Light-Triggered Reversible Slimming of Azobenzene-Containing

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Letter Cite This: ACS Macro Lett. 2019, 8, 460−465

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Light-Triggered Reversible Slimming of Azobenzene-Containing Wormlike Nanoparticles Synthesized by Polymerization-Induced Self-Assembly for Nanofiltration Switches Song Guan,† Zichao Deng,† Tianyu Huang,† Wei Wen,† Yongbin Zhao,‡ and Aihua Chen*,†,§

ACS Macro Lett. Downloaded from pubs.acs.org by UNIV AUTONOMA DE COAHUILA on 04/03/19. For personal use only.

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School of Materials Science and Engineering and §Beijing Advanced Innovation Centre for Biomedical Engineering, Beihang University, Beijing 100191, People’s Republic of China ‡ Shandong Oubo New Material Co. Ltd., Shandong 257088, People’s Republic of China S Supporting Information *

ABSTRACT: Photoresponsive wormlike block copolymer nanoparticles (NPs) have potential applications in versatile fields, but their preparation suffers from narrow worm phase region and tedious approaches. In this work, azobenzene-containing wormlike NPs based on poly(methylacrylic acid)-bpoly(4-((4-butylphenyl)diazenyl)phenyl methacrylate) are prepared via polymerization-induced self-assembly at high solids concentration in ethanol. The pure wormlike NPs occupy a remarkably broad region in the morphological phase diagram because of the rigid nature of the core-forming block. These wormlike NPs expand resulting from trans−cis transformation upon UV irradiation, and slim near to the original state via visible light irradiation. The diameter and its variation amplitude of worms increase with the chain length of core-forming block. Moreover, a nanofiltration switch for rhodamine B is assembled to illustrate one of its potential applications by remote trigger using light.

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deformation for one-dimensional (1D) NPs has been rarely reported to date. Polymerization-induced self-assembly (PISA) is a highly efficient and powerful technique to prepare polymeric NPs at high solid concentration with reliable morphology controlling.20 Taking advantage of PISA, wormlike NPs in large scale have been prepared by Armes,21 An,22 Pan,23 Cai,24 Zhang,25 Boyer,26 Tan,27 and others.28−32 Moreover, particular attention is paid to reversible transition, such as worm-to-sphere. For instance, thermoresponsive behavior has been reported for the nano-objects obtained from poly(glycerol monomethacrylate)b-poly(2-hydroxypropyl methacrylate) PISA formulation, which undergoes reversible worm-to-sphere transition.33 Recently, we reported the preparation of azobenzenecontaining BCP NPs with anisotropic morphologies via polymerization-induced hierarchical self-assembly.34 Cuboids and ellipsoidal vesicles transformed to spherical assemblies upon UV irradiation, but failed to recover when irradiated by visible light subsequently. Yuan et al. have reported that the micelles of poly(N,N-dimethylaminoethyl methacrylate)-bpoly(benzyl methacrylate) decorated by small amount (10 mol %) of 4-phenylazophenyl methacrylate underwent reversible worm-to-vesicle transition upon alternative UV/

timuli-responsive block copolymer (BCP) nanoparticles (NPs), which show sharp and reversible responses to external stimuli, such as temperature,1 pH,2 gas,3 redox,4 enzyme,5 voltage,6 and light,7 have received considerable attention due to their applications in various fields including smart nanoreactors,8 controlled drug delivery,9 and logic gates.10 Among these, light is a rapid and remote stimulus that does not require any change in the chemical environment and can be precisely controlled in specific time and space. Since the first report in 2004, BCP micelles responsive to light have been broadly investigated due to the potential application for controlled drug delivery.11 Reversible photochromic moieties, such as azobenzene,12−15 spiropyran,16 dithienylethene,17 and diazonaphthoquinone,18 have been frequently incorporated into the BCP structures as side groups of the hydrophobic block. Based on this, a series of BCP spherical aggregates with reversible dissociation-to-association triggered via alternative UV/visible light irradiation have been reported.11 Very recently, we have reported that snowmanlike amphiphilic Janus NPs with azobenzene-containing poly(methacrylate) (PMAAz) as the hydrophobic head via the approach of emulsion solvent evaporation.19 PMAAz heads expand to envelop the hydrophilic heads upon UV irradiation and then recover completely with liquid-crystal (LC) ordering interior by visible light irradiation. However, the reported fabrication process is mostly based on conventional BCP selfassembly in dilute solution, suffering from low-throughput and tedious approaches. Moreover, light-triggered reversible © XXXX American Chemical Society

Received: February 26, 2019 Accepted: April 1, 2019

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DOI: 10.1021/acsmacrolett.9b00146 ACS Macro Lett. 2019, 8, 460−465

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ACS Macro Letters

Scheme 1. (a) Synthetic Method to Azobenzene-Containing Wormlike NPs via RAFT Dispersion Polymerization in Ethanol at 70 °C; (b) Light-Triggered Reversible Slimming of One Example Wormlike NP

Figure 1. (a−h) Morphologies of PMAA90-b-PMA(0C)Azn NPs prepared via RAFT dispersion polymerization in ethanol at 70 °C. “d” denotes the diameter of wormlike NPs. (i) Phase diagram of the self-assembled morphology with different DP of PMA(0C)Az block and solids concentration.

visible light irradiation.35 However, the homogeneity of the morphologies is desired to enhance for the transition research.35,36 Herein, we propose the formation of wormlike NPs of poly(methylacrylic acid)-b-poly(4-((4-butylphenyl)diazenyl)phenyl methacrylate) (denoted as PMAA-b-PMA(0C)Az) via PISA (Scheme 1) in ethanol. The aspect ratio of wormlike NPs can be tuned via controlling the feed ratio of MA(0C)Az/ PMAA macromolecular chain transfer agent (macro-CTA) and solids concentration. A morphological phase diagram was constructed and the pure wormlike NPs occupy a substantial broad region. Because of the trans−cis photoisomerization of azobenzene moieties, the wormlike NPs expand upon UV light irradiation, and then slim near to the original state when illuminated by visible light. To the best of our knowledge, it is the first report about the reversible slimming behavior of wormlike BCP NPs. Furthermore, a light-trigger nanofiltration switch for rhodamine B is assembled to illustrate one of its potential applications. Scheme 1a outlines the synthetic method of PMAA90-bPMA(0C)Azn NPs by PISA. PMAA90 macro-CTA was synthesized via RAFT solution polymerization. Subsquently, chain extension of the soluble PMAA90 macro-CTAs was carried out by RAFT dispersion polymerization of MA(0C)Az

monomers in ethanol initiated by 4,4′-azobis(4-cyanovaleric acid) (ACVA) at 70 °C. The DP of PMAA macro-CTA and PMAA-b-PMA(0C)Az were measured from 1H NMR spectra. Typical 1H NMR spectra of MA(0C)Az monomer, PMAA90 macro-CTA, and PMAA90-b-PMA(0C)Az BCP were shown in Figure S1. The number-average molar mass (Mn) and the dispersity (Mw/Mn) of PMAA90 macro-CTA and series of the PMAA90-b-PMA(0C)Azn were measured by gel permeation chromatography (GPC), showing in Figure S2. Low dispersity (Mw/Mn < 1.50) was observed in all samples. The dispersion polymerization kinetics of PMAA90-b-PMA(0C)Az80 (target composition) at 15% w/w solids was investigated in detail (Figures S3 and S4). Monomer conversion of 71% was achieved within 11 h (Figure S4a), lower than that of 11-[4-(4butylphenylazo)phenoxy]undecyl methacrylate (MAAz), 82% conversion within 12 h,34 which can be attributable to higher steric hindrance in the absence of flexible spacer. The semilogarithmic plot shows three stages (Figure S4a), which corresponds to the initial homogeneous polymerization, a subsequent slightly accelerated polymerization after the onset of nucleation, and the final decelerated polymerization resulting from the deactivation of the higher steric hindrance monomers in the late stage of polymerization, respectively. Compared with other common coil−coil PISA formula461

DOI: 10.1021/acsmacrolett.9b00146 ACS Macro Lett. 2019, 8, 460−465

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ACS Macro Letters

Figure 2. UV−vis spectra of PMAA90-b-PMA(0C)Az56 NPs upon (a) UV irradiation, (b) visible light irradiation, and (c) five irradiation cycles. A cycle consists of 1 min of UV irradiation and 6 min of visible light irradiation.

tion,37,38 the difference of polymerization rate in the first two stages is weaker also due to the higher steric hindrance. Figure S4b shows a linear relationship between Mn and monomer conversion, which indicates a well-controlled RAFT polymerization. The morphology and size of BCP nano-objects were characterized by TEM and DLS under varying DP of coreforming block, PMA(0C)Az, and solids concentration. When DP is very low, such as 80 (Figure S6b,g,l), whereas no lamellae or vesicles were observed. A phase diagram was constructed accordingly, as shown in Figure 1i. It is emphasized that the pure worm phase covers a remarkably broad region in the phase diagram, which is not common in other coil−coil BCP PISA formulations. We have previously reported the formation of various nonspherical NPs from azobenzene-containing BCPs via PISA formulation.34 Azobenzene mesogens are incorporated onto poly(methacrylate) main chains via 11 methylenes as flexible spacers (PMAAz). LC smectic ordering has been formed as an internal driving force, assisting to form unique nonspherical NPs. In the current report, the molecular structure of coreforming block PMA(0C)Az is different from that of PMAAz with no flexible spacers between azobenzene unit and the main chain. Therefore, the segments of PMA(0C)Az become more rigid because of π−π stacking, leading to a relatively high Tg (∼198 °C; Figure S7). Here, PMAA90-b-PMA(0C)Azn was polymerized at 70 °C in ethanol. The formed worms are rather stiff under this condition, impeding the fusion to lamellae or vesicles. Moreover, DSC analysis indicates the LC property was lost for both PMA(0C)Az homopolymer and PMAA90-bPMA(0C)Azn BCPs (Figure S7). Just as previously reported, worm-like NPs can be prepared over a wide range of BCP compositions when core-forming block is relatively rigid.28,39 The unique phase diagram in this formulation is possibly due to the π−π stacking and the stiff nature of PMA(0C)Az, and the precise reason is in further study. Azobenzene-containing polymers display photoresponsive behavior upon UV/vis light irradiation due to reversible trans−

cis photoisomerization. Figures 2 and S8 show the UV−vis spectra of PMAA90-b-PMA(0C)Az56 NPs dispersed in ethanol and dissolved in solution of 75/25% w/w CHCl3/DMSO mixtures, respectively. As shown in Figure S8a, the absorption peak at 337 nm assigning to the π−π* transition of the transazobenzene, shifted to 328 nm in Figure 2a, implying a Haggregation of the azobenzene chromophore in the core of NPs. This indicated that the azobenzene moieties arranged more orderly in the core of wormlike NPs than in solution.40 Moreover, when the NPs dispersion were irradiated by 360 nm UV, the absorption intensity at 328 nm became weak while the peak intensity at 439 nm corresponding to the n−π* transition of cis-azobenzene increased, indicating the trans−cis isomerization of azobenzene chromophores. The intensity decreased fast at the initial stage, and then leveled off to a plateau at 50 s corresponding to 33% of pristine intensity (Figure 2a). Afterward, the absorption intensity at 328 nm recovered to ∼85% of that of the pristine sample upon irradiation by 520 nm visible light for 300 s. Nevertheless, the intensity can not completely recover although the irradiation time was prolonged (Figure 2b), which is quite different from the behavior of the solution sample (Figure S8). It can be explained that for wormlike NPs, the molecular motion of azobenzene moiety in the core part is hindered sterically. Interestingly, the absorption intensity decreases/recovers completely in the subsequent cycles upon alternative UV/vis light irradiation (Figure 2c, a cycle consists of 1 min UV irradiation and 6 min visible light irradiation). The morphology evolution of PMAA90-b-PMA(0C)Azn NPs under different irradiation conditions was recorded by TEM, AFM images, and DLS analysis, as shown in Figures 3 and S9− S12. For PMAA90-b-PMA(0C)Az56 NPs (the pristine diameter is 20 ± 3 nm, Figure 1e), worms expanded gradually with the diameter increasing to du = 31 ± 4 nm after UV irradiation time for 1 min, which slimmed to dv = 23 ± 3 nm upon visible light irradiation time to 6 min subsequently. Moreover, there was no significant difference in length during the whole cycles. The height of worms from AFM images and the hydrodynamic diameters from DLS analysis display the same tendency upon alternative UV/vis light irradiation (Figures S9b and S10). The similar tendency was observed for the samples with different DPs, as shown in Figure S11. Azobenzene units undergo isomerization between elongated trans and contracted cis states when irradiated by UV and visible light, accompanied by the polarity change.11,12 Azobenzene molecules are prone to forming ordered arrangement in their elongated trans state, and become random when transferred into contracted cis state. 462

DOI: 10.1021/acsmacrolett.9b00146 ACS Macro Lett. 2019, 8, 460−465

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Figure 4. SEM images of the composite membrane: (a) Crosssectional images, top surfaces of (b) pristine and (c) after UV irradiation for 2 min. (d) Rejection performance of composite membrane under different irradiation conditions.

Figure 3. (a−c) TEM images of PMAA90-b-PMA(0C)Az56 (15% w/ w) NPs under different conditions (du and dv denote the diameter of wormlike NPs after UV and visible light irradiation, respectively). (d) Plot of the diameter and its variation amplitude of the wormlike NPs as a function of mass fraction of PMA(0C)Az.

to 52 L/m2h and the rejection decreased to 82% after visible light irradiation for 10 min (Figures 4d and S14). The variation tendency is similar in the later four cycles. It is obvious that the flux and rhodamine B rejection display a reversible variation upon alternative UV/vis light irradiation, demonstrating a periodically adjustable rejection performance, similar to a FAST/SLOW state of a switch. Generally, the nanofiltration performance primarily depends on the porous structure of the membrane.43 As aforementioned, worms expand/slim accordingly upon UV/vis light irradiation when dispersed in solution. For the constructed NFS, worms cross-linked together to form a membrane, where pores are inevitable. Compared with Figure 4b, it can be seen that worms became slightly larger and the pores became unclear after UV irradiation (Figure 4c). The color change of the membrane also reflected its different states (Figure S16).44 In other words, due to the photoresponsive behavior, the lighttriggered reversible slimming of worms induces the reversible increase of the pores of the cross-linked membrance, resulting in the FAST/SLOW state of the NFS. Additionally, it is reasonable to expect the better rejection performance based on the advanced membrane fabrication process. In summary, we have demonstrated the effective formation of azobenzene-containing wormlike NPs by PISA in ethanol. The diameter of worms can be tuned by varying the chain length of PMA(0C)Az and the solids concentration. Worms aggregate together under the high DP of PMA(0C)Az. The pure worm phase covers a remarkably broad region in the phase diagram because of the rather rigid nature of the coreforming block PMA(0C)Az. Worms become expanded and slim under alternative UV/visible light irradiation. Except the pristine state, the completely reversible deformation of worms in the UV state and visible light state can be obtained. The diameter variation amplitude increases with the volume fraction of the core-forming block. A nanofiltration switch is assembled based on the photoresponsive wormlike NPs, which displays accordingly variable rejection performance for rhodamine B upon alternative UV/visible light irradiation, illustrating one of potential applications by remote light control.

Therefore, azobenzene-containing hydrophobic core part expands when converted to cis state upon UV irradition.19 Moreover, the polarity increases when transferred into cis state, which further induces the expansion of the core part when the worms disperse in ethanol. Conversely, the volume of hydrophobic core and the hydrophilicity of core blocks decrease upon visible light irradiation, resulting in the slimming of the worms. It should be pointed out that the worms can not recover to the pristine state although further increasing visible light irradiation time. This can be attributed to the incomplete trans−cis photoisomerization of azobenzene in the core part of worms due to steric hindrance effect, in good accordance with the UV−vis spectra result (Figure 2). Similarly, the deformation is reversible between dv and du in the next five cycles, as shown in Figure S12. The diameter variation amplitude of worms between different irradiation state increases with core-forming PMA(0C)Az chain fraction of the BCPs (Figure 3d). Benefiting from the high-throughput approach of PISA to BCP NPs,41,42 a light-triggered nanofiltration switch (NFS) was constructed by assembling these wormlike NPs on the surface of a commercial polypropylene (PP) support membrane with pore size of 0.22 μm for rhodamine B rejection, as shown in Scheme S1. The switch is expected to be achieved by alternating UV/vis irradiation due to the reversible slimming behavior of worms. Here, 5 mL ethanol dispersion of wormlike NPs (1% w/w) was filtered onto the PP support membrane with 13.4 cm2 in effective area, and then the composite membrane was cross-linked in excess calcium acetate solution to form stable porous composite membrane, as a NFS. Figures 4a and S13 show cross-sectional and topsurface SEM images of the NFS. The composite membrane shows a dense surface with no interfacial defects/gaps, and the thickness of the worm layer is about 1 μm. The nanofiltration experiments were conducted under 0.10 MPa at room temperature with rhodamine B solution (10−5 mol/L) as the feed. The flux is 57 L/m2h and the rejection of rhodamine B is 80% under this state, which changed to 32 L/m2h and 93%, respectively, upon UV irradiation for 2 min. The flux increased 463

DOI: 10.1021/acsmacrolett.9b00146 ACS Macro Lett. 2019, 8, 460−465

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.9b00146.

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Materials and methods, characterization, and supporting figures and table (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Aihua Chen: 0000-0002-9609-988X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Nos. 51272010 and 51472018), Beijing Nova Program (No. XX2013009), and the Fundamental Research Funds for the Central Universities.

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