Synthetic Materials: Polymers Grafted

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Hybrid Rigid/Soft and Biologic/Synthetic Materials: Polymers Grafted onto Cellulose Microcrystals Simon Harrisson,*,†,§ Glenna L. Drisko,†,^ Eva Malmstr€om,‡ Anders Hult,‡ and Karen L. Wooley*,†,|| †

Center for Materials Innovation and Department of Chemistry, Washington University in St. Louis, St. Louis, Missouri 63130, United States ‡ KTH Royal Institute of Technology, Department of Fibre and Polymer Technology, SE-100 44 Stockholm, Sweden ABSTRACT: Rigid nanoscale polymer rods were prepared by grafting preformed amine-terminated poly(styrene) and poly(tert-butyl acrylate) onto oxidized cellulose microcrystals. Low polydispersity polymers, grown using atom transfer radical polymerization, were characterized and purified prior to cellulose attachment. Oxidation of the cellulose microcrystal led to the formation of carboxylic acids on the surface of the microcrystals. Covalent attachment of the polymers onto the cellulose microcrystals was achieved via a carbodiimide-mediated amidation reaction. The length and diameter of the polymer-cellulose composites increased upon surface modification. Typically, polymer-cellulose composites are synthesized by a grafting-from method because it can be difficult to obtain sufficient graft density using a grafting-to preparation. However, the composites reported here comprised 60-64% grafted polymer by mass. This degree of grafting-to allowed the composite to form stable suspensions in organic solvents.

’ INTRODUCTION The preparation of objects with defined shape and submicrometer size domains is a major goal of nanotechnology. An impressive degree of geometric control is attainable when working with crystalline materials: rods, cubes, spheres, wires, and more complex 3D shapes have been reported.1 The inherent lack of rigidity of soft materials places severe restrictions on the variety of shapes that can be produced. As a result, the vast majority of discrete nanoscale structures formed from soft materials exhibit spherical symmetry (e.g., micelles, vesicles). When cylindrical (1D) or lamellar (2D) structures are obtained, they are highly deformable, which may limit their use as building blocks in more complex superstructures.2 An attractive alternative to the de novo production of 1D or 2D nanostructures is to make composite materials that incorporate a rigid scaffold that can impart its shape to the total assembly. This approach allows the wide variety of functionalities associated with soft materials to be married to the diversity of shapes exhibited by hard materials.3 Cellulose is a ubiquitous crystalline material that can be readily broken down into rod-shaped nanocrystals.4 It is an excellent stiff substrate because it contains surface hydroxyl groups that can be used as attachment points for an assortment of organic functionalities.5 Cellulose is biosynthesized in the form of microfibrils with nanoscale diameters and alternating crystalline and noncrystalline (amorphous) domains.5 Treatment with warm, concentrated acid results in the preferential hydrolysis of the amorphous domains, leading to the formation of rigid, rod-shaped crystalline particles, known as cellulose microcrystals.4 The dimensions of these particles depend on the source of the cellulose but are typically tens of nanometers in diameter and range from 100 nm to several micrometers in length.6 Cellulose microcrystal suspensions r 2011 American Chemical Society

have been studied since 1959,7 with interest largely focused on their ability to form colloidal liquid crystalline phases.8-12 More recently, interest in nanocomposite materials has prompted research into the use of cellulose microcrystals as inexpensive, biorenewable, rigid, nanoscale fillers in composites with synthetic and natural polymers.1,5 Current limitations of unmodified cellulose microcrystals include poor solubility in common solvents, a lack of thermoplasticity, and poor dimensional stability. Their inability to form stable colloidal suspensions in nonaqueous solvents forces the use of matrices that are either water-soluble (e.g., starch,13-15 polyethylene glycol,16-19 ethylenediamine/thiocyanate salt20) or water-dispersible (e.g., styrene-butyl acrylate emulsions,21 epoxy emulsions,22 poly(β-hydroxyoctanoate) latices23). The highly hydrophilic nature of the cellulose microcrystal surfaces will result in poor adhesion to hydrophobic matrices. By grafting polymers with differing hydrophobicity and elasticity onto the surface of cellulose, their dispersibility can be increased. Many attempts have been made to modify the surface of cellulose microcrystal suspensions to improve their dispersibility in nonaqueous solvents.4,5 The most successful strategies for doing so have been (a) grafting PEG chains onto the cellulose surface,24,25 (b) silylation of the surface cellulose chains,26-28 (c) polymerization from the cellulose surface,29-38 and (d) adsorption of a polyethylene oxide nonyl phenyl phosphate surfactant.39,40 In all cases, the electrostatic stabilization of the original microcrystals is converted to steric stabilization, by either grafted or adsorbed polymer chains (a, c, and d) or by partially solvated silylated cellulose chains (b). Silylation has limited utility as a Received: December 13, 2010 Revised: January 18, 2011 Published: March 07, 2011 1214

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Biomacromolecules means of dispersion: the cellulose can partially dissolve as the silylation progresses, which may limit the degree of silylation that can be achieved.27 The crystallinity and stiffness of the silylated microcrystals are substantially decreased. By contrast, stabilization by grafting polymers or by adsorbing surfactant proceeds without detrimental effects on the morphology of the cellulose microcrystals. The vast majority of the polymers have been grafted-from, rather than grafted-to, the cellulose backbone.5 This approach allows high graft densities to be achieved, but the polymerization is difficult to control and the resulting materials are poorly characterized. The grafted-to method is a modular approach in which the polymers are fully characterized and purified before attachment to cellulose. Moreover, the graftedto approach allows the polymers to be prepared via a wider variety of techniques, as long as there is a linkable functionality. In this Article, we present a method for the production of cellulose microcrystals grafted with hydrophobic polymers, such as poly(styrene) and poly(t-butyl acrylate). The materials are prepared via amidation reactions between the polymers’ terminal amine functionalities and carboxylic acid groups on the surface of the oxidized cellulose microcrystals. The polymer-grafted microcrystals are readily dispersed in nonpolar solvents such as acetone and toluene and form an interesting class of rigid, rod-shaped, polymeric nanoparticles, which may have applications in the production of nanocomposite materials with hydrophobic matrices such as natural rubber. The hydrophobic polymers are produced via a quasi-living radical polymerization process, which can be used to form well-defined polymers with a wide range of functionalities and architectures. Polymer grafting could allow this rigid biomaterial to be integrated into an assortment of composites for use in packaging, coatings, adhesives, biomedical polymers, tissue engineering, active surfaces, and engineering applications.

’ EXPERIMENTAL SECTION Materials. Fibrous cellulose powder (Whatman CF11 chromatography grade) was obtained from Whatman International (Clifton, NJ). Styrene (99%) and tert-butyl acrylate (99%) were purchased from Sigma Aldrich (St. Louis, MO); inhibitor was removed by passing the monomer through a column of basic alumina prior to use. Copper(I) bromide (99.9995%), N,N,N0 ,N00 ,N00 -pentamethyl diethylene triamine (PMDETA, 99%), 2-(2-aminoethoxy) ethanol (98%), phthalic anhydride (99%) triethylamine (99%), 2-bromoisobutyryl bromide (99%), 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO, 98%), 1% phosphotungstic acid (PTA), methanol (MeOH), ethanol (EtOH), tetrahydrofuran (THF), dimethyl formamide (DMF), 2-(2-aminoethoxy)ethanol, phthalic anhydride, N-hydroxy succinimide (NHS), 1,3-diisopropyl carbodiimide (DIC), and toluene were obtained from Sigma Aldrich and used without further purification. Sodium hypochlorite solution (available chlorine 10-13%) was obtained from Sigma Aldrich. Its concentration was determined by the addition of excess potassium iodide in the presence of sulfuric acid, followed by back-titration with sodium thiosulfate solution using a starch indicator.24 Measurements. 1H NMR (300 MHz) and 13C NMR (75 MHz) spectra were recorded on a Varian Mercury 300 MHz spectrometer. Infrared (IR) spectra were obtained on a Perkin-Elmer BX FT-IR system equipped with a diffuse reflectance accessory. Elemental analysis was performed by M-H-W Laboratories (Phoenix, AZ). Thermogravimetric analyses (TGAs) were performed at a heating rate of 10 C min-1 on a Mettler Toledo TGA822e using Mettler Toledo Star SW 7.01 software. Gel Permeation Chromatography (GPC). Polymer molecular weight distributions were determined by GPC, conducted on a Waters

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Chromatography model 150-CV, equipped with a model 410 DRI detector, a Precision Detectors PD2040 dual angle (15 and 90) light scattering detector, and a three-column series of Polymer Laboratories PLgel 10 μm mixed B 300 7.5 mm columns. The system was equilibrated at 35 C in anhydrous THF, which served as the polymer solvent and eluent (flow rate 1.00 mL min-1). Data collection and analysis were performed using Precision Detectors software. Interdetector delay volume and the light scattering detector calibration constant were determined using a nearly monodisperse poly(styrene) calibrant (Pressure Chemical, Mp = 90 000 g mol-1, Mw/Mn < 1.04). The differential refractometer was calibrated with standard poly(styrene) reference material (SRM 706 NIST) of a known specific refractive index increment (dn/dc = 0.184 mL g-1). The dn/dc values of the analyzed polymers were then determined from the differential refractometer response. Transmission Electron Microscopy (TEM). Carbon-coated copper grids were prepared by oxygen plasma treatment to make the surface hydrophilic. Samples were diluted 9:1 in water. A drop of diluted sample was deposited onto a grid; after 1 min excess solvent was wicked away. A drop of 1% PTA stain was then added and left for 1 min before excess stain was wicked away. Particle diameters and standard deviations were calculated from measurements of at least 100 particles from three TEM micrographs from different regions. Atomic Force Microscopy (AFM). Tapping-mode atomic force microscopy measurements were conducted in air with a Nanoscope III BioScope system (Digital Instruments) operated under ambient conditions with standard silicon tips (type: OTESPA-70; L: 160 μm; normal spring constant: 50 N m-1; resonance frequency 246-282 kHz). Samples were prepared by spin coating a dilute suspension of cellulose microcrystals in water (pregraft) or acetone (postgraft) onto a freshly cleaved mica surface.

Synthesis of N-(2-(2-Bromoisobutyryl)ethoxy)ethyl phthalimide (1). 2-(2-Aminoethoxy)ethanol (10.5 g, 0.10 mol), phtha-

lic anhydride (14.8 g, 0.10 mol), and THF (50 mL) were mixed and refluxed together for 1 h, by which time all phthalic anhydride had dissolved. THF was removed in vacuo, and the product was refluxed overnight in toluene using a Dean-Stark trap. The biphasic mixture became homogeneous as the reaction proceeded. The solution was cooled to 0 C, triethylamine (10.1 g, 0.10 mol) and 2-bromoisobutyryl bromide (23.0 g, 0.10 mol) were added, and the solution was allowed to warm to room temperature and stirred for 1 h. Triethylamine hydrobromide was precipitated and was removed by filtration. The solution was washed with water (3 30 mL) and saturated NaCl solution (1 30 mL), then filtered through a short plug of basic Al2O3. Removal of solvent in vacuo gave the desired product, which was further dried under high vacuum to remove the last traces of toluene. Yield: 32.93 g (85.7 mmol, 85.7%) as a yellow oil. 1 H NMR (CDCl3): δ 1.85 (s, 6H, -(CH3)2CBr), 3.72 (t, 2H, J = 4.8 Hz, -CH2CH2OC(O)-), 3.76 (t, 2H, J = 5.8 Hz, -OCH2CH2N-), 3.90 (t, 2H, J = 5.8 Hz, -NCH2-), 4.27 (t, 2H, J = 4.8 Hz, CH2OC(O)-), 7.70-7.86 (m, 4H, aromatic protons). 13C NMR (CDCl3): δ 30.8 (CH3-), 37.5 (>NCH2-), 55.8 (BrC(CH3)2-), 65.2 (-C(O)OCH2-), 68.2 (>NCH2CH2O-), 68.4 (-C(O)OCH2CH2O-), 123.5, 132.3, 134.2 (aromatic carbons), 168.4 (phthalimide CdO), 171.7 (ester CdO). IR (diffuse reflectance): 2945, 2871, 1775 (phthalimide CdO stretch), 1725 (ester CdO stretch), 1467, 1430, 1400, 1281, 1174, 1028, 725, 530 cm-1. Elemental analysis: C: 49.85% H: 4.70% N: 3.44% Br: 21.03%. Theoretical C: 50.02% H: 4.72% N: 3.65% Br: 20.80%. MS (HRFAB, 3-NBA/Li matrix): m/z 390.0528 ([M þ Li]þ, theoretical mass 390.0528). Preparation of r-Phthalimidyl poly(styrene). Kinetics: Initiator (1, 503 mg, 1.31 mmol), styrene (14.6 g, 140 mmol), CuBr (200 mg, 1.4 mmol), and PMDETA (290 μL, 1.4 mmol) were mixed in a round-bottomed flask and freeze-thaw degassed (three cycles) before 1215

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Scheme 1. Preparation of Polymer-Grafted Cellulose Microcrystals

being backfilled with N2 and placed in an 80 C oil bath. Samples (∼0.5 mL) were taken every 30 min to track conversion (1H NMR) and molecular weight (GPC) (Figure 4). At 210 min, the reaction was stopped by pouring the reaction mixture into 200 mL of MeOH. The polymer was dissolved in THF and filtered through a short plug of basic Al2O3 to remove copper, then reprecipitated from MeOH and dried overnight in a vacuum oven at 70 C. We recovered 3.07 g of white polymer (53% based on 36% final conversion). Mn = 4840 g mol-1. PDI = 1.08 (GPC, PS stds). Theoretical Mn: 4390 g mol-1. Poly(styrene) for grafting: initiator (1, 500 mg, 1.31 mmol), styrene (7.70 g, 68 mmol), CuBr (200 mg, 1.4 mmol), and PMDETA (280 μL, 1.4 mmol) were degassed and heated at 100 C for 2 h before precipitation from MeOH and drying under vacuum. Conversion 97% (1H NMR). Yield: 5.32 g (71%). Mn = 8950 g mol-1 (GPC, PS stds), PDI = 1.14. Theoretical Mn: 5540 g mol-1. Preparation of r-Phthalimidyl poly(t-butyl acrylate). Initiator (1, 503 mg, 1.31 mmol), tert-butyl acrylate (16.99 g, 133 mmol), CuBr (200 mg, 1.4 mmol), and PMDETA (290 μL, 1.4 mmol) were mixed, degassed and placed in an oil bath at 60 C. After 3 h (81% conversion), the reaction was stopped by precipitating the polymer in 200 mL of 75:25 MeOH/H2O. The polymer was redissolved in THF, filtered through a short plug of basic Al2O3, and dried thoroughly in vacuo to remove THF and residual monomer. We recovered 7.01 g of polymer (47.3% based on 81% conversion). Mn = 10 530 g mol-1. PDI = 1.10 (GPC, molecular weight by multiangle light scattering). Theoretical Mn: 10 960 g mol-1. Deprotection of r-Phthalimidyl Poly(styrene). Phthalimidylfunctionalized poly(styrene) (5.87 g, Mn = 8950 g mol-1, PDI = 1.14 0.66

mmol) was dissolved in 20 mL of THF. Hydrazine hydrate (1 mL) was added, and the solution was refluxed for 2 h. A white precipitate of phthalhydrazide formed. The polymer was precipitated in 100 mL of MeOH and dried overnight in a vacuum oven (70 C). Yield: 5.22 g (88.9%). Mn = 9870 g mol-1. PDI = 1.32.

Deprotection of r-Phthalimidyl Poly(t-butyl acrylate).

Phthalimidyl-functionalized poly(t-butyl acrylate) (5.00 g, Mn = 10 530 g mol-1, PDI = 1.10, 0.48 mmol) was dissolved in 20 mL of EtOH. Hydrazine hydrate (1 mL) was added, and the solution was heated at reflux for 2 h. A white precipitate of phthalhydrazide formed. The polymer was precipitated in 50 mL of water and dried in a vacuum oven (70 C, overnight). Yield: 4.48 g (89.6%). Mn = 10 800 g mol-1. PDI = 1.09. Preparation of Cellulose Microcrystal Suspension. Cellulose microcrystal suspensions were produced according to the method of Dong et al.41 In a typical procedure, 10 g of powdered cellulose was dispersed in 100 mL of 64% H2SO4 and stirred for 45 min at 45 C. The resulting viscous, slightly yellow suspension was poured in 1 L of H2O to quench the reaction. The cellulose was separated by centrifugation (3000 rpm, 5 min), then resuspended in H2O. This treatment was repeated until the supernatant remained cloudy after centrifugation. At this point, the entire mixture was homogenized using an Osterizer food processor set to “liquefy” for 30 min, then centrifuged once more. The cloudy supernatant was poured into dialysis bags and dialyzed against DI H2O for 3 days to remove the last traces of H2SO4. A white suspension was obtained. Volume: 535 mL. Solids content: 1.08 w/w% (determined by lyophilization). pH 4. Yield: 58%. 1216


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Figure 1. TEM micrographs and length and diameter population distributions for cellulose microcrystals (A, unfilled bars) and oxidized cellulose microcrystals (B, filled bars).

Oxidation of Cellulose Microcrystals. Cellulose microcrystal suspensions were oxidized according to the method of Araki, Kuga, and Wada.24 In a typical procedure, 1.73 g TEMPO and 4.32 g NaBr were added to 400 mL of cellulose suspension (1.08% solids, 4.32 g cellulose). The pH was adjusted to approximately 11 through the addition of a few drops aqueous KOH and 22.6 mL of NaOCl solution (86 g L-1, 2.16 g NaOCl). The yellow-orange mixture was stirred at room temperature for 4 h. After this time, 30 g NaCl was added to flocculate the suspension, and the cellulose was separated by centrifugation (3000 rpm, 5 min). The cellulose was washed three times with 1 M NaCl (by suspension and centrifugation), three times with 0.1 M HCl, and a final time with DI H2O before being transferred to dialysis bags and dialyzed against DI H2O for 3 days. Yield: 183 mL of white suspension, solids content 1.63% (69% yield). Carboxylic acid content was determined by titration to be 0.76 mmol g-1 cellulose by following the procedure in ref 24. IR analysis showed a strong absorption at 1732 cm-1, indicating the formation of carboxylic acid groups. Suspension of Oxidized Cellulose in DMF. In a typical procedure, 50 mL of oxidized cellulose suspension (1.63%, 0.82 g cellulose) was flocculated by the addition of 3 mL of saturated NaCl, separated by centrifugation and rinsed (by redispersion, followed by centrifugation at 3000 rpm for 5 min) three times with 50 mL of acetone, then two times with 50 mL of DMF. The pellet was dispersed in 50 mL of DMF and subjected to 5 min of sonication, then centrifuged (5 min, 3000 rpm). The turbid supernatant was decanted. The residue was dispersed in a further 50 mL of DMF, given a further 5 min ultrasound treatment and centrifuged (5 min, 3000 rpm). All of the cellulose remained in suspension. The two portions were combined, yielding 100 mL of DMF suspension containing 0.82 g oxidized cellulose. Poly(styrene) Grafted (PS-Grafted) onto Oxidized Cellulose. Amine-functionalized poly(styrene) (2.0 g, 9 870 g mol-1, 0.2

mmol) was dissolved in 10 mL of DMF and added to a 50 mL suspension of oxidized cellulose in DMF (0.41 g cellulose, 0.31 mmol COOH). NHS (100 mg, 0.87 mmol) and DIC (172 mg, 1.37 mmol) were added to this mixture. The suspension was stirred at room temperature for 16 h. The cellulose and poly(styrene) were precipitated by the addition of 50 mL of H2O and separated by centrifugation. The pellet was washed three times with acetone by resuspension, ultrasonication (5 min), and centrifugation to dissolve the excess poly(styrene). The pellet was then dispersed in toluene (50 mL) using an iterative process: 10 min of ultrasound treatment, centrifugation at 3000 rpm for 5 min, decantation of the liquid, and addition of 25-50 mL toluene. Each time, a turbid supernatant was collected. After three iterations, the small amount of solid material remaining was discarded. The final volume was 121 mL. The toluene was removed through evaporation from a 10 mL portion of the suspension, and the solid residue was heated to 60 C for 2 h under vacuum to remove any remaining solvent. Final mass: 63 mg (7.3 mg g-1, equivalent to 0.76 g total solids). Mass yield: 186% relative to oxidized cellulose.

Poly(t-butyl acrylate) Grafted (PtBA-Grafted) onto Oxidized Cellulose. Amine-functionalized poly(t-butyl acrylate) (3.0 g, 0.28 mmol) was dissolved in 10 mL of DMF and added to 50 mL of a suspension of oxidized cellulose in DMF (0.65 g cellulose, 0.52 mmol COOH). DIC (320 μL, 2 mmol) and NHS (140 mg, 1.2 mmol) were added to the resulting suspension, which was then stirred for 13 h at room temperature. The mixture was centrifuged, and the pellet was twice suspended in acetone and separated by centrifugation. It was then resuspended in acetone and subjected to a 10 min ultrasound treatment. This resulted in a cloudy suspension that did not completely separate on centrifugation. The cloudy supernatant was decanted, and the ultrasound and centrifugation treatment was repeated until no further cloudy suspension was obtained. A total of 100 mL (86 g) of suspension was obtained, with solids content of 1217


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Figure 2. (A,C) AFM micrographs and sections of cellulose microcrystals and (B,D) oxidized cellulose microcrystals. White lines in A and B indicate the position of sections C and D. The vertical distance between the arrowheads in C and D is 18 and 19 nm, respectively.

Table 1. Dimensions of PS- and PtBA-Grafted, Oxidized, and Untreated Microcrystals from Analysis of TEM Data PS-grafted XN (nm)a

s.d. (nm)b

XW (nm)c

PDId

XN (nm)a

1.15 1.16

340 24

cellulose microcrystals

length diameter

300 17

110 7.0

341 20

oxidized cellulose

length

250

91

280

1.13

310

25

1.09

18

diameter polymer-grafted cellulose

length diameter

a

PtBA-grafted

23

7.0

s.d. (nm)b

XW (nm)c

PDId

140 12

390 31

1.16 1.27

120

350

1.15

20

1.13

6.0

301

99

334

1.11

440

136

482

1.10

44

12

47

1.07

79

17

82

1.05

Number average. b Standard deviation. c Weighted average. d Polydispersity index (XW/XN).

30 mg g-1 (total solids 2.58 g, 397% mass yield relative to cellulose). A small amount of material remained precipitated. The suspension was stable for more than 6 months of storage under ambient conditions.

’ RESULTS AND DISCUSSION The synthetic strategy for preparation of the polymer-grafted cellulose microcrystals is shown in Scheme 1. It involves (a) the preparation of a cellulose microcrystal suspension, (b) the oxidation of the cellulose to produce carboxylic acid groups at the microcrystal surface, (c) the polymerization of amine-functionalized polymers, and (d) the attachment of the polymers to the cellulose surface via carbodiimide-mediated amidation. The amine-functionalized polymers were synthesized via atom transfer radical polymerization (ATRP) using initiator 1, which carries latent amine functionality in the form of a phthalimidyl group. This group is quantitatively converted to a primary amine by reaction with hydrazine. Formation and Oxidation of Cellulose Microcrystal Suspensions. Cellulose microcrystal suspensions were produced

according to the method of Dong et al.41 by hydrolysis of Whatman CF11 powdered cellulose (derived from cotton) in warm 64% H2SO4. During this process, the amorphous regions of the cellulose microfibrils were converted to soluble monomers and oligomers, leaving a suspension of crystalline material. The suspended cellulose consisted of rigid rod-shaped particles, which were observed by TEM (Figure 1A) and AFM (Figure 2A). By removing the flexible amorphous matter, we produced individual crystalline rods. Suspensions of the microcrystals exhibited flow

Figure 3. Typical potentiometric titration curve for a suspension of oxidized cellulose microcrystals (gray line) and its first derivative (black line). The weak (cellulose-COOH) and the strong (HCl) acid end points are indicated with arrows. The first derivative curve is a threepoint smoothed curve of point estimates of first derivative.

birefringence when shaken between crossed polarizers, providing further confirmation of the rod morphology. The suspensions were subsequently oxidized, to convert some of the primary alcohols attached to the C6 carbon of the anhydroglucose units into carboxylic acids.24 Oxidation proceeded without loss of the rod-shaped morphology or significant changes in the nanoparticle dimensions, as measured by TEM (Figure 1B, Table 1) and AFM (Figure 2B). The diffuse reflectance IR spectrum of the lyophilized particles showed a strong absorption at 1732 cm-1 due to the CdO stretch. The concentration of carboxylic acid groups could be determined by 1218


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Figure 4. (A) Conversion versus time and (B) molecular weight/PDI versus conversion plots for the atom transfer radical polymerization of styrene initiated by 1. Straight line in (A) shows linear fit to -ln(1-conversion) data, demonstrating first-order kinetics after a short induction period. Straight line in (B) represents the theoretical molecular weight (= MW(1) þ MW(STY) [M]/[I] conversion). The line connecting the polydispersity data is a guide for the eye only. Data points represented by closed circles relate to the y axis on the left, where open circles correspond to the y axis on the right.

Figure 5. 1H NMR spectra of R-phthalimido (bottom) and R-amino (top) (A,C) poly(styrene) and (B,D) poly(t-butyl acrylate).

potentiometric titration of the microcrystals (Figure 3) and was found to be 0.76 mmol g-1 of oxidized cellulose. Formation and Deprotection of Amine-Terminated Polymers. Amine-terminated polymers were formed in a two-step process from a phthalimidyl-terminated initiator, N-(2-(2-bromoisobutyryl)ethoxy)ethyl phthalimide (1). The use of the phthalimide protecting group as a latent amine functionality in ATRP has previously been suggested by Haddleton and coworkers42

using N-(2-bromoisobutyryl)ethyl phthalimide initiator. The initiator used in this work, 1, was designed with a longer spacer between the phthalimide and ester moieties to prevent the irreversible isomerization of aminoethyl ester to hydroxyethyl amide upon deprotection.43 The utility of 1 as an initiator for ATRP was demonstrated here in the polymerization of styrene. The rate of polymerization was first-order in monomer concentration after a short induction period, showing that insignificant 1219


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Figure 6. Molecular weight distributions of (A) poly(styrene) and (B) poly(t-butyl acrylate), before (- 3 - -) and after (—) hydrazinolysis.

levels of termination took place. Low polydispersity polymer was produced, with a molecular weight proportional to conversion and close to the theoretical value (Figure 4). 1 H NMR spectroscopy confirms the deprotection of the amine after polymerization of styrene and t-butyl acrylate. Resonances due to the phthalimide and the aliphatic protons of the initiatorderived end group are clearly visible in the 1H NMR spectra of the polymers (Figure 5A,B). The R-phthalimido polymers were converted to their R-amino analogs by heating with excess hydrazine for 2 h. The reaction was accompanied by almost complete (93-100%) disappearance of the aromatic resonances of the phthalimidyl moiety in the 1H NMR spectrum and the appearance of a new resonance (at 2.9 ppm for poly(styrene) or 3.0 ppm for poly(t-butyl acrylate)), assigned to the methylene protons R to the amine group (Figure 5C,D). The resonance position of the methylene protons R to the ester was essentially unaffected by deprotection, indicating that this functionality survived deprotection without isomerization to the hydroxyalkyl amide. No other changes to the polymer spectra were observed, indicating that the hydrazinolysis was specific to the phthalimide group. In particular, the relative integration ratios of the initiator to chain resonances were preserved during the deprotection, suggesting that hydrazinolysis of the ester linkage in the initiator did not occur to a significant extent. The molecular weight distribution of the poly(styrene), as observed by size exclusion chromatography, exhibited a highmolecular-weight shoulder after deprotection, indicative of some chain-chain coupling during deprotection (Figure 6A). The mechanism of the coupling reaction is not well-understood. However, as shown in Figure 5, the amine-carrying end group is not affected, suggesting that the Br terminus is involved. Possible mechanisms for coupling might involve radical formation by loss of bromine, followed by radical-radical coupling or reaction of the bromine terminus with hydrazine to form a substituted hydrazine. The molecular weight distribution of the poly(t-butyl acrylate) was essentially unaffected by the deprotection process (Figure 6B). Grafting Polymers onto Cellulose. Poly(styrene) and poly(t-butyl acrylate) were grafted onto microcrystalline cellulose. DMF was used as a compatibilizing solvent to allow the hydrophobic

Figure 7. IR spectra of (A) oxidized cellulose, (B) cellulose-PS graft, and (C) cellulose-PtBA graft. Characteristic absorptions of the grafted polymers are labeled with asterisks (*): (B) aromatic C-H str., 3050 cm-1; aromatic C-C str., 1500 cm-1, out of plane C-H bend, 700 cm-1; (C) methyl CH str., 3000 cm-1, CdO str., 1730 cm-1, t-butyl umbrella bend, 800 cm-1. The absorptions marked with @ at 1650 cm-1 are due to adsorbed DMF.

polymers and hydrophilic cellulose to coexist in solution. Grafting took place via a carbodiimide-mediated amidation reaction between the carboxylic acid moieties of the cellulose and the terminal amines of the polymer chains. Unreacted polymer chains were removed by repeated sonication of the grafted cellulose in acetone, followed by separation of the cellulose by centrifugation. Under normal conditions of storage, the suspensions were stable for more than 6 months; however, the cellulose could be precipitated by centrifugation due to the difference in the specific gravity between the acetone and the microcrystals. Both the poly(styrene)-cellulose and poly(t-butyl acrylate)cellulose grafts formed stable suspensions in the nonpolar solvents tested: chloroform, acetone, and toluene. Neither polymer-cellulose composite could be dispersed in water. Diffuse reflec1220


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Figure 8. Thermogravimetric analysis traces of (A) cellulose-graft-poly(styrene) (- - -), oxidized cellulose (gray - - -, normalized to 36% of graft initial mass), poly(styrene) (—, normalized to 56% of graft initial mass) and sum of oxidized cellulose and poly(styrene) traces (gray —); and (B) cellulosegraft-poly(t-butyl acrylate) (- - -), oxidized cellulose (gray - - -), normalized to 34% of graft initial mass), poly(t-butyl acrylate) (—, normalized to 60% of graft initial mass) and sum of oxidized cellulose and poly(t-butyl acrylate) traces (gray —).

Figure 9. (A,B) Atomic force micrographs and (C,D) sections of (A,C) PS-grafted and (B,D) PtBA-grafted cellulose microcrystals. White lines on A and B indicate the positions of C and D. The vertical distances between the arrowheads in C and D are 27 and 40 nm, respectively.

tance IR analysis of the polymer-cellulose grafts showed absorbances characteristic of both cellulose and the grafted polymer (Figure 7). Thermogravimetric analysis of the polymer-cellulose grafts (Figure 8) provided a means of estimating the extent to which grafting had taken place. In the case of poly(styrene)-grafted cellulose, the cellulose and poly(styrene) decompositions were clearly separated. Superposition of a poly(styrene) decomposition curve onto the composite decomposition curve indicated that the graft comprised 56% by mass poly(styrene) and 38% by mass cellulose. This corresponds to an attachment of poly(styrene) to ca. 20% of the cellulose carboxylic acid groups, calculated from n(polymer)/n(COOH) = m(polymer)/m(cellulose) 1/[n(COOH)/g cellulose] 1/[Mn (polymer)], where n is the quantity (mol) and m is the mass (g). A further 6% mass loss occurred in the range 120-200 C due to desorption of DMF, which binds strongly to cellulose. Poly(t-butyl acrylate) shows a very sharp mass loss at 240 C due to conversion of the t-butyl ester to a carboxylic acid, with accompanying loss of isobutene: this transition results in the loss of 43.7% of the mass of poly(t-butyl acrylate). A corresponding mass loss was observed in the cellulose-t-butyl acrylate graft and was used to estimate the t-butyl acrylate content of the graft at 60 wt %, with 6 wt % DMF and 34 wt % cellulose. This corresponds to the attachment of poly(t-butyl acrylate) to ca. 22% of the cellulose carboxylic acid groups. For comparison, Araki, Wada, and Kuga24 reported grafting densities of 20-30%

(depending on the method of analysis) in the production of PEG-grafted cellulose by amidation in aqueous suspension. For both cellulose-polymer grafts, the maximum cellulose decomposition rate was observed at 350 C, compared with 325 C for oxidized cellulose or a blend of oxidized cellulose and poly(styrene). This may be a result of the covalent grafting reaction and the intimate contact between the cellulose surface and the grafted polymer. The cellulose-polymer grafts were also characterized by AFM (Figure 9) and TEM (Figure 10). These techniques revealed substantial increases in the diameters and lengths of the cellulose microcrystals on grafting, with retention of their rod-shaped morphologies. Some grafting techniques lead to decreases in the particle length, even at low grafting densities.44 Because the length/ width aspect is highly related to the performance of the particles as reinforcers, this is a desirable grafting technique because it does not compromise the morphology of the cellulose microcrystal. In the case of poly(styrene)-grafted cellulose microcrystals, attachment of poly(styrene) resulted in an increase in average diameter from 23 ( 7 to 44 ( 12 nm (a difference of three standard deviations relative to the substrate), as measured by TEM (Table 1). The diameter distribution of the poly(styrene)-grafted microcrystals was clearly shifted toward larger diameters relative to the substrate (Figure 10). AFM measurements were in broad agreement with this value, with typical particle heights of ∼30 nm (Figure 9C), compared with ca. 20 nm for oxidized cellulose (Figure 2). For 1221


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Figure 10. TEM images and histograms (filled bars) of (A) PS-grafted and (B) PtBA-grafted cellulose. The unfilled bars in histograms represent the substrate oxidized cellulose size distributions.

comparison, the contour length of a polystyrene molecule with MW 9600 is ca. 20 nm. The length distribution of the poly(styrene)-grafted microcrystals is similar to that of the substrate, with an increase in average length from 250 ( 91 to 300 ( 99 nm (a difference of 0.55 standard deviations). Whereas the absolute increase in average length is larger than the increase in diameter, it is associated with a high degree of uncertainty due to the inherent variability in the length of the cellulose microcrystals. Therefore, a large proportion of the observed difference in length may be due to sampling error. Poly(t-butyl acrylate) grafted microcrystals displayed a larger increase in average diameter (18 ( 6 to 79 ( 17 nm) and average length (310 ( 120 to 440 ( 136 nm) than poly(styrene) grafted microcrystals, as observed by TEM. Because both poly(styrene)grafted and poly(t-butyl acrylate)-grafted microcrystals contained similar amounts of polymer (∼60% in each case) of similar molecular weight (Mn ≈ 10 000 g mol-1) and contour length

(∼20 nm), the larger increase in size observed for poly(t-butyl acrylate)-grafted particles cannot solely be attributed to differences in intramolecular interactions. The diameter distribution of PtBA-grafted cellulose (Figure 10) is distinctly multimodal, with peaks at 55-60, 70-75, 85-90, and 110-115 nm, separated by approximately the diameter of one cellulose microcrystal (18 nm). This unusual distribution suggests that the many of the particles observed by TEM comprise bundles of several microcrystals. Such aggregations of a few microcrystals would also be longer than a single microcrystal, explaining the increase in observed average length of the poly(t-butyl acrylate)-grafted microcrystals. Whereas bundle-like aggregates of PS-grafted cellulose are also observed, they appear to be much more prevalent in PtBA-grafted cellulose, to the point where a typical particle contains several cellulose microcrystals. Typical microcrystal heights of ∼40 nm were obtained by AFM (Figure 9D), similar to the heights of PS-grafted microcrystals. 1222


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’ CONCLUSIONS We have demonstrated a method for the production of polymer-grafted cellulose microcrystals, making use of amineterminated polymers produced via ATRP. The grafting-to approach used here allows for well-defined polymers of a variety of molecular weights and functionalities to be covalently attached to the cellulose surface, without degradation of the support. The success of grafting the two polymer examples on to cellulose microcrystals suggests that this approach could be extended to many other amine-functionalized polymers. The resulting rigid, rod-shaped nanoparticles are readily dispersible in a range of organic solvents, including acetone, chloroform, and toluene. Therefore, the grafting density of 60-64% (polymer/composite) is sufficient to overcome the dispersibility challenges of unmodified cellulose. ATRP can be used to produce a wide range of polymers with different functionalities and architectures, which should allow substantial modification of the surface properties of the cellulosebased nanoparticles while conserving the underlying rod-shaped morphology. The particles that result may be useful as the reinforcing phase in nanocomposites and would be expected to exhibit greater compatibility with hydrophobic matrices than do unmodified cellulose microcrystals. The particles are also of interest as precursors to multifunctional 1D nanoparticles. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: (S.H.) [email protected]. (K.L.W.) E-mail: [email protected]. Present Addresses §

)

Faculte de Pharmacie, Universite Paris-Sud XI, 5 rue JeanBaptiste Clement, 92296 Châtenay-Malabry, France. Department of Chemistry, Texas A&M University, P.O. Box 30012, College Station, TX 77842, United States. ^ Laboratoire de Chimie de la Matiere Condensee de Paris, College de France, 11 place Marcelin Berthelot, 75005 Paris, France.

’ ACKNOWLEDGMENT The material presented in this publication is based on work supported by the National Science Foundation under grant no. 0301833. We are grateful to Mr. G. Michael Veith for assistance with transmission electron microscopy.

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