Effect of Topology and Composition on Liquid Crystal Order and Self

Oct 30, 2017 - State Key Laboratory of Fine Chemicals, Department of Polymer Science and Engineering, Liaoning key Laboratory of Polymer Science and E...
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Effect of Topology and Composition on Liquid Crystal Order and Self-Assembly Performances Driven by Asynchronously Controlled Grafting Density Li Han, Hongwei Ma,* Siqi Zhu, Pibo Liu, Heyu Shen, Lincan Yang, Rui Tan, Wei Huang, and Yang Li* State Key Laboratory of Fine Chemicals, Department of Polymer Science and Engineering, Liaoning key Laboratory of Polymer Science and Engineering, School of Chemical Engineering, Dalian University of Technology, Dalian Liaoning 116024, China S Supporting Information *

ABSTRACT: A series of thermo-tunable liquid crystal block copolymers (LCBCs) with well-designed architectures were successfully synthesized. Linear/star poly[4-(4-vinylphenyl)-1butene]-block-polybutadiene (PVSt-co-PB) moieties were obtained using living anionic copolymerization of 4-(4vinylphenyl)-1-butene with butadiene, and topological [PVSt-co-PB]-LCBCs were generated through the adherence of mesogenic moiety via facile hydrosilylation. The PVSt LC block had well-defined grafting densities of approximately 100%, 70%, and 40%, whereas the PB LC block had an asynchronously tunable grafting density. This work included comprehensive studies on their self-assembly and yielded some interesting results. The influences of topologies and compositions on the phase transition behaviors and polarized optical performances of the resulting LCBCs that were driven by asynchronously controlled grafting density were carefully illustrated. The LCBCs with controlled molecular weight (MW) and narrow PDI showed wider LC phase ranges (ΔT) and a high tunability was added into the construction to aid thermos-responsive devices. The wide ΔT and high thermo-stability were demonstrated to be complementary between two LC blocks. However, the response-time and aggregation morphology in POM showed close similarity to LC blocks and showed a gradient in temperature-dependent changes with the PB LC block at a lower temperature and the PVSt LC block at a higher temperature. It is common for LC texture to change with varying temperature, whereas the gradient switching process was unique to LC blocks, which was further confirmed by temperature-dependent WAXD. In particular, the structural reorganization was determined to be driven by asynchronous grafting density by measuring the temperature-variation AFM, in that the asynchronous-tunable motion between LC blocks facilitates small phase separation.

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INTRODUCTION Since liquid crystal (LC) and block copolymers (BCs) are representative of self-organized systems, LC block copolymers (LCBCs) that are composed of amorphous and mesogenic side groups, which can have spontaneous ordering and various selfassembly, have attracted considerable attention in both industrial and academic research.1−4 In the past few decades, the preparation of finely structured LCBCs has allowed for remarkable applications in multidisciplinary areas.5−9 Major investigations have demonstrated that the primary and significant properties of polymers are inherently linked to their architectures.10−13 The precision control that is available in polymers has become increasingly important for determining the structural essence and precise structure−property relationshios.14−16 Therefore, tremendous efforts have been made for the fabrication of templates for self-assembly, but they were greatly limited in their precise manipulation, especially regarding high tunability. With the development of new materials that have innovative functions, many studies have found that the microphase © XXXX American Chemical Society

separation-induced morphology and the LC order can influence each other due to the immiscibility of the different blocks;17 for example, LCBCs that have cooperative or competitive motion have been designed,2 and a variety of synthetic methodologies have been developed to fabricate LCBCs that have novel functions and well-organized structures.18,19 Generally, LCBCs were synthesized through random generation when LC moiety was adhered to the polymer matrixes,9 whereas with recent advances, controlled fabrication is formed using living polymerization, highly efficient clickable reactions and other processes.16,20 BCs can form spontaneous ordered morphologies, such as spheres, cylinders, and lamellar phases Studies have shown that the phase-separated morphology typically depends on the degree of polymerization, mole fraction and interaction parameter between blocks that occur in a variety of manners.21−23 Currently, the precision polymers that have Received: September 8, 2017 Revised: October 23, 2017

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DOI: 10.1021/acs.macromol.7b01952 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Brief synthetic routes of PVSt-co-PBs and [PVSt-co-PB]-LCBCs in combination living anionic copolymerization with hydrosilylation methodology.

ing,40,41 in which both PVSt and PB blocks conveniently possessed functional sites without any further modification. Then LC moiety Chol-SiH (M) was attached to rubbery PB and rigid PVSt blocks via highly efficient hydrosilylation to yield [PVSt-co-PB]-LCBCs with well-designed architectures. PVSt and PB LC blocks achieve an asynchronous grafting density by varying the SiH to Vinyl feed molar ratio. This work describes comprehensive studies regarding the self-assembly of the resulting LCBCs, along with the influences of topologies and compositions on the phase transition behaviors and optical performances. The phenomenon of [PVSt-co-PB]-LCBCs that is demonstrated in this study highlighted the asynchronous motion driven by well-controlled grafting density, suggesting new potential applications. Furthermore, it is highly desired to add tunability to the LCBCs using facile fabrication.

control over density of their functional groups are being widely investigated. The effect of grafting density and topology on phase-separation has also been described.24−26 Moreover, the significant influence of various interaction forces and grafting density on polymer order has been confirmed by DeLongchamp et al.27 As proposed by Zhou et al., mesogen-jacketed side chain LC polymers (SCLCPs) with bulky pendants around the backbone succeed in packing densely.28 Since interactions between LC blocks could arise from dense or sparse grafting, molecular interaction has been proven to be closely associated with grafting density. As a result, the interaction parameters are well accounted-for in self-organized SCLCPs and BCs.29 LCBCs that have cooperative motion have been designed.2 However, LCBCs with asynchronous-tunable motion (for example asynchronous grafting density between LC blocks) have rarely been reported. In general, the precision preparations of LCBCs are not trivial, nor are the well-controlled LC blocks with different grafting density. Therefore, it is very challenging but valuable to develop a detailed understanding of LCBCs. Precision functionalization of universal polymer materials has a predominant tendency to achieve high performance. To the best of our knowledge, BCs that have rigid and flexible compositions tend to promote fascinating phase-separation due to their thermodynamic property differences.30−33 Phaseseparated styrene−butadiene (SB) and styrene−butadiene− styrene (SBS) block copolymers, which are excellent universal polymer materials with characteristic rigid-flexible components, are well-studied.33 In-chain LC functions have been incorporated into rubbery PB blocks, where only PB blocks were modified and PS blocks were not functionalized.34−39 Little is known about the coupled LC modification of PB and PS blocks because PS blocks do not contain functional groups. As such, they are first functionalized and then postfunctionalized in a multistep process. In this study, linear and star PVSt-co-PBs as BCs templates were synthesized using living anionic copolymerization according to previous methods for preparation and process-

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RESULTS AND DISCUSSION Polymers Design. Tailoring polymer architectures with innovations is regarded as a superior procedure for a wide array of applications.42−46 In general, the tailoring of SCLCPs can be categorized into three parts including the polymeric matrix, LC moiety and flexible spacer. Recently, there have been contributions to the rational design of polymer matrixes to determine precise structure−property relationships in LC functional materials.47−51 Polymeric matrixes are divided into many categories according to their composition, including homopolymers, copolymers and complexes and others. The use of copolymers instead of homopolymers as polymeric matrixes could facilitate major specific advantages. In previous studies, our group has synthesized the most copolymers through living anionic polymerization,52 where the synthesis of BCs was relatively simple. PB that has a low glass transition temperature (Tg) was generally adopted as a flexible matrix and was applied to room temperature LC. Meanwhile, PS was used as a stiffening phase and was always introduced into diverse copolymers to improve photoelectric properties and achieve the necessary coupling of B

DOI: 10.1021/acs.macromol.7b01952 Macromolecules XXXX, XXX, XXX−XXX

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Figure 2. (a) 1H NMR absorption attribution of Chol-SiH (M), compared with representative PVSt-co-PB and [PVSt-co-PB]-LCBC, and (b) GPC comparison of PVSt-co-PBs with [PVSt-co-PB]-LCBCs.

mechanical stability and processability in the system.53,54 Because the constructions of SCLCPs with well-defined high 1,4-PB or PVSt homopolymer matrixes were considered,40,41 LCBCs that had precision functionalization based on wellcontrolled PVSt-co-PB BCs were prepared as shown in Figure 1, with the PVSt block in the minority mole fraction and the PB block in the majority mole fraction. Because high incompatibility between siloxanes and many organic components facilitates ordered phase separation, siloxane-containing BCs are desirable.55−57 Additionally, the conventional cholesterol units that offer biological compatibility, were widely used in the mesogens.58,59 As the Wang− Warner and Maier−Saupe theory available to SCLCPs underlined the primary effect of molecular interaction from main chains and side chains on LC phase conformation,60 LC moiety Chol-SiH (M) that synchronously contains siloxane and

cholesterol groups in the side chain and PVSt-co-PB in the main chains has characteristic rigid-flexible components, which were possibly responsible for the phase-separated LCBCs. However, the cooperative effect of various architectures on properties and ordering, especially the structural precision control was not distinctive, and further research is necessary. In the field of topological functional polymers, our group has performed related research and has reached many surprising conclusions.61,62 Although topological polymers with low intrinsic viscosity prove to be particularly versatile, the issue remains controversial.63 Many have argued that topological structures were disadvantageous in stabilizing the LC phase. However, LC properties and molecular order are determined by many factors (not only topologies), which has been confirmed in major cases.64,65 After we designed topological SCLCPs with well-defined grafting density the unique effects of C

DOI: 10.1021/acs.macromol.7b01952 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules Table 1. Molecular Compositions and Thermal Property Characterizations of [PVSt-co-PB]-LCBCs EA (mol %)e [S-co-B]-LCBCs

a b

S-[S1.25k-B1.25k] S-[S1.25k-B4.0k] S-[S1.25k-B8.0k] S-[S2.5k-B8.0k] L-[S1.25k-PB4.0k] L-[S1.25k-B8.0k] L-[S2.5k-B2.5k] L-[S2.5k-B8.0k] L-[S5.0k-B16.0k] S-[S1.25k-B1.25k]c L-[S2.5k-B8.0k] S-[S1.25k-B8.0k]c L-[S1.25k-B4.0k]

Mnd

d

(kg/mol)

PDI

EA(total)

45.0 81.2 172.9 168.5 30.3 60.1 28.4 68.0 125.1 31.0 47.1 151.1 25.2

1.24 1.21 1.25 1.28 1.10 1.14 1.15 1.21 1.27 1.28 1.28 1.21 1.27

55.5 57.5 59.2 59.1 58.1 54.7 68.6 61.2 57.3 33.4 36.1 40.8 42.4

e

phase transition

EA(PVSt)f

EA(PB)g

Tgh (°C)

98.5 99.4 100 100 100 99.3 100 100 100 69.2 61.6 38.1 43.1

37.6 38.9 44.9 44.5 46.0 45.3 48.8 47.6 44.5 21.5 30.3 41.2 42.4

38 38 38 36 38 36 40 37 35 22 24 24 18

Tii (°C) [ΔHi (J/g)] 169 179 184 186 183 183 192 190 188 123 120 134 115

[4.5] [4.4] [4.8] [4.4] [4.3] [4.7] [5.0] [4.6] [4.2] [2.6] [1.7] [3.2] [2.2]

ΔTj (°C)

Tdk (°C)

131 141 146 150 145 147 152 153 153 101 96 110 97

l 350 − − − 348 − 348 341 − − − −

a

[PVSt-co-PB]-LCBCs with linear (L)/star (S) and varying compositions: S, PVSt; B, PB. bLCBCs with different grafting density, EA(total) = 60%. LCBCs with different grafting density, namely EA(total) = 40%. dNumber-average molecular weight (Mn) and polydispersity index (PDI), both determined by GPC. eGrafting density, the mole content of reactive double bonds occupied in total double bonds, calculated by B1−B3. fGrafting density, the mole content of reactive vinyl groups occupied in total vinyl groups of PVSt blocks, calculated by B1−B3. gGrafting density, the mole content ofreactive double bonds occupied in total double bonds of PB blocks, calculated by B1−B3. hGlass transition temperature on the second cycle heating. iIsotropic transition temperature and thermal transition absorption enthalpy, all determined by DSC. jLC phase ranges (ΔT = Ti − Tg). kThermal decomposition temperature at 5% weight loss. lNo characterization. c

all the block copolymers backbones showed well-designed molecular weight and narrow PDI. Without going into detail regarding the exact synthesis procedures (see the Supporting Information), it is important to discuss the grafting density. In this work, LC moieties Chol-SiH (M) were introduced into both PVSt and PB blocks from welldesigned PVSt-co-PBs that are available for asynchronous grafting density for PVSt and PB LC blocks. On the basis of the operations [double bonds (PVSt+PB):SiH (Chol-SiH (M)) =1:1, input mole ratios], the 1H NMR of [PVSt-co-PB]-LCBCs compared with PVSt-co-PB and Chol-SiH (M) are shown in Figure 2a. Since EA(1,2-PB) = 100% was proposed according to previously established designs40,41 and the peak δ = 3.92 (p) assigned to −CH2−CH2−O−Ar (2H) of Chol-SiH (M) served as an integral standard in 1H NMR spectra of [PVSt-co-PB]LCBCs, it was possible to determine the grafting density of [PVSt-co-PB]-LCBCs. For example, as shown in Figure S7, the disappearance of δ = 6.0 (a) assigned to −CHCH2 (PVSt) demonstrated the EA(PVSt) = 100% of [PVSt-co-PB]-LCBCs. Detailed calculation methods are described in the Supporting Information. The basic molecular compositions of [PVSt-coPB]-SCLCPs were summarized in Table 1. As a result, all the total densities were appropriate (60%), denoted as EA(total) = 60%. Additionally, PB LC blocks and PVSt LC blocks formed approximately 40% and 100% of the grafting density, respectively, and are denoted as EA(PB) = 40% and EA(PVSt) = 100%. Accounting for the above results, the same units of Chol-SiH (M) were separately incorporated into the PVSt and PB blocks, leading to the formation of [PVSt-co-PB]-SCLCPs. The PVSt LC block and the PB LC block had an asynchronous grafting density. To target [PVSt-co-PB]-LCBCs with different grafting densities than those mentioned above, various molar ratios [double bonds (PVSt):SiH (M1) = 1:0.7 and double bonds (PB):SiH (M2) = 1:0.4, M1 + M2 = M] were performed. The presence of a remnant characteristic peak δ = 6.0 (a) in Figure S6a suggests the incomplete graft of the PVSt LC block. According to Table 1, all the total grafting densities of [PVSt-

topologies and grafting density on LC properties and molecular order were investigated.40,41 While the topologies had obvious effects on LC properties and molecular order, there was also a significant relevance to the various grafting densities. As is already known, LC elastomers have multiple potential applications due to the coupled rubber elasticity and LC orientation, which has resulted in a strong correlation between macroscopic deformation and molecular order.66,67 Since star PVSt-co-PB is similar to triblock SBS and can be used as the matrix of LC elastomers, in particular for high performance materials, we performed further systematic research on star LCBCs in which PVSt and PB blocks were both modified. Characterization. As shown in Figure S1, the 1H NMR spectra of PVSt-co-PBs showed characteristic peaks and integrations that are in good agreement with previously reported PVSt and PB homopolymers,40,41 where several peaks overlapped. For example, both −CHCH2 (PVSt, 2H) and −CHCH2 (PB, 2H) appeared in the integral area of δ = 4.9−5.0 ppm. The mole fractions of 1,4-olefins (C1,4) and 1,2-olefins (C1,2) in PB blocks among PVSt-co-PBs were calculated according to Supporting Information. As summarized in Table S1, the 1,4-olefins content in PB blocks indicated that the synthesized PVSt-co-PBs contained 90 mol % 1,4olefins microstructures. As indicated by the results in Figure S2, the presence of two characteristic peaks with an equal integral in the benzene area between δ = 6.2 to δ = 7.5, denoted as h = k, suggested that the BCs were obtained via living anionic polymerization. The GPC traces of the fractionated PVSt-co-PBs displayed a narrow GPC distribution, shown in Figure 2b. Figure S3 displays the GPC comparison of L-[PVSt-co-PB]s and the PVSt precursors. The PVSt-co-PBs with a monomodal distribution and complete VSt conversion were obtained at low PVSt molecular weights of 1.2 kg/mol, 2.5 kg/mol, and 5.0 kg/mol according to previous studies.40,41 Figure S4 shows the typical bimodal distribution of S-[PVSt-co-PB]s, as well as the precursors PVSt and L-[PVSt-co-PB] for comparison. The basic molecular compositions are summarized in Table S1, and D

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Figure 3. Representative temperature-dependent (a) WAXD traces and (b) POM-50 μm textures of [PVSt-co-PB]-LCBCs (EA(total) ≈ 100%, EA(PVSt) ≈ 100%, and EA(PB) ≈ 40%), compared with PVSt-SCLCPs (EA(PVSt) ≈ 100%) and PB-SCLCPs (EA(PB) ≈ 40%).

Figure 4. Typical temperature-dependent WAXD traces of [PVSt-co-PB]-LCBCs with EA(total) = 60%.

PVSt-SCLCPs more than 130 °C. As seen in Figure 3b, regarding PVSt-SCLCPs, the rigid black patterns were maintained even through increases in external pressure, and the typical LC textures were not observed until the temperature exceeded 130 °C, which probably subjected to be rigid PVSt. The images gradually generated bright colors depending on the increasing temperature. When nearing an isotropic state, distinct focal conic textures were observed. Alternatively, LC textures of PB-SCLCPs were exhibited at approximately 30 °C, and the images gradually generated bright colors as temperature increased, especially for the well-formed fan-shaped textures near an isotropic state. The polarized optical performances (Figure 3b) of [PVSt-coPB]-LCBCs were investigated, with a goal to investigate advantageous complementation between PVSt and PB LC blocks. Similar to PVSt-SCLCPs and PB-SCLCPs, the focal conics gradually became well-formed as temperature increased. The response-time and aggregation morphology in the gradual process were closely related to the LC blocks, but they were independently identified. The LCBCs were clearly similar to the PB LC block at lower temperature and to the PVSt LC block as they reached an isotropic state. Accordingly, the wide LC phase formation was demonstrated to be complementary between two LC blocks, and the thermo-tunable polarized optical performances of [PVSt-co-PB]-LCBCs behaved similarly to the LC blocks. It is worth noting that two phase transitions were observed in DSC of [PVSt-co-PB]-LCBCs, including Tg

co-PB]-LCBCs were well-controlled (40%), whereas two kinds of EA (PVSt) values, namely, EA(PVSt) = 40% and EA (PVSt) = 70%, were exhibited. This possibly resulted from the incomplete design of PVSt LC blocks and the nonsite-specific hydrosilylation between PVSt and PB blocks. As shown in Figure S6b, the narrow GPC distribution of [PVSt-co-PB]LCBCs was described. All the GPC traces of [PVSt-co-PB]LCBCs, including EA(total) = 40% and EA(total) = 60%, showed a well-designed molecular weight and PDI, and the retention time advanced slightly according to different molecular weights in analogous systems that are in line with the structural design. Polarized Optical Performance and LC Order Behavior. The typical fan-shaped and focal conic textures of CholSiH (M) are shown in Figure S8. The SmA phase was verified using POM and LC order changes due to the introduction of Si−O−Si groups into the traditional cholesteric LC.40,41 In this study, LC moieties Chol-SiH (M) were attached to PVSt-PBs, leading to [PVSt-co-PB]-LCBCs having flexible Si−O−Si spacers. On one hand, using Si−O−Si as LC spacers may generate different LC orders; on the other hand, using Si−O− Si as additive groups adhered to LCBCs may affect the phase separation due to the high incompatibility between siloxanes and many organic components.55−57 According to our previous report,40,41 the Tg values of PVStSCLCPs and PB-SCLCPs were both approximate 30−40 °C. PB-SCLCPs exhibited LC textures just near to 30 °C, whereas E

DOI: 10.1021/acs.macromol.7b01952 Macromolecules XXXX, XXX, XXX−XXX

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Figure 5. POM-50 μm textures near to an isotropic state (Ti) of [PVSt-co-PB]-LCBCs with varying architectures, (a-1−a-4) EA(total) = 60%, EA(PVSt) = 100% and (a-5) EA(total) = 40%, EA(PVSt) = 40%: star-[PVSt-co-PB]-LCBCs; (b-1−b-4) EA(total) = 60%, EA(PVSt) = 100% and (b-5) EA(total) = 40%, EA(PVSt)=70%. Linear-[PVSt-co-PB]-LCBCs: S, PVSt block; B, PB block.

co-PB]-LCBCs exhibited a better POM than S-[PVSt-co-PB]LCBCs, in accordance with PB-SCLCPs, whereas that of the PVSt-SCLCPs was not obviously controlled by topologies. Powder WAXD measurements were essential for identifying the mesophases. WAXD for macroscopically oriented samples indicated a layer-like correlation according to the detected correlation period. Hence, the samples were tested at a fixed temperature (70 °C, drawing near to Tg) within the wide range of mesophasic formation. [PVSt-co-PB]-LCBCs in Figure S9 exhibited similar WAXD patterns. The sharpest first-order peak was observed and kept constant at low angles, detected equally at d = 38.4 Å. In addition, a very broad distribution peak was observed at wide angles, which was best defined for the longrange order and the SmA lamellar order. The microstructures of [PVSt-co-PB]-LCBCs are summarized in Table S2, and all showed the essential SmA phase with lamellae structures. To investigate the microarrangement of PVSt and PB LC blocks, the d-spacing between [PVSt-co-PB]-LCBCs and side chains was compared, with d = 38.4 Å > 2L1 (PB LC moiety L1 = 15.8 Å) and L2 < d = 38.4 Å < 2L2 (PVSt LC moiety L2 = 23.3 Å). The comparison suggested a typical double-molecular layer-like arrangement for the PB LC block and a monolayer or interpenetrative double-molecular layer-like arrangement for the PVSt LC block. The d-spacing values of [PVSt-co-PB]LCBCs were slightly similar to that of PB-SCLCPs and were much less similar to that of PVSt-SCLCPs, which is attributed to the larger molecular weight of PB-SCLCPs. Since PBSCLCPs and PVSt-SCLCPs were proven to be SmA with a double-molecular layer-like arrangement, LC side chains of the PVSt LC block were probably interpenetrative. In addition, SAXS was performed to determine more precise structures, as shown in Figures S11 and S12. These profiles show that the obvious halo on the meridian and the sharp peaks at small angles are in agreement with the aforementioned interpretation. Thermal Behaviors. The synthetic [PVSt-co-PB]-LCBCs were expected to have a room-temperature Tg, because both PVSt-SCLCPs and PB-SCLCPs exhibited approximate roomtemperature Tg due to the highly flexible siloxane linkages that were introduced.40,41 The thermal properties of [PVSt-co-PB]LCBCs were investigated. The results from DSC and TGA are summarized in Table 1, with respect to the curves shown in Figure 6. By comparing the 60% and 40% grafting density of the [PVSt-co-PB]-LCBCs, the shorter ΔT and lower Tg of the LCBCs with a 40% grafting density were determined. LCBCs with a 60% grafting density showed an approximately equivalent Tg (36−40 °C), and the Ti and ΔT values varied slightly along with an increasing PB or PVSt molar fraction,

and Ti, which displayed the existence of a single LC phase without obvious LC phase changes. As a result, although it is common that LC textures changed with varying temperature, the gradient switching process followed by two LC blocks was unique. This was further confirmed by the temperaturedependent WAXD. As shown in Figure 3a, the WAXD trace of [PVSt2.5k-co-PB2.5k]-LCBCs exhibited two sharp diffraction peaks within 2θ