Photoresponsive behavior and self-organization of azobenzene-containing block copolymers

Abstract

Photoresponsive behavior and self-organization of poly(tert-butyl acrylate)-block-poly(6-[4-(4-methoxyphenylazo)phenoxy]hexyl methacrylate) (PtBA-PAzoMA) diblock copolymers in solvents and thin films were investigated using UV-vis spectroscopy, transmission electron microscopy, and small angle X-ray scattering. When PtBA-PAzoMA is dissolved in a solvent, the photoresponsive behavior strongly depends on the solvency. In a PtBA-selective solvent, PtBA-PAzoMA formed a micellar structure because of the unfavorable interaction between solvent molecules and PAzoMA blocks. The confinement of PAzoMA in the core of micelles reduced the kinetics of photoisomerization when compared with the PtBA-PAzoMA in a neutral solvent. Additionally, the confinement promoted the organization of azobenzene moieties into the H- and J-type aggregates. By contrast, PtBA-PAzoMA in neutral solvent adopted a fully extended conformation, resulting in a reduced association of azobenzene mesogens. When PtBA-PAzoMA was prepared in a thin-film architecture, the PtBA-PAzoMA phase separated into both lamellar and cylindrical structures, depending on the volume fraction of the azobenzenes. The phase-separated microdomains provided a confined geometry that led to a substantial reduction of both the photoisomerization rate and the cis isomer content in the photostationary state.

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1. Introduction

The molecular design and synthesis of multifunctional polymeric materials have attracted considerable attention due to their unique properties in multidisciplinary fields of nanotechnology. Among these materials, novel liquid-crystalline polymers carrying azobenzene moieties have been the primary focus because of their photoresponsive behaviors including photoisomerization, photocrosslinking, photoalignment, and photoinduced cooperative motions.¹ The most crucial characteristic of azobenzene molecules is their ability to transform from a stable trans state to a metastable cis state after UV irradiation and the reversible process can be triggered by visible light or heat. This photoswitchable property makes azobenzene-containing polymers potentially feasible for a variety of practical applications, such as optical data recording and storage, optoelectronics, holography, and photoinduced patterning.^2–8

A substantial amount of research has demonstrated the photoresponsive characters of azobenzene-containing polymers.^9–13 Among them, block copolymers that contain azobenzenes are practically intriguing because new functions can be generated from the interaction between photoactivity and self-organized nanostructures in block copolymers. In azobenzene-containing block copolymers, the azobenzene molecules with reversible photoisomerization in one block act as both a mesogen and a photoresponsive moiety, whereas the other sequence is composed of traditional coil-like chains. Therefore, the interplay between the microphase separation induced by the incompatibility of two blocks and the liquid-crystalline order formed by azobenzenes exhibits more distinct features than their amorphous counterparts do. Additionally, Tong and coworkers found that the confinement of azobenzene molecules in the microdomains of a block copolymer reduced the ability of azobenzene mesogens to self-organize compared with unconfined azobenzenes in a homopolymer.¹⁴ Furthermore, the transformation from a liquid-crystalline phase to the isotropic state and their recovery during photoisomerization slowed when the azobenzenes were confined. Therefore, a detailed understanding of the photoresponsive behavior of azobenzenes in a block copolymer and their effect on the microphase separation (kinetics and order) of a block copolymer is crucial for tailoring the phototunable properties of these nanomaterials.

Systematic studies have demonstrated the correlation between the cooperative ordering of azobenzene-containing block copolymers on different length scales and morphologies. Unlike coil–coil block copolymers forming a wide range of morphologies, including lamellae, hexagonally perforated layers, gyroids, hexagonally packed cylinders, and spheres, a new morphology of a wormlike mesogenic domain was found when the azobenzene segment was in the minority phase.¹⁵ The formation of this nanostructure was attributed to the difference in the elastic modulus between two constituents. Anthamatten et al. reported disk-like domains arranged hexagonally between the smectic bilayers of liquid crystals.¹⁶ These new structures imply that mesogens can influence the microphase-segregated morphologies through the interplay process between the microphase separation and the regular periodicity of liquid-crystalline ordering. Ikeda et al. controlled the orientation of the nanostructures of azobenzene-containing block copolymer thin films by using an optical approach.¹⁷ When a block copolymer with a cylindrical structure parallel to a substrate was applied to an actinic light, nanocylinders reoriented perpendicular to the polarization direction according to the cooperative motions between the ordered azobenzene liquid crystals and the microphase separation of the parent block copolymer. The microphase-separated morphologies also influence the liquid-crystalline properties of the photoresponsive blocks.^14,18 For example, the nanocylindrical structures in the photoresponsive block copolymer stabilized the smectic mesophase within it more than the lamellar morphology did.¹⁸ These studies have revealed that the incorporation of ordered azobenzene molecules into the microphase-separated domains of a block copolymer can transfer molecular ordering to the supramolecular level.

Regarding the photoisomerization of azobenzene-containing polymers, Sin et al. synthesized the cyano- and butoxy-substituted azobenzenes in a block copolymer.¹⁹ The presence of a bulky butoxy group in side-chain liquid-crystalline molecules slowed the photoisomerization rate because of steric hindrance. When the azobenzene polymer contained long terminal alkyl chains, the photoisomerization rate decreased because of the entanglement of the alkyl chains that restricted the motion of azo groups.²⁰ The pH value also affects the kinetics of photoisomerization. For an amphiphilic diblock copolymer composed of quaternized poly(4-vinyl pyridine) and polymethacrylate bearing azobenzene side groups in an aqueous solution, the high pH value displayed a lower rate of photoisomerization.²¹ Because the size of the micellar aggregates of this block copolymer decreased with the pH value, the different photoisomerization rates were attributed to changes in the packing of azobenzene groups in the core region as the pH level varied.

The azobenzene moieties typically organize themselves in the liquid-crystalline phase in different types of aggregates including side-by-side (H-type), head-to-head (J-type), and non-associated aggregates.¹⁴ The different aggregation state of azobenzenes determines the ability of fluorescence emission from the azobenzene-containing polymer. Bo and Zhao reported that the fluorescence was contributed chiefly by azobenzene groups absorbing at a wavelength larger than 350 nm, at which non-associated and J-type aggregates dominated.²¹ On the other hand, the H-type aggregates of most dyes were nonfluorescent.²² Frenz et al. found that H-type aggregates dominated in the azobenzene-containing homopolymer, whereas the azobenzene-containing random copolymer reflected the presence of the non-associated form of cis azobenzene moieties.²³ For block copolymers, the microstructures generated by the self-assembly of block copolymers hindered the organization of azobenzene molecules, and thus, fewer H-type aggregates were found in the block copolymer than in the homopolymer.²⁴ The H-type aggregates were observed with an increase in the azobenzene fraction, and H-type aggregates accumulated with a change in morphology from miscible to spherical, and finally, to cylindrical. These findings confirm that microphase separation provides confined domains that enable manipulating the aggregation of azobenzenes.

The present study systematically investigated the effects of the molecular properties of block copolymers (e.g., composition) and the solvent selectivity on the photoresponsive behavior and aggregation state of azobenzene-containing block copolymers. Our model system is poly(tert-butyl acrylate)-block-poly(6-[4-(4-methoxyphenylazo)phenoxy]hexyl methacrylate) (PtBA-PAzoMA), which was synthesized employing atomic transfer radical polymerization by using PtBA-Br as the macroinitiator. This study clearly shows the effects of the solvency in creating confined regions that reduce the kinetics of photoisomerization and render the packing of azobenzene mesogens (H- and J-type aggregates). Similar behavior can be achieved by preparing PtBA-PAzoMA in a thin-film architecture. We present a generalized experimental model of the relationship between block copolymer nanostructures and the aggregation state of azobenzene moieties. Because the literature presenting experimental data on both the kinetics of photoisomerization and azobenzene aggregation is scant, this paper will prove to be a valuable reference for manipulating the structure and photoresponsive behavior of azobenzene-containing polymers for potential use in nanotechnology.

2. Experimental section

2.1. Materials

tert-Butyl acrylate (tBA, 99%) was provided by Acros Organics. N,N,N′,N′,N′′-Pentamethyldiethylenetriamine (PMDETA, 98%), p-anisidine (99%), methacryloyl chloride (97%), 6-chlorehexanol (97%), and phenol (99%) were purchased from Alfa Aesar and used as received. Triethylamine (99.9%), ethyl 2-bromopropionate (2-EBP, 99%), and copper bromide (CuBr, 99.999%) were obtained from Aldrich and used without further purification. Methanol (99.9%) and tetrahydrofuran (THF, 99.3%) were purchased from Echo Chemical Co.

2.2. Synthesis of macroinitiator

The macroinitiator of bromide-terminated poly(tert-butyl acrylate) (PtBA-Br) was prepared following the method in the literature, by using tBA as a monomer, CuBr complexed with PMDETA as a catalyst, and 2-EBP as the initiator.²⁵ The mixture was reacted at 60 °C for 24 h, and the obtained macroinitiator was purified by filtering the solution through a column of alumina to remove the complex of CuBr and PMDETA. The PtBA-Br was then precipitated into 500 mL of methanol. After decanting off the solvent, the polymer was redissolved in THF, and the precipitation procedure was repeated three times to remove the unreacted monomer. Table 1 lists detailed information on the different reactions and the molecular weight of the synthesized macroinitiators.

Table 1 The synthesis of PtBA-Br using the CuBr/PMDETA catalyst system under various conditions^a

Entry	[M] : [I] : [C] : [L]	Mn (g mol^−1)	Mw/Mn	Yield
a [M]: monomer; [I]: initiator; [C]: catalyst; [L]: ligand.
1	100 : 1 : 1 : 1	14 400	1.17	58%
2	200 : 1 : 1 : 1	21 500	1.17	66%
3	400 : 1 : 1 : 1	39 400	1.17	67%

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2.3. Synthesis of 6-[4-(4′-methoxyphenylazo)phenoxy]hexyl methacrylate (AzoMA)

The azobenzene-containing monomer 6-[4-(4′-methoxyphenylazo)phenoxy]hexyl methacrylate (AzoMA) was synthesized by following the documented procedure.^26,27 In brief, 4-methoxy-4′-hydroxyazobenzene was synthesized through diazotation of p-anisidine with phenol. The compound was mixed with 6-chlorohexanol in dimethylformamide to obtain 4-methoxy-4′-(6-hydroxyhexyloxy)azobenzene. Further reaction of 4-methoxy-4′-(6-hydroxyhexyloxy)azobenzene with triethylamine and methacryloyl chloride led to the azobenzene-containing monomer AzoMA. The characterizations of the azobenzene-containing monomers are included in the ESI.†

2.4. Preparation of PtBA-PAzoMA block copolymer

The PtBA-PAzoMA block copolymer was obtained through atom transfer radical polymerization by using PtBA-Br as the macroinitiator. PtBA-Br, AzoMA, and CuBr were introduced into a dry round-bottom flask with a stir bar. The flask was degassed and then filled with nitrogen three times. THF was added to the flask to dissolve the compounds, and the mixture was stirred until a homogeneous solution formed. Afterward, PMDETA was added to the mixture to produce the Cu complex. The flask was frozen in liquid nitrogen, and the mixture was placed under a vacuum condition. The flask was then placed in an oil bath at 60 °C for 24 h. The reaction mixture was diluted with THF and passed through an alumina column. The resultant polymer was purified through precipitation in methanol three times, and then dried in a vacuum oven.

2.5. Characterization

The number-averaged molecular weight and polydispersity of the macroinitiators and copolymers were determined using gel permeation chromatography (GPC, Schambeck RI2000). GPC measurements were conducted using THF as the elution solvent at an elution rate of 0.5 mL min⁻¹. The instrument was calibrated with a polystyrene standard. The ¹H NMR spectra were recorded in CDCl₃ on a Bruker AV-500 Spectrometer. Fourier transform infrared spectroscopy (FTIR) was preformed on a Scinco/Nicolet 5700 spectrometer at a resolution of 2 cm⁻¹. Thermal phase transitions of PtBA-PAzoMA were obtained using a Perkin-Elmer DSC-7 differential scanning calorimeter at a heating rate of 5 °C min⁻¹ under a nitrogen atmosphere. UV-vis spectra were obtained using a Jasco V-550 spectrophotometer. The concentration of the polymer solution was 0.05 mg mL⁻¹. For the experiment on the photochemical phase transition, UV irradiation was performed at 365 nm with a light intensity of 2.2 mW cm⁻² and visible light irradiation was conducted at 430 nm with a light intensity of 13.3 mW cm⁻². The structure of PtBA-PAzoMA in a solvent was characterized using transmission electron microscopy (TEM), operated on a Hitachi H7500 electron microscope. Samples for TEM analysis were prepared by leaving a drop of the polymer solution on a copper grid coated with carbon film.

To prepare bulk samples, PtBA-PAzoMA was dissolved in THF. The polymer solution was stored in an ambient condition for 2 days. The polymer was then embedded in epoxy and cast on Kapton. After drying in vacuum, the specimens were annealed at 150 °C for 5 days. For TEM measurements, the PtBA domains were selectively stained using OsO₄. An Ultracut R microtome (Reichert, Leica, MI) was used to section the embedded samples in epoxy to a thickness of approximately 80 nm. The morphology of PtBA-PAzoMA was then examined using TEM. To complement the TEM results, small angle X-ray scattering (SAXS) measurements were performed using a Bruker diffractometer (NanoSTAR U System, Bruker AXS Gmbh, Karlsruhe, Germany). The samples on Kapton were measured at room temperature with a sample-to-detector distance of 2 m. The data were corrected for incident flux, absorption, detector sensitivity variation, and dark current.

3. Results and discussion

3.1. Synthesis and characterization of PtBA-PAzoMA

For this study, we used the PtBA-Br macroinitiator to polymerize the azobenzene monomer. We manipulated PtBA-PAzoMA diblock copolymers with various volume fractions of the azobenzene block via the amount of the added azobenzene monomer for extension polymerization. Fig. 1 shows the representative ¹H-NMR spectrum of PtBA-PAzoMA. The composition of PtBA-PAzoMA was determined from the peak assignment and with the GPC value for the PtBA block, the molecular weight of the azobenzene block was obtained. Table 2 lists in summary form the molecular weight of the series of PtBA-PAzoMA with various compositions.

Fig. 1 ¹H NMR spectrum of the PtBA₃₀₀-PAzoMA₄₀ block copolymer.

Table 2 Characteristics of the synthesized PtBA-PAzoMA

Polymer^d	Mn of PtBA-PAzoMA	Mn of PtBA^a	Mn of PAzoMA^b	fPtBA^c	fPAzoMA^c	Mw/Mn
a The Mn of PtBA was measured by GPC using a polystyrene standard.b The Mn of PAzoMA was calculated from ^1H NMR.c fi: the volume fraction of i block.d PtBAm-PAzoMAn where m and n are the degree of polymerization of PtBA and PAzoMA, respectively. The m and n values are calculated by the total molecular weight of each block divided by the monomer molecular weight.
PtBA110-PAzoMA128	65.0k	14.4k	50.7k	0.22	0.78	1.88
PtBA170-PAzoMA99	60.7k	21.5k	39.2k	0.35	0.65	1.66
PtBA170-PAzoMA49	40.9k	21.5k	19.4k	0.53	0.47	1.65
PtBA300-PAzoMA42	56.0k	39.4k	16.6k	0.70	0.30	1.60
PtBA300-PAzoMA40	55.2k	39.4k	15.8k	0.72	0.28	1.31

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Fig. 2 shows the DSC traces of the PtBA-PAzoMA block copolymers. The PtBA block has a glass transition temperature (T_g) of approximately 40 °C. The PAzoMA is known to have both smectic and nematic phases.^28,29 The formation of the block copolymer does not alter the mesophase transition of PAzoMA. The presence of the peak at a lower temperature in DSC traces indicated the smectic-to-nematic phase transition, and the peak that occurred at a higher temperature presented the nematic-to-isotropic transition. Table 3 lists the phase-transition temperatures of various PtBA-PAzoMA block copolymers observed using DSC. All the PtBA-PAzoMA block copolymers exhibited liquid-crystalline properties. Furthermore, the nematic-to-isotropic transition shifted slightly from 129 to 118 °C when the volume fraction of the azobenzene block decreased. The endothermic transitions of PtBA₃₀₀-PAzoMA₄₀ were less discernible because of its low PAzoMA fraction.

Fig. 2 DSC traces of PtBA-PAzoMA with various compositions.

Table 3 Thermal transitions of PtBA-PAzoMA

Polymer	Phase transition temperature (°C)
PtBA110-PAzoMA128	g41 S89 N129 I
PtBA170-PAzoMA99	g40 S85 N124 I
PtBA170-PAzoMA49	g44 S87 N127 I
PtBA300-PAzoMA42	g42 S89 N125 I
PtBA300-PAzoMA40	g40 S78 N118 I

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Fig. 3 shows the SAXS patterns of PtBA-PAzoMA block copolymers. Distinct peaks in the low q region (q = 4π sin(θ/2)/λ, where θ is the scattering angle, and λ is the incident radiation wavelength) correspond to the ordered phase of copolymers. Regarding PtBA₁₇₀-PAzoMA₉₉ as an example, the ratio of the higher-order peaks to the first-order peak (q/q*, where q* is the scattering vector of the first-order peak) is 1 : 2 : 3 : 4, suggesting the formation of a lamellar structure. The absence of even-numbered peaks for the SAXS pattern of PtBA₁₇₀-PAzoMA₄₉ was presumably due to its convolution with a minimum in the form factor of the lamellae. The morphology of PtBA-PAzoMA was identical for f_PAzoMA (the volume fraction of PAzoMA in the block copolymer) between 0.30 and 0.78. Regarding PtBA₃₀₀-PAzoMA₄₀ for which f_PAzoMA was 0.28, the SAXS pattern exhibited peaks with q/q* of 1 : √4 : √7, indicating the presence of hexagonally packed cylinders. To complement these results, we used TEM to characterize the structural features of these block copolymers. In Fig. 4, the dark areas of the micrographs are selectively stained PtBA phases, and the white areas are segregated PAzoMA sequences. Qualitatively, the TEM images for PtBA-PAzoMA ideally match the structure obtained using SAXS. When f_PAzoMA was 0.78, PAzoMA exhibited thick layers, whereas PtBA presented only thin layers (Fig. 4(a)). The layer thickness of PtBA gradually increased with a decrease of f_PAzoMA. The decreasing f_PAzoMA again trigger changes to the polymer morphology. As shown in Fig. 4(e), PtBA-PAzoMA with an f_PAzoMA of 0.28 exhibited closely packed cylinders, with PAzoMA as the minority phase forming dispersed cylinders, and PtBA as the majority phase forming the continuous phase.

Fig. 3 SAXS profiles of PtBA-PAzoMA. (Scattering patterns have been shifted to increase clarity, and the arrows and numbers in the plots show the ratio of the peak positions to the value of the first-order peak.)

Fig. 4 TEM images of (a) PtBA₁₁₀-PAzoMA₁₂₈; (b) PtBA₁₇₀-PAzoMA₉₉; (c) PtBA₁₇₀-PAzoMA₄₉; (d) PtBA₃₀₀-PAzoMA₄₂; and (e) PtBA₃₀₀-PAzoMA₄₀.

It is widely understood that the phase diagram of a coil–coil diblock copolymer in a segregation limit exhibits favorable symmetry on the phase behavior with lamellae for 0.35 < f (a volume fraction of one component) < 0.65 and hexagonally packed cylinders for f > 0.65 and f < 0.35.³⁰ In our case, the region of the lamellar phase was enhanced and the order–order transition occurred when f_PAzoMA was smaller than 0.30. Studies have reported similar results on shifting in the composition range in liquid crystal-coil diblock copolymers with azobenzene side groups.^16,18,31 For instance, in one study, the liquid crystal-coil diblock copolymer exhibited a cylindrical structure at f_LC (a volume fraction of liquid-crystalline phase) = 83.7% and 23.8%, and a bicontinuous structure at f_LC = 30.7%.¹⁸ The origin of these changes to the phase stability is attributed to the molecular structure of azobenzenes. The bulky mesogenic unit of azobenzenes in the side chains impedes conformational flexibility, leading to the stretched chain extension. This prevents a change to the interfacial curvature. Thus, the planar interface between two microdomains is energetically favored and the phase diagram of these rod–coil diblock copolymers exhibits rich lamellar regimes.

3.2. Photoinduced isomerization of PtBA-PAzoMA in a solvent

The most intriguing characteristic of PtBA-PAzoMA is its photoinduced response after UV irradiation. Fig. 5 shows the variations in the UV-vis spectra of PtBA-PAzoMA in THF during UV irradiation and its recovery in visible light (also see Fig. S3 in ESI†). Without UV irradiation, a highly structured spectrum is visible with a peak at approximately 358 nm, which is associated with the π–π* absorption of trans-azobenzene. After UV irradiation, a considerable feature of the absorption spectrum is the progressive increase in peak intensity at approximately 450 nm in addition to the extensive reduction of the trans π–π* transition absorption (Fig. 5(a)). The evolving peak at 450 nm is assigned as the cis n–π* transition absorption. The presence of isosbestic points at approximately 318 and 425 nm indicates that the two distinct absorbing species are in equilibrium with each other and no side reaction occurs during photoisomerization.²⁰ The disappearance of the absorption of trans-azobenzenes for a long UV exposure time is due to the nearly complete trans-to-cis photoisomerization. The percentage of cis isomers calculated using eqn (1),²⁹ where A_o and A_s are the absorbance at 358 nm before and after UV irradiation, respectively, yields over 85% cis-azobenzenes for all PtBA-PAzoMA in THF after 40 s.

Fig. 5 The variations in the UV-vis spectra of PtBA₃₀₀-PAzoMA₄₂ in (a and b) THF; (c and d) ethyl acetate; (e and f) ethanol; and (g and h) n-butanol. (a, c, e and g) Irradiated with UV light; and (b, d, f and h) recovered with visible light for different times. The concentration of polymer solution was 0.05 mg mL⁻¹.

To gain more insight into the kinetics of photoisomerization as a function of the molecular properties of PtBA-PAzoMA, we fitted the UV-vis data with the first-order kinetic equation^19,21

ln(A) = −kt (2)

where A = (A_∞ − A_t)/(A_∞ − A_o); A_∞ and A_t are the absorbance at the photostationary state and after UV irradiation for time t, respectively; and k is the rate constant. Fig. 6 shows a comparison of the plots of ln(A) versus the UV irradiation time for different PtBA-PAzoMA block copolymers in THF. The rate constant obtained based on the slope of the linear fit is listed in summary form in Table 4. The results showed that the rate of photoisomerization increased as the molecular weight of the azobenzenes decreased. As indicated by Rameshbabu and Kannan,³² the switching time for photoisomerization was a strong function of the terminal pendant substituents of the azobenzene moieties. This was associated with the size of the terminal substituents and the effect of the withdrawing/donating nature of the resonance. In our work, the synthesized PtBA-PAzoMA block copolymers were identical, and they were dissolved in the same solvent. The discrepancy between the kinetics of photoisomerization was not caused by the structural differences of the polymers in a solvent. Instead, the small molecular weight of PAzoMA facilitated the kinetics of photoisomerization because of its fast relaxation.

Fig. 6 Plots of the first-order trans-to-cis photoisomerization after UV irradiation for various PtBA-PAzoMA in THF. □: PtBA₁₁₀-PAzoMA₁₂₈; ■: PtBA₁₇₀-PAzoMA₉₉; ▲: PtBA₁₇₀-PAzoMA₄₉; ●: PtBA₃₀₀-PAzoMA₄₂; and ◆: PtBA₃₀₀-PAzoMA₄₀.

Table 4 The wavelength of the maximum absorption peak and the rate constant of various PtBA-PAzoMA block copolymers in THF obtained by UV-vis spectroscopy

Polymer	Maximum peak position (nm)	Rate constant (1/s)
PtBA110-PAzoMA128	358	0.0956
PtBA170-PAzoMA99	358	0.1337
PtBA170-PAzoMA49	358	0.2087
PtBA300-PAzoMA42	358	0.2815
PtBA300-PAzoMA40	358	0.3773

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Conversely, the cis-to-trans isomerization could be activated through visible light irradiation, and the unstable cis isomers gradually converted to the stable trans isomers, which was demonstrated by the evolution of the intensity of the π–π* transition of azobenzene moieties, as shown in Fig. 5(b). Eventually, the spectrum was nearly identical to that before UV irradiation. We also found the cis-to-trans photoisomerization rate to be dependent on the molecular weight of azobenzene moieties, again with PtBA₁₁₀-PAzoMA₁₂₈ being the slowest, and PtBA₃₀₀-PAzoMA₄₀ being the fastest. The photoisomerization was complete after approximately 90 and 60 s for PtBA₁₁₀-PAzoMA₁₂₈ and PtBA₃₀₀-PAzoMA₄₀, respectively. One study reported that the recovery of cis to trans isomers could also be triggered in the dark, but that it requires hours to days to complete.¹⁹

Table 4 lists the wavelength of the maximum absorption peak. A study proposed that the aggregation state of azobenzene moieties determines the wavelength of the maximum absorption peak, and that the maximum absorption occurs at 384 nm, 360 nm, and 334 nm for J-type, non-associated, and H-type azobenzene molecules, respectively.³³ According to the wavelengths, we deconvoluted the UV-vis spectra of PtBA-PAzoMA in THF, and we used the area of the deconvoluted peaks to calculate the fraction of the different aggregation states of azobenzenes. The results are shown in Fig. 7. THF is a good solvent for both PtBA and PAzoMA blocks, and yields the random-coiled conformation of PtBA-PAzoMA in THF. Because the molecules are fully extended and well dispersed in THF, the self-organization of mesogens was prohibited, and the non-associated azobenzenes dominated, regardless of the volume fraction of PAzoMA in the block copolymer.

Fig. 7 The population of different organizations of azobenzenes in THF as a function of f_PAzoMA (●: H-aggregated; ○: non-associated; ▼: J-aggregated).

We also investigated the photoisomerization and the packing of azobenzenes as a function of the solvency by dissolving PtBA-PAzoMA in various solvents. In Fig. 5, ethyl acetate is also a good solvent, but ethanol and n-butanol are selective solvents for PtBA-PAzoMA. When PtBA-PAzoMA in these solvents was exposed to UV irradiation, the photoinduced trans-to-cis isomerization of azobenzene mesogens occurred in all of these solvents, regardless of the solvency. Table 5 lists the rate constants of PtBA₃₀₀-PAzoMA₄₂ in various solvents after UV irradiation, which were obtained by assuming the first-order kinetics. We observe a reduction in the photoisomerization rate when PtBA₃₀₀-PAzoMA₄₂ was dissolved in a selective solvent. Because both ethanol and n-butanol are good solvents for PtBA, but poor solvents for PAzoMA, the increased enthalpic interaction between the solvent molecules and PAzoMA forced PAzoMA blocks to aggregate, forming a micellar structure with PtBA in the shell and PAzoMA in the core. The reduced photoisomerization rate of PtBA-PAzoMA in a selective solvent indicated that the confinement of azobenzene molecules inside the core of the micelles hindered the trans-to-cis photoisomerization. This result is consistent with the findings reported by Bo and Zhao, in which the photoisomerization rate of the azobenzene-containing block copolymer in a mixed neutral/selective solvent decreased considerably compared with that in a neutral solvent.²¹

Table 5 The wavelength of the maximum absorption peak and the rate constant of PtBA₃₀₀-PAzoMA₄₂ in various solvents obtained by UV-vis spectroscopy

Solvent	Maximum peak position (nm)	Rate constant (1/s)
THF	358	0.2768
Ethyl acetate	356	0.1395
Ethanol	342	0.0957
n-Butanol	340	0.0690

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In addition to the variation in the kinetics of photoisomerization, Table 5 also shows the shifted wavelength of the maximum absorption from 358 and 356 nm for THF and ethyl acetate, respectively, to 343 and 340 nm for ethanol and n-butanol, respectively. The variation in the peak position suggested the varied organization of azobenzene molecules. Similarly, the UV-vis spectra of PtBA-PAzoMA in different solvents were deconvoluted to determine the aggregation state of azobenzenes, and the results are shown in Fig. 8. When PtBA-PAzoMA was dissolved in a good solvent (THF and ethyl acetate), polymer chains formed a relaxed conformation. The formation of the aggregates was severely hindered, and the absorption maximum at approximately 358 nm was mostly from non-associated azobenzene groups. On the contrary, when PtBA-PAzoMA was dissolved in a selective solvent, micelles formed. The micellar structure provided confined domains, which rendered the packing of azobenzene molecules. Therefore, the organization of azobenzenes was in favor of H- and J-aggregations. Similar behavior was obtained for the other copolymers, such as PtBA₃₀₀-PAzoMA₄₀ shown in Fig. S4.†

Fig. 8 The population of different organizations of azobenzenes in PtBA₃₀₀-PAzoMA₄₂ as a function of the solvency.

To further understand the effect of solvency on the aggregation state of azobenzenes, we dissolved PtBA-PAzoMA in a mixed THF–ethanol solvent, and manipulated the solvency according to the relative amount of THF in the mixed solvent. Fig. 9(a) shows the UV-vis spectra of PtBA₃₀₀-PAzoMA₄₀ in mixed solvents with different solvent contents, and the corresponding aggregation state of azobenzenes in these solvents are shown in Fig. 9(b). In THF, both blocks demonstrated good compatibility with solvent molecules, and the azobenzenes were not associated. With the addition of ethanol, micellization occurred because ethanol is a nonsolvent for the PAzoMA block. This resulted in a gradual blue-shift of the absorption peak from a wavelength of 358 nm in THF to that of 342 nm in ethanol. This was associated to the changes in the organization of azobenzene molecules from non-association to H-aggregation. The TEM image in Fig. 10(a) shows that PtBA₃₀₀-PAzoMA₄₀ formed spherical micelles in 75/25 THF–ethanol. Because PAzoMA was confined to the micellar core, the interparticle distance between the azobenzene molecules was small, which facilitated the organization of azobenzenes. Therefore, azobenzenes became packed in H- and J-type forms. The solvent selectivity increased with the ethanol content, and the spherical micelles were maintained (Fig. 10(b) and (c)). The PtBA₃₀₀-PAzoMA₄₀ transformed to rod-like micelles when dissolved in ethanol (Fig. 10(d)), and both H- and J-aggregation dominated in the organization of azobenzenes.

Fig. 9 (a) UV-vis spectra of PtBA₃₀₀-PAzoMA₄₀ in a mixed THF–ethanol solvent with different solvent contents; and (b) the population of different organizations of azobenzenes in these solvents. The concentration of polymer solution was 1 mg mL⁻¹.

Fig. 10 TEM images of PtBA₃₀₀-PAzoMA₄₀ in a mixed THF–ethanol solvent with different solvent contents. (a) 75/25 THF–ethanol; (b) 50/50 THF–ethanol; (c) 25/75 THF–ethanol; and (d) ethanol.

3.3. Photoinduced isomerization of PtBA-PAzoMA in thin film

We also investigated the photoisomerization of PtBA-PAzoMA in a thin-film architecture. Fig. 11 shows the UV-vis spectra of spin-coated PtBA-PAzoMA thin films. A transformation from trans to cis isomerization similarly occurred when PtBA-PAzoMA was irradiated with UV light, and the reverse reaction was initiated when PtBA-PAzoMA was exposed to visible light. The PtBA-PAzoMA in a solid state showed a lower conversion from the trans to the cis isomer, compared with that in a solution state. The percentage of cis isomers could reach only 50–70% after UV irradiation for 120 s, which was substantially smaller than that in the solution state. Prolonged UV exposure time did not further increase the conversion, indicating complete photoisomerization. The reduced conversion was attributed to the existence of the steric hindrance of molecules in the thin film.³⁴ Table 6 lists the rate constant of photoisomerization extracted from the plot of ln(A) versus UV exposure time. Similar to the PtBA-PAzoMA in solution, the photoisomerization rate of PtBA-PAzoMA in the solid state showed molecular weight dependence, with PtBA₁₁₀-PAzoMA₁₂₈ being the slowest, and PtBA₃₀₀-PAzoMA₄₀ being the fastest. The kinetics of photoisomerization slowed in the thin film because of both the reduced dynamics of polymers in the solid state and the geometrical restriction of azobenzene mesogens in the phase-separated microdomains. The reduced photoisomerization rate in the thin film is in agreement with the findings reported by Yu et al.³⁴

Fig. 11 The variations in the UV-vis spectra of PtBA₃₀₀-PAzoMA₄₂ in thin films. (a) Irradiated with UV light; and (b) recovered with visible light for different times.

Table 6 The wavelength of the maximum absorption peak and the rate constant of various PtBA-PAzoMA block copolymers in thin films obtained by UV-vis spectroscopy

Polymer	Maximum peak position (nm)	Rate constant (1/s)
PtBA110-PAzoMA128	346	0.0247
PtBA170-PAzoMA99	344	0.0394
PtBA170-PAzoMA49	346	0.0451
PtBA300-PAzoMA42	346	0.0477
PtBA300-PAzoMA40	344	0.0479

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In addition to the slow photoisomerization rate of PtBA-PAzoMA in the solid state, the absorption peak blue-shifted slightly to 346 nm compared with that of the PtBA-PAzoMA in THF. We observed similar behavior when PtBA-PAzoMA block copolymers with different f_PAzoMA were confined in a thin-film architecture, as listed in Table 6. The blue shift of the absorption peak indicated increased numbers of aggregated azobenzenes. The population of the aggregation state of azobenzenes clearly showed the dominant H- and J-type aggregations, with few non-associated azobenzene molecules (Fig. 12). Unlike the diluted polymer solution in which molecules are well separated, the distance between neighboring azobenzene molecules in the thin film decreased considerably. With intermolecular interaction, azobenzenes tend to form aggregates, gathering into a compact structure. Consequently, H- and J-aggregates existed mainly in the thin film.

Fig. 12 The population of different organizations of azobenzenes in thin films as a function of f_PAzoMA (●: H-aggregated; ○: non-associated; ▼: J-aggregated).

Based on these observations, the molecular organization of azobenzene moieties regarding the environment can be described using the model shown in Fig. 13. When azobenzene molecules are dissolved in a neutral solvent, solvent molecules swell two blocks, and polymer chains form a random-coiled conformation. The azobenzene mesogens are less associated, and the aggregation of azobenzenes is minimized. When the azobenzene-containing block copolymer is in a selective solvent or a solid state, the azobenzene moieties are confined in restricted microdomains. In these two cases, the azobenzene groups in proximity may associate with each other to form aggregates, resulting in a substantial increase in the population of H- and J-type aggregates.

Fig. 13 Schematics of molecular organization of azobenzene moieties in various environments.

4. Conclusions

We investigated the phase and photoresponsive behaviors of the azobenzene-containing block copolymer PtBA-PAzoMA by using DSC, SAXS, TEM, and UV-vis spectroscopy. The morphology of the block copolymer showed strong dependence on the composition of PtBA-PAzoMA. When the volume fraction of PAzoMA in PtBA-PAzoMA was between 0.3 and 0.78, a lamellar structure was obtained. An order–order transition occurred when the volume fraction of PAzoMA was less than 0.3. The lamellar region in PtBA-PAzoMA was enhanced by the architectural asymmetry in which the bulky mesogenic unit of azobenzenes in the side chains impeded conformational flexibility, leading to a stretched chain extension. This hindered the change in the interfacial curvature, and prolonged phase stability. The photoresponsive behavior of PtBA-PAzoMA depends on the solvency. When PtBA-PAzoMA was dissolved in a neutral solvent, over 85% azobenzenes were transformed to cis isomers within 40 s. By contrast, the kinetics of photoisomerization decreased considerably when the solvent selectivity increased. In a PtBA-selective solvent, the unfavorable interaction between the solvent and PAzoMA promoted the self-assembly of PtBA-PAzoMA molecules, forming a micellar structure with PAzoMA in a core and PtBA as a corona. The micellar structure provides the confined geometries for PAzoMA, in which the motion and rearrangement of azobenzene groups are hindered, thereby restricting their photoisomerization process. The PtBA-PAzoMA in a thin-film state exhibited an even slower photoreaction compared with that in a solvent. The confinement affects not only the kinetics of the photo-processes but also the stacking of azobenzene moieties. In a neutral solvent, PtBA-PAzoMA molecules were well separated, and the azobenzene moieties were less associated. By contrast, a selective solvent and a thin film provided confined regions that facilitated the assembly of the azobenzene groups in proximity, resulting in the self-organization of azobenzenes and the domination of H- and J-type aggregates in the confined geometries.

Acknowledgements

This work was financially supported by National Science Council in Republic of China under Grant no. NSC 101-2628-E-006-001 and NSC 102-2221-E-006-018-MY3.

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Footnote

† Electronic supplementary information (ESI) available: ¹H NMR and infrared spectra of azobenzene-containing monomers, UV-vis spectra of the PtBA-PAzoMA block copolymers with different compositions, and the aggregation state of PtBA₃₀₀-PAzoMA₄₀ in different solvents. See DOI: 10.1039/c4ra03724c

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