Effects of Some Nonionic Polymeric Additives on the Crystallization of

Effects of Some Nonionic Polymeric Additives on the Crystallization of Calcium Carbonate Il Won Kim,†,¶ Richard E. Robertson,*,†,‡ and Robert Zand†,§ Macromolecular Science & Engineering Center, Department of Materials Science & Engineering, Biophysics Research Division, and Department of Biological Chemistry, University of Michigan, 2300 Hayward Street, Ann Arbor, Michigan 48109-2136 Received August 9, 2004;

CRYSTAL GROWTH & DESIGN 2005 VOL. 5, NO. 2 513-522

Revised Manuscript Received December 3, 2004

ABSTRACT: The effect of water-soluble, nonionic polymers on the crystallization of calcium carbonate is reported. The polymers used were poly(vinyl alcohol) (PVA), polyacrylamide, poly(N-isopropyl acrylamide), poly(N-vinyl pyrrolidone), and poly(ethylene oxide). In addition, several smaller molecules containing hydroxyl groups (methanol, ethanol, ethylene glycol, glycerol, dextran, and maltodextran) were also examined. Crystallization began by mixing together, with or without a polymer or other additive, calcium chloride and ammonium carbonate. The first crystals to form in all cases were vaterite, possibly with a very small amount of calcite. Subsequent behavior depended on the polymer or other additive present. With most additives, the vaterite transformed within ca. 1 h to calcite, probably by dissolution and recrystallization. However, with PVA, the formation of calcite was inhibited, and when enough PVA was added, the vaterite remained. With less PVA, aragonite formed. After ca. 9 days, aragonite was the exclusive crystalline phase, except for a very small amount of early-forming calcite. The distinguishing characteristics of PVA seem to be its ability to hydrogen bond and its forming nearly theta solutions in water over a range of temperatures including room temperature. From a theta solution, the polymer tends to adsorb nonspecifically on all solid surfaces. The common formation of aragonite in biological organisms may arise from the organism’s proteins acting in a way similar to that of PVA. 1. Introduction Biologically mineralized materials have attracted great attention because of their unusual properties arising from the orientation, morphology, and polymorphs of the constituent minerals.1 Among the biological manipulations of minerals, polymorph selection is the least understood. The controlled formation of aragonite, a metastable polymorph of calcium carbonate, has been especially problematic.2-5 Aragonite is an ideal reinforcement in composite materials because of its strength and needle-shaped morphology. It is important as a biomaterial because of its hydrothermal transformation into biocompatible hydroxyapatite.6,7 Also, biogenic aragonite itself has shown osteoconductive properties in vitro and in vivo.8,9 Although many biological organisms routinely form aragonite with biomacromolecules, its synthesis outside of biological organisms has been difficult under ambient conditions.5 Not only is nucleation without forming other polymorphs difficult, but once formed, aragonite easily changes into thermodynamically stable calcite. Among the attempts to mimic the biological synthesis, insoluble substrates, such as a Langmuir monolayer, self-assembled monolayers, aragonite-imprinted polymers, and other polymer surfaces have been tried.10-14 Also, many soluble additives have been employed to try to influence the crystallization of calcium carbonate in general.13-16 * To whom correspondence should be addressed. E-mail: [email protected]; phone: 734-763-9867; fax: 734-763-4788. † Macromolecular Science & Engineering Center. ‡ Department of Materials Science & Engineering. § Biophysics Research Division and Department of Biological Chemistry. ¶ Present address: Laboratory for Chemical Physics, New York University, 345 E. 24th Street, New York, NY 10010.

These attempts at the exclusive formation of aragonite have had mixed success. Perhaps most successful have been the recently reported uses of a polymer substrate (crystalline poly(vinyl alcohol)13 and a treated polyamide14) in the presence of soluble polymers (poly(acrylic acid)13 and poly(vinyl alcohol)14). For the present study, nonionic, water-soluble polymers with different polar groups and solution properties were tested, as well as small molecules containing hydroxyl groups, to observe their effect on the crystallization of calcium carbonate. Calcium carbonate crystallization was conducted at concentrations of 0.020, 0.10, and 0.50 M at room temperature (23-24 °C). 2. Experimental Section 2.1 Materials. Calcium chloride (Mallinckrodt) and ammonium carbonate (Baker) were used to form calcium carbonate. Deionized water (RO/DI, Crown Engineering) was used to make all aqueous solutions. Polymeric additives studied were poly(vinyl alcohol) (PVA) (Air Product), polyacrylamide (Aldrich), poly(N-isopropyl acrylamide) (Aldrich), poly(N-vinyl pyrrolidone) (Aldrich), and poly(ethylene oxide) (Aldrich). The chemical structures of these polymeric additives are shown in Figure 1. Other additives used include methanol, ethanol, ethylene glycol, glycerol, dextran, and maltodextran. 2.2 Crystallization of Calcium Carbonate. The crystallization was conducted at concentrations of 0.020, 0.10, and 0.50 M calcium carbonate at room temperature (23-24 °C). Typical calcium carbonate crystallization was started by mixing aqueous solutions of CaCl2 with (NH4)2CO3 in a sealed 20-mL glass vial with a Teflon-coated magnetic stir bar. For a 0.020 M calcium carbonate solution, for example, equal amounts of 0.040 M CaCl2 and 0.040 M (NH4)2CO3 were mixed. During mixing, the solution was rapidly stirred with a magnetic stirrer, and stirring was maintained for 2 days.

10.1021/cg049721q CCC: $30.25 © 2005 American Chemical Society Published on Web 02/15/2005

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Kim et al. The relative amounts of vaterite, aragonite, and calcite were quantified by the relative areas of characteristic X-ray peaks of the polymorphs.

3. Results

Figure 1. Chemical structures of the polymeric additives. Before mixing, the vial and stir bar had been washed with 0.1 M HCl (aq) and deionized water, and then dried. Polymer additives, when used, were mixed with the ammonium carbonate solution, by adding a small amount of a concentrated polymer solution. The concentrated polymer solutions were prepared in deionized water by dissolving powders of PVA, poly(N-vinyl pyrrolidone), poly(N-isopropyl acrylamide), and poly(ethylene oxide) at 90 °C. The polyacrylamide solution was prepared by diluting a 50% solution. The concentrated polymer solutions were cooled to room temperature before using for the calcium carbonate crystallization. Crystallization was stopped by removing the precipitated particles from solution by centrifugation. Most of the crystallizations were continued for up to 9 days, with small amounts being removed during this period to study the result at intermediate times. The removed particles were washed with deionized water (20 mL for a whole batch), and then the water was exchanged with acetone (10 times with 20 mL each for a whole batch) to prevent further change. To ensure uniform water/acetone exchanges, vortex mixing was employed each time acetone was added. Also, to retain all crystals during water/acetone removal, thorough centrifugation was performed after each vortex mixing. 2.3 Characterization. The crystals were studied with scanning electron microscopy (SEM) (Philips XL30FEG) at 5 kV. A small quantity of the crystals in acetone was placed on an aluminum holder, and the acetone was evaporated at room temperature. Then, a thin Au/Pd coating was applied by sputtering in an argon-flushed vacuum chamber. X-ray powder diffraction (XRD) studies (Rigaku Miniflex) were performed from θ ) 20° to 50° at a scanning rate of 0.3°/ min with Cu radiation generated at 30 kV and 15 mA. The crystals in acetone were dried and then mixed with a small amount of vacuum grease to hold them on glass substrates.

All of the calcium carbonate polymorphs described in the following, aragonite, calcite, and vaterite, nucleated and grew in the bulk solution, not on the walls of the vessel. 3.1. Crystallization in the Presence of Poly(vinyl alcohol) (PVA). Three different PVAs were studied: 98%-hydrolyzed with molecular weight (MW) of ca. 18 000 g/mol, 88%-hydrolyzed with MW of ca. 18 000 g/mol, and 88%-hydrolyzed with MW of ca. 120 000 g/mol. (Poly(vinyl alcohol) (PVA) is derived from poly(vinyl acetate) by removing acetate groups by hydrolysis, and the hydrolysis percent corresponds to the fraction of free hydroxyl groups, with the rest being the original acetate groups.) 88%-Hydrolyzed PVA is generally more soluble in water than is 98%-hydrolyzed PVA. The effect of 98%-hydrolyzed, 18 000 g/mol PVA on the crystallization of calcium carbonate was studied with three different concentrations of calcium carbonate, 0.020, 0.10, and 0.50 M. Crystallization with 0.020 M calcium carbonate was observed for 9 days without PVA and with 0.0005, 0.0050, and 0.050% PVA. The crystal morphologies resulting after 9 days with these four concentrations of PVA are shown in Figure 2. Without PVA and with 0.0005% PVA, crystals with the distinctive appearance of calcite were formed (Figures 2a,b). However, with 0.0005% PVA, the calcite crystals were only about one-fifth the size of those forming without PVA. With 0.0050% PVA, crystals with the distinctive appearance of aragonite were formed almost exclusively (Figure 2c). These crystals were needle-shaped and of high aspect ratio. With 0.050% PVA, the crystals found had the distinctive appearance of polycrystalline vaterite, along with a very small number of needle-shaped aragonite crystals (Figure 2d). The individual vaterite crystals were ca. 0.5 µm in diameter, although the vaterite polycrystalline spherical clusters were as large as 5 µm in diameter. Crystallization with 0.10 M calcium carbonate was also observed for 9 days. By the end of the 9 days without PVA and with 0.050% PVA, only crystals with the distinctive appearance of calcite were present, and these were confirmed by X-ray analysis to be calcite. The calcite crystals that formed with 0.050% PVA were slightly smaller and better defined than those formed without PVA. With 0.50% PVA, crystals with the distinctive appearance of aragonite were formed nearly exclusively. Again the crystals were needle-shaped and of high aspect ratio, and they were confirmed by X-ray analysis to be aragonite. With 1.0% PVA, crystals suggesting a mixture of aragonite and vaterite were found, and X-ray analysis confirmed this. Crystallization with 0.50 M calcium carbonate was again observed for 9 days, and at the end of the 9 days, without PVA and with 0.050% PVA, only crystals with the distinctive appearance of calcite were found. Again, the calcite crystals that formed with PVA were smaller and better-defined than those forming without PVA. With 0.50% PVA, crystals suggesting a mixture of aragonite and calcite formed. With 5.0% PVA, crystals

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Figure 2. Crystallization at 23-24 °C from 0.020 M calcium carbonate with 98%-hydrolyzed, 18 000 g/mol PVA after 9 days. (a) No PVA, (b) 0.0005% PVA, (c) 0.0050% PVA, and (d) 0.050% PVA.

suggesting a mixture of vaterite, aragonite, and calcite were found. The aragonite crystals were again needleshaped and of high aspect ratio, and the crystalline clusters of vaterite had an overall spherical morphology, though some of the clusters were only spherical sectors. The effect of the more soluble 88%-hydrolyzed, 18 000 g/mol PVA on the crystallization from 0.10 M calcium carbonate was also observed at the end of 9 days. With 0.50% PVA, only crystals with the distinctive appearance of calcite were found. With 5.0% PVA, mostly crystals with the distinctive high aspect ratio needleshaped appearance of aragonite were found. The calcite crystals that formed with 0.50% PVA were again betterdefined than those forming without PVA. The effect of 88%-hydrolyzed, 120 000 g/mol PVA on the crystallization from 0.10 M calcium carbonate was also observed at the end of 9 days. With 0.0005 and 0.0050% PVA, only crystals with the appearance of calcite were found. With 0.050 and 0.50% PVA, crystals with the appearance of high aspect ratio needle-shaped of aragonite were found. The calcite crystals that formed with 0.0050% PVA were better shaped than those that formed with 0.0005% PVA and without PVA. The aragonite crystals that formed with 0.50% PVA were thicker than those formed with 0.050% PVA. The above crystal type observations are collected in Table 1, where A ) aragonite, C ) calcite, and V ) vaterite. More than one symbol indicates roughly equal amounts were present, though a symbol in parentheses indicates a minor amount of that polymorph.

Table 1. Calcium Carbonate Polymorph Resulting after 9 Days at 23-24 °C in the Presence of Different Amounts and Types of Poly(vinyl alcohol) CaCO3 conc (M) PVA mol wt (g/mol) % hydrolysis of PVAc PVA (%) 0 0.0005 0.0050 0.050 0.50 1.0 5.0

0.02 0.10 0.10 0.10 0.50 18000 18000 18000 120000 18000 98 98 88 88 98 C C C C A V, (A) C C A C A, (V) A, (C)

C C C A A

C C C, A V, C, A

The results of the crystallization were also observed, and confirmed by X-ray analysis, at intermediate times with 0.10 M calcium carbonate and 0.50% of 98% hydrolyzed, MW 18 000 g/mol PVA. Nearly exclusive aragonite was ultimately formed, but it did not form immediately, as shown by the sequence of micrographs in Figure 3. The first crystalline form observed was polycrystalline vaterite having an overall spherical morphology (Figure 3a). The spherical morphology may have originated as amorphous calcium carbonate.18 Individual vaterite crystals were initially in the size range of 20-50 nm and then matured with time to larger hexagonal crystals (Figure 3b). Needle-shaped aragonite crystals became noticeable after 8 h, although the vaterite-to-aragonite transformation was not complete until after ca. 9 days (Figure 3c). This vaterite-to-aragonite transformation was also followed by X-ray analysis, and the results are shown

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Figure 4. XRD patterns showing vaterite-to-aragonite transformation with time for a solution initially containing 0.10 M calcium carbonate and 0.50% 98%-hydrolyzed, 18 000 g/mol PVA. Vaterite {110}, calcite {104}, and aragonite {221} peaks were used to calculate the fraction of these polymorphs.

Figure 3. Crystallization at 23-24 °C from 0.10 M calcium carbonate with 0.05% of 98%-hydrolyzed, 18 000 g/mol PVA. (a) After 3 min, spherical polycrystalline vaterite was the initial polymorph observed. (b) After 8 h, vaterite matured to much larger individual vaterite crystals (around 0.5 µm). Many of them were of hexagonal shape, and some crystals arranged to larger hexagons. A few aragonite needles are also apparent. (c) After 9 days, the vaterite-to-aragonite transformation seemed to be complete.

in Figure 4. Although only four of the peaks are labeled in Figure 4 (two vaterite peaks and one each of calcite and aragonite), all peaks that decreased with time arose from vaterite, those that grew with time arose from aragonite, and those that remained constant arose from calcite. The aragonite is seen to have increased at the expense of vaterite, whereas the calcite present after 9 days had formed in the first 3 min and remained constant thereafter. Moreover, the aragonite that was produced showed no signs of transforming into calcite even over the following six months. X-ray analysis also confirmed, by peak sharpening, the apparent maturing

Figure 5. Effect of the concentration of 98%-hydrolyzed, 18 000 g/mol PVA on the kinetics of the vaterite transformation with time at 23-24 °C for a solution initially containing the PVA in 0.10 M calcium carbonate. Filled symbols are for calcite formation, and open symbols are for aragonite formation.

of the vaterite seen in Figure 3. From crystals present after 3 min to those present after 8 h, the vaterite {114} peak had sharpened ca. 30%. The final composition after 9 days, based on the X-ray analysis in Figure 4, was ca. 98% aragonite, with the rest being calcite. The rates of crystallization from 0.10 M calcium carbonate solutions with different concentrations of PVA (18 000 g/mol, 98% hydrolyzed) are compared in Figure 5. The relative amounts of the polymorphs were calculated from the relative areas of the vaterite {110}, calcite {104}, and aragonite {221} peaks in Figure 4.19 Figure 5 shows the transformation rate of vaterite, the left two curves to calcite, the right three to aragonite. Except for the curve on the far right, the ultimate points are where the vaterite was exhausted, and the differences between these and 1.0 represents the calcite present after 3 min. (The short leftward-jutting line segments for 0.25% and 0.50% PVA indicate that these ultimate values may have been, and probably were,



Figure 7. XRD patterns showing vaterite-to-calcite transformation with time for a solution initially containing 0.10 M calcium carbonate, without additives. Vaterite {114} and calcite {104} peaks were used to calculate polymorph fractions.

Figure 6. Crystallization at 23-24 °C from a 0.10 M calcium carbonate solution without any additives. (a) After 3 min, initial formation of spherical vaterite was similar to that with PVA. (b) After 30 min, calcite fraction had increased with dissolution of vaterite, having become ca. 70%, based on X-ray analysis. (c) After 1 h, vaterite-to-calcite transformation appeared to be complete.

reached earlier than their locations along the abscissa.) The rate of crystallization is seen to have decreased as the concentration of PVA was increased. The calcite fraction also decreased: 100% calcite with 0.050% PVA, ca. 5.9% calcite with 0.25% PVA, ca. 2.2% calcite with 0.50% PVA, and ca. 1.7% calcite with 1.0% PVA. The results of the crystallization at intermediate times were also studied with 0.10 M calcium carbonate without a polymeric additive. Micrographs of the morphology at three different times are shown in Figure 6. Spherical vaterite again formed initially (Figure 6a shows this morphology after 3 min), but rather than maturing, it quickly transformed, probably by dissolving

and recrystallizing,20 to stable calcite. Figure 6b shows the mixture of vaterite and calcite present after 30 min. The recrystallization was essentially complete within an hour, as seen in Figure 6c. A quantitative monitoring of the vaterite-to-calcite transformation was provided by XRD, and the diffraction patterns are shown at four times in Figure 7. The mole fraction of each polymorph was calculated based on the area of the calcite {104} and vaterite {114} peaks.21 The amount of calcite present (with the remainder being vaterite) was ca. 15% after 3 min, ca. 25% after 10 min, ca. 70% after 30 min, and essentially 100% after 1 h. 3.2. Crystallization in the Presence of Other Nonionic Polymers. After the presence of PVA was discovered to cause the formation of aragonite instead of calcite, other nonionic polymers were examined, first, to see if they had the same effect as PVA on the crystallization of calcium carbonate, and second, to try to determine why the presence of PVA behaved as it did. The other polymers examined were polyacrylamide (PAAm), poly(N-isopropyl acrylamide) (PiPAAm), poly(N-vinyl pyrrolidone) (PVP), and poly(ethylene oxide) (PEO). These polymers were chosen to vary hydrogen bonding tendency and polymer-solvent interaction. The effect of polyacrylamide (MW 10 000 g/mol) on CaCO3 crystallization was studied both in water and in mixtures of water and methanol. Methanol was added to vary the quality of the PAAm solutions. As the amount of methanol increases, the solubility of PAAm decreases; a 4:6 methanol/water (by volume) mixture becomes a theta (poor) solvent at 20 °C for PAAm.22 From water solutions of 0.10 M calcium carbonate with 0.50, 1.0, and 5.0% PAAm, only calcite was formed. The calcite crystals formed with 0.50% PAAm are shown in Figure 8a. The morphology of these crystals, which were confirmed to be calcite by X-ray analysis, differs from the more typical rhombohedral shapes that are usually enclosed by {104} faces. The crystals obtained with PAAm seem to have other faces as well. Although a regular morphology is not obvious, some crystals have truncated rhombohedral shapes, and others even have inward concave surfaces. Also, the crystal faces have a rough texture, rather than the smooth surfaces more

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Figure 8. Crystallization at 23-24 °C from a 0.10 M calcium carbonate solution with 0.50% of the following polymers, after 9 days: (a) PAAm, (b) PiPAAm, (c) PVP, and (d) PEO. Table 2. Crystallization of Calcium Carbonate (0.10 M) in the Presence of Polyacrylamide (PAAm) from Methanol/Water Solutions at 23-24 °C after 9 Daysa methanol/water PAAm (%) 0 0.50 1.0 5.0

0:10 (water only) C C C C

1:9

2:8

3:7

4:6

C, (V)

C, (V)

V, (C)

C V

a A ) aragonite, C ) calcite, V ) vaterite; symbol in parentheses indicates minor amounts.

Figure 9. Effects of polymers on the kinetics of calcite crystallization (0.10 M). With 0.050% PVA (98% hydrolysis, 18 000 g/mol), the vaterite-to-calcite transformation took nearly 100 times longer than with 0.50% of any of the other polymers and without an additive.

commonly seen. The initial vaterite was the same as that without polymeric additive, and the rate of the vaterite-to-calcite transformation was similar to that without polymeric additive, as shown in Figure 9. To study the effect of solution quality for PAAm on CaCO3 crystallization, 1:9, 2:8, 3:7, and 4:6 methanol/

water solutions were made with 0.50% PAAm and 0.10 M CaCO3. In the 4:6 methanol/water mixture, only vaterite was present after 9 days of crystallization; without PAAm, calcite had formed within this time span. As the amount of methanol decreased, calcite started to form even in the presence of PAAm. In the 3:7 methanol/water mixture, a mixture of vaterite and calcite were formed within 9 days, and in the 2:8 and 1:9 methanol/water mixtures, mostly calcite was formed within 9 days. This behavior is summarized in Table 2. The effect of PAAm concentration on CaCO3 crystallization in a 4:6 methanol/water mixture was studied with 0.020 M calcium carbonate. Crystallization was observed at the end of 9 days with 0.0005, 0.0050, 0.050, and 0.50% PAAm. With 0.0005 and 0.0050% PAAm, only calcite was formed. With 0.050% PAAm, a mixture of vaterite and calcite was formed. With 0.50% PAAm, only vaterite was found after 9 days of crystallization. With 0.0005% PAAm, more or less typical rhombohedral calcite crystals were formed, whereas with 0.0050%

Effects of Nonionic Polymeric Additives on Crystallization Table 3. Crystallization of Calcium Carbonate (0.10 M) in the Presence of Polyacrylamide (PAAm), Poly(N-i-propyl Acrylamide) (PiPAAm), Poly(vinyl Pyrolodone) (PVP), and Polyoxyethylene (PEO) from Water Solutions at 23-24 °C, after 9 Daysa polymer polymer amt (%) 0 0.50 1.0 5.0 a

PAAm

PiPAAm

PVP

PEO

C C C C

C C

C C

C C

C

C

C

Crystal Growth & Design, Vol. 5, No. 2, 2005 519 Table 4. Two Characteristics of the Polymer Additives adsorption from water proton donor yes no

PAAm, the calcite crystals that formed had truncated rhombohedral shapes with a complex surface texture, and some even with concave inward surfaces. Poly(N-isopropyl acrylamide) (PiPAAm), poly(N-vinyl pyrrolidone) (PVP), and poly(ethylene oxide) were each used in water solutions with 0.10 M calcium carbonate. Their effect on crystallization was small, even when used at relatively high concentrations. With both 0.50 and 5.0% PiPAAm (MW 20 000-25 000 g/mol), only calcite was formed; no aragonite was observed at any stage of the crystallization. The calcite crystals that formed, however, had an irregular morphology, as if they had been crushed (Figure 8b). With 0.50 and 5.0% PVP (MW 55 000 g/mol), again only calcite was formed, with no aragonite being observed at any stage of the crystallization. These calcite crystals also had an irregular morphology (Figure 8c), although less so than with PiPAAm. With 0.50 and 5.0% poly(ethylene oxide) (PEO) (MW 300 000 g/mol), again only calcite was formed, with no aragonite being observed at any stage of the crystallization. But these crystals were mostly rhombohedral (Figure 8d). The observations with these polymers and concentrations are summarized in Table 3. As occurred without polymeric additive, vaterite formed first, having its characteristic morphology, with each of these polymers, PiPAAm, PVP, and PEO. The vaterite then transformed to calcite (again, presumably by dissolution and recrystallization) at about the same rate as without these polymers. The rates based on X-ray analysis are shown in Figure 9. 3.3. Crystallization in the Presence of Other Hydroxyl-Containing Substances. Smaller molecules containing hydroxyl groups were also examined. These included common alcohols (methanol and ethanol), ethylene glycol, and glycerol, as well as dextran and maltodextran. Only calcite was obtained at the end of crystallization, and aragonite was not observed at any stage of the crystallization. 4. Discussion The first crystalline form of calcium carbonate seen in every case examined was vaterite, irrespective of the type of polymer present, its concentration, or even without a polymer being present. The differences observed occurred after this initial vaterite formation. Of all of the additives studied, only PVA significantly guided the final calcium carbonate polymorph away from calcite and had a strong effect on the transformation rate. Without an additive and with all other additives, only calcite formed in water solutions. As mentioned, all crystals described nucleated and grew in the bulk solutions, away from the vessel walls.

good

PAAm PEO

PVA, (PiPAAm) PVP, (PiPAAm)

Table 5. Flory-Huggins Interaction Parameters (χ) with Watera polymer χ a

C ) calcite.

poor

PAAm ≈0

PEO ≈0.4b

PiPAAm 0.50c

PVA 0.49

PVP ≈0.5

25 °C. b Room temperature. c 31 °C.

4.1. Crystal Growth in the Presence of PVA. In aqueous solutions, only PVA affected the final polymorph of calcium carbonate after 9 days or more, stabilizing metastable aragonite or vaterite over calcite. Two pertinent characteristics of PVA in water are its ability to hydrogen bond and its tendency to adsorb nonspecifically onto solid surfaces. The latter occurs from water being a relatively poor solvent for PVA, with the solutions being close to a theta condition (with the Flory-Huggins solution parameter, χ, being close to 0.5) over a range of temperatures that includes room temperature. The ability to hydrogen bond and the tendency to adsorb onto surfaces is not shared by any of the other additives studied (Tables 4 and 5). A possible exception is PiPAAm, although steric hindrance around the active hydrogen of PiPAAm makes the interactive ability of PiPAAm questionable. Of the polymers examined, PVA, PiPAAm, and PVP share the characteristic of having Flory-Huggins χ values equal to or nearly equal to 0.5 (Table 5), and therefore each tends to extensively adsorb nonspecifically onto solid surfaces from water.23 A dense polymer layer is adsorbed under these conditions.24 This is likely related to the small radius of gyration of the polymer in a poor solvent.25 For the other polymers, water is a good solvent, and there is little tendency for extensive nonspecific adsorption onto solid substrates.26 Also, the specific interaction between PVA and calcium carbonate through hydrogen-bonding, probably to the carbonate ions, allows a high density of polymer chains right near the interface by increasing the number of segments that are intimately attached to the solid.26 To further test the significance of polymer adsorption, the solution state of PVA was varied by changing its degree of hydrolysis and molecular weight.27 Increased solubility in water is obtained by a slight lowering of the degree of hydrolysis, from 98 to 88%. With less degree of hydrolysis, some of the intersegmental hydrogen bonding within PVA is reduced, and the PVA becomes more soluble. Forming a better solution in water, 88%-hydrolyzed PVA is therefore expected to have less tendency to adsorb on solid substrates. And indeed, aragonite did not formed from a 0.10 M CaCO3 solution containing an 88%-hydrolyzed PVA, although this PVA had the same molecular weight (ca. 18 000 g/mol) and concentration (0.50%) as a nearly fully hydrolyzed (98% hydrolysis) PVA that did form aragonite. When the molecular weight was increased to ca. 120 000 g/mol, which reduced solubility, the 88%hydrolyzed PVA was then able to form aragonite. The long induction period required to form aragonite suggests an inhibition of calcite crystal growth, and this was confirmed by monitoring calcite formation (0.10 M) with different amounts of PVA (18 000 g/mol, 98% hydrolyzed). Thus, 0.050% PVA, which did not inhibit

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Figure 10. Calcite “staircase” dendrites formed with 0.50% PVA after 30 min. Surrounding crystals are parts of vaterite spheres.

calcite formation completely, did slow the crystallization rate considerably, as can be seen in Figure 9. Complete transformation of vaterite into calcite took about 4 days, rather than an hour without PVA. With higher PVA concentrations, the fraction of calcite also decreased: ca. 5.9% calcite with 0.25% PVA, ca. 2.2% calcite with 0.50% PVA, and ca. 1.7% calcite with 1.0% PVA. These are the amounts of calcite that formed early in the crystallization, within the first 3 min. They then remained constant throughout the vaterite-to-aragonite transformation, with further growth seeming to be completely inhibited. In addition, the calcite crystals that formed had an unusual stairsteplike dendritic morphology, as seen in Figure 10. This morphology appears when impurities inhibit the growth of crystals in normal directions.28 The corners between faces have a greater chance to break out and resume normal growth until impurities block the growth again. The repetition of this process yields these stairstep crystals. Aragonite formation was also retarded with PVA, as shown in Figure 5. After 5 days, more than half of the vaterite had converted to aragonite with 0.25% PVA, but only about 7% had converted to aragonite with 1.0% PVA. Although aragonite crystal growth was inhibited by PVA, it was less so than that of calcite. This difference may arise from the dynamic nature of polymer adsorption and crystal growth. In particular, desorption of polymer molecules is facilitated by the dissolution of the crystal surfaces. Therefore, inhibition by polymer adsorption (without specific motif match) was less effective with the more soluble aragonite. Similar behavior is seen in other systems. For example, the more soluble phases of calcium phosphate are also less inhibited during growth by various geological and biological inhibitors,29,30 and the more soluble phase of lead azide is found to form as the major phase in the presence of organic dyes, probably again through inhibition of the more insoluble phase.31 Certainly to the extent that the vaterite was able to redissolve, and to coarsen, if that occurred by dissolution and recrystallization, its crystallization seemed least inhibited by PVA. The formation of vaterite even in the midst of a high concentration of PVA may also indicate this, although the vaterite may have formed before inhibition by polymer adsorption could occur. Also, the vaterite

Kim et al.

may have arisen from the direct transformation from amorphous calcium carbonate.18 Inhibition by PVA adsorption was also observed with the other two concentrations of CaCO3, 0.020 and 0.50 M. With 0.020 M CaCO3 and 0.0005% PVA (98% hydrolysis), calcite growth was inhibited enough to reduce the size of the calcite crystals that formed. With 0.0050% PVA, calcite growth was further inhibited, and aragonite formed nearly exclusively. With 0.050% PVA, both calcite and aragonite formation were inhibited, leaving mostly the vaterite. Another effect of increased PVA concentration is the change of solution quality. The solution approaches closer to a theta-condition as the polymer concentration increased.23 This seemed to have occurred with 88%hydrolyzed PVA (MW 18 000 g/mol). The adsorption (and inhibition) ability increased sufficiently on increasing the polymer concentration from 0.50 to 5.0% to allow formation of aragonite. As the concentration of calcium carbonate increased, the amount of PVA needed to inhibit calcite and to form aragonite increased. To form nearly 100% aragonite, roughly 0.0050% PVA was needed with 0.020 M CaCO3, 0.50% PVA was needed with 0.10 M CaCO3, and not even 5.0% PVA was sufficient with 0.50 M CaCO3. With 0.50 M CaCO3 and 5.0% PVA, a substantial amount of calcite was present along with the aragonite after 9 days, as well as a substantial amount of vaterite. This could have resulted from the very large number of nuclei/crystals that PVA should adsorb onto. But it is also possible, even likely, that the calcite had formed directly, without going through vaterite, because of the high calcium carbonate supersaturation. If formed very soon after mixing the calcium and carbonate solutions, well before the first observation at 3 min, this formation would have been less easily inhibited by polymer adsorption. When calcite forms through vaterite, the process is necessarily slower because the amount of calcium carbonate in solution is that in equilibrium (at least approximately) with vaterite. A byproduct of increasing polymer concentration is an increase in viscosity, which seemed to affect crystal size. The aragonite needles obtained with 0.50% PVA (88%-hydrolysis, MW 120 000 g/mol) were two to three times thicker than those with 0.050% PVA. Also, when 0.050% PVA (88%-hydrolysis, MW 120 000 g/mol) was added together with 5.0% PEO, aragonite needles became 10-20 times thicker than those with only 0.050% PVA. Most studies into the formation of the calcium carbonate polymorphs have been concerned with nucleation rather than with growth inhibition. Nucleation was not specifically studied in the present work. Nucleation in the present work appeared to have occurred on unseen particles that seemed not to have been very specific to any particular polymorph, especially between aragonite and calcite. However, we did note that without the stirring during the first 2 days, little aragonite formed, leaving the vaterite intact. Presumably, the stirring multiplied the number of aragonite nuclei, a common phenomenon observed with other crystals. 4.2 Crystal Growth in the Presence of Polyacrylamide (PAAm). In water solutions of PAAm, only calcite was formed. Extensive adsorption on solid sub-


strates is not expected because water is a very good solvent for PAAm. However, a 4:6 methanol/water mixture is a theta solvent at 20 °C for PAAm, and extensive adsorption of PAAm on solid surfaces was expected, similar to PVA in aqueous solution.22 And indeed, calcite formation was completely inhibited with 0.50% PAAm. However, over the course of 4 months, no aragonite had formed either. The methanol itself may have significantly reduced the transformation rate by reducing the dielectric constant of the mixed solvent. Then, the dissolution of the initially formed phase (vaterite), which is a prerequisite of the solvent-mediated transformation, would have been drastically retarded. Even without PAAm, the vaterite-to-calcite transformation took nearly 10 times longer (9 h) than in pure water (1 h). Another possible source of retardation could arise from the amino group, which may significantly reduce the rate of desorption of polymer segments due to its bidendate interaction ability. This would reduce the rate of vaterite dissolution further. In contrast to 40 vol % methanol, calcite did form when the methanol fraction was 30%, probably due to this being a better solvent for PAAm. Also, when the concentration of PAAm in the 4:6 methanol-water mixture was decreased from 0.50 to 0.050% and to 0.0050%, the calcite inhibition progressively disappeared. When PAAm was further decreased to 0.0005%, the calcite morphology was unaffected. 4.3. Calcite Growth and Morphology in the Presence of Nonionic Polymers. Although only PVA was able to steer crystallization toward aragonite, the other polymers did affect crystallization, often both the morphology and the growth rate of the calcite that formed. Even PVA reduced the calcite crystal size when less was added than needed to inhibit calcite formation. When enough PVA was added to inhibit calcite, the small fraction of calcite that did form had a stairstep morphology, as explained above. PAAm dramatically changed the calcite crystal morphology (Figure 8a), probably due to a strong specific interaction with calcium carbonate. Although the interaction was enough to disturb the formation of typical calcite rhombohedra, the high water solubility of PAAm reduced the tendency to adsorb and inhibit calcite growth. Besides complex crystal shapes and textures being obtained, parts of the crystals showed a stairstep character similar to that obtained with PVA. Also, during the vaterite-to-calcite transformation, some vaterite crystals seemed to be closely associated with the growing calcite crystals, leaving concave imprints in the calcite crystals after dissolution of the vaterite. PiPAAm (Figure 8b) and PVP (Figure 8c) formed very irregular (although still more or less rhombohedral) calcite crystals, probably due to uneven secondary nucleation as well as an aggregation of nuclei/crystals during crystal growth. This may have originated from these polymers’ abilities to adsorb nonspecifically onto solid substrates, although a lack of specific interaction reduced their ability to inhibit calcite formation. In contrast, PEO allowed the formation of large, welldefined calcite crystals while seeming to speed their growth. PEO has neither specific interaction nor nonspecific adsorption ability. The large, well-defined crystals may simply be an effect of viscosity. The PEO used


had the highest molecular weight (300 000 g/mol) among the polymers studied. The elevated solution viscosity could have reduced secondary nucleation that might arise from crystal-crystal or crystal-stirrer/wall contacts, allowing the formation of better-shaped crystals of increased size. A similar effect of viscosity on size was observed in the morphology of aragonite with PVA. The effect of these polymers on calcite growth rate (Figure 9) shows little correlation with the morphology. Instead, growth rate, except for PVA, seems more dependent on the molecular weight of the polymer, and probably therefore solution viscosity. The higher the molecular weight, the more the rate was accelerated. Thus, growth rate was highest with PEO, next highest for PVP, and relatively unaffected by PAAm and PiPAm, the two polymers having the lowest molecular weight. 4.4 Implication in Biomineralization. The biomacromolecules involved in the formation of aragonite in biological systems may behave in a similar way to that of PVA. Studies with biomolecules have shown the same inhibition of calcite crystal growth as seen with PVA. When soluble proteins derived from the nacre of the red abalone were added to supersaturated calcium carbonate solution with seed calcite crystals, the calcite growth steps became roughened and their rate of growth was reduced. With further addition of the proteins, calcite growth ceased, and aragonite crystals began to grow on top of the seed calcite crystals.4,32 Also, the nacreous proteins from the Japanese pearl oyster and sawtooth penshell showed similar concentration-dependent calcite inhibition. The amount of calcite crystals formed was reduced as the protein concentration in the supersaturated calcium carbonate solution increased.33,34 Interestingly, the morphology of the calcite crystals formed in the presence of synthetic peptides representing N- and C-terminal sequences (30 amino acids each) of the Japanese pearl oyster proteins was similar to the stairstep dendritic structure arising with PVA (Figure 10).35 Another piece of evidence supporting the growth of aragonite through inhibition involves the repair of the aragonitic shell of a freshwater snail. The repair took about 10 days,36 which is about the same time as for the inhibited vaterite-to-aragonite transformation that was seen with PVA. The role of the biomacromolecules in all of these observations has been suggested to be the nucleation of aragonite through epitaxy.2,33,37 However, one suggested epitaxy has a mismatch of ca. 8.8%.2 In a recent study of the epitaxial precision needed to grow aragonite under calcite-favorable conditions, we found the maximum epitaxial strain allowing aragonite to form was less than 7.1%, and this was when the nucleating substrates were rigid inorganic crystals isomorphous to aragonite.38 Hence, the more likely or equally likely role played by the biomacromolecules would seem to be that of inhibiting the growth of calcite. Like PVA, biomacromolecules contain hydroxyl, amino, and amide groups that can donate protons for hydrogen bonding (and additional acidic groups for electrostatic interaction), and the solution properties of biomacromolecules in water can be such as to allow extensive adsorption onto any solid surfaces that form. 5. Conclusions In summary, near exclusive formation of aragonite was attained through growth inhibition of the more

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stable calcite with PVA. The contributing characteristics of PVA seemed to be its extensive adsorption and its ability to hydrogen bond. Growth of nascent calcite nuclei was probably prevented by the PVA forming an ion diffusion barrier at least in part by way of its hydrophobic moiety. Similar inhibitory activity is suggested for various biomacromolecules involved in biogenic aragonite formation of mollusks, with the biomacromolecules suggested to act as does PVA. The control of phase transformation is a common phenomenon in biologically mineralized systems, and many or most of these may arise from inhibition.39 Also, as in the present study, polymer adsorption seems to be useful as a general strategy to modulate the kinetics of solvent-mediated polymorphic transformation. References (1) Mann, S.; Archibald, D. D.; Didymus, J. M.; Douglas, T.; Heywood, B. R.; Meldrum, F. C.; Reeves, N. J. Science 1993, 261, 1286. (2) Weiner, S.; Traub, W. FEBS Lett. 1980, 111, 311. (3) Falini, G.; Albeck, S.; Weiner, S.; Addadi, L. Science 1996, 271, 67. (4) Belcher, A. M.; Wu, X. H.; Christensen, R. J.; Hansma, P. K.; Stucky, G. D.; Morse, D. E. Nature 1996, 381, 56. (5) Weiner, S.; Addadi, L. J. Mater. Chem. 1997, 7, 689. (6) Cardarelli, F. Materials Handbook: A Concise Desktop Reference. Springer-Verlag: London, 2000; Chapter 11. (7) Zaremba, C. M.; Morse, D. E.; Mann, S.; Hansma, P. K.; Stucky, G. D. Chem. Mater. 1998, 10, 3813. (8) Lopez, E.; Vidal, B.; Berland, S.; Camprasse, S.; Camprasse, G.; Silve, C. Tissue Cell 1992, 24, 667. (9) Atlan, G.; Balmain, N.; Berland, S.; Vidal, B.; Lopez, E. C. R. Acad. Sci. Paris/Life Sci. 1997, 320, 253. (10) Litvin, A. L.; Valiyaveettil, S.; Kaplan, D. L.; Mann, S. Adv. Mater. 1997, 9, 124. (11) Ku¨ther, J.; Nelles, G.; Seshadri, R.; Schaub, M.; Butt, H.J.; Tremel, W. Chem. Eur. J. 1998, 4, 1834. (12) D’Souza, S. M.; Alexander, C.; Carr, S. W.; Waller, A. M.; Whitcombe, M. J.; Vulfson, E. N. Nature 1999, 398, 312. (13) Hosoda, N.; Sugawara, A.; Kato, T. Macromolecules 2003, 36, 6449. (14) Lakshminarayanan, R.; Valiyaveettil, S.; Loy, G. L. Cryst. Growth Des. 2003, 3, 953. (15) Naka, K.; Keum, D.-K.; Tanaka, Y.; Chujo, Y. Chem. Comm. 2000, 1537. (16) Naka, K.; Chujo, Y. Chem. Mater. 2001, 13, 3245.

Kim et al. (17) Lippmann, F. Sedimentary Carbonate Minerals; SpringerVerlag: Berlin, 1973. (18) Addadi, L.; Raz, S.; Weiner, S. Adv. Mater. 2003, 15, 959. (19) Kontoyannis, C. G.; Vagenas, N. V. Analyst 2000, 125, 251. (20) Kawano, J.; Shimobayashi, N.; Kitamura, M.; Shinoda, K.; Aikawa, N. J. Cryst. Growth 2002, 237-239, 419. (21) Rao, M. S. Bull. Chem. Soc. Jpn. 1973, 46, 1414. (22) Elias, H. G. in Polymer Handbook; Brandup, J.; Immergut, E. H., Eds.; John Wiley & Sons: New York, 1989; p 216. (23) Barton, A. F. M. CRC Handbook of Polymer-Liquid Interaction Parameters and Solubility Parameters; CRC Press: Boca Raton, FL, 1990. (24) Kawaguchi, M.; Hayakawa, K.; Takahashi, A. Macromolecules 1983, 16, 631. (25) Doi, M.; Edwards, S. F. The Theory of Polymer Dynamics; Oxford University Press: New York, 1986; Chapter 2. (26) Fleer, G. J.; Scheutjens, J. M. H. M.; Cohen Stuart, M. A. Colloids Surf. 1988, 31, 1. (27) Toyoshima, K. in Polyvinyl Alcohol: Properties and Applications; Finch, C. A., Eds.; John Wiley & Sons: New York, 1973; Chapter 2. (28) Walton, A. G. The Formation and Properties of Precipitates; Interscience Publishers: New York, 1967; Chapter 5. (29) Grossl, P. R.; Inskeep, W. P. Soil Sci. Soc. Am. J. 1991, 55, 670. (30) Eidelman, N.; Brown, W. E.; Meyer, J. L. J. Cryst. Growth 1991, 13, 643. (31) Miles, F. D. Philos. Trans. R. Soc. London, Ser. A 1935, 235, 125. (32) Thompson, J. B.; Paloczi, G. T.; Kindt , J. H.; Michenfelder, M.; Smith, B. L.; Stucky, G.; Morse, D. E.; Hansma, P. K. Biophys. J. 2000, 79, 3307. (33) Samata, T.; Hayashi, N.; Kono, M.; Hasegawa, K.; Horita, C.; Akera, S. FEBS Lett. 1999, 462, 225. (34) Levi, Y.; Albeck, S.; Brack, A.; Weiner, S.; Addadi, L. Chem. Eur. J. 1998, 4, 389. (35) Kim, I. W.; DiMasi, E.; Evans, J. S. Cryst. Growth Des. 2004, 4, 1113. (36) Wilbur, K. M.; Watabe, N. Ann. N. Y. Acad. Sci. 1963, 109, 82. (37) Morse, D. E.; Cariolou, M. A.; Stucky, G. D.; Zaremba, C. M.; Hansma, P. K. Mater. Res. Soc. Symp. Proc. 1993, 292, 59. (38) Kim, I. W.; Robertson, R. E.; Zand, R. Adv. Mater. 2003, 15, 709. (39) Mann, S.; Webb, J.; Williams, R. J. P., Eds.; Biomineralization: Chemical and Biochemical Perspectives; VCH: Weinheim, Germany, 1989.

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