Letter pubs.acs.org/macroletters
Temperature-Directed Self-Assembly: from Tadpole to Multi-Arm Polymer Nanostructures Directly in Water Valentin A. Bobrin, Sung-Po R. Chen, Zhongfan Jia, and Michael J. Monteiro* Australian Institute for Bioengineering and Nanotechnology, The University of Queensland, Brisbane QLD 4072, Australia S Supporting Information *
ABSTRACT: Driving amphiphilic block copolymers to selfassemble into asymmetric and equilibrium nanostructures remains a challenge. Here, we use the temperature-directed morphology transformation (TDMT) method to tailor the self-assembly of block copolymers into asymmetric nanoparticles with either a single (i.e., tadpole) or multi-arm geometry directly in water and at scale (>10 wt % of polymer). These nanostructures were close to or at their equilibrium morphology and not a transient kinetically trapped structure since they did not change with the addition of high amounts of plasticizer, could be freeze-dried and rehydrated without any structural rearrangement.
T
PNIPAM block copolymer remained as a sphere (head). The resultant tadpole structure was stable and could be freeze-dried and rehydrated without altering the tadpole structure. The other advantage, apart from the direct one-pot synthesis of tadpoles at high weight fractions of polymer, was that the tail surface could be decorated with multiple and orthogonal chemical functionality to that placed within the head.7 The aim of this work was to develop a strategy to produce asymmetric equilibrium nanostructures using the TDMT method. A key parameter to constructing the tadpole structure was the large difference between the LCSTs of the two MacroCTAs (ΔLCST was close to 44 °C).7 By changing the ΔLCST, could control over the nanostructure be achieved? If successful, this would make the TDMT method a powerful selfassembly strategy for the creation of equilibrium structures with asymmetric geometries. Scheme 1 shows the TDMT methodology and summarizes the findings when changing the ΔLCST between the two MacroCTAs. The PNIPAM MacroCTAs (A−F) were synthesized using RAFT-mediated polymerization in DMSO at 60 °C. The molecular weight distributions (MWDs) for the MacroCTAs were well controlled, with good agreement between the number-average molecular weights (Mns) from size exclusion chromatography (SEC) and 1H NMR (see Table S2 in SI). It was also found that the dispersities (ĐMs) were below 1.2, representing narrow MWDs. By incorporating a few hydrophobic styrene units into the copolymer with NIPAM, the LCST decreased,8 and by increasing the amount of hydrophilic dimethylacrylamide (DMA) into the copolymer with NIPAM,
here are many examples of transient (i.e., kinetically trapped) block micelles self-assembled nanostructures formed through solvent exchange1 (400 nm); however, the particle size distributions for all MacroCTAs were broad with polydispersities (PDIDLSs) much greater than 0.4, in which a dispersity below 0.1 represents a narrow particle size distribution. When mixing two PNIPAM chains in water with a large difference between the Mns, cooperative behavior produced only one observable LCST.9 Cooperative behavior between the two MacroCTAs in our work was determined by mixing MacroCTA A with either B, C, D, E, or F at a 1:1 weight ratio, SDS and water at ∼3 °C, and slowly heating (0.6 °C/min) in the DLS until a temperature of either 70 or 80 °C (the latter temperature was only for the mixture of MacroCTAs A and F, as F had an LCST of 68 °C). Figures S6A,B showed that, for the mixtures of A/B and A/C, there was only one observable LCST corresponding to the low LCST MacroCTA A (∼19 °C), and the final Dh for the mixtures was also close to that for only MacroCTA A. For cooperative behavior to dominate, a single LCST transition at an intermediated temperature between the two MacroCTAs is required. For the other three mixtures (A/D, A/E, and A/F), two LCST transitions were observed corresponding to the individual MacroCTAs. This slow heating for all the MacroCTA mixtures in the DLS produced very broad particle size distributions with the PDIDLSs much greater than 0.4. Taken together, the data supports that the mixing of A with one of the other MacroCTAs produced noncooperative behavior, presumably due to the different copolymer compositions of either styrene or DMA, leading to different interaction parameters (χ). The initial step in the TDMT process was to mix all components (i.e., MacroCTAs, initiator, SDS, monomer, and water) at close to 3 °C and then rapidly heat the mixture to 70 °C. The following experiments to measure the size and polydispersity for the individual or mixed pairs of MacroCTAs with SDS in water were given in Figure 1. Rapid heating over a 5 min period showed that MacroCTA A alone produced nanoparticles of 120 nm and a narrow particle size distribution (PDIDLS = 0.028). There was an increase from 120 to 1268 nm
Figure 1. Hydrodynamic diameters (Dh) and PDIDLS in parentheses of (A) individual MacroCTAs and (B) their mixtures (1:1 wt ratio), determined by DLS during rapid heating (from 3 to 70 °C or 80 °C over 5 min). Total polymer concentration of 53.8 mg/mL (5.1 wt ratio) and SDS concentration of 2.23 mg/mL (0.22 wt ratio). MacroCTA F and A/F mixture were heated to 80 °C; all other MacroCTAs and their mixtures were heated to 70 °C.
in size with an increase in LCST to MacroCTA D, after which the size decreased to 516 and 825 nm for E and F, respectively. Only MacroCTAs A, B, and F had narrow distributions compared to the broad distributions found for C, D, and E. Mixing the various MacroCTAs with A produced particle sizes increasing from 142 to 193 nm when paired with B−F. More importantly, the distributions were all narrow, with PDIDLS values all below 0.062. The results support that MacroCTA A was an essential component in the mixture for generating narrow particle size distributions and collocating the other MacroCTA pair within the same nanoparticle, a prerequisite for tadpole or multi-arm formation. 1048
DOI: 10.1021/acsmacrolett.7b00589 ACS Macro Lett. 2017, 6, 1047−1051
Letter
ACS Macro Letters
Table 1. RAFT-Mediated Emulsion Polymerization of STY with Two Thermoresponsive MacroCTAs (at the Weight Ratio 1:1) in Water for 4 h Using SDS as a Surfactant and AIBN as an Initiator DLSc
SEC (DMAc) TD
a
expt
MacroCTAs
ΔLCST (°C)
conv (%)
1 2 3 4 5
A/B A/C A/D A/E A/F
21 26 36 44 49
75 68 69 75 82
a
Mnb
(theory)
Mn ( H NMR)
STY units ( H NMR)
Mn
ĐM,TD
Dh (nm)
PDIDLS
10220 10000 10160 10890 11270
45 42 42 45 47
11700 12000 12100 11900 12670
1.06 1.09 1.11 1.1 1.1
137 144 140 115 114
0.097 0.066 0.043 0.09 0.074
9915 9600 9820 10580 11160
1
1
Conversion by gravimetry. bCalculated using eq S4 in SI. cDLS at 70 °C (expts 1−4) and 80 °C (expt 5), immediately after stopping the reaction.
The mixing of all components (pairs of MacroCTAs, SDS, STY, and AIBN), rapid heating from 3 to 70 °C (only expt 5 was carried out at 80 °C) and then polymerizing for 4 h produced block copolymers with relatively narrow MWDs (Table 1). The Mn values for all five emulsion polymerizations were close to theory and thus well-controlled by the RAFTpolymerization. The resultant latex particles ranged in size from 114 to 144 nm and, similar to the seed particle, had a narrow particle size distribution (i.e., PDIDLS < 0.097). There was excellent agreement between the DLS size data and that found from the TEM images of the latex particles at 70 °C (see Figures S8 and S9 in SI). The 1H NMR data given in Table 1 also showed that the number of styrene units in the second block was ∼45 and close to the number of PNIPAM units of 47 in the first block. The data demonstrated the remarkable control of both the MWD and particle size distribution when using the TDMT method, supporting previous work on using PNIPAM MacroCTAs as seed particles10 and the choice of experimental conditions.11 The next step in the TDMT process was to first open the polymerization reaction mixture to remove residual styrene over a 4 h period.4 The latex was then mixed with a small amount of toluene (40 μL/mL; to plasticize the PSTY blocks to induce self-assembly) and cooled to 34 °C, a temperature between the LCST of the two MacroCTAs. The TEM images (Figure 2) showed that for the MacroCTA pairs of A/E (expt 4) and A/F (expt 5) tadpole structures formed with the tail consisting of the high LCST block copolymer and the head of the low LCST block copolymer. These two experiments used MacroCTA pairs with an LCST difference (ΔLCST) of greater than 44 °C. The TEMs clearly showed that the population of structures were nearly all tadpoles with little or no contamination of spheres or worms. The tail distribution from expt 4 (A/E) ranged from 200 nm to over 1 μm with an average length of 372 nm and polydispersity index (Lw/Ln) of 1.31 (see Figure S14 in SI). The tail distribution for expt 5 (A/ F) had a similar average length of 371 nm but with a narrow distribution of 1.20. Decreasing the ΔLCST to below 44 °C, the TEM images in Figure 2A−C showed a population of multi-arm structures protruding from the head (expts 1−3). It can be seen that in addition to the linear arms others structures consisting of loops were observed, suggesting that the self-assembly to the cylindrical arms was governed by the initial particle morphology, most possibly core−shell or raspberry-types. The decrease in volume of the initial sphere to the volume found in the head after cooling for all experiments correlated with the volume extruded by the high LCST block into the arms as theoretically calculated on a mass basis (see Table S4 in SI). The data above strongly support that the structures observed at 34 °C (see Figure 2) were equilibrium structures as
Figure 2. TEM images of nanostructures upon cooling from 70 to 34 °C with 40 μL/mL toluene: (A) expt 1, (B) expt 2, (C) expt 3, (D) expt 4, and (E) expt 5 (Table 1).
they did not change their structure even at high amounts of toluene (∼80 μL/mL) as shown in Figure S9 in SI. These structures were stable in water at 34 °C for many weeks and could be freeze-dried and rehydrated without change in conformation (Figure S16).6,7 To explore the formation of both the multi-arm and tadpoles, TEM images were taken by cooling to different temperatures along the temperature profile as shown in Figure 3A. Maintaining the reaction at 70 °C for 16 h, the TEM image showed spherical particles (Figure 3B), and upon decreasing the temperature to 50 °C (Figure 3C), spherical particles were also the main population but with a small amount of tadpoles (single-arm structures). At lower temperatures, the arms became more predominant (Figure 3D,E); in particular, at these low temperatures, some of these arms consisted of loops, possibly suggesting that the linear multi-arms formed from these loops. In contrast, the formation of the tadpole structures were quite different and changed from spheres at 45 °C directly to all tadpoles at 40 °C (Figure 4), a sharp transformation. The 1049
DOI: 10.1021/acsmacrolett.7b00589 ACS Macro Lett. 2017, 6, 1047−1051
Letter
ACS Macro Letters
mechanism of multi-arm versus tadpole formation during the cooling process seems to be controlled by the initial particle morphology, which then directs the self-assembly process. However, all characterization techniques to determine the initial morphology at 70 °C was not successful, as the MacroCTAs were nearly identical in composition, complicated by the chain extension with styrene. In summary, rapid heating of mixtures of MacroCTAs with different LCSTs with SDS in water produced a narrow seed distribution for the emulsion block copolymerization with styrene. Both the MWD and particle size distributions were narrow, demonstrating the excellent control by using the TDMT technique. Upon cooling, the emulsion latexes to 34 °C, the spheres transformed into a spherical head with either one or multiple worm-arms protruding. Tadpoles (single arm structures) formed by mixing pairs of MacroCTAs with a large difference in their LCSTs (i.e., ΔLCST > 36 °C), while multiarm structures formed when ΔLCST ≤ 36 °C. We found that the spheres transformed to tadpoles at a sharp temperature of 44 °C, whereas there was a gradual formation of multi-arm structures upon cooling. The TDMT method provides a powerful self-assembly strategy to produce close to equilibrium structures with asymmetric geometries at high weight fractions of polymer directly in water, an advantage over other selfassembly techniques.
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ASSOCIATED CONTENT
S Supporting Information *
Figure 3. (A) LCST profiles of MacroCTA A (blue curve) and MacroCTA D (red curve). TEM images of nanoparticles from expt 3 via slow cooling from 70 °C with 40 μL/mL of toluene after 16 h at the temperature: (B) 70 °C, (C) 50 °C, (D) 40 °C, and (E) 34 °C.
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00589. Experimental details and NMRs and SEC traces (PDF).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Zhongfan Jia: 0000-0001-9690-7288 Michael J. Monteiro: 0000-0001-5624-7115 Funding
M.J.M. acknowledges financial support from the ARC Discovery Grant (DP140103497). Z.J. acknowledges the financial support from the ARC Future Fellowship (FT130101442). Notes
The authors declare no competing financial interest.
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REFERENCES
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