N diblock copolymer, it is expected to preferentially locate in the N blocks. We thus utilize the selectivity of 5CB to tune the self-assembly of the S−N diblock copolymer. The concentration of the S−N in 5CB is made in the range 24−100 wt %. The resulting blends are homogeneous macroscopically, yet different ordered structures form on the nanometer scale, as shown in the SAXS experiments below. In order to investigate their phase behaviors, DSC measurements are performed on the S−N/5CB blends in various compositions. No N−I transition attributed from the pristine 5CB is observed in the studied concentration range, indicating the absence of macrophase separation. In POM image (Figure S7), a grainy texture typical of S−N/5CB blends is observed. As shown in Figure 5, the introduction of 5CB in the blends reduces the Tg and the Ti of the systems, similar to the dilute effects observed in the LCP/5CB blends. The melting of the blends broadens as the content of 5CB increases. When the content of 5CB is larger than 39 wt %, two transitions are discernible. With further increase of the 5CB content, the transition at lower temperature becomes dominant and D

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Figure 7. Temperature-dependent SAXS profiles of the S−N/5CB blends at (a) 54 wt %, (b) 47 wt %, (c) 40 wt %, and (d) 32 wt % of the S−N diblock copolymers. The cooling processes are also demonstrated in (a−c), denoted with a prefix of C.

Figure 6. Temperature-dependent SAXS profiles of the neat S−N (a) and S−N/5CB blends at (b) 96 wt %, (c) 83 wt %, and (d) 61 wt % of the S−N diblock copolymers. The typical temperature regions are indicated by different colors.

nanostructures. The characteristics of the ordered structures of S−N/5CB blends at 30 °C are summarized in the Table 1.

strength between the two blocks. It can be partitioned into the S domain even the distribution in the N domains is preferable. With further decrease of the S−N content to 40−54 wt %, the transformation of nanophase-separated structures of the blends takes place, as shown in Figure 7. In the SAXS profiles of the blends at low temperature (blue curves), the scattering at the low angle was found to locate at the peak ratios of 1:√3:2, suggesting the cylindrical nanophase-separated structure, as also demonstrated in the AFM image (Figure S8). Besides, the satellite peak near the scattering of the smectic layers at q ∼ 1.45 nm−1 is characteristic of the Ho superstructure. Considering the preferential distribution of 5CB in the N domain, the effective volume fraction of the N domain increases as 5CB increasingly added, eventually leading to the cylindrical structure. When heating these binary blends, OOT from the Ho to the Lo structure is generally observed. The most distinct features associated with the OOT are the extinction of the (101) reflection belonging to the Ho structure and the formation of the (310) reflection characteristic of the Lo structure, and the peak ratios of the reflections at the low angle change from 1:√3:2 to 1:2. This thermotropic OOT between the Lo and the Ho structures is also reversible during the cooling process as illustrated in Figures 7a−c. When the content of S−N in 5CB is as low as 24.5 wt %, we can barely identify the ordered structure, as demonstrated in Figure S11. This might be attributed to the severe swollen state of the N blocks, in which the solvent could weaken the correlation and distort the ordered packing of the resulting

Table 1. Characteristics of the S−N/5CB Blends S−N (wt %)

volume fraction ΦP

Tg (oC)

TOOT (oC)

Ti (oC)

q* (nm−1)a

d (nm)b

qSm (nm−1)

100 96 83 71 61 54 47 40 32

1 0.96 0.82 0.70 0.60 0.53 0.46 0.39 0.31

21 6.3 −5 −28 −33 −42 −44 −48 −28

141 128 119 102 95 62 60 59 54

141 128 119 102 100 86 84 80 71

0.538 0.542 0.549 0.555 0.566 0.520 0.539 0.543 0.496

11.67 11.59 11.44 11.32 11.10 12.08 11.65 11.56 12.66

1.388 1.407 1.420 1.437 1.447 1.455 1.456 1.455 1.455

a Primary reflection in the SAXS profiles at 30 °C. bDomain size of the superstructure, calculated from the equation d = 2π/q*.

Figure 8 summarizes the lyotropic effects of 5CB on the hierarchical structures of the S−N/5CB blends. Figure 8a illustrates the domain size of the nanophase-separated structures (d-spacing), deduced from the primary peak position q* (d = 2π/q*), as a function of the volume fraction of the S− N polymer (ΦP) at 30 °C. Figure 8b is regarding the thickness of the smectic layers (dSm). As shown in Figure 8a, the d-spacing of the Lo structure decreases as ΦP decreases. The concentration dependence of the d-spacing of the lamellar phase-separated domain can be dramatically influenced by the selectivity of solvent and its E

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Figure 8. Dependence of (a) the d-spacing of the lamellar and cylinder nanophase-separated structures and (b) the thickness of the S layers (dSm) at 30 °C as a function of the volume fraction of the S−N diblock copolymer (ΦP).

partition within the two blocks. The reduction of the d-spacing of the nanophase-separated structures could be related to the attenuated incompatibility between the S and the N blocks due to the introduction of 5CB, as 5CB is a selective solvent yet it still can enter the S domain. It screens the unfavorable interactions between the S and the N blocks. Meanwhile, the partition of the 5CB into the S domain reduces the thickness of the S layers dSm as compared to that of the S homopolymer (Figure 4) or the neat S−N diblock copolymer (Figure 8b). In Figure 8b, it is found that the dSm in the Lo structure continuously decreases from 4.5 to 4.3 nm as ΦP decreases to 0.53. Whereas, the reduced dSm keeps nearly identical as ΦP decreases into the Ho phase region. This may suggest that a saturation point be reached with the introduction of 5CB since the macrophase separation of the SP/5CB blend can take place when the mole ratio of 5CB to the mesogen of SP is more than 0.74 (67 wt % SP). Considering the large miscibility difference of 5CB in SP and NP, once the partition of 5CB in S block approaches the saturation value, the dSm would not change much anymore. The majority of 5CB would locate in the N domain. In that case, by assuming that 5CB in the S−N diblock copolymer would follow the same partition trend as in the SP/ 5CB blends, and particularly the saturation value (0.74) of 5CB in SP is taken, the partitioned 5CB in the N domain would be nearly 2.7 times that in the S block at ΦP ∼ 0.53. Nonetheless, it is intriguing to observe the coincidence of the turning point of dSm with the OOT. This may need further theoretical understanding of the balance of entropic and enthalpic effects including the mixing entropy to determine the overall free energy of the system thermodynamically. Considering the thickness decrease of the S layer in S−N with increasing 5CB content (Figure 8b), the S chains are more stretched by the insertion of 5CB in the smectic layer, resulting in a slight increase of its chain length in the S block. However, 5CB solvates the N blocks much more and would cause a larger chain length decrease and thus compensate the increase of the S blocks and make the overall d-spacing of nanophase-separated structures decrease. 3.2.3. Phase Diagram. According to the temperaturedependent structural studies of the S−N/5CB blends at different fractions of 5CB, the phase diagram can be constructed as demonstrated in Figure 9. The lamellar nanostructures are dominant in the concentrations of the S− N diblock copolymers higher than 61 wt %, while the

Figure 9. Phase diagram of the S−N/5CB blends in terms of the polymer concentration and temperature. The dashed lines are drawn as the guides, and the black and red lines represent the ODT and OOT of the blends, respectively.

cylindrical nanophase-separated structures are stable at the low temperatures in the concentration range of 32−54 wt %. Further decrease of ΦP in the blends ultimately leads to the disordered state. The TODT of the system decreases as the content of polymer reduces. Thermotropic OOTs from the Ho to the Lo nanostructures and from the Lo to the Lam nanostructures are observed in the low and high polymer concentration regions, respectively. The decrease of TOOT and TODT are related to the segregation strength of the building blocks, in which the incompatibility is continuously decreased as more 5CB molecules solvate both blocks. This attenuated compatibility of the system results in the absence of OOT from the Lo to the Lam nanostructures in the high fraction of 5CB in comparison with the neat S−N diblock copolymer or the concentrated S−N/5CB blends. Generally, the distribution of solvent molecules in the block copolymers plays a key role in determining the microstructures.48−50 The partition of 5CB in each block depends on its selectivity as well as the phase state. According to the studies of the SP/5CB and the NP/5CB systems, we can deduce that the anisotropic 5CB is highly selective and prefers to locate F

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at the N domain. In the S−N/5CB blends, a significantly more amount of 5CB molecules can solvate the N blocks compared with the S blocks and thus increase the effective volume fraction of the N domain. Therefore, the flat interface in the lamellae is prone to be bended by the solvated NP chains and result in the lyotropic OOT from the lamellar to the cylinder structure with increase of 5CB content. It should note that the high temperature adopted in the experiments potentially leads to the melting of 5CB into the isotropic state that has a poorer selectivity and can redistribute in both the S and the N blocks. The solvated N blocks seem to have a lower transition temperature, separated from that of the S blocks in the low concentration region of polymer. We propose that it is the partial melting of the 5CB molecules in the solvated shell of the NP chains that results in the thermotropic OOT from the Ho to the Lo structures, considering the coincidence of the OOT with the start of the melting process observed in the DSC measurements. Further increasing the temperature into the isotropic state of the whole system leads to the ODT. Our work has demonstrated that 5CB solvent can be utilized to effectively induce the lyotropic and thermotropic transformations of this fully LC featured complex fluid, making it a promising candidate for switchable or multiresponsive devices in the future.

Huiming Xiong: 0000-0001-6628-8519 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We appreciate BL14B and BL16B beamlines in SSRF. This research is supported by the National Natural Science Foundation of China (No. 21574082 and No. 21374063).

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4. CONCLUSION The miscibility and phase behavior of the nematic and smectic LC polymers (NP and SP) in a nematic LC solvent (5CB) have been examined first in order to investigate the phase structures of the S−N/5CB binary blends. The 5CB molecules are found to preferentially solvate the NP chains but exhibit a limited miscibility with SP. The distinct miscibilities and LC interactions have been shown to drive the S−N/5CB mixing systems to form hierarchical nanostructures dependent on the concentration and the temperature. From the phase diagram in terms of polymer composition and temperature, we can identify several phase transformations including the thermotropic Lo− Lam−Disorder and Ho−Lo−Disorder transitions and the lyotropic Lo−Ho−Disorder transition. We have also revealed the influence of the introduction of 5CB on the dimension sizes of the phase-separated structure and the smectic layers, which is related to the partition of the LC solvent into the LC phases of distinct nature. Our work demonstrates the self-assemblies of hierarchical structures of LC block copolymers and their tunable transformations by using selective LC solvent, particularly in concentrated solutions. We hope that our work could advance our understanding of the complex LC interactions and provide a strategy to design multifunctional materials based on block copolymer solutions.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.7b01669. Chemical characterization, POM experiment and structural characterization, and AFM image (PDF)

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*E-mail [email protected] (H.X.). G

DOI: 10.1021/acs.macromol.7b01669 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.7b01669 Macromolecules XXXX, XXX, XXX−XXX