a giant capsule from the self-assembly of a penta-telechelic hybrid poly(acrylic acid) based on...

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MacromolecularChemistry and Physics

DOI: 10.1002/macp.201400012

A Giant Capsule from the Self-Assembly of a Penta-Telechelic Hybrid Poly(acrylic acid) Based on Polyhedral Oligomeric Silsesquioxane

Weian Zhang , * Lizhi Hong , Patrick C. McGowan *

An amphiphilic penta-telechelic polyhedral oligomeric silsesquioxane (POSS)-containing inor-ganic/organic hybrid poly(acrylic acid), (Glu-PAA-POSS 5 ), is prepared by hydrolysis of penta-telechelic poly( tert -butyl acrylate) (Glu-P t BA-POSS 5 ), synthesized by the combination of atom transfer radical polymerization (ATRP) and a “click” reaction. The self-assembly behavior of Glu-PAA-POSS 5 in aqueous solution at pH 8.5 is investigated by using transmission elec-tron microscopy (TEM), scanning electronic microscopy (SEM), atomic force microscopy (AFM), and dynamic light scattering (DLS). The results show that Glu-PAA-POSS 5 , with a long poly(acrylic acid) (PAA) chain, can self-assemble in water into giant capsules, which provides an optional approach in the con-struction of capsules.

Prof. W. Zhang, Dr. L. Hong Shanghai Key Laboratory of Functional Materials Chemistry, School of Chemistry and Molecular Engineering, East China University of Science and Technology, 130 Meilong Road , Shanghai 200237 , China E-mail: [email protected] Prof. P. C. McGowan Department of Chemistry, University of Leeds , Leeds, LS2 9JT , UK E-mail: [email protected]

process parameters including concentration, temperature, pH, medium and additives, etc. [ 5–7 ] Among these reported morphologies, capsules have been recognized of great interest due to their powerful loading capability, especially for drug, biomacromolecules, lipids, dyes as well as nano-particles. [ 8,9 ] In the previous reports, capsules based on polymers have been fabricated mostly using polyelectro-lytes via layer-by-layer (LbL) self-assembly technique. [ 10,11 ]

Recently, the self-assembly behavior of hybrid copoly-mers containing inorganic components such as fullerene and carbon nanotubes has also attracted increasing atten-tion, since it can offer different assembly morphologies and functionalities, compared to the conventional pure organic copolymers. [ 12 ] Polyhedral oligomeric silsesqui-oxanes (POSS), with a well-defi ned nanostructure, have been widely used to produce novel polymer hybrids. The common variety consists of eight RSiO 1.5 units forming a cage-like inorganic core surrounded by eight organic corner groups, which offer POSS molecules with higher solubility in organic solvents and reactivity via func-tional corner groups. [ 13,14 ] So POSS can be easily incorpo-rated into polymeric matrices to prepare polymer hybrids

1. Introduction

In past several decades, self-assembly of amphiphilic copolymers in selective solution has attracted great interest since they demonstrate the potential to form well-defi ned morphologies, which offers the promising appli-cation in many fi elds, including drug delivery, catalysis, medical diagnostics, electronics, and sensors, etc. [ 1–4 ] The self-assembly can be tuned into a variety of morphologies such as micelles, vesicles, worms, nanorods and capsules, by changing the amphiphilic copolymer structure and

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A Giant Capsule from the Self-Assembly of a Penta-Telechelic Hybrid Poly(acrylic acid) . . .

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by physical blending or chemical copoly merization. [ 15–24 ] More recently, POSS molecules have been applied in construction of well-defi ned POSS-containing amphiphilic copolymers using POSS as monomers or initiators, and the self-assembly behavior of these hybrid amphiphilic copolymers has also been studied. [ 20–24 ] For example, Cheng et al. studied the self-assembly behavior of polystyrene-(carboxylic acid)-functionalized POSS conjugate (PS-APOSS) with “a hydrophilic head” and “a hydrophobic polymeric tail” in selective solvents, and the self-assembly morphology of PS-APOSS in solution can be tuned from vesicles to worm-like cylinders and further to spheres by varying the degree of ionization of the carboxylic acid groups of POSS moiety. [ 25,26 ] We also contributed some works to the self-assembly of well-defi ned POSS-containing hybrid polymers and obtained a lot of unexpected assembled morphologies in recent years. [ 27 ] We prepared “tadpole-shaped” (hemi-telechelic) hybrid poly( N -isopropylacrylamide) and poly(acrylic acid) (PAA) using reversible addition–fragmentation transfer polymerization, and found the tadpole-shaped POSS–PAAs self-assemble into rather large aggregates where the POSS moieties are dispersed in the particle, which is different with the typical core–shell micellar structure. [ 28,29 ] The telechelic POSS-containing PAA (POSS-PAA-POSS) was pre-pared by the combination of atom transfer radical poly-merization (ATRP) and “click chemistry,” and the results show that POSS-PAA-POSS with short PAA chain can self-assemble in water into ellipsoidal aggregates with a moderately uniform size, whereas POSS-PAA-POSS with a long PAA chain only self-assembles into aggregates with a broad size distribution. [ 30 ] To our best knowledge, there is no report on the capsules prepared from POSS containing hybrid polymers, especially for a giant capsule.

In this contribution, we continue to explore the self-assembly behavior of POSS-containing polyelectrolytes, and apply in the construction of capsules, which would provide a novel approach for preparation of capsules. We synthesized penta-telechelic Glu-PAA-POSS 5 , where the ends of the star-shaped PAA chain are attached by POSS. The synthetic strategy is shown in Scheme 1 . The syn-thesis was performed in four main steps. Penta-telechelic bromo-terminated P t BA (Glu-P t BA-Br 5 ) with different molecular weights were fi rstly prepared via ATRP using a penta-bromo-functionalized initiator (PBBG); subse-quently, the terminal bromo groups were transformed to azido groups using sodium azide in DMF via nucleophilic

substitution. Then the click coupling occurred between the penta-azido-P t BA (Glu-P t BA-(N 3 ) 5 ) and alkyne-func-tionalized POSS (alkyne-POSS) to afford penta-telechelic Glu-P t BA-POSS 5 . Finally, the POSS-containing polyelectro-lyte, telechelic Glu-PAA-POSS 5 , was obtained by hydrol-ysis of Glu-P t BA-POSS 5 using trifl uoroacetic acid (TFA). The self-assembly behavior of Glu-PAA-POSS 5 in aqueous solution was further investigated by using transmission electron microscopy (TEM), scanning electron microscopy (SEM), atomic force microscopy (AFM), and dynamic light scattering (DLS).

2. Results and Discussion

The penta-telechelic Glu-P t BA-Br 5 was synthesized at 60 °C by ATRP using CuBr/PMEDTA as the catalyst. The reaction conversion was determined by 1 H NMR by comparing the peaks of the double-bond protons at 5.75 ppm to those of aromatic protons at 7.29 ppm. The reaction was quenched at a desired conversion of t BA. The molecular weights and PDIs of the P t BA polymers are listed in Table 1 . The low polydispersity and well-symmetrical GPC curves (Figure S1, Supporting Information) indicate that the polymerization of t BA is well controlled using a penta-bromo-functional-ized ATRP initiator (PBBG).

The terminal bromo groups of Glu-P t BA-Br 5 were trans-formed into azido groups to afford telechelic Glu-P t BA-(N 3 ) 5 by nucleophilic substitution. The reaction was performed in DMF for 24 h at room temperature. Fourier transform IR (FTIR) spectroscopy was used to characterize the formation of terminal azido groups of Glu-P t BA-(N 3 ) 5



Scheme 1. Synthesis of penta-telechelic POSS–PAA by a combination of ATRP and “Click chemistry.”

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(Figure S2, Supporting Information). Compared with the FTIR spectrum of Glu-P t BA 218 -Br 5 , a peak at 2111 cm −1 appears in the spectrum of Glu-P t BA-(N 3 ) 5 , which is assigned to the stretching vibration of terminal azido groups. This indicates that the nucleophilic substitu-tion was effectively conducted, and the terminal bromo groups were transformed into azido groups.

The azide/alkyne “click” coupling between Glu-P t BA-(N 3 ) 5 and alkyne–POSS was performed at 80 °C for 24 h in DMF using CuBr/PMDETA as the catalyst. To complete the coupling reaction, the initial molar ratio of alkyne-POSS to azido groups of Glu-P t BA-(N 3 ) 5 was set to be 2/1. The unreacted excess alkyne-POSS was removed via dialysis against THF. The 1 H NMR spectrum of Glu-P t BA 218 -POSS 5 was shown in Figure S4 (Supporting Information). Com-pared with 1 H NMR spectrum of Glu-P t BA 218 -Br 5 (Figure S3, Supporting Information), except for the characteristic signals at δ = 2.25 ppm (d) and 1.45 ppm (f), respectively, ascribed to the methine protons of the backbone and the methyl protons of the tert -butyl groups of P t BA, the sig-nals at δ = 0.98 and 0.64 ppm (c) are, respectively, assigned to the methyl protons ( Si CH 2 CH(C H 3 ) 2 ) and methylene protons ( Si C H 2 CH(CH 3 ) 2 and Si C H 2 CH 2 CH 2 NH ) originating from alkyne-POSS. Moreover, it still can be discerned that the signal at δ = 5.17 (g) is ascribed to the methylene protons adjacent triazole ring in the enlarged image of 1 H NMR spectrum. Additionally, the FT-IR spectrum was also used to confi rm the click reac-tion (Figure S5, Supporting Information). Compared to the FT-IR spectrum of Glu-P t BA 218 -(N 3 ) 5 , the peak at 2111 cm −1 of azido stretching vibration completely disappeared and a new peak appeared at 1114 cm −1 in the spectrum of Glu-P t BA 218 -POSS 5 , which is assigned to the stretching vibra-tion of Si O Si. The GPC traces of Glu-P t BA-POSS 5 with different molecular weights are shown in Figure S1 (Sup-porting Information). All the curves of Glu-P t BA-POSS 5 shift to lower elution volume compared to those of their maternal Glu-P t BA-Br 5 , which means the alkyne-POSS has been successfully attached to star-shaped Glu-P t BAs by click coupling.

Glu-PAA 218 -POSS 5 was obtained by hydrolysis of the tert -butyl ester groups of Glu-P t BA 218 -POSS 5 in dichloromethane using TFA. The FTIR spectrum of

Glu-PAA 218 -POSS 5 is shown in Figure S5 (Supporting Infor-mation). Compared to the FTIR spectrum of Glu-P t BA 218 -POSS 5 , we can fi nd a broad absorption band appearing in the range from 2260 to 3662 cm −1 in the spectrum of Glu-PAA 218 -POSS 5 . The carbonyl peak at 1729 cm −1 is slightly shifted to lower wave numbers at 1701 cm −1 , and the peak also becomes broader. These are due to the forma-tion of carboxylic acid groups from the tert -butyl ester of Glu-P t BA 218 -POSS 5 .

In previous reports on the self-assembly of amphiphilic copolymers in solution, a variety of assembled morpholo-gies such as micelles, vesicles, worms, nanorods, and capsules, have been obtained using pure organic amphi-philic copolymers, [ 31 ] whereas the self-assembly behavior of amphiphilic hybrid copolymers containing inorganic components has rarely been studied. In recent several years, we prepared a series of POSS-containing amphiph-ilic hybrid copolymers, and systematically studied their self-assembly behavior in aqueous solution, and we got many interesting self-assembled morphologies, which are different from the conventional pure organic amphi-philic copolymers.

For example, we synthesized hemi-telechelic POSS-end-functional amphiphilic polymers such as poly(ethylene oxide) (PEO) and PAA, and found the self-assembly behavior to be different from that of the typical amphiph-ilic polystyrene- block -poly(acrylic acid) (PS- b -PAA) block copolymers. Hemi-telechelic POSS-containing PAA or PEO in water only formed rather large, spherical aggregates, a self-assembly structure obviously different from the well-known core–shell micelles with the hydrophobic block as the core and the hydrophilic block as the shell. [ 29,32 ] We also prepared the telechelic POSS-containing amphiphilic hybrid POSS-PAA-POSS and POSS-PEO-POSS, respectively, and interestingly these two telechelic POSS-containing hybrid polymers both can self-assemble into ellipsoidal aggregates. [ 30 ] Here, we synthesized penta-telechelic Glu-PAA-POSS 5 , and expected to obtain unusual self-assembly morphologies. The critical micelle concentration of Glu-PAA 418 -POSS in water is 0.0044 mg mL −1 , which was deter-mined by fl uorescent probe techniques with pyrene as a probe (Figure S6 and S7, Supporting Information). Trans-mission electron microscopy (TEM) was carried out to



Table 1. Results of Polymerization of t BA at 60 °C via ATRP.

Sample [M] 0 /[I] 0 MMnn, GPC a) [g mol −1 ]

MMnn, th b) [g mol −1 ]

MMww/MMnn a) x [%]

Glu-P t BA 218 -Br 5 250 24 600 28 800 1.15 90

Glu-P t BA 418 -Br 5 500 37 900 54 500 1.11 85

Glu-P t BA 630 -Br 5 750 56 800 81 700 1.06 84

a) Determined by GPC using polystyrene standards; b) Theoretical number-average molecular weight, nM , th = [M] 0 /[I] 0 × M t BA × x + M init., where M t BA and M init. are the molecular weights of t BA and initiator, respectively, and x is the conversion, as measured by 1 H NMR.

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characterize the self-assembly morphologies of the Glu-PAA-POSS 5 in aqueous solution. The samples for TEM were prepared by applying a drop of the polymer solution on a carbon-coated copper grid without additional staining or etching.

Figure 1 shows TEM images obtained from aqueous solutions of Glu-PAA-POSS 5 at pH 8.5. It is very intriguing that self-assembled capsules were obtained for these three samples, which are obviously different in shape from the previous self-assem-bled hemi-telechelic and telechelic POSS–PAA. The assembled aggregates from Glu-PAA 218 -POSS 5 are not perfect capsules, and the density of the aggre-gate is not uniform (Figure 1 a). From the single enlarged aggregate (Figure 1 d), clearly it is not a typical capsule, and it has some fractures marked by arrows. We can also fi nd the POSS aggregates are not well dispersed. Some dark domains exist in the capsules, where POSS has a higher content, due to the higher aggregation of POSS units. For Glu-PAA 418 -POSS 5 , there are typical capsules in Figure 1 b. These capsules are not quite spherical, and they are polydisperse in size (1–2 μm). From its enlarged image (Figure 1 e), the surface of the Glu-PAA 418 -POSS 5 is relatively homogeneous, except for several dark dots. This indicates that POSS does not form some bigger aggregates in the membrane of the capsule. Additionally, the folds and creases also can be found

in the capsule, which are resulted from the solvent evaporation during TEM samples preparation. Thus, this is a typ-ical polymer capsule, which is a similar to the conventional one prepared by LbL assembly technique. When the PAA chain was further extended, Glu-PAA 630 -POSS 5 also showed giant capsules in Figure 1 c,f, but their sizes did not obvi-ously increase with the increased length of PAA chain compared to that of Glu-PAA 418 -POSS 5 .

The self-assembled morphologies of penta-telechelic Glu-PAA 630 -POSS 5 were also characterized by atomic force microscopy (AFM, Figure 2 a) and scanning electronic microscopy (SEM, Figure 3 A). The shape of the capsules from AFM is similar to that of TEM

result, which shows the shape of the capsule is tending to be spherical, but the size of AFM and SEM is slightly different from the TEM. It is perhaps because the dif-ferent preparation methods for measurements, and cap-sules should have a larger spreading area on the mica for AFM and silicon wafer for SEM than that of copper grids



Figure 1. TEM of penta-telechelic Glu-PAA-POSS 5 self-assembly aggregates drop-coated from water at pH 8.5. a,d) Glu-PAA 218 -POSS 5 , b,e) Glu-PAA 418 -POSS 5, c,f) Glu-PAA 630 -POSS 5 .

Figure 2. Tapping-mode AFM image (a) and its height profi le along the line (b) of penta-telechelic Glu-PAA 630 -POSS 5 self-assembly aggregates from water at pH = 8.5.

Figure 3. A) SEM of Glu-PAA 630 -POSS 5 and B) DLS of Glu-PAA-POSS 5 self-assembly aggre-gates from water at pH = 8.5.

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for TEM. The height was also analyzed by AFM and it is about 60 nm, which is much smaller than their diameter (Figure 2 b). This suggests that the capsules collapsed on the surface of the mica.

The self-assembly aggregate of Glu-PAA-POSS 5 was also measured by DLS (Figure 3 B), and the result shows the apparent hydrodynamic diameters, D h , of Glu-PAA-POSS 5 are roughly in agreement with the size of capsules meas-ured by TEM. Moreover, the D h of Glu-PAA-POSS 5 slightly increases with the chain length of PAA, and the cumulant analysis renders rather large polydispersity, which also agrees with the results of TEM and AFM. Additionally, small aggregates of Glu-PAA 630 -POSS 5 in the range of 100–200 nm were also detected by DLS.

In our previous studies of POSS-containing hemi-tele-chelic and telechelic amphiphilic hybrid PAA, we did not fi nd the assembled capsule. [ 29,30 ] In present work, the for-mation of capsule should be attributed to the structure of penta-telechelic Glu-PAA-POSS 5 . The hydrophobic POSS as the end groups of star-shaped PAA is tending to aggre-gate in water, and they are much easier to aggregate in the dilute solution of Glu-PAA-POSS 5 , due to the penta-telechelic structure. It can be assumed that several macro-molecules of Glu-PAA-POSS 5 could be formed a big assem-bled aggregate, which was achieved by the aggregation between intramolecular POSS units. Correspondingly, a schematic illustration is shown in Scheme 2 . We com-pared the single enlarged capsule from the three sam-ples, and we found that it is diffi cult to form the perfect capsule from Glu-PAA 218 -POSS 5 with a short PAA chain, whereas for Glu-PAA 418 -POSS 5 and Glu-PAA 630 -POSS 5 , they have a long PAA chain, and the intramolecular aggrega-tion is easy to form. So the formation of a giant capsule from Glu-PAA-POSS 5 is also dependent on the length PAA chain.

3. Conclusion

Inorganic/organic hybrid telechelic Glu-PAA-POSS 5 was successfully prepared by the combination of ATRP and “click chemistry.” The self-assembly behavior of Glu-PAA-POSS 5 with three different molecular weights in aqueous

solution at pH 8.5 was investigated by using TEM, SEM, AFM, and DLS. The results showed that Glu-PAA-POSS 5 with a long PAA chain can self-assemble in water into giant capsules, which provide an optional approach in con-struction of capsules.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements: This work was fi nancially supported by the National Natural Science Foundation of China (Nos. 21074035 and 51173044), Research Innovation Program of SMEC (No.14ZZ065), and the Project-sponsored by SRF for ROCS, SEM. W.Z. also acknowledges support by the Marie Curie Fellowship (No. 2009-252943), and the Fundamental Research Funds for the Central Universities.

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Received: January 8, 2014 ; Revised: February 17, 2014 ; Published online: March 28, 2014; DOI: 10.1002/macp.201400012

Keywords: atom transfer radical polymerization (ATRP) ; capsule ; click chemistry ; self-assembly ; polyhedral oligomeric silsesquioxane (POSS)



Scheme 2. Self-assembly process of penta-telechelic Glu-PAA-POSS 5 in water.

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