A Strategy to Prepare Anemone-Shaped Polymer Brush by Controlled

Chapter 24

Downloaded by CORNELL UNIV on August 8, 2012 | http://pubs.acs.org Publication Date: June 26, 2003 | doi: 10.1021/bk-2003-0854.ch024

A Strategy to Prepare Anemone-Shaped Polymer Brush by Controlled/Living Radical Polymerization Fu-Mian Li, Ming-Qiang Zhu, Xin Zhang, Liu-He Wei, Fu-Sheng Du, and Zi-Chen Li Department of Polymer Science and Engineering, College of Chemistry, Peking University, Beijing 100871, China

A well-defined diblock copolymer containing a block of alternately structured maleic anhydride (MAn) with 4-vinylbenzyl chloride ( V B C ) and P V B C block, P(MAn-alt- V B C ) - b - P V B C ,was prepared by a one-pot reversible addition-fragmentation chain transfer polymerization (RAFT) of M A n and V B C . This block copolymer can form stable inverse micelles in tetrahydrofuran after the M A n moieties being reacted with 2-mercaptoethyl amine. Thus silver ions were embedded in the inner core of the micelle, which were in-situ reduced to silver in the existence of N a B H to obtain stable silver nanoparticles. The benzyl chloride groups on the nanoparticle surfaces were able to initiate the atom transfer radical polymerization (ATRP) of St to achieve a new type of Anemone-shaped polymer brush. 4

342

© 2003 American Chemical Society

In Advances in Controlled/Living Radical Polymerization; Matyjaszewski, K.; ACS Symposium Series; American Chemical Society: Washington, DC, 2003.


343 There has been increasing interest in the development of new strategies to prepare a polymer brush on a solid surface such as silica or quartz wafer and noble metals or inorganic nanoscale particles(7, 2). There are two major currents to prepare polymer brush on the solid surface. One is to assemble a block copolymer whose segments possess distinct solubility in solution onto the solid surface (so called "grafting to") (3,4). Another way is via graft polymerization of vinyl or cyclic monomers on the solid surface (so called "grafting from") (5-7). The latter is capable of providing polymer hairs with high graft density on the solid surface. Furthermore, i f a controlled radical polymerization technique including atom transfer radical polymerization (ATRP), nitroxide-mediated polymerization (NMP) and radical reversible addition-fragmentation chain transfer polymerization (RAFT) is employed, not only a well-defined polymer brush can be achieved, but also can be performed under conventional radical polymerization conditions even in aqueous milieu (8). Most of the surfaceinitiated controlled radical graft polymerizations reported are routinely carried out on the halomethylphenyl- (or α-haloester) and alkoxyamine-modified surface of silica, quartz wafers and particles as initiating sites for ATRP(7, 9, 10) and N M P (77), respectively. In this paper, we describe a strategy to prepare a new type of anemoneshaped polymer brush (ASPB) whose hairs are tethered on the nanoscale silver particles. The whole process is outlined in Scheme 1. R A F T was used to synthesize a well-defined diblock copolymer composed by a block of alternately structured maleic anhydride (MAn) with 4-vinylbenzyl chloride (VBC) and a block of P V B C , P(MAn-aft-VBC) -ô-PVBC (I), in the first step, then, this block copolymer was allowed to react with 2-mercaptoethylamine to obtain a amphiphilic diblock copolymer (II) which can form stable inverse micelles in tetrahydrofuran with P V B C as the corona. m

n

i: BTBA/AIBN at 60°C;

I: HSCH CH NH /NEt /THF

iii: AgN0 /NaBH

iV. CuBr/bipy, monomer

3

4

2

2

2

3

Scheme 1


344 Thus silver ions can be embedded in the inner core of the micelle, and after the in-situ reduction with NaBH4, stable silver nanoparticles were obtained. The ATRP of St was successively carried out on the surface of the A g nanoparticles by using densely surface-anchored benzyl chloride groups in the P V B C segment to achieve an anemone-shaped polymer brush (ASPB).


Experimental

Materials. Azobisisobutylnitrile (AIBN, Aldrich) was recrystallized from methanol. Styrene (St, Beijing Chemicals Co.) and 4-vinylbenzyl chloride ( V B C , Aldrich) were dried and redistilled before use. Maleic anhydride (MAn, Lushun Chemicals Co.) was recrystallized from benzene. S-benzyl dithiobenzoate (BTBA)was synthesized according to a literature method (12) CuBr was purified according to published procedure. 2-Mecaptoethylamine hydrochloride, 2,2'bipyridine, A g N 0 , and NaBH4 are used as received. 3

Characterization. The molecular weights and molecular weight distributions (My/Μη) were measured on a Waters 401 gel permeation chromatography at 35°C. T H F was used as an eluent and polystyrene standards were used for calibration. The copolymer composition was determined by *H-NMR recorded on a Varian Gemini 200 M H z spectrometer in CDC1 . UV-vis absorption spectra were recorded on a Schmadzu U V 2101PC spectrometer. 3

Dynamic Light Scattering (DLS). The average hydrodynamic radius (Rh) and Rh distribution function, f(R ) of these aggregates was determined using a selfregulating light scattering spectrometer (ALV/DLS/-5000) with a light wavelength of 514.5 nm. For each measurement, the solutions were allowed to stand at room temperature for 1 day to ensure that the solutions had reached equilibrium The measurements of each solution sample were repeated for three times. The concentration of diblock copolymer in THF was 0.5 mg/mL. h

Transmission Electron Microscopy (TEM). A solution of aggregates in T H F (0.5mg/mL) was deposited onto the surface of 200 mesh Formvar-Carbon filmcoated copper grids at 25°C. Excess solvent was removed with a filter paper. The samples were examined with a J E O L 1210TEM at a lOOkv accelerating voltage. Atomic Force Microscopy (AFM). Tapping-mode atomic force microscopy ( A F M ) observations were carried out in air with a commercial system (Seiko Instruments Inc., SPA300HV, Japan) operated under ambient conditions with SIDF20 silicon cantilever. The samples for A F M analysis were prepared by depositing tyL of the nanoparticle solution (0.5 mg/mL in THF) onto mica and allowing it to dry freely in air.


345


R A F T copolymerization of M A n with V B C . The R A F T polymerization was conducted in a sealed glass tube. S-Benzyl dithiobenzoate ( B T B A ) was used as one of the components of R A F T initiating system. In a typical run, M A n , V B C , B T B A and A I B N were charged in the glass tube in the molar ratio of 100: 900: 10: 1. After the mixture was degassed three times, the tube was sealed under vacuum, then was kept in an oil bath of 60 °C to conduct the polymerization. After 8 hr, the tube was broken and T H F was added to cease the polymerization. The polymer was obtained by reprecipitation from methanol. The powdery pink polymer was dried under vacuum at 60 °C. The conversion of M A n is 100%, and the conversion of V B C is about 40%. Preparation of polymer protected A g nanoparticles. Block copolymer, P(MAn-alt-VBC) -b-PVBC (20 mg) was dissolved in 40 mL of THF, and 10 mg of 2-mercaptoethylamine hydrochloride was added to the solution. The mixture was stirred at room temperature for 3 days to obtain a clear solution. To this solution was added 40 μΐ of 1 M A g N 0 aqueous solution and again the mixture was stirred at room temperature for 2 days. Then, 40 mg of N a B H was added, the clear solution change to dark blue after stirring at room temperature for 30 min. Stirring was continued for 2 days until a dark brown solution was obtained. 10

36

3

4

In-situ A T R P of St on the surface of A g nanoparticles. The solvent in the above-prepared A g nanoparticle solution was removed by evaporation. Then CuBr (4 mg), 2,2'-bipyridine (15 mg) and St (1 mL) was added to the dried A g nanoparticles. The mixture was degassed for three times, the tube was sealed under vacuum, then was kept in an oil bath of 110 °C to conduct the polymerization. After 8 hr, the tube was broken and the polymer was obtained by precipitation from methanol. The A S P B sample was dried under vacuum at 60°C. The polymer yield is 80%.

Results and Discussion Preparation of P(MAn-aft-VBC) -*-PVBC„ It is known that the copolymerization of M A n with St via transition metalmediated radical polymerization to directly prepare a well-defined alternating copolymer hardly takes place since the active anhydride residue would damage the catalytic system (13,14). Even though the N M P of M A n with St takes place at higher temperature, it does not provide an alternately structured copolymer due to high polymerization temperature performed (15) We have successfully m


346 synthesized a well-defined diblock copolymer of M A n with St composed by MAn-flft-St block and PSt block via R A F T copolymerization of M A n with excess amount of St initiating with ABBN and in the presence of S-benzyl dithiobenzoate ( B T B A ) at 60°C, and the resulting block copolymer self-assemble into well-defined nanoscale particles with narrow size distribution in aqueous medium after hydrolysis (16). Besides St in the MAn/St system, we demonstrated that this method could also be used for the preparation of P(MAn-aft-VBC) -6P V B C through the R A F T copolymerization of M A n with an excess amount of V B C . A s a representative run, the diblock copolymer (I) constituted by a block with alternately structured M A n / V B C copolymer block and P V B C block was also accomplished by simple one-pot procedure of M A n and V B C in the molar ratio of A I B N : B T B A : M A n : VBC=1:10:100:900 via R A F T at a moderate temperature of 60°C. S E C result indicated that the number average molecular weight of the polymer was 7,900 and the molecular weight distribution was 1.37. Based on the analysis of both gas chromatography (GC) and *H N M R , a diblock copolymer, Ρ(ΜΑη-α/ί-VBC) -6-PV B C , was successfully prepared, which was further hydrolyzed to form an amphiphilic diblock copolymer P(MA-aft-VBC)i 6-PVBC (II).


m

l0

3 6

0

36

In situ formation of polymer-wrapped Ag nanoparticles To improve the stability and interaction between metallic A g and the diblock copolymer (II), mercapto groups were introduced to the diblock copolymer (I) as shown in Scheme 1. The mercaptonization of P(MAn-aft-VBC)i -6-PVBC was carried out in T H F with 2-mercaptoethylamine. Then, A g N 0 aqueous solution was added slowly to the above mercapto-modified amphiphilic diblock copolymer solution under agitation, yielding a nano-droplet of A g N 0 aqueous solution shelled by the diblock copolymer via inverse micellization. The A g in the inner core of reverse micelles was in situ reduced by N a B H to get black colored metallic A g particles out-shelled by a corona with high density of benzyl chloride groups. Shown in Figure 1 are the U V - V i s absorption spectra of the A g N 0 aqueous solution shelled by the diblock copolymer (II) via inverse micellization before and after the addition of N a B H . The absorption peak appeared at 415 nm after adding N a B H to the solution indicated the formation of nanoscale A g particles. The T E M photographs of the inverse micelles loading A g and A g nanoparticles after reduction are shown in Figure 2. It can be seen that before reduction, the average diameters of the inverse micelles are in the range of 30 nm; when the loaded A g was reduced to metallic A g , the apparent diameters of the A g nanoparticle decreased to about 20 nm. This may be caused by the shrinkage of the inner core of the inverse micelle after the formation metallic A g . Both of them are stable under the present conditions even when the solvent was removed. Therefore, they may be used for the subsequently surface initiated A T R P of St. 0

36

3

3

+

4

3

4

4

+

+



347

-1

i

300

!

400

«

1

«

500

1

·

1

600

700

Wavelength (nm) Figure 1 UV-vis absorption spectra of the AgNOs aqueous solution shelled by the diblock copolymer via inverse micellization after the addition ofNaBH* I: 5min after adding NaBH ; 2:10min after adding NaBH . 4

(a)

4

(b)

Figure 2 TEM images of diblock copolymer inverse micelles loading Ag* (a) and Ag nanoparticles prepared φ).


348


Preparation of A S P B via A T R P using surface-anchored benzyl chloride groups as the multi-initiators. As mentioned hereinbefore, the benzyl chloride groups surface-anchored A g nanoparticles can be used as the multi-initiators for surface-grafted ATRP of vinyl monomers such as St to prepare a polymer brush on the A g nanoparticle. Since the ATRP should be conducted at higher temperature, whether the sphereshaped A g nanoparticles still keep their proper shapes during the ATRP is of our concern. Additionally, whether the amide and mercapto group embedded in the particle would damage the ATRP is also of our subsequent interest. In the present experiment, the surface-initiated graft ATRP of St was conducted at 110°C for 8 hours by using CuBr and 2,2'-bipyridine as the catalytic system and the benzyl chloride groups anchored onto the surface of A g nanoparticles were acted as the multi-initiators. The monomer conversion was 80% as detected by G C , meaning that the amide and S H - groups did not affect the ATRP. This may be ascribed to the fact that amide and -SH groups inside the particle. The A S P B thus obtained was characterized by means of atomic force microscopy (AFM), size-exclusion chromatography (SEC) and dynamic light scattering (DLS). A F M images are capable of providing the direct information about the shapes and sizes of particles in solid state. Thus, the changes of shape and size for (Il)-shelled A g nanoparticles before and after graft ATRP of St can be directly observed from the A F M images. It is seen from Figure 3 that the A g nanoparticles still kept their regular spheres, there is almost no change in shape. However, the size of the grafted one became larger, that is, the average diameters of (Il)-shelled A g nanoparticles was ca. 19 nm, it was increased to ca. 30 nm

Figure 3 AFM images ofAg nanoparticles before (a) and after (b) grafting PSt.


349 after grafting. This indicates that the (Il)-shelled A g nanoparticles are stable enough to keep their proper spherical shape during the ATRP process. This may be attributed to the double protection, that is, Ag-S binding plus encapsulating by polymer layer (17). The fluid average hydrodynamic radius (R ) of started (Il)-shelled A g nanoparticles (a) and A S P B (b) were determined by SEC method. Figure 4 shows the S E C profiles for (a) and (b) by using THF as an eluent and linear PSt as a standard. The apparent number average molecular weight " M , " of A g nanoparticles-nucleated A S P B remarkably increased compared to that of the (a). The "M„" of (a) was only about 4,000 with polydispersity of 1.40, whereas after grafting, the "M„" of A S P B increased to as high as 180,000 with polydispersity of 1.42. This means that even though the size of polymer brush increased remarkably, the polydispersity did not change. Thus, the shapes of A g nanoparticles and A S P B are still spherical. It should be pointed out that according to the particle size, the " M " of (Il)-shelled A g particle before and after grafted should be in the order of 10 -10 . However, we did not obtained such high "Λί„", since the calibration standard was linear PSt not suitable for starshaped PSt.


av

M

5

6

A

I

J I 15

M

I ; ι 20

I I I I ι 25

M

I

Figure 4 SEC profiles of Ag nanoparticles before (a) and after being grafted (b). (a) Ag-(Il) M =3950, MJM„=1.40; (b) ASPB M =183900, MJM =1.42. n

M

n

Dynamic light scattering (DLS) was then used to measure the size and size distribution of (a) and A S P B (b) in solution state. Shown in Figure 5 are the distribution profiles of R of (a) and A S P B (b). D L S results confirmed that the surface-grafted ATRP of St proceeded undoubtedly and R increased after surface-grafted ATRP. The R of (a) was 9.5 nm. After surface-grafted ATRP of St, the R increased to 20.4 nm with a slight increase in size distribution width. The small peaks in the range of 1-5 nm may be from single P V B C chains or a v

a v

a v

a v


350


PVBC-graft-PSt chains which did not assemble with Ag. This indicates that the grafting of St via ATRP on the surface of A g nanoparticles resulted in the welldistributed increase of particles. In fact, the grafted (Il)-shelled A g particle can take its A S P B shape conformationally only in solution due to the flexibility of PSt hair. In solid state, the (II) forms an ultra thin film, consisting of a uniform sphere-shaped A g nanoparticle core with polymer shell.

(b)

(a)

Li

0.01 0.1 1 10 100 1E3 1E4 1E5

0.01 0.1 1 10 100 1E3 1E4 1E5 1E6

(nm) (nm) Figure 5 Dynamic light scattering analysis of Ag nanoparticles before (a) and after grafted (b).

In conclusion, ATRP and R A F T techniques were used to prepare well-defined A S P B in the successive steps. This new strategy may be extended to other noble metals such as gold, inorganic sulfides such as CdS, ZnS etc. The properties and detailed characterization of those ASPBs are under way.

Acknowledgement This work was partially supported by the National Natural Science Foundation of China (under the contract No. 29992590-4).

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A Strategy to Prepare Anemone-Shaped Polymer Brush by Controlled

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