Photoresponsive Phase Separation of a Poly(NIPAAm-co-SPO-co

Jul 21, 2014 - The separate effects of DEAP and Span 80 were also investigated by UV spectrophotometric analysis of the rate coefficient of the revers...
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Photoresponsive Phase Separation of a Poly(NIPAAm-co-SPO-cofluorophore) Random Copolymer in W/O Droplet Saifullah Lone,† Jeong In Ahn,† Mi Ri Kim,† Hyang Moo Lee,† Sung Hoon Kim,‡ Timothy P. Lodge,§ and In Woo Cheong*,† †

Department of Applied Chemistry and ‡Department of Textile System Engineering, Kyungpook National University, Daegu 702-701, South Korea § Department of Chemistry and Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota 55455-0431, United States S Supporting Information *

ABSTRACT: The photoresponsive phase separation of a poly(N-isopropylacrylamide-co-spironaphthoxazine methacryloylco-allyl-2-(2,6-bis((E)-4-(diphenylamino)styryl)-4H-pyran-4-ylidene)-2-cyanoacetate) random copolymer, i.e., poly(NIPAAm-coSPO-co-fluorophore), in water-in-oil (W/O) droplets is described. The photoresponsive aqueous droplets were generated in the coflow regime of a simple tubular microfluidic device. The phase separation of the copolymer in the W/O droplets was induced by UV light at 365 nm and was affected significantly by the presence of 2,2-diethoxyacetophenone (DEAP) and sorbitan monooleate (Span 80). When the droplets were subjected to UV irradiation for more than 2 min, the phase-separated copolymer was transferred completely from the aqueous droplet to the continuous phase of hexadecane. The phase separation arises from the photoisomerization shifting the spiro to the merocyanine form of the SPO pendant group in the copolymer, which in turn reduces the hydrophilicity of the copolymer via attractive hydrogen-bonding interactions between the merocyanine group and hydrophobic additives, i.e., Span 80, DEAP, and some stable fragments derived from the photocleavage of DEAP under UV irradiation. These interactions cause the copolymer to associate with the additives and then accelerate the phase separation of the copolymer and subsequent phase transfer of copolymer aggregates. The separate effects of DEAP and Span 80 were also investigated by UV spectrophotometric analysis of the rate coefficient of the reverse transformation (merocyanine to spiro) of the photochromic monomer. We propose a mechanism of phase separation of the copolymer in the W/O droplet based on the NMR and GC-MASS analyses of DEAP.

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INTRODUCTION Micron-sized and monodisperse droplets have been exploited to design functional particles/capsules through a range of templating processes. The art of developing “smart” droplets from materials that can respond to external stimuli, such as stress, temperature, pH, and electric or magnetic fields, has been established as a new dimension in microchemical reactions and biological processing.1−5 The manipulation of smart droplets can be tuned more easily by the surrounding stimuli in a sensitive and active manner, compared to conventional emulsions. Recently, a variety of stimuliresponsive polymers6−8 triggered by temperature, pH, light, chemicals, biomolecular interactions, and ultrasound have been highlighted9,10 and studied intensively, leading to the development of smart materials and devices.11 These studies are also useful for a wide range of applications, such as biomimetic interfaces and membranes, highly efficient sensing and diagnosis, controlled drug delivery, and smart actuators inspired by living systems in nature.12−21 While several types of stimuli © 2014 American Chemical Society

have been reported, light is the most attractive candidate due to the advantages it holds. The intensity, wavelength, spatial localization, and rapid cycle of light stimulus can be controlled easily.22−25 Spironaphthoxazine (SPO) or spiropyran derivatives are well-known photochromic compounds that offer a range of potential applications26−31 and exhibit excellent light-fatigue resistance.32,33 SPOs undergo photocleavage of the spiro bond (closed ring) when subjected to UV irradiation, creating a merocyanine (open ring) form with a deep blue color, which has a broad absorption band at approximately 610 nm. The merocyanine can be converted reversibly back to the spiro form under visible light or dark conditions (or upon heating).34 The unidirectional aggregation and internal phase separation of SPO-containing random copolymers35,36 in water-in-oil (W/O) Received: June 8, 2014 Revised: July 18, 2014 Published: July 21, 2014 9577

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Figure 1. Chemical structure of the poly(NIPAAm-co-SPO-co-fluorophore) random copolymer used in this study: (a) spiro (closed) form and (b) merocyanine (open) form.36

Figure 2. (a) Schematic diagram of the tubular microfluidic device to examine the phase separation of poly(NIPAAm-co-SPO-co-fluorophore) in a W/O droplet. The resulting droplets on the glass slide were subjected to UV irradiation to undergo phase separation, (b) photographic image of W/ O droplet formation in a simplified tubular microfluidic device, and (c) corresponding single W/O droplet on the slide glass prior to UV exposure.

spectroscopic analyses with NMR and GC-MASS regarding the additives.

droplets under 365 nm UV irradiation were recently reported.37,38 The droplets containing the copolymer were then exploited to control the morphology from symmetric to asymmetric Janus particles by the combined addition of N,Nmethylenebis(acrylamide) (MBA, water-soluble monomer), DEAP (oil-soluble photoinitiator), and Span 80 (nonionic surfactant). This strategy could overcome the cross-mixing problems39,40 arising in the coflow stream composed of two immiscible fluids for a biphasic (Janus) morphology.38 More recently, Choi, et al. reported the morphological control of aqueous droplets consisting of poly(ethylene glycol) diacrylate in a microfluidic channel by adopting the present approach.41 However, the role of certain additives (particularly, photoinitiator and surfactant) was not fully explained. This study therefore examines the impact of these additives on the phase separation of a poly(NIPAAm-co-SPO-co-fluorophore) random copolymer in the W/O droplet. An association mechanism between SPO pendant group and the additives is proposed based on UV spectrophotometric analysis of the reverse transformation (from merocyanine to spiro) with

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EXPERIMENTAL SECTION

Materials. N-Isopropylacrylamide (NIPAAm), DEAP, and hexadecane were purchased from Sigma-Aldrich (St. Louis, MO). Span 80 (sorbitan monooleate, C24H44O6, HLB = 4.3) was obtained from Duksan Pure Chemical (Seoul, Korea). Ultrapure water (resistivity >18.2 MΩ·cm, Direct-Q, Millipore Co.) was degassed and used for all experiments. The photoresponsive random copolymer was prepared with a fluorescent dye (as a comonomer) for the visualization of the copolymer in the W/O droplet phase.36 In the preparation of the copolymer, NIPAAm (27 mmol), SPO methacryloyl monomer (0.13 mmol), and fluorescent monomer (0.13 mmol, allyl-2-(2,6-bis((E)-4(diphenylamino)styryl)-4H-pyran-4-ylidene)-2-cyanoacetate) were dissolved in anhydrous tetrahydrofuran (THF, 20 mL). Copolymerization was initiated by adding 2,2′-azobis(isobutyronitrile) (AIBN, 0.4 mmol) under a N2 atmosphere. The product consisted of three different functional units: NIPAAm, SPO methacryloyl, and fluorescent monomer as the thermosensitive, photoresponsive, and fluorophore, respectively. Gel permeation chromatography (GPC) showed that the number- and weight-average molecular weights were 9578

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Mn = 19 800 g/mol and Mw = 22 000 g/mol, respectively, based on polystyrene standards (SL-105, Shodex, Japan). Figure 1 shows the molecular structure of the random copolymer used in this study. W/O Droplet Formation. A simplified microfluidic device was used to examine the phase separation of the random copolymer in the W/O droplet. Figure 2 presents a schematic diagram of the preparation of the W/O droplets. The device was prepared by inserting a 30 GB needle (NanoNC, Seoul, Korea) into the Tygon microbore tubing (i.d. = 515 μm, Cole-Parmer, Vernon Hills, IL). The flow rates of the continuous and dispersed phases were maintained using two microsyringe pumps (Legato 200, KD Scientific Inc., Holliston, MA). A probe-type UV beam (SPOT UV/Inno-cure 100N, 365 nm, 100 W, 2000 mW/cm2, Lichtzen, Seoul, Korea) was used as the light source. For W/O droplet formation, the dispersed phase (5 wt % copolymer + 95 wt % water) was injected through the needle connected to the Tygon tube. The disperse phase was coflowed with the continuous phase (5 wt % Span 80 + 5 wt % DEAP + 90 wt % hexadecane). The W/O droplets were generated at the tip of the needle in a typical coflow regime with flow rates of 10 and 0.2 μL/s for the continuous and dispersed phases, respectively. Once the W/O droplets were produced, they were deposited on a glass slide for offdevice UV exposure. Characterization. The different morphologies of the W/O droplets were visualized under an optical microscope (Nikon, ECLIPSE LV 100D, Tokyo, Japan) equipped with a video camera (Moticam 2300, Motic, Beijing, China) and a thermostage (HG-SZ002, Live Cell Instrument Co.) maintained at 20 °C. The effects of Span 80 and DEAP on the UV-triggered phase separation in a W/O emulsion were also examined by fluorescence microscopy (FM, Axioplan-2, ZEISS, Germany). The W/O droplets received from the microfluidic device were taken on a glass slide (76 × 26 × 1 mm, Marienfeld, Germany) for post-UV irradiation. In order to investigate the effects of Span 80 and DEAP on the photoisomerization of SPO, the rate of reverse transformation (or relaxation) from merocyanine (open) to spiro (closed) form was measured by UV spectrophotometry (S-3100, Scinco, Seoul, Korea) at 10 s intervals immediately after 5 s of UV irradiation (SPOT UV/ Inno-cure 100N). For the sample preparation, the copolymer was first dissolved in pure water at 1 wt % due to the upper limit in UV spectrophotometer, then an excess amount of DEAP or Span 80 was added to the solution, and then the supernatant was removed due to the low water solubility of DEAP and Span 80. Direct spectroscopic analysis of the W/O droplet system was not possible due to the scattering of the UV−vis beam. The association between merocyanine of SPO and DEAP (or Span 80) in aqueous phase was observed in terms of the rate of relaxation. The rate coefficient of relaxation (k) was calculated using the equation

ln

It − I∞ = − kt I0 − I∞

Figure 3. Optical images of the 5 wt % poly(NIPAAm-co-SPO-cofluorophore) random copolymer in a W/O droplet in the presence of both Span 80 (5 wt %, in continuous phase) and DEAP (5 wt %, in continuous phase) under (a) visible light and (b−e) 365 nm UV light. (f) Corresponding fluorescent image of the droplet in (e).

presence of both Span 80 and DEAP under 365 nm UV irradiation. The copolymer under visible light is hydrophilic due to the large portion (99 mol %) of NIPAAm units below their lower critical solution temperature (LCST). Therefore, it is soluble within the aqueous droplet (colorless, Figure 3a). As shown in Figure 3b, the droplet color first turns dark brown (this is not a true color due to the illumination of the microscope), and the copolymer in the droplet begins to separate to form a dense aggregate that protrudes from the aqueous droplet (Figure 3c). Under UV light, the spiro (closed) moieties of the copolymer were transformed to merocyanine (open) isomers, which produce hydrogenbonding interactions with Span 80, DEAP, and the fragmented species from the UV photocleavage of DEAP. The copolymer becomes more hydrophobic (deep blue color, Figure 3(b→e)) due to the association with these hydrophobic molecules. These phenomena, from the color change to the phase separation, occurred within 10 s. In this section, we attempt to determine the roles of the photoinitiator (DEAP) and nonionic surfactant (Span 80) in the phase separation process. Therefore, aqueous droplets of the random copolymer (5 wt %) were first prepared without DEAP. Figures 4a and 4b show optical images of the droplets

Figure 4. Optical images of 5 wt % poly(NIPAAm-co-SPO-cofluorophore) random copolymer in W/O droplet prepared with Span 80 (5 wt %, in continuous phase) in the absence of DEAP: (a) under visible light and (b) under 365 nm UV light.

(1)

stabilized by 5 wt % Span 80 before and after 365 nm UV light exposure for 1 min, respectively. In the absence of DEAP, no phase separation was observed regardless of UV exposure time. This suggests that there is neither an interaction nor association between the random copolymer of merocyanine and the Span 80 molecule. This is presumably due to the large difference in water solubility of the copolymer and Span 80. Span 80 is an oil-soluble surfactant that is often used to stabilize W/O droplets due to the low HLB (∼4.3). The effect of DEAP was then examined by preparing W/O droplets with the copolymer (5 wt %, in water) and DEAP (5 wt %, in hexadecane) without Span 80. Figure 5 shows the phase separation behavior of the copolymer in the presence of only DEAP under the same UV exposure conditions. The results confirm that the DEAP causes phase separation. Compared to the results in Figure 3, the phase separation was much slower and required 4 times the UV irradiation time

where It is UV absorption intensity (λI = 610 nm) at t, I0 the absorption intensity at t = 0 (immediately after turning the UV beam off), I∞ the absorption intensity at infinite time (before UV irradiation), and t the time for the relaxation (0−40 s). 1 H NMR (CDCl3, Avance-400 MHz, Bruker) and GC-MASS (GC/ MSD, 7890A-5975C, Agilent) analyses of DEAP before/after UV irradiation were performed to confirm the chemical structure of DEAP-derived species as a hydrogen-bonding donor (refer to Supporting Information).

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RESULTS AND DISCUSSION Phase Separation Behavior. A previous study reported that the phase separation of poly(NIPAAm-co-SPO-cofluorophore) was induced by UV irradiation and the subsequent asymmetric Janus morphology developed within several seconds.38 Figure 3 shows the typical phase separation behavior of the random copolymer in the W/O droplet in the 9579

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height was rapid and exponential with time; however, the rate gradually became slower. This photoisomerization was found to be reversible. If DEAP or Span 80 somehow affects the phase separation of the copolymer, then the rate of transformation from merocyanine to spiro may also be affected. Figure 7 shows Figure 5. Optical images of the 5 wt % poly(NIPAAm-co-SPO-cofluorophore) random copolymer in the W/O droplet in the absence of Span 80 under (a) visible light and (b−e) 365 nm UV light. (f) Corresponding fluorescence image of the droplet in (e).

(∼45 s) to drive the same degree of phase separation than in the presence of Span 80. Second, phase separation was confined within the droplet, with initially less dense and irregularly shaped domains (Figure 5b). As a result, the process did not yield a perfect Janus morphology, as in the presence of Span 80 (Figure 3e). Therefore, Span 80 not only accelerates the UVtriggered phase separation in the W/O droplet but also transforms the simple phase separated domain into asymmetric (i.e., acorn-shaped) Janus droplets, presumably by decreasing the interfacial tension between the aqueous and oil phases. During the examination, the phase separation of the copolymer was found to require DEAP, whether it is pristine or denatured by UV. The same phase separation experiment was carried out with DEAP (5 wt %, in hexadecane) after a pretreatment with UV irradiation for 6 h (to complete the UV photocleavage reaction of DEAP), and the same results of phase separation were observed as with the DEAP. DEAP is an oil-soluble photoinitiator that is fairly water-soluble (0.16 g/L water, 25 °C) and readily undergoes UV-induced decomposition via competitive reactions.42 As most of photoreactions are fast, the phase separation in Figure 3 also occurred within 10 s and which seems fairly fast, although the detailed mechanism was not elucidated completely. Effects of DEAP and Span 80. Figure 6 shows the UV absorption spectra of 1 wt % copolymer aqueous solutions with and without DEAP or Span 80 and which are comparable to Figures 3, 4, and 5. As shown in Figure 6, the spiro form of the SPO group is transformed to merocyanine under the UV irradiation (before UV → 0 s in the legend), yielding a strong absorption peak at 610 nm. When the UV is turned off, the peak begins to decrease. The initial reduction rate of peak

Figure 7. Semilog plots of the peak intensity variation (measured at 610 nm) of (a) 1 wt % poly(NIPAAm-co-SPO-co-fluorophore) random copolymer aqueous solution, (b) 1 wt % copolymer aqueous solution with a saturated concentration of DEAP, and (c) 1 wt % copolymer aqueous solution with a saturated concentration of Span 80.

the decay curves of UV absorption peak intensity (I) measured at 610 nm in semilogarithmic format. Based on the UV intensity changes in Figure 6, the reverse transformation rate (k) was calculated by using eq 1. The k values for the samples (a), (b), and (c) were 0.037, 0.027, and 0.027 s−1, respectively. The results clearly showed that the transformation rate from open to close was reduced by DEAP. The effect of Span 80 appears to be insignificant, as compared with DEAP; however, this might be underestimated due to the lower water solubility of Span 80. From the decay curves of (b) and (c) in Figure 7, one can see that the rate is affected by Span 80. Several models have been proposed to describe the interactions between polymers and surfactants;43−46 however,

Figure 6. UV absorption spectra of (a) 1 wt % poly(NIPAAm-co-SPO-co-fluorophore) random copolymer aqueous solution, (b) 1 wt % copolymer aqueous solution with a saturated concentration of DEAP, and (c) 1 wt % copolymer aqueous solution with a saturated concentration of Span 80. Each spectrum was recorded with 10 s intervals at 20 °C. The small peak around 450 nm is attributed to the fluorescent dye. 9580

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Figure 8. Chemical structures of DEAP, fragmented species from the UV photocleavage of DEAP, and Span 80. Hydrogen bonding takes place between the hydrogen-bonding donor (blue) and acceptor (red). The UV photocleavage reaction of DEAP was modified from ref 48.

UV-induced hydrogen bonding would be a plausible mechanism for explaining the phase separation of the copolymer under UV exposure. Lee et al. also suggested that the spiropyran form of a merocyanine was associated with lecithin and sodium deoxycholate via hydrogen bonding,47 although they could not present direct evidence of a hydrogen-bonding mechanism. In Figure 8, the UV photocleavage reaction of DEAP and resulting chemical species potentially involved in hydrogen bonding are illustrated.48 When the excited DEAP follows Norrish I cleavage, it yields a benzoyl radical and a ketyl (1,1-diethoxymethane) radical. The former is relatively stable but the latter is quite unstable, so this undergoes a further waste reaction yielding an alkyl radical and ethyl methanoate (i) (hydrogen bond donor, in blue). The radicals from Norrish I undergo recombination to yield benzil (a photoinitiator) and 1phenylpropan-1-one. The Norrish II cleavage of DEAP would yield a 1,4-radical intermediate that could form a 1,3-propylene oxide ring (ii) by cyclization or decomposes to an acetophenone derivative (iii) and aldehyde (iv) by elimination.48 Since the SPO only provides hydrogen bond acceptors (nitrogen and oxygen atoms), we propose that some of the DEAP-derived compounds (i−iv) participate in hydrogen bonding with the SPO unit of the random copolymer after UV irradiation. These compounds were confirmed by NMR and GC-MASS analyses (see Supporting Information). There could also be other interactions, e.g., ionic37 or cation−π49 interactions, that play a role; however, a more detailed view of the interactions involved in the phase separation is not described in this article because of the complexity of the UV irradiation process including photocleavage reaction. Reversibility of Phase Separation. To explore the reversibility of the phase separation and corresponding change in the interfacial energy in the W/O droplet state, a meniscus interface was created between the aqueous and oil phases (at the same composition as in Figure 3) in a microcapillary glass tube. As shown in Figure 9a, the concave (upper, aqueous) phase contains the copolymer, and the convex (lower, hexadecane) phase contains Span 80 and DEAP. After being subjected to UV irradiation, the copolymer in the aqueous phase became hydrophobic due to the association between the merocyanine form of the SPO units and Span 80, DEAP, and the other species. As a result, the meniscus height of the oil phase decreased, as shown in the sequence Figure 9(a → b → c). The meniscus interface appears diffuse, and the copolymer

Figure 9. Optical images of the interface between the aqueous and oil phases in a glass microcapillary tube: (a) under visible light, (b−d) under 365 nm UV light, and (e−h) under visible light. The aqueous phase is composed of 5 wt % poly(NIPAAm-co-SPO-co-fluorophore) random copolymer in pure water, and the oil phase is composed of 90 wt % hexadecane, 5 wt % Span 80, and 5 wt % DEAP.

aggregates even cause significant movement into the oil phase (Figure 9d). The meniscus height returned to its original position when the UV light was switched off, as shown in Figure 9(e → f → g → h), which indicates that the process is reversible. The results confirmed that a decrease in interfacial tension facilitates the protrusion of the copolymer aggregates (as in Figure 3(c → d → e)). UV-Triggered Transport. In the PDMS-based microfluidic preparation of Janus particles,38 the W/O droplets bearing the copolymer in the aqueous mixture pass through a limited region of UV irradiation. Consequently, the UV exposure time for each droplet is restricted to a few seconds. Herein, the UV irradiation time was prolonged to an additional 2 min after the complete formation of the Janus morphology. During the course of UV exposure, the formation of a Janus morphology via phase separation occurred on an average of 10 s, as found in the previous case of the W/O droplets. On the other hand, the additional UV exposure for 2 min resulted in complete release of the copolymer from the aqueous droplet, which was confirmed by optical and fluorescent microscopy as shown in Figure 10. The photoinduced phase separation in these results suggests that phase separation has very high likelihood of being controllable (i.e., arrested or accelerated) at any time by 9581

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ASSOCIATED CONTENT

S Supporting Information *

1

H NMR spectra and GC-MASS data for DEAP before and after UV irradiation. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Tel +82 53 950 7590; Fax +82 53 950 6594 (I.W.C.). Author Contributions

Figure 10. Optical images of the stepwise phase separation of poly(NIPAAm-co-SPO-co-fluorophore) in W/O (a) under visible light, (b−d) under UV light (365 nm) undergoes phase separation in the presence of both Span 80 and DEAP to form Janus morphology, (e) complete release of fluorescent poly(NIPAAm-co-SPO-co-fluorophore) copolymer from aqueous phase of W/O emulsion, and (f−j) fluorescent micrographs of the respective morphologies in panels a−e.

S.L. and J.I.A. contributed to this work equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by Kyungpook National University Research Fund (2011) and Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. 2014R1A1A4A01007436).

switching the UV source on or off; even manipulating the time scale of UV irradiation can control the droplet morphology. This facile control over the stepwise phase separation of the copolymer can be used to produce asymmetric particles with various halves (snowman shape, acorn shape, Janus, and dumbbell particle). In addition, the complete interfacial migration of the copolymer from the aqueous mixture into the outer continuous phase as a function of the UV irradiation time might have significant applications in droplet microfluidics for high-throughput drug/cell-based screening and bioassays because droplets allow minimal volume (∼picoliter), high surface area, and kilohertz speed of manipulation/measurement, as compared to conventional methods.50

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CONCLUSIONS This study examined the nature of the photoresponsive phase separation of poly(NIPAAm-co-SPO-co-fluorophore) random copolymer in a W/O emulsion in relation to the presence and absence of Span 80 and DEAP in the oil phase, the difference in the interfacial tension variations between the two phases, and the phase-separated copolymer release from the W/O emulsion, all as a function of the UV irradiation time length. While investigating the UV-triggered phase separation, both DEAP and Span 80 were found to play important roles in the phase separation of the copolymer via attractive interactions, i.e., hydrogen bonding between the merocyanine form (hydrogen bond acceptor) of the random copolymer and the hydrogen bond donors derived from the photocleavage reaction of DEAP under UV irradiation. The donors render the copolymer hydrophobic via association leading to the phase separation of copolymer, while Span 80 (hydrogen bond donor) accelerates the phase separation and protrusion (i.e., phase transfer) toward oil phase due to the low HLB of Span 80. This study presents an interesting insight into understanding the phase separation behavior of SPOs undergoing the photocleavage of the spiro bond (closed ring) in the presence of rendering additives with hydrogen-bonding acceptor/donor when subjected to UV/vis light in W/O droplet. These results highlight potential and interesting applications in smart dropletbased bioassays for the controlled release of encapsulated materials. 9582

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dx.doi.org/10.1021/la5022005 | Langmuir 2014, 30, 9577−9583