Diblock copolymers composed of a liquid crystalline azo block and a poly(dimethylsiloxane) block: synthesis, morphology and photoresponsive properties

Abstract

A series of diblock copolymers, composed of a soft poly(dimethylsiloxane) (PDMS) block with a defined length and a liquid crystalline poly(6-(4-(4′-cyanophenylazo)phenoxy)hexyl acrylate) (POCN) block with different lengths, was synthesized by the atom transfer radical polymerization (ATRP). A homopolymer POCN was also prepared by the same method for comparison. The polymer structures, liquid crystalline phase, microphase-separated morphology, and photoisomerization were then investigated by ¹H NMR, FT-IR, GPC, UV-Vis, POM, DSC, XRD and TEM. The results showed that the well-defined diblock copolymers (PDMS_n-b-POCN_m) possess four different soft/rigid ratios (n = 58, m = 8, 15, 24, 36) and relatively narrow molecular distributions (PDI ≤ 1.30). POCN blocks of the copolymers form a smectic-A sub-phase, which is identical to the mesomorphic behaviour of POCN. After being annealed at 140 °C in a vacuum for 24 h, the copolymers form a lamellar morphology when W_PDMS is 42.3–57.8% and a morphology of PDMS spheres embedded in an azo matrix when W_PDMS is 23.3–31.4%. POCN and PDMS_n-b-POCN_m in solution show typical photoisomerization behaviour of the azobenzene moieties. Although H-aggregation is observed to a certain extent, photochemical processes of the as-cast films of the polymers are similar to those of the solutions. On the other hand, the photoisomerization behaviour of annealed PDMS_n-b-POCN_m films is significantly different from that of the annealed POCN film, which is closely correlated with the phase-separated morphologies. The observations can provide deep understanding of the phase-separated structures and be used to develop new materials.

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1. Introduction

Block copolymers can self-assemble to form ordered structures on the nano-scale and offer attractive patterning technology for applications in the microelectronics industry and others.^1–6 The final morphologies of block copolymers depend on the volume fraction 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.^5–7 The coil–coil diblock copolymers can self-assemble to form a variety of morphological structures such as lamellae, bicontinuous double gyroid, hexagonally packed cylinders, and body centered cubic arrays of spheres depending on these parameters.^8–13 Block copolymers composed of both liquid crystalline (LC) and isotropic (I) blocks have attracted considerable recent research interest because of the ordering at two different scales and unique functions.^14–29 LC/I block copolymers can form microphase-separated morphologies with coexisting anisotropic and isotropic phases. The LC blocks of the copolymers form the ordered sub-phase because of anisotropic alignment of the mesogens. As isotropic blocks are highly immiscible in the LC sub-phase, the systems are usually strongly segregated to show well-defined morphologies and high order in LC sub-phase. The competition between the LC order and phase-separated confinement can result in a rich variety of morphologies and nanostructures.^{10,11,16–32}

Azobenzene and its derivatives have been widely used as functional groups to develop materials with various photoresponsive properties.^33–42 Upon irradiation with light at appropriate wavelengths, polymers containing azo functional groups (azo polymers for short) can show a variety of photoresponsive properties related to the reversible trans–cis isomerization of azo chromophores. The repeated photo-isomerization of the azo chromophores will cause motions at different levels.^33–35 The photoresponsive properties and possible applications of azo polymers include photoinduced dichroism and birefringence,^33,43 photoinduced phase transition,⁴⁴ surface-relief-grating (SRG),^45,46 two-dimensional surface quasi-crystal structure,⁴⁷ spontaneous surface pattern,⁴⁸ photo-mechanical thin film contraction and bending,^49–51 light-responsive block copolymer micelles,³⁶ photoinduced colloidal deformation,⁵² and many others. Azo polymers with novel molecular architectures and aggregated structures are of particular interest in material development and device applications.

LC/I azo block copolymer can incorporate the attractive features of block copolymers and azo polymers. When the azo blocks bear mesogens, the LC ordering will be introduced into the microphase-separated morphological structures.²¹ The liquid crystallinity of the azo blocks and correlative motions will bring significant property changes.^22–25,28 Therefore, the obtained block copolymers can be expected to incorporate and enhance the photoresponsive features of azo polymers within the well-defined nanostructures.^34,35 Recently, LC/I block copolymers bearing poly(ethylene glycol) (PEG) and LC azo blocks have been prepared.^21–25 The PEG block forms well-organized nanocylinders embedded in a matrix of azobenzene-containing LC block. The alignment of PEG nanocylinder array can be controlled by irradiation with a polarized laser beam.²⁴ Soft poly(dimethyl-siloxane) (PDMS) is another interesting component used to build up LC/I block copolymers.^11,26–28 PDMS has the extremely flexible Si–O backbone and can easily separate from mesogenic LC sub-phase to form a rich variety of nanostructures.¹¹ A diblock copolymer composed of PDMS and azobenzene-containing LC blocks has been synthesized and the light-directed anisotropic reorientation in monolayer has been demonstrated.²⁸ To understand the microphase-separated morphology and its correlation with photoresponsive properties of this type of azo block copolymers, a systematic investigation is currently needed. However, to our knowledge, no such study has been reported concerning the synthesis of block copolymer with different PDMS and azo block ratios, their mesomorphic state, morphology, and corresponding photoresponsive properties.

In this study, a series of LC/I diblock copolymers containing both PDMS and LC azo blocks was synthesized. The diblock copolymers, poly(dimethylsiloxane)-block-poly(6-(4-(4′-cyano-phenylazo)phenoxy)hexyl acrylate) (PDMS_n-b-POCN_m), were synthesized by atom transfer radical polymerization (ATRP). The diblock copolymers show LC order and two different phase-separated morphologies with variation of the compositions. The photoisomerization behaviour of the block copolymers was investigated and its correlation with the microphase-separated morphologies was investigated. The syntheses, LC mesophase, morphology, and photochemical isomerization of the block copolymers are reported below.

2. Experimental section

2.1 Materials

2-Bromo-2-methylpropionyl bromide (98%) and 4-amino-benzonitrile (98%) were purchased from Alfa Aesar and used as received. Monohydroxy-terminated PDMS (PDMS-OH) with a Mn of 4700 and methyl 2-bromo-2-methylpropionate (98%) were obtained from Sigma Aldrich and used as received. 1,1,4,7,10,10-Hexamethyltriethylenetetramine (HMTETA, 97%) was purchased from J&K chemical and used as received. Copper(I) bromide (98%, Alfa Aesar) was washed with acetic acid, ethanol and diethyl ether at least for 5 times, respectively, and then dried in vacuum. Tetrahydrofuran (THF) was purified by distillation with sodium and benzophenone. Deionized water (resistivity >18 MΩ cm) was obtained from a Milli-Q water purification system. All other reagents were commercially available products and used as received without further purification.

2.2 Synthesis

4-Cyano-4′-hydroxy-azobenzene. 4-Aminobenzonitrile (11.8 g, 0.10 mol) was dissolved in a mixture of hydrochloric acid (25 mL) and water (50 mL) at 0 °C. NaNO₂ (8.3 g, 0.12 mol) dissolved in water (20 mL) was slowly dropped into the 4-aminobenzonitrile solution. The mixture was stirred with the ice bath cooling for 10 min and filtered to obtain the diazonium salt solution. The diazonium salt solution was added dropwise into a water solution (200 mL) of a mixture of phenol (9.4 g, 0.10 mol) and NaOH (8.0 g, 0.20 mol), where the temperature was controlled to be 0 °C. After reaction for 2 h, the raw product was obtained by pouring the reaction solution into an excess of water. The precipitate was collected by filtration and washed with plenty of water. After drying, the residue was purified by column chromatography on silica gel with hexane/AcOEt (5/1, v/v) as eluting solvent to afford red solid (90%). ¹H NMR (600 MHz, d₆-DMSO), δ (ppm): 10.55 (s, 1H, OH), 8.03 (d, 2H, ArH), 7.94 (d, 2H, ArH), 7.86 (d, 2H, ArH), 6.99 (d, 2H, ArH).

4-Cyano-4′-(6-hydroxyhexyloxy)azobenzene. 6-Chloro-1-hexanol (13.7 g, 0.10 mol), K₂CO₃ (13.8 g, 0.10 mol) and KI (1.7 g, 0.01 mol) were added into a DMF (200 mL) solution of 4-cyano-4′-hydroxy-azobenzene (17.8 g, 0.08 mol). After reaction at 110 °C for 12 h, the reaction solution was poured into excess of water. The precipitate was collected by filtration and washed with plenty of water. After drying, the residue was subjected to column chromatography on silica gel with DCM as eluting solvent to yield red powder (90%). ¹H NMR (400 MHz, d₆-DMSO), δ (ppm): 8.05 (d, 2H, ArH), 7.95 (m, 4H, ArH), 7.15 (d, 2H, ArH), 4.38 (t, 1H, OH), 4.08 (t, 2H, OCH₂), 3.42 (t, 2H, OCH₂), 1.73 (m, 2H, CH₂), 1.42 (m, 6H, CH₂). ¹³C NMR (100 MHz, d₆-DMSO) δ (ppm): 25.8, 25.9, 33.0, 61.2, 68.7, 113.0, 115.7, 119.1, 123.4, 125.8, 134.3, 146.5, 154.7, 163.0.

6-(4-(4-Cyanophenylazo)phenoxy)hexyl methacrylate (OCN). 4-Cyano-4′-(6-hydroxyhexyloxy)azobenzene (16.3 g, 0.05 mol) and triethylamine (12.3 g, 0.12 mol) in DCM (200 mL) was stirred in an ice bath. Methacryloyl chloride (6 mL, 0.06 mol) was added dropwise into the above solution. After the methacryloyl chloride was added, the reaction was carried out overnight. Triethylamine hydrochloride was filtered and the filtrate was washed with water (150 mL), 5% NaHCO₃ solution (150 mL), and water (150 mL), and dried over Na₂SO₄. After evaporation of the solvents, the residue was purified by column chromatography on silica gel with DCM as eluting solvent to yield red powder (90%). ¹H NMR (400 MHz, CDCl₃), δ (ppm): 7.94 (m, 4H, ArH), 7.78 (d, 2H, ArH), 7.01 (d, 2H, ArH), 6.10 (s, 1H, C=CH₂), 5.55 (s, 1H, C=CH₂), 4.17 (t, 2H, OCH₂), 4.07 (t, 2H, OCH₂), 1.95 (s, 3H, CH₃), 1.87–1.48 (m, 8H, CH₂). ¹³C NMR (100 MHz, CDCl₃) δ (ppm): 18.4, 25.8, 25.9, 64.7, 68.3, 113.2, 123.2, 125.3, 125.6, 133.2, 136.6, 146.8, 154.9, 162.6, 167.8. IR (KBr; cm⁻¹): 2941 (C–H; s), 2226 (C≡N; s), 1727 (C=O; s), 1632 (C=C; s), 1601, 1580, 1500 (Benz. ring, s), 1255 (C–O–C, s). MS (ESI): calcd for C₂₂H₂₅O₃N₃ [M + Na]⁺: m/z = 414.1788, found: 414.1786. UV-Vis: λ_max = 363 nm (chloroform).

POCN. A typical ATRP of the monomer (OCN) was carried out as follows: OCN (3.1 g, 8 mmol), CuBr (57.6 mg, 0.4 mmol), HMTETA (109 μL, 0.4 mmol), 2-bromo-2-methylpropionyl bromide (36.2 mg, 0.2 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 polymerization was carried out in an oil bath of 90 °C for 20 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 solvent under reduced pressure. The crude product was purified by washing it with hot ethanol at least for 5 times and dried in a vacuum oven overnight at room temperature. Yield: 80%. ¹H NMR (400 MHz, CDCl₃), δ (ppm): 7.82 (m, 4H, ArH), 7.68 (m, 2H, ArH), 6.90 (m, 2H, ArH), 3.94 (m, 4H, CH₂), 2.03–0.91 (m, 13H, CH₂, CH₃). IR (KBr; cm⁻¹): 2941, 2862 (C–H; s), 2225 (C≡N; s), 1728 (C=O; s), 1599, 1581, 1500 (Benz. ring, s), 1255 (C–O–C, s). GPC: Mn = 2.5 × 10⁴, Mw/Mn = 1.17. DSC: T_g = 48.9 °C, T_LC-I = 156.8 °C (heating). UV-Vis: λ_max = 363 nm (chloroform).

PDMS-Br. PDMS-Br was prepared according to literature procedure.²⁶ PDMS-OH (10.0 g, 2.1 mmol) and triethylamine (4.1 g, 40 mmol) were dissolved in anhydrous THF (100 mL) in an ice bath. 2-Bromo-2-methylpropionyl bromide (6 mL, 48.5 mmol) was added dropwise into the solution with stirring. After the 2-bromo-2-methylpropionyl bromide was added, the reaction was carried out overnight. Triethylamine salt was filtered and the filtrate was evaporated under vacuum. The resulting oil was dissolved into dichloromethane (200 mL) and washed with saturated NaHCO₃ solution for 3 times, dried over Na₂SO₄ and then filtered. After evaporation of the solvent, the residue was dissolved in THF, and then methanol was added dropwise to precipitate the product. PDMS-Br was obtained by evaporating of the remaining solvent. Yield: 25%. ¹H NMR (400 MHz, CDCl₃), δ (ppm): 4.32 (t, 2H, CH₂), 3.67 (t, 2H, CH₂), 3.45 (t, 2H, CH₂), 1.97 (d, 6H, CH₃), 1.86 (m, 2H, CH₂), 1.60 (m, 4H, CH₂), 0.54 (m, 2H, CH₂), 0.33–0.03 (m, 350H, Si–CH₃). IR (KBr; cm⁻¹): 2963, 2905 (C–H; s), 1260 (Si–C, s), 1094, 1020 (Si–O–Si, s), 800 (Si–C, δ). GPC: Mn = 5.9 × 10³, Mw/Mn = 1.16. HRMS (MALDI TOF mass): Mn = 4.3 × 10³, Mw/Mn = 1.02.

PDMS_n-b-POCN_m. The block copolymers were prepared through ATRP of OCN by using PDMS-Br as the macroinitiator. Block copolymers PDMS_n-b-POCN_m were obtained to possess a soft PDMS block and rigid POCN block with four different lengths. The synthesis of PDMS₅₈-b-POCN₈ is given as a typical example. Other copolymers were prepared through a similar procedure by changing the amount of OCN. The monomer OCN (0.18 g, 0.5 mmol), CuBr (26.4 mg, 0.2 mmol), HMTETA (50 μL, 0.2 mmol), PDMS-Br (0.2 g, 0.05 mmol) and anisole (0.8 mL) were placed in a 50 mL Schlenk flask. The mixture was degassed by three freeze–pump–thaw cycles and sealed under vacuum. The polymerization was then carried out in an oil bath of 90 °C for 20 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 under reduced pressure. The crude product was further purified by washing it with hot ethanol at least for 5 times and dried in a vacuum oven overnight at room temperature. Yield: 80%. ¹H NMR (400 MHz, CDCl₃), δ (ppm): 7.84 (m, 4H, ArH), 7.71 (m, 2H, ArH), 6.90 (m, 2H, ArH), 3.94 (m, 4H, CH₂), 1.76–0.92 (m, 13H, CH₂, CH₃), 0.11–0.03 (m, 58H, Si–CH₃). IR (KBr; cm⁻¹): 2963 (C–H; s), 2228 (C≡N; s), 1729 (C=O; s), 1501, 1582, 1502 (Benz. ring, s), 1261 (Si–C, s), 1140 (C–O–C, s), 1099, 1020 (Si–O–Si, s), 800 (Si–C, δ). GPC: Mn = 3.4 × 10⁴, Mw/Mn = 1.24.

Other PDMS_n-b-POCN_m polymers were prepared by using the similar procedure and conditions. The amounts of OCN used in polymerizations were 0.35 g (0.9 mmol) for PDMS₅₈-b-POCN₁₅, 0.55 g (1.4 mmol) for PDMS₅₈-b-POCN₂₄, and 0.75 g (1.9 mmol) for PDMS₅₈-b-POCN₃₆.

2.3 Characterization

¹H NMR and ¹³C NMR spectra were recorded on a JEOL JNM-ECA400 spectrometer (400 MHz) or JNM-ECA600 spectrometer (600 MHz) by using d₆-DMSO or CDCl₃ as the solvent and tetramethylsilane as internal standard. Fourier transform infrared (FT-IR) measurements were carried out on a Nicolet 560-IR spectrophotometer by incorporating the samples in the KBr pellets. UV-Vis spectra were obtained with an Agilent 8453 UV-vis spectrophotometer. Polarizing microscopic (POM) observations were conducted on a Nikon LV 1000 POL microscope equipped with a Nikon DS-Fi2 CCD camera, Nikon DS-U3 digital sight and a Linkam LTS420E hot stage. Thermal analyses were carried out using TA Q2000 system with a heating rate of 10 °C min⁻¹ in a nitrogen atmosphere. The X-ray diffraction measurements were performed on a RINT2000 vertical goniometer, to record the diffraction patterns, the powder sample were grinded, and put in a standard glass sample holder. 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. THF was used as the eluent and the flow rate was 1.0 mL min⁻¹. MALDI TOF HRMS experiments were performed on a Shimadzu Biotech Axima Performance by dissolving PDMS-Br in chloroform. Transmission electron microscopy (TEM) was performed on a Hitachi H-7650B electron microscope operated at 100 kV. Bulk samples of the block copolymers as thick films 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 140 °C under vacuum for 24 h and embedded into epoxy resin. Ultrathin sections of about 70 nm were cut from the bulk samples on an ultramicrotome (Lecia EM UC6).

2.4 Photoresponsive property measurements

The trans–cis photoisomerization of the samples was recorded in situ by a UV-vis spectrophotometer equipped with an Iwata LED UV light as the irradiation light (365 nm). The measurements were performed at different time intervals until the photo-stationary states were reached. The light intensity used in this experiment was 0.4 mW cm⁻² at the top of the sample slot. The solutions of the samples were obtained by dissolving the corresponding polymers in chloroform with the concentrations of 0.02 mg mL⁻¹. For preparing polymer films for the measurements, chloroform solutions of the polymers with a concentration of 2.5 wt% were spin-coated on quartz slides and dried properly. When measuring, the polymer films were put inside of the sample slot of the spectrophotometer with a tilting angle about 15° off the vertical. The films were uniformly exposed to the irradiation light.

3. Results and discussion

3.1 Synthesis and characterization

The series of azo block copolymers PDMS_n-b-POCN_m was synthesized by ATRP to contain covalently linked PDMS and POCN blocks. In order to study the influence of the ratios of isotropic block to LC block on the morphology and photoinduced isomerization properties, four PDMS_n-b-POCN_m samples with the same PDMS block length and four different POCN block lengths were prepared. The copolymers and related homopolymer were characterized by spectral methods, GPC and thermal analysis. The analytical results are given in Experimental section and ESI (Fig. S1–S7†). Some main results are discussed below.

The synthetic route of the azo diblock copolymers is illustrated as Fig. 1. Firstly, the azo monomer OCN and the macroinitiator PDMS-Br were prepared, respectively. Then, by using PDMS-Br as the macroinitiator, PDMS_n-b-POCN_m was synthesized through ATRP of OCN. The polymerization was carried out at 90 °C by using anisole as the solvent in the presence of CuBr/HMTETA as the catalyst. Four block copolymers with the same PDMS block length and different OCN block lengths were obtained by adjusting the ratios of OCN to PDMS-Br. MALDI TOF HRMS measurement shows that PDMS-Br has M_n = 4.3 × 10³ and M_w/M_n = 1.02 (Fig. 2a). Therefore, the degree of polymerization is estimated to be 58 for the PDMS block (n = 58). Fig. 2b gives GPC traces of macroinitiator (PDMS-Br), azo homopolymer (POCN), and block copolymers (PDMS_n-b-POCN_m). The GPC results clearly demonstrate the successful propagation of the OCN chains initiated by the macroinitiator. It can be observed that the GPC curves shift to the higher molecular weight side compared to the peak of PDMS-Br after introducing the POCN blocks. As expected, the copolymers with the longer POCN blocks show the higher molecular weights. The molecular weight distributions of PDMS_n-b-POCN_m become slightly broader with the growth of the OCN block.

Fig. 1 Synthetic route of the azo diblock copolymers.

Fig. 2 (A) MALDI-TOF MS results of the macroinitiator (PDMS-Br); (B) GPC traces of the polymers: (1) PDMS-Br, (2) POCN, (3) PDMS₅₈-b-POCN₈, (4) PDMS₅₈-b-POCN₁₅, (5) PDMS₅₈-b-POCN₂₄, (6) PDMS₅₈-b-POCN₃₆.

Fig. 3 gives the ¹H NMR spectra of the macroinitiator PDMS-Br, the azo homopolymer (POCN) and the azo copolymers in chloroform-d. Fig. 3A shows the assignment of the ¹H NMR resonance signals for PDMS₅₈-b-POCN₃₆ as a typical example. The characteristic resonances from the POCN blocks can be clearly identified and the signal intensities increase with the block length increases (Fig. 3B). The repeat unit number of the POCN blocks can be calculated according to the equation:

m = (I_3.93 × 6 × n)/(I_0.04 × 4) (1)

where the repeat unit number of PDMS is n = 58 as calculated from the MALDI-TOF MS result. I_3.93 and I_0.04 are the integrated areas of the resonances for the methylene protons in POCN block at 3.93 ppm and the protons of the methyl group in PDMS block at 0.04 ppm, respectively. The polymerization degree ratios of PDMS to POCN are calculated to be 58 : 8, 58 : 15, 58 : 24 and 58 : 36. The block copolymers are denoted as PDMS₅₈-b-POCN₈, PDMS₅₈-b-POCN₁₅, PDMS₅₈-b-POCN₂₄ and PDMS₅₈-b-POCN₃₆. The molecular weights and molecular weight distributions of the copolymers and related polymers are summarized in Table 1. Owing to the advantages of controlled radical polymerization, the copolymers possess the relatively narrow molecular weight distributions.

Fig. 3 ¹H NMR spectra of the polymers. (A) The assignment of the ¹H NMR spectrum for PDMS₅₈-b-POCN₃₆ as a typical example; (B) ¹H NMR spectra of the polymers, (1) POCN, (2) PDMS₅₈-b-POCN₃₆, (3) PDMS₅₈-b-POCN₂₄, (4) PDMS₅₈-b-POCN₁₅, (5) PDMS₅₈-b-POCN₈, (6) PDMS-Br.

Table 1 Molecular weights and molecular weight distributions of the macroinitiator (PDMS-Br), homopolymer (POCN) and copolymers

Sample	Mn (GPC)	Mw/Mn (GPC)	PMDS content (wt%)^a
a Determined by ^1H NMR.
PDMS-Br	5.9 × 10^3	1.16	100
POCN	2.5 × 10^4	1.17	0
PDMS58-b-POCN8	3.4 × 10^4	1.24	57.8
PDMS58-b-POCN15	4.6 × 10^4	1.33	42.3
PDMS58-b-POCN24	5.4 × 10^4	1.28	31.4
PDMS58-b-POCN36	5.8 × 10^4	1.20	23.3

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Fig. 4 shows the DSC curves of PDMS_n-b-POCN_m on the second heating and second cooling scans. The block copolymers show the glass transition temperature (T_g) around 60 °C and an endothermic transition around 160 °C on the heating scan. As PDMS has an extremely low T_g (−123 °C),^26,28 the above transitions are all related with POCN blocks. By POM observation and comparing with the thermal transitions of POCN (Table 2), it can be confirmed that the endothermic transitions correspond the transitions from LC to the isotropic state for the POCN blocks. The exothermic transitions on cooling scan, corresponding to the isotropic to LC transitions, shift to lower temperature side due to the supercooling effect. The T_g and LC-isotropic transition temperature (T_LC-I) were obtained from the DSC heating and cooling curves. The values of T_g and T_LC-I of the copolymers and POCN are summarized in Table 2. For the block copolymers, the transition temperatures decline as the POCN block length decreases. T_LC-I reduces to 149 °C (heating scan) for PDMS₅₈-b-POCN₈ as the lowest in the series. On the other hand, the transition temperatures of PDMS₅₈-b-POCN₃₆ are similar to those of POCN. The similarity between transition temperatures of the copolymers and POCN reveal that the LC block form a separated sub-phase in the systems.^14–18

Fig. 4 DSC curves of the diblock copolymers PDMS_n-b-POCN_m: (A) heating scanning, (B) cooling scanning. (1) PDMS₅₈-b-POCN₈, (2) PDMS₅₈-b-POCN₁₅, (3) PDMS₅₈-b-POCN₂₄, (4) PDMS₅₈-b-POCN₃₆.

Table 2 Thermal transition temperatures of the polymers

Sample	Heating		Cooling
	Tg/°C	TLC-I/°C	Tg/°C	TLC-I/°C
POCN	60.3	157.2	56.2	153.6
PDMS58-b-POCN8	55.1	149.2	51.3	145.6
PDMS58-b-POCN15	56.8	153.9	53.6	151.9
PDMS58-b-POCN24	58.9	155.7	55.1	154.2
PDMS58-b-POCN36	61.6	156.6	57.1	155.4

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3.2 Mesophase structure

POM and XRD were used to identify the mesophase structure of the liquid crystal components. For POCN, POM observation indicates that the isotropic-LC transition occurs at around 164 °C. When cooling from the isotropic phase, batonnet texture first appears at that temperature and gradually develops into a typical fan-shaped texture as the temperature decreases (Fig. 5). The texture suggests a smectic phase formed in the temperature range. The XRD pattern of the polymers shows a diffraction peaks in the small angle region (Fig. 6), which corresponds to periodic spacing of 5.04 nm. In the wide angle region, the broad scattering peak evidences the disordered arrangement of the mesogens in the smectic layers. Considering the periodic space, POCN should form the smectic-A (SmA) bilayer structure.¹⁸ The diffraction patterns of the diblock copolymers are almost the same as that of the homopolymer POCN. It indicates that the mesomorphic structures of LC sub-phase of the diblock copolymers are identical to that of the homopolymer POCN.

Fig. 5 Polarizing optical micrographs of POCN cooling from 165 °C (isotropic phase) to (A) 164 °C, (B) 163 °C, (C) 160 °C, and (D) 100 °C.

Fig. 6 X-ray diffraction curves of the polymers at room temperature. (1) POCN, (2) PDMS₅₈-b-POCN₈, (3) PDMS₅₈-b-POCN₁₅, (4) PDMS₅₈-b-POCN₂₄, (5) PDMS₅₈-b-POCN₃₆.

3.3 Microphase-separated morphology

To investigate the morphological structures, the azo diblock copolymer films were annealed at 140 °C (below T_LC-I) under vacuum for 24 h and observed by TEM. Ultrathin films about 70 nm were obtained from the bulk samples by ultramicrotome cutting. Due to the electron density difference between the two polymer blocks, no staining treatment was needed for TEM observation. Fig. 7 gives typical TEM images of microphase-separated morphologies of the diblock copolymers after thermal annealing. In TEM images, the dark regions represent the PDMS sub-phase and the gray regions are from the POCN sub-phase. For PDMS₅₈-b-POCN₈ and PDMS₅₈-b-POCN₁₅, the lamellar phase morphology is clearly visible. Thickness of POCN lamellae of PDMS₅₈-b-POCN₈ is about 4.0 nm and the thickness of PDMS lamellae is about 10.8 nm. For PDMS₅₈-b-POCN₁₅, the thickness of POCN and PDMS lamellae are 11.0 nm and 8.0 nm respectively. In the cases of PDMS₅₈-b-POCN₂₄ and PDMS₅₈-b-POCN₃₆, spherical phase with average diameters of 17.0 nm and 15.8 nm is found for the PDMS sub-phase dispersed in continuous POCN matrix. More TEM images and small-angel X-ray scattering (SAXS) profiles of the copolymers are given in ESI (Fig. S8–S10†). Above results indicate that along with the decrease of PDMS block content from 42.3 wt% to 31.4 wt%, the microphase-separated morphology exhibits a transition from lamellar phase to spherical phase.

Fig. 7 Typical TEM images of the copolymers: (A) PDMS₅₈-b-POCN₈, (B) PDMS₅₈-b-POCN₁₅, (C) PDMS₅₈-b-POCN₂₄, (D) PDMS₅₈-b-POCN₃₆.

For typical coil–coil block copolymers, the phase-separated structure formation is determined by the competition between the minimization of the interfacial energy and the maximization of the coil entropy. As LC block usually needs higher surface area per chain than the isotropic block, a highly curved interface is favorable when the joints of the blocks are in a narrow interface.¹⁴ Therefore, the stabilization of isotropic spheres or cylinders in the LC matrix could be expected. On the other hand, for block copolymers composed of LC blocks and isotropic blocks, the anisotropic LC packing has a significant influence on the phase separation of the block copolymers, which can lead to some unique characteristics. The smectic LC sub-phases will favor the formation of lamellar morphology as a result of compatible symmetry.^17,18 The distorted director field of the mesogens will lead to an increase of the elastic energy.¹⁴ Above understanding can rationalize the observed morphologies, where the distinct morphologies are the spherical and lamellar phases. When the POCN block is long, the interface curvature should be a dominant factor to result in the spherical morphology. This can be seen for the case for PDMS₅₈-b-POCN₂₄ and PDMS₅₈-b-POCN₃₆. On the other hand, when the POCN block becomes shorter, the interface crowd effect is reduced and the elastic energy effect of LC sub-phase could play an important role to stabilize the lamellar morphology. This is the case for PDMS₅₈-b-POCN₈ and PDMS₅₈-b-POCN₁₅ to form a lamellar morphology. Generally, the smectic layers could lie perpendicular or parallel to the microphase separation interface.^16–18 Considering the narrow thickness of the POCN sub-phase observed here, a perpendicular orientation of the smectic layers to the lamellae should be a reasonable inference.

3.4 Photoisomerizaiton in solutions

Photoinduced isomerization is one of the most fundamental and important functions of azo polymers. According to the spectral feature and isomerization behavior, azobenzene derivatives can be classified into azobenzene type, aminoazobenzene type, and pseudo-stilbene type.⁵³ For the azobenzene type molecules, the absorption spectra show a low-intensity n → π* transition band in the visible region and a high-intensity π → π* band in the UV region. The cis–trans back relaxation is relatively slow for this type of molecules and can be easily monitored by UV-vis spectroscopy. Besides photo-switching functions, this type of azo moieties in polymers can be used as a molecular probe to investigate the local packing and free volume of the systems.³³

To characterize the photoisomerization in solutions, the polymers were dissolved in chloroform with the concentration of 0.02 mg mL⁻¹. The UV-vis spectra were recorded after irradiation with UV light (365 nm) for different time periods. Fig. 8A shows the UV-vis spectral variation of PDMS₅₈-b-POCN₈ solution given here as a typical example. UV-vis spectrum of this polymer, as well as other azo polymers studied here, shows two absorption bands for the trans-azobenzene moieties. The absorption band around 363 nm is attributed to the π → π* transition and the absorption band around 450 nm is due to the n → π* transition. Upon irradiation with UV light at 365 nm, the π → π* absorption band (λ_max = 363 nm) gradually decreases and the n → π* transition (λ_max = 460 nm) gradually increases for all these diblock copolymers. The spectral variation evidences the trans–cis isomerization of the azobenzene moieties. After irradiation with the UV light for 150 s, the spectral change is saturated as the photostationary state is reached. The azobenzene moieties in POCN blocks show features similar to the azobenzene type molecules as revealed by the UV-vis spectroscopy. Fig. 8B shows the relative absorbance at λ_max for the solutions of the azo polymers measured after irradiation with UV light at 365 nm for different time periods, which reflect the photoinduced trans–cis isomerization kinetics. In the figure, A₀ means the maximum absorbance before the light irradiation and A_t is the maximum absorbance after the irradiation for t seconds. The results given in Fig. 8B can be best fitted by the following first-order exponential decay function,

A_t/A₀ = A + B exp(−t/T) (2)

where A and B are two constants, T is the characteristic time, t is the irradiation time. The corresponding fitted curves for the solutions of azo block copolymers and POCN are also given in the figure. The parameters obtained from the curve-fitting are given in the ESI (Table S1†). The isomerization degree, defined as (A₀ − A_t)/A₀ 100%, can reach over 80% for POCN and copolymers. The spectral variations of the cis–trans back isomerization for the azo block copolymers in solutions are given in ESI (Fig. S11†).

Fig. 8 (A) The UV-vis spectra for the solution of PDMS₅₈-b-POCN₈ measured after the irradiation with UV light at 365 nm for different time periods; (B) The relative absorbance at λ_max for the solutions of the azo polymers measured after irradiation with UV light at 365 nm for different time periods and the corresponding fitted curves.

3.5 Photoisomerization of solid films

To understand the influences of the nano-scaled structures on the photoresponsive properties, the photoisomerization of azo copolymers was studied by using two types of the solid films. The polymer films were prepared by spin-coating chloroform solutions of the polymers (2.5 wt%) on quartz slides. The as-cast films were the films obtained after properly drying but without annealing. The annealed films with the fully developed smectic phase structures and microphase-separated morphologies were obtained by annealing at 140 °C for 24 h.

For the as-cast films of the azo polymers, λ_max of the π → π* absorption bands exhibit a blue-shift about 6 nm compared with the corresponding ones of the solutions. It can be attributed to the H-aggregation (face-to-face stacking) of the azobenzene moieties. Fig. 9A shows the typical spectral variation of the as-cast film of PDMS₅₈-b-POCN₁₅. The solid film exhibits the trans–cis isomerization pattern similar to that of the PDMS₅₈-b-POCN₁₅ solution. It shows a successive decrease of the π → π* absorption band (λ_max = 357 nm) and gradual increase of the n → π* absorbance (λ_max = 460 nm) after the irradiation with 365 nm light. On the other hand, a significant difference can also be observed from the comparison. The photoisomerization is obviously blocked and obstructed for the solid film compared with the solution. Even after irradiation for 300 s, the isomerization degree is still obviously lower compared with that of the solution, which is 30% versus 80%. Similar results were also observed for the other azo block copolymers.

Fig. 9 The UV-vis spectral variations of the PDMS₅₈-b-POCN₁₅ films under the irradiation with UV light at 365 nm: (A) as-cast film, (B) annealed film. The inset is the relative absorbance at λ_max measured at different time intervals and the corresponding fitted curves.

For the homopolymer POCN film, the annealing shows a significant effect on the photoisomerization behaviour. The maximum absorbance of the annealed film is further blue-shifted compared with that of the as-cast film and the absorbance maximum decreases more than 60% after annealing. These spectral variations indicate the strong H-aggregation of the azobenene moieties. The absorption intensity decrease can also be attributed to the alignment of the azobenzene moieties perpendicular to the substrate surface due to the compatible stack of smectic layers on the substrate. Upon the irradiation with the 365 nm light, a complicated spectral variation can be observed for the annealed POCN film. In the initial stage of the irradiation, the absorbance at 357 nm decreases successively, indicating the trans–cis isomerization. However, with further irradiation, the absorbance significantly increases and finally reaches its photostationary state (Fig. S12 in ESI†). The low isomerization degree of the annealed POCN film is due to insufficient free volume for the strong H-aggregation system and perpendicular alignment of the azo chromophores.²¹ The absorbance increase after irradiation is caused by the disruption of the H-aggregation and reorientation of the azobenzene moieties during the trans–cis isomerization. This unusual spectral variation has been observed for other smectic azo polymers.²¹ The spectral variation corresponding to the thermal cis–trans relaxation of POCN in dark also shows unusual behaviour owing to the aggregation and alignment effect. The UV-vis absorbance significantly increases to reach a highest value first and then gradually decreases with the time increase (Fig. S13 in ESI†).

The annealed azo diblock copolymer films show obviously different spectral feature and isomerization behaviour compared with those of annealed POCN film. Only a slight decrease of π → π* absorbance and no blue-shift are observed for the annealed copolymer films. These differences are related to the microphase-separated morphologies formed by the PDMS and POCN blocks in the annealed films. The morphological structures hinder both the formation of larger LC domain and the chromophore H-aggregation within the LC domains. Fig. 9B shows the UV-vis spectrum of PDMS₅₈-b-POCN₁₅ and its variation after irradiation with the UV light. Upon the irradiation with the 365 nm light, the annealed film shows the photoisomerization pattern similar to its as-cast film. However, the degree of isomerization, characterized by (A₀ − A_t)/A₀ 100%, decreases from 30% to 18% for the annealing film. The annealed films of the other azo diblock copolymers show similar spectral feature and isomerization behaviour as that of PDMS₅₈-b-POCN₁₅. Fig. 10 gives the relative absorbance at λ_max after irradiating the annealed films with 365 nm light for different time periods. The saturated isomerization degrees are 10%, 16%, 18% and 21% for PDMS₅₈-b-POCN₃₆, PDMS₅₈-b-POCN₂₄, PDMS₅₈-b-POCN₁₅ and PDMS₅₈-b-POCN₈, respectively.

Fig. 10 The relative absorbance at λ_max of the annealed films of the diblock copolymers after irradiation with UV light at 365 nm for different time periods and the corresponding fitted curves.

Above results indicate that the films with the lamellar morphology possess the higher isomerization degree. It can be attributed to the influence of the morphological structures on the mesomorphic packing of the POCN block. When the LC sub-phase is not continuous, the mesogen alignment will be significantly influenced by the morphological confinement.³⁰ In the current case, as POCN lamellae are relative thin, the tight packing of mesogens in smectic layers is disrupted, which favours the photoisomerization for PDMS₅₈-b-POCN₁₅ and PDMS₅₈-b-POCN₈. On the other hand, continuous LC sub-phase is formed for PDMS₅₈-b-POCN₃₆ and PDMS₅₈-b-POCN₂₄, which show relatively lower isomerization degrees. Moreover, PDMS₅₈-b-POCN₃₆ and PDMS₅₈-b-POCN₂₄ show much faster thermal cis–trans isomerization rates compared with those of PDMS₅₈-b-POCN₁₅ and PDMS₅₈-b-POCN₈ (Fig. S14 in ESI†). It could be attributed to the local tension caused by the photoisomerization in the less disrupted smectic layers of the POCN continuous sub-phase. The results obtained from the isomerization investigations are consistent with the morphology observations.

4. Conclusions

A series of LC azo diblock copolymers (PDMS_n-b-POCN_m) with the well-defined structures and relatively narrow molecular weight distributions was synthesized by ATRP method. After annealed at 140 °C for 24 h, the diblock copolymers form two types of the microphase-separated morphologies, i.e. PDMS/POCN lamellae, and PDMS spheres embedded in POCN matrix. The LC blocks possess the smectic-A phase in the morphological structures. The photoresponsive properties of the diblock copolymers are closely related with the condensation states. In solutions, the diblock copolymers (PDMS_n-b-POCN_m) exhibit photoisomerization behaviour similar to that of the homopolymer POCN. The as-cast films of the polymers show similar photochemical processes as solutions except that H-aggregation can be observed in certain extent. Photoisomerization of annealed POCN films is influenced by strong H-aggregation and layer stacks. The photoisomerization behaviour of the annealed block copolymer films depend on the microphase-separated morphologies. The photochemical process resembles that of the as-cast films, especially for those with lamellar morphology. The significantly different photoisomerization behaviour of the block copolymers compared to that of azo homopolymers can be attributed to the influence of the morphological structures on the mesogen alignment of the block copolymers.

Acknowledgements

The financial support from the NSFC under Projects 51233002 is gratefully acknowledged.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: More results of the syntheses, characterization, TEM images and the cis–trans isomerization kinetics of the solution and films of the azo polymers can be seen there. See DOI: 10.1039/c4ra09863c

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