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Langmuir 2006, 22, 4804-4810
Starch Nanocrystals with Large Chain Surface Modifications Wim Thielemans, Mohamed Naceur Belgacem, and Alain Dufresne* Ecole Franc¸ aise de Papeterie et des Industries Graphiques, Institut National Polytechnique de Grenoble (EFPG-INPG) BP65, 38402 Saint-Martin d’He` res Ce´ dex, France ReceiVed December 15, 2005. In Final Form: March 16, 2006 Nanoscale monocrystalline starch particles were successfully modified using stearic acid chloride and poly(ethylene glycol) methyl ether. Surface modification was confirmed using FTIR, XPS spectroscopy, and contact angle measurements. X-ray diffraction and DSC analysis confirmed that there was no alteration of the starch crystalline structure due to the surface modification. The grafts at the starch surface were also found to crystallize on the surface. TEM showed the individualization of nanoparticles as a result of the reduction of polar and hydrogen bonding forces. These results show our ability to modify the starch nanocrystal surface with plasticizing chains. Modified nanoparticles can find applications as compatibilized polymer additives, surface-active particles, and co-continuous nanocomposite precursors.
Introduction Bionanocomposites, or econanocomposites, are novel materials born out of the growing interest in nanomaterials and in the development of materials derived from renewable sources.1 The renewable component can be the matrix phase, the nanoparticles, or both. Reported examples of renewable polymer nanocomposites can be found in the use of carbon nanotubes in polymers from renewable materials.2,3 Polysaccharides such as cellulose, starch, and chitin are a potential renewable source of nanosized reinforcements. They are naturally found in a semicrystalline state, and aqueous acids can be employed to hydrolyze the amorphous sections of the polymer. Consequentially, the crystalline sections found in these naturally occurring polysaccharides are released, resulting in individual monocrystalline nanoparticles.4 The obtained particles display different shapes depending on the polysaccharide source: rigid rodlike particles for cellulose and chitin and platelets for starch. Aspect ratios of the rigid rod cellulose and chitin particles are found to vary with the source material: values are reported between 10 and 65 for cellulose and up to 120 for chitin.4-6 The use of starch nanoparticles is receiving a significant amount of attention because of the abundant availability of starch, low cost, renewability, biocompatibility, biodegradability, and nontoxicity.7 The latter properties make them excellent candidates for implant materials and drug carriers. Starch nanocrystals have also been found to be excellent reinforcements.8 The work presented here involves the chemical modification of starch-derived nanocrystals with poly(ethylene glycol) methyl ether (PEGME) and stearic acid chloride. Longer-chain surface modifications can yield some extraordinary possibilities. The * Corresponding author. E-mail:
[email protected]. Phone: +33 476 82 69 95. Fax: +33 476 82 69 33. (1) Dorgan, J. R.; Braun, B. PMSE Prepr. 2005, 93, 954-955. (2) Moon, S.-I.; Jin, F.; Lee, C.; Tsutsumi, S.; Hyon, S.-H. Macromol. Symp. 2005, 224, 287-295. (3) Thielemans, W.; McAninch, I. M.; Barron, V.; Blau, W. J.; Wool, R. P. J. Appl. Polym. Sci. 2005, 98, 1325-1338. (4) Samir, M. A. S. A.; Alloin, F.; Dufresne, A. Biomacromolecules 2005, 6, 612-626. (5) Garcia de Rodriguez, N. L.; Thielemans, W.; Dufresne, A. Cellulose, in press. (6) Morin, A.; Dufresne, A. Macromolecules 2002, 35, 2190-2199. (7) Chakraborty, S.; Sahoo, B.; Teraoka, I.; Miller, L. M.; Gross, R. A. Macromolecules 2005, 38, 61-68. (8) Angellier, H.; Molina-Boisseau, S.; Dufresne, A. Macromolecules 2005, 38, 9161-9170.
surface modifications can act as binding sites for active agents in drug delivery systems or for toxins in purifying and treatment systems. These surface modifications may also be able to interdiffuse, upon heating, to form the polymer matrix phase. The covalent linkage between the reinforcement and matrix will result in near-perfect stress transfer at the interface with exceptional mechanical properties of the composite as a result. The large surface area, inherent to the small size of nanoparticles, guarantees a large surface activity and a high grafting per unit mass of particles. Their small size also reduces the required chain length for interdiffusion to ensure sufficient matrix-phase cohesion. The described surface modifications have a layer thickness on the order of magnitude of the thickness of starch nanoparticles. PEGME was chosen for the excellent compatibility of starch with poly(ethylene glycol)9 and its applicability to drug and other medical products. The stearate modification was chosen for its ease of crystallization and its efficiency as a recipient for organic hydrophobic agents.10 Experimental Section Materials. Amylopectin-rich waxy maize starch (trade name Waxylis 200) was obtained from Roquette S. A. (Lestrem, France) and used as received. Sulfuric acid (g95%), stearoyl chloride (99%), toluene 2,4-diisocyanate (2,4-TDI, 95%), poly(ethylene glycol) methyl ether (Mn ) 550 g/mol), triethylamine (99.5%), dibutyltin dilaureate (95%), methyl ethyl ketone (99+%), acetone (99%), toluene (anhydrous, 99.8%), dichloromethane (99.5%), and trimethyl orthoformate (anhydrous, 99.8%) were all obtained from SigmaAldrich. All solvents were dried over molecular sieves (3 Å, 4-8 mesh beads, Sigma-Aldrich) for 48 h. Poly(ethylene glycol) methyl ether (PEGME) was dried using trimethyl orthoformate (TMOF): a mixture of 10 wt % TMOF in PEGME was kept at 40 °C for 3 h to allow TMOF to react with sample moisture to form volatile alcohols. The formed alcohols and remaining unreacted trimethyl orthoformate were removed by evaporation under vacuum at 60 °C for 4 h. Electron Microscopy. Transmission electron micrographs of starch nanocrystals were taken with a Philips CM200 transmission electron microscope at an acceleration voltage of 80 kV. Deposited starch nanocrystals were negatively stained with an aqueous 2% solution of uranyl acetate. Scanning electron micrographs were obtained with a Quanta 200 environmental scanning electron microscope under vacuum at an (9) Samir, M. A. S. A.; Alloin, F.; Sanchez, J.-Y.; Dufresne, A. Polymer 2004, 45, 4149-4157. (10) Boufi, S.; Belgacem, M. N. Cellulose, in press.
10.1021/la053394m CCC: $33.50 © 2006 American Chemical Society Published on Web 04/08/2006
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Figure 1. Transmission electron micrographs of (a) unmodified starch nanocrystals and starch nanocrystals surface-modified with (b) stearates and (c) poly(ethylene glycol) methyl ether. Table 1. Reaction Time, Temperature, and Reactant Amounts for Starch Nanocrystal Modification Reactions starch nanocrystals solvent catalyst grafting agent
reaction temperature reaction time
stearate modification
PEG modification
2.0 g 40 mL of methyl ethyl ketone 7 mL of triethylamine 13 mL of stearic acid chloride
2.0 g 80 mL of toluene 1 mL of dibutyl tin dilaureate 11.8 mL of 2,4-TDI 39.2 g of PEGME 70 °C 7 days
60 °C 4 days
operating voltage of 10 kV. Dried starch nanoparticles were gold coated by sputtering for 15 s. Starch Nanocrystals. Starch nanocrystals were obtained by acid hydrolysis of waxy maize starch for 5 days at 40 °C in a 3.16 M aqueous H2SO4 solution while stirring constantly, as described in detail elsewhere.11 Complete hydrolysis was confirmed by transmission electron microscopy as shown in Figure 1a and by agreement of the final product yield with earlier results.11 A homogeneous dispersion of starch nanocrystals was obtained by using an Ultra Turrax T25 homogenizer for 3 min at 13 500 rpm. This dispersion was frozen immediately by immersing the container in liquid nitrogen. Subsequent lyophilization of the frozen dispersion resulted in dried nanocrystals. Surface Modification Reactions. Chemical modification of the nanoparticles was performed in a round-bottomed reaction flask under a nitrogen atmosphere while constantly stirring with a magnetic stir bar. Reactant amounts and reaction time and temperature are recorded in Table 1. The reaction schemes are shown in Scheme 1. The stearate modification was performed by the reaction of dry starch nanocrystals with stearic acid chloride in methyl ethyl ketone (11) Angellier, H.; Choisnard, L.; Molina-Boisseau, S.; Ozil, P.; Dufresne, A. Biomacromolecules 2004, 5, 1545-1551.
Scheme 1. Reaction Scheme for the Grafting of (a) Stearates and (b) PEGME to the Starch Nanocrystal Surface
(MEK). All reactants and solvent (Table 1) are combined and left to react for 4 days. Triethylamine (TEA) is added to the reaction mixture to catalyze the reaction and to act as a complexing agent for HCl formed during the reaction. The TEA/HCl complex is insoluble in MEK and sediments from the reaction solution. After the completion of the reaction, the reaction mixture is filtered over an extraction thimble to separate the modified starch and the TEA/ HCl complex from the liquid reaction mixture. Distilled water (500 mL) was subsequently passed through the extraction thimble to dissolve and remove the TEA/HCl complex. A subsequent wash with a 1% urea-in-water solution was performed to ensure the removal of potential physically adsorbed stearic acid, after which a dimethyl formamide wash removed any retained urea. The remaining modified starch nanocrystals were Soxhlet extracted first with acetone and then with dichloromethane. Each extraction lasted for 24 h.
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Table 2. Surface Tension Contributions for Liquids Used for Contact Angle Measurements
D
2
γ (mJ/m ) γP (mJ/m2) γtotal (mJ/m2)
water29
N,N-dimethyl formamide30
diiodomethane29
21.8 51.0 72.8
39.0 19.0 58.0
49.5 1.3 50.8
Poly(ethylene glycol) methyl ether (PEGME, Mw ) 550 g/mol) modification involves a two-step process. The first step requires the reaction of PEGME with one isocyanate functionality of 2,4-toluene diisocyanate (2,4-TDI). During the second step, the unreacted second isocyanate of 2,4-TDI is then reacted with the surface hydroxyl groups of the starch nanoparticles to graft the PEGME chain onto the particles. It has been shown that the isocyanate at the 4 position is 7 times more reactive than that at the 2 position, such that the 4 position is presumed to react first, in agreement with the literature.12 Dry toluene (80 mL) is placed into a three-necked round-bottomed flask equipped with a reflux condenser, kept under a nitrogen atmosphere, and heated to the reaction temperature. 2,4-TDI (11.8 mL) is added while stirring vigorously. Catalyst dibutyl tin dilaureate (1 mL) is subsequently added. The reaction is started by the dropwise addition of a stoichiometric amount of PEGME (39.2 g) in order to react a single isocyanate of 2,4-TDI with the remaining PEGME hydroxyl group. The reaction is followed by FTIR spectroscopy, confirming the formation of a urethane linkage and the conservation of an equal number of isocyanates (FTIR spectra not shown). This reaction is complete after 24 h. Dry starch nanocrystals are placed in a separate three-necked round-bottomed flask equipped with a reflux condenser and heated to the reaction temperature under a nitrogen atmosphere. Addition of the previous reaction mixture (PEGME-TDI adduct, toluene, and catalyst) starts the grafting reaction. After 7 days, the mixture is filtered over an extraction thimble to strain the modified starch crystals, after which they are washed with 250 mL of dimethyl formamide to remove any formed urea. Urea may have been formed in small quantities because of residual moisture in the reaction mixture. Urea formation is kinetically favored over urethane formation and more so in our case because it takes place in the homogeneous reaction mixture, whereas urethane formation occurs at the solid-liquid interface. However, after 7 days of reaction, a large number of isocyanates were unreacted, and no measurable amount of urea was detected by FTIR spectroscopy. Therefore, urea could have been formed in only extremely small quantities. The dimethyl formamide wash was performed nonetheless to ensure the accuracy of the obtained surface characterization. The washed nanocrystals are subsequently Soxhlet extracted with dichloromethane for 24 h before drying at 40 °C under vacuum. Modification Characterization. Brunauer-Emmet-Teller (BET) adsorption isotherms were measured to determine the specific surface area of lyophilised starch nanocrystals on a Micromeritics surface and porosity analyzer (ASAP2020, Norcross, GA) using nitrogen as the adsorption gas. FTIR spectrograms were obtained by the inclusion of dried powder in KBr pellets in a ratio of 99:1 KBr/starch. Sixty-four scans were taken between 4400 and 450 cm-1 in 2 cm-1 intervals on a PerkinElmer Paragon 1000 FTIR spectrometer. Spectra were analyzed using SpectrumNT. Contact angle measurements were performed at room temperature using a dynamic drop tensiometer (ITConcept, Longessaine, France). The contact angle and drop volume were monitored as a function of time using WINDROP software. Three different liquids, with differing dispersive and polar surface tensions, were used to determine the surface energy of the starch nanocrystals (Table 2). The drop volume was between 5 and 10 µL, and smooth surface starch samples were obtained by compacting the powder under a pressure of 10 metric tons using a KBr press. Contact angle measurements were carried out on starch nanocrystal samples before and after surface modification. (12) Belgacem, M. N.; Quillerou, J.; Gandini, A. Eur. Polym. J. 1993, 29, 1217-1224.
The Owens-Wendt approach was used to relate the dispersive and polar contributions to the surface energy of the starch samples to the dispersive and polar contributions of the surface tension of the liquids used and to their equilibrium contact angle with the starch surface (where the work of adhesion is replaced by the Young equation)13 γL(1 + cos θ) ) 2xγDL γDS + 2xγPLγPS
(1)
with γ, γD, and γP being the total, dispersive, and polar surface energy, respectively. Subscripts L and S refer to the liquid drop (L) and the solid surface (S), and θ denotes the contact angle between the solid substrate and the liquid drop. The liquid surface tensions were taken from the literature and are combined in Table 2. X-ray photoelectron spectroscopy (XPS) experiments were carried out using an XR3E2 apparatus (Vacuum Generators, U.K.) equipped with an unmonochromated Mg KR X-ray source (1253.6 eV) and operated at 15 kV under a current of 20 mA. Samples were placed in an ultrahigh vacuum chamber (10-8 mbar) with electron collection by a hemispherical analyzer at a 90° angle. Signal decomposition was done using Spectrum NT, and the overall spectrum was shifted to ensure that the C-C/C-H contribution to the C 1s signal occurred at 285.0 kV. Comparison of the elementary surface composition was performed using I1/s1 I2/s2
(2)
where Ii is the intensity of signal i (carbon, oxygen or nitrogen) and si (sC ) 0.00170, sO ) 0.00477, sN ) 0.00299) denotes the atomic sensitivity factor whose values were calculated from si )
Tiλiσi 4π
(3)
with Ti, λi, and σi denoting the transmission energy, the electron inelastic mean free path, and the photoionization cross section for the X-ray source, respectively. Ti depends on the atomic kinetic 0.7 with Ekin ) 966.6 eV, energy Ekin (eV) following Ti ) 1/(Ekin i i ) C kin kin EO ) 722.6 eV, and EN ) 851.6 eV. The Penn algorithm14 was used to calculate the electron inelastic mean free path λi (λC ) 2.63 nm, λO ) 2.11, λN ) 2.39 nm), and σi values were taken from Scofield15 (σC ) 1, σO ) 2.85, σN ) 1.77). X-ray diffraction data were recorded for dry starch powder at ambient temperature with a scattering angle step size of 0.02° between 4 and 30° on a Siemens D500 diffractometer equipped with a Cu KR anode with λ ) 1.5406 Å. Differential scanning calorimetry (DSC) was performed on a DSC Q100 (TA Instruments, New Castle, DE) fitted with a manual liquidnitrogen cooling system. Starch powder, conditioned at 0% relative humidity, was placed in a hermetically closed DSC crimp sealed pan. Samples were tested in the range of -100 to 350 °C at a heating rate of 10 °C/min under a nitrogen atmosphere. The sample weight was between 2 and 5 mg. FTIR and XPS spectroscopy, contact angle measurements, and X-ray diffraction were performed on the dried powder before and after surface modification. TEM was performed on modified starch nanocrystals deposited on a carbon-coated grid from a dispersion using a 2% uranyl acetate solution as a negative staining liquid.
Results and Discussion Dried Starch Nanocrystals. BET adsorption isotherms were used to determine the structure of the lyophilised starch powder. The specific surface area was determined to be 7.4 m2/g. This (13) Adamson, A. W. Physical Chemistry of Surfaces; John Wiley: New York, 1990. (14) Tanuma, S.; Powell, C. J.; Penn, D. R. Surf. Sci. 1987, 192, L849-L857. (15) Scofield, J. H. J. Electron. Spectrosc. Relat. Phenom. 1976, 8, 129-137.
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Figure 2. Scanning electron micrographs of lyophilized starch nanoparticles.
value is roughly twice an earlier reported value16 of 3.2 m2/g and points toward loose aggregates of nanocrystals in the dried state. Storing the dried nanoparticles at a relative humidity higher than 0% was found to have a significant effect on the measured specific surface area because of a compacting effect on the aggregates. This may explain significant differences in the reported values for the specific surface area. SEM micrographs (Figure 2) of the lyophilized starch nanocrystals also show the existence of aggregates with a large specific area due to a porous structure and a fractal-like base structure. The existence of nanocrystal aggregates, however, did not appear to influence the surface reactivity of the dry powder when redispersed in toluene. This was apparent from similar substitution results obtained by using high-surface-area lyophilized starch nanocrystals and highly aggregated nanocrystals dried under vacuum while heating. FTIR Spectroscopy. FTIR spectroscopy of the dry modified particles by inclusion in KBr pellets shows clear traces of modification (Figure 3). Modification with stearic acid chloride results in the appearance of signals at 2954, 2918, and 2850 cm-1 due to the grafted alkane chain.17 Ester signals appear at 1706 and 1752 cm-1, with the 2954 and 1706 cm-1 signal due to hydrogen bond formation of esters adjacent to a hydroxyl group. While the CdO acid stretch for stearic acid also appears around 1705 cm-1, a small number of modified nanocrystals were washed several times with aqueous NaOH and urea to remove physisorbed stearates. The 1706 cm-1 signal is correctly attributed to stearate esters because the signal intensity did not decrease after these washes. It must also be noted that no acid signal appears around 3000 cm-1, further confirming the absence of physisorbed stearic (16) Angellier, H.; Molina-Boisseau, S.; Belgacem, M. N.; Dufresne, A. Langmuir 2005, 21, 2425-2433. (17) http://www.aist.go.jp/RIODB/SDBS, 2005.
Figure 3. FTIR spectra of starch nanoplatelets (a) unmodified and surface modified with (b) PEGME-TDI and (c) stearate.
acid. The FTIR spectrum of PEGME-TDI modified starch is less conspicuous because there is no clear additional signal. PEGME has similar backbone features to starch. Comparison with the unmodified starch spectrum does reveal small, yet important changes: a decrease in the hydroxyl signal at 3370 cm-1, a relative increase in the 2880 and 2923 cm-1 signals pointing toward an increase in CH2 units adjacent to an ether group, a signal increase around 1100 cm-1 pointing toward an increase in C-O units, and a small shoulder appearing at 1721 cm-1 suggesting the formation of urethane bonds. Considering the small size of the starch nanoparticles and the high absorbance of urethane linkages, it appears that the surface coverage is limited and involves only a fraction of surface hydroxyl groups. XPS Spectroscopy. X-ray photoelectron spectroscopy provides a more quantitative idea of the level of surface modification. A
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Table 3. Elemental Surface Composition (%) and Composition Ratios of Unmodified and Modified Starch Nanocrystals as Determined from XPSa unmodified stearate PEG
O
N
C
O/C
N/C
35.8 5.0 33.8
0 0 4.0
64.2 95.0 62.2
0.56 (0.83) 0.05 (0.0)b 0.54 (0.5)
0 0 0.06
a Numbers in parentheses denote theoretical values for the pure surface compound with only the top layer measured. b Considering only the aliphatic chain end.
Figure 4. General XPS spectra of (a) unmodified and (b) stearateand (c) PEGME-modified starch nanocrystal surfaces with signal assignments. Table 4. Surface Functional Group Composition as Obtained from the Decomposition of the C 1s Signal C1 C2 C-C/C-H C-N BE (eV)a unmodified stearate PEG
285 25.0 92.0 24.9
286.2 0.0 0.0 4.8
C3 C-O
C4 O-C-O/CdO
C5 O-CdO
286.5 ( 0.1 58.2 6.4 55.8
288.05 ( 0.05 14.9 0.96 10.8
289.0 ( 0.1 1.9 0.66 3.7
a
The binding energy for each signal is given with variation seen between different samples.
determination of the elemental surface composition (Table 3 and Figure 4) of unmodified starch nanocrystals reveals a higherthan-expected carbon content. The theoretically calculated value for the oxygen/carbon (O/C) ratio from the starch chemical structure is significantly larger than the experimental ratio. This lower value is in agreement with earlier results where the high carbon content was attributed to hydrocarbon impurities.16,18 No significant surface nitrogen was found, unlike some previous studies that found small amounts due to residual proteins.16,19 Decomposition of the C 1s signal of unmodified starch (Table 4 and Figure 5a) resulted in carbon contributions in line with other reported values.16,19 Stearate modification was quite effective, increasing the surface carbon content to 92%. Decomposition of the C 1s signal (Table 4 and Figure 5b) confirms the replacement of the oxygen-rich starch surface by an aliphatic surface. The C 1s signal is almost completely composed of the C-C/C-H contribution (C1). Oxygen-carbon linkages (contributions C2 to C5) are found in only small quantities. Compared to the unmodified starch decomposition results, the ester carbon contribution (contribution C5) increases relative to the ether and alcohol carbon contributions (C4 and C3, respectively). This provides additional proof of successful grafting of stearate to starch. (18) Varma, A. J. Carbohydr. Polym. 1984, 4, 473-480. (19) Rindlav,-Westling, A.; Gatenholm, P. Biomacromolecules 2003, 4, 166172.
Figure 5. Decomposition of the C 1s signal into its constituent contributions for (a) unmodified and (b) stearate- and (c) PEGTDI-modified starch nanocrystals.
The starch surface appears to be completely covered and shielded by stearate groups. The high carbon content is also an indication of the outward orientation of the surface-grafted stearates, giving rise to brushlike surface coverage. This has been confirmed by X-ray diffraction, as described later. Surface grafting of PEGME using 2,4-TDI is confirmed by the appearance of a significant nitrogen signal (4% of the elemental surface composition) in the overall XPS spectrum, pointing to around 80% surface coverage by the PEGME-TDI adduct. Additional proof is found in the decomposition of the C 1s signal, confirming the existence of carbon-nitrogen bonds and thus 2,4-TDI at the surface. Direct determination of PEGME on the surface is less evident because its backbone structure contains chemical bonds that are similar to those of starch. However, a significant reduction in the C4 contribution, found only in starch and not PEGME, provides confirmation. Additionally, the confirmed PEGME-TDI reaction before grafting implies that
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Figure 7. X-ray diffraction patterns of (a) unmodified (b) PEGMETDI- and (c) stearate-modified starch nanoparticles. Figure 6. Water contact angle with time for (a) unmodified and (b) stearate- and (c) PEGME-TDI-modified starch nanocrystals. Table 5. Calculated Surface-Energy Contributions and Contact Angles for the Tested Liquids on Unmodified and Modified Starch Nanocrystals
water formamide diiodomethane surface energy (mJ/m2) dispersive (γd) polar (γP)
unmodified starch
starch stearate
starch PEGME-TDI
35 20.5 52
102 85 61
66 39 38
31.3 29.3
32.9 0.0
38.0 9.6
the addition of nitrogen to the surface includes the grafting of surface PEGME. Trace amounts of tin are also found and are due to residual catalyst that was not completely removed during Soxhlet extraction. Contact Angle Measurements. Table 5 combines the calculated contributions and the contact angles for the tested liquids on the three solid substrates (unmodified starch nanocrystals and both modified substrates). The contact angle used was the equilibrium contact angle found at longer times (Figure 6). This guarantees the absence of drop deposit and initial spreading effects. The polar contribution to the surface energy, before modification and similar in magnitude to that of the dispersive contribution, is reduced significantly after reaction. The disappearance of polarity from the starch-stearate surface is a clear signal of efficient surface coverage by the stearate aliphatic chain ends. In addition, the dispersive surface energy of the starch-stearate surface is within the range found for low-molecular-weight polyethylene, which is further proof of effective surface coverage.20 The lesser reduction in γP for PEGME-grafted starch is expected because of the ether linkages present in the poly(ethylene glycol) backbone. However, its total surface tension is found to approach the upper range values reported for PEG, and its γP/γ is in agreement with poly(ethylene glycol) surface energy data.20 This indicates efficient surface coverage, even though FTIR and XPS spectroscopy pointed to limited reaction of the starch surface. PEGME chains are thus expected to be lying flat on the starch surface rather than being assembled in a brushlike structure. This flat structure provides a PEGME surface to the outside with limited surface grafting. The polymer coating of the starch nanocrystals can be attributed to the compatibility of starch and PEGME.9 X-ray Diffraction. The hydrolyzed unmodified waxy maize starch nanoparticles (Figure 7a) show the expected scattering pattern for the A allomorph:21,22,24 the 18° (equator d spacing of
0.49 nm) signature peak, always found as a doublet with a signal at 17.2° (d ) 0.52 nm) and at 23° (d ) 0.39 nm) for the A allomorph, is clearly visible. There is also no signal at 5.5° (d ) 1.16 nm), a signature signal for the B allomorph.22-24 Acid hydrolysis did not destroy or transform the native crystalline state of the nanocrystals. After modification with both stearate and PEGME-TDI, it is apparent that the crystalline nanoparticles structure is kept intact. The basic A allomorph diffraction pattern is still clearly visible in the diffraction pattern of the modified particles (Figure 7b and c). PEGME-TDI modification does not appear to have any pronounced effect on the diffraction pattern, other than the appearance of a broad halo at 8° (1.1 nm) centered around 5.68° (d ) 1.57 nm) as well as small shoulders or sharp protruding signals at 15.7° (d ) 0.57 nm), 18° (d ) 0.5 nm), and 23.3° (d ) 3.9 nm). The positions of these sharp signals are in agreement with the PEG crystallization diffraction pattern.25 However, their low intensity points to very limited surface crystallization, either owing to the relatively short chain length coupled with limited mobility due to grafting restraints, or due to insufficient surface modification, to give rise to considerable crystallization of the chains into polymer brushes. The small halo at low scattering angles can be attributed to the formation of a PEGME coating on the starch surface. It is expected that hydrogen bonding between the PEGME ether groups and unreacted starch hydroxyl groups provides ample interactions to bend the surface-grafted chains onto the surface, as mentioned earlier. Significant crystallization of the stearate surface modification is evident from its diffraction pattern (Figure 7c). The stearate diffraction signals are cleanly superimposed over the starch pattern. The stearate signals are found at 5.76° (d ) 1.53 nm), 9.6° (d ) 0.92 nm), 13.44° (d ) 0.66 nm), 21.56° (0.41 nm), and 23.64° (0.376 nm). The latter two are attributed to the respective (110) and (020) reflections in an orthorhombic subcell crystal structure for paraffins and paraffinic derivatives.26 The first three spacings are similar to values reported for the vinyl stearate crystal structure.26,27 It does not appear that the crystal (20) Wu, S. In Polymer Handbook; Brandrup, J., Immergut E. H., Eds.; Interscience Publishers: New York, 1966; pp VI411-VI434. (21) Matveev, Y. I.; van Soest, J. J. G.; Nieman, C.; Wasserman, L. A.; Protservo, V. A.; Ezernitskaja, M.; Yuryev, V. P. Carbohydr. Polym. 2001, 44, 151-160. (22) Katopo, H.; Song, Y.; Jne, J.-L. Carbohydr. Polym. 2002, 47, 233-244. (23) Bule´on, A.; Ge´rard, C.; Riekel, C.; Vuong, R.; Chanzy, H. Macromolecules 1998, 31, 6605-6610. (24) van Soest, J. J. G.; Hulleman, S. H. D.; de Wit, D.; Vliegenthart, J. F. G. Ind. Crops Prod. 1996, 5, 11-22. (25) Fuller, C. S.; MacRae, R. J.; Walther, M.; Cameron, R. E. Polymer 2001, 42, 9583-9592. (26) Morosoff, N.; Morawetz, H.; Post, B. J. Am. Chem. Soc. 1965, 87, 30353040.
4810 Langmuir, Vol. 22, No. 10, 2006
Figure 8. DSC traces of (a) unmodified and (b) PEGME- and (c) stearic acid chloride-modified starch molecules. Positive values denote exothermicity. Traces have been shifted vertically.
structure has been affected by the constraints imposed by surface grafting as has been found for poly(vinyl stearate).26 From these results, it is evident that the covalently linked stearates crystallize at the starch nanocrystal surface in a way that is similar to that of the aliphatic ends of other stearates. Combined with the high level of surface grafting determined before, the stearates are believed to take on a crystalline brushlike structure from the starch surface outward. Dynamic Scanning Calorimetry. The large stearate content was also confirmed by DSC analysis, where a distinct melting region appears between 35 and 110 °C with a maximum at 86 °C attributed to the stearate surface phase and an integrated value of 143.9 J/g between 46 and 106 °C (Figure 8). Smaller melting regions at higher temperature are attributed to starch melting (between 112 and 240 °C) with starch decomposition occurring above 240 °C. Integration of the unmodified starch melting signals between 112 and 232 °C (Figure 8a) results in a heat of fusion of 104.5 J/g, whereas the starch stearate upper signals give a value of 49.9 J/g. This results in a concentration of 48% crystalline starch in the starch-stearate sample. Using the heat of fusion of crystalline polyethylene (293.1 J/g),28 the number of crystalline stearate chains is calculated to be approximately 50%. These values are in agreement with the earlier stated assumption of an almost complete surface modification and inherent crystallization of the polymer brushes. The stretched stearate length at the nanocrystal surface is roughly 2.5 nm. Complete surface coverage of the starch nanocrystals by stearate chains gives rise to a total stearate layer of 5 nm thickness (2.5 nm on both sides), whereas the starch nanocrystals display a thickness of 5-7 nm. Completely surfacemodified starch nanocrystals can thus be expected to display approximately equal parts of stearate and starch, as confirmed by the calculations based on the melting regions area. It is also very important to note that starch degradation is held up by more than 100 °C because of the protective crystalline layer formed by the oxygen-poor stearate surface. PEGME modified starch did not show a separate melting signal in its DSC trace even though the melting point of poly(ethylene glycol) is significantly lower than any starch signal. The absence of a clear PEGME signal is an indication of the absence of a crystalline PEGME phase at the starch nanocrystal surface. The (27) Lutz, D. A.; Witnauer, L. P. J. Polym. Sci. 1964, B2, 31-33. (28) Khabbaz, F.; Albertsson, A.-C.; Karlsson, S. Polym. Degrad. Stab. 1999, 63, 127-138. (29) Ada˜o, M. H. V. C.; Saramago, B. J. V.; Fernandes, A. C. J. Colloid Interface Sci. 1999, 217, 94-106. (30) Michalski, M.-C.; Hardi, J.; Saramago, B. J. V. J. Colloid Interface Sci. 1998, 208, 319-328.
Thielemans et al.
start of starch degradation (occurring around 240 °C) is not delayed. This is expected considering the abundant availability of oxygen in the PEGME chain, providing ample oxygen for low-temperature degradation under a nitrogen atmosphere. The fast degradation seen for PEGME-grafted particles, however, indicates a loose surface structure that degrades more quickly than the crystalline starch found at the core of the grafted nanoparticles. Transmission Electron Microscopy. Transmission electron micrographs of the modified nanocrystals are shown in Figure 1b and c. For both modifications, the structural integrity of the nanoparticles does not appear to have been affected. Moreover, the particles are more individualized than their unmodified counterparts. The starch nanoplatelets are believed to aggregate as a result of hydrogen bond interactions due to the surface hydroxyl groups.11 Blocking these interactions by relatively large molecular weight molecules obviously improves the individualization of the nanoparticles. The decrease in the polar contribution to the surface energy, quantified using contact angle measurements, reduces the strength of the interparticle interactions and results in the individualization of the nanoplatelets. The presented surface modifications are much more efficient in individualizing the nanoparticles than earlier reported modifications with alkenyl succenic anhydride and phenyl isocyanate.16 These modifications did not appear to have a pronounced effect because agglomerates were still visible after surface modification. Earlier reported modifications formed only a small shell around the nanocrystal surface and did not hinder the interactions enough to prevent aggregate formation.
Conclusions Starch nanoparticles were successfully modified with two different agents: stearic acid chloride and poly(ethylene glycol) methyl ether. The latter used 2,4-TDI to graft the PEGME to the starch surface. Successful surface modification was confirmed using FTIR and XPS spectroscopy and DSC as well as contact angle measurements. Efficient, complete surface coverage was confirmed, but the starch crystalline structure was not affected. X-ray diffraction showed extensive crystallization of the stearate moieties grafted to the starch nanoparticle surface, forming a crystalline hydrophobic shell around the hydrophilic starch nanocrystal. Crystallization of the PEGME chains at the surface is more ambiguous. Small signals indicate PEGME crystallization but only to a limited extent. Strong interactions between the grafted PEGME and the starch particle are the most likely reason. These interactions cause the PEGME chains to align themselves with the nanocrystal surface. Both modifications, however, exhibit a large effect on the individualization of the nanocrystals because of reduced hydrogen bonding and polar interactions between the individual particles. The described surface modifications clearly change the character of the nanocrystal surfaces and their activity. These modified nanocrystals will be tested as toxic agent trapping devices and in co-continuous composites. Acknowledgment. We thank Isabelle Paintrand (CERMAV) for her help with the TEM imaging of the starch nanocrystals, Gre´gory Berthome´ (LTPCM-ENSEEG) for his help with the XPS experiments, Herve´ Roussel and Ste´phane Coindeau (CMTC-ENSEEG) for their help with X-ray diffraction, and Roquette S.A. (Lestrem, France) for supplying the waxy maize starch. Financial support for this work was provided by ADEME (Agence Franc¸ aise de l’Environnement et de la Maıˆtrise de l’Energie, convention 0401C0011). LA053394M
