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DOI: 10.1039/C4RA09347J (Communication) RSC Adv., 2014, 4, 45890-45894

Photoinduced deformation of hollow nanospheres formed by the self-assembly of amphiphilic random copolymers and small azo molecules†

Cheng Jina, Taoran Zhanga, Lingyu Wanga, Meiying Hea, Tinglian Yuana, Bo Jianga, Dequan Xiaob and Qinjian Yin*a
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 27th August 2014 , Accepted 4th September 2014

First published on 11th September 2014

Abstract

Size controllable hollow nanospheres have been fabricated through self-assembly of polymer/azo-compound pairs in aqueous media. A unique photoinduced deformation behavior of nanospheres was observed, when the nanospheres were irradiated by a linearly polarized Ar+ laser beam.

The structures with hollow nanospheres have attracted much attention owing to their unique architecture and broad applications in drug-delivery,1 catalysis,2 and diagnostics.3 Polymeric hollow nanospheres have been generally prepared from amphiphilic block copolymers4 (with a few exceptions5). Because the synthesis conditions of block copolymers are complex and demanding, large-scale production remains difficult. 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 preparation hollow nanospheres based on random copolymer and small molecular pairs. Whether hollow structures can be formed by self assembly based on random copolymer and small molecular pairs is an question.

Polymers that contain azobenzene chromophores have attracted great interest in recent years for their unique photoresponsive properties induced by the photochemical E/Z isomerization of the azobenzene units, which have great potential applications in many fields such as information storage,6 waveguide switch,7 light-controllable artificial muscles8 etc. Design of photosensitive polymers with optimizing properties to satisfy different applications is an important task. Presently, various photosensitive materials have been obtained by doping low-molecular-weight azo-dyes in polymers or covalently attaching azo-moiety to polymer side chain, polymer backbone, or dendrimer polymers. However, the doping material usually has low azo content due to poor compatibility. The chemical synthesis processes of the photosensitive materials are usually rigorous, fussy and involved many steps with low yield.9 Hence design of photoresponsive azo-polymers with different structures remains an attractive challenge. Previously, we reported a novel method to fabricate polymeric hollow nanospheres via the self-assembly of side-chain azocomplex.10 Firstly, PAN-stat-P4VP was synthesized by free radical copolymerization. Second, side-chain azo complex was fabricated by the ionic self-assembly of protonated PAN-stat-P4VP+ and anionic azobenzene dye metanil yellow (MY) in aqueous solution. After that, the hollow nanospheres were formed through gradual hydrophobic aggregation of the azocomplexes in mixed aqueous–organic solvents. In the preparation process of azo complexes, PH and the dropping speed of metanil yellow must be controlled accurately. Whether the PH was too high or too low, PAN-stat-P4VP and MY cannot form azo complexes. The preparation process is fussy and rigorous. Moreover, the toxicity of AN is so high that it is not suitable in practical application. So we want to find a route that are not just more convenient but also greener. In this article, the hollow nanospheres are obtained by one-step synthesis method. PMMA-stat-P4VP and AZO were first dissolved in ethanol, then water was added to the ethanol solution, and finally hollow nanosphere were obtained. In order to verify whether this azo material has light response or not, we studied the photoinduced deformation behaviors of azo aggregates. The findings show that although azo molecules connect to polymer by a weak hydrogen-bond interaction in comparison to covalent bond and ionic bond, the azo aggregates still possess light response. This discovery provides a new horizon for researchers on optical material.

The poly(methyl methacrylate-stat-poly(4-vinylpyridine)) copolymer (PMMA-stat-P4VP) and the azo-compound 4-penylazophenol (AZO) were selected due to the amphipathy of PMMA-stat-P4VP and that the 4VP segment can associate with AZO by hydrogen-bonding. The copolymer PMMA-stat-P4VP was obtained via free radical reaction (denoted as free-PMMA42-stat-P4VP58). The number-average molecular weight of the copolymer is 53[thin space (1/6-em)]600 with the polydispersity index (PDI) of 1.8 measured by gel permeation chromatography (GPC) and the molar ratio of VP[thin space (1/6-em)]:[thin space (1/6-em)]MMA estimated from 1H-NMR spectrum is 58[thin space (1/6-em)]:[thin space (1/6-em)]42, which is described in Table S1.†

For fabrication of hollow nanospheres, the free-PMMA42-stat-P4VP58 and AZO were first dissolved in ethanol to form a homogeneous solution of azocomplex with the concentration of copolymer at 0.5 mg ml−1 (AZO/4VP = 0.7). Then deionized water (1.5 ml, in a typical case) was added dropwise into the ethanol solution (say 1.0 ml). The hollow nanospheres were finally obtained by slow evaporation of liquid, which was carried out at room temperature for about 24 h. FT-IR confirms that hydrogen bonds are formed between P4VP-stat-MMA and Azo (described in Fig. S3 and S5†).

Fig. 1a displays TEM photographs of aggregates formed by free-PMMA42-stat-P4VP58/AZO0.7, in which three kinds of aggregates are observed. The size of solid nanosphere is concentrated at 200 and 700 nm. The size of hollow nanospheres is about 300–500 nm. The rest are small disorder aggregates. The average size of the aggregates is about 600 nm, and the proportion of hollow nanospheres is about 50% (estimated statistically from TEM images). The inside diameter of hollow nanospheres is about 100 nm. The size distribution of the nanospheres was obtained by the dynamic light scattering (DLS). As shown in Fig. 1c, the PDI of the aggregates is 0.44 and average size is 600 nm, which is consistent with the result by TEM. The non-uniformity of copolymer molecules may lead to the broad distribution of aggregates. Due to the heterogeneity of the copolymer composition, some polymer chains may have more MMA than 42% (average content of MMA in free-PMMA42-stat-P4VP58), and other may have more VP than 58% (average content of VP in free-PMMA42-stat-P4VP58). We expect that, increasing the content of MMA units in the copolymer free-PMMA42-stat-P4VP58 will favour the formation of small solid nanospheres. Which may be further gathered to form large solid nanospheres. On the other hand, when the amount of VP units becomes more than average content of VP in free-PMMA42-stat-P4VP58, the hollow nanospheres and small disorder aggregates could be formed due to increase of hydrophily. Hence, we observed three kinds of aggregates in experiments. In order to prove our hypothesis, we synthesized homogeneous raft-PMMA-stat-P4VP via reversible addition–fragmentation chain-transfer (RAFT) copolymerization and studied the self-assembly process of the aggregates.


image file: c4ra09347j-f1.tif
Fig. 1 (a) TEM images of polymer nanospheres formed by the self-assembly of free-PMMA42-stat-P4VP58/AZO (b) the self-assembly of raft-PMMA45-stat-P4VP55-AZO0.7 (c) Size distribution profile for the nanospheres based on the azocomplex free-PMMA42-stat-P4VP58/AZO0.7 measured by dynamic light scattering analysis when the water content was 60% (d) relationship between the size of the nanospheres and their initial polymer concentrations in ethanol (the initial concentration of raft-PMMA45-stat-P4VP55-AZO0.7 in ethanol was: (d-a) 1.0 mg mL−1, (d-b) 0.75 mg ml−1, (d-c) 0.50 mg mL−1, (d-d) 0.25 mg mL−1).

In conventional free-radical copolymerization, the monomers are consumed at different rates dictated by the steric and electronic properties of reactants. Consequently, conventional copolymers are generally not homogeneous in composition at the molecular level. In RAFT polymerization processes, where all chains grow throughout the polymerization chains have similar composition and are called gradient or tapered copolymers. Thus, the RAFT polymerization processes allow us to obtain copolymers with homogeneous chemical composition per chain.

We first synthesized raft-PMMA45-stat-P4VP55 using CDB as a RAFT agent, 2,2′-azobisisobutyronitrile (AIBN) as initiator and DMF as solvent. The number-average molecular weight and PDI were measured by GPC and the molar ratio of VP[thin space (1/6-em)]:[thin space (1/6-em)]MMA estimated from 1H-NMR is shown in Table S1† raft-PMMA45-stat-P4VP55 and AZO are self-assembled for preparing hollow nanospheres. The conditions of self assembly are consistent with free-PMMA42-stat-P4VP58-AZO0.7 (the initial concentration of raft-PMMA45-stat-P4VP55 is 0.5 mg ml−1, AZO/4VP = 0.7). The aggregate morphology and PDI of raft-PMMA45-stat-P4VP55-AZO0.7 are shown in Fig. 1b and (d-c). The size of aggregate is about 600 nm with PDI of 0.24 when the water content reaches 60 vol%. Compared with free-PMMA42-stat-P4VP58-AZO0.7 the content of hollow nanospheres increases dramatically and the PDI decreases from 0.44 to 0.24. Thus, the content and uniformity of hollow nanospheres are increased by RAFT copolymerization.

To study the effects of AZO/VP ratios, two series of raft-PMMA45-stat-P4VP55-AZO1 and raft-PMMA45-stat-P4VP55-AZO0.4 azocomplexes were prepared with the AZO[thin space (1/6-em)]:[thin space (1/6-em)]VP ratios of 1[thin space (1/6-em)]:[thin space (1/6-em)]1 and 0.4[thin space (1/6-em)]:[thin space (1/6-em)]1 respectively based on raft-PMMA45-stat-P4VP55. TEM images for all the polymeric hollow nanospheres were taken with the water content of 60%. As shown in Fig. 2a and b and S2,† when the AZO[thin space (1/6-em)]:[thin space (1/6-em)]VP ratio is 1[thin space (1/6-em)]:[thin space (1/6-em)]1, the solid nanospheres are obtained with the diameter of 854 nm. When the AZO[thin space (1/6-em)]:[thin space (1/6-em)]VP ratio decreases to 0.4[thin space (1/6-em)]:[thin space (1/6-em)]1, the hollow nanospheres and small disorder aggregate with average size of 496 nm are obtained primarily. The volume of hollow nanosphere is about 150 nm (estimated statistically from TEM images), which is bigger than raft-PMMA45-stat-P4VP55/AZO0.7. The effects of MMA[thin space (1/6-em)]:[thin space (1/6-em)]VP ratio are also studied. Two different raft-copolymer raft-PMMA31-stat-P4VP69 and raft-PMMA57-stat-P4VP43 were synthesized as shown in Table S1,† raft-PMMA31-stat-P4VP69-AZO0.7 and raft-PMMA57-stat-P4VP43-AZO0.7 azocomplexes were also synthesized with the MMA[thin space (1/6-em)]:[thin space (1/6-em)]VP ratios of 31[thin space (1/6-em)]:[thin space (1/6-em)]69, 57[thin space (1/6-em)]:[thin space (1/6-em)]43, where the ratio of AZO[thin space (1/6-em)]:[thin space (1/6-em)]VP is fixed at 0.7[thin space (1/6-em)]:[thin space (1/6-em)]1. As shown in Fig. 2c and d and S2,† as MMA[thin space (1/6-em)]:[thin space (1/6-em)]VP ratio decreased to 31[thin space (1/6-em)]:[thin space (1/6-em)]69 the hollow nanospheres and a mass of small disorder aggregates with average size of 768 nm are obtained. When the MMA[thin space (1/6-em)]:[thin space (1/6-em)]VP ratio increased to 57[thin space (1/6-em)]:[thin space (1/6-em)]43, the hollow sphere disappears and solid sphere with the size of 394 nm appears. The volume of hollow nanosphere is about 160 nm, which is bigger than raft-PMMA45-stat-P4VP55/AZO0.7.


image file: c4ra09347j-f2.tif
Fig. 2 TEM images of polymer nanospheres formed by the self-assembly of (a) raft-PMMA45-stat-P4VP55-AZO1 (b) raft-PMMA45-stat-P4VP55-AZO0.4 (c) raft-PMMA57-stat-P4VP43-AZO0.7 (d) raft-PMMA31-stat-P4VP69-AZO0.7.

In order to understand the formation process of the hollow spheres, the aggregates were prepared by adding deionized water dropwise into the ethanol solutions of raft-PMMA45-stat-P4VP55/AZO0.7 to reach the preset water content (the initial concentration of raft-PMMA45-stat-P4VP55 is 0.5 mg ml−1). Then, the sizes and PDI of the nanospheres are obtained by DLS. With increasing of water content from 40 vol% to 60 vol%, the average size of polymeric aggregate decreases from 645 to 600 nm. When the water content increases from 60 vol% to 90 vol%, the average size of polymeric aggregates undergoes a significant decrease from 600 nm to 250 nm with a changed PDI in the range of 0.24–0.08 as show in Fig. 3a and b. It suggests that the polymeric aggregates, existing in the dispersion with water content of 40–60 vol%, have characteristics of loose aggregates. If excess water is added at this stage, the loosely associated polymer chains collapse to colloids with solid interiors through hydrophobic interactions.


image file: c4ra09347j-f3.tif
Fig. 3 Sizes and its distribution for polymer nanospheres formed in H2O/ethanol media with different water contents.

The morphology and size of aggregate are governed mainly by the collective effect of the hydrophilic/hydrophobic interactions between azocomplex and solvents. Increasing the MMA and AZO content leads to formation of solid nanospheres because of the enhanced hydrophobic interaction among polymer chains. Increasing the content of VP leads to (i) increase of hollow nanospheres size, (ii) formation of small disorder aggregates that could not form hollow nanospheres because of the enhancement of hydrophilic interaction.

The size of the hollow spheres could be adjusted by changing the initial polymer concentration in ethanol. The relationship between the sizes of aggregates and the concentration was studied by DLS. Fig. 1d shows sizes of aggregates as a function of the initial polymer concentration in ethanol. When the initial concentration of raft-PMMA45-stat-P4VP55 changes from 0.25 to 1.00 mg mL−1, in which the ratio of AZO/VP is 0.7 for all the samples, the size of aggregates could be adjusted from 531 to 1210 nm (i.e., 531, 615, 906, 1210 nm). The data of polydispersity index are given in Fig. 1d (0.20, 0.24, 0.33 and 0.41).

For the light irradiation test, the raft-PMMA45-stat-P4VP55-AZO0.7 hollow nanospheres were cast on copper sheet and dried carefully under ambient conditions. A linearly polarized Ar+ laser beam (405 nm, 100 mW cm−2) was used as the light source. The hollow nanospheres were exposed to the laser beam for different time periods followed by SEM study. It was found that the nanospheres are significantly deformed after the laser irradiation.

Fig. 4 shows the SEM images of the hollow nanospheres before and after the light irradiation for different time periods. Fig. 4b–d shows SEM images of the hollow nanospheres after exposure to a linearly polarized light for 0.5 h, 1.0 and 1.5 h, respectively. The deformation behavior can be obtained through the statistical method. The l/m (l represent long axis of ellipsoid and m represent minor axis) of the hollow nanospheres (estimated statistically from SEM images of 20 spheres) was used to characterize the deformation of the aggregates. The average axial ratio is 1.5, 2.0 and 3.0 aggregates after irradiation for 0.5, 1.0 and 1.5 hours respectively. It can be found that the l/m ratio increases almost linearly with the irradiation time in this range. It indicates that the hollow nanospheres with different l/m ratios can be prepared by simply adjusting the irradiation time.


image file: c4ra09347j-f4.tif
Fig. 4 Typical SEM images of the colloidal spheres composed of raft-PMMA45-stat-P4VP55/AZO0.7,which were irradiated with a linearly polarized Ar+ laser beam (405 nm, 100 mW cm−2) for (a) 0 min; (b) 30 min; (c) 60 min; (d) 90 min.

In the polymers containing azo dyes, anisotropy arises from two mainly contrasting mechanisms, photo-induced softening mechanism and photochemical reorientation mechanism. M. Saphiannikova11 presents that light-induced softening is a very weak accompanying effect rather than a necessary condition for the formation of surface relief gratings. They use molecular dynamics simulations to explain that the chromophore reorientation alone is capable of deformation under homogeneous illumination. In the reorientation mechanism the influence of the linearly polarized light leads to orientation of the chromospheres as shown in Scheme 1. Maximal probability of the transition from the rod-like trans-state to the bent cis-state is achieved at such orientation of the rod-like chromospheres, when its long axis is parallel to the electric vector of the light E.12 As a result, after multiple trans–cis–trans photoisomerization cycles the number of rod-like chromospheres that are arranged parallel to the vector E, becomes lower than the number of chromospheres which are oriented in perpendicular direction. In order to study the arrangement of AZO in the aggregate, UV-visible spectra were used. Interesting phenomena in aggregates containing azobenzenes are J-aggregation and H-aggregation.13 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).


image file: c4ra09347j-s1.tif
Scheme 1 Schematic illustration for the formation process of PMMA-stat-P4VP/AZO hollow nanospheres and elongated hollow nanospheres.

The H-aggregate is also 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. As show in Fig. S4,† the maximum absorption peaks of hollow nanosphere at 354 nm and 365 nm before and after irradiation its blueshift 21 nm and 10 nm respectively when compared with 375 nm. Our previous studies show that H-aggregates exist in hollow nanospheres which are obtained by self assembly of azocomplex using the same approach.10,14 Hence, the phenomenon of blueshift is observed before irradiation. After undergoing multiple trans–cis–trans photoisomerization cycles in the process of irradiation, azo chromophores are aligned parallel to each other, which makes it easier to form H-aggregates. Hence the phenomenon of blueshift is also observed after irradiation. This phenomenon is consistent with reorientation mechanism. It is believed that the deformation effect demonstrated in this article will shed new light on the understanding of the nature of photoinduced deformation in general and can be used as a new way to prepare deformable hollow nanospheres.

Conclusions

In summary, we firstly report here that the hollow microspheres can be formed based on the self-assembly of amphiphilic random copolymer PMMA-stat-P4VP and AZO compound 4-penylazophenol. The ratio and uniformity of the polymer nanospheres were improved using PMMA-stat-P4VP from the RAFT copolymerization rather than from the free radical polymerization. The size and content of hollow nanospheres can be controlled by adjusting the composition of copolymer and the conditions of self-assembly. Secondly, a unique photoinduced deformation behavior of the polymer nanospheres was observed, upon the irradiation of a linearly polarized Ar+ laser beam. The hollow microspheres were stretched to elongated hollow structures. The deformation degree increases as the irradiation time increases.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra09347j
This journal is © The Royal Society of Chemistry 2014
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