Temperature and pH-Dual Responsive AIE-Active Core Crosslinked

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Letter Cite This: ACS Macro Lett. 2018, 7, 886−891

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Temperature and pH-Dual Responsive AIE-Active Core Crosslinked Polyethylene−Poly(methacrylic acid) Multimiktoarm Star Copolymers Zhen Zhang*,†,‡ and Nikos Hadjichristidis*,‡ †

School of Chemical Engineering and Light Industry, Guangdong University of Technology, Guangzhou 510006, China Physical Sciences and Engineering Division, KAUST Catalysis Center, Polymer Synthesis Laboratory, King Abdullah University of Science and Technology (KAUST), Thuwal 23955, Saudi Arabia

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‡

S Supporting Information *

ABSTRACT: A series of aggregation-induced emission (AIE) active core crosslinked miktoarm star copolymers, having multi polyethylene (PE) and poly(methacrylic acid) (PMAA) arms, were synthesized and their thermal/pH responsive properties were studied. The procedure involves (a) the synthesis of PE-Br by polyhomologation of dimethylsulfoxonium methylide with triethylborane as initiator, followed by oxidation-hydrolysis/esterification reactions and of poly(tert-butyl methacrylate) (PtBMA-Br) by atom transfer radical polymerization (ATRP) of tert-butyl methacrylate, (b) the synthesis of (PE)n-(PtBMA)m-P(TPE-2St) by ATRP of a double styrene-functionalized tetraphenylethene (TPE-2St) with PE-Br and PtBMABr macroinitiators, and (c) the hydrolysis of (PE)n-(PtBMA)m-P(TPE-2St) to afford the amphiphilic miktoarm star copolymers (PE)n-(PMMA)m-P(TPE-2St). Due to their spherical core−shell structure (temperature-responsive) and the presence of hydrophilic PMAA (pH-responsive) and TPE-2St (AIE), these miktoarm star copolymers are AIE materials with temperature/pH-dual responsivity. In addition, thanks to the coexistence of hydrophilic and hydrophobic arms, these materials promote stable water-in-oil emulsions. timuli-responsive or “smart” polymers have attracted significant attention in recent years due to their broad applications in drug delivery, tissue engineering, sensors, and nanodevices.1−4 Numerous stimuli-sensitive polymers have been developed responding to various stimuli, such as temperature, pH, light, ultrasound, electron transfer (redox variations), and host−guest interactions,5 with temperature and pH to be the most commonly employed stimuli. The conventional temperature-sensitive polymers, such as poly(Nisopropylacrylamide) (PNIPAM), 6 are based on their amphiphilic character and changes of hydrogen bonding. pHsensitive polymers possess ionizable acidic or basic groups that can accept/release protons in response to a change in the solution pH.7 Although the conventional temperature and pHresponsive polymers have been well developed, novel materials with unique responsive properties, such as fluorescence, are still highly desirable. Since Tang’s discovery of the aggregation-induced emission (AIE) phenomenon in 2001,8 stimuli-responsive polymers with AIE properties have attracted remarkable research attention and various “smart” polymers have been synthesized and reported. 9,10 The AIE fluorogens usually show weak fluorescence or nonemissivity in solution but are highly emissive when aggregate due to the restriction of the intramolecular rotation in the aggregate state leading the excitations to decay radiatively.11 By incorporating AIE fluorogens, for example, tetraphenylethene (TPE) into

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polymer chains, polymer properties can be combined with AIE characteristics to emit in response to external stimuli. For example, water-soluble poly(N-isopropylacrylamide)-tetraphenylthiophene (PNIPAM-TP) polymers exhibiting thermoresponsive photoluminescence emission have been reported.12 A core crosslinked poly(ethylene glycol) star polymer with phosphonic acid groups and TPE fluorophores in the core, synthesized by ring-opening metathesis polymerization (ROMP),13 demonstrated strong fluorescence in water when pH < 4 and weak when pH > 5. Recently, our group synthesized AIE-active polyethylene-based core crosslinked multiarm star polymers by atom transfer radical polymerization (ATRP),14 exhibiting temperature-responsive fluorescence caused by their spherical shape and thus differ from the conventional temperature-responsive polymers with amphiphilic character (e.g., PNIPAM). The development of living radical polymerization techniques, such as ATRP,15 nitroxide mediated polymerization (NMP),16 and reversible addition−fragmentation chain transfer (RAFT),17 have enabled the facile synthesis of multiarm star homo/copolymers with a wide range of monomers.18,19 Herein, with the expectation that the three-dimensional (3D) Received: May 1, 2018 Accepted: June 27, 2018

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DOI: 10.1021/acsmacrolett.8b00329 ACS Macro Lett. 2018, 7, 886−891

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ACS Macro Letters Scheme 1. Synthetic Route for the AIE-Active Core Crosslinked Multimiktoarm Star Copolymers

Table 1. Synthesis of Core Crosslinked Miktoarm Star Copolymers by the “Arm-First” Method Narmd entrya

MI-1

[MI] (mM)

time (h)

[TPE-2St]:[MI-1]:[MI-2]

Mnb (kg mol−1)

Đb

Mwc (kg mol−1)

Đc

Rhc (nm)

PE

PtBMA

1 2 3 4 5

PE2.5k-Br PE1.2k-Br PE1.6k-Br PE1.6k-Br PE1.6k-Br

16.1 18.8 25.0 16.1 14.3

24 26 17 25 26

28:1:2.6 20:2:1 15:1:1 30:1:3 15:1:1

27.5 12.4 31.4 16.2 14.3

1.51 1.93 2.95 1.90 1.73

252.9 178.9

1.52 1.94

6.6 5.8

14.2 25.5

22.8 8.6

e

e

e

e

e

519.0 218.0

2.22 1.52

6.3 7.8

29.0 19.4

42.6 14.5

Reaction condition: toluene, 90 °C, [CuBr]:[PMDETA]:[MI] = 1:2:1. bWeight-average molecular weight (Mn) and polydispersity (Đ) determined by GPC RI detector. cAbsolute molecular weight (Mw), polydispersity (Đ), and hydrodynamic radius (Rh) measured by GPC triple detector. dAverage number of arms: Narm = (Mw,star × armwt%)/Mn,NMR,arm. eNot determined due to solubility reasons. a

Figure 1. 1H NMR spectra of (a) (PE)n-(PtBMA)m-P(TPE-2St) (600 MHz, 1,1,2,2-tetrachloroethane-d2, 90 °C), and (c) (PE)n-(PMAA)mP(TPE-2St) (600 MHz, THF-d8, 25 °C); (b) HT-GPC (TCB at 150 °C) traces of linear PE1.6k-Br and the corresponding (PE)n-(PtBMA)mP(TPE-2St) stars (the negative peak of PE1.6k-Br is due to the negative DRI of PE); (d) FT-IR spectra of (PE)n-(PtBMA)m-P(TPE-2St) and (PE)n(PMAA)m-P(TPE-2St).

globular structure of miktoarm star copolymers will lead to unique optical and responsive properties, we have synthesized miktoarm star copolymers with pH-responsive arms using ATRP. Despite the fact that pH-responsive core crosslinked multiarm star polymers with phosphonic acid and TPE in the core have already been reported,13 multiarm stars containing acid groups on the arms and fluorogens in the core have never

been presented. Therefore, in this work, we report the synthesis and stimuli responsive behavior of AIE-active core crosslinked multimiktoarm star copolymers having multi polyethylene (PE) and poly(methacrylic acid) (PMAA) arms. In addition to their AIE characteristics and dual responsivity, these amphiphilic star copolymers can stabilize water-in-oil emulsions at low surfactant concentration. 887

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Figure 2. (a) Photoluminescence (PL) spectra of (PE)19.4-P(MAA)14.5-P(TPE-2St) in THF/n-hexane mixtures with different n-hexane fractions measured at room temperature (23 °C). (b) Plot of I/I0 vs water fraction in the THF/n-hexane mixture (I is PL intensity in THF/n-hexane mixtures with different n-hexane fractions; I0 is PL intensity in pure THF solution; concentration, 0.1 g/L). Inset: photographs of THF/n-hexane mixtures of (PE)19.4-P(MAA)14.5-P(TPE-2St) with 0, 30, 60, and 90 vol % n-hexane fractions taken under 365 nm UV illumination.

Figure 3. PL spectra of (a) (PE)19.4-(PMAA)14.5-P(TPE-2St) and (b) (PE)19.4-(PtBMA)14.5-P(TPE-2St) in 50/50 vol % THF/n-hexane solution with temperature decreasing from 55 to 5 °C by a step size of 5 °C, polymer concentration, 0.1 g/L; (c) Plot of PL intensity vs temperature; (d) PL spectra of (PE)19.4-(PMAA)14.5-P(TPE-2St) in THF-buffer mixture (10/90 vol %, polymer concentration: 0.05 g/L); (e) Plot of I/I0 vs pH value, I0 is PL intensity at pH 5.0; (f) Plot of transmittance vs pH value. Inset: photographs of the star polymer in THF-buffer (pH 1.1, 7.2, and 13.3) mixture taken under 365 nm UV illumination.

ethylene having two styrene moieties (crosslinker, TPE-2St), following typical ATRP procedures. Due to the poor solubility of polyethylene at low temperature, the polymerization reaction was performed at high temperature (100 °C). By employing different ratios of crosslinker to macroinitiators (Table 1), five miktoarm stars with different chemical compositions were synthesized. To make sure all the PE-Br will be consumed, since it is difficult to remove the unreacted PE-Br from the star, the influence of concentration of macroinitiators [MI] on the polymerization of TPE-2St was investigated. When [MI] was higher than 14.3 but lower than 25.0 mM, practically all PE arms were incorporated into the

As shown in Scheme 1, core crosslinked miktoarm star copolymers (PE)n-(PtBMA)m-P(TPE-2St) were synthesized via the “arm-first” method using PE-Br and PtBMA-Br initiators simultaneously to polymerize the double styrenefunctionalized tetraphenylethene (TPE-2St). Linear PE with Br-chain-end sites (PE-Br) was prepared in two steps: (1) polyhomologation of dimethylsulfoxonium methylide leading to PE with hydroxy end-group (PE-OH), and (2) esterification of hydroxy group with 2-bromoisobutyryl bromide. Linear PtBMA-Br was synthesized by ATRP of tBMA (for GPC trace, see Figure S1). PE-Br and PtBMA-Br were utilized, simultaneously, as macroinitiators to polymerize tetraphenyl888

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between carboxylic groups when temperature is decreasing.20,21 The effect of pH on the optical properties of (PE)n(PMAA)m-P(TPE-2St) was then investigated. The fluorescence properties of the miktoarm star copolymers in THFbuffer mixtures (10/90 vol %, pH ranging from 1.1 to 13.3) were determined (Figure 3d−f). With an increase in pH from 1.1 to 5.0, the PL intensity of the solutions increased gradually. Since the pKa of PMAA is 4.28, at pH < 4.2, e.g. 1.1−4.1, the PMAA arms are protonated, resulting in the decrease of the hydrophilicity of carboxyl groups (−COOH) and consequently the copolymer micelles are contracted, exhibit poor solubility as confirmed by transmittance (51−71%, Figure 3f) and DLS measurements (Figures S6 and S7). From pH 5.0 to 7.2 the PL intensity decreased due to the carboxyl groups dissociation in the aqueous solution leading to the swelling state of the star polymers. The emission remained almost unaltered at pH 7.2−10.0 and decreased dramatically at pH 10.0−13.3 with a weak emission with a weak emission observed at pH 13.3 (Figure 3e). As the pH increases, the charge density and hydrophilicity of the PMAA chains increase, causing the PMAA arms to adopt a more extended conformation, resulting in an increase of the space between the TPE molecules separated by water and THF.22 Thus, the TPE molecules are prevented from approaching each other, causing the fluorescence of the star to decrease with the increase in pH from 10.0 to 13.3. The polymers showed good solubility in pH ranging from 5.0 to 12.2, where transmittance >92% is observed (Figure 3f). At pH = 13.3, the transmittance of the solution dropped to 67% with the I/I0 0.39. In a recent paper, the pH responsive properties of 4-arm PMMA stars with one TPE molecule in the core showed a different tendency in the plot of PL intensity vs pH.23 We can conclude that the microstructure of the polymer plays a key role for the responsive properties and further work is in progress. Since the synthesized miktoarm star copolymers contain hydrophilic PMAA arms and hydrophobic PE arms, their use as surfactants for emulsion applications was then briefly investigated in water (pH buffer)/oil (toluene) ratio at 1:1 by weight. The star polymer (0.1 or 0.05 wt %) was added to the mixture followed by mixing using a vortex mixer. No emulsions were formed at room temperature due to the poor solubility of PE chains. After heating the mixture in closed vials to around 80 °C with stirring, emulsions were formed when the temperature reduced to room temperature. As shown in Figure 4, water-in-toluene emulsions were successfully prepared at concentrations as low as 0.05 wt % vs total weight of pH buffer and toluene solution. A neat toluene layer was observed on top of the emulsion and emulsions were stable for more than two months at room temperature. Changing the pH value from 4.9 to 7.1 and 11 with a concentration of star polymer 0.1 wt %, no change in emulsion was observed. When the concentration of star polymer was 0.05 wt %, the volume fraction of emulsion phase was reduced at pH 7.1. The possible reason is that there has no enough star polymers available to stabilize the large surface area when pH is 7.1. An additional reason is the change of the packing/adsorption of star polymers at the interphases under different pH values and thus of the emulsion.24,25 In summary, core crosslinked miktoarm star copolymer (PE)n-(PMAA)m-P(TPE-2St) have been successfully synthesized by ATRP of AIE TPE-2St (cross-linker) with PE-Br and PtBMA-Br macroinitiators, followed by hydrolysis of PtBMA

star polymer in less than 26 h. At concentration 25.0 mM, a star polymer with higher molecular weight and poor solubility in 1,3,5-trichlorobenzene (TCB) was formed in 17 h (entry 3 in Table 1). The successful synthesis of miktoarm stars was confirmed by (a) NMR characterization, where all fingerprints of TPE-2St, PtBMA, and PE were present in the spectrum (Figures 1a, S2, and S3) and (b) HT-GPC, where all chromatograms were monomodal with traces of PE-Br (Figures 1b, S4, and S5). The reactivity of MI-1 (PE-Br) and MI-2 (PtBMA-Br) were expected to be different due to their different chemistry and molecular weights, resulting in a higher incorporation of one type of arms. The absolute molecular weight (Mw,star) and hydrodynamic radius (Rh) of the star polymers were determined by triple-detection GPC (Table 1). The molecular weight and weight fraction of arms (Mn,NMR,arm and armwt %) were determined by 1H NMR analysis; the average number of arms was calculated using the following equation: Narm = (Mw,star × armwt%)/Mn,NMR,arm. The experimental results (Table 1) indicate that more PE than PtBMA arms are incorporated into the miktoarm star polymer. For example, a feed ratio of PE-Br to PtBMA-Br 1:3 led to 29.0 PE and 42.6 PtBMA arms (PE/PtBMA = 2/3) and a feed ratio of PE-Br to PtBMA-Br 1/ 1 resulted in 19.4 PE and 14.5 PtBMA arms (PE:PtBMA = 1.3/1; entries 4 and 5 in Table 1). The hydrolysis of PtBMA to PMAA was conducted in 1,4dioxane with excess hydrochloric acid under reflux overnight. The hydrolyzed polymers were precipitated in methanol and used, after drying, for characterization by 1H NMR and FT-IR spectroscopies. Figure 1c illustrates the 1H NMR spectrum of hydrolyzed polymer where the complete disappearance of tertbutyl resonance at around δ = 1.5 ppm, demonstrates the quantitative hydrolysis of the tert-butyl ester groups. In the FTIR spectrum (Figure 1d), the broad absorbance characteristic of carboxylic acid groups is present from 3100 to 3700 cm−1, while the characteristic absorbance of the tert-butyl group at 1369 and 1391 cm−1 disappeared. The AIE behavior of miktoarm star copolymers (PE)n(PMAA)m-P(TPE-2St) was then investigated in dilute THF/nhexane mixture at a concentration of 0.1 g/L. As shown in Figure 2, addition of n-hexane into the THF solution aggregates the molecules and gradually enhanced the PL intensity. The highest emission intensity was observed in 90 vol % mixtures of THF with a 11-fold increase as compared with that of pure THF solutions. As we described in our previous work,14 due to the restriction of intramolecular rotation caused by the core crosslinked structure, the miktoarm stars are emissive in pure THF or in THF/n-hexane mixtures with low fraction of the nonsolvent n-hexane. The effect of temperature on the optical properties of miktoarm stars was also investigated. As shown in Figure 3a−c, PL intensity of both miktoarm star copolymers containing PMAA or PtBMA arms increased when the temperature decreased from 55 to 5 °C. The temperature-responsive fluorescence could be attributed to the shrinkage of the shell restricting the motions of TPE molecules in the core when temperature decreases. A linear relationship between PL intensity and temperature from 40 to 5 °C was found for (PE)n-(PtBMA)m-P(TPE-2St) miktoarm stars. The same linear relationship was observed for PE stars in our previous work.14 However, in the case (PE)n-(PMAA)m-P(TPE-2St) stars a nonlinear relationship between PL intensity and temperature was found, probably due to the formation of hydrogen bonding 889

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arms. The synthesized star copolymers exhibited AIE characteristics and temperature-responsive emission in THF/ n-hexane mixtures. Oppositely to the PE stars and (PE)n(PtBMA)m-P(TPE-2St) stars, the fluorescence intensity of (PE)n-(PMAA)m-P(TPE-2St) stars increased nonlinearly with decreasing temperature from 40 to 5 °C. In addition, due to the hydrophilic PMAA arms, the star copolymers also exhibited changes in fluorescence intensity due to the different aggregation states of the TPE moieties in the core at different pH. Moreover, the miktoarm star copolymers could be utilized as stabilizer for generation of water-in-oil emulsions with longterm stability at low polymer concentrations. We anticipate that these miktoarm star polymers will provide a new avenue toward materials with potential applications ranging from porous membranes to vehicles for drug delivery and pHresponsive nanofilms.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.8b00329. Additional figures, Experimental section, GPC traces, 1 NMR spectra, and DLS measurements (PDF).

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REFERENCES

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Figure 4. (a) Illustration of water-in-oil emulsion formation; (b) images of water (pH buffer)-in-oil (toluene) emulsions stabilized by (PE)19.4-(PMAA)14.5-P(TPE-2St).

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Letter

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Zhen Zhang: 0000-0002-6512-8561 Nikos Hadjichristidis: 0000-0003-1442-1714 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Research reported in this publication was supported by King Abdullah University of Science and Technology (KAUST). 890

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ACS Macro Letters (22) Chen, F.; Li, C.; Wang, X.; Liu, G.; Zhang, G. pH and IonSpecies Sensitive Fluorescence Properties of Star Polyelectrolytes Containing A Triphenylene Core. Soft Matter 2012, 8, 6364−6370. (23) Guan, X.; Zhang, D.; Meng, L.; Zhang, Y.; Jia, T.; Jin, Q.; Wei, Q.; Lu, D.; Ma, H. Various Tetraphenylethene-Based AIEgens with Four Functional Polymer Arms: Versatile Synthetic Approach and Photophysical Properties. Ind. Eng. Chem. Res. 2017, 56, 680−686. (24) Li, W.; Yu, Y.; Lamson, M.; Silverstein, M. S.; Tilton, R. D.; Matyjaszewski, K. PEO-Based Star Copolymers as Stabilizers for Water-in-Oil or Oil-in-Water Emulsions. Macromolecules 2012, 45, 9419−9426. (25) Xie, G.; Krys, P.; Tilton, R. D.; Matyjaszewski, K. Heterografted Molecular Brushes as Stabilizers for Water-in-Oil Emulsions. Macromolecules 2017, 50, 2942−2950.

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