In Situ Cross-Linking of Vesicles in Polymerization-Induced Self

Letter pubs.acs.org/macroletters

In Situ Cross-Linking of Vesicles in Polymerization-Induced SelfAssembly Qingwu Qu,‡ Guangyao Liu,‡ Xiaoqing Lv, 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: In situ cross-linking of nano-objects with controllable morphologies in polymerization-induced self-assembly (PISA) has been a challenge because cross-linking lowers chain mobility and hence inhibits morphology transition. Herein, we propose a novel strategy that allows in situ cross-linking of vesicles in PISA in an aqueous dispersion polymerization formulation. This is realized by utilizing an asymmetric crosslinker bearing two vinyl groups of differing reactivities such that cross-linking is delayed to the late stage of polymerization when morphology transition has completed. Cross-linked vesicles with varying degrees (1−5 mol %) of cross-links were prepared, and their resistance to solvent dissolution and surfactant disruption was investigated. It was found that vesicles with ≥2 mol % cross-links were able to retain their structural integrity and colloidal stability when dispersed in DMF or in the presence of 1% of an anionic surfactant sodium dodecyl sulfate.

P

multivinyl comonomers (cross-linkers) in PISA may seem to be a more straightforward method to directly afford stabilized nano-objects.34,35 However, because PISA involves simultaneous polymerization, self-assembly, and reorganization steps to reach higher-order morphologies,36,37 in situ cross-linking reduces chain mobility of the produced block copolymers, thus hindering morphology transition in PISA. Armes and coworkers have studied in situ cross-linking of nano-objects in an aqueous PISA formulation based on poly(2-hydroxypropyl methacrylate) (PHPMA).38 “Lumpy rods” were observed when ethylene glycol dimethacrylate (EGDMA) was used as a crosslinker. These lumpy rods were composed of spherical particles of similar diameters to those of the primary particles synthesized without EGDMA, suggesting interparticle crosslinking is responsible for the formation of such an unusual morphology. Cross-linked nano-objects with higher-order morphologies have been prepared by introducing cross-linkers at high or complete monomer conversions in PISA to confine cross-links to the end of the produced block copolymers (as a short third block); however, this inevitably leads to heterogeneities in cross-link distribution within the nano-object structure.39,40 Therefore, producing in situ cross-linked, higherorder morphologies with a uniform cross-link distribution in PISA has remained a challenge. (Co)polymerization of multivinyl (co)monomers has been an important method for the synthesis of networks, branched

olymerization-induced self-assembly (PISA) has been established as a robust and efficient method for the synthesis of block copolymer nano-objects with predictable morphologies.1−4 Recently, PISA has been intensively investigated by using different combinations of stabilizing and coreforming blocks under either emulsion or dispersion polymerization conditions. 5−17 Although several polymerization techniques have been explored for PISA,18−20 reversible addition−fragmentation chain transfer (RAFT) has been the polymerization technique of choice in most PISA studies, in which a solvophilic macromolecular chain transfer agent (Macro-CTA) is used as a stabilizer and polymerization control agent.1,3 Chain extension to Macro-CTAs in situ produces block copolymers that self-assemble into nano-objects. PISA is typically conducted at high concentrations,21,22 and one-pot methods have also been developed to simplify the synthesis,23−27 making it realistic for large-scale production of block copolymer nano-objects. Although block copolymer nano-objects have enhanced structural stability in comparison with those formed by smallmolecule surfactants, high dilution, high mechanical stress, change of stimuli, and existence of oil, surfactants, or serum proteins can have detrimental effects on their structural integrity.28,29 Both postpolymerization and in situ cross-linking approaches have been exploited to improve structural stability of PISA-generated block copolymer nano-objects. Postpolymerization cross-linking involves two steps: (co)polymerization of functional (co)monomers via PISA to generate nano-objects with functional handles and subsequent reactions with complementary multifunctional compounds to effect crosslinking.30−33 In situ cross-linking via copolymerization with © XXXX American Chemical Society

Received: January 24, 2016 Accepted: February 9, 2016

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ACS Macro Letters Scheme 1. Structure of Cross-Linkers and Synthesis of in Situ Cross-Linked Vesicles

double bonds reacts, the reactivity of the other is reduced, but its reactivity is still higher than the allyl group. PDMA30 with a low dispersity (Đ = 1.10) was synthesized by RAFT polymerization in DMF and was subsequently used as a Macro-CTA for RAFT-mediated aqueous dispersion polymerization of DAAM at 70 °C. Kinetic analysis of a polymerization targeting DP 300 at 20% solids in a linear PDMA30−PDAAM300 vesicle synthesis indicated minimal Macro-CTA residue and thus high blocking efficiency, linear evolution of molecular weights, and low dispersities (≤1.23) (Figure S5). As previously reported,33 this dispersion polymerization exhibited a two-stage polymerization kinetics corresponding to solution polymerization (kp1 = 0.148 h−1) and polymerization in monomer-swollen particles (kp2 = 0.715 h−1) (Figure 1). To in situ cross-link the formed vesicles in this PISA formulation, BIS was initially used as a cross-linker. When 1 mol % (relative to DAAM) BIS was used, vesicles with a Dh 543 nm by dynamic light scattering (DLS) were observed at full conversion (Figure S6). However, 1.25 mol % BIS led to formation of precipitate, and higher amounts of BIS resulted in complete gelation. The vesicles cross-linked with 1 mol % BIS were subject to N,N-dimethylformamide (DMF) dissolution and surfactant disruption studies following the protocol reported by Armes and co-workers.30 In DMF, the vesicles were significantly swelled to more than 2 μm. In attempts to redisperse the vesicles into water via dialysis, however, precipitate was formed, indicating low colloidal stability of the vesicles during such transferring processes. Addition of an anionic surfactant sodium dodecyl sulfate (SDS) (1% in solution) to an initially nontransparent vesicle dispersion (1%) caused a significant increase in transparency (40% transmittance) (Figure S7), suggesting partial disruption of the vesicles by SDS. Therefore, 1% BIS was insufficient for the vesicles to fully resist DMF and SDS challenges.

polymers, core cross-linked star polymers, and cyclized polymers.41−44 Conventional free radical polymerization in the presence of multivinyl monomers results in gelation at low monomer conversions.41 With controlled radical polymerization techniques, it has become possible to delay gelation to relatively high monomer conversions and to synthesize polymers with sophisticated topologies.43,44 Despite significant progress, suppressing gelation and control of structure topology in the presence of multivinyl (co)monomers continues to represent a forefront in polymer chemistry. Although it can be easily envisaged that in situ cross-linking of nano-objects in PISA in the presence of multivinyl comonomers is a more efficient route to cross-linked nano-objects compared to postpolymerization cross-linking methods, this seemingly simple goal has yet to be realized for the above-mentioned reasons. We hypothesized that if we could delay the crosslinking process to the late stage of polymerization in PISA when morphology transition has already completed we would be able to in situ cross-link nano-objects with controlled morphologies similar to those obtained in equivalent synthesis without cross-linkers. Herein, we report our study on in situ cross-linking of nano-objects utilizing an asymmetric crosslinker,45−47 in aqueous dispersion polymerization of diacetone acrylamide (DAAM) using poly(N,N-dimethylacrylamide) (PDMA30) as a stabilizer.33 Unlike the symmetric cross-linker methylene bis(acrylamide) (BIS), the asymmetric cross-linker allyl acrylamide (ALAM) used in this study has an acrylamido moiety with a similar reactivity to DAAM and an allyl moiety with a lower reactivity (Scheme 1). Therefore, it is expected that at the early stage of polymerization the acrylamido moiety would copolymerize uniformly with DAAM with minimal reaction on the allyl moiety; however, at the late polymerization stage the allyl moiety would mainly participate in radical reactions leading to cross-linking of the nano-objects. It is worth pointing out that even with BIS, after one of the two 317

DOI: 10.1021/acsmacrolett.6b00066 ACS Macro Lett. 2016, 5, 316−320

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cross-linked vesicles were formed after 4 h. These results are consistent with the GPC measurements; soluble branched polymers were formed within 3 h, and afterward the polymers became insoluble in DMF. Also noteworthy is that the Dh measured in ethanol after 4 h exhibited a slight reduction, presumably due to further cross-linking leading to more compact structures. Colloidally stable vesicles were synthesized when targeting DP 300 at 20% solids with 1−3 mol % ALAM, but precipitate formation was observed with ≥4 mol % ALAM. Successful vesicle synthesis with 5 mol % ALAM was realized when targeting DP 400 at a lower solids (10%). The vesicles (VDP−solids−x) were designated according to their synthetic conditions with the subscripts representing target DP, solids (%), and amount of ALAM (mol %). TEM micrographs and intensity-average size distributions of the linear and cross-linked vesicles are shown in Figure 2.

Figure 1. (A) Kinetic data for dispersion polymerization of DAAM in the presence of varying amounts of ALAM ([PDMA30]/[DAAM]/[V50] = 1/300/0.02, solids 20%, 70 °C). GPC traces (B), DLS size evolution (C), and TEM micrograph for 4 h of polymerization (D) for the dispersion polymerization in the presence of 3 mol % ALAM.

Next, the ability of the asymmetric cross-linker ALAM to in situ cross-link vesicles was investigated. Kinetic analysis of dispersion polymerization in the presence of varying amounts of ALAM is shown in Figure 1A. In comparison with the synthesis without any cross-linker, a gradual reduction in polymerization rate is evident when the amount of ALAM is increased from 1 to 5 mol % (relative to DAAM). In branching solution polymerization in the presence of cross-linkers, Armes and co-workers observed a similar rate reduction caused by increased viscosities due to formation of branched polymers.44,48 Because the two vinyl groups of ALAM have different reactivities, as confirmed by 1H NMR analysis during polymerization (Figure S9), light branching, instead of crosslinking, is expected to occur at low monomer conversions. GPC analysis was used to follow the formed polymers. The shape of the GPC traces becomes progressively asymmetric with increasing monomer conversions, and the dispersities significantly increase from 1.15 (8% conv.) to 1.37 (17% conv.) to 3.23 (37% conv.) for the synthesis with 3 mol % ALAM (Figure 1B). GPC analysis for higher conversions became impossible due to formation of insoluble polymer networks. Higher amounts of ALAM caused more significant branching (Figure S10), as expected. DLS was used to follow the diameters of the generated nano-objects during the synthesis with 3 mol % ALAM (Figure 1C). A gradual increase in Dh was observed when measured in water. When the same series of samples were measured in ethanol, which is a good solvent for the PDMA− PDAAM block copolymer, Dh remained smaller than 17 nm up to 3 h of polymerization (37% conv.), which is in stark contrast to the Dh 296 nm measured in water for the same sample. However, a drastic surge in Dh (844 nm) was seen for the 4 h sample (50% conv.) measured in ethanol, and from this point on the Dh measured in ethanol was consistently larger than the Dh measured in water. Transmission electron microscopy (TEM) analysis indicated vesicle formation for the 4 h sample (Figure 1D). These results suggest that the nano-objects formed within 3 h of polymerization do not have a sufficiently cross-linked network to prevent dissolution in ethanol, and fully

Figure 2. TEM micrographs for V300−20−0 (A), V300−20−1 (B), V300−20−2 (C), V300−20−3 (D), V400−10−5 (E), and the corresponding DLS size distributions (F).

The ability of these linear and cross-linked vesicles to resist solvent dissolution and to maintain colloidal stability during solvent switching was evaluated. As shown in Figure 3, V300−20−0 quickly dissolved into block copolymer unimers in DMF as evidenced by a dramatic size reduction from 499 nm in water to 12 nm in DMF. Solvent switching from DMF to water via dialysis resulted in poorly dispersed particles that finally precipitated. In contrast, the Dh of V300−20−3 increased from 413 nm in water to 826 nm in DMF; DMF is a good solvent and thus swells the network of cross-linked PDAAM without dissolution. Upon redispersion from DMF to water via dialysis, V300−20−3 almost restored its original size (443 nm) as well as the vesicular morphology without losing colloidal stability. The small peaks at around 5−6 μm were possibly due to formation of a small fraction of agglomerates during the synthesis, but no sign of instability of the vesicle dispersions was observed. For 318

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increased within minutes upon addition of SDS, and the final solution became almost totally transparent (T = 92.4%), suggesting the linear vesicles were disrupted by SDS. The relatively large size (Dh 112 nm) after disruption of the vesicles was probably due to scattering by some loose structures (PDI = 0.564) formed by complexation of SDS with PDMA−PDAAM. Dialysis to remove SDS only resulted in a transparent solution, and no particles/vesicles were recovered (Figure S13). While V300−20−1 cross-linked with the lowest amount of ALAM showed limited resistance to SDS disruption (T = 44.7%), V300−20−3 and V400−10−5 were more robust with respect to SDS challenge showing minimal changes in transparency. In general, it was difficult to obtain high-quality TEM images for the dispersions with added SDS; however, some successfully obtained TEM images showed that the vesicles transformed into solid particles in the presence of SDS (Figure S13), suggesting that insertion of SDS into the membrane of vesicles resulted in expansion of the membrane and finally filling of the vesicle lumen. Nevertheless, all the vesicles cross-linked with ALAM restored their vesicular morphology and size after removal of SDS via dialysis (Figures S13 and S15). A cationic surfactant hexadecyl trimethylammonium bromide (HDTAB) and a nonionic surfactant Tween-20 were also used in surfactant challenge studies, but these two types of surfactants were essentially nondestructive to the vesicles (Table S5 and Figure S16). In summary, we have developed a novel strategy to realize in situ cross-linking of vesicles in PISA. Although cross-linked vesicles can be synthesized in the presence of 1 mol % BIS, they show poor colloidal and structural stabilities in solvent switching and surfactant challenge studies. The use of an asymmetric cross-linker ALAM with two vinyl groups of differing reactivities can delay the cross-linking process to the late stage of polymerization, supported by GPC, DLS, and TEM analysis. One of the main advantages of this novel strategy is that a higher amount of ALAM can be introduced in the synthesis, leading to a higher cross-link density in the final vesicles. As a result, enhanced colloidal and structural stabilities are observed for the vesicles cross-linked with 2−5 mol % ALAM. This in situ cross-linking strategy is expected to expand the use of PISA nano-objects under circumstances where colloidal and structural stabilities are required. Extension of this strategy to in situ cross-link higher-order morphologies in other dispersion formulations is under way in our laboratory.

Figure 3. DLS size distributions for V300−20−0 (A) and V300−20−3 (C) in water, DMF, and redispersion from DMF to water via dialysis. TEM micrographs for V300−20−0 (B) and V300−20−3 (D) after redispersion from DMF to water.

the vesicles synthesized under the same conditions but with different amounts of ALAM, their ability to resist swelling by DMF is consistent with the amounts of ALAM used in the synthesis (Figures S11−12 and Table S4), as expected. Interestingly, the vesicles cross-linked by 1−5 mol % ALAM were more resistant to DMF swelling than the vesicles crosslinked by 1 mol % BIS. It is worth noting that in DMF the lumen of the vesicles was filled due to swelling and softening of the vesicle membrane by DMF, and generally the TEM images showed less defined interfaces with lower amounts of ALAM. Except for V300−20−1 cross-linked with the lowest amount of ALAM, which formed precipitate upon solvent switching from DMF to water, all other vesicles cross-linked with higher amounts of ALAM were able to regain their stable vesicular morphology with a similar size to their original value. In surfactant challenge studies, vesicle dispersions at a concentration of 1% were treated with SDS. Table 1 summarizes the results of Dh and transmittance (T) measured at 600 nm by UV−vis spectroscopy before and after addition of SDS (SDS concentration in the dispersions is 1%). The transparency of the linear V300−20−0 dispersion quickly

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsmacrolett.6b00066.

Table 1. Summary of Surfactant Challenge Studies.a surfactantb V300−20−0 V300−20−1 V300−20−3 V400−10−5

none SDS none SDS none SDS none SDS

Dh (nm) (PDI)c 451 112 473 409 431 402 247 249

(0.138) (0.564) (0.068) (0.132) (0.176) (0.184) (0.030) (0.002)

ASSOCIATED CONTENT

Td (%) 0 92.4 0 44.7 0 0.4 0 0

Experimental details and supplementary data (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions ‡

These authors contributed equally.

a

Vesicle concentration 1%. bSDS concentration either 0 or 1%. c Measured on a Malvern ZEN 3600 without dilution. dTransmittance measured at 600 nm by UV−vis spectroscopy.

Notes

The authors declare no competing financial interest. 319

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ACKNOWLEDGMENTS The authors are grateful for financial support by National Natural Science Foundation of China (21274084) and assistance of Instrumental Analysis and Research Center, Shanghai University.

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DOI: 10.1021/acsmacrolett.6b00066 ACS Macro Lett. 2016, 5, 316−320