Preparation of Well-Defined Star Polymer from Hyperbranched

Ind. Eng. Chem. Res. 2007, 46, 97-102

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MATERIALS AND INTERFACES Preparation of Well-Defined Star Polymer from Hyperbranched Macroinitiator Based Attapulgite by Surface-Initiated Atom Transfer Radical Polymerization Technique Peng Liu*,† and Tingmei Wang‡ State Key Laboratory of Applied Organic Chemistry and Institute of Polymer Science and Engineering, College of Chemistry and Chemical Engineering, Lanzhou UniVersity, Lanzhou, Gansu 730000, China, and State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, China

To prepare the well-defined star polymer grafted attapulgite nanofibrillar clay (WSP-ATP) with hydrophilic core and hydrophobic shell and higher percentage of grafting, the postgraft polymerization of vinyl polymer from hyperbranched macroinitiator based attapulgite (HMI-ATP) via the surface-initiated atom transfer radical polymerization (SI-ATRP) technique was investigated for the first time. The HMI-ATP, with bromoacetic ester surface groups, was prepared by the bromoacetylation of the surface hydroxyl groups of the hyperbranched aliphatic polyester grafted attapulgite (HAPE-ATP). The HAPE-ATP was prepared by the one-pot polycondensation of the AB2-type monomer 2,2-bis(hydroxymethyl)propionic acid (bis-MPA), from the surfaces of the amino group modified attapulgite nanofibrillar clay (A-ATP) with p-toluenesulfonic acid (p-TSA) as catalyst. Then the SI-ATRP of methyl methacrylate (MMA) was conducted from the HMI-ATP made in the presence of 1,10-phenanthroline and Cu(I)Br as catalyst in toluene. The postgraft polymerization exhibited the characteristics of controlled/“living” radical polymerization. The product was characterized by Fourier transform infrared (FT-IR) spectroscopy, thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and transmission electron microscopy (TEM). Introduction The surface functionalizations of nanomaterials by grafting of polymer are expected to play important roles in the designing of novel organic/inorganic nanocomposite materials. The grafting of polymer onto nanosurfaces by “grafting from” and “grafting onto” methods has been widely investigated.1-4 The grafting of polymer onto inorganic particles, such as silica, clay, carbon black, and ferrite, is an effective means to improve surface properties because the surface grafted polymer chain can interfere with the aggregation of these particles and increase their surface affinity for organic solvents and polymer matrixes.5 Linear,6-19 hyperbranched, or dendrimer-like polymers20-23 have been successfully grafted onto various nanosurfaces. Mostly, the grafting polymerization was conducted via two main routes. (1) The first is the “grafting to” method: The readymade polymers with reactive end groups reacted with the functional groups on the nanosurfaces. The percentages of grafting (PG%’s) were no higher than 30% because of the more pronounced space hindrance of the readymade polymers and the cross-linking reactions that might have occurred.6-10,20,21 (2) The second is the “grafting from” method: Polymerizable groups such as carbon-carbon double bonds,11,12 chain-transfer groups,13 and initiator groups14-19 were covalently attached to the nanosurfaces and then the polymerization reactions (chain * To whom correspondence should be addressed. Tel.: 86-9318912516. Fax: 86-931-8912582. E-mail: [email protected]. † Lanzhou University. ‡ Chinese Academy of Sciences.

polymerization11-18 or step polymerization19,22,23) took place. For the “grafting from” method, the percentages of grafting (PG%’s) were not higher than 50%, except for the surfaceinitiated living/controlled radical polymerization (SI-LCRP) from inorganic nanoparticles.24-29 To obtain much higher grafting density, a new effective approach was developed in which the hyperbranched polymer grafted surfaces with higher percentage of grafting can be obtained by the postgraft polymerization initiated by the pendant initiating groups of the grafted polymer on the surfaces.30-35 Attapulgite (ATP) is a type of natural fibrillar silicate clay mineral and is mainly used as an absorbent, catalyst carrier, densifying agent, adhesive, and food additive.36 It has been classified as a three-layer inserted mineral of fibrous habit whose structure comprises a complete planar sheet of oxygen atoms arranged in exactly the same manner as that in micas and other clay minerals. A fibrillar single crystal is the smallest structural unit with a length of 500-2000 nm and diameter of 10-25 nm. Attapulgite has been shown to possess dioctahedral character. The chemical composition of attapulgite usually deviates from that associated with the ideal, electrically neutral, structural formula ((OH2)4(OH)2Mg5Si8O20‚4H2O) as a result of isomorphous replacement of Al for Si in the tetrahedral site and Al, Fe(II), Fe(III), and Ca for Mg in octahedral sites.37 In the present work, a novel method was developed for the postgraft polymerization of the vinyl monomer methyl methacrylate (MMA) from the hyperbranched polymer grafted attapulgite nanofibrillar clay by the surface-initiated atom transfer radical polymerization (SI-ATRP) technique (Scheme

10.1021/ie060504r CCC: $37.00 © 2007 American Chemical Society Published on Web 12/06/2006

98 Ind. Eng. Chem. Res., Vol. 46, No. 1, 2007 Scheme 1. Preparation Procedure for Well-Defined Star Polymer Grafted Attapulgite Nanofibrillar Clay

1). Compared with the work of Taniguchi et al. mentioned above,35 the controlled well-defined hydrophobic polymer shell was achieved in the proposed method. Experimental Section Raw Materials and Reagents. Attapulgite nanofibrillar clay (ATP) with an average diameter of 325 mesh was provided by Gansu ATP Co. Ltd., Gansu, China. It was dried in a vacuum at 110 °C for 48 h before use. γ-Aminopropyltriethoxysilane (APTES) (Gaizhou Chemical Industrial Co. Ltd., Liaoning, China) was used as received. p-Toluenesulfonic acid (p-TSA) and 2,2-bis(hydroxymethyl)propionic acid (bis-MPA) were of analytical grade and used as received from Tianjin Chemical Co., Tianjin, China. Bromoacetyl bromide was an analytical reagent grade from ACROS ORGANICS. Both 1,10-phenanthroline and Cu(I)Br were of analytical reagent grade and recrystallized from ethanol before using. Methyl methacrylate (MMA) and toluene (Tianjin Chemical Reagent Co., Tianjin, China) were of analytical reagent grade and used after being stirred overnight over CaH2 and distilled under reduced pressure. Triethylamine (TEA) was of analytical reagent grade. Ethanol, tetrahydrofuran (THF), dimethylformamide (DMF), and other solvents used were of analytical grade. Hyperbranched Aliphatic Polyester Grafted Attapulgite (HAPE-ATP). γ-Aminopropyltriethoxysilane (APTES) was first assembled onto the surface of the attapulgite nanofibrillar clay by the following procedure: 3.0 g of ATP and 5.0 mL of APTES were dispersed into 100 mL of dried toluene with ultrasonic agitation for 30 min in a 250 mL flask; the mixture was refluxed for 8 h with electromagnetic stirring. After cooling to room temperature, the product, aminopropyl modified attapulgite (A-ATP), was filtered and thereafter thoroughly washed with ethanol and dried in a vacuum at 40 °C. Then the AB2 monomer, bis-MPA, was solution polycondened according to the method reported previously,38 cored with the γ-aminopropyl attapulgite (A-ATP) with surface active sites amino groups: 2.0 g of γ-aminopropyl attapulgite (A-ATP), 5.0 g of bis-MPA, and 0.50 g of p-TSA were added into 40

mL of DMF. The mixture was irradiated ultrasonically for 30 min and refluxed for 8 h with N2 bubbling throughout. The hyperbranched aliphatic polyester grafted attapulgite (HAPEATP) was separated from the nongrafted hyperbranched aliphatic polyester by several cycles of dispersion in dimethylformamide (DMF) with ultrasonic vibrations for 30 min and precipitated by centrifugation at 104 rpm for 30 min. It was then washed throughout with ethanol and dried in a vacuum at 40 °C. HMI-ATP. The hyperbranched macroinitiator based attapulgite (HMI-ATP), bromoacetic ester modified hyperbranched aliphatic polyester grafted attapulgite (HAPE-ATP), was prepared by the procedure similar to that reported previously:39 2.0 g of HAPE-ATP and a catalyst amount of TEA were dispersed into 100 mL of THF with ultrasonic agitation for 30 min, 2.0 mL bromoacetyl bromide was added into the dispersion, and the mixture was stirred with an electromagnetic stirrer for 12 h at room temperature. Then the HMI-ATP was filtered and thereafter thoroughly washed with dichloromethane and ethanol. The HMI-ATP was dried in a vacuum at 40 °C. Well-Defined Star Polymer Grafted Attapulgite Nanofibrillar Clay (WSP-ATP). A 2.0 g sample of the HMI-ATP, 0.72 g (5.0 mmol) of Cu(I)Br, 1.80 g (10.0 mmol) of 1,10phenanthroline, 20.0 mL of MMA, and 100 mL of toluene were charged into a 250 mL flask. The mixture was irradiated with

Figure 1. FT-IR spectra of ATP, A-ATP, HAPE-ATP, and WSP-ATP.

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Figure 2. Effect of polymerizing time on C% and PG% values.

Figure 3. TGA curves of bare ATP, HAPE-ATP, and WSP-ATP.

ultrasonic vibrations for 30 min, bubbling with nitrogen (N2). Then the mixture was refluxed with electromagnetic stirring for 8 h. N2 was bubbled throughout the polymerizing period. Parts of the mixture were taken out after every 2 h. The parts of the reacting solution taken out were poured into ethanol to precipitation, respectively. The precipitants were dried in a vacuum at 40 °C. To remove the possible nongrafted free poly(methyl methacrylate) (PMMA), the products were washed thoroughly with toluene. Removal of Copper Contaminants.40 The crude product was greenish in color due to copper residues that were retained. To remove the copper contaminants from the crude product, the powder was extracted with acetylacetone-ethanol solution (100 mL, volume ratio of 1:5) for 24 h. The product was filtered and washed with ethanol. In this way, a gray powder was obtained. Analytical Methods. Elemental analysis (EA) of C, N, and H was performed on an Elementar vario EL instrument. A Bruker IFS 66 v/s infrared spectrometer was used for the Fourier transform infrared (FT-IR) spectroscopic analysis. Thermogravimetric analysis (TGA) was performed with a Perkin-Elmer TGA-7 system (Perkin-Elmer Corporation) at a scan rate of 10 °C min-1 to 800 °C in N2 atmosphere. X-ray photoelectron spectroscopy (XPS) was accomplished using a PHI-5702 multifunctional X-ray photoelectron spectrometer with a pass energy of 29.35 eV and an Mg KR line excitation source. The binding energy of C 1s (284.6 eV) was used as a reference. X-ray diffraction (XRD) analysis was carried out with a Shimadu XRD 6000 with Cu KR radiation, operated at 50 kV and 80 mA over the range 10° < 2θ < 100°. The morphologies of the nanofibrillar clays were characterized with a JEM-1200 EX/S transmission electron microscope (TEM). The powders were respectively dispersed in their appropriate solvents in an ultrasonic bath for 5 min, and then deposited on a copper grid covered with a perforated carbon film. The conversion of the monomer (MMA) (C%) and the percentage of grafting (PG%) were calculated according to the following relationships from the results of carbon elemental analyses:

solution polycondensation of the AB2-type monomer 2,2-bis(hydroxymethyl)propionic acid (bis-MPA), in the presence of the amino group modified attapulgite nanofibrillar clay (A-ATP) with p-toluenesulfonic acid (p-TSA) as catalyst. The reaction of the carboxyl groups of the AB2-type monomer and the amino groups of the A-ATP has precedence over the reaction of the carboxyl and hydroxyl groups of the AB2-type monomer because of the higher reactive activity of amino groups. Therefore, a higher grafting efficiency is expected. After the assembly of γ-aminopropyltriethoxysilane onto the surfaces of attapulgite, the 2941 cm-1 band of methyl and methylene groups and the absorbance bands at about 3556 and 3620 cm-1 of amino groups were found in the FT-IR spectrum of the product (Figure 1). The amino group content was found to be 0.6 mmol/g, calculated from the C and N elemental analyses. After the “grafting from” polycondensation of the AB2 monomer, the absorbance band at 1734 cm-1 of ester groups was found in the FT-IR spectrum of the HAPE-ATP (Figure 1). This showed the hyperbranched aliphatic polyester had been successfully grafted onto the surfaces of the ATP nanofibrillar clay by the proposed method. The presence of the characteristic band at about 1655 cm-1 of amide groups also showed that the hyperbranched aliphatic polyester was grafted onto the surfaces of the ATP nanofibrillar clay via the amidation reaction between the carboxyl group of the monomer and the amino groups on the A-ATP. The presence of the bands at about 3556 and 3620 cm-1 of amino groups also showed that the amino groups of the A-ATP had not completely reacted with the carboxyl groups of the monomer or the nongrafted HAPE molecules with smaller molecular weights because of space hindrance. A PG% of 5.6% was found to be obtained from the elemental analysis results. The introduction of the bromoacetic ester surface groups onto the ATP nanofibrillar clay was achieved by the bromoacetylation of the surface hydroxyl groups of the HAPE-ATP with bromoacetyl bromide as shown in Scheme 1. The weight of the product, the hyperbranched macroinitiator modified attapulgite (HMI-ATP), was 2.28 g. The content of the initiating groups for ATRP, bromoacetic ester, was found to be about 2.3 mmol/g HMI-ATP. SI-ATRP. After the surface-initiated atom transfer radical polymerization (SI-ATRP) of methyl methacrylate (MMA) from the surfaces of the macroinitiators, the hyperbranched macroinitiator modified attapulgite (HMI-ATP), the characteristic absorbance band at 1734 cm-1 of ester groups was found to be enhanced in the FT-IR spectrum of the products (Figure 1). This indicated that poly(methyl methacrylate) (PMMA) chains had been successfully grafted from the surfaces of the macroinitiator. However, the FT-IR band of ester groups at 1734 cm-1 seems

C% ) total PMMA (g)/MMA charged (g) × 100% PG% ) well-defined star polymer grafted (g)/HMI-ATP charged (g) × 100% Results and Discussion Hyperbranched Macroinitiator Based Attapulgite (HMIATP). The grafting of the hyperbranched aliphatic polyester onto the attapulgite nanofibrillar clay was conducted by the

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Table 1. Surface Compositional Data from XPS surface element analysis (atom %) sample

Si

O

Al

C

N

Br

Cu

ATP A-ATP HAPE-ATP HMI-ATP WSP-ATP

10.41 8.14 7.66 5.62 2.10

44.52 32.95 29.83 26.37 29.42

6.28 2.95 1.80 0.95 0.04

38.16 51.55 59.11 63.51 66.05

0.63 4.41 1.60 0.90 0.50

0.00 0.00 0.00 2.65 1.87

0.00 0.00 0.00 0.00 0.02

Figure 4. XPS survey spectra of ATP, A-ATP, HAPE-ATP, HMI-ATP, and WSP-ATP.

Figure 5. XRD patterns of ATP, HAPE-ATP, and WSP-ATP.

weaker because of the strong Si-O stretching absorbance band at 1030 cm-1 of attapulgite.

The products of the SI-ATRP were washed throughout with toluene to remove the possible free nongrafted poly(methyl methacrylate) (PMMA). The washing solutions were poured into ethanol and no precipitant was observed. This showed that all of the PMMA formed was chemically bonded onto the surfaces of the ATP nanofibrillar clay and no free nongrafted PMMA was brought. It is one of the important strong points of the SIATRP technique.25 The kinetics of the SI-ATRP of MMA was studied by monitoring the changes of the carbon element content in the well-defined star polymer grafted attapulgite nanofibrillar clay (WSP-ATP) by carbon elemental analysis (EA) as a function of time. To prove the controlled nature of the polymerization initiated from the surfaces of the hyperbranched macroinitiator, the effect of the polymerizing time on the PG% (overall percentage of grafting of the well-defined star polymer) and the C% were investigated (Figure 2). They increased linearly with increasing polymerizing time, and reached 113.4% and 10.7%, respectively, after a polymerizing time of 8 h, calculated from the EA results. This indicated that the proposed method showed the characteristics of the controlled/“living” radical polymerization.26,34,40,41 At higher C%, departures of the C% and PG% values from the linear function were found. This might have resulted from the wrapping of some initiator groups and the decrease of the monomer concentration at higher C% values. In the TGA curve of the well-defined star polymer grafted attapulgite nanofibrillar clay (WSP-ATP) (Figure 3), the weight loss at nearly 100 °C could be assigned to the release of solvent adsorbed. The main weight losses at 200 °C were assigned to the release of the structure water of ATP and thermal degradation of the grafted well-defined star polymer. The weight losses of ATP and WSP-ATP were found to be 8.77% and 53.23%,

Figure 6. TEM images of (a) bare ATP (in water), (b) HAPE-ATP (in DMF), and (c) WSP-ATP (in toluene).

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respectively. Almost all the weight losses took place in the temperature range of 200-800 °C. The PG% of the WSP-ATP was found to be about 110%, calculated from the TGA results. This is in good agreement with that (113.4%) calculated from the carbon elemental analysis. Surface Analysis. Surface analysis by X-ray photoelectron spectroscopy (XPS) also indicated that the well-defined star polymer had been successfully grafted from the surface of the ATP nanofibrillar clay with the procedure shown in Scheme 1 (Table 1, Figure 4). It was evident that the surface element contents of Si and Al decreased throughout the four steps of reaction. The surface element content of O decreased and those of C and N increased after the assembly of the functional silane onto the attapulgite. The surface element content of C increased and those of N and O increased after the hyperbranched aliphatic polyester was grafted. The trends of the surface element contents of C, O, and N were same after the bromoacetylation of the surface hydroxyl groups with bromoacetyl bromide and Br element appeared. After the SI-ATRP of MMA from the HMIATP, the surface element contents of C and O increased and those of N and Br increased. Cu element was found due to the incomplete removal of the catalyst. XRD and TEM Analyses. The XRD patterns of the bare ATP, the HAPE-ATP, and the WSP-ATP are given in Figure 5. There was no change of the XRD patterns of the ATP after surface modifications with the hyperbranched aliphatic polyester, and the well-defined star polymer subsequently, except for the changes of their intensities. It could be concluded that the welldefined copolymer was grafted from the surfaces of the nanofibrillar clay by the proposed method and it had no effect on the crystal structure of the nanofibril. The TEM images of the bare ATP (in water), the HAPEATP (in DMF), and the WSP-ATP (in toluene) are shown in Figure 6. It could be concluded that the grafting of the hyperbranched or star polymers had markedly improved the dispersibility of the nanofibrillar clay in organic solvents. Conclusion A new preparation method for well-defined star polymer grafted nanosurfaces was developed by the surface-initiated atom transfer radical polymerization (SI-ATRP) technique using a grafted hyperbranched polymer as macroinitiator. The SI-ATRP procedure showed the characteristics of the controlled/“living” radical polymerization. Another advantage can be concluded that the controlled well-defined hydrophobic polymer shell was achieved in the proposed method. Acknowledgment This work was supported by the Natural Science Foundation of Gansu Province (3ZS041-A25-002) and the Interdisciplinary Innovation Research Fund For Young Scholars, Lanzhou University (LZU200302). Literature Cited (1) Yoshinaga, K. Functionalization of inorganic colloidal particles by polymer modification. Bull. Chem. Soc. Jpn. 2002, 75, 2349. (2) Liu, P. Carbon-chain polymers “grafting from” inorganic nanoparticles. e-Polymers 2005, 070. (3) Liu, P. Preparation of dendrimer-like or hyperbranched polymers grafted inorganic nanoparticles: A literature review. Mater. Res. InnoV. 2005, 9, 103. (4) Liu, P. Modifications of carbon nanotubes with polymers. Eur. Polym. J. 2005, 41, 2693. (5) Liu, P. In Polymeric Nanostructures and Their Applications; Nalwa, H. S., Ed.; American Scientific Publishers: 2006.

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ReceiVed for reView April 21, 2006 ReVised manuscript receiVed October 29, 2006 Accepted November 2, 2006 IE060504R