Letter Cite This: ACS Macro Lett. 2018, 7, 358−363
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Formation of Anisotropic Liquid Crystalline Nanoparticles via Polymerization-Induced Hierarchical Self-Assembly Song Guan,†,§ Chen Zhang,† Wei Wen,† Ting Qu,† Xiaoxiong Zheng,† Yongbin Zhao,‡ and Aihua Chen*,†,§ †
School of Materials Science and Engineering, Beihang University, Beijing 100191, People’s Republic of China Shandong Oubo New Material Co. Ltd., Shandong 257088, People’s Republic of China § Beijing Advanced Innovation Centre for Biomedical Engineering, Beihang University, Beijing 100191, People’s Republic of China ‡
S Supporting Information *
ABSTRACT: Polymeric nanoparticles (NPs) containing liquid crystalline (LC) mesogens with tunable anisotropic morphologies have applications in various fields, but their preparation typically suffers from tedious and lowthroughput approaches. Here we present an efficient route to the preparation of anisotropic morphologies of azobenzene-containing block copolymers (BCPs) at high solids content via a polymerization-induced hierarchical self-assembly in ethanol. Various anisotropic NPs, including cuboids, short belts, lamellae, and ellipsoidal vesicles, have been obtained in a remarkably broad range of BCP compositions. The NPs exhibit a smectic phase with ordered stripes when observed under TEM. This internal LC ordering plays a significant role on the formation of these intriguing anisotropic morphologies. Morphological transitions from anisotropic to isotropic spheres can be obtained upon UV illumination due to the photoresponsive properties of the azobenzene mesogens. This work significantly expands the scope of accessible morphologies in PISA and suggests that the under explored LC BCPs may have an impactful role in the PISA field.
P
type of anisotropic morphologies of polymeric NPs, such as cuboids and short belts, via this scalable PISA method combining with other interactions and to explore their functionality, which would enable new materials applications. Azobenzene is a widely studied LC unit because of its interesting trans-cis photoisomerization properties. Azobenzene-containing BCPs show LC behavior in their trans state, which diminishes when a change to cis form is induced by UV light.36−38 Previously, we have reported highly ordered cylindrical microdomians within azobeneze-containing BCP thin films through a hierarchical self-assembly, consisting of microphase separation and LC ordering.39−42 Here, we demonstrate the formation of polymeric NPs with tunable anisotropic morphologies using poly(methacrylate) bearing azobenzene segments in the side chain as the core-forming block (denoted as PMAAz) and poly(methylacrylic acid) (PMAA) as the stabilizer block via PISA in ethanol (Scheme 1). During PISA, the LC ordering in the core-forming block will also take place, thus, providing a hierarchical self-assembly system where both the assembly of the BCPs and the internal LC ordering occur simultaneously. We found that besides ellipsoidal vesicles, lamellae, NPs with some unusual shapes,
olymeric nanoparticles (NPs) with various morphologies have received considerable attention due to their applications in various fields, such as biomedicine,1,2 stimuliresponsive materials,3,4 and controlled drug delivery systems.5 Particularly, particles of various shapes differ in their potential biological impacts in vivo.6−12 For instance, polymeric cubes exhibit improved performance compared to spheres as drug deliverers for breast cancer cells.11 Block copolymer (BCP) selfassembly in solution is an effective approach to polymeric NPs with different morphologies.13,14 Moreover, additional interactions, such as crystallization,15,16 liquid crystalline (LC) ordering,17,18 and poly(ionic liquid),19 were introduced into BCP for the design and preparation of anisotropic morphologies. However, a scalable synthetic method that can enable the synthesis of anisotropic NPs at high solids remains a great challenge via this approach. Recently, polymerization-induced self-assembly (PISA) has provided a more convenient and efficient route for the synthesis of discrete polymeric NPs at high solid contents.20−23 Various morphologies of BCP polymeric NPs have been fabricated via PISA, including spheres, worms, lamellae, vesicles, and so on.24−28 To date, studies in PISA have been focused primarily on coil−coil BCPs,29−33 whereas rod−coil systems, such as LC-containing BCPs, have been seldom reported due to the critical requirement on the respective solubilities of the monomer and the produced polymer in a dispersion polymerization system.34,35 Furthermore, it is desirable to enrich the © XXXX American Chemical Society
Received: January 29, 2018 Accepted: March 1, 2018
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DOI: 10.1021/acsmacrolett.8b00082 ACS Macro Lett. 2018, 7, 358−363
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ACS Macro Letters
dithiobenzoate (CPADB) as the chain transfer agent and 4,4′-azobis (4-cyanovaleric acid) (ACVA) as the initiator. The soluble PMAA macro-CTAs were then chain-extended through RAFT dispersion polymerization of MAAz in ethanol initiated by ACVA at 70 °C and 20% w/w solids. A series of PMAAm-bPMAAzn with varying degrees of polymerizations (DPs) were synthesized by controlling the feed ratio of MAAz/macro-CTA to systematically investigate the self-assembly behavior. Typical 1 H NMR spectra of CPADB, PMAA macro-CTA, MAAz monomer, and PMAA-b-PMAAz are shown in Figure S1. The actual DPs of the stabilizer block and the core-forming one were calculated from 1H NMR spectra, as listed in Table S1. Gel permeation chromatography (GPC) results of PMAA112 macro-CTA and a series of the corresponding PMAA112-bPMAAzn for which the carboxylic acid units were fully esterified using benzyl chloride with 1,1,3,3-tetramethylguanidine as a promoter, are shown in Figure S2. The Mn and Mw/Mn of all polymers are listed in Table S1. All samples show a relatively low dispersity (Mw/Mn < 1.50). The polymerization of PMAA75-b-PMAAz150 (target composition) was chosen to investigate the polymerization kinetics in detail. Time-dependent 1H NMR spectra at different polymerization time are shown in Figure S3. Kinetics studies (Figures S4 and S5) indicate a high monomer conversion (>80%) was achieved within 12 h and the semilogarithmic plot shows typical twostage polymerization kinetics, corresponding to an initial solution polymerization and a subsequent accelerated polymerization in swollen particles after nucleation.25 The plot of Mn-
Scheme 1. (a) Synthetic Route to the PMAA-b-PMAAz BCP NPs via RAFT Dispersion Polymerization in Ethanol; (b) Scheme Illustrating the Formation of Anisotropic Morphologies
such as cuboids and short belts, can be obtained by tuning the volume fraction of the blocks, which has not been previously observed in other PISA formulations. These PISA-generated anisotropic NPs exhibit a clear LC phase with smectic stripes and UV-light-responsive morphological transitions. Scheme 1a illustrates the synthetic route to PMAA-b-PMAAz NPs via PISA. PMAA macromolecular chain transfer agents (PMAA macro-CTAs) were first synthesized via reversible addition−fragmentation chain transfer (RAFT) solution polymerization in ethanol using 4-cyanopentanoic acid
Figure 1. TEM, SEM, AFM images, and the corresponding height profiles of PMAA112-b-PMAAzn NPs with different DPPMAAz dispersed in ethanol at 20% w/w solids, respectively: (a) PMAA112-b-PMAAz66, (b) PMAA112-b-PMAAz89, (c) PMAA112-b-PMAAz115, (d) PMAA112-b-PMAAz142. 359
DOI: 10.1021/acsmacrolett.8b00082 ACS Macro Lett. 2018, 7, 358−363
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ACS Macro Letters conversion shows a linear evolution, indicating a wellcontrolled RAFT polymerization. Figure 1 shows representative transmission electron microscopy (TEM), scanning electron microscope (SEM) and atomic force microscopy (AFM) images of PMAA112-b-PMAAzn with different DPs of PMAAz at 20% w/w solids. Remarkably, a large population of cuboidal NPs with edge sizes in the range 50−150 nm and heights of around 40 nm formed when the DP of PMAAz (DPPMAAz) is 66 (Figure 1a). To the best of our knowledge, cuboidal NPs have not been reported in other PISA formulations. The NPs grew up in a 1D way to form short belt structures with the width and height of around 100 and 40 nm, respectively, when the DPPMAAz increased to 89 (Figure 1b). Lamellae were obtained in the case of PMAA112-b-PMAAz115 (Figure 1c), while the thickness remained unchanged. With the DPPMAAz increased continuously to 142, ellipsoidal vesicles with the long axis of ∼450 nm, the short axis of ∼150 nm were formed, as shown in Figure 1d. From the AFM image, it is clear that the height is changed to about 100 nm. It can be concluded that the morphology of the NPs evolves from cuboids to short belts to lamellae to ellipsoidal vesicles as the DPPMAAz increases for PMAA112-b-PMAAzn. The effect of the length of the stabilizer block, PMAA, on the nanoparticle morphology was also investigated (Figure 2). With PMAA75 was used as a stabilizer block, a similar morphological evolution from cuboids to short belts to lamellae to vesicles was observed, with increasing DPPMAAz, as shown in Figure 2a−d and Figure S6. The size of the cuboids has no obvious difference with that in Figure 1a. However, when the DPPMAA decreased to 38, ellipsoidal vesicles and a mixture of lamellae and worm-like NPs, were obtained with decreasing the DPPMAAz (Figures 2e and S7). The major-to-minor axis ratio of the PMAA38-b-PMAAz44 vesicles in Figure 2e is obviously smaller than that obtained by PMAA112-b-PMAAz142 in Figure 1d. When PMAA176 was used as the soluble stabilizer block, wormlike morphologies were obtained over a wide range of DPPMAAz (from 87 to 134) (Figures 2f and S8). This 1D morphology is quite different from the short belt-like NPs of PMAA112-bPMAAz89 (Figure 1b) and PMAA75-b-PMAAz65 (Figure 2b). When DPPMAAz increased from 134 to 190, besides worm-like NPs, lamellae were also obtained. When DPPMAAz is 75, a mixture of spherical, ellipsoidal, and worm-like NPs were observed (Figure S9). A phase diagram can be constructed according to the above results, as shown in Figure 2g. The morphology evolves from cuboids to short belts to lamellae to vesicles with the increase of DPPMAAz when the stabilizer block chain length located in the suitable range. In this work, the evolution covers a remarkably large range in the phase diagram. Lamellae and vesicles appeared in the system of PMAA with short chain length, whereas 1D worms are the main structures in the case of long PMAAz chain length systems. To the best of our knowledge, nanoscale polymeric cuboids have rarely been reported to date. Very recently, Matyjaszewski and co-workers reported that polymeric nanocubes with an internal bicontinuous cubic phase could be realized through a hierarchical self-assembly of BCPs containing a poly(ionic liquid) block.19 Waymouth et al. prepared nanocubes via selfassembly combined with crystallization of dithiolane-containing BCPs.16 In these systems, the nanocubes were obtained through traditional BCP self-assembly in solution coupled with additional interactions such as crystallization. In our work, the polymeric cuboids were formed in large synthetic scale in a
Figure 2. (a) SEM image of the PMAA75-b-PMAAz48 NPs. TEM images of the (b) PMAA75-b-PMAAz65, (c) PMAA75-b-PMAAz84, (d) PMAA75-b-PMAAz123, (e) PMAA38-b-PMAAz44, (f) PMAA176-bPMAAz87 NPs. The insets in (e) and (f) are the corresponding SEM images of the NPs, respectively. (g) Phase diagram of the selfassembled morphology with different DPPMAA and mass fraction of PMAAz.
PISA formulation, in which azobenzene-containing monomers form a LC core-forming block. These cuboids as well as other unique anisotropic NPs observed for PMAA−PMAAz may originate from the unique LC properties of the PMAAz coreforming block. Mesomorphic properties of PMAA-b-PMAAz were investigated by using differential scanning calorimetry (DSC), polarized optical microscopic (POM), and small-angle X-ray scattering (SAXS). Figures 3a and S10 show the typical DSC curves of PMAA75-b-PMAAzn on the second heating and the first cooling procedures, respectively. In Figure 3a, two clear endothermic transitions in the range of 66−72 °C and 114− 121 °C were observed, which were attributed to two phase transitions. According to our previous work, for homopolymer PMAAz and PEO-b-PMAAz, the phase transition from isotropic phase (I) to smectic A (SmA) to smectic X (SmX) can be observed clearly upon cooling.43,44 Here, these two peaks can be assigned as the transition between the two smectic phases and the transition of Sm−I, respectively. The transition 360
DOI: 10.1021/acsmacrolett.8b00082 ACS Macro Lett. 2018, 7, 358−363
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Based on the above analysis, it can be proposed that the morphologies controlled by the balance of the internal LC packing and the surface energy, which results in the excellent stability of these anisotropic NPs (Figures S12 and S13). To prove this standpoint, the internal LC ordering in NPs was removed by UV light irradiation of the as-prepared samples in ethanol at room temperature for 1 h. The original cuboids in PMAA112-b-PMAAz66 (Figure 1a) changed to spherical particles with relatively bigger size after UV irradiation (Figure 4a),
Figure 3. (a) DSC curves of the PMAA75-b-PMAAzn on the second heating process. (b) SAXS pattern and (c) POM image of the PMAA112-b-PMAAz142 NPs (as-prepared samples without heat treatment). (d) A magnified TEM image of the PMAA112-b-PMAAz142 NPs, which clearly shows the smectic stripes.
Figure 4. TEM images of the PMAA112-b-PMAAz66 NPs (a) and PMAA112-b-PMAAz142 NPs (b) after UV irradiation.
confirmed by dynamic light scattering (DLS) analysis (Figure S14c). Moreover, the LC ordering stripes disappeared. After UV illumination, the azobenzene moieties changed from trans to cis form, resulting in the LC-to-isotropic phase transition inside the cores, losing the LC packing energy.37 Thus, the balance of hierarchical driving forces in the original PISA process was destroyed, leading to the transition to isotropic morphologies. Similar morphological changes were also observed in the aggregates of PMAA112-b-PMAAz142 (Figure 4b). The anisotropic ellipsoidal micelles diminished, replaced by the spherical large compound micelles (Figure S14d−f). Moreover, the transfer from trans to cis not only disturbs the LC packing, but also increases the hydrophilicity of core blocks. The associated core expansion induces the bigger size of the aggregates.46,47 From the other standpoint, this result also indicates that these anisotropic NPs are photoresponsive. In summary, we have successfully demonstrated the scale-up formation of polymeric NPs with anisotropic morphologies for azobenzene-containing PMAA-b-PMAAz via a polymerizationinduced hierarchical self-assembly in ethanol, combining PISA and LC ordering. LC-containing polymeric nanocuboids were obtained in large synthetic scale for the first time. The unconventional morphology evolution from cuboids to short belts to lamellae and to ellipsoidal vesicles as the increase of the PMAAz chain length was reported over a remarkable broad composition range of the BCPs. The visible ordered stripes on these anisotropic NPs indicate the internal smectic phase, confirmed by SAXS and POM. The balance of the surface energy and internal LC packing energy leads to the formation of these anisotropic structures. LC ordering can be removed by UV irradiation, resulting in the formation of spherical NPs, which shows that these NPs are photoresponsive. This work enriches the anisotropic morphologies of polymeric NPs in large synthesis scales, which may be useful for applications in areas such as actuators and drug delivery with photo control.
temperature increases with increasing PMAAz fractions. For POM and SAXS measurements, the as-prepared samples obtained by PISA were dried at room temperature for 48 h to remove the solvent completely without any heat treatment. In Figure 3b, two peaks at q = 1.94 and 3.87 nm−1 appeared in the typical SAXS pattern of PMAA112-b-PMAAz142, assigned as [001] and [002] scattering peaks derived from the layered structure of smectic phases, respectively. The calculated periodicity of the layered structure is 3.24 nm, which is similar to the lamellar spacing of fully extended side chain length (3.29 nm calculated by MM2 force field method), suggesting the titled smectic phase.43,44 Figure 3c shows that a number of bright aggregates can be spotted on POM at room temperature, indicating that the LC ordering in the NPs leads to birefringence. Figure 3d is a representative magnified TEM image of as-prepared PMAA112-b-PMAAz142 NPs. It is worth noting that clear stripes appeared with a spacing of 3−4 nm, which is in good accordance with the calculated periodicity from SAXS data. Therefore, it can be concluded that these lamellar stripes originated from the smectic phase of the LC structures in PMAAz domains,43−45 which were observed on all samples, as shown in Figure S11. In addition, the morphological evolution was also affected by molecular compositions. With the increase of DPPMAAz, the initial cuboids fuse to form short belts, and to lamellae, subsequently, the lamellae curve and close to form ellipsoidal vesicles so as to minimize the total surface energy of the system. On the other hand, when the stabilizer PMAA block is very short, the products cannot be obtained until the volume fraction of PMAAz block is high enough. Therefore, the cuboid phase is jumped and enter the worm and lamella phase directly. While, the LC packing energy will be screened to some extent when the stabilizer PMAA block is very long, leading to the formation of spheres or ellipsoids instead of cuboids if the volume fraction of PMAAz block is low. These spheres or ellipsoids fuse to form 1D cylinders with increasing of DPPMAAz, covering a very wide space on the phase diagram in this case, which is consistent with other references.34 361
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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/acsmacrolett.8b00082. Materials and methods, characterization, and supporting figures and table (PDF).
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Aihua Chen: 0000-0002-9609-988X Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Prof. Mingjiang Zhong in Yale University for disscussions. This work was financially supported by the National Natural Science Foundation of China (Nos. 51272010 and 51472018), Beijing Nova Program (No. XX2013009), and the Fundamental Research Funds for the Central Universities.
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