Sono-RAFT Polymerization-Induced Self-Assembly in Aqueous

Oct 29, 2018 - The first sonochemically induced reversible addition–fragmentation chain transfer by the polymerization-induced self-assembly process...
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Article Cite This: Macromolecules 2018, 51, 8862−8869

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Sono-RAFT Polymerization-Induced Self-Assembly in Aqueous Dispersion: Synthesis of LCST-type Thermosensitive Nanogels Sandie Pioge,́ *,† Thi Nga Tran,† Thomas G. McKenzie,‡ Sagrario Pascual,† Muthupandian Ashokkumar,§ Laurent Fontaine,† and Greg Qiao*,‡

Macromolecules 2018.51:8862-8869. Downloaded from pubs.acs.org by UNIV OF LOUISIANA AT LAFAYETTE on 11/29/18. For personal use only.

†

Institut des Molécules et Matériaux du Mans, UMR 6283 CNRS − Le Mans Université, Av. O. Messiaen, 72085 Le Mans cedex 9, France ‡ Polymer Science Group, Department of Chemical and Biomolecular Engineering, The University of Melbourne, Melbourne 3010, Australia § Sonochemistry Research Team, School of Chemistry, The University of Melbourne, Melbourne 3010 Australia S Supporting Information *

ABSTRACT: The first sonochemically induced reversible addition− fragmentation chain transfer by the polymerization-induced selfassembly process (Sono-RAFT-PISA) has been investigated. Highfrequency ultrasound (490 kHz, 40 W) was applied for the controlled polymerization of N-isopropylacrylamide (NIPAM) in aqueous dispersion via the Sono-RAFT-PISA technique using poly(poly(ethylene glycol) methyl ether acrylate) (PPEGA) as both macromolecular chain transfer agent and surfactant (macro-transurf). The synthesis of PPEGA-b-PNIPAM copolymers in aqueous media at 20 °C (DPn,NIPAM = 204) was found to reach total NIPAM conversion in a short time (60 min.) with narrow molecular weight distribution (Đ < 1.26). Furthermore, PNIPAM-based spherical nanogels (Dh ≤ 69 nm, pdi ≤ 0.26) were successfully synthesized by Sono-RAFT-PISA (aqueous dispersion, 45 °C), qualifying as a highly “green” method due to the complete monomer conversion, absence of organic initiator, of residues, and to the unique use of water as initiator and solvent (inisolv).

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INTRODUCTION In recent years, polymerization-induced self-assembly (PISA) has become a well-established methodology to synthesize in situ self-assembled block copolymer nanomaterials.1−4 This process involves the chain extension of a living solvophilic polymer precursor with a monomer that forms a solvophobic polymer under dispersion or emulsion polymerization. Selfassembled block copolymer nanomaterials, used in a broad range of applications (carrier for drug delivery,5−8 imaging agent,9−11 pickering emulsifier,12,13 smart nanomaterials,14 and so forth) have been produced via this PISA process performed under concentrated conditions (10−50 wt %) with a facile access to a large range of morphologies.15−18 A number of parameters are known to strongly influence the morphology and recently, various morphology transitions have been induced by the temperature19,20 or by supramolecular interactions.21−23 PISA processes can be conducted using any type of controlled/living polymerization; one of the most studied is reversible addition−fragmentation chain transfer (RAFT) polymerization using traditional thermally initiated systems in aqueous dispersion.24,25 Thermally initiated RAFT-PISA is conducted using a macromolecular chain transfer agent and surfactant (macro-transurf) at high reaction temperature (T ≥ 50 °C) and in the presence of a water-soluble azo-type initiator.26−32 The exploitation of mild, low-temperature © 2018 American Chemical Society

polymerization conditions can enable for example the encapsulation of sensitive molecules such as therapeutic enzymes without compromising their biological properties.33 Efforts have been devoted to carrying out aqueous PISA at low reaction temperatures including photoinitiated RAFT-PISA which often involves the addition of a photoinitiator.5,34−36 One promising strategy never explored is the RAFT-PISA activated by ultrasounds. High-frequency ultrasounds are successfully used to prepare well-defined homopolymers with total monomer conversion by RAFT polymerization in water without the use of radical initiators at low temperature without polymer degradation.37 In such systems, the ultrasounds generate radicals that act as initiators of the RAFT polymerization, directly from the homolysis of water molecules. Therefore, the RAFT-PISA process activated with highfrequency ultrasounds is promising as it will bring the following benefits: (i) low reaction temperature, (ii) complete monomer conversion, and (iii) absence of additional initiator, leading to self-assembled copolymer materials free of volatile organic compounds. Among the self-assembled copolymers nanomaterials, spherical nanogels, defined as chemically or physically cross-linked hydrogel particles with a nanoscale size, Received: July 26, 2018 Revised: October 12, 2018 Published: October 29, 2018 8862

DOI: 10.1021/acs.macromol.8b01606 Macromolecules 2018, 51, 8862−8869

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Macromolecules Scheme 1. Synthesis of Thermosensitive Nanogels Based on PPEGA-b-P(NIPAM-co-MBA)a

a

Via Sono-RAFT-PISA in dispersion at low temperature and in the presence of water as inisolv. to monitor the progress of the reaction via 1H NMR (to determine the NIPAM conversion) and GPC in DMF with LiBr (to determine Mn,GPC and Đ). After 6.5 h, the polymerization was stopped by cooling of the solution in liquid nitrogen. 4. “ON/OFF” Experiment. The “ON/OFF” experiment was conducted as per a typical Sono-RAFT-PISA reaction ([PPEGA20]0/ [NIPAM]0 = 1/204; [NIPAM]0 = 0.15 M). The glass vial containing the reaction mixture was sparged with argon for 30 min and then submerged in the ultrasonic water bath at 45 °C. The ultrasonic plate was then switched ON to mark the start of the reaction, and turned OFF after various time intervals to assert the ON/OFF temporal control. Samples were extracted throughout for 1H NMR and GPC analysis. During the OFF periods the vial remained in the ultrasonic bath (i.e., at 45 °C). 5. Typical synthesis of PNIPAM Nanogel by Sono-RAFTPISA. In a 14 mL glass vial, 46 mg of NIPAM (204 equiv), 25.7 mg of PPEGA26 (1 equiv), and a solution of MBA in water (8 equiv) were dissolved in 2.68 g of water to give a solid content of 2.6% ([PPEGA26]0/[NIPAM]0/[MBA]0 = 1/204/8 ; [NIPAM] = 0.15M). The glass vial was sparged with argon for 30 min and then submerged in the ultrasonic water bath fitted with circulaint cooling water operated at 45 °C. The ultrasonic plate was then switched on (490 kHz, 40W) to mark the start of the reaction. After 90 min of continuous ultrasonic irradiation the polymerization reaction was stopped. After checking a total conversion by 1H NMR, the reaction media was freeze-dried. 6. Characterizations. Nuclear magnetic resonance (NMR) spectra were recorded on a Varian Unity 400 MHz spectrometer operating at 400 MHz at ambient temperature. Chemical shifts were reported in ppm relative to the deuterated solvent resonances. The average molecular weight (number-average molecular weight Mn,GPC, weight-average molecular weight Mw,GPC) and dispersity (Đ) values were measured by gel permeation chromatography (GPC) using N,Ndimethylformamide (with LiBr at 1g·L−1) as an eluent and carried out using a system equipped with a guard column (Polymer Laboratories, PL gel 5 μm) followed by two columns (2 Phenomenex Phenogel 5 μm columns, 500 Å and 104 Å porosity) with a Shimadzu RID-10A differential refractometer (DRI) and a Shimadzu APD-20A UV detector operating at 633 and 285 nm, respectively. The instrument operated at a flow rate of 1.0 mL·min−1 at 50 °C and was calibrated with narrow linear polystyrene (PS) standards ranging in molecular weight from 1120 to 537 000 g·mol−1. Differential scanning calorimetry (DSC) measurement was performed on a TA Instruments Q100 connected to a computer in aluminum pans under nitrogen otherwise noted. The DSC instrument was calibrated using an indium standard. Sample was heated from 15 to 50 °C at a heating rate of 20 °C.min−1 and under a static nitrogen atmosphere, followed by cooling to 15 °C at the same rate after an isotherm at 50 °C during 2 min. Six cycles of heating and cooling experiments were realized. Thermal transitions were obtained from the maximum of the endotherm (heating scan). Dynamic light scattering (DLS) measurements were performed on a Malvern Instruments Nanosizer fitted with a 4 mW He−Ne laser operating at 633 nm with an angle detection (173°). The data were processed using the CONTIN method of analysis. Individual measurements were made at 25 and 45 °C after an

represent interesting soft nanomaterials as they are able to uptake a large amount of water with relatively low surface tension.38,39 These nanomaterials can be designed to respond to a wide variety of stimuli, such as temperature.40−43 Thermosensitive nanogels were prepared via PISA process by taking advantage of the solubility change in water at high temperature (above the LCST) during the polymerization. The majority of literature examples reporting the thermosensitive nanogel elaboration are based on thermally initiated RAFT-PISA. The purpose of this work is therefore to develop a new highly “green” method using sonochemically induced RAFTPISA under high-frequency (490 kHz) ultrasounds at moderate temperature in the absence of organic compounds (initiator and residues) and using water as initiator and solvent, named here for the first time, inisolv, to target thermosensitive nanogels based on poly(N′-isopropylacrylamide-co-N,N′methylenebis(acrylamide)) P(NIPAM-co-MBA) core and poly(poly(ethylene glycol) methyl ether acrylate) PPEGA shell (Scheme 1) .

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EXPERIMENTAL SECTION

1. Reagents. All chemicals were purchased from Aldrich. Nisopropylacrylamide (NIPAM, >99%) and N,N′-methylenebis(acrylamide) (MBA, >99%) were used after recrystallization in methanol. Pure water was obtained from a Milli-Q system and had a conductivity of 18.2 MΩ.cm at 25 °C. The macro-transurf PPEGA were synthesized by thermally initiated RAFT polymerization of poly(ethylene glycol) methyl ether acrylate (PEGA, 480 g·mol−1) using the 2-cyano-5-oxo-5-(prop2-yn-1-ylamino)pentan-2-yldodecylcarbonotrithioate (COPYDC) as RAFT agent.44 2. Typical Sono-RAFT Polymerization Reaction in Water. In a 14 mL glass vial, 46.2 mg of NIPAM (204 equiv) and 25.3 mg of PPEGA26 (1 equiv) were dissolved in 2.63 g of water to give a solid content of 2.6% ([PPEGA26]0/[NIPAM]0 = 1/204 ; [NIPAM]0 = 0.15 M in water). The glass vial was sparged with argon for 30 min then submerged in the ultrasonic water bath fitted with circulaint cooling water operated at 20 °C for Sono-RAFT in aqueous media and at 45 °C for Sono-RAFT-PISA in aqueous dispersion. The ultrasonic plate was then switched on (490 kHz, 40W) to mark the start of the reaction with samples extracted periodically to monitor the progress of the reaction via 1H NMR (to determine the NIPAM conversion) and GPC in DMF with LiBr (to determine Mn,GPC and Đ). After 60 min of continuous ultrasonic irradiation, the kinetic was stopped. 3. Thermal-RAFT-PISA in Aqueous Dispersion. In a 14 mL glass vial was added 44.4 mg of NIPAM (204 equiv), 24.7 mg of PPEGA26 (1 equiv), and 45 μL of initiator solution (34.8 mg of V50 in 10 mL of water, (0.1 equiv)) in 2.62 g of water to give a solid content of 2.6% ([PPEGA26]0/[NIPAM]0 = 1/204 ; [NIPAM]0 = 0.15 M in water). The solution was degassed during 30 min with argon then heated at 70 °C. The samples were extracted periodically 8863

DOI: 10.1021/acs.macromol.8b01606 Macromolecules 2018, 51, 8862−8869

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Figure 1. (a) Reaction kinetics under continuous ultrasonic irradiation (f = 490 kHz, P = 40 W) of Sono-RAFT polymerization of NIPAM (204 eq. per PPEGA, [NIPAM]0 = 0.15 M in water) using a PPEGA with DPn of 26 as macro-transurf PPEGA. (b) Molecular weight characterization data of formed PPEGA-b-PNIPAM (dotted line represents theoretical molecular weight). (c) GPC in DMF (LiBr) chromatograms showing polymer growth with time of applied ultrasound. equilibration time of 2 min. The sample was prepared by dissolving copolymer or nanogels in pure water (5 g·L−1) and the solution was filtered using nylon membrane filter to 0.45 μm porosity. Transmission electronic microscopy (TEM) measurement was carried out using a JEOL2100 at an accelerating voltage of 120 kV. To prepare the TEM samples, a dilute aqueous solution (0.5% w/v) of nanogels was dropped onto a carbon-coated copper grid. Excess nanogels solution was gently removed using absorbent paper. In the experimental setup for the Sono-RAFT experiment, an RF generator (AG series amplifier LVG 60-10 produced by T&C Power Conversion Inc.) was used operating at RF applied powers of 20 or 40W connected to an ultrasonic plate transducer (490 kHz, Honda Electronics Co. Ltd.) that made up the bottom of a jacketed water bath with a circulating water heating system to maintain a constant bath temperature.

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copolymer, the molar ratio between macro-transurf PPEGA26 and NIPAM ([PPEGA26]0/[NIPAM]0) was fixed at 204. The 1 H NMR spectrum (Figure S2) of the resulting PPEGA26-bPNIPAM obtained after purification shows signals at 3.35 ppm ((CH2CH2O)7−8CH3) and 4.15 ppm (C(O)OCH2CH2O(CH2CH2O)7−8) characteristic of the PPEGA block and a signal at 3.95 ppm (−NH−CH(CH3)2) characteristic of the PNIPAM block. These signals were used to determine the molar composition of the resulting copolymers by 1H NMR spectroscopy (DPn,NIPAM,NMR = 204, DPn,PPEGA,NMR = 26). A kinetic plot for the Sono-RAFT of NIPAM at 20 °C using macro-transurf PPEGA26 was obtained by with-drawing samples at different time intervals. As shown in Figure 1a, a very fast initial rate of polymerization was observed which reached approximately 80% conversion after 15 min under ultrasonic irradiation. Moreover, the reaction proceeded without any induction period, and a quantitative NIPAM conversion was observed within 60 min under continuous ultrasonic irradiation as determined by 1H NMR spectroscopy. The gel permeation chromatography (GPC) analysis of the resulting polymers showed narrow monomodal peaks that linearly shift to higher molecular weights with increasing NIPAM conversion (Figure 1b,c). It is noteworthy to mention that the nonzero intercept was due to the PPEGA26 with Mn,GPC of 20.3 kg mol−1 and the difference between theoretical and experimental Mn,GPC values were related to the use of PS calibration. Dispersity values of the obtained diblock copolymers were relatively low (Đ ≤ 1.26). These results strongly indicate that the polymerization proceeds via the RAFT mechanism following initiation by sonochemically generated radicals. When the same PPEGA26-b-PNIPAM block copolymers were targeted using thermal-RAFT polymerization (V50 as hydrosoluble initiator), 6.5 h was required to achieve complete NIPAM conversion instead of 60 min by Sono-RAFT polymerization and a higher dispersity value (Đ ≤ 1.32) was obtained (Figure S3). This result showed the potential of Sono-RAFT polymerization in comparison to thermal-RAFT polymerization. Synthesis of Diblock Copolymer Based on PPEGA-bPNIPAM by Sono-RAFT-PISA in Aqueous Dispersion. For the synthesis of nano-objects by the PISA process the second block, here the PNIPAM, must precipitate out from the reaction media (water) and self-assemble into nanodomains stabilized by the macro-transurf PPEGA during the polymerization. With the LCST of PNIPAM48 being 32 °C, we have studied the Sono-RAFT-PISA of NIPAM at 45 °C (490 kHz,

RESULTS AND DISCUSSION

Synthesis of Diblock Copolymer Based on PPEGA-bPNIPAM by Sono-RAFT in Water at 20 °C. The radical formation by sonochemical reactions in water is the result of “acoustic cavitation” which occurs during ultrasound treatment.45,46 Acoustic cavitation refers to the vaporization of water into gaseous bubbles, followed by the expansion and the implosion of bubbles giving localized regions of extreme pressure and heat which can degrade the water into hydroxyl and hydrogen radicals. The sonochemical generation of radicals is dependent on the volume fraction, vapor pressure, and surface activity of the monomer, all of which can cause changes in acoustic cavitation events. 47 The optimal conditions, frequency (f) and power (P), to obtain the highest rate of hydroxyl radicals inducing controlled polymerization via Sono-RAFT were determined previously: f = 414 kHz, P = 40 W, T = 21 °C, producing hydroxyl radicals at a rate of about 15 μM.min−1.37 Moreover the optimal monomer concentration was found to be in the range 0.15−1.5 M. At lower concentration (0.1 M), the rate of polymerization was slow and at higher concentration (5 M) no polymerization was observed. In this work, similar conditions were used for the Sono-RAFT of NIPAM in aqueous media, at low temperature (T = 20 °C) and at low concentration (0.15 M). The aqueous Sono-RAFT of NIPAM was carried out in the presence of PPEGA with DPn = 26 as a macro-transurf and water as an inisolv. The PPEGA26 was synthesized by RAFT polymerization of PEGA with a molecular weight of 480 g·mol−1 using the 2-cyano-5-oxo-5-(prop2-yn-1-ylamino)pentan-2-yldodecylcarbonotrithioate (named COPYDC)44 as RAFT agent (Table S1, Figure S1). For the synthesis of PPEGA-b-PNIPAM 8864

DOI: 10.1021/acs.macromol.8b01606 Macromolecules 2018, 51, 8862−8869

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Figure 2. Effect of power (▲, Δ, P = 20 W; ■, □, P = 40 W, at the same [NIPAM]0 = 0.15 M in water) on (a) reaction kinetics and (b) molecular weight characterization data of formed PPEGA-b-PNIPAM via the Sono-RAFT-PISA at 45 °C under continuous ultrasonic irradiation (f = 490 kHz) of NIPAM (204 eq. per PPEGA) from macro-transurf PPEGA26. (c) Monomer conversion with alternating ON/OFF periods of applied ultrasound ([NIPAM]0/[PPEGA]0 = 204/1, [NIPAM]0 = 0.15 M, f = 490 kHz, P = 40 W, 45 °C). (d) “ON/OFF” experiment GPC chromatograms showing no polymer growth when ultrasound is switched OFF.

(collapse of PNIPAM at T ≥ LCST). The power of the ultrasound treatment was varied to examine its effect on the rate and control of the Sono-RAFT-PISA. The power was reduced from 40 to 20 W while maintaining the frequency at 490 kHz. The reduction in the ultrasonic irradiation power resulted in a slower rate of polymerization as shown by the decrease of the slope of ln[M]0/[M]t versus time (Figure 2a). This is consistent with a lower concentration of propagating radicals probably due to the reduced rate of radical production expected under these conditions.37,50 Moreover, using 20 W as ultrasonic irradiation power, the decrease of propagating radicals during the polymerization was not observed contrary to the system at 40 W (Figure 2a,b). This shows the potential for reaction rates to be additionally controlled via the applied power. Another significant advantage of Sono-RAFT-PISA is the capability of activation or deactivation by ultrasound. We performed controlled Sono-RAFT-PISA by exposing the polymerization to an alternating ON/OFF periods of applied ultrasound, as shown in Figure 2c. During the initial ON periods, the rate of polymerization was rapid, consistent with results discussed earlier. As the reaction proceeds and conversion increases past 50%, the rate of polymerization slows down, as expected due to increase in viscosity. NIPAM conversion stopped completely in the absence of ultrasound, while restarting efficiently when it was turned back on. The polymerization seemed to reach a lower limiting monomer conversion (60%). The fast cycling time indicates the rapid switch ability of this activation/deactivation, as previously

40 W) in the presence of PPEGA26 ([PPEGA26]0/[NIPAM]0 = 1/204) and water as inisolv ([NIPAM]0 = 0.15 M in water) and compared with the studies on the Sono-RAFT polymerization at 20 °C (Figure 1) and the thermal-RAFT-PISA at 70 °C (Figure S3). Figure S4 shows that very quickly the reaction medium of the Sono-RAFT polymerization of NIPAM at 45 °C is opalescent, highlighting the formation of nano-objects based on a PNIPAM core surrounded by PPEGA shell via the PISA process. This first example of Sono-RAFT-PISA (f = 490 kHz, P = 40 W) exhibits a fast reaction rate and a relatively good control of molar masses and dispersities (Figure 2 and Figure S5). At the beginning of the reaction, the NIPAM conversion for the Sono-RAFT-PISA (t = 10 min, ρ = 44%) is less than that of the Sono-RAFT performed at 20 °C (t = 10 min, ρ = 60%, Figure 1). As shown by Ashokkumar’s team, the influence of temperature on the cavitation (viscosity, surface tension, and the vapor pressure of the solution) impacts the rate of radical production.49 The Sono-RAFT-PISA at 45 °C is less well controlled than the Sono-RAFT at 20 °C for NIPAM conversions superior to 80%. Indeed, dispersity values increase from 1.35 at 80% of NIPAM conversion to 1.53 at a quasi-total conversion (Figure 2b). However, the molar masses and dispersities of the Sono-RAFT-PISA at 45 °C are similar to those obtained by thermal-RAFT-PISA (Figure S3). The irreversible termination reactions observed at high monomer conversions, as evidenced by the sharp increase in polymer molecular weight and dispersity in the case of polymerization at high temperature (45 °C for Sono-RAFT-PISA and 70 °C for thermal-RAFT-PISA), are related to the dispersion process 8865

DOI: 10.1021/acs.macromol.8b01606 Macromolecules 2018, 51, 8862−8869

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Table 1. Synthesis, Hydrodynamic Size (Dh), and Thermoresponsive Properties of PPEGA-b-P(NIPAM-co-MBA) Nanogels Synthesized by Sono-RAFT-PISA in Aqueous Dispersiona nanogel

DPn,PPEGA,NMR

[PPEGA]0/[NIPAM]0/[MBA]0

Dh (nm)b 25 °C

pdib 25 °C

Dh (nm)b 45 °C

pdib 45 °C

fc

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

7 11 20 26 26 26 33 49

1/204/8 1/204/8 1/204/8 1/204/8 1/300/8 1/500/8 1/204/8 1/204/8

d 272 65 51 128 222 42 33

d 0.291 0.222 0.234 0.214 0.285 0.259 0.236

d 175 41 35 61 99 30 22

d 0.179 0.117 0.122 0.176 0.236 0.119 0.128

d 3.7 3.9 3.1 9.3 11.3 2.7 3.4

Polymerization conditions: [NIPAM]0 = 0.15 M, 45 °C, 90 min; the monomer conversions were all quantitative. bThe hydrodynamic diameters (Dh) and polydispersities (pdi) of nanogels were obtained from DLS measurements conducted with 5g·L−1 Milli-Q water solutions at 25 and 45 °C. cThe volume swelling ratios (v) were obtained by v = (Dh,25 °C/Dh,45 °C).3 dDestabilization of the reaction mixture during the polymerization with the formation of aggregates. a

Figure 3. (a) DLS number-average diameter distributions of the nanogel NG-4 in water at 25 °C (―) and at 45 °C (---). (b) The Dh values of NG-4 at 25 and 45 °C from repeated heating and cooling experiments. (c) Overlaid DLS number-average diameter distributions of nanogels based on different PPEGAx in water at 25 °C (gray ---, NG-2; gray ―, NG-3; ···, NG-4; ―, NG-7; ---, NG-8). (d) Evolution of the nanogel size (Dh) versus polymerization degree of PPEGA macro-transurf at 25 °C (▲) and 45 °C (Δ).

observed in aqueous solution at 20 °C49 and explained by the short lifetime of the active hydroxyl radicals as well as the rapid time scale of sonochemical events. GPC analysis of the PPEGA-b-PNIPAM formed during this experiment clearly shows no polymer growth during the OFF periods (Figure 2d). Elaboration of LCST-Thermosensitive Nanogels by Sono-RAFT-PISA in Aqueous Dispersion. LCST-type thermosensitive nanogels were prepared by Sono-RAFT-PISA of NIPAM at 45 °C using PPEGA with different DPn,PPEGA,NMR (7 ≤ DPn,PPEGA,NMR ≤ 49) as macro-transurf (Table S1, Figure S1), difunctional monomer MBA as cross-linker, and water as inisolv (Table 1). The solid content was 2.6% and the molar ratios [PPEGA]0/[NIPAM]0/[MBA]0 were 1/204/8. After 90 min at 490 kHz, 40 W, and 45 °C, the monomer was

completely consumed as revealed by 1H NMR analysis (Figure S6). The nanogel NG-4 was lyophilized to remove water and redispersed in Milli-Q water to obtain a desired concentration (5 g·L−1). DLS measurements were realized on of NG-4 in water at 25 °C, where the PPEGA block and the P(NIPAM-coMBA) block are soluble. DLS study of NG-4 aqueous solution showed a single size distribution at 25 °C with the hydrodynamic diameter (Dh) of 51 nm and a polydispersity (pdi) of 0.234 (Table 1 and Figure 3a). Compared to the DLS analysis at 25 °C of PPEGA26-b-PNIPAM copolymer obtained by Sono-RAFT-PISA without MBA (Dh,DLS,25 °C = 11 nm, Figure S7), the hydrodynamic diameter of nanogel NG-4 is much larger. This highlights the formation of a cross-linked core based on P(NIPAM-co-MBA) during the PISA process. 8866

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significantly by the DP n,PPEGA (Table 1). We further investigated the effect of the DPn of PNIPAM block on the size, polydispersity, and volume swelling ratio of the nanogels. Using the PPEGA26, Sono-RAFT-PISA was performed in the presence of 204 up to 500 molar ratio between NIPAM and macro-transurf PPEGA26 ([PPEGA26]0/[NIPAM]0) and a constant [PPEGA26]0/[MBA]0 ratio (1/8). With a molar ratio of 300 ([PPEGA26]0/[NIPAM]0 = 1/300), well-defined nanometric gel particles were obtained (NG-5). With a molar ratio of 500 ([PPEGA26]0/[NIPAM]0 = 1/500) a stable milky solution was formed during polymerization (45 °C) (Figure 4a, NG-6). The pdi obtained by DLS analysis revealed that this sample was relatively homogeneous (Table 1), containing stabilized microgels. TEM image of NG-6 confirmed the formation of spherical objects with a size consistent with the DLS study of the shrinked object (Figure 4c). As shown in Table 1, for NG-4, -5, and -6, the volume swelling ratio upon heating from 25 to 45 °C increased from 3 to 9 and 11, respectively, with increasing NIPAM from 204 to 300 and 500 mol equiv with respect to macro-transurf PPEGA26. This is reasonable because of the lower quantity of MBA cross-linker for the higher amount of NIPAM.

To study the thermosensitive property, the NG-4 aqueous solution was analyzed by DLS at 45 °C, where the P(NIPAMco-MBA) block is insoluble; the Dh was 35 nm, and the pdi was 0.122 (Table 1, Figure 3a). The Dh and pdi decreased upon heating related to the collapse of the cross-linked core based on P(NIPAM-co-MBA); opalescence was observed at 45 °C (Figure 4a, NG-4). This behavior was reversible and

Figure 4. (a) Photos and (b) TEM images of PPEGA26-b-P(NIPAMco-MBA) nanogel NG-4 synthesized using molar ratio [PPEGA26]0/ [NIPAM]0 = 1/204. (c) TEM images of PPEGA26-b-P(NIPAM-coMBA) nanogels (NG-5 and NG-6) synthesized using different molar ratio [PPEGA26]0/[NIPAM]0.

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CONCLUSIONS In summary, we have expanded the promise of RAFT aqueous dispersion polymerization by combining the PISA process with sonochemistry to target spherical nanogels. This work demonstrates the great attractiveness of the Sono-RAFTPISA approach owing to relatively low temperature, total monomer conversion, higher rate of polymerization, and temporal control over the polymerization. The major advantages of Sono-RAFT-PISA highlighted are the use of water as inisolv and the absence of residual organic compound in the nanogels making this novel process highly “green”. We believe that Sono-RAFT-PISA offers considerable scope for the preparation of various self-assembly copolymers nanomaterials even in a large scale. Indeed, the scaling-up of sonochemical reactors is foreseeable.52

reproducible; the nanogel regained the same Dh over successive shrink/swell cycles (heating (45 °C)/cooling (25 °C) cycles (Figure 3b)). This corresponds to a swelling volume ratio (v) of 3.1 (Table 1). The LCST of nanogel NG-4 aqueous solution was measured by differential scanning calorimetry (DSC, repeated heating/cooling experiments, rate: 20 °C.min−1, Figure S8) and was found to be 35 °C. Transmission electronic microscopy (TEM) was used to see the morphology of the nanogel NG-4 (Figure 4b). TEM images of the NG-4 indicated the formation of spherical nanoobjects, which is consistent with literature27,51 with a size consistent with the DLS study of the shrinked nanogel. Having succeeded in the synthesis of a LCST-type thermosensitive nanogels via Sono-RAFT-PISA using PPEGA26, we then investigated how the DPn,PPEGA and the DPn,PNIPAM affected the size, dispersity, swelling, and stability of nanogels. With the macro-transurfs PPEGA with DPn of 11, 20, 26, 33, and 49, stable thermosensitive nanogels (NG-2, -3, -4, -7, and -8) were obtained (Figure S9). However, with PPEGA7, a heterogenous dispersion was obtained containing aggregates. A minimum DPn of the PPEGA block is thus necessary to reach effective stabilization of the nano-objects formed in situ during the PISA process, as already observed before.27,29,30 This stabilization was confirmed by the evaluation of micelle formation based on PPEGA (at the concentration of nanogel synthesis, 8 g·L−1) at using steadystate fluorescence spectroscopy of pyrene (Figure S10). Unlike the other PPEGA macro-transurfs, the one with a DPn,PPEGA of 7 does not form micelles in aqueous solution. The Dh values and thermosensitive properties at 25 and 45 °C of the obtained nanogels were characterized by DLS, and the results are shown in Table 1 and in Figure 3c,d. At 25 °C, with increasing DPn of the macro-transurf PPEGA, Dh of the particles decreased from 272 to 33 nm (Table 1). Figure 3d shows the evolution of Dh at 25 and 45 °C with the DPn,PPEGA. The Dh at 25 and 45 °C decreased with the DPn,PPEGA and then reached a plateau. The polydispersity and the volume swelling ratio were not affected

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.8b01606. Additional polymer characterization data (NMR, GPC, DSC) and DLS analysis (PDF)

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AUTHOR INFORMATION

Corresponding Authors

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

Sandie Piogé: 0000-0002-5932-2368 Sagrario Pascual: 0000-0002-9890-0095 Laurent Fontaine: 0000-0003-0043-1508 Greg Qiao: 0000-0003-2771-9675 Notes

The authors declare no competing financial interest. 8867

DOI: 10.1021/acs.macromol.8b01606 Macromolecules 2018, 51, 8862−8869

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ACKNOWLEDGMENTS ̈ We thank Anthony Rousseau for TEM images and Héloise Loget for DSC analysis.

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DOI: 10.1021/acs.macromol.8b01606 Macromolecules 2018, 51, 8862−8869