Influence of the Mole Ratio of the Interacting to the Stabilizing Portion

Article pubs.acs.org/crystal

Influence of the Mole Ratio of the Interacting to the Stabilizing Portion (RI/S) in Hyperbranched Polymers on CaCO3 Crystallization: Synthesis of Highly Monodisperse Microspheres Wenyong Dong,† Chunlai Tu,‡ Wei Tao,§ Yongfeng Zhou,‡ Gangsheng Tong,‡ Yongli Zheng,‡ Yongjin Li,*,† and Deyue Yan*,†,‡ †

College of Material, Chemistry and Chemical Engineering, Hangzhou Normal University, No. 16 Xuelin Road, Hangzhou 310036, People's Republic of China ‡ College of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, 800 Dongchuan Road, Shanghai 200240, People's Republic of China § School of Medical Engineering, Hefei University of Technology, 193 Tunxi Road, Hefei, Anhui 230009, People's Republic of China S Supporting Information *

ABSTRACT: This work investigates the influence of the mole ratio of the interacting to the stabilizing portion (RI/S) in hyperbranched polymers on the morphology and the polymorph evolution of CaCO3. The RI/S is defined as the mole ratio of the carboxyl units (interacting portion) to the glycerol units (stabilizing portion) in carboxyl-terminated hyperbranched polyglycerol (HPG-COOH) and can be changed systematically from 0.1 to 0.9. Especially, when it is above 0.5, highly monodisperse CaCO3 microspheres are first prepared by direct mixing of Ca2+ and CO32− in the presence of HPG-COOH. Our results demonstrate the topology effect of the hyperbranched polymers on the crystallization of CaCO3 and suggest that both the conformation of the stabilizing portion and the density and the distribution of the interacting portion determine the roles of HPG-COOH in CaCO3 crystallization, such as the calcium-binding ability, the face-selective interaction with crystals, and the inhibition of CaCO3 nucleation and growth.

1. INTRODUCTION Biomimetic synthesis of calcium carbonates with controlled morphology and polymorph is an active area of research during the past few years for their importance in industry and nature.1 Among the various artificial additives applied for the controlled synthesis of calcium carbonate, the newly appeared doublehydrophilic block copolymer (DHBC) is superior, and a series of calcium carbonates with interesting shapes and complex forms have been obtained.2 The classical DHBC polymer has a linear block structure, in which one hydrophilic block interacts with mineral surfaces while the other provides the stabilization of the mineral, and it can be inferred that the mole ratio of the interacting to the stabilizing portion (RI/S) in DHBC polymers may have some influence on the crystal morphology, polymorph, size, etc.3 Recently, investigations of natural glycoproteins on biomineralization revealed that the secondary structure of the polypeptides could exhibit a strong influence on the morphology control and phase transformation of CaCO3.4 Hyperbranched polymers, which are different in topology compared with their linear analogues, have a three-dimensional highly branched structure and many chain end functional groups for modification.5 In the present study, a hydrophilic hyperbranched polyether was selected as the stabilizing portion and its chain end groups were converted into the interacting portion by a facile end-capping method. In the obtained © 2012 American Chemical Society

branched molecules, the mole ratio of the carboxyl units (interacting portion) to the glycerol units (stabilizing portion) could be easily changed by the conversion of the chain end groups of hyperbranched polyglycerol. By a systematic investigation, it has been found that the RI/S could exert a strong influence on both the morphology and the polymorph of CaCO3. Upon increase of the RI/S from 0.1 to 0.9, the morphology changed from pinecone-like to olive-like and finally to highly monodisperse spherical morphology when the RI/S exceeded 0.5. The morphology evolution of CaCO3 was accompanied by the polymorph changing from pure calcite through mixed calcite and vaterite, and finally to pure vaterite. The synthesis of monodisperse microspheres has attracted extensive attention for their applications in ceramics, catalysis, pigments, separation, etc.6 To the best of our knowledge, the synthesis of such monodisperse CaCO3 microspheres with size distribution less than 5% has only been accomplished by a gasdiffusion method, which controlled the crystallization by a linear block copolymer in a mixed solution of DMF and water.7 Our results demonstrated that the hyperbranched polymers could also mediate the crystallization of CaCO3 to form highly monodisperse microspheres in a pure-water system, and the Received: April 19, 2012 Revised: June 12, 2012 Published: June 26, 2012 4053

dx.doi.org/10.1021/cg3005344 | Cryst. Growth Des. 2012, 12, 4053−4059

Crystal Growth & Design

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samples were measured at room temperature and at a scattering angle of 173°, known as backscatter detection. Steady-state fluorescence spectra were obtained on a FLS-920 (Edinburg Instruments) luminescence spectrometer. The excitation wavelength was set at 335 nm, which was chosen according to the maximum intensity obtained in the excitation spectra. Step increment was set as 4 nm, and scan speed was set as 480 nm min−1. The sample solution was prepared as follows: 6 μL of an acetone solution of pyrene (0.1 mmol L−1) was added into the test tube, and then the acetone was blown away by nitrogen gas; 1 mL of polymer solution (1 g L−1) was added into the test tube at different concentrations of CaCl2. The I1/I3 emission intensity ratios of pyrene (I1 and I3 are the intensities of the first and third bands in the pyrene fluorescence spectrum, respectively) were recorded at room temperature. Thermogravimetric analysis (TGA) was conducted under nitrogen gas at a heating rate of 10 °C min−1 on a Perkin-Elmer TGA-7 thermogravimetric analyzer from room temperature to 800 °C.

systematic investigation in this work indicated that the appropriate RI/S value and the branched character of the hyperbranched polymers played a pivotal role on the narrow size distribution of CaCO3.

2. EXPERIMENTAL SECTION 2.1. Materials. Glycidol (96%, Acros) and BF3·Et2O (A.R. grade) were refluxed with CaH2 and then distilled under reduced pressure prior to use. Pyridine (A.R. grade, Sinopharm Chemical Reagent Co., Ltd.) was refluxed with CaH2 and then distilled prior to use. Succinic anhydride (A.R. grade) was recrystallized from chloroform before use. All other chemicals were of analytical grade and used as received. 2.2. Synthesis of Carboxyl-Terminated Hyperbranched Polyglycerol (HPG-COOH). Hyperbranched polyglycerol (HPG) was synthesized by the cationic ring-opening polymerization of glycidol initiated by BF3·Et2O, as reported by Penczek.8 The synthetic procedure for carboxyl-terminated hyperbranched polyglycerol (HPGCOOH) is outlined in Scheme S1, Supporting Information. For the synthesis of the three-dimensional crystal growth modifier HPGCOOH, the hydroxyl-terminated HPG was dissolved in anhydrous pyridine, then quantitative succinic anhydride was added after complete dissolution of HPG, and the reaction was conducted at 60 °C for about 24 h. After removal of most of the solvent, K2CO3 was added, and the residual pyridine was removed by azeotropic distillation with toluene. The crude product was further dialyzed against distilled water for three days and lyophilized by a freeze-dryer system (Martin Christ, α1-4, Germany) at −50 °C, and some viscous product was obtained finally. A series of HPG-COOHs with different RI/S values from 0.1 to 0.9 was obtained by changing the amount of the added succinic anhydride, and they were denoted as HPG-COOH0.1, HPGCOOH0.3, HPG-COOH0.5, HPG-COOH0.6, HPG-COOH0.7, and HPG-COOH0.9, as summarized in Table 1. For example, HPGCOOH0.1 means 10% of the terminal hydroxyl groups were converted to carboxyl groups. 2.3. Mineralization of CaCO3. All glassware (glass bottles and small pieces of glass substrate) was first immersed in piranha solution overnight at room temperature, then rinsed with distilled water, and finally dried with acetone. The mineralization of CaCO3 was carried out by the direct mixing of CaCl2 solution with Na2CO3 solution in the presence of HPG-COOH with different RI/S. In a typical synthesis, an aqueous solution of Na2CO3 (0.5 M, 0.32 mL) was injected into an aqueous solution of HPG-COOH (1 g L−1, 20 mL), the pH of the solution was adjusted to 10 by HCl or NaOH solution, and finally an aqueous solution of CaCl2 (0.5 M, 0.32 mL) was added under vigorous stirring. After stirring for about 1 min, the solution was allowed to stand quiescently for 24 h before the product was collected. 2.4. Characterization. Nuclear magnetic resonance (NMR) spectra were recorded with a Varian Mercury Plus 400-MHz spectrometer at room temperature, with deuterated water (D2O) or deuterated methanol (CD3OD) as the solvent. X-ray diffraction (XRD) patterns were recorded on a Rigaku D/ MAX-2550/PC spectrometer with Cu Kα radiation (λ = 1.54056 Å). Data were collected at a scan step of 0.02° and at conditions of 40 kV and 300 mA. Fourier transformed infrared (FTIR) measurements were recorded on an FT-IR spectrometer (Perkin-Elmer Paragon 1000) by KBr sample holder method. Scanning electron microscopy (SEM) images were recorded using a field emission scanning electron microscope (Sirion 200 SEM, Philips, 5 kV or 7401F SEM, JEOL, 1 kV). Prior to imaging by SEM, the samples were sputtered with a thin layer of gold. High-resolution scanning electron microscopy (HR-SEM) images were recorded using a field emission scanning electron microscope (S4800 SEM, Hitachi, 5 kV). Prior to imaging by SEM, the samples were sputtered with a thin layer of gold. Dynamic light scattering (DLS) measurements were performed with a Malvern Zetasizer Nano S apparatus (Malvern Instruments Ltd.) equipped with a 3.0 mW He−Ne laser operating at 633 nm. All

3. RESULTS 3.1. Synthesis of a Series of HPG-COOH with RI/S from 0.1 to 0.9. The synthesis of a series of polymers with different RI/S can be accomplished easily by converting the terminal hydroxyl groups of hyperbranched polyglycerol into carboxyl groups, as indicated in Scheme S1, Supporting Information.9 Figure S1, Supporting Information, shows the 1H NMR results of four kinds of hyperbranched polymers with a variable composition of carboxyl-terminated groups. Compared with the spectrum of HPG, several new peaks appear in the spectra of HPG-COOH, and the peaks between 2.3 and 2.4 ppm are ascribed to the methylene groups (-OCH2CH2COOH) in succinic anhydride. It can be found that the peaks of the methylene groups increase significantly from HPG-COOH0.1 to HPG-COOH0.9, which indicates the successful synthesis of a series of polymers with different RI/S. Compared with their linear analogues, such as the typical DHBC polymer poly(ethylene glycol)-b-poly(methacrylic acid) (PEG-b-PMAA), HPG-COOH has a different topology and exhibits a core− shell structure with the stabilizing portion as the core and the interacting portion as the shell. 3.2. Crystallization at RI/S = 0.5. The mineralization of CaCO3 was carried out by a direct mixing method, during which process the aqueous solution of CaCl2 was added quickly into the aqueous solution of Na2CO3 and HPG-COOH with different RI/S at pH 10.10 As shown in Figure 1, highly

Figure 1. CaCO3 particles obtained in the presence of HPG-COOH0.5. [HPG-COOH0.5] = 1 g L−1, pH = 10. The scale bar in the inset is 10 μm. 4054



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monodisperse CaCO3 microspheres with a diameter of about 2.2 μm were obtained when RI/S was 0.5, with a polymer concentration of 1.0 g L−1, and the microspheres were so uniform that they could form hexagonally ordered structures in some regions. The corresponding XRD pattern in Figure 2e

Figure 3. CaCO3 particles obtained in the presence of HPG-COOH with RI/S 0.1 and 0.3: (A) [HPG-COOH0.3] = 1 g L−1; (B) [HPGCOOH0.3] = 0.5 g L−1; (C) [HPG-COOH0.3] = 2 g L−1; (D) [HPGCOOH0.1] = 1 g L−1. The scale bar is 1 μm in panels A−D and 2 μm in the inset of panel A.

morph at RI/S 0.3. As shown in Figure 3B, when the concentration decreased to 0.5 g L−1, uniform peanut-like particles with rough surfaces were obtained. As the concentration increased to 2 g L−1, the obtained morphology was a mixture of rods and spheres (Figure 3C).12 The XRD analysis indicated that the CaCO3 samples obtained at these three concentrations are all mixtures of calcite and vaterite, and the content of vaterite in the mixture increased from 15% to 55% as the concentration increased from 0.5 to 2 g L−1 (Figure 2b− d).13 It can be deduced from the XRD and SEM results that the spheres in Figure 3C are vaterite phase for they often exhibit a spherical morphology; the peanut-like, olive-like, and rod-like particles are all oriented along the [001] direction, and the morphological evolution as the concentration increasing can be due to the different amounts of HPG-COOH0.3 interacting with the faces parallel to the [001] direction of calcite. Furthermore, as the RI/S decreased to 0.1, the CaCO3 mineralization product exhibited a pinecone-like morphology with stepped surfaces, and the XRD result showed that this sample was pure calcite (Figure 2a).14 A closer investigation of the pinecone-like particles indicated that they were constituted by rhombohedral calcite with well-defined {104} faces stacking along the [001] direction. 3.4. Crystallization at RI/S > 0.5. Interestingly, when the RI/S was above 0.5, both the morphology and the polymorph of the mineralization products were significantly different from those with RI/S below 0.5. As shown in Figure 4A−C, from RI/S 0.6 to 0.9 with a polymer concentration of 1 g L−1, all of the mineralization products displayed a spherical shape and a highly monodisperse character. It can be found that the sizes of the spheres obtained at these three RI/S values do not differ significantly from each other. For example, at RI/S 0.6, the diameter of the spheres was about 3.8 μm. As the RI/S increased to 0.7, the diameter increased accordingly to 4.1 μm. The high monodispersity of these spheres indicated that they were potential candidates to form two-dimensional or three-dimensional self-assembled structures. Figure 4E showed the largescale fabrication of the hexagonally close-packed monolayer by precipitating a certain concentration of CaCO3 (the product in

Figure 2. XRD patterns of CaCO3 obtained in the presence of HPGCOOH with RI/S 0.1, 0.3, and 0.5: (a) [HPG-COOH0.1] = 1 g L−1; (b) [HPG-COOH0.3] = 0.5 g L−1; (c) [HPG-COOH0.3] = 1 g L−1; (d) [HPG-COOH0.3] = 2 g L−1; (e) [HPG-COOH0.5] = 1 g L−1. Note that C denotes calcite (JCPDS 47-1743) and V denotes vaterite (JCPDS 33-0268).

indicated that the mineralization product was mainly vaterite, with a trace amount of calcite. In the absence of HPG-COOH, only the typical rhombohedral calcite crystals were formed under the same reaction conditions. The synthesis of such uniform microspheres has only been reported by Yu et al.7 They obtained a similar result by the slow gas−liquid diffusion method, mediated by a linear block copolymer poly(ethylene glycol)-b-poly(L-glutamic acid) (PEG-b-pGlu) in a mixed solvent of water and DMF. They indicated that it was mainly the characters of the mixed solvents that determined the uniformity of the spheres. In our reaction system, the synthesis of highly monodisperse microspheres in pure water is an environmentally benign method, and the monodispersity may be due to both the three-dimensional topology of HPG-COOH and the moderate mole ratio of the interacting to the stabilizing portion (RI/S). To prove this hypothesis, the relative mole ratio of the interacting to the stabilizing portion in hyperbranched polymers HPG-COOH was varied systematically from 0.1 to 0.9. 3.3. Crystallization at RI/S < 0.5. When the RI/S decreased to 0.3, olive-like particles11 with a length of 11.3 ± 2.1 μm and a width of 6.6 ± 1.3 μm were obtained, as shown in Figure 3A. The corresponding XRD pattern indicated that this sample was mainly calcite (Figure 2c). The inset in Figure 3A shows a freestanding particle with three remnant {104} faces at the tip; hence the olive-like particles elongated along the [001] direction could be ascribed to the HPG-COOH0.3 polymers absorbing at and inhibiting the faces roughly parallel to the [001] direction of calcite. We further investigated the concentration effect on the CaCO3 morphology and poly4055



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further. Finally, when the RI/S increased to the maximum of 0.9, compared with RI/S 0.6 and 0.7, though the average diameter almost did not change, the uniformity of the spheres deteriorated a little. We also investigated the concentration effect of HPG-COOH on the size and uniformity of CaCO3 and found that for both HPG-COOH0.6 and HPG-COOH0.7, as the concentration increased from 1 to 2 g L−1, the uniformity was retained while the size decreased from 3.8 to 3.2 μm (Figure 4A,D) for HPG-COOH0.6 and from 4.1 to 2.8 μm for HPG-COOH0.7. Decreasing the concentration of HPG-COOH from 1 g L−1 inevitably led to the formation of polydisperse spheres (data not shown). Figure S2, Supporting Information, presents the XRD patterns of the samples prepared at RI/S above 0.5. All the peaks could be indexed readily to a pure vaterite phase of CaCO3. In the mean time, the broad diffraction peaks of the monodisperse spheres implied that they were constituted by small crystalline subunits and the average subunit size could be estimated by calculating at 2θ = 26.5° and 32.5°, based on the Scherrer equation. Taking the spheres in Figure S2d, Supporting Information, for example, the analysis showed that the subunit size was about 60 nm.

4. DISCUSSION The experimental conditions and the results above are summarized in Table 1. An interesting phenomenon in this table is that at RI/S = 0.3, even if the [COOH] increased to 5.8 mM (column 3, row 5, in Table 1), which is a little higher than that for the formation of monodisperse spheres at RI/S 0.9 (column 3, row 11, in Table 1), the obtained morphology and polymorph are mainly rod-like particles and calcite, respectively. It can be concluded from Table 1 that RI/S value is the decisive factor for the formation of the monodisperse spheres and RI/S 0.5 is a transition point for morphological evolution from elongated particles to monodisperse spheres and for polymorph from calcite to vaterite. This means that the topology of the hyperbranched polymers, including the density and the distribution of the carboxyl groups on the surface of the hyperbranched polymers, the aggregation of HPG-COOH induced by calcium ions in alkaline solution, etc., may have some relationship with the nucleation and the crystal growth of CaCO3. We used pyrene fluorescence spectroscopy to investigate the properties of the aggregates formed by HPG-COOH and calcium ions at pH 10 in solution. Generally, the fluorescence

Figure 4. CaCO3 particles obtained in the presence of HPG-COOH with RI/S 0.6, 0.7, and 0.9: (A) [HPG-COOH0.6] = 1 g L−1; (B) [HPGCOOH0.7] = 1 g L−1; (C) [HPG-COOH0.9] = 1 g L−1; (D) [HPGCOOH0.6] = 2 g L−1; (E) 2-D self-assembly of monodisperse CaCO3 microspheres.

Figure 4A) on a glass slide. By optimization of some factors, such as the evaporation rate of the solvent and the concentration, the quality of the monolayer may be improved Table 1. The Results of CaCO3 Crystallization RI/Sa

[polymer] (g L−1)

[COOH]b (mM)

morphology

polymorphc

0.1

1

1.2

pinecone

C

0.3 0.3

0.5 1

1.4 2.9

peanut olive

C, (V) C, V

0.3 0.5 0.6 0.6 0.7 0.7 0.9

2 1 1 2 1 2 1

5.8 4.0 4.5 9.0 4.9 9.8 5.5

rod, sphere monodisperse monodisperse monodisperse monodisperse monodisperse monodisperse

sphere sphere sphere sphere sphere sphere

C, V V, (C) V V V V V

sized (μm) 3.3 ± 0.3 (L) 1.4 ± 0.1 (W) 11.3 ± 2.1(L) 6.6 ± 1.3 (W) 2.2 3.8 3.2 4.1 2.8 3.8

a

RI/S, the mole ratio of the interacting to the stabilizing portion in a hyperbranched polymer. b[COOH] was calculated depending on the molecular weight of HPG-COOH with different RI/S. cC = calcite, V = vaterite; symbol in parentheses indicates a trace amount. dL = length, W = width. 4056



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spectrum of pyrene in water exhibits five predominant peaks, and the intensity ratio of the first to the third vibronic peak, known as an I1/I3 ratio is a sensitive parameter characterizing the polarity of the probe’s environment (the inset in Figure 5).

Figure 5. The change of I1/I3 value of 6 × 10−7 M pyrene with the addition of CaCl2 in HPG-COOH0.1, HPG-COOH0.5, and HPGCOOH0.9, [HPG-COOH] = 1 g L−1; all samples were prepared at pH = 10.

I1/I3 ratio is in the vicinity of 1.8 in pure water and decreased as the hydrophobicity increased in the environment.15 Figure 5 showed the change of the I1/I3 ratio as the amount of the calcium ions increased in the HPG-COOH solutions. The results for three typical HPG-COOH polymers with RI/S 0.1, 0.5, and 0.9 indicated that the complexation of the calcium ions with HPG-COOH almost did not increase the hydrophobicity of the aggregates. The results also implied that these three aggregates adopted a loose internal structure in solution, and the aggregates formed by HPG-COOH0.9 with calcium ions might be a little bigger and denser than those with HPGCOOH0.1. After the calcium ions (0.16 mmol) were added into the solution (20 mL) of carbonate ions (0.16 mmol) and HPGCOOH with a concentration of 1 g L−1 and stirred for a minute, it was observed that the turbidity of the solution decreased from opaque to transparent as the RI/S increased from 0.1 to 0.9 (Figure 6A−E). This could be ascribed to more calcium ions being bound tightly to HPG-COOH and stronger inhibition of HPG-COOH on nucleation of CaCO3 as the RI/S increased, both of which prolonged the induction time of CaCO3 crystallization.16 Figure 6F displays the corresponding DLS measurements of the samples in Figure 6A−E. It is obvious that the size and the size distribution could be divided into two regions by RI/S 0.5 (Figure S3, Supporting Information, and Figure 6F). When the RI/S was above 0.5, both the particle size and the size distribution were significantly decreased compare with those with the RI/S below 0.5. Comparison of the DLS results with the Scherrer analysis on the XRD patterns of the monodisperse spheres (Figure S2d, Supporting Information) demonstrated that at RI/S 0.9, the particle size after reaction for a minute was 43 ± 15 nm, which was nearly the same as the highly monodisperse microspheres. High-resolution scanning electron microscopy was applied to investigate the inner structure of a broken sphere, and the result revealed that the highly monodisperse microspheres were constituted by nanoparticles with sizes from 25 to 50 nm (Figure S4, Supporting Information), which is in accordance with the Scherrer analysis of the XRD results. So the mechanism for the formation of the monodisperse spheres

Figure 6. (A−E) Digital photo of CaCO3 solutions at the start of the crystallization mediated by a series of 1 g L−1 HPG-COOH with different RI/S: 0.1, 0.3, 0.5, 0.7, and 0.9, respectively. (F) DLS plots of CaCO3 solutions mediated by HPG-COOH with different RI/S.

can be explained by the Matijević’s model: the first step involved the nucleation of the nearly uniform CaCO 3 nanoparticles; then the primary nanoparticles aggregated to form the highly monodisperse spheres.17 The hyperbranched polymers HPG-COOH played a pivotal role during this process, and this could be supported by the thermal gravimetrical analysis of the mineralization products. As shown in Figure S5, Supporting Information, 3 wt % HPGCOOH0.1 was enclosed in the pinecone-like particles, while in the monodisperse spheres, 7 wt % HPG-COOH0.5 and 10 wt % HPG-COOH0.9 were enclosed, respectively. FT-IR measurements of these three mineralization products are shown in Figure S6, Supporting Information. The absorption bands at 745 and 713 cm−1 are characteristic of vaterite and calcite, respectively. The FT-IR results are consistent with XRD patterns that the polymorphs are pure calcite at RI/S 0.1, majority vaterite with a trace of calcite at RI/S 0.5, and pure vaterite at RI/S 0.9. Apart from the bands of CaCO3, the FT-IR results also corroborate the existence of HPG-COOH, such as the CH2 groups (2925, 2860 cm−1) and the C−O groups (1170, 1084 cm−1). The trend of the FT-IR results is in accordance with the TGA results that as the RI/S increased, the intensity of the peaks ascribed to HPG-COOH increased accordingly, indicating more HPG-COOH was enclosed in CaCO3. Now, one question may naturally arise: why could HPGCOOH with different RI/S (Figure 7A) determine the morphology and the polymorph of CaCO3? We tentatively explain this based on the contribution of Tsukruk.18 The flexible and water-swollen hyperbranched polyglycerol deforms in solution in order to accommodate the carboxyl groups to 4057



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AUTHOR INFORMATION

Corresponding Author

*E-mail addresses: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program (Grant 2007CB808000), National Natural Science Foundation of China (Grant No. 50633010, No. 51173036 and No. 20874060) and Shanghai Leading Academic Discipline Project (Grant No. B202).

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Figure 7. Models of HPG-COOH with low (C) and high (D) RI/S interacting with CaCO3.

interact effectively with CaCO3 (Figure 7B), so the number of the carboxyl groups will influence the topology of HPGCOOH. At low RI/S values, the hyperbranched polymers adopt a prolate shape with loose packing and randomly oriented carboxyl groups (Figure 7C), while at high RI/S values, the higher number of carboxyl groups force the hyperbranched polymers to flatten and the carboxyl groups pack more orderly and densely (Figure 7D). That is why the HPG-COOH with RI/S over 0.5 could stabilize the primary CaCO3 nanoparticles at the start of the reaction, and then the nearly uniform nanoparticles aggregated to form the highly monodisperse spheres.

5. CONCLUSIONS A series of DHBC-type hyperbranched polymers with different RI/S values were employed in this work to control the morphology and the polymorph of CaCO3; especially when the RI/S was over 0.5, monodisperse vaterite microspheres were first obtained in an environmentally benign pure water system. The monodisperse microspheres may have applications as hard templates to synthesize capsules or as precursors to construct photonic crystals. In addition, the morphology and the polymorph selections of HPG-COOH further confirmed the topology effect of the hyperbranched polymers on biomineralization.

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REFERENCES

ASSOCIATED CONTENT

S Supporting Information *

The synthetic scheme for HPG-COOH, the NMR spectra of HPG and HPG-COOH with different RI/S, the DLS analysis results of CaCO3, the HR-SEM image of a broken sphere, the XRD patterns of CaCO3 in the presence of HPG-COOH with RI/S over 0.5, and the TGA and FT-IR results of CaCO3. This material is available free of charge via the Internet at http:// pubs.acs.org. 4058



Article

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Influence of the Mole Ratio of the Interacting to the Stabilizing Portion

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