Letter pubs.acs.org/macroletters
Polymerization-Induced Cooperative Assembly of Block Copolymer and Homopolymer via RAFT Dispersion Polymerization Anqi Zhu, Xiaoqing Lv, Liangliang Shen, Baohua Zhang, and Zesheng An* Institute of Nanochemistry and Nanobiology, College of Environmental and Chemical Engineering, Shanghai University, Shanghai 200444, China S Supporting Information *
ABSTRACT: Polymerization-induced cooperative assembly (PICA) is developed to promote morphological transitions at high solids via RAFT dispersion polymerization, using both a macromolecular chain transfer agent (macro-CTA) and a small molecule chain transfer agent (CTA) to generate nano-objects consisting of well-defined block copolymer and homopolymer. PICA is demonstrated to promote morphological transitions under various conditions. Elemental mapping provides unambiguous evidence for the uniform distribution of the homopolymer within the core of the nano-objects. It is proposed that the growing homopolymer first reaches its solubility limit and forms aggregates, which induce the adsorption of the growing block copolymer. This effective and robust PICA approach significantly expands the capability to promote morphological transitions in RAFT dispersion polymerization and will facilitate the efficient synthesis of various higher-order morphologies at high solids.
B
(PS) to poly(styrene-b-acrylic acid) (PS-b-PAA) spheres only resulted in an increase in the size and polydispersity without changing the morphology. However, the addition of PS to PSb-PAA vesicles or cylinders led to morphological transitions to spheres. This transition to lower-order morphologies was explained by the phase separation of PS homopolymer to the center of the micellar core of PS-b-PAA, but no evidence was provided regarding the relative distribution of the PS homopolymer in the PS microdomain of PS-b-PAA. An opposite observation was made by Ouarti et al.42 in their study of the effect of PS addition on the morphology of linear and cyclic PS-b-polyisoprene (PS-b-PI) block copolymer micelles. With the addition of PS, linear PS-b-PI spheres underwent a morphological transition to worms, and cyclic PSb-PI worms experienced a morphological transition to vesicles. In Eisenberg’s study the micelles were prepared by first dissolving the polymers in N,N-dimethylformamide (DMF) followed by the addition of water, while in Ouarti’s case the polymers were directly dissolved in heptane at an elevated temperature. Thus, these opposite results may reflect the sensitivity of the morphologies to the processing conditions.2 Despite these studies on the traditional micellization of blends of block copolymer and homopolymer in dilute solution, the “blending” of block copolymer with homopolymer in heterogeneous polymerization at high solids without postpolymerization processing has not been reported. We present
lock copolymer self-assembly has been extensively studied for the production of nanostructured materials.1,2 Solution micellization of amphiphilic block copolymers has resulted in the formation of nano-objects with a wide range of morphologies.3−8 Recently, polymerization-induced self-assembly (PISA) has emerged as an efficient alternative method for the synthesis of block copolymer nano-objects in concentrated solution (≥10 wt %).9−11 PISA affords robust control over morphology and avoids postpolymerization processing steps. PISA has been conducted in both dispersion and emulsion heterogeneous polymerization systems using water and polar and nonpolar organic solvents.12−31 Chain extension from a solvophilic stabilizer block produces a block copolymer with a growing solvophobic core-forming block, which in situ selfassembles into block copolymer nano-objects with controllable morphologies. Block ratio and solids are the two main synthetic parameters that are currently being exploited to tune nanoobject morphologies, which are dictated by the packing parameter (p);32 a higher solvophobic/solvophilic ratio lowers the curvature (higher p), and a higher solid increases the probability of inelastic collisions between particles, thus driving morphological transition toward higher-order morphologies. Blending a block copolymer with a homopolymer corresponding to the solvophobic block of the block copolymer has been shown to influence the morphologies in the bulk,33−36 thin films,37−40 as well as solution.41−44 For amorphous polymers, the homopolymer needs to have a lower molecular weight than the corresponding core-forming block of the block copolymer to allow for solubilization of the homopolymer in the core of the micelles. In the first solution blending study reported by Zhang and Eisenberg,41 the addition of polystyrene © XXXX American Chemical Society
Received: January 30, 2017 Accepted: March 2, 2017
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Scheme 1. RAFT Ethanolic Dispersion Polymerization of BnMA Mediated by PDMAEMA Macro-CTA and CTA (A) and CTA Structures Used in This Study (B)
herein a polymerization-induced cooperative assembly (PICA) of block copolymer and homopolymer to realize the remarkable modulation of nano-object morphologies in concentrated solution (10−20 wt %). Significantly, the PICA process can effectively and reliably promote morphological transitions toward higher-order morphologies. The PICA process is established using ethanolic dispersion polymerization of benzyl methacrylate (BnMA)45 as a model formulation, mediated by reversible addition−fragmentation chain transfer (RAFT) polymerization.46 Both a solvophilic macromolecular chain transfer agent (macro-CTA) and a smallmolecule CTA are simultaneously employed to control the RAFT polymerization of BnMA, which affords colloidally stable nano-objects consisting of a mixture of well-defined block copolymer and homopolymer. As shown in Scheme 1, when x = 0 (x is the molar fraction of CTA relative to the sum of macro-CTA and CTA), the RAFT dispersion polymerization of BnMA is solely mediated by poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) macro-CTA, representing a PISA process; when 0 < x < 1, the RAFT dispersion polymerization of BnMA is comediated by PDMAEMA macro-CTA and a small-molecule CTA, representing a PICA process. The feasibility of conducting a stable PICA process (CTA1, x = 0.6) was first investigated in comparison with a PISA process (x = 0), using PDMAEMA38 at a solid of 20 wt % and 70 °C. To our delight, the PICA synthesis proceeded smoothly and produced a colloidally stable dispersion in 6.5 h when the conversion reached 94% using 0.3 equiv of AIBN (relative to the sum of PDMAEMA38 and CTA1), which was in close analogy to the PISA synthesis using PDMAEMA38 alone. As shown in Figure 1, both the PISA and PICA syntheses show a linear ln([M]0/[M]) vs time plot, suggesting pseudo-first-order polymerization kinetics in both cases. In a previous report on PISA synthesis of PDMAEMA-bPBnMA, Armes et al.45 observed enhanced polymerization rate on micellar nucleation after 2 h. In our case, a higher molar ratio of AIBN/(macro-CTA+CTA) (0.3 vs 0.2) and higher solids (20 wt % vs 17 wt %) were used, which resulted in an overall higher polymerization rate. It is possible that the
Figure 1. Polymerization kinetic data (A) and macromolecular parameters (B) for RAFT ethanolic dispersion polymerization of BnMA, mediated by PDMAEMA38 (x = 0) or PDMAEMA38 and CTA1 (x = 0.6). TEM micrographs for x = 0 (C) and x = 0.6 (D). Polymerization conditions: [PDMAEMA38 + CTA1]/[BnMA]/ [AIBN] = 1:60:0.3, solids = 20 wt %, 70 °C.
micellar nucleation point occurred prior to the first sampling point (1 h) due to the overall higher polymerization rate in our study. It is also noticeable that the apparent polymerization rate constant for the PICA synthesis (kPICA = 0.356 h−1) is slightly higher than that for the PISA synthesis (kPISA = 0.290 h−1). However, these syntheses were conducted at the same solids (20 wt %) and AIBN/(macro-CTA+CTA) ratio, which means a slightly higher concentration of AIBN was used in the PICA (4.5 mmol/L) than in the PISA (3.6 mmol/L) synthesis due to the simultaneous use of macro-CTA and CTA1 in the PICA synthesis. After normalization of the polymerization rate constant to the AIBN concentration, quite similar values were obtained for both systems, suggesting that the macro-CTA and CTA1 have similar reactivities toward RAFT polymerization and therefore the block copolymer and homopolymer have a similar PBnMA degree of polymerization (DP). For both PICA and PISA syntheses, the molecular weights were well-controlled 305
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ACS Macro Letters
the solution was adjusted such that the DP and solids were the same as those in the PISA synthesis. The PICA synthesis with x = 0.2 led to the formation of some fused spheres. Large perforated lamellae predominated the morphology obtained for x = 0.3. While regular vesicles were observed for x = 0.4, the vesicular morphology was distorted to “bean pod” vesicles for x = 0.6. Colloidally stable dispersions could be produced with x being up to 0.8. Generally, the colloids were very stable with only small amounts of precipitate being formed after storage for several months. It should be emphasized that a minor fraction of homopolymers may be produced in PISA synthesis due to the use of externally added radical initiators, but this minimal fraction of homopolymers typically does not significantly affect the nano-object morphologies. Clearly, tuning the molar ratio of CTA/macro-CTA and thus the molar ratio of the resultant homopolymer/block copolymer effectively promotes morphological transitions at the same target DP and solids. Control experiments were also conducted using only macro-CTA (1 − x) targeting the same DP of PBnMA and at the same solids, but lower-order morphologies were observed (Figure S13). In PISA synthesis morphological transitions are more favored at higher solids. However, when the PICA synthesis (x = 0.6) was conducted at progressively lower solids from 20 to 15 to 10 wt %, the vesicular morphology was retained in all cases without transitioning to lower-order morphologies (Figure 3E−G). These results convincingly prove the strong capability of PICA to promote morphological transitions even at solids lower than that in PISA. The PISA synthesis targeting PBnMA DP 80 using PDMAEMA32 at 20 wt % produced a worm phase, and the morphological modulation was investigated for equivalent PICA synthesis by varying x. Clear morphological transitions from worms to perforated lamellae (x = 0.2−0.3) to lamellae (x = 0.4) to “bean pod” vesicles (x = 0.6) were again observed (Figure 3H−L). These results demonstrate that effective morphological modulation can also start from an intermediate morphology (worms) via the PICA approach at a constant DP and solids. In order to illustrate the general applicability of this PICA approach, PICA synthesis was further explored using PDMAEMA of different molecular weights and CTAs of different structures. While the control experiments for the equivalent PISA syntheses resulted in the formation of spheres (Figures S14 and S16), obvious morphological transitions were discernible for all the PICA syntheses (x = 0.6) conducted at 20 wt % (Figure S15). When PDMAEMA38 and PBnMA DP 70 were employed, perforated lamellae were present for CTA1, while rods started to form for CTA2 and both rods and vesicles were produced for CTA3. In addition, fused rods dominated the morphology for CTA4 when PDMAEMA32 and PBnMA DP 60 were employed for the PICA synthesis. These results indicate that the PICA process is highly robust in promoting morphological transitions in dispersion polymerization. It should be emphasized that the different morphologies observed using different CTAs under similar conditions were primarily due to the subtle differences of the impact of their structures on the solubility of the growing homopolymers. The block copolymer nano-objects synthesized in PISA are stabilized by the solvophilic stabilizer block. However, PICA produces both block copolymer and homopolymer, and the homopolymer cannot be self-stabilized. It is expected that the growing homopolymer would become insoluble first when a critical DP is reached, while the growing block copolymer is still
with low dispersities (Đ < 1.3) being observed over the entire polymerization. The overall lower molecular weights in the PICA synthesis can be explained by the coexistence of block copolymers and homopolymers. However, the final nanoobjects have quite different morphologies with spheres and fused rods being observed for the PISA and PICA synthesis, respectively (Figure 1C,D). This remarkable morphology difference suggests that PICA involving cooperative assembly of block copolymer and homopolymer indeed can promote morphological transitions because the packing parameter p is increased. Separation of the block copolymer and homopolymer was realized for the PICA synthesis using a longer PDMAEMA56 by repeated protonation of the PDMAEMA block and toluene extraction of the PBnMA homopolymer (see Supporting Information). GPC analysis (Figure 2A) indicated that after
Figure 2. GPC traces (A) and 1H NMR spectra in DMSO-d6 (B) of PDMAEMA56-b-PBnMA100, PBnMA100, and their blend.
separation the block copolymer had a higher molecular weight and the homopolymer had a lower molecular weight than the blend, as expected, and both the block copolymer and homopolymer had a low Đ ∼ 1.3. 1H NMR spectroscopy analysis (Figure 2B) confirmed that the blend was successfully separated into the corresponding block copolymer and homopolymer. The ester CH2 of the PDMAEMA block shifted from 4.0 to 4.3 ppm after protonation, while no PDMAEMA ester group was observed in the separated homopolymer. Comparison of the integrals of a, b, and a′ indicated that the PBnMA DP for the block copolymer and homopolymer was 123 and 115, respectively, confirming comparable polymerization rates for the block copolymer and homopolymer. Next, the capability of PICA to promote morphological transitions at different x targeting BnMA DP 60 was explored. As shown in Figure 3A−E, when x = 0, the PISA synthesis using PDMAEMA32 at 20 wt % resulted in the formation of uniform spheres. In the PICA syntheses, the concentration of 306
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Figure 3. TEM micrographs of nano-objects synthesized using PDMAEMA32 and CTA1, [PDMAEMA32 + CTA1]/[AIBN] = 1:0.3, 70 °C, targeting PBnMA DP 60 (A−G) and 80 (H−L) at indicated solids.
soluble. The fact that PICA produces stable nano-objects suggests that the insoluble homopolymer should be stabilized by the amphiphilic block copolymer. Therefore, it is reasoned that once homopolymer aggregates are formed upon reaching its solubility limit they will induce adsorption of the block copolymer “surfactant” onto the nascent hydrophobic homopolymer aggregates, although the block copolymer has not reached its critical length to self-assemble. In reasonable analogy to the PISA process, the nanoparticles consisting of the coassembled block copolymer and homopolymer are expected to be swollen by the monomer such that the solvophobic block of the block copolymer and the homopolymer are intermixing with each other, leading to the formation of a uniform “solution” in the core during the polymerization. Effective morphological promotion requires uniform distribution of the homopolymer within the core of the nano-objects. To probe the relative distribution of the PBnMA homopolymer and the PBnMA block of PDMAEMA-b-PBnMA within the nano-objects, the nano-objects were subjected to elemental mapping via energy-dispersive X-ray spectroscopy (EDS) on a high-resolution TEM (HRTEM). Because both the PBnMA homopolymer and the PBnMA block of the block copolymer carry trithiocarbonate as the end group, sulfur distribution is expected to be uniform within the core of the nano-objects (Figure 4A and Figure S17). In order to differentiate the PBnMA homopolymer from the PBnMA block of PDMAEMAb-PBnMA within the same nano-objects, a chlorine-labeled CTA4 was designed such that the PBnMA homopolymer carrying a dichlorobenzene unit could be exclusively mapped out using the chlorine element. As shown in Figure 4, the sulfur and chlorine distribution patterns of an enlarged part of the fused worms (also see Figure S15D) are quite similar, which explicitly confirms that the PBnMA homopolymer and the PBnMA block of PDMAEMA-b-PBnMA are uniformly intermixed within the PICA nano-objects. This result is different from Eisenberg’s study where the homopolymer
Figure 4. Sulfur (A) and chlorine (B) elemental mapping via EDSHRTEM for nano-objects synthesized using PDMAEMA32 and CTA4 (x = 0.6), targeting PBnMA DP 60 at 20 wt % and 70 °C.
phase-segregated to the center of the micelles, and thus the morphology order was lowered.41 Although higher-order morphologies were observed in Ouarti’s study, the molecular weight of the homopolymer was lower than that for the solvophobic block of the block copolymer.42 In this regard, the PICA process exhibits highly cooperative assembly of the growing homopolymer and block copolymer that have similar molecular weights for the PBnMA, and importantly, their uniform distribution within the core of the nano-objects is the key to the effective promotion of morphological transition at high solids. On the basis of the above analysis, a PICA mechanism is proposed as schematically illustrated in Scheme 2. An effective and robust PICA approach has been developed to facilitate morphological transitions at high solids in dispersion polymerization. The PICA process shows similar polymerization kinetics to the PISA process, and thus the homopolymer and the solvophobic block of the block copolymer have similar DPs. Morphological transitions from either spheres (x = 0) or worms (x = 0) in the presence of only macro-CTA to vesicles with the addition of CTAs have been observed on increasing the molar fraction of CTAs. The effect of PICA for morphology promotion is generic with respect to CTA structures, and higher-order morphologies have been observed under various synthetic conditions. Elemental 307
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ACS Macro Letters Scheme 2. Schematic Representation of the Proposed PICA Mechanisma
a
(a) Initiation from a homogeneous solution of macro-CTA, CTA, and monomer results in chain growth of homopolymer and block copolymer. (b) The homopolymer first reaches its solubility limit and precipitates to form nascent aggregates, which induce adsorption of the block copolymer. (c) The nanoparticles consisting of the growing homopolymers and block copolymers are swollen by monomer to form a uniform “solution” of the homopolymer and the solvophobic block of the block copolymer, leading to uniform distribution of the homopolymer in the core of the final nanoobjects of various morphologies.
It has come to our attention that another group has contemporaneously conducted a related study.48
mapping for the nano-objects synthesized using the chlorinelabeled CTA4 provides direct evidence for the uniform distribution of the homopolymer within the core of the nanoobjects. It is proposed that the growing homopolymer first forms aggregates which induce the adsorption of the block copolymer to stabilize the nascent nanoparticles, and subsequently both the homopolymer and block copolymer grow at a similar rate within the monomer-swollen nanoparticles, leading to the uniform distribution of the homopolymer within the core of the nano-objects. This simple PICA process significantly expands the currently available parameters, mainly DP and solids, with the hydrophobicity of the coreforming block having been only recently reported,47 that have been used to tune the morphologies in RAFT-mediated dispersion polymerization. Although a minor fraction of homopolymers may be produced in PISA, this work represents the study of PICA in the presence of significant amounts of homopolymers. Although in this work an ethanolic dispersion polymerization formulation is studied as the model system, it is expected that PICA can be similarly employed in other dispersion polymerization systems, allowing for facile access to potentially new, higher-order morphologies at high solids.
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ACKNOWLEDGMENTS
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REFERENCES
We thank financial support by National Natural Science Foundation of China (21674059, 21604050).
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.7b00069. Experimental details, NMR and MS spectra, TEM micrographs, polymerization kinetic data, and tables of summary of dispersion polymerization (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Zesheng An: 0000-0002-2064-4132 Notes
The authors declare no competing financial interest. 308
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