Calcium Carbonate Crystallization on a Polyethylene Surface

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Calcium Carbonate Crystallization on a Polyethylene Surface Containing Ultrathin Layers of Hydrophilic Polymers Rafael Silva, Guilherme M. Pereira, Edvani C. Muniz, and Adley F. Rubira* Grupo de Materiais Polime´ricos e Compo´sitos, Departamento de Quı´mica, UniVersidade Estadual de Maringa´, AVenida Colombo 5790, CEP: 87020-900 - Maringa´, Parana´, Brazil

CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 7 3307–3312

ReceiVed January 30, 2009; ReVised Manuscript ReceiVed May 15, 2009

ABSTRACT: Inorganic/polymer composite materials with controlled phases organized in layers with part of the inorganic material hosted by a hydrophilic polymer layer covalently bound to a hydrophobic polymer layer and the other part of the inorganic material in crystalline phases adsorbed onto the composite surface were prepared. An ultrathin layer of poly(vinyl alcohol) (PVA) and a poly(acrylic acid) (PAA) layer were prepared on polyethylene films, which were used in the crystallization of CaCO3 by a sequential immersion method in a dip-coating process. The number of sequential immersions and the addition of PVA as a possible stabilizer were analyzed. The data suggest that the nucleation and growth of the inorganic material started on the hydrophilic polymer layer, which was composed mainly of PAA, and a mixed inorganic/organic layer was formed. The shape and the proportions of the CaCO3 phases on the composite surface depend on the number of immersion cycles and the addition of PVA. Introduction Nature is rich in composite materials containing crystalline phases and organic biopolymers, such as teeth, bones, shells, corals, and natural fibers (rice husk, sisal fibers, and others).1 The intimate mineral-organic association/interaction and architecture of biomineralized materials are responsible for providing excellent biocompatibility and enhanced mechanical properties.2 These composite materials are formed by biomineralization, the nucleation and growth of inorganic crystals being controlled by interactions with the organic materials.3,4 The mechanism of control of nucleation and growth of inorganic phases in polymers is directly related to the action of hydrophilic functional groups, such as carboxyl and hydroxyl groups.5,6 Biocompatible materials with good mechanical properties can be prepared by mimicking the natural biomineralization process in the formation of inorganic crystalline materials in the presence of synthetic polymers.7 These materials can be applied in technological and biomedical areas, such as in bone substitution or to make catheters.8,9 Calcium carbonate (CaCO3) is one of the most abundant minerals in the world. It has three anhydrous crystalline polymorphs: vaterite, aragonite, and calcite. The calcite phase has the highest thermodynamic stability. The selective formation of the metastable polymorphs using organic agents has been studied to develop artificial biomimetic systems.10-14 Polymers containing specific moieties have been found to nucleate calcium carbonate.15-19 The results obtained by using a number of polymeric substrates were explained by assuming that the nucleation of the crystalline phases on a polymer is preceded by the formation of surface ion pairs. The use of organic molecules such as collagen,20 cholesterol,21 a stearic acid monolayer,22 chitin,23 and poly(acrylic acid) (PAA),19,24-26 has shown significant effects on the synthesis of polymerCaCO3 composites. Zhang and Gonsalves24 indicated that the nucleation of CaCO3 was initiated from the protonated nitrogen and carboxylate anions that had been produced on a chitosan membrane surface by introducing PAA into a supersaturated CaCO3 solution. On the other hand, Kato and co-workers19,26 * To whom correspondence should be addressed. E-mail: [email protected]. Phone: +55 44 3261 4332. Fax: +55 44 3261 4125.

indicated that PAA was adsorbed onto a chitosan membrane by interactions between the COO- moieties of PAA and either the OH or the NH groups of the chitosan membrane. They observed that these bonds resulted in the strong binding of Ca2+ ions, and vaterite crystals were predominantly formed in accordance with Ostwald’s rule. According to Westin and Rasmuson,27,28 the crystal growth rate is significantly reduced in the presence of calcium complexing substances, depending on both their concentration and polymorphism. The relative crystal growth rate was correlated to the total concentration of the complexing agent. A controlled deposition process on specific substrates and the stability of the deposited material are important factors that should be investigated for technological application of mineralized materials. Polyethylene (PE) substrate is an attractive material for the preparation of composites aimed at application in the biomedical area. PE presents interesting bulk properties such as malleability. In addition, the easy processing of PE allows low cost fabrication of devices with controlled dimensions and shape. Biocompatible materials with suitable mechanical properties have been obtained by using low cost raw materials and polyethylene substrate with chemically modified surfaces. Silva et al. have reported simple procedures to prepare ultrathin hydrophilic polymer layers of poly(vinyl alcohol) (PVA) and PAA covalently immobilized onto PE surfaces. The polymer surface containing a high density of functional groups can be used as a substrate for the formation of inorganic crystalline compounds, such as calcium carbonate. This property is related to the possibility of interaction between the moieties of the polymers immobilized onto the PE and the ions of the inorganic compound. Scheme 1 shows the schematic immobilization of the hydrophilic polymers on the surface of polyethylene. The deposition of calcium carbonate onto modified PE films involves simple alternate immersion of the films into aqueous solutions containing Ca2+ and CO32- ions. During the alternate immersion of the materials, an insoluble CaCO3 phase is produced in situ on the films. This work reports the crystallization of calcium carbonate on the surface of PE films containing ultrathin PVA and PAA layers. The influence of the number of soaking cycles and the addition of PVA in each cycle is studied. The presence of ultrathin hydrophilic polymer layers

10.1021/cg900106s CCC: $40.75  2009 American Chemical Society Published on Web 06/04/2009

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Scheme 1. Schematic Representation of the Chemical Reactions Proposed for the PE Modification Procedures

linked by covalent bonds to the surface of the PE film may warrant carbonate recovery with good adhesion to the film.30 We have investigated the growth of carbonate calcium in and on PAA thin layers with or without the addition of PVA. Experimental Section The substrate was prepared according to a procedure presented in our previous study.10 PE films were functionalized by radical grafting of maleic anhydride to produce surfaces containing anhydride groups. The anhydride groups formed in the functionalization were used to anchor a PVA ultrathin layer by thermal esterification. After anchoring the PVA layer, a second ultrathin layer constituted of PAA was also anchored by thermal esterification between PVA hydroxyl groups and PAA carboxylic acid groups. The procedure used allows the formation of an ultrathin hydrophilic layer on the surface of PE. Crystallization through Dip-Coating Cycles. The crystallization process was performed using a dip-coater. The equipment has a mechanical arm with movements controlled in two axes as a function of time. Both the movements and time in each position are controlled by a microcomputer. The dip-coater was maintained inside a closed acrylic box. The temperature of the solutions was controlled by a heating system. The modified PE film was fixed to the dip-coater mechanical arm and alternately soaked in different solutions according the following steps: Step 1. The film was soaked in a CaCl2 solution (200 mmol) for 60 s, removed from the solution, and maintained in the box atmosphere immediately above the CaCl2 solution for 30 s. Step 2. The film was soaked in deionized water for 120 s, removed from the deionized water, and maintained in the box atmosphere immediately above the deionized water for 30 s. Step 3. The film was immersed in acetone for 30 s, removed from the acetone, and maintained in box atmosphere immediately above the acetone for 180 s, which corresponds to the time necessary to dry the film. Step 4. The film was soaked in a Na2CO3 solution (200 mmol) for 60 s, removed from the solution, and maintained in the box atmosphere immediately above the Na2CO3 solution for 30 s. Steps 2 and 3 were repeated. Step 5. The film was soaked in a 1% (w/v) PVA solution for 60 s and exposed to the box atmosphere immediately above the PVA solution for 30 s. Steps 2 and 3 were repeated. A cycle comprises all the steps described above. The influence of the addition of PVA to the cycle was also tested. The cycle for the sample prepared without the addition of PVA consists of steps 1-4. The samples were prepared with 10, 20, and 30 cycles. The temperature of the system (solutions, water, and propanone) was maintained constant at 37 °C (human physiological temperature) during the experiment. The solutions were prepared using deionized water. The amount of propanone in the respective recipient was maintained constant during the experiment. Thermal Treatment. After the crystallization stage, the films were thermally treated at 140 °C for 2 h. At this temperature we can expect the thermal esterification of PVA hydroxyl groups deposited during the process with the PAA carboxylic acid groups of the substrate surface.

Characterization. The chemical characterization was performed by attenuated total reflection in Fourier transform infrared (ATR-FTIR) in Bomem model MB-100 with a Pike MIRacle ATR accessory at an incident angle of 45° with a ZnSe crystal and nitrogen purge. The film surface morphologies were analyzed by scanning electron microscopy (SEM) in SHIMADZU SS-550. X-ray diffractometry (XRD) were obtained in D-6000 Shimadzu using a copper tube with KR 1.5406 Å. The diffractograms were obtained in the following conditions: 2θ ) 25-75°, 35 kV, 18 mA, counting time of 1 s, 0.5° min-1, and slit width of 0.05 mm.

Results and Discussion The substrate used in the calcium carbonate crystallization has a high density of carboxylic groups on its surface as evidenced by the band at 1717 cm-1 in the ATR-FTIR spectrum, Figure 1. This band is attributed to the carbonyls of PAA and the carbonyls of the ester groups of the cross-linkage between PAA and PVA, as well as to the ester groups formed by the linkage between PVA and the functional groups on the surface of PE. The film was first soaked in the calcium solution to partially neutralize the carboxylic groups of the PAA layer. Next, it was washed with water and afterward with propanone. The immersion in water aims to remove the excess salt on the substrate. The immersion in propanone seeks to remove the excess water before the next immersion. The substrate containing calcium ions was immersed in the solution containing carbonate ions, Na2CO3. The formation of

Figure 1. FTIR-ATR spectrum of the modified PE film containing an immobilized PAA surface layer.

CaCO3 Crystallization on a Polyethylene

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Scheme 2. Deposition of CaCO3 onto PE Containing a Hydrophilic Polymer Thin Layer and the Formation of the Multilayer Composite

an insoluble CaCO3 phase took place due to the low solubility of the compound. The crystallization process setup aims to start the formation of inorganic material in the PAA layer. Xu et al.32 demonstrated that PAA has an important effect in the transformation of amorphous calcium carbonate (ACC) into crystalline CaCO3 phases. According to Xu et al., PAA has a strong inhibitory effect in the transformation of ACC. They verified that the transformation of ACC samples containing PAA was significantly delayed when compared to that of pure ACC. The deposition of PVA by immersion in the PVA solution seeks to limit the growth of the CaCO3 particles and increase the adhesion of the particles to the substrate surface, as the PVA solution can penetrate into the immobilized PAA layer. The thermal treatment was used to remove water from the surface layer and the inorganic phase and promote the esterification between the deposited PVA and the surface carboxyl groups. The temperature does not cause structural changes in the inorganic phases other than the water release. The schematic representation of the composites and the start material can be seen in Scheme 2. The inorganic material grows in two chemical environments, in the polymeric layer and on the film surface. Two systems were tested: the hydrophilic polymeric layer composed of only PAA and the hydrophilic polymeric layer composed of PAA and the PVA added during the immersions cycles. Crystallization of CaCO3. Figures 2 and 3 present the ATRFTIR spectra of the modified PE films containing the mineralized CaCO3. Comparing Figures 2 and 3, it can be seen that the addition of PVA influences the chemical characteristics of the formed surface. In relation to the samples added with PVA, Figure 2, in the composite submitted to 10 cycles, the intensity of the band at 1717 cm-1 decreased and a band appeared at 1567 cm-1 when compared with the spectrum in Figure 1. The signal at 1567 cm-1 can be attributed to the carboxylate groups produced in the neutralization of the carboxyl groups of the immobilized PAA. The remaining signal at 1717 cm-1 can be attributed to ester groups. The spectrum of the sample prepared with 20 cycles, Figure 2b, presents small alterations in relation to that of the sample prepared with 10 cycles. The ATR-FTIR of the sample prepared with 30 cycles, Figure 2c, show the characteristic bands of carbonate ions at 1433 cm-1, which overlap the PE signal, making it broader and more intense. The narrow signal that appeared at around 875 cm-1 is

attributed to the carbonate ions in the crystalline structures.8,33 The broad signal at 3400 cm-1 relative to OH groups is difficult to be observed in the spectra of the sample prepared with 10 and 20 cycles, but it appears in the spectrum of the sample prepared with 30 cycles. The ATR-FTIR spectra of the composites prepared without the addition of PVA are presented in Figure 3. These spectra show that the signal relative to PE is detected only for the film prepared with 10 cycles, showing a double signal for the C-H bond at 2900 cm-1. The PE signal suppression in the ATRFTIR spectra indicates the formation of a layer almost exclusively constituted of CaCO3 on the surface of the substrate thicker than 1 µm (average value of the deep penetration of the IR radiation in the ATR technique), indicating that the amount of inorganic material formed is higher in the process without the addition of PVA than with added PVA. The ATR-FTIR spectra of the films without PVA for 10, 20, and 30 cycles show an intense and broad peak at 1433 cm-1 characteristic of the carbonate ion and a narrow peak at around 875 cm-1, which is attributed to the vibrational transition of carbonate ions in crystalline structure (ν2CO3).33 For the composite prepared with

Figure 2. FTIR-ATR spectra of the PE composites with CaCO3 added with PVA in (a) 10 cycles, (b) 20 cycles, and (c) 30 cycles.

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Figure 3. FTIR-ATR spectra of the composites of PE with CaCO3 without the addition of PVA: (a) 10 cycles, (b) 20 cycles, and (c) 30 cycles.

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Figure 5. SEM micrographs of (a) virgin PE and (b) PAA anchored to modified PE film.

Figure 4. XRD diffractograms of the composites prepared with 30 cycles (A ) aragonite, C ) calcite, and V ) vaterite).

30 cycles, the width of the ν2CO3 signal increased and it shifted to low wavenumber values, as can be seen in the inset in Figure 3. The presence of the crystalline phases on the surface of PE after the crystallization process was confirmed by X-ray diffraction analysis, as shown in Figure 4. The X-ray diffraction analysis of the composites prepared with 30 cycles revealed the formation of the CaCO3 crystalline phases: calcite, vaterite, and aragonite. The crystalline planes were attributed by comparison with the standard obtained from JCPDS cards (vaterite JCPDS-33-268, calcite JCPDS-5-586, and aragonite JCPDS 41-1475). Comparing the diffractograms of the samples, it can be noted that for the composite prepared with the addition of PVA, the XRD peaks are broader and shifted to higher 2θ values than those of the composite prepared without PVA, indicating that its addition provokes a decrease in the crystallite size and the interplanar space. To identify the modification caused by the CaCO3 crystallization process, scanning electron microscopy (SEM) micrographs of virgin PE films and with PAA anchored to the surface of modified PE are presented in Figure 5. The SEM micrographs reveal that the virgin PE and PE coated with a covalently anchored PAA layer have homogeneous flat surfaces.

Figure 6. SEM micrographs of the PE and CaCO3 composites with the addition of PVA in (a) 10 cycles, (b) 20 cycles, and (c) 30 cycles.

SEM micrographs of the surface of the PE film after the CaCO3 crystallization by immersion in the PVA solution are

CaCO3 Crystallization on a Polyethylene

Crystal Growth & Design, Vol. 9, No. 7, 2009 3311 Table 1. Deconvoluted Signals of the Different Crystalline Phases Formed in the Composites Prepared with 30 Immersion Cycles signal vaterite and calcite

Figure 7. SEM micrographs of the PE/CaCO3 composites prepared without the addition of PVA: (a) 10 cycles, (b) 20 cycles, and (c) 30 cycles.

presented in Figure 6. The micrographs of the composites with PVA added in the crystallization process, Figure 6, have different characteristics as a function of the number of immersion cycles. The composite prepared with 10 cycles has several spherical particles visually buried in the polymeric matrix. The particles have some nanometers in diameter and are uniformly distributed on the film surface. For the composite prepared with 20 cycles of immersions, it can be seen an enlargement of the

composite

(cm-1)

(cm-1)

area ratio

PE/CaCO3 PE/CaCO3 added with PVA

872 874

875 875

0.795 0.937

aragonite area (cm-1) ratio 858 861

0.205 0.063

spherical particles. In both composites described above, it is evident that the inorganic phase formed on the surface is mixed with the PVA deposited in the same crystallization process and the PAA previously anchored onto the modified PE surface. The spherical particles buried in the polymeric matrix may be attributed to the ACC phases formed in the early stages of the crystallization process. The morphology of the composite prepared with 30 cycles was analyzed using the SEM micrograph in Figure 6c. Crystals can be observed on the film surface. Different morphologies can be identified, such as cauliflower-like (vaterite), rhombohedral-like particles (calcite), and dendrite (aragonite). The morphologies observed match well with the XRD data, which identified the formation of the three crystalline structures in the 30-cycle sample. From the micrograph in Figure 6c, it can be inferred that the main crystalline structure obtained on the surface of the polymeric film is the cauliflower-like structure, which is attributed to the vaterite phase. Examining the surface of the composite, besides the crystals, spherical particles can be observed buried in the polymeric matrix. SEM micrographs of the composites prepared without the addition of PVA are presented in Figure 7. Different from the composite with PVA added during the crystallization process, cauliflower-like (vaterite) and rhombohedral-like particles (calcite) are observed in the composite prepared with 10 and 20 cycles. The SEM micrographs show only vaterite and calcite and that the amount of crystals of vaterite is much higher than that of calcite. The morphology of the composite prepared with 30 cycles without the addition of PVA presents an accentuated modification when compared to the others. Elongated aragonite crystals covering practically all the surface of the composite and a few rhombohedral-like particles appeared in this composite. Vaterite crystals were not observed; however, the presence of the vaterite phase was demonstrated by the XRD diffractograms. A possible explanation for this fact is the growth of aragonite crystals on the surface of the vaterite crystal, forming a core-shell structure, where the core is formed by the vaterite phase and the shell is composed of the aragonite phase.

Figure 8. ATR-FTIR spectra of the composites prepared with 30 cycles: (a) with added PVA and (b) without PVA addition, and the deconvoluted signals of the different crystalline phases (V ) vaterite, C ) calcite, and A ) aragonite).

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The CaCO3 polymorphism can be quantified by exploring the IR band relative to the carbonate ion (ν2) in the different crystalline structures at around 875 cm-1. Vaterite and calcite show the ν2CO3 band at around 875 cm-1, while aragonite crystals show the ν2CO3 band at 855 cm-1. The XRD data cannot be used in these determinations due the too low signalto-noise ratio. Most of the IR methodologies presented in the literature8 were restricted to binary mixtures because of overlapping vaterite and calcite bands. In the present study, this method was applied to determine the amount of aragonite in relation to the vaterite and calcite phases. The ATR-FTIR spectra of the composites prepared with 30 cycles with and without the addition of PVA were decomposed with the same parameters according to the following protocol. First, a baseline correction was performed on each spectrum. Peak positions and some curve parameters (Lorentzian peaks) were set and used as initial input in a curve-fitting program. The iterations were continued until the best fit was obtained. The deconvoluted signals are presented in Figure 8. The output of this analysis is expressed as relative peak area and peak position, Table 1. The data obtained in the deconvolution of the ν2CO3 signal indicate that aragonite is present in the composite prepared with 30 cycles added with PVA; however, it is the minor phase, corresponding only to 6.3% of the ν2CO3 signal. This result is in agreement with the SEM micrograph, Figure 6c, which showed only a few dendritic structures. The SEM micrograph of the composite prepared with 30 cycles without the addition of PVA demonstrates that the entire film surface is covered with dendrite-like crystals, a few rhombohedral-like particles, and no cauliflower-like particles characteristic of the vaterite phase. However, the deconvolution data of the ν2CO3 signal indicate that the aragonite phase corresponds to only 20.5% of the ν2CO3 signal. The formation of a core-shell structure, whose core is probably vaterite and the shell is constituted of aragonite crystal, is a possible explanation for this fact. Conclusion Calcium carbonate was deposited on the surface of modified PE containing an ultrathin layer of PAA immobilized on its surface. The composites were prepared by a sequential immersion procedure of the films in solutions of soluble calcium and carbonate salts. The formation of the inorganic phase starts in the PAA layer with the neutralization of the PAA acid groups. In the composite prepared with the addition of PVA in 10 and 20 immersion cycles, it was not verified the formation of crystalline phases on the surface of the composite. However, both composites presented small particles and light spots on the polymeric matrix. In contrast, the composite prepared in 30 immersion cycles with the addition of PVA formed different crystalline phases, vaterite, calcite, and aragonite, with vaterite being the major phase.

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The addition of PVA plays an important role in the formation of the crystalline material. Crystalline phases were observed on the surface of all composites prepared without the addition of PVA. For the films prepared in 10 and 20 cycles, the crystalline phases observed were vaterite and calcite. The presence of three different phases (vaterite, calcite, and aragonite) in the composite prepared with 30 cycles was confirmed.

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