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DOI: 10.1039/C3RA46242K (Paper) RSC Adv., 2014, 4, 8216-8223

Fabrication of size controllable polymeric hollow nanospheres containing azo functional groups via ionic self-assembly†

Cheng Jina, Taoran Zhanga, Fangzhuan Liua, Lingyu Wanga, Qinjian Yin*a and Dequan Xiao*b
aKey Laboratory of Green Chemistry and Technology and College of Chemistry, Sichuan University, Chengdu, China. E-mail: changer@scu.edu.cn; Fax: +86 28 85412907; Tel: +86 28 85418112
bDepartment of Chemistry of Chemical Engineering, University of New Haven, West Haven, USA. E-mail: dxiao@newhaven.edu

Received 30th October 2013 , Accepted 9th January 2014

First published on 10th January 2014

Abstract

Fabrication of size controllable polymeric hollow nanospheres with azo functional groups is of great interest for applications in biomedical engineering, electronics, optics, and diagnostics. We report here a facile and economic way of fabricating polymeric hollow nanospheres with azo functional groups using ionic self-assembly of random copolymers poly(acrylonitrile)-stat-poly(4-vinyl-pyridine) and azobenzene dye metanil yellow (MY). The size of the hollow nanospheres is homogeneous and can be conveniently controlled by varying molar ratios of the copolymers to MY and molar ratios of monomers of the copolymers. The composition of the polymeric nanospheres and the self-assembly behaviors were characterized by 1H-NMR, FTIR, UV-Vis spectrophotometry, TEM, dynamic light scattering and elemental analysis. The formation process of hollow nanospheres was considered based on the results from UV-Vis spectrophotometry and TEM. Finally, the effects of azo complex composition on the morphology of the nanospheres were discussed, and the formation mechanism of polymeric hollow nanospheres was proposed.

Introduction

Azobenzene-containing polymers have photoresponsive properties, such as phase transition,1 chromophore orientation,2 surface relief grating formation,3 optical nonlinearity,4 and photoinduced deformation,5 which are caused by the trans-cis photoisomerization of azobenzenes. Azo polymers with different structures have been prepared to develop or optimize the photoresponsive function,6 by doping low molecular weight azo-dyes into a polymeric matrix or covalently attaching azo groups to the polymer side-chains or backbones. The doping azo-polymers usually have low azo content due to the poor compatibility, and the photoinduction property of the polymers is proved to be unstable.7 The azo-polymers obtained from covalent attachment have better photoinduction stability. However, the synthetic processes were usually rigorous and fussy and involved many steps with low-yield.8,9 Hence, the design of photoresponsive azo-polymers with different structures remains an attractive challenge. In this paper, we use ionic self-assembly to fabricate size-controllable polymeric hollow nanospheres that contain azo-dye functional groups.

Self-assembled polymeric materials with well-defined structures such as spheres, rods, vesicles, lamellas and other nanostructures have attracted increasing interests recently due to their potential applications in biomedical engineering, electronics, optics and diagnostics. In particular, polymeric hollow nanospheres have been used as encapsulates for drugs,10 enzymes,11,12 protein13 and catalysts.14,15 The hollow polymeric nanospheres are expected to have much higher efficiency of encapsulation compared with hollow nanospheres from small-molecule surfactants which have been used as transducers,16 absorbent materials for sound and microwaves,17 nanoreactors for the fabrication of nanodevices18 and label-free chip sensors.19 Polymeric hollow nanospheres have been generally prepared from amphiphilic block copolymer20,21 (with few exceptions),22 and these polymeric nanospheres show versatile nanostructures and stimuli-sensitive release behavior.23

Our group has been following a block-copolymer free strategy for making polymeric micelles and hollow spheres. However, to the best of our knowledge, there are few reports on random copolymer based hollow nanospheres. Previously, we reported a facile method to fabricate polymeric hollow nanospheres via the self-assembly of side-chain azo complexes.24 The random copolymer was synthesized by free radical copolymerization, and was then complexed with azo-dyes. The hollow nanospheres were formed through gradual hydrophobic aggregation of the polymeric chains in mixed aqueous-organic solvents, induced by a continuous increase of the water content. In order to control the size or volume of the polymeric hollow nanospheres, here we aim to study the influences of chemical composition (i.e., polymer structures and ratio of polymers and organic dyes) on the morphology of hollow nanospheres, based on gradient copolymers obtained from reversible addition-fragmentation chain-transfer (RAFT) polymerization. In conventional free-radical copolymerization, the monomers are consumed at different rates dictated by the steric and electronic properties of reactants. Consequently, both the monomer feed and copolymer composition drift with conversion. Conventional copolymers are generally not homogeneous in composition at the molecular level. In RAFT polymerization processes, where all chains grow throughout the polymerization, the composition drift is captured within the chain structures. All chains have similar composition and are called gradient or tapered copolymers. Thus, the RAFT polymerization will afford each copolymer chain with homogeneous chemical compositions. Since the RAFT copolymers used here are still statistical copolymers, they are suitable for synthesizing in a facile and economic way for large-scale production.

This paper is organized as the following. Firstly, a series of poly(acrylonitrile)-stat-poly(4-vinyl-pyridine) (designated as PAN-stat-P4VP) with different monomer ratios were synthesized by RAFT polymerization. Second, a series of side-chain azo complexes, which can be used in future studies and applications of organic polymer optical materials, were fabricated by the ionic self-assembly of protonated poly(acrylonitrile)-stat-poly(4-vinyl-pyridine) (designated as PAN-stat-P4VP+) and anionic azobenzene dye metanil yellow (MY, 3-[[4-(phenylamino) phenyl]azo]-benzene sulfuric acid monosodium salt) in aqueous solution. After that, a facile way (i.e., by mixing organic/aqueous solvents) was applied to obtain polymeric hollow nanospheres using the self-assembled azo complexes. It was found that the size or volume of polymeric hollow nanospheres can be controlled by adjusting the monomer ratios in the copolymers and the relative ratios of copolymers and organic dyes. Finally, the effects of azo complex composition on the morphology of nanospheres were discussed, and the formation mechanism of polymeric hollow nanospheres was proposed.

Experimental section

Materials

4-Vinyl pyridine (4-VP) (95%, Acros) and acrylonitrile (AN) (>99%, Chengdu Kelong Chemical Reagent Co.) were dried over calcium hydride and then distilled under reduced pressure prior to use. Azobisisobutyronitrile (AIBN) (C.P., Chengdu Kelong Chemical Reagent Co.) was recrystallized twice using ethanol. Deionized water (resistivity > 18 MΩ) was obtained from a millipore water purification system and used for the following experiments. Metanil yellow (MY) was obtained from Fluka. All the other chemical reagents were purchased from Chengdu Kelong Chemical Reagent Co. and used as received.

Synthesis of cumyl dithiobenzoate (CDB)25

CDB was synthesized in yield of 30.2% according to the method described in ref. 25. The 1H NMR spectrum of CDB is shown in the ESI (Fig. S1†), and the characterization peaks are assigned as the following: d (ppm): 2.02 (6H, s, CH3), 7.20–7.60 (8H, m, meta, ortho-ArH) and 7.86 (2H, m, para-ArH). The 13C NMR spectrum of CDB is shown in the ESI (Fig. S2†) and the characterization peaks are assigned as the following δ (ppm): 28.3, 56.5, 126.6, 128.2, 131.8, 144.2, and 146.3. The C[double bond, length as m-dash]S signal (δ > 220.0 ppm) was beyond the frequency range of the NMR spectrum. By FI-IR spectrum characterization, the characterization peaks of C[double bond, length as m-dash]S appear at 1042 cm−1 and 1221 cm−1 (see Fig. S3† in the ESI).

RAFT copolymerization of 4-vinylpyridine and acrylonitrile

RAFT of 4-vinylpyridine and acrylonitrile was performed using CDB as a macro-RAFT agent and 2, 2′-azobisisobutyronitrile (AIBN) as initiator. The molar ratio was [CDB-RAFT][thin space (1/6-em)]:[thin space (1/6-em)][AIBN] = 4[thin space (1/6-em)]:[thin space (1/6-em)]1. A dry Schlenk flask was charged with CDB macro-RAFT agent, 4-vinylpyridine, acrylonitrile, dimethylformamide (DMF), and AIBN. After three freeze–pump–thaw cycles, the reaction mixture was immersed in a thermostat oil bath at 70 °C. After the polymerization was carried out for 40 h, the reaction mixture was cooled to −40 °C. The polymer was precipitated by pouring the polymer solution into excess water while stirring. The precipitate was collected by filtration, and then dried in a vacuum oven at 60 °C overnight.

Synthesis of PAN-stat-P4VP/MY

PAN-stat-P4VP was neutralized with concentrated hydrochloric acid. An aqueous solution of 1.0 g L−1 copolymer was prepared. The pH was controlled at 2.5. A MY aqueous solution of 1.0 g L−1 was prepared. The titration of the polymer solution by metanil yellow was then conducted slowly with a speed of about 2–3 drops per second. The PAN-stat-P4VP/MY precipitates were filtered out, washed twice with copious amounts of hot water for removing any unbound MY from the complex, and dried under vacuum at 60 °C for 2 days.

Characterization

1H-NMR spectra were recorded by a Bruker AV II-400 NMR spectrometer at room temperature. Infrared spectra were recorded on Bruker Tensor 27 infrared spectrophotometer. UV-Vis spectra were recorded by Unico UV-2800A spectrophotometer from 300 nm to 600 nm at 25 °C. Transmission electron micrographs (TEM) were obtained on a JEM-100CX instrument. The molecular weight and molecular weight distribution were determined on a Waters 150C gel permeation chromatography (GPC) equipped with three Ultrastyragel columns (500, 103, 104 Å) in series and RI 2414 detector at 30 °C, and DMF was used as eluent at a flow rate of 1.0 mL min−1. To determine the size and distribution of polymeric hollow nanospheres, the system was analyzed by the dynamic light scattering instrument (DLS, Malvern Nano-ZS, and wavelength of 632.8 nm) at 25 °C.

Results and discussion

Synthesis of PAN-stat-P4VP copolymers

Numerous reports have shown that the RAFT process is applicable to statistical copolymerization.26–30 The copolymers via conventional free-radical polymerization are generally not homogeneous in composition at the single-chain level. In contrast, the gradient (or tapered) copolymers via the RAFT polymerization have similar composition in polymer chains and are homogeneous.

We synthesized four different gradient copolymers polyacrylonitrile-stat-poly(4-vinylpyridine) (denoted PANm-stat-P4VPn) via RAFT copolymerization in DMF using CDB as a macro-RAFT agent and 2,2-azobisisobutyronitrile (AIBN) as an initiator according to Scheme 1, while m and n represent the number of repeating units in PANm-stat-P4VPn. These gradient copolymers are different in terms of the monomer ratios AN[thin space (1/6-em)]:[thin space (1/6-em)]VP in the polymer chains, enabling us to conduct a systematic study of the influence of the monomer ratios on the structure of polymeric hollow nanospheres. Table 1 shows the specific AN[thin space (1/6-em)]:[thin space (1/6-em)]VP ratios for the copolymers. Fig. 1 shows the molecular weight distribution of the copolymers by GPC traces.


image file: c3ra46242k-s1.tif
Scheme 1 Synthesis routes for PAN-stat-P4VP copolymers and PAN-stat-P4VP/MY complexes.
Table 1 Molecular weight (Mn) and chemical compositions of the copolymers prepared by RAFT polymerization. PDI is the polydispersity index
PANm-stat-P4VPn Mn (g mol−1) VP(%) AN:VP PDI
PAN31-stat-P4VP23 4005 43% 57[thin space (1/6-em)]:[thin space (1/6-em)]43 1.26
PAN25-stat-P4VP30 4596 55% 45[thin space (1/6-em)]:[thin space (1/6-em)]55 1.19
PAN18-stat-P4VP40 4129 68% 32[thin space (1/6-em)]:[thin space (1/6-em)]68 1.18
PAN12-stat-P4VP46 5486 79% 21[thin space (1/6-em)]:[thin space (1/6-em)]79 1.15


image file: c3ra46242k-f1.tif
Fig. 1 GPC traces of four different PAN-stat-P4VP gradient copolymers: (a) PAN31-stat-P4VP23, (b) PAN25-stat-P4VP30, (c) PAN18-stat-P4VP40 and (d) PAN12-stat-P4VP46.

The characteristic H peaks of VP and the polymer backbone are at δ = 8.5 (H in the o-position of the N-atom) and δ = 1.0–3.2, respectively. Their relative ratios of AN[thin space (1/6-em)]:[thin space (1/6-em)]VP were determined by analyzing the ratio of the characteristic peak areas of VP and the polymer backbone by the 1H NMR analysis (see Fig. S4† in ESI). The molecular weights of the copolymers are in a range of 4000–6500 g mol−1 with a polydispersity around 1.2, indicating that the copolymers are monodispersive.

Preparation of PAN-stat-P4VP/MY azo complexes

Two series of PANm-stat-P4VPn/MYx azo complexes (see Tables 2 and 3) were synthesized by the electrostatic interactions between protonated PAN-stat-P4VP and anionic MY, where x represent the number of MY in the P4VPm-stat-ANn/MYx complex. The ratios of AN[thin space (1/6-em)]:[thin space (1/6-em)]VP was determined by 1H NMR analysis. In order to obtain the content of MY, the elemental analysis was used. PAN-stat-P4VP+ contains N element while MY contains not only N element but also O element. Consequently, the ratio of PAN-stat-P4VP+ unit to MY can be determined through the element analysis of O and N elements. In Table 2, the azo complexes with different VP[thin space (1/6-em)]:[thin space (1/6-em)]MY ratios (from 49% to 29% of MY) were synthesized using the copolymer PAN18-stat-P4VP40 where the AN[thin space (1/6-em)]:[thin space (1/6-em)]VP ratio is fixed at 68[thin space (1/6-em)]:[thin space (1/6-em)]32. In contrast, for the azo complexes in Table 3, the AN[thin space (1/6-em)]:[thin space (1/6-em)]VP ratios are varied (from 57[thin space (1/6-em)]:[thin space (1/6-em)]43 to 21[thin space (1/6-em)]:[thin space (1/6-em)]79), but the VP[thin space (1/6-em)]:[thin space (1/6-em)]MY ratio is fixed at ∼55[thin space (1/6-em)]:[thin space (1/6-em)]45.
Table 2 Molecular characterization of azo complexes with different VP[thin space (1/6-em)]:[thin space (1/6-em)]MY ratios. The inside and outside diameters and shell thickness for polymeric hollow nanospheres made by the corresponding azo complexes are obtained from the TEM analysis
PANm-stat-P4VPn/MYx AN[thin space (1/6-em)]:[thin space (1/6-em)]VP/1HNMR VP[thin space (1/6-em)]:[thin space (1/6-em)]MY/EA Inside diameter/nm Outside diameter/nm
PAN18-stat-P4VP40/MY38 32[thin space (1/6-em)]:[thin space (1/6-em)]68 51[thin space (1/6-em)]:[thin space (1/6-em)]49 68 131
PAN18-stat-P4VP40/MY31 32[thin space (1/6-em)]:[thin space (1/6-em)]68 56[thin space (1/6-em)]:[thin space (1/6-em)]44 55 108
PAN18-stat-P4VP40/MY26 32[thin space (1/6-em)]:[thin space (1/6-em)]68 61[thin space (1/6-em)]:[thin space (1/6-em)]39 31 93
PAN18-stat-P4VP40/MY16 32[thin space (1/6-em)]:[thin space (1/6-em)]68 71[thin space (1/6-em)]:[thin space (1/6-em)]29 23 77

Table 3 Composition characterization for the azo complexes with different AN[thin space (1/6-em)]:[thin space (1/6-em)]VP ratios. The inside and outside diameters and shell thickness for polymeric hollow nanospheres made by the corresponding azo complexes are obtained from the TEM analysis
PANm-stat-P4VPn/MYx AN[thin space (1/6-em)]:[thin space (1/6-em)]VP/1H-NMR VP[thin space (1/6-em)]:[thin space (1/6-em)]MY/EA Inside diameter/nm Outside diameter/nm
PAN31-stat-P4VP23/MY18 57[thin space (1/6-em)]:[thin space (1/6-em)]43 56[thin space (1/6-em)]:[thin space (1/6-em)]44 10 60
PAN25-stat-P4VP30/MY24 45[thin space (1/6-em)]:[thin space (1/6-em)]55 55[thin space (1/6-em)]:[thin space (1/6-em)]45 30 80
PAN18-stat-P4VP40/MY31 32[thin space (1/6-em)]:[thin space (1/6-em)]68 56[thin space (1/6-em)]:[thin space (1/6-em)]44 55 108
PAN12-stat-P4VP46/MY37 21[thin space (1/6-em)]:[thin space (1/6-em)]79 55[thin space (1/6-em)]:[thin space (1/6-em)]45 89 140

Synthesis and UV-Vis characterization of PAN-stat-P4VP/MY hollow nanospheres

The formation process of polymeric hollow nanospheres was investigated by monitoring the aggregation of azo-dyes using a UV-Vis spectrometer. Early studies indicated that ionic dyes could form different dye–dye aggregates and show metachromacy in aqueous or mixed aqueous-organic solutions.31,32 the π–π aggregation has been named by previous researchers according to the relative position of the aggregate absorption band to the molecular absorption band (M-band). H-aggregation denotes the aggregation showing a blue-shifted band (hypsochromic band or H-band) to the M-band and J-aggregation (after Jelley and Scheibe) refers to the aggregation exhibiting a red-shifted band and resonance fluorescence. Interesting phenomena in aggregates containing azobenzenes are J-aggregation and H-aggregation.33

As shown in Fig. 2, the J-aggregate is a one-dimensional molecular arrangement in which the transition moments of individual monomers are aligned parallel to the line joining their centers (end-to-end arrangement). The H-aggregate is a one-dimensional array of molecules in which the transition moments of individual monomers are aligned parallel to each other but perpendicular to the line joining their centers (face-to-face arrangement). The most characteristic feature of J-aggregate is that they exhibit redshift in the absorption spectrum with respect to the monomer absorption. The absorption spectrum of the H-aggregate consists of a blueshifted band with respect to the monomer absorption. The energy shift of the absorption bands of the aggregates has been explained by exciton theory. Hence the spectroscopic characteristics of azo chromophores in different states can be regarded as a probe to study the arrangement of MY in aggregate.


image file: c3ra46242k-f2.tif
Fig. 2 Schematic representation of the different arrangement of MY dyes.

To study the formation of polymeric nanospheres for PAN-stat-P4VP/MY in the DMF/H2O solvent, a homogeneous solution was prepared by dissolving the PAN18-stat-P4VP40/MY31 azo complex in DMF (a good solvent). Deionized water (a selective solvent for pyridium moieties) was gradually added into the complex solution under whisk. As the water content increases, the solubility of the PAN18-stat-P4VP40/MY31 azo complex becomes poorer for the hydrophobic groups (i.e., MY and AN), inducing the aggregation of the polymer chains and MY moieties. The pack arrangement of MY changes with the aggregates of polymer chains. The UV-Vis spectra of the MY chromophores in different stages are shown in Fig. 3a. As expected, the UV-Vis absorption intensity decreases as the water content increase because the concentration of the solutions decreases. Fig. 3b shows the relationship between the maximum UV-Vis absorbance peaks (λmax) and water contents in the DMF/H2O mixed solvents. As shown in Fig. 3b, λmax of PAN18-stat-P4VP40/MY31 in DMF/H2O redshifts from 419 nm to 440 nm as the water content increases from 0 to 80%. We attribute this phenomenon to the change of the solvation polarity due to the mixing of solvents and also the effect of J-aggregation of dyes. λmax is 434 nm in pure H2O (Fig. S6b,† Supporting Information) and is 419 nm in pure DMF (Fig. S5b,† Supporting Information). When the water content reaches 65%, λmax of PAN18-stat-P4VP40/MY31 is 435 nm, which is slightly longer than that of the PAN18-stat-P4VP40/MY31 in pure water, suggesting the presence of J-aggregation of dyes for the further red-shift of the absorption wavelength.


image file: c3ra46242k-f3.tif
Fig. 3 (a) UV-Vis spectra of the PAN-stat-P4VP/MY azo complexes in DMF/H2O solutions and (b) the change of λmax, with the increase of water content (v%) in the DMF/H2O mixed solvents.

The morphologies of polymeric hollow nanospheres were studied using transmission electron microscopy. Fig. 4 shows that the morphology of the aggregates changes from disordered phase to vesicles with the addition of water to the solution. Both disordered phase and vesicles were observed when the water content was in the range of 20–60 v% (Fig. 4a–b). Hollow nanospheres became the only morphology (Fig. 4c) when the water content reached 60 v%. The morphology and sizes of hollow nanospheres did not change any more when the water content increased to the range of 60–80 v%. But, the hollow nanospheres tend to stay together under the condition of high of water contents (60–80 v%). As show in Fig. 4c and d, the sizes of hollow nanospheres reached about 100 nm in diameter when the water content was 60%, and did not increase any more with the increase of water content. The sizes and distribution of hollow nanospheres were investigated by DLS and are shown in Fig. 5. According to the Malvern specification, samples with PDI≈0.2 are considered to be monodisperse.34 Thus, our hollow nanospheres are monodisperse. The hydrodynamic diameters estimated for our hollow nanospheres by the DLS analysis (Fig. 5) was found to be 80–120 nm, which is consistent with the TEM results. In order to study the formation mechanism of hollow nanospheres ulteriorly and the effects of the composition on the nanospheres morphology, we discuss the effect of VP: MY and AN: VP ratios on the aggregate morphology of the polymeric hollow nanospheres.


image file: c3ra46242k-f4.tif
Fig. 4 TEM images of polymeric hollow nanospheres formed by the self-assembly of PAN18-stat-P4VP40/MY26 in the DMF/H2O mixed solvent with different water contents (v%) of (a) 20%, (b) 40%, (c) 60%, (d) 80%.

image file: c3ra46242k-f5.tif
Fig. 5 Size distribution profile for hollow nanospheres based on the azo complex PAN18-stat-P4VP40/MY31 measured by dynamic light scattering analysis when the water content was 60%.

Effects of the 4VP[thin space (1/6-em)]:[thin space (1/6-em)]MY and AN[thin space (1/6-em)]:[thin space (1/6-em)]VP ratios on the nanosphere morphology

To study the effect of VP[thin space (1/6-em)]:[thin space (1/6-em)]MY ratios, a series of PAN-stat-P4VP/MY azo complexes (in Table 2) were synthesized with the VP[thin space (1/6-em)]:[thin space (1/6-em)]MY ratios of 51[thin space (1/6-em)]:[thin space (1/6-em)]49, 56[thin space (1/6-em)]:[thin space (1/6-em)]44, 61[thin space (1/6-em)]:[thin space (1/6-em)]39, and 71[thin space (1/6-em)]:[thin space (1/6-em)]29, respectively. The UV-Vis absorption λmax for these azo complexes in pure water and in pure DMF are shown in the ESI. In the solution of DMF/H2O, when the water content reached 80.0%, λmax of all the samples reached a maximum and did not increase any more with the increase of water content. To eliminate the influence of the mixing of solvents, TEM images for all the polymeric hollow nanospheres were taken with the water content of 80%. As shown in Fig. 6 and 7, for the azo complexes with VP[thin space (1/6-em)]:[thin space (1/6-em)]MY = 51[thin space (1/6-em)]:[thin space (1/6-em)]49, 56[thin space (1/6-em)]:[thin space (1/6-em)]44, 61[thin space (1/6-em)]:[thin space (1/6-em)]39, and 71[thin space (1/6-em)]:[thin space (1/6-em)]29, the outer diameters of hollow nanospheres are 131 nm, 108 nm, 93 nm and 77 nm, respectively, and the inner diameters are 68, 55, 31 and 23 nm, respectively. The wall thicknesses are nearly the same for all the hollow nanospheres with diameters larger than 30 nm.
image file: c3ra46242k-f6.tif
Fig. 6 TEM images of the polymeric hollow nanospheres made by the azo complexes of (a) PAN18-stat-P4VP40/MY38, (b) PAN18-stat-P4VP40/MY31, (c) PAN18-stat-P4VP40/MY26, and (d) PAN18-stat-P4VP40/MY16 with the water content of 80% for all sample.

image file: c3ra46242k-f7.tif
Fig. 7 Diameters of the polymeric hollow nanospheres as a function of the VP[thin space (1/6-em)]:[thin space (1/6-em)]MY ratios.

Hence, by increasing of the VP[thin space (1/6-em)]:[thin space (1/6-em)]MY ratios, the diameters of nanospheres decrease. Increasing the MY content in the vesicle wall leads to the increasing rigidity of the wall, this is because MY is a rigid chemical group. It will take more energy (i.e. bending energy) to bend the bilayer for forming a nanosphere with the same diameter. A nanosphere with a larger diameter is formed because it requests less bending (thus reduces the overall bending energy) of the bilayer. Therefore, the diameters of the polymeric hollow nanospheres decrease as the VP[thin space (1/6-em)]:[thin space (1/6-em)]MY ratios increase.

To study the effect of AN[thin space (1/6-em)]:[thin space (1/6-em)]VP ratio on nanosphere morphology, a series of PAN-stat-P4VP/MY azo complexes were synthesized with the AN[thin space (1/6-em)]:[thin space (1/6-em)]VP ratios of 57[thin space (1/6-em)]:[thin space (1/6-em)]43, 45[thin space (1/6-em)]:[thin space (1/6-em)]55, 32[thin space (1/6-em)]:[thin space (1/6-em)]68, and 21[thin space (1/6-em)]:[thin space (1/6-em)]79, where the ratio of VP[thin space (1/6-em)]:[thin space (1/6-em)]MY is fixed at ∼55[thin space (1/6-em)]:[thin space (1/6-em)]45.

The maximum UV-Vis absorption peaks of these azo complexes in pure water and pure DMF are shown the ESI (Fig. S8† and Fig. S9†). In the DMF/H2O solution, when the water content reaches 80.0% (Fig. S10†), the λmax for all the samples reached a maximum. To eliminate the influence of the mixing of solvents, TEM images were taken with the water content of 80% for all the samples. As shown in Fig. 8 and 9, for the azo complexes with AN[thin space (1/6-em)]:[thin space (1/6-em)]VP ratios of 57[thin space (1/6-em)]:[thin space (1/6-em)]43, 45[thin space (1/6-em)]:[thin space (1/6-em)]55, 32[thin space (1/6-em)]:[thin space (1/6-em)]68 and 21[thin space (1/6-em)]:[thin space (1/6-em)]79, the outer diameters of hollow nanospheres are 60 nm, 80 nm, 108 nm and 140 nm, respectively, and the inner diameters are 10 nm, 30 nm, 55 nm and 89 nm, respectively. The wall thicknesses are about 25–30 nm for all the samples. Hence, decrease in the AN[thin space (1/6-em)]:[thin space (1/6-em)]VP ratio (i.e. decreasing the AN content) causes the increase of both the outer and inner diameters of the nanospheres. We envision that, by increasing the content of protonated VP in the system, the bent bilayers (Scheme 2) tend to expend more due to the hydrophilic interaction between the pyridium groups in the outer sphere and water molecules, leading to the increase of the outer diameters. Because the MY content is fixed, the bilayer thickness does not change much, leading to the increase of the inner diameters.


image file: c3ra46242k-f8.tif
Fig. 8 TEM images of the polymeric hollow nanospheres made by the azocomplexes of (a) PAN31-stat-P4VP23/MY18, (b) PAN25-stat-P4VP30/MY24, (c) PAN18-stat-P4VP40/MY31, and (d) PAN12-stat-P4VP46/MY37 with the water content of 80% for all sample.

image file: c3ra46242k-f9.tif
Fig. 9 Diameters of the polymeric hollow nanospheres as a function of the AN[thin space (1/6-em)]:[thin space (1/6-em)]VP ratios.

image file: c3ra46242k-s2.tif
Scheme 2 Schematic illustration for the formation process of PAN-stat-P4VP/MY hollow nanospheres.

A schematic illustration of self-assembly process is shown in Scheme 2, which is a net result of the hydrophilic and hydrophobic interactions between the azo complexes and solvent molecules, and also the bending energy of bilayers (i.e., the energy cost for bending the bilayers). At the beginning, no obvious aggregation is formed when PAN-stat-P4VP/MY is dissolved in pure DMF, because DMF is a good solvent for PAN-stat-P4VP/MY. When water is added into the polymeric solution, the hydrophobic interactions between MY and water and the Van der Waals interactions among MY moieties induce the formation of a bilayer-like intermediate structure (see Scheme 2) of the azo complexes, with the hydrophilic pyridium groups staying on the two surfaces of the bilayers. With the further addition of water, the bilayers are bended over to form hollow nanospheres, where the hydrophilic groups (i.e., uncomplexed pyridium groups) are located in the outer and inner surfaces of the hollow nanospheres. The bilayers have boundaries where MY moieties are exposed to water (i.e., the end boundaries, see Scheme 2), which are not energetically favoured. Thus, it is feasible for the bilayers to bend into hollow nanospheres where such energetically disfavoured end boundaries do not exist any more. In this process of bending, the overall free energy decrease due to the diminishing of the end boundaries should overcome the bending energy of the bilayers. The redshift of λmax for the azo-dyes was observed due to the J-aggregation of MY in the bilayer interior. In real systems, the shell is thicker than bilayer model due to the rigidity structure of MY which impede the close contact of bilayer. So the phenomenon of redshift is not obvious.

The size of hollow nanospheres was governed mainly by the collective effect of the hydrophilic–hydrophobic interactions between azo complex and solvents and the mechanic bending energy of the bilayers. To control the sizes of hollow nanospheres, (i) increasing the AN content leads to smaller diameters because of the enhanced hydrophobic interaction among polymer chains, and (ii) increasing the MY content leads to larger diameters because of the enhanced bending energy of bilayers due to the structural rigidity of MY.

Conclusion

In this work, a new method based on ionic self-assembly technology was provided to prepare side chain azo complexes to fabricate polymeric hollow nanospheres with azo functional groups. By gradually adding water into the pure DMF solution of the PAN-stat-P4VP/MY, the homogeneous polymeric hollow nanospheres were obtained. By tuning the ratios of VP[thin space (1/6-em)]:[thin space (1/6-em)]MY and AN[thin space (1/6-em)]:[thin space (1/6-em)]VP, the feasibility of controlling the sizes of the polymeric hollow nanospheres was successfully demonstrated. The effects of azo complex composition on the morphology of nanospheres were discussed. Specifically, decreasing the VP[thin space (1/6-em)]:[thin space (1/6-em)]MY or AN[thin space (1/6-em)]:[thin space (1/6-em)]VP ratio leads to the increasing diameters and volume of polymeric hollow nanospheres. This work offers a new possibility for the design of complexed functional materials, which are difficult to synthesize by traditional chemical approaches. In addition, the hollow nanospheres with azo functional groups are anticipated be new basis material for the preparation of smart photoelectric nanomaterials in optical information storage, diffractive optical elements, and nonlinear optical devices.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c3ra46242k
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