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Mar 20, 2014 - (Figure S4C). □ DISCUSSION. The experiments demonstrate that the effects of polyamines on. CaCO3 precipitation depend upon many ...
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Systematic Study of the Effects of Polyamines on Calcium Carbonate Precipitation Anna S. Schenk,† Bram Cantaert,† Yi-Yeoun Kim,† Yuting Li,‡ Elizabeth S. Read,‡ Mona Semsarilar,‡ Steven P. Armes,‡ and Fiona C. Meldrum*,† †

School of Chemistry, University of Leeds, Woodhouse Lane, Leeds, LS2 9JT, U.K. Department of Chemistry, University of Sheffield, Brook Hill, Sheffield, S3 7HF, U.K.

‡

S Supporting Information *

ABSTRACT: While negatively charged organic additives are widely used as an effective means to control CaCO 3 precipitation, positively charged additives are generally considered to be much less active. Nevertheless, the cationic polyelectrolyte poly(allylamine hydrochloride) has recently been shown to exert significant control over CaCO 3 precipitation, driving the formation of thin films and fibers, and other examples suggest that many positively charged additives promote vaterite formation. This article aims to bring together these sometimes conflicting views of the activity of positively charged additives. The effect of a series of polyamines on CaCO3 precipitation was studied, where the polyamines were selected such that the amine group type, the pKa value (of the corresponding conjugated acid), the molecular weight, and the side chain length of the polymers could be evaluated. The results unambiguously demonstrate that polyamines carrying primary amine groups are capable of exerting a significant effect and that the activity of this class of polyamines is strongly dependent on the length of the side chain. In contrast, polyamines comprising with quaternary amines have negligible effect, despite carrying a permanent positive charge. The activity of the most active polyamines therefore depends on their ability to complex with carbonate ions present in solution, and electrostatic attraction alone is not sufficient.

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precipitation.16−18 Indeed, the basic amino acids lysine and arginine are also commonly found in mineralization-directing biomacromolecules, and the terminal sequences of nacreoccluded biomacromolecules can exhibit either net negative or positive charges.16 Many synthetic positively charged additives have little effect on CaCO3 crystallization. A few polymers with amine side groups have been reported to stabilize vaterite with respect to calcite,19,20 while poly-L-lysine21,22 and a cationic 16residue peptide23 caused relatively minor changes in calcite morphology. In contrast, modeling work has suggested that arginine residues are the most important binders of the chicken eggshell protein ovocleidin-17 to calcite,24 and aggregates of poly(propylene imine) dendrimers that were modified with octadecylamine stabilized amorphous calcium carbonate (ACC), with this behavior being attributed to their structural rigidity.25 We have also recently demonstrated that the synthetic polyelectrolyte, poly(allylamine hydrochloride) (PAH), can induce the formation of CaCO3-based thin films and fibers.26,27 These observations were attributed to phase separation based on the electrostatic interaction between the protonated amine

INTRODUCTION The sophisticated architectures and superior mechanical performance of many biominerals have inspired efforts to translate the design principles of biomineralization to synthetic crystallization processes.1,2 The use of soluble and insoluble biopolymers to control mineralization is arguably the most important strategy that has arisen from this approach.3−5 Water-soluble organic molecules that mimic key features of the biomolecules associated with biominerals are often employed to control the crystallization of inorganic materials. In some cases these organic additives have provided excellent control of the size, polymorph, morphology, and nanostructure of the mineral phase.6 Polyelectrolytes such as poly(aspartic acid) and copolymers with anionic blocks have proven to be particularly potent additives for such bioinspired mineralization studies,7−12 due in part to their ability to sequester cations and to associate with mineral surfaces. In the case of calcium carbonate, an abundant biomineral, research has principally focused on the influence of anionic additives as crystal habit modifiers.1 Synthetic polyanions bear many similarities to the naturally occurring biomacromolecules occluded within CaCO3 biominerals, which are typically highly acidic because of their high aspartic and glutamic acid contents.13−15 A small number of studies have also addressed the potential role of cationic additives in controlling CaCO3 © 2014 American Chemical Society

Received: February 12, 2014 Revised: March 19, 2014 Published: March 20, 2014 2703

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CaCO3 particles was rinsed in first water and then ethanol before being left to air-dry. Control experiments were carried out under the same general conditions but in the absence of polymeric additives. Overgrowth Experiments. Glass slides supporting calcite crystals precipitated in the presence of poly(vinylamine) were immersed in 10 mL of a fresh 10 mM CaCl2 solution in the absence of any polyamine additive. After 1 day of exposure to ammonium carbonate vapor, the overgrown particles were removed from the reaction solution and rinsed with excess water and ethanol prior to characterization studies. Characterization of Calcium Carbonate. The CaCO3 crystals were investigated using a range of microscopy techniques. Light microscopy and polarized optical microscopy (POM) were used to probe the crystallinity and particle morphology using a Nikon Eclipse LV 100 microscope. The crystal morphology and surface texture were studied by scanning electron microscopy (SEM) using a LEO 1530 Gemini FEG-SEM instrument. Secondary electron imaging was performed in the in-lense detector mode at an acceleration voltage of 3 kV, while images based on contrast generated by backscattered electrons were recorded using the BSD detector mode at an acceleration voltage of 25 kV. The crystals were generally examined on glass substrates, which were attached to aluminum sample holders with adhesive carbon pads after coating with a 10 nm Pd/Pt overlayer. SEM was also used to image cross sections of crystals, which had been fractured between the glass substrate they were grown on and another clean glass cover slide pressed down from the top. Cross sections were also prepared by embedding glass slides supporting crystals in TEM epoxy resin (Gatan G1) cured at 145 °C and subsequently mechanically polishing them using diamond lapping pads with grain sizes ranging from 3 μm to 500 nm; such samples were examined using backscattered SEM and polarized light microscopy. Crystals were also analyzed using Raman spectroscopy, where measurements were performed using an inVia Raman microscope equipped with a 785 nm diode laser (Renishaw Raman 2000). The laser was focused onto individual crystals using a 50× objective lens (numerical aperture NA 0.75), and 20 randomly selected crystals were typically analyzed for each sample.

groups on the polymer and the negatively charged carbonate ions in the solution. PAH therefore acts in a similar way to anionic polyelectrolytes such as poly(aspartic acid) (PAsp)7 and poly(acrylic acid) (PAA),28 where a fluid-like phase separates from the reactant solution as a result of the interaction between the carboxylate groups on the polymer and the Ca2+ cations.29,30 The current literature therefore presents no consensus regarding the effects of positively charged additives on CaCO3 precipitation. This article addresses this issue and presents a study of the influence of a wide range of synthetic polyamines varying in terms of their molecular weight, side chain length, and nature of the amine groups on CaCO3 precipitation. Particular emphasis was placed on identification of the essential molecular features that promote the formation of mineral thin films and fibers. Our results clearly demonstrate that macromolecules comprising primary amine groups are capable of exerting strong effects and promote the deposition of mineral crystals with nonequilibrium morphologies. Tertiary and quaternary amines, in contrast, have little or no effect. The activity of polyamines is therefore dependent on their ability to complex with carbonate ions present in solution, and an electrostatic attraction alone is not sufficient.

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EXPERIMENTAL SECTION

As only a limited number of polyamines are commercially available, our selection was made based on which polyamines could be purchased and readily synthesized (using existing protocols), as described below. Chemicals. All aqueous solutions were prepared using deionized water (Milli-Q Standard, resistivity of 18.2 MΩ cm). If not specified otherwise, commercially available reagents were purchased as analytical grade and were used without further purification. CaCl2·2H2O, MgCl2· 6H2O, (NH4)2CO3, and poly(diallyldimethylammonium chloride) (PDADMAC, Mw < 100 000 g mol−1, 35 wt % aqueous solution) were ordered from Sigma-Aldrich. Poly(vinylamine) (PVAm, Mw = 340 000 g mol−1) was obtained from BASF (trade name, Lupamin) and was dialyzed in water for 2 days to remove any residual impurities, followed by freeze-drying from water overnight. As a result of this cleanup protocol, the salt content bound by the polymer, and thus the amount of free amine groups, is unknown. An equivalent weight (EW = mass of polymer per mole of titratable amine) of EW = 83.3 g mol−1 was determined via conductometric titration by Wen et al. for freeze-dried PVAm obtained from the same source.31 The concentrations in our standard reaction setup therefore correspond to an amine/calcium ratio of [NH2]/[Ca2+] = 1.2. Poly(ethylene imine) (PEI; linear form, Mw = 25 000 g mol−1 and branched form either Mw = 1200 g mol−1 or 50 000−100 000 g mol−1) were obtained from Alfa Aesar. Homopolymers and block copolymers based on 2-aminoethyl methacrylate (AMA) (see Table 1) were prepared according to previously published protocols.32−34 The structures of the additives used in this study are summarized in Table 1, and a brief description of the synthesis and characterization of the bespoke polyamines is given in the Supporting Information. Note that the (poly(tert-butylaminoethyl methacrylate) was considered as a polyamine with secondary amine functionality, but owing to its insolubility in water at pH 8 or above, it was not suitable for this study. Precipitation of Calcium Carbonate in the Presence of Polyamines. Calcium carbonate was precipitated in turn in the presence of each of the polyamines shown in Table 1 using the wellknown ammonium carbonate diffusion technique.40 Briefly, a solution of CaCl2·2H2O and polymer was placed in a Petri dish, into which a clean glass substrate was inserted. Subsequently, the dishes were covered with Parafilm and placed in a sealed desiccator, where they were exposed to ammonium carbonate vapor. The reactions were typically allowed to proceed for 4 days. When removed from the reaction solution, each glass slide supporting the freshly formed

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RESULTS Calcium carbonate was precipitated in the presence of the water-soluble polyamines shown in Table 1 using the ammonium carbonate diffusion method40 under the following fixed conditions: [Ca2+] = 10 mM and [polymer] = 1 g L−1. Control experiments performed under identical conditions but in the absence of organic additives yielded rhombohedral calcite crystals (Figure S1 in Supporting Information). As a comparison, poly(allylamine hydrochloride) (PAH) induced the formation of single as well as polycrystalline calcite films and fibers (Figure 1), with little or no effect of molecular weight being observed for PAH samples of 15 000 g mol−1 and 56 000 g mol−1. This system has been extensively described in previous publications,26,27 and images are presented here as a reference to enable convenient comparison with the entire set of poly(amines) employed here. Experiments were then conducted with poly(vinylamine) (PVAm), a primary amine functionalized polymer that differs from PAH by having just one fewer methylene unit in the side chain. In common with PAH, PVAm caused dramatic morphological effects (Figure 2). Spherical CaCO3 particles were formed from which fibers subsequently grew to produce a thistle-like morphology (Figure 2A and Figure 2B). SEM studies indicated that most fibers measured up to 40 μm in length and were straight, but bent fibers (and also fibers that were branched or intergrown) were also frequently observed (Figure 2C). Raman spectroscopy confirmed that all CaCO3 crystals precipitated in the presence of PVAm were calcite (Figure 3) as identified by the characteristic vibration bands 2704

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Table 1. Structures and Molecular Weights of the Poly(amines) Used in the Studya

a

Reference data for the pKa values of the polymers were obtained from the following literature sources: PAH,35 PVAm,36 PAMA,32 PAMA-PEO,32 PEI (linear),37 PDMAEMA,38 and PEI (branched).37 Polymers comprising amine-functionalized methacrylate subunits can undergo degradation by either inter- or intramolecular amidation reactions or alkaline ester hydrololysis under alkaline conditions. However, a detailed study by Thompson et al. addressing the degradation of PAMA in aqueous alkaline solutions has clearly shown that a 2.0 wt % solution of PAMA49 ([PAMA] = 20 g L−1) was stable for a period of at least a week at a temperature of 25 °C and pH 9 39 and that the degradation rate was accelerated when higher amounts of polymer were dissolved. These observations show that minimal degradation of the polymer would be expected under the experimental conditions applied here, where these employ a much lower polymer concentration and the reaction is completed after 4 days. 2705

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Figure 3. Raman spectra of representative CaCO3 particles precipitated from a 10 mM Ca2+ solution in the presence of 1 g L−1 PAMA (n = 200) or PVAm (calcite = C, vaterite = V).

Figure 1. Polarized light microscopy image and scanning electron micrograph (inset) of a polycrystalline CaCO3 film and protruding fibers obtained under conditions of [Ca2+] = 10 mM and 1 g L−1 PAH after 1 day. The white arrows indicate small fibers growing from the polycrystalline film.

minor degree of misorientation (Figure 4B), suggesting that the fibrous protrusions were predominantly single crystals. Cross sections of the calcite/PVAm particles were also examined by embedding them in epoxy resin followed by mechanical thinning and polishing (Figure 4C). Backscattered electron microscopy (BSE) of these embedded sections showed variations in contrast that were consistent with the presence of polymer-enriched domains within the CaCO3/PVAm crystals (Figure 2F (overview) and Figure S2). It is interesting to note that those particles that appeared more faceted by morphology showed contrast variations in sectors while others were seemingly spherulitic exhibiting concentric laminated structures with alternating layers of higher and lower polymer concentration. Similar crystal textures have previously been observed by Dai et al.44 Finally, the influence of the [Ca2+]/[PVAm] molar ratio on the nature of the precipitated CaCO3 was also investigated, albeit using [Ca2+] = 5 mM because of solubility limitations associated with this particular polymer. At [PVAm] = 1 g L−1,

appearing at 1089 cm−1 (ν1 CO32−), 716 cm−1 (ν4 CO32), 284 cm−1, and 158 cm−1 (lattice modes).41 In addition to these particles, small, isolated patches of a crystalline film were also observed (Figure 2D), although these were far less extensive than those produced using PAH26 or the anionic polyelectrolytes PAsp and PAA.7,28,30 To compare with the effects of PAH, the structures of the rounded particles and the fibers they supported were investigated using a range of methods. Both the external particle surface and the internal fracture surfaces (prepared by gently crushing the crystals) (Figure 2E) appeared to be rough and granular, suggesting an aggregation-based growth mechanism. Crystal overgrowth was used to examine the crystallographic structure,42,43 where the orientations of the epitaxially grown, overgrowth calcite rhombohedra demonstrated that the spherical particles were polycrystalline (Figure 4A). In contrast, the crystals deposited on the fibers were coaligned with only a

Figure 2. CaCO3 precipitated in the presence of poly(vinylamine) (PVAm). (A) Light microscopy shows thistle-like mineral particles with pronounced fibrous outgrowths. (B) SEM image of a CaCO3/PVAm particle. (C) A higher magnification electron micrograph of a bundle of intergrown fibers. (D) In some cases isolated, flat, crystalline patches were observed on the substrate. (E) A fracture surface of a spherical calcite/ PVAm particle showing pronounced roughness. (F) Back-scattered electron microscopy image recorded for calcite/PVAm particles, which were embedded in epoxy resin and finely polished prior to inspection. Areas exhibiting an accumulation of elements with a high atomic number (here calcium) appear bright. 2706

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In order to obtain further information regarding the ability of primary amine based polymers to control CaCO3 precipitation, PAMA200 and the related diblock copolymer, PAMA60-PEO45, were also employed under the same reaction conditions ([Ca2+] = 10 mM, [polymer] = 1 g L−1). Such polymers exhibit longer side chains than either PAH or poly(vinylamine) (Table 1). The sample prepared in the presence of the PAMA200 homopolymer comprised a 1:4 mixture of 30 μm spherical vaterite particles and 30−40 μm elongated, cigarshaped calcite crystals (Figure 5A and Figure 5B), where the

Figure 4. SEM images of the overgrowth of CaCO3/PVAm particles, as carried out in a reaction solution with [Ca2+] = 10 mM. (A) The particles are covered by calcite crystals without an obvious mutual orientation. (B) Overgrowth of the fibers results in largely aligned rhombohedral calcite crystals. (C) Under crossed polarizers the cores of finely polished calcite/PVAm particles embedded in epoxy resin are clearly polycrystalline.

NH2/Ca2+ = 2.4, the resulting calcite particles were clearly aggregated and exhibited irregular, rounded morphologies (Figure S3A) and markedly fewer associated fibers, compared with particles generated under the conditions shown in Figure 2A. Calcite particles prepared in the presence of 10-fold less polymer (i.e., [PVAm] = 0.1 g L−1, NH2/Ca2+ = 0.24) also appeared aggregated and showed irregular, segmented structures, but there was also some evidence for crystal faces (Figure S3B). Fiber formation was not observed under these conditions.

Figure 5. Calcium carbonate precipitated in the presence of PAMA and the related block copolymer PAMA60-PEO45 at [Ca2+] = 10 mM. (A) Light microscopy image of CaCO3 particles formed with 1 g L−1 PAMA (Mw = 33 150 g mol−1, n = 200): V = vaterite and C = calcite. (B) SEM image of calcite particles precipitated as in (A). (C) SEM images of calcite particles precipitated in the presence of 1 g L−1 PAMA60-PEO45. Inset, scale bar = 10 μm. 2707

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different polymorphs were again distinguished by Raman microscopy (Figure 3). On lowering the polymer concentration to [PAMA200] = 0.5 g L−1, the degree of elongation of the calcite particles was reduced and the crystals became more faceted, while a further reduction to [PAMA200] = 0.1 g L−1 led to rhombohedral calcite crystals with pronounced surface roughness. The proportion of vaterite was also systematically decreased on lowering the PAMA/Ca2+ molar ratio. PAMA60PEO45 similarly led to the formation of elongated calcite crystals, where these were capped with well-defined facets (Figure 5C). In a second series of experiments, the PAMA molecular weight was varied while the NH2/Ca2+ molar ratio remained constant. While PAMA200 generated cigar-shaped crystals, PAMA42 and PAMA34 had relatively little effect, yielding only rhombohedral calcite crystals with very slightly truncated edges and roughened surfaces (Figure S4). This clearly shows that a sufficiently long polymer chain is essential to exert a significant influence over CaCO3 growth, suggesting that the primary amine groups may act cooperatively in this context. The influence of amine group substituents (i.e., secondary, tertiary, or quaternary amines vs primary amines) on the ability to control in situ CaCO3 precipitation was then investigated. Poly(ethylene imine) (PEI) is a linear polymer exhibiting secondary amine groups. Addition of this macromolecule to the reactant solution at [Ca2+] = 10 mM and [polymer] = 1 g L−1 led to the formation of aggregated calcite rhombohedra, together with agglomerated vaterite platelets (∼40%, Figure S5). Vaterite became the predominant phase (90%) at low polymer concentrations (e.g., [PEI] = 0.1 g/L). The behavior of this polymer is consistent with previous observations that some cationic additives19,20,22 induce initial vaterite formation, which slowly transforms into the more stable calcite over time.45 CaCO3 crystals precipitated in the presence of polyamines possessing either tertiary amine (e.g., PDMAEMA, Figure 6A) or quaternized amine groups (e.g., PQDMA or PDADMAC, Figure 6B and Figure 6C, respectively) also exhibited only very minor deviations from calcite grown in the absence of any additives: rhombohedra were obtained with cavities located in the center of the faces. While such hoppertype morphologies are frequently observed under diffusionlimited conditions, their occurrence in the presence of low amounts of anionic polymers has been previously suggested to be the result of nonuniform adsorption of these macromolecules onto the facets of the growing crystals.8 As a final type of polyelectrolyte, branched PEI, which comprises a number of different types of amine side chains, was investigated. This polymer exhibits a mixture of primary, secondary, and tertiary amine groups but only exerts a minor effect on the fundamental calcite morphology, generating distorted rhombohedra, together with some vaterite structures (Figure S4C).

Figure 6. Light microscopy images of calcite crystals grown in the presence of (A) PDMAEMA (tertiary amine), (B) PQDMA (quaternized amine), and (C) PDADMAC (quaternized amine). All particles were isolated from reactant solutions with concentrations of [Ca2+] = 10 mM and [polymer] = 1 g L−1.

study; poly(tert-butylaminoethyl methacrylate) proved to be water-insoluble at pH 8 while linear PEI, which has secondary amines along the chain, promoted the formation of significant amounts of vaterite. Branched PEI, which contains some secondary amines (along with primary amines and tertiary amines), supported the formation of aggregated calcite rhombohedra and vaterite particles. All tertiary amine and quaternary amine functional polymers examined exerted a negligible effect on CaCO3 precipitation, yielding calcite rhombohedra with smooth surfaces in all cases. Depending on their degree of protonation, which is determined by the pH of the reaction solution, primary amines can bind ions from the surrounding solution either by electrostatic interaction between cationic −NH3+ groups and carbonate anions or by chelation of Ca2+ cations to the neutral NH2 groups. Given that the ammonium carbonate diffusion

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DISCUSSION The experiments demonstrate that the effects of polyamines on CaCO3 precipitation depend upon many parameters including the nature of the amine group, the polymer molecular weight, and the precise chemical structure (which in turn dictates the cationic charge density). However, the nature of the amine groups appears to be the dominant effect, with only primary amine based polymers being capable of exerting a significant influence on CaCO3 morphologies. Unfortunately, no suitable secondary amine functionalized polymers were available for 2708

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technique leads to mineral deposition at around pH 9.0−9.5,40 the primary amine groups can be either partially or in some cases almost fully deprotonated. However, our data conclusively demonstrate that the variation in pKa is not the dominant factor in dictating the activity of a polyamine in modifying CaCO3 precipitation (Table 1). Thus, while PAH, PVAm, and PEI all have pKa values of ∼8.5 and therefore possess considerable positive charge at pH 9, the primary amine based polymers, PAH and PVAm, both promote the formation of nonequilibrium morphologies, while the secondary amine functional PEI remains inactive. Similarly, with pKa values of ∼7− 7.6, which render the polymers almost completely uncharged under the alkaline pH conditions at which CaCO3 is precipitated, the presence of primary amine based PAMA leads to precipitation of highly elongated calcite crystals, while the tertiary amine based PDMAEMA has a negligible effect. In contrast, polyamines carrying trimethyl quaternary groups (e.g., −N(CH3)3+) possess a pH-independent positive charge on each repeat unit but exert little or no influence over the calcite morphology. A crystal growth mechanism based on a purely electrostatic interaction between the polycation and charged faces of the forming mineral particles would suggest that quaternized polyamines should be the most efficient in altering the CaCO3 crystal habit. As this was clearly not observed in the present study, we suggest that the capacity for ion complexation, which is associated with the degree of functionalization and the steric accessibility of the (primary) amine groups, is a crucial factor in determining the efficacy of the polyamine additive. Thus, while PAMA and PDMAEMA have similar pKa values and identical side chain lengths, the steric hindrance and the absence of hydrogen bonding of the tertiary amines make it an inferior σ donor, with a reduced capacity for coordination to Ca2+ ions.46 PDMAEMA is thus ineffective as a crystal habit modifier for CaCO3. The coordination of calcium ions therefore appears to be the principal source of interaction of the primary amines with CaCO3, and a merely electrostatic interaction between cationic additives and growing CaCO3 crystals has little effect. In addition to the amine group substituents, the polymer molecular weight and the nature of the side chain (spacer units) also exert a strong effect on the final CaCO3 crystals. Thus, while PAMA200 generated cigar-shaped crystals, neither PAMA42 nor PAMA34 had any significant effect apart from edge truncation and surface roughening. This suggests that the interaction of the polyamine with a growing crystal is promoted by the increased cooperativity afforded by longer chains, and thus depends on the number of amine groups on the chain. The amine groups on longer chains act cooperatively to afford a stronger interaction, therefore causing a greater effect on morphology. The effect of varying the spacer group can be assessed by comparing the performance of PAH with that of PVAm (which has just one fewer methylene group in its side chain). PVAm is less effective than PAH but nevertheless promoted the formation of fibers and small patches of thin films. In contrast, PAMA homopolymers, which possess longer side chains, failed to induce formation of thin films and fibers. Indeed, elongated calcite crystals with morphologies comparable to calcite/PAMA200 and calcite/PAMA60-PEO45 have previously been obtained in the presence of malic acid or citric acid, which are both low molecular weight carboxylic acids.47 Interactions between neighboring amine groups on the polymer chains may also play a role in determining how they

influence CaCO3 precipitation, by limiting their ability to bind to ions or onto the growing crystal surface. For example, while there are very strong interactions between the primary amine groups for PVAm, PAH apparently exhibits weaker interactions.36 The ability of PAH to generate noncrystallographic CaCO3 morphologies has been attributed to phase separation. PAH is known to undergo microphase separation in the presence of carbonate ions (and a range of other anions48) due to the formation and coexistence of R-NH3+ and R-NHCO2− groups in the pH range from 7.4 to 10.49 This also occurs in the presence of Ca2+ ions, as supported by the observation of amorphous calcium carbonate (ACC) during the early stages of CaCO3 precipitation in the presence of PAH using cryo-TEM coupled with electron diffraction.26 Comparable morphologies have also been obtained with highly acidic polyelectrolytes such as poly(aspartic acid) (PAsp),7 poly(acrylic acid) (PAA),28 DNA,50 and the glycoprotein ovalbumin.51 These anionic polyelectrolytes direct mineral precipitation via a comparable polymer-induced liquid precursor “PILP” mechanism, whereby a fluid-like amorphous phase separates from the reaction solution because of the attractive interaction between anionic carboxylate groups on the polymer chains and Ca2+ cations.29,30 Droplets of this highly hydrated complex can then accumulate and coalesce to form thin films and fibers of CaCO3 before eventually crystallizing.52 The liquid-like characteristics of the precursor allow the mineral to be molded into dedicated shapes53,54 and to enter small pores via capillary action.55,56 As an alternative mechanism, a condensed phase of liquid-like droplets of CaCO3 has also been observed to form in the absence of additives at a near neutral pH, where it is stabilized by bicarbonate ions.29 It was then proposed that the activity of negatively charged polymers such as PAsp and PAA in forming PILP can be attributed to a kinetic stabilization of this condensed phase. Clearly, further work is required to investigate whether such a mechanism could operate with cationic polyelectrolytes such as PAH. The general similarities in the CaCO3 structures generated by PVAm and PAH strongly suggest a similar phase separation mechanism. The dependence on the polymer molecular weight and the side chain spacer also supports this hypothesis, as phase separation is governed by the aggregation behavior of the polymer and its capacity to chelate carbonate anions rather than face-selective adsorption onto nascent mineral particles. Indeed, poly(glutamic acid), which differs from poly(aspartic acid) only in its side chain spacer, is less effective than the latter polymer in generating thin films and fibers.7,52 Experiments conducted with the series of PAMA homopolymers of differing molecular weights, in which a much stronger effect was obtained with the longest chains, despite a fixed NH2/Ca2+ molar ratio, also indicated that their efficacy relied on a cooperative effect between the amine groups. Finally, it is interesting to consider further the observed activity of the PAMA homopolymers. These primary amine functionalized chains supported the formation of elongated calcite crystals with stepped faces rather than the characteristic films or fibers observed when using PAH or PVAm. While these latter cationic polyelectrolytes can undergo phase separation driven by interactions between the cationic amine groups and carbonate anions, its relatively low pKa of 7.6 ensures that PAMA chains are essentially neutral (rather than cationic) in the alkaline reaction solution. Thus, unlike PAH or PVAm, PAMA cannot induce phase separation and its lack of 2709

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charge prevents electrostatic interactions with the growing calcite crystals. The effect that PAMA has on the calcite morphology is therefore quite surprising and may indicate that hydrogen bonding interactions play an important role.

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CONCLUSIONS In summary, our results demonstrate that morphological variations in calcium carbonate can be achieved with polyamines, with interactions ranging from mild to dramatic depending on the molecular weight, the side chain length, pKa value, and most notably, the nature of the amine group. Importantly, primary amine functionalized polymers can exhibit strong interactions and therefore constitute an effective class of growth additives for CaCO3. Polyamines functionalized with tertiary and quaternary amines, in contrast, proved to be inactive as growth additives. In common with the effects observed for poly(allylamine hydrochloride) (PAH), crystallization products indicative of a liquid-like precursor phase were obtained with poly(vinylamine) (PVAm). This corroborates the recent observation that the ability of polyelectrolytes to strongly affect CaCO3 precipitation is not confined to anionic species.26 These results also provide useful insights regarding the likely role played by basic amino acids in mineralizationdirecting naturally occurring biomacromolecules. Indeed, it is noted that the amino acids histidine, lysine, and arginine often appear as short clusters in nacre protein terminal sequences,16 where both lysine and arginine contribute primary amine groups to proteins.

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ASSOCIATED CONTENT

S Supporting Information *

Synthesis and characterization of polymers and additional images of precipitated crystals. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Engineering and Physical Sciences Research Council [Grant EP/I001514/1] (A.S.S. and F.C.M.), where this program grant funds the Materials in Biology (MIB) consortium. B.C., Y.-Y.K., and F.C.M. were also funded by an EPSRC Leadership Fellowship (Grant EP/ H005374/1). The authors are grateful to Stuart Micklethwaite (Leeds Electron Microscopy and Spectroscopy Centre) for assistance with back-scattered electron microscopy. BASF (Ludwigshafen, Germany) is thanked for the gift of the poly(vinylamine).

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