Synthesis of Core− Corona Polymer Hybrids with a Raspberry-like

Feb 21, 2008 - A core−corona polymer hybrid with a raspberry-like structure was ... the pyridyl group of poly(ethylene glycol dimethacrylate-co-meth...
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Langmuir 2008, 24, 3358-3364

Synthesis of Core-Corona Polymer Hybrids with a Raspberry-like Structure by the Heterocoagulated Pyridinium Reaction Junyou Wang and Xinlin Yang* Key Laboratory of Functional Polymer Materials, Ministry of Education, Institute of Polymer Chemistry, Nankai UniVersity, Tianjin 300071, China ReceiVed NoVember 21, 2007. In Final Form: January 9, 2008 A core-corona polymer hybrid with a raspberry-like structure was synthesized via the heterocoagulated pyridium reaction between the pyridyl group of poly(ethylene glycol dimethacrylate-co-methacrylic acid)@poly(ethylene glycol dimethacrylate-co- vinylpyridine) (poly(EGDMA-co-MAA)@poly(EGDMA-co-VPy)) core-shell small microspheres and the chloromethyl group of poly(divinylbenzene-co-chloromethyl styrene) (poly(DVB-co-CMSt)) microspheres, in which poly(EGDMA-co-MAA)@poly(EGDMA-co-VPy) acted as the corona and poly(DVB-co-CMSt) behaved as the core. The control coverage of the corona particles on the surfaces of core microspheres for the polymer hybrid was studied in detail through the adjustment of the mass ratio between the core and corona particles. The effects of the pH and solvents on the stability of the raspberry-like core-corona hybrids were investigated. The water static angle on the surface of polycarbonate (PC) film was studied using a contact angle system. The polymer particles and the resultant heterocoagulated raspberry-like hybrids were characterized with scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The nature of the heterocoagulation between the core microspheres and corona particles was identified as the covalent pyridium reaction with Fourier transform infrared (FT-IR) spectroscopy.

Introduction In recent years, much work has been done in the field of polymer colloidal materials with different compositions and complicated shapes,1-3 in which interesting properties, such as optical, electrical, and chemical, can be varied with their size, dimension, composition, and structure. Further, the core-corona polymer composites/hybrids are currently studied due to their wide applications in various fields, including biotechnology, medicine, electronics, chemical industry, pharmacology, and photographic industry, and as stabilizers of nanometallic colloids.4-7 Techniques for the preparation of the core-corona composite particles have been reported in the literature, including seeded emulsion polymerization,8 surface modification polymerization,9,10 surfactant-free emulsion polymerization,11-13 and miniemulsion polymerization.14-16 The other technique for the preparation of composite aggregates is the heterocoagulation of * To whom correspondence should be addressed. Telephone: +86-2223502023. Fax: +86-22-23503510. E-mail: [email protected]. (1) Sanchez, C.; Soler-Illia, G. J. A. A.; Ribot, F.; Lalot, T.; Mayer, C. R.; Cabuil, V. Chem. Mater. 2001, 13, 3061. (2) Caruso, F. AdV. Mater. 2001, 13, 11. (3) Reculusa, S.; Poncet-Legrand, C.; Ravaine, S.; Mingotaud, C.; Doguet, E.; Bourgeat-Lami, E. Chem. Mater. 2002, 14, 2354. (4) Okubo, M.; Nakagawa, T. Colloid Polym. Sci. 1994, 272, 530. (5) Zhang, S. H.; Huang, X.; Yao, N. S.; Horvath, C. J. Chromatogr., A 2002, 948, 193. (6) Reynolds, C. H.; Annan, N.; Beshah, K.; Huber, J. H.; Shaber, S. H.; Lenkinski, R. E.; Wortman, J. A. J. Am. Chem. Soc. 2000, 122, 8940. (7) Davies, R.; Schurr, G. A.; Meenan, P.; Nelson, R. D.; Bergna, H. E.; Brevett, C. A. S.; Goldbaum, R. H. AdV. Mater. 1998, 10, 1264. (8) Okubo, M.; Kanada, K.; Matsumoto, T. Colloid Polym. Sci. 1987, 265, 876. (9) Bourgeat-Lami, E.; Jacques, L. J. Colloid Interface Sci. 1999, 210, 281. (10) Zeng, Z.; Yu, J.; Guo, Z. J. Polym. Sci., Part A: Polym. Chem. 2005, 43, 2826. (11) Barthet, C.; Hickey, A. J.; Carins, D. B.; Armes, S. P. AdV. Mater. 1999, 11, 408. (12) Chen, M.; Zhou, S. X.; You, B.; Wu, L. M. Macromolecules 2005, 38, 6411. (13) Chen, M.; Wu, L. M.; Zhou, S. X.; You, B. Macromolecules 2004, 37, 9613. (14) Landfester, K. Macromol. Symp. 2000, 150, 171. (15) Tiarks, F.; Landfester, K.; Antonietti, M. Langnuir 2001, 17, 5775.

large and small oppositely charged colloidal particles via electrostatic interaction in the presence of other auxiliaries.17,18 Positively charged oxidized-polyferrocentylsilane particles (OPFSMs) underwent electrostatically driven self-assembly with negatively charged silica microspheres to result in core-corona composite particles.19 Walt et al. introduced the heterocoagulation technique through specific chemical (amine-aldehyde) and biochemical (avidin-biotin) interactions as a way to control the structure of the particle assembly.20 These anomalous polymer particles have some attractive characteristics, such as unique morphology, designed surface properties, large surface area, and high light-scattering ability, compared to the spherical polymer microspheres. Recent advances of the kinetic and structural aspects of heterocoagulation processes for the stability of binary colloids have been well-summarized in a review by HidalgoAlvarez et al.21 In our previous work, we reported anomalous polymer composites with a raspberry-like structure having uneven surfaces afforded by self-assembled heterocoagulation based on a charge compensation mechanism through the affinity complex between the carboxylic acid group and pyridinium group,22 a hydrogenbonding interaction process between the carboxylic acid group and pyridyl group,23 and a synergic hydrogen-bonding interaction between the carboxylic acid group and amide group.24 Here, we describe the synthesis of core-corona polymer hybrids with a (16) Schrock, F. J.; Poehlein, G. W.; Wang, S.; Reimers, J.; Rodrigues, J.; Samer, C. Colloids Surf., A 1999, 153, 39. (17) Yang, B.; Matsumura, H.; Katoh, K.; Kise, H.; Furusawa, K. Langmuir 2001, 17, 2283. (18) Caruso, F.; Lichtenfeld, H.; Giersig, M.; Mo¨hwald, H. J. Am. Chem. Soc. 1998, 120, 8523. (19) Li, H.; Han, J.; Panioukhnie, A.; Kumacheva, E. J. Colloid Interface Sci. 2002, 255, 119. (20) Fleming, M. S.; Mandal, T. K.; Walt, D. R. Chem. Mater. 2001, 13, 2210. (21) Lopez-Lopez, J. M.; Schmitt, A.; Moncho-Jorda, A.; Hidalgo-Alvarez, R. Soft Mater. 2006, 2, 1025. (22) Li, G. L.; Yang, X. L.; Bai, F.; Huang, W. Q. J. Colloid Interface Sci. 2006, 297, 705. (23) Li, R.; Yang, X. L.; Li, G. L.; Li, S. N.; Huang, W. Q. Langmuir 2006, 22, 8127. (24) Li, G. L.; Yang, X. L.; Huang, W. Q. J. Appl. Polym. Sci. 2007, 104, 1350.

10.1021/la7036394 CCC: $40.75 © 2008 American Chemical Society Published on Web 02/21/2008

Synthesis of Raspberry-like Core-Corona Hybrids

raspberry-like structure via the heterocoagulated pyridinium reaction between the chloromethyl group and pyridyl group, as the raspberry-like composites were formed with the aid of the secondary level interaction in our previous work.22-24 The study on the interaction/reaction between the polymer microspheres for the construction of raspberry-like composites/hybrids with uneven surfaces may be significant for the development of lotuslike superhydrophobic materials and for understanding the interactions in basic-life processes for biological systems. Experimental Section Materials. Divinylbenzene (DVB80, 80% DVB isomers, Shengli Chemical Technical Factory, Shandong, China) was washed with 5% aqueous sodium hydroxide and water and then dried over anhydrous magnesium sulfate prior to use. 4-Chloromethylstyrene (CMSt) and ethylene glycol dimethacrylate (EGDMA) were purchased from Aldrich Chemical Co. and used without further treatment. 4-Vinylpyridine (4-VPy) was purchased from Acros and distilled under vacuum. Methacrylic acid (MAA) was provided by Tianjin Chemical Reagent II Co. and purified by vacuum distillation. Both 2,2′-azobisisobutyronitrile (AIBN) and benzoyl peroxide (BPO) were obtained from Chemical Factory of Nankai University and recrystallized from methanol. Acetonitrile (analytical grade, Tianjin Chemical Reagent II Co.) was dried over calcium hydride and purified by distillation before use. The other reagents were analytical grade and used without any further purification. Preparation of Poly(DVB-co-CMSt) Microspheres. The synthesis of poly(divinylbenzene-co-chloromethylstyrene) (poly(DVBco-CMSt)) by distillation precipitation polymerization was reported in detail in our previous paper.25 A typical procedure for such polymerization was as follows: DVB (1.2 mL, 1.1 g, 8.5 mmol), CMSt (0.80 mL, 0.87 g, 5.2 mmol), and AIBN (0.04 g, 0.24 mmol, 2 wt % relative to the comonomers) were dissolved in 80 mL of acetonitrile in a dried 100 mL two-necked flask attached to a fractionating column, a Liebig condenser, and a receiver. The flask was submerged in a heating mantle, the reaction mixture was heated from ambient temperature until boiling state within 15 min, and the reaction system was kept under reflux for an additional 5 min. The solvent was then distilled off the reaction system, and the reaction was stopped after 40 mL of acetonitrile was distilled from the reaction system within 1.5 h. After polymerization, the resulting poly(DVBco-CMSt) microspheres were purified by vacuum filtration over a G-5 sintered glass filter with subsequent washing with THF and acetone for three times. The procedure for determining the loading capacity of the accessible chloromethyl groups on the surfaces of poly(DVB-coCMSt) microspheres was as follows: An excessive amount of pyridine was added to the suspension of poly(DVB-co-CMSt) microspheres in N,N-dimethylformamide (DMF), and the mixture was kept at 60 °C for 24 h with magnetic stirring. The loading capacity of the reactive chloromethyl groups was determined from the increasing content of nitrogen for the final poly(DVB-costyrylmethyl pyridinium chloride) (poly(DVB-co-StMPyC1)) microspheres after the pyridinium reaction compared to the nitrogen content of the corresponding poly(DVB-co-CMSt) microspheres (originating from the residual part of the AIBN initiator). Preparation of Monodisperse Poly(EGDMA-co-MAA)@poly(EGDMA-co-VPy) Core-Shell Microspheres by Two-Stage Distillation Precipitation Polymerization. The preparation of poly(ethylene glycol dimethacrylate-co-methacrylic acid) (poly(EGDMAco-MAA)) core particles by distillation precipitation polymerization was similar to the technique reported in our previous paper,26 in which the cross-linking degree of EGDMA used was 0.60 and BPO was used as the initiator instead of AIBN. The loading capacity of the accessible carboxylic acid groups on the surfaces of poly(EGDMA-co-MAA) cores was 3.20 mmol/g as determined by (25) Li, S. F.; Yang, X. L.; Huang, W. Q. Chin. J. Polym. Sci. 2005, 23, 197. (26) Bai, F.; Yang, X. L.; Li, R.; Huang, B.; Huang, W. Q. Polymer 2006, 47, 5775.

Langmuir, Vol. 24, No. 7, 2008 3359 acid-base titration. The poly(ethylene glycol dimethacrylate-comethacrylic acid)@poly(ethylene glycol dimethacrylate-covinylpyridine) (poly(EGDMA-co-MAA)@poly(EGDMA-co-VPy)) core-shell microspheres were synthesized by second-stage distillation precipitation polymerization with the aid of hydrogen-bonding interaction between the carboxylic acid groups and the pyridyl groups of VPy. In a typical experiment, 0.20 g of poly(EGDMA-co-MAA) core particles, 0.25 mL (0.26 g, 1.3 mmol) of EGDMA, 0.25 mL (0.25 g, 2.25 mmol) of 4-VPy and, 0.01 g (2 wt % relative to the comonomers) of BPO as the initiator were dispersed in 80 mL of acetonitrile with ultrasonic irradiation at room temperature in a 100 mL two-neck flask equipped with a fractionating column, a Liebig condenser, and a receiver. The flask was submerged in a heating mantle, the reaction mixture was heated from ambient temperature until the boiling state within 15 min, and the reaction mixture was kept under reflux for an additional 5 min. The solvent was then distilled off the reaction system, and the polymerization was stopped after 40 mL of acetonitrile was distilled from the reaction system within 1.5 h. After polymerization, the resultant poly(EGDMAco-MAA)@poly(EGDMA-co-VPy) core-shell microspheres were purified by three cycles of ultracentrifugation, decanting, and resuspension in acetonitrile with ultrasonic irradiation. The loading capacity of the pyridyl groups on the surfaces of core-shell particles was determined by an acid-base titration similar to the procedure in our previous work.27 Preparation of Core-Corona Polymer Hybrids with a Raspberry-like Structure by the Heterocoagulated Pyridinium Reaction. The heterocoagulated pyridinium reaction between the pyridyl groups of poly(EGDMA-co-MAA)@poly(EGDMA-co-VPy) particles and the chloromethyl groups of poly(DVB-co-CMSt) microspheres was performed as as follows: A volume of 200 µL of the cross-linked poly(DVB-co-CMSt) suspension (10 mg/mL in DMF) was dispersed in 5 mL of a supspension of poly(EGDMAco-MAA)@poly(EGDMA-co-VPy) microspheres (5 mg/mL) in DMF with different concentrations on a SHA-B shaker. The heterocoagulated pyridinium reaction for the self-assembly was carried out with gentle agitation by rolling the bottles in a horizontal position to approximately 60 rpm at 60 °C for 48 h. The resulting heterocoagulates were purified by centrifugation, decanting, and resuspension in DMF three times and then dried in a vacuum oven. The effect of pH on the morphology of the polymer hybrids was determined by using either 0.1 M HCl or 0.1 M NaOH aqueous solution to adjust the pH in the range of 1-13. The effect of solvent on the morphology of the raspberry-like polymer hybrids was carried out by suspending the resultant hybrids with ultrasonic irradiation in different solvents including DMF, methanol, water, acetonitrile, tetrahydrofuran (THF), and hexane. Determination of the Static Contact Angle of Polycarbonate Films Containing Raspberry-like Polymer Hybrids. Polycarbonate (PC) was dissolved in THF at concentration of 5 wt % for the preparation of the polymer films containing raspberry-like hybrids on a clean glass filter. After that, 1.0 mL of the suspension containing raspberry-like polymer hybrids was added to 4.0 mL of PC/THF (5 wt %) solution and placed on a SHA-B shaker with gentle agitation by rolling the bottles on a horizontal position around 40 rpm for 5 h. The suspension with raspberry-like polymer hybrids was then directly cast-coated on a clean glass plate and dried under ambient temperature. The resulting films were used for the determination of the static contact angle. Characterization. The morphology of the core and corona polymer microspheres and the resultant core-corona polymer hybrids with a raspberry-like structure was studied by scanning electron microscopy (SEM, Philips XL-30) and transmission electron microscopy (TEM, TECNAI G2 2 200 S-TWIN). Fourier transform infrared (FT-IR) analysis was performed on a Bio-Rad FT-135 FT-IR spectrometer, and the diffuse reflectance spectra were scanned over the range of 400-4000 cm-1. Elemental analysis (EA) was carried out on a Perkin-Elmer-2400 instrument to determine the nitrogen content of the polymer particles. (27) Li, S. N.; Yang, X. L.; Huang, W. Q. Chin. J. Polym. Sci. 2007, 25, 555.

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Figure 1. Morphology of the polymer particles: (A) SEM of poly(DVB-co-CMSt); (B) TEM of poly(EGDMA-co-MAA); (C) TEM of core-shell poly(EGDMA-co-MAA)@poly(EGDMA-co-VPy); and (D) SEM of core-corona hybrids (from corona/core as 1/10 in mass ratio). The static contact angle of water on the polymer film was measured on a JC-2000C1 instrument (Chenguang Co., China) at room temperature.

Table 1. Size and Size Distribution of the Polymer Particles (Number-Average Diameter (Dn), Weight-Average Diameter (Dw), and Polydispersity Index (U)), and Loading Capacity of the Accessible Functional Groups of the Polymer Particles

entry

Dn (µm)

Dw (µm)

U

functional group content (mmol/g)

poly(DVB-co-CMSt) poly(DVB-co-MAA) poly(EGDMA-co-MAA)@ poly(EGDMA-co-VPy)

2.96 0.112

2.97 0.113

1.003 1.007

1.02 3.20

0.133

0.134

1.008

1.91

Results and Discussion Distillation precipitation polymerization has been proven to be a useful and facile technique for the synthesis of monodisperse polymer microspheres with various functional groups on the gel layer and surface.24-28 The SEM micrograph of poly(DVB-coCMSt) microspheres in Figure 1A indicates that the polymer particles have a spherical shape with a smooth surface. The average diameter of the poly(DVB-co-CMSt) microspheres was 2.96 µm with a monodispersity of 1.003. The loading capacity of the accessible chloromethyl groups on the surfaces of poly(DVB-co-CMSt) microspheres was 1.02 mmol/g determined from the difference of the nitrogen content of the polymer particles before (1.03%) and after (2.46%) the pyridinium reaction between the chloromethyl group and pyridine by EA. These results demonstrate that the chloromethyl groups on the surfaces of poly(DVB-co-CMSt) microspheres were easily accessible with high reactivity, which permitted further construction of the complicated core-corona polymer hybrids via the heterocoagulated pyridinium reaction. Preparation of Monodisperse Poly(EGDMA-co-MAA)@ poly(EGDMA-co-VPy) Core-Shell Microspheres. In our previous paper, we reported core-shell structure polymer microspheres with different functional groups on the shell layer formed by two-stage distillation precipitation polymerization, in which the functional shell was formed during the second-stage polymerization through the capture of the monomers from solution with the aid of the residual vinyl groups on the surfaces of the core particles due to the poor solvent and much low monomer (28) Bai, F.; Yang, X. L.; Huang, W. Q. Macromolecules 2004, 37, 9746.

loading used for the first-stage polymerization.29 The TEM micrograph of poly(EGDMA-co-MAA) microspheres by distillation precipitation polymerization is shown in Figure 1B, which shows polymer particles of a spherical shape with the average size of 112 nm and a narrow dispersion (U) of 1.007 (summarized in Table 1), which were used as seeds for the second-stage distillation precipitation polymerization. The loading capacity of the accessible carboxylic acid groups on the surfaces of poly(EGDMA-co-MAA) microspheres with the cross-linking degree of 0.60 was 3.20 mmol/g determined by acid-base titration, which permitted the occurrence of the efficient hydrogen-bonding interaction between the carboxylic acid groups and the other functional groups with strong polarity, such as pyridyl groups. In the present work, the reactive vinyl groups were introduced onto the surfaces of the poly(EGDMA-co-MAA) nanocores through the efficient hydrogen-bonding interaction between the carboxylic acid group on the surface of the polymer core and the pyridyl group of VPy. The mechanism for the hydrogen-bonding interaction between the pyridyl group and the carboxylic acid group aided the formation of core-shell poly(methacrylic (29) Qi, D. L.; Bai, F.; Yang, X. L.; Huang, W. Q. Eur. Polym. J. 2005, 41, 2320.

Synthesis of Raspberry-like Core-Corona Hybrids

Langmuir, Vol. 24, No. 7, 2008 3361 Scheme 1. Synthesis of the Core-Corona Polymer Hybrids with a Raspberry-like Structure

Figure 2. FT-IR spectra of the polymer particles: (a) poly(EGDMAco-MAA); (b) core-shell poly(EGDMA-co-MAA)@poly(EGDMAco-VPy); (c) core-corona hybrids; and (d) poly(DVB-co-CMSt).

acid)@polydivinylbenzene) (PMAA@PDVB) microspheres, and further development of PDVB hollow microspheres with pyridyl group on the interior surface has been studied in detail in our previous work.30 These results imply that the addition of the VPy monomer together with the EGDMA cross-linker during the second-stage polymerization could then lead to the desired coreshell microspheres with pyridyl groups on the shell layer via the efficient hydrogen-bonding interaction between the carboxylic acid group and pyridyl group. A typical TEM micrograph of the resultant core-shell poly(EGDMA-co-MAA)@poly(EGDMA-co-VPy) microspheres is shown in Figure 1C. The results demonstrate that the core-shell structure polymer microspheres had a spherical shape with a nonsegmented surface in the absence of any second-initiated small particles. The size of the core-shell particles as summarized in Table 1 is 133 nm with a narrow dispersion (U) of 1.008. This meant that the thickness of the poly(EGDMA-co-VPy) shell was around 10 nm, which was calculated as half of the difference between the diameter of the poly(EGDMA-co-MAA) core and that of the resultant poly(EGDMA-co-MAA)@poly(EGDMAco-VPy) core-shell microspheres. The loading capacity of the accessible pyridyl groups on the surfaces of the core-shell microspheres was 1.91 mmol/g as determined by acid-base titration. The preparation of poly(EGDMA-co-MAA)@poly(EGDMAco-VPy) core-shell microspheres via two-stage distillation precipitation polymerization was studied further by FT-IR spectroscopy as shown in Figure 2a and b. For both the poly(EGDMA-co-MAA) core and core-shell particles, the FT-IR spectra had a strong peak at 1726 cm-1 corresponding to the characteristic stretching vibration of the carbonyl component of the ester group. Comparing with the FT-IR spectrum of the poly(EGDMA-co-MAA) core in Figure 2a, the FT-IR spectrum of the core-shell particles (Figure 2b) showed an absorption peak at 1599 cm-1 assigned to the typical vibration of the pyridyl groups. In other words, the cross-linked poly(EGDMA-co-VPy) shell was successfully incorporated onto the surface of the poly(EGDMA-co-MAA) core via the second-stage distillation precipitation polymerization to result in the core-shell polymer microspheres with the aid of the efficient hydrogen-bonding interaction between the carboxylic acid group and pyridyl group. All these results provide the possibility for the construction of the core-corona polymer hybrids with a raspberry-like structure (30) Li, G. L.; Yang, X. L.; Bai, F. Polymer 2007, 48, 3074.

via the heterocoagulated pyridinium reaction between the reactive chloromethyl group and pyridyl group. Heterocoagulated Pyridinium Reaction for the Construction of Core-Corona Polymer Hybrids with a Raspberrylike Structure. It is crucial to work with polymer particles rather than small molecules with only one limited spot for the reaction because the polymer microspheres provide many spots for interparticle reaction for the heterocoagulation of a single integral polymer hybrid consisting of two different polymer microspheres. The synthesis of core-corona polymer hybrids with a raspberrylike structure via a heterocoagulated pyridinium reaction between the chloromethyl group and pyridyl group from two different polymer microspheres is illustrated in Scheme 1. As is well-known, pyridyl groups and chloromethyl groups are good reactive partners and they have been successfully used to covalently bond viologen moieties onto the surfaces of lowdensity polyethylene (LDPE) films via reaction with 4,4′bipyridine followed by benzyl chloride or other alkyl halides.31-33 In the present work, the core-corona polymer hybrid particles with a raspberry-like structure were synthesized via the heterocogulated pyridinium reaction between the chloromethyl groups on the surfaces of the poly(DVB-co-CMSt) core particles and the pyridyl groups on the shell layers of the poly(EGDMAco-MAA)@poly(EGDMA-co-VPy) corona particles as shown in Scheme 1. The typical SEM image of the core-corona polymer hybrids in Figure 1D indicates that the small corona particles are homogeneously deposited on the surfaces of the large core micorspheres in the absence of any coagulum to afford integral polymer hybrids with a raspberry-like structure. The raspberry-like structure particles have provided characteristics uniquely different from those in the homogeneous media and film states, such as uneven surfaces with a large surface area. To understand the heterocoagulated pyridinium reaction between the chloromethyl groups of the host poly(DVB-co-CMSt) core and the pyridyl groups of the guest poly(EGDMA-co-MAA)@ poly(EGDMA-co-VPy) corona, FT-IR spectra were measured for these two polymer particles and the raspberry-like polymer core-corona hybrids as shown in Figure 2. For both the corona and the resultant core-corona polymer hybrids, the FT-IR spectra in Figure 2b and c have a middle peak at 1599 cm-1 corresponding to the typical vibration of the pyridyl group. For poly(DVBco-CMSt) core microspheres, the FT-IR spectrum in Figure 2d show a characteristic peak at 1265 and 835 cm-1 assigned to the stretching vibration of the chloromethyl groups, which was (31) Liu, X.; Neoh, K. G.; Kang, E. T. Macromolecules 2003, 36, 8361. (32) Sampanthar, J. K.; Neoh, K. G.; Ng, S. W.; Kang, E. T.; Tan, K. L. AdV. Mater. 2000, 12, 1536. (33) Yu, W. H.; Kang, E. T.; Neoh, K. G. Ind. Eng. Chem. Res. 2004, 43, 5194.

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consistent with the results in the literature.22,34 In the FT-IR spectrum for the core-corona polymer hybrids in Figure 2c, a new peak appeared at 1652 cm-1 corresponding to the stretching vibration of the pyridinium group and the peak at 835 cm-1 relating to the vibration of the C-C1 bond in the chloromethyl group disappeared simultaneously. All these results demonstrate that the heterocoagulated pyridinium reaction between the chloromethyl groups of the cores and the pyridyl groups of the corona particles played an essential role as the interparticle affinity for the forming core-corona polymer hybrids with a raspberrylike structure as illustrated in Scheme 1, which was much different from the previous results for the formation of the raspberry-like structure polymer composites via the second-level interaction in the literature.16-18,22-24 However, the intensity of the peak at 1599 cm-1 for the pyridyl group in the FT-IR spectrum in Figure 2c for the core-corona hybrids was not reduced drastically compared to that in Figure 2b for the corona particles. These results suggest that a considerable number of pyridyl groups were not involved in the interparticle pyridinium reaction during the heterocoagulation, which was due to the strong steric hindrance of the rigid surfaces of the poly(DVB-co-CMSt) core and poly(EGDMA-co-MAA)@poly(EGDMA-co-VPy) corona. The formation of a pyridinium bond between the chloromethyl groups of poly(DVB-co-CMSt) microspheres and pyridine has been investigated in detail by FT-IR spectroscopy in our previous paper.24 Here, the poly(EGDMA-co-MAA)@poly(EGDMA-co-VPy) small corona particles were adsorbed on the surfaces of the poly(DVB-co-CMSt) core microspheres via covalent bonding in the heterocoagulated pyridinium reaction. The mass ratio of the small corona particles to the large core microspheres should have a crucial impact on the morphology of the resultant raspberry-like polymer hybrids. Figure 3 shows SEM micrographs of the corecorona polymer hybrids obtained with different mass ratios of corona particles to core microspheres ranging from 1/1 to 1/15. The results indicate that the core-corona polymer hybrids had a high coverage with a mass ratio in the range from 1/1 to 1/10 (Figure 3A and B), while the coverage decreased significantly with the mass ratio decreasing to less than 1/15 (Figure 3C). The mass ratio of poly(EGDMA-co-MAA)@poly(EGDMAco-VPy) small corona particles to poly(DVB-co-CMSt) core microspheres was kept at 1/10 for all the following experiments for self-assembled heterocoagulation via the pyridinium reaction in the present work. Stability of the Raspberry-like Core-Corona Polymer Hybrids. The stability of materials with complicated structures has been a particular concern for their application, especially for the core-corona polymer particles with a raspberry-like structure. In our previous work,23,24 the coverage of the polymer corona particles on the core polymer microspheres for the raspberrylike polymer composites via the interparticle hydrogen-bonding interaction decreased significantly after the corona particles were separated from the composites by ultracentrifugation, which was due to the fact that hydrogen-bonding as a driving force for the construction of the raspberry-like polymer composites was in a dynamic equilibrium state. This would limit further application of these polymer composites. Here, the resulting core-corona polymer hybrids with a raspberry-like structure were formed by the interparticle pyridinium reaction as a result of covalent bonds, which were conveniently purified by repeating centrifugation, decanting, and resuspension in DMF three times. The typical SEM micrograph in Figure 4A demonstrates that the core-corona (34) Akelah, A.; Rehab, A.; Agag, T.; Betiha, M. J. Appl. Polym. Sci. 2007, 103, 3739.

Wang and Yang

Figure 3. SEM micrographs of core-corona hybrids with different mass ratios of corona particle to core microsphere: (A) 1:1; (B) 1:5; and (C) 1:15.

polymer hybrids have satisfactory coverage after complete removal of the suspended small corona particles from the heterocoagulated system. In short, the core-corona polymer hybrids via the linkage between the core and corona components (covalent bonds) were stable enough and were easily separated from the reaction mixture without destruction of the raspberrylike structure and the morphology of the resultant polymer hybrids. The pH in the environment and solvent had spherical effects on the structure and stability of the morphology of the polymer composites in our previous work,22-24 which were due to the formation of heterocoagulates via charge compensation22 and hydrogen-bonding interaction23,24 as the weak secondary affinities. Here, the effects of pH and solvents on the stability and the morphology of the resultant raspberry-like polymer hybrids were studied in detail by means of transferring the self-heterocoagulated polymer hybrids to different suspensions with different pH values and various solvents. Figure 4B and C shows the typical SEM micrographs of the polymer hybrids in an acidic (pH 2.2) suspension and a basic (pH 12.4) suspension. These results imply

Synthesis of Raspberry-like Core-Corona Hybrids

Langmuir, Vol. 24, No. 7, 2008 3363

Figure 4. Stability of the core-corona hybrids under various environments: (A) after removal of corona particles; (B) pH ) 2.2; (C) pH ) 12.4; (D) MeOH; and (E) hexane.

that the raspberry-like polymer hybrids were highly stable over the entire pH range studied. The solvent effect on the morphology and stability of the polymer hybrids was investigated in various solvents, including water, methanol, ethanol, THF, hexane, and toluene. Typical SEM micrographs of the resultant polymer hybrids from different solvents such as those in methanol and hexane are shown in Figure 4D and E, respectively. The results indicate that the solvents had little effect on the structure and morphology of the resultant raspberry-like polymer hybrids for the solvent with strong polarity, including DMF, water, and alcohol (Figure 4A-C). The roughness of the surface of the core-corona hybrids decreased in the solvent with weak polarity, including THF and hexane, which was due to the highly swollen states for both of the core and corona components with EGDMA as the cross-linker. This may be a way to prepare core-shell hybrids with a smooth surface with an even lower cross-linking degree of EGDMA. The high stability of the core-corona polymer hybrids originated from the nature of the interparticle pyridinium reaction as a covalent bond rather than the polymer composites with the aid

of the secondary-level interaction in the literature,16-18,22-24 which was consistent with the results of the FT-IR spectra as discussed above. Hydrophobic Coating Surface with Raspberry-like Polymer Hybrids. To fully utilize the unique characteristics of the raspberry-like polymer hybrids, such as uneven surfaces with a large surface area, the hydrophobic property of the polymer film containing raspberry-like hybrids was preliminarily investigated. The dual-size hierachical structure on the surface of the lotus leaf exhibits super-water-repellent properties.35 The contact angle (CA) on a polycarbonate (PC) film coated with raspberry-like polymer hybrids was 101° as shown in Figure 5C, while the CAs on neat PC film and PC film coated with poly(DVB-co-CMSt) particles were 77° (Figure 5A) and 85° (Figure 5B), respectively. These results indicated that the PC film coated with raspberrylike polymer hybrids had better hydrophobicity than the neat PC film and PC film coated with poly(DVB-co-CMSt) particles. In (35) Feng, L.; Li, S.; Li, H.; Zhang, L.; Zhai, J.; Song, Y.; Liu, B.; Jiang, L.; Zhu, D. B. AdV. Mater. 2002, 14, 1857.

3364 Langmuir, Vol. 24, No. 7, 2008

Wang and Yang

polymer hybrids. A detailed investigation about the hydrophobicity of the film containing these raspberry-like polymer hybrids is being conducted by our group.

Conclusion Core-corona polymer hybrids with a raspberry-like structure were synthesized by a self-assembly process via the heterocoagulated pyridinium reaction between the chloromethyl groups of poly(DVB-co-CMSt) core microspheres and the pyridyl groups of poly(EGDMA-co-MAA)@poly(EGDMA-co-VPy) small corona particles. The resultant core-corona polymer hybrids had a high coverage when the mass ratio of corona particles to core microspheres was higher than 1/10. FT-IR spectra demonstrated that the nature of the construction for raspberry-like polymer hybrids was the heterocoagulated pyridinium reaction between the chloromethyl group of the core microsphere and the pyridyl group of the corona particle rather than the other affinities. The raspberry-like polymer hybrids were highly stable under the investigated pH ranging from 2.2 to 12.4 and in various solvents, which originates from the covalent bond nature of the linkage between the core and corona components. The unique characteristics of the raspberry-like polymer hybrids may be expected to extend their application to afford hydrophobic polymer films due to the surface roughness of the core-corona hybrids. Figure 5. Shapes of water droplets on polycarbonate films: (A) neat PC film; (B) PC film coated with poly(DVB-co-CMSt) microspheres; and (C) PC film coated with raspberry-like polymer hybrids.

other words, the polymer surface produced by the present technique possessed a bionic “lotus effect”, which may originate from the rough micro-nano binary structure of the raspberry-like

Acknowledgment. This work was supported in part by the National Science Foundation of China (Project No. 20504015) and the Opening Research Fund from the State Key Laboratory of Polymer Chemistry and Physics, Chinese Academy of Sciences (Project No. 200613). LA7036394