Controllable and Reversible Dimple-Shaped ... - ACS Publications

Nov 26, 2015 - Controllable and Reversible Dimple-Shaped Aggregates Induced by. Macrocyclic Recognition Effect. Ming Zhang, Lingyan Liu,* Weixing ...
2 downloads 0 Views 2MB Size
Download PDF
Article pubs.acs.org/Langmuir

Controllable and Reversible Dimple-Shaped Aggregates Induced by Macrocyclic Recognition Effect Ming Zhang, Lingyan Liu,* Weixing Chang, and Jing Li* The State Key Laboratory of Elemento-Organic Chemistry, Nankai University, Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Weijin Road 94#, Nankai District, Tianjin, P. R. China S Supporting Information *

ABSTRACT: A novel dimethyl acrylate 18-membered macrocycle (DMECE), acting as both bifunctional monomer and cross-linker, was designed and synthesized, and thus employed to construct a series of macrocycle-containing amphiphilic hyperbranched polymers (HBPs). The macrocyclic recognition effect between the HBPs and alkali metal ions showed that Na+ was introduced in 1:1 interactive mode, whereas K+ and Rb+ were in 2:1 ratio. Through the formation of the DMECE/K+ = 2:1 rigid “sandwich” complex of amphiphilic hyperbranched polymers, dimple-shaped aggregates were observed by TEM, SEM and AFM. Moreover, the initial concentration, the nature of solvent, the mode and affinity of the macrocyclic recognition effect as well as the amount of K+, were essential control factors for the formation of dimpleshaped aggregates. Most importantly, the macrocyclic recognition effect endows the reversibility of the dimpleshaped aggregates and the size controllability of its circular opening, which provides a new strategy for design novel macrocycle-containing HBPs and great potential application in the field of capture and release.

■

HBPs.25,26 In this work, we designed and synthesized a series of various topology polymers via RAFT polymerization of methyl methacrylate (MMA) and 2-(2-methoxyethoxy) ethyl methacrylate (MEO2MA) in dilute solution through using a dimethyl acrylate cross-linker containing 18-membered macrocycle (DMECE). The ion recognition property of these macrocycle-containing hyperbranched polymers with various alkali metal ions was investigated by 1H NMR titration experiments, and their self-assembly behaviors in THF/H2O mixture were given a focus of attention by TEM, SEM, AFM, and DLS. Unexpectedly, a kind of dimple-shaped aggregate was observed induced by the macrocyclic recognition effect between K+ and hyperbranched polymers containing macrocyclic crown ether. More importantly, the dimple-aggregates were controllable and reversible by changing the ratio of block and introducing the 18-crown-6 and K+ alternatively, respectively (Scheme 1). We believe that the macrocyclic recognition effect would pave a new way for designing novel HBPs, both in structure and morphology, as well as functions.

INTRODUCTION Hyperbranched polymers (HBPs) have been used in special coatings, tissue engineering, nanomedicine and controlled drug as well as gene delivery,1,2 due to the unique physical properties, such as compact shape resulting in low viscosity in solution and in the molten state compared to linear counterparts, and large number of functional groups on their periphery.3−7 And recently, HBPs have demonstrated great potential in biomedical applications, which has stimulated the rapid development of a new research area of the self-assembly of HBPs. With Yan and Zhou’s pioneering work reported on the self-assembly for HBPs in 2004,8,9 many delicate structures at some scales and dimensions have been subsequently developed by direct solution self-assembly,10,11 interfacial selfassembly,12−14 and hybrid self-assembly15−17 of amphiphilic HBPs. All these driving forces of self-assembly8−17 are mainly attributed to microphase separation, hydrogen bonding or electrostatic adsorption, etc. Meanwhile, the marriage of supramolecular chemistry and polymer chemistry is an emerging strategy for the efficient and selective synthesis of designed material.18,19 Some recent examples of stimuliresponsive polymers have also been developed based on macrocycle units,20−24 but these polymers are generally linear copolymers. In comparison, few macrocycle-containing HBPs are reported, especially the smart morphology transition of © XXXX American Chemical Society

Received: July 25, 2015 Revised: November 24, 2015

A

DOI: 10.1021/acs.langmuir.5b03865 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir Scheme 1. Reversibility and Controllability of Dimple-Shaped Aggregates

Scheme 2. Synthesis of DMECE and Polymerization Methods of Hyperbranched Copolymer by “Arm-First” And “Core-First”

■

pellets of solid samples on a Bruker Tensor 27. Mass spectroscopy (MS) was analyzed by quatropde-time-of-flight mass spectrometer (QTOF-MS) measurement, which was performed on Agilent 6520 QTOF LC/MS with acetonitrile as the solvent. The number (Mn) and weight-average molecular weights (Mw) and the polydispersity index (PDI) of the polymers were determined by size exclusion chromatography (SEC) on an Agilent Technologies 1200 series SEC equipped with a G1362A differential refractive index detector, with THF as eluent at a flow rate of 1.0 mL/min. Polystyrene standards were used for the calibration of molecular weight. The hydrodynamic diameter (Dh) of the hyperbranched copolymer samples were determined by Nano ZS Zetasizer (Malvern Instruments, UK) equipped with a He/Ne laser light source (633 nm, 4.0 mW). The measurements were made at a scattering angle of 90° (back scattering detection). The concentrations of the copolymers are all at 0.5 mg/mL in THF/water (v: v = 1:1) mixture. Before the data were collected, the samples were allowed to equilibrate for 2 min at each temperature. The stock solutions were filtered through a Millipore 0.20 μ m PVDF filter into a dust-free vial. The mean particle size was approximated by the intensity of particles and the width of the distribution as the polydispersity index (PDI) obtained by the cumulants method assuming a spherical shape with THF. Transmission electron microscopy (TEM) images were taken using a JEOL JEM-2100F transmission electron microscope operating at 200 kV. Samples were prepared by drop-casting micelle solutions onto carbon-coated copper grids, and then air-drying at room temperature before measurement. Scanning electron microscopy (SEM) image were taken using a FEI

EXPERIMENTAL SECTION

Materials. 2-(acetoacetyoxy) ethyl methacrylate, 2-hydroxyethyl methacrylate, rhodium(II) octanoate dimer (Rh2(Oct)4), tosyl azide (TsN3) and methyl acetoacetate were purchased from Aladdin and used without further purification. Triethylamine (Et3N, Tianjin Chemical Co., 99%) and methyl methacrylate (MMA, Tianjin chemical, 99%) were refluxed over CaH2 for 24 h and distilled before use. 2-(2-methoxyethoxy) ethyl methacrylate (MEO2MA) was purchased from TCI and purified by basic alumina column chromatography to remove the inhibitors and antioxidants. 2-(2cyanopropyl) dithiobenzoate (CPDB) was synthesized according to the literature procedure.27 2,2-Azodiisobutyronitrile (AIBN) (>98%, Tianjin Chemical Co.) was recrystallized from ethanol and stored in refrigerator before use. N, N-dimethylformamide (DMF) was purchased from J&K with 10 Å molecular sieve. 1-ethyl-3-[3(dimethylamino)-propyl] carbodiimide hydrochloride (EDCI), 4dimethylaminopyridine (DMAP) were purchased from Heowns and used as received without further purification. Deuterated chloroform (99.9%), deuterated dimethyl sulfoxide (99.9%) and deuterated acetone (99.9%) were purchased from Cambridge Isotope Laboratories. Sodium hexafluorophosphate (NaPF6, 99%), potassium hexafluorophosphate (KPF6, 99%), rubidium iodide (RbI, 99%) were purchased from TCI and used without further purification. Instrumentation. 1H and 13C NMR spectra were recorded on a 400 MHz Bruker Avance spectrometer at room temperature, with TMS as an internal standard. FT-IR spectra were recorded with KBr B

DOI: 10.1021/acs.langmuir.5b03865 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir Table 1. Synthesis of Polymer by RAFT Polymerization entry 1 2 3 4 5 6 7 8 9

polymer PMMA85 PMEO2MA54 PMMA85-b-PMEO2MA80, PI PMMA70-co-PDMECE5, PII PMMA24-co-PDMECE18 PMEO2MA32-co-PDMECE8 PMMA85-b-(PMEO2MA42-co-PDMECE8), PIII-1 PMMA85-b-(PMEO2MA82-co-PDMECE10), PIII-2 (PMMA70-co-PDMECE5)-b-PMEO2MA35, PIV

conv. (%)

Mn, NMRb (104g/mol)

Mn, GPC (104g/mol)

Mw, GPC (104g/mol)

PDIc

0.87 1.04 2.48 1.00 1.34 1.10 2.14 3.02 1.68

0.85 0.83 2.24 2.17 1.09 0.74 1.73 2.32 1.55

0.94 0.96 2.60 3.93 1.67 1.20 4.33 5.67 2.79

1.09 1.16 1.18 1.81 1.53 1.62 2.51 2.45 1.80

a

71 72

58d 48 53

a The conversion of monomer MMA was determined by 1H NMR; bMn,NMR was determined by the conversion: Mn,NMR = M0 × ([M0])/ ([CPDB]) × conversion(%) + 221.34 (CPDB) g/mol cPDI was determined by GPC. dMMA conversion; and the ratio of MMA to DMECE was determined by the integral the proton of OCH3 in MMA and CH3 in the macrocycle of DMECE.

Nova NanoSEM 230 operating at 50 kV. Atom force microscope (AFM) images were taken using a Nanoscope IV controller (Bruker, Veeco) in tapping mode onto freshly cleaved mica surfaces using standard Si probe. 1. Hyperbranched Copolymers via RAFT Copolymerization. In a typical reaction, CPDB (22.1 mg, 0.1 mmol), AIBN (3.3 mg, 0.02 mmol), MMA (1201 mg, 12 mmol), DMECE (600.6 mg, 1 mmol) and DMF (10 mL) were added into a dry 50 mL Schlenk tube equipped with a stirring bar under argon atmosphere. The tube was sealed with a septum, and freezed-pump-thawed for three cycles. Subsequently, the reaction mixture was allowed to thaw and immersed in a preheated oil bath at 60 °C for 48h. After polymerization, the reaction was stopped by rapid cooling in liquid nitrogen. The crude polymer was purified by reprecipitation in a large excess of cold ether, and washing with cold ether (three times). Afterward, the resulting product was dried under vacuum at room temperature to yield PMMA70-co-PDMECE5 (865 mg, yield 48%, PII). 2. Hyperbranched Copolymer via “Arm-First” RAFT Copolymerization. In a typical reaction, Macro-CTA (PMMA85) (218 mg, 0.025 mmol), AIBN (0.8 mg, 0.005 mmol), MEO2MA (282 mg, 1.5 mmol), DMECE (300.3 mg, 0.5 mmol), and DMF (10 mL) were added into a dry 50 mL Schlenk tube equipped with a stirring bar under nitrogen. The following procedure was the same as above synthesis of PII. The PMMA85-b-(PMEO2MA42-co-PDMECE8) was prepared (305 mg, yield 38%, PIII-1). 3. Hyperbranched Copolymer via “Core-First” RAFT Copolymerization. In a typical reaction, Macro-CTA (PMMA70-co-PDMECE5) (205 mg, 0.02 mmol), AIBN (0.7 mg, 0.004 mmol), MEO2MA (376 mg, 2 mmol) and DMF (10 mL) were added into a dry 50 mL Schlenk tube equipped with a stirring bar under nitrogen. The following procedure was the same as above synthesis of PII. The (PMMA70-coPDMECE5)-b-PMEO2MA42 was obtained (225 mg, yield 39%, PIV).

was investigated with high [DMECE]/[CTA] ratio (PII, Entry 4 in Table 1 [DMECE]/[CTA] = 15 and [MMA] = 11 wt %) in dilute solution by 1H NMR spectra. The conversion of MMA showed a good linear correlation with time in relatively low MMA conversion (up to 60−70%), which demonstrated an ideal RAFT copolymerization (up to 60−70%, Figure S7). The PII was then used as a macromolecular initiator in the subsequent RAFT polymerization of MEO2MA, and thus resulting in the star-like hyperbranched copolymer PIV (Entry 9 in Table 1), which was named as “core-first” method (Method A). Through this method, the macrocycle units were successfully incorporated into the hydrophobic PMMA moiety. Alternatively, PMMA85 was used as a macro-CTA in the subsequent RAFT copolymerization of MEO 2 MA and DMECE, which was named as “arm-first” method,38 to obtain PIII (Method D). In Method D, the macrocycle units were introduced into the hydrophilic PMEO2MA moiety. Notably, we presented four routes to synthesize the star-like hyperbranched polymers (Scheme 2), but the Methods B and C were discarded due to the gelation during the polymerization. In addition, block copolymer PI (Entry 3 in Table 1 and Figure S8) was synthesized by using macro-CTA (PMMA85) as a contrast. The detailed polymerization conditions and results are listed in Table 1. In 1H NMR spectrum of PIII-1, the proton resonance signals of the 18-membered macrocycle methyl (e) in DMECE appeared in the range of 2.25−2.49 ppm, methoxy group (c) of MEO2MA at 3.40 ppm, and the methoxy group (a) of PMMA, ethylene oxide group of MEO2MA (b, b’) and DMECE (d, d’, h, g, g’) in 3.45−4.70 ppm (Figures S9, 10), respectively. Therefore, the block ratio of PII was calculated by the integral ratios in 1H NMR. Similarly, the ratios of these three monomers in PII, PIII-2, and PIV were also determined by 1H NMR spectra. Flory developed a so-called “mean field” theory for an idealized formulation,39,40 whereby a divinyl cross-linker reacts statistically with the monovinyl monomer to link up perfectly monodisperse primary chains. However, subsequent experimental studies indicate a significant deviation from the theory.41 This phenomenon is generally ascribed to the intramolecular cyclization of divinyl group. Armes and coworkers have also shown that intramolecular cyclization of divinyl cross-linker becomes much more prevalent during the course of synthesizing branched copolymers in dilute solution (MMA = 10 wt %).42,43 Similarly, in our case of the [MMA] = 11 wt %, the intramolecular cyclization of DMECE cross-linker might also be dominant in polymerization. Moreover, the good

■

RESULTS AND DISCUSSION 1. Synthesis of Cross-Linker DMECE and the Hyperbranched Polymers. The essence for developing novel macrocycle-containing monomer is to incorporate polymerizable groups into the macrocycle-containing compound. Based on previous reports,28 we herein designed and synthesized a cross-linker DMECE as shown in Scheme 2. Namely, the dimethyl acetate compound (M1) was saponified and acidized to give dicarboxyl 18-membered macrocyclo compound (M2). Using M2 and 2-hydroxyethyl methacrylate, DMECE was then synthesized in relative high yield in the presence of EDCI/ DMAP catalytic system. The synthesized M2 and DMECE were characterized by NMR and IR (Figure S1−S6, see the Supporting Information). Next, the hyperbranched polymer PII was synthesized by RAFT polymerization directly using DMECE and MMA.29−37 The kinetics of RAFT copolymerization of MMA and DMECE C

DOI: 10.1021/acs.langmuir.5b03865 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir Scheme 3. Polymers with Different Compositions and Topologies

the macrocyclic recognition effect with K+ and Rb+ also showed distinction as mentioned above. 3. Morphology. Amphiphilic block copolymer can selfassemble to form the ordered structures in a wide range of morphologies through the immiscibility of the segments and/or the immiscibility of one of the segments in certain solvent. Additionally, topology and composition of the copolymer play an extremely important role in determining micellar morphology, since they affect the conformational packing geometry of micellar structure.45−53 Herein, we chose five polymers with different compositions and topologies to investigate their morphology: (1) linear block copolymer PMMA 85 -bPMEO2MA80, PI; (2) hyperbranched polymer PMMA70-coPDMECE5, PII; (3) star-like hyperbranched polymer with hydrophilic core”, PMMA85-b- (PMEO2MA42-co-PDMECE8), PIII-1, and (4) PMMA85-b-(PMEO2MA72-co- PDMECE10), PIII-2 with longer hydrophilic segment; (5) star-like hyperbranched polymer with “hydrophobic core”, (PMMA70-coPDMECE5)-b-PMEO2MA35, PIV (Scheme 3). 3.1. Self-Assembly Behavior of Hyperbranched Polymers in THF/H2O Mixture. The self-assembly behaviors of the hyperbranched polymers were investigated in 0.5 mg/mL THF/water (v/v = 1:1) mixture. For example, the polymers were first dissolved in THF with 1.0 mg/mL, the equal volume of redistilled water was then slowly added into the solution (0.8 mL/h) by a syringe pump to give 0.5 mg/mL polymer THF/ water mixture. As a result, plenty of spheres were observed with the sizes about 80 and 100 nm from the TEM images of PII and PIII-1 in Figures 1a and b, 114 and 132 nm of apparent hydrodynamic diameters from DLS measurement in Figures S32 and S34, respectively. 3.2. Self-Assembly Behavior of Polymers with the Addition of NaPF6 in THF/H2O mixture. Next, the self-assembly behaviors of the polymers with the addition of NaPF6 in 0.5 mg/mL THF/water (v/v = 1:1) mixture were investigated by TEM (Figures 1c and d) and DLS (Figures S32, S34, and S41). To determine the “salt-out” effect with the addition of Na+ on the morphology of polymers, the polymer PI without macrocycle was employed as a model (Figures S30 and S31). It was observed that no obvious morphology transformation before and after addition of Na + . This phenomenon demonstrated that the “salting-out” effect54 are negligible in low concentration (10−4 M). Hence, for PII and PIII-1, induced by the “1:1” the macrocyclic recognition effect,

solubility of the hyperbranched polymer and the relatively high [DMECE]/[CTA] ratio also demonstrated that a great amount of DMECE conducted intramolecular cyclization, especially in relatively low monomer concentration. 2. Stoichiometric Chemistry. The monomer DMECE contains an 18-membered macrocycle, thus the interaction between DMECE and metal cations was first evaluated. But due to the low solubility of DMECE in d6-acetone, the similar 18membered macrocycle compound M1 was chosen as an alternative. Namely, M1 was mixed with NaPF6, KPF6 or RbI in d6-acetone to analyze the interaction by 1H NMR spectroscopy, respectively. As a result, the protons (a, c, c’, d, e) assigned to the 18-membered macrocycle segment shifted to downfield about 0.08−0.15 ppm as shown in Figures S12, S14, and S16, respectively, which was larger than that accompanying shifted to downfield in the chain-end methoxycarbonyl groups (b) by 0.02 ppm. These results indicated that M1 could efficiently coordinate with these alkali metal ions. Based on the chemical shift change of the 18-membered macrocycle proton and the molar fraction (χ) of M1 (Figure S13 and Table S1), the Job Plot42 peaks at χ = 0.5 in Figure S13, which means the stoichiometric chemistry of the interaction is 1:1 (one Na+ per M1). K+ or Rb+ was captured by M1 via 2:1 interaction mode determined by similar method, which showed a maximum at χ = 0.67 in Figures S14−17 and Tables S2 and S3, respectively. By 1H NMR titration experiment, the association constants (Ka)44 of M1 with Na+, K+ and Rb+ were also estimated to be approximately 1.6 × 105 (M−1) (Figures S18, S19 and Tables S4, S5), 4.8 × 105 and 7.2 × 104 (M−2, Figures S20−23 and Tables S6−9), respectively. It was obvious that K+ has a stronger cation affinity for the DMECE than Rb+. Afterward, the interactions between the hyperbranched copolymers and alkali metal cations were also examined. As for PIII-1, the Job’s Plots peak at the same position as DMECE (Na+ (χ = 0.5), K+ and Rb+ (χ = 0.67)). These results demonstrated a similar 1:1 stoichiometeric interaction between a Na+ guest and an in-chain DMECE unit of PIII-1 (Table S10, Figures S24 and 25, while the adjacent DMECE receptors of the intra/inter polymeric chains captured K+ or Rb+ to form stable and relatively rigid 2:1 “sandwich” complexes (Tables S11 and S12, Figures S26−S29), thus confined the rigidity of the polymer segments. Summarily, the modes of the macrocyclic recognition effect were different between Na+ or K+ and the macrocycle units in the polymer. Meanwhile, the affinity of D

DOI: 10.1021/acs.langmuir.5b03865 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

SEM images (Figures 2a,b and S37) for the PIII-1. The size of the aggregates was in the range of 200−400 nm, the diameter of the circular opening was estimated between 100 and 160 nm, and the depth of the dimple was evaluated about 200 nm when they are on the lateral surface, respectively. Additionally, an obvious difference was found in turbidity from the optical photograph when KPF6 was added into the solution of PIII-1 compared to NaPF6 (Figure S32). Dimple-shaped aggregate is a kind of unsymmetrical LCMs with a void space locating in the edge of the aggregate. Some “shadow” areas were located in the edge of the aggregates in tapping mode AFM height image (Figure 3c), indicative of the “void space” of the aggregates. Moreover, a double peak of the height analysis could be seen in Figure S38, which means the existence of a “dimple” toward us. However, for the PIV, only spheres were observed after the addition of KPF6 in Figure 2d (Figure S44, S45). For the reason, we attributed to the totally different topology between PIV and PIII-1. Namely, as for PIV, the DMECE units located in the hydrophobic PMMA segments and act as the “core”, while they located in the hydrophilic PMEO2MA segments and act as the “shell” in the case of PIII-1. Eisenberg and co-workers55,56 have ever reported the bowlshaped aggregates and a plausible interior viscosity control mechanism was proposed. In their cases, the supplemental viscosity control was provided by physical cross-linking and hydrogen bonding. Jiang’s group also prepared dimple-like micelles from polyimide with two carboxyl ends, and they believed that hydrogen-bond action between carboxyl ends and the rigidity of the backbone provided extra viscosity to control the mechanism57 In our system, the “void spaces” are small and shallow compared to the aggregates, we preferred to describe the aggregates as “dimples”, and the main driving force for the formation of the dimple-shaped aggregates was ascribed to the macrocyclic recognition effect. In detail, a possible formation mechanism was proposed as shown in Scheme 4. When water

Figure 1. TEM images of aggregates formed by (a) PII; (b) PIII-1; (c) PII + NaPF6; (d) PIII-1 + NaPF6 with 0.5 mg/mL in THF/water (v/v = 1:1) mixture solvent.

polydisperse LCMs (large compound micelles45−53) were formed with the size of 200−300 nm after the addition of Na+. For the reason, we surmised that when NaPF6 was added, the macrocycle recognition effect (macrocycle/Na+ = 1:1) changed effective charge of “corona-forming” segments and enhanced the electrostatic repulsion from the corona, rendering the size of the aggregate increased. 3.3. Self-Assembly Behavior of Polymers in THF/H2O Mixture with the Addition of KPF6. In comparison, the “dimple”-type aggregates were unexpectedly observed when KPF6 was added into the solution of PII or PIII-1. For example, in the case of PII, it is manifested that the size of the aggregates was 140−240 nm, and the circular opening was smaller than 50 nm in Figure S32 and S33. Moreover, dimpleshaped aggregates were also clearly observed in both TEM and

Figure 2. (a,b) TEM and SEM images of aggregates formed by PIII-1 + KPF6 in 0.5 mg/mL THF/water (v/v = 1:1) mixture solvent; (c) Height image of tapping mode AFM images of aggregates formed by PIII-1 + KPF6 in 0.5 mg/mL THF/water (v/v = 1:1) mixture solvent; (d) TEM images of aggregates formed by PIV + KPF6 in 0.5 mg/mL THF/water (v/v = 1:1) mixture solvent. E

DOI: 10.1021/acs.langmuir.5b03865 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

Figure 3. TEM images of aggregates formed by (a) PIII-1 + RbI in 0.5 mg/mL THF/water (v/v = 1:1) mixture; (b) PIII-1 + KI in 0.5 mg/mL THF/water (v/v = 1:1) mixture; (c) PIII-1 + KPF6 in 0.25 mg/mL THF/water (v/v = 1:1) mixture; (d) PIII-1 + KPF6 in 0.50 mg/mL THF/water (v/v = 1:1) mixture; (e) PIII-1 + KPF6 in 0.75 mg/mL THF/water (v/v = 1:1) mixture; (f) PIII-1 + KPF6 in 0.50 mg/mL aqueous solution.

Scheme 4. Schematic Illustration of the Dimple-Shaped Aggregates

was initially added to the THF solution of PIII-1, THF was drawn out gradually and the mixture solvent became progressively less favorable for the hydrophobic segments of the PMMA. At critical water content (Figure S46), the polymer segments lost their solubility and associated with each other. Therefore, the spheres were observed (Figure 1a and b) because of the immiscibility of hydrophobic PMMA segments in the mixture solvent.43−51With the further addition of KPF6 into the THF/H2O solution of PIII-1, the adjacent DMECE receptors of intra/inter segments captured K+ to form relatively stable and rigid 2:1 “sandwich” complexes, thus leading to the restricted mobility of polymers segments. As a result, LCMs were formed with hydrophobic islands in the continuous matrix formed by hydrophilic segments. Additionally, some hydrophilic segments located on the surface of the LCMs could stabilize these aggregates. Meanwhile, more extraction of THF from the LCMs led to liquid−liquid phase separation, and some tiny THF/water-filled bubbles formed within the aggregates. Afterward, these bubbles coalesced into a large one, and subsequently “dimples” were formed when the large bubble

broke from the weakest part of the surface. For this to occur, the supplement viscosity should not only be high enough to form the “bubbles”, but also low enough to allow coalescence of the bubbles. Moreover, the results of viscosity measurement were also agreed with the observed electron microscopic results (Figure S39a). Obviously, K+ is the best ion for sharply increasing the apparent viscosity. Besides, the amount of K+ was also the key effect point for the solution viscosity and the morphology (Figure S39b, S40). In addition, given the fact that no dimple-shaped aggregate was formed in the case of the addition of NaPF6 into the PIII-1 solution (Figure 1d), it could be explained that the 1:1 DMECE/Na+ mode of macrocyclic recognition effect did not even confined chains mobility or enhanced the viscosity during the micellization process (Scheme 4). Surprisingly, the same amount of RbI was added under the same condition, there was no any dimple-shaped aggregate even with the same coordination mode of 2:1 DMECE/Rb+ as well (Figure 3a). It may be due to the relatively weak affinity of the macrocyclic recognition effect (Figure S39). Namely, the supplement F

DOI: 10.1021/acs.langmuir.5b03865 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir

be used for the field of capture and release, especially the burst release of hydrophilic cargo.58−60 Finally, increasing the length of hydrophilic PMEO2MA segment (PIII-2), dimple-shaped aggregates was still observed in TEM images (Figures 4c,d). And the size of the aggregates was significantly increased from 200 to 400 nm to 500−600 nm, which may be caused by the enhancement of hydrophilic nature of the polymer. Specifically, with the increase of relative content of hydrophilic PMEO2MA segment, more confined water droplets were formed inside the aggregate during the formation of LCMs upon water addition, leading to a larger structure.43−55 These results demonstrated that the ratio of hydrophobic−hydrophilic moieties of polymers influenced the self-assembly behavior as well. At another point of view, a facile strategy to control the size of circular opening was developed.

viscosity was not high enough to inhibit chains mobility effectively. Additionally, adding the same amount of KI to the solution of PIII-1 instead of KPF6, the dimple-shaped aggregates were observed as well (Figures 3b and S43). It demonstrated that the mode of macrocyclic recognition effect between K+ and polymers was independent of counteranion. Thus, it was concluded that the macrocycle recognition effect (mode and affinity) played an essential role in the formation process of the dimple-shaped aggregates. Beyond that, the interior viscosity is very sensitive to the concentration of polymer and the inherent nature of solvent.43−55 Aliquots with other two concentrations of 0.25 and 0.75 mg/mL THF/aqueous mixture of PIII-1 and KPF6 were also examined by TEM (Figures 3c,e), respectively. Consequently, no dimple-shaped aggregate was observed in these two cases. Moreover, in aqueous solution of PIII-1 and KPF6, only spheres were observed (Figure 3f). Summarily, the viscosity control mechanism for the formation of dimpleshaped aggregates is very sensitive to external environment. The initial concentration of copolymer, the nature of solvent, the mode and affinity of the macrocyclic recognition effect are all the essential viscosity-controlled factors. 3.4. The Reversibility of the Dimple-Shaped Aggregate and the Controllability of the Circular Opening. In addition, the reversibility of the dimple-shaped aggregates was also investigated. When 18-crown-6 was added to the solution of K+ and PIII-1, the dimple-like aggregates collapsed and the morphology returned to the spheres mentioned as shown in Figure 4a. The phenomenon may be due to the stronger affinity

■

CONCLUSION In summary, a series of macrocycle-containing hyperbranched polymers were facilely constructed by RAFT polymerization of a novel monomer DMECE. Herein the DMECE plays a triple role, acting as both monomer and cross-linker in the polymerization, as well as molecule recognize host in the selfassembly process. Beside, given the fact that alkali metal ions could be incorporated into the aggregates through their coordination with macrocycle units (DMECE), a novel kind of dimple-shaped aggregate was observed in the solution of hyperbranched polymer PMMA-b-(PMEO2MA-co-PDMECE) and K+, which may be induced by the macrocyclic recognition effect from the formation of the DMECE/K+ = 2:1 rigid “sandwich” complex. The initial concentration, the nature of solvent, the mode and affinity of the macrocyclic recognition effect as well as the amount of K+, were essential viscositycontrol factors. To the best of our knowledge, it was the first report about the macrocycle-containing hyperbranched polymers with the dimple-shaped aggregates formed via macrocyclic recognition effect. The macrocyclic recognition effect provides a new efficient and convenient strategy to realize the controllability and reversibility of the dimple-shaped aggregates besides the previous reported physical cross-linking and hydrogen bonding. Meanwhile, the distinction of “on the surface” of the aggregates and “in the void space” is “singlepoint touching mode” convexity and “multi-point touching mode” concavity, respectively. Therefore, this kind of aggregates has potential application in the field of targeted binding, storage and release device. Moreover, the macrocyclecontaining hyperbranched polymer systems help to understand the guidelines the elaborate polymer topology, have an insight into complicated self-assembling process and furthermore, design fascinating molecular devices.

Figure 4. TEM images of aggregates formed by (a) PIII-1 + KPF6 (1eq ) + 18-crown-6 (1 equiv) in 0.5 mg/mL THF/water (v/v = 1:1) mixture solvent; (b) PIII-1 + KPF6 (1 equiv) + 18-crown-6 (1 equiv) + KPF6 (2 equiv) in 0.5 mg/mL THF/water (v/v = 1:1) mixture solvent; (c,d) PIII-2 + KPF6 at 0.50 mg/mL THF/water (v/v = 1:1) mixture solvent.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.langmuir.5b03865.

between 18-crown-6 and K+. As a result, with the degradation of the DMECE/K+ “sandwich” complex, the interior viscosity returned back to initial state, which was insufficient to maintain the dimple-shape. To our delight, when a considerable amount of K+ was added to the same samples again, the interior viscosity was enhanced once again, and the similar dimpleshaped aggregates reappeared (Figures 4b, S47 and S48). This facile reversible method provides a feasible strategy which may

■

Additional information as noted in the text (PDF)

AUTHOR INFORMATION

Corresponding Authors

*(L.L.) Tel: +86 22 23501410. E-mail: [email protected]. *(J.L.) [email protected]. G

DOI: 10.1021/acs.langmuir.5b03865 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir Notes

(18) Meeuwissen, J.; Reek, J. N. H. Supramolecular catalysis beyond enzyme mimics. Nat. Chem. 2010, 2, 615−621. (19) Suzuki, K.; Sato, S.; Fujita, M. Template synthesis of precisely monodisperse silica nanoparticles within self-assembled organometallic spheres. Nat. Chem. 2010, 2, 25−29. (20) Jia, Y. G.; Liu, L. Y.; Lei, B.; Li, J.; Zhu, X. X. Crown Ether Cavity-Containing Copolymers via Controlled Alternating Cyclocopolymerization. Macromolecules 2011, 44, 6311−6317. (21) Jia, Y. G.; Zhu, X. X.; Liu, L. Y.; Li, J. Multi-responsive properties of a poly(ethylene glycol)-grafted alternating copolymers of distyrenic monomer with maleic anhydride. Langmuir 2012, 28, 4500− 4506. (22) Yin, J.; Li, C.; Wang, D.; Liu, S. FRET-derived ratiometric fluorescent K+ sensors fabricated from thermoresponsive poly(Nisopropylacrylamide) microgels labeled with crown ether moieties. J. Phys. Chem. B 2010, 114, 12213−12220. (23) Yu, H. R.; Ju, X. J.; Xie, R.; Wang, W.; Zhang, B.; Chu, L. Y. Portable diagnosis method of hyperkalemia using potassiumrecognizable poly(N-isopropylacrylamide- co-benzo-15-crown-5-acrylamide) copolymers. Anal. Chem. 2013, 85, 6477−6484. (24) Liu, Z.; Liu, L.; Ju, X. J.; Xie, R.; Zhang, B.; Chu, L. Y. K(+)recognition capsules with squirting release mechanisms. Chem. Commun. 2011, 47, 12283−12285. (25) Jia, Z. F.; Chen, H.; Zhu, X. Y.; Yan, D. Y. BackboneThermoresponsive Hyperbranched Polyethers. J. Am. Chem. Soc. 2006, 128, 8144−8145. (26) Jin, H.; Huang, W.; Zhu, X.; Zhou, Y.; Yan, D. Biocompatible or biodegradable hyperbranched polymers: from self-assembly to cytomimetic applications. Chem. Soc. Rev. 2012, 41, 5986−5997. (27) Johnston-Hall, G.; Theis, A.; Monteiro, M. J.; Davis, T. P.; Stenzel, M. H.; Barner-Kowollik, C. Accessing Chain Length Dependent Termination Rate Coefficients of Methyl Methacrylate (MMA) via the Reversible Addition Fragmentation Chain Transfer (RAFT) Process. Macromol. Chem. Phys. 2005, 206, 2047−2053. (28) Zeghida, W.; Besnard, C.; Lacour, J. Rhodium(II)-catalyzed onepot four-component synthesis of functionalized polyether macrocycles at high concentration. Angew. Chem., Int. Ed. 2010, 49, 7253−7256. (29) Bütün, V.; Bannister, I.; Billingham, N. C.; Sherrington, D. C.; Armes, S. P. Synthesis and Characterization of Branched WaterSoluble Homopolymers and Diblock Copolymers Using Group Transfer Polymerization. Macromolecules 2005, 38, 4977−4982. (30) Koh, M. L.; Konkolewicz, D.; Perrier, S. A Simple Route to Functional Highly Branched Structures: RAFT Homopolymerization of Divinylbenzene. Macromolecules 2011, 44, 2715−2724. (31) Bütün, V.; Bannister, I.; Billingham, N. C.; Sherrington, D. C.; Armes, S. P. Synthesis and Characterization of Branched WaterSoluble Homopolymers and Diblock Copolymers Using Group Transfer Polymerization. Macromolecules 2005, 38, 4977−4982. (32) Gao, H.; Min, K.; Matyjaszewski, K. Determination of gel point during atom transfer radical copolymerization with cross-linker. Macromolecules 2007, 40, 7763−7770. (33) Gao, H. F.; Miasnikova, A.; Matyjaszewski, K. Effect of CrossLinker Reactivity on Experimental Gel Points during ATRcP of Monomer and Cross-Linker. Macromolecules 2008, 41, 7843−7849. (34) Rosselgong, J.; Armes, S. P.; Barton, W. R. S.; Price, D. Synthesis of Branched Methacrylic Copolymers: Comparison between RAFT and ATRP and Effect of Varying the Monomer Concentration. Macromolecules 2010, 43, 2145−2156. (35) Vo, C. D.; Rosselgong, J.; Armes, S. P.; Billingham, N. C. RAFT synthesis of branched acrylic copolymers. Macromolecules 2007, 40, 7119−7125. (36) Tai, H. Y.; Duvall, C. L.; Hoffman, A. S.; Stayton, P. S.; Wang, W. X. pH-Responsive Hyperbranched Copolymers from One-Pot RAFT Copolymerization. Macromol. Mater. Eng. 2012, 297, 1175− 1183. (37) Kennedy, R.; Ul Hassan, W.; Tochwin, A.; Zhao, T. Y.; Dong, Y. X.; Wang, Q.; Tai, H. Y.; Wang, W. X. In situ formed hybrid hydrogels from PEG based multifunctional hyperbranched copolymers: a RAFT approach. Polym. Chem. 2014, 5, 1838−1842.

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We are grateful for financial support from the National Natural Science Foundation of China (21202084 and 21372120). We also thank the Associate Professor Qian Wang, Yongliang Cui and Professor Husheng Yan for the helps in SEM, TEM, and DLS measurements.

■

REFERENCES

(1) Mori, H.; Müller, A. H. E. Hyperbranched (Meth)acrylates in Solution, Melt, and Grafted From Surfaces. Top. Curr. Chem. 2003, 228, 1−37. (2) Yates, C. R.; Hayes, W. Synthesis and applications of hyperbranched polymers. Eur. Polym. J. 2004, 40, 1257−1281. (3) Carlmark, A.; Hawker, C.; Hult, A.; Malkoch, M. New methodologies in the construction of dendritic materials. Chem. Soc. Rev. 2009, 38, 352−362. (4) Konkolewicz, D.; Monteiro, M. J.; Perrier, S. Dendritic and Hyperbranched Polymers from Macromolecular Units: Elegant Approaches to the Synthesis of Functional Polymers. Macromolecules 2011, 44, 7067−7087. (5) Dong, R.; Zhou, Y.; Zhu, X. Supramolecular dendritic polymers: from synthesis to applications. Acc. Chem. Res. 2014, 47, 2006−2016. (6) Astruc, D.; Lu, F.; Aranzaes, J. R. Nanoparticles as recyclable catalysts: the frontier between homogeneous and heterogeneous catalysis. Angew. Chem., Int. Ed. 2005, 44, 7852−7872. (7) Boas, U.; Heegaard, P. M. Dendrimers in drug research. Chem. Soc. Rev. 2004, 33, 43−63. (8) Yan, D. Y.; Zhou, Y. F.; Hou, J. Supramolecular self-assembly of macroscopic tubes. Science 2004, 303, 65−67. (9) Zhou, Y. F.; Yan, D. Y. Supramolecular self-assembly of amphiphilic hyperbranched polymers at all scales and dimensions: progress, characteristics and perspectives. Chem. Commun. 2009, 1172−1188. (10) Rodríguez-Hernán dez, J.; Chéc ot, F.; Gnanou, Y.; Lecommandoux, S. Toward “smart” nano-objects by self-assembly of block copolymers in solution. Prog. Polym. Sci. 2005, 30, 691−724. (11) Chen, D. Y.; Jiang, M. Strategies for Constructing Polymeric Micelles and Hollow Spheres in Solution via Specific Intermolecular Interactions. Acc. Chem. Res. 2005, 38, 494−502. (12) Rybak, B. M.; Ornatska, M.; Bergman, N. K.; Genson, L. K.; Tsukruk, V. V. Formation of Silver Nanoparticles at the Air-Water Interface Mediated by a Monolayer of Functionalized Hyperbranched Molecules. Langmuir 2006, 22, 1027−1037. (13) Peleshankoa, S.; Tsukruk, V. V. The architectures and surface behavior of highly branched molecules. Prog. Polym. Sci. 2008, 33, 523−580. (14) Yeh, J. P.-Y.; Kainthan, K. R.; Zou, Y. Q.; Chiao, M.; Kizhakkedathu, N. J. Self-Assembled Monothiol-Terminated Hyperbranched Polyglycerols on a Gold Surface: A Comparative Study on the Structure, Morphology, and Protein Adsorption Characteristics with Linear Poly(ethylene glycol)s. Langmuir 2008, 24, 4907−4916. (15) Garamus, M. V.; Maksimova, T.; Richtering, W.; Aymonier, C.; Thomann, R.; Antonietti, L.; Mecking, S. Solution Structure of Metal Particles Prepared in Unimolecular Reactors of Amphiphilic Hyperbranched Macromolecules. Macromolecules 2004, 37, 7893−7900. (16) Kuo, P. L.; Liang, W. J.; Wang, F. Y. Hyperbranchpolyethyleniminated functional polymers. I. Synthesis, characterization of novel ABA type of dumbbell-like polyethyleniminated polyoxypropylenediamines, and their complexing properties with copper(II) ions in aqueous solution. J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 1360−1370. (17) Balz, M.; Barriau, E.; Istratov, V.; Frey, H.; Tremel, W. Controlled Crystallization of CaCO3 on Hyperbranched Polyglycerol Adsorbed to Self-Assembled Monolayers. Langmuir 2005, 21, 3987− 3991. H

DOI: 10.1021/acs.langmuir.5b03865 Langmuir XXXX, XXX, XXX−XXX

Article

Langmuir (38) Matyjaszewski, K.; Tsarevsky, N. V. Macromolecular engineering by atom transfer radical polymerization. J. Am. Chem. Soc. 2014, 136, 6513−6533. (39) Flory, P. J. Molecular Size Distribution in Three Dimensional Polymers. I. Gelation1. J. Am. Chem. Soc. 1941, 63, 3083−3090. (40) Stockmayer, W. H. Theory of Molecular Size Distribution and Gel Formation in Branched-Chain Polymers. J. Chem. Phys. 1943, 11, 45. (41) Rosselgong, J.; Armes, S. P.; Barton, W.; Price, D. Synthesis of Highly Branched Methacrylic Copolymers: Observation of Near-Ideal Behavior using RAFT Polymerization. Macromolecules 2009, 42, 5919−5924. (42) Li, Y.; Armes, S. P. Synthesis of Model Primary Amine-Based Branched Copolymers by Pseudo-Living Radical Copolymerization and Post-polymerization Coupling of Homopolymers. Macromolecules 2009, 42, 939−945. (43) Rosselgong, J.; Armes, S. P. Quantification of Intramolecular Cyclization in Branched Copolymers by 1H NMR Spectroscopy. Macromolecules 2012, 45, 2731−2737. (44) Terashima, T.; Kawabe, M.; Miyabara, Y.; Yoda, H.; Sawamoto, M. Polymeric pseudo-crown ether for cation recognition via cation template-assisted cyclopolymerization. Nat. Commun. 2013, DOI: 10.1038/ncomms3321. (45) Zhang, L.; Eisenberg, A. Multiple Morphologies of ″Crew-Cut″ Aggregates of Polystyrene-b-poly(acrylic acid) Block Copolymers. Science 1995, 268, 1728−1731. (46) Discher, D. E.; Eisenberg, A. Polymer vesicles. Science 2002, 297, 967−973. (47) Yin, L. G.; Hillmyer, M. A. Disklike Micelles in Water from Polyethylene-Containing Diblock Copolymers. Macromolecules 2011, 44, 3021−3028. (48) Pochan, D. J.; Zhu, J. H.; Zhang, K.; Wooley, K. L.; Miesch, C.; Emrick, T. Multicompartment and multigeometry nanoparticle assembly. Soft Matter 2011, 7, 2500−2506. (49) Gonzalez, D. C.; Savariar, E. N.; Thayumanavan, S. Fluorescence patterns from supramolecular polymer assembly and disassembly for sensing metallo- and nonmetalloproteins. J. Am. Chem. Soc. 2009, 131, 7708−7716. (50) Arumugam, S.; Vutukuri, D. R.; Thayumanavan, S.; Ramamurthy, V. Amphiphilic homopolymer as a reaction medium in water: product selectivity within polymeric nanopockets. J. Am. Chem. Soc. 2005, 127, 13200−13206. (51) Liu, J.; Huang, W.; Pang, Y.; Huang, P.; Zhu, X.; Zhou, Y.; Yan, D. Molecular self-assembly of a homopolymer: an alternative to fabricate drug-delivery platforms for cancer therapy. Angew. Chem., Int. Ed. 2011, 50, 9162−9166. (52) Yu, Y.; Zhang, L.; Eisenberg, A. Multiple Morphologies of CrewCut Aggregates of Polybutadiene-b-poly(acrylic acid) Diblocks with Low Tg Cores. Langmuir 1997, 13, 2578−2581. (53) Yu, K.; Zhang, L.; Eisenberg, A. Novel Morphologies of “CrewCut” Aggregates of Amphiphilic Diblock Copolymers in Dilute Solution. Langmuir 1996, 12, 5980−5984. (54) Zhang, L.; Yu, K.; Eisenberg, A. Ion-Induced Morphological Changes in ″Crew-Cut’’ Aggregates of Amphiphilic Block Copolymers. Science 1996, 272, 1777−1779. (55) Liu, X. Y.; Kim, J. S.; Wu, J.; Eisenberg, A. Bowl-shaped aggregates from the self-assembly of an amphiphilic random copolymer of poly(styrene-co-methacrylic acid). Macromolecules 2005, 38, 6749−6751. (56) Riegel, I. C.; Eisenberg, A.; Petzhold, C. L.; Samios, D. Novel bowl-shaped morphology of crew-cut aggregates from Amphiphilic block copolymers of styrene and 5-(N,N-diethylamino)isoprene. Langmuir 2002, 18, 3358−3363. (57) Wang, J.; Kuang, M.; Duan, H.; Chen, D.; Jiang, M. pHdependent multiple morphologies of novel aggregates of carboxylterminated polymide in water. Eur. Phys. J. E: Soft Matter Biol. Phys. 2004, 15, 211−215.

(58) Roberts, D. A.; Crossley, M. J.; Perrier, S. Fluorescent bowlshaped nanoparticles from ‘clicked’ porphyrin−polymer conjugates. Polym. Chem. 2014, 5, 4016−4021. (59) Yang, Z.; Wang, X.; Yang, Y.; Liao, Y.; Wei, Y.; Xie, X. Synthesis of Electroactive Tetraaniline−PEO−Tetraaniline Triblock Copolymer and Its Self-Assembled Vesicle with Acidity Response. Langmuir 2010, 26, 9386−9392. (60) Wang, W.; Zhang, M. J.; Chu, L. Y. Functional polymeric microparticles engineered from controllable microfluidic emulsions. Acc. Chem. Res. 2014, 47, 373−384.

I

DOI: 10.1021/acs.langmuir.5b03865 Langmuir XXXX, XXX, XXX−XXX