Visible Light Initiated Thermoresponsive Aqueous Dispersion

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Visible Light Initiated Thermoresponsive Aqueous Dispersion Polymerization-Induced Self-Assembly Yajie Ma, Pan Gao, Yi Ding, Leilei Huang, Lei Wang, Xinhua Lu, and Yuanli Cai*

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State-Local Joint Engineering Laboratory of Novel Functional Polymeric Materials, Suzhou Key Laboratory of Macromolecular Design and Precision Synthesis, Department of Polymer Science and Engineering, College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China ABSTRACT: We report an updated polymerization-induced thermal self-assembly (PITSA) [Figg, C. A.; et al. Chem. Sci. 2015, 6, 1230]. The concept is validated using visible light initiated RAFT aqueous dispersion polymerization of diacetone acrylamide monomer at 25 − 70 °C. This PITSA formulation produces block copolymer lamellae at 25 °C while the copolymer morphology evolves from spheres to worms to vesicles during polymerization at 60 °C, which is above the lower critical solution chain length (LCSCL) of the coreforming block. Particle shape and size uniformity can be controlled by reaction temperature using a single photo-PISA formulation. Vesicles-to-lamellae and vesicles-to-worms transitions are achieved in situ upon cooling reaction dispersions (70 °C) to 25 °C, leading to the transformation of initially free-flowing liquids to physical hydrogels. Moreover, reversible thermoresponsive lamellae-to-vesicles-to-lamellae and worms-tovesicles-to-worms transitions of as-synthesized nanoparticles are achieved in dilution in a heating−cooling cycle. This thermoresponsive photo-PISA formulation updates Figg’s PITSA protocol mainly in three aspects: (1) the absence of LCST limitation, (2) user-friendly control of particle shape and size uniformity by reaction temperature using a single photo-PISA formulation, and (3) reversible thermoresponsive transition of the ketone-functionalized vesicles to customer-guided lamellae or worms.

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INTRODUCTION Polymerization-induced self-assembly (PISA) represents a robust method for the scalable synthesis of the block copolymer (BCP) nanoparticles of defined shape, size, surface chemistry, and tunable properties.1−6 It relies on the solvophobic interaction of the core-forming block that leads to shape changes during polymerization in a selective solvent.7−10 The versatility has been already demonstrated by the stimulus-responsive nanoparticles.11−20 In this regard, the supramolecular interactions are crucial. Sumerlin and coworkers21 developed polymerization-induced thermal selfassembly (PITSA) via the hydrogen-bonded poly(N-isopropylacrylamide) (PNIPAM) phase separation from the aqueous reaction solution above the lower critical solution temperature (LCST). Diacetone acrylamide (DAAM) is a water-soluble monomer that consists of a hydrogen-bonding donator (amide) and an acceptor (ketone). We16 have demonstrated that the oligomer (DP = 28, 52) is water-insoluble at 5 − 85 °C, but poly(diacetone acrylamide-co-N,N-dimethylacrylamide) shows distinct thermoresponsive LCST behavior,22 whose assembly and reaction-induced reorganization relied on the block, gradient, or block-gradient comonomer distribution. An and co-workers23 reported the unique thermoresponsive LCST and UCST (upper critical solution temperature) behaviors of poly(diacetone acrylamide-co-acryloylglycinamide) relying on the composition, DP, concentration, and added electrolyte. © XXXX American Chemical Society

Basically, these thermoresponsive properties resulted from temperature-variable hydrogen bonding of the PDAAM segments.16,22,23 Recently, we24 developed a fast room-temperature PISA method using visible light initiated RAFT25−29 aqueous dispersion polymerization of DAAM monomer16,24 and ionic monomers30−32 in the presence of hydrophilic macro-chaintransfer agent (macro-CTA) and sodium phenyl-2,4,6trimethylbenzoylphosphinate (SPTP) photoinitiator. Tan, Sumerlin, and co-workers33 soon named it photo-PISA. The robustness was evidenced by PET-RAFT34−37 and photoiniferter38,39 mediated photo-PISA. By using our photo-PISA, we16 synthesized the ketone-functionalized nanotubes with adaptive porous membranes, in which ribbon and lamellar nanostructures formed at extremely low conversions.16 Thermo-initiated DAAM-monomer PISA formulation was explored by An,19,40−43 Sumerlin,44 Armes,45 and other research groups.46 The thermoresponsive transitions of the worms to short worms/vesicles45 and the lamellae to worms/ spheres19 have been demonstrated. Herein, we report a new PITSA21 method, in which particle shape and size uniformity can be controlled by a wide range of reaction temperature without LCST limitation. We evaluated Received: November 21, 2018 Revised: January 10, 2019

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DOI: 10.1021/acs.macromol.8b02490 Macromolecules XXXX, XXX, XXX−XXX

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Figure 1. Digital photographs of the reaction dispersions (top). 1H NMR spectra of the obtained PHPMA38−PDAAMn block copolymers (arrow: n = 10 → 100; asterisk: 1,3,5-trioxane reference) in (A) methanol-d4 and (B) D2O. (C) Polymer dehydration evaluated by the analysis in D2O. for varying feed ratio, using a PHPMA30 macro-CTA, and/or changing reaction temperature. Instrumentation. A visible light source at a 0.20 mW/cm2 in intensity was obtained from homemade photoreaction system equipped with a 400 W mercury lamp, a JB400 filter, a ventilator, and a UV-A radiometer (420 nm detector). The polymer product was dried in a Labconco Freezone 2.5 L freeze-drier. The slution pH was probed using an OHAUS ST3100 digital pH-meter at 25 °C. Characterizations. 1H NMR spectra were recorded on an INOVA 400 MHz NMR instrument. Size exclusion chromatography (SEC) was conducted on a PL GPC220 integrated system equipped with a refractive index detector and a column set (2 × PLGel MIXEDB + 1 × PLGel MIXED-D). N,N-Dimethylformamide (DMF) that contained 10 mM LiBr was used as an eluent. PMMA standards (Agilent, 7.36−2136 kg/mol) were used for calibration. The calibration and analysis proceeded at a flow rate of 1.0 mL/min at 80 °C. The sample was filtered using a 0.2 μm Supor filter before SEC. Dynamic light scattering (DLS) was performed on a Brookhaven BI-200SM setup equipped with a 35 mW He−Ne laser (λ = 633 nm), a BI-200SM goniometer, and a BI-TurboCorr digital correlator. The dispersion was diluted to 1.0 mg/mL, moved to a sample vial, and placed in the cell at 25 °C. Data recorded at 90° were evaluated using cumulants analysis in CONTIN routine, averaged over five runs. Transmission electron microscopy (TEM) was performed on a Hitachi HT 7700 transmission electron microscope at a 100 kV accelerating voltage. The final dispersion was diluted in water at the polymerization temperature to 0.5 mg/mL. A 10 μL aliquot was dripped onto a hydrophilic carbon film coated copper grid, immersed in liquid nitrogen, frozen to −170 °C, lyophilized under reduced pressure, and placed in a drying desiccator for TEM studies.

the lower critical solution chain length (LCSCL) of PDAAM block at 25 °C and phase transition in photo-PISA using poly(2-hydropropylmethacrylamide) (PHPMA) macro-CTA and SPTP photoinitiator at 25 and 60 °C. The temperature effect on particle shape and size uniformity was investigated using a single photo-PISA formulation. Finally, the thermoresponsive order-to-order transition of the ketone-functionalized nanoparticles was studied upon directly cooling the hot reaction dispersion or using a heating−cooling cycle in the dilution.

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EXPERIMENTAL SECTION

Materials. DAAM monomer (donated by Hipro Polymer Materials) was recrystallized. The syntheses of HPMA,47 4-cyano-4ethylsulfanylthiocarbonylsulfanylpentanoic acid (CEP),48 and SPTP49 were described elsewhere. Deuterium oxide (D2O) and methanol-d4 were purchased from J&K, and deuteriochloric acid (20% in D2O) was from Adamas. Deionized (DI) water at a resistivity of 18.2 MΩ cm was obtained from a Direct-Q 5 UV Millipore system. Synthesis of PHPMA Macro-CTA. HPMA monomer (6.44 g, 45.0 mmol), CEP chain transfer agent (394.5 mg, 1.50 mmol), and 2butoxyethanol/water (30:70 w/w, 6.50 g) were added into a 50 mL flask. The solution was adjusted to pH 3.0. SPTP photoinitiator (116.3 mg, 375 μmol) was added into the flask. The flask was sealed with rubber septa and immersed in a water bath at 25 °C. The solution was bubbled with argon in the dark for 1 h and exposed to visible light for 1.5 h. 1H NMR: 78% conversion. The polymer product was precipitated into acetone, washed with acetone, and dried in a vacuum oven. Yield: 5.70 g, 80.3%. 1H NMR (400 MHz, in D2O, δ/ppm): 3.88 (1H, CH(OH)CH3), 3.33−2.95 (2H, CONHCH2), 2.06−1.60 (2H, CH2C(CH3)), 1.22−0.77 (6H, CH2CCH3 + CH(OH)CH3). DP = 30 was determined by DP = (4 × I3.88)/I2.50, in which I3.88 and I2.50 are integral signals CH(OH)CH3 of HPMA units at δ = 3.88 ppm and CH2CH2COOH of RAFT chain-ends at 2.50 ppm. SEC analysis gave a number-average molecular weight (Mn) of 3.5 kg/mol and dispersity (Đ) of 1.17. PHPMA38 macroCTA was synthesized under the above conditions at [HPMA]0/ [CEP]0/[SPTP]0 = 40:1:0.25. 1H NMR: 80% conversion. DP = 38; SEC: Mn = 4.2 kg/mol, Đ = 1.16. Photo-PISA Protocol. Typically, DAAM monomer (162.2 mg, 0.96 mmol), PHPMA38 macro-CTA (136.7 mg, 24 μmol), and water (1.122 g) were added into a 5 mL flask. The solution was adjusted to pH 3.0. SPTP photoinitiator (1.0% w/w in water, 74.4 mg, 2.4 μmol) was added into the flask. The flask was sealed, immersed in a water bath at 25 °C, and bubbled with argon in the dark for 1 h. The mixture was exposed to visible light for 4.5 h and studied without further purification. 1H NMR: >99% conversion. PHPMA38− PDAAM40 (subscript: DP); SEC: Mn = 9.3 kg/mol, Đ = 1.18. Other nanoparticles were synthesized under above conditions except

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RESULTS AND DISCUSSION Thermoresponsive Photo-PISA. First, we synthesized PHPMA38−PDAAMn block copolymers via photo-PISA using PHPMA38 macro-CTA and SPTP initiator at SPTP/PHPMA = 0.1 in water at 20% w/w solids, targeting the DP values of the core-forming block (n = DAAM/PHPMA) of 10, 15, 20, 30, 35, 40, 50, 55, 60, 70, 80, and 100 under visible light at 25 °C for 10, 8, 7, 6, 5, 4.5, 4, 3.5, 3.5, 3, 3, and 2 h, respectively. All the aqueous solution polymerization and dispersion polymerization were performed at pH 3.0, such that the RAFT chain-ends could maintain intact without decomposition. Figure 1 top displays (a, b) transparent or (c−e) turbid freeflowing liquids, (f, g) viscous liquids, and (h−k) transparent or (l) milky hydrogels, suggesting the aqueous dispersion polymerization. Figure 1 bottom shows that vinyl signals at 5.5 − 6.3 ppm disappeared in methanol-d4 (A) and D2O (B), B

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Figure 2. (A) SEC traces, and (B) Mn and Đ (Mw/Mn) versus the n values of PHPMA38−PDAAMn block copolymers.

Figure 3. DLS results of PHPMA38−PDAAMn nanoparticles synthesized via photo-PISA at 25 °C. (A) DLS intensity and (B) diameter profile of as-synthesized particles at labeled n value. (C) DLS diameter profiles of the particles at n = 40 at labeled polymer concentrations.

Figure 4. TEM images of the PHPMA38−PDAAMn nanoparticles at n = 20 (A), 30 (B), 35 (C), 40 (D), 50 (E), 55 (F), 60 (G), 70 (H), 80 (I), and 100 (J). Scale bar: 5 μm.

suggesting that >99% conversions have been reached. Analyzing the spectra in methanol-d4 confirmed the intact repeating units. Moreover, for those in D2O, signals h, j of PDAAM and signals d, e of PHPMA appeared at n = 10, 15, which attenuated at n > 20. The signal attenuation facilitated the evaluation of polymer dehydration11,50−53 using an added 1,3,5-trioxane reference at [trioxane]/[HPMA] = 1:12 according to eqs 1 and 2, in which Id, Ij, and I* are integral signals CH(OH) of PHPMA, COCH3 of PDAAM, and CH2 of trioxane, respectively, and 38/n is the target DP ratio of HPMA to DAAM. Figure 1C suggests that both the coreforming and the stabilizer blocks dehydrate on increase in n value, suggesting that the chain mobility of the stabilizer block has been reduced by the significant interfacial tension induced by the dehydration of the core-forming block, which decreased

the copolymer curvature and in turn favored the formation of below-discussed higher-order nanostructures.11 SEC results (Figure 2) indicate the well-defined block copolymers. In contrast to the water-soluble PDAAM blocks at n = 10 and 15, those at n ≥ 20 are aqueous insoluble. As complementary to the known LCST/UCST,22,23 we defined this critical point (n = 20) as lower critical solution chain length (LCSCL). ij Id yzz dehydration of PHPMA = jjj1 − z × 100% j 2 × I {zz k *

Ij 38 zy ij zz × 100% dehydration of PDAAM = jjj1 − j 6I n zz{ k * C

(1)

(2)

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Macromolecules Table 1. Photo-PISA Reaction Conditionsa and Molecular Parameters of PHPMA−PDAAM Block Copolymers entry

productb

fc (%)

feed ratiod

solids (wt %)

T (°C)

te (min)

αf (%)

Mnf (kDa)

Mng (kDa)

Đg

A1 A2 A3 A4 A5 A6 A7 A8 B1 B2 B3 B4 B5 B6 B7 C1 C2

H30D40 H30D70 H30D100 H30D200 H30D300 H30D400 H30D450 H30D600 H38D560 H38D560 H38D560 H38D560 H38D560 H38D560 H38D560 H38D90 H38D100

40.2 27.8 21.2 11.9 8.2 6.3 5.7 4.3 5.7 5.7 5.7 5.7 5.7 5.7 5.7 27.2 25.2

40:1:0.1 70:1:0.1 100:1:0.1 200:1:0.1 300:1:0.1 400:1:0.1 450:1:0.1 600:1:0.1 560:1:0.1 560:1:0.1 560:1:0.1 560:1:0.1 560:1:0.1 560:1:0.1 560:1:0.1 90:1:0.1 100:1:0.1

20 20 20 20 20 20 20 20 15 15 15 15 15 15 15 15 15

60 60 60 60 60 60 60 60 25 35 45 50 53 55 60 70 70

120 110 105 100 90 75 70 60 120 115 100 95 80 75 70 75 65

>99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99 >99

11.3 16.4 21.5 38.4 55.3 72.2 80.6 106.0 100.3 100.3 100.3 100.3 100.3 100.3 100.3 20.9 22.6

7.7 11.8 16.3 30.1 44.1 57.2 64.8 85.2 77.4 77.8 79.1 79.2 79.3 79.3 78.6 14.5 15.9

1.14 1.13 1.13 1.18 1.23 1.27 1.27 1.28 1.36 1.24 1.27 1.26 1.25 1.26 1.26 1.17 1.18

a

Photo-PISA at pH 3.0. bComposition (H: PHPMA; D: PDAAM; number: DP). cThe weight fraction of PHPMA (f). dDAAM/PHPMA/SPTP molar ratio. eReaction time (t). fConversion (α) and Mn values determined by 1H NMR. gMn and Đ values determined by SEC.

Figure 5. TEM images of the PHPMA30−PDAAMn nanoparticles at n = 40 (A), 70 (B), 100 (C), 200 (D), 300 (E), 400 (F), 450 (G), and 600 (H) synthesized via photo-PISA at 60 °C. Scale bar: 1 μm.

because lamellar nanostructures increased at 30 (B) and 35 (C). Large-area flexible lamellae (>30 μm in size) are observed at 40 (D), 50 (E) and 55 (F). Film curvatures are evident at 60 (G), 70 (H) and 80 (I), and film clustering occurred at 100 (J). Consequently, the photo-PISA at 25 °C led to exclusively lamellae formation rather than the classical evolution in copolymer morphology from spheres to worms to vesicles. To elucidate temperature effect, we synthesized the PHPMA30−PDAAMn nanoparticles (n = 40−600) via photoPISA at 20% w/w solids at 60 °C. The shorter stabilizer block (PHPMA30 . PHPMA38) favored the inelastic particle collision41,54 prerequisite for the higher-order nanostructure transition. The relatively low weight fractions (f = 4.3−40.2%) favor the formation of higher-order structures compared to those for the PHPMA38−PDAAMn particles (f = 25.2−62.8%). 1 H NMR and SEC studies demonstrate >99% conversions and high blocking efficiencies (Table 1, entries A1−A8).

The actual size in solution was studied using DLS analysis. Figure 3 shows that for those at n = 10 and 15 the intensities are comparable to that of water (1.40 kcps) and the diameters are less than 10 nm. Nevertheless, for those at n = 20 − 100, the intensity increased and the diameter profile broadened. Moreover, the particles at n = 40 always show a broad diameter profile at 0.2 − 10.0 mg/mL (Figure 3C). These results suggest the formation of anomalous nanostructures above the LCSCL value. To look into the nanostructures, we inspected these particles using a cryo-fixation TEM method. That is, reaction dispersion was diluted in water at 25 °C. Aliquot was dripped on a hydrophilic carbon film coated copper grid, deeply frozen into liquid nitrogen, lyophilized, and used for TEM studies. Figure 4 shows solely ribbon and lamellar nanostructures rather than spheres. The population of ribbons are predominant, but minor lamellae coexist at n = 20 (A). These ribbon structures formed by agitation induced deformation of fragile lamellae D

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Figure 6. TEM images of the PHPMA30−PDAAM450 nanoparticles synthesized via photo-PISA at 20% w/w solids at 25 (A), 35 (B), 45 (C), and 60 °C (D). Those at 15% w/w solids at 25 (E), 35 (F), 45 (G), and 60 °C (H). Scale bar: 1 μm.

Figure 7. (A) 1H NMR spectra, (B) SEC traces, and (C) DLS diameter profiles of the PHPMA38−PDAAM560 nanoparticles synthesized via photoPISA at 15% w/w solids and labeled temperatures.

Figure 8. TEM images of the PHPMA38−PDAAM560 nanoparticles synthesized via photo-PISA at 15% w/w solids at 25 (A), 35 (B), 45 (C), 50 (D), 53 (E), 55 (F), and 60 °C (G). Scale bar: 1 μm.

As-synthesized nanostructures were studied using the abovementioned cryo-fixation TEM method, in which reaction dispersion was diluted in water at 60 °C. Figure 5 suggests the

evolution from spheres to worms to vesicles during the DAAM polymerization. The absence of pure worm phase is attributed to the narrow worm space demonstrated by Armes and coE

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Figure 9. Changes in digital photographs of reaction dispersions and TEM images of the (A) PHPMA38−PDAAM90 and (B) PHPMA38− PDAAM100 nanoparticles upon directly cooling the dispersions (70 °C) to 25 °C.

PDAAM 560 vesicles are smaller than the PHPMA 30 − PDAAM450 counterparts (Figure 6G), suggesting that the steric shell played a key role in the inelastic particle collision. Thus, the shape and size uniformity of spheres, lamellae, and vesicles are indeed temperature-tunable in this photo-PISA. Reversible Thermoresponsive Vesicles−Lamellae and Vesicles−Worms Transitions. Basically, the PDAAMbased nanoparticles are insensitive to temperature.16,19,24,41−43,45 Exceptionally, Armes45 and An19 separately discovered thermal transitions of the worms to short worms/ vesicles and the lamellae to worms/spheres. We16 have demonstrated that PDAAM oligomer (DP = 28) is waterinsoluble at 5−85 °C. Aqueous insolubility was evidenced by temperature-variable 1H NMR studies of the BCP nanoparticles.19 It was proposed that the order-to-order transitions originated from the plasticization induced by the temperaturevariable hydration of minor PDAAM units on the core−shell interface.19 We expected that, aside from as-reported worms and lamellae, the vesicles should be also thermoresponsive in terms of the order-to-order transition. To achieve this goal, we synthesized the PHPMA38−PDAAM90 and PHPMA38− PDAAM100 vesicles via photo-PISA at 15% w/w solids at 70 °C (see Table 1, entries C1 and C2). The reaction dispersion was diluted in water at 70 °C to 0.5 mg/mL, and the nanostructure was visualized by cryo-fixation TEM studies. Residual dispersion was directly placed in an oven at 25 °C for 2 days to inspect the reassembled nanostructures using cryo-fixation TEM studies. Figure 9 clearly suggests that the vesicles evolved separately to lamellae (A: PHPMA38−PDAAM90) and worms (B: PHPMA38− PDAAM100), leading to the transformation of initially freeflowing liquids to (A) milky and (B) turbid hydrogels. Both the vesicles-to-lamellae and vesicles-to-worms transitions are reverse to those observed in PISA but like those upon chain extension of a hydrophilic block onto a vesicle-based macroCTA.56 To the best of our knowledge, this is the first report of the thermal transition of vesicles to either lamellae or worms. Moreover, these transformations occur in situ in reaction dispersion at high solids, satisfying the criteria of user-friendly scalable preparation. To elucidate the reversibility of the as-discussed order-toorder transition, the reaction dispersion was diluted in water to 1.0 mg/mL in a sample vial at 25 °C and sampled for DLS and TEM studies. This vial was sealed and immersed in a bath at

workers.45 The vesicle membrane becomes thicker on increase in n value because of the inward membrane-growth mechanism.55 Curled films exist in the vesicle phase (C), suggesting that high temperature favors the lamellae-to-vesicles transition. Unlike the evolution from spheres to worms to vesicles during photo-PISA of 2-hydroxypropyl methacrylate that occurs at either 25 or 70 °C,54 this PITSA formulation produces solely lamellae at 25 °C, but an evolution from spheres to worms to vesicles occurs at 60 °C. It is also unlike Figg’s PITSA21 in which particles dissociated below the LCST of the PNIPAM core-forming block and cross-linking was needed to stabilize the structures. In this sense, this is a new PITSA without LCST limitation. To reveal the thermoresponsive behavior, we synthesized the PHPMA30−PDAAM450 nanoparticles via photo-PISA at 20% w/w solids at 25, 35, 45, and 60 °C. Particles were inspected using the cryo-fixation TEM method. Note that the dispersion was diluted in water at the polymerization temperature. Figure 6 shows sphere clusters (A) at 25 °C, tubes/vesicles (B, C) at 35 and 45 °C, and thick-membrane vesicles (D) at 60 °C. To decrease the inelastic particle collision,41,54 we synthesized the PHPMA30−PDAAM450 particles in dilution at 15% w/w solids. Spheres (E) are more discriminable than those of (A) at 25 °C. Unlike vesicles (B), the population of spheres (F) is predominant at 35 °C. Moreover, vesicles (G) are smaller and more uniform in size than those prepared at 45 °C (C). The vesicle membrane (H) is thinner than that of (D) at 60 °C. Vesicles (H) are larger than vesicles (G), like the literature results.41,54 The results demonstrate that the shape and size uniformity can be controlled by reaction temperature in a single photo-PISA formulation. Furthermore, we synthesized the PHPMA38−PDAAM560 particles via photo-PISA at 15% w/w solids at 25 − 60 °C, using a longer stabilizer block (PHPMA38 vs. PHPMA30) but maintaining a f = 5.7%. Figure 7A indicates >99% conversions. Figure 7B suggests high blocking efficiencies and low Đ values (Table 1, entries B1−B7). Figure 7C displays micrometersized particles at 25 and 35 °C, but the diameters decreased dramatically at 45 °C. The sizes increase whereas dispersities broaden at 50, 53, 55, and 60 °C. Figure 8 clearly shows large clusters A (25 °C) and B (35 °C) but ∼200 nm vesicles C (45 °C). Further increase in temperature yields the larger vesicles at the expense of the structural inhomogeneity (C → G in Figure 8). Albeit both obtained at 45 °C, PHPMA38− F

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Figure 10. TEM and DLS results for reversible lamellae-to-vesicles-to-lamellae transition of PHPMA38−PDAAM90 (top) and worms-to-vesicles-toworms transition of PHPMA38−PDAAM100 (bottom) in a heating (70 °C)−cooling (25 °C) cycle in the dilution (1.0 mg/mL in water).

70 °C under gentle stirring for 2 days. Sample was measured by DLS at 70 °C and taken immediately for the cryo-TEM sample preparation. Residual dispersion was placed in an oven at 25 °C for 2 days for DLS and TEM studies at 25 °C. Figure 10 exhibits a reversible lamellae-to-vesicles-to-lamellae transition of PHPMA38−PDAAM90 (top) and a reversible wormsto-vesicles-to-worms transition of PHPMA38−PDAAM100 (bottom). The reversible order-to-order transition in this heating−cooling cycle is evident from roughly identical DLS results at 25 °C but different from those at 70 °C (data in Figure 10). Together, the results indicate the perfect reversibility of thermoresponsive vesicles−lamellae and vesicles−worms transitions of the ketone-functional nanoparticles synthesized using this thermoresponsive photo-PISA formulation.

uniformity by reaction temperature in a single photo-PISA formulation, and (3) reversible thermoresponsive transition of ketone-functional vesicles to customer-guided lamellae or worms.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Y.C.). ORCID

Yuanli Cai: 0000-0001-5473-485X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This research work was supported by National Natural Science Foundation of China (21774083, 21474069), Priority Academic Program Development of Jiangsu Higher Education Institutions.

CONCLUSIONS We have described a new PITSA21 in which particle shape and size uniformity can be controlled by reaction temperature without LCST limitation. To this end, we studied photo-PISA of DAAM monomer using PHPMA macro-CTA and SPTP initiator in water at 25 − 70 °C. 1H NMR results confirmed >99% conversions. SEC studies suggested high blocking efficiency and low Đ values. DLS and cryo-fixation TEM results evidenced that such a PITSA formulation produced exclusively block copolymer lamellae at 25 °C while the copolymer morphology evolved from spheres to worms to vesicles during polymerization at 60 °C, which is above the LCSCL of the core-forming block. The particle shape and size uniformity could be controlled by reaction temperature. Vesicles-to-lamellae and vesicles-to-worms transitions were achieved upon directly cooling the reaction dispersions (70 °C) to 25 °C, leading to transformation of initially free-flowing liquids to physical hydrogels. Moreover, reversible thermoresponsive lamellae-to-vesicles-to-lamellae and worms-to-vesiclesto-worms transitions of as-synthesized particles were observed upon a heating−cooling cycle in the dilution at 1.0 mg/mL. This thermoresponsive photo-PISA formulation updated Figg’s PITSA protocol21 mainly in three aspects: (1) without LCST limitation, (2) user-friendly control of the shape and size

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DOI: 10.1021/acs.macromol.8b02490 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.8b02490 Macromolecules XXXX, XXX, XXX−XXX