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Langmuir 1998, 14, 582-589
Crystal Design of Calcium Carbonate Microparticles Using Double-Hydrophilic Block Copolymers Helmut Co¨lfen* and Markus Antonietti Max-Planck-Institut fu¨ r Kolloid- und Grenzfla¨ chenforschung, Kolloidchemie, Kantstrasse 55, D-14513 Teltow, Germany Received July 9, 1997 Small block copolymers, consisting of a hydrophilic poly(ethylene glycol) block and a second hydrophilic moiety which strongly interacts with alkaline earth ions, were employed to template the precipitation of calcium carbonate from aqueous solution. Depending on the type of polymeric functionalization pattern used, it was possible to grow two of the three different crystal modifications of CaCO3 (vaterite and calcite) under similar conditions. Vaterite could be successfully stabilized for more than 1 year by the polymer under conditions where it would normally transform into calcite within 80 h. Also the crystal size and shape respond to the polymeric template; for example, it was possible to grow monodisperse spherical particles with a diameter of ca. 2 µm or hollow spheres both consisting of aggregated nanocrystallites. The effect of different functional groups and of variable block length on the size, shape, and morphology of CaCO3 is discussed. It must be emphasized that these particles sediment, but are stabilized and protected by a dense layer of double-hydrophilic block copolymers, which enables direct use as pigments and fillers or in ceramic processing.
1. Introduction In a previous paper we described the synthesis of a series of so-called “double-hydrophilic” block copolymers and showed their capability to serve as efficient builders for calcium-binding and precipitation inhibition of CaCO3.1 Such polymers consist of one hydrophilic block which is designed to interact with inorganic salts and surfaces, whereas the other hydrophilic block just promotes dissolution in water, but does not interact (or just weakly interacts) with the dissolved ions. It was speculated that these double-hydrophilic block copolymers, due to their strong interaction with inorganic surfaces, also have the potential to control the growth of inorganic crystallites, such as those of calcium carbonate or hydroxyapatite, by acting as a template during nucleation and growth. This process is called “crystal design/ engineering”2-4 and was pioneered by Buckley, Raistrick, and Whetstone;5-7 it enables, in principle, a direct handling of the size, the shape, and the crystal structure of the material (in cases where more than one modification exists). In the case of CaCO3, successful control of the modification of crystals grown under Langmuir monolayers could be achieved. All three crystal modifications of CaCO3 could be obtained, namely, the thermodynamically stable calcite8 and the two other polymorphs, vaterite8 and aragonite.9 Using classical means to control the size, morphology, and polymorphic expression of a crystal, one changes (1) Sedlak, M.; Antonietti, M.; Co¨lfen, H. Macromol. Chem. Phys., in press. (2) Addadi, L.; Berkovitch-Yellin, Z.; Weissbuch, I.; Lahav, M.; Leiserowitz, L. Top. Stereochem. 1986, 16, 1. (3) Black, S.; Bromley, L.; Cottier, D.; Davey, R. J.; Dobbs, B.; Rout, J. J. Chem. Soc., Faraday Trans. 1991, 87, 3409. (4) Weissbuch, I.; Popovitz-Biro, R.; Leiserowitz, L.; Lahav, M. In Perspectives in Supramolecular Chemistry; Behr, J. M., Ed.; John Wiley: Chichester, U.K. 1994; Vol. 1, p 173. (5) Buckley, H. E. Z. Kristallogr., 1935, 91, 375. (6) Raistrick, B. Discuss. Faraday Soc. 1949, 40, 234. (7) Whetstone, J.; Discuss. Faraday Soc. 1954, 16, 132. (8) Rajam, S.; Heywood, B. R.; Walker, J. B. A.; Mann, S.; Davey, R. J.; Birchall, J. D. J. Chem. Soc., Faraday Trans. 1991, 81, 727. (9) Heywood, B. R.; Mann, S. Adv. Mater. 1992, 4, 278.
variables like temperature, pH, supersaturation, and solvent quality. This proved sufficient for size control10 but had limited influence on the crystal morphology or texture control of a growing crystal.11 From the viewpoint of classical colloid chemistry, Matijevich has shown in a number of impressive papers reviewed in ref 12 that the synthesis of monodisperse crystals with various shapes, such as spheres,13 needles,14 plates,15 cereals, or stars, can be handled on the micrometer scale. In these cases, the mineral formation is mainly controlled by the precipitation conditions in combination with low molecular weight counterions as also reported in many papers about the influence of ions on the crystallization of CaCO3 (e.g., aragonite formation in the presence of Mg2+ ions). Biological systems, such as mollusk shells, sea urchins, or cocoliths, possess a still better command of crystal design, as shown in some recent advisable work about “biomineralization” (for example, see refs 16-19). Here, amphiphilic biopolymers (proteins and polysaccharides) with their complicated functionalization pattern play the template part. There are also some biomimetic approaches toward crystal design, mainly applying surfactant monolayers as templates (for a review, see ref 20 and the references cited therein). Furthermore, there is an excellent example of the control of the CaCO3-crystal modification by macromolecules extracted from mollusk shells.21 Existing crystal modifications can also be stabilized by additives as shown by the successful blocking (10) Sohnel, O.; Garside, J. Precipitation: Basic Principles and Industrial Applications; Butterworth-Heinemann: London, 1992. (11) Matijevic, E. MRS Bull. 1989, 14 (2), 18. (12) Matijevic, E. Chem. Mater. 1993, 5, 412. (13) Brace, R.; Matijevic, E. J. Inorg. Nucl. Chem. 1973, 35, 3691. (14) Chittofrati, A.; Matijevic, E. Colloids Surf. 1990, 48, 65. (15) Sapieszko, R. S.; Matijevic, E. J. Colloid Interface Sci. 1980, 74, 405. (16) Weiner, S. Crit. Rev. Biochem. 1986, 20 (4), 365. (17) Mann, S.; Webb, J.; Williams, R. J. P. Biomineralization, Chemical and Biochemical Perspectives; VCH: Weinheim, 1989. (18) Mann, S.; Perry, C. C. Adv. Inorg. Chem. 1991, 36, 137. (19) Mann, S.; Ozin, G. A. Nature 1996, 382 (6589), 313. (20) Heywood, B. R. In Biomimetic materials chemistry; Mann, S., Ed.; VCH: Weinheim, 1996; Chapter 6, p 143. (21) Falini, G.; Albeck, S.; Weiner, S.; Addadi, L. Science 1996, 271, 67.
S0743-7463(97)00765-8 CCC: $15.00 © 1998 American Chemical Society Published on Web 01/17/1998
Crystal Design of Calcium Carbonate Microparticles
of the solvent-mediated vaterite-calcite transformation.22 Furthermore, the crystal polymorphs of CaCO3 can be influenced by organic additives like ethylenediamine or glutamate/aspartate (vaterite instead of calcite formation) or surfactants.23,24 The present as well as the future use of such particles is manifold: They can be applied as pigments, fillers, pastes, and abrasives and also in the construction of new biomimetic material hybrids consisting as the seashell (volume material) or mother of pearl (coating) of organic and inorganic components, arranged and assembled on the nanometer scale. Since it was previously shown that double-hydrophilic block copolymers are superb builders,1 in combination with the knowledge of the morphology-directing influence of amines and certain amino acids on CaCO3,23,24 it is straightforward to use these functional polymers for the templated precipitation of CaCO3 too. Also for the purpose of crystal design, we expect some advantages in using the architectural principle of double-hydrophilic block copolymers: Due to the separation of the binding moiety on one side (the “head” of the polymeric tool) and the solvating but non-interacting moiety on the other (the “handle” of the polymeric tool), rather complex functionalization patterns can be realized, preserving both functions of the polymeric template, binding and dissolution, also in the loaded state. Another advantage of using a functionalized polymer chain for crystal design compared to monolayers is the pattern of functional groups which can be varied over a wide range and thus provides a tool for molecular patterning. 2. Experimental Section 2.1. Synthesis of Polymers and Calcium Carbonate Precipitation. The synthesis of the double-hydrophilic block copolymers was already partly described in the previous paper.1 Additional polymers were prepared either by using the chemistry described in ref 1 for the attachment of simple functional groups (amino acids etc.) to poly(ethylene glycol) or by modification of a technical poly(ethylene glycol)-block-poly(methacrylic acid)25 supplied by Th. Goldschmidt AG. The increase of the COOH density by reaction of the methacrylic acid groups with aspartic acid was performed via the acid anhydride which was derived by standard treatment with SOCl2. Thus every second methacrylic acid group can be statistically connected to an aspartic acid. The phosphonation of COOH groups was performed according to ref 26. Table 1 summarizes the formula as well as some of the molecular parameters of the polymers used. Abbreviations are as follows: PEG ) poly(ethylene glycol), EDTA ) ethylenediaminetetraacetic acid, PEIPA ) poly(ethylenimine)-poly(acetic acid), pAsp ) polyaspartic acid (2300 g/mol), PMAA ) poly(methacrylic acid), PMAA-Asp ) poly(methacrylic acid) modified with aspartic acid statistically at every second group, PMAA-PO3H2 ) monophosphonated poly(methacrylic acid) according to ref 26, and CP2O7HNa4 ) sodium salt of a bisphosphonated COOH group according to ref 26. The PEG blocks have a molecular weight of 1000 and 3000 g/mol, and in the case of CH3OPEG-OH, 2000 and 5000 g/mol. (22) Lopezmacipe, A.; Gomezmorales, J.; Rodriguezclemente, R. J. Cryst. Growth 1996, 166 (1-4), 1015. (23) Matsushita, I.; Hamada, Y.; Moriga, T.; Ashida, T.; Nakabayashi, I.; J. Ceram. Soc. Jpn 1996, 104 (11), 1081. (24) Sugihara, H.; Ono, K. I.; Adachi, K.; Setoguchi, Y.; Ishihara, T.; Takita, Y. J. Ceram. Soc. Jpn 1996, 104 (9), 832. (25) Esselborn, E.; Fock, J.; Knebelkamp, A. Macromol. Chem. Phys., Macromol. Symp. 1996, 102, 91. (26) Kieczykowski, G. R.; Jobson, R. B.; Melillo, D. G.; Reinhold, D. F.; Grenda, V. J.; Shinkai, I. J. Org. Chem. 1995, 60, 8310.
Langmuir, Vol. 14, No. 3, 1998 583 Table 1: Polymers Used for the Mineralization of CaCO3a sample 1: CH3O-PEG-COOH 2: CH3O-PEG-COOH 3: CH3O-PEG-EDTA 4: CH3O-PEG-EDTA3 5: CH3O-PEG-b-PEIPA 6: CH3O-PEG-b-PEIPA 7: CH3O-PEG-Asp 8: CH3O-PEG-b-pAsp 9: PEG-b-PMAA 10: PEG-b-PMAA 11: PEG-b-PMAA-Asp 12: CH3O-PEG-CP2O7HNa4 13: CH3O-PEG-CP2O7HNa4 14: PEG-b-PMAA-PO3H2
% COOH, Mtheor Mw (g/mol) (g/mol) Mw/Mn % PO3H2 2060 5060 5285 2925 3840 6840 2190 7400 1700 3700 4620 2295 5295 4250
ndb ndb 5300 2400 2430 4550 2010 4615 ndb ndb 2980 1190 ndb ndb
ndb ndb 1.05 1.19 1.24 1.16 1.08 1.14 ndb ndb 1.12 1.20 ndb ndb
100 93 75 90 53 70 14 96 ndb 100 27 71, 29 57, 43 79, 21
a The COOH and PO H content relates to the theoretically 3 2 expected amount. Polymers 9 and 10 were characterized by the manufacturer, and sample 14 did not dissolve in DMA. b Not determined.
The precipitation of CaCO3 in the presence of the various synthesized block copolymers was carried out in a double jet reactor as described in a previous paper1 to prevent heterogeneous nucleation at the glass walls. The reactor consists of a thermostated vessel (25 °C) with a solution of 2 mmol of block copolymer (corrected for COOH content) in 300 mL of distilled water (adjusted to pH 8.5). This solution is kept under N2 to prevent the dissolution of CO2 from the air. Na2CO3 in water (0.5 mol/L, pH ) 11.57) and 0.5 mol/L CaCl2 (adjusted to pH 8.5) were injected via capillaries into the reaction vessel with vigorous stirring and with a reactant supply of 10 mL/h. The formation rate of CaCO3 was 1.39 µmol/s. The pH was monitored continuously by a Metrohm (Herisau, Switzerland) Titrino 702 SM. The reactant supply was stopped after macroscopic crystal formation could be observed. The crystals were kept under the saturated CaCO3 and polymer solution for a period of 12 months to allow further crystal growth by Ostwald ripening and to maximize the visualization of the polymer template effects. Afterward, the crystals in the supernatant solution were exhaustively dialyzed against saturated CaCO3 under N2 to remove excess polymer which was not bound to the crystals. 2.2. Analytical Techniques. Molar masses were characterized using GPC with dimethylacetamide as the solvent. The column was calibrated with PEG standards in the required range. Analysis of the effectiveness of the phosphonation reactions was done using quantitative 31P NMR, which was capable of distinguishing between inorganic phosphorus, and mono- and bisphosphonate groups, respectively. Wide-angle X-ray scattering (WAXS) was performed on dried powder samples using a PDS 120 (Nonius GmbH, Solingen) with Cu KR radiation (λ ) 1.54 Å). In the case of mixed crystal modifications in one sample, the amount of the individual modifications was estimated from the height ratio of the strongest peaks giving the volume ratio of the corresponding modifications under the assumption that the crystals in the scattering volume are representative of the whole sample. Furthermore, the peak widths were used for the determination of the crystallite sizes. Scanning electron microscopy was done on a DSM 940 A (Carl Zeiss, Jena). The crystals were taken from the solution with a tip, and then most of the solution was soaked off using filter paper in order to minimize crystallization resulting from the drying process of the saturated
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CaCO3 solution. In the case of polycrystallinity, electron diffraction was performed using a Zeiss EM 912 Omega transmission electron microscope. 3. Results 3.1. Polymer Characterization. The molar mass determination of the polymers listed in Table 1 shows that these polymers are rather well-defined species (Mw/ Mn ) 1.05-1.24). The determined molar mass almost equals the theoretical value in cases where only one or two functional groups are attached to the PEG chain (samples 3 and 7) with the exception of sample 12. If functional blocks are connected to the PEG chain, the determined molar mass is systematically too low compared to the theoretical value because the GPC column was calibrated with PEG, and furthermore, an interaction of the functional blocks with the column material cannot be excluded (samples 4-6, 8, 11, and 12). This is visualized by a “tailing” of the peaks. In the case of high functional group densities (polymer 11), the determined molar mass can even drop below that of the precursor polymer (3700 g/mol). However, GPC proved to be the only analytical method yielding information about the molar mass and the formation of block copolymers as discussed in ref 1. The yields of the functionalization reactions as determined by titration or 31P-NMR were usually good for the COOH functionalities (with the exception of polymers 5 and 7) but low for the phosphonation of the COOH groups (polymers 12-14). 3.2. Influence of the Functionalized Groups/ Blocks on CaCO3 Crystallization. The influence of the different polymers in Table 1 on the crystallization of CaCO3 is already obvious from the settling time of the macroscopic crystals after shaking the sample, indicating large differences in particle size, shape, or density. Therefore, the crystals were characterized both with optical phase contrast microscopy in solution and with scanning electron microscopy (SEM), thus allowing the precise characterization of size and shape. In almost every case, the crystal sizes and shapes observed with optical microscopy (crystals in solution) correspond to those which were observed using SEM. Thus, drying artifacts during sample preparation can be excluded, unless explicitly stated. From these pictures, it can be seen that for all PEG polymers which contain only one terminal functional unit (COOH, Asp, EDTA), the crystal size as well as the shape is not well-defined. In all cases, big crystals and crystal aggregates exceeding 20 µm are obtained. These crystals are synthetic calcite as shown by WAXS. The calcite crystal faces can be seen in the SEM images, which furthermore show the polycrystalline nature of the investigated crystals. An example is given in Figure 1 for the precipitation in the presence of sample 7. The only exceptions are the CP2O7HNa4 groups which lead to comparatively smaller crystals, partly with a spherical shape (sample 13). These macrocrystals consist of vaterite and calcite (see also Table 2). This shows that the effectiveness of the functional groups with respect to the control of the particle size differs. The best growth inhibitor of the monofunctional derivatives is the bisphosphonate group followed by EDTA, whereas simple COOH functionalities show the worst inhibition effect. On the other hand, all double-hydrophilic block copolymers listed in Table 1 indeed act as templates in the mineralization process, that is, the CaCO3 precipitates have a completely different appearance as compared to those obtained under similar conditions, but without added
Figure 1. SEM picture of a synthetic calcite crystal grown in the presence of CH3O-PEG-Asp (sample 7).
polymeric template (e.g., rhombohedral calcite) or with only PEG added. It is seen that all crystals have sizes in the micrometer range. The particles are often close to spherical. One example is the PEG-b-PMAA (sample 10). In Figure 2 it can be seen that almost spherical, synthetic calcite (from WAXS) particles with very broad particle size distributions between 1 and 10 µm are obtained. In this case, only the particle shape is determined by the CaCO3 surface active block. It is remarkable that several crystals which have grown together can be recognized in Figure 2. From the different stages of this growing process, one sees that finally a spherical crystal is formed again. This underlines the activity of the block copolymer to the inorganic surface. The appearance of these calcite crystals matured for 12 months differs very much from those investigated directly after crystallization. In a recent paper, the same block copolymers (with slightly different block lengths) were found to produce elongated rhombohedral calcite with aspect ratios of about 2.27 Obviously, important timedependent changes of the crystal shape occur which will be the subject of further studies. If the COOH functional density of this block copolymer is increased by coupling aspartic acid to 27% of the COOH groups of the PMAA block (polymer 11), one still obtains mainly spherical particles in the optical micrograph (synthetic calcite from WAXS), but here, they are mainly interconnected as twins or aggregates thereof. One can clearly see the crystal faces in the SEM picture (see Figure 3). The particle size of the single “spheres” ranges between 8 and 10 µm. Again, the crystals show a tendency to grow together which probably leads to the minor amount of rod-like crystals in Figure 3. Obviously, this polymer is less surface active on CaCO3 than the unmodified PEGPMAA. Monophosphonation (21% yield) of the COOH groups of PEG-PMAA leads to very well-defined spherical particles. Processing the SEM pictures and optical micrographs, one derives a particle size of 2.41 ( 0.54 µm (27) Marentette, J. M.; Norwig, J.; Sto¨ckelmann, E.; Meyer, W. H.; Wegner, G. Adv. Mater. 1997, 9 (8), 647.
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Table 2: Summary of the Experimental Results of the Crystallization of CaCO3 in the Presence of the Block Copolymers Listed in Table 1 size template
CaCO3 macrocrystal
nanocrystallites
3: CH3O-PEG-EDTA 4: CH3O-PEG-EDTA3
aggr cryst >15 µm aggr cryst 14-20 µm
5: CH3O-PEG-b-PEIPA
4-15 µm
calcite 40.3 nm vaterite 20.0 nm, calcite 33.8 nm vaterite 18.0 nm
6: CH3O-PEG-b-PEIPA 7: CH3O-PEG-Asp 8: CH3O-PEG-b-pAsp
4-18 µm >30 µm 12.9 ( 3.7 µm
9: PEG-b-PMAA 10: PEG-b-PMAA 11: PEG-b-PMAA-Asp 12: CH3O-PEG-CP2O7HNa4
2-8 µm 2-8 µm 2 aggr cryst 5-8 µm 10-25 µm
13: CH3O-PEG-CP2O7HNa4
5-20 µm
14: PEG-b-PMAA-PO3H2
2.4 ( 0.5 µm
vaterite 27.9 nm calcite 36.9 nm vaterite 9.0 nm, calcite 11.0 nm calcite 34.5 nm calcite 22.2 nm calcite 35.0 nm vaterite 17.8 nm, calcite 40.2 nm vaterite 27.0 nm, calcite 45.9 nm vaterite 9.5 nm, calcite 12.7 nm
shape irregular irregular hollow spheres and some large cryst hollow spheres cryst faces spherical almost spherical spherical artichoke-like double spheres irregular mainly spherical, partly hollow spherical
cryst modificatn synth calcite vaterite (15%), synth calcite (85%) vaterite vaterite synth calcite vaterite (55%), synth calcite (45%) synth calcite synth calcite synth calcite vaterite (50%), synth calcite (50%) vaterite (65%), synth calcite (35%) vaterite (75%), synth calcite (25%)
Figure 2. SEM picture of calcite crystals grown in the presence of PEG-b-PMAA (sample 10).
Figure 3. SEM picture of calcite crystals grown in the presence of PEG-b-PMAA-Asp (sample 11).
(standard deviation 22%). This is already a rather monodisperse sample (see Figure 4). WAXS analysis shows that these crystals consist of vaterite (75%), a thermodynamically unstable CaCO3 modification, and (25%) calcite which is thermodynamically stable. These findings hint at polycrystallinity, which is furthermore indicated by line width analysis of the WAXS spectra yielding 9.5 nm vaterite and 12.7 nm calcite nanocrystallites. The origin of the calcite cannot be given without ambiguity but might be explained by partial vaterite-calcite transformation. This is supported by the finding that the analogous polymer with a 1000 g/mol PEG block formed pure vaterite which was stable over a period of 1 year. It is therefore important in future studies to also look at time-dependent changes of the crystal shape and modification. The above example demonstrates a control of the particle size, shape, and in parts even the crystal modification by the polymeric template. Although spherical vaterite crystals have already been reported in the literature,28 these particles have not been templated
by a polymer but consisted of aggregated 25-35 nm nanocrystallites formed in the presence of various bivalent cations. This result is suprising due to the fact that vaterite could be stabilized for more than 1 year because it is well-known that vaterite transforms into the stable calcite via a solvent-mediated process. As far as we are aware there was just one reported case where vaterite could be successfully stabilized by a total coverage of the vaterite surface by a surfactant (AOT ) bis(2-ethylhexyl) sulfosuccinate).22 This is not the only example of obtaining CaCO3 as vaterite in a spherical colloid morphology. If CH3O-PEGb-pAsp (sample 8) is applied as a template, the CaCO3 crystals formed are vaterite (55%) and calcite, again hinting at polycrystalline samples. These particles are bigger than those grown in the presence of PEG-PMAAPO3H2 but also rather well-defined in particle size (12.9 ( 3.7 µm, 29% SD). The crystal surface is smooth (see Figure 5), which is indicative of particle formation via the aggregation of nanocrystallites. This is another example of controlling the crystal size, shape, and morphology by using a polymeric template.
(28) Brecevic, L.; Nothiglaslo, V.; Kralj, D.; Popovic, S. J. Chem. Soc., Faraday Trans. 1996, 92 (6), 1017.
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Figure 4. SEM picture of vaterite/calcite crystals grown in the presence of PEG-b-PMAA-PO3H2 (sample 14).
Figure 6. (a, top) Optical micrograph of vaterite hollow spheres grown in the presence of CH3O-PEG-b-PEIPA (samples 5 and 6). (b, bottom) SEM picture.
Figure 5. SEM picture of vaterite/calcite crystals grown in the presence of CH3O-PEG-b-pAsp (sample 8).
Spherical vaterite crystals with a very special morphology are obtained if CH3O-PEG-b-PEIPA is applied as a template. In this case, pure vaterite is obtained (WAXS) which could be kept stable for more than 1 year. The striking feature of these colloids is the fact that they adopt a hollow sphere morphology (Figure 6). Although these vaterite particles are not very well controlled in size, their morphology and shape are well-defined. The formation and further characterization of these hollow inorganic particles will be the subject of another study.29 3.3. Influence of the Block Length. The block length of the functionalized block has a significant influence on (29) Co¨lfen, H.; Antonietti, M. to be published.
the control of CaCO3 mineralization as already became clear in an earlier study on the effectiveness of different block copolymers as inhibitors/stabilizers of CaCO3.1 However, the best CaCO3 binding efficiency does not necessarily lead to the best control of mineralization as is demonstrated for the example of EDTA and PEIPA blocks. If only one EDTA group is connected to the PEG block, one can clearly see that a single EDTA group is not sufficient to control the mineralization of CaCO3, although EDTA is well-known to be an excellent chelate complexer for bivalent cations, especially Ca2+, and thus one can definitely expect an interaction. Nevertheless, irregular large calcite crystals are obtained. If an EDTA trimer (sample 4) is connected to a PEG chain, one obtains partial control over the crystal modification (15% vaterite, 85% calcite), but the crystals are rather undefined polycrystalline samples which show a strong aggregation tendency. In the SEM picture calcite crystal faces are obvious (see Figure 6a). The aggregation seems to be further promoted by the drying process because the optical micrographs show species between 14-20 µm; that is, the aggregation is just weakly confined by the polymer. A macrocrystal consists of small aggregated 20-30 nm nanocrystallites as seen in the TEM picture (Figure 7b). This is also confirmed by WAXS line width analysis (see Table 2). The size of the calcite nanocrystallites was found to be 33.8 nm, whereas vaterite was a bit smaller at 20.0
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but also the shape and the crystal modification. As in the literature example of AOT,22 the vaterite surfaces seem to be stabilized by the PEIPA moiety for a period of at least 1 year. This period is very long if one bears in mind that vaterite transforms into calcite under the chosen conditions within 80 h and even faster in the presence of NaCl, the by-product of the CaCO3 precipitation.30 These three examples illustrate the molecular weight dependent transition between no control and a pronounced templating effect in an illustrative fashion. Similar considerations can be made for the Asp functional group (sample 7) and the pAsp block (sample 8). The PEG block on the other hand has no detectable influence on the mineralization control in the tested range between 1000 and 5000 g/mol, as can be deduced from a comparison of samples 5 and 6 or 9 and 10. The obtained CaCO3 particles depend only on the type and length of the functionalized block. As expected, the PEG block does not show a significant interaction with the growing mineral and thus serves only as a stabilizing moiety. 3.4. Summary of the Characterization of CaCO3 Microparticles. Table 2 summarizes the structural characterization of all CaCO3 colloids by listing their size, shape, and crystal modification as obtained from WAXS. For all samples, we obtain sets of narrow scattering peaks of purely crystalline material which can be assigned to two of the three crystal modifications of calcium carbonate, calcite, and vaterite. In every case, nanocrystallites were detected. When the SEM pictures show a much larger particle size than the optical micrographs in solution, the latter are used for the size determination.
Figure 7. (a, top) SEM and (b, bottom) TEM pictures of vaterite/ calcite grown in the presence of CH3O-PEG-EDTA3 (sample 4).
nm. Electron diffraction on various colloids confirms that the material is in general polycrystalline. Calcite and vaterite coexist as aggregated nanocrystallites in one macrocrystal as can also be deduced from electron diffraction patterns detected at different locations in the particle. If the 85% calcite results from a vaterite-calcite transformation over the investigated time period of a year or if calcite was nucleated simultaneously with vaterite remains unknown yet because this requires time-dependent studies. If a PEIPA block, which is essentially a poly-EDTA block with a molar mass of 1800 g/mol, is connected to PEG (samples 5 and 6), one obtains the hollow vaterite spheres as shown in Figure 6. Here the functional block determines not only the particle size (though rather imperfect)
4. Discussion From the results given above, it is obvious that doublehydrophilic block copolymers can have a dramatic effect on CaCO3 mineralization. Although only the functionalized block determines the crystal size, shape, and modification, the PEG block is also necessary to keep the growing crystal nuclei in solution. In the range 10005000 g/mol, the PEG block has no influence on the formed crystallites at all. From our experiences with noble metal formation in aqueous systems in the presence of doublehydrophilic block copolymers, we know that a single functionalized block alone cannot stabilize the formed microcrystal.31 The same is valid for the polymers in this study as shown in previous work1 for the case of PEGb-pAsp in comparison with a pAsp homopolymer. Therefore, the concept of separating the stabilizing moiety from the interacting moiety by “double-hydrophilic” blocks is important. This becomes directly obvious to the naked eye after the crystallization experiment by comparing the product of CaCO3 crystallization in the presence of one of the block copolymers in Table 1 (macroscopic crystals) to the flakes which are derived in the presence of a statistically functionalized copolymer or homopolymer through crosslinking via Ca2+. From these considerations, it is clear that an optimum block length of the functionalized block exists, balancing interparticle cross-linking with the polymer Ca2+ interaction, leading to a stabilization even at high CaCO3 concentrations. This is indeed found and demonstrated in section 3.3. for the PEG-(EDTA)n example. The optimum length depends on the targeted concentration regime. Using very low copolymer concentrations in the ppm regime, successful crystallization (30) Richter, A.; Petzold, D.; Hofmann, H.; Ulrich, B. Chem. Tech. 1996, 48, Heft 5, 271. (31) Bronstein, L. M.; Sidorov, S. N.; Gourkova, A. Y.; Valetsky, P. M.; Hartmann, J.; Breulmann, M.; Co¨lfen, H.; Antonietti, M. Inorg. Chim. Acta, submitted for publication.
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inhibition was observed with (polycarboxylate carboxymethyl)inulin as with the industrial acrylate-maleate standard copolymer because, in such a concentration regime, the interparticle cross-linking is suppressed.32 The formation of the hollow spheres (see Figure 6) is yet unexplored, and with the present experimental material, one can only speculate. One explanation for the growth of these hollow spheres can be the initial formation of an unstable CaCO3 modification, such as amorphous CaCO3. With time, the amorphous CaCO3 will be transformed to more stable modifications. Due to the template effect of the PEIPA block, this results in the formation of vaterite crystal nuclei on the surface of the primary colloid which then grow on the outside at the cost of the particle core thus leading to a hollow sphere. The explanation of this structure-directing mechanism will be the subject of future work.29 Another functional group which strongly promotes vaterite formation is phosphonate (samples 12-14). Even with the low degree of COOH transformation into a bisphosphonate group (or in the case of block copolymers, a monophosphonate group due to steric hindrance) using the chemistry described in ref 26, significant effects are observed. Comparison of samples 12 and 13 in Table 1 which differ only by the PEG block length (no influence on mineralization) and the phosphonation degree (31PNMR) allows elaboration on the influence of the phosphonation degree and the number of phosphonate groups on the CaCO3 mineralization. Sample 12 has only a low phosphonate content of 29%. Here, only big, irregular calcite/vaterite crystals (50%) are obtained. On the other hand, sample 13 with 43% phosphonate content leads to mainly spherical crystals (not shown) which mainly consist of vaterite (65%). These particles are partly hollow. In the case of a maximally phosphonated COOH block (21% monophosphonation), one gets similar results with respect to the crystal modification (75% vaterite), but the spheres are much more defined in size and shape due to the block copolymer character of the template (see Figure 4). These results show that the phosphonate group is a strong templating group even if present in only small quantities. From Table 2, it can further be seen that the increase in COOH density of a PEG-PMAA block copolymer by connecting aspartic acid to the COOH functions (sample 11, yield 27%) also leads to a change in CaCO3 crystallization. Here, one obtains mainly twins of two spheres which clearly show calcite crystal faces at the outer range in a staggered prismatic arrangement (see Figure 8 and also Figure 3 for a representative picture). The closeto-exclusive twin formation is also a fact which can be explained by special interaction of the polymer with the growing nucleus as described below. Interestingly, Figure 8 shows similar 11 h 0 calcite faces analogous to those which were stabilized by γ-carboxyglutamate or aspartate.33 These faces were not expressed in the absence of the additive. More generally, all R,ωdicarboxylic acids (CH2)n(CO2H)2 with ionized COOH groups and n < 3 were found to stabilize the 11h 0 calcite crystal face.34 The more hydrophobic these molecules were (increasing n), the less was their ability to induce morphological changes during crystallization. This functionality pattern is closely matched by the COOH/ aspartate block in polymer 11. Hence the analogy is not (32) Verraest, D. L.; Peters, J. A.; Vanbekkum, H.; Vanrosmalen, G. M. J. Am. Oil Chem. Soc. 1996, 73 (1), 55. (33) Mann, S.; Archibald, D. A.; Didymus, J. M.; Heywood, B. R.; Meldrum, F. C.; Wade, V. J. MRS Bull. 1992, 32. (34) Mann, S.; Heywood, B. R.; Rajam, S.; Walker, J. B. A. J. Appl. Phys. 1991, 24, 154.
Co¨ lfen and Antonietti
Figure 8. SEM photo of calcite nucleated in the presence of PEG-b-PMAA-Asp (polymer 11).
too suprising with the exception that the double-hydrophilic block copolymer leads to a three-dimensional body with expressed crystal faces, whereas the literature example shows a more or less two-dimensional plate-like structure due to the dimensional confinements of these experiments.33 From Figure 8, it seems that indeed a disk-like primary crystallite has started the templated crystallization on opposite sites leading to an almost equal particle size of the two counterparts of the twin particle. This is supported by the optical micrographs which also predominately show twins or aggregates thereof. As in the case of polymers 14, 5, or 6, a closer look at the initial crystals formed in the presence of the polymeric template would allow characterization of the formation mechanism. A similar result was reported in the literature where double-ellipsoid R-Fe2O3 particles were formed by the hydrolysis of FeCl3.35 It was stated that initially an akaganeite rod formed, serving as a template for the hematite growth. However, it still remains unclear how the disk-like primary crystallite in the CaCO3 in this study is formed. Obviously, the polymeric aspartate block acts, in the stabilization of calcite crystal faces, in a manner similar to that of its low molecular weight counterpart. Due to the polymeric template structure, not only was the match of unit cell motifs to spatial patterns of functional groups at the crystal interface of importance but also crystallization is influenced by the formation of three-dimensional superstructures in the presence of growing crystals, due to the high surface activities of the functionalized polymer blocks. Hence it is of paramount importance for the discussion of the templated mineralization presented above to know whether the double-hydrophilic block copolymers already form superstructures upon addition of Ca2+ ions. This question is negotiated by means of light scattering for all polymers presented in Table 1. Addition of a calcium salt to a double-hydrophilic block copolymer solution does not result in a remarkable increase of scattered light, as it is characteristic for polymeric superstructure formation. Thus, template effects of the polymer only take place after the critical (35) Bailey, J. K.; Brinker, C. J.; Mecartney, M. L. J. Colloid Interface Sci. 1993, 157, 1.
Crystal Design of Calcium Carbonate Microparticles
crystal nucleus has been formed. Just after that, the double-hydrophilic block copolymers become amphiphilic with respect to some of the CaCO3 interfaces of the growing crystals. The template mechanism of the double-hydrophilic block copolymers seems to be of a complex nature because simultaneous CaCO3 nucleation and interaction with the polymer can be expected. An interesting feature of many of the CaCO3 particles investigated in this study is that they consist of aggregated nanocrystals (see Figure 7b or Table 2). This fact is known for purely inorganic colloids from the newer literature, and the interesting feature of most of these colloids is that the macrocrystals are monodisperse. Examples are CaCO3,28 CuO,36 MgF2,37 SnO2,38 ZnO,39 R-Fe2O3,35,40-42 CeO2,43 SiO2,44 TiO2,45 and R-FeOOH.46 Although these nanocrystals usually aggregate into monodisperse macrocrystals without external forces, very little has been suggested concerning the formation mechanism of such particles. The most comprehensive study/summary of the formation mechanisms of uniform colloids via the aggregation of nanocrystallites is that of Ocana et al.,47 who distinguish between unidirectional (aggregation of primary particles and subsequent internal cementation) and directional aggregation. The latter mechanism can only work if structure-directing factors are present which, interestingly, is shown for the example of a template mechanism of phosphate ions on the formation of R-Fe2O3 ellipsoids. These anisotropic primary particles then ripen to asymmetric macrocrystals. However, these plausible mechanisms do not answer the question of why the formed macrocrystals are monodisperse. Some factors controlling the aggregation process to produce monodisperse particles are suggested to be the colloid stability (destabilization by approach to the isoelectric point) associated with pH changes during colloid formation,36 dissolution-reprecipitation-aggregation mechanisms,48 uniformity of the nanocrystallites with high collision numbers and chemical surface reactions,43 or entropy increase by release of structured water (bound water molecules) upon aggregation of the primary nanocrystallites.37 In general, it can be stated that monodisperse particles can only be obtained from aggregation processes of nanocrystallites if the aggregation is stopped at a certain particle size due to specific reasons. These may vary significantly for different systems. As the underlying mechanism of the aggregation of polymer-templated nanocrystals into monodisperse bodies is considered to be complicated due to partial stabilization of the nanocrystallites, a conclusive model of the aggregation process with experimental evidence is (36) Lee, S. H.; Her, Y. S.; Matijevic, E. J. Colloid Interface Sci. 1997, 186, 193. (37) Hsu, W. P.; Zhong, Q.; Matijevic, E. J. Colloid Interface Sci. 1996, 181, 142. (38) Ocana, M.; Serna, C. J.; Matijevic, E. Colloid Polym. Sci. 1995, 273, 681. (39) Jezequel, D.; Guenot, J.; Jouini, N.; Fievet, F. J. Mater. Res. 1995, 10 (1), 77. (40) Ocana, M.; Morales, M. P.; Serna, C. J. J. Colloid Interface Sci. 1995, 171, 85. (41) Sugimoto, T.; Muramatsu, A.; Sakata, K.; Shindo, D. J. Colloid Interface Sci. 1993, 158, 420. (42) Kandori, K.; Kawashima, Y.; Ishikawa, T. J. Chem. Soc., Faraday Trans. 1991, 87, 2241. (43) Hsu, W. P.; Ro¨nnquist, L.; Matijevic, E. Langmuir 1988, 4, 31. (44) Bogush, G. H.; Zukowski, C. F. In Ultrastructure Processing of Advanced Ceramics; Mackenzie, J. D., Ulrich, D. R., Eds.; John Wiley & Sons: New York, 1988; p 477. (45) Edelson, L. H.; Glaeser, A. M. J. Am. Ceram. Soc. 1988, 71, 225. (46) Murphy, P. J.; Posner, A. M.; Quirk, J. P. J. Colloid Interface Sci. 1976, 56, 284. (47) Ocana, M.; Rodriguez-Clemente, R.; Serna, C. J. Adv. Mater. 1995, 7, 212. (48) Candal, R. J.; Regazzoni, A. E.; Blesa, M. A. J. Mater. Chem. 1992, 2, 657.
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out of the scope of this paper and will be the subject of a forthcoming publication.29 It is worth mentioning that growth of all three CaCO3 modifications, calcite, aragonite and vaterite, is wellknown in biological systems. Even amorphous CaCO3 occurs as a biomineral.49 In natural systems, the crystal modifications are well stabilized too, so that a transformation into calcite is prevented. This is achieved by the functional polymer as was proven for the example of mollusk shell proteins.21 This at least parallels our finding with PEG-PEIPA samples (samples 5 and 6) which stabilize the pure vaterite. 5. Conclusion and Outlook We have shown that the application of double-hydrophilic block copolymers as templates during precipitation indeed enables the control of crystal size, shape, and modification, that is, the crystal design of calcium carbonate. We could precipitate purely crystalline calcite and vaterite depending on the functionalized block and the pattern of functional groups. These crystals show interesting morphologies (monodisperse spheres, hollow spheres, twins) which would not occur without the template and which are polycrystalline. For the first time, we could stabilize vaterite made from a simple precipitation process in aqueous solution with a synthetic polymer for more than 1 year under conditions where the complete phase transformation into the thermodynamically stable calcite occurs within 80 h, usually much faster. The strong dependence of the mineralized particle on the functional pattern of the polymer makes it necessary to find modular synthesis strategies for the defined design of the functionalized block. The kind and location of the functionalities in the polymer must be variable to a maximum extent. Such an approach simplifies the optimization of the described double-hydrophilic block copolymers with respect to crystal design. This opens a wide field for future studies. Particles like the monodisperse spheres might have some potential as fillers or for the production of defectfree ceramics at low sintering temperatures. Thus, it is interesting to extend the present work to other inorganic materials where colloidal systems have some special technological importance, i.e., TiO2 (including adjustment of the modifications rutile and anatase) or hydroxyapatite (for high-performance ceramics and hybrids). In addition, the mechanism of the templated particle formation of the CaCO3 must be investigated in more detail as this was possible in this first study. Time-resolved investigations are considered to be especially helpful to shed light on the aggregation mechanism of nanocrystallites to defined macrocrystals in the presence of block copolymers. Acknowledgment. We thank Ingrid Zenke and MarcAndree Micha for help during X-ray analysis and NMR, Dr. Sean Davis, Dr. J. Hartmann, and H. P. Hentze for the SEM pictures, Johann Moskalenko for GPC measurements, Dr. Milos Sedlak for polymer synthesis work, and Erich C. for encouragement during some long windings. We also thank Th. Goldschmidt AG for the PEG-PMAA block copolymers. Financial support by the Max Planck Society is gratefully acknowledged. LA970765T (49) Mann, S. J. Chem. Soc., Dalton Trans. 1993, 1.
