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Reversible photo- and thermoresponsive, selfassembling azobenzene containing zwitterionic polymers Vivek Arjunan Vasantha, Jun-Hui Chen, Wenguang Zhao, Alexander M. van Herk, and Anbanandam Parthiban Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01820 • Publication Date (Web): 13 Aug 2018 Downloaded from http://pubs.acs.org on August 17, 2018

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Reversible Photo- and Thermoresponsive, SelfAssembling Azobenzene Containing Zwitterionic Polymers Vivek Arjunan Vasantha,* Chen Junhui, Zhao Wenguang, Alexander M. van Herk and Anbanandam Parthiban* Institute of Chemical and Engineering Sciences, Agency of Science, Technology and Research (A*STAR), 1 Pesek Road, Jurong Island, Singapore 627833.

zwitterion, sulfobetaine, azobenzene, UCST, light responsive, photoswitching, trans-cis isomerization

ACS Paragon Plus Environment

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Abstract

Commercially available azo dyes bearing amino groups were grafted to zwitterionic copolymers composed of cyclic anhydride functionality. The zwitterionic copolymers were prepared for the first time by polymerizing sulfobetaine (SB) monomer with maleic anhydride (MA) under conventional free radical polymerization as well as reversible addition-fragmentation chain transfer (RAFT) polymerization. Poly(SB-co-MA) self-assembled in deionized water. Azobenzene grafted zwitterionic poly((SB-co-MA)-g-Azo) exhibited multi-responsive behavior. As confirmed by UV-Vis spectroscopy, trans → cis isomerization of the azo group was responsible for the photo- and thermal response. The photoisomerization was reversible, and no photoaging was detected during the repeated exposure to UV and visible light. The water-soluble nature of photoresponsive azo dye grafted copolymers makes it suitable for applications in biological systems.


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Introduction Stimuli-responsive materials showing dynamic functional properties have received much attention recently. Many properties of these materials could change with external stimuli such as pH, temperature, salt, light, magnetic and electric field and redox potential, etc.1-3 The response to these stimuli is accompanied by features such as enhanced stabilization, triggered release, switching of solubility, phase transition, etc. which are advantageous for designing functional materials.4-5 Of late, light responsive smart materials of natural and synthetic origin are receiving greater attention, due to the wide range of potential application in biomedicine, nanodevices, tissue engineering, pharmaceuticals, and drug delivery.6-18 Among all possible external stimuli, light is unique because of its non-invasive nature and thus can remotely induce changes.19-22 Photoresponsive moieties such as azobenzene and spiropyran have diverse industrial applications because of their unique characteristic photoswitching behavior.23-24 As far as photoswitching is concerned azobenzene is the most active chromophore. The photoswitching behavior of azobenzene revolves around trans → cis isomerization of substituents around the azo (-N=N-) group. Azobenzene polymers have been actively pursued for optical and other applications in recent years. A considerable number of strategies have been developed for preparing azobenzene polymers in a variety of architectures such as block, graft, hyperbranched polymers, and dendrimers.25-26 Most of the reported azobenzene polymers are typically waterinsoluble. Also, most of these approaches use hazardous solvents and suffer from other limitations such as poor solubility, aggregation, constraints to chain motion, suppressed and poor control in obtaining reproducible phototropic effect.27 Water-soluble amphiphilic azobenzenes have been used in self-assembly process to produce thin films, micelles, vesicles, and hydrogels.28-30 The photoswitching induced isomerization in


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solution is accompanied by change in solubility.31-32 The light-responsive nature could influence characteristics such as deformation of colloid particles, surface-relief-grating, gel-sol transitions, association-dissociation of micelles, and triggered release.15,

33-36

These unique properties are

promising for the applications in optical devices, sensors, photoprobes, surface mass transport, self-cleaning, smart microfluidic controller, and self-healing.37-40 More recently, applications involving photoisomerization of azobenzenes have been extended to encapsulation and drug delivery.41-42

Photochromic switches derived from water-soluble azobenzenes are often

disregarded due to reasons like phase separation or aggregation or precipitation accompanying photoisomerization. To overcome these problems, azobenzene could be modified with compatible and flexible hydrophilic units. Such a modification is likely to enhance their appeal for biomedical applications. Zwitterionic polymers provide a very convenient way of achieving this goal, due to their inherent hydrophilic nature as well as other aqueous solution behavior.43 Zwitterions can be designed through a judicious combination of anions and cations. Polysulfobetaines (PSB) have attracted considerable interest in areas such as antifouling, membrane, coatings, stabilization, encapsulation and oil-field applications.44-52 Polyzwitterions that reduce nonspecific protein fouling upon UV irradiation has been reported recently.53-58 Therefore, the use of the polyzwitterions in conjunction with the photoresponsive moiety such as azo group could open up new possibilities to construct the highly stable photochromic system for biomedical and other applications.59-60 In this approach, the key to obtaining photoresponsive zwitterionic polymer is to incorporate the azobenzene functionality into the zwitterionic polymer. Hence, we first designed and synthesized alternative sulfobetaine-maleic anhydride (P(SB-co-MA)) copolymers (Scheme 1). To the best of our knowledge, polymers of the type poly(sulfobetaine-co-maleic anhydride) have


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not been reported. The ability of maleic anhydride to form alternating copolymers results in the availability of reactive anhydride functionality for further modifications at regular intervals in the polymer backbone. The anhydride functionality enables the introduction of azobenzene through functional group transformations involving appropriate reactive species. Here we report a new series of photoresponsive zwitterionic copolymers. These copolymers possessing azobenzene moieties exhibit unique phase transition and photoswitching activity.


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Experimental Section Materials All reactions and polymerizations were performed with a Schlenk technique under an argon atmosphere. N-(4-vinylbenzyl)-N, N-dimethylamine, (90%) was purchased from ACROS. Maleic anhydride (MA, 98%) obtained from Fluka was recrystallized from chloroform before use. 1, 3-propane sultone (99%) and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIPA, 99%) were purchased from TCI chemicals and used as received. The initiators 4, 4′-azobis(4-cyanovaleric acid) (ACVA, 98%) and reversible addition-fragmentation chain transfer (RAFT) polymerization chain transfer agent (CTA), 4-cyano-4-(dodecylsulfanylthiocarbonyl)sulfanyl]pentanoic acid (97%) were used as received from Sigma Aldrich. Dichloromethane (DCM) was freshly dispensed from Glass Contour - solvent purification system. Other solvents and reagents were of analytical grade and purchased from Merck, VWR and Fisher Scientific. For the imidization reaction, aminoazobenzene derivatives such as 4-aminoazobenzene (Azo 1, 98%), 2,2′-[4-(4aminophenylazo)phenylimino]diethanol (Disperse black 9, Azo 2, dye content 97%) and 4-(4nitrophenylazo)aniline (Disperse orange 3, Azo 3, dye content 90%) were purchased from Sigma Aldrich and used as received. The dialysis tubing (Spectra/Por®) regenerated cellulose membrane with a molecular weight cutoff (MWCO) of 1000 and 3500 Da were purchased from Spectrum Laboratories. Deionized water (DI water) was purified using a Millipore water purification system and filtered by 0.45 µm filter before use.


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General procedure for free radical copolymerization Poly-[3-(dimethyl(4-vinylbenzyl)ammonio)propane-1-sulfonate-co-maleic anhydride] (P(SBco-MA) 6; Table S1, Entry 3) The monomer, 3-(dimethyl(4-vinylbenzyl)ammonio)propane-1-sulfonate (SB) was synthesized using

N-(4-vinylbenzyl)-N,N-dimethylamine

and

1,3-propanesultone

in

refluxing

dichloromethane for 24 h in good yield as described earlier.61 In a dry Schlenk flask (100 mL) equipped with a magnetic stirring bar with an argon inlet the monomers, SB (5 g; 17.6 mmol), MA (2.2 g; 22.1 mmol) and initiator, ACVA (25 mg; 0.08 mmol) were added followed by DI water (20% w/v, 35 mL). The solution was stirred and purged with argon gas for 30 minutes. The flask was placed under argon atmosphere and sealed. The reaction flask was then immersed in a constant temperature bath maintained at 70 °C for the desired time. The reaction mixture became highly viscous during this period. The resulting viscous polymer solution was cooled and diluted with 10 mL of 0.5 M NaBr. The dilute polymer solution was precipitated in methanol. The resulting polymer was then dialyzed against deionized water for 3 days (MWCO = 1000) to remove traces of unreacted monomers and salt. The resulting copolymer was lyophilized and dried in vacuo at 50 °C for 24 h. Yield: 5.56 g (77.6%). The reaction conditions used in the free radical copolymerization of poly(SB-co-MA) copolymers are summarized in Table S1. 1

H NMR (NaCl/D2O) δ: 7.68–7.17 (m, 4H) CHaromatic; 4.98 (s, 2H) CH2 benzyl; 3.76 (m, 2 H)

*CH2N(CH3)2; 3.34 (m, 10 H) ((6 H–N(*CH3)2, 2–H, S–*CH2 and 2H –CH- (backbone, MA)); 2.62 (m, 3H) ((1H, –CH– (backbone) and ( 2H, N–CH2–*CH2–CH2–S)); 2.05 (m, 2H) –CH2– (backbone, SB).


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C NMR (101 MHz, D2O) δ: 177.61 (C=O), 146.86 (CHaromatic), 143.42 (CHaromatic), 133.50

(CHaromatic), 129.63 (CHaromatic), 126.46 (CHaromatic), 67.54 (CHbenzyl), 63.29 (*CH2N(CH3)2), 54.71- 53.13 (–CH–(backbone, MA)), 50.05 N(*CH3)2, 48.04 S-CH2, 43.99- 41.08 (–CH2– (backbone, SB)), 18.93 N–CH2–*CH2–CH2–S. General procedure for RAFT copolymerization Poly-[3-(dimethyl(4-vinylbenzyl)ammonio)propane-1-sulfonate-co-maleic anhydride] (P(SBco-MA) 6; Table 1, Entry 1) In a dry Schlenk flask (100 mL) equipped with a magnetic stirring bar was charged with SB (4 g; 14.1 mmol), MA (1.38 g; 14.1 mmol) and CTA (0.11 g; 0.282 mmol) followed by 0.5 M NaBr solution (22% w/v, 25 mL). The reaction mixture was stirred and purged with argon gas for 30 minutes. The initiator ACVA (20 mg; 0.07 mmol) was then added. The flask was placed under argon atmosphere and sealed. The reaction flask was then immersed in an oil bath kept at 90 °C for the desired time. The reaction mixture became highly viscous. The resulting viscous polymer solution was cooled and then precipitated in methanol. Methanol is a good solvent for removing any residual unreacted monomer. The resulted polymer was then dialyzed against deionized water for 3 days (MWCO = 1000) to remove traces of salt and monomers if any. The resulting copolymer was lyophilized and dried in vacuo at 50 °C for 24 h. Yield: 3.1 g (62.1%). Other copolymers, poly(SB-co-MA) prepared by RAFT technique are summarized in Table 1. 1

H NMR (NaCl/D2O) δ: 7.72–7.14 (m, 4H) CHaromatic, 4.60 (s, 2H) CH2 benzyl, 3.82 (m, 2 H)

*CH2N(CH3)2, 3.40 (m, 10 H) ( 6H –N(*CH3)2, 2H, S–*CH2, and 2H –CH– (backbone, MA)), 2.69 (m, 3H) ((1H, –CH– (backbone), and ( 2H, N–CH2–*CH2–CH2–S)), 2.11 (m, 2H) –CH2– (backbone, SB), 1.67 (m, 20H) CTA-(CH2)10, 1.25 (s, 3H) CTA-(CH3).


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C NMR (101 MHz, D2O) δ: 178.84 (C=O), 146.54 (CHaromatic), 143.42 (CHaromatic), 132.56

(CHaromatic), 130.47 (CHaromatic), 118.67 (CHaromatic), 67.24 (CHbenzyl),, 63.46 (*CH2N(CH3)2),, 53.50 (–CH– (backbone, MA)), 49.79 (N(*CH3)2), 48.15 (S-CH2), 44.10-38.62 (–CH2– (backbone, SB)), 18.83 (N–CH2–*CH2–CH2–S), 11.85 (CTA– (CH2)10). General producer for conjugation of 4-aminoazobenzene derivatives imidization reaction Poly-[3-(dimethyl(4-vinylbenzyl)ammonio)propane-1-sulfonate-co-maleicanhydride-coaminoazobenzene] (P((SB-co-MA)-g-Azo 1); Table 2, Entry 2) The

copolymer

poly-[3-(dimethyl(4-vinylbenzyl)ammonio)propane-1-sulfonate-co-maleic

anhydride-co-aminoazobenzene], abbreviated as P((SB-co-MA)-g-Azo 1). Poly(SB-co-MA) copolymer was dried at 110 °C for 20 h under vacuum before the imidization. In a 100 mL two neck flask, 1.02 g (2.7 mmol, equivalent of MA and SB) of P(SB-co-MA) copolymer and 20 mL of HFIPA was taken. 4-Aminoazobenzene (0.52 g; 2.6 mmol) was then added. The reaction mixture was heated gently to 80 °C for 2 hours to form the amic acid (ring-opening reaction, Scheme 1) and then cooled to room temperature. To the mixture, 50 mL of toluene was added, and the flask was fitted with a Dean-Stark apparatus. The reaction mixture was refluxed for 18 h with continuous removal of water in the form of its azeotrope with toluene. Then the reaction mixture was cooled and poured into methanol, to precipitate the polymer. After filtration, the polymer was dialyzed. The azobenzene copolymer was then dried under vacuum to obtain zwitterionic poly((SB-co-MA)-g-Azo 1). The same procedure was followed to prepare conjugated disperse orange 3 and disperse black 9 (Table 2). 1

H NMR (400 MHz, NaCl/D2O) δ: 8.19 (b, 4H) CHAzo-aromatic, 7.62–7.06 and (b, 8H) CHAzo-

aromatic

and CH–aromatic, 5.10(s, 2H) CH2

benzyl,

3.79 (m, 2 H) *CH2N(CH3)2, 3.37 (m, 10 H)


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((6H- N(*CH3)2), 2H, S–*CH2 and 2H, –CH– (backbone, MA)), 2.67 (m, 3H) ((1H, –CH– (backbone), and (2H, N–CH2–*CH2–CH2–S)), 2.01 11 (m, 2H) –CH2– (backbone, SB). Characterization Methods Instrumentation 1

H- and 13C NMR experiments were performed at room temperature in NaCl/D2O solution on a

Bruker UltraShield AVANCE 400SB spectrometer. The signal arising from the residual proton of deuterated solvent was used as a reference. The size exclusion chromatography was performed at 40 °C using an aqueous GPC system that was equipped with a Delta 600 pump, a 600 controller, a 717plus autosampler, a 2414 refractive-index detector, from Waters using Empower 3 system. The eluent was 0.1 M NaNO3 in deionized water with the flow rate of 0.7 mL/min. The column used in series were Waters Ultrahydrogel 6x40 mm Guard Column, Ultrahydrogel Linear 7.8x300 mm, and Ultrahhydrogel 120 7.8x300 mm. The molecular weight and molecular weight distribution (PDI) were obtained using poly(ethylene oxide) (PEO) standard in the range of 454 to 0.1 kDa. Fourier transform infrared (FT-IR) spectroscopy was performed on a PerkinElmer Frontier NIR spectrometer using KBr from 400 to 4000 cm-1. The sample measurements were performed with 64 scans per spectrum with the resolution of 4 cm-1. The aggregate sizes of copolymers were evaluated by a Zetasizer Nano ZS dynamic light scattering (DLS) instrument (Malvern, UK) with Peltier heating system. The wavelength of 633 nm and the scattering angle of 173° were fixed at room temperature. Transmittance measurement was made using 3.5 ml quartz cell with a UV2700 spectrophotometer (Shimadzu) and measured in a 1 cm path length cell at a wavelength of 500 nm at room temperature. The absorbance of copolymer was negligible at 500 nm. Thermogravimetric analysis (TGA) was performed on a TA instrument Q600 simultaneous TGA/DTA/DSC, model Q600/Nicolet 6700 under nitrogen


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atmosphere. In order to remove the moisture, the samples were heated to 110 °C followed by an isothermal step for 30 minutes. The heating was set at the rate of 10 °C per min for temperature between 30 and 600 °C. Photoswitching studies The photoswitching experiments were carried out at room temperature using a 9Wx1 UV 365nm lamp to irradiate the sample. The in-house UV chamber with reflective aluminum reflection was used. The copolymer stock solution in DI water was heated at 50 °C and filtered through a 0.45 µm filter. The concentrations were adjusted to the absorbance between 0.2 and 0.8 (a.u). The solution was kept under dark before irradiation. The irradiation distance was kept constant at 10 cm. The samples were irradiated for 30 sec to 300 sec. Similar studies were conducted for visible light using Philips PL-S 2P lamp (9W x 1). At defined time intervals, the samples were taken to measure UV absorption (within 30 sec) using UV 2700 (Shimadzu) spectrophotometer in the range of 200-800 nm with the path length 1 cm of quartz cuvette at a resolution of 1 nm. DI water was used as a reference for the background. RESULTS AND DISCUSSION


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Scheme 1. Synthesis of copolymers poly(SB-co-MA) and grafted copolymers poly((SB-co-MA)g-Azo) Synthesis of poly(SB-co-MA) copolymers Free radical copolymerization Reactive zwitterionic copolymers were prepared by polymerizing sulfobetaine (SB) and maleic anhydride (MA) by free radical polymerization (Scheme 1). MA as comonomer has many advantages. The anhydride functionality enables the introduction of functional components because of the facile reaction of cyclic anhydride functionality with amines and alcohols. It also enhances the polarity of polymers. Literature is abundant with examples where anhydride moiety has been exploited to form polyelectrolytes because of its ease of reaction with bases. Thus in the copolymer, MA provided the reactive component while SB contributed in terms of hydrophilic and halophilic characteristics. We herein followed a bottom-up approach in contrast to the topdown processes reported in the literature. Accordingly, in the literature, zwitterionic copolymers were prepared through post-polymerization modification of styrene-maleic anhydride, acrylate, and acrylamide-maleic anhydride copolymers.62-63 The direct alternative copolymerization of zwitterionic maleimide and allylamine-based sulfoxide has also been reported.59, 64-66 It is also well known that the radical polymerization of maleic anhydride (electron acceptor) with other electron-rich vinyl monomers results in alternating copolymers. Similarly, the electron-rich 3(dimethyl (4-vinylbenzyl) ammonio) propane-1-sulfonate (SB) was copolymerized with maleic anhydride (MA) through conventional free radical polymerization (Scheme 1). As given in detail in Table S1, three solvents were studied as copolymerization medium viz., acetic acid, deionized water and the aqueous solution of sodium bromide. The reaction proceeded poorly in acetic acid with both yield and molecular weight of copolymer being low


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(Table S1; Entry 2). In deionized water, though the yield of copolymers and their molecular weight were high so also the polydispersity. Electrolytes like the aqueous solution of sodium bromide (NaBr) yielded copolymers with high molecular weights (Figure S1). In both deionized water and electrolyte, in general, polydispersity increased with conversion (Table S1; Entry 3 and 6). The trend in molecular weight was as expected, i.e. initiator at low concentrations provided copolymers with high molecular weight. However, the yield of copolymers formed in deionized water was high in shorter reaction time. 1

H NMR

The 1H–NMR spectra of zwitterionic poly(SB-co-MA) copolymer and homopolymer of poly(SB) are shown in Figure 1a. The signals corresponding to the aromatic protons of sulfobetaine and CH protons of cyclic anhydride were observed at 6.8-7.8 ppm and 2.8-3.6 ppm, respectively. The other characteristic signals of sulfobetaine observed at 4.91, 3.77, 3.35, 2.63 and 2.20 ppm corresponded to -CH2– (benzylic), –CH2–N, N–(CH3)2, –CH2–SO3-, (–CH2–CH2– CH2) and backbone methylene,

respectively (Figure 1a). The copolymer composition was

determined by comparing the signal intensity of aromatic protons with other respective protons in the polymer chain as detailed in supporting information. 1H-NMR confirmed the incorporation of maleic anhydride into the polymer chain as represented by the signals at 3.35 ppm corresponding to the methine protons (>CH–) of cyclic anhydride moiety. The ratio of integrated areas of respective aliphatic and aromatic protons also confirmed the composition of the copolymer to be 50.1:49.9. 13

C NMR

The 13C–NMR spectra of poly(SB-co-MA) copolymer and the homopolymer of poly(SB) were recorded using NaCl/D2O as the solvent. The assignment of the signals is shown in Figure 1b.


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The high signal to noise ratio is because of the viscous nature of copolymer solution. The characteristic resonance signal of carbonyl carbons of cyclic anhydride / carboxylic acid moiety was observed at 177.61 ppm. The backbone methine carbon of cyclic anhydride / carboxylic acid was observed at 54–53 ppm. The signals corresponding to aromatic carbons were observed at 146.9, 143.4, 133.5, 129.6 and 126.5 ppm. The methylene carbons –CH2– (benzylic) and –CH2– N of zwitterion were found at 67.5 and 63.3 ppm respectively. The N, N–disubstituted methyl, N– (CH3)2 carbon and methylene carbon –CH2–SO3- were noticed at 50.1 and 48.0 ppm, respectively. The backbone methylene and methine carbons of zwitterion were located at 44 and 41.1 ppm respectively. The methylene carbons (–CH2–) situated at β-position of anion was identified at 18.93 ppm.


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(a)

(b)

Figure 1. (a) 1H-NMR and (b)

13

C-NMR of poly(SB-co-MA) copolymer and poly(SB)

homopolymer in NaCl/D2O.


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RAFT copolymerization Copolymerization under RAFT conditions was performed with two objectives. First and foremost to bring down the high polydispersity of copolymers obtained by conventional free radical polymerization. Secondly, in order to lower the molecular weight of copolymers so that handling of copolymer solutions for polymer-analogous reactions become convenient. The zwitterionic monomers were directly polymerized by RAFT in the aqueous medium. Accordingly,

RAFT

copolymerization

of

SB

and

MA

mediated

by

4-cyano-4-

[(dodecylsulfanylthiocarbonyl)sulfanyl]pentanoic acid as a chain transfer agent (CTA) and 4,4′azobis(4-cyanovaleric acid) (ACVA)

as an initiator was performed at 90 °C. The

copolymerization was conducted in the aqueous solution of NaBr. The results of copolymerization are summarized in Table 1. Table 1. Zwitterionic poly(SB-co-MA) copolymers via RAFT copolymerization conver sion %

b

50

20

62.1

50

20

100 200

Dp

P(SB-coMA) 6 P(SB-coMA) 7 P(SB-coMA) 8 P(SB-coMA) 9

1:1:0.02:0. 005 1:0.75:0.0 2:0.005 1:1:0.01:0. 003 1:1:0.005: 0.001

a

time (h)

polymers SB: MA:CTA: initiator (mmol)

Mn, Th

c

Mn,

c

Mw,

PDIS

SEC

SEC

EC

12200

10200

19700

1.94

85.3

15600

11300

17400

1.85

18

40.4

15800

14200

23800

1.67

18

34.3

26600

16000

25200

1.57

d

SB:

MA

50.3: 49.7 51.0: 49.0 51.0: 49.0 50.1: 49.9

a

conversion determined by gravimetry; bMn, th = ([SB]/[CTA]xMw,SB )+ ([MA]/[CTA]xMw, MA ) x conversion + Mw, CTA; cDetermined by SEC using 0.1 M NaNO3 solution as the eluent, PEG as a standard; dcopolymer composition determined by 1H NMR; solvent 0.5 M NaBr (21% w/v). 2% w/w of P(SB-co-MA) obtained by RAFT polymerization is soluble completely in DI water and does not show any phase transition.


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Self-assembly of poly(SB-co-MA) copolymers

(a)

(b)

Figure 2. (a) The effect of poly(SB-co-MA) 2 copolymer concentration on size and transmittance (2% w/w in DI water); inset: transition from translucent (higher concentration) to clear (lower concentration) appearance on dilution; (b) Temperature responsive behavior of poly(SB-co-MA) 2 copolymer (2% w/w in DI water) by cooling and heating cycles as determined by DLS; inset: transition from transparent (heating) to translucent appearance (cooling).


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Interestingly, poly(SB-co-MA) 2 (Mn, SEC = 54500; 1% w/w) (Table S1) was soluble in deionized water (pH =7.35) whereas the homopolymer poly(SB) only swelled in water. However, the pH went down to 5.75 after dissolution as expected due to the ring opening of anhydride to acid caused by DI water. Even though designating the copolymer as poly(SB-coMA) is not accurate, the designation can still be retained. This is because of the fact that the degree of hydrolysis may not be quantitative due to chain conformation and hence it is at best a mixture of acid and anhydride containing copolymer. Therefore, the structure prevailing is not straightforward to term it as poly(sulfobetaine-co-maleic acid). However, this particular aspect cannot be ignored. The corresponding homopolymer poly(SB) (Mn, SEC = 66500; 1% w/w) was soluble only in aqueous >0.9% w/w NaCl solution. In copolymers, in addition to the polarity caused by ions of sulfobetaine units, the polarity is also introduced by way of cyclic anhydride groups. The introduction of cyclic anhydride groups also interrupts with any ionic interaction that may have existed between two adjacent units of the zwitterionic repeat unit. This in effect could lead to reduced charged interactions which explain the difference in behavior between homo and copolymers noticed in the aqueous medium.51,

67-68

Like, the homopolymer of

poly(SB), poly(SB-co-MA)s were also soluble in trifluoroethanol, hexafluoroisopropanol, and formic acid. The copolymers were insoluble in most of the common organic solvents. The selfassociation and phase transition in the aqueous solution of poly(SB-co-MA)s obtained by free radical polymerization with high molecular weight were studied using DLS and transmittance measurements. The change in aggregate size and transmittance of poly(SB-co-MA) 2 copolymer in deionized water are shown in Figure 2a. At concentrations of 2% w/w in DI water, poly(SB-co-MA) 2 formed a translucent dispersion with a bluish tinge free of precipitates at 25 °C (Figure 2a


18

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Langmuir

(inset)). In general, the particle size increased with concentration until reaching a plateau within the range studied. At room temperature, ionic interactions prevailed over hydration as indicated by the large particle size of above 200 nm. This was also reflected in the appearance of polymers in the aqueous medium where the transmittance was low. However, with increasing temperature, the enhanced polymer chain mobility disrupted the interchain ionic interactions thereby enabling hydration to predominate (Figure 2b). This led to the reduction in particle size, 50 nm thereby increasing the transmittance (Figure 2b (inset)). The transitions begin to occur above 30 °C. Thus poly(SB-co-MA) 2 polymers exhibited reversible UCST behavior. The minor hysteresis between the heating and cooling process as observed in Figure 2b is in contrast to the behavior of other polysulfobetaines. This could have been the result of ionic interactions being diluted by the MA comonomer. It may also be noted that the comonomer brings in additional H-bonding interactions. The loosely packed self-assembled structures formed through the aforementioned interactions below UCST could be disrupted upon heating. In contrast, the homopolymer, poly(SB) showed such temperature sensitivity only in the presence of salt, due to the stronger inter-chain ionic interactions.69 Factors such as polymer structure, concentration, molecular weight, ionic strength, and the spacer between counterions affect the temperature dependent solution behavior of the polysulfobetaines.70-72 Based on the polysulfobetaines reported here, there is a threshold molecular weight to observe the selfassembled phase transition. Therefore, the transitions that were noticed in the high molecular weight poly(SB-co-MA) copolymers (>30,000, g/mol) prepared by conventional free radical polymerization was absent in those polymers obtained by RAFT polymerization. The polymers obtained by RAFT polymerization were of comparatively lower molecular weight (

Reversible Photo- and Thermoresponsive, Self ... - ACS Publications

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