UCST-Type Thermosensitive Hairy Nanogels Synthesized by RAFT

Jan 23, 2017 - PISA is a versatile method widely used for synthesizing polymer nano-objects(47-49) with various shapes and sizes, including LCST nanog...
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UCST-Type Thermosensitive Hairy Nanogels Synthesized by RAFT Polymerization-Induced Self-Assembly Wenxin Fu,*,† Chunhui Luo,† Emily A. Morin,‡ Wei He,‡,§,∥ Zhibo Li,⊥ and Bin Zhao*,† †

Department of Chemistry, ‡Department of Mechanical, Aerospace and Biomedical Engineering, and §Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, United States ∥ Department of Polymer Science and Engineering, Dalian University of Technology, Dalian, Liaoning 116023, China ⊥ School of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao, Shandong 266042, China S Supporting Information *

ABSTRACT: While lower critical solution temperature (LCST)type thermosensitive nanogels have been intensively studied, the upper critical solution temperature (UCST)-type versions are much less explored. This communication reports a method for the synthesis of zwitterionic UCST nanogels by reversible addition−fragmentation chain transfer (RAFT) polymerizationinduced self-assembly in water−organic solvent mixtures. The nanogels were prepared by RAFT polymerization of 3-dimethyl(methacryloyloxyethyl)ammonium propanesulfonate, whose polymer is known to exhibit UCST behavior in water, conducted in ethanol−water mixtures at 70 °C using poly(poly(ethylene glycol) methyl ether methacrylate) as a macro-chain transfer agent (CTA) and a difunctional monomer as cross-linker. At a sufficiently high ethanol content in reaction media, spherical hairy nanogels with a single size distribution were obtained. These nanogels exhibited reversible heating-induced swelling and cooling-induced shrinking, consistent with the expected UCST behavior. The hydrodynamic size, volume changing ratio, and transition temperature of nanogels can be tuned by varying ethanol content in solvent mixtures, molar ratio of monomer-to-macro-CTA, and amount of cross-linker. Hairy nanogels were also successfully synthesized using a water−THF mixture as medium. The use of water−organic solvent mixtures as reaction media allowed for facile incorporation of a hydrophobic fluorescent monomer to make functional UCST nanogels.

N

(acrylamide-co-acrylonitrile)36−38 as a representative for the latter. The cloud points of UCST polymers in water are usually more sensitive to polymer concentration, end groups, and polymer microstructures compared with LCST polymers and can be tuned by copolymerization with a second monomer, either hydrophilic or hydrophobic.31,34,39 Compared with LCST-type thermosensitive nano- or microgels, the UCST-type versions are much less explored, and the very limited examples in the literature are either based on inter/intrachain hydrogen bonding of random copolymers of acrylamide and acrylic acid synthesized by an inverse emulsion polymerization method40−44 or obtained from polyelectrolytes in the presence of multivalent counterions.45 There has been no report on the UCST thermosensitive properties of PDMAPS nano- or microgels. Willcock et al. synthesized branched PDMAPS polymers by reversible addition−fragmentation chain transfer (RAFT) polymerization in 0.5 M NaCl aqueous solutions with ethylene glycol dimethacrylate as branching monomer46 and observed that the high molecular weight (500 kDa) branched PDMAPS exhibited thermally induced reversible size changes between ∼20 nm (at 25 °C) and ∼25 nm (at 90 °C). Considering the

anogels, which are cross-linked hydrophilic polymeric nanoparticles containing a significant amount of water in the polymer networks,1,2 have received considerable interest due to their potential applications in drug delivery,3,4 sensing,5,6 imaging,7−9 etc. These soft nanomaterials can be designed to respond to a wide variety of stimuli, such as temperature,10−12 pH,13−16 light,17,18 and enzyme.4,19 Among all stimuliresponsive nanogels, thermosensitive nanogels, which undergo temperature-induced reversible swelling and shrinking in water, are probably the most studied. To date, these nanogels are overwhelmingly based on lower critical solution temperature (LCST)-type thermosensitive polymers and are usually prepared via precipitation polymerization,20,21 dispersion polymerization,22,23 and more recently polymerization-induced self-assembly (PISA)24−27 by taking advantage of the solubility change in reaction media during the polymerization. Another class of thermosensitive water-soluble polymers exhibits upper critical solution temperature (UCST) behavior in water, which is opposite to that of LCST-type polymers (i.e., clouding upon cooling). These polymers have received growing interest in recent years from fundamental understanding to exploration of possible applications.28−30 Their UCST behavior stems from either electrostatic interactions or hydrogen bonding, 3 1 − 3 3 with zwitterionic poly(3-dimethyl(methacryloyloxyethyl)ammonium propanesulfonate) (PDMAPS)34,35 as an example for the former and poly© XXXX American Chemical Society

Received: November 18, 2016 Accepted: January 12, 2017

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DOI: 10.1021/acsmacrolett.6b00888 ACS Macro Lett. 2017, 6, 127−133

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Scheme 1. Synthesis of UCST-Type Thermosensitive Nanogels by RAFT Polymerization-Induced Self-Assembly (PISA) in Water−Ethanol or −THF Mixtures and Schematic Illustration of Thermally Induced Reversible Swelling and Shrinking of Nanogels in Water

Figure 1. (A) Apparent hydrodynamic diameter (Dh) distribution for a 0.05 mg/g NG-1 aqueous solution at 25 °C obtained from a DLS study. (B) AFM of NG-1 spin-cast onto freshly cleaved mica from a 0.1 mg/g aqueous solution: height image (top) and cross-sectional profile (bottom).

water-miscible organic solvent.51 Therefore, we first studied the effect of adding ethanol into water on cloud point of PDMAPS. A PDMAPS homopolymer with a degree of polymerization (DP) of 240, PDMAPS240, was synthesized by RAFT polymerization of DMAPS in 2,2,2-trifluoroethanol (TFE) at 60 °C using 2-cyanoprop-2-yl dithiobenzoate (CPDB) as CTA (Scheme S1), where TFE is a good solvent for both monomer and polymer. The obtained polymer was purified by dialysis against Milli-Q water and freeze-dried, yielding a pink powder. The cloud points of PDMAPS240 in water and water−ethanol mixtures were measured by dynamic light scattering (DLS). Figure S4A shows the plots of scattering intensity versus temperature for PDMAPS240 in water with concentrations of 1.0, 2.0, and 5.0 mg/g. The onset transition temperatures (Ttransition) as shown in the plots were used to represent the cloud points. As expected, with the PDMAPS concentration increasing from 1.0 to 2.0 and 5.0 mg/g, the Ttransition increased from 28.0 to 32.1 and 35.0 °C, respectively. Adding ethanol into water increased the cloud point of PDMAPS. At a polymer concentration of 1.0 mg/g, with increasing ethanol content from 0 to 6.25 and 10 wt %, the Ttransition increased from 28.0 to 44.0 and 53.7 °C, respectively. Further increasing the ethanol content in the solvent mixture to 20 wt %, the 1.0 mg/g PDMAPS240 solution was still cloudy at 85 °C. Thus, considering that the cloud point of PDMAPS increases with increasing concentration, the ethanol−water mixtures with an ethanol content of ≥20 wt % should be suitable for RAFT PISA of PDMAPS at 70 °C. To ensure that the RAFT PISA

unique thermoresponsive properties of UCST nanogels, one can envision many possible applications, e.g., heatingaccelerated release of substances. Thus, it is of great interest to develop simple and robust methods to synthesize this type of nanogels and investigate their UCST behavior. Herein, we report the use of RAFT polymerization-induced self-assembly (PISA) for the synthesis of hairy nanogels composed of a crosslinked PDMAPS core and a poly(poly(ethylene glycol) methyl ether methacrylate)) (PPEGMMA) corona (Scheme 1). PISA is a versatile method widely used for synthesizing polymer nano-objects47−49 with various shapes and sizes, including LCST nanogels.50 PPEGMMA was used as a macro-chain transfer agent (CTA) to polymerize DMAPS in mixtures of H2O and a water-miscible organic solvent (ethanol or THF), with N,N′-methylenebis(acrylamide) (MBAA) as cross-linker; it also acted as a stabilizer for the self-assembled PDMAPS core. We show that the size, volume swelling ratio, and transition temperature of nanogels can be tuned by changing the polymerization parameters. One critical requirement for the synthesis of nano-objects by PISA is that the second block must precipitate out from the reaction media and self-assemble into nanodomains, which are stabilized by the macro-CTA during the polymerization. Since RAFT polymerization is usually conducted at 60−70 °C using an azo-type initiator and UCST-type thermosensitive polymers show a better solubility in water at higher temperatures, we first made an effort to search for polymerization media suitable for RAFT PISA of UCST-type polymers. The solubility of a zwitterionic polymer in water can be modified by adding a 128

DOI: 10.1021/acsmacrolett.6b00888 ACS Macro Lett. 2017, 6, 127−133

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Figure 2. Plots of apparent hydrodynamic size (Dh) and polydispersity of NG-1 in water at a concentration of 0.05 mg/g, obtained from a DLS study, versus temperature upon cooling (A) and heating (B) in the temperature range of 70 to 10 °C. (C) The Dh values of NG-1 at 70 and 10 °C in water at a concentration of 0.05 mg/g from repeated heating and cooling experiments. The standard deviation for each data point is also shown in the plots.

freshly cleaved mica at room temperature. Spherical nanoobjects were observed by AFM (Figure 1B), and the size was in the range of ∼140 to ∼300 nm, consistent with the DLS result. We also characterized the nanogel by 1H NMR spectroscopy analysis. The 1H NMR spectrum of NG-1 in 0.5 M NaCl D2O showed the characteristic peaks of PPEGMMA corona (hairy layer) and PDMAPS core (Figure S7); the addition of NaCl enhanced the solubility of PDMAPS in water, making the peaks of the cross-linked PDMAPS core visible in the 1H NMR spectrum. We confirmed that the RAFT polymerization for the synthesis of NG-1 was chain extended from PPEGMMA40 by carrying out a polymerization under the same conditions except without adding the cross-linker. The polymerization went to completion after 16 h, and aqueous SEC analysis showed that the peak shifted to the high molecular weight side and remained monomodal (Figure S8), testifying the successful chain extension. We note here that we also used two poly(N,Ndimethylacrylamide) macro-CTAs with DPs of 38 (PDMA38) and 116 (PDMA116) to attempt to synthesize PDMAPS nanogels under the similar conditions for the synthesis of NG-1, but both were unsuccessful; precipitation was observed during the polymerization. Although the reason was unclear, PPEGMMA40, with 40 oligo(ethylene glycol) side chains per polymer molecule, appeared to have a better ability to stabilize the insoluble PDMAPS block in the reaction mixture. To study the thermoresponsive property, a 0.05 mg/g NG-1 aqueous dispersion was first equilibrated at 70 °C for 5 min; the apparent Dh was 278.3 nm, and the polydispersity was 0.083. The temperature was then lowered in a 5 °C step, and at each temperature the sample was equilibrated for 5 min. The DLS

occurred, we used water−ethanol mixtures with an ethanol content of 40 or 60 wt % in most cases. UCST nanogels were then prepared by RAFT polymerization of DMAPS in water−ethanol mixtures at 70 °C using PPEGMMA with a DP of 40 (PPEGMMA40) as macro-CTA and stabilizer, difunctional monomer N,N′-methylenebis(acrylamide) (MBAA) as cross-linker, and 4,4′-azobis(4cyanovaleric acid) (ACVA) as initiator. PPEGMMA40 was synthesized by RAFT polymerization of PEGMMA with a molecular weight of 500 g/mol using CPDB as CTA (Scheme S2, Figures S5 and S6). A typical process for the synthesis of PPEGMMA/PDMAPS hairy nanogel NG-1 is briefly described below. PPEGMMA40, DMAPS, and stock solutions of crosslinker and initiator in ethanol were added into a flask with the molar ratios of 1:200:3:0.4, respectively, followed by the addition of calculated amounts of ethanol and water to obtain an ethanol-to-water mass ratio of 6:4 for the solvent mixture. The weight ratio of DMAPS to the solvent mixture was set at 1:10. The homogeneous and clear polymerization mixture was degassed by bubbling with N2 for 30 min in an ice/water bath, followed by placing the flask in a 70 °C oil bath. After 16 h, the monomer was completely consumed as revealed by 1H NMR analysis; the reaction mixture was cloudy but stable. The hairy nanogel was dried with air flow to remove the solvents and redispersed in Milli-Q water to obtain a desired concentration. DLS study of NG-1 in Milli-Q water at a concentration of 0.05 mg/g showed a single size distribution at 25 °C with the average apparent hydrodynamic size (Dh) of 244 nm and a polydispersity of 0.053 (Figure 1A). Atomic force microscopy (AFM) was used to characterize the morphology of the nanogel. A 0.1 mg/g NG-1 aqueous solution was spin-cast onto 129

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Table 1. Synthesis, Hydrodynamic Size (Dh), and Thermoresponsive Properties of PPEGMMA−PDMAPS Hairy Nanogels Synthesized by RAFT PISA in Water−Ethanol Mixturesa nanogel

feeding molar ratio of CTA/DMAPS/ MBAA

ethanol/ water

NG-1 NG-2 NG-3 NG-4 NG-5 NG-6 NG-7 NG-8

1/200/3 1/200/3 1/200/3 1/50/3 1/100/3 1/300/3 1/200/2 1/200/5

6/4 4/6 2/8 6/4 6/4 6/4 4/6 4/6

Dh (nm)b 10 °C PDIb 10 °C Dh (nm)b 70 °C PDIb 70 °C 226.3 125.3 93.2d 143.0 171.9 365.7 -e 133.6

0.058 0.144 -d 0.195 0.152 0.088 -e 0.185

278.3 176.2 -d 152.3 201.4 465.4 -e 152.9

0.083 0.077 -d 0.186 0.145 0.133 -e 0.180

volume swelling ratio ( f)c 1.86 2.78 -d 1.21 1.61 2.06 -e 1.50

a Polymerization conditions: monomer/solvent = 1/10 (w/w), 70 °C, 16 h, ACVA as initiator; the monomer conversions were all 100%. bThe hydrodynamic diameters (Dh) and polydispersities (PDI) of nanogels were obtained from DLS measurements conducted with 0.05 mg/g Milli-Q water solutions at different temperatures. cf = (Dh,70°C/Dh,10°C)3. dMultiple distributions were observed from DLS; the mean size of the biggest peak based on intensity was taken as Dh; the PDI and swelling ratio were not listed. eMultiple size distributions were observed at 10 °C from DLS and dissociation to small sizes occurred upon heating.

Figure 3. Plots of apparent hydrodynamic size (Dh) and polydispersity (PDI), obtained from DLS study of a 0.05 mg/g dispersion in Milli-Q water of (A) NG-2, (B) NG-4, (C) NG-5, (D) NG-6, and (E) NG-8 versus temperature upon cooling from 70 to 10 °C.

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Figure 4. (A) Photoluminescence spectra of a 0.1 mg/g NG-12 aqueous solution at different temperatures upon cooling from 70 to 10 °C and a 0.1 mg/g NG-2 aqueous solution at 10, 40, and 70 °C with an excitation wavelength of 330 nm. (B) Photos of a 2 mg/g NG-12 aqueous solution under 365 nm UV light at 20 and 70 °C.

10 °C decreased from 226.3 to 125.3 and 93.2 nm, respectively. Note that when the ethanol/water weight ratio was decreased to 2/8 the obtained nanogel, NG-3, showed multiple size distributions at both 10 and 70 °C (Figure S11). Moreover, for NG-1 and -2, the volume swelling ratio upon heating from 10 to 70 °C increased from 1.86 to 2.78. These are likely because a higher water content in the medium increased the solubility of PDMAPS. When the water content was increased to 80%, although still a very poor medium for PDMAPS, the selfassemblies during the RAFT polymerization may be irregular, and the nanodomains may not be well stabilized, likely because the mobility of PDMAPS chains increased with increasing water content. (ii) Effect of monomer-to-macro-CTA molar ratio. We also synthesized a series of nanogels using the same molar ratio of macro-CTA to cross-linker MBAA (1/3) in the 6:4 ethanol− water mixture but different monomer-to-CTA molar ratios (NG-1, -4, -5, and -6). With increasing DMAPS from 50 (NG4) to 100 (NG-5), 200 (NG-1), and 300 equiv (NG-6) with respect to PPEGMMA40, the nanogel’s size increased from 152.3 to 201.4, 278.3, and 465.4 nm, respectively, at 70 °C, and the same trend was observed for the sizes at 10 °C. The Ttransition values of NG-4, -5, and -1 in water upon cooling were 40, 41.6, and 46.6 °C, respectively, which were in the same order of increasing DMAPS amount in the synthesis of nanogels. Differently, NG-6 underwent steady shrinking upon cooling, with no obvious transition in the studied temperature range, implying a much higher Ttransition likely because of the higher molecular weight. Furthermore, as shown in Table 1, for NG-4, -5, -1, and -6, the volume swelling ratio upon heating from 10 to 70 °C increased from 1.21 to 1.61, 1.86, and 2.06, respectively, with increasing DMAPS from 50 to 100, 200, and 300 equiv with respect to macro-CTA. This is reasonable because of the lower degree of cross-linking for the higher amount of PDMAPS. (iii) Effect of amount of cross-linker. NG-2, -7, and -8 were made by using the same conditions in a 4:6 ethanol−water mixture but different amounts of cross-linker MBAA. With decreasing molar ratio of MBAA to macro-CTA from 5/1 to 3/ 1, the nanogel’s Dh did not change much at 10 °C, 133.6 nm for NG-8 and 125.3 nm for NG-2, but the volume swelling ratio increased from 1.50 to 2.78, which was apparently due to the lower degree of cross-linking of NG-2. Further decreasing the molar ratio of MBAA to CTA to 2/1, the obtained nanogel, NG-7, exhibited multiple size distributions at 10 °C, and

results are summarized in Figure 2A. The Dh decreased upon cooling, reaching 226.3 nm at 10 °C. This corresponds to a volume shrinkage of 46.2%. Interestingly, the decrease of Dh became faster below 46.6 °C, which is taken as the onset temperature of the UCST volume transition of NG-1 (Ttransition). The cooling-induced shrinking was reversible; heating caused the nanogel to swell, and a similar UCST volume transition was observed (Figure 2B). However, the transition temperature, 40 °C, was slightly lower. The UCST transition of NG-1 in water was also observed from variabletemperature 1H NMR analysis (Figure S10A). A 1.0 mg/g dispersion of NG-1 in D2O was studied by 1H NMR spectroscopy from 25 to 48 °C, and at each temperature, the sample was equilibrated for 5 min. The peaks from the PDMAPS core were small and broadened at