Synthesis, liquid crystalline mesophases and morphologies of diblock copolymers composed of a poly(dimethylsiloxane) block and a nematic liquid crystalline block

ABSTRACT In this study, a series of liquid crystalline diblock copolymers, composed of a soft poly(dimethylsiloxane) (PDMS) block with a deﬁned length and a side-on liquid crystalline poly(3ʹʹ-acryloyloxypropyl 2,5-di(4ʹ-butyloxybenzoyloxy) benzoate) (P3ADBB) block with different lengths, are synthesised by the atom transfer radical polymerisation. The macromolecular structures, liquid crystalline properties and the microphase-separated morphologies of the diblock copolymer are investigated by 1H NMR, FT-IR, GPC, POM, DSC and TEM. The results show that the well-deﬁned diblock copolymers (PDMSn-b-P3ADBBm) possess four different soft/rigid ratios (n = 58, m = 10, 25, 42, 66) and relatively narrow molecular distributions (PDI ≤ 1.30). P3ADBB blocks of the copolymers show nematic sub-phases, which are identical to the mesomorphic behaviour of the homopolymer P3ADBB. After being annealed at 90°C in a vacuum oven for 48 h, the copolymers form a lamellar morphology when m = 10 and morphologies of PDMS spheres embedded in P3ADBB matrix when m = 25, 42 and 66. Graphical Abstract


Introduction
Block copolymers, which can self-assemble to form different kinds of ordered structures at the nanoscale, have received extensive attention for their applications in the microelectronics industry and others [1][2][3][4]. The final microphase separation structures, including bicontinuous double gyroid, lamellae, body-centred cubic arrays of spheres and hexagonally packed cylinders [5,6], are controlled by the volume fraction of each block and χN, where χ is the Flory-Huggins interaction parameter, N is the degree of the polymerisation of both blocks [7]. By changing the ratio of the blocks for a given copolymer, these microphase separation structures have been obtained [8,9]. By introducing a liquid crystalline segment as the rod block into the block copolymer, a bigger χ will be obtained. With the anisotropic alignment of the mesogens, the liquid crystalline block in the copolymer forms ordered subphase; while the isotropic block is usually highly immiscible with the liquid crystalline phase. The competition between the liquid crystalline order and phase separated confinement make the system to form microphase-separated morphologies easily [10][11][12][13][14][15]. In addition, the changing of the orientation of the liquid crystals will lead to a transition of the microphaseseparated structures from one to another [16][17][18]. For example, Shi et al. [18] reported the lamellar phase to hexagonally phase transition of a rod-coil liquid crystalline diblock copolymer when being annealed at temperature higher than its T g .
On the other hand, a bigger χ will also be obtained by incorporating a softer block, such as poly(dimethylsiloxane) (PDMS). PDMS has the extremely flexible Si-O backbone, which exhibit comparatively strong thermodynamic incompatibility when connect with conventional aliphatic polymers [19]. As a result, this block can easily separate from the other sub-phases to form a rich variety of nanostructures [8,10,20]. Owing to the strong segregation of the PS-PDMS system, Lo et al. [8] obtained different microphase-separated morphologies, including double gyroid, hexagonally packed cylinders and randomly distributed spheres. Shi et al. [10] also reported the fabrication of various nanostructures with low-MW PDMS-b-PMPCS rodcoil diblock copolymers.
An optimised block copolymer, which is designed to form microphase-separated nanostructure, is composed of a liquid crystalline block and a PDMS block. Depending on the mesogenic units in the polymer, liquid crystalline polymers are divided into main-and side-chain liquid crystalline polymers [21][22][23][24][25]. The side-chain liquid crystalline polymers are further divided into 'side-on' and 'end-on' types depending on whether the mesogenic groups are attached laterally or terminally to the polymer backbone via a flexible spacer [26,27]. For liquid crystalline polymers with the 'end-on' structure, they usually tend to form smectic phases [27,28], while the nematic phase is usually obtained in 'side-on' liquid crystalline polymer [29,30]. In recent years, block copolymers composed of a smectic liquid crystalline block and a PDMS block have been widely investigated [10,18,20,31], while block copolymers containing both a PDMS block and a side-on nematic block is still lacking in the literature.
In this study, a series of diblock copolymers containing both a PDMS block and a side-on nematic block are prepared. The diblock copolymers, poly(dimethylsiloxane)-block-poly(3ʹʹ-acryloyloxypropyl 2,5-di(4ʹ-butyloxybenzoyloxy)benzoate (PDMS n -b-P3ADBB m ), are synthesised by atom transfer radical polymerisation (ATRP). The synthesis, characterisation, liquid crystalline properties as well as the microphase-separated morphologies of the block copolymers are studied.
3ʹ-bromopropyl 2,5-dihydroxybenzoate KHCO 3 (10.6 g, 106 mmol) was added to a stirred mixture of 2,5-dihydroxybenzoic acid (6.2 g, 40 mmol) and 1,3-dibromopropane (18 mL, 175 mmol) in DMF (100 mL). The mixture was heated to 70°C and stirred for 20 h. After that, the reaction mixture was cooled down to room temperature, diluted with water (300 mL) and extracted twice with 80 mL dichloromethane (DCM). The organic phase was washed twice with water (150 mL) and dried over MgSO 4 . After evaporation of the solvent, the residue was subjected to column chromatography on silica gel with DCM as eluting solvent to yield white powder (75%). 1  3ʹ-acryloyloxypropyl 2,5-dihydroxybenzoate KHCO 3 (3.0 g, 30 mmol) was added to a stirred mixture of 3ʹ-bromopropyl 2,5-dihydroxybenzoate (5.5 g, 20 mmol) and acrylic acid (5.0 g, 69 mmol) in DMF (120 mL). The mixture was heated to 50°C and stirred for 20 h. The reaction mixture was cooled down to room temperature, diluted with water (150 mL) and extracted twice with 80 mL DCM. The organic phase was washed twice with water (80 mL) and dried over MgSO 4 . After evaporation of the solvent, the residue was subjected to column chromatography on silica gel with DCM as eluting solvent to yield white powder (92%). 1

P3ADBB
A typical ATRP of the monomer was carried out as follows: 3-ADBB (0,93 g, 1.5 mmol), HMTETA (54 μL, 0.2 mmol), CuBr (29 mg, 0.2 mmol), methyl 2-bromo-2-methylpropionate (18 mg, 0.1 mmol) and anisole (2 mL) were placed in a 50 mL Schlenk flask. The mixture was degassed by three freeze-pump-thaw cycles and sealed under vacuum. The polymerisation was carried out in an oil bath of 90°C for 24 h and then cooled down to room temperature. The mixture was further diluted with THF, passed through an alumina column to remove the catalyst. Crude product was obtained by evaporating of the solvent under reduced pressure and then purified by washing it with hot ethanol at least for 5 times, the product was dried in a vacuum oven overnight at room temperature. GPC: M n = 7.9 × 10 3

PDMS-Br
PDMS-Br was prepared according to literature procedure [18]. The PDMS-OH obtained from Sigma Aldrich was first dissolved in THF, and then methanol was added dropwise, leading to the precipitation of molecules with high molecular weights. After some precipitation was observed, the supernatant was poured away and the PDMS-OH with a narrow molecular weight distribution was obtained by evaporating of the solvent. PDMS-OH with a narrow molecular weight distribution (1.0 g, 0.21 mmol) and triethylamine (0.4 g, 4 mmol) were dissolved in anhydrous THF (50 mL) in an ice bath. 2-Bromo-2-methylpropionyl bromide (1 mL, 8 mmol) in 5 mL THF was added dropwise into the solution with stirring. The reaction was carried out overnight after 2-bromo-2-methylpropionyl bromide was added. After the reaction, triethylamine salt was filtered and the filtrate was evaporated under vacuum. The resulting oil product was dissolved in DCM (100 mL) and washed with saturated NaHCO 3 solution for 3 times, then dried over MgSO 4 . PDMS-Br with a narrow molecular weight distribution was obtained by evaporating of the remaining solvent. Yield: 20%. 1 The block copolymers PDMS n -b-P3ADBB m were prepared through ATRP of 3-ADBB by using PDMS-Br as the macroinitiator. PDMS n -b-P3ADBB m was obtained to possess a soft PDMS block and rigid P3ADBB liquid crystalline blocks with four different lengths. The synthesis of PDMS 58 -b-P3ADBB 10 was given as a typical example. Other copolymers were prepared through a similar procedure by changing the amount of 3-ADBB. The monomer 3-ADBB (0.37 g, 0.6 mmol), CuBr (26.3 mg, 0.2 mmol), HMTETA (51 μL, 0.2 mmol), PDMS-Br (0.2 g, 0.05 mmol) and anisole (1 mL) were placed in a 50 mL Schlenk flask. The mixture was degassed by three freeze-pump-thaw cycles and sealed under vacuum. The polymerisation was then carried out in an oil bath of 90°C for 24 h and then cooled down to room temperature. The mixture was further diluted with THF, passed through an alumina column to remove the catalyst. Crude product was obtained by evaporating the solvent under reduced pressure and then purified by washing it with hot ethanol at least for 5 times, the product was dried in a vacuum oven overnight at room temperature. Yield: 80%. 1  Other block polymers were prepared by using the similar procedure and condition. The amounts of reaction used in polymerisations were 0.93 g (1.5 mmol) 3-ADBB and 0.2 g (0.05 mmol) PDMS-Br for PDMS 58 -b-P3ADBB 25 , 0.31 g (0.5 mmol) 3-ADBB and 0.04 g (0.01 mmol) PDMS-Br for PDMS 58 -b-P3ADBB 42 and 0.49 g (0.8 mmol) 3-ADBB and 0.04 g (0.01 mmol) PDMS-Br for PDMS 58 -b-P3ADBB 66 , respectively. Characterisation 1 H NMR spectra were recorded on a JEOL JNM-AL spectrometer (300 MHz) by using d 6 -DMSO or CDCl 3 as the solvent. Fourier transform infrared (FT-IR) measurements were carried out on a Nicolet 560-IR spectrophotometer by incorporating the samples in the KBr pellets. Polarising microscopic (POM) observations were conducted on a Nikon LV 1000 POL microscope equipped with a Nikon DS-U3 digital sight, a Nikon DS-Fi2 CCD camera and a Linkam LTS420E hot stage. Thermal analyses were carried out using TA Q2000 system with a heating rate of 10°C/min in a nitrogen atmosphere. The molecular weights and molecular weight distributions were measured using a gel permeation chromatographic (GPC) instrument equipped with a PLgel 5 μm mixed-D column and a refractive index (RI) detector (Wyatt Optilab rEX). The measurements were carried out at 35°C and the molecular weights were calibrated with polystyrene standards. MALDI TOF HRMS experiments were performed on a Shimadzu Biotech Axima Performance. Wide-angle X-ray diffraction (WAXD) was recorded on a Rigaku RINT 2400 vertical goniometer with Cu K α radiation. Small-angel X-ray scattering (SAXS) experiment was performed with a high-flux SAXS instrument (SAXSess, Anton Paar) equipped with Kratky blockcollimation system and a Philips PW3830 sealed-tube X-ray generator (Cu K α ). Transmission electron microscopy (TEM) was performed on a Hitachi H-7650B electron microscope operated at 80 kV. Bulk samples of the block copolymers were prepared by solution-cast method using THF as the solvent at room temperature and drying for 1 week. The samples for TEM observation were prepared by annealing the bulk samples at 90°C under vacuum for 48 h and embedded into epoxy resin. Ultrathin sections of about 70 nm were cut from a thin film on an ultramicrotome (Lecia EM UC6).

Results and discussion
A series of side-on liquid crystalline diblock copolymers PDMS n -b-P3ADBB m , containing covalently linked soft PDMS blocks and liquid crystalline P3ADBB blocks, were synthesised by ATRP. Four PDMS n -b-P3ADBB m samples with the same PDMS block length and four different P3ADBB block lengths were prepared to study the influence of the ratios of isotropic block to liquid crystalline block on the liquid crystalline and morphology properties. The copolymers and related homopolymer were characterised by different methods, including 1 H NMR, GPC, POM and thermal analysis.
The synthetic route of the liquid crystalline diblock copolymers PDMS n -b-P3ADBB m is presented in Scheme 1. Firstly, the side-on liquid crystalline monomer 3-ADBB together with the macro-initiator PDMS-Br is prepared. Then, block copolymers PDMS n -b-P3ADBB m are synthesised through ATRP of 3-ADBB by using PDMS-Br as the macro-initiator and CuBr/ HMTETA as the catalyst at 90°C. Four block copolymers with the same PDMS block length and different P3ADBB block lengths are prepared by adjusting the ratios of 3-ADBB to PDMS-Br.
The synthetic route used in this paper to prepare the side-on liquid crystalline monomers (3-ADBB) is also shown in Scheme 1, which includes three reactions. In the first step, the carboxyl group of 2,5dihydroxybenzoic acid reacts with one of the bromine in 1,3-dibromopropane under the presence of KHCO 3 . Then, the second bromine is substituted with the acrylate group, which is suitable for the polymerisation, using the same procedure. Finally, under the catalyst of pyrrolidinopyridine, the liquid crystalline monomer 3-ADBB is obtained by esterification between the previous intermediate and 4-n-butyloxybenzoic acid under the presence of dicyclohexylcarbodiimide. Figure 1 presents the COSY spectrum of the liquid crystalline monomer 3-ADBB with the resonance signal assignments, which indicates the successfully synthesis of this monomer.
With the prepared liquid crystalline monomer 3-ADBB, the corresponding liquid crystalline homopolymer P3ADBB is polymerised by using methyl 2bromo-2-methylpropionate as the initiator and CuBr/ HMTETA as the catalyst at 90°C. For the controlled/ living radical polymerisation, the average molecular weights of the obtained polymer can be calculated by the feeding ratio ([monomer]/[initiator]) and the monomer conversion, which can be obtained from  the 1 H NMR spectrum of the raw experimental sample by comparing the integration of protons of the residual monomer to the integration of the protons of the obtained polymer [32]. In this study, the conversion estimated by this method is approximately 80% and the feeding ratio is 15. As a result, the calculated degree of polymerisation of P3ADBB is 12. The 1 H NMR spectrum of the purified homopolymer P3ADBB synthesised by ATRP is shown in Figure 2(b) (line 1). The resonance signals of the spectrum can be readily assigned to the protons of the polymer. Comparing with the spectrum of the liquid crystalline monomer 3-ADBB, the peaks in the low magnetic field range from 6.5 ppm to 8.5 ppm are coming from the mesogenic core on the phenyl rings. All of the monomer have been polymerised or washed away as the peaks of the protons of the double bonds from the residual monomer are disappeared. The ATRP process is well controlled as the molecular weight distribution of P3ADBB is only 1.22 and M n is 7.9 × 10 3 (Table 1), according to the GPC results.
The PDMS-OH purchased from Sigma Aldrich has an asymmetric and broad molecular weight distribution with a polydispersity of 1.31, which needs to be fractionated prior to be used. The fractionation of PDMS-OH is done by adding methanol into its THF solution, leading to the precipitation of PDMS-OH with higher molecular weight. Through this method, PDMS-OH with narrower molecular weight distribution can be easily obtained. Similar procedure has also been reported by Shi et al. [18]. PDMS-Br with a narrow molecular weight distribution is finally obtained by reacting the fractionated PDMS-OH with 2-bromo-2methylpropionyl bromide. Figure 3 presents the   MALDI TOF HRMS result of PDMS-Br. According to the intensity of every m/z, the calculated molecular weight and molecular weight distribution are M n = 4.3 × 10 3 and M w /M n = 1.02. As the molecular weight of the repeat unit of PDMS-Br is M = 74, the degree of the polymerisation of PDMS-Br is estimated to be M n /M, which is 58 for the PDMS block (n = 58). After the synthesis and characterisation of the liquid crystalline monomer 3-ADBB and the macroinitiator PDMS-Br, block copolymers PDMS n -b-P3ADBB m are synthesised through ATRP with them. Figure 2(a) shows the 1 H NMR spectrum of PDMS 58 -b-P3ADBB 42 with the assignment of the resonance signals as a typical example. Comparing with the 1 H NMR spectrum of PDMS-Br (Figure 2(b) (line 6), new resonance signals which located at the same positions that P3ADBB 12 appears are observed, indicating the successful synthesis of the block copolymer. The degree of the polymerisation of P3ADBB block can be calculated according to the equation: where I f and I a are the integration areas of the resonances for the protons on the phenyl ring in P3ADBB block and the protons on the methyl groups in PDMS block, as shown in Figure 2(a); n is the degree of polymerisation of PDMS block, which is 58. Through this method, the calculated m is 42 in this diblock copolymer. By changing the ratio of the monomer and the macroinitiator used in the ATRP processes, three other liquid crystalline diblock copolymers, namely PDMS 58 -b-P3ADBB 10 , PDMS 58 -b-P3ADBB 25 and PDMS 58 -b-P3ADBB 66 are also prepared. The 1 H NMR spectra of the obtained copolymers are shown in Figure 2(b), it is worth noting that the characteristic resonance from the P3ADBB block increases with the increment of the block length. The degrees of the polymerisation of P3ADBB block and PDMS block as well as the molecular weights of the copolymers are summarised in Table 1.
The typical GPC traces of PDMS-Br and the block copolymers are shown in Figure 4. The average molecular weight and molecular weight distribution of PDMS-Br calculated from the GPC result are 5.8 × 10 4 and 1.15. Comparing with PDMS-Br, the GPC curves of the obtained copolymers shift towards a higher molecular weight after the ATRP, indicating the formation of P3ADBB chains on the macroinitiator. As expected, the copolymer with longer P3ADBB block length shows higher molecular weight. The molecular weights are 3.2 × 10 4 , 5.2 × 10 4 , 6.4 × 10 4 and 8.6 × 10 4 for PDMS 58 -b-P3ADBB 10 , PDMS 58 -b-P3ADBB 25 , PDMS 58 -b-P3ADBB 42 and PDMS 58 -b-P3ADBB 66 , respectively. The molecular weight distributions of the obtained liquid crystalline block copolymers are slightly broader than that of the macroinitiator PDMS-Br, but are still below 1.3, indicating that the ATRP processes are well controlled. The average molecular weights and molecular weight distributions of the polymers obtained from GPC are also listed in Table 1.
The liquid crystallinity of the homopolymer P3ADBB 12 and the diblock copolymer PDMS n -b-P3ADBB m is studied by POM, XRD and DSC after the characterisation of the structures of them. Figure 5 shows the typical POM images of 3-ADBB and P3ADBB 12 , which are the typical schlieren textures [25]. XRD measurements could provide more detailed information on the liquid crystalline phase structures. For smectic liquid crystal, a strong peak at low angle (1°< 2θ < 5°) on its SAXS curve and a broad peak at higher angle (2θ ≈ 20°) on its WAXD curve would be observed. While for nematic and cholesteric liquid crystals, only the broad peak at higher angle (2θ ≈ 20°) on their WAXD curves can be observed [33]. The SAXS and WAXD curves of the homopolymer P3ADBB are shown in Figure S1 and S2 (supplementary information). As no peak is observed on their SAXS curves, the smectic phase of these polymers can be excluded. Together with the POM observation, the nematic phases of the series of polymer are confirmed. Cooling from the isotropic state, POM observation indicated that the isotropic-nematic transition of P3ADBB 12 occurred at about 106°C. When connected with the PDMS block, the diblock copolymers PDMS n -b-P3ADBB m also show nematic phase, as shown in Figure 6, Figure S1 and S2. We have reported similar results for the block copolymers containing azo blocks [19]. The isotropic-nematic transition temperatures obtained from POM observation are 110°C, 111°C, 113°C and 114°C for PDMS 58 -b-P3ADBB 10 , PDMS 58 -b-P3ADBB 25 , PDMS 58 -b-P3ADBB 42 and PDMS 58 -b-P3ADBB 66 , respectively. Figure 7 shows the DSC curves of diblock copolymers PDMS n -b-P3ADBB m on the second heating and second cooling scans. On the heating scan, the diblock   copolymer PDMS 58 -b-P3ADBB 10 shows a glass transition temperature (T g ) at 50.9°C and an endothermic transition at 107.9°C (peak value). As PDMS has an extremely low T g (−123°C) [18], both of the above transitions are related with P3ADBB block. Comparing the transition temperature based on DSC with the result from POM observation, it can be deduced that the endothermic transition on the DSC curve of PDMS 58 -b-P3ADBB 10 corresponds to the transition from the nematic phase to isotropic state (T NI ). With the increasing length of P3ADBB block, the glass transition temperature, the nematic to isotropic transition temperature and the phase transition enthalpy of the diblock copolymer increases. The glass transition temperatures and the nematic to isotropic transition temperatures obtained from the heating scan are 50.9°C, 55.9°C, 58.  Table 2. The results show that the T g and T NI of the block copolymer are close to that of the homopolymer P3ADBB 12 , revealing that the liquid crystalline block form a separated sub-phase in these systems [1,12,19].
The microphase-separated morphologies of the diblock copolymers are studied by TEM observation. Before the TEM observation, the film of the liquid crystalline diblock copolymers was annealed at 90°C (below the T NI ) under vacuum for 48 h. And then the bulk films were further cut to ultrathin films of about 70 nm by ultramicrotome cutting. Due to the significant mass thickness contrast from the strong scattering of the silicon-containing microdomains, the ultrathin films of the block copolymers were used for the TEM observation directly without staining treatment. Figure 8 shows the typical TEM images of the microphase-separated morphologies of the block copolymer after thermally annealed at 90°C for 48 h. In these images, the grey regions represent the P3ADBB sub-phases while the dark regions are from the PDMS sub-phases as the result of the electron density differences [19]. The diblock copolymers may form microphase separation morphologies such as bicontinuous double gyroid, lamellae, body centred cubic arrays of spheres and hexagonally packed cylinders after the selfassembling process. For PDMS 58 -b-P3ADBB 10 , lamellar phase morphology is observed (Figure 8(a)). The thickness of P3ADBB lamellae in PDMS 58 -b-P3ADBB 10 is measured to 12.6 nm and the thickness of PDMS lamellae is about 7.3 nm. For PDMS 58 -b-P3ADBB 25 , PDMS 58 -b-P3ADBB 42 and PDMS 58 -b-P3ADBB 66 , hexagonally packed cylinders and/or spherical phase might be formed according to the results of Figure 8(b-d). As there are many microphases in the sample, hexagonally packed cylinders should be observed if the sample is large enough. Figure S4, S5 and S6 in the supporting information show the TEM images of PDMS 58 -b-P3ADBB 25 , PDMS 58 -b-P3ADBB 42 and PDMS 58 -b-P3ADBB 66 in a larger scale, respectively. Only spherical phases are observed in these figures, indicating that PDMS 58 -b-P3ADBB 25 , PDMS 58 -b-P3ADBB 42 and PDMS 58 -b-P3ADBB 66 form spherical phases. We have reported that the transition from lamellar phase to spherical phase for PDMS n -b-POCN m occurs when the content of PDMS changed from 42.3 wt% to 31.4 wt% [19]. It is reasonable for the PDMS 58 -b-P3ADBB 25 , PDMS 58 -b-P3ADBB 42 and PDMS 58 -b-P3ADBB 66 to form the spherical phase as the content of PDMS are lower than 21.8 wt%. The

Conclusions
In conclusion, a series of liquid crystalline diblock copolymers (PDMS n -b-P3ADBB m ) with well-defined structures and narrow molecular weight distributions were prepared by ATRP. MALDI-TOF MS showed that the degree of polymerisation of PDMS block is 58. 1 H NMR spectra of the diblock copolymers indicated that the degree of polymerisation of P3ADBB blocks are 10, 25, 42 and 66, respectively. POM and XRD revealed that the diblock copolymers show nematic phase when cooling down from the isotropic state. The T g and T NI of the diblock copolymers increases with the increment of the length of P3ADBB block. After annealed at 90°C for 48 h, PDMS 58 -b-P3ADBB 10 formed lamellar microphase-separated nanostructure. While for the other three diblock copolymers, spherical PDMS sub-phase dispersed in continuous P3ADBB matrix were observed.

Disclosure statement
No potential conflict of interest was reported by the authors.